Circulating neoantigen- and viral oncoprotein-specific CD8+ T cells share a transcriptional signature.

Tumor-specific CD8+ T cells in blood appear to be important for and predictive of response to anti-PD-1 therapies. However, as most tumor antigens are unique to a given patient, identification of tumor-specific CD8+ T cells is not routinely feasible. Here, we characterized polyomavirus-specific CD8+ T cells from blood of 17 patients with virus-driven Merkel cell carcinoma (MCC). We identified a 98-gene signature, SPoTT (Signature of Peripheral Tumor-specific CD8+ T cells), that discriminated circulating tumor-specific CD8+ T cells from other T cells in immunotherapy-naïve patients. We observed profound transcriptomic differences among tumor-specific CD8+ T cells from blood versus from tumor. In validation cohorts of MCC, as well as neoantigen-driven cancers, SPoTT was able to identify viral oncoprotein- and neoantigen-specific CD8+ T cells with both sensitivity and specificity above 75%. We also tested a previously described 151-gene signature (NeoTCR_PBL) trained on neoantigen-specific CD8+ T cells and found it was able to recognize MCPyV-specific T cells with sensitivity of 66% and a specificity of 88%. These findings show that circulating tumor-specific CD8+ T cells share fundamental characteristics across diverse tumor antigen types. More broadly, insights into antitumor T cells gained from virus-driven cancers are also likely to be relevant in mutationally-driven cancers.