Ligand-receptor interactions of V-domain Ig-containing suppressor of T cell activation and programmed death-1 suppress the anticancer activities of T cells.

Background: V-domain immunoglobulin-containing suppressor of T cell activation (VISTA) is a unique multifunctional immune checkpoint protein, which can display both receptor and ligand properties. It plays a crucial role in the cancer immune evasion machinery operated by a wide range of human malignancies and may thus be considered as a potential target for immunotherapy of cancer. Receptors of VISTA through which this protein transmits immunosuppressive signals under various normal and pathological conditions remain to be identified. Materials and methods: To conduct the study, we used human recombinant proteins and various human cell lines as well as primary T cells. A wide range of techniques including tissue culture and co-cultures, Western blot analysis, on-cell Western, ELISA, co-immunoprecipitation, biochemical assays and synchrotron radiation circular dichroism spectroscopy were employed. Results: Here we report for the first time that VISTA has affinity to programmed cell death protein 1 (PD-1) and binds it as a ligand. We found that when interacting with PD-1, VISTA suppresses interleukin 2 production by T helper cells. These effects were confirmed in the in vitro and ex vivo experiments. Affinity of VISTA to PD-1 was also characterised and found to be moderate, with a K d of ∼2.3 μM detected by synchrotron radiation circular dichroism spectroscopy. Conclusions: These results open a completely new chapter in our understanding of the concept of immune checkpoint proteins, where some of them clearly show both ligand and receptor activities and display multifunctional properties.