P-Selectin on activated platelets inhibits the anti-tumor activity of CD8+ T cells through PSGL-1 binding

Platelets are anucleated blood cells that are crucial in hemostasis and thrombosis. They interact with cancer cells to facilitate tumor progression and enhance malignancy. However, their direct effects on the anti-tumor activity of CD8 + T cells remain not yet fully clarified. Herein, we first confirmed that the combination therapy of anti-platelet agents and PD-L1 blockade immune checkpoint inhibitory antibody resulted in a synergistic anti-tumor effect, and found that activated platelets adhered to the tumor-infiltrating CD8 + T cells. The binding was reduced by the treatment of anti-platelet agent with anti-PD-L1 antibody, thereby enhancing CD8 + T cell activation. Then, we examined whether platelets directly affect the antigen-specific cancer-killing activity of CD8 + T cells using an in vitro co-culture system consisting of cancer cells and iPSC-derived cytotoxic T lymphocytes (CTL3-3) that specifically recognize the WT1 antigen expressed on cancer cells. Here, platelets suppressed the cytotoxic activity of CTL3-3. Although supernatants from activated platelets partially suppressed CTL3-3 cytotoxicity, this effect was less effective than the suppression caused by intact platelets. Flow cytometry analysis revealed that activated platelets, expressing P-selectin (CD62P), directly interact with CTL3-3, which was further confirmed by immunofluorescence staining and electron microscopy. Blocking P-selectin or its counterpart PSGL-1 on CTL3-3 with neutralizing antibodies inhibited this adhesion and restored CTL3-3 cytotoxic activity in the presence of platelets. These results suggest that platelets bind to and suppress the WT1-specific activity of CTL3-3 via P-selectin and PSGL-1 binding without affecting CTL3-3 proliferation.