Zika virus is an arbovirus (arthropod-borne virus, i.e. virus transmitted by arthropod vectors such as insects) that belongs to the Flaviviridae family of viruses and is spread by the Egyptian and the Asian mosquito strains Aedes aegypti and Aedes albopictus. In the years 2013 and 2014, the incidence of Zika infections in French Polynesia escalated, followed by large outbreaks in Latin America since 2015, peaking epidemic-like at the time around the Olympic Games in Brazil in 2016. So far, efficacious medical treatment or vaccination is lacking. In healthy persons an infection with Zika virus usually leads to no or only mild symptoms. However, for pregnant women the situation is different. The Zika virus can be transmitted from a pregnant woman to her unborn child and is suspected to cause severe brain malformations and defects such as microcephaly. Microcephaly (derived from Greek for "small head") is a condition in which a baby's brain does not develop properly, and thus its head has a smaller size than normal. Infection in adults may lead to the so-called Guillan-Barre syndrome (muscle weakness caused by the immune system which attacks the peripheral nervous system). Efficacious medical treatment after infection is lacking. In order to receive timely supportive treatment it is crucial to know if a patient has been infected with Zika virus. In particular, highly specific immunoassays are utterly needed in regions with high prevalence of multiple flaviviral infections. As a matter of course, it is a daunting task to reliably diagnose, e.g. , a recent Zika infection in an individual that has undergone other flaviviral infections such as Dengue or yellow fever in the past and whose serum is characterized by polyclonal antibodies against the main immunogens of Dengue and yellow fever virus.

Therefore, apart from a vaccination and medication development strategy, also highly sensitive and specific serological diagnostics need to be developed in order to reliably confirm or rule out an infection of a patient with Zika virus. Since 2016 a couple of ELISA-based immunoassays detecting antibodies against the Zika virus NS 1 antigen are commercially available {e.g. Huzly et ah, Euro Surveill. 2016; 21 (16) pii=30203, 1-4). However, these assays are suspected to cross- react with antibodies that have been raised against related viruses such as Dengue virus and other arboviruses that belong to the family of flaviviruses like West Nile virus, yellow fever virus, tick-borne encephalitis virus (FSME) or Japanese encephalitis virus. This cross-reactivity would lead to false positive results and erroneous interpretation of a patient's immune status and seems to be due to the structural and sequence homologies of Zika NS1 (non-structural antigen) and its counterparts in other flaviviruses such as West Nile and Dengue viruses (Hilgenfeld 27 Oct 2016, Embo J. 1-3). It is conceivable that the incidence and prevalence data that have been reported for the Zika epidemic in Brazil in 2015/2016, have suffered the flaw of an exaggeration bias due to the limited specificity of the immunoassays that have been used at the time. In order to evaluate prevalence and incidence data and to assess the objective risk of an emerging epidemic, clear-cut diagnostics are an indispensable prerequisite. Serologic assays with poor specificity would overemphasize the extent of an epidemic and therefore lead to panic-driven decisions not only by the authorities, but also by unsettled and frightened individuals. For instance, it has been reported that the number of abortions in Brazil has significantly increased in the wake of the medial reporting on the true or ostensible Zika epidemic. The crystal structure of full-length, glycosylated NS1 from West Nile and Dengue virus has been solved at high resolution and reveals distinct domains and a rather complex protein topology (Akey et al., Science (2014; 343 (6173): 881-885). Recently, the crystal structure of a C-terminal fragment (amino acid residues 172-352) of the Zika virus nonstructural protein 1 (NS1) has been published, revealing a head-to-head dimer and confirming the oligomeric character of NS1 (Song et al. Nature Struct. Mol. Biol. 2016 (23) 5, 456-459). In addition, the complete three- dimensional structure of full-length Zika virus NS 1 has been published by Brown et al. Nature Struct. Mol. Biol. 2016 (23) 9, 865-868, describing the NS1 structure in further detail. Thus, NS1 turns out to be a very intricate and complex protein due to its oligomeric state, its glycosylation pattern and its abundance in cysteine residues. Publications on dedicated Zika antigens for diagnostic purposes are therefore scarce, and reliable information on linear or conformational B- cell epitopes that are unique in Zika antigens and would allow for a clear-cut discrimination between Zika virus and related flaviviruses have not been available so far.

International patent applications WO2017/144173 and WO2017/144173 describe variants of the full length Zika NS1 antigen of 352 amino acids, listing several essential epitopes, wherein a β- ladder domain epitope (positions 322 to 326) in the C-terminal part of the NS1 antigen is regarded as essential for reactivity.

The problem underlying the invention therefore is the limited specificity of the hitherto available immunoassays detecting antibodies against Zika virus. The problem is solved by the current invention as specified in the claims. Summary of the invention:

The invention relates to a polypeptide, and its use in immununoassays, that is suitable for detecting antibodies against Zika virus in an isolated biological sample comprising a Zika virus NSl domain specific amino acid sequence and variants thereof, wherein no amino acid sequences from the Zika virus NSl B-ladder domain are present in said polypeptide. In an embodiment said Zika virus NSl domain specific amino acid sequence consists essentially of SEQ ID NO. 1 or 2. In another embodiment, no further Zika virus specific amino acid sequences are present in said polypeptide. This polypeptide does not immunologically cross-react with antibodies raised against structurally related antigens from tick-borne encephalitis virus comprising NSl polypeptides of Dengue virus 1-4, West Nile virus, yellow fever virus or Japanese encephalitis virus, but immunologically reacts with antibodies raised against full length Zika virus NSl antigen according to SEQ ID NO:3.

In an embodiment, this polypeptide does not immunologically cross-react with antibodies raised against structurally related antigens from tick-borne encephalitis virus comprising any of SEQ ID NOs:5 or 6, and/or from Dengue virus 1-4 comprising any of SEQ ID NOs:7 to 14, and/or from West Nile virus comprising any of SEQ ID NOs:15 or 16, and/or from yellow fever virus comprising any of SEQ ID NOs: 17 or 18, and/or from Japanese encephalitis virus comprising any of SEQ ID NOs: 19 to 20, but immunologically reacts with antibodies raised against full length Zika virus NSl antigen according to SEQ ID NO:3.

Also disclosed is a method of producing a soluble and immunoreactive Zika virus NSl polypeptide as well as methods for detecting antibodies specific for Zika virus in an isolated sample using a Zika virus NS 1 polypeptide as described above as binding partner for anti-Zika virus antibodies. These methods detect either IgG or IgM class antibodies or both. Further disclosed is the use of said NS 1 Zika virus polypeptide in an in vitro diagnostic test for detecting anti-Zika virus antibodies.

Further aspects are reagent kits for the detection of anti-Zika virus antibodies, comprising a Zika virus polypeptide as specified before.

Another aspect is a method for detecting antibodies against a Zika virus in an isolated biological sample presumed to contain antibodies against at least one other non-Zika flavivirus, by using a Zika virus NS 1 polypeptide comprising a complete or partial sequence of the β-ladder domain as specific binding partner, wherein the cross-reactivity against said non-Zika flavivirus is eliminated by adding a polypeptide comprising the NSl B-ladder domain of said Zika virus in an unlabeled form, in an embodiment adding said polypeptide comprising the β-ladder domain as a quencher. Legend to the disclosed amino acid sequences:

The mature NSl protein comprises 352 amino acid residues (NSl, 1-352). Within the NSl protein, the wing domain comprises 151 amino acid residues and spans the NSl amino acid region 30-180. Thus, the wing domain positions (1-151) easily translate into the NSl numbering by adding 29 amino acid positions. Vice versa, the NSl amino acid positions easily translate into wing domain numbering by subtracting as many as 29 amino acid positions; aa 1 (wing) = aa 30 (NSl), aa 2 (wing) = aa 31 (NSl), aa 3 (wing) = aa 32 (NSl) and so forth. In an analogous way the B-ladder domain positions (1-162) easily translate into the NSl numbering by adding 190 amino acid positions as the NSl positions 191-352 correspond to the B-ladder domain.

SEP ID NO: l. Zika virus NSl wing domain aa 30-180 with position 179 X = A or S or C

SEP ID NO:2, Zika virus NSl wing domain aa 30-180 with C55, C143, C179A

SEQ ID NO:3, Zika virus NSl full length aa 1-352; this sequence is also published as strain MR 766, UniProt ID W8Q7Q3; the corresponding numbering within the full length Zika precursor polyprotein is aa 795-1146

SEQ ID NO:4, Zika virus NSl B-ladder domain aa 191-352 SEQ ID NO:5, Tick-borne encephalitis (FSME) virus NSl wing domain aa 30-180 according to

UniProt ID P I 4336; European subtype s rain Neudoerfl; X = A or C or S

SEQ ID NO:6: Tick-borne encephalitis (FSME) virus NS l full length aa 777-1128 according to

UniProt I D P I 4336; European subtype strain Neudoerfl; 352 amino acids

SEQ ID NO:7, Dengue virus type 1 NS l wing domain aa 30-180; X = A or C or S

SEQ ID NO:8. Dengue virus type 1 NSl full length aa 776-1127 according to UniProt ID W8FUV0, 352 amino acids

SEQ ID NO:9, Dengue virus type 2 NSl wing domain aa 30-180; X = A or C or S SEQ ID NO: 10. Dengue virus type 2 NSl full length aa 776-1127 according to UniProt ID P29990, strain Thailand/16881/1984, 352 amino acids

SEP ID NO: 11. Dengue virus type 3 NS l wing domain aa 30-180; X = A or C or S)

SEP ID NO: 12. Dengue virus type 3 NSl full length aa 774-1125 according to UniProt ID W8FRG8, 352 amino acids

SEP ID NO: 13. Dengue virus type 4 NSl wing domain aa 30-180; X = A or C or S)

SEP ID NO: 14. Dengue virus type 4 NSl full length aa 775-1126 according to UniProt ID

Q58HT7, strain Philippines/H241/1956, 352 amino acids

SEP ID NO: 15. West-Nile virus NSl wing domain aa 30-180; X = A or C or S

SEP ID NO: 16. West-Nile virus NSl full length aa 788-1139 according to UniProt ID P06935, 352 amino acids

SEQ ID NO: 17, Yellow fever virus NS l wing domain aa 30-180; X = A or C or S

SEQ ID NO: 18, Yellow fever virus NSl full length aa 779-1130 according to UniProt ID P03314, strain 17D vaccine, 352 amino acids

SEQ ID NO: 19, Japanese encephalitis virus NS l wing domain a a 30-180; X = A or C or S

SEQ ID NO:20, Japanese encephalitis virus NSl full length aa 795-1146 according to UniProt ID Q9YJ16, 352 amino acids

SEQ ID NO:21, Fusion protein of tandem E. coli SlyD with Zika virus NS 1 wing domain aa 30-

180

SEQ ID NO:22, Fusion protein of tandem E. coli SlyD with Zika virus NSl β-ladder domain (aa

191-352, strain Mr 766)

SEQ ID NO:23, Fusion protein of E. coli SlyD with Zika virus NSl B-ladder domain (aa 191- 352, strain Mr 766)

Detailed description of the invention

Currently available ELISA format immunoassays for detecting anti-Zika virus antibodies employ an NS 1 antigen as immunoreactive reagent. However, we find that these assays lack specificity, leading to a fairly high number of false positive results. Surprisingly, by confining the Zika NSl antigen to its wing domain as explained further below, the number of erroneously reactive samples can be considerably reduced while the high sensitivity of the assay is maintained.

When samples positive for anti-Zika virus antibodies were tested with two different fragments of the NSl antigen of Zika, i.e. with the so-called "wing" domain antigen and the so-called "B- ladder" domain antigen it became evident that both antigens are able to detect anti-Zika antibodies. However, we found out that the wing domain antigen does not cross-react with Dengue antibody positive samples whereas the Zika NS 1 β-ladder domain antigen does cross- react with Dengue antibody positive samples, leading to false positive results and to erroneous conclusions. Additional blocking experiments with anti-Zika positive sera and NSl antigens of related arboviruses conclusively show that the Zika NS 1 wing domain antigen signal can hardly be quenched by these related arboviral NSl antigens whereas the Zika NSl B-ladder domain antigen signal is quenched significantly. We infer that the B-ladder antigen is considerably blocked from binding to the immunoglobulins by the competing related arboviral NSl antigens. Thus, we were able to show that the Zika NSl wing domain antigen is less susceptible to immunological cross-reactivity with other arboviral NSl homologues. As a consequence, the Zika NSl wing domain enables an immunoassay for anti-Zika antibodies with superior specificity that is capable of diagnosing Zika virus infections in the presence of other (recent or past) arbovirus infections, in an embodiment discriminating Zika virus infections from Dengue virus infections.

The invention therefore concerns a polypeptide suitable for detecting antibodies against Zika virus in an isolated biological sample comprising a Zika antigen with a Zika virus NS1 wing domain specific amino acid sequence wherein no amino acid sequences from the Zika virus NS 1 β-ladder domain are present in said polypeptide. In an embodiment, no further Zika virus NS1 specific amino acid sequences are present in this polypeptide, in an embodiment no further Zika virus specific amino acid sequences are present in said polypeptide. In an embodiment, said Zika virus NS1 domain specific amino acid sequence consists essentially of SEQ ID NOs. 1 or 2, in an embodiment consists of SEQ ID NOs 1 or 2. In particular, no amino acid sequences from the C-terminal so-called B-ladder domain are present in the polypeptide of the invention. In an embodiment SEQ ID NO:4 is not present in said polypeptide.

In an embodiment, the invention concerns the use of said polypeptide in an immunoassay method for detecting anti-Zika virus antibodies. The term "Zika virus" is equivalent to the shorter forms "Zika" or "ZIKV"; these acronyms refer to the same virus.

The terms "NS1", "NS1 antigen", NS1 polypeptide" are used synonymously and refer to the non-structural antigen no. 1 within the viral precursor polyprotein and relate (unless specified differently) to the full length antigen NS1. The structure of this protein has been described for West Nile virus and Dengue virus 2 in Akey et al, supra. For Zika NS1 the structure of the C- terminal domain (amino acid residues 172-352) has been described by Song et al. (supra) and the complete NS1 three-dimensional structure has been described in further detail by Brown et al. (supra). The Zika NS1 sequence comprises 352 amino acids and is shown in SEQ ID NO: 3. The term "NS1 wing" or "NS1 wing domain", "variant of NS1 wing" or "NS1 wing region" refers to a domain within the NS1 polypeptide and is therefore a partial sequence of NS1. For the Zika NS1 wing domain this is exemplified in SEQ ID NOs: l and 2. Also the terms "polypeptide", "polypeptides", "antigen" and "antigens" are understood as synonyms unless further specified.

The synonymous terms "B-ladder", "B-ladder domain" or "ladder tip antigen" or "ladder tip", "ladder tip domain", "ladder tip polypeptide", "ladder tip antigen" refer to a NS1 domain located C-terminally adjacent to the wing domain. This domain has been described for West Nile virus and Dengue virus 2 in Akey et al, supra. For Zika virus this NS1 domain has been described by Song et al., supra, and Brown et al., supra. For the Zika NS1 B-ladder domain the amino acid sequence is exemplified in SEQ ID NO:4.

These definitions are applicable to all arboviruses within this specification. The following arboviruses that belong to the flavivirus family can be abbreviated as follows: West Nile virus (West Nile, WNV), tick-borne encephalitis virus (TBEV or FSME), Dengue virus 1-4 (Dengue, the four virus strains of Dengue: DENV 1-4), yellow fever virus (YFV), Japanese encephalitis virus (JEV).

According to the invention, a Zika virus NS 1 domain specific amino acid sequence is an amino acid sequence wherein no amino acid sequences from the Zika virus NS1 β-ladder domain are present in said polypeptide. In an embodiment, no further Zika virus specific amino acid sequences are present in such polypeptide. For example a Zika NS1 wing domain polypeptide contains only a wing domain sequence, in an embodiment contains SEQ ID NOs. 1 or 2. In an embodiment, SEQ ID NO:4 is not present in said polypeptide sequence. In another embodiment, no further Zika virus specific amino acid sequences are present in this sequence. The absence of NS1 B-ladder domain specific sequences and in an embodiment the absence of further Zika NS1 specific sequences or of further Zika specific sequences supports the aim to either reduce or completely avoid cross-reactivities with antibodies raised against other arboviruses.

However, variants of the Zika NS1 wing domain polypeptide are encompassed as well. These variants may easily be created by a person skilled in the art by conservative or homologous substitutions of the disclosed amino acid sequences (such as e.g. substitutions of a cysteine by alanine or serine, or substitutions of isoleucine by valine, or vice versa). The term "variants" in this context also relates to a protein or a protein fragment {i.e. a polypeptide or peptide) substantially similar to said protein. For example, modifications such as C- or N-terminal truncations at one end or at both ends by 1 to 10 amino acids, in an embodiment by 1 to 5 amino acids, are within the scope of the claimed Zika NS1 wing domain antigens. In particular, a variant may be an isoform which shows amino acid exchanges, deletions or insertions compared to the amino acid sequence of the most prevalent protein isoform. In one embodiment, such a substantially similar protein has a sequence similarity to the most prevalent isoform of the protein of at least 80%, in another embodiment at least 85% or at least 90%, in yet another embodiment at least 95%. The term "variant" also relates to a post-translationally modifed protein such as a glycosylated or phosphorylated protein. According to the invention a variant classifies as a Zika NS1 wing domain variant as long as the immunoreactivity in an in vitro diagnostic immunoassay is unchanged or largely maintained, i.e. the variant is still able to bind and detect anti-Zika antibodies present in an isolated sample while antibodies raised against other arboviruses are not detected or detected to a much lower extent. In addition, the overall three-dimensional structure of said Zika polypeptide remains unaltered, so that epitopes that were previously (i.e. in the wild type) present and accessible for binding to antibodies are still present and accessible in the variant.

A "variant" is also a protein or antigen which has been modified for example by covalent or non- covalent attachment of a label or carrier moiety to the protein or antigen. Possible labels are radioactive, fluorescent, chemiluminescent, electrochemiluminescent, enzymes or others e.g. like digoxin, digoxigenin or biotin. These labels are known to a person skilled in the art. When a provided polypeptide sequence information specified in the form of SEQ ID NOs is described by the term "consisting essentially of (i.e., said sequence) this means that the sequence is present as literally listed but can also be present as variants that do not materially affect the basic characteristics of this polypeptide in terms of immunological binding to antibodies. An example of this would be the deletion or addition of only few amino acids at the N- and/or C-terminal end of this peptide as well as the exchange of a similar amino acid as, e.g., alanine for serine, isoleucine for valine, and vice versa.

The Zika NS1 wing domain antigens of the current invention are soluble, stable and immunoreactive, i.e. they are suitable as antigens for use in an immunological assay. This means that the antigens according to the invention are soluble under physiological buffer conditions, for example in a phosphate buffer system at ambient temperature without addition of detergents. The antigens are also capable of binding to or being recognized and bound by antibodies specific for Zika NS1 wing domain, like e.g. anti-Zika antibodies present in an isolated sample such as human sera.

In an embodiment, the addition of non-Zika-specific linker or peptidic fusion amino acid sequences to the Zika NS1 wing domain polypeptides is possible as these sequences are not specific for anti-Zika virus antibodies and would not interfere with the in vitro diagnostic immunoassay.

In an embodiment the Zika NS1 wing domain antigens may be fused to a chaperone. The term "fusion protein", "fusion polypeptide" or "fusion antigen" refers to a protein comprising a Zika NS1 wing domain polypeptide and at least one protein part derived from a chaperone that serves the role of a fusion partner. Chaperones are well-known folding helper proteins that assist the folding and maintenance of the structural integrity of other proteins. Examples of folding helpers are described in detail in WO 03/000877. According to the invention chaperones of the peptidyl prolyl isomerase class such as chaperones of the FKBP family can be used for fusion to the Zika NSl wing domain antigen variants. Examples of FKBP chaperones suitable as fusion partners are FkpA, SlyD and SlpA. A further chaperone suitable as a fusion partner for the Zika NSl wing antigen is Skp, a trimeric chaperone from the periplasm of E.coli, not belonging to the FKBP family. It is not always necessary to use the complete sequence of a chaperone. Functional fragments of chaperones (so- called binding-competent modules or polypeptide-binding motifs) which still possess the required abilities and functions may also be used (cf. WO 98/13496).

In a further embodiment of the invention at least one or at least two modules of an FKBP chaperone such as e.g. E. coli SlyD, SlpA or FkpA are used as fusion moieties for expression of the Zika NSl wing domain antigen. The chaperone Skp may be used as a fusion partner as well. The fusion of two FKBP-chaperone domains results in improved solubility of the resulting fusion polypeptide. The fusion moieties may be located at the N-terminus or at the C-terminus or at both ends (sandwich- like) of the Zika NS 1 wing domain antigen.

In an embodiment the Zika NSl wing domain antigen is fused to an oligomeric chaperone. Oligomeric chaperones are chaperones that naturally form dimers, trimers or even higher multimers so that a plurality of monomeric subunits are assembled into a well-defined functional quaternary structure by specific non-covalent interactions. Thereby, the covalently fused antigens are coerced into a higher epitope density as well. Preferred oligomeric chaperones are FkpA and Skp. Multimerized antigens are particularly useful in detecting IgM antibodies and hence early immune responses immediately after infections.

In an embodiment, the Zika NSl wing domain polypeptide is fused to one, two or more chaperone molecules of bacterial SlyD, SlpA, FkpA or Skp, in an embodiment of E. coli SlyD, SlpA, FkpA or Skp. In a further embodiment the Zika NSl wing domain polypeptide consists of SEQ ID NO:21.

Another embodiment of the invention is a Zika NSl antigen that does not immunologically cross-react with antibodies raised against structurally related antigens from tick-borne encephalitis virus comprising any of SEQ ID NOs:5 or 6, and/or from Dengue virus 1-4 comprising any of SEQ ID NOs:7 to 14, and/or from West Nile virus comprising any of SEQ ID NOs: 15 or 16, and/or from yellow fever virus comprising any of SEQ ID NOs: 17 or 18, and/or from Japanese encephalitis virus comprising any of SEQ ID NOs: 19 to 20, but immunologically reacts with antibodies raised against full length Zika virus NSl antigen according to SEQ ID NO:3. In a further embodiment said Zika NSl antigen is a Zika NSl wing domain antigen wherein the Zika-specific sequence consists essentially of SEQ ID NO: 1 or 2, in an embodiment consists of SEQ ID NO: 1 or 2.

The term "does not immunologically cross-react" designates a strongly reduced or completely abolished undesired immunological reactivity. The term "immunological cross-reactivity" has been coined to illustrate an unwanted binding of immunoglobulins which is due to similarities in sequence or structure of an antigen with the immunogen, against which the antibodies have originally been mounted. In an embodiment, Zika virus NSl wing domain polypeptides show a completely abolished or strongly reduced immunological reactivity towards antibodies or towards a subset of antibodies raised against homologous or related the corresponding arboviral NSl antigens named above as compared to the full length Zika virus NSl polypeptide. In yet another embodiment Zika virus NSl wing domain polypeptides show a strongly reduced immunological cross-reactivity towards antibodies or towards a subset of antibodies raised against Dengue virus, in an embodiment towards antibodies raised against Dengue virus types 1, 2, 3, 4. In a further embodiment, a strongly reduced immunological cross-reactivity of the Zika virus NS 1 wing domain polypeptides applies also to antibodies or a subset of antibodies raised against yellow fever virus. The expression "does not immunologically cross-react" also refers to a situation where in an immunoassay in a double antigen sandwich format for detecting antibodies the sample antibodies (i.e., the analyte antibodies) are bound by two specific antigens: one is capable of being bound to a solid phase and the other carries a label, the sample antibody is sandwiched between both antigens. In the presence of analyte antibodies the labeled antigen is recruited - within the resulting ternary immune complex - to the solid phase and yields a signal. In the present case, a Zika NSl wing domain polypeptide is labeled and the measured signal is set as 100%. In a parallel or subsequent experiment, the same assay is run with another aliquot of the same (positive) sample and in addition a non-labeled antigen with an amino acid sequence suspected to compete with the labeled antigen is added to the mixture. In the present case, full length NSl polypeptides of either TBEV, DENV1-4, WNV, YFV or JEV are added. In another embodiment, NSl polypeptides consisting of or comprising only the NSl wing domain of either TBEV, DENV1-4, WNV, YFV or JEV are added. When the signal obtained after measurement is maintained at about at least 70% signal recovery, in an embodiment at least 80% signal recovery, in an embodiment at least 85% signal recovery, in an embodiment at least 90% signal recovery of the original signal, the flaviviral (in this example: Zika) NS1 polypeptide is not prone to signal quenching. It is not outcompeted by the added antigens and therefore withstands potentially cross-reactive substances. For illustration, such blocking experiments are described in example 2 (table 2).

In an embodiment, NS 1 full length or NS 1 wing domain peptides from tick-borne encephalitis virus comprising any of SEQ ID NOs:5 or 6, and/or from Dengue virus 1-4 comprising any of SEQ ID NOs:7 to 14, and/or from West Nile virus comprising any of SEQ ID NOs: 15 or 16, and/or from yellow fever virus comprising any of SEQ ID NOs: 17 or 18, and/or from Japanese encephalitis virus comprising any of SEQ ID NOs: 19 to 20, are added in the above-described blocking experiment.

In another embodiment, the Zika NS1 wing domain polypeptide immunologically reacts with antibodies, or a subset of antibodies, raised against full-length NS1 antigen as shown in SEQ ID NO:3. This means that in the above-disclosed assay setup, the signal of the Zika NS1 wing domain polypeptide should be fully quenched (100%) upon addition of the full-length Zika NS1 polypeptide.

The approach how to determine immunological cross-reactivity to Zika-related viruses is further described in example 2.

The Zika NS1 polypeptides (both wing domain and B-ladder domain) as well as the polypeptides applied for the blocking experiments of example 2 can be generated and prepared by means of recombinant DNA techniques and protein purification techniques as known in the art. Another aspect of the invention therefore is a recombinant DNA molecule encoding a Zika NS1 wing domain antigen, in an embodiment an antigen according to SEQ ID NOs 1, 2 or 21 and variants thereof as defined further above. The term "recombinant DNA molecule" refers to a molecule which is made by the combination of two otherwise separated segments of DNA sequence accomplished by the artificial manipulation of isolated segments of polynucleotides by genetic engineering techniques or by chemical synthesis. In doing so one may join together polynucleotide segments of desired functions to generate a desired combination of functions. Recombinant DNA techniques for expression of proteins in prokaryotic or lower or higher eukaryotic host cells are well known in the art. They have been described e.g. by Sambrook et al., (1989, Molecular Cloning: A Laboratory Manual) The recombinant DNA molecules according to the invention may also contain sequences encoding linker peptides of 5 to 100 amino acid residues in between the Zika virus NSl wing domain antigen and the fusion moieties and also between several fusion moieties. Such a linker sequence may for example harbor a proteolytic cleavage site. A further aspect of the invention is an expression vector comprising operably linked a recombinant DNA molecule according to the present invention, i.e., a recombinant DNA molecule encoding a Zika virus NSl wing domain antigen and optionally a peptidyl prolyl isomerase chaperone, such as an FKBP-chaperone, wherein the FKBP-chaperone is selected from FkpA, SlyD and SlpA. In an alternative embodiment the recombinant DNA molecule encodes a fusion protein comprising a Zika virus NSl wing domain antigen and Skp. The expression vector comprising a recombinant DNA according to the present invention may be used to express the Zika virus NS 1 wing domain antigen in a cell free translation system or may be used to transform a host cell for expression of the Zika virus NSl wing domain antigen according to methods well known in the art. Another aspect of the invention therefore relates to a host cell transformed with an expression vector according to the present invention. In one embodiment of the current invention the recombinant Zika virus NS 1 wing domain antigens are produced in E. coli cells.

An additional aspect is a method for producing a soluble, stable and immunoreactive Zika virus NSl wing domain antigen. Said Zika virus NSl wing domain antigen may be produced as a fusion protein containing the Zika virus NSl wing domain antigen and a chaperone. Preferably, a chaperone such as Skp or a peptidyl prolyl isomerase class chaperone like an FKBP chaperone is used. In a further embodiment of the invention said FKBP chaperone is selected from the group consisting of SlyD, FkpA and SlpA.

This method comprises the steps of

a) culturing host cells transformed with the above-described expression vector containing a gene encoding an Zika virus NS 1 wing domain antigen

b) expression of the gene encoding said Zika virus NSl wing domain antigen

c) purification of said Zika virus NSl wing domain antigen.

Optionally, as an additional step d), functional solubilization needs to be carried out so that the Zika virus NSl wing domain antigen is brought into a soluble and immunoreactive conformation by means of refolding techniques known in the art. Yet another embodiment is a method for producing a soluble, stable and immunoreactive Zika virus NS 1 wing domain antigen in a cell- free in vitro translation system.

An additional aspect of the present invention concerns a method for the detection of anti-Zika antibodies in an isolated human sample wherein a Zika virus NS1 wing domain antigen according to the invention is used as a binding partner for the antibodies. The invention thus covers a method for the detection of antibodies specific for Zika virus in an isolated sample, said method comprising a) forming an immunoreaction admixture by admixing a body fluid sample with an Zika virus NS1 wing domain antigen according to the invention b) maintaining said immunoreaction admixture for a time period sufficient for allowing antibodies against said Zika virus NS1 wing domain antigen present in the body fluid sample to immunoreact with said Zika virus NS1 wing domain antigen to form an immunoreaction product; and c) detecting the presence and/or the concentration of any of said immunoreaction product.

In a further aspect said method is suitable for detecting Zika virus antibodies of the IgG and the IgM subclass or of both classes in the same immunoassay.

Immunoassays for detection of antibodies are well known in the art, and so are methods for carrying out such assays and practical applications and procedures. The Zika NS1 antigens according to the invention can be used to improve assays for the detection of anti-Zika antibodies independently of the labels used and independently of the mode of detection {e.g., radioisotope assay, enzyme immunoassay, electrochemiluminescence assay, etc.) or the assay principle {e.g., test strip assay, sandwich assay, indirect test concept or homogenous assay, etc.).

In an embodiment of the invention the immunoassay is a particle-based immunoassay applying microparticles as solid phase. A "particle" as used herein means a small, localized object to which can be ascribed a physical property such as volume, mass or average size. Microparticles may accordingly be of a symmetrical, globular, essentially globular or spherical shape, or be of an irregular, asymmetric shape or form. The size of a particle envisaged by the present invention may vary. In one embodiment the microparticles used are of globular shape, e.g. microparticles with a diameter in the nanometer and micrometer range. In one embodiment the microparticles used in a method according to the present disclosure have a diameter of 50 nanometers to 20 micrometers. In a further embodiment the microparticles have a diameter of between 100 nm and 10 μιη. In one embodiment the microparticles used in a method according to the present disclosure have a diameter of 200 nm to 5 μιη or from 750 nm to 5 μιη. Microparticles as defined herein above may comprise or consist of any suitable material known to the person skilled in the art, e.g. they may comprise or consist of or essentially consist of inorganic or organic material. Typically, they may comprise or consist of or essentially consist of metal or an alloy of metals, or an organic material, or comprise or consist of or essentially consist of carbohydrate elements. Examples of envisaged material for microparticles include agarose, polystyrene, latex, polyvinyl alcohol, silica and ferromagnetic metals, alloys or composition materials. In one embodiment the microparticles are magnetic or ferromagnetic metals, alloys or compositions. In further embodiments, the material may have specific properties and e.g. be hydrophobic, or hydrophilic. Such microparticles typically are dispersed in aqueous solutions and retain a small negative surface charge keeping the microparticles separated and avoiding non-specific clustering.

In one embodiment of the present invention, the microparticles are paramagnetic microparticles and the separation of such particles in the measurement method according to the present disclosure is facilitated by magnetic forces. Magnetic forces are applied to pull the paramagnetic or magnetic particles out of the solution/suspension and to retain them as desired while liquid of the solution/suspension can be removed and the particles can e.g. be washed.

All biological liquids known to the expert can be used as isolated samples for the detection of anti-Zika antibodies. The samples usually used are bodily liquids like whole blood, blood serum, blood plasma, urine or saliva, in an embodiment blood serum or plasma. A further embodiment of the invention is an immunoassay for detecting anti-Zika antibodies in an isolated sample performed according to the so-called double antigen sandwich concept (DAGS). Sometimes this assay concept is also termed double antigen bridge concept, because the two antigens are bridged by an antibody analyte. In such an assay the ability of an antibody to bind at least two different molecules of a given antigen with its two (IgG, IgE), four (IgA) or ten (IgM) paratopes is required and utilized.

In more detail, an immunoassay for the determination of anti-Zika antibodies according to the double antigen bridge format is carried out by incubating a sample containing the anti-Zika antibodies with two different Zika virus NS1 wing domain antigens, i.e. a first ("solid phase'Or "capture") Zika virus NS1 wing domain antigen and a second Zika virus NS1 wing domain ("detection" or "reporter") antigen, wherein each of the said antigens binds specifically to said anti-Zika antibodies. The first antigen can be bound directly or indirectly to a solid phase and usually carries an effector group which is part of a bioaffine binding pair. One type of a bioaffine binding pair which is suitable for the method according to the present invention is a hapten and anti-hapten antibody binding pair. A hapten is an organic molecule with a molecular weight of 100 to 2000 Dalton, preferably of 150 to 1000 Dalton. Such small molecule can be rendered immunogenic by coupling it to a carrier molecule and anti-hapten antibodies can be generated according to standard procedures. The hapten may be selected from the group comprising sterols, bile acids, sexual hormones, corticoids, cardenolides, cardenolide- glycosides, bufadienolides, steroid-sapogenines and steroid alkaloids, cardenolides and cardenolide-glycosides. Representatives of these substance classes are digoxigenin, digitoxigenin, gitoxigenin, strophanthidin, digoxin, digitoxin, ditoxin, and strophanthin. Another suitable hapten is for example fluorescein. In an embodiment, a bioaffine binding pair comprises biotin and avidin/streptavidin or digoxin and anti-digoxin.

In yet another embodiment, the first antigen is conjugated to biotin and the complementary solid phase is coated with either avidin or streptavidin. The second antigen carries a label that confers specific detectability to this antigen molecule, either alone or in complex with other molecules. Thus an immunoreaction admixture is formed comprising the first antigen, the sample antibody and the second antigen. This ternary complex consisting of analyte antibody sandwiched in between two antigen molecules is termed immunocomplex or immunoreaction product. A solid phase to which the first antigen can be bound is added either before the addition of the sample to said antigens or after the immunoreaction admixture is formed. This immunoreaction admixture is maintained for a time period sufficient for allowing anti-Zika antibodies against said Zika virus NS 1 wing domain antigens in the body fluid sample to immunoreact with said Zika virus NS1 wing domain antigens to form an immunoreaction product. Next step is a separation step wherein the liquid phase is separated from the solid phase. Finally, the presence of any of said immunoreaction product is detected in the solid or liquid phase or both. In said DAGS immunoassay the basic structures of the "solid phase antigen" and the "detection antigen" are essentially the same. It is also possible to use, in a double antigen bridge assay, similar but different Zika virus NS1 wing domain antigens, which are immunologically cross- reactive. The essential requirement for performing such assays is that the relevant epitope or the relevant epitopes are present on both antigens. According to the invention it is possible to use the same or different fusion moieties for each Zika virus NS1 wing domain antigen (e.g. SlyD fused to Zika virus NS1 wing domain antigen on the solid phase side and, e.g., FkpA fused to Zika virus NS1 wing domain antigen on the detection side) as such variations significantly alleviate the problem of non-specific binding and thus mitigate the risk of false-positive results. A further embodiment is a method for detecting anti-Zika virus antibodies (i.e., immunoglobulins) of the M class (IgM detection). In an embodiment of this method a Zika NS1 wing domain polypeptide as disclosed further above is applied in such a way that the multivalent IgM antibodies present in a sample specifically bind to the Zika NS1 wing domain antigen. In an embodiment, the Zika NS1 wing domain antigen is provided in a multimeric form by either chemically cross-linking the antigen or by fusing the antigen to an oligomerizing molecule such as an oligomeric chaperone, in an embodiment to FkpA or Skp. In another embodiment a Zika wing domain antigen is present in multiple form by connecting individual antigens in series, adjacent to each other. These individual antigen moieties can also be separated by linker molecules that are not Zika specific. In a further embodiment the multiple Zika antigens connected in series can additionally be multimerized by an oligomerization molecule such as an oligomeric chaperone like e.g. FkpA or Skp. In yet another embodiment the Zika NS1 wing domain polypeptide is used in a multimeric form wherein each polypeptide is present at least in duplicate form, in an embodiment it is present three to ten times. In yet another embodiment of the IgM detection method of Zika-antibodies the IgM class antibodies present in the sample are bound to a solid phase by a so-called μ-capture component which usually is a binding partner or an antibody or antibody fragment that specifically binds to the Fc part of human IgM molecules, independently of the specificity of the IgM molecule. Said μ-capture component carries an effector group (such as biotin) which is part of a bioaffme pair with avidin or streptavidin. In an embodiment also other bioaffme pairs such as e.g. digoxin and anti-digoxin or further hapten and anti-haptens as described further above can be used. In an embodiment, a solid phase covered with avidin or streptavidin then attracts and binds the μ- capture component. In order to specifically detect the Zika-specific antibodies a Zika NS 1 wing domain polypeptide as described is used in a labeled form to detect the anti-Zika antibodies of the IgM class.

Another embodiment is the use of a Zika NS 1 wing domain polypeptide as detailed above in an in vitro diagnostic test, in an embodiment an immunoassay method as defined above, for the detection of anti-Zika virus antibodies.

As a further embodiment the maximal total duration of the immunoassay method for detecting Zika virus antibodies is less than one hour, i.e. less than 60 minutes, in an embodiment less than 30 minutes, in a further embodiment less than 20 minutes, in an embodiment between 15 and 30 minutes, in an embodiment between 15 to 20 minutes. The duration includes pipetting the sample and the reagents necessary to carry out the assay as well as incubation time, optional washing steps, the detection step and also the final output of the result.

An additional subject matter of the invention is a reagent kit for the detection of antibodies against Zika virus that comprises a Zika NS1 wing domain polypeptide disclosed above. In an embodiment a reagent kit comprises in separate containers or in separated compartments of a single container unit at least microparticles coated with avidin or streptavidin, and a Zika NS 1 wing domain polypeptide as detailed before. In another embodiment, said microparticles are coated with one partner of other bioaffme pairs as described further above, such as e.g. digoxin and anti-digoxin, hapten and anti-hapten. In an embodiment, said Zika NS1 wing domain polypeptide is covalently coupled to biotin. In an embodiment, said Zika NS1 wing domain is covalently coupled to the second partner of other bioaffme pairs such as e.g. digoxin and anti- digoxin, hapten and anti-hapten. In another embodiment, said Zika NS1 wing domain polypeptide is covalently coupled to a detectable label, in an embodiment to an electrochemiluminescent complex. In a further embodiment also chemiluminescent labels such as e.g. acridinium ester or radioactive or fluorescent compounds or enzymes can be applied as label. In yet another embodiment, said reagent kit comprises in separate containers or in separated compartments of a single container unit at least microparticles coated with avidin or streptavidin, a first Zika NS 1 wing domain polypeptide covalently coupled to biotin and a second Zika NS1 wing domain polypeptide that is covalently coupled to a detectable label, e.g. to an electrochemiluminescent ruthenium complex or an electrochemiluminescent iridium complex.

A further embodiment is a reagent kit for detecting anti-Zika antibodies of the IgM class, comprising in separate containers or in separated compartments of a single container unit at least microparticles coated with avidin or streptavidin, and a μ-capture binding partner that is covalently coupled to biotin. In a further embodiment said IgM detection reagent kit additionally contains Zika NS 1 wing domain polypeptide which is covalently coupled to a detectable label, in an embodiment to an electrochemiluminescent complex.

The term single container unit relates to the fact that for many automatic analyzers, like the Elecsys® analyzer series from Roche diagnostics, the reagents required to measure a certain analyte are provided in the form of a "reagent pack", i.e. as one container unit fitting on the analyzer and containing in different compartments all the key reagents required for measurement of the analyte of interest. In addition, the reagent kits defined above contain controls and standard solutions as well as reagents in one or more solutions with the common additives, buffers, salts, detergents and the like as used by the average man skilled in the art along with instructions for use.

In yet another aspect, the invention concerns a method for detecting antibodies against a Zika virus in an isolated biological sample that is presumed to contain antibodies against at least one other non-Zika flavivirus such as e.g. Dengue virus. In this method, Zika virus NSl polypeptide comprising a complete or partial sequence of the B-ladder domain is applied as specific binding partner, i.e. not only the NSl wing domain. In this setup cross-reactivity against other non-Zika flaviviruses can be expected due to the presence of β-ladder domain peptide sequences in the specific binding partner. In order to eliminate this interference a polypeptide comprising solely the NSl B-ladder domain of said Zika virus is added in an unlabeled form so that the cross- reacting antibodies of non-Zika origin are bound and quenched. In an embodiment, the B-ladder domain is added as a quencher, in a further embodiment said B-ladder domain polypeptide consists essentially of SEQ ID NO:4, in an embodiment consists of SEQ ID NO:4.

The following embodiments are also part of the invention:

1. A polypeptide suitable for detecting antibodies against Zika virus in an isolated

biological sample comprising a Zika virus NS 1 wing domain specific amino acid sequence, wherein no amino acid sequences from the Zika virus NS 1 B-ladder domain are present in said polypeptide

2. A polypeptide according to embodiment 1, wherein no further Zika virus specific amino acid sequences are present in said polypeptide.

3. A polypeptide according to any of embodiments 1 or 2, wherein said Zika virus NSl wing domain specific amino acid sequence consists essentially of SEQ ID NO. 1 or 2

4. A polypeptide according to any of embodiments 1 to 3, wherein said Zika virus NSl wing domain specific amino acid sequence consists of SEQ ID NO. 1 or 2.

5. A polypeptide according to embodiment 1 to 4, wherein said Zika virus NSl wing

domain specific amino acid sequence can be truncated by 1 to 5 amino acids at its N- terminal or C-terminal end or at both ends. 6. A polypeptide according to embodiments 1 to 5, wherein said Zika virus NSl wing

domain specific amino acid sequence can be modified by conservative amino acid substitutions so that the immunoreactivity of said Zika polypeptide remains unchanged or is largely maintained, in an embodiment so that the three-dimensional structure of said Zika polypeptide remains unchanged. 7. A polypeptide according to any of embodiments 1 to 6 wherein said Zika polypeptide is fused to a chaperone. 8. A polypeptide according to any of embodiments 1 to 7 wherein said chaperone is selected from the group consisting of SlyD, SlpA, FkpA and Skp. 9. A polypeptide according to embodiment 8 consisting essentially of SEQ ID NO:21, in an embodiment consisting of SEQ ID NO:21. 10. A polypeptide according to any of embodiments 1-9, wherein in an immunoassay for detecting antibodies against Zika virus, said Zika NSl polypeptide does not

immunologically cross-react with antibodies raised against structurally related antigens from tick-borne encephalitis virus comprising any of SEQ ID NOs:5 or 6, and/or from Dengue virus 1-4 comprising any of SEQ ID NOs:7 to 14, and/or from West Nile virus comprising any of SEQ ID NOs: 15 or 16, and/or from yellow fever virus comprising any of SEQ ID NOs: 17 or 18, and/or from Japanese encephalitis virus comprising any of SEQ ID NOs: 19 to 20, but immunologically reacts with antibodies raised against full length Zika virus NSl polypeptide according to SEQ ID NO:3. 11. A method of producing a soluble and immunoreactive Zika virus NSl wing domain

polypeptide, said method comprising the steps of

a) culturing host cells transformed with an expression vector comprising operably linked a recombinant DNA molecule encoding a Zika virus NSl polypeptide according to any of embodiments 1 to 10,

b) expression of said Zika virus NS 1 polypeptide and c) purification of said Zika virus NS1 polypeptide. 12. A method for detecting antibodies specific for Zika virus in an isolated sample wherein a Zika virus polypeptide according to any of embodiments 1 to 10 or a Zika virus polypeptide obtained by a method according to embodiment 11 is used as a capture reagent and/or as a binding partner for said anti-Zika virus antibodies. 13. A method for detecting antibodies specific for Zika virus in an isolated sample said

method comprising

a) forming an immunoreaction admixture by admixing a body fluid sample with a Zika virus polypeptide according to any of embodiments 1 to 10 or a Zika virus polypeptide obtained by the method of embodiment 11

b) maintaining said immunoreaction admixture for a time period sufficient for allowing antibodies present in the body fluid sample against said Zika virus polypeptide to immunoreact with said Zika virus polypeptide to form an immunoreaction product; and c) detecting the presence and/or the concentration of any of said immunoreaction product. 14. A method for detecting antibodies specific for Zika virus in an isolated sample according to any of embodiments 12 or 13, wherein the detected antibody is an IgG antibody. 15. A method for detecting antibodies specific for Zika virus in an isolated sample according to embodiment 13 or 14 wherein said immunoreaction is carried out in a double antigen sandwich format comprising

a) adding to said sample a first Zika virus polypeptide according to any of embodiments 1 to 10 or a polypeptide produced by a method according to embodiment 11, which can be bound directly or indirectly to a solid phase and said first Zika virus polypeptide carries an effector group which is part of a bioaffine binding pair, and a second Zika virus polypeptide according to any of embodiments 1 to 10 or produced by a method according to embodiment 11 , and said second Zika virus polypeptide carries a detectable label, wherein said first and second Zika virus polypeptides bind specifically to said anti-Zika virus antibodies,

b) forming an immunoreaction admixture comprising said first Zika virus polypeptide, said sample antibody and said second Zika virus polypeptide wherein a solid phase carrying a corresponding effector group of said bioaffine binding pair is added before, during or after forming the immunoreaction admixture,

c) maintaining said immunoreaction admixture for a time period sufficient for allowing Zika virus antibodies against said first and second Zika virus polypeptides in the body fluid sample to immunoreact with said first and second Zika virus polypeptides to form an immunoreaction product,

d) separating the liquid phase from the solid phase

e) detecting the presence of any of said immunoreaction product in the solid or liquid phase or both. A method for detecting antibodies specific for Zika virus according to embodiment 15, wherein said first Zika virus polypeptide carries a biotin moiety, and said second Zika virus polypeptide is labeled with an electrochemiluminescent moiety, in an embodiment with a ruthenium or iridium complex. A method for detecting antibodies specific for Zika virus in an isolated sample according to any of embodiments 12 or 13, wherein the detected antibody is an IgM antibody. A method for detecting antibodies specific for Zika virus of the IgM class according to embodiment 17, wherein the Zika NS1 wing domain polypeptide is used in a multimeric form, in an embodiment wherein said polypeptide is present at least in duplicate form, in an embodiment three to ten times. A method according to any of embodiments 12 or 13, wherein said detected antibodies are IgM antibodies and wherein said IgM antibodies are captured on a solid phase by a μ- capture binding partner. A method for detecting IgM antibodies specific for Zika virus in an isolated sample according to any of embodiments 17 to 19 wherein said immunoreaction is carried out in a μ-capture format comprising

a) adding to said sample a μ-capture binding partner which can be bound directly or indirectly to a solid phase and said μ-capture binding partner carries an effector group which is part of a bioaffine binding pair,

and a Zika virus polypeptide according to any of embodiments 1 to 10 or produced by a method according to embodiment 11 , and said Zika virus polypeptide carries a detectable label,

wherein said μ-capture binding partner specifically binds to the Fc part of human IgM antibodies and said Zika virus polypeptide binds specifically to said anti-Zika virus antibodies,

b) forming an immunoreaction admixture comprising said μ-capture binding partner, said sample antibodies and said Zika virus polypeptide wherein a solid phase carrying a corresponding effector group of said bioaffme binding pair is added before, during or after forming the immunoreaction admixture,

c) maintaining said immunoreaction admixture for a time period sufficient for allowing IgM antibodies against said Zika virus polypeptide in the body fluid sample to immunoreact with said Zika virus polypeptide to form an immunoreaction product, d) separating the liquid phase from the solid phase

e) detecting the presence of any of said immunoreaction product in the solid or liquid phase or both. 21. A method according to any of embodiments 12 to 20, wherein said method does not use Zika NS 1 antigens from the B-ladder domain, in an embodiment does not use a polypeptide comprising an amino acid sequence according to SEQ ID NO:4. 22. Use of a Zika virus polypeptide according to any of embodiments 1 to 10 or of a Zika virus polypeptide obtained by the method of embodiment 11 in an in vitro diagnostic test for the detection of anti-Zika virus antibodies. 23. Use of a Zika virus polypeptide according to any of embodiments 1 to 10 or of a Zika virus polypeptide obtained by the method of embodiment 11 in an in vitro diagnostic test for the detection of anti-Zika virus antibodies according to any of the methods of embodiments 12 to 21. 24. A reagent kit for the detection of anti-Zika virus antibodies, comprising a Zika virus polypeptide according to any of embodiments 1 to 10 or of a Zika virus polypeptide obtained by the method of embodiment 11. 25. A reagent kit according to embodiment 24 comprising in separate containers or in

separated compartments of a single container unit at least microparticles coated with avidin or streptavidin, and a polypeptide according to any of embodiments 1 to 10 or obtained by a method according to embodiment 11 that is covalently coupled to biotin. 26. A reagent kit according to embodiment 24 comprising in separate containers or in

separated compartments of a single container unit at least microparticles coated with one partner of a bioaffine pair such as hapten/anti-hapten, and a polypeptide according to any of embodiments 1 to 10 or obtained by a method according to embodiment 11, wherein said bioaffine pair is hapten/anti-hapten, in an embodiment digoxin/anti-digoxin. 27. A reagent kit according to embodiment 24 additionally comprising a second polypeptide according to any of embodiments 1 to 10 or obtained by a method according to embodiment 11 that carries a detectable label. 28. A reagent kit according to embodiment 24, comprising in separate containers or in

separated compartments of a single container unit at least microparticles coated with avidin or streptavidin, and a μ-capture binding partner that is covalently coupled to biotin. 29. A reagent kit according to embodiment 24, wherein said Zika virus polypeptide carries a detectable label. 30. A method for detecting antibodies against a Zika virus in an isolated biological sample presumed to contain antibodies against at least one other non-Zika flavivirus, by using a Zika virus NS 1 polypeptide comprising a complete or partial sequence of the B-ladder domain as specific binding partner, wherein the cross-reactivity against said non-Zika flavivirus is eliminated by adding a polypeptide comprising the NS1 β-ladder domain of said Zika virus in an unlabeled form, in an embodiment adding said polypeptide comprising the β-ladder domain as a quencher. 31. A method according to embodiment 30, wherein said added polypeptide comprising the Zika virus NS1 B-ladder domain consists essentially of SEQ ID NO:4, in an embodiment consists of SEQ ID NO:4. 32. Use of Zika virus NSl B-ladder domain as a reagent for reducing interference in an immunoassay for detecting anti-Zika virus antibodies, in an embodiment said NS 1 B- ladder domain consisting essentially of SEQ ID NO:4, in an embodiment consisting of SEQ ID NO:4.

The invention is further illustrated by the Examples.

Examples

Example 1: Immunological reactivity (i.e., antigenicity) of different Zika NS 1 antigen variants in a Zika IgG immunoassay

NSl antigens and variants were essentially cloned, expressed, purified and labeled as described in WO2014054990A1 or by Scholz et al., J. Mol. Biol. (2005) 345, 1229-1241. The purified and solubilized gene products were subsequently coupled to either biotin or to an electro- chemiluminescent ruthenium label.

The immunological reactivity (i.e., antigenicity) of the polypeptide fusion variants of Zika NSl antigens was assessed in automated Elecsys ^® 2010 and cobas e 411 analyzers (Roche Diagnostics GmbH). Elecsys ^® is a registered trademark of the Roche group. Measurements were carried out in the double antigen sandwich format. Signal detection in Elecsys ^® 2010 and cobas e 411 is based on electrochemiluminescence. The biotin-conjugate (i.e. the capture-antigen) is immobilized on the surface of a streptavidin coated magnetic bead whereas the detection-antigen bears a complexed Ruthenium cation (switching between the redox states 2+ and 3+) as the signaling moiety. In the presence of a specific immunoglobulin analyte, the chromogenic ruthenium complex is bridged to the solid phase and emits light at 620 nm after excitation at a platinum electrode. The signal output is in relative light units. Typically, the total duration of an assay is 18 minutes.

The recombinant Zika NSl antigen fusion polypeptides were assessed in a double antigen sandwich (DAGS) immunoassay format. To this end, recombinant Zika NS 1 antigen was used as a biotin and a ruthenium conjugate pair, respectively, to detect anti-Zika NSl antibodies in human sera. NSl is one of the immunodominant antigens of Zika Virus, and soluble variants of NSl antigen - as disclosed in this patent application - are invaluable tools for the detection of Zika virus infections. In all measurements, chemically polymerized and unlabeled SlyD-SlyD was implemented in large excess (~ 8 μg/ml) in the reaction buffer as anti-interference substances to avoid immunological cross reactions via the chaperone fusion and linker units.

In particular, two Zika NSl variants were scrutinized in this study, namely the Zika NSl "wing" domain (see SEQ ID NOs: 1 and 2) and the Zika NSl "B-ladder" domain (see SEQ ID NO:4). In order to detect anti-Zika NSl IgG molecules, SlyD-SlyD-Zika NSl-biotin and SlyD-SlyD-Zika NSl -ruthenium were used in Rl (reagent buffer 1) and R2 (reagent buffer 2), respectively. The concentrations of the antigen conjugates in Rl and R2, respectively, were 500 ng/ml each.

Human serum samples negative for both Zika and Dengue IgG antibodies, human serum samples positive for Zika IgG antibodies and human serum samples positive for Dengue IgG antibodies were tested with both of the Zika NSl recombinant antigens ("wing" and "B-ladder") in comparison with a commercially available state of the art immuno assay (indirect ELISA applying a recombinant NSl antigen coated to a microtiter plate, detection of the sample antibodies against Zika virus by addition of an enzyme-labeled anti-human IgG conjugate), as described by Huzly et ah, supra.

In this experiment, the human samples described above were assessed with the aforementioned DAGS immunoassay setup. The unavoidable system-inherent signal is around 500 counts. Low background signals for human serum samples negative for Zika IgG and Dengue IgG antibodies are indicative of high solubility and generally benign physicochemical properties of the respective antigen ruthenium conjugates. Hydrophobic or, generally spoken, "sticky" antigen-ruthenium conjugates tend to interact with the bead surface and thereby increase the background signal. From table 1 we can infer that the physicochemical properties of Zika NSl "wing" are excellent (column 1). This holds true for Zika NSl "B-ladder" as well (data column 2): When Zika NS 1 "wing" (SEQ ID NO:21) is used as an antigen pair in biotinylated form and in ruthenylated form in the DAGS format, or Zika NS 1 "B-ladder" (SEQ ID NO:22) is used as an antigen pair in biotinylated and ruthenylated form in the DAGS format, both antigen pairs yield a signal background of ~ 600- 900 counts with negative human sera, which clearly points to good solubility properties of the antigen conjugates. However, it becomes evident at first glance that the Zika NSl "wing" and the Zika NSl "B-ladder", although being equivalent in their capability to detect anti-Zika antibodies, largely differ in their cross-reactivity with anti-Dengue antibodies as shown in Table 1. Having a closer look at the Zika IgG positive samples, we find that both Zika NS1 "wing" and Zika NS1 "β-ladder" detect all Zika IgG samples as positive (> 2000 counts). Importantly, Zika NS1 "wing" does not cross-react with Dengue IgG positive samples, since all Dengue IgG samples are found negative (« 2000 counts) with the Zika NS1 wing antigen. In marked contrast, Zika NS1 "β-ladder" detects 9 out of 20 Dengue IgG samples as reactive (> 2000 counts), which points to a considerable degree of cross-reactivity. A very similar reactivity pattern with the Zika IgG and the Dengue IgG positive samples is observed with the commercially available Zika IgG assay described by Huzly et al. (Euro Surveill. 2016; 21(16), pii=30203, 1-4), indicating that the commercially available Zika IgG assay suffers from considerable immunological cross-reactivity (i.e., the commercially available Zika IgG assay provokes quite a few false positive Zika results) with Dengue IgG positive samples. In conclusion, both engineered variants of the Zika NS1 antigen (Zika NS1 "wing" and Zika NS1 "B-ladder") possess outstanding physicochemical and antigenic properties. Yet, the Zika NS1 "wing" antigen outperforms the β-ladder domain in that it displays superior specificity for Zika IgG antibodies and significantly reduced immunological cross reactivity with Dengue positive sera.

The engineered recombinant Zika NS1 "wing" domain therefore constitutes a superior NS1 variant for specific determination of Zika antibodies as compared to the state of the art commercially available Zika IgG immunoassay (which presumably is based on the full-length NS1) as described by Huzly et al., supra.

Table 1 shows the superior specificity (i.e., the strongly reduced cross-reactivity with anti- Dengue antibodies) of Zika NS1 "wing" as compared to Zika NS1 "B-ladder" and commercially available state of the art Zika IgG assay.

Table 1

Example 2: Blocking experiment to corroborate the superior specificity of the Zika NSl „wing" antigen as compared to Zika NSl "β-ladder" antigen

The immunological reactivity (i.e., the antigenicity) of the polypeptide fusion variants of Zika NSl antigens was assessed in automated Elecsys ^® 2010 and cobas e 411 analyzers (Roche Diagnostics GmbH) as described in Example 1.

Three human serum samples positive for Zika IgG antibodies were tested with both of the engineered Zika NSl recombinant antigens ("wing" and "B-ladder"). In parallel, these human serum samples positive for Zika IgG antibodies were individually spiked with full-length flavivirus NSl antigen preparations of either TBEV (tick-borne encephalitis virus, FSME, SEQ ID NO:6), or one antigen of DENVl-4 (Dengue virus 1-4, SEQ ID NOs:8, 10, 12, 14), or WNV (West-Nile virus, SEQ ID NO: 16), or YFV (yellow fever virus, SEQ ID NO: 18), or JEV (Japanese encephalitis virus, SEQ ID NO:20) or ZIKV (Zika virus, SEQ ID NO:3) (tables 2a and 2b).

In this experiment, the human samples (spiked and unspiked) as described above were assessed with the aforementioned DAGS immunoassay setup.

Samples were pre-diluted to a reactivity level (titer) needed for the blocking experiment. This step was needed as the concentration of the flavivirus NS-1 preparations prepared for the blocking experiment and the amount of antibodies in the sample need to be within a reasonable concentration ratio in order to yield a clear blocking result. Then the reactivity of the pre-diluted human anti-Zika IgG positive sample was compared to the signals achieved with the same sample when spiked with full-length flavivirus NS-1 of either TBEV, or DENVl-4, or WNV, or YFV, or JEV or ZIKV. The extent of signal reduction due to competition by the different full- length flavivirus NS-1 preparations of TBEV, DENVl-4, WNV, YFV, JEV and ZIKV was calculated. The signal reduction/extent of blocking was normalized to the maximal blocking achieved with full length Zika NSl, which, as expected, shows the strongest quenching of the signal with both assays (irrespective of the use of either Zika NSl "wing" or Zika NSl "B- ladder"). The degree of the capacity to compete was calculated for all other full-length flavivirus NSl preparations (TBEV, DENVl-4, WNV, YFV, JEV). It is evident at first glance that the signal of the assay based on the Zika NSl "wing" antigen is only weakly quenched by the non- Zika NSl antigens, whereas the signal of the assay based on the Zika NSl "B-ladder" antigen is markedly reduced by TBEV, DENVl-4 and JEV. This finding compellingly indicates that the NSl wing domain and the NSl B-ladder domain strongly differ in their structural uniqueness among the arboviral NSl homologues. Obviously, the wing part of the non-Zika NSl antigens is not able to efficiently compete with the engineered Zika wing antigen for binding to the anti- Zika analyte antibodies. Conversely, the β-ladder part of the non-Zika NSl antigens is perfectly able to efficiently compete with the engineered Zika β-ladder antigen for binding to the anti-Zika analyte antibodies.

In conclusion, this experiment is perfectly in line with Example 1 as it also demonstrates that the assay based on Zika NS 1 "wing" antigen largely detects those anti-Zika IgG antibodies, which are not cross-reactive (or prone to cross-reactivity) with flavivirus NS-1 preparations of other arboviruses (TBEV, DENV1-4, WNV, YFV, JEV). In contrast, the assay based on Zika NSl "B- ladder" antigen binds anti-Zika antibodies, which are, in part, cross-reactive (or prone to cross- reactivity) with TBEV, DENV1-4 and JEV (i.e., they can be blocked by TBEV, DENV1-4 and JEV NS-1 preparations). In other words, the B-ladder domain of the Zika NSl antigen seems to share significant structural and sequence homology with the B-ladder domains of related arboviruses, thereby evoking false positive results in immunoassays when used as an antigen. In contrast, the wing domain of the Zika NSl antigen seems to be rather unique and seems to share little structural homology with its NS counterparts from other arboviruses.

Our data indicate that this principle holds true not only in the discrimination of Zika virus infections from other flaviviral infections, but also in the discrimination of other flaviviral infections from each other. We conclude that the specific detection of antibodies against the wing domain of the NSl antigen may be an excellent means to clearly distinguish flaviviral infections even on a multiple flaviviral infection background. We reason that our findings bear promise to markedly improve the current serology and to obtain a more appropriate view on prevalence and incidence data in the flavivirus field.

In summary, the Zika NSl "wing" antigen displays outstanding and superior specificity for Zika IgG, is less susceptible to immunological cross-reactivity with antibodies raised against NSl homologues from other arboviruses (here: family of flaviviruses) and is therefore much more suitable for specific testing for anti-Zika IgG antibodies. Table 2a

Ta ble 2b

Explanation of acronyms:

ZIKV = Zika virus

DENV1 = Dengue Virus Type 1

DENV2 = Dengue Virus Type 2

DENV3 = Dengue Virus Type 3

DENV4 = Dengue Virus Type 4

WNV = West Nile Virus

JEV = Japanese Encephalitis Virus

YFV = Yellow Fever Virus

TBEV = Tick Borne Encephalitis Virus (= FSME Virus)