Antibodies to Feline McDonough Sarcoma (FMS)-like tyrosine kinase 3 receptor ligand (FLT3L) and uses thereof for treating autoimmune and inflammatory diseases

1. An antibody or antigen-binding fragment thereof that specifically binds to Feline McDonough Sarcoma (FMS)-like tyrosine kinase 3 receptor (FLT3) ligand (FLT3L), comprising a set of Complementarity-Determining Regions (CDRs): HCDR1, HCDR2, HCDR3, LCDR1, LCDR2, and LCDR3, wherein the HCDR1, HCDR2, HCDR3, LCDR1, LCDR2, and LCDR3 comprise the amino acid sequences of

SEQ ID NOs: 29, 30, 31, 32, 33, and 34, respectively.

2. The antibody or antigen-binding fragment thereof of claim 1 , comprising a heavy chain variable region (VH) and light chain variable region (VL), wherein the VH and VL regions comprise an amino acid sequence having at least 95%, 96%, 97%, 98%, or 99% sequence identity to

SEQ ID NO: 1 and SEQ ID NO: 2, respectively.

3. The antibody or antigen-binding fragment thereof of claim 2 , wherein the VH and VL comprise the amino acid sequence of:

SEQ ID NO: 1 and SEQ ID NO: 2, respectively.

4. The antibody or antigen-binding fragment thereof according to claim 1 , which is a monoclonal antibody, a recombinant antibody, a human antibody, a humanized antibody, or a chimeric antibody.

5. The antibody or antigen-binding fragment thereof according to claim 1 , wherein the antibody or antigen-binding fragment comprises a heavy chain immunoglobulin constant domain selected from the group consisting of:

(a) an IgA constant domain;

(b) an IgD constant domain;

(c) an IgE constant domain;

(d) an IgG1 constant domain;

(e) an IgG2 constant domain;

(f) an IgG3 constant domain;

(g) an IgG4 constant domain; and

(h) an IgM constant domain.

6. The antibody or antigen-binding fragment thereof according to claim 1 , wherein the antibody or antigen-binding fragment comprises a light chain immunoglobulin constant domain selected from the group consisting of:

(a) an Ig kappa constant domain; and

(b) an Ig lambda constant domain.

7. An isolated nucleic acid molecule encoding the antibody or antigen-binding fragment thereof according to claim 1 .

8. A host cell transformed with the nucleic acid of molecule of claim 7 .

9. A hybridoma producing the antibody or antigen-binding fragment of claim 1 .

10. A method of making the antibody or antigen-binding fragment thereof according to claim 1 , comprising (a) culturing a host cell expressing said antibody or antigen-binding fragment thereof; and (b) isolating said antibody or antigen-binding fragment thereof from said cultured host cell.

11. A method for treating an autoimmune disease, comprising:

administering to a subject in need thereof a pharmaceutically effective amount of the antibody or antigen-binding fragment thereof according to claim 1 .

12. A The method of claim 11 , wherein the autoimmune disease is for Sjögren's Syndrome, nephritis, myositis or systemic lupus erythematosus (SLE).

13. A method of neutralizing membrane bound FLT3L in a subject in need thereof, the method comprising administering to the subject a pharmaceutically effective amount of an antibody or antigen-binding fragment thereof according to claim 1 .

14. A method of reducing populations of circulating classical dendritic cells (cDCs) and plasmacytoid dendritic cells (pDCs) in a subject in need thereof, the method comprising administering to the subject a pharmaceutically effective amount of an antibody or antigen-binding fragment thereof according to claim 1 .

15. A method of reducing FLT3L expression on CD4+T cells, comprising administering to a subject in need thereof a pharmaceutically effective amount of an antibody or antigen-binding fragment thereof according to claim 1 .

16. A method of reducing the percentage of CD4+T cells expressing FLT3L, comprising administering to a subject in need thereof a pharmaceutically effective amount of an antibody or antigen-binding fragment thereof according to claim 1 .

17. A method of reducing ERK signaling in a lymphoblast, comprising contacting the lymphoblast with an antibody or antigen-binding fragment thereof according to claim 1 .

18. A method of reducing MEK 1/2 phosphorylation in primary CD133+human stem cells, comprising contacting the stem cells with an antibody or antigen-*binding fragment thereof according to claim 1 .

19. A human, monoclonal antibody that specifically binds to FLT3L, the antibody comprising:

i. complementarity-Determining Regions (CDRs) HCDR1, HCDR2, HCDR3, LCDR1, LCDR2, and LCDR3, wherein the HCDR1, HCDR2, HCDR3, LCDR1, LCDR2, and LCDR3 comprise the amino acid sequences of SEQ ID NOs:29, 30, 31, 32, 33, and 34, respectively;

ii. a human IgG1 heavy chain constant domain; and

iii. a human Ig lambda light chain constant domain.

20. The human, monoclonal antibody of claim 19 , comprising a heavy chain comprising the amino acid sequence of SEQ ID NO:61 and a light chain comprising the amino acid sequence of SEQ ID NO:62.

21. A pharmaceutical composition comprising the antibody of claim 20 .