Background Merkel cell carcinoma (MCC) is an aggressive human skin cancer primarily induced by Merkel Cell Polyomavirus (MCPyV) driven by expression of the oncogenic T antigens (T-Ags): Large T and Small T antigen. Checkpoint inhibition therapy blocking the programmed cell death protein-1 (PD-1) pathway has proven effective with a clinical response rate up to 58%,¹ highlighting the critical role of immune surveillance for tumor control. Yet, evidence for the impact of T-Ag-specific T cells following immunotherapy is still limited.

Methods Potential CD8+ T cell epitopes derived from the T-Ags and the Viral capsid protein 1 (VP1) were predicted with netMHCpan 4.0 as 9- and 10-mer peptides with a rank score <2 for binding to 33 different HLA class I types. Peripheral blood mononuclear cells (PBMC) were obtained from 24 MCPyV+ MCC patients prior and post anti-PD-1 therapy initiation (CITN-09/Keynote-017). T cell recognition of the MCPyV-derived peptides during therapy was evaluated using the high-throughput technology of DNA barcode labeled pMHC multimers. Phenotypic characteristics of multimer-binding T cells were evaluated for selected patients.

Results Across all patients, 40 T-Ag-specific CD8+ T-cell populations were detected recognizing 31 epitopes restriction to 14 different HLA class I types. T-Ag-specific CD8+ T cells were detected in responders (complete and partial response, n=17) during therapy with a trend of increased number of responses observed after therapy initiation. Whereas only a single T-Ag-specific population was detectable in non-responders (stable and progressive disease, n=7). Moreover, the T cell repertoire after therapy initiation was significantly increased in the responder group compared to non-responder with the T-Ag-specific T cells showing an activated effector memory phenotype.

Conclusions The current study indicates that the T-Ag-specific T cells are associated with clinical benefit to checkpoint inhibitor therapy. Furthermore, the broad identification of novel T-Ag-derived T cell epitopes could potentially facilitate the use of targeted T cell therapy to enhance T cell recognition and clearance of MCC in combination with checkpoint inhibition.

Acknowledgements Supported by the Cancer Immunotherapy Trials Network (CITN)

Reference

1. Nghiem P, Bhatia S, Lipson E. Three-year survival, correlates and salvage therapies in patients receiving first-line pembrolizumab for advanced Merkel cell carcinoma. J Immunother cancer 2021;9:e002478.