The spectrum of Merkel cell polyomavirus expression in Merkel cell carcinoma, in a variety of cutaneous neoplasms, and in neuroendocrine carcinomas from different anatomical sites.

Most Merkel cell carcinomas display pure neuroendocrine differentiation (pure Merkel cell carcinoma), whereas a minority show combined neuroendocrine and nonneuroendocrine elements (combined Merkel cell carcinoma). Recent identification of Merkel cell polyomavirus DNA and Merkel cell polyomavirus large T antigen expression in a proportion of Merkel cell carcinomas has suggested viral-induced oncogenesis. To date, Merkel cell polyomavirus immunohistochemistry has shown an absence of viral large T antigen expression in combined Merkel cell carcinoma as well as select non-Merkel cell carcinoma cutaneous lesions and visceral neuroendocrine tumors. In our series, we aimed to further characterize the frequency and pattern of Merkel cell polyomavirus large T antigen expression by CM2B4 immunohistochemistry in primary and metastatic Merkel cell carcinoma (pure Merkel cell carcinoma and combined Merkel cell carcinoma) and various non-Merkel cell carcinoma lesions from patients with Merkel cell carcinoma, patients without Merkel cell carcinoma, and individuals with altered immune function. Merkel cell polyomavirus large T antigen was detected in 17 (63%) of 27 pure Merkel cell carcinomas and absent in all 15 (0%) combined Merkel cell carcinomas. Furthermore, complete concordance (100%) of Merkel cell polyomavirus large T antigen expression was observed in 10 cases of primary Merkel cell carcinoma and subsequent tumor metastases. We also evaluated 70 non-Merkel cell carcinoma lesions including 15 cases each of pulmonary and gastrointestinal neuroendocrine tumors. All 70 non-Merkel cell carcinoma lesions were negative for Merkel cell polyomavirus by CM2B4 immunohistochemistry, irrespective of any known Merkel cell carcinoma diagnosis and immune status. In summary, our identification of Merkel cell polyomavirus large T antigen expression in a subset of Merkel cell carcinoma and lack of findings in combined Merkel cell carcinomas and non-Merkel cell carcinoma lesions concur with earlier findings and implicate Merkel cell polyomavirus-independent pathogenesis in these cases. Overall, CM2B4 immunohistochemistry appears to be a specific method for Merkel cell polyomavirus detection and has the potential to play an important role in the diagnosis and classification of Merkel cell carcinoma in the future.