The Inhibitory Signaling Receptor Protocadherin-18 Regulates Tumor-Infiltrating CD8+ T-cell Function.

Cancers are infiltrated with antitumor CD8+ T cells that arise during tumor growth, but are defective in effector phase functions because of the suppressive microenvironment. The reactivation of TILs can result in tumor destruction, showing that lytic dysfunction in CD8+ tumor-infiltrating lymphocytes (TIL) permits tumor growth. Like all memory T cells, TILs express inhibitory signaling receptors (aka checkpoint inhibitor molecules) that downregulate TCR-mediated signal transduction upon TIL interaction with cells expressing cognate ligands, thereby restricting cell activation and preventing the effector phase. Previously, we identified a novel murine CD8+ TIL inhibitory signaling receptor, protocadherin-18, and showed that it interacts with p56lck kinase to abrogate proximal TCR signaling. Here, we show that TILs from mice deleted in protocadherin-18 had enhanced antitumor activity and that coblockade of PD-1 and protocadherin-18 in wild-type mice significantly enhanced TIL effector phase function. These results define an important role for protocadherin-18 in antitumor T-cell activity. Cancer Immunol Res; 5(10); 920-8. ©2017 AACR.