Data from Human CD4<sup>+</sup> T Cells Specific for Merkel Cell Polyomavirus Localize to Merkel Cell Carcinomas and Target a Required Oncogenic Domain

Abstract

<div>Abstract<p>Although CD4<sup>+</sup> T cells likely play key roles in antitumor immune responses, most immuno-oncology studies have been limited to CD8<sup>+</sup> T-cell responses due to multiple technical barriers and a lack of shared antigens across patients. Merkel cell carcinoma (MCC) is an aggressive skin cancer caused by Merkel cell polyomavirus (MCPyV) oncoproteins in 80% of cases. Because MCPyV oncoproteins are shared across most patients with MCC, it is unusually feasible to identify, characterize, and potentially augment tumor-specific CD4<sup>+</sup> T cells. Here, we report the identification of CD4<sup>+</sup> T-cell responses against six MCPyV epitopes, one of which included a conserved, essential viral oncogenic domain that binds/disables the cellular retinoblastoma (Rb) tumor suppressor. We found that this epitope (WED<sub>LT209-228</sub>) could be presented by three population-prevalent HLA class II alleles, making it a relevant target in 64% of virus-positive MCC patients. Cellular staining with a WED<sub>LT209-228</sub>–HLA-DRB1*0401 tetramer indicated that specific CD4<sup>+</sup> T cells were detectable in 78% (14 of 18) of evaluable MCC patients, were 250-fold enriched within MCC tumors relative to peripheral blood, and had diverse T-cell receptor sequences. We also identified a modification of this domain that still allowed recognition by these CD4<sup>+</sup> T cells but disabled binding to the Rb tumor suppressor, a key step in the detoxification of a possible therapeutic vaccine. The use of these new tools for deeper study of MCPyV-specific CD4<sup>+</sup> T cells may provide broader insight into cancer-specific CD4<sup>+</sup> T-cell responses.</p></div>