Method of identifying anti-VISTA antibodies

1. A method of identifying an antibody or antigen-binding fragment thereof that binds a V-domain Ig Suppressor of T cell Activation (VISTA) protein, wherein the antibody or antigen-binding fragment thereof elicits a biological response, comprising:

a) providing different antibodies that each bind VISTA, or an antigen-binding fragment thereof, wherein the different antibodies or antigen-binding fragments each comprise an antibody VH domain comprising a VH CDR1 having the amino acid sequence of SEQ NO:25, a VH CDR2 having the amino acid sequence of SEQ ID NO:26, and a VH CDR3 having the amino acid sequence of SEQ ID NO:27, and which each further comprise an antibody VL domain comprising a VL CDR1 having the amino acid sequence of SEQ ID NO:28, a VL CDR2 having the amino acid sequence of SEQ ID NO:29, and a VL CDR3 having the amino acid sequence of SEQ ID NO:30;

b) determining which of these different antibodies or antigen-binding fragments thereof induces a biological response, wherein the biological response is selected from the group consisting of:

i) activation of monocytes;

ii) activation of T cells;

iii) a decrease in the number of circulating immune cells;

iv) a decrease in the number of granulocytes in bone marrow and spleen;

v) an increase in the number of neutrophils, macrophages or both in a tumor microenvironment;

vi) an increase in the level of one or more cytokines; and

vii) a combination of any of (i)-(vi); and

further wherein antibodies which elicit said biological response are detected using an in vitro assay that detects macrophage-mediated phagocytosis of cells expressing VISTA; and

c) selecting an antibody or antigen-binding fragment thereof of (b) from among said different antibodies each possessing the VH CDR polypeptides of SEQ ID NO: 25, 26 and 27 and the VL CDR polypeptides of SEQ ID NO: 28, 29 and 30 that induce said biological response.

2. The method of claim 1 , wherein the different antibodies each comprise an Fc region that binds to an Fc receptor on immune cells.

3. The method of claim 2 , wherein the Fc receptor is a CD16 receptor.

4. The method of claim 1 , wherein the antibody or antigen-binding fragment thereof comprises an antibody VH domain comprising the amino acid sequence of SEQ ID NO:37, and/or wherein the antibody or antigen-binding fragment thereof comprises an antibody VL domain comprising the amino acid sequence of SEQ ID NO:44.

5. The method of claim 1 , wherein the method further comprises an in vitro assay selected from the group consisting of an immunohistochemical (IHC) staining assay, a cytokine release assay, a chemokine release assay, a cell activation assay, a cell proliferation assay, a cell migration assay, and a flow cytometry assay.