Regulation 3.2 AUSTRALIA Patents Act 1990 COMPLETE SPECIFICATION STANDARD PATENT APPLICANT: Bavarian Nordic A/S Invention Title: INDUCTION OF DENDRITIC CELL DEVELOPMENT WITH MACROPHAGE-COLONY STIMULATING FACTOR (M-CSF) The following statement is a full description of this invention, including the best method of performing it known to me: CROSS REFERENCE TO RELATED APPLICATIONS [001] This application is a continuation-in-part of U.S. Application No. 11/790,798 filed April 27, 2007, which is incorporated by reference herein. BACKGROUND OF THE INVENTION [002] This invention relates to methods of inducing dendritic cell formation and methods of using induced dendritic cells as therapeutic agents. [003] Dendritic cells (DCs) are important decision makers within the immune system. For example, DCs initiate adaptive immune responses such as antibody production and killer cell formation. DCs also direct the quantity and quality of immune responses, for example determining whether an allergic, inflammatory, or tolerogenic immune response is to be mounted. [004] Many phenotypically and functionally distinct subsets of DCs exist (1). Though they are rare in the blood and immune organs, DCs include two major subgroups of different subsets, the plasmacytoid DCs (pDCs) and the conventional DCs (cDCs) (2). The cDCs in mouse include at least 3 subsets: CD4*CD8~, CD4*CD8 and CD4~CD8-. The CD8' cDCs express the surface marker CD8aX and are the most important cells for cross-presentation of antigens, which allows for killer cell induction against viral infection. The CD8+ cDCs can also produce large amounts of interleukin-12, an essential cytokine for inflammatory immune responses. The CD8~ cDC populations are known to produce large amounts of chemokines and to be better at MHCII presentation of antigens to T cells. pDCs are anti-viral cells, that produce large amounts of the anti-viral and immune protecting cytokines, including type I Interferons (IFN-l) in response to viral DNA or viral RNA. [005] Like other immune cells, DCs develop from haematopoietic stem cells and later stage precursors under the influence of growth factors and cytokines. 1 Granulocyte-Macrophage-Colony Stimulating Factor (GM-CSF) induces haematopoietic precursor cells and monocytes to develop into DCs, called GM-DCs (3-5). GM-DCs are not the majority of steady state DC subsets in lymphoid organs, though, since mice deficient for either GM-CSF or the GM-CSF receptor do not demonstrate much impairment in DC numbers (6). However, application of stabilized GM-CSF to mice in vivo results in increased levels of CD8- cDCs, but not pDCs (10). Moreover, GM-CSF has been shown to block generation of pDCs in vitro (7). [006] Fms-like-Tyrosine-Kinase 3-Ligand (FL) also induces development of DCs, including both cDCs and pDCs, from bone marrow (BM) precursor cells (8,9), both in vitro and in vivo (10-13). The role of FL in development of DCs (called FL DCs) has been definitively demonstrated by the drastically reduced number of both pDCs (9) and cDCs (14) in the lymphoid organs of mice deficient in FL (FLKO). [007] Ex vivo isolated or FL-generated pDCs respond to direct stimulation via the Toll like receptors (TLR) 7 and 9 and their respective ligands, RNA and DNA, by producing high levels of Interferon-alpha (IFN-a). Other cell types, including cDCs, can be induced to produce IFN-a in response to active viruses or transfected DNA or RNA. IFN-a production by cDCs, though, is mediated via TLR7 and TLR9 independent pathways, including PKR, RIG-1, MDA5 and TLR-3 and as yet unidentified cytoplasmic DNA-recognition complexes (15-17). Thus, pDCs are the only cells that employ TLR7 and 9 for the high level production of IFN-a. Furthermore, certain nucleic acid molecules, such as CpG-motif containing oligonucleotides (CpG-ODN A-type), induce extremely high levels of IFN-a solely in pDCs (18). Therefore, IFN-a production in response to A-type CpG-ODN is a functional test for the presence of pDCs in mixed cell populations (17). 2 [008] From studies employing GFP encoded downstream of the receptor for Macrophage Colony Stimulating Factor (M-CSF; also called CSF-1), it is clear that during differentiation of pDCs and cDC subsets the M-CSF receptor is transcribed (26). In addition, there is a report that mice deficient in M-CSF (op/op mice) have reduced numbers of DC subsets (26). Furthermore, DCs and macrophages may develop from a common progenitor cell (41). Other reports show that some DCs, though not pDCs, develop under the influence of a combination of growth factors, including M-CSF (42). Nevertheless, while GM-CSF and FL have been shown to induce development of DCs, M-CSF has always been considered to induce development of monocytes and differentiation of macrophages, not DCs (34). [009] The effect of M-CSF as a therapeutic treatment has previously been investigated, though in limited situations, such as in the treatment of invasive fungal infections (39). It is not known if the patients treated with M-CSF displayed any changes in their DCs. [010] An increase in the number of DCs could be therapeutic in certain situations. For instance, more DCs would be helpful in fighting infections in neonatal individuals. Similarly, individuals who suffer from HIV infection, certain cancers, allergies, have received transplants or who are immunocompromised due to radiation therapy or chemotherapy or from taking certain drugs may have reduced numbers of DCs. In these individuals, it would be desirable to increase the numbers of DCs, including pDCs and cDCs. [011] A new cytokine was identified, termed IL-34 (51). IL-34 was shown to function as an alternative ligand for the M-CSF-R. Id. [012] Thus, there is a need in the art for methods of DC induction, either in vitro or in vivo. 3 BRIEF SUMMARY OF THE INVENTION [013] The invention provides induction of DCs from precursor hematopoietic cells by M-CSF receptor ligands. This induction is independent of induction by FL. DCs induced by the methods of the invention are able to regulate immune responses, for example, by producing anti-viral cytokines, such as IFN-l. [014] The invention is based on the unexpected observation that M-CSF and IL-34, two M-CSF receptor ligands, induce BM cultures to produce IFN-a in response to CpG. These M-CSF receptor ligand-induced BM cells are by phenotype and function similar to pDCs and cDCs and are called pDC and cDC. [015] The receptors for FL and M-CSF, Flt3 and c-fms respectively, are Group Ill receptor tyrosine kinases and share structural identity. Induction of DC differentiation by M-CSF, though, does not involve cross-reaction with the FL receptor, as demonstrated by experiments in which either the FL receptor or the M CSF receptor were inhibited. Furthermore, both pDC and cDC populations developed in M-CSF BM cultures from FL knockout mice (FLKO), ruling out that M CSF induction of these cells requires an indirect contribution of endogenous FL. Finally, when either wild type or FLKO mice were treated with M-CSF, pDCs and cDCs developed in vivo. [016] These results indicate that signaling through the M-CSF receptor can induce the development of pDCs and cDCs. It is possible that in natural, in vivo settings both FL and M-CSF work together to induce and regulate normal DC homeostasis. This is because infections that enhance DCs and immune conditions that result in increases or decreases in DC levels are often associated with increased levels of circulating FL and some of these same conditions have also been reported to enhance circulating M-CSF levels. For example, Langerhans cell 4 histiocytosis demonstrates increased FL and M-CSF in the serum of patients (28), viral infections shown to increase circulating FL also increase M-CSF (29-32), the serum of Systemic Lupus Erythematosus (SLE) patients has increased FL (33), and animal models of SLE demonstrate elevated M-CSF levels (34). However, as shown here, M-CSF can clearly act independently of FL to induce DC development. It is possible that if conditions exist where M-CSF is induced but not FL, or vice versa. Thus, under some conditions DC may be induced that are derived from M-CSF or FL, without the influence of the other growth factor. [017] The methods of the invention allow for increased levels of pDCs and cDCs after being applied to individuals in vivo. These cells can be used to defend against infections, and mount or direct immune responses. In addition, the invention provides for therapeutic and prophylactic treatments against cancers that do not involve csf-1 mutations, for example, but not limited to, Acute Myelogenous Leukemia (AML, also known as Acute Myelogenous Leukemia) of the type still responsive to M-CSF treatment and including those patients that are receiving therapy for overactive Flt3. The invention also extends to therapeutic and prophylactic treatment against autoimmune diseases, such as SLE. [018] In an embodiment of the invention, DCs can be produced in vitro by culturing hematopoietic precursor cells in the presence of an M-CSF receptor ligand. Preferably, the M-CF receptor ligand is M-CSF or IL-34. The hematopoietic precursor cells that can be induced are, but are not limited to, bone marrow cells. [019] In a further embodiment of the invention, DCs induced by M-CSF receptor signaling in vitro can be used to produce cytokines. These cytokines include, but are not limited to IFN-1, IL-1, IL-6, IL-8, IL-10, IL-12, IL-15, IL-16, IL-18, IL-23, IL-27, IL-28, IL-29, TNF-a, TNF-p and chemokines. The cytokines can be 5 produced either in vitro or in vivo after the M-CSF receptor ligand induced DCs are introduced into an animal. In vivo, the induced DCs stimulate either innate immune responses or adaptive immune responses. [020] In yet other embodiments of the invention, DCs induced by an M-CSF receptor ligand in vitro can be exposed to antigens to stimulate specific immune responses. [021] In further embodiments of the invention, the DCs generated by an M CSF receptor ligand can be used to stimulate immune responses in other immune cells. [022] The invention includes a method of increasing dendritic cells (DCs) in vitro, comprising culturing hematopoietic precursor cells; quantitating the number of DCs or eliminating any DCs within the bone marrow; administering an Macrophage Colony Stimulating Factor (M-CSF) receptor ligand; quantitating the number of dendritic cells present after administration of the M-CSF receptor ligand; wherein the number of DCs after the M-CSF receptor ligand administration is increased over the number of DCs before administration of the M-CSF receptor ligand. In embodiments of the invention, the precursor cells are bone marrow cells. In other embodiments of the invention, the DCs are plasmacytoid dendritic cells (pDCs) and the number of pDCs is quantitated by measuring the level of at least one cell surface marker, including but not limited to, CD11c, CD45R, CD45RA, PDCA-1, CCR9, Ly49Q, Ly6C, Siglec-H, HLA-DR, CD4, CD123, BDCA-2, BDCA-4. In other embodiments of the invention, DCs are conventional dendritic cells (cDCs), which are quantitated by measuring the level of at least one cell surface marker, including but not limited to CD11c, CD11b, CD4, CD8, Sirp-alpha, DEC-205, MHCII, 33D1, HLA-DR, and BDCA-1. In other embodiments of the invention, an M-CSF receptor ligand is 6 administered in an MVA vector. In other embodiments of the invention, an M-CSF receptor ligand is administered in another viral vector. In yet other embodiments of the invention, the method of increasing DCs further comprises stimulating the DCs by exposing them to a stimulatory agent, wherein the stimulatory agent includes, but is not limited to, a TLR-agonist, virus, bacteria, fungi, plant, or parts thereof, or cytokines including but not limited to IFN-l, IL-10, IL-12, IL-6, and TNF-a. Embodiments of the invention also include a step of reintroducing the harvested dendritic cells into the animal. In yet other embodiments of the invention, the DCs can be pDCs or cDCs or both. [023] Another embodiment of the invention is a method of increasing dendritic cells in an animal, comprising co-administering an M-CSF receptor ligand with an antigen to the animal; wherein the co-administration results in an increase in the number of DCs in the animal. Further embodiments of the invention include antigens that are derived from tumors, viruses, bacteria, fungi, parasites, prions, plants, molluscs, arthropods, or vertebrate toxins. In other embodiments of the invention, the animal is a mouse or a human. The invention encompasses DCs that are pDCs or cDCs. In other embodiments of the invention, an M-CSF receptor ligand is administered in an MVA vector. In other embodiments of the invention, an M-CSF receptor ligand is administered in another viral vector. In other embodiments of the invention, the M-CSF receptor ligand is administered in a plasmid or via RNA. [024] Yet another embodiment of the invention is a method of producing dendritic cells comprising culturing hematopoietic precursor cells; administering an M-CSF receptor ligand to the cultured cells; generating dendritic cells; and harvesting the dendritic cells. In some embodiments of the invention, the method can further comprise exposing the dendritic cells to an antigen. The invention 7 encompasses antigens including, but not limited to antigens that are derived from tumors, viruses, bacteria, fungi, parasites, prions, plants, molluscs, arthropods, or vertebrate toxins. Embodiments of the invention also include removing the precursor cells from an animal and reintroducing the harvested dendritic cells into the animal. In yet other embodiments of the invention, the DCs can be pDCs or cDCs. The invention also includes embodiments in which the M-CSF receptor ligand is administered to the cultured cells as a polypeptide or as a nucleic acid that is expressed in the cultured cells wherein the nucleic acid is DNA or RNA. The invention also includes methods wherein the M-CSF receptor ligand is administered to the cultured cells in a Modified Vaccinia virus Ankara (MVA) viral vector. [025] A further embodiment of the invention includes a method of inducing an immune response to one or more antigens in an animal, comprising removing hematopoietic precursor cells from an animal; culturing the precursor cells; administering an M-CSF receptor ligand to the cultured cells; generating dendritic cells; exposing the dendritic cells to antigens; harvesting the primed dendritic cells; and reintroducing the primed dendritic cells into the animal. In the invention, the antigens are derived from tumors, viruses, bacteria, fungi, parasites, prions, plants, molluscs, arthropods, or vertebrates, including toxins. Embodiments of the invention also include administering the tumor antigen to the animal. In yet other embodiments of the invention the animal is a human patient suffering from Systemic Lupus Erythematosus (SLE) and the antigens include peptides based on the anti idiotypic or CDR regions of SLE-associated autoantibodies (45, 46). [026] Embodiments of the invention also include an animal that is a human patient suffering from Acute Myeloid Leukemia (AML) or ALL undergoing chemotherapy to inhibit FIt3 or c-kit and the antigen could include novel peptides 8 derived from the patients mutated or duplicated Flt3 or c-kit that are not present in the normal form of these receptors (47). [027] Another embodiment of the invention is a method of producing interferon-a (IFN-a), comprising culturing hematopoietic precursor cells; administering an M-CSF receptor ligand to the cultured cells; and collecting the IFN a. [028] Yet another embodiment of the invention is a method of treating a patient suffering from AML, comprising administering an M-CSF receptor ligand to the patient suffering from AML and increasing the number of DCs in the patient. [029] A further embodiment of the invention is a method of treating a patient suffering from SLE, comprising administering an M-CSF receptor ligand to the patient suffering from SLE and increasing the number of DCs in the patient. [030] Yet another embodiment of the invention is a method of stimulating an immune response, comprising culturing a hematopoietic precursor cell; administering an M-CSF receptor ligand to the cultured cell; generating a DC; and exposing the DC to an immune cell, wherein the immune cell is stimulated to produce an immune response. The immune cells can be T-cells (including, but not limited to regulatory T-cells, suppressor T-cells, or Killer T-cells), T-helper cells (including, but not limited to, regulatory T-cell, suppressor T-cell, or Killer T-cell), B-cells, Natural Killer Cells, or macrophages. Stimulation of an immune response can be achieved in vitro or in vivo. Furthermore, the immune response can be an anti-allergic immune response, an anti-septic immune response, an anti-graft immune response, an anti-tumor immune response, an anti-autoimmune response, a tolerogenic immune response, an anti-pathogen immune response, or a regulatory immune response. 9 BRIEF DESCRIPTION OF THE DRAWINGS [031] Figure 1 depicts that M-CSF- and FL-derived BM, but not GM-CSF derived BM, promote IFN-a production to CpG2216. BM cells were incubated for six days with FL (35 ng/ml), mouse GM-CSF (50ng/ml), or M-CSF (50 ng/ml). Mouse FL was used in these experiments; however, similar experiments with similar results have been obtained with human FL. Human M-CSF was used in these experiments; however, similar results have been obtained with murine M-CSF. M-CSF and GM CSF were added at day 0 and again at day 5. On day 6, CpG2216 (500nM) was added to the total cultures or left without stimulation (media) for another day. Cell culture supernatant was analyzed for IFN-a content by ELISA. [032] Figure 2 depicts M-CSF-induced pDCs and cDCs development from BM cells, even in the absence of FL. (A) C57BL/6 BM cells depleted of B220+ and CD11 c+ cells were incubated for 6 days with M-CSF (20 ng/ml) added at day 0 and again at day 3. M-CSF was routinely human M-CSF, but murine M-CSF also shows a similar effect. On day 6, the cells were harvested and stained with antibodies to detect CD1 1c and CD45RA expression. Cells with the phenotype of pDCs and cDCs populations are shown boxed in the upper panel. The number of cells in each of the pDC and cDC populations is shown in the lower panel and compared to numbers obtained from day 6 FL-generated DCs also using BM cells depleted of B220+ and CD1 1c+ cells. Data shown are from one experiment representative of three experiments of day 6 FL cultures and more than five experiments of day 6 M CSF cultures. (B) BM cells from mice lacking FL were similarly depleted of B220+ and CD11 c+ cells and incubated for six days with FL or M-CSF (with additional feeding at day 3). The pDC and cDC populations were also evident in these cultures, indicating that M-CSF could drive the generation of cells with pDC and cDC 10 phenotype, independently of FL. Data from FLKO mice from one experiment that is representative of four experiments. [033] Figure 3 depicts the surface phenotype of M-CSF-derived pDCs (M pDCs) compared to FL-derived pDCs (FL-pDCs) and ex-vivo isolated spleen pDC. Stained cells from day 6 M-CSF cultures (filled histograms), FL cultures (grey open histograms), or freshly isolated spleen DCs (black histograms) were gated on pDCs by selecting for the expression of CD1 1c and CD45RA or CD45R amongst the PI negative cells. The expression of a range of surface markers on the pDC surface is shown. The light grey histograms represent the background staining of the M-pDCs within each stain. All M-pDCs also lacked expression of CD3, CD19, CD49b and NK1.1. The surface phenotypes shown are from one experiment representative of two to five experiments for M-pDC, two to three experiments for FL-pDCs and two experiments for spleen pDCs. [034] Figure 4 depicts production of IFN-a by M-pDCs in response to TLR9 stimulation and production of other cytokines in response to TLR7 stimulation. Highly purified, sorted M-pDCs were incubated for 18 hrs with the TLR ligands shown and the supernatants were assayed by ELISA for the presence of IFN-a (A) or by Cytometric Bead Assay (CBA) for the production of IL-6 and TNF-a (B). No IFN-y, IL-12p70 or MCP-1 were detected by CBA in the M-pDC supernatants. The data shown are from one experiment representative of five experiments (IFN-a ELISA) and three experiments (CBA assay). Error bars represent the range of duplicate samples. [035] Figure 5 depicts surface phenotype of M-CSF-derived cDCs (M-cDCs) compared to FL-derived cDCs (FL-cDCs) and ex-vivo isolated spleen cDCs. Stained cells from day 6 M-CSF cultures (filled histograms), FL cultures (grey open 11 histograms) or freshly isolated spleen DCs (black histograms) were gated on cDCs by selecting for the expression of CD1 1c and lack of CD45RA or CD45R amongst the PI negative cells. The expression of a range of surface markers on the cDC surface is shown. The light grey histograms represent the background staining of the M-cDCs within each stain. All M-cDCs also lacked expression of CD3, CD19, CD49b and NK1.1. The surface phenotypes shown are from one experiment representative of two to five experiments for M-cDC, two to three experiments for FL cDCs and two experiments for spleen cDCs. [036] Figure 6 depicts the inhibition of M-DC development by a c-FMS inhibitor. Replicate M-CSF and FL cultures were conducted in parallel in the presence or absence of a range of cFMS Receptor Tyrosine Kinase Inhibitor concentrations. At the end of the culture period all cells were counted. The number of cells harvested from cultures without inhibitor was set at 100%. Cells from cultures containing inhibitor were expressed as a percentage of cells obtained in the absence of inhibitor. The data shown are from one experiment, representative of two experiments. Routinely, murine FL and human M-CSF were used in these experiments, but similar results were also obtained with human FL and murine M CSF. [037] Figure 7 depicts DC increase due to M-CSF treatment in vivo. FLKO mice were treated for five consecutive days intraperitoneally (ip) with 10 pg/day human M-CSF in 0.01% BSA in PBS or with vehicle alone (Control). DCs were purified from FLKO spleens and stained with CD1 1c and CD45RA. The pDC and cDC populations were gated (A) and enumerated (B). Data are pooled from three individual mice within two separate experiments compared to control mice analyzed the same day. 12 [038] Figure 8 depicts bone marrow cultures stimulated with human M-CSF or mouse IL-34 that were then stimulated after 0 or 6 days with CpG-2216. The cell culture supernatants were assayed for IFN-alpha production. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS [039] The invention is supported by experimental results showing that human M-CSF, mouse M-CSF, and mouse IL-34, M-CSF receptor ligands, induce IFN-a production (Figures 1 and 8) and the development of pDCs and cDCs from hematopoietic precursor cells in vitro (Figure 2), in the absence of FL. These M-CSF induced pDCs and cDCs are phenotypically identical to pDCs and cDCs induced by FL or those that develop in vivo, (Figures 3 and 5) and produce antiviral cytokines, such as IFN-I (Figure 4). Furthermore, inhibition of the M-CSF receptor, demonstrates that M-CSF inducing activity occurs through its own receptor and not as a cross-reaction with the FL receptor, fIt3 (Figure 6). Finally, M-CSF has been shown to induce pDCs and cDCs in vivo, in an FL-independent pathway (Figure 7). [040] In describing the embodiments of the invention, the term "induction" refers to the introduction of a signal that results in a change in the morphology and or physiology of a cell. The term "development" refers to the alteration of cell morphology and physiology along a genetically determined pathway. The term "differentiation" refers to the development of a cell from a precursor cell to a more specialized cell type. The term "cell surface marker" refers to a protein or other molecule on the surface of a cell that is specific for that cell, either from any other cell or from other cells in the developmental pathway of the cell. The term "precursor cell" refers to a less differentiated cell that has the ability to develop into a more differentiated cell after induction by some signal. Hematopoietic precursor cells include, but are not limited to, bone marrow cells. 13 [041] In an embodiment of the invention, DCs are produced in vitro. Hematopoietic precursor cells can be cultured and DCs harvested by techniques known to those of skill in the art, as exemplified, but not limited to, the procedures described in Example 1 below. The number of DCs can be quantitated directly. For example, the number of DCs can be quantitated by measuring DC cell surface antigens, for example, Ly49Q, CD4, MHCII, B7H1, CD81, CD62L, and CD11b, CD45RA, and F4/80, by techniques known to those of skill in the art. These techniques include, but are not limited to, surface staining and fluorescence activated cell sorting faces) , for example by the methods described in Example 1 below. The number of DCs can also be quantitated indirectly. For example, the number of DCs can be quantitated by measuring DC-specific increases in cytokine production. Quantitation of cytokine production, IFN-l, IFN-a, IL-12 p70, IL-6, TNF-a, MCP-1 and IFN-y, can be achieved with techniques known to those of skill in the art. These techniques include, but are not limited to, ELISA, as described in Example 1, below. Fold increases in cytokine production can include, but are not limited to, greater than or equal to 1.2 fold, greater than or equal to 1.5 fold, greater than or equal to 2 fold, greater than or equal to 3 fold, greater than or equal to 4 fold, greater than or equal to 5 fold, or greater than or equal to 10 fold. [042] In embodiments of the invention, an M-CSF receptor ligand and/or antigen can be administered to cultured cells as a protein. The M-CSF receptor ligand can be produced by methods known to those of skill in the art, including, but not limited to, in vitro, prokaryotic, and eukaryotic expression systems. [043} The invention encompasses M-CSF receptor ligand polypeptides and fragments thereof in various forms, including those that are naturally occurring or produced through various techniques such as procedures involving recombinant 14 DNA technology. In preferred embodiments, the M-CSF receptor ligand is M-CSF or IL-34. Within the context of this invention, an "M-CSF receptor ligand" means any polypeptide capable of binding to the M-CSF receptor on hematopoietic precursor cells and stimulating the production of DCs. In a preferred embodiment, the M-CSF receptor ligand is a human or mouse protein. Polypeptides derived from a native M CSF or IL-34 by in vitro mutagenesis, are included in the invention. Such forms include, but are not limited to, derivatives, variants, and oligomers, as well as fusion proteins or fragments of M-CSF or IL-34. These include polypeptides containing conservative amino acid changes from a native M-CSF or IL-34 protein. [044] Thus, the invention provides polypeptides and fragments of an M-CSF receptor ligand that are capable of binding to the M-CSF receptor on hematopoietic precursor cells and stimulating the production of DCs. Such a fragment is preferably a soluble polypeptide. Polypeptides and fragments of an M-CSF receptor ligand that are capable of binding to the M-CSF receptor on hematopoietic precursor cells and stimulating the production of DCs can be screened using assays, such as those described in the examples. [045] M-CSF receptor ligands can be screened using a variety of well-known techniques in the art. For example, purified M-CSF receptor or cells expressing the M-CSF receptor can be contacted with a suspected M-CSF receptor ligand or cells suspected of expressing an M-CSF receptor ligand The reaction of the M-CSF receptor with the M-CSF receptor ligand is then detected, for example, using standard binding assays. [046] In various embodiments, phages or vectors expressing a library of mutated M-CSF or IL-34, or a random library containing suspected m-CSF receptor 15 ligands are selected using purified M-CSF receptor on a solid substrate or cells expressing the M-CSF receptor. [047] Combinatorial libraries, such as a phage display, using M-CSF receptor ligand cDNAs can be screened for pages capable of interacting with a M-CSF receptor. See, e.g., McCafferty et al. PCT publication WO 92/01047; Marks et al. (1991) J. Mol. Biol. 222:581 597; and Griffths et al. (1993) EMBO J 12:725 734. A combinatorial library of M-CSF receptor ligand mutants can be generated by mutating a M-CSF receptor ligand. For example, a DNA sequence encoding a native M-CSF or IL-34 can be mutated by, for example, using randomly altered mutagenized oligonucleotides, to generate a library of phage comprising mutated M CSF or IL-34 sequences, which can then be screened to bind to a M-CSF receptor attached to a solid substrate or cells expressing an M-CSF receptor. Methods of inducing random mutagenesis and screening methods can be found in, for example, Barbas et al. PCT publication WO 96/07754; Barbas et al. (1992) Proc. Nat'l Acad. Sci. USA 89:4457 4461. [0481 A library containing an M-CSF receptor ligand can be expressed by a population of display packages, preferably derived from filamentous phage, to form a display library. Examples of methods and reagents particularly amenable for use in generating a display library can be found in, for example, Ladner et al. U.S. Pat. No. 5,223,409; Kang et al. PCT publication WO 92/18619; Dower et al. PCT publication WO 91/17271; Winter et al. PCT publication WO 92/20791; Markland et al. PCT publication WO 92/15679; Breitling et al. PCT publication WO 93/01288; McCafferty et al. PCT publication WO 92/01047; Garrard et al. PCT publication WO 92/09690; Ladner et al. PCT publication WO 90/02809; Fuchs et al. (1991) Bio/Technology 9:1370 1372; Hay et al. (1992) Hum Antibod Hybridomas 3:81 85; Huse et al. (1989) 16 Science 246:1275 1281; Griffths et al. (1993) supra; Hawkins et al. (1992) J Mol Biol 226:889 896; Clackson et al. (1991) Nature 352:624 628; Gram et al. (1992) PNAS 89:3576 3580; Garrad et al. (1991) Bio/Technology 9:1373 1377; Hoogenboom et al. (1991) Nuc Acid Res 19:4133 4137; and Barbas et al. (1991) PNAS 88:7978 7982. Once displayed on the surface of a display package (e.g., filamentous phage), the library is screened to identify and isolate packages that express a protein that binds an M-CSF receptor. In a preferred embodiment, the primary screening of the library involves panning with an immobilized M-CSF receptor and display packages expressing polypeptides that bind immobilized M-CSF receptor are selected. [049] To identify M-CSF receptor ligands, soluble forms of M-CSF receptor fused to a mutated Fc can be used (52). A cDNA library can be prepared by isolating RNA from cells, and M-CSF receptor ligands can be cloned by a slide binding autoradiographic method (Id.). The cloning can involve transfection of cells on slides with DNA from clone pools and detection of M-CSF receptor ligand expression by a two-step binding method utilizing M-CSF receptor as the first step. Positive pools can be subdivided and the procedure repeated with sub-pool DNAs until single cDNAs conferring binding are identified. M-CSF receptor ligands can be screened for proteins that bind specifically to the receptor with reasonably high affinity. Various constructs are made available which allow either labeling of the receptor to detect its ligand. For example, directly labeling an M-CSF receptor, fusing onto it markers for secondary labeling, e.g., FLAG or other epitope tags, etc., will allow detection of receptor. This can be histological, as an affinity method for biochemical purification, or labeling or selection in an expression cloning approach. 17 [050] A two-hybrid selection system may also be applied by making appropriate constructs with the M-CSF receptor sequences and a library containing suspected M-CSFreceptor ligand encoding sequences.. See, e.g., 53. [051] In embodiments of the invention, the M-CSF receptor ligand can be administered in vitro to cultured cells at levels including, but not limited to, 1-100 ng/ml, 1-75 ng/ml, 1-50 ng/ml, 1-25 ng/ml, 1-10 ng/ml, 10-100 ng/ml, 10-75 ng/ml, 25-100 ng/ml, 50-100 ng/ml, 75-100 ng/ml, 25-75 ng/ml, or 50-75, ng/ml, preferably at 10-50 ng/ml, and most preferably at 20 ng/ml. [052] An M-CSF receptor ligand and/or antigen can also be administered to cultured cells by introduction of a DNA or RNA that encodes the M-CSF receptor ligand and directs its expression within the cultured cell. Techniques for this method of administration include, but are not limited to, techniques for transfection, dipofection, electroporation, and transduction. The M-CSF receptor ligand can also be administered to a cell by infection with a virus that carries the genetic information to produce the M-CSF receptor ligand. Non-limiting examples of such a virus are DISC-Herpes virus and Modified Vaccinia virus Ankara (MVA). A strain of MVA, MVA-BN is deposited at the European Collection of Animal Cell Cultures (ECACC) with the deposit number V00083008. Those of skill in the art are knowledgeable of these methods of administration. Administration of the M-CSF receptor ligand or antigen to the DC "exposes" the DC to the the M-CSF receptor ligand or antigen. [053] In other embodiments of the invention, antigens can be co administered to DCs with the M-CSF receptor ligand. These antigens include, but are not limited to, antigens present on viruses (in non-limiting example, influenza, HIV, CMV, EBV, human papilloma virus, adenovirus, HBV, HCV and vaccinia), bacteria, fungi, parasites, prions, and tumor cells (tumor antigens), as well as toxin 18 antigens from viruses, bacteria, fungi, parasites, mollosucs, arthropods, and vertebrates. In embodiments of the invention, antigens can also include peptides from autoantibodies which can be antigens for the treatment of SLE, and peptides corresponding to the mutant forms of FIt3 or c-kit which can be antigens for the treatment of AML. [054] The term "co-administration" refers to the administration of more than one substance to an animal or to cultured cells. Co-administration can occur simultaneously or in series, with one substance administered before the other. When administered in series, the second substance can be, but is not limited to, within 1 minute, 2, minutes, 5 minutes, 10 minutes, 30 minutes, 1 hour, 2 hours, 8 hours, 12 hours, 24 hours, 2 days, 3 days, 7 days, 14 days, or 1 month. In embodiments of the invention, DCs generated by administration of an M-CSF receptor ligand are also "exposed" to antigen by the co-administration of the antigen. [055] The term "tumor antigen" refers to an antigen associated with certain tumoral diseases. Tumor antigens are most often antigens encoded by the genome of the host that develops the tumor. Thus, in a strict sense tumor antigens are not foreign antigens. However, tumor antigens are found in significant amounts in tumors; whereas, the amount of tumor antigens in normal tissues is significantly lower, and most often no tumor antigens are found at all in normal tissue. Examples of tumor antigens include gp75 antigen for melanoma papilloma virus proteins for cervical cancer, and tumor specific idiotypic proteins for B cell lymphomas. [056] In further embodiments of the invention, M-CSF receptor ligand generated DCs are used to stimulate immune responses in other immune cells in vivo or in vitro. These immune cells include, but are not limited to, T-cells (including, but not limited to, regulatory or suppressor T-cells, Killer T-cells (CTLs), and T 19 Helper cells (including, but not limited to Th1, Th2, and Th17), B cells, Natural Killer cells (NK cells), and macrophages. The stimulated cells can be introduced into an animal in vivo to mount an immune response. Such immune responses include, but are not limited to, anti-allergic responses, anti-septic responses, anti-graft rejection responses, anti-tumor responses, anti-autoimmune disease responses, tolerogenic immune responses, anti-pathogenic immune responses, and regulatory immune responses. [057] M-CSF receptor ligand generated DCs can also be exposed to stimulatory agents, wherein "stimulatory agents" are proteins and other molecules that induce a specific response from DCs. Stimulatory agents of the invention include, but are not limited to, TLR-agonists, viruses, bacteria, fungi, plants, parasites or parts thereof, or cytokines including but not limited to IFN-l, IL-6, IL-10, IL-12 and TNF-a. [058] In other embodiments of the invention, an M-CSF receptor ligand is administered to an animal. The term "animal" includes, but is not limited to vertebrates, most preferably mammals, including, but not limited to humans, horses, cows, pigs, sheep, goats, llamas, cats, dogs, mice, and rats. [059] In other embodiments of the invention, antigens can be co administered with an M-CSF receptor ligand. There antigens include, but are not limited to, antigens present on viruses (in non-limiting example, influenza, HIV, CMV, EBV, human papilloma virus, adenovirus, HBV, HCV and vaccinia), bacteria, fungi, parasites, prions, and tumor cells (tumor antigens), as well as toxin antigens from viruses, bacteria, fungi, parasites, mollosucs, arthropods, and vertebrates. In embodiments of the invention antigens can also include peptides from autoantibodies which can be antigens for the treatment of SLE, and peptides 20 corresponding to the mutant forms of Flt3 or c-kit, which can be antigens for the treatment of AML. [060] An M-CSF receptor ligand and/or antigen can be administered to an animal as a protein, DNA, RNA, or virus. Administration of a protein to an animal can be achieved by, but is not limited to, oral, transdermal, transmucosal administration, or by injection (parenteral). The dose administered can vary depending on which type of administration is used. Pharmaceutically acceptable formulations of an M-CSF receptor ligand and antigen are known in the art. Carriers or excipients can be used to produce pharmaceutical compositions. Examples of carriers include, but are not limited to, calcium carbonate, calcium phosphate, various sugars such as lactose, glucose, or sucrose, or types of starch, cellulose derivatives, gelatin, vegetable oils, polyethylene glycols, and physiologically compatible solvents. Examples of physiologically compatible solvents include, but are not limited to sterile solutions of water for injection (WFI), saline solution, and dextrose. The M-CSF receptor ligand can be administered by different routes, including but not limited to, intravenous, intraperitoneal, subcutaneous, intramuscular, oral, transmucosal, rectal, or transdermal. [061] In vivo, an M-CSF receptor ligand and/or antigen is administered to an animal at levels of 0.01 pg - 100 mg/day, 0.1 pg-100 mg/day, 1 pg-100 mg/day, 10 pg-1 00 mg/day, 100 pg-100 mg/day, 1 mg-1 00 mg/day, 10 mg-1 00 mg/day, 50-100 mg/ day, 0.01 pg-10 mg/day, 0.1 pg-10 mg/day, 1 pg-10 mg/day, 10 pg-10 mg/day, 100 pg-10 mg/day, 1-10 mg/day, 10-50 mg/ day, 0.01 pg-1 mg/day, 0.1 pg-1 mg/day, 1 pg-1 mg/day, 10 pg-1 mg/day, 100 pg-1 mg/day, 1-10 mg/day, or 1-50 mg/ day. Levels of 1-20 pg/day are preferable and 10 pg/day most preferable for administration to rodents. Levels of 1-50 mg/day are preferable, and 25 mg/day 21 most preferable, for humans. An M-CSF receptor ligand can also be administered to animals on a per weight basis, including, but not limited to, 0.5 pg-10 g/g weight/day, 1 pg-10 g/g weight/day, 10 pg-10 g/g weight/day, 100 pg-10 g/g weight/day, 1g-10 g/g weight/day, 0.5 pg-1 g/g weight/day, 1 pg-1 g/g weight/day, 10 pg-1 g/g weight/day, or 100 pg-1 g/g weight/day, preferably 0.5 pg/g weight/day. Other dosages are contemplated by the invention, and can be determined using assays known to the skilled artisan. [062] Further embodiments of the invention include administration of an M CSF receptor ligand to precursor cells that have been isolated from an animal. These cells are induced by the M-CSF receptor ligand in vitro, exposed to antigen, and returned to the animal for a therapeutic or prophylactic effect. Techniques for such "ex vivo" therapies are known to those in the art, as described (36,37, 44). Other techniques for ex vivo therapy are also contemplated for the invention. [063] To induce hematopoietic precursor cells in vitro, the cells can be cultured and DCs harvested by techniques known to those of skill in the art, as exemplified, but not limited to, the procedures described in Example 1 below. In this embodiment the DCs are characterized by observing DC cell surface antigens, for example CD11c, Ly49Q, CD4, CD8, CD22, DEC-205, 33D1, PDCA-1, BDCA-1, BDCA-2, BDCA-4, CD25, CD80, CD86, CD40, CD69, Siglec-H, Ly6C, CCR9, HLA DR, CD123, MHCII, B7H1, CD81, CD62L, CD11b, CD45R, CD45RA, and F4/80, by techniques known to those of skill in the art. These techniques include, but are not limited to, surface staining and fluorescence activated cell sorting faces) , for example by the methods described in Example 1 below. Quantitation of cytokine production can also be used, including, but not limited to, IFN-1, IFN-a, IL-12 p70, IL 6, TNF-a, MCP-1 and IFN-y. This is achieved with techniques known to those of skill 22 in the art. These techniques include, but are not limited to, ELISA, as described in Example 1, below. [064] In embodiments of the invention involving ex vivo therapies, an M-CSF receptor ligand and/or antigen can be administered to cultured cells as a protein in vitro. The M-CSF receptor ligand protein can be produced by methods known to those of skill in the art, including, but not limited to, in vitro, prokaryotic, and eukaryotic expression systems. [065] Some autoimmune diseases, for example, but not limited to SLE have been shown to be mediated by the Toll-like receptor 9 (TLR9) (38). TLR9 recognizes DNA and, under certain conditions, can recognize self-DNA in autoimmune disease. In these diseases, B-cells that express TLR9 will proliferate when TLR9 binds to self DNA. In addition, pDCs are also activated by the TLR9 DNA complexes and produce increased levels of IFN-l, which further aggravates the disease. FL induction of pDCs contributes to this aggravation by adding constant stimulation when it induces pDC development. An embodiment of the invention provides a better therapeutic regime, wherein an M-CSF receptor ligand is administered to a patient suffering from an autoimmune disease, for example SLE, which acts to prime the new DCs induced by the M-CSF receptor ligand. In contrast to treatment with FL, though, not only are DCs induced, but the M-CSF receptor ligand also down-regulates TLR9 and its responses to self-DNA complexes (23). Thus, M-CSF receptor ligand -induced DCs can promote specific immune responses in the patient, including but not limited to, down modulation of exaggerated auto immune reactions, without triggering additional TLR9 IFN-l production and B-cell stimulation. 23 [066] In another embodiment of the invention, a therapeutic regime for leukemias is provided. These leukemias include, but are not limited to, AML. AML and other leukemias are mediated by activation of FIt3, the receptor for FL (39, 48 50). Thus, in this embodiment of the invention, administration of FL to a patient to induce development of DCs would aggravate the disease. In contrast, the invention provides for administration of an M-CSF receptor ligand, along with a tumor antigen, to a patient with leukemia, including, but not limited to AML, so that DCs can be induced to provide an immune response against the tumor cells, without further stimulation of the tumor cells with FL. An inhibitor of Ft13 can also be used together with an M-CSF receptor ligand to treat the leukemias. [067] Embodiments of the invention are also directed to the treatment of other proliferative disorders including, but not limited to, hematopoietic neoplastic disorders involving hyperplastic/neoplastic cells of hematopoietic origin arising from myeloid, lymphoid or erythroid lineages, or precursor cells thereof. These include, but are not limited to erythroblastic leukemia, acute promyeloid leukemia (APML), chronic myelogenous leukemia (CML), lymphoid malignancies, including, but not limited to, acute lymphoblastic leukemia (ALL), which includes B-lineage ALL and T lineage ALL, chronic lymphocytic leukemia (CLL), prolymphocytic leukemia (PLL), hairy cell leukemia (HLL) and Waldenstrom's macroglobulinemia (WM). Additional forms of malignant lymphomas include, but are not limited to, non-Hodgkin lymphoma and variants thereof, peripheral T cell lymphomas, adult T cell leukemia/lymphoma (ATL), cutaneous T-cell lymphoma (CTCL), large granular lymphocytic leukemia (LGF), Hodgkin's disease and Reed-Sternberg disease. [068] In addition, embodiments of the invention include, but are not limited to, the treatment of malignancies of epithelial or endocrine tissues, including respiratory 24 system carcinomas, gastrointestinal system carcinomas, genitourinary system carcinomas, testicular carcinomas, breast carcinomas, prostatic carcinomas, endocrine system carcinomas, and melanomas. Exemplary carcinomas include, but are not limited to, those forming from tissue of the cervix, lung, prostate, breast, head and neck, colon and ovary. [069] This invention will be described in greater detail in the following Examples. 25 EXAMPLE I Materials and Methods [070] Mice - C57BL/6 mice were obtained from Harlan Winkelmann (Borchen, Germany) and used at 6-10 weeks of age. FLKO mice were developed as described (14) and bred at the Institute of Labortierkunde (University of Zurich). [071] Antibodies and Reagents - Recombinant (rec) flag-tagged murine (mu) FL was expressed in CHO cells and purified in house as previously described (10). recmuM-CSF and rechuM-CSF were obtained from Tebu-Bio (Frankfurt, Germany) and rechuM-CSF from R&D Systems (Wiesbaden, Germany). The cFMS Receptor Tyrosine Kinase Inhibitor (Cat. No. 344036) was obtained from EMD Biosciences (Darmstadt, Germany). Oligonucleotides containing CpG motifs (CpG2216 and CpG1668) were synthesized by TIB MOLBIOL (Berlin, Germany) according to published sequences (35). Imiquimod (R837) and palmitoyl-3-cysteine-serine-lysine 4 (Pam-3-Cys) were purchased from InvivoGen (San Diego, USA). Poly(cytidylic inosinic) acid (poly 1:C)), lipopolysaccharide (LPS) and 7-Allyl-7,8-dihydro-8 oxoguanosine (Loxoribine) were purchased from Sigma-Aldrich (Taufkirchen, Germany). All antibodies, unless otherwise stated, were obtained from Becton Dickinson, Germany, with the following exceptions: purified and FITC-conjugated anti-CD11c (rat clone 223H7) and anti-Ly49Q (Biozol Diagnostica Vertrieb GmbH, Eching, Germany), anti-mPDCA-1 (Miltenyi Biotec, Bergisch Gladbach, Germany) and anti-F4/80 (NatuTec GmbH, Frankfurt, Germany). Hybridomas, the supernatants of which were used in the depletion cocktail for ex vivo DC purification (21), were provided by Professor Ken Shortman, WEHI, Melbourne, Australia. [072] M-CSF and FL BM cultures - BM cells were flushed from femurs and tibiae of mice. Red cell lysis buffer (Sigma-Aldrich) was used to deplete red blood cells from the BM cell suspensions. BM cells were then either cultured directly or 26 after depletion. To deplete, BM cells were incubated for 30 min with rat antibodies to CD11c and CD45R (B220) followed by 30 min incubation with goat anti-rat magnetic beads (Qiagen, Hilden, Germany). The depletion procedure routinely removed 65 80% of total BM cells. It should be noted that depletion with the beads only, in the absence of the rat antibodies, also depleted about 50% of the BM cells, presumably via FcR/lg interactions. Total BM cells or depleted BM cells were cultured at 1.5 x 106 cells/mI in RPMI-1640 media (Gibco) supplemented with 10% FCS, 50pM beta mercaptoethanol, 100 IU/ml penicillin/streptomycin (complete media) and either 20 ng/ml recmuM-CSF or rechuM-CSF or 35 ng/ml recmuFL for 6-8 days at 37 0 C in a humidified atmosphere containing 5% C02. The M-CSF cultures were fed with fresh M-CSF every 3 days, without media change. [073] Surface staining of M-CSF or FL BM cultures - Harvested cells were washed in PBS containing 2% FCS and 2mM EDTA (FACS buffer). FcR binding was then blocked by incubation with 1 mg/ml purified anti CD16/32 monoclonal antibody (clone 2.4G2) for 20 mins on ice. An equal volume of 2 x concentrated specific antibody stain was then added to the cell suspension and incubated for a further 20 mins. Cells were washed in FACS buffer and resuspended in FACS buffer containing 1pg/ml propidium iodide. [074] Activation of DC subsets and analysis of cytokine production by ELISA - Unsorted M-CSF or FL BM cultures or sorted DCs (0.25- 0.5 x 106 cells/ml) were stimulated for 18-24 hrs in complete media with or without an added stimulus. The stimuli used were as follows: 1 ptg/ml Pam-3-Cys, 100 pg/mI poly(1:C), 1 pg/ml LPS, 1 pg/ml R837, 1 mM Loxoribine, 0.5 pM CpG2216, 0.5 pM CpG1668. Culture supernatants were assayed for the presence of IFN-a by two-site ELISA as previously described (21). Other cytokines (IL-12 p70, IL-6, TNF-a, MCP-1 and IFN 27 y) were measured using the Cytometric Bead Array, Mouse Inflammation Kit (Becton Dickinson). Stimulated DCs were blocked as stated above and stained with antibodies directed to CD8a, CD40, CD69, CD80 and CD86. [075] In vivo M-CSF treatment - Wild type and FLKO mice were treated ip with 10 pg of human M-CSF in 0.01% BSA (100 pl volume), or with vehicle alone, for five consecutive days. At the end of five days mice were sacrificed. The peritoneum was flushed 3 times with complete media and then organs were collected for DC purification. [076] Ex vivo DC purification - DCs were purified from spleens of M-CSF or vehicle treated mice essentially as previously described (21), using FACS buffer, RPMI and 1.077A Nycodenz (Progen Biotechnik GmbH, Heidelberg, Germany) that was adjusted to mouse osmolarity (308 mOsm). EXAMPLE2 Total BM cells cultured with M-CSF are potent producers of IFN-a in response to CpG-ODN [077] Culture of mouse bone marrow (BM) cells with FL for 8-10 days has been reported to lead to the generation of millions of highly pure pDCs and cDCs that closely resemble the DC populations of steady state mouse spleen (19). To routinely test the kinetics of pDC development within these FL cultures, the IFN-a producing capacity of total BM cells incubated with FL in a multiwell format over a time course of 0-7 days was analyzed. A parallel culture of total BM cells with human M-CSF was included as a negative control, because M-CSF is routinely used to generate macrophages from BM cells. Usually, the media and M-CSF are replaced every few days and only adherent cells are harvested at the end of a culture period of 7 days or longer (17). Instead, M-CSF cultures were treated exactly as the FL cultures, and wells of total M-CSF cultures (including adherent and non 28 adherent cells) were analyzed for IFN-a production in response to CpG-2216. Unexpectedly, IFN-a was induced to high levels in the M-CSF cultures (Figure 1). Moreover, the IFN-a produced in response to CpG-2216 increased with culture time, implying that IFN-ax producing cells were being generated in the course of the M-CSF culture. EXAMPLE 3 The IFN-a producers induced in M-CSF BM cultures display characteristics of pDC, but develop without the influence of FL [078] Depletion of pDCs and cDCs from total BM cells depleted the CpG induced IFN-a producing capacity of BM cells. When the DC-depleted BM cells were cultured with human M-CSF for 6 days, potent IFN-a producing capacity was detected again in the non-adherent cells of the M-CSF culture. [079] To determine whether any of the non-adherent M-CSF-generated cells displayed the phenotype expected of a pDCs they were stained with CD1 1 c and CD45RA. Indeed, a population of 10-20% of cells within the cultures expressed high levels of CD45RA and medium levels of CD1 1 c, together with the lack of CD3, CD1 9 or CD49b or NK1.1 expression, low side scatter and forward scatter this was commensurate with the phenotype of pDCs (Figure 2a). [080] It was clear that M-CSF could drive pDC development yet the yield of pDCs was substantially less then that obtained with FL. When the two cultures were compared side by side after six days of culture, M-CSF was approximately 10-fold less efficient than FL. Day 6 was chosen for comparison because after this stage the M-CSF cultures become very acidic and sorted DC populations from the cultures died much more rapidly in culture and failed to produce cytokines. [081] To determine if M-CSF induction of pDCs requires endogenous FL, replicate BM cultures were studied from mice in which the FL gene had been ablated 29 (FLKO mice). Total cell numbers obtained from the BM cultures of FLKO mice were reduced whether cultures were conducted with M-CSF addition or FL addition (Figure 1 b), indicating that pDCs developed even in the absence of FL. It was clear that cells with the phenotype and morphology of pDCs were produced by culture of BM first depleted of any DC populations and in the presence of only exogenous M CSF, without the potential influence of any FL. These M-CSF generated pDCs are referred to as M-pDC. EXAMPLE 4 Detailed surface phenotype of M-pDC [082] Extensive phenotyping of the M-pDCs from wild type and FLKO mice shows that the elicited pDCs displayed an identical phenotype for over 40 surface markers. When compared to pDCs generated in vitro with FL (FL-pDC), numerous cell surface markers were different between the pDCs generated with the two different cytokines. In fact, as seen in Figure 3, the M-pDCs displayed a phenotype that appeared for many markers to be intermediate between FL-pDC and ex-vivo isolated spleen pDC. Molecules that are recognized as differentiation markers of pDC; Ly49Q (20), CD4 (21) and MHCII, are all higher on M-pDCs than on FL-pDCs and are very similar to the levels on spleen pDCs. The M-pDCs express a similarly high level of Ly6C as the ex vivo spleen pDCs. Because it has recently been shown that pDC develop from Ly6C- precursors, M-pDCs represent a further differentiated state in pDC development than the FL-pDC (22). [083] The surface expression of B7H1, CD81, CD62L and CD11b on the M pDCs also more closely resemble the ex-vivo pDCs than the FL-pDCs. In contrast, M-pDCs express the lowest levels of CD44 and a spread of surface CD24 bridging the high levels expressed by spleen pDCs and the low levels expressed by FL pDCs. Also, M-pDCs express low levels of F4/80. The low F4/80 expression lends 30 a "myeloid" phenotype to the M-pDCs. M-pDCs express very low levels of CD1 1b (in the order of 10-fold lower than FL-pDC) and do not express Ly6G. EXAMPLE 5 Highly purified M-pDCs are activated by ligands for TLR7 and TLR9 [084] Similar to FL-pDCs and ex-vivo pDCs, sorted M-pDC were activated with ligands for TLR7 and 9. In response to a type A CpG-ODN (CpG2216), M pDCs produced high levels of IFN-a. In response to the TLR7 ligand R837, the M pDCs produced low but substantial levels of IFN-a (Figure 4A). Other cytokines induced by these TLR ligands included IL-6 and TNF-a (Figure 4B), with highest levels induced by R837. [085] After overnight stimulation, the M-pDCs were examined for changes in phenotype. The M-pDCs, similarly to FL-pDCs and ex-vivo pDCs, expressed elevated levels of CD8a, CD69, CD86 and CD40 upon TLR stimulation. EXAMPLE 6 Conventional DCs also develop in M-CSF BM cultures [086] As shown in Figure 1, CD11c' cells that did not concomitantly express CD45RA or T, B or NK cell markers were also induced in the M-CSF BM cultures. Like the pDCs, these cells also developed in BM cultures of FLKO mice. Surface phenotype analyses of these cells reveals that they resemble conventional (c) DC, expressing costimulation markers and MHCII (Figure 5). These cDCs were of quite a mature phenotype, with higher levels of surface MHCII and co-stimulation markers than the cDCs induced in FL cultures. Upon TLR stimulation the cDC were activated to resemble mature, ex-vivo activated cDCs. [087] Of interest in the activation of the M-cDCs and M-pDCs is that they respond well to TLR9 ligands. This is quite a different scenario from macrophages 31 within M-CSF cultures that downregulate TLR9 and consequently respond poorly to TLR9 ligands (23). EXAMPLE 7 M-pDC generation is dependent upon active c-fms [088] It has previously been shown that Flt3+ cells within the BM common myeloid progenitors (CMP) and common lymphoid progenitors (CLP) are the precursors of cDCs and pDCs within mouse lymphoid organs (24,25) and it has been assumed that FL is essential for DC development. Thus the generation of DCs, particularly the generation of pDCs with a typical monocytic poietin, in the absence of FL, was unexpected. As shown in Figure 1 the M-pDCs clearly develop in the absence of FL, yet they also show many similarities to FL-pDCs. Given that M-CSF and FL, as well as their respective receptors, Flt3 and c-fms, have structural similarities, whether M-CSF was signalling through Flt3 was investigated. That is, whether M-CSF would act as a FL surrogate that also signalled via Flt3, generating M-CSF-induced "FL-DCs" was examined. [089] There are numerous inhibitors of receptor tyrosine kinases, with varying levels of cross-reactivity. The inhibitor cFMS Receptor Tyrosine Kinase Inhibitor (Calbiochem), which is reported to be a highly specific c-fms inhibitor, was used. Indeed, cFMS Receptor Tyrosine Kinase Inhibitor entirely blocked the haematopoietic effects of M-CSF over a broad concentration range (Figure 6). The highest concentrations of inhibitor also blocked the generation of DCs by FL. However, cFMS inhibitor used in the range of 0.63-1.3 pM had only minor effects on FL-mediated FL-DC generation (Figure 6). Since these same low concentrations still blocked M-DC generation, M-pDC generation does not involve M-CSF acting through the Flt3 receptor tyrosine kinase. Although the c-fms inhibitor is likely a promiscuous receptor tyrosine kinase inhibitor at high concentrations, at low 32 concentrations it did not inhibit FL-DC generation, demonstrating that it is, indeed, c fms specific. Thus, M-pDCs and M-cDCs can be generated by M-CSF via c-FMS signalling, independently of Flt3 and FL in vitro. EXAMPLE 8 Administration of M-CSF in vivo [090] The FLKO mice display a major defect in pDC and cDC numbers. In the spleen they have only about 12% of the DC numbers of wild type C57BL/6 mice. These mice also have reduced CFU-GM cells within BM (14). As FL acts upon early hematopoietic stem cells, as well as later stage DC precursors, it is possible that the major reduction of DCs within these mice is a due to a combination of a lack of DCs that require FL for later development and of DCs that require FL for early precursor development, but are independent of FL once committed to the DC lineage. On the other hand, it has recently been reported that op/op mice that carry a mutation in the gene for M-CSF and thus lack functional M-CSF, exhibit reduced numbers of splenic DC (26). Specifically, cDCs were reduced about 2-fold and pDCs about 3-fold. This was substantially less of an effect than that seen in mice lacking FL, but nevertheless, the lack of M-CSF did result in reduced cell DC numbers, a result consistent with the in vitro data. [091] Whether exogenously applied M-CSF increases DC numbers in mice was tested. Previously, exogenous M-CSF concentrations in the range of 10 to 200 tg/day. The source and specific activity of M-CSF used has varied widely and consequently and it has been reported that M-CSF has an extremely short half-life of only 10 mins in circulation (27). Here, M-CSF was administered at 10 pig/day of M-CSF to C57BL/6 or FLKO mice for 5 days. After the 5 day application period, a huge increase in F4/80hi cells was observed in peritoneal lavages of the one treated 33 wild type and treated FLKO mice that we examined. These observations indicate that indeed M-CSF was inducing an effect in vivo. Analysis of DC populations in the spleen revealed that M-CSF was inducing a reproducible increase of about 2-fold in both pDC and cDC numbers. This increase was evident in both the single C57BL/6 and all 3 FLKO mice (Figure 7). A more extensive DC increase (greater than 6-fold) was evident in the peritoneal lavage of the C57BL/6 mouse examined. EXAMPLE9 Administration of another M-CSF receptor ligand, IL-34 [092] To determine whether other M-CSF receptor ligands could have the same effect as M-CSF, depleted mouse bone marrow cells of cDC and pDC (via rat monoclonal antibodies to CD11c and B220 and anti rat igG magnetic bead depletion) were incubated with 50ng/ml each of either human M-CSF or mouse IL-34 (R&D systems, Wiesbaden, Germany), [093] On day 6, the cells were analyzed by FACS for the presence of pDC and cDC. FACS analysis of bone marrow cultures after 6 days with M-CSF or IL-34 revealed that both factors promoted the development of cells with the phenotype of pDC and cDC, [094] In addition, the depleted bone marrow cells cultured with the corresponding growth factors for 0 or 6 days were stimulated with the TLR9 ligand CpG-2216. The analysis of the cell culture supernatant revealed that IFN-alpha was produced in response to CpG-2216, a functional feature of pDC (figure 1). IFN-alpha production in response to CpG-2216 is a functional feature indicative of the presence of pDC. Thus, the pDC that developed under the influence of either M-CSF or IL-34 are functional. The data shows that IL-34, like M-CSF, is capable of generating pDC and cDC after 6 days culture of bone marrow cells. Thus, signaling through the M CSF receptor by an M-CSF- receptor ligand can cause the generation of DCs. 34 References [095] The following references are cited herein. 1. Steinman,R.M. and K.Inaba. 1999. Myeloid dendritic cells. J Leukoc. Biol. 66:205-208. 2. Shortman,K. and Y.J.Liu. 2002. Mouse and human dendritic cell subtypes. Nat. Rev. Immunol. 2:151-161. 3. Scheicher,C., M.Mehlig, R.Zecher, and K.Reske. 1992. Dendritic cells from mouse bone marrow: in vitro differentiation using low doses of recombinant granulocyte-macrophage colony-stimulating factor. J Immunol Methods 154:253-264. 4. 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