Prevalent and Diverse Intratumoral Oncoprotein-Specific CD8⁺ T Cells within Polyomavirus-Driven Merkel Cell Carcinomas

Merkel cell carcinoma (MCC) is often caused by persistent expression of Merkel cell polyomavirus (MCPyV) T-antigen (T-Ag). These non-self proteins comprise about 400 amino acids (AA). Clinical responses to immune checkpoint inhibitors, seen in about half of patients, may relate to T-Ag–specific T cells. Strategies to increase CD8⁺ T-cell number, breadth, or function could augment checkpoint inhibition, but vaccines to augment immunity must avoid delivery of oncogenic T-antigen domains. We probed MCC tumor-infiltrating lymphocytes (TIL) with an artificial antigen-presenting cell (aAPC) system and confirmed T-Ag recognition with synthetic peptides, HLA-peptide tetramers, and dendritic cells (DC). TILs from 9 of 12 (75%) subjects contained CD8⁺ T cells recognizing 1–8 MCPyV epitopes per person. Analysis of 16 MCPyV CD8⁺ TIL epitopes and prior TIL data indicated that 97% of patients with MCPyV⁺ MCC had HLA alleles with the genetic potential that restrict CD8⁺ T-cell responses to MCPyV T-Ag. The LT AA 70–110 region was epitope rich, whereas the oncogenic domains of T-Ag were not commonly recognized. Specific recognition of T-Ag–expressing DCs was documented. Recovery of MCPyV oncoprotein–specific CD8⁺ TILs from most tumors indicated that antigen indifference was unlikely to be a major cause of checkpoint inhibition failure. The myriad of epitopes restricted by diverse HLA alleles indicates that vaccination can be a rational component of immunotherapy if tumor immune suppression can be overcome, and the oncogenic regions of T-Ag can be modified without impacting immunogenicity.