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J Immunother Cancer. 2018 Nov 27;6(1):131. doi: 10.1186/s40425-018-0450-7.

Merkel cell polyomavirus-specific immune responses in patients with Merkel cell carcinoma receiving anti-PD-1 therapy.

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Department of Medicine, Divisions of Dermatology and Medical Oncology, University of Washington, 850 Republican Street, Seattle, WA, 98109, USA.
Cancer Immunotherapy Trials Network, Fred Hutchinson Cancer Research Center, Seattle, WA, USA.
Comprehensive Cancer Center, Section of Medical Oncology, Yale University School of Medicine, New Haven, CT, USA.
Division of Medical Oncology, Department of Medicine, University of Washington, Seattle, WA, USA.
Department of Surgery, Johns Hopkins University School of Medicine, and Johns Hopkins Bloomberg~Kimmel Institute for Cancer Immunotherapy, Baltimore, MD, USA.
Department of Medicine, Divisions of Dermatology and Medical Oncology, University of Washington, 850 Republican Street, Seattle, WA, 98109, USA.



Merkel cell carcinoma (MCC) is an aggressive skin cancer that frequently responds to anti-PD-1 therapy. MCC is associated with sun exposure and, in 80% of cases, Merkel cell polyomavirus (MCPyV). MCPyV-specific T and B cell responses provide a unique opportunity to study cancer-specific immunity throughout PD-1 blockade therapy.


Immune responses were assessed in patients (n = 26) with advanced MCC receiving pembrolizumab. Peripheral blood mononuclear cells (PBMC) were collected at baseline and throughout treatment. MCPyV-oncoprotein antibodies were quantified and T cells were assessed for MCPyV-specificity via tetramer staining and/or cytokine secretion. Pre-treatment tumor biopsies were analyzed for T cell receptor clonality.


MCPyV oncoprotein antibodies were detectable in 15 of 17 (88%) of virus-positive MCC (VP-MCC) patients. Antibodies decreased in 10 of 11 (91%) patients with responding tumors. Virus-specific T cells decreased over time in patients who had a complete response, and increased in patients who had persistent disease. Tumors that were MCPyV(+) had a strikingly more clonal (less diverse) intratumoral TCR repertoire than virus-negative tumors (p = 0.0001).


Cancer-specific T and B cell responses generally track with disease burden during PD-1 blockade, in proportion to presence of antigen. Intratumoral TCR clonality was significantly greater in VP-MCC than VN-MCC tumors, suggesting expansion of a limited number of dominant clones in response to fewer immunogenic MCPyV antigens. In contrast, VN-MCC tumors had lower clonality, suggesting a diverse T cell response to numerous neoantigens. These findings reveal differences in tumor-specific immunity for VP-MCC and VN-MCC, both of which often respond to anti-PD-1 therapy.


Anti-PD-1; Immunotherapy; Merkel cell carcinoma; Merkel cell polyomavirus; Pembrolizumab; T cell; Viral cancer antigen

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