MuSK activation

Novel methods for treating a condition or disorder associated with impaired neuromuscular transmission, such as MuSK myasthenia gravis, in a subject are provided herein. Novel methods for treating a symptom associated with impaired neuromuscular transmission in a subject are also provided. The invention also provides multivalent binding agents for use in treating the same.

Background

The neuromuscular junction (NMJ) is the location where the motor neuron makes contact with the muscle fibre. At this site, the motor neuron and muscle fibre together create a specialized structure that enables communication between the two cells in both directions. The motor neuron instructs the muscle to contract. Several signal transduction cascades are involved to regulate this communication process. At the centre of neuromuscular transmission is the release of acetylcholine (ACh) by the motor neuron ending. ACh diffuses through the synaptic cleft to the muscle fibre. There it binds ACh receptors (AChR), ligand-gated ion channels that are located at the muscle fibre membrane. Their opening triggers a muscle membrane action potential that ultimately evokes muscle fibre contraction.

The release of sufficient ACh and the presence of densely clustered AChR are prerequisites for successful neuromuscular transmission and subsequent muscle contraction. The agrin- low density lipoprotein receptor-related protein 4 (Lrp4) - muscle- specific kinase (MuSK) signalling cascade is an important regulator of AChR clustering and essential for establishing and maintaining neuromuscular synapses. MuSK transduces the extracellular agrin signal internally to facilitate AChR clustering. Successful ACh release and binding to the AChR is necessary for muscle contraction.

Impaired neuromuscular transmission in a subject is typically characterised by fatigable muscle weakness. Neuromuscular transmission may be impaired for a number of different reasons, including the presence of a neuromuscular transmission disorder such as autoimmune or congenital myasthenia gravis (autoimmune or congenital MG), muscular dystrophy, amyotrophic lateral sclerosis (ALS) or due to the presence of toxins, like organophosphate poisoning. Alternatively, neuromuscular transmission may become impaired due to muscle loss or disuse, or as a consequence of nerve dysfunction, both of which are suggested to play a role in muscle atrophy during ageing.

Autoimmune MG is the most common disorder of the neuromuscular synapse, affecting 10 to 20 per 100,000 people in the US. It is a debilitating autoimmune disease where autoantibodies against NMJ proteins impair neuromuscular transmission and cause fatigable muscle weakness. All skeletal muscles can be affected although, depending on MG subtype, specific subsets of muscles are more sensitive to the autoimmune attack. Approximately 80% of patients carry autoantibodies against muscle nicotinic AChR, resulting in AChR MG. In most patients the first symptom is extraocular muscle weakness, and when the disease progresses, also bulbar or generalized weakness of skeletal muscles occurs. The pathomechanism by which AChR autoantibodies cause MG is tightly related to the autoantibody isotype; immunoglobulin (IgG) 1 and lgG3. lgG1 and lgG3 are pro-inflammatory antibodies which can activate complement, bind Fc receptors on immune cells and crosslink and internalize the antigen. In addition these AChR antibodies can crosslink AChRs and cause antigen modulation of the AChRs, block the binding site of ACh, or change the affinity for ACh by inducing structural changes of the AChR. These effector functions are all contributing to the disease in AChR MG by diminishing the availability of functional AChRs and result in functional impairment and disassembly of the NMJ.

Approximately 5% of MG patients have autoantibodies against MuSK, resulting in MuSK MG (Hoch et al., 2001 ). MuSK orchestrates AChR clustering and sub-synaptic gene expression and is therefore essential for NMJ formation and maintenance (Burden et al., 2018). Bulbar and respiratory muscles are particularly affected by MuSK autoantibodies which can lead to respiratory crisis in approximately 40% of these patients (Evoli et al., 2003). In contrast to AChR MG, the autoantibodies in MuSK MG are predominantly of the lgG4 isotype (McConville et al., 2004). Epitope mapping with polyclonal serum antibodies showed that disease severity correlates with lgG4 reactivity against the N-terminal Ig-like 1 domain of MuSK (Huijbers ef al., 2016). Furthermore, passive transfer of purified human polyclonal lgG4, but not lgG1-3, from MuSK MG patients confirmed the pathogenic nature of these autoantibodies as they induced MG in immunocompromised mice (Klooster et al., 2012). In vitro studies showed that MuSK lgG4 autoantibodies purified from plasma, block the interaction between MuSK and Lrp4, thereby inducing AChR declustering which culminates in impaired neuromuscular transmission and MG (Huijbers & Zhang 2013, Koneczny et al., 2013, Otsuka ef al., 2015).

Symptomatic treatment with acetylcholinesterase inhibition provides some symptomatic relief in a minority of patients with MuSK MG, but the use is restricted due to side-effects. The majority of patients require immunosuppressive treatments such as prednisone or azathioprine. However, approximately 10-20% of patients do not benefit sufficiently from these therapeutics and require plasmapheresis, intravenous immunoglobulin (Ivlg) treatment, or rituximab treatment. In addition, for some patients none of the available treatments lead to a sustained remission and they remain dependent on chronic plasmapheresis or Ivlg. There is no cure or disease-specific therapy available for MuSK MG.

There is a need for better therapies for treating a condition or disorder associated with impaired neuromuscular transmission such as MuSK MG.

Brief summary of the disclosure

The inventors have generated recombinant MuSK antibodies from clonal MuSK-specific memory B cell cultures from MuSK MG patients to characterize them on a genetic and functional level. These included lgG1 , lgG3 and lgG4 antibodies that used different heavy and light chain variable region genes that had undergone high levels of affinity maturation in their complementary-determining regions (CDRs), consistent with antigenic selection. Binding experiments confirmed their specificity for the Ig-like 1 domain of MuSK and their affinity for mouse NMJs. It is known that lgG4 autoantibodies that bind to the Ig-like 1 domain of MuSK can cause MuSK MG. lgG4 is initially generated in vivo in bivalent, monospecific form (i.e. with two variable regions specific for the same target antigen). lgG4 then undergoes exchange of half-lgGs over time (a process called Fab-arm exchange in the literature and herein, see below), resulting in a pool of bivalent bi-specific lgG4 molecules in vivo (i.e. with two variable regions specific for different target antigens). Over time in vivo, lgG4 therefore becomes functionally monovalent for its original target antigen (i.e. although it still has two variable regions, it retains only one variable region that specifically binds to the original target; the Ig-like 1 domain of MuSK in this instance). This is a rapid process occurring within 24 hours for >99% of the lgG4 pool (provided that all requirements are met for Fab-arm exchange among which the required residues are present in the lgG4 Fc tail).

The inventors have surprisingly found that MuSK antibodies only inhibit agrin-induced MuSK phosphorylation (and thus inhibit MuSK dimerization and activation) when they are functionally monovalent for the Ig-like 1 domain of MuSK (i.e. when they retain only one variable region that specifically binds to the Ig-like 1 domain of MuSK). The data provided herein explains, for the first time, that the lgG4 autoantibodies observed in MuSK MG patients only become inhibitory in nature when they have become functionally monovalent (i.e. after they have undergone Fab-arm exchange in vivo). The ability of patient lgG4 MuSK antibodies to undergo Fab-arm exchange seems therefore crucial for the pathogenesis of disease as Fab-arm exchange renders endogenous MuSK lgG4 bi- specific and functionally monovalent for the MuSK Ig-like 1 domain. Only in this state does the lgG4 act as an inhibitor of MuSK activity, causing loss of AChR clustering and, consequently, muscle weakness.

The inventors have also shown that agrin-independent MuSK phosphorylation is, surprisingly, induced or increased when the MuSK antibodies are bivalent and monospecific for the Ig-like 1 domain of MuSK (i.e. when they have two variable regions that both specifically bind to the Ig-like 1 domain of MuSK). The inventors have therefore shown that the valency of an antibody for the Ig-like 1 domain of MuSK determines whether the antibody is either a MuSK antagonist (e.g. bivalent bi-specific antibody, or monovalent monospecific antibody fragment etc) or a MuSK agonist (e.g. the bivalent, monospecific antibody). The data generated herein indicates that any binding agent with only one binding region specific for the Ig-like 1 domain of MuSK can act as a MuSK antagonist, while binding agents with at least two binding regions specific for the Ig-like 1 domain of MuSK can act as a MuSK agonist (by inducing MuSK dimerization, phosphorylation and activation). The invention is completely surprising, as the data presented herein shows that antibodies considered to be causative of neuromuscular failure and disease (MuSK Ig- like 1 domain lgG4 antibodies) can in fact be used as an effective treatment for the same neuromuscular disease when forced to retain bivalent monospecific form. MuSK agonists described herein can therefore advantageously be used to treat a condition or disorder associated with impaired neuromuscular transmission (such as MuSK MG), or a symptom following from impaired neuromuscular transmission (i.e. muscle weakness), in a subject.

Although the data used herein has exemplified the invention using lgG1 , and lgG4 and Fab, the invention also encompasses the use of other IgG isotypes (e.g. lgG1 , lgG2, lgG3), other antibodies (e.g. IgM), and other binding agents comprising at least two binding regions that are specific for the MuSK Ig-like 1 domain. Advantageously, any of the described binding agents with at least two binding regions that are specific for the MuSK Ig-like 1 domain can act as MuSK agonists by inducing MuSK phosphorylation and/or dimerization and/or activation.

In one aspect, a multivalent binding agent comprising at least two binding regions that specifically bind to an Ig-like 1 domain of a MuSK protein is provided for use in treating a symptom, condition and/or disorder associated with impaired neuromuscular transmission, in a subject.

Advantageously, the binding agent described herein induces (i.e. is capable of inducing) MuSK phosphorylation and/or dimerisation (and/or activation).

Suitably, the binding agent is a binding protein.

Suitably, the binding region that specifically binds to an Ig-like 1 domain of the MuSK protein is a variable region. Suitably, the MuSK protein is a human MuSK protein.

Suitably, the binding agent is bivalent or trivalent.

Suitably, the binding agent is an antibody.

Suitably, the antibody is a monoclonal antibody. Suitably, the antibody is:

(a) a human antibody or a humanised antibody; or

(b) a chimeric antibody.

Suitably, the antibody is selected from a Fab2, Fab3 _, di-scFv, scFv-Fc, duobody, camelid antibody or a minibody.

Suitably, the antibody is an IgG. The IgG may be selected from lgG1 , lgG2 or lgG3. Alternatively, the IgG may be an lgG4 variant with a reduced ability or an inability for Fab- arm exchange in vivo.

Suitably, the lgG4 variant comprises an lgG4 constant region comprising one or more amino acid substitutions that reduce the ability for or prevent Fab-arm exchange in vivo.

Suitably, the lgG4 variant comprises an lgG4 constant region comprising an amino acid substitution at amino acid position 228 and/or an amino acid substitution at amino acid position 409 and/or an amino acid substitution at amino acid position 405 of the heavy chain numbered according to the EU index.

Suitably, the subject is a human.

Suitably, the symptom, condition and/or disorder associated with impaired neuromuscular transmission is selected from MuSK MG, autoimmune MG, congenital myasthenic syndrome (CMS), muscular dystrophy, motor neuron disease, sarcopenia, muscle disuse or sports injury.

Suitably, the symptom, condition and/or disorder associated with impaired neuromuscular transmission is MuSK MG; wherein optionally the MuSK MG is selected from refractory generalised MG or severe MG; and/or alternatively wherein the MuSK MG is any MG patient undergoing treatment with corticosteroids or immunosuppressive drugs.

Suitably, the binding agent is recombinant and the at least two binding regions are variable regions derived from a MuSK antibody obtained from the subject.

Suitably, the binding agent is an antibody and at least one or at least two of the binding regions that specifically bind to an Ig-like 1 domain of a MuSK protein has a sequence selected from:

a) a VH CDR1 comprising SEQ ID NO: 10, a VH CDR2 comprising SEQ ID NO: 1 1 , a VH CDR3 comprising SEQ ID NO: 12, a VL CDR1 comprising SEQ ID NO: 14, a VL CDR2 comprising SEQ ID NO: 15, and a VL CDR3 comprising SEQ ID NO: 16; optionally wherein the binding region comprises a heavy chain variable domain comprising SEQ ID NO: 9 and a light chain variable domain comprising SEQ ID NO: 13;

b) a VH CDR1 comprising SEQ ID NO:18, a VH CDR2 comprising SEQ ID NO: 19, a VH CDR3 comprising SEQ ID NO:20, a VL CDR1 comprising SEQ ID NO:22, a VL CDR2 comprising SEQ ID NO:23, and a VL CDR3 comprising SEQ ID NO: 24, optionally wherein the binding region comprises a heavy chain variable domain comprising SEQ ID NO: 17 and a light chain variable domain comprising SEQ ID NO:21 ;

c) a VH CDR1 comprising SEQ ID NO:26, a VH CDR2 comprising SEQ ID NO:27, and a VH CDR3 comprising SEQ ID NO:28, optionally wherein the binding region comprises a heavy chain variable domain comprising SEQ ID NO: 25;

d) a VH CDR1 comprising SEQ ID NO:30, a VH CDR2 comprising SEQ ID NO:31 , a VH CDR3 comprising SEQ ID NO:32, a VL CDR1 comprising SEQ ID NO:34, a VL CDR2 comprising SEQ ID NO:35, and a VL CDR3 comprising SEQ ID NO: 36, optionally wherein the binding region comprises a heavy chain variable domain comprising SEQ ID NO: 29 and a light chain variable domain comprising SEQ ID NO: 33; e) a VH CDR1 comprising SEQ ID NO:38, a VH CDR2 comprising SEQ ID NO:39, a VH CDR3 comprising SEQ ID NO:40, a VL CDR1 comprising SEQ ID NO:42, a VL CDR2 comprising SEQ ID NO:43, and a VL CDR3 comprising SEQ ID NO: 44, optionally wherein the binding region comprises a heavy chain variable domain comprising SEQ ID NO: 37 and a light chain variable domain comprising SEQ ID NO:41 ;

f) a VH CDR1 comprising SEQ ID NO:46, a VH CDR2 comprising SEQ ID NO:47, a VH CDR3 comprising SEQ ID NO:48, a VL CDR1 comprising SEQ ID NO:50, a VL CDR2 comprising SEQ ID NO:51 , and a VL CDR3 comprising SEQ ID NO: 52, optionally wherein the binding region comprises a heavy chain variable domain comprising SEQ ID NO: 45 and a light chain variable domain comprising SEQ ID NO: 49;

g) a VH CDR1 comprising SEQ ID NO:54, a VH CDR2 comprising SEQ ID NO:55, a VH CDR3 comprising SEQ ID NO:56, a VL CDR1 comprising SEQ ID NO:58, a VL CDR2 comprising SEQ ID NO:59, and a VL CDR3 comprising SEQ ID NO: 60, optionally wherein the binding region comprises a heavy chain variable domain comprising SEQ ID NO: 53 and a light chain variable domain comprising SEQ ID NO: 57; or

h) combinations of any of the above.

Suitably, the at least two binding regions that specifically bind to an Ig-like 1 domain of a MuSK protein:

i) each comprise the sequence provided in a);

ii) each comprise the sequence provided in b);

iii) each comprise the sequence provided in c);

iv) each comprise the sequence provided in d);

v) each comprise the sequence provided in e);

vi) each comprise the sequence provided in f); or

vii) each comprise the sequence provided in g).

A method of treating a subject having a symptom, condition and/or disorder associated with impaired neuromuscular transmission is also provided, the method comprising administering to the subject a multivalent binding agent comprising at least two binding regions that specifically bind to an Ig-like 1 domain of a MuSK protein. A method for inducing or increasing MuSK activity in a subject in need thereof is also provided, comprising administering to the subject a multivalent binding agent comprising at least two binding regions that specifically bind to an Ig-like 1 domain of a MuSK protein. Suitably, the subject has a symptom, condition and/or disorder associated with impaired neuromuscular transmission.

Advantageously, the binding agent described herein induces (i.e. is capable of inducing) MuSK phosphorylation and/or dimerization (and/or activation).

Suitably, the binding agent is a binding protein.

Suitably, the region that specifically binds to an Ig-like 1 domain of the MuSK protein is a variable region.

Suitably, the MuSK protein is a human MuSK protein.

Suitably, the binding agent is bivalent or trivalent. Suitably, the binding agent is an antibody.

Suitably, the antibody is a monoclonal antibody.

Suitably, the antibody is:

(a) a human antibody or a humanised antibody; or

(b) a chimeric antibody.

Suitably, the antibody is selected from a Fab2, Fab _3, di-scFv, scFv-Fc, duobody, camelid antibody, or a minibody. Suitably, the antibody is an IgG. The IgG may be selected from lgG1 , lgG2 or lgG3. Alternatively, the IgG may be an lgG4 variant with a reduced ability or an inability for Fab- arm exchange in vivo. Suitably, the lgG4 variant comprises an lgG4 constant region comprising one or more amino acid substitutions that reduce the ability for or prevent Fab-arm exchange in vivo.

Suitably, the lgG4 variant comprises an lgG4 constant region comprising an amino acid substitution at amino acid position 228 and/or an amino acid substitution at amino acid position 409 and/or an amino acid substitution at amino acid position 405 of the heavy chain numbered according to the EU index.

Suitably, the subject is a human. Suitably, the symptom, condition and/or disorder associated with impaired neuromuscular transmission is selected from MuSK MG, any form of autoimmune MG, congenital myasthenic syndrome (CMS), muscular dystrophy, motor neuron disease, sarcopenia, muscle disuse or sports injury. Suitably, the symptom, condition and/or disorder associated with impaired neuromuscular transmission is MuSK MG; wherein optionally the MuSK MG is selected from refractory generalised MG or severe MG; and/or alternatively wherein the MuSK MG is any MG patient undergoing treatment with corticosteroids or immunosuppressive drugs. Suitably, the binding agent is recombinant and the at least two binding regions are variable regions derived from a MuSK antibody obtained from the subject.

Suitably, the binding agent is an antibody and at least one or at least two of the binding regions that specifically bind to an Ig-like 1 domain of a MuSK protein has a sequence selected from:

a) a VH CDR1 comprising SEQ ID NO: 10, a VH CDR2 comprising SEQ ID NO:1 1 , a VH CDR3 comprising SEQ ID NO: 12; and/or a VL CDR1 comprising SEQ ID NO: 14, a VL CDR2 comprising SEQ ID NO: 15, and a VL CDR3 comprising SEQ ID NO: 16; optionally wherein the binding region comprises a heavy chain variable domain comprising SEQ ID NO: 9 and/or a light chain variable domain comprising SEQ ID NO: 13 respectively; b) a VH CDR1 comprising SEQ ID NO:18, a VH CDR2 comprising SEQ ID NO: 19, a VH CDR3 comprising SEQ ID NO:20; and/or a VL CDR1 comprising SEQ ID NO:22, a VL

CDR2 comprising SEQ ID NO:23 and a VL CDR3 comprising SEQ ID NO: 24, optionally wherein the binding region comprises a heavy chain variable domain comprising SEQ ID NO: 17 and/or a light chain variable domain comprising SEQ ID NO:21 respectively; c) a VH CDR1 comprising SEQ ID NO:26, a VH CDR2 comprising SEQ ID NO:27, and a VH CDR3 comprising SEQ ID NO:28, optionally wherein the binding region comprises a heavy chain variable domain comprising SEQ ID NO: 25;

d) a VH CDR1 comprising SEQ ID NO:30, a VH CDR2 comprising SEQ ID NO:31 , a VH CDR3 comprising SEQ ID NO:32; and/or a VL CDR1 comprising SEQ ID NO:34, a VL CDR2 comprising SEQ ID NO:35, and a VL CDR3 comprising SEQ ID NO: 36, optionally wherein the binding region comprises a heavy chain variable domain comprising SEQ ID NO: 29 and/or a light chain variable domain comprising SEQ ID NO: 33 respectively; e) a VH CDR1 comprising SEQ ID NO:38, a VH CDR2 comprising SEQ ID NO:39, a VH CDR3 comprising SEQ ID NO:40; and/or a VL CDR1 comprising SEQ ID NO:42, a VL CDR2 comprising SEQ ID NO:43, and a VL CDR3 comprising SEQ ID NO: 44, optionally wherein the binding region comprises a heavy chain variable domain comprising SEQ ID NO: 37 and/or a light chain variable domain comprising SEQ ID NO:41 respectively; f) a VH CDR1 comprising SEQ ID NO:46, a VH CDR2 comprising SEQ ID NO:47, a VH CDR3 comprising SEQ ID NO:48; and/or a VL CDR1 comprising SEQ ID NO:50, a VL CDR2 comprising SEQ ID NO:51 , and a VL CDR3 comprising SEQ ID NO: 52, optionally wherein the binding region comprises a heavy chain variable domain comprising SEQ ID NO: 45 and a light chain variable domain comprising SEQ ID NO: 49 respectively;

g) a VH CDR1 comprising SEQ ID NO:54, a VH CDR2 comprising SEQ ID NO:55, a VH CDR3 comprising SEQ ID NO:56; and/or a VL CDR1 comprising SEQ ID NO:58, a VL CDR2 comprising SEQ ID NO:59, and a VL CDR3 comprising SEQ ID NO: 60, optionally wherein the binding region comprises a heavy chain variable domain comprising SEQ ID NO: 53 and/or a light chain variable domain comprising SEQ ID NO: 57; or

h) combinations of any of the above. Suitably, the at least two binding regions that specifically bind to an Ig-like 1 domain of a MuSK protein:

i) each comprise the sequence provided in a);

ii) each comprise the sequence provided in b);

iii) each comprise the sequence provided in c);

iv) each comprise the sequence provided in d);

v) each comprise the sequence provided in e);

vi) each comprise the sequence provided in f); or

vii) each comprise the sequence provided in g).

In one aspect, an antibody is provided comprising at least two binding regions that specifically bind to an Ig-like 1 domain of a MuSK protein, wherein at least one or at least two of the binding regions that specifically bind to an Ig-like 1 domain of a MuSK protein has a sequence selected from:

a) a VH CDR1 comprising SEQ ID NO: 10, a VH CDR2 comprising SEQ ID NO: 1 1 , a VH CDR3 comprising SEQ ID NO: 12; and/or a VL CDR1 comprising SEQ ID NO: 14, a VL CDR2 comprising SEQ ID NO:15, and a VL CDR3 comprising SEQ ID NO: 16; optionally wherein the binding region comprises a heavy chain variable domain comprising SEQ ID NO: 9 and/or a light chain variable domain comprising SEQ ID NO: 13 respectively;

b) a VH CDR1 comprising SEQ ID NO:18, a VH CDR2 comprising SEQ ID NO: 19, a VH CDR3 comprising SEQ ID NO:20; and/or a VL CDR1 comprising SEQ ID NO:22, a VL CDR2 comprising SEQ ID NO:23, and a VL CDR3 comprising SEQ ID NO: 24, optionally wherein the binding region comprises a heavy chain variable domain comprising SEQ ID NO: 17 and/or a light chain variable domain comprising SEQ ID NO:21 respectively; c) a VH CDR1 comprising SEQ ID NO:26, a VH CDR2 comprising SEQ ID NO:27, and a VH CDR3 comprising SEQ ID NO:28, optionally wherein the binding region comprises a heavy chain variable domain comprising SEQ ID NO: 25;

d) a VH CDR1 comprising SEQ ID NO:30, a VH CDR2 comprising SEQ ID NO:31 , a VH CDR3 comprising SEQ ID NO:3; and/or a VL CDR1 comprising SEQ ID NO:34, a VL

CDR2 comprising SEQ ID NO:35, and a VL CDR3 comprising SEQ ID NO: 36, optionally wherein the binding region comprises a heavy chain variable domain comprising SEQ ID NO: 29 and/or a light chain variable domain comprising SEQ ID NO: 33 respectively; e) a VH CDR1 comprising SEQ ID NO:38, a VH CDR2 comprising SEQ ID NO:39, a VH CDR3 comprising SEQ ID NO:40 and/or a VL CDR1 comprising SEQ ID NO:42, a VL CDR2 comprising SEQ ID NO:43, and a VL CDR3 comprising SEQ ID NO: 44, optionally wherein the binding region comprises a heavy chain variable domain comprising SEQ ID NO: 37 and/or a light chain variable domain comprising SEQ ID NO:41 respectively; f) a VH CDR1 comprising SEQ ID NO:46, a VH CDR2 comprising SEQ ID NO:47, a VH CDR3 comprising SEQ ID NO:48; and/or a VL CDR1 comprising SEQ ID NO:50, a VL CDR2 comprising SEQ ID NO:51 , and a VL CDR3 comprising SEQ ID NO: 52, optionally wherein the binding region comprises a heavy chain variable domain comprising SEQ ID NO: 45 and/or a light chain variable domain comprising SEQ ID NO: 49 respectively; g) a VH CDR1 comprising SEQ ID NO:54, a VH CDR2 comprising SEQ ID NO:55, a VH CDR3 comprising SEQ ID NO:56; and/or a VL CDR1 comprising SEQ ID NO:58, a VL CDR2 comprising SEQ ID NO:59, and a VL CDR3 comprising SEQ ID NO: 60, optionally wherein the binding region comprises a heavy chain variable domain comprising SEQ ID NO: 53 and/or a light chain variable domain comprising SEQ ID NO: 57 respectively; or h) combinations of any of the above. Suitably, the at least two binding regions that specifically bind to an Ig-like 1 domain of a MuSK protein:

i) each comprise the sequence provided in a);

ii) each comprise the sequence provided in b);

iii) each comprise the sequence provided in c);

iv) each comprise the sequence provided in d);

v) each comprise the sequence provided in e);

vi) each comprise the sequence provided in f); or

vii) each comprise the sequence provided in g). Throughout the description and claims of this specification, the words“comprise” and “contain” and variations of them mean“including but not limited to”, and they are not intended to (and do not) exclude other moieties, additives, components, integers or steps. Throughout the description and claims of this specification, the singular encompasses the plural unless the context otherwise requires. In particular, where the indefinite article is used, the specification is to be understood as contemplating plurality as well as singularity, unless the context requires otherwise.

Features, integers, characteristics, compounds, chemical moieties or groups described in conjunction with a particular aspect, embodiment or example of the invention are to be understood to be applicable to any other aspect, embodiment or example described herein unless incompatible therewith.

Various aspects of the invention are described in further detail below.

Brief description of the drawings

Embodiments of the invention are further described hereinafter with reference to the accompanying drawings, in which:

Figure 1 shows that patient derived- recombinant MuSK antibodies bind mouse NMJs in whole mount levator auris longus muscle. Scale marker is 25 urn.

Figure 2 shows that patient-derived recombinant MuSK antibodies can activate or inhibit MuSK phosphorylation and AChR clustering depending on the antibody valency. Bivalent monospecific recombinant MuSK antibodies (Clone# 1 1 -3F6 and 13-3B5) activated MuSK phosphorylation in the presence and absence of agrin (A). Activation of MuSK phosphorylation was dose-dependent (B). Clone 13-3B5 was slightly more potent compared to 1 1 -3F6. A biotin control antibody did not affect (agrin-dependent) MuSK phosphorylation. Monovalent Fab fragments generated from these recombinant MuSK monoclonals inhibited MuSK phosphorylation (C). Agrin-dependent AChR clustering was unaffected when exposed to a biotin control antibody or Fab fragments thereof (D). Bivalent monospecific recombinant lgG4 monoclonal MuSK antibodies significantly increased AChR clustering compared to Fab fragments derived from the same monoclonal, although agrin-dependent AChR clustering remained lower than the anti- biotin control (for lgG4: P=0.0004, for Fab fragments: P=0.0001 , one-way ANOVA Dunnett corrected, 11-3F6 lgG4 vs Fab: P=0.0003, 13-3B5 lgG4 vs Fab: P= 0.03, anti- biotin lgG4 vs Fab: P= 0.3, unpaired t test,). Fab fragments reduced AChR cluster to the level of purified patient lgG4 and the “no agrin” condition. Bivalent monospecific antibodies furthermore increased AChR clustering independent from agrin (11 -3F6: P= 0.03, 13-3B5: P= 0.2 one-way ANOVA Dunnett corrected). Data represent mean ± SEM. Scale bar represents 50pm.

Figure 3 shows MuSK reactivity results for 8 B cell clones isolated from MuSK MG patients. MuSK was produced in yeast and E. Coli. MuSK reactive B cell clones isolated from MuSK MG patients and healthy donors were single cell sorted and differentiated to plasma cells. Medium from these single cell cultures was tested in ELISA for MuSK (produced in E. Coli.) reactivity using subclass-(a)specific (anti-lgG1 or anti-lgG4 or anti- IgG total) secondary antibodies. In these experiments 8 MuSK reactive B cell clones were isolated, of which 7 yielded a B cell receptor (BCR) sequence. The level of reactivity against the MuSK protein varied between clones. A large number of wells with MuSK non-reactive clones derived from both the healthy donor cohort and MuSK MG patients were also tested. One clone from the healthy donor seemed to have some reactivity to MuSK in this assay (11 -1 B3). Figure 4 shows that the reactivity observed for MuSK clones isolated from MuSK MG patients were specific for MuSK, whereas the reactivity observed for the MuSK clone (11 - 1 B3) that was isolated from a healthy donor was not. The specificity of MuSK reactivity was also tested for 4 MuSK MG clones using MuSK produced in yeast and compared to a control antigen (the acetylcholine receptor alpha subunit) produced in the same system. The“MuSK” positive clone from the healthy donor (11 -1 B3) did not show reactivity to MuSK produced in yeast suggesting that this antibody/BCR sequence recognized an E. Coli. related protein rather than being MuSK-specific. The MuSK clones isolated from MuSK MG patients (11 -3F6, 11-7C5, 11-8G4, and 11 -3D9) showed clear reactivity against yeast produced MuSK.

Figure 5 shows sequence alignments using Clustal W alignment program. Figure 6 - anti-MuSK/Anti-HIV-b12 exchanged heterospecific lgG4 antibodies were generated using methods detailed in Labrijn, 2014, Nature Protocols and Genmab, Labrijn, 2013, PNAS, which were adjusted to lgG4. These antibodies were tested for MuSK binding in mouse neuromuscular junction (NMJ) in ex vivo levator auris longus muscle. Equivalent anti-MuSK homospecific antibodies were also tested in parallel. The data confirms that both heterospecific MuSK lgG4 antibodies (13-3B5/HIV lgG4 and 11 - 3F6/HIV lgG4) and homospecific MuSK lgG4 antibodies (13-3B5 lgG4 and 11-3F6 lgG4) bind to MuSK at the NMJ. Figure 7 - heterospecific MuSK lgG4 antibodies cause inhibition of agrin induced MuSK phosphorylation whereas homospecific MuSK lgG4 antibodies activate phosphorylation in absence of agrin.

Figure 8 shows that homospecific versus heterospecific musk antibodies have differential effects on in vivo neuromuscular performance. Nod/scid mice were i.p. Injected with 5ug/gbw recombinant antibody on day 0, 3 and 7. The heterospecific (functionally monovalent) version of both clones induce quick and severe muscle weakness and body weight loss. Homospecific 11 -3F6 lgG4 did not induce muscle weakness or body weight loss. I.p. = intraperitoneal, gbw = gram body weight.

Detailed Description

The inventors have made recombinant lgG1 and lgG4 antibodies that are bivalent and monospecific for the Ig-like 1 domain of MuSK, using the variable domains of lgG4 MuSK antibodies obtained from MuSK MG patients.

Surprisingly, patient-derived recombinant MuSK monoclonal antibodies (both lgG1 and lgG4) activated rather than inhibited MuSK phosphorylation (Figure 2A). This effect was observed in both the absence and presence of agrin. Activation of MuSK phosphorylation was concentration-dependent (Figure 2B) and differed slightly between the two clones. This suggests that patient-derived bivalent monospecific MuSK antibodies binding the Ig- like 1 domain facilitate dimerization and activation of MuSK in vitro. Recombinant monoclonal lgG1 and lgG4 however both engage in bivalent monospecific antibody-antigen interactions. To investigate the functional effects of the bispecificity and functional monovalency of Fab-arm exchanged lgG4 MuSK antibodies in patients, the inventors generated monovalent Fab fragments from these recombinant antibodies by papain digestion. In vitro, these Fab fragments inhibited agrin-dependent MuSK phosphorylation (Figure 2C) and AChR clustering similar to patient serum-derived anti- MuSK lgG4 (Figure 2D). In contrast, (and in line with activating MuSK phosphorylation in vitro), bivalent monospecific monoclonal MuSK antibodies activated agrin-dependent AChR clustering compared to Fab fragments from the same monoclonal. Furthermore, AChR clustering could be partially induced using bivalent monospecific antibodies independent from agrin (Figure 2D). Thus, monovalent MuSK binding blocks the AChR clustering pathway, whereas bivalent monospecific MuSK antibodies stimulate MuSK, and can facilitate or induce AChR clustering in this tissue culture model. AChR clustering at the NMJ is critical for successful neuromuscular transmission and muscle contraction (Burden et al., 2018). Lower levels of AChR clustering are tolerated in patients until they reach a critical threshold. For example, for MuSK MG there is a dose dependent decrease in AChR clustering in mice passively transferred with MuSK antibodies (Klooster et al., 2012). Increasing AChR clustering will therefore result in improved neuromuscular transmission. Small increases in neuromuscular transmission can be therapeutically effective in patients (as exemplified by, for example, acetylcholine esterase treatment which is the first line symptomatic treatment for AChR MG patients). Advantageously, the bivalent monospecific MuSK antibodies described herein may therefore be used as a therapeutic agent to improve AChR clustering and synaptic stability at the NMJ.

The invention has been exemplified using antibodies. However, the general concept also applies to other binding agents, with binding regions that are specific for the MuSK Ig-like 1 domain. As an example, the data provided herein demonstrate that bivalent monospecific MuSK Ig-like 1 binding agents (i.e. binding agents with two binding regions, both of which are specific for the MuSK Ig-like 1 domain) can act as MuSK agonists (by inducing MuSK dimerization and/or phosphorylation and/or activation), whereas bivalent bi-specific MuSK Ig-like 1 binding agents (i.e. binding agents with two binding regions, only one of which is specific for the MuSK Ig-like 1 domain) can act as MuSK antagonists (by preventing MuSK dimerization and/or MuSK phosphorylation and/or MuSK activation).

The invention described herein is based on the finding that a multivalent binding agent comprising at least two binding regions that specifically bind to an Ig-like 1 domain of a MuSK protein can be used as a MuSK agonist at the NM J by inducing MuSK dimerization. The binding agents described herein may therefore be used for treating a symptom, condition and/or disorder associated with impaired neuromuscular transmission in a subject.

Binding agents

A multivalent binding agent comprising at least two binding regions that specifically bind to an Ig-like 1 domain of a MuSK protein is described herein.

Advantageously, the binding agent induces MuSK dimerization (which in turn induces the phosphorylation and activation of MuSK). In other words, the binding agents described herein induce (i.e. are capable of inducing) MuSK dimerization and/or MuSK phosphorylation and/or MuSK activation.

The binding agent is useful for treating a symptom, condition and/or disorder associated with impaired neuromuscular transmission, in a subject. As defined in more detail below,“induce” or“induction” of MuSK dimerization (and/or MuSK phosphorylation and/or MuSK activation) refers to an increase in MuSK dimerization etc in the subject and thus encompasses partial induction (e.g. induction of some but not all MuSK dimerization etc in a subject). In one specific example, the multivalent binding agent is a binding protein comprising at least two binding regions that specifically bind to an Ig-like 1 domain of a MuSK protein. In this context, the binding protein may be an antibody as described in more detail below, wherein the binding regions are variable regions. In this example, the antibody may comprise at least two variable regions, wherein each of the two variable regions specifically binds to an Ig-like 1 domain of a MuSK protein. As described herein, the binding agent (e.g. binding protein, e.g. antibody) is multivalent. As used herein, a“multivalent binding agent” refers to a binding agent with a plurality of (i.e. more than one) binding regions (i.e. a plurality of antigen-binding sites). A multivalent binding agent may therefore have two, three, four, five, six or more binding regions/antigen-binding sites. As an example a“bivalent” binding agent is a multivalent binding protein having two binding regions/antigen-binding sites, whereas a“trivalent” binding protein is a multivalent binding protein having three binding regions/antigen- binding sites. By contrast, a “monovalent” binding protein has only one binding region/antigen-binding site. The terms“binding region”,“antigen-binding site” and“epitope-binding site” are used interchangeably herein unless the context indicates otherwise. As an example, the binding agent may have only two binding regions (i.e. two binding regions that specifically bind to an Ig-like 1 domain of a MuSK protein), and no other binding regions. In this context, the binding agent is bivalent.

For binding agents (e.g. binding proteins, e.g. antibodies) described herein that have more than two binding regions, at least two of these binding regions must specifically bind to an Ig-like 1 domain of a MuSK protein. The remaining binding region(s) of the multivalent binding agent (i.e. the third, fourth, fifth etc binding region of the multivalent binding agent) may also specifically bind to an Ig-like 1 domain of a MuSK protein but they may also alternatively specifically bind to a different antigen/epitope. Typically, the remaining binding region(s) of the multivalent binding agent (i.e. the third, fourth, fifth etc binding region of the multivalent binding agent) specifically bind to a different antigen (e.g. they do not specifically bind to MuSK). However, for the avoidance of doubt, in an alternative example, the remaining binding region(s) of the multivalent binding agent (i.e. the third, fourth, fifth etc binding region of the multivalent binding agent) may also specifically bind to the Ig-like 1 domain of a MuSK protein or they may specifically bind to another region of the MuSK protein (e.g. the Ig-like 2 domain, Ig-like 3 domain, Frizzled domain etc) provided that the multivalent binding agent remains able to (e.g. is capable of) inducing MuSK dimerization and/or phosphorylation and/or activation and thus does not otherwise interfere with MuSK function. Methods for identifying whether or not MuSK dimerization and/or phosphorylation and/or activation occurs are well known in the art.

In one particular example, the binding agents described herein are antibodies. In this context, the binding regions of the antibody may be variable regions. The term "variable region" refers to the region of an immunoglobulin that comprises one or more Ig domains substantially encoded by any of the VK, VA, and/or VH genes that make up the kappa, lambda, and heavy chain immunoglobulin genetic loci respectively. More specifically, the term refers to the domain of an antibody heavy or light chain that is involved in binding the antibody to an antigen. The variable region of an immunoglobulin is therefore typically made up of two variable domains (i.e. the variable domain of the heavy chain and the variable domain of the light chain). The variable domains of the heavy chain and light chain (VH and VL, respectively) of a native antibody generally have similar structures, with each domain comprising four conserved framework regions (FRs) and three hypervariable regions (HVRs). (See, e.g., Kindt et al. Kuby Immunology, 6th ed., W.H. Freeman and Co., page 91 (2007).). A single VH or VL domain may be sufficient to confer antigen-binding specificity.

The term "hypervariable region" refers to the amino acid residues of an antibody which are responsible for antigen-binding. The hypervariable region generally comprises amino acid residues from a "complementarity determining region" or "CDR" (e.g. residues 24- 34 (L1 ), 50-56 (L2) and 89-97 (L3) of the light chain variable domain and 31-35 (H1 ), 50- 65 (H2) and 95-102 (H3) of the heavy chain variable domain according to Kabat et al, Sequences of Proteins of Immunological Interest, 5th Ed. Public Health Service, National Institutes of Health, Bethesda, MD. (1991 )) and/or those residues from a "hypervariable loop" (e.g. residues 26-32 (L1 ), 50-52 (L2) and 91-96 (L3) in the light chain variable domain and 26-32 (H1 ), 53-55 (H2) and 96-101 (H3) in the heavy chain variable domain according to Chothia and Lesk J. Mol. Biol. 196:901 -917 (1987)). The amino-terminal portion of each chain includes a variable region of about 100 to 1 10 or more amino acids primarily responsible for antigen recognition, generally referred to in the art and herein as the "Fv domain" or "Fv region". In the variable region, three loops are gathered for each of the V domains of the heavy chain and light chain to form an antigen-binding site. Each of the loops is referred to as a complementarity-determining region (hereinafter referred to as a "CDR"), in which the variation in the amino acid sequence is most significant. "Variable" refers to the fact that certain segments of the variable region differ extensively in sequence among antibodies. Variability within the variable region is not evenly distributed. Instead, the V regions consist of relatively invariant stretches called framework regions (FRs) of 15-30 amino acids separated by shorter regions of extreme variability called "hypervariable regions" that are each 9-15 amino acids long or longer. Each VH and VL is composed of three hypervariable regions ("complementary determining regions," "CDRs") and four FRs, arranged from amino- terminus to carboxy-terminus in the following order: FR1 -CDR1 -FR2-CDR2-FR3- CDR3-FR4.

An antibody of the invention may comprise at least two binding regions (i.e. variable regions) that specifically bind to an Ig-like 1 domain of a MuSK protein. In the context of an antibody, the terms variable region and binding region are used interchangeably herein. Each variable region may comprise the CDRs of a light chain variable domain (VL) (i.e. VL CDR1 , VL CDR2, and VL CDR3) and/or the CDRs of a heavy chain variable domain (VH) (i.e. VH CDR1 , VH CDR2, and VH CDR3). As stated elsewhere herein, the CDRs from one of VL or VH may be sufficient to confer antigen binding specificity (i.e. specific binding to the Ig-like 1 domain of MuSK). In other cases, antigen binding specificity (i.e. specific binding to the Ig-like 1 domain of MuSK) may be obtained by the presence of CDRs 1 , 2, and 3, from both VL and VH.

Specific examples of combinations of CDRs that confer antigen binding specificity to the Ig-like 1 domain of MuSK are provided below. The CDR sequences have been identified using IMGT/V-QUEST program version: 3.4.17 (19 February 2019) - IMGT/V-QUEST reference directory release: 201910-2 (5 March 2019) (http://imgt.org/IMGT_vquest/vquest), selecting for homo sapiens sequences. These CDR combinations form a binding region (i.e. variable region) of an antibody (including a Fab2, Fab3, di-scFv, scFV-Fc, duobody, camelid antibody or minibody) that specifically binds to an Ig-like 1 domain of a MuSK protein:

1 ) CDR1 comprising GFNFSTYT (SEQ ID NO:10), CDR2 comprising ISSRSAYK (SEQ ID NO: 11 ) and CDR3 comprising ARDFFQLGPPRFDS (SEQ ID NO:12). These CDRs may optionally be in the context of a VH, e.g. a VH comprising the sequence of SEQ ID NO:9.

2) CDR1 comprising QRISSF (SEQ ID NO:14), CDR2 comprising GAS (SEQ ID NO:

15) and CDR3 comprising QQSYSPMYT (SEQ ID NO: 16). These CDRs may optionally be in the context of a VL, e.g. a VL comprising the sequence of SEQ ID NO: 13. 3) CDRs of SEQ ID NO: 10, 11 and 12 (heavy chain) and CDRs of SEQ ID NO: 14, 15 and 16 (light chain) e.g. in the context of a VH and VL. For example, antigen specificity may be obtained by a combination of variable domains of SEQ ID NO: 9 (heavy chain) and SEQ: 13 (light chain). 4) CDR1 comprising GFTFSSYT (SEQ ID NO:18), CDR2 comprising IGSNGDYI (SEQ

ID NO: 19) and CDR3 comprising ARGQLAVAGTHFDY (SEQ ID NO:20). These CDRs may optionally be in the context of a VH e.g. a VH comprising the sequence of SEQ ID NO:17. 5) CDR1 comprising QKVNKY (SEQ ID NO:22), CDR2 comprising AAS (SEQ ID NO:

23) and CDR3 comprising QQSYSPLCT (SEQ ID NO:24). These CDRs may optionally be in the context of a VL, e.g. a VL comprising the sequence of SEQ ID NO:21.

6) CDRs of SEQ ID NO: 18, 19 and 20 (heavy chain) and CDRs of SEQ ID NO: NO: 22, 23 and 24 (light chain) e.g. in the context of a VH and VL. For example, antigen specificity may be obtained by a combination of variable domains of SEQ ID NO: 17 (heavy chain) and SEQ: 21 (light chain). 7) CDR1 comprising GFTFSDFT (SEQ ID NO:26), CDR2 comprising IGSSGTFI (SEQ ID NO: 27) and CDR3 comprising ARGRIAVAGTHFDL (SEQ ID NO:28). These CDRs may optionally be in the context of a VH, e.g. a VH comprising the sequence of SEQ ID NO:25.

8) CDR1 comprising GYTFTGQY (SEQ ID NO:30), CDR2 comprising INPSSGVT (SEQ ID NO: 31 ) and CDR3 comprising ATLSLGVYYVGMVA (SEQ ID NO:32). These CDRs may optionally be in the context of a VH, e.g. a VH comprising the sequence of SEQ ID NO:29.

9) CDR1 comprising GLAQQH (SEQ ID NO:34), CDR2 comprising KDI (SEQ ID NO: 35) and CDR3 comprising QSGDRTATSVL (SEQ ID NO:36). These CDRs may optionally be in the context of a VL, e.g. a VL comprising the sequence of SEQ ID NO:33.

10) CDRs of SEQ ID NO: 30, 31 and 32 (heavy chain) and CDRs of SEQ ID NO: 34, 35 and 36 (light chain) e.g. in the context of a VH and VL. For example, antigen specificity may be obtained by a combination of variable domains of SEQ ID NO: 29 (heavy chain) and SEQ: 33 (light chain).

11 ) CDR1 comprising GFDFSAST (SEQ ID NO:38), CDR2 comprising VSGDSHHI (SEQ ID NO: 39) and CDR3 comprising ARERLLRLGVGFDS (SEQ ID NO:40). These CDRs may optionally be in the context of a VH, e.g. a VH comprising the sequence of SEQ ID NO:37.

12) CDR1 comprising QRISGF (SEQ ID NO:42), CDR2 comprising AAS (SEQ ID NO: 43) and CDR3 comprising QQSYSPLYT (SEQ ID NO:44). These CDRs may optionally be in the context of a VL, e.g. a VL comprising the sequence of SEQ ID NO:41.

13) CDRs of SEQ ID NO: 38, 39 and 40 (heavy chain) and CDRs of SEQ ID NO: 42, 43 and 44 (light chain) e.g. in the context of a VH and VL. For example, antigen specificity may be obtained by a combination of variable domains of SEQ ID NO: 37 (heavy chain) and SEQ: 41 (light chain).

14) CDRs may be as follows: CDR1 comprising GFTFSSYT (SEQ ID NO:46), CDR2 comprising ISSGGHYI (SEQ ID NO: 47) and CDR3 comprising ARERLLRLGVGFDF

(SEQ ID NO:48). These CDRs may optionally be in the context of a VH., e.g. a VH comprising the sequence of SEQ ID NO:45.

15) CDR1 comprising QSISGY (SEQ ID NO:50), CDR2 comprising AAS (SEQ ID NO: 51 ) and CDR3 comprising QQSYSALYT (SEQ ID NO:52). These CDRs may optionally be in the context of a VL, e.g. a VL comprising the sequence of SEQ ID NO:49.

16) CDRs of SEQ ID NO: 46, 47 and 48 (heavy chain) and CDRs of SEQ ID NO: 50, 51 and 52 (light chain) e.g. in the context of a VH and VL. For example, antigen specificity may be obtained by a combination of variable domains of SEQ ID NO: 45 (heavy chain) and SEQ: 49 (light chain).

17) CDR1 comprising GFTFSSYW (SEQ ID NO:54), CDR2 comprising LNEDGSTT (SEQ ID NO: 55) and CDR3 comprising VSDLSGKDEH (SEQ ID NO:56). These CDRs may optionally be in the context of a VH, e.g. a VH comprising the sequence of SEQ ID NO:53.

18) CDR1 comprising QSLLHSNGYYW (SEQ ID NO:58), CDR2 comprising LGF (SEQ ID NO: 59) and CDR3 comprising MQGLQTPYT (SEQ ID NO:60). These CDRs may optionally be in the context of a VL, e.g. a VL comprising the sequence of SEQ ID NO:57.

19) CDRs of SEQ ID NO: 54, 55 and 56 (heavy chain) and CDRs of SEQ ID NO: 58, 59 and 60 (light chain) e.g. in the context of a VH and VL. For example, antigen specificity may be obtained by a combination of variable domains of SEQ ID NO: 53 (heavy chain) and SEQ: 57 (light chain). The antibody comprises at least two binding regions (variable regions) that specifically bind to an Ig-like 1 domain of a MuSK protein. The at least two binding regions (e.g. CDR sequences, VL, VH sequences, variable regions etc) may be the same (in other words, the at least two binding regions that specifically bind to the Ig-like 1 domain of the MuSK protein may both comprise the sequences set out in 1 , both comprise the sequences set out in 2, both comprise the sequences set out in 3, both comprise the sequences set out in 4, both comprise the sequences set out in 5, both comprise the sequences set out in 6, both comprise the sequences set out in 7, both comprise the sequences set out in 8, both comprise the sequences set out in 9, both comprise the sequences set out in 10, both comprise the sequences set out in 1 1 , both comprise the sequences set out in 12, both comprise the sequences set out in 13, both comprise the sequences set out in 14, both comprise the sequences set out in 15. both comprise the sequences set out in 16, both comprise the sequences set out in 17, both comprise the sequences set out in 18, both comprise the sequences set out in 19 etc).

The above CDRs may be in the context of an antibody, e.g. an IgG. For example, the IgG may be selected from lgG1 , lgG2, lgG3, or lgG4.

For example, the CDR and variable domain sequences provided in options 1 , 2, 3, 7,

1 1 , 12, or 13 above may be present in the context of an lgG1 antibody Fc sequence. An exemplary sequence may be that of SEQ ID NO: 61 or a variant thereof (see for example the Fc region of SEQ ID NO: 65, 66 or 67).

For example, the CDR and variable domain sequences provided in options 8, 9 and 10 above may be present in the context of an lgG4 antibody Fc sequence. An exemplary sequence may be that of SEQ ID NO: 64 or a variant thereof (see for example the Fc region of SEQ ID NO: 69).

For example, the CDR and variable domain sequences provided in options 14, 15 or 16 above may be present in the context of an lgG3 antibody Fc sequence. An exemplary sequence may be that of SEQ ID NO: 63 or a variant thereof (see for example the Fc region of SEQ ID NO: 68). As described in detail above, antibodies described herein (including a Fab2, Fab3, di- scFv, scFV-Fc, duobody, camelid antibody or minibody) comprise at least two binding regions (i.e. variable regions) that specifically bind to an Ig-like 1 domain of a MuSK protein.

In one embodiment, a binding region that specifically binds to the Ig-like 1 domain of a MuSK protein comprises : a heavy chain variable domain having an amino acid sequence that is 85%, 86%, 87%, 88%, 89%, 90%, 91 %, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99% or 100% identical to the polypeptide of SEQ ID NO:9; and a light chain variable domain an amino acid sequence that is 85%, 86%, 87%, 88%, 89%, 90%,

91 %, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99% or 100% identical to the polypeptide of SEQ ID NO: 13. Preferably, the antibody is selected from an antibody, a Fab ₂ , Fab3 _, di-scFv, scFv-Fc, duobody, camelid antibody or a minibody. The antibody comprises at least two binding regions that specifically bind to an Ig-like 1 domain of a MuSK protein. The at least two binding regions (e.g. CDR sequences, VL, VH sequences etc) may be the same (i.e. they may both comprise the sequences set out above).

Preferably, the heavy chain variable domain has an amino acid sequence that is 85%, 86%, 87%, 88%, 89%, 90%, 91 %, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99% or 100% identical to the polypeptide of SEQ ID NO:9 and comprises at least one, particularly at least two, more particularly at least 3 of the heavy chain CDR’s: a VH CDR1 having SEQ ID NO:10, a VH CDR2 having SEQ ID NO:11 and a VH CDR3 having SEQ ID NO: 12; and light chain variable domain an amino acid sequence that is 85%, 86%, 87%, 88%, 89%, 90%, 91 %, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99% or 100% identical to the polypeptide of SEQ ID NO: 13 and comprises at least one, particularly at least two, more particularly at least 3 of the light chain CDR’s: a VL CDR1 having SEQ ID NO:14, a VL CDR2 having SEQ ID NO:15 and a VL CDR3 having SEQ ID NO: 16. Preferably, the antibody is selected from an antibody, a Fab ₂ , Fab3 _, di-scFv, scFv-Fc, duobody, camelid antibody or a minibody. The antibody comprises at least two binding regions that specifically bind to an Ig-like 1 domain of a MuSK protein. The at least two binding regions (e.g. CDR sequences, VL, VH sequences etc) may be the same (i.e. they may both comprise the sequences set out above). Most preferably, the binding region comprises a heavy chain variable domain of SEQ ID NO:9 and a light chain variable domain of SEQ ID NO: 13. Preferably, the antibody is selected from an antibody, a Fab2, Fab _3, di-scFv, scFv-Fc, duobody, camelid antibody or a minibody. The antibody comprises at least two binding regions that specifically bind to an Ig-like 1 domain of a MuSK protein. The at least two binding regions (e.g. CDR sequences, VL, VH sequences etc) may be the same (i.e. they may both comprise the sequences set out above). In one embodiment, a binding region that specifically binds to the Ig-like 1 domain of a MuSK protein comprises a heavy chain variable domain comprising a polypeptide sequence having at least 1 , 2, 3, 4 or 5 conservative substitutions compared to a polypeptide sequence of SEQ ID NO: 9; and a light chain variable domain comprising a polypeptide sequence having at least 1 , 2, 3, 4 or 5 conservative substitutions compared to a polypeptide sequence of SEQ ID NO: 13. Preferably, the binding region that specifically binds to the Ig-like 1 domain of a MuSK protein comprises at least one, particularly at least two, more particularly at least 3 of the heavy chain CDR’s: a VH CDR1 having SEQ ID NO:10, a VH CDR2 having SEQ ID NO:11 and a VH CDR3 having SEQ ID NO: 12. Preferably, the binding region that specifically binds to the Ig-like 1 domain of a MuSK protein comprises at least one, particularly at least two, more particularly at least 3 of the light chain CDR’s: a VL CDR1 having SEQ ID NO: 14, a VL CDR2 having SEQ ID NO: 15 and a VL CDR3 having SEQ ID NO: 16. Preferably, the binding region that specifically binds to the Ig-like 1 domain of a MuSK protein comprises at least one, particularly at least two, more particularly at least 3 of the heavy chain CDR’s: a VH CDR1 having SEQ ID NO:10, a VH CDR2 having SEQ ID NO:11 and a VH CDR3 having SEQ ID NO: 12 and at least one, particularly at least two, more particularly at least 3 of the light chain CDR’s: a VL CDR1 having SEQ ID NO:14, a VL CDR2 having SEQ ID NO: 15 and a VL CDR3 having SEQ ID NO: 16. Preferably, the antibody is selected from an antibody, a Fab2, Fab _3, di-scFv, scFv-Fc, duobody, camelid antibody or a minibody. The antibody comprises at least two binding regions that specifically bind to an Ig-like 1 domain of a MuSK protein. The at least two binding regions (e.g. CDR sequences, VL, VH sequences etc) may be the same (i.e. they may both comprise the sequences set out above).

In one embodiment, a binding region that specifically binds to the Ig-like 1 domain of a MuSK protein comprises a heavy chain variable domain comprising a polypeptide sequence having at least 1 , 2, 3, 4 or 5 conservative substitutions compared to a polypeptide sequence of SEQ ID NO: 9; and a light chain variable domain of SEQ ID NO: 13. Preferably, the binding region that specifically binds to the Ig-like 1 domain of a MuSK protein comprises at least one, particularly at least two, more particularly at least 3 of the heavy chain CDR’s: a VH CDR1 having SEQ ID NO: 10, a VH CDR2 having

SEQ ID NO:11 and a VH CDR3 having SEQ ID NO:12. Preferably, the antibody is selected from an antibody, a Fab2, Fab3 _, di-scFv, scFv-Fc, duobody, camelid antibody or a minibody. The antibody comprises at least two binding regions that specifically bind to an Ig-like 1 domain of a MuSK protein. The at least two binding regions (e.g. CDR sequences, VL, VH sequences etc) may be the same (i.e. they may both comprise the sequences set out above).

In one embodiment, a binding region that specifically binds to the Ig-like 1 domain of a MuSK protein comprises a heavy chain variable domain of SEQ ID NO: 9; and a light chain variable domain comprising a polypeptide sequence having at least 1 , 2, 3, 4 or 5 conservative substitutions compared to a polypeptide sequence of SEQ ID NO: 13. Preferably, the binding region that specifically binds to the Ig-like 1 domain of a MuSK protein comprises at least one, particularly at least two, more particularly at least 3 of the light chain CDR’s: a VL CDR1 having SEQ ID NO:14, a VL CDR2 having SEQ ID NO: 15 and a VL CDR3 having SEQ ID NO: 16. Preferably, the antibody is selected from an antibody, a Fab2, Fab3 _, di-scFv, scFv-Fc, duobody, camelid antibody or a minibody. The antibody comprises at least two binding regions that specifically bind to an Ig-like 1 domain of a MuSK protein. The at least two binding regions (e.g. CDR sequences, VL, VH sequences etc) may be the same (i.e. they may both comprise the sequences set out above). In one embodiment, a binding region that specifically binds to the Ig-like 1 domain of a MuSK protein comprises at least one, particularly at least two, more particularly at least 3 of the heavy chain CDR’s: a VH CDR1 having SEQ ID NO: 10, a VH CDR2 having SEQ ID NO: 11 and a VH CDR3 having SEQ ID NO:12. Preferably, the antibody is selected from an antibody, a Fab2, Fab _3, di-scFv, scFv-Fc, duobody, camelid antibody or a minibody. The antibody comprises at least two binding regions that specifically bind to an Ig-like 1 domain of a MuSK protein. The at least two binding regions (e.g. CDR sequences, VL, VH sequences etc) may be the same (i.e. they may both comprise the sequences set out above).

In one embodiment, a binding region that specifically binds to the Ig-like 1 domain of a MuSK protein comprises at least one, particularly at least two, more particularly at least 3 of the light chain CDR’s: a VL CDR1 having SEQ ID NO:14, a VL CDR2 having SEQ ID NO: 15 and a VL CDR3 having SEQ ID NO: 16. Preferably, the antibody is selected from an antibody, a Fab2, Fab _3, di-scFv, scFv-Fc, duobody, camelid antibody or a minibody. The antibody comprises at least two binding regions that specifically bind to an Ig-like 1 domain of a MuSK protein. The at least two binding regions (e.g. CDR sequences, VL, VH sequences etc) may be the same (i.e. they may both comprise the sequences set out above).

In one embodiment, a binding region that specifically binds to the Ig-like 1 domain of a MuSK protein comprises at least one, particularly at least two, more particularly at least 3 of the heavy chain CDR’s: a VH CDR1 having SEQ ID NO: 10, a VH CDR2 having SEQ ID NO:11 and a VH CDR3 having SEQ ID NO: 12 and at least one, particularly at least two, more particularly at least 3 of the light chain CDR’s: a VL CDR1 having SEQ ID NO:14, a VL CDR2 having SEQ ID NO:15 and a VL CDR3 having SEQ ID NO:16. Preferably, the antibody is selected from an antibody, a Fab2, Fab _3, di-scFv, scFv-Fc, duobody, camelid antibody or a minibody. The antibody comprises at least two binding regions that specifically bind to an Ig-like 1 domain of a MuSK protein. The at least two binding regions (e.g. CDR sequences, VL, VH sequences etc) may be the same (i.e. they may both comprise the sequences set out above). In one embodiment, a binding region that specifically binds to the Ig-like 1 domain of a MuSK protein comprises the heavy chain CDR’s: a VH CDR1 having SEQ ID NO: 10, a VH CDR2 having SEQ ID NO: 11 and a VH CDR3 having SEQ ID NO:12 and the light chain CDR’s: a VL CDR1 having SEQ ID NO:14, a VL CDR2 having SEQ ID NO:15 and a VL CDR3 having SEQ ID NO: 16. Preferably, the antibody is selected from an antibody, a Fab2, Fab _3, di-scFv, scFv-Fc, duobody, camelid antibody or a minibody. The antibody comprises at least two binding regions that specifically bind to an Ig-like 1 domain of a MuSK protein. The at least two binding regions (e.g. CDR sequences, VL, VH sequences etc) may be the same (i.e. they may both comprise the sequences set out above).

In one embodiment an antibody binding region that specifically binds to the Ig-like 1 domain of a MuSK protein comprises: a heavy chain variable domain having an amino acid sequence that is 85%, 86%, 87%, 88%, 89%, 90%, 91 %, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99% or 100% identical to the polypeptide of SEQ ID NO: 17; and a light chain variable domain an amino acid sequence that is 85%, 86%, 87%, 88%, 89%,

90%, 91 %, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99% or 100% identical to the polypeptide of SEQ ID NO:21. Preferably, the antibody is selected from an antibody, a Fab2, Fab _3, di-scFv, scFv-Fc, duobody, camelid antibody or a minibody. The antibody comprises at least two binding regions that specifically bind to an Ig-like 1 domain of a MuSK protein. The at least two binding regions (e.g. CDR sequences, VL, VH

sequences etc) may be the same (i.e. they may both comprise the sequences set out above). Preferably, the heavy chain variable domain has an amino acid sequence that is 85%, 86%, 87%, 88%, 89%, 90%, 91 %, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99% or 100% identical to the polypeptide of SEQ ID NO: 17 and comprises at least one, particularly at least two, more particularly at least 3 of the heavy chain CDR’s: a VH CDR1 having SEQ ID NO:18, a VH CDR2 having SEQ ID NO:19 and a VH CDR3 having SEQ ID NO:20; and light chain variable domain an amino acid sequence that is 85%, 86%, 87%, 88%, 89%, 90%, 91 %, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99% or 100% identical to the polypeptide of SEQ ID NO:21 and comprises at least one, particularly at least two, more particularly at least 3 of the light chain CDR’s: a VL CDR1 having SEQ ID NO:22, a VL CDR2 having SEQ ID NO:23 and a VL CDR3 having SEQ ID NO:24. Preferably, the antibody is selected from an antibody, a Fab2, Fab _3, di-scFv, scFv-Fc, duobody, camelid antibody or a minibody. The antibody comprises at least two binding regions that specifically bind to an Ig-like 1 domain of a MuSK protein. The at least two binding regions (e.g. CDR sequences, VL, VH sequences etc) may be the same (i.e. they may both comprise the sequences set out above).

Most preferably, the the binding region that specifically binds to the Ig-like 1 domain of a MuSK protein comprises a heavy chain variable domain of SEQ ID NO: 17 and a light chain variable domain of SEQ ID NO:21. Preferably, the antibody is selected from an antibody, a Fab2, Fab _3, di-scFv, scFv-Fc, duobody, camelid antibody or a minibody. The antibody comprises at least two binding regions that specifically bind to an Ig-like 1 domain of a MuSK protein. The at least two binding regions (e.g. CDR sequences, VL, VH sequences etc) may be the same (i.e. they may both comprise the sequences set out above).

In one embodiment, a binding region that specifically binds to the Ig-like 1 domain of a MuSK protein comprises a heavy chain variable domain comprising a polypeptide sequence having at least 1 , 2, 3, 4 or 5 conservative substitutions compared to a polypeptide sequence of SEQ ID NO: 17; and a light chain variable domain comprising a polypeptide sequence having at least 1 , 2, 3, 4 or 5 conservative substitutions compared to a polypeptide sequence of SEQ ID NO: 21 . Preferably, the binding region that specifically binds to the Ig-like 1 domain of a MuSK protein comprises at least one, particularly at least two, more particularly at least 3 of the heavy chain CDR’s: a VH CDR1 having SEQ ID NO: 18, a VH CDR2 having SEQ ID NO: 19 and a VH CDR3 having SEQ ID NO:20. Preferably, the binding region that specifically binds to the Ig-like 1 domain of a MuSK protein comprises at least one, particularly at least two, more particularly at least 3 of the light chain CDR’s: a VL CDR1 having SEQ ID NO:22, a VL CDR2 having SEQ ID NO:23 and a VL CDR3 having SEQ ID NO:24. Preferably, the binding region that specifically binds to the Ig-like 1 domain of a MuSK protein comprises at least one, particularly at least two, more particularly at least 3 of the heavy chain CDR’s: a VH CDR1 having SEQ ID N0: 18, a VH CDR2 having SEQ ID N0:19 and a VH CDR3 having SEQ ID NO:20 and at least one, particularly at least two, more particularly at least 3 of the light chain CDR’s: a VL CDR1 having SEQ ID NO:22, a VL CDR2 having SEQ ID NO:23 and a VL CDR3 having SEQ ID NO:24. Preferably, the antibody is selected from an antibody, a Fab2, Fab _3, di-scFv, scFv-Fc, duobody, camelid antibody or a minibody. The antibody comprises at least two binding regions that specifically bind to an Ig-like 1 domain of a MuSK protein. The at least two binding regions (e.g. CDR sequences, VL, VH sequences etc) may be the same (i.e. they may both comprise the sequences set out above).

In one embodiment, a binding region that specifically binds to the Ig-like 1 domain of a MuSK protein comprises a heavy chain variable domain comprising a polypeptide sequence having at least 1 , 2, 3, 4 or 5 conservative substitutions compared to a polypeptide sequence of SEQ ID NO: 17; and a light chain variable domain of SEQ ID NO:21 . Preferably, the binding region that specifically binds to the Ig-like 1 domain of a

MuSK protein comprises at least one, particularly at least two, more particularly at least 3 of the heavy chain CDR’s: a VH CDR1 having SEQ ID NO: 18, a VH CDR2 having SEQ ID NO: 19 and a VH CDR3 having SEQ ID NO:20. Preferably, the antibody is selected from an antibody, a Fab2, Fab _3, di-scFv, scFv-Fc, duobody, camelid antibody or a minibody. The antibody comprises at least two binding regions that specifically bind to an Ig-like 1 domain of a MuSK protein. The at least two binding regions (e.g. CDR sequences, VL, VH sequences etc) may be the same (i.e. they may both comprise the sequences set out above). In one embodiment, a binding region that specifically binds to the Ig-like 1 domain of a MuSK protein comprises a heavy chain variable domain of SEQ ID NO: 17; and a light chain variable domain comprising a polypeptide sequence having at least 1 , 2, 3, 4 or 5 conservative substitutions compared to a polypeptide sequence of SEQ ID NO: 21 . Preferably, the binding region that specifically binds to the Ig-like 1 domain of a MuSK protein comprises at least one, particularly at least two, more particularly at least 3 of the light chain CDR’s: a VL CDR1 having SEQ ID NO:22, a VL CDR2 having SEQ ID NO:23 and a VL CDR3 having SEQ ID NO:24. Preferably, the antibody is selected from an antibody, a Fab2, Fab3 _, di-scFv, scFv-Fc, duobody, camelid antibody or a minibody. The antibody comprises at least two binding regions that specifically bind to an Ig-like 1 domain of a MuSK protein. The at least two binding regions (e.g. CDR sequences, VL, VH sequences etc) may be the same (i.e. they may both comprise the sequences set out above).

In one embodiment, a binding region that specifically binds to the Ig-like 1 domain of a MuSK protein comprises at least one, particularly at least two, more particularly at least 3 of the heavy chain CDR’s: a VH CDR1 having SEQ ID NO: 18, a VH CDR2 having SEQ ID NO: 19 and a VH CDR3 having SEQ ID NO:20. Preferably, the antibody is selected from an antibody, a Fab ₂ , Fab3 _, di-scFv, scFv-Fc, duobody, camelid antibody or a minibody. The antibody comprises at least two binding regions that specifically bind to an Ig-like 1 domain of a MuSK protein. The at least two binding regions (e.g. CDR sequences, VL, VH sequences etc) may be the same (i.e. they may both comprise the sequences set out above).

In one embodiment, a binding region that specifically binds to the Ig-like 1 domain of a MuSK protein comprises at least one, particularly at least two, more particularly at least 3 of the light chain CDR’s: a VL CDR1 having SEQ ID NO:22, a VL CDR2 having SEQ ID NO:23 and a VL CDR3 having SEQ ID NO:24. Preferably, the antibody is selected from an antibody, a Fab ₂ , Fab3 _, di-scFv, scFv-Fc, duobody, camelid antibody or a minibody. The antibody comprises at least two binding regions that specifically bind to an Ig-like 1 domain of a MuSK protein. The at least two binding regions (e.g. CDR sequences, VL, VH sequences etc) may be the same (i.e. they may both comprise the sequences set out above).

In one embodiment, a binding region that specifically binds to the Ig-like 1 domain of a MuSK protein comprises at least one, particularly at least two, more particularly at least 3 of the heavy chain CDR’s: a VH CDR1 having SEQ ID NO: 18, a VH CDR2 having SEQ ID NO: 19 and a VH CDR3 having SEQ ID NO:20 and at least one, particularly at least two, more particularly at least 3 of the light chain CDR’s: a VL CDR1 having SEQ ID NO:22, a VL CDR2 having SEQ ID NO:23 and a VL CDR3 having SEQ ID NO:24. Preferably, the antibody is selected from an antibody, a Fab2, Fab _3, di-scFv, scFv-Fc, duobody, camelid antibody or a minibody. The antibody comprises at least two binding regions that specifically bind to an Ig-like 1 domain of a MuSK protein. The at least two binding regions (e.g. CDR sequences, VL, VH sequences etc) may be the same (i.e. they may both comprise the sequences set out above).

In one embodiment, a binding region that specifically binds to the Ig-like 1 domain of a MuSK protein comprises the heavy chain CDR’s: a VH CDR1 having SEQ ID NO: 18, a VH CDR2 having SEQ ID NO:19 and a VH CDR3 having SEQ ID NO:20 and the light chain CDR’s: a VL CDR1 having SEQ ID NO:22, a VL CDR2 having SEQ ID NO:23 and a VL CDR3 having SEQ ID NO:24. Preferably, the antibody is selected from an antibody, a Fab2, Fab _3, di-scFv, scFv-Fc, duobody, camelid antibody or a minibody. The antibody comprises at least two binding regions that specifically bind to an Ig-like 1 domain of a MuSK protein. The at least two binding regions (e.g. CDR sequences, VL, VH sequences etc) may be the same (i.e. they may both comprise the sequences set out above).

In one embodiment, a binding region that specifically binds to the Ig-like 1 domain of a MuSK protein comprises: a heavy chain variable domain having an amino acid sequence that is 85%, 86%, 87%, 88%, 89%, 90%, 91 %, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99% or 100% identical to the polypeptide of SEQ ID NO:25. Preferably, the antibody is selected from an antibody, a Fab2, Fab _3, di-scFv, scFv-Fc, duobody, camelid antibody or a minibody. The antibody comprises at least two binding regions that specifically bind to an Ig-like 1 domain of a MuSK protein. The at least two binding regions (e.g. CDR sequences, VL, VH sequences etc) may be the same (i.e. they may both comprise the sequences set out above).

Preferably, the heavy chain variable domain has an amino acid sequence that is 85%, 86%, 87%, 88%, 89%, 90%, 91 %, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99% or 100% identical to the polypeptide of SEQ ID NO:25 and comprises at least one, particularly at least two, more particularly at least 3 of the heavy chain CDR’s: a VH CDR1 having SEQ ID NO:26, a VH CDR2 having SEQ ID NO:27 and a VH CDR3 having SEQ ID NO:28. Preferably, the antibody is selected from an antibody, a Fab2, Fab3 _, di-scFv, scFv-Fc, duobody, camelid antibody or a minibody. The antibody comprises at least two binding regions that specifically bind to an Ig-like 1 domain of a MuSK protein. The at least two binding regions (e.g. CDR sequences, VL, VH sequences etc) may be the same (i.e. they may both comprise the sequences set out above).

Most preferably, the binding region that specifically binds to the Ig-like 1 domain of a MuSK protein comprises a heavy chain variable domain of SEQ ID NO:25. Preferably, the antibody is selected from an antibody, a Fab2, Fab3 _, di-scFv, scFv-Fc, duobody, camelid antibody or a minibody. The antibody comprises at least two binding regions that specifically bind to an Ig-like 1 domain of a MuSK protein. The at least two binding regions (e.g. CDR sequences, VL, VH sequences etc) may be the same (i.e. they may both comprise the sequences set out above).

In one embodiment, a binding region that specifically binds to the Ig-like 1 domain of a MuSK protein comprises a heavy chain variable domain comprising a polypeptide sequence having at least 1 , 2, 3, 4 or 5 conservative substitutions compared to a polypeptide sequence of SEQ ID NO: 25. Preferably, the binding region that specifically binds to the Ig-like 1 domain of a MuSK protein comprises at least one, particularly at least two, more particularly at least 3 of the heavy chain CDR’s: a VH CDR1 having SEQ ID NO:26, a VH CDR2 having SEQ ID NO:27 and a VH CDR3 having SEQ ID NO:28. Preferably, the antibody is selected from an antibody, a Fab2, Fab3 _, di-scFv, scFv-Fc, duobody, camelid antibody or a minibody. The antibody comprises at least two binding regions that specifically bind to an Ig-like 1 domain of a MuSK protein. The at least two binding regions (e.g. CDR sequences, VL, VH sequences etc) may be the same (i.e. they may both comprise the sequences set out above).

In one embodiment, a binding region that specifically binds to the Ig-like 1 domain of a MuSK protein comprises at least one, particularly at least two, more particularly at least 3 of the heavy chain CDR’s: a VH CDR1 having SEQ ID NO:26, a VH CDR2 having SEQ ID NO:27 and a VH CDR3 having SEQ ID NO:28. Preferably, the antibody is selected from an antibody, a Fab2, Fab3 _, di-scFv, scFv-Fc, duobody, camelid antibody or a minibody. The antibody comprises at least two binding regions that specifically bind to an Ig-like 1 domain of a MuSK protein. The at least two binding regions (e.g. CDR sequences, VL, VH sequences etc) may be the same (i.e. they may both comprise the sequences set out above).

In one embodiment, a binding region that specifically binds to the Ig-like 1 domain of a MuSK protein comprises the heavy chain CDR’s: a VH CDR1 having SEQ ID NO:26, a VH CDR2 having SEQ ID NO:27 and a VH CDR3 having SEQ ID NO:28. Preferably, the antibody is selected from an antibody, a Fab ₂ , Fab3 _, di-scFv, scFv-Fc, duobody, camelid antibody or a minibody. The antibody comprises at least two binding regions that specifically bind to an Ig-like 1 domain of a MuSK protein. The at least two binding regions (e.g. CDR sequences, VL, VH sequences etc) may be the same (i.e. they may both comprise the sequences set out above).

In one embodiment the binding region that specifically binds to the Ig-like 1 domain of a MuSK protein comprises: a heavy chain variable domain having an amino acid sequence that is 85%, 86%, 87%, 88%, 89%, 90%, 91 %, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99% or 100% identical to the polypeptide of SEQ ID NO:29; and a light chain variable domain an amino acid sequence that is 85%, 86%, 87%, 88%, 89%,

90%, 91 %, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99% or 100% identical to the polypeptide of SEQ ID NO:33. Preferably, the antibody is selected from an antibody, a Fab ₂ , Fab3 _, di-scFv, scFv-Fc, duobody, camelid antibody or a minibody. The antibody comprises at least two binding regions that specifically bind to an Ig-like 1 domain of a MuSK protein. The at least two binding regions (e.g. CDR sequences, VL, VH

sequences etc) may be the same (i.e. they may both comprise the sequences set out above).

Preferably, the heavy chain variable domain has an amino acid sequence that is 85%, 86%, 87%, 88%, 89%, 90%, 91 %, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99% or

100% identical to the polypeptide of SEQ ID NO:29 and comprises at least one, particularly at least two, more particularly at least 3 of the heavy chain CDR’s: a VH CDR1 having SEQ ID NO:30, a VH CDR2 having SEQ ID N0:31 and a VH CDR3 having SEQ ID NO:32; and light chain variable domain an amino acid sequence that is 85%, 86%, 87%, 88%, 89%, 90%, 91 %, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99% or 100% identical to the polypeptide of SEQ ID NO:33 and comprises at least one, particularly at least two, more particularly at least 3 of the light chain CDR’s: a VL CDR1 having SEQ ID NO:34, a VL CDR2 having SEQ ID NO:35 and a VL CDR3 having SEQ ID NO:36. Preferably, the antibody is selected from an antibody, a Fab2, Fab _3, di-scFv, scFv-Fc, duobody, camelid antibody or a minibody. The antibody comprises at least two binding regions that specifically bind to an Ig-like 1 domain of a MuSK protein. The at least two binding regions (e.g. CDR sequences, VL, VH sequences etc) may be the same (i.e. they may both comprise the sequences set out above).

Most preferably, the binding region that specifically binds to the Ig-like 1 domain of a MuSK protein comprises a heavy chain variable domain of SEQ ID NO:29 and a light chain variable domain of SEQ ID NO:33. Preferably, the antibody is selected from an antibody, a Fab2, Fab _3, di-scFv, scFv-Fc, duobody, camelid antibody or a minibody. The antibody comprises at least two binding regions that specifically bind to an Ig-like 1 domain of a MuSK protein. The at least two binding regions (e.g. CDR sequences, VL, VH sequences etc) may be the same (i.e. they may both comprise the sequences set out above).

In one embodiment, a binding region that specifically binds to the Ig-like 1 domain of a MuSK protein comprises a heavy chain variable domain comprising a polypeptide sequence having at least 1 , 2, 3, 4 or 5 conservative substitutions compared to a polypeptide sequence of SEQ ID NO: 29; and a light chain variable domain comprising a polypeptide sequence having at least 1 , 2, 3, 4 or 5 conservative substitutions compared to a polypeptide sequence of SEQ ID NO: 33. Preferably, the binding region that specifically binds to the Ig-like 1 domain of a MuSK protein comprises at least one, particularly at least two, more particularly at least 3 of the heavy chain CDR’s: a VH CDR1 having SEQ ID NO:30, a VH CDR2 having SEQ ID NO:31 and a VH CDR3 having SEQ ID NO:32. Preferably, the binding region that specifically binds to the Ig-like 1 domain of a MuSK protein comprises at least one, particularly at least two, more particularly at least 3 of the light chain CDR’s: a VL CDR1 having SEQ ID NO:34, a VL CDR2 having SEQ ID NO:35 and a VL CDR3 having SEQ ID NO:36. Preferably, the binding region that specifically binds to the Ig-like 1 domain of a MuSK protein comprises at least one, particularly at least two, more particularly at least 3 of the heavy chain CDR’s: a VH CDR1 having SEQ ID NO:30, a VH CDR2 having SEQ ID NO:31 and a VH CDR3 having SEQ ID NO:32 and at least one, particularly at least two, more particularly at least 3 of the light chain CDR’s: a VL CDR1 having SEQ ID NO:34, a VL CDR2 having SEQ ID NO:35 and a VL CDR3 having SEQ ID NO:36. Preferably, the antibody is selected from an antibody, a Fab2, Fab3 _, di-scFv, scFv-Fc, duobody, camelid antibody or a minibody. The antibody comprises at least two binding regions that specifically bind to an Ig-like 1 domain of a MuSK protein. The at least two binding regions (e.g. CDR sequences, VL, VH sequences etc) may be the same (i.e. they may both comprise the sequences set out above). In one embodiment, a binding region that specifically binds to the Ig-like 1 domain of a MuSK protein comprises a heavy chain variable domain comprising a polypeptide sequence having at least 1 , 2, 3, 4 or 5 conservative substitutions compared to a polypeptide sequence of SEQ ID NO: 29; and a light chain variable domain of SEQ ID NO:33. Preferably, the binding region that specifically binds to the Ig-like 1 domain of a MuSK protein comprises at least one, particularly at least two, more particularly at least 3 of the heavy chain CDR’s: a VH CDR1 having SEQ ID NO:30, a VH CDR2 having SEQ ID NO:31 and a VH CDR3 having SEQ ID NO:32. Preferably, the antibody is selected from an antibody, a Fab2, Fab3 _, di-scFv, scFv-Fc, duobody, camelid antibody or a minibody. The antibody comprises at least two binding regions that specifically bind to an Ig-like 1 domain of a MuSK protein. The at least two binding regions (e.g. CDR sequences, VL, VH sequences etc) may be the same (i.e. they may both comprise the sequences set out above).

In one embodiment, a binding region that specifically binds to the Ig-like 1 domain of a MuSK protein comprises a heavy chain variable domain of SEQ ID NO: 29; and a light chain variable domain comprising a polypeptide sequence having at least 1 , 2, 3, 4 or 5 conservative substitutions compared to a polypeptide sequence of SEQ ID NO: 33. Preferably, the binding region that specifically binds to the Ig-like 1 domain of a MuSK protein comprises at least one, particularly at least two, more particularly at least 3 of the light chain CDR’s: a VL CDR1 having SEQ ID NO:34, a VL CDR2 having SEQ ID NO:35 and a VL CDR3 having SEQ ID NO:36. Preferably, the antibody is selected from an antibody, a Fab2, Fab3 _, di-scFv, scFv-Fc, duobody, camelid antibody or a minibody. The antibody comprises at least two binding regions that specifically bind to an Ig-like 1 domain of a MuSK protein. The at least two binding regions (e.g. CDR sequences, VL, VH sequences etc) may be the same (i.e. they may both comprise the sequences set out above).

In one embodiment, a binding region that specifically binds to the Ig-like 1 domain of a MuSK protein comprises at least one, particularly at least two, more particularly at least 3 of the heavy chain CDR’s: a VH CDR1 having SEQ ID NO:30, a VH CDR2 having SEQ ID NO:31 and a VH CDR3 having SEQ ID NO:32. Preferably, the antibody is selected from an antibody, a Fab2, Fab3 _, di-scFv, scFv-Fc, duobody, camelid antibody or a minibody. The antibody comprises at least two binding regions that specifically bind to an Ig-like 1 domain of a MuSK protein. The at least two binding regions (e.g. CDR sequences, VL, VH sequences etc) may be the same (i.e. they may both comprise the sequences set out above).

In one embodiment, a binding region that specifically binds to the Ig-like 1 domain of a MuSK protein comprises at least one, particularly at least two, more particularly at least 3 of the light chain CDR’s: a VL CDR1 having SEQ ID NO:34, a VL CDR2 having SEQ ID NO:35 and a VL CDR3 having SEQ ID NO:36. Preferably, the antibody is selected from an antibody, a Fab2, Fab3 _, di-scFv, scFv-Fc, duobody, camelid antibody or a minibody. The antibody comprises at least two binding regions that specifically bind to an Ig-like 1 domain of a MuSK protein. The at least two binding regions (e.g. CDR sequences, VL, VH sequences etc) may be the same (i.e. they may both comprise the sequences set out above).

In one embodiment, a binding region that specifically binds to the Ig-like 1 domain of a MuSK protein comprises at least one, particularly at least two, more particularly at least 3 of the heavy chain CDR’s: a VH CDR1 having SEQ ID NO:30, a VH CDR2 having SEQ ID NO:31 and a VH CDR3 having SEQ ID NO:32 and at least one, particularly at least two, more particularly at least 3 of the light chain CDR’s: a VL CDR1 having SEQ ID NO:34, a VL CDR2 having SEQ ID NO:35 and a VL CDR3 having SEQ ID NO:36. Preferably, the antibody is selected from an antibody, a Fab2, Fab _3, di-scFv, scFv-Fc, duobody, camelid antibody or a minibody. The antibody comprises at least two binding regions that specifically bind to an Ig-like 1 domain of a MuSK protein. The at least two binding regions (e.g. CDR sequences, VL, VH sequences etc) may be the same (i.e. they may both comprise the sequences set out above).

In one embodiment, a binding region that specifically binds to the Ig-like 1 domain of a MuSK protein comprises the heavy chain CDR’s: a VH CDR1 having SEQ ID NO:30, a VH CDR2 having SEQ ID NO:31 and a VH CDR3 having SEQ ID NO:32 and the light chain CDR’s: a VL CDR1 having SEQ ID NO:34, a VL CDR2 having SEQ ID NO:35 and a VL CDR3 having SEQ ID NO:36. Preferably, the antibody is selected from an antibody, a Fab2, Fab _3, di-scFv, scFv-Fc, duobody, camelid antibody or a minibody. The antibody comprises at least two binding regions that specifically bind to an Ig-like 1 domain of a MuSK protein. The at least two binding regions (e.g. CDR sequences, VL, VH sequences etc) may be the same (i.e. they may both comprise the sequences set out above).

In one embodiment, a binding region that specifically binds to the Ig-like 1 domain of a MuSK protein comprises : a heavy chain variable domain having an amino acid sequence that is 85%, 86%, 87%, 88%, 89%, 90%, 91 %, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99% or 100% identical to the polypeptide of SEQ ID NO:37; and a light chain variable domain an amino acid sequence that is 85%, 86%, 87%, 88%, 89%,

90%, 91 %, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99% or 100% identical to the polypeptide of SEQ ID NO:41. Preferably, the antibody is selected from an antibody, a Fab2, Fab _3, di-scFv, scFv-Fc, duobody, camelid antibody or a minibody. The antibody comprises at least two binding regions that specifically bind to an Ig-like 1 domain of a MuSK protein. The at least two binding regions (e.g. CDR sequences, VL, VH sequences etc) may be the same (i.e. they may both comprise the sequences set out above).

Preferably, the heavy chain variable domain has an amino acid sequence that is 85%, 86%, 87%, 88%, 89%, 90%, 91 %, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99% or

100% identical to the polypeptide of SEQ ID NO:37 and comprises at least one, particularly at least two, more particularly at least 3 of the heavy chain CDR’s: a VH CDR1 having SEQ ID NO:38, a VH CDR2 having SEQ ID NO:39 and a VH CDR3 having SEQ ID NO:40; and light chain variable domain an amino acid sequence that is 85%, 86%, 87%, 88%, 89%, 90%, 91 %, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99% or

100% identical to the polypeptide of SEQ ID NO:41 and comprises at least one, particularly at least two, more particularly at least 3 of the light chain CDR’s: a VL CDR1 having SEQ ID NO:42, a VL CDR2 having SEQ ID NO:43 and a VL CDR3 having SEQ ID NO:44. Preferably, the antibody is selected from an antibody, a Fab2, Fab3 _, di-scFv, scFv-Fc, duobody, camelid antibody or a minibody. The antibody comprises at least two binding regions that specifically bind to an Ig-like 1 domain of a MuSK protein. The at least two binding regions (e.g. CDR sequences, VL, VH sequences etc) may be the same (i.e. they may both comprise the sequences set out above). Most preferably, the binding region that specifically binds to the Ig-like 1 domain of a MuSK protein comprises a heavy chain variable domain of SEQ ID NO:37 and a light chain variable domain of SEQ ID NO:41. Preferably, the antibody is selected from an antibody, a Fab2, Fab3 _, di-scFv, scFv-Fc, duobody, camelid antibody or a minibody. The antibody comprises at least two binding regions that specifically bind to an Ig-like 1 domain of a MuSK protein. The at least two binding regions (e.g. CDR sequences, VL, VH sequences etc) may be the same (i.e. they may both comprise the sequences set out above).

In one embodiment, a binding region that specifically binds to the Ig-like 1 domain of a MuSK protein comprises a heavy chain variable domain comprising a polypeptide sequence having at least 1 , 2, 3, 4 or 5 conservative substitutions compared to a polypeptide sequence of SEQ ID NO: 37; and a light chain variable domain comprising a polypeptide sequence having at least 1 , 2, 3, 4 or 5 conservative substitutions compared to a polypeptide sequence of SEQ ID NO: 41 . Preferably, the binding region that specifically binds to the Ig-like 1 domain of a MuSK protein comprises at least one, particularly at least two, more particularly at least 3 of the heavy chain CDR’s: a VH CDR1 having SEQ ID NO:38, a VH CDR2 having SEQ ID NO:39 and a VH CDR3 having SEQ ID NO:40. Preferably, the binding region that specifically binds to the Ig-like 1 domain of a MuSK protein comprises at least one, particularly at least two, more particularly at least 3 of the light chain CDR’s: a VL CDR1 having SEQ ID NO:42, a VL CDR2 having SEQ ID NO:43 and a VL CDR3 having SEQ ID NO:44. Preferably, the binding region that specifically binds to the Ig-like 1 domain of a MuSK protein comprises at least one, particularly at least two, more particularly at least 3 of the heavy chain CDR’s: a VH CDR1 having SEQ ID NO:38, a VH CDR2 having SEQ ID NO:39 and a VH CDR3 having SEQ ID NO:40 and at least one, particularly at least two, more particularly at least 3 of the light chain CDR’s: a VL CDR1 having SEQ ID NO:42, a VL CDR2 having SEQ ID NO:43 and a VL CDR3 having SEQ ID NO:44. Preferably, the antibody is selected from an antibody, a Fab2, Fab3 _, di-scFv, scFv-Fc, duobody, camelid antibody or a minibody. The antibody comprises at least two binding regions that specifically bind to an Ig-like 1 domain of a MuSK protein. The at least two binding regions (e.g. CDR sequences, VL, VH sequences etc) may be the same (i.e. they may both comprise the sequences set out above).

In one embodiment, a binding region that specifically binds to the Ig-like 1 domain of a MuSK protein comprises a heavy chain variable domain comprising a polypeptide sequence having at least 1 , 2, 3, 4 or 5 conservative substitutions compared to a polypeptide sequence of SEQ ID NO: 37; and a light chain variable domain of SEQ ID NO:41 . Preferably, the binding region that specifically binds to the Ig-like 1 domain of a MuSK protein comprises at least one, particularly at least two, more particularly at least 3 of the heavy chain CDR’s: a VH CDR1 having SEQ ID NO:38, a VH CDR2 having SEQ ID NO:39 and a VH CDR3 having SEQ ID NO:40. Preferably, the antibody is selected from an antibody, a Fab2, Fab3 _, di-scFv, scFv-Fc, duobody, camelid antibody or a minibody. The antibody comprises at least two binding regions that specifically bind to an Ig-like 1 domain of a MuSK protein. The at least two binding regions (e.g. CDR sequences, VL, VH sequences etc) may be the same (i.e. they may both comprise the sequences set out above).

In one embodiment, a binding region that specifically binds to the Ig-like 1 domain of a MuSK protein comprises a heavy chain variable domain of SEQ ID NO: 37; and a light chain variable domain comprising a polypeptide sequence having at least 1 , 2, 3, 4 or 5 conservative substitutions compared to a polypeptide sequence of SEQ ID NO: 41 . Preferably, the binding region that specifically binds to the Ig-like 1 domain of a MuSK protein comprises at least one, particularly at least two, more particularly at least 3 of the light chain CDR’s: a VL CDR1 having SEQ ID NO:42, a VL CDR2 having SEQ ID NO:43 and a VL CDR3 having SEQ ID NO:44. Preferably, the antibody is selected from an antibody, a Fab2, Fab3 _, di-scFv, scFv-Fc, duobody, camelid antibody or a minibody. The antibody comprises at least two binding regions that specifically bind to an Ig-like 1 domain of a MuSK protein. The at least two binding regions (e.g. CDR sequences, VL, VH sequences etc) may be the same (i.e. they may both comprise the sequences set out above).

In one embodiment, a binding region that specifically binds to the Ig-like 1 domain of a MuSK protein comprises at least one, particularly at least two, more particularly at least 3 of the heavy chain CDR’s: a VH CDR1 having SEQ ID NO:38, a VH CDR2 having

SEQ ID NO:39 and a VH CDR3 having SEQ ID NO:40. Preferably, the antibody is selected from an antibody, a Fab2, Fab3 _, di-scFv, scFv-Fc, duobody, camelid antibody or a minibody. The antibody comprises at least two binding regions that specifically bind to an Ig-like 1 domain of a MuSK protein. The at least two binding regions (e.g. CDR sequences, VL, VH sequences etc) may be the same (i.e. they may both comprise the sequences set out above).

In one embodiment, a binding region that specifically binds to the Ig-like 1 domain of a MuSK protein comprises at least one, particularly at least two, more particularly at least 3 of the light chain CDR’s: a VL CDR1 having SEQ ID NO:42, a VL CDR2 having SEQ

ID NO:43 and a VL CDR3 having SEQ ID NO:44. Preferably, the antibody is selected from an antibody, a Fab2, Fab3 _, di-scFv, scFv-Fc, duobody, camelid antibody or a minibody. The antibody comprises at least two binding regions that specifically bind to an Ig-like 1 domain of a MuSK protein. The at least two binding regions (e.g. CDR sequences, VL, VH sequences etc) may be the same (i.e. they may both comprise the sequences set out above).

In one embodiment, a binding region that specifically binds to the Ig-like 1 domain of a MuSK protein comprises at least one, particularly at least two, more particularly at least 3 of the heavy chain CDR’s: a VH CDR1 having SEQ ID NO:38, a VH CDR2 having SEQ ID NO:39 and a VH CDR3 having SEQ ID NO:40 and at least one, particularly at least two, more particularly at least 3 of the light chain CDR’s: a VL CDR1 having SEQ ID NO:42, a VL CDR2 having SEQ ID NO:43 and a VL CDR3 having SEQ ID NO:44. Preferably, the antibody is selected from an antibody, a Fab2, Fab _3, di-scFv, scFv-Fc, duobody, camelid antibody or a minibody. The antibody comprises at least two binding regions that specifically bind to an Ig-like 1 domain of a MuSK protein. The at least two binding regions (e.g. CDR sequences, VL, VH sequences etc) may be the same (i.e. they may both comprise the sequences set out above).

In one embodiment, a binding region that specifically binds to the Ig-like 1 domain of a MuSK protein comprises the heavy chain CDR’s: a VH CDR1 having SEQ ID NO:38, a VH CDR2 having SEQ ID NO:39 and a VH CDR3 having SEQ ID NO:40 and the light chain CDR’s: a VL CDR1 having SEQ ID NO:42, a VL CDR2 having SEQ ID NO:43 and a VL CDR3 having SEQ ID NO:44. Preferably, the antibody is selected from an antibody, a Fab2, Fab _3, di-scFv, scFv-Fc, duobody, camelid antibody or a minibody. The antibody comprises at least two binding regions that specifically bind to an Ig-like 1 domain of a MuSK protein. The at least two binding regions (e.g. CDR sequences, VL, VH sequences etc) may be the same (i.e. they may both comprise the sequences set out above).

In one embodiment, a binding region that specifically binds to the Ig-like 1 domain of a MuSK protein comprises: a heavy chain variable domain having an amino acid sequence that is 85%, 86%, 87%, 88%, 89%, 90%, 91 %, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99% or 100% identical to the polypeptide of SEQ ID NO:45; and a light chain variable domain an amino acid sequence that is 85%, 86%, 87%, 88%, 89%,

90%, 91 %, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99% or 100% identical to the polypeptide of SEQ ID NO:49. Preferably, the antibody is selected from an antibody, a Fab2, Fab3 _, di-scFv, scFv-Fc, duobody, camelid antibody or a minibody. The antibody comprises at least two binding regions that specifically bind to an Ig-like 1 domain of a MuSK protein. The at least two binding regions (e.g. CDR sequences, VL, VH

sequences etc) may be the same (i.e. they may both comprise the sequences set out above). Preferably, the heavy chain variable domain has an amino acid sequence that is 85%, 86%, 87%, 88%, 89%, 90%, 91 %, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99% or 100% identical to the polypeptide of SEQ ID NO:45 and comprises at least one, particularly at least two, more particularly at least 3 of the heavy chain CDR’s: a VH CDR1 having SEQ ID NO:46, a VH CDR2 having SEQ ID NO:47 and a VH CDR3 having SEQ ID NO:48; and light chain variable domain an amino acid sequence that is 85%, 86%, 87%, 88%, 89%, 90%, 91 %, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99% or 100% identical to the polypeptide of SEQ ID NO:49 and comprises at least one, particularly at least two, more particularly at least 3 of the light chain CDR’s: a VL CDR1 having SEQ ID NO:50, a VL CDR2 having SEQ ID NO:51 and a VL CDR3 having SEQ ID NO:52. Preferably, the antibody is selected from an antibody, a Fab2, Fab3 _, di-scFv, scFv-Fc, duobody, camelid antibody or a minibody. The antibody comprises at least two binding regions that specifically bind to an Ig-like 1 domain of a MuSK protein. The at least two binding regions (e.g. CDR sequences, VL, VH sequences etc) may be the same (i.e. they may both comprise the sequences set out above).

Most preferably, the binding region that specifically binds to the Ig-like 1 domain of a MuSK protein comprises a heavy chain variable domain of SEQ ID NO:45 and a light chain variable domain of SEQ ID NO:49. Preferably, the antibody is selected from an antibody, a Fab2, Fab3 _, di-scFv, scFv-Fc, duobody, camelid antibody or a minibody. The antibody comprises at least two binding regions that specifically bind to an Ig-like 1 domain of a MuSK protein. The at least two binding regions (e.g. CDR sequences, VL, VH sequences etc) may be the same (i.e. they may both comprise the sequences set out above).

In one embodiment, a binding region that specifically binds to the Ig-like 1 domain of a MuSK protein comprises a heavy chain variable domain comprising a polypeptide sequence having at least 1 , 2, 3, 4 or 5 conservative substitutions compared to a polypeptide sequence of SEQ ID NO: 45; and a light chain variable domain comprising a polypeptide sequence having at least 1 , 2, 3, 4 or 5 conservative substitutions compared to a polypeptide sequence of SEQ ID NO: 49. Preferably, the binding region that specifically binds to the Ig-like 1 domain of a MuSK protein comprises at least one, particularly at least two, more particularly at least 3 of the heavy chain CDR’s: a VH CDR1 having SEQ ID NO:46, a VH CDR2 having SEQ ID NO:47 and a VH CDR3 having SEQ ID NO:48. Preferably, the binding region that specifically binds to the Ig-like 1 domain of a MuSK protein comprises at least one, particularly at least two, more particularly at least 3 of the light chain CDR’s: a VL CDR1 having SEQ ID NO:50, a VL CDR2 having SEQ ID NO:51 and a VL CDR3 having SEQ ID NO:52. Preferably, the binding region that specifically binds to the Ig-like 1 domain of a MuSK protein comprises at least one, particularly at least two, more particularly at least 3 of the heavy chain CDR’s: a VH CDR1 having SEQ ID NO:46, a VH CDR2 having SEQ ID NO:47 and a VH CDR3 having SEQ ID NO:48 and at least one, particularly at least two, more particularly at least 3 of the light chain CDR’s: a VL CDR1 having SEQ ID NO:50, a VL CDR2 having SEQ ID NO:51 and a VL CDR3 having SEQ ID NO:52. Preferably, the antibody is selected from an antibody, a Fab2, Fab3 _, di-scFv, scFv-Fc, duobody, camelid antibody or a minibody. The antibody comprises at least two binding regions that specifically bind to an Ig-like 1 domain of a MuSK protein. The at least two binding regions (e.g. CDR sequences, VL, VH sequences etc) may be the same (i.e. they may both comprise the sequences set out above).

In one embodiment, a binding region that specifically binds to the Ig-like 1 domain of a MuSK protein comprises a heavy chain variable domain comprising a polypeptide sequence having at least 1 , 2, 3, 4 or 5 conservative substitutions compared to a polypeptide sequence of SEQ ID NO: 45; and a light chain variable domain of SEQ ID NO:49. Preferably, the binding region that specifically binds to the Ig-like 1 domain of a MuSK protein comprises at least one, particularly at least two, more particularly at least 3 of the heavy chain CDR’s: a VH CDR1 having SEQ ID NO:46, a VH CDR2 having SEQ ID NO:47 and a VH CDR3 having SEQ ID NO:48. Preferably, the antibody is selected from an antibody, a Fab2, Fab3 _, di-scFv, scFv-Fc, duobody, camelid antibody or a minibody. The antibody comprises at least two binding regions that specifically bind to an Ig-like 1 domain of a MuSK protein. The at least two binding regions (e.g. CDR sequences, VL, VH sequences etc) may be the same (i.e. they may both comprise the sequences set out above).

In one embodiment, a binding region that specifically binds to the Ig-like 1 domain of a MuSK protein comprises a heavy chain variable domain of SEQ ID NO: 45; and a light chain variable domain comprising a polypeptide sequence having at least 1 , 2, 3, 4 or 5 conservative substitutions compared to a polypeptide sequence of SEQ ID NO: 49. Preferably, the binding region that specifically binds to the Ig-like 1 domain of a MuSK protein comprises at least one, particularly at least two, more particularly at least 3 of the light chain CDR’s: a VL CDR1 having SEQ ID NO:50, a VL CDR2 having SEQ ID NO:51 and a VL CDR3 having SEQ ID NO:52. Preferably, the antibody is selected from an antibody, a Fab2, Fab3 _, di-scFv, scFv-Fc, duobody, camelid antibody or a minibody. The antibody comprises at least two binding regions that specifically bind to an Ig-like 1 domain of a MuSK protein. The at least two binding regions (e.g. CDR sequences, VL, VH sequences etc) may be the same (i.e. they may both comprise the sequences set out above). In one embodiment, a binding region that specifically binds to the Ig-like 1 domain of a MuSK protein comprises at least one, particularly at least two, more particularly at least 3 of the heavy chain CDR’s: a VH CDR1 having SEQ ID NO:46, a VH CDR2 having SEQ ID NO:47 and a VH CDR3 having SEQ ID NO:48. Preferably, the antibody is selected from an antibody, a Fab2, Fab3 _, di-scFv, scFv-Fc, duobody, camelid antibody or a minibody. The antibody comprises at least two binding regions that specifically bind to an Ig-like 1 domain of a MuSK protein. The at least two binding regions (e.g. CDR sequences, VL, VH sequences etc) may be the same (i.e. they may both comprise the sequences set out above).

In one embodiment, a binding region that specifically binds to the Ig-like 1 domain of a MuSK protein comprises at least one, particularly at least two, more particularly at least 3 of the light chain CDR’s: a VL CDR1 having SEQ ID NO:50, a VL CDR2 having SEQ ID NO:51 and a VL CDR3 having SEQ ID NO:52. Preferably, the antibody is selected from an antibody, a Fab2, Fab _3, di-scFv, scFv-Fc, duobody, camelid antibody or a minibody. The antibody comprises at least two binding regions that specifically bind to an Ig-like 1 domain of a MuSK protein. The at least two binding regions (e.g. CDR sequences, VL, VH sequences etc) may be the same (i.e. they may both comprise the sequences set out above).

In one embodiment, a binding region that specifically binds to the Ig-like 1 domain of a MuSK protein comprises at least one, particularly at least two, more particularly at least 3 of the heavy chain CDR’s: a VH CDR1 having SEQ ID NO:46, a VH CDR2 having SEQ ID NO:47 and a VH CDR3 having SEQ ID NO:48 and at least one, particularly at least two, more particularly at least 3 of the light chain CDR’s: a VL CDR1 having SEQ ID NO:50, a VL CDR2 having SEQ ID NO:51 and a VL CDR3 having SEQ ID NO:52. Preferably, the antibody is selected from an antibody, a Fab2, Fab _3, di-scFv, scFv-Fc, duobody, camelid antibody or a minibody. The antibody comprises at least two binding regions that specifically bind to an Ig-like 1 domain of a MuSK protein. The at least two binding regions (e.g. CDR sequences, VL, VH sequences etc) may be the same (i.e. they may both comprise the sequences set out above).

In one embodiment, a binding region that specifically binds to the Ig-like 1 domain of a MuSK protein comprises the heavy chain CDR’s: a VH CDR1 having SEQ ID NO:46, a VH CDR2 having SEQ ID NO:47 and a VH CDR3 having SEQ ID NO:48 and the light chain CDR’s: a VL CDR1 having SEQ ID NO:50, a VL CDR2 having SEQ ID NO:51 and a VL CDR3 having SEQ ID NO:52. Preferably, the antibody is selected from an antibody, a Fab2, Fab _3, di-scFv, scFv-Fc, duobody, camelid antibody or a minibody. The antibody comprises at least two binding regions that specifically bind to an Ig-like 1 domain of a MuSK protein. The at least two binding regions (e.g. CDR sequences, VL, VH sequences etc) may be the same (i.e. they may both comprise the sequences set out above). In one embodiment, a binding region that specifically binds to the Ig-like 1 domain of a MuSK protein comprises: a heavy chain variable domain having an amino acid sequence that is 85%, 86%, 87%, 88%, 89%, 90%, 91 %, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99% or 100% identical to the polypeptide of SEQ ID NO:53; and a light chain variable domain an amino acid sequence that is 85%, 86%, 87%, 88%, 89%, 90%, 91 %, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99% or 100% identical to the polypeptide of SEQ ID NO:57. Preferably, the antibody is selected from an antibody, a Fab2, Fab3 _, di-scFv, scFv-Fc, duobody, camelid antibody or a minibody. The antibody comprises at least two binding regions that specifically bind to an Ig-like 1 domain of a MuSK protein. The at least two binding regions (e.g. CDR sequences, VL, VH

sequences etc) may be the same (i.e. they may both comprise the sequences set out above).

Preferably, the heavy chain variable domain has an amino acid sequence that is 85%, 86%, 87%, 88%, 89%, 90%, 91 %, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99% or 100% identical to the polypeptide of SEQ ID NO:53 and comprises at least one, particularly at least two, more particularly at least 3 of the heavy chain CDR’s: a VH CDR1 having SEQ ID NO:54, a VH CDR2 having SEQ ID NO:55 and a VH CDR3 having SEQ ID NO:56; and light chain variable domain an amino acid sequence that is 85%, 86%, 87%, 88%, 89%, 90%, 91 %, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99% or 100% identical to the polypeptide of SEQ ID NO:57 and comprises at least one, particularly at least two, more particularly at least 3 of the light chain CDR’s: a VL CDR1 having SEQ ID NO:58, a VL CDR2 having SEQ ID NO:59 and a VL CDR3 having SEQ ID NO:60. Preferably, the antibody is selected from an antibody, a Fab2, Fab3 _, di-scFv, scFv-Fc, duobody, camelid antibody or a minibody. The antibody comprises at least two binding regions that specifically bind to an Ig-like 1 domain of a MuSK protein. The at least two binding regions (e.g. CDR sequences, VL, VH sequences etc) may be the same (i.e. they may both comprise the sequences set out above). Most preferably, the binding region that specifically binds to the Ig-like 1 domain of a MuSK protein comprises a heavy chain variable domain of SEQ ID NO:53 and a light chain variable domain of SEQ ID NO:57. Preferably, the antibody is selected from an antibody, a Fab2, Fab _3, di-scFv, scFv-Fc, duobody, camelid antibody or a minibody. The antibody comprises at least two binding regions that specifically bind to an Ig-like 1 domain of a MuSK protein. The at least two binding regions (e.g. CDR sequences, VL, VH sequences etc) may be the same (i.e. they may both comprise the sequences set out above).

In one embodiment, a binding region that specifically binds to the Ig-like 1 domain of a MuSK protein comprises a heavy chain variable domain comprising a polypeptide sequence having at least 1 , 2, 3, 4 or 5 conservative substitutions compared to a polypeptide sequence of SEQ ID NO: 53; and a light chain variable domain comprising a polypeptide sequence having at least 1 , 2, 3, 4 or 5 conservative substitutions compared to a polypeptide sequence of SEQ ID NO: 57. Preferably, the binding region that specifically binds to the Ig-like 1 domain of a MuSK protein comprises at least one, particularly at least two, more particularly at least 3 of the heavy chain CDR’s: a VH CDR1 having SEQ ID NO:54, a VH CDR2 having SEQ ID NO:55 and a VH CDR3 having SEQ ID N056. Preferably, the binding region that specifically binds to the Ig-like 1 domain of a MuSK protein comprises at least one, particularly at least two, more particularly at least 3 of the light chain CDR’s: a VL CDR1 having SEQ ID NO:58, a VL CDR2 having SEQ ID NO:59 and a VL CDR3 having SEQ ID NO:60. Preferably, the binding region that specifically binds to the Ig-like 1 domain of a MuSK protein comprises at least one, particularly at least two, more particularly at least 3 of the heavy chain CDR’s: a VH CDR1 having SEQ ID NO:54, a VH CDR2 having SEQ ID NO:55 and a VH CDR3 having SEQ ID NO:56 and at least one, particularly at least two, more particularly at least 3 of the light chain CDR’s: a VL CDR1 having SEQ ID NO:58, a VL CDR2 having SEQ ID NO:59 and a VL CDR3 having SEQ ID NO:60. Preferably, the antibody is selected from an antibody, a Fab2, Fab _3, di-scFv, scFv-Fc, duobody, camelid antibody or a minibody. The antibody comprises at least two binding regions that specifically bind to an Ig-like 1 domain of a MuSK protein. The at least two binding regions (e.g. CDR sequences, VL, VH sequences etc) may be the same (i.e. they may both comprise the sequences set out above).

In one embodiment, a binding region that specifically binds to the Ig-like 1 domain of a MuSK protein comprises a heavy chain variable domain comprising a polypeptide sequence having at least 1 , 2, 3, 4 or 5 conservative substitutions compared to a polypeptide sequence of SEQ ID NO: 53; and a light chain variable domain of SEQ ID NO:57. Preferably, the binding region that specifically binds to the Ig-like 1 domain of a MuSK protein comprises at least one, particularly at least two, more particularly at least 3 of the heavy chain CDR’s: a VH CDR1 having SEQ ID NO:54, a VH CDR2 having

SEQ ID NO:55 and a VH CDR3 having SEQ ID NO:56. Preferably, the antibody is selected from an antibody, a Fab2, Fab3 _, di-scFv, scFv-Fc, duobody, camelid antibody or a minibody. The antibody comprises at least two binding regions that specifically bind to an Ig-like 1 domain of a MuSK protein. The at least two binding regions (e.g. CDR sequences, VL, VH sequences etc) may be the same (i.e. they may both comprise the sequences set out above).

In one embodiment, a binding region that specifically binds to the Ig-like 1 domain of a MuSK protein comprises a heavy chain variable domain of SEQ ID NO: 53; and a light chain variable domain comprising a polypeptide sequence having at least 1 , 2, 3, 4 or 5 conservative substitutions compared to a polypeptide sequence of SEQ ID NO: 57. Preferably, the binding region that specifically binds to the Ig-like 1 domain of a MuSK protein comprises at least one, particularly at least two, more particularly at least 3 of the light chain CDR’s: a VL CDR1 having SEQ ID NO:58, a VL CDR2 having SEQ ID NO:59 and a VL CDR3 having SEQ ID NO:60. Preferably, the antibody is selected from an antibody, a Fab2, Fab3 _, di-scFv, scFv-Fc, duobody, camelid antibody or a minibody. The antibody comprises at least two binding regions that specifically bind to an Ig-like 1 domain of a MuSK protein. The at least two binding regions (e.g. CDR sequences, VL, VH sequences etc) may be the same (i.e. they may both comprise the sequences set out above). In one embodiment, a binding region that specifically binds to the Ig-like 1 domain of a MuSK protein comprises at least one, particularly at least two, more particularly at least 3 of the heavy chain CDR’s: a VH CDR1 having SEQ ID NO:54, a VH CDR2 having SEQ ID NO:55 and a VH CDR3 having SEQ ID NO:56. Preferably, the antibody is selected from an antibody, a Fab2, Fab _3, di-scFv, scFv-Fc, duobody, camelid antibody or a minibody. The antibody comprises at least two binding regions that specifically bind to an Ig-like 1 domain of a MuSK protein. The at least two binding regions (e.g. CDR sequences, VL, VH sequences etc) may be the same (i.e. they may both comprise the sequences set out above).

In one embodiment, a binding region that specifically binds to the Ig-like 1 domain of a MuSK protein comprises at least one, particularly at least two, more particularly at least 3 of the light chain CDR’s: a VL CDR1 having SEQ ID NO:58, a VL CDR2 having SEQ ID NO:59 and a VL CDR3 having SEQ ID NO:60. Preferably, the antibody is selected from an antibody, a Fab2, Fab _3, di-scFv, scFv-Fc, duobody, camelid antibody or a minibody. The antibody comprises at least two binding regions that specifically bind to an Ig-like 1 domain of a MuSK protein. The at least two binding regions (e.g. CDR sequences, VL, VH sequences etc) may be the same (i.e. they may both comprise the sequences set out above).

In one embodiment, a binding region that specifically binds to the Ig-like 1 domain of a MuSK protein comprises at least one, particularly at least two, more particularly at least 3 of the heavy chain CDR’s: a VH CDR1 having SEQ ID NO:54, a VH CDR2 having SEQ ID NO:55 and a VH CDR3 having SEQ ID NO:56 and at least one, particularly at least two, more particularly at least 3 of the light chain CDR’s: a VL CDR1 having SEQ ID NO:58, a VL CDR2 having SEQ ID NO:59 and a VL CDR3 having SEQ ID NO:60. Preferably, the antibody is selected from an antibody, a Fab2, Fab _3, di-scFv, scFv-Fc, duobody, camelid antibody or a minibody. The antibody comprises at least two binding regions that specifically bind to an Ig-like 1 domain of a MuSK protein. The at least two binding regions (e.g. CDR sequences, VL, VH sequences etc) may be the same (i.e. they may both comprise the sequences set out above). In one embodiment, a binding region that specifically binds to the Ig-like 1 domain of a MuSK protein comprises the heavy chain CDR’s: a VH CDR1 having SEQ ID NO:54, a VH CDR2 having SEQ ID NO:55 and a VH CDR3 having SEQ ID NO:56 and the light chain CDR’s: a VL CDR1 having SEQ ID NO:58, a VL CDR2 having SEQ ID NO:59 and a VL CDR3 having SEQ ID NO:60. Preferably, the antibody is selected from an antibody, a Fab2, Fab _3, di-scFv, scFv-Fc, duobody, camelid antibody or a minibody. The antibody comprises at least two binding regions that specifically bind to an Ig-like 1 domain of a MuSK protein. The at least two binding regions (e.g. CDR sequences, VL, VH sequences etc) may be the same (i.e. they may both comprise the sequences set out above).

As used herein, the terms“homology” and“identity” are used interchangeably.

Calculations of sequence homology or identity between sequences are performed as follows.

To determine the percent identity of two amino acid sequences, or of two nucleic acid sequences, the sequences are aligned for optimal comparison purposes (e.g., gaps can be introduced in one or both of a first and a second amino acid or nucleic acid sequence for optimal alignment and non-homologous sequences can be disregarded for comparison purposes). In a preferred embodiment, the length of a reference sequence aligned for comparison purposes is at least 30%, preferably at least 40%, more preferably at least 50%, even more preferably at least 60%, and even more preferably at least 70%, 75%, 80%, 82%, 84%, 85%, 86%, 87%, 88%, 89%, 90%, 91 %, 92%,

93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% of the length of the reference sequence. The amino acid residues or nucleotides at corresponding amino acid positions or nucleotide positions are then compared. When a position in the first sequence is occupied by the same amino acid residue or nucleotide as the

corresponding position in the second sequence, then the molecules are identical at that position (as used herein amino acid or nucleic acid“identity” is equivalent to amino acid or nucleic acid“homology”). The percent identity between the two sequences is a function of the number of identical positions shared by the sequences, taking into account the number of gaps, and the length of each gap, which need to be introduced for optimal alignment of the two sequences.

The comparison of sequences and determination of percent identity between two sequences can be accomplished using a mathematical algorithm. In a preferred embodiment, the percent identity between two amino acid sequences is determined using the Needleman et al. (1970) J. Mol. Biol. 48:444-453) algorithm which has been incorporated into the GAP program in the GCG software package (available at http://www.gcg.com), using either a BLOSUM 62 matrix or a PAM250 matrix, and a gap weight of 16, 14, 12, 10, 8, 6, or 4 and a length weight of 1 , 2, 3, 4, 5, or 6. In yet another preferred embodiment, the percent identity between two nucleotide sequences is determined using the GAP program in the GCG software package (available at http://www.gcg.com), using a NWSgapdna.CMP matrix and a gap weight of 40, 50, 60, 70, or 80 and a length weight of 1 , 2, 3, 4, 5, or 6. A particularly preferred set of parameters (and the one that should be used if the practitioner is uncertain about what parameters should be applied to determine if a molecule is within a sequence identity or homology limitation of the invention) are a BLOSUM 62 scoring matrix with a gap penalty of 12, a gap extend penalty of 4, and a frameshift gap penalty of 5. Alternatively, the percent identity between two amino acid or nucleotide sequences can be determined using the algorithm of Meyers et al. (1989) CABIOS 4:11-17) which has been incorporated into the ALIGN program (version 2.0), using a PAM 120 weight residue table, a gap length penalty of 12 and a gap penalty of 4. As used herein the term "conservative amino acid substitution" refers to replacement of an amino acid residue with an amino acid residue having a similar side chain. Families of amino acid residues having similar side chains have been defined in the art. These families include amino acids with basic side chains (e.g., lysine, arginine, histidine), acidic side chains (e.g., aspartic acid, glutamic acid), uncharged polar side chains (e.g., glycine, asparagine, glutamine, serine, threonine, tyrosine, cysteine), nonpolar side chains (e.g., alanine, valine, leucine, isoleucine, proline, phenylalanine, methionine, tryptophan), beta-branched side chains ( e.g ., threonine, valine, isoleucine) and aromatic side chains {e.g., tyrosine, phenylalanine, tryptophan, histidine).

In the context of binding, the terms “specific” and “specifically” are used herein interchangeably to indicate that other biomolecules do not significantly bind to the binding region (e.g. CDR, variable region etc.) that is specifically binding to the biomolecule of interest (where the biomolecule of interest is the Ig-like 1 domain of a MuSK protein). In some examples, the level of binding to a biomolecule other than the Ig-like 1 domain of a MuSK protein results in a negligible (e.g., not determinable) binding affinity by means of ELISA or an affinity determination.

By“negligible binding” a binding is meant, which is at least about 85%, particularly at least about 90%, more particularly at least about 95%, even more particularly at least about 98%, but especially at least about 99% and up to 100% less than the binding to an Ig-like 1 domain of a MuSK protein.

The binding affinity of a binding region with an Ig-like 1 domain of a MuSK protein may be determined using a standard binding assay, such as surface plasmon resonance technique (BIAcore®, GE-Healthcare Uppsala, Sweden). The term "surface plasmon resonance," as used herein, refers to an optical phenomenon that allows for the analysis of real-time biospecific interactions by detection of alterations in protein concentrations within a biosensor matrix, for example using the BIAcore system (Pharmacia Biosensor AB, Uppsala, Sweden and Piscataway, N.J.). For further descriptions, see Jonsson, U., et al. (1993) Ann. Biol. Clin. 51 : 19-26; Jonsson, U., et al. (1991 ) Biotechniques 1 1 :620- 627; Johnsson, B., et al. (1995) J. Mol. Recognit. 8: 125-131 ; and Johnnson, B., et al.

(1991 ) Anal. Biochem. 198:268-277.

For example, “specifically binding” in the context of the binding of an antibody to a predetermined antigen/epitope means binding with an affinity corresponding to a KD (equilibrium dissociation constant) of about 10 ⁷ M or less, such as about 10 ⁸ M or less, such as about 10 ^-9 M or less, about 10 ¹⁰ M or less, or about 10 ^-11 M or even less when determined for instance by surface plasmon resonance (SPR) technology in a BIAcore 3000 instrument using the antibody as the analyte (wherein a low KD indicates a high affinity). This term also means that the antibody binds to the predetermined antigen/epitope with an affinity corresponding to a KD that is at least ten-fold lower, such as at least 100 fold lower, for instance at least 1000 fold lower, such as at least 10,000 fold lower, for instance at least 100,000 fold lower than its affinity for binding to a non- specific antigen (e.g. bovine serum albumin, casein) other than the predetermined antigen or a closely-related antigen. The amount with which the affinity is lower is dependent on the KD of the antibody, so that when the KD of the antibody is very low (that is, the antibody is very specific and binds very well), then the amount with which the affinity for the antigen is lower than the affinity for a non-specific antigen may be at least 10,000 fold. The term“KD” as used herein, means the dissociation rate constant of a particular antibody-antigen interaction.

Equivalent affinity values for“specific binding” of other binding agents described herein are well known in the art.

A binding region described herein may be specific for (i.e. specifically bind to) an epitope within the Ig-like 1 domain of a MuSK protein. As used herein the term“epitope” refers to a site on a target molecule (e.g., an antigen, such as a protein, for example an Ig-like 1 domain of a MuSK protein) to which a binding agent (e.g., a binding protein such as an antibody or antibody fragment) binds. Epitopes are groupings of molecules such as amino acids or sugar side chains and usually have specific structural characteristics, as well as specific charge characteristics. A single antigen may have more than one epitope. Epitopes can be formed both from contiguous or adjacent noncontiguous residues (e.g., amino acid residues) of the target molecule. Epitopes formed from contiguous residues (e.g., amino acid residues) typically are also called linear epitopes. An epitope typically includes at least 5 and up to about 12 residues, mostly between 6 and 10 residues (e.g. amino acid residues). Epitopes may also be conformational (i.e. non-linear). In one example, the MuSK protein is a human MuSK protein. The MuSK protein in humans is well characterized. The MuSK protein has been sequenced and the protein characterized recently by Valenzuela et at. (see W097/21811 ). It is a receptor tyrosine kinase (RTK) located on the cell surface of muscle cells at the NMJ and has the sequence shown in SEQ ID NO: 1 . The human MuSK protein can be identified by UniprotKB identifier: 015146. Human MuSK gene information can be found at Ensembl ref: ENSG00000030304. The gene has seven transcripts, as outlined below:

Table 1 : The seven ranscripts of human MuSK.

The extracellular region of MuSK contains three Ig-like domains and a Frizzled-like domain. The first N-terminal Ig-like domain (also known as the Ig-like 1 domain herein) in MuSK is required for MuSK to bind Lrp4 (Zhang et al., 201 1 ). Mutation of a single residue, I96, on a solvent exposed surface of the first Ig-like 1 domain, prevents MuSK from binding Lrp4 and responding to agrin (Stiegler et al., 2006; Zhang et at., 201 1 ). A hydrophobic surface on the opposite side of the first Ig-like 1 domain mediates MuSK homodimerization, essential for MuSK trans-phosphorylation. Although MuSK is expressed by muscle and not by motor neurons, MuSK is essential for presynaptic as well as postsynaptic differentiation (Burden et al., 2013). MuSK regulates presynaptic differentiation by clustering Lrp4 in muscle, which functions bi-directionally by serving not only as a receptor for agrin and a ligand for MuSK, but also as a direct retrograde signal for presynaptic differentiation. In addition to its role during synapse formation, MuSK is also required to maintain adult synapses, as inhibition of MuSK expression in adult muscle leads to profound defects in presynaptic and postsynaptic differentiation (Hesser et al., 2006). As described herein, at least two binding regions of the multivalent binding agent are specific for (i.e. specifically bind to) an epitope within the Ig-like 1 domain of a human MuSK protein (also referred to herein as the Immunoglobulin-like 1 domain of MuSK; the MuSK Ig-like 1 domain; or the first Ig-like 1 domain of MuSK etc). The Ig-like 1 domain of the human MuSK protein has the sequence shown in SEQ ID NO:2:

PVITTPLETVDALVEEVATFMCAVESYPQPEISWTRNKILIKLFDTRYSIRENGQLL TILSV EDSDDGIYCCTANNGVGGAVESCGALQV

Binding agents that are multivalent may be monospecific. As used herein, a“monospecific multivalent” binding agent contains a plurality of (e.g. two, three, four, five etc) binding regions/antigen-binding sites (e.g. variable regions) each of which are identical (or at least, each of which bind to the same epitope in the target antigen).

Binding agents that are multivalent may also be“multi-specific” (e.g. bi-specific, or tri- specific etc). This is may be the case when the binding agents described herein as MuSK agonists have three or more binding regions (i.e. two binding regions that are specific for the Ig-like 1 domain of a MuSK protein, and one or more additional binding regions that may be specific for a different antigen/epitope. A“bi-specific” binding agent binds to two different antigens/epitopes etc.

The binding agent may be any appropriate binding agent known in the art. Non-limiting examples of binding agents described herein include, but are not limited to, aptamers (e.g. nucleic acid aptamers, peptide aptamers, aptabodies, affimers), binding proteins (e.g. antibodies, antibody mimetics, camelid antibodies, duobodies etc) and small molecules. Other appropriate binding agents are also well known and readily identifiable to a person of skill in the art using routine experimental procedures.

As used herein, “aptamer” refers to nucleic acid aptamers and/or peptide aptamers. Examples of aptamers include affimers (an evolution of peptide aptamers) and aptabodies (formed by hybridisation of two DNA aptamers), which are also well known and readily identifiable to a person of skill in the art using routine experimental procedures.

In one example, the binding agent is a binding protein. Examples of appropriate binding proteins include antibodies and antibody mimetics. Examples of antibodies are provided elsewhere herein. Examples of appropriate antibody mimetics include affibody molecules (including affimabs) affilins, peptide aptamers (including affimers), affitins, alphabodies, anticalins, avimers, DARPins, Fynomers, Kunitz domain peptides, monobodies, nanoCLAMPs etc., which are also well known and readily identifiable to a person of skill in the art using routine experimental procedures. As stated elsewhere herein, and as shown in the examples, the binding agent may be an antibody. The terms“antibody” or“antibodies” as used herein refer to molecules or active (multivalent) fragments of molecules that bind to known antigens, particularly to immunoglobulin molecules and to immunologically active portions of immunoglobulin molecules, i.e. molecules that contain two binding sites that immunospecifically binds to the target antigen (the Ig-like 1 domain of MuSK in this instance). The immunoglobulin according to the invention can be of any class (IgG, IgM, IgD, IgE, IgA and IgY) or subclass (e.g. lgG1 , lgG2, lgG3, lgG4, lgA1 and lgA2) of immunoglobulin molecule and based on heavy chain sequences from any species. For example, the species may be, but not limited to dogs, cats, horses, cows, pigs, guinea pigs, mice, rats and the like. The species may be a primate (e.g. a non-human primate). In a preferred example, the species is a human.

The term“antibody” or“antibodies” include monoclonal, polyclonal, chimeric, single chain, bi-specific, human and humanized antibodies as well as active multivalent fragments thereof. Examples of active multivalent fragments of molecules that bind to known antigens and are useful in the present invention include F(ab') ₂ , F(ab')3, diabodies, triabodies, scFv-Fc and di-scFv and minibodies, including the products of an Fab immunoglobulin expression library and epitope-binding multivalent fragments of any of the antibodies and multivalent fragments mentioned above.

In a particular example, the antibody may be a monoclonal antibody. As used herein, the term“monoclonal antibody” refers to an antibody that is mass produced in the laboratory from a single clone and that recognizes only one antigen. Monoclonal antibodies may be generated by any appropriate technique known in the art (e.g. by production in HEK or insect cells, or by generation of B cell hybridomas). As used herein, the term“chimeric antibody” refers to a monoclonal antibody comprising a variable region, i.e. , binding region, from one source or species, (i.e. non-human primates, humans, dogs, cats, horses, cows, pigs, guinea pigs, mice, rats and the like ) and at least a portion of a constant region derived from a different source or species, usually prepared by recombinant DNA techniques. Chimeric antibodies comprising a mouse variable region and a human constant region are examples. Such chimeric antibodies are the product of expressed immunoglobulin genes comprising DNA segments encoding mouse immunoglobulin variable regions and DNA segments encoding human immunoglobulin constant regions. Other forms of“chimeric antibodies” encompassed by the present disclosure are those in which the class or subclass has been modified or changed from that of the original antibody, for example to

alter them so that they are complement and Fc receptor binding deficient. Methods for producing chimeric antibodies involve conventional recombinant DNA and gene transfection techniques now well known in the art. See, e.g., Morrison, S. L., et al., Proc. Natl. Acad Sci. USA 81 (1984) 6851 -6855; U.S. Pat. No. 5,202,238 and U.S. Pat. No. 5,204,244. In a particular example, the antibodies described herein are complement and Fc receptor binding deficient.

In a particular example, the antibody may be a human antibody or a humanized antibody. As used herein the term“humanized antibody” or“humanized version of an antibody” refers to antibodies in which the framework or“complementarity determining regions” (CDR) have been modified to comprise the CDR of an immunoglobulin of different specificity as compared to that of the parent immunoglobulin. In some examples, the CDRs of the VH and VL are grafted into the framework region of human antibody to prepare the“humanized antibody.” See e.g. Riechmann, L., etal., Nature 332 (1988) 323- 327; and Neuberger, M. S., et ai, Nature 314 (1985) 268-270. The heavy and light chain variable framework regions can be derived from the same or different human antibody sequences. Both the heavy chain and the light chain may be required for effective antigen (MuSK) binding. The human antibody sequences can be the sequences of naturally occurring human antibodies. Human heavy and light chain variable framework regions are listed e.g. in Lefranc, M.-P., Current Protocols in Immunology (2000)— Appendix 1 P A.1 P.1 -A.1 P.37 and are accessible via IMGT, the international ImMunoGeneTics information System® (http://imgt.cines.fr) or via http://vbase.mrc-cpe.cam.ac.uk, for example. Optionally the framework region can be modified by further mutations. Exemplary CDRs correspond to those representing sequences recognizing the antigens noted above for chimeric antibodies. In some examples, such humanized version is chimerized with a human constant region.

As used herein the term “human antibody” is intended to include antibodies having variable and constant regions derived from human germ line immunoglobulin sequences. Human antibodies are well-known in the state of the art (van Dijk, M. A., and van de Winkel, J. G., Curr. Opin. Chem. Biol. 5 (2001 ) 368-374). Human antibodies can also be produced in transgenic animals (e.g., mice) that are capable, upon immunization, of producing a full repertoire or a selection of human antibodies in the absence of endogenous immunoglobulin production. Transfer of the human germ-line immunoglobulin gene array in such germ-line mutant mice results in the production of human antibodies upon antigen challenge (see, e.g., Jakobovits, A., et ai, Proc. Natl. Acad. Sci. USA 90 (1993) 2551 -2555; Jakobovits, A., et ai, Nature 362 (1993) 255-258; Brueggemann, M. D., et ai, Year Immunol. 7 (1993) 33-40). Human antibodies can also be produced in phage display libraries (Hoogenboom, H. R., and Winter, G., J. Mol. Biol. 227 (1992) 381 -388; Marks, J. D., et ai, J. Mol. Biol. 222 (1991 ) 581-597). The techniques of Cole, A., et al. and Boerner, P., et al. are also available for the preparation of human monoclonal antibodies (Cole, A., et ai, Monoclonal Antibodies and Cancer Therapy, Liss, A. R. (1985) p. 77; and Boerner, P., et ai, J. Immunol. 147 (1991 ) 86-95).

In a particular example, the antibody may be selected from a Fab2, Fab3 _, di-scFv, scFv- Fc, duobody, or a minibody.

Other examples include single domain antibodies such as those found in camelids including, but not limited to, llamas and alpacas; and cartilaginous fish including, but not limited to, sharks which are widely known in the art.

As used herein“single chain antibody” refers to single chain Fv molecules (scFv), wherein a VH domain and a VL domain are linked by a peptide linker which allows the two domains to associate to form an antigen binding site (Bird et ai, 1988, Science 242:423-426, Huston et ai, 1988, Proc. Natl. Acad. Sci. U.S.A. 85:5879-5883 or a bi-specific single chain Fv (WO 03/11161 ). Typical scFv linkers are well known in the art, are generally 10 to 25 amino acids in length and include glycines and serines.

Single domain antibody fragments (VHH or nanobodies) e.g. the functional antibodies produced by camelids that are devoid of light chains and wherein a single N-terminal domain is fully capable of antigen binding are also an example of a binding agent described herein. Such fragments can also form bivalent VHHH, or pentabodies (i.e. with 5 VHH domains). As used herein, a“di-ScFv” refers to a dimerized scFV.

As used herein, “minibodies” are minimized antibody-like proteins comprising a scFv joined to a CH3 domain. See Hu et at., 1996, Cancer Res. 56:3055-3061. In some cases, the scFv can be joined to the Fc region, and may include some or the entire hinge region.

By "Fab" or "Fab region" as used herein is meant the polypeptides that comprise the VH, CH1 , VH, and CL immunoglobulin domains. Fab may refer to this region in isolation, or this region in the context of a full-length antibody or antibody fragment or fab fusion protein.

The terms“Fab”,“Fab region”,“Fab portion” or“Fab fragment” are understood to define a polypeptide that includes a VH, a CH1 , a VL, and a CL immunoglobulin domain. Fab may refer to this region in isolation, or this region in the context of an antibody molecule according to the invention, as well as a full-length immunoglobulin or immunoglobulin fragment. Typically a Fab region contains an entire light chain of an antibody. A Fab region can be taken to define“an arm” of an immunoglobulin molecule. It contains the epitope-binding portion of that Ig. The Fab region of a naturally occurring immunoglobulin can be obtained as a proteolytic fragment by a papain-digestion. A“F(ab')2 portion” is the proteolytic fragment of a pepsin-digested immunoglobulin. A“Fab' portion” is the product resulting from reducing the disulfide bonds of an F(ab')2 portion. As used herein the terms “Fab”,“Fab region”,“Fab portion” or“Fab fragment” may further include a hinge region that defines the C-terminal end of the antibody arm (of. above). This hinge region corresponds to the hinge region found C-terminally of the CH1 domain within a full-length immunoglobulin at which the arms of the antibody molecule can be taken to define a Y. The term hinge region is used in the art because an immunoglobulin has some flexibility at this region. By "Fc fusion" as used herein is meant a protein wherein one or more polypeptides is operably linked to Fc. Fc fusion is herein meant to be synonymous with the terms "immunoadhesin", "Ig fusion", "Ig chimera", and "receptor globulin" (sometimes with dashes) as used in the prior art (Chamow et at., 1996, Trends Biotechnol 14:52-60; Ashkenazi et at., 1997, Curr Opin Immunol 9: 195-200). An Fc fusion combines the Fc region of an immunoglobulin with a fusion partner, which in general may be any protein, polypeptide or small molecule. The role of the non-Fc part of an Fc fusion, i.e. , the fusion partner, is to mediate target binding, and thus it is functionally analogous to the variable regions of an antibody. Virtually any protein or small molecule may be linked to Fc to generate an Fc fusion. Protein fusion partners may include, but are not limited to, the target-binding region of a receptor, an adhesion molecule, a ligand, an enzyme, a cytokine, a chemokine, or some other protein or protein domain. Small molecule fusion partners may include any therapeutic agent that directs the Fc fusion to a therapeutic target. Such targets may be any molecule, e.g., an extracellular receptor that is implicated in disease.

As used herein the term“antibody fragments” refers to a portion of a full-length antibody, for example possibly a variable domain thereof, or at least an antigen binding site thereof. Examples of antibody fragments include diabodies, single-chain antibody molecules, and multispecific antibodies formed from antibody fragments. scFv antibodies are, e.g., described in Huston, J. S., Methods in Enzymol. 203 (1991 ) 46-88. Antibody fragments can be derived from an antibody of the present invention by a number of art-known techniques. For example, purified monoclonal antibodies can be cleaved with an enzyme, such as pepsin, and subjected to HPLC gel filtration. The appropriate fraction containing Fab fragments can then be collected and concentrated by membrane filtration and the like. For further description of general techniques for the isolation of active fragments of antibodies, see for example, Khaw, B. A. et at. J. Nucl. Med. 23: 101 1 -1019 (1982); Rousseaux et al. Methods Enzymology, 121 :663-69, Academic Press, 1986. Antibody fragments useful to the invention are multivalent antibody fragments.

The binding proteins provided herein may be encoded by a nucleic acid molecule that has been codon optimized to increase protein expression in the appropriate host (e.g. HEK cells or insect cells). Codon optimization and protein production techniques are well known in the art and can readily be adapted for the production of binding proteins described herein. In a particular example, the binding protein described herein may be an IgG. The IgG may be selected from lgG1 , lgG2 or lgG3. In an alternative example, the binding protein described herein may be an lgG4 variant (described in more detail below). In vivo, lgG4 naturally undergoes Fab-arm exchange which renders it functionally bi- specific preventing crosslinking and internalization of the antigen (van der Neut Kolfschoten et ai, 2007). The term“Fab-arm exchange” or“FAE” as used herein means the exchange and recombination of one complete heavy-light chain pair (half-molecule or half-lgG) of one antibody with that of another antibody. In other words, “Fab-arm exchange” is used herein (and in the wider literature) to refer to the process leading to the exchange of human lgG4“half molecules” (half IgG exchange - see for example: Schuurman et ai, mAbs 4:6, 636-636 Nov/Dec 2012). Fab-arm exchange is also known as lgG4 shuffling, in which increased susceptibility of native lgG4 hinge disulfide bonds to reduction allows the heavy chains to separate and randomly re-associate to produce a mixed population of lgG4 molecules with randomized heavy-chain and light-chain pairs (Aalberse et ai, 1999. Int Arch Allergy Immunol 118: 187-189; Labrijn, et al, 2009, Nat Biotechnol 27:767-771 ; Schuurman et al, 2001. Mol Immunol 38: 1 -8; van der Neut Kolfschoten et al, 2007. Science 317: 1554-1557). See also Makaraviciute A, Jackson CD, Millner PA, Ramanaviciene A. J Immunol Methods. 2016 Feb;429:50-6. The terms “Fab-arm exchange”,“half-antibody exchange” and“half-lgG exchange” are therefore used interchangeably herein.

In the context of the invention, it is advantageous to reduce or remove the ability of an lgG4 to undergo Fab-arm exchange (e.g. in order to retain the at least two binding regions that specifically bind to the Ig-like 1 domain of a MuSK protein in the multivalent binding agent).

The binding protein described herein may therefore be an lgG4 variant with reduced ability for Fab-arm exchange in vivo (e.g. the lgG4 may be incapable of undergoing Fab- arm exchange). This type of binding protein is particularly useful as lgG4 antibodies are considered as “anti-inflammatory” antibodies because they are unable to bind complement or recruit immune cells (Lighaam et al., 2016). Accordingly, lgG4 variants described herein are less likely to induce an inflammatory response when administered to a subject as described herein (compared to an equivalent lgG1 , lgG2 or lgG3).

A (recombinant) lgG4 antibody variant with reduced ability for Fab-arm exchange in vivo may therefore be generated for use in the methods described herein, wherein the lgG4 variant comprises at least two variable regions that specifically bind to an Ig-like 1 domain of a MuSK protein.

As used herein, an lgG4 variant with“reduced ability for Fab-arm exchange in vivo " refers to an lgG4 that has a lower rate of Fab-arm exchange than a reference lgG4 antibody, under the same {in vivo) conditions. A“reduced ability” refers to a reduction in the Fab- arm exchange rate (e.g. at least a 5%, at least 10%, at least 15%, at least 20% at least 30%, at least 40%, at least 50%, at least 60%, at least 70%, at least 80%, at least 85%, at least 90%, at least 95% reduction in the rate of Fab-arm exchange over a defined time period (set from the point of the start of the test, e.g. administration of the lgG4 to a subject like a mouse or human subject)). Methods for measuring the rate of Fab-arm exchange are known in the art.

A reduction in lgG4 shuffling/Fab-arm exchange may be determined by detecting of a lower amount of half antibody molecules, detection of new bi-specific antibodies or of arm exchange produced from an lgG4 variant described herein (which e.g. contains one or more amino acid substitutions in the hinge or CH3 region), as compared to the amount of half antibody molecules or of arm exchange produced from an lgG4 molecule containing an lgG4 hinge or CH3 region not comprising said one or more amino acid substitutions. Any assay well-known in the art can be used to detect half antibody production and bi- specific antibody molecules. See, e.g., Van der Neut Kolfschoten et al, 2007, Science, 317: 1554-1557, for examples of assays to detect production of bi- specific antibodies.

As used herein, an lgG4 variant that is“incapable of undergoing Fab-arm exchange” means that the lgG4 variant is not able to exchange and recombine one complete heavy- light chain pair (half-molecule) of one antibody with that of another antibody in vivo (e.g. when measured over a 24 hour period set from the point of administration of the lgG4 to a subject e.g. a mouse subject or a human subject) or in vitro (e.g. when using a reducing agent such as glutathione). In other words, in this context an lgG4 variant that is bivalent and monospecific for the Ig-like 1 domain of a MuSK protein would not be capable of Fab- arm exchange and thus would not become bi-specific in vivo (e.g. when measured over a defined time period set from the point of start of the test, e.g. administration of the lgG4 to a subject like a mouse subject or a human subject). Methods for determining whether an lgG4 undergoes Fab-arm exchange are known in the art and are described above.

In one example, the lgG4 variant comprises an lgG4 constant region comprising one or more amino acid substitutions that reduce the ability for Fab-arm exchange in vivo.

For example, the lgG4 variant is incapable of undergoing Fab-arm exchange because it comprises an lgG4 constant region comprising an amino acid substitution at amino acid position 228 and/or an amino acid substitution at amino acid position 405 and/or an amino acid substitution at amino acid position 409 of the heavy chain numbered according to the EU index (See, Edelman et al. , 1969, Proc. Natl. Acad. Sci. USA, 63(1 ): 78-85 and Huijbers 2016). One or more of these substitutions in the hinge or CH3 region of human lgG4 results in considerable reduction of intra-chain disulfide bond formation, resulting in the reduction of lgG4 "half-antibody" molecules and reduced heterogeneity/shuffling of lgG4 molecules (Bloom et al. 1997, Protein Sci, 6:407- 415; Angal et al, 1993, Molecular Immunology, 30(1 ): 105-108)). There are also published reports that these hinge or CH3 mutations (or mutant forms that lack the residues required for Fab-arm exchange) may decrease lgG4 shuffling and increase the half-life of the lgG4 molecules in vivo (Labrijn, et al, 2009, Nat Biotechnol 27:767-771 ; Stubenrauch, et al, 2010, Drug Metab Dispos 38:84-91 ).

The lgG4 variant described herein may therefore contain one or more amino acid substitutions in the lgG4 hinge or CH3 region, wherein lgG4 shuffling is reduced relative to an antibody comprising an lgG4 hinge or CH3 region not comprising said one or more amino acid substitutions. In a specific example, the lgG4 hinge or CH3 region comprises a single amino acid substitution (either at position 228 (e.g. S228P), or at amino acid position 409 (e.g. R409K), or at amino acid position 405 (e.g. F405L) of the heavy chain numbered according to the EU index). In another specific example, the lgG4 hinge or CH3 region comprises two amino acid substitutions (at position 228 (e.g. S228P) and at position 409 (e.g. R409K); or at position 228 (e.g. S228P) and at position 405 (e.g. F405L); or at position 405 (e.g. F405L) and at position 409 (e.g. R409K)) of the heavy chain numbered according to the EU index). In another specific example, the lgG4 hinge or CH3 region comprises three amino acid substitutions (at position 228 (e.g. S228P), at position 405 (e.g. F405L) and at position 409 (e.g. R409K) of the heavy chain numbered according to the EU index). In other specific examples, the lgG4 region additional includes other amino acid substitutions; appropriate substitutions are well known in the art.

Treatment of a subject

The multivalent binding agent (e.g. binding protein such as an antibody) described herein is for use in treating a symptom, condition and/or disorder associated with impaired neuromuscular transmission, in a subject.

The terms“subject”,“patient” and“individual” are used interchangeably herein.

In certain examples, the subject is a mammal. A subject therefore refers to, for example, dogs, cats, horses, cows, pigs, guinea pigs, mice, rats and the like. The subject may be a primate (e.g. a non-human primate). In a preferred example, the subject is a human. The subject is typically a subject in need of treatment for a symptom, condition and/or disorder associated with impaired neuromuscular transmission.

A condition and/or disorder associated with impaired neuromuscular transmission is also referred to as “a neuromuscular transmission disorder” herein. A neuromuscular transmission disorder is a medical condition where the normal transmission through the NMJ fails to function correctly. It therefore encompasses any neuromuscular diseases that would benefit from strengthening of neuromuscular synapses and transmission or communication. Identifying neuromuscular transmission diseases that would benefit from strengthening of neuromuscular synapses (for e.g. increasing or improving neuromuscular transmission via strengthening of the synapses) is routine in the art (see e.g. the methods described below). In this context, the terms“condition”,“disorder” and“disease” are used interchangeably, unless specified otherwise. The multivalent binding agent (e.g. binding protein such as an antibody) described herein may therefore be useful for strengthening of neuromuscular synapses (for e.g. increasing or improving neuromuscular transmission via strengthening of the synapses) in a subject.

Appropriate neuromuscular transmission disorders may be characterized by fatigable muscle weakness and may be confirmed, for example, by a typical distribution of muscle weakness, or by electromyographic evidence of a defect in neuromuscular transmission (for example, a decrease of more than about 10% in the amplitude of the compound muscle action potential on repetitive nerve stimulation at 3 Hz and/or an increase in jitter on single fiber studies), or by a positive response to anticholinesterase medication (edrophonium or pyridostigmine). Such methods are well known in the art.

Appropriate neuromuscular transmission disorders may additionally or alternatively be detected by the presence of autoantibodies to the MuSK protein, see for example U.S. Pat. No. 7,267,820, or the AChR, orLrp4, or agrin, or the presynaptic voltage-gated calcium channels, or another protein or molecule at the neuromuscular synapse. Alternatively, a neuromuscular transmission disorder may be seronegative for autoantibodies to all known NMJ-related antigens. Such a neurotransmission disorder may be moderate or severe (generalized) MG in which a standard radio- immunoprecipitation assay for all known NMJ-related antibodies is negative on several occasions, but MG is suspected due to phenotypical characteristics or EMG results that reflect MG.

Appropriate neuromuscular transmission disorders may additionally or alternatively be characterised by clinical signs and symptoms as seen the subject, including but not limited to, muscle weakness or atrophy of the external eye muscles, eyelid muscles, bulbar and neck muscles, muscles involved in respiratory functions, as well as other skeletal muscles. Neuromuscular transmission disorders include, but are not limited to, autoimmune MG, Lambert-Eaton myasthenic syndrome (LEMS), congenital myasthenic syndrome (CMS), ALS (amyotrophic lateral sclerosis), or other forms of motor neuron disease, or some forms of muscular dystrophy. Congenital myasthenic syndromes are heterogeneous disorders in which neuromuscular transmission is compromised by one or more specific mechanisms. (See, for example, Engel, 2008, Handb Clin Neurol; 91 :285-331 ).

In a particularly preferred example, the symptom, condition and/or disorder associated with impaired neuromuscular transmission is selected from MuSK MG, any form of autoimmune MG, congenital myasthenic syndrome (CMS), muscular dystrophy, motor neuron disease, sarcopenia, muscle disuse or sports injury.

MG may include refractory generalized MG. Refractory generalized MG may be characterized as including subjects or patients positive for autoantibodies binding to nicotinic acetylcholine receptor (anti-AChR) or MuSK (anti-MuSK) who continue to show marked generalized, bulbar or ocular weakness or persistent symptoms of MG while receiving current standard of care for MG such as cholinesterase inhibitor therapy and immunosuppressant therapy (1ST) or who require chronic plasma exchange or chronic IVIg to maintain clinical stability. In other embodiments, refractory generalized MGis characterized as including subjects or patients who continue to show marked generalized, bulbar or ocular weakness or persistent symptoms of MG while receiving current standard of care for MG such as cholinesterase inhibitor therapy and immunosuppressant therapy (1ST) or who require chronic plasma exchange or chronic IVIg to maintain clinical stability. As used herein, the phrase“requires chronic plasma exchange” to maintain clinical stability refers to the use of plasma exchange therapy on a patient on a regular basis for the management of muscle weakness at least every 4 months over the last 12 months. As used herein, the phrase“requires chronic IVIg” to maintain clinical stability refers to the use of IVIg therapy on a patient on a regular basis for the management of muscle weakness at least every 3 months over the last 12 months. In a one example, the multivalent binding agent (e.g. binding protein such as an antibody) described herein is for use in treating MuSK MG in a subject.

In one example, the MuSK MG subject is selected from subjects with refractory generalised MG or severe MG.

In one example, the MuSK MG subject is selected from subjects in need of treatment with corticosteroids or immunosuppressive drugs. The multivalent binding agent (e.g. binding protein such as an antibody) described herein may also be used to treat a symptom associated with impaired neuromuscular transmission in a subject.

Symptoms associated with impaired neuromuscular transmission include muscle weakness and fatigability. Other symptoms associated with impaired neuromuscular transmission are readily identifiable to a person of skill in the art (e.g. using electromyographical tests with repetitive nerve stimulation or single fibre electromyography or pharmacological tests using acetylcholinesterase inhibitors (i.e. neostigmine)). Symptoms associated with chronically impaired or blocked neuromuscular transmission include muscle loss or atrophy.

Muscle weakness and/or fatigability may occur in a subject due to muscle loss as a consequence of aging or as a consequence of muscle disuse. Muscle weakness and/or fatigability may also occur in a subject as a consequence of sports injury. In such cases, the muscle weakness and/or fatigability is considered to be a symptom associated with impaired neuromuscular transmission.

As a non-limiting example, symptoms associated with MG (e.g. MuSK or AChR MG) include muscle weakness and fatigability. Muscles primarily affected by MG include muscles that control eye and eyelid movement, facial expressions, chewing, talking, swallowing, breathing, neck movements, and limb movements. By way of example, a symptom of MG is often ocular weakness, causing ptosis (drooping eyelids) and/or diplopia (double vision). Other symptoms include leg weakness, dysphagia and slurred or nasal speech. Symptoms of weakness tend to worsen with various stressors such as exertion, heat and infection. The multivalent binding agent (e.g. binding protein such as an antibody) described herein may be used to treat muscle loss in a subject. Methods for identifying loss of muscle are well known in the art and include physical examination, muscle echography or magnetic resonance imaging (MRI) or computer tomography (CT) of muscle. The multivalent binding agent (e.g. binding protein such as an antibody) described herein is for treating a subject. As used herein, the term 'treating' or 'treatment' of any disease or symptom refers, in one embodiment, to ameliorating the disease or symptom (i.e. , arresting the disease, extent or severity of at least one of the clinical symptoms thereof). In another embodiment 'treating' or 'treatment' refers to ameliorating at least one physical parameter, which may not be discernible by the subject. In yet another embodiment, 'treating' or 'treatment' refers to modulating the disease or symptom, either physically (e.g., stabilization of a discernible symptom), physiologically, (e.g., stabilization of a physical parameter), or both. In a further embodiment, 'treating' or 'treatment' relates to slowing the progression of a disease or reducing the effects of a symptom.

In some examples, treatment of a neuromuscular transmission disorder (such as MG, e.g. MuSK MG) includes delaying the progressive motor dysfunction in the subject, or preserving neuromuscular synapses in the subject. The multivalent binding agent (e.g. binding protein such as an antibody) described herein may therefore be used to increase MuSK activity in the subject and thereby improve motor function in the subject or preserve neuromuscular synapses (for e.g. increasing or improving neuromuscular transmission via strengthening of the synapses) in the subject.

An improvement in motor function is measurable by determining strength or muscle function. For example, motor function and/or muscle strength can be assessed by physical examination using for example the Medical Research Council (MRC) scale or by using an instrument or apparatus (i.e. a handheld dynamometer or Quantitative Muscle Assessment (QMA) apparatus).

An improvement in motor function is measurable by determining electrophysiological characteristics. For example, EMG can be used to assess motor function and/or muscle innervation.

In some examples the multivalent binding agent (e.g. binding protein such as an antibody) described herein maintains the innervation levels in the subject. In other examples, it stabilises motor axon synapses or increases the number of motor axon synapses in the subject.

In other examples, treatment of MG includes the improvement of a clinical marker for MG severity. These markers include MG activity of daily living profile (MG-ADL), quantitative Myasthenia Gravis (QMG) score for disease severity, Myasthenia Gravis composite (MGC), negative inspiratory force (NIF), forced vital capacity, MGFA post-intervention status, and other quality of life measurements. In certain examples, MG-ADL is the primary score for measuring improvement after therapeutic interventions in MG. The current QMG scoring system consists of 13 items: ocular (2 items), facial (1 item), bulbar (2 items), gross motor (6 items), axial (1 item), and respiratory (1 item); each graded 0 to 3, with 3 being the most severe. The range of total QMG score is 0 -39. The QMG scoring system is considered to be an objective evaluation of therapy for MG and is based on quantitative testing of sentinel muscle groups. The MGFA task force has recommended that the QMG score be used in prospective studies of therapy for MG. A clinically meaningful improvement in a patient's QMG would be a e.g. 2.3 point or greater reduction in score (see N Engl J Med. 2016 Aug 11 ;375(6):511 -22).

The MG-ADL is an 8-point questionnaire that focuses on relevant symptoms and functional performance of activities of daily living (ADL) in MG subjects. The 8 items of the MG-ADL were derived from symptom-based components of the original 13-item QMG to assess disability secondary to ocular (2 items), bulbar (3 items), respiratory (1 item), and gross motor or limb (2 items) impairment related to effects from MG. In this functional status instrument, each response is graded 0 (normal) to 3 (most severe). The range of total MG-ADL score is 0-24. A clinically meaningful improvement in a patient's MG-ADL would be a 3 point or greater reduction in score after 26 weeks of treatment. The multivalent binding agent comprising at least two binding regions that specifically bind to an Ig-like 1 domain of a MuSK protein may be recombinant, wherein the at least two binding regions are variable regions derived from a MuSK antibody obtained from the subject. In other words, an autoantibody (e.g. an lgG4 autoantibody) for an Ig-like 1 domain of MuSK may be obtained from a subject (e.g. a subject having MuSK MG). The variable regions specifically bind to an Ig-like 1 domain of a MuSK protein of the antibody obtained from the subject may be used to generate a recombinant multivalent binding protein comprising at least two variable regions that specifically bind to an Ig-like 1 domain of a MuSK protein. The recombinant antibody may then be administered to the subject as an (autologous) treatment. Advantageously, use of autologous treatment may increase the chance of tolerance by the patient and minimalize the induction of neutralizing antibodies.

The multivalent binding agent (e.g. binding protein such as an antibody) described herein may be for administering to a subject in a therapeutically effective amount. “Therapeutically effective amount” means that amount of a drug, compound, antibody, or pharmaceutical agent that will elicit the biological or medical response of a subject that is being sought. Accordingly, in therapeutic applications described herein, agents and/or compositions thereof are administered to a subject in an amount sufficient to at least partially arrest the symptoms or disease and its complications. An amount adequate to accomplish this is defined as a“therapeutically effective amount or dose.” Amounts effective for this use will depend on the severity of the disease and the weight and general state of the patient.

A multivalent binding agent (e.g. binding protein such as an antibody) described herein may be part of a composition (e.g. a pharmaceutical composition) that comprises the binding protein and one or more other components. A composition may be a composition that comprises a multivalent binding agent described herein and a pharmaceutically acceptable excipient, adjuvant, diluent and/or carrier. Compositions may routinely contain pharmaceutically acceptable concentrations of salt, buffering agents, preservatives, compatible carriers, supplementary immune potentiating agents such as adjuvants and cytokines and optionally other therapeutic agents or compounds.

As used herein, "pharmaceutically acceptable" refers to a material that is not biologically or otherwise undesirable, i.e. , the material may be administered to an individual along with the selected binding protein without causing any undesirable biological effects or interacting in a deleterious manner with any of the other components of the pharmaceutical composition in which it is contained.

Excipients are natural or synthetic substances formulated alongside an active ingredient (e.g. a binding protein provided herein), included for the purpose of bulking-up the formulation or to confer a therapeutic enhancement on the active ingredient in the final dosage form, such as facilitating drug absorption or solubility. Excipients can also be useful in the manufacturing process, to aid in the handling of the active substance concerned such as by facilitating powder flowability or non-stick properties, in addition to aiding in vitro stability such as prevention of denaturation over the expected shelf life. Pharmaceutically acceptable excipients are well known in the art. A suitable excipient is therefore easily identifiable by one of ordinary skill in the art. By way of example, suitable pharmaceutically acceptable excipients include water, saline, aqueous dextrose, glycerol, ethanol, and the like.

Adjuvants are pharmacological and/or immunological agents that modify the effect of other agents in a formulation. Pharmaceutically acceptable adjuvants are well known in the art. A suitable adjuvant is therefore easily identifiable by one of ordinary skill in the art.

Diluents are diluting agents. Pharmaceutically acceptable diluents are well known in the art. A suitable diluent is therefore easily identifiable by one of ordinary skill in the art. Carriers are non-toxic to recipients at the dosages and concentrations employed and are compatible with other ingredients of the formulation. The term“carrier” denotes an organic or inorganic ingredient, natural or synthetic, with which the active ingredient is combined to facilitate the application. Pharmaceutically acceptable carriers are well known in the art. A suitable carrier is therefore easily identifiable by one of ordinary skill in the art. Generally, the multivalent binding agents (e.g. binding proteins such as antibodies) are administered in a pharmaceutically effective amount. The amount of multivalent binding agent actually administered will typically be determined by a physician, in the light of the relevant circumstances, including the condition to be treated, the chosen route of administration, the actual compound administered, the age, weight, and response of the individual patient, the severity of the patient's symptoms, and the like.

The pharmaceutical compositions can be administered by a variety of routes including oral, rectal, transdermal, subcutaneous, intravenous, intramuscular, and intranasal. The compositions are typically presented in unit dosage forms to facilitate accurate dosing. The term "unit dosage forms" refers to physically discrete units suitable as unitary dosages for human subjects and other mammals, each unit containing a predetermined quantity of active material calculated to produce the desired therapeutic effect, in association with a suitable pharmaceutical excipient. Typical unit dosage forms include prefilled, premeasured ampules or syringes of the liquid compositions or pills, tablets, capsules or the like in the case of solid compositions. In such compositions, the furansulfonic acid compound is usually a minor component (from about 0.1 to about 50% by weight or preferably from about 1 to about 40% by weight) with the remainder being various vehicles or carriers and processing aids helpful for forming the desired dosing form.

Liquid forms suitable for oral administration may include a suitable aqueous or nonaqueous vehicle with buffers, suspending and dispensing agents, colorants, flavors and the like. Solid forms may include, for example, any of the following ingredients, or compounds of a similar nature: a binder such as microcrystalline cellulose, gum tragacanth or gelatin; an excipient such as starch or lactose, a disintegrating agent such as alginic acid, Primogel, or corn starch; a lubricant such as magnesium stearate; a glidant such as colloidal silicon dioxide; a sweetening agent such as sucrose or saccharin; or a flavoring agent such as peppermint, methyl salicylate, or orange flavoring.

Injectable compositions are typically based upon injectable sterile saline or phosphate- buffered saline or other injectable carriers known in the art. As before, the active compound in such compositions is typically a minor component, often being from about 0.05 to 10% by weight with the remainder being the injectable carrier and the like.

Transdermal compositions are typically formulated as a topical ointment or cream containing the active ingredient(s), generally in an amount ranging from about 0.01 to about 20% by weight, preferably from about 0.1 to about 20% by weight, preferably from about 0.1 to about 10% by weight, and more preferably from about 0.5 to about 15% by weight. When formulated as an ointment, the active ingredients will typically be combined with either a paraffinic or a water-miscible ointment base.

The above-described components for pharmaceutical compositions are merely representative. Other materials as well as processing techniques and the like are set forth in Part 8 of Remington's Pharmaceutical Sciences, 17th edition, 1985, Mack Publishing Company, Easton, Pennsylvania.

The compounds described herein can also be administered in sustained release forms or from sustained release drug delivery systems. A description of representative sustained release materials can be found in Remington's Pharmaceutical Sciences. The compounds can be administered as the sole active agent or they can be administered in combination with other agents, including other compounds that demonstrate the same or a similar therapeutic activity and are determined to be safe and efficacious for such combined administration. Methods of treatment

A method of treating a subject having a symptom, condition and/or disorder associated with impaired neuromuscular transmission is also provided, the method comprising administering to the subject a multivalent binding agent comprising at least two binding regions that specifically bind to an Ig-like 1 domain of a MuSK protein.

The binding agent (e.g. binding protein such as an antibody) is for administration to an appropriate subject in a therapeutically effective amount.

The therapeutically effective amount may be sufficient to increase MuSK activity in the subject and thereby improve or preserve neuromuscular transmission and/or motor function in the subject.

The therapeutically effective amount may be sufficient to increase MuSK activity in the subject and thereby improve the function of neuromuscular synapses (for e.g. increasing or improving neuromuscular transmission via strengthening of the synapses) or preserve neuromuscular synapses in the subject.

The therapeutically effective amount may be sufficient to increase MuSK activity in the subject and thereby treats MuSK MG in the subject.

Features of appropriate binding agents, subjects, disorders and symptoms, pharmaceutical compositions, means for administration and tests for determining successful treatment are described in detail above and apply equally to this aspect.

A method for inducing or increasing MuSK activity in a subject in need thereof is also provided, the method comprising administering to the subject a multivalent binding agent (e.g. binding protein such as an antibody) comprising at least two binding regions that specifically bind to an Ig-like 1 domain of a MuSK protein.

The binding agent is for administration to an appropriate subject in a therapeutically effective amount.

As used herein,“inducing” or“increasing” refers to enhancing or up-regulating. For the avoidance of doubt,“inducing” or“increasing” the equivalent terms are all used herein generally to mean an increase in (e.g. MuSK dimerization) by a statistically significant amount.“Inducing” or increasing” therefore typically means an increase by at least 10% as compared to a control (i.e. in the absence of binding agent), for example an increase by at least about 20%, or at least about 30%, or at least about 40%, or at least about 50%, or at least about 60%, or at least about 70%, or at least about 80%, or at least about

90% or up to and including a 100% decrease, or any decrease between 10-100% as compared to a control (i.e. in the absence of binding agent), or at least about a 0.5-fold, or at least about a 1.0-fold, or at least about a 1.2-fold, or at least about a 1.5-fold, or at least about a 2-fold, or at least about a 3 -fold, or at least about a 4-fold, or at least about a 5-fold or at least about a 10-fold increase, or any increase between 1.0-fold and 10-fold or greater as compared to a control (i.e. in the absence of binding agent).

As used herein“MuSK activity” refers to both biological activity and functional activity of a MuSK protein, polypeptide or nucleic acid molecule (e. g., in a MuSK expressing cell or tissue), as determined in vivo, or in vitro, according to standard techniques. MuSK activity includes phosphorylation of MuSK or stimulation of aggregation of AChR on a cell membrane at a NMJ. MuSK activation may result in induction or maintenance of postsynaptic differentiation. As used herein a“subject in need thereof” refers to a subject that would benefit from an increase in MuSK activity. For example, the subject may be a subject having a symptom, condition and/or disorder associated with impaired neuromuscular transmission. Alternatively, the subject may be a subject at risk of developing a symptom, condition and/or disorder associated with impaired neuromuscular transmission. In a further example, the subject may have a desire to increase their MuSK activity (i.e. increase neuromuscular transmission) e.g. prior to or during rehabilitation, or prior to, during, or after sport. The methods of the invention may therefore also be useful for rehabilitation, or for increasing neuromuscular transmission in subjects (e.g. sports persons or sports animals such as race horses) that are undertaking, at about to undertake, or have just undertaken a sporting activity. The therapeutically effective amount may be sufficient to increase MuSK activity in the subject and thereby improve or preserve motor function in the subject.

The therapeutically effective amount may be sufficient to increase MuSK activity in the subject and thereby improve the function of neuromuscular synapses (for e.g. increasing or improving neuromuscular transmission via strengthening of the synapses) or preserve neuromuscular synapses in the subject.

The therapeutically effective amount may be sufficient to increase MuSK activity in the subject and thereby treats MuSK MG in the subject.

Features of appropriate binding agent, subjects, disorders and symptoms, pharmaceutical compositions, means for administration and tests for determining successful treatment are described in detail above and apply equally to this aspect.

Preparation of a medicament

In another aspect, the use of a multivalent binding agent (e.g. binding protein such as an antibody) comprising at least two binding regions that specifically bind to an Ig-like 1 domain of a MuSK protein in the preparation of a medicament for treating a symptom, condition and/or disorder associated with impaired neuromuscular transmission, is also provided. The medicament is for administration to an appropriate subject in a therapeutically effective amount.

The medicament is for administration to an appropriate subject in a therapeutically effective amount.

The therapeutically effective amount may be sufficient to increase MuSK activity in the subject and thereby improve or preserve motor function in the subject. The therapeutically effective amount may be sufficient to increase MuSK activity in the subject and thereby improve the function of neuromuscular synapses (for e.g. increasing or improving neuromuscular transmission via strengthening of the synapses) or preserve neuromuscular synapses in the subject.

The therapeutically effective amount may be sufficient to increase MuSK activity in the subject and thereby treats MuSK MG in the subject.

Features of appropriate binding agents, subjects, disorders and symptoms, pharmaceutical compositions, means for administration, and tests for determining successful treatment are described in detail above and apply equally to this aspect.

In another aspect, the use of a multivalent binding agent (e.g. binding protein such as an antibody) comprising at least two binding regions that specifically bind to an Ig-like 1 domain of a MuSK protein in the preparation of a medicament for inducing or increasing MuSK activity in a subject in need thereof is also provided. The medicament is for administration to an appropriate subject in a therapeutically effective amount.

The therapeutically effective amount may be sufficient to increase MuSK activity in the subject and thereby improve or preserve motor function in the subject. The therapeutically effective amount may be sufficient to increase MuSK activity in the subject and thereby improve the function of neuromuscular synapses (for e.g. increasing or improving neuromuscular transmission via strengthening of the synapses) or preserve neuromuscular synapses in the subject. The therapeutically effective amount may be sufficient to increase MuSK activity in the subject and thereby treats MuSK MG in the subject.

Features of appropriate binding proteins, subjects, disorders and symptoms, pharmaceutical compositions, means for administration, and tests for determining successful treatment are described in detail above and apply equally to this aspect.

The invention may be better understood by reference to the following non-limiting examples, which are provided as exemplary of the invention. The following examples are presented in order to more fully illustrate the preferred embodiments of the invention and should in no way be construed, however, as limiting the broad scope of the invention. Examples

The data provided below demonstrates that functionally monovalent MuSK antibodies (isolated from patients) binding the Ig-like 1 domain of MuSK act as antagonists. Conversely, the same antibodies in a bivalent monospecific form act as agonists. As serum lgG4, due to Fab-arm exchange, is also functionally monovalent, and lgG4 patient MuSK antibodies cause MG in animal models and in humans, the monovalency of these antibodies is considered the determining factor in the pathophysiology of MuSK MG.

Results

Description of MuSK MG patients included

Table 2: Clinical characteristics from study subjec s at the moment of PBMC withdrawal. Isolation and genetic characterization of MuSK autoantibodies from MuSK MG patients Antigen-specific single cell sorting yielded eight MuSK-binding B cells from the two MuSK MG patients. The frequency of circulating MuSK clones was ~7 per 100 million PBMC for patient 1 , and 2.5 per 100 million PBMC for patient 2. An overview of the MuSK autoantibody characteristics is given in Table 3. For six out of the eight MuSK autoantibody producing clones the inventors could isolate the variable-region sequences of the heavy chain (VH) and light chain (VL). For one clone only the VH variable-region could be sequenced.

Table 3: MuSK autoantibody characteristics

Interestingly, the majority of the antibodies isolated were of the lgG1 isotype. In addition, the inventors identified one lgG4 clone and one lgG3 clone. Fab-arm exchange is an important feature of lgG4 and might affect the functional characteristics of MuSK autoantibodies (Koneczny et al., 2017). Previous work on polyclonal purified fractions suggested that MuSK MG lgG4 has the ability to undergo Fab-arm exchange (Koneczny et at., 2017). The inventors sequenced the VH of the isolated MuSK monoclonal antibodies and confirmed that these antibodies indeed possess the serine at position 228 and the arginine at position 409 required to undergo Fab-arm exchange (data not shown).

N-linked glycosylation of the Fab can be important for antigen binding, e.g. in rheumatoid arthritis ACPA autoantibodies (van de Bovenkamp et at., 2018). Therefore, the inventors assessed the presence of the NXST (where X can’t be a P) motif in the monoclonal MuSK autoantibodies variable regions. In only one clone, such a motif was found, suggesting that glycosylation of the Fab is not essential for all MuSK autoantibodies. Functional characteristics of recombinant MuSK monoclonal autoantibodies

Autoantibody binding epitopes are important determinants of the pathomechanism in autoimmune diseases. The Ig-like 1 domain of MuSK was previously recognized as the main immunogenic region of MuSK in polyclonal lgG4 patient antibody fractions and serum (Huijbers et al., 2016). For five of the patient-derived MuSK antibodies the epitope could be mapped to the first Ig-like 1 domain of MuSK (Table 3). No monoclonal autoantibodies against other domains have thus far been identified. To establish the functional characteristics of the MuSK autoantibodies, recombinant antibodies were produced from an lgG1 isolated clone and an lgG4 isolated clone. To assess the importance of autoantibody isotype in MuSK MG each of these variable-regions were cloned in both an lgG1 and lgG4 backbone.

To assess the ability of recombinant patient-derived lgG1 and lgG4 MuSK antibodies to bind to MuSK at the postsynaptic membrane of NMJs, the inventors performed immunostaining on isolated mouse skeletal muscle. Both the IgG 1 and lgG4 versions of the recombinant monoclonal antibodies clearly bound to NMJs (Figure 1 , data shown for the lgG4 recombinant antibodies). Successful neuromuscular transmission depends on properly clustered AChR and is therefore strictly orchestrated through the agrin-Lrp4-MuSK signaling cascade (Burden et al., 2018). Agrin is released by the motor nerve terminal, binds Lrp4 and Lrp4 subsequently binds and stimulates MuSK dimerization and transphosphorylation. Activation of MuSK phosphorylation stimulates a variety of intracellular signaling cascades of which one culminates in AChR clustering. Purified patient lgG4 MuSK autoantibodies inhibit MuSK-Lrp4 interaction, MuSK phosphorylation and thereby reduce AChR clustering in C2C12 myotube cultures and mice (Huijbers & Zhang 2013, Koneczny et at., 2013). Interestingly, when the effect of patient-derived MuSK monoclonal antibodies was assessed on MuSK phosphorylation both recombinant lgG1 and lgG4 (bivalent monospecific) antibodies strongly activated rather than inhibited MuSK phosphorylation (Figure 2A). This effect was observed in both absence and presence of agrin. Activation of MuSK phosphorylation was furthermore concentration-dependent (Figure 2B) and differed slightly between the two clones. Monovalent Fab-antibody fragments produced by papain digestion from the same antibodies inhibited MuSK phosphorylation (Figure 2C). Without wishing to be bound to a particular theory, it appears that consistent with the observed activation or inhibition of MuSK phosphorylation respectively, bivalent monospecific MuSK antibodies induced agrin-independent AChR clustering whereas bivalent bi-specific MuSK antibodies or monovalent monospecific Fab fragments inhibited AChR clustering similar to MuSK serum-derived lgG4 (Koneczny et at, 2013) (Figure 2D).

Recombinant monoclonal lgG1 and lgG4 however both engage in bivalent monospecific antibody-antigen interactions. To investigate the functional effects of the bispecificity and functional monovalency of Fab-arm exchanged lgG4 MuSK antibodies in patients, the inventors generated monovalent Fab fragments from these recombinant antibodies by papain digestion. In vitro, these Fab fragments inhibited agrin-dependent MuSK phosphorylation (Figure 2C) and AChR clustering similar to patient serum-derived anti- MuSK IgG4 (Figure 2D).

In contrast, (and in line with activating MuSK phosphorylation in vitro), bivalent monospecific monoclonal MuSK antibodies activated agrin-dependent AChR clustering compared to Fab fragments derived from the same monoclonal. Furthermore, AChR clustering could be partially induced using bivalent monospecific antibodies independent from agrin (figure 2D). Thus, monovalent MuSK binding blocks the AChR clustering pathway, whereas bivalent monospecific MuSK antibodies stimulate MuSK, and can facilitate or induce AChR clustering in this tissue culture model.

These observations suggest that an agent with two MuSK Ig-like 1 domain binding sites has the ability to activate MuSK, whereas agents with only one MuSK Ig-like 1 domain binding site (similar to what was seen with patient purified lgG4 or Fab fragments) inhibit MuSK activation.

MuSK antibody characterisation

MuSK antibodies (isolated from myasthenia gravis patients with MuSK serum antibodies) that bind to the Ig-like 1 domain of MuSK have been isolated and sequenced as detailed below. As shown in Figures 3 and 4, a total of eight MuSK Ig-like 1 domain reactive antibodies (11 -3D9, 11 -3F6, 11 -8G4, 13-3B5, 13-3D10, 13-4D3, 14-2E9 and 11-7C5) were obtained from MuSK MG patients.

Variable region sequences/binding regions specific for binding to an Ig-like 1 domain of a MuSK protein

The variable region amino acid sequence for seven of these clones was obtained (11- 3D9, 11 -3F6, 11 -8G4, 13-3B5, 13-3D10, 13-4D3 and 14-2E9). The variable region of an antibody is typically made up of the heavy chain variable domain (VH) and the light chain variable domain (VL). Each of VH and VL typically have a CDR1 , a CDR2 and a CDR3 sequence that, together, confer specificity for the target antigen (i.e. VH CDR1 , VH CDR2, VH CDR3, VL CDR1 , VL CDR2 and VL CDR3 together confer specificity for the target antigen). However, as stated previously herein, a single VH or VL domain (i.e. VH CDR1 , VH CDR2 and VH CDR3; or VL CDR1 , VL CDR2 and VL CDR3) may also be sufficient to confer antigen-binding specificity.

For six clones, both the heavy chain and light chain variable domains were successfully sequenced - for one clone (1 1 -8G4) only the heavy chain variable domain sequence was successfully obtained. The heavy chain and light chain variable domain sequences are provided for each clone below. The type of light chain is also indicated below. HC = Heavy chain; LC = Light chain; VH = heavy chain variable domain; VL = light chain variable domain. Amino acid sequences that are underlined represent CDRs (from N to C terminus of the amino acid sequence: CDR1 , CDR2, CDR3).

The antibody sequences have been analysed using IMGT/V-QUEST program version: 3.4.17 (19 February 2019) - IMGT/V-QUEST reference directory release: 201910-2 (5 March 2019) (http://imgt.org/IMGT_vquest/vquest) Selecting for homo sapiens sequences.

1 1 -3D9 VH (SEQ ID NO:9)

EVQLVESGGGLVKPGESLRLSCAASGFNFSTYTMHWVRQAPGKGLEWVSSISSRSAY

KYY ADSVKGRFTISRDIAKNSLYLQMNSLRAEDTAVYYCARDFFQLGPPRFDSWGQGTLVT VS S

In other words, the heavy chain variable domain (VH) CDRs may be as follows: CDR1 comprising GFNFSTYT (SEQ ID NO:10), CDR2 comprising ISSRSAYK (SEQ ID NO:

1 1 ) and CDR3 comprising ARDFFQLGPPRFDS (SEQ ID NO: 12). These CDRs may optionally be in the context of a VH comprising the sequence of SEQ ID NO:9.

1 1 -3D9 VLk (SEQ ID NO: 13):

DIQMTQSPSSLSASIGDRVTISCRASQRISSFLNWYQQKPGKPPNLLIYGASNLQSG VP S

RFSGSGSGTDFTLTITSLQPEDYAIYYCQQSYSPMYTFGQGTSLEIK

In other words, the light chain variable domain (VL) CDRs may be as follows: CDR1 comprising QRISSF (SEQ ID NO: 14), CDR2 comprising GAS (SEQ ID NO: 15) and CDR3 comprising QQSYSPMYT (SEQ ID NO: 16). These CDRs may optionally be in the context of a VL comprising the sequence of SEQ ID NO: 13. Antigen specificity may be obtained by a combination of CDRs of SEQ ID NO: 10, 1 1 and 12 (heavy chain) and CDRs of SEQ ID NO: 14, 15 and 16 (light chain). For example, antigen specificity may be obtained by a combination of variable domains of SEQ ID NO: 9 (heavy chain) and SEQ: 13 (light chain).

1 1 -3F6 VH (SEQ ID NO: 17):

EVQLVESGGGLVKPGGSLRLSCAASGFTFSSYTMDWVRQAPGKGLEWVSSIGSNGD

YIYY ADSVRGRFTISRDNAKNSLYLQMNSLRPDDTADYYCARGQLAVAGTHFDYWGRGSLV TVS S

In other words, the heavy chain variable domain CDRs may be as follows: CDR1 comprising GFTFSSYT (SEQ ID NO: 18), CDR2 comprising IGSNGDYI (SEQ ID NO:

19) and CDR3 comprising ARGQLAVAGTHFDY (SEQ ID NO:20). These CDRs may optionally be in the context of a VFI comprising the sequence of SEQ ID NO: 17.

1 1 -3F6 VLk (SEQ ID NO: 21 ):

DIQMTQSPSSLSASVGDRVTISCRASQKVNKYVNWYQQTPGKAPRLLIYAASTLQSG V PS

RFSGSGSGA/VFTLTISGLQPEDFAIYFCQQSYSPLCTFGQGTKLEIK

In other words, the light chain variable domain CDRs may be as follows: CDR1 comprising QKVNKY (SEQ ID NO:22), CDR2 comprising AAS (SEQ ID NO: 23) and CDR3 comprising QQSYSPLCT (SEQ ID NO:24). These CDRs may optionally be in the context of a VL comprising the sequence of SEQ ID NO:21 .

Antigen specificity may be obtained by a combination of CDRs of SEQ ID NO: 18, 19 and 20 (heavy chain) and CDRs of SEQ ID NO: NO: 22, 23 and 24 (light chain). For example, antigen specificity may be obtained by a combination of variable domains of SEQ ID NO: 17 (heavy chain) and SEQ: 21 (light chain). 1 1 -8G4 VH (SEQ ID NO: 25) (No VL sequence available for this antibody)

EVQLVESGGGLVKPGGSLRLSCAASGFTFSDFTMNWVRQAPGQGLEWVSSIGSSGT

FIYY AASVRGRFTISRDNAQDLLSLQMNSLRADDTATYFCARGRIAVAGTHFDLWGQGTLVT VS S

In other words, the heavy chain variable domain CDRs may be as follows: CDR1 comprising GFTFSDFT (SEQ ID NO:26), CDR2 comprising IGSSGTFI (SEQ ID NO: 27) and CDR3 comprising ARGRIAVAGTFIFDL (SEQ ID NO:28). These CDRs may optionally be in the context of a VFI comprising the sequence of SEQ ID NO:25.

13-3B5 VH (SEQ ID NO: 29):

QVQLVQSGAWAKPGASVQVSCKTSGYTFTGQYLHWVRQAPGQGLEWIGWINPSSG VTKFAEKFEGRATMTRDTSITTAYIDLRSLRSDDTATYYCATLSLGVYYVGMVAWGQG TLVTVS S In other words, the heavy chain variable domain CDRs may be as follows: CDR1 comprising GYTFTGQY (SEQ ID NO:30), CDR2 comprising INPSSGVT (SEQ ID NO: 31 ) and CDR3 comprising ATLSLGVYYVGMVA (SEQ ID NO:32). These CDRs may optionally be in the context of a VFI comprising the sequence of SEQ ID NO:29. 13-3B5 VLI variable region (SEQ ID NO: 33)

SYELTQPPSVSVSPGQTATITCSGDGLAQQHVYWFQQRPGQAPLLIIYKDIERPSGIPE

R

FSGSSSGTTAMLTISGVQAEDEADYYCQSGDRTATSVLFGGGTKMTVL In other words, the light chain variable domain CDRs may be as follows: CDR1 comprising GLAQQFI (SEQ ID NO:34), CDR2 comprising KDI (SEQ ID NO: 35) and CDR3 comprising QSGDRTATSVL (SEQ ID NO:36). These CDRs may optionally be in the context of a VL comprising the sequence of SEQ ID NO:33.

Antigen specificity may be obtained by a combination of CDRs of SEQ ID NO: 30, 31 and 32 (heavy chain) and CDRs of SEQ ID NO: 34, 35 and 36 (light chain). For example, antigen specificity may be obtained by a combination of variable domains of SEQ ID NO: 29 (heavy chain) and SEQ: 33 (light chain).

13-3D10 VH (SEQ ID NO: 37)

EDLLVESGGGLVKPGQSLTLSCAASGFDFSASTMAWVRQAPGKGLEWVASVSGDSH HIYY

ADSLKGRFTLSRDNARNSFFLEMNSLRAEDTAVYYCARERLLRLGVGFDSWGQGSLV

AVS

S

In other words, the heavy chain variable domain CDRs may be as follows: CDR1 comprising GFDFSAST (SEQ ID NO:38), CDR2 comprising VSGDSHHI (SEQ ID NO: 39) and CDR3 comprising ARERLLRLGVGFDS (SEQ ID NO:40). These CDRs may optionally be in the context of a VH comprising the sequence of SEQ ID NO:37.

13-3D10 VLk (SEQ ID NO: 41 )

DIHMTQSPSSLSSSVGDRVTMTCRASQRISGFVNWYQQKPGRAPTLLISAASTLQSGV

PS

RFSGSASGTDFTLTISGLQPEDSAIYYCQQSYSPLYTFGQGTKVEIK

In other words, the light chain variable domain CDRs may be as follows: CDR1 comprising QRISGF (SEQ ID NO:42), CDR2 comprising AAS (SEQ ID NO: 43) and CDR3 comprising QQSYSPLYT (SEQ ID NO:44). These CDRs may optionally be in the context of a VL comprising the sequence of SEQ ID NO:41.

Antigen specificity may be obtained by a combination of CDRs of SEQ ID NO: 38, 39 and 40 (heavy chain) and CDRs of SEQ ID NO: 42, 43 and 44 (light chain). For example, antigen specificity may be obtained by a combination of variable domains of SEQ ID NO: 37 (heavy chain) and SEQ: 41 (light chain).

13-4D3 VH (SEQ ID NO: 45)

EVQLMESGGGLVKPGGSLRLSCTASGFTFSSYTMTWVRQAPGKGLEWVSSISSGGH YIYY

TDSLKGRFIISRDNAKNSVFLQMNNLRAEDTATYYCARERLLRLGVGFDFWGQGSLV T

VS

S

In other words, the heavy chain variable domain CDRs may be as follows: CDR1 comprising GFTFSSYT (SEQ ID NO:46), CDR2 comprising ISSGGHYI (SEQ ID NO:

47) and CDR3 comprising ARERLLRLGVGFDF (SEQ ID NO:48). These CDRs may optionally be in the context of a VH comprising the sequence of SEQ ID NO:45.

13-3D4 VLk (SEQ ID NO: 49)

DIQMTQSPSSLSASEGDRVTMTCRASQSISGYLNWYQQKPGKAPKLLIYAASTLQSGV

PS

RFSGSSSGTEFTLSISSLQPEDFATYYCQQSYSALYTFGQGTRVEIK

In other words, the light chain variable domain CDRs may be as follows: CDR1 comprising QSISGY (SEQ ID NO:50), CDR2 comprising AAS (SEQ ID NO: 51 ) and CDR3 comprising QQSYSALYT (SEQ ID NO:52). These CDRs may optionally be in the context of a VL comprising the sequence of SEQ ID NO:49.

Antigen specificity may be obtained by a combination of CDRs of SEQ ID NO: 46, 47 and 48 (heavy chain) and CDRs of SEQ ID NO: 50, 51 and 52 (light chain). For example, antigen specificity may be obtained by a combination of variable domains of SEQ ID NO: 45 (heavy chain) and SEQ: 49 (light chain). 14-2E9 VH (SEQ ID NO:53)

EVQLVESGGGLVQPGGSLRLSCAASGFTFSSYWMHWVRQVPGKGLVWVSRLNEDG

STTNYADSVKGRFTISRDNAKYTLYLQMNSLRFEDTAVYYCVSDLSGKDEHWGQGTL

VTVSS

In other words, the heavy chain variable domain CDRs may be as follows: CDR1 comprising GFTFSSYW (SEQ ID NO:54), CDR2 comprising LNEDGSTT (SEQ ID NO: 55) and CDR3 comprising VSDLSGKDEFI (SEQ ID NO:56). These CDRs may optionally be in the context of a VFI comprising the sequence of SEQ ID NO:53.

14-2E9 LCk fSEQ ID NO: 57)

DIVMTQSPLSLPVTPGEPASISCRSTQSLLHSNGYYWLDWYLQKPGQSPQLLVYLGFN

RASGVPDRFSGSGSGTDFTLKISRVEAEDVGVYYCMQGLQTPYTFGQGTTLEIK In other words, the light chain variable domain CDRs may be as follows: CDR1 comprising QSLLHSNGYYW (SEQ ID NO:58), CDR2 comprising LGF (SEQ ID NO: 59) and CDR3 comprising MQGLQTPYT (SEQ ID NO:60). These CDRs may optionally be in the context of a VL comprising the sequence of SEQ ID NO:57. CDRs of SEQ ID NO: 54, 55 and 56 (heavy chain) may be together with CDRs of SEQ ID NO: 58, 59 and 60 (light chain). Variable domains of SEQ ID NO: 53 (heavy chain) and SEQ: 57 (light chain) may also be combined.

Antigen specificity may be obtained by a combination of CDRs of SEQ ID NO: 54, 55 and 56 (heavy chain) and CDRs of SEQ ID NO: 58, 59 and 60 (light chain). For example, antigen specificity may be obtained by a combination of variable domains of SEQ ID NO: 53 (heavy chain) and SEQ: 57 (light chain).

Fc region sequences

From 5 clones sufficient cDNA was left to determine the Fc sequence of the antibody. See below for the subclass analysis. The reference sequences were obtained from ensemble.org. Reference sequences

lgG1 (SEQ ID NO:61)

ASTKGPSVFPLAPSSKSTSGGTAALGCLVKDYFPEPVTVSWNSGALTSGVHTFPAVLQ

ss

GLYSLSSWTVPSSSLGTQTYICNVNHKPSNTKVDKKVEPKSCDKTHTCPPCPAPELL GG

PSVFLFPPKPKDTLMISRTPEVTCVWDVSHEDPEVKFNWYVDGVEVHNAKTKPREEQ

YN

STYRWSVLTVLHQDWLNGKEYKCKVSNKALPAPIEKTISKAKGQPREPQVYTLPPSR DE

LTKNQVSLTCLVKGFYPSDIAVEWESNGQPENNYKTTPPVLDSDGSFFLYSKLTVDK S

RW

QQGNVFSCSVMHEALHNHYTQKSLSLSPELQLEESCAEAQDGELDGLWTTITIFITL FL

L SVCYSATVTFFKVKWIFSSVVDLKQTIIPDYRNMIGQGA lgG2 (SEQ ID NO: 62)

ASTKGPSVFPLAPCSRSTSESTAALGCLVKDYFPEPVTVSWNSGALTSGVHTFPAVLQ

SS GLYSLSSWTVPSSNFGTQTYTCNVDHKPSNTKVDKTVERKCCVECPPCPAPPVAGP SVF

LFPPKPKDTLMISRTPEVTCVWDVSHEDPEVQFNWYVDGVEVHNAKTKPREEQFNS

TFR

WSVLTVVHQDWLNGKEYKCKVSNKGLPAPIEKTISKTKGQPREPQVYTLPPSREEMT KN

QVSLTCLVKGFYPSDISVEWESNGQPENNYKTTPPMLDSDGSFFLYSKLTVDKSRWQ

QGN

VFSCSVMHEALHNHYTQKSLSLSPGK lgG3 (SEQ ID NO: 63)

XSTKGPSVFPLAPCSRSTSGGTAALGCLVKDYFPEPVTVSWNSGALTSGVHTFPAVL

QSS GLYSLSSWTVPSSSLGTQTYTCNVNHKPSNTKVDKRVELKTPLGDTTHTCPRCPEPK

SC

DTPPPCPRCPEPKSCDTPPPCPRCPEPKSCDTPPPCPRCPAPELLGGPSVFLFPPKP

KDT LMISRTPEVTCVWDVSHEDPEVQFKWYVDGVEVHNAKTKPREEQYNSTFRWSVLT VLH

QDWLNGKEYKCKVSNKALPAPIEKTISKTKGQPREPQVYTLPPSREEMTKNQVSLTC L

VK

GFYPSDIAVEWESSGQPENNYNTTPPMLDSDGSFFLYSKLTVDKSRWQQGNIFSCSV MHE

ALHNRFTQKSLSLSPGK lgG4 (SEQ ID NO: 64)

ASTKGPSVFPLAPCSRSTSESTAALGCLVKDYFPEPVTVSWNSGALTSGVHTFPAVLQ SS

GLYSLSSWTVPSSSLGTKTYTCNVDHKPSNTKVDKRVESKYGPPCPSCPAPEFLGGP

SV

FLFPPKPKDTLMISRTPEVTCWVDVSQEDPEVQFNWYVDGVEVHNAKTKPREEQFN

STY RWSVLTVLHQDWLNGKEYKCKVSNKGLPSSIEKTISKAKGQPREPQVYTLPPSQEEM TK

NQVSLTCLVKGFYPSDIAVEWESNGQPENNYKTTPPVLDSDGSFFLYSRLTVDKSRW

QEG

NVFSCSVMHEALHNHYTQKSLSLSLGK cDNA generated from the B cell clones was sent for sanger sequencing. This gave clear data with regard to the antibody Fc sequence. The DNA sequence was translated using https://web.expasy.org/translate/. 11-3D9 clone amino acid sequence (translated from cDNA) SEQ ID NO: 65

GDKRVEPKSCDKTHTCPPCPAPELLGGPSVFLFPPKPKDTLMISRTPEVTCVWDVSH

ED PEVKFNWYVDGVEVHNAKTKPREEQYNSTYRWSVLTVLHQDWLNGKEYKCKVSNK

ALPA

PIEKTISKAKGQPREPQVYTLPPSREEMTKNQVSLTCLVKGFYPSDIAVEWESNGQP E

NN YKTTPPVLDSDGSFFLYSKLTVTR

11-8G4 clone amino acid sequence (translated from cDNA) - SEQ ID NO: 66

GDKRVEPKSCDKTHTCPPCPAPELLGGPSVFLFPPKPKDTLMISRTPEVTCVWDVSH

ED PEVKFNWYVDGVEVHNAKTKPREEQYNSTYRWSVLTVLHQDWLNGKEYKCKVSNK

ALPA

PIEKTISKAKGQPREPQVYTLPPSREEMTKNQVSLTCLVKGFYPSDIAVEWESNGQP E

NN

YKTTPPVLDSDGSFFLYSKLTV

13-3D10 clone amino acid sequence (translated from cDNA) - SEQ ID NO: 67

KRVEPKSCDKTHTCPPCPAPELLGGPSVFLFPPKPKDTLMISRTPEVTCWVDVSHED

PE

VKFNWYVDGVEVHNAKTKPREEQYNSTYRWSVLTVLHQDWLNGKEYKCKVSNKAL

EKTISKAKGQPREPQVYTLPPSREEMTKNQVSLTCLVKGFYPSDIAVEWESNGQPEN N

YK

TTPPVLDSDGSFFLYSKLTVT 13-4D3 clone amino acid sequence (translated from cDNA) SEQ ID NO: 68

RVELKTPLGDTTHTCPRCPEPKSCDTPPPCPRCPEPKSCDTPPPCPRCPEPKSCDTP

PPC

PRCPAPELLGGPSVFLFPPKPKDTLMISRTPEVTCWVDVSHEDPEVQFKWYVDGVEV

HN AKTKPREEQFNSTFRVVSVLTVLHQDWLNGKEYKCKVSNKALPAPIEKTISKTKGQPR EP QVYTLPPSREEMTKNQVSLTCLVKGFYPSDIAVEWESSGQPENNYNTTPPMLDSDGS

FFL

YSKLTV 13-3B5 clone amino acid sequence (translated from cDNA) - SEQ ID NO: 69

DKRVESKYGPPCPSCPAPEFLGGPSVFLFPPKPKDTLMISRTPEVTCVWDVSQEDPE

VQ

FNWYVDGVEVHNAKTKPREEQFNSTYRVVSVLTVLHQDWLNGKEYKCKVSNKGLPS

SIEK TISKAKGQPREPQVYTLPPSQEEMTKNQVSLTCLVKGFYPSDIAVEWESNGQPENNY

KTT

PPVLDSDGSFFLYSRLTVTR

Using the multiple sequence alignment program Clustal W (1.83), the inventors conclude that 1 1 -3D9, 1 1 -8G4, 13-3D10 all have a standard lgG1 sequence (See Figure 5). 13-3B5 has a standard lgG4 Fc tail and 13-4D3 has a lgG3 Fc sequence. All variants for 13-4D3 are known on ensemble.org. The colour coding is to indicate similarities between different antibodies and their matching subclass reference sequence. The subclass specificity of 1 1 -3F6 and 14-2E9 was also determined in ELISA with lgG1 specific antibody (see Figure 3).

Discussion

One of the most striking and surprising findings of this study is the fact that (patient- derived) MuSK antibody valency and consequently MuSK autoantibody isotype are key determinants in their ability to inhibit MuSK function and induce MG. Bivalent monospecific MuSK recombinant antibodies binding the Ig-like 1 domain of MuSK acted as agonists and activate MuSK phosphorylation (and AChR clustering) in myotube cultures independent from agrin. Both original lgG1 and lgG4 antibodies showed this effect. In contrast, functionally monovalent Fab fragments generated from these antibodies inhibit agrin-dependent MuSK phosphorylation (and AChR clustering) similar to purified patient lgG4 and IgGtotal (Huijbers & Zhang 2013, Koneczny et ai, 2013). The ability of patient lgG4 to undergo Fab-arm exchange thus seems crucial for the pathogenesis of disease as Fab-arm exchange renders endogenously produced MuSK lgG4 bi-specific and functionally monovalent for the MuSK antigen. Previous research has suggested patient lgG4 has the ability to undergo Fab-arm exchange (Koneczny 2017 J autoimmunity) and the sequences of the MuSK monoclonals described herein confirm that the required residues for this process are present. The data suggests that bivalent monospecific MuSK antibodies binding the MuSK Ig-like 1 domain have the ability to dimerize and activate MuSK, whereas functionally monovalent MuSK Ig-like 1 domain antibodies have the ability to inhibit MuSK dimerization and thus inhibit MuSK activation. To summarize, MuSK antibody valency is a key determinant for the functional effects. Bivalent monospecific antibodies, with two MuSK Ig-like 1 domain binding sites, stimulate MuSK, while monovalent or bi-specific antibodies, with only one MuSK Ig-like 1 domain binding site, block MuSK function. In conclusion, the data suggest that functional monovalency of MuSK antibodies is crucial for the induction of myasthenic features.

Material & methods

Patient selection

MuSK MG patients were recruited in our MG outpatient clinic at the Leiden University Medical Center and were selected based on the presence of a positive MuSK antibody test (RSR ltd) and clinical characteristics consistent with MG. The study was conducted in accordance with the Declaration of Helsinki, was approved by the local medical ethics committee and all patients signed informed consent.

Isolation of monoclonal autoantibodies from MuSK MG patients

MuSK-binding memory B cells were isolated from cryopreserved PBMC selecting for CD19 ⁺ , CD20 ⁺ , CD27 ⁺ cells (CD19-BV421 HIB19 BD, CD20-AF700 2H7 BD, CD27- APCHy7 M-T271 BD, 0,1 %BSA, 2mM EDTA/dPBS, all monoclonals were mouse anti human). To remove dead cells and non-B cells a dump channel was included (7-AAD,00- 6993-50, CD3/FITC UCHT1 BD and CD14/FITC M5E2, BD and CD56/FITC HCD56 Biolegend). Selection for antigen specific cells was done by using recombinant MuSK produced in E. Coli (Huijbers et al., 2016) labeled with R-PE (AS-721 13, Anaspec) and MuSK produced in yeast tagged with DyLight 650 (a kind gift of Kostas Lazardinis and Socrates Tzartos, Dylight650 NHS ester, Thermofisher). Single cells were cultured on irradiated L-CD40L feeder cells (a kind gift from Kees van Kooten) in a 96 wells plate in RPMI medium supplemented with 1 % pencilline, streptomycin, - 50 uM beta- mercaptoethanol (M3148, Sigma), 20 ug/ml human apo-transferrin, (T2036, Sigma- Aldrich, depleted for human IgG with protein A sepharose [GE Healthcare (1 ng/ml IL1 b (201 -LB-005, R&D), 50 ng/ml IL-21 (PHC0215, Thermofisher), 0,3 ng/ml TNFa (210-TA- 005, Thermofisher), 0,5 ug/ml R848 (SML0196, Sigma) (Lighaam et al., 2014). On average 50 million PBMC were used per patients to isolate between 1 -6 antigen-specific cells. After two weeks, the medium was tested for MuSK reactive antibodies using the MuSK ELISA described previously (Huijbers et al., 2016). In short, recombinant MuSK was coated overnight on maxisorp plates (NUNC). The next day the plates were washed with PBS and blocked with 2% casein 0.01 % tween in PBS. To detect MuSK-binding antibodies culture medium was incubated 1 :10 in block buffer and bound antibodies were detected using alkaline phosphatase labelled goat anti-human antibody (Rabbit anti human IgG AP, 309-055-008 Jackson Laboratories) and PNP as a substrate. Plates were read on a Biotek plate reader at 405 nm. RNA isolation, cDNA production and antibody sequence isolation

After two weeks, plates were spun and supernatant was removed to perform the antibody analysis described above. Single wells containing MuSK antibody producing cells were lysed with 150 pi Qiazol and RNA was isolated using standard chloroform extraction and isopropanol/ethanol precipitation. RNA was rehydrated in 8mI H ₂ 0 and stored at -80 °C until further use. cDNA was directly synthesized (without pre-amplification or purification) using Smartscribe reverse transcriptase (Takara Bio Europe) an OligodT40 primer and the template switching oligo for 10 rounds of amplification (primer sequences in table 4). Full length V(D)J were obtained by ARTISAN PCR. Koning et al., 2017. HC primer stands for heavy chain.

Table 4: Primer sequences. Where“dT40” stands for 40 T nucleotides followed by VN where“V” stands for either an A, C or G nucleotide and“N” stands for any nucleotide (see https://www.mathworks.com/help/bioinfo/ref/baselookup.html). In the BC-TSO “rG” stands for riboguanosines and the G+ stands for one LNA-modified guanosine. In the hkappaLC rev primer“M” stands for an A or C nucleotide and in the hlambdaLC rev primer Ύ” stands for C or T nucleotide.

Recombinant antibody production, purification and characterization

Heavy and light chain sequences were ordered at Geneart (Thermofisher) in an lgG1 and lgG4 backbone pcDNA3.1 vector and transfected in suspension Freestyle HEK293-F cells (R790-07, Thermofisher) using Fectin (12347-019, Thermofisher) in FreeStyleTM 293 Expression Medium (12338, ThermoFisher). To increase transfection and production efficiency the cells were co-transfected with SV40 large T antigen, hp21 and hp27 (A kind gift from Theo Rispens). After 6 days culture medium was collected, cell debris was removed by centrifugation and IgG was purified using a HiTrapTM Protein A affinity column (17-0402-01 , GE healthcare) on an Akta pure (GE Healthcare). Antibodies were dialyzed to PBS, filter-sterilized and stored at -20°C. Fab fragments were generated from these recombinant antibodies using papain according to manufacturer’s instructions (20341 , ThermoFisher) with the following adjustments: Input concentration of the antibodies was 0,5 mg/mL and the duration of the digest was 1 hour for 13-3B5 and 3 hours for 1 1 -3F6 and anti-biotin (13-3B5 was lost when digest took longer than 1 hour). Total IgG and Fc fragment depletion was performed for 2 hours head over head at RT with protein A agarose beads (1 1 134515001 , Roche). To determine that equal amounts of MuSK-specific Fabs were used, concentration was measured compared to their parent antibody in the MuSK ELISA replacing the above described antibody combinations with a mouse anti-human Fab (1 : 1000, SA1 -19255, Pierce) and a rabbit anti-mouse alkaline phosphatase antibody (1 :750, D0314, Dako). Biotin Fab concentration was determined comparing MuSK Fab concentration on western blot using the same primary antibody and a donkey anti-mouse 800CW (1 : 10.000, 925- 32212, Licor). Bound antibodies were detected using the Odyssey CLx (Licor).

To determine the binding characteristics of the recombinant and medium produced antibodies we performed an epitope mapping ELISA as described previously (Huijbers et at., 2016). Truncated versions of the MuSK proteins were immobilized on maxisorp plates and exposed to either antibodies from the single cell cultures 1 : 10 or to recombinant antibodies. Recombinant antibodies were also used to immunostain mouse levator auris longus muscle overnight at 4 °C at 1 : 100. AChRs in synaptic regions were labelled with AlexaFluor488 conjugated alpha-bungarotoxin (Life technologies) and bound recombinant antibodies were detected with AlexaFluor594 conjugated goat anti-human IgG (Life technologies). Muscles were imaged on a Leica SP8 confocal laser-scanning microscope and analyzed using Las X software.

To determine the ability of the antibodies to inhibit MuSK phosphorylation and AChR clustering we exposed C2C12 myotubes cultures (Cell lines service) to them as described previously (Huijbers & Zhang et al 2013 PNAS). Differentiated myotubes were stimulated with 0.1 nM agrin (550-AG-100, R&D systems) in the presence or absence of 100ng/mL recombinant antibodies or Fab fragments. For MuSK phosphorylation data, immunoprecipitation of MuSK was initiated after 30 minutes of exposure. Then the myotubes were lysed and MuSK was precipitated using 5pL/sample rabbit anti MuSK polyclonal serum (ab94276, or ab94277 a kind gift of Steve Burden) during an overnight incubation at 4 °C (Huijbers & Zhang et al. 2013). Bound antigen-antibody complexes were precipitated using protein A agarose beads (1 1 134515001 , Roche) which were extensively washed. Samples were subsequently ran on SDS-PAGE gel and transferred to PDVF membrane. MuSK and phosphorylated MuSK was detected using goat anti-rat MuSK (1 :2000, AF562, R&D systems) and mouse anti-phosphotyrosine clone 4G10 (1 : 1000, 05-321 , Millipore) as primary antibodies, and donkey anti-mouse-680RD

(1 : 10,000, 926-68072, Licor) and donkey anti-goat 800CW (1 : 10,000, 926-32214, Licor). Bound antibodies were detected using the Odyssey CLx (Licor).

AChR clustering was studied after 16 hours of exposing myotubes to 100ng/mL recombinant antibodies or Fab fragments in absence or presence of 0, 1 nM agrin. Subsequently the cells were washed three times with differentiation medium (DMEM, 31966 Gibco, 2% heat-inactivated horse serum 26050-088, Gibco, 1 % pen/strep and 1 % L-glutamine) and incubated with 0,5pg/mL AlexaFluor488 conjugated a-bungarotoxin (B13422, ThermoFisher) in differentiation medium for 30 minutes at 37°C. After staining cells were fixed in 4% formalin solution for 5 minutes, washed with PBS and mounted using hardset mounting medium (H-1500, Vector laboratories). Twenty fields divided over two coverslips per condition were randomly selected, and imaged with Leica DM5500 microscope. AChR cluster count and size were analyzed using ImageJ (1 48v).

Statistics

All data are expressed as mean ± standard error of the mean (SEM). Statistical significance of differences between treatment groups were tested with Student's t-tests, with corrections for multiple testing wherever appropriate. Differences with P-values <0.05 were considered statistically significant. Further examples Non-limiting examples of a multivalent binding agent comprising at least two binding regions that specifically bind to an Ig-like 1 domain of a MuSK protein are listed in the tables below:

Table 5A

Table 5B

Table 5C

Table 5D

Table 5E

Table 5F

Table 5G

Unless defined otherwise herein, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. For example, Singleton and Sainsbury, Dictionary of Microbiology and Molecular Biology, 2d Ed., John Wiley and Sons, NY (1 94); and Hale and Marham, The Harper Collins Dictionary of Biology, Harper Perennial, NY (1991 ) provide those of skill in the art with a general dictionary of many of the terms used in the invention. Although any methods and materials similar or equivalent to those described herein find use in the practice of the present invention, the preferred methods and materials are described herein. Accordingly, the terms defined immediately below are more fully described by reference to the Specification as a whole. Also, as used herein, the singular terms "a", "an," and "the" include the plural reference unless the context clearly indicates otherwise. Unless otherwise indicated, nucleic acids are written left to right in 5' to 3' orientation; amino acid sequences are written left to right in amino to carboxy orientation, respectively. It is to be understood that this invention is not limited to the particular methodology, protocols, and reagents described, as these may vary, depending upon the context they are used by those of skill in the art. The reader's attention is directed to all papers and documents which are filed concurrently with or previous to this specification in connection with this application and which are open to public inspection with this specification, and the contents of all such papers and documents are incorporated herein by reference. All of the features disclosed in this specification (including any accompanying claims, abstract and drawings), and/or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive. Each feature disclosed in this specification (including any accompanying claims, abstract and drawings), may be replaced by alternative features serving the same, equivalent, or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features.

The invention is not restricted to the details of any foregoing embodiments. The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed.

Sequences:

SEQ ID NO: 1 MuSK (human - isoform 205):

MRELVNIPLVHILTLVAFSGTEKLPKAPVITTPLETVDALVEEVATFMCAVESYPQPEIS WTRNKILIKLFDTRYSIRENGQLLTILSVEDSDDGIYCCTANNGVGGAVESCGALQVKM K

PKITRPPINVKIIEGLKAVLPCTTMGNPKPSVSWIKGDSPLRENSRIAVLESGSLRI HNV

QKEDAGQYRCVAKNSLGTAYSKVVKLEVEVFARILRAPESHNVTFGSFVTLHCTATG IP

V PTITWIENGNAVSSGSIQESVKDRVIDSRLQLFITKPGLYTCIATNKHGEKFSTAKAAAT ISIAEWSKPQKDNKGYCAQYRGEVCNAVLAKDALVFLNTSYADPEEAQELLVHTAWNE LK

WSPVCRPAAEALLCNHIFQECSPGWPTPIPICREYCLAVKELFCAKEWLVMEEKTHR

G LYRSEMHLLSVPECSKLPSMHWDPTACARLPHLDYNKENLKTFPPMTSSKPSVDIPNL PS

SSSSSFSVSPTYSMTVIISIMSSFAIFVLLTITTLYCCRRRKQWKNKKRESAAVTLT TLP

SELLLDRLHPNPMYQRMPLLLNPKLLSLEYPRNNIEYVRDIGEGAFGRVFQARAPGL LP

Y EPFTMVAVKMLKEEASADMQADFQREAALMAEFDNPNIVKLLGVCAVGKPMCLLFEY MAY GDLNEFLRSMSPHTVCSLSHSDLSMRAQVSSPGPPPLSCAEQLCIARQVAAGMAYLS

ERK

FVHRDLATRNCLVGENMVVKIADFGLSRNIYSADYYKANENDAIPIRWMPPESIFYN RY

T TESDVWAYGWLWEIFSYGLQPYYGMAHEEVIYYVRDGNILSCPENCPVELYNLMRLC WS

KLPADRPSFTSIHRILERMCERAEGTVSV

SEQ ID NO:2 The Ig-like 1 domain of the human MuSK protein:

PVITTPLETVDALVEEVATFMCAVESYPQPEISWTRNKILIKLFDTRYSIRENGQLLTIL SV EDSDDGIYCCTANNGVGGAVESCGALQV

in

For sequences 9 to 69, see the examples section above. For sequences 70 to 78 see Figure 5.

References:

“Adaptive antibody diversification through /V-linked glycosylation of the immunoglobulin variable region.” van de Bovenkamp FS, Derksen NIL, Ooijevaar-de Heer P, van Schie KA, Kruithof S, Berkowska MA, van der Schoot CE, IJspeert H, van der Burg M, Gils A, Hafkenscheid L, Toes REM, Rombouts Y, Plomp R, Wuhrer M, van Ham SM, Vidarsson G, Rispens T. Proc Natl Acad Sci U S A. 2018 Feb 20; 1 15(8): 1901 -1906.

“The role of MuSK in synapse formation and neuromuscular disease.” Burden SJ, Yumoto N, Zhang W. Cold Spring Harb Perspect Biol. 2013 May 1 ;5(5):a009167

“Fundamental Molecules and Mechanisms for Forming and Maintaining Neuromuscular Synapses.” Burden SJ, Huijbers MG, Remedio L. lnt J Mol Sci. 2018 Feb 6;19(2). “Clinical correlates with anti-MuSK antibodies in generalized seronegative myasthenia gravis.”

Evoli A, Tonali PA, Padua L, Monaco ML, Scuderi F, Batocchi AP, Marino M, Bartoccioni E.

Brain. 2003 Oct; 126(Pt 10):2304-1 1 .

“Synapse disassembly and formation of new synapses in postnatal muscle upon conditional inactivation of MuSK.” Hesser BA, Henschel O, Witzemann V. Mol Cell Neurosci. 2006 Mar;31 (3):470-80. “Auto-antibodies to the receptor tyrosine kinase MuSK in patients with myasthenia gravis without acetylcholine receptor antibodies.” Hoch W, McConville J, Helms S, Newsom- Davis J, Melms A, Vincent A. Nat Med. 2001 Mar;7(3):365-8. “Longitudinal epitope mapping in MuSK myasthenia gravis: implications for disease severity.” Huijbers MG, Vink AF, Niks EH, Westhuis RH, van Zwet EW, de Meel RH, Rojas-Garcia R, Diaz-Manera J, Kuks JB, Klooster R, Straasheijm K, Evoli A, Ilia I, van der Maarel SM, Verschuuren JJ. J Neuroimmunol. 2016 Feb 15;291 :82-8

“MuSK lgG4 autoantibodies cause myasthenia gravis by inhibiting binding between MuSK and Lrp4.” Huijbers MG, Zhang W, Klooster R, Niks EH, Friese MB, Straasheijm KR, Thijssen PE, Vrolijk H, Plomp JJ, Vogels P, Losen M, Van der Maarel SM, Burden SJ, Verschuuren JJ.

Proc Natl Acad Sci U S A. 2013 Dec 17; 1 10(51 ):20783-8.

“Muscle-specific kinase myasthenia gravis lgG4 autoantibodies cause severe neuromuscular junction dysfunction in mice.” Klooster R, Plomp JJ, Huijbers MG, Niks EH, Straasheijm KR, Detmers FJ, Hermans PW, Sleijpen K, Verrips A, Losen M, Martinez-Martinez P, De Baets MH, van der Maarel SM, Verschuuren JJ. Brain. 2012 Apr; 135(Pt 4): 1081 -101

“MuSK myasthenia gravis lgG4 disrupts the interaction of LRP4 with MuSK but both lgG4 and lgG1 -3 can disperse preformed agrin-independent AChR clusters.” Koneczny I, Cossins J, Waters P, Beeson D, Vincent A. PLoS One. 2013 Nov 7;8(1 1 ):e80695.

“lgG4 autoantibodies against muscle-specific kinase undergo Fab-arm exchange in myasthenia gravis patients.” Koneczny I, Stevens JA, De Rosa A, Huda S, Huijbers MG, Saxena A, Maestri M, Lazaridis K, Zisimopoulou P, Tzartos S, Verschuuren J, van der Maarel SM, van Damme P, De Baets MH, Molenaar PC, Vincent A, Ricciardi R, Martinez- Martinez P, Losen M. J Autoimmun. 2017 Feb;77: 104-1 15.

“ARTISAN PCR: rapid identification of full-length immunoglobulin rearrangements without primer binding bias.” Koning MT, Kietbasa SM, Boersma V, Buermans HPJ, van der Zeeuw SAJ, van Bergen CAM, eleven AHG, Kluin PM, Griffioen M, Navarrete MA, Veelken H. Br J Haematol. 2017 Sep;178(6):983-986.

“The Immunobiology of Immunoglobulin G4.” Lighaam LC, Rispens T. Semin Liver Dis. 2016 Aug;36(3):200-15.

“Phenotypic differences between lgG4+ and lgG1 + B cells point to distinct regulation of the lgG4 response.” Lighaam LC, Vermeulen E, Bleker Td, Meijlink KJ, Aalberse RC, Barnes E, Culver EL, van Ham SM, Rispens T. J Allergy Clin Immunol. 2014 Jan;133(1 ):267-70.e1 -6.

“Detection and characterization of MuSK antibodies in seronegative myasthenia gravis.” McConville J, Farrugia ME, Beeson D, Kishore U, Metcalfe R, Newsom-Davis J, Vincent A.

Ann Neurol. 2004 Apr;55(4):580-4.

“Anti-inflammatory activity of human lgG4 antibodies by dynamic Fab arm exchange.” van der Neut Kolfschoten M, Schuurman J, Losen M, Bleeker WK, Martinez-Martinez P, Vermeulen E, den Bleker TH, Wiegman L, Vink T, Aarden LA, De Baets MH, van de Winkel JG, Aalberse RC, Parren PW. Science. 2007 Sep 14;317(5844): 1554-7.

“Collagen Q and anti-MuSK autoantibody competitively suppress agrin/LRP4/MuSK signaling.” Otsuka K, Ito M, Ohkawara B, Masuda A, Kawakami Y, Sahashi K, Nishida H, Mabuchi N, Takano A, Engel AG, Ohno K. Sci Rep. 2015 Sep 10;5: 13928.

“Crystal structure of the agrin-responsive immunoglobulin-like domains 1 and 2 of the receptor tyrosine kinase MuSK.” Stiegler AL, Burden SJ, Hubbard SR. J Mol Biol. 2006 Dec 1 ;364(3):424-33. “Agrin binds to the N-terminal region of Lrp4 protein and stimulates association between Lrp4 and the first immunoglobulin-like domain in muscle-specific kinase (MuSK).” Zhang W, Coldefy AS, Hubbard SR, Burden SJ. J Biol Chem. 201 1 Nov 25;286(47):40624-30.