Ligand-Receptor Interactions of V-domain Ig-Containing Suppressor of T Cell Activation and Programmed Death-1 Suppress the Anti-Cancer Activities of T Cells

Background 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 PD-1 and binds it as a ligand. We found that when interacting with PD-1, VISTA suppresses IL-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 Kd of approximately 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.