We read with interest the article by Bechert et al illustrating the challenge in diagnosing Merkel cell carcinoma (MCC) from fine-needle aspiration (FNA) samples.[1] The authors discuss the role of immunohistochemistry in differentiating MCC from its mimics, and mention that immunostains may not help distinguish MCC from extrapulmonary small cell carcinomas that share a similar immunophenotype because both express dot-like cytokeratin 20 (CK20) positivity. We would like to add that since Merkel cell polyomavirus (MCPyV) is now known to be present in approximately 80% of cases of MCC,[2, 3] the antibody CM2B4 that recognizes the large T antigen of MCPyV in MCC tissue specimens has proven to be diagnostically helpful.[4] Indeed, CM2B4 has even been shown to distinguish MCC from high-grade primary parotid neuroendocrine carcinomas, which are similar with regard to cytomorphology and immunostaining pattern with dot-like expression of CK20.[5] To the best of our knowledge, CM2B4 immunocytochemistry findings have not yet been reported in cytology material. Therefore, we would like to take this opportunity to briefly share our experience with CM2B4 immunocytochemistry.

We initially used MCPyV-positive cell line xenograph induced MCC in mice to optimize immunohistochemistry. Immunostaining using 1:50 CM2B4 anti-MCPyV Large T antigen antibody (mouse monoclonal immunoglobulin G; Santa Cruz Biotechnology Inc, Dallas, Tex) was performed after heat-induced epitope retrieval (ethylenediamine tetraacetic acid buffer [pH 8.0]). Archival formalin-fixed paraffin-embedded cell block material from 6 FNA cases of MCC and 18 FNA controls (small cell carcinoma, non-MCC neuroendocrine tumor, small cell non-Hodgkin lymphoma, basaloid carcinoma, Ewing sarcoma, and melanoma) were used. Tissue resections in 10 cases of MCC were also studied. Clinical findings and other immunostaining results (specifically keratin, synaptophysin, and CK20) also were recorded.

MCPyV T antigen staining was seen as staining was observed as nuclear positivity. All MCC cytology cases (average patient age, 66 years; 6 men and 1 woman) were obtained from metastases that demonstrated synaptophysin and CK20 dot-like immunoreactivity, and were positive for CM2B4 in 5 of 6 cell blocks (83%). The tumor cells also were positive in 5 of 10 tumor resections (50%). All control cases (average patient age, 63 years; 11 men and 7 women) were negative for CM2B4 except for staining of a few lymphoma cells in a lymphoplasmacytic non-Hodgkin lymphoma and infiltrating reactive lymphocytes in a high-grade non-MCC neuroendocrine tumor.

These findings demonstrate that CM2B4 immunocytochemistry to detect MCPyV can be helpful to confirm the diagnosis of MCC. However, staining should be interpreted with caution because not all MCC tumor cells are immunoreactive. Moreover, CM2B4 staining may occasionally be detected in small reactive lymphocytes and some non-Hodgkin lymphomas, as has been previously reported.[6]


  1. Top of page

Dr. Pantanowitz has received royalties from Springer for a textbook entitled Cytopathology of Infectious Diseases. Dr. Pantanowitz has also received reimbursement from CAP for travel to the United States and Canadian Academy of Pathology, College of American Pathologists, and from ASCP for travel to American Society for Clinical Pathology meetings.

  • Liron Pantanowitz, MD

  • Sarah Navina, MD

  • Sara E. Monaco, MD

  • Department of Pathology

  • University of Pittsburgh Medical Center

  • Pittsburgh, Pennsylvania


  1. Top of page