Lymphatic PD-L1 Expression Restricts Tumor-Specific CD8+ T-cell Responses.

Lymph node (LN)-resident lymphatic endothelial cells (LEC) mediate peripheral tolerance by self-antigen presentation on MHC-I and constitutive expression of T-cell inhibitory molecules, including PD-L1 (CD274). Tumor-associated LECs also upregulate PD-L1, but the specific role of lymphatic PD-L1 in tumor immunity is not well understood. In this study, we generated a mouse model lacking lymphatic PD-L1 expression and challenged these mice with two orthotopic tumor models, B16F10 melanoma and MC38 colorectal carcinoma. Lymphatic PD-L1 deficiency resulted in consistent expansion of tumor-specific CD8+ T cells in tumor-draining LNs in both tumor models, reduced primary tumor growth in the MC38 model, and increased efficacy of adoptive T-cell therapy in the B16F10 model. Strikingly, lymphatic PD-L1 acted primarily by inducing apoptosis in tumor-specific CD8+ central memory T cells. Overall, these findings demonstrate that LECs restrain tumor-specific immunity via PD-L1, which may explain why some patients with cancer without PD-L1 expression in the tumor microenvironment still respond to PD-L1/PD-1-targeted immunotherapy. SIGNIFICANCE: A new lymphatic-specific PD-L1 knockout mouse model reveals that lymphatic endothelial PD-L1 expression reduces tumor immunity, inducing apoptosis in tumor-specific CD8+ central memory cells in tumor-draining lymph nodes.