The receptor for VISTA is identified (VSIG8) as well as the use of this receptor in the identification or synthesis of agonist or antagonist compounds, preferably antibodies, polypeptides and fusion proteins which agonize or antagonize the effects of VSIG8 and/or VISTA and/or the VSIG8/VISTA binding interaction. These antagonists may be used to suppress VISTA's suppressive effects on T cell immunity, and more particularly used in the treatment of cancer, or infectious disease. These agonist compounds may be used to potentiate or enhance VISTA's suppressive effects on T cell immunity and thereby suppress T cell immunity, such as in the treatment of autoimmunity, allergy or inflammatory conditions. Screening assays for identifying these agonists and antagonist compounds are also provided.