A small molecule inhibitor of VSIG-8 prevents its binding to VISTA

The V-region immunoglobulin-containing suppressor of T cell activation (VISTA), a unique B7 family member, is an attractive immunotherapeutic target for cancer, autoimmune and inflammatory diseases. In 2016, a patent demonstrated V-Set and Immunoglobulin domain containing 8 (VSIG-8) was the putative VISTA receptor. Antagonistic or agonistic agents can conceivably modulate VISTA and its interacting partners, which will greatly benefit the treatment of many diseases. The interaction of VISTA and VSIG-8 were measured by Enzyme-linked Immuno Sorbent Assay (ELISA), Microscale Thermophoresis (MST) and coimmunoprecipitation (Co-IP) experiments. The bioactivity of VSIG-8 inhibitor L557-0155 was evaluated in vitro and in vivo. K_d value of human VISTA binding to human VSIG-8 was 1.58 ± 0.44 μM by MST. When the related amino acid binding site of VISTA was mutated to alanine, the interaction between VISTA and VSIG-8 disappeared. VSIG-8 protein induced an decrease in the level of IL-2 in VISTA-overexpressing cells but a increase in VISTA^-/- cells. Furthermore, VSIG-8 inhibitor L557-0155 promoted cytokine production and cell proliferation in PBMCs and suppressed melanoma growth. VSIG-8/VISTA coinhibitory pathway may provide a novel strategy for the treatment of human cancers, and VSIG-8 blockade may increase antitumor immunity. This study was the first time to report that VSIG-8 interacts with VISTA, and inhibited T cell function.