Method Field of the Invention

The invention relates to a method for identifying a modulator of a ce|l Signalling pathway* jri particular a method for identifying a modulator of a cell signalling pathway that triggers differentiation in cells. Modulators may be identified by exposing progenitor cells to candidate modulators. Ih one aspect, the progenitor cells are multipoint or unipotent stern cells which are isolated at various stages of differentiation, and further cultured together with a potential modulator of the differentiation process that acts by promoting or inhibiting the effett of cell signalling pathways oh differentiation.

Background to the Invention

The field of regenerative medicine holds the realistic prc-mise of regenerating damaged tissues and organs in vivo, in patients with conditions sucfi as cardiovascular disease, neurodegenerative disease, musculoskeletal disease, |iver disease, br diabetes. Techniques for regeneration of darnaged tissue involve either {he repair of existing diseased tissue in vivo using regenerative drugs or by replacement of such tissue using cells first prepared in vitro with or withøiit the use of regenerative drug's and then transplanted in vivo.

|n either case, the goals of regenerative medicine can only be realised if specific genes, factors or modulators controlling cell signalling pathways a,nd the downstream cellular processes they regulate can be identified. It is thought ihat the controlled differentiation of stem cells in vitro for example, may provide a source of replacement dells for transplantation. The pluripotency and plasticity of stem cells allows them to be committed to- a particular cell type following- treatment certain cLjIture. conditipηs. However such an approach relies on the prior identification of factors or modulators that control the cellular and molecular events of (mepge differentiation. Use of these factors or modulators on stem cells ex vivo could reduce thp likelihood of ^' spontaneous differentiation of stem cells into divergent lineages upon ⁽ . transplantation, as well as reduce the risk of teratoma formation in t(ie case of embryonic stern cells.

Such factors or modulators could also form the basis of therapies that aim td mobilise endogenous stem cells in vivo, or to trigger their differentiation into a cej| type that

can amplify, repair, restore, replace or otherwise benefit a damaged tissue'. An example df a factor that affects cell differentiation and is Used in thprapy is erythropoietin (EPO). EPO is a naturally occurring protein factor that promotes the differentiation of haematopoietic precursors into erythrocytes. Recombinant EPO is use,d to treat anaemia and has a global market of approximately US$10 billion.

In addition to naturally occurring molecules or factors, it is possiblβ to affect differentiation of cells using synthetic modulators of signalling pathways, in particular rhodulatόrs of pathways controlling differentiation. SB-49711J5 is a small molecule drug being developed by GSK that mimics the activity of thrombopoieftn (TPO), a protein factor that promotes growth and production of blodd pjgtelets. TfJe drug could bβ used to treat thrombocytopenia: the inability to produce platelets, which are critically required as components of the clotting process during bleeding. It is estimated that the market opportunity is approximately USf 4-5 billion.

Though regenerative drugs such as EPO or SB-497115 act on stem cells, a general method for the discovery of regenerative drugs using stern cells, iri particular embryonic or fpetal stem cells, has not been proposed.

Naive attempts at using stem cells in drug discovery have involved experiments in which pluripotent embryonic stem cells, self-renewing adult stem cells or cell lines have been used in cell-based phenόtypic &nd pathway-specific screens of natural products or synthetic compounds to discover agents capable of affecting differentiation of these cells (see review by Ding & Shultz (2004) Nature Biotechnology 22: 833-840). One of the reasons for using these bells js that they can be easily amplified to yield the quantities required for a screen. However the approaches in the prior art, in which self-renewing, undifferentiated sφm cells or cell lines are used, are found lacking as. drug discovery methodologies for a numbe'r of reasons discussed below.

Firstly, the cell types used in the prior art (particularly ES cells and cell lines) may not be physiologically relevant targets suitable for pharmacological intervention in vivo. For example, while a factor which is able to cause differentiation of ES cells ^' to e.g. cardiomyocytes (Wu et a!., J Am Chem Soc. 2004:126(6): 159Q-1) mdy be of use in allowing the creation of cardiomyocytes in vitro from ES cells, the ES cell is not present in the adult and any agent which has activity specifically on the ES cell would likely not act as a regenerative medicine in vivo.

Secondly the cell types used in the prior art (particularly ES cells and self-renewing adult stem ¹ cells) lie too far upstream of the target dell lineage in the developmental pathway to undergo directed differentiation to that lineage in response to a single application of a single agent. For instance it is known th^t timed application of numerous factor cocktails in series are required to differehtiate ES cells into specific lineages, particularly those which are specified relatively late in development as a result of a relatively complicated process of tissue specification.

Thirdly, the cell types used in the prior art (particularly primary adult stem cells) may be difficult to obtain in sufficient quantities to carry out large sea e rjigh-throughput drug screening.

Finally, fhe cell types used in the prior art (particularly primary adult stem cells) may exhibit highly variable effects in response to drugs, depending on the source or the method of isolation and preparation.

WO2004/031369 describes methods and cell signalling pathways which pδrmit differentiation of cells - such as stem cells. In the technique described in W020047031369 cells are cultured under multiple culture steps under a plurality of conditions to modulate cellular pathways and the method provides a rrteahs of determining the effect of diverse multiple culture step regimes on cellular processes such as differentiation.

Existing approaches to discovering regenerative drugs are suboptjmal and there exists a need for improved methods to discover such drugs. The present invention offers solutions to these problems and provides improved methods for the discovery of drugs - such as regenerative drugs.

Summary of the Invention

The present invention is based, at least in part, on the finding jhat it Is possible to arrest stem cells in a differentiation pathway thus isolating cel|s of another type suitable for screening. In one embodiment, this is achieved by obtaining cells of a first type and determining a differentiation protocol which løads to the appearance of a given target phenotype of interest via a progenitor bell, and optionally further modifying this protocol, typically by varying the cell culturing media, such that the

differentiation process is stalled at a stage in which cells of another type, preferably progenitor cells, are present. The identity of the cells of the second type eg. progenitor cells need not be known but their existence in the preparation can be inferred frorn the fact that the phenotype of interest will appear i^ trie original differentiation protocol is followed to completion) Thus, in one embodiment, the methods described herein can be used to produce developmental progenitors, of ce|l types whose progenitors in vivo are not yet known.

One Way of prpducing such cells is to use the method of WO2004/031369 in order to discover a series of culture steps leading to differentiation and subsequently modifying or truhcating the process leading to the isolation of progenitor cells.. The ihventioη' recognises that by sequential exposure to selected agents, cells may be isolated in a partially differentiated state which is substantially identical to the progenitor pool in vivo, and then screened for factors that induce further differentiation.

The present invention also recognises that regenerative drug discovery screening assays are more likely to identify effective drug pandidates if a physiologically relevant progenitor cell is used in a cell based screen of natural products or synthetic compourids. The physiologically relevant progenitor cells have less developmental potential compared to self-renewing embryonic pr adult Stem cβlls and lie downstream of these multipotent cells in the developmental pathway. For example, the, target cell type for EPO in the clinical treatment of anaemia is not the embryonjc stem cell, nor the self-renewing adult haematopoietic; stem cell, but rather a more committed erythrocyte progenitor cell which lies furthef along the developmental pathway and whose developmental potential is far more restricted (Fisher, Exp Biol Med 20C)3 Jan;228(1):1-14). If this progenitor population werp available! for use in a drug discovery screening assay, the effect of EPO on regpnerating erythrocytes would be readily apparent. On the other hand, this regenerative effect of EPJD js not as readily apparent if undifferentiated ES cells are used in a screen, even though these ceils can be differentiated to erythrocytes using suitable protocols. Tfierefqre k physiologically relevant progenitor cell, rather than a multipotent self-refiewϊng stem dell, should be used in a drug screen to identify regenerative drujgs.

However an important problern of using progenitor populations is that they are φxtremely difficult to source, they have limited amplification capacity and in some instances they are completely uncharacteriβed. For instance, certain neural

progenitors are well kηown but reside in the living bφin and kre thus difficult to source for drug screening assays. Furthermore, the liver is an organ capable of rapid regeneration in vitro, and live!r biopsies are relatively easier to source compared to brain biopsies, but the progenitor cell population responsible for liver regeneration has not yet been conclusively identified and therefore cannot b,e isolated, cultured and used in drug screening assays.

The present invention also recognises that physiologically relevant progenitor cells suitable for regenerative drug discovery screening assays cah be derived from self- renewing embryonic dr adult stem cells, if these cells can be made to differentiate up to, but no further than, the relevant progenitor stage. There is therefore a need ψ the art for improved techpiques to isolate partially differentiated cells in order subsequently to screen for factors that could be used to modulate their djffererjtiation.

The present invention involves methods of isolating partially differentiated cell types which comprise physiologically relevant progenitor cells. Ir'i one, embodiment, this is achieved b,y obtaining stem cells and subjecting them to the techniques described in in WO2004/031369 to dispover a differentiation protocol, and further niodifying this pVotocόl such' that the differentiation process is stalled at, or riot progressecj past, a stage in w^ich target prpgenitor cells are present. Ih another embodirήlept, it involves isolating them from the adult stem cell pool, developing foetus or animal irj various staqes o'f development and optionally modifying them such thiat they can be arήplified ip vitro, lη addition, these prόgeηitor cells may be used in assays in which natural products and candidate small molecule modulators of cell signaling are screened to identify agents which affect cell signalling in ^" the progenitor cells, causing, for example, mobilization, amplification or differentiation, and which can then be developed into regenerative medicines

Aspects and embodiments of the present invention

In a first aspect, there is provided a method for identifying a potential modulator of a cell signalling pathway, comprising the steps of: (a) providing a cell of a first cell type, Wherein said first cell type may be differentiated to a second cdll type via a progenitor ce|l by sequentially exposing said first cell type to one or niore, preferably two or more reaction conditions; (b) adding to or replacing at least one of said two or more reaction conditions to which the progenitor cell has been exposed with exposure fo orte or more different reaction conditions compirising s&id, pptpritiail

modulator; and (c) monitoring the differentiation of the first cell type to deterrηine formation of the second cell type.

In a further aspect, there is provided a method for identifying a potential modulator of a cell signalling pathway, comprising the steps of; (a) providing a cell of $ firsf cell type, wherein said first cell type may be differentiated to a sepond ceil type via a progenitor cell by sequentially exposing said first cell tyRe to two or more reaction conditions; (b) adding to or replacing at least one qf said two; or more reaction conditions tCr which the progenitor cell has been exposed with exposure to one or more different- reaction conditions comprising said potential modulatpr; and (c) monitoring the- differentiation of the first cell type to determine formation of the Second cell type, wherein differentiation of the cells to the second cell type is indicative that said potential modulator modulates the cell differentiation pathway.

The invention also recognises that by derivation from an organis|(τi in an eWly state of development, cells may be isolated in a partially differentiated state and optionally dmpjified, optionally further differentiated, and ihen screened for modulators that induce differentiation to a target cell lineage or phenotypei

Accordingly) in one embodiment, the first cell type is obtained or obtainable from an embryo of foetus and optionally modified to allow amplification.

In one embodiment, the progenitor cell is ¹ derived [n ψitro from a first cell type by exposure (eg. sequential exposure) to one or more (eg. two or more) reaction conditions.

In another embodiment, the cells are monitored "for cell death instead of cell differentiation. Accordingly, the screen may be a toxicity screen.

In one embodiment, the reaction conditions comprise a screen of potential mqdulators (e.g. a screen of at least 100 potential modulators, at least Ip'ob potential modulators, orgt least 10,000 potential modulators).

In another embodiment, the first cell type is a self-renewing stem cell.

The invention employs cell units. Such units may be single cells, but are advantageously colonies of two or more cells, which are arranged in such a form that

they are resistant to disruption even during split pool procedures. For instance, the cells may be cultured on a solid substrate, such, as beads, as described in more detail bfelow. In the present invention cell units can be isolated at any stage of the differentiation process triggered by the sequential abditibn of pgents intp the culture medium. Accordingly, there is provided a method for determining the effect of a plurality of culture conditions on a cell as described above, whereiη cell units that are partially differentiated are then isolated and used in the rnethod described for identifying the effect of modulatorsxin cell signalling pathways.

Typically the cell units used are microearriers which are. small enough to be transferred to a HTS screehing system in liquid phase and without substantial disruption of the cell unit. This system greatly facilitates the production of quantities of differentiated cells for screening, and also the set up of the HTS as^y- In particular it also allows the use of cells for HTS without any prior disruption of the cells, such as by proteolytic digestion to allow transfer of cells from one vessel to another, which is advantageous since such processus may affect trie immediate differentiation status or downstream differentiation ability of cells. Furthermore it allows the automated transfer of cells using robotic systems.

Thus an impprtant advantage of using cell units - for example cell uηifs, grown on microcariiers -, to prepare the cellulaf material prior .to screening is that it preserves the cellular niche that has arisen iη the preparation process a|nd which fnay be important for the downstream differentiation screen. If the cellϋlaf material were prepared in the conventional way and disrupted for dispensing into we; lls, theh the niche would be disrupted. If on the other hand the cellular material were prepared for the screen in separate wells, then the resulting- well-to'-well variation in the preparation would make drug screening difficult to interpret

Advantageously, in certain applications the cell units rrjay be labelled. Labelling allows the following of the culture ponditions to which the cells have been exposed; thus, any given cell unit can have its label read in order to determine how it has ¹ been derived from the starter cell pool or culture. Labelling rnay taktø kny c}f a variety qf forms, including nucleic acid labels, radiofrequertcy encoded tags, microsphere tags, barcoded tags and spatial encoding of cell units on a surface or ¹ matrix.

The label may be selected from the group consisting of a virus, an oligonucleotide, a peptide, a fluorescent compound, a secondary amine, a halpcarbon, a mixture of

stable isotopes, a bar ςode, an optical tag, a bead, a quanturη dot and a radiofrequency encoding tag. Two or more labels may everi be selected |rom this tjroup and used in combination to label a cell unit, for instance a bead cprηprising fluorescent compounds and/or quantum dots. Labelling and specific labels to be used with cell units are further discussed in our co-pencjing application GβO15173|32;β incorporated herein by reference.

Cells may be cultured jn cell units, each cell unit comprising one or mjore cells. |n another embodiment, the dell units are single cells. The cell unit may comprise one or more cells adherent to or bounded by a solid substrate. In a further pmbodiment, the solid substrate is a microcarrier or bead. In a still further embodiment {he solid substrate is a well or medium-permeable barrier. The culture conditions may be media to which the cell is exposed. The media may contain one or more specific agents, which influence a cellular process.

In one embodiment, the reaction conditions include any physical or chemical mødium in which cells are isolated and manipulated but suitably the reaction condition is a culture condition to which cells are exposed. Culture conditipns include growth rnedia, temperature regimes, substrates, atmospheric conditions, physical cell handling arid the like. Growth media comprise natural and synthetic substances that nourish and affect the cells including but not limited to basal media, growth factors, nutrients, buffers, chemicals, drugs and the like. The reaction conditions may even comprise a screen of potential modulators of a cell signalling pathway.

The invention may be usedio monitor differentiation of a first cell type at any-stgge of development but the inventors recognise that due to their relative ease of culture, pluripotency and therapeutic- potential, stem cells mpy bje particularly- suitabj§ as a first cejl type. Thus, in one- Embodiment, thp invention provides a method- as described above wherein the first cell type is a cell which has been arrested alphg a differentiation pathway between a stem cell and a differentiated cell typ!e.

lri one embodiment, the cell type is a primary cell, cell line; ør turtiqur derived cell line. Tthe tissuβ of origin of the cell type may be selected frbm a group consisting of brain, heart, liver, lung, hair, eye, gut, blood, ear, kidney, skiη, todt|i, pancreas, rηuscle, borle and vasculature.

In another embodiment, there is provided for use Qf a partially differentiated, or progenitor, cell type. The said cell type may be isolated frpm an organism or produced from pluripotent cells using a method of determining the effect pf cμlture conditions on differentiation; and further subjected to a jnethod of identifying the effect of modulators on cell signalling pathways affecting differentiation. The method of identifying the effect of modulators on cell signalling pathways^ Effecting differentiation may involve screening potential modulators using the drug cjiscovery techniques commonly employed by pharmaceutical companies (Reinventing Drug Discovery, Executive Briefing, Accenture, 1997; High Performance Drug Discovery, Executive Briefing, Accenture, 2001).

In another embodiment, there is provided a method for exposing progenitor cells or partially differentiated cells to a potential modulator and then monitoring the effe'ct Of the modulator on the process of differentiation. In one embodiment, the potential modulator is an inhibitor of a cell signalling pathway. In anoiher embodjrrient, the potential modulator is a promoter of a cell signalling pathway. The effect of a modulator to promote or inhibit cell signalling pathways affecting differentiation, may be assessed by a suitable assay including but not limited to monitoring phenotype, reporter gerie expression, genotype, molecule production, viability, metabolic changes or the proliferative ability of cells.

The invention provides a method for obtaining progenitor cells or partly differeniiated cells from tissues in developing embryos and foetuses, or inde'ecj adults. As tissues develop through the foetal and adult stages, they develop stem cells, which are progressively restricted in developmental capacity, ultimately becoming adult stem cells. Thus at any point in development or an organism, progenitor ¹ cells may be excised from tissues, for instance from the foetus of am animaj or a jwman foefus pbtained immediately following an elective abortion. For instance, cells which comprise the precursors to dopamine-producing cells may be isolated from spe'cific regions of the developing central nervous system of the foetus. Since cells derived from foetal material are scarce, it may be necessary to ampljfy these pells. In this case it is possible to do this without affecting their differentiation state by transforming these cells, for instance using an όhcogene such as l-myc. Suitably, the transformaiion is reversible and does not lead to differentiation qf the progenitor cell. The invention recognises that foetal or adult stern, cell rriaterial may require' further differentiatipn in order to produce progenitor cells, and this can be achieved by the method disclpsed below fqr other stem cells, such as ES; cellsi

The invention also prpvides a method for obtaining progenitor cells dr partly differentiated pells from pluπpotent cell lines, including but not limited to lines of embryonic stem ceNs, by determining a differentiation protocol and performing this in part. In this case the resulting cell population will comprise one or more progenitor cells: it is not necessary to know which proportion of the cells in the population are progenitor cells, nor to be able to identify these, however their presence may be inferred from the fact that fully differentiated cells would, arise from these if the said differentiation prptocol is donpluded (instead of being arrested).

Differentiation protocols typically involve subjecting the cells to a temporally specified series of apprppiiiate culture! conditions. Cells giving rise; to progenitpr cells may be induced to differentiate along a desired developmental pathway using ¹ this method of βerial cell culture. Cells may be arrested at any stage of that differentiation process, thus obtaining progenitor cells, by interrupting or modifying the series of appropriate culture conditions. For instance, if a ten day differentiation protocol cpmprisihg a series of five qell culture steps is required to differentiate ES ce'lls to macrophages, theh fully performing only three of the steps in this series wil| resijlt in the partial differentiation of the ES cells along that lineage and will allow isolation bf macrophage progenitor cells.

The method described for identifying a plurality of cultμre conditions allows thousands or miflions of ceH culture conditions and reagents to be tested, in a multiplexed high-throughput assay, to determine the conditions necessary to achieve the differentiation of cells.

In another aspect of the invention, there is provided a method for Identifying modulators of cell signalling as previously described, wherein a- first .cell is differentiated to a second cell type by modulating cell signalling and/or the expression of one or more genes in the cell.

Cell signalling and/or the expression of the genes in the cell can be mocjulated by, for example, addition of biomolecules such as factors, grPwth fgctors, morphogens, hormones, receptor agonists and antagonists, lipids, antibodies, drugs and tfie like; or by addition of synthetic drugs, chemicals, small molecules and the like. Suitably, the above mpdulators are added ih combinations, such as from a cell extract, from a fco-culture, In animal serum, or a cocktail prepared in vitro.

Cell signalling artd/or the expression of a gene can also be modulated by traήsfeςting or otherwise transferring a geηe into the cell such th^t jt is expressed or over- expressed in a transiehtj ligand-induced or permanent manner. Alternatively, the expression of the endogenous gene may be altered, έuch as by targeted enhancer insertion or the administration of exogenous agents which cause an increase or {decrease in expression of the gene. Moreover, the product of fhe gene may itself be administered to the cell, or its activity eliminated from the cell, to achieve the sεfme result. Modulators capable of decreasing the expression of a gene include interfering RNA or antisense compounds, while modulators capable pf decreasing the activity of a proteih include ¹ drugs, antibodies, aptamers and the like.

In one aspect the differentiation of the cell is monitored by observing the phenotype of the cell or detecting the modulation of expression of one or rhore genes jn g cell, thereby determining the state of differentiation of said cell. Phenotype determinatiop can be carried but by a variety of techniques, for instance by visual inspection of the cell units under a microscope, or using high content screening and analysis instrumentation (see Cellomics Inc; www.cellomics.com). Alternatively differentiation can be detected by observing a marker product characteristic of the differentiate^ cell. This may be an endogenous marker such as a particular DNA or RNA sequence, or a cell protein which can be detected by a ligand, conversion df an enzyme substrate, or antibody that recognises a particular p'fφnotypic marker. A differentiation marker may also be exogenous, i.e. one that hψ bpen rhtroduced-into the cell population, for example by transfection or viral transduction. Example's of exogenous markers are the fluorescent proteins ^" (e.cj. GFP) br cell surface antigens whjch a're not normally expressed in a particular cell lineage or which are ep|tόpe- modified; or from a different species. A trdnsgene or exogenous marker g^ne with associated transcriptional control elements can be expres$ed in a manner that reflects a pattern representative of an endogenous gene(s) indicative of phenotype or differentiation state. This can be achieved by associating the gene with a cell type- specific promoter, or by integrating the transgene into a particular locuδ (e.g. sβe European patent No. EP 0695351). The markers indicative of differentiatiqh m^y be detected by a variety of techniques, both manual and automated, including observation under a microscope, affinity purification ('panning'), or by fluorescence activated cell sorting (FACS). Accordingly, the present invention provides a method of moniWing differentiation wherein the modulation of expression of one or more

reporter gehes is observed wherein the reporter gene(s) respond(s) to one or more differentiation states of said cell.

In. a further embodiment, the expression of genes involved is monitored on a^ gene chip. Gerie expression may conveniently be analysed using a genfej chip qr array technology, which is widely available from suppliers such a ^* s Affymetrix.

Advantageously, the genes employed in this analysis, encode extracellular markers, which may be detected for Instance by immunoassays

In another embodiment of the invention the differehfiation of a cell is monjtored by joss of proliferative ability.

The invention moreover provides methods of culturing stem ceils, and differentiated cells derived from stem cells in vitro, adherent to microcaYriers, such as beads, /vlicrocarrier culture has significant advantages, including the scale-up of ςultdres, and also allows units of stem cells to be exposed tq selected culture conditions as required in order to obtain the desired growth and/or differentiation coπldijtioηs.

The potential modulator may comprise an organic or inorganic small molecule, a rjatural or derjvatised carbohydrate, protein, polypeptide, peptide, glycoprotein, nucleic acid, DNA, RNA, oligonucleotide or protein-nucleic add (PNA). In another embodiment, the potential modulator is obtained froήi a ¹ library of small molecules with drug like properties,

In a. further aspect, there is- provided a modulator of a cell signalling pathway obtained or obtainable by the methods described herein.

In another aspect there is provided a pharmaceutical composition comprising the modulator together with a pharmaceutically acceptable carrier; diluent or expient.

In a further aspect, there is provided a partially differentiated cell, which has been differentiated in vitro from a stem cell and arrested alorig a differentiation pathway between a stem cell and a differentiated cell type. The partially differentiated cell may be a neuronal or haematopoietic cell. The partially differentiated cell may be a bipotent cell. The partially differentiated cell may be a unipotent cell.

In a further aspect, there is provided a method for identifying a modulator of a cell signalling pathway (eg. a regenerative drug) comprising the use of a progenitor cell.

In a further aspect, there is provided a method for identifying a modulator Of & cell signalling pathway (eg. a regenerative drug)- comprising the use of a partially differentiated cell, which has been differentiated in vitro from a stein cell and arrested along a differentiation pathway between a stem cell and a differentiated cell type

In a further aspect, there is provided \he use of a progenitor ςell Hi a drug screening assay to jdentify-a modulator of β cell signalling pathway (eg. a regenerative drug).

Ih a ¹ further aspect, there is provided the use of a partially differentiated ce,li, vytficH has been differentiated in vitro from a stem cell and arrested along a differentiation pathway between a stem cejl and a ¹ differentiated cell typ!e in a drug screening assay to identify a rtiodulator of a cell signalling pathway (eg. a regenerative drug).

n a further aspect, there is provided a method for differentiating ύn embryonic stem cell into a progenitor of the myeloid lineage, Comprising the use of a gelatin rnicrocarrier (eg. a CultiSpher microcarrier).

In a further aspect, there is provided the use of a gelatin microcarrier (eg. a CultiSpher mtcrocarrier) for differentiating embryonic stem cells into haematopoietic progenitors.

In a further aspect, there is provided a method for producing a haematopoietic cell ffom a stem cell in vitro comprising exposing said stem cell to one or mpre, preferably, two or more, reaction conditions, wherein sajd reaction copdjtions comprise incubating said stem cell with: (a) retinoie acid, tøimethylsulpj-ibxide (DMSO) and/or stem cell factor (SCF); and (b) insμlin, stem cell factor (SCF ¹ J, TGF Bejta 1, PMP2, BMP4 and/or TPO; and (c) IL-3, IL-6, TPO, EPO aηd/or M-CSF.

In one embodiment, the sterri cell is seeded on a microcarrier - such as a cjelatin microcarrier.

In One embodiment, the stem cell is contained in an IMpM basal medium or a Streamline Haematopoietic Expansion Mediμm.

Ih one embodiment, in step (b) insulin alone is used.

In one embodiment, in step (b) SCF, JGF beta 1, BMt ³ J-* and TF ¹ O is used.

Ih one embodiment, in step (c) 1L-3 and IL-6 are used. In another embodiment, TPO, EPO and/or M-GSF are a|sp used.

In one embodiment, the step (a) is performed on day 1.

Ih dpe embodiment, the step (b) is performed on day 4.

In one embodiment, the step (c) is performed on day 6.

Detailed Description of the Invention

Definitions

Culture Cqnditions As used herein, the term "culture conditions" refers to the environment which cells are placed in or are exposed to in order to prtimote growth or differentiation of said cells. Thus, the term refers to the raediuhn, teφp^rature, atmospheric conditions, substrate, stirring conditions and the like which may affect the growth arid/ or differentiation of cells. More particularly, the term refers to specific agents which may be incorporated into culture^ media and which may influence the grpwth arid/or differentiation of cells.

Cell A cell, as referred- to herein, is defined as the smallest Structural unjt pf an organism that is capable of independent functioning, or a single-celled organism, consisting of pne or more nuclei, cytoplasm, and various organelles, all surrounded by a semipermeable cell membrane or cell wall. The cell may be prokaryotic, eukaryotip or archaebacterial. For example, the cell rhay be a eμkaryotic, cell. Mammalian cells are suitable, especially human cells, CeIIs ₁ may be; natural or modified, such as by genetic manipulation or passaging jn culture, to achieve desired properties. A stem cell is defined in more detail bølotø, and is a totipotent, pluripoterij: or multipotent cell capable of giving rise to more than one differentiate'd cell type. Stem cells may be differentiated in vitro to give rise t6 differentiated cells, which may themselves be multipotent, or may be terminally differentiated. ¹ CeJIs

differentiatecl in vitro are cells which have been created artificially by exposing stem cells to one or rhore agents which promote cell differentiation.

Fjrst cell type In one embodiment, the first cell type is a cell that retains ffie ability to renew itself through cell division and can differerittøte into a wide range of specialized cell types. In another embodiment, the first cell type is a cell that is less differentiated than a progenitor cell. In another embodi|nent, the first qell type is a stem cell, as described herein helow. The stem cell may be, for example, an embryonic stem cell or an adult stem cell. The cells of the first type rriay be differentiated into a certain lineage of a second cell type before the cells øf the second cell type are screened. Thus, in one embodiment, the firs'f ce|| type is differentiated to a second cell type by exposing (eg. sequentially exposing) the first cell type to two or more (eg. three or more, four or more, or five, or mpre) reaction conditions.

Second cell iype In one embodiment, the second cell type is cøll that can only differentiate, but it cannot renew itself anymore. The second cell type rfiay be more limited in the kinds of cells it can become than the first cell type. The second cell may be more differentiated than the first cell type. The second cell may be rhore differentiated than the progenitor cell. In one embodiment, the second cell type is a partially differentiated cell, which has been differentiated in vitro from the first cell $p£ and arrested along a differentiation pathway between a cψ of the first type (eg. a stem cell) and a differentiated cell type. In another embodiment, the second tell type is a c'ell with a target cell phenotype.

Regenerative medicine or drug The terrη 'regenerative drug' refers to a natural dr Synthetic substance which- acts on a- stem cell or progenitor cell and is ihfJs able to regenerate or repair a tissue or organ of the body. 'Regenerative medicine' refers to the samJ3, or to the discipline of regenerating tissues or organs as μ medical freatmerit, Regenerative medicine encompasses cell replacement therapies, and/or the administration of regenerative drugs to patients.

Progenitor A 'progenitor' or 'progenitor cell' is a cell type whiψ lies upstre'am of a more differentiated cell, but downstream of a true stem ce'll. Progenito'r's are not typically capable of long term self-renewal as are true stem cells, ahd their developmental potential is more limited than is that of stem cells. For instance CFU-E and BFU-E are erythrocyte-committed progenitor dell populations, whereas the LT-

HSC is a self^newing and multipotent haematopoietic stem cell and the ES cell is a self-renewing arid pluripOtent stem cell. Differentiated erythrocytes can be derived from CFU-E which in turn can be derived from LT-HSC which in turn cah b'e derived from ES cells.

Cell signalling The term "cell signalling" refers to the, molecular mechanisms whereby Ce ₁ IIs detect and respond to external stimuli and send messages to other cells. Cell signalling therefore includes -transcriptional and translational controls and mechanisms as well as signal transduction mechanisms,.

Modulator The term "modulator" refers to any factor that can vary the state of a cell, changing it from one state to another. In the context of the invention thus, refers to modulation of cell signalling processes. Modulators may irihibijt or promote particular cell signalling pathways. They may take the form of natural products or ¹ chemically synthesised molecules; for example, an organic or inorganic small molecule, a natural or derivatised carbohydrate, protein, polypeptide, peptide, glyqoprdtein, nucleic acid, DMA, RNA, oligonucleotide or protein-nucleic acid (PNA). Modulators also Include agonists or antagonists. Modulators that are inhibitors include bμt are not limited to: nonspecific, irreversible, reversible - competitive and noncompetitive inhibitors. Modulators that promote cell signalling stimulate or enhance the effect of a particular mplecular pathway on the cell and include but are hdt limited t|D: agonists, agonist mimetics, co-factors, promoters and the like.

Amplification "amplification" refers to a process by which an increase in magnitude or number of cells, cellular components or cellular process s occurs: In particular amplification refers to a process of increasing cell numbers in a cell culture system, this may occur by increasing the rate of proliferation .or survival of cells in the system.

Compound The term "compound" is used herein in accordance with the meaning normally assigned thereto in the art. The term compound is us'ed in its broadest sense i.e. a substance comprising two or more elements in fi>|ced proportions, including molecules and supramolecular complexes. This definition includes s'mall molecules (typically <500 Daltons) which make up the majority of pharmaceuticals. However, the definition also includes larger molecules, including polymers, for example polypeptides, nucleic acids and carbohydrates, and supramolecular complexes thereof.

High-throughput screening The term "high- throughput sbree!ning refers to the large-scale, trial-and-error evaluation of compounds in a parallel target-basecj or fcell- based assay.

Qompound library A "compound library" is a group of diverse compounds that cari be used to identify new lead candidates in the drug discovery process. Compound libraries may be generated by any means known in the art, including cjarnbinatorial chemistry, compound evolution, or purchased from commercial sources such as Sigma Aldrich, Discovery Partners International, Maybridge and Tripos. A repertoire advantageously comprises at least 10 ² , 10 ³ , 10 ⁴ , 10 ⁵ , 10 ⁶ , 10 ⁷ , ]θ ⁸ , 10 ⁹ _? 10 ¹⁰ , 10 ¹¹ or more different compounds, which may be related or Unrelated in structure or function.

Modulation The term modulation is used to signify an increase and/or decrease in the parameter being modulated. Thus, modulation of gene expression includes both increasing gene expression and decreasing gene expression.

Cellular prpcess A cellular process is any characteristic, function, process, event, cause or effect, intracellular or extracellular, which occurs or is observed or which can be attributed to a cøll. Examples of cellular processes include,, but are not limited tp, viability, senescence, death, pluripotency, morphology, signalling, binding, recognition, molecule production or destruction (degradation), mutation, protein folding, transcription, translation, catalysis, synaptic transmission _! vesicular transport, organelle function, cell cycle, metabolism, proliferation, division, differentiation, pheηotype', genotype, gene expression, or the control of these processes.

CeIi ^" unit A group of cells, which may be a group of oηe. Pools of cell units may be sorted, subdivided and handled, without substantially dissociating the; cell units themselves, such) that the cell unit behaves as a colony of cells and each cell iη the cell unit ip exposed to the same culture conditions. For example, a peli unit may comprise a bead to which Is adhered a group of cells.

Totipoteht A totipotent cell is a cell with the potential to differentiate into any type of somatic or germ cell found in the organism. Thus, apy desired ce,ll rrφy bύ derived, by gome means, from a totipotent cell.

Pluripotent or Multipotent A pluripotent or multipotent cell is a cell which may differentiate into more than one, but probably not all, cell types.

Label A label or tag, as used herein, is a means to identify a cell unit and/or determine a culture condition, or a sequence of culture conditions, tc> which the cell unit has been exposed. Thus, a label may be a group of labels, each adided at a specific cultύring step; or a label added at the beginning or the experirrjjen't Which is modified according to, or tracked during, the culturing steps to which the cell unit is exposed; or simply a positional reference, which allows the culturihg steps used tb bfe deduced. A label or tag may also be a device that reports pr records the location or fhe identity αf a cell unit at any one time, or assigns a unique identifier to thje eel) unit. Examples of labejs or tags are molecules of unique sequence, structure or mas|s; or fluqrescent rηolecules or objects such as beads; or radiojrequehcy and other traηsponders; or objects with unique markings or colours or sh^pes^

Exposure to culture conditions A cell is exposed to culture conditions when tt is placed in contact with a medium, or grown under conditions which affecit one or more cellular processes) such as the growth, differentiation, or metabolic state of the cell. Thus, if the culture conditions comprise culturing the cell in a medium, the cell is placed in the medium for a sufficient period of time for it to have an effect. Likeyvise, if the conditions are temperature conditions, the cells are cultured at the desired temperature.

Pooling The pooling of one or more groups of cell units involves the admixture of the groups to create a single group or pool which comprises ^" cell units of fnore than One background) that is, that have been exposed to more than one different sets of culture-conditions. A pool may be subdivided further irito groups.) either randomly or non-raήdomly; such groups are not themselves "pools" for the present purposes, but rnεiy themselves be pooled by combination, for example after exposμre to different sets of culture conditions.

Proliferation Cell growth and cell proliferation are Used interchangeably herein to denote multiplication of cell numbers without differentiation into different cell types or lineages. In other words, the terms denote increase df viable cell numbers. In one embodiment, proliferation is not accompanied by appreciable! changes in phenόtype or genotype.

Differentiation Cell differentiation is the development, from a cell type, ft a different cell type ¹ . For example, a bipotent, pluripotent or totipotent cell may differentiate into a neural cell. Differentiation may be accompanied by proliferation, or may be independent thereof. The term 'differentiation' generally refers to the acquisitipn of a phenotype of a mature celt type from a tess developmentally dόffned cell typie, e _f g. a neuron, or a lymphocyte, but does not preclude transdϊfferentiation, whereby one mature cell type may convert to another mature cell type e;,g. a neurbri to a lymphocyte. Ih this application 'differentiation' will b'e taken to mean 'de^- differentiation' and vice versa. Commonly, 'de-differentiatidn' refers to the acquisition Of a phenotype of a less mature cell type from a more develpprnentally cjefihed cell type,, e.g. a myocyte becoming a myogenic precursor, pe-differerψation may be followed by further differentiation to revert to the original (Je ₁ II type, or to a further ce'JI type [Ding & Shultz (2004) Nature Biotechnology 22: 83(^-840, and references therein].

Differentiation state The differentiation state of a pell is the level to which a cell has differentiated along a particular pathway or lineage.

State of a cellular process The state of a cellular process refers to whether a cellular process is occurring or not and in complex cellular processes, bah denote a particular step or stage in that cellular process. For example, ia cellular differentiation pathway in a cell may be inactive or may have been induced and may comprise a number of discrete steps or components such as signalling events characterised- by the preseηce of a characteristic set of enzymes or intermediates.

Gene A gene is_a nucleic acid which encodes a gene product, be it_a polypeptide or an RNA gene product. As used herein, a gene includes at least trje codjng sequence Which encodes, the gene product; it may, optionally, include one or rηore regulatory regions nepessaryϊbrthe transcription and/or translation of the coding sequence.

Gene Product A gehe product is typically a protein encoded by a gene in the conventional manner. However, the term also encompasses non-polypeptide gene products, such as ribonucleic acids, which are encoded by the gene.

Nucleic apid synthesis Nucleic acids may be synthesized according p aηy available technique. In one embodiment, nucleic acid synthesis is automated. Moreover, nucleic acids may be produced by biojogical replication, suclf as by

cloning and Replication in bacterial or eukaryotic celfe, according to procedure's known in th<3 art.

Differential Expression Genes which are expressed at different levels iη response to cell culture ¹ conditions can be identified by gene expression analysis, such as on a gene array, by methods known in the art. Genes whiph are differentially øxpressed display a greater or lesser quantity of mRNA or gene product in the cell under the test conditions than under alternative conditions, relalive-to overall gerje expression levels.

Transfection Genes may be transfected into cells by any appropriate meεjns. The terrh is Used herein to signify conventional transfection, for example Using calcium phosphate, but also to include other techniques for transferring nucleic acjds into a ciell, including transformation, viral transduction, electropόration and the lilje.

Cell-based assays

Cell-based assays are an important part of modern biomedical sciences and comprise any assay that involves a step in which a cell is used. Cell-based assays can be used across nearly all stages of the pharmaceutical drug discovery and development process, and are valuable in providing information about how a compound is likely to interact in a biological systepi, not just dbout how it intferacts. with a potential drug target in isolation.

For example, cell-based assays can be used to- identify and Validate poteritiaj drug targets. Cell-based assays have been developed that ca,n ^~ be used to identify genes or cellular pathways involved in .disease processes, to determine the functions of target genes, or to measure phenotypic changes that may be induced upon activation of certain genes or their products.

Cell-based assays can also be used in drug discovery for lead-ςompόund discovery, selection, and optimization, LJnlike the biochemical assays that are often used in traditional high-throughput-screening assays, cell-based assays can provide information relating to drug properties such as absorption, permeability, selectivity, specificity, and metabolism. As a result, lead compounds that kre selected after cell- based screening are better characterized, more likely to provide, valuable leads and less likely to be eliminated in subsequent phases of the drug discovery process.

A major application of cell-based assays is in toxicity screening. A crucial p'βrt of drug discovery ¹ and development is the screening of driφ candidates to eliminate cbmpourids that will cause side effects. However, current methodologies, are- largely inadequate, and in particular the use of animal rnodels for toxicity screening is expensive and time-consuming. In addition, animal models pan be unreliable, pecause results in these models do not always accurately prjedict how) a compound will perform in humans. Thus human cell-based screening is ^~ suitable.

Screening assays and HTS

Hiqh throughput screens (HTS) can be used in the present invention in order to screen for new drug targets. The emphasis of pharmaceutical research activities has shifted toward the purposeful discovery of novel chemical classes and novel molecular targets. This change in emphasis, and timely technological breaktnrough _, s (e.g molecular biology, laboratory automation, combinatorial chemistry) gave bifth to high throughput screening, or HTS, which is now widespread throughout the biophariηaceutical industry.

High throughput screening involves several steps: creating an pssay that i$ predictive of a particular physiological response; automating fhe assay so that it can be reprpducibly performed a large number of times; and, sequentially "festing samples from a chemical library to identify chemical structures able to "hit ¹ ' the- asjSay, suggesting that such structures might be capable of provoking the intended physiological response. Hits from the high throughput screen are foildwed up in a variety of secondary assays to eliminate artifactual results, particularly toxic compounds. Thus, the assays used jn high throughput screens are ipteπded to detect the presence of chemical samples (e.g. compounds, substances, molecules) possessing specific biological or biochemical properties. These propertiesj are chosen to identify compounds with the potential to elicit a specjfic biological response when applied in vivo. High throughput screens identify both agents thjat can be used as drugs themselves and in addition, drug candidates that will ultimately be used as drugs. A compound bf a certain chemical class that is fdund to have some level of desired Ipiqlogical property in a high-throughput assay can then be the basis for synthesis of derivative compounds. Cell based assays utilise intact cells in culture. Examples of such assay include luciferase reporter gene assays and calciup'i flux assays.

A particularly powerful method of performing a cell-teased assay suitable for the identification of differentiation modulators and regenerative drugs, jas describe^ in the -present inveηtibn, is to determine the differentiatipn status of progenitor cplls by monitoring a genetic marker of differentiation. The marker may be endogenous, such as ah expressed nucleotide sequence or protein that is present specifically in the differentiated cell type but not in its progenitor, nor in any other cell present in the cell population used for screening. The marker may also be-ah exogenous marker (i.e. a reporter gene:) which is introduced intp the cell population used for screening in such a Way that it is expressed specifically in t(ie differentiated cell type bjut not in its progenitor, nor in any other cell present in the cell population used for spreeηing. Examples of exogenous markers are enzymes such as' Lac Z, or non-enzymic markers sUch as the fluofescent proteins (e.g. GFP; YFP etc.'), or cell surface antigens which are not normally expressed in a particular cell lineage or which are epitope-m'odified or from a different species. A tfansgene or exogenous marker gene {with associated transcriptional control elements can be expressed in a manned that reflects a! pattern representative of ah endogenous gerie(s). This can be achieved by ^associating the gene with elements of a cell type-specific promoter, or by integrating the transgene ihto a particular locus which is expressed in a cell-type speqific manner (e.g. see European patent No. EP 0695351).

In qne embodiment, the undifferentiated cell type used as the starting material in the present invention is modified to express two markers/repisrters (e.g. botH GFP and YFP) such that one marker indicates the presence of the progenitor cell type submitted to trje HTS process, and the other marker indicates the presence of the differentiated cell type produced by the addition of- a suitable modulator.

A number of suitable reporter gene systems and cellular screening assays, including dual reporter systems, are disclosed in. reviews (and referenciss therejinj by Hill et al. [Current Opinion in Pharmacology (2001) 1:526-532] and by Blak'e [Current Opinion in Pharmacology (2001) 1:533-539] all of which are incorporated herein in their entirety.

Progenitor cell

A progenitor cell is any somatic cell which has the capacity to ge'rierate fully differentiated, functional progeny by differentiation and proliferation. Progenitor cells

include progenitors from any tissue or organ system, including, but not limited to, blood, nerve, muscle, sKin, gut, bone, kidney, liver, pancreas, thymus, arid the like.

Progenitor cells (nay be distinguished from differentiated cells (Ze. thos'e cells whjch (nay or may not have the capacity to proliferate, i.e., self-feplicate, but which are unable to undergo further differentiation to a different cell type under ηόrmal physiologϊpal conditions). Progenitor cells may be further distinguished from abnormal cells - such as cancer cells, especially leukemia cells, which proliferate (self-replicaie) but which generally do not further differentiate, despite appealing to be immature or undifferentiated.

Progenitor σells include all the cells in a lineage of differentiation and proliferation prior to the most differentiated or the fully mature cell.

Recording to one embodiment of the present invention _} a distinction is drawn Between a 'stem cell' and & 'progenitor ¹ cel|. Stem cells are typically pluripplent and mu|tipoterιt ahd can give rise to a number of lineages. Progenitor pells, eind in particular lineage-committed progenitor cells, are only capable pf producing the' cells of a, single lineage. Hønce the developmental potential of progenitors is typically more restricted than that of stem cells, A second irnportarit difference between stem pelfs and progenitor cells is that stem cells are capable of significant amplification - under tr|e correct culiuring conditions they can divide indefinitely - whereas progenitor cells have a limited proliferation capacity. These differences mean that, for instance; it is pOssible to reconstitute tire haematopoietic system of an animal using, haematopoietic stem cells but not haematopoietic progenitor cells.

By way of example, production of mature, functional red blood cells ¹ results from proliferation and differentiation of "unipotential progenitors," i.e., those progenitors which have the capacity to make only one type of one type of blood cell. Various other hematopoietic progenitors (HPCs) have been characterized.

HPCs consist of many subclasses including pluripotent stem cells, lymphbid stem cells, CFU-GEMM colpny forming unit granulocyte, erythroid, maproprjage qr megakaryocyte, BFU-E, CFU-E, CFU-Meg, CFU-GM colony φrming uni{ granulocyte pr macrophage, CFU-EoS colony forming uniteosinophil, progenitor B cells and progenitor T cells.

Stem Cells

Stem cells are described in detail in Stem Cells: Scientific Progress and Future Research Directions. Department of Health and Human Services. June 2001. http://wvyw.nih. gov/news/sterηcell/spireport.htm. The contents of the report are herein incorporated by reference.

Stem cells are cells that are capable of differentiating to form at least one and sometimes many specialised cell types. The repertoire of the different cells that can be formed from stem cells is thought to be exhaustive; that is to say it includes all the different cell types that make up the organism. Stem cells are present throughout the lifetime of an organism, from the early embryo where they are relatively abundant, to the adult where they are relatively rare. Stem cells present in many tissues of adult animate afe important in normal tissue repair and, homeostasis.

The existence of these cells has raised the possibility fhat they could provide a means of generating specialised functional cells in vitro that can be transplanted into humans and replace dead or non-functioning cells in djs,eased tissues. The list of diseases for which this may provide therapies iheludes Parkinson's disease, diabetes, spinal cord injury, stroke, chronic heart disease, end-ptage kidney disease, |ivdr failure and cancer. In order for cell replacement therapy to becorhe feasible a number of major breakthroughs in stem bell res ^' earcfi are required, including improvemeiits in the growth of stem cells, differentiation of stem cells and av-oidance of immunological rejection of stem cells.

For this reasOri, alternative approachesio exploiting stem celJs for therapy are being considered. As described herein, methods are disclosed for the discovery of cørnpounds which may be developed into drugs (eg. regenerative drugs) that cayse endogenous sfem cells to regenerate tissues of the body,

Types of stem cell

There is, still cbnsiderable debate about what constitutes a stem pell, however for the purposes of this discussion a key characteristic is the ability to differentiate into a different cell type. An optional characteristic is the ability to self-renej/v, in certain cas.es indefinitely, allowing amplification of the cell numbers. Examples of stern cells are given beloW.

Different stem cells have differing potential tp form various cell types: sperrhatogonial stem cells are unipotent as they naturally produce only spermatozoa, whereas haematopoietic stem cells are multipotent, and embryonic stern ¹ cells are thought to be able to give rise to all cell types and are said to be totipotent or pluripotent.

To date three types of mammalian pluripotent stem cell have been isolated. These cells-can, give rise to cell types that are normally derived from all three gerrh laye'rs of the embryo (endoderm, mesoderm and ectoderm). The three types of stem cell are; e,mbryonal carcinoma (EC) cells, derived from testicular tumours; erribrypnlφ stem (ES) cells, -derived from the pre-implantation embryo (normally the blastocyst); and embryonic germ (EG) cells derived from the post-implantation embryo (normally cells of the foetus destined to become part of the gonads). These cells are receiving particular attention in the effort to direct differentiation, precisqly because they are pluripotent

Stem cells are also present in the adult organism. An adult stem cell is an undifferentiated cell that pccurs in a differentiated (specialised) tissue, renews itself, and can differentiate to yield more specialised cfells. Recently it h^s been shown that adijlt stem cells are capable of plasticity, that is to say they can differentiate to yield cell type^s that are not characteristic of the tissue iri which they reside, nor indeed of the germ layer frpm which that tissue originates For example, it has been showri that blood stem cells (derived from mesoderm) can differentiate into neurons, (normally derived from ectoderm). Toriia et a/. (2001 , Nature Ce ₁ // BbI. 3, p778-784) have recently described the identificaiion and isolation of a nέw type of stern icell that was -derived from- the dermis of the skin. These stem- cells were termed skin _r derived precursor (SKP) cells. The! SKPπcells could be induced to differentiate by cuiturihg ¹ on .poly-lysirie and varying the concentration of serum in the culture medium, In the absenpe of serum they differentiate into neurons arid glial cells; with' addition of 3% serum they differentiate into smoothrmusole cells; and increasing the serum to 10 ^d /o causes tHe SKP cells to differentiate into adipocytes. Adult stem cells havfe so far been reported in tissues as diverse as the nervous system, the bone rnarrow and blood, thle liver, skeletal muscle, the skin and digestive system.

In addition to the adult stem cells there are numerous types of progenitor or precursor cells, as described herein. These are cells that an? partially restricted in their differentiative potential and occur in probably all of the tissues of the body - they

are capable of differentiating but differ from ste(n ce|Is in that their repertoire is ritit as broad, and by definition they are not capable of self-renewal.

Recent evidence even suggests that differentiated cell types afe capable of changing phenotype. This phenomenon, -termed traπsdiffereήtiation, is- the conversion of or)e differentiated ceil type to another, with or without an intervening cell divisior). It was previously generally accepted that the terrninal differentiated statjs is fixed, but it is now clear thatdifferentiation ^~ can sometimes be reversed or altered. In vitro protocols are now available in whiόh cell lines can be induced to trahsdifferentiate (se& Shen, Slack & Tosh, 2000, Nature Cell Biol .vol 2, p. 879-887; Horb et al, 12003, Current Biol. VoI 13, p105-115). Finally, there have been reports of specialised cell types that can de-differentiate to yield stem-like cells with the potential to differentiate into further cell types.

Stem cell growth and differentiation

An important property of stem cells is their ability to divide symmetrically in culture, giving rise to two daughter cells t|iat are exact copies of the stem cell frcjm Which they were derived. This allows stem cells to be expanded ir| culture jn their undifferentiated state, producing enough material for screening purposes, biological studies or even cell therapy. The means by which stem cells are able to do jtiis is understandably the subject of intensive research, yetfefy of the factors that promote stem celj renewal are known. Typically, pluripotent stem cell lines are maintained on mitotically inactive feeder layers of fibroblasts, ϋr mediurn conditioned by suc.hr cells. It jg assumed that feeder cells- remove/neutralise some unknown factor from the culture medium, and/or they_provide a factor that suppresses the differentiation, and promotes the self-renewal of stem cells. One such factor is leukaemia, inhibitory factor (LIF), a member of the cytokine family related to IL--6, which is Capable of promoting mouse ES cell self-renewal in the absence bf feeder cells. Stem cells grown iη the absence of feeder cells (and/or LIF) oftefi differentiate spontaneously and haphazardly, producing a mixture of differentiated pe(l types'. Mpre reqently, ES cell lines have been produced that are able to grow in feeder free culture arid uhder defined conditions.

The factor^ that influence stem cell self-renewal may eithel" be stimulatory or inhibitory and may function extracellularly or intracellular^. In the ca$e of the secreted factor LIF, it is known that its extracellular receptor is gp 130, and that

activation of this protein is sufficient for inhibiting murine ES cell differentiation. Within the cell, a crucial downstream effector of gp130 is the signal trahsducer aήd activator of transcription-3 (STAT-3). Another molecule which is particularly important in maintaining stem cell pluripotency is the transcription factor Oct-4, which when downrequlated artificially leads to the loss of the pluripoterit state In ES cells cir mice. Other signalling molecules that naturally inhibit ES cell self-renewal, such as the mitogen-activated protein kinases, have also been elucidated. A rnajof §oal _O f stem cell research will be the discovery of- natural and synthetic factprs, drugs, polypeptides, genes, oligonucleotides, tissue culture media and conditions, Specific Conditioned media, feeder cells, and other stimuli that havd the effect of promotiηg the expansion and retaining the differentiation potential ef various types of stem celL Thjs includes adult stem cells, which at present have ηot beeη expanded appreciably jn cell culture.

The second great challenge of stem cell research is tq direct the differentiation of stem cells to particular cell types which are functional, can replace cells lost in various disease states, and result in a positive clinical outcome. Coaxing stem cells to begin differentiating is actually a fairly straightforward process. For insfance^ ES cells removed from feeder cultures and grown to cohfluønpe! on an adherent substrate will begin to differentiate spontaneously. Similarly, ES cells removed from feeder cultures and grown on a non-adherent substrate will form embrydid, bodies - clusters of μndifferentiated and partially differentiatβd cells from all three germ layers. These dells can be subsequently dissociated and plated, in monolayer culture, and exposed tp factors that promote directed differentiation, pulture^ exposed to. these factors ^" are more likely to be populated byiewer types of differentiated cell, øompared to embryoid bodies or untreated cultures of differentiating cells which Comprise mixtures of many different cell types. Nevertheless, few if any conditions have-been devised thus far that produce substantially pure cultures_of djfferenyøted cells. |n addition it is not clear if dny of the prbtocols devised for stem cell differentiation yield cells th,at pre suitable tor cell replacement therapy - it may be that the cells ftøve ndt terminally differentiated into the precise phenotype required, or that the differentiated cells are no longer viable in vivo.

The factprs that have been Used to induce directed differentiation bf stem cells include; retirioic acid, epidermal growth factor (EGF) ₁ bone morphogenic proteins (BMPs), bjgsic f broblast growth factor (bFGF), actiγin-A, transfqrming growth factor beia-1 (TFG β-1), hepatocyte growth factor, nerve growth fέctor, sonic hedgehog

(SHH), interleukin-3 (IL-3), interleukin-6 (IL-6), granulocyte macrophage, colony stimulating factor (GM-CSF), erythropoietin, vitamin D3j dexamethasohe, β- mercaptoethanol, butylated hydroxyanisole, 5-azacytidlήe, DMSO, insulin, thyroid hormone (T3), LIF, foetal caJf serum, vascular endothelial growth factor (VEpF) ₁ steel factor, variations in oxygen concentration, ascorbic acid, β-glycerQphosJDhate, nicotinamide, platelet derived growth factor (PDGF), cAMP, various cell adhesion molecules and substrates, and others. In addition to these defined factors, it is likely that undefined extracts, such as conditioned media, humpn and ariima} tissue homogenates, or plant extracts can be used to direct stem cell differentiation. Progressive fractionation of these undefined extracts may yield active fractions- or fsven pure components with high potency. These factors can be added tbihe growth medium used in a particular experiment, either alone, or in combination, cV in a defined order which is crucial to the experimental result.

Many systems that have been devised for the differentiation of stem cells in vitro; are complex multi-stage procedures, in which the precise nature of the various stetøs, as well as the chronology of the various steps, are irhportant. For instanqe, Lee et al (2000, Nature Biotechnology, vol. 18, p 675-679) used a five sfage protocol to deViv^ dopaminergic neurons from mouse ES cells: 1) undifferentiated ES cells were expanded on gelatin-coated tissue culture surface in ES cell medium in the, presence of LIF ; 2) ernbryoid bodies were generated in suspension cultures for 4 days in ES dell medium; 3) nestin-positive celJs were selected Tram embryoid bodies iri IfSFn medium for 8 days after plating on tissue culture surface; 4) nes'tip-positive fce'lls were expanded for 6 days in N2 medium containing bFGF/la}πinin; 5) finall^ the expanded neuronal precursor cells were induced to differentiate by withdrawing bFGF from N2 ^~ medium containing laminin.

In a second -example of serial cell culture, Bonnrer-Weir et al. jProc. Natl. Acad. Sci. (200Q) 97: 7999-8004] derived insulin producing cells from human pancreatic ductal cells by: 1) selecting ductal cells over islet cells by selective jadhesiqn on a solid surface in the presence of serum for 2-4 days; 2) sub'se'quently withdfawipg serum and adding keratinocyte growth factor to select for ductal epithelial cells over fibroblasts for 5-10 days; and 3) overlaying the cells with the extracellular matrix preparation 'IVJatrigel' for 3-6 weeks.

In a further example of Serial cell culture, Lurhelsky et al. [Science (2001) 2$2: 1389- 1394] derived insulin secreting cells by directed differentiation of mouse embryonic stem (ES) cells by: 1) expanding ES cells in the presence of LIF for 2-3 Hays,; 2) generating erhbryoid bodies in the absence of LIF over 4 days; 3) selecting nestin- positive cells using ITSFn medium for 6-7 days; 4) expanding pancreatic endocrine precursors in N2 medium containing B27 media supplement and bFGF for 6 days; and 5) inducing differentiation to insulin secreting cells by withdrawing bFGF and adding nicotinamide.

However it is not only the sequence and duration of the various 'steps or the series of addition of different factors that is important in the determination of cell differentiation. As embryonic development is regulated by the action of gradients of signalling factors that impart positional information, it is tp be expected that the concentration of a single signalling factor, aηd also the relative conceηtration of two or more faptars, will be important in specifying the fate of a cell population in vitro and in vivo. Factor coπcentrdtiphs vary during development and stem cells respond differently to different concentrations of the saryie molecule. For instance, Stem cells isdlate'd from the CNS of late stage embryos respond differently to different cqncentrations of JEGF: low concentrations of EGF result in a signal to proliferate, while rjigher conqentrations of EGF- result in proliferation and differentiation to astrocytes.

Many of the factors that have been found to influenpe self-renewal and differentiation of stem cells in vitro are naturally-occurrjng molecules^ This is fo- be expected, as differentiation is induced and controlled by signalling molecules-and -receptors that act along signal transduction pathways. However, by the same, ^" token, it is likely that many synthetic, compounds will have an effect on stem cell differentiation. Such synthetic compounds that have high probability of interacting with cellular targets within signalling and signal transduction pathways, (so called drugable targets) are routinely synthesised, for instance for drug screening by pharmaceutical ςpmpjanies. Once knowrr, these compounds can be used to direct the differentiation of stem cells ex vivo, or can be administered in vivo in which case they woMId εict on resident stem cells in the target orgah of a patient.

Common variables in tissue culture

Ir) developing conditions for the successful culture of a particular cell typδ, or in order to achieve or modulate-a cellular process,, it is often Important to consider a Variety of factors.

One important factor is the decision of whether to propagate the cells in suspension or as a monolayer attached to a substrate. Most cells, prefer to adhere to a substrate ^though some, including transformed cells, haematopoietic cells, and cells from ascites, dan be propagated in suspension.

Assuming the culture is of adherent cells, an important factor is the choice of adhesion substrate. Most laboratories use disposable plastics as substrates for tissue culture. The plastics that have been used include pό|ystyrehe (Ihe most common type), polyethylehe, polycarbonate, Perspex, PVC, Teflon, cejlojahane and cellulose acetate. It is likely that any plastic can be used, but many of these will need to be treated to make them wettable and suitable for cell attachment. Furthermore it is very likely that any suitably prepared solid substrate can be used to provide a support for cells, and the substrates that have been used to date include glass (e.g. alum-borosjlicate and soda-lime glasses), rubber, synthetic fibres, polymerised dextrans, metal (e.g. stainless steel and titanium) and others.

Some cell types, such as bronchial epithelium, vascular endothelium, skeletal muscle and neurons require the growth substrate to be Ooated wjth biological products, usually extracellular matri>rmateria!s such as fibronectjn, ccillageh, laminin, polylysine or others. Trie growth substrate and the method of application (wet or dry-poating, or gelling) can have ari effect on cellular processes such as the growth and differerjtiation characteristics of bells, and these must be dete'rrnine^ djnpiricaJly as disbussed above.

Probably the most obviously important of the variables in cell culture is the choice of culture medium and supplements such as serum. These provide an aqueous compartment for cell growth, complete with nutrients and various factors, some of which have been listed above, others of which are poorly defined. Some of jihese factors are essential for adhesion, others for conveying information (e.g. hormones, mitogens, cytokines) and others as detoXificants. CorrirnOnly gsed npedia include RPMI 1640; MEM/Hank's salts, MEM/Earle's salts, F12, DME|WF12, μi5, MCDB

153, and cithers. The various media can differ wjdely in their constituents - soffie of the Qommoή differences include sodium bicarbonate concentration, concentration of divalent ions such as Ca and Mg, buffer composition, antibiotics, trade, elements, nucleosides, polypeptides, synthetic compounds, drugs, etc. It is well known that different media are selective, meaning they promote the growth of ϋnly sqme cell types. Me _, dia supplements such as serum, pituitary brain ari'd other extracts, are qften essential for the growth of cβlls in culture, and in addition are frequently responsible for determining the phenotype of cells in culture, i.e. they are capable of determining cell survival ^" or directing differentiation. The role of supplements in cell processes such as differentiation is complex and depends on their concentration, tfie time point at VvhJch they are added to the culture, the cell type and fhddiurή used. The updefirted nature of these supplernerits, and their potential to affect thje cell phendtype ¹ , have motivated the development of serum-free media. As with all media, their development has come about largely by trial and erroη as has beeri dis issed above.

The gas phase of the tissue culture is also important and its composition and volume which should be used can depend on the type of medjum used, the amount of buffering required, whether the culture vessel is open or sealed, and whether a particular cellular process needs to be modulated. Common variables include concentration of carbon dioxide and oxygen.

Other conditiops important to tissue culture include the chøicp of culture vessel, amount of headspace, inoculation density, temperature, frequency, of medja changes, treatment with enzymes, rate and mode ^" of agitation or stirring.

Varying the cell ^" culture conditions is therefore a method of achieving a djbsired cellular process. One aspect of the invention recognises that variation of the cell culture conditions in a serial manner can be a highly effective method for achieving. a cellular effect. In various applications, for instance in studies, of cell differentiation, it will often be ¹ the case that a specific series of different tissue culture conditions are required to effect a cellular process. The different conditions may include additions or withdrawals to/from the media or the change of media at sjpeclfit timlb obints. βuch a set of conditions, examples of which are given below, are commonly developed by trial end error as has been discussed above.

Formation of dell units

An important aspect of the present invention is that groups of cells (cell colonies) can be grow,η in cell culture under various conditions and that the colony c£n largely rnaintain its integrity undervarious conditions, when disturbed, knd when mixed with othfer colonies. Such groups or colonies are referred to herein as cell units. Formation of cell units may be achieved, by way of illustration, by growing cells as adherent cultures-on-soϋd substrates such as carriers. If ceil proliferation opcurs after seeding on the carriers, the daughter cells will attach on the same caYrier and form part of- the same colony... |n general, live- adherent cells do not readily dissociate from their growth substrate, and so the integrity of the cell cøjony persists despite any mechanical manipulation of the carrier, agitation of the culture medjum, or transfer into another tissue culture system. Similarly, if at any time multiple carriers are placed in the same vfessel (e.g. beads are pooled) then there will be no substantial transfer of cfells from one bead to another.

One ad^ant^ge of growing ¹ pells in units or polonies is that if a unijt is placed serially in a set qf different tissue culture media, then all the cells comprising ¹ the colony will have been exposed to the same series of culture conditions, in the same order and for the same period of time.

Another of the advantages of this method is that tissue culture can be miniaturised: relatively few cells are required to colonise a micrdcarrier bead (see below) compared to even the smallest tissue culture flask.

Growing; cells in units that are not necessarily themselves adherent to the tissue culture vessel has the further advantage thatindividϋal polonjes can be removed at will ^" and transferred to a different culture vessel, this is particularly important in the present invention as it allows for the transfer of differentiate^ cells pomprising progenitors to rriicrotitre plates suitable for drug screening. Thus the pirogøηiϊors can be prepared in bulk - such as by a method previously deterrτ|med using the technique disclosed in WO2004/031369, and then the same cell units transfdrred essentially by liquid transfer into the HTS platform, This (i) greatiy facilitates thje HTS procedure, (ii) maintains the 3D (multi-) cellular organisation of units which may be integral to further differentiation, (iii) and obviates any dissociation of cellular organisation which may in itself cause cellular differentiation.

A further advantage of growing cell units formed pn carriers is thai cell culture ςa'n be scaled up. Growth of stem cells on carriers offers a way of scaling up production to provide enough material for high throughput screening. Scale up of such cell cultures may require at least 1 g (dry weight) of microcarrier - such as 10g, 5Og, or more.

Another important advantage of forming cell units on solid substrates is that the substrate - and therefore the attached cells by reason of association -. can be labelled by various-means:

Glass spheres of 3mm and 5mm have been widely used as cell adhesiόη substrates, particularly in glass bead bioreactoτs ^" (e _: g. such as manufactured by Meredos Grfibh) used for the scale-up of cell cultures. These beads are typically us'ed in packed beds rather than batch culture', to avoid mechanical damage to fhe adhereht cells. Though such substrates are suitable for the purposes of this invention, other carriers described below may be even more suitable.

In particular when cells are grown on smaller carriers they can be treated as a Suspension culture. Importantly, a common method of growing cells on small carriers is referred to as microcarrier cell culture (see 'Microcaxrie'r cell pulture, Principles and Methods', Editidn AA, available from Amersham Bipsciences (18-1140-6..); herein incorporated in its entirety by reference). Miprocarrier cultύφs a're used, commercially for antibody and interferon production in fermenters of jjp to 4θθd. litre?. A Variety of microcarπers is available, ranging in shape and size and made of different materials. Microcarrier beads made of polystyrene (Biosilon, |Munc), glass (Bioglass, Solohill εng), cpilagøn (Biospheres, Solohill Eng), DEAE sephadex (Cytddex-1, Pharrriacia), dextran (Dormacell, Pfeifer & Langen), cellulose (DE-53, Whatman), gefatih (Geηbead,, l-lazelton Lab), and DEAE dextran (Microde'x, Dextran Prod.) amongst others a're commercially available. These_carriers are well characterisedϊn-terms of the specific gravity oflhe beads, the diameter and the surface, area available for cell growth. In addition a number of porous (micro) carriers are available wifή greatly increased surface area for cell growth. A further characteristic of these porous carriers is that they are suitable for growth of both anchorage dependent ρe|ls, as well as for suspension cells which are carried by entraprnέnt in the network of ppen, interconnecting pores. Porous carriers are available in materials such, as ge'latin (Cultispher G, Percell), cellulose (Cytocell, Pharmacia), po|ye|thylyne (Cytolir'ie 1 arid, 2, Pharmapia), silicone rubber (Immobasil, Ashby Sciehtifjc); collagen (Microsphere,

Cellex Biosciences), alrid glass (Siran, Schott Glassware), these carrers are variously suited to stirred, fluidised or fixed bed culture systems.

As the pihysipal properties of carriers are well known jt is easy to calculate the number of carriers used in an experiment. Some of the carrier? described-and many besides are available as dried products which can be accurately weighed, and subsequently prepared by swelling in liquid medium. In addition the number øf cells used to inoculate a microcarrier culture pan be-worked ^~ out«nd variedr For instance, a culture of Cytodex 3 (2 g/ITtre) inoculated at 6 cells per bead will give a culture- containing 8 million microcarriers, on which 48 million cells/litre are grown at a density of 5x10 ⁴ cells/cm ^z .

If required, harvesting of cel|s grown on microcarriers, or liberation of labels from microcarriers (see below), can be achieved by enzymatic detachment of cells, and/of by digestion of the carrier where applicable: gelatin carriers can be solubilised by trypsin and/or EDTA, collagen carriers using collagenase and dextran carriers using dextranase.

In addition to solid or porous microcarriers, cells rhay be! grouped by immurement, i.e. confined within a medium permeable barrier. Membrane culture systems have been developed where a permeable dialysis membrane retains a group of cells,, but allows the culture medium and its constituents to exchange freely with the inner and outer compartments. Cell culture in hollow fibre cartridges hajs also been developed, and a multitude of fibres-and even turnkey systems are corf|meroial|y available (e.g. from Amicon, Cellex Biosciences). Cell encapsulation in semi-solid matrices has also been developed, where cells are immobilised by adsorption, covalent bonding, cfasslirikirrg of entrapment in a polymeric matrix. Materials that have been used include gelatin, pόlylysine, alginate arid agarose. A typical protocol, is to mix 5% agarose at 40 ^D C- with a suspension of cells in their normal growth medium, to emulsify the mixture using an equal volume of paraffin oil and to cool in an ice bath, producing spheres of 80-200μm diameter. These spheres can be separated frorη the oil and transferred to medium in μ tissue culture vessel.

Cell entrapment is a simple method for the immobilisation of groups of cells, akin to the use of microcarriers or porous substrates. A simple technique is Io enrhesh cells in cellulose fibres such as DEAE, TLC, QAE, TEAE (a|| available frorn. Siqrna). Other

more sophisticated devices are ceramic cartridges whjch are suitable for suspension cells, as in the Opticel culture system (Cellex Biosciences).

One skilled in the art will envisage, in addition to the above φethods of creatjng cell units, other methods of creating groupings of cells including forming 3D cultures of cells such as neural spheres or embryoid bodies, or using tissues! and indeed whole organisms such ^as Drosόphila or C. elegans.

Cell units, or the substrates of which they are comprised, can be associated with a particular factor including, .but without limitation, .proteins, nucleic acids or other chemicals such as drugs. Pre-conditioning of substrates can be achieved in. φany ways, for instance simply by incubating the substrate with the factor of interest or by attaching the factor cϋvalently or non-covalently to the subέtrat.e' . Solubl^ factors can be incorporated, into dry materials by impregnation. This technique relies on thjs rapid ingress of ljquid, carrying soluble factors, into dry porous material that concomitantly becomes swollen and ready for use. Solid factors can be incorporated for example by mixing the faςtor in fibrinogen with thrombin solution, at which poinj a fibrin clot containing the factor is formed. Multiple other ways can be envisaged of associating factor(s) with a cell group, in addition to impregnating, entrapping or encapsulating the factor together with cells.

A method for associating a cell group with a number of different factors is to pre-form cαcktails of factors which are subsequently associated with a parficular cell group. A second method would be to serially condition cell groups in a number of factors. Using dry- formulations of cell group growth substrates as an example, this method would involve firstly partially swelling the substrate in a solution, containing a ¹ first factor and subsequently further swelling the-same in a splutipji containing a secohd factor, resulting in a substrate to- which both factors ^' have become associated. By Revising a έysfematic protocol of associating cell grό'ups with different combinations of different factors it will be possible to sample the effect on the cell group of any coφbinatipn of a set of factors.

Regardless of the method used to condition cell units, with factors, the factors are taken up by cells that comprise that cell unit. Factors, leaking into the growth medium are diluted to such an extent that their concentration falls below physiόlogjcally relevant limits and they have no effect on any additional cell group to which they are exposed. The diffusion of the factor out of e.g. the substrate forming p#rt of the! cell

unit js governed by parameters such as the nature and dimensions of tfife material, the megn pore diameter, and the. molecular weight and poncentration of the fa'ctor. Ta dalibrate the process if necessary, factor release can be measurecj by physical assays suoti as HPLC analysis or release of labelled factor into the medium, or by biological assays such as the dorsal root ganglion outgrowth bioass∑ty for neurotrophic factors.

Combinatorial serial culture of cells

The invention further addresses the problem that cell cultLJre techniqμes involving a plurality of steps and agents are difficult if not impossible to determine by conventional experimentation, which in the prior art has involved trial and error: Empirical determination of tissue culture conditions in complex, multi-stage procedures is not feasible in practice. Advantageously, the culture conditions required tύ differentiate a cell type may first be identified using the methods described herein below.

(a) Split-pool cell culture

Split-popl culturing allows cells to be subjected to a series of culture conditions, and exposed to a series of agents in culture media, in a systematic; and highly productive manner and is described in detail in WO2004/031369.

The first -step in split-pool cell culture is to form cell units (particularly rηicrosi_©pic cell units) as ^~ each cell unit constitutes an easily handled unit that can be exposed to a variety, of cell culture conditions. For simplicity, in this discussion we will assume thdt cell groupings are produced by growing cells in microcarrier culture, and the terms cell unit, cell group, colony and bead are used interchangeably.- However, the methods desbribed are equally applicable to any cell ψnit. A particularly efficient method for sampling a large number of cell culture conditions is referred to as Combinatorial GeII Culture or split-pool cell culture and in one embodiment involves the serial subdividing and combining of groups pf cell units in order tq sdrηple multiple combinations of cell culture conditions. In one aspecj 6f the invention the method operates by taking an initial starter culture (or djfferent starter cultures) of cell uηits divided into Xi number ¹ of aliquots each containing multiple beads (grqups/coloήies/carriers) which are grown separately under different ςd|ture conditions. Following pell culture for a given time, the cell units can be pooled by

combining and mixing the beads from the different aljquόts. This pool cah be split again into X ₂ number of aliquots, each of which is cultured under different conditions for a period of time, and subsequently also pooled. This iterative procedure, of splitting, culturing and pooling (or pooling, splitting and culturing; depending bn Where one enters the cycle) cell units allows systematic sampling of many different combination's of cell culture conditions. The complexity of the experiment, or in other words the number of different combinations of cell culture conditions tested, is equal to the prpdύct of the number of different conditions (X ₁ x X ₂ X ...X _n ) sampled- at each round. h|ote that the step of pooling all the cell units prior to a subsequent split can be optional — a step in which a limited number of cell units are pooled can hatye the same effect. The invention therefore embodies a number of related melhod^ of -systematically sampling multiple combinations of cell culture conditions where groups of ςell units are handled in bulk.

Regardless of the precise manner in which a diversity of cell culture conditions is sarhpled by this means the procedure is efficient because multiple cell units can share a single vessel, where they are cultured under identical conditions;, and it cpn be carried out using only a few culture vessels at any one time ¹ (the number of culture vessels in use is equal to the number of split samples), iri many respects the principle of this procedure resembles that of split synthesis of large chemical libraries (known as, combinatorial chemistry), which samples all possible combinations bf linkage between chemical building block groups (see for example: Combinatorial Chemistry, Oxford University Press (2000), Hicham Fenriiri (Editor)). .Split-pool cell culture- can be repeated over -any number of rounds, and any number of conditions can be sampled at each round. So -long as the number of cell units (dr colonised beads in trjis example) is greater than or equal to the number of- different conditions sampled over all rounds, and assuming that the splitting of cell units occurs- totally randomly, it is expected that there-will be at least one cell unit that has beeji cultured according to each of the various combinations of culture conditions sampled by the experiment. This procedure can be used to sample growth or differentiation conditions for any cell type, or the efficiency of bjomolecule production (e.g. production of erythropoeitjn or interferon) by any cell type. Because tH^ procedure is iterative, it is ideally suited to testing multistep tissue culture protocols - for instance those described above in connection with stem cell differentiation, the Variables which can be sampled using this technique include cell type, cell grouping (e.g. microcgrrier culture, ceil encapsulation, whole organism), growth substrate (e.g. fibronectin bn microcarrier), duration of cell culture round, temperature, different

culture media (including different concentrations of constituents), growth factors, conditioned mddia, co-culture with various cell types (e.g. feede'r cells), animal or plant extracts, drugs, other synthetic chemicals, infection with viruses (incl. transgenic viruses), addition of transgenes, addition pf a'ntiaense or anti-gene rnolecules (e.g. RNAi, triple helix), sensory inputs (in the case of organisms), electrical, light, pr red-ox stimuli and others.

Injone embodiment, the culture conditions required to differentiate th^ firsϊf cell type are first identified in a method comprising the steps of: (a) providing a first set of groups of cell units each comprising one or more cells, and exposing said groups to cjesired culture conditions; (b) subdividing one or more of said groups to create a further set of groups of cell units; (c) exposing said further groups to furtrjer desired culture conditions; (d) optionally, repeating steps (b)-(ό); and (e) assessing the effect on a given cell Unit of the culture condition^ to which it has been exposed.

In another embodiment, the culture conditions required to partially of fully differentiate the first cell type are first identified in a method corήprising the steps of: (a) providing a first set of groups of cell units each comprisipg one or more cells, and exposing said groups to desired culture conditions; (b) pooljng two or more o'f sajd groups to form at least one second pool; (c) subdividing the βecond pool to create a further set of groups- of ce|l units; (d) exposing said further groμps to desired cu|ture conditions; (e) optionally, repeating steps (b) - (d); and (f) assessing thjes pffeci on & given cell unit of the culture conditions to which it has been -exposed.

Suitably, cell units that are partially differentiated are isolated "ana used in the methods described herein.

Suitably, cell units are labelled and the label(s) reflect(s) the culture conditions to which the cell unit has been exposed. The label may be spatially encoded. The label may be selected from the group consisting of a virus, an oligonucleotide, a peptide,, a fluorescent compound, a secondary amine, a halocarbon, a mixture of stable isotopes, a bar code, an optical tag, a bead, a quanfum dot and a radiofrequency encoding tag or combinations comprising at least two of these labels. Two or more labels may be selected and used in combination to label a cell unil

Suitably, the cells are cultured in cell units, each cell unit comprising one or more cells, the cell units may be single cells. Each cell unit may comprise one or more

cells adherent to or bounded by a solid substrate. The solid substrate may be a mmiiccrrooccaarrier of bead. The solid substrate rηay even be a wel| or mediurh-permeable barrier.

In One embodiment, the culture conditions are media to which fhe cell is expqsed. Suitably, the media contain one or more specific agents which influence a cellular process.

The cell culture conditions may comprise culturing at one or rrlόre spepific temperatures or partial pressures of oxygen or carbon dioxide. Th^s cell culture pαnditioris may comprise culturing on one or more specific substrates.

(b) Split-split cell culture

The purpose of performing split-pool processes on pell units is to systematically expose' these to a pre-defined combination of conditions. The person skillpd in the art will conceive of many different means of achieving this outcorjie. In addition to split- pool processes and variations thereof, it is worthwhile briefly discussirjig ^.plit-split processes. A Split-split process involves subdividing a group of Ce ₁ II units at least twice, without intervening pooling of cell units. If split-split processes are used ovfer a large number of rounds, the number of separate samples that arje generated increases exponentially. In this case it is important to employ some levjel pf automation, for example the use of a robotic platform' and sophisticated Sample tracking systems. The advantage of split-split steps is that (since cell upits are hot combined) it is possible to segregate lineages of the various cell units based on ^" their cell culture history. CoήSequently- split-split steps can be used to if a particular cell culture condition is responsible for amy-gjven cellular process and therefore used to deduce the culture history of cell urrits (explained in detail under 'Determination of culture history of a cell unit').

Predetermined protocols

The splitting and/or pooling of cell units may be accomplished totally randoφly or may follow a predetermined protocol. Where cell uriits are split andior pooled randomly, the segregation of a given cell unit into any group is lϊbt predetermined or prejudiced ifi any way. In order to result in a high probability that at least one ceil unit has been exposed to each of the possible combinations of cell culture conditions, it is

I

advantageous to employ a larger number of cell units than thø total number of combinations of cell culture conditions that are bejrjg tested. Under certain circumstances it is therefore advantageous to. split arid/όr pool cell units according to a predetermined protocol, the overall effect being, that adventitious duplications or omissions pf combinations are prevented. Predeterminecj handling ^s of cell units can fcjie optippally planned in advance and logged on at spreadsheet or computer programrpe, and splitting and/or pooling operations executed using; automated protocols, for instance robotics: Labelling of ceil units-(see-beJoW-) can be by any of a purhber of means, for instance labelling by RFID, optical tagging or spatial encoding. Robotic devices capable of determining the- identity of a sample, a'hd therefore partitioning the samples according to a predetermined protocol, have been described (see 'Corhbϊnatorial Chemistry, A practical Approach', Oxford University Press (2000), Ed H. Fenniri). Alternatively, standard laboratory liquid hahdling aηd/or tissue culture robotics (for example such as manufactured by: Beckman Coulter Inc, pullerton, CA; The Automation Partnership, Royston, UK) is capable of spatially encoding the identity of multiple samples and of adding, removing or translocating these according to pre-programmed protocols.

Analysis and/or separation of cell units

Following each round of cell culture, or after a defined number of rounds!, the cell units can be studied to observe any given cellular propesfe that may have, b'een affected by the tissue culture conditions. The examples below are illustrative arid nbt i tended to limit the scope of the invention.

Following each round of cell culture, or after a defined, nurrjber-of rounds, tP|e qell units can be assayed to determine Whether there are members displaying mcredsed cell proliferation. This- can be achieved by a variety_αf techpiques, for instance by visual inspection of the cell units under a microscope, oτ b^ quantitating a marker product characteristic of the cell. This may be an endogenous marker such as a particujar DNA sequenpe, or a cell protein which cah ^" be detected by a Ijgahd or anlibody, Alternatively an exogenous marker, such as gredn fludreόce,nt protein (GFP), can be introduced into the cell units being assayed to provide εj specific readout of (living) cells. Live cells can be visualised using a variety of vital stains, or conversely dead cells can be labelled using a variety of methods, for instance using propidium iodide. Furthermore the labelled eel! units can be separated from unlabelled ones by a variety of techniques, both manual and automated, including

affiriity purification ('panning'), or by fluorescence activated pell sorting (FACS) or broadly similar techniques. Depending on the application it may be possible to use standard, laboratory equipment, or it may be advantageous to use specialised, ipstrumentation. For instance, certain analysis and sorting instruments (e.g. se&- Union Biometrica Inc., Somerville MA, USA) have flpw cell diameters of up tp one millimeter, whjch allows flow sorting of beads with diameters μp to 500 rriicrons. Theise instruments provide a reading of bead size and optical density as well as two fluorescent emission wavelengths- from tags such as GFP-, YFP or QS-red. Sorting speeds of 180,000 beads per hour and dispensing into multi-well plates or into a bulk receptor ar-β possible.

Following each round of cell cujture, or after a defined number of rounds, the cell units can be assayed to deteπήine whether there are members displaying a particular genptype or μhenotype. Genotype determination can be' carried out usjng ^ell known techniques such as the polymerase chain reaction (PGR), fluorescence in situ hybridisatiop (FISH), DNA sequencing, and others. Phenotype deterrniriajiό ^' n can be carried out by a variety of techniques, for instance by visual inspection of the cell units under a microscope, or by detecting a marker product characteristic of the cell. This may be an endogenous marker such as a particular DNA or RNA sequence, or a cell protein which can be detected by a Iigand, conversion of an enzyrhe substrate, or antibody that recognises a particular phenotypic marker (For instance see Appendix E of Stem Cells: Scientific Progress and Future- Research Directions. Department of Health and Human Services. June 2001; appendices incorporated herein by reference). A genetic marker may also be exogenous, i.e. one that has been introduced into the cell population, for example by transfectjon pr viral transduction. Examples of exogenous markers; -are the fluorescent proteins (ag. GFP) or bell surface antigens- which are not normally expre^ed in a particular cell lineage or which are .epitope-modified, or from a-different spefcies. A transgene_xιr exogenous rηafker gene with associated transcriptional control elemelhts can be expressed in a manner that reflects a pattern representative of jgn endogenous gene(sj. This can be achieved by associating the gene with a minimal cfeir type- specific promoter, or by integrating the tranSgene intp a particular locus (e.g. see European patent No. EP 0695351). The labelled cell units call be separated from unlabelled ones by a variety of techniques, both manual and Automated, including affinity purification ('panning'), or by fluorescence activated c,ell sorting (FACiS). Nishikawa et a| (1998, Development vol 125 _t p1747-1757) used cell sijrface markers recogrtispd by antibodies to follow the differentiation of totipotent murine p-S cells.

Using FACS they were able to identify and purify cells of thø haematopoietic lineage at various stages in their differentiation.

An alternative or complementary technique for enriching cell units of a particular genotype or phenotype is to genetically select the desired groups. This can be achieved for instance by introducing a selectable marker into the cell units, and to assay for viability under selective conditions, for instance see Soria et al (2000, Diabetes vol 49, p1-6) who used Such a system to select insulin secreting cells from differentiated ES cells. Li et al (1998, Curr Biol vol 8, p 971 _T Si74) identified heural progenitors by integrating the bifunctional selection rnarker/reporter βgeo (which provides for β-galactosiiJase activity and G418 resistance) iήtq me Spx2 lotus by homologous recombination in murine ES cells. Since one of the characteristics of neural progenitors is expression of Sox2, and therefore the integrated marker genes, these cells could be selected from non-neuronal lineages by addition of (3418 after inducing differentiation using retinoic acid. Cell viability could be determined by inspkption under a microscope, or by monitoring β-gal activity. Unjike phehotype- based selection approaches, which can be limited by the' availability of ah appropriate ligand or antibody, genetic selection can be applied to any differentially expressed genfe,

Microcarriers

A variety of microcarriers are available, ranging |n shape and size and made of different materjals.

By way o|f example, the microcarrier may-be a porous microcarrier selected from the group consisting of Cytopore microcarrier (eg. a Cytopore 1 micrόcaπrfer or a Cytopore 2 microcarrier), a. Cultispher microcarrier, a Cultlspher-G micrjύcarrier, a Cultispher-GL microcarrier and a Cultispher-S- miqrocarrier, an Informatrix microcarrier, a Microsphere microcarrier, a Siran microcarrier., and a Mitiropqrous MG microcarrier.

By way of further example, the microcarrier may be a solid microcarrier - such as a pytodex microcarrier (eg. a Cytodβx 1, Cytodex 2 or Cytodex 3 microcarrier) a Biosilon microcarrier, a Bioglass microcarrier, a FACT Ul micfiJfiajrriet" or a DE 52/53 microcarrier.

Microcarrier culture has significant advantages, including the scale-up of cultures, and also allows units of cells to be exposed to selected culture conditions as required in order to obtain the desired growth and/or differentiation conditions.

The surfaces pf the microcarriers may be further (modified by physical or cherjnical treatments, such as adsorption or covalent cross-linking of molecular entities with a desired charge or other desired characteristic.

Cultispher microcarriers

CultiSpher is manufactured from pharmaceutical grade pofcine .gelatin via a process which yields a highly crpss-linked gelatin matrix with high mechanical and thermal stability-. When used in ceH cultures, cells can attach to both the externa! and the internal surfaces of the mafrix. The increased surface area of φe matrix together with the protection from stress afforded to the cells in the interior of the matrix results in enhanced ceil production capabilities. An additional advantage of the pVόduct js that the matrix can be dissolved with proteolytic enzyme,s resulting in the harvesting of cells with almost 100% viability.

In one embodiment, the microcarrier is a CultispherrG mjcrocarrier. Cultlspher-G has a particle diameter of 130-380 μin, a volume ύf 12-18 mVg dry, a density of 1.04 g/ml with an average pore diameter of 20 μm.

In order to prepare and use Cμltispher-G microcarriers, reference can be made to inter alia Biotech. Bioeng. (2000) β8, 1 p59-70; Brit, J. Cancer. Suppl. XXVII, S-78- S82 (1996); and the manufacturer's website at www.percell.se.

Cyfopore, 2 microcarrier

Cytopόre rjnicrocarriers are available from GE Heaithcare .(previously ¹ Amersham) (www.microcarriers.comV Cytopore is made of 100% cellulose, which is nontoxic to the cells and biodegradable. It is positively charged, due 1cr the Kl ₁ N,- diethylafninoethyl groups. It has a very precise particle s;ize distribϋtiofi and a network structure, the ratio of surface area to particle material is more than 95 to 1. The network structure enables stained cells to be cldsely obβerved while they grow inside the microcarriers. The typical particle diameter is ¹ 200-280 μm and effective surface area is 1.1 m2/g dry. The relative density is 1.03 g/ml, the average diameter of pore openings is 30 μm and the volume is 40 ml/g dry. In order to preipar^ and usό Cytopore microcarriers reference is made to inter alia Applied Microbiology and Biotechήology (1997) 47, 4 p352-7; Cytotechnology (1999) 30 p143-147; Chinese

Joufnal of Biotechnology (1999) 15, 4 p239-44 and Ada Otό _t -Laryngόlogica (2002) 122, 5 p541-S.

Cytppore 2 has been Optimised for anchorage-dependent cel|s requiring a charge density around 1.8 meq/g.

In some embodiments, the microcarrier is a porcine gelatjn micrqca'rrier. in some embodiments, the microcarrier is made of 100% cellulose.

In one embodiment, differentiated cells may be obtained from stem cells in vitro by a method comprising the steps of: (a) growing stem cells adherent to microcajriers; j _n a culture medium; (b). transferring the microcarriers from one culture medium tq another; (c) optionally repeating step (b) as required; and (d) obtaining the- differentiated cells attached to the microcarrier.

The stem ce|ls or partially differentiated cells may be exposed to a potential modulator whilst still attached to said microcarrier. The invention thus provides a method for using these cells in HTS while still attached to the microcarrier, such as carrying out cel| transfer steps by Using robotics. The differentiated cells may be isolated by enzymatic detachment from the microcarrier. The differentiated cells may be isolated .by digestion of the microcarrier.

The method? of the invention may be practised using more than 5Og dry wøight of microcarrier.

Pluripotent stem cells may be grown in vitro by a method comprising the steps of: (a) seeding said cells on microcarriers; and ^" (b) propagating the pells while jattached-to the carriers.

Determination of the identity or cell culture history of a cell unit

When hand|irig large numbers of cell units, their identity and/or cell culture history (for example the chronology and the exact nature of a series of cijlture conditions that any one group or unit may have been exposed to) ban become coηfusied. For instanqe, the split-pool protocol of cell culture necessarily involves mixing cell units in each røund, making it difficult to follow individual units,. Determirjing the ceil culture history of a cell unit in a mixture of cell units which have been subjected to multiple culture conditions is sometimes referred to as 'deøonyolutjύn' of thp cell culture

history. One method of doing this is to label cell units and it is therefore advantageous to label the cell units. Labelling may be performed at .the begϊηhihg of an experiment, or during each round of an experiment and may involve a unique label (which may or may not be modified in the course of an experiment) or a series of labels which comprise a unique aggregate. Similarly, reading ¹ of the label(s) may take place during each round or simply at the end of the (experiment. In one embodiment, unique labels such as RFID labels are read durinjg each rpund, whjereas labels-added serially at each round are read at the end of an experimeiit.

Labelling of cell units may be achieved by a variety of means, foi: instance labelling either- the cells themselves, or any material to which the cells are attached or otherwise associated with. Any of the chemical εind ndn-chemical methods qsed to encode synthetic combinatorial libraries can be adapted for mis purpose and some of these are described in Methods in Enzymology VoI 267 (1996), 'Qombifratorial Chemistry', John IM. Abelson (Editor); Combinatorial Chemistryi Oxford University Press (2000), Hicham Fenniri (Editor); K. Braeckmans et al., 'βcanning the code ¹ , Modern Drug Discovery (Feb. 2003); K. Braeckmans et al,i εnώding micrcjcarriers: Present a'nd Future Technologies'; Nature Reviewss Drug Discovery _r γbl. 1 , p. 447- 456 (2002) all of which are herein incorporated by reference, isome examples of labelling methods follow.

One method of labelling cell units involves associating cell units with a tag that becomes sequentially modified as it is placed in different culture conditions. This may involve for instance the addition or subtraction of. further units to the tqg such that its stereochemistry, sequence or mass is altered; or the alteration of eleqtronic rηemory as in reacj-write RF transponders:(see below).

Another method of labelling cell units involves sequentially asβόpiating unique tags with the cell units whenever they are cultured under different conditions, sucn that subsequent detection and identification of the tags provides for an unambiguous record of the chronology and identity of the cell culture conditions to which the cell unjt has been exposed. Tags can be taken up by cells, or attached to the cell surface by adsorption, or a suitable ligand pr antibody, or conjugated to a cell-asSociated mεitrix such as a carrier by adsorption, colloidal forces of a variety df linkεiges such as covalent linkage or rton-covalent link&ge, e.g. biotin-streptavidin linkage. For instance, one simple tag that can be introduced to cel|s qr attached to a matrix associated with cells is an oligonucleotide of defined length arid/jor sequence.

Oligonucleotides may comprise any class of nucleic acid (e.g. RNA, DNA, PNA, linear, circular pr viral) and may contain specific sequences for amplification (e.g. primer sequences for PCR) or labels for detection (e.g. fluorophores or quenchers, or isotopic tags). The detection of these may be direct, for instance by sequencing the oligos or by hybridising them to complementary sequences (e.g. on an array or chip), or indirect as by monitoring an oligonucleotide-encoded gene product, or the interference of the nucleotide with a cellular activity (e.g. antisense inhibition of a particular gene). An ^~ advantageous-method_of amplifying nuclejc acids is by rolling Circle amplification (RCA; 2002, V. Demidoy, Expert f?ev. MoI, Qiagn. 2(6), p.89-95) where nucleic acid-tags can comprise RCA templates, elongation primers, or S ¹ IrUtS that aid the circularizatiorrof rrrinicircle templates).

Any molecular or macromolecular tag can be used so long as it can be detected, including peptide tags, coloured or fluorescent pompounds, secondary amines, halόcarbonSj mixtures of stableisotopes etc. Tags may attach to cell ύnjts directly or via an intermediary, for instance an antibody raised against a component of the cell unit, or via an interacting pair such as biotin-steptavidin. In addition tags can be protected against degradation by the components of thq cell culture, for example by chemical or other modification or by encapsulation. Encapsulation of taqs pan take place in many different media, for example in beads many types of which are available from suppliers such as Bangs Laboratories Inc. (Fishers IN, LjSA)j and encapsulation may be used to standardise tag dosage in addition to providing cornponenrts for tag arhplification and/or detection (for example by providing PCR primers for uέe with a DNA lag). One method of labelling cell units employs fluorescent beads-such as those- manufactured by Luminex Corporation (Austin, TX, USA). The Luminex system comprises polystyrene beads which may or may not be externally derivatised (e.g-. with avidin or anibσdy) and are internally dyed -with differing- ratios of two spectrally distinct fluorophores ₇ and a re^denwhich ^' ή capable of characterising the spectral signature of each bead. A further method employs beads such as those manufactured by Bangs Laboratories Inø. (Fishers IN, USA). The Bangs system comprises bead sets which can be distinguished based on differing sizes (e.g. bead sets of 4.4μrn and 5.5μm tfiameter). Beads within eaόh set can be furthermore distinguished from each other based or't differing fluorescence intensity owing to differential loading with a single fluorescent dye. It is possjble to use many different dyes with different absorption or emission characteristics, which can be internally loaded or attached externally to carriers by a multiplicity of mea;ns. It

is furthermore possible to use 'quantum dots' to obtain a veryf high number of different fluorescent labels which can be read conveniently.

Cell grovvth substrates such as those described in connection with forirjing cell units can be derivatised or coated with substances that faciljfatie tagging and dσ not interfere with cell growth. One method of derivatisjng carriers is to modify them covalently or non-covalently with biotin, to which a fag can be) attached via streptavidin or avidln. in general it will be-- important to us!e a tgtø that-wilf-not itself induce a cellular effect (I.e. an inert tag), and thai can be distinguished from molecules Rresent in cell units or the culture medial, and that can be attached Io its target and subsequently detected in the background of such ^" molecules. Tb facilitate detection, it may be advantageous to selectively elute tags from cell units or to strip off the cells from cell units using selective conditions. Mβre complicated molecular tagging strategies can also be envisaged, including the strategy of 'binary encc-ding' where information is recorded by a set of binary codes assigned to a set of rηolecμlar tags and their mixtures.

Detection of tags can be accomplished by a variety of rnethόds familiar to those skilled iη the art. Methods include mass spectrometry, nuclear magnetic resonance, secjuenping, hybridisation, antigen detection, electrophoresis, sbectrospppy, microscopy, image analysis, fluorescence detection, etc.

Of particular interest are labelling or encoding strategies in which labelling is carried out only once-or where labelling and/or detection are non-physical and therefore noninvasive. Radiόfrequency Identification (RFID) is- an example of a system exhibiting these properties. RFID empioys transponders (RF tags), antennae and readers. An RF tag is a -small electronic circuit, usually encased, irj glass or plastic, which in its -simplest form provides access to- a unique identification code that may b,e 'read', without confact or line of sight, by suitable electronics. Tags may Bilsό store - information generated by the user, agaih without contacf or line of sight. A 'reader' is an electronic unit that transfers information to and from one or more tags (it should be noted that the term reader is used interchangeably to mean both a read only and read/write unit). The size and features of a reader may vary considerably, and jt may operate in isolation, or be connected to a remote computer system. An a,htenηa is used to transmit information from a reader to a tag, and to receive information sent by an RF tag. The size and format of an antenna will reflect the specific application, and may range from a small circular coil to large planar structures. An F?F|D system

may operate in isolation, or be connected to a remote computer for more comprehensive interpretation and manipulation of identification and associated, d,ata derived from, a tag. One RF|D strategy used in combinatorial chemistry is described in Nicolaou e,t al (1995, Angew Cherri lntl Ed Engl, vol. 34 j p. 2289) cpmprjses: (i) a porous enclosure containing a synthesis substrate arid the semiconductor tag; (ii) the solid phase synthesis resin; (Hi) a glass- encased Single of Multiple Addressable Radiofrequency Tag semiconductor unit capable of receivihg, storing and emitting radiofrequency signals. A similar- device could be adapted- to growjng and following cell units simply by replacing the solid phase synthesis re^in with tissue culture microcarriers or- suitable-cell units. More variations of this can be envisaged including but not limited to (coated Όγ uncoated) RF tags on which cells, are grown ^" directly, or RF tags implanted into cell units or organisms.

Thus tags do not necessarily have to be distinguished βy their chemical or molecular structure in the first instance. Multiple variations of the non-chemical tagging strategy can be devised to determine Jhe identity of a given pell unit in a mixture or of deducing the identity of the different cell units that comprise a mixture. For ipstance optical or visual methods of tagging have been described jwhere different shaped objects, graphically encoded objects or different colours denote the identity <pf a sample (for example see 1998, Guiles et al, Angew. Chem. lritl Ed Engl, vbϊ. 37, p926; Luminex Corp, Austin TX, USA; BD Bioscieηces; Merηobead Technologies, Ghent, Belgium), or where a pattern or bar code is etched onto a _, substrate such as a ceramic- bar or nanowire and recognised using pattern recognition technology (for example see 1997, Xiao et al, Angew. Chem. lntl Ed Engl, vol 3B, p780; SmartBead Technologies, Babraharh, UK; Oxonjca Ltd, Kidlington, UK).

A further method of tracking or labelling cell ^' units is to encode their identity spatially, i.e. by their pøsition in space. In this method- different eejll units iare segregated in defined relative positions, and these positions denote or encode the identity of the units. For instance, cell units may be cultured in ah array, whereby the identity and/or culture history of each unit is known and is associated to a particular position in the array. In their simplest forms such arrays can comprise collections qf tissue culture flasks, wells of a multi-well plate, or locations on a glass slide or other surface. Examples of positional encoding strategies can be found in Geysen et at (1984, Prϋc Natl Acad Sci USA vol. 81 , p. 3998-4002), Fodor et al. (1991 , Science vol. 251 , p. 767-773), Ziauddin and Sabatini (2001 , Nature, Vol. 411 , p. 1|b7-110), and VVu et al. (2002, Trends Cell Biol. Vol. 12(10), p.485-8).

The Invention hds many facets, each of which may have many forms that may be combined to form numerous permutations of the invention. It yvil| be appareht that it is not necessary to label all of the cell Units in order to be able to deduce information qbout thβ outcomes resultant from a combination of cell culture protocols. Thus Without labelling of cell units it would still be possible to assjay large corpbinations of cell Culture conditions accbrding to, the invention, and to determine whether oiiø or more of tr}ese was capable of resulting in a particular celiular effect. embodiment, cell units are labelled. Labelling of a cell unit alldwfc useful information from the. experiment regarding the outcome of the pήrticula'r conditions sampled by the labelled cell unit, as opposed to all the cell upits. Alternatively it is sometimes advantageous to label one or a few group(s) pf cell units which haVe all been exposed to a certain culture protocol, for instance a group of cell μnits which have been segregated into the same medium during a particular split or pool step. It will also be apparent that labelling certain cell units allows pne to infer the identity of other (perhaps unlabelled) cell units.

Similarly, it will be clear that performing cell culture experiments irj which various conditiqris are pmitted can give information regarding the utility of those conditions with respect to a particular experimental outcome. It would therefore be possible to evaluate each of the conditions sampled in a manner acdordihg to the invention by repeating, the experiment a number of times, each time omjittirig a different set of conditions.

Split-split cell culture steps can also be used to deteπηine the effect Of ^~ a particular set of conditions on -experimental outcome. In effect split-split bteps result it] the formation of particular lineages of cell units which have each beerj- exposed to a. unique cell culture conditions at the time of branching. By sludying the different lineages it is possible to determine the utility of the tissue culture cdnd)tjt)ns studied at the branching point, with respect, to a particular experimøntal outcome.

Haematopoietic cell

In a further aspect, there is provided a method for producing a haematopoietic cell from a s,tem cell - such as pn embryonic stem cell - in vitto comprising exposing said stem cell to one or more, preferably, two or more, reaction conditions, wherein said

reaction conditions comprise incubating said stem cell With: (a) retinoic acid, dirriethylsύlphoxide (DMSO) and/or stem cell factor (SCF); and (b) insulin, stem' cell factor (SCF), TGF beta 1 , BMP2, BMP4 and/or TPO; and (c) IL-3, IL-6, TP-O, EPO and/or M-CSF.

the stem cell may be seeded on a microcarrier - such a? a gelatin microca'rrier.

Typically, the stem cells are contained a medium - such as ^n IMDM based (medium or a Streamline Haematopoietic Expansion Medium.

Suitably, in step (a) retjnoic acid or dimethylsulphoxide (DMSO) or stem pell factor (SCF) are us&d; suitably in step- (b) insulin alone is used. Suitably, in step (b) SCF, TGp beta, 1 , BMP2 and TPO is used. Suitably, in step (c) IL-3 and IL-6 are usdd and optionally TPO, EPO and/ύr M-CSF are used.

Typically, step (a) is performed on day 1. Typically, step (b) is performed pri day 4. Typicaliy _s step (c) is performed on day 6.

|n one specjfic embodiment, the conditions are to incubate the ^tem cell with retinoic acid, dirhethylsulphoxide (DMSO), or stem cell factor (SCF); then incubate the stem cell with insulin, stem cell factor (SCF), TGF beta 1 , BMP2 or 4 and TPO or incubate the stem cell. wjth insulin alone and/or a combination of SCF, JC;F beta 1 , B.MP2 arid TPO; and then incubate the stem cell with IL-3, IL-6, and- optionally TPO, EPO and/or M-CSF.

|n another specific embodiment, the conditions are on day 1 jncubate the stem cell with retinoic acid or dimethylsulphoxide (DMSO), or stern cell facfor (SCF); on day 4 incubate the Stem cell with stem cell factor (SCF), JGF beta 1 , BMP2, BNλP4 and TPO or jncub'ate the stem cell with insulin alone and/or a combination of S ^{^} CF, TGF beta 1 , BMP2 and TPO; on day 6 incubate the stem cell with |L-3, IL-6, and optionally TPO, EPO and/or M-CSF.

Further aspects

Further aspects of this invention are' presented in the accompanying paragraphs:

1. A method for identifying a potential modulator of a cell signalling pathway, comprisfήg the steps bf:

(a) providing a cell of a first cell type wherein said first cell type may be differentiated to a second cell type by sequentially exposing said firstcell type to one or more reaction conditions;

(b) adding to or replacing at least one of said one or more reaction conditions with exposure to one or more different reaction conditions comprising said _, potential modulator; and

(c) monitoring the differentiation of t(ie first cell type to determine formation of the second cell type.

2. A method for identifying a potential modulator of a -cell signalling pathway, comprising the steps of:

(a) providing a cell of a first cell type, wherein said first ce|l type is obtained from an e,mbryo or foetus and may be differentiated to a sefcond cell type;

(b) optionally amplifying the sajd first cell type;

(c) further, differentiating the said first cell type b,y sequentially exposing said first cell type to one or more reaction conditions;

(d) exposing the first cell type to one or more different reaction conditions corrtpriεjing said potential modqlator; and

(e) monitoring the differentiation of the first cell type cell to determine formation of the second cell type.

3. A method according to paragraph 1 or 2, wherein the reaction conditions are the culture conditions ^" to which cells are exposed.

4. A method according to paragraph 2 or 3, wherein the first cell type is a cell which has beeη arrested along a differentiation pathway -between a stem cell and a differentiated cell type.

5. A rηethod according to paragraph 1 or 3 wherein the cell type is a primary cell, cell line or tumour derived cell line.

<δ. A method according tq paragraph 4 or 5 wherein the tissue of Origin of the cell type is selected from a group consisting of brain, heart, liver ¹ , lung, kidney, pkih, hair, eye, tqoth, pancreas, muscle, bone and vasculature.

7. A method according to any preceding paragraph, wherein the potential modulator is an inhibitor of a cell signalling pathway.

8. A method according to paragraphs 1 to 6, wherein the poteptial modulator is a promoter of a ceii'signallrng pathway.

9. A method according to paragraphs 3 to 8 wherein the culture conditions required to differentiatera cell-type-are first identified in a method-comprising the-steps of:

(a) providing a first set of groups of cell units each comprising one or more cells, and exposing-said groups to desired culture conditions;

(b) subdividing dne or more of said groups to create a ¹ further set of groups of pell units;

(c) exposing said further groups to further desired culture conditions;

(d) optionally, repeating steps (b)-(c) iteratively, as requjred; and

(e) assessing Ihe effect on a given cell unit of the culture conditions to which it has been exposed.

10. A method according to paragraphs 3 to 8 wherein the culture conditions required to partially or fully differentiate a cell type are first identified jn a method comprising the steps of:

(a) providing a firstset of groups of cβil units each comrirising one or more cejls, and exposing said groups to desired culture..conditions;

(b) pooling two or more of said groups to form at least one second pool;

(c) subdividing the second pool to create a further set of groups of cell upits;

(d) exposing said furthør groups to desired culture conditions;

(e) øptionally, repeating steps (b) - (d) iteratively as required; and

(f) assessing, the effect on a given celJ unit of thexulture conditions to Which it has been exppsed..

11. A rηet.hoς) according to paragraph 9 or 10 wherein cell units that are partially differentiated are then isolated and used in the method of any of claims 1 to 8.

12. Use of a partially differentiated cell type identified in paragraph 9 or 1ύ ih the method of any preceding claim.

13. A method according to paragraphs 9 to 12 wherein cell units are labelled and the label(s) reflect(s) the culture conditions to which the ce|l unit has been exposed.

14. A method according to paragraph 13, wherein the label js spatially encodqd.

15. A method according to paragraph 13 or 14, wherein the label is selected from the group consisting of a virus, an oligonucleotide, a peptide, a fluorescent compound, a secondary amine, a halocarbon, a mixture of stable isOtopes, a bar code, art optical tag, a bead, a quantum dot and a radipfrequency encoding tag.

16. A method according to paragraph 1-5 wherein" two or more labels ar^ selected and used in combination to label a cell-unit.

17. A method -according to paragraphs 9 to 16, wherein the cells are cultured in cell units, each ςell unit comprising one or more cells.

18. A method acpording to paragraph 9 to 16, Wherein the cell units are single cells.

19. A method according to paragraph 9 to 17, wherein each cell unit comprises one or rriore cells adherent to or bounded by a solid substrate.

20. A method according to paragraph 19, wherein the solid substrate is a microέiarrier or bead.

21. A metfiod according to paragraph 19, wherein the solid substrate is a wpll or medium-permeable barrier.

22. A method according to paragraphs 9 Io 2-1 , wherejn the- culture conditons are mediarto which the cell is. exposed.

23 _: A method accordiηg to paragraph 22, wherein the media contain one or more specific} agents which influence a cellular process,

24. A rnethpd according to paragraphs 3 to 23, wherein the cell culture conditions comprise ¹ culturing at one or more specific temperatures or 'partial pressures of oxygen or carbon dioxide.

25. A method according to paragraphs 3 to 24, wherein the cell culture conditions cornprise culturing on one or more specific substrates.

26. A methocj according to any preceding paragraph wherein a firkt cell is differentiated to a second cell type by modulating cell signalling and/or the expression of one or more genes ifi the cell.

27. A method according to paragraph 26, wherein modulation of gene expression in the cell comprises transfection of said one or mqre genes into the cell.

28. A method according to paragraph 26, wherein modμlation of gene expression comprises the exogenous- administration of a gene-product.

29. A method according to any preceding paragraph wherein the differentiation of the cell is monitored by observing the phenotype of the cell or by detecting the modulation of expression of one or more genes in a cell, thereby deterriiining the state of differentiation of said cell.

30. A method accordirig to paragraph 29, wherein the modulation of e>φfession of one or more reporter genes is observed wherein the reporter gene(s) respond(s) to one ύr more differentiation states of said cell.

81. A methqd according to paragraph 29 or 30 wherein the expression of genes involved is monitored qn a gene phip-

32. A method according to paragraph 29, wherein said one or fnore genjas encode a- marker.

33. A method according to paragraph 32, wherein said rnarker may- be detected by an irnmurroassay.

34. A rriethod according to any preceding method paragraph wfhβrein the differentiation of a cell is monitored by loss of proliferative ability.

35. A method according to paragraphs 4 or 5 wherein stem cells or cells that have bepn defived from stem cells in vitro, are cultured by a method comprising the steps of: a) combining one or more cultures of cells grown under different conditions; and

b) culturing the cells.

36. A method apcording to paragraph 35, wherein said stem cells are subjected tf> at least one qhange of culture conditions.

37. λ method according to paragraph 36, wherein said change of cuiiύre conditions Comprises a change of medium.

38. A method according to paragraph 4 wherein differentiated cells Have been obtained from stern cells in vitro by a method comprisihg theiteps of:

(a) Growing stem cells adherent to microcarriers in a culture rηedium;

(b) Transferring the microcarriers from one cujture medium to another;

(c) Optionally repeating step (b) as required; and

(d) Obtaining the differentiated cells attached to the microcarrier.

39. A method according to paragraph 38, wherein stem cells or partially differentiated cells are exposed to said potential modulator whilst still attached to said microcarrier.

40. A method according to paragraph 38, wherein the differentiated cells are isdlated by enzymatic detachment from the micracarrier.

41. A method according to paragraphs 38, 33 or-40 wherein the process is scaled up such thatdt-least 5Og (dry weight) of microcarrier is^ernployed.

42. A method- according to paragraph 38 or 41 , whereirj the differentiated dβl|s are isolated by digestion ofthe microcarrier.

43. The metήpd according to paragraph 4 wherein pluripotent stem cells have been grown in vjtro by a method comprising the steps of: '

(a) seeding said cells on microcarriers; and

(b) propagatirig the cells, while attached to the carriers.

44. A method according to any preceding paragraph in which the potential modulator comprises an organic or inorganic small molecule, a natural or derivatis'ed carbohydrate, protein, polypeptide, peptide, glycoprotein, nucleic acid, DNA, RNA, oligonucleotide or prptein-nucleiς acid (PNA)-

45. A method according to any preceding paragraph wherein the potential modulator is obtained from -a library of small molecules with drug like properties.

46. Use of a compound library to identify a potential modulator according to any preceding paragraph.

47. A pharmaceutical composition comprising a modulator identified accόrdihg4o any preceding paragraph together with a pharmaceutically acceptable carrier; cjiluent or expient.

48. Use of a modulator identified in any preceding paragraph in the manufacture of a medicament for treatment of a disease.

49. Use of a modulator according to paragraph 48 Wherein the treatmenf is a ¹ cell replacement therapy.

50. A partially differentiated cell, which has bee,n differentiated in vitro from a stem cell and arrested along a differentiation pathway between a sie,rή dell and a differentiated cell type.

The invention will now be further described by way .of an Example, whiph is meant to serve to assist one of ordinary skill in the art in carrying out the invention, and is not intended- in any way to limit the scope of the invention.

Example

A screen was performed to identify regenerative drugs capable of stimulating the myeloid lineages of the haematopoietic system. The screqn was perfαjrned by using a series of bulture steps to differentiate mouse embryonip stem cells (mESC) seeded on microparriers into progenitors of the myeloid lineages, and subsequently subjecting these to culture conditions in the presence or abέence of thie lineage specific haematopoietic growth factors TPO, M-CSF and IL-5, or the control compound Vitamin C Which is reported t6 affect cell survival but is not a haematopoietic, growth factor. After 48 hours the effect on myeloid ceil differentiation was assessed using the appearance of macrophages as a surrogate assay. This

screen identified TPO and WhCSF as haematopoietic regenerative drugs capable of stimulating the myeloid lineage.

Materials arid methods

Reagents

murine stem-cell factor (SCF) (R&D Systems) murine thrombopoietin (TPO) (R&D Systems) human erythropoietin (EPO) (R&D Systems) human interleukin 6 (IL-6) (R&D Systems) human fransforming growth. factor β1 (TGFβi) (R&D Systems) murine macrophage colony stimulating factpr (M-CSF) (R&D Systems) murine interleukin 3 (IL-3) (R&D Systems) retinoio acid (Sigma) human bone morphogenetic protein 2 (BMP2) (R&D Systeφs) mouse interleukin 5 (IL-5) ((R&D Systems)

Ascorbic acid (Vitamin C) (Sigma)

Microculture of m ESC

CultiSpher-d microcarriers (Percell Biolytica AE) were hydτated aήd sterilized according to the rnanufactureKs recommendations.

D3 ES cells (ATTC no. CRC-1934) were grown on gelatine-cohted plastic in KQ- DMEM containing 15%- knock-out -serum replacement (KOSR), 1% -non-essential, amino acid& (WEAA)J % Glutamax, 0:5% penicillin/streptomycin, OλrnM D- mercaptoethanol (β.-ME; Sigma) and 1000U/ml Leukemia Inhibitory Factor (LIF; Chemicon); all from Invitrogen unless indicated otherwise.

On the day preceding day 1 of the experiment, approximately 1.5 x 10 ⁴ biotinylated micocarriers equilibrated in medium A (IMDM (Gibco), 'IH KOSR, 1% NEAA, 0.5% pen/strep, O.irriM β-ME, 1000U/ml LIF and 1.SxIQ ^-4 M ₁ 1-thioglyoerol (M ^' fG; Sigma)) were added to 100ml of medium A containing approximately 4.5 x 10 ⁶ ES cells, split into three equai aliquots placed in wells of a 100mm square petri dish (Bibby Sterilin) and incubated overnight.

Preparation of progenitors from mESC

In order tq prepare myeloid progenitors a two step method was empJbyed. Beacjs seeded with rfiESC as described- above were incubated in IMDM containing 10 ^"8 M retinoic acid for 72h. The beads were then washed jn PBS and transferηed, to Stemline™ (Haematopoietic Expansion Medium (Sigma) containing 40ng/ml SCF, 2.5ng TGFβl, 5ng/ml BMf ³ O and 20nd,/ml TPO and incubated for fSh.

Celi-based assay for regenerative drugs

Beads tøeariηg myeloid progenitors prepared as described above were mbfed, washed in PBS and transferred to the Test Growth Medium (Stemline™ (Haematopoietic Expansion Medium (Sigma) supplemented with 30ng/m! IL-3 arid 20rιg/ml IL-6). Equal aliquots of approximately 1 QO beads were dispensed ifotό the wells of a 48 well plate and incubated in the presence or absence of 50μg/rrjl Vitamin C, iOng/ml IL-5, 20ng/ml TPO and 20ng/ml M-CSF. The screen was carried out in triplicate In wells of separate plates placed in the same incubatpr.

After 48h, 1mg of the macrophage assay reagent DQ-ovalbuφirj (Molecular Probes) was- made up in 0.4ml PBS and added to each well at a dilution of 1 :100.. Following incubation Ion at least 4h, the medium was aspirated and replaced with P _] BS _, . The -samples were examined on a Nikon TE2000-S inverted epifluϋrescent mjcrosdόpe using a -FITC filter set to quahtitate microcarriørs bearing large, round cells internally labelled With green fluorescence.

The. results of the screen are reported as the average nuηnber (per cejrj;) of -micro-carriers that/ were decorated with macrophage. In Test Growth iyiedium alone (i.e. in the absence of any test reagents) the average conversion to macrophage^ was 1%, which was taken as the basal level of conversion qwing to spontaneous differentiation. When the ¹ negative control compouηd Vitamin C was used as the test reagent the average conversion to macrophage was Q%, i.e. below the basal level, indicating it had no influence on differentiation. When the haematqpdietic growth factor IL-5 was used as the test reagent the average conversion to macrophage was 0%, i.e. also below the basal level, indicating it had no influence on differentiation of myeloid cells in this assay. (L-5 is known to influence the lymphoid haerfiatopjoietic

lineage but has no notable effects on the rnyeloid branch. However, whφn either TPO or M-CSF was used as the test reagent, the average conversion to macrophage was inqreased to 6%, representing a significant increase over background.

This mESC-based assay is therefore capable of identifying reagents Which act to differentiate myeloid progenitors and could therefore be used to screen libraries of chemical cornpounds to identify novel regenerative drugs,.

All publications mentioned in the above specification are herein incorporated by reference. Various modifications and variations of the described methods and system of the indention will be apparent to those skilled in the &rt without departing from the scope dnd spirit of the invention. Although the ini/entiøn has beieri described in connection with specific embodiments, it should be understood that the ^' invention as claimed should not be unduly limited to such specific embodiments. Indeed, various modifications of the described modes for carrying out the inventioη which are obvious to4hose skilled in molecular biology or re|ated fields-are intended to be within the scope of the following claims.