Fig. 1

Current immune checkpoint receptors and their respective ligands. Many immune checkpoints expressed on the surface of T cells, such as PD-1, CTLA-4, LAG-3, TIGIT, VISTA, and TIM-3, bind to their respective ligands on APCs and/or tumor cells, eliciting positive and/or negative activity in the T cell response. TIM-3 also participates in associated signaling through PtdSer, HMGB-1 and Gal-9 in dying tumor cells. Notably, checkpoints such as PD-L1, CD80, CD226, and VISTA (B7-H5) are expressed on both T cells and APC/tumor cells. B7-H3 is also expressed on the surface of both T cells and APC/tumor cells, but its receptors have not been clearly elucidated, which has engendered great enthusiasm in cancer immunology investigators. In this article, we identify TLT-2, IL20RA, and PLA2R1 as three potential receptors for B7-H3. “+” in green indicates the immunostimulatory (positive) signal, and “−” in red indicates the immunosuppressive (negative) signal. PtdSer, phosphatidylserine; HMGB-1, high-mobility group protein B1; Gal-9, galectin-9