Association of Merkel Cell Polyomavirus Status With p53, RB1, and PD-L1 Expression and Patient Prognosis in Merkel Cell Carcinomas: Clinical, Morphologic, and Immunohistochemical Evaluation of 17 Cases.

BACKGROUND: Merkel cell carcinoma (MCC) is a rare, aggressive, primary neuroendocrine carcinoma of the skin whose main risk factors are immunosuppression, UV radiation exposure, and Merkel cell polyomavirus. Programmed death-1/programmed death ligand-1 (PD-L1)-based immunotherapy is currently the first choice for treating patients with metastatic MCC. METHODS: MCC biopsies (17) were evaluated for their nucleus and cytoplasm characteristics and growth patterns, as well as for intratumor lymphocytes, mitotic number, and lymphovascular invasion. Paraffin-embedded tissue samples of the biopsies were stained with MCPyV large T-antigen (LTag), RB1, p53, and PD-L1. RESULTS: We observed MCPyV LTag expression in 9 out of the 17 tumors, and all 9 cases were positive for RB1 ( P <0.000). p53 staining was not significantly correlated with MCPyV LTag. We observed no relationship between p53 expression and any other parameters, and PD-L1 expression was low in the MCC samples. We evaluated PD-L1 using both the combined positive score and tumor proportion score (TPS), and found that TPS was correlated with MCPyV LTag expression ( P =0.016). Tumors with tumor-infiltrating lymphocytes showed a better prognosis than those without these lymphocytes ( P =0.006). DISCUSSION: Our data demonstrated that RB1 was effective for immunohistochemically investigating the MCPyV status of tumors. TPS was superior to the combined positive score in evaluating PD-L1 in MCC. Tumor-infiltrating lymphocytes were the only parameters that were associated with survival. Further studies with larger series are required to confirm these results.