Background/Aim: Merkel cell carcinoma (MCC) is an aggressive cutaneous neuroendocrine carcinoma. Although recent reports suggest that tumor-infiltrating leukocytes (TILs), especially CD8+ T-cells, contribute to the pathogenesis of MCC, it is difficult for a single Institute with a small number of patients with MCC to determine the threshold number of CD8+ cells. Therefore, clearer and easier methods of evaluating prognostic factors of MCC are needed. Patients and Methods: In order to identify the prognostic factors of 24 cases of MCC, we employed immuno histochemical staining of phospho-signal transducer and activator of transcription 5B (pSTAT5B), which has been reported to be a prognostic marker for several types of cancers. Results: All MCC cases with a good outcome (n=16) expressed pSTAT5B, whereas all MCC cases with a poor outcome (n=8) did not express pSTAT5B. Moreover, we additionally employed immunohistochemical staining of periostin (POSTN) and interleukin-4, as well as subpopulations of TILs (granulysin-bearing cells, regulatory Tcells, CD163+ cells, and CD206+ cells), and the deposition of matrix metalloproteinase 12 in the lesional skin of patients with MCC. The results suggested that there is no significant difference in stromal factors between MCC cases with a good and those with a poor outcome. Conclusion: pSTAT5B expression may be an indicator of positive prognosis in patients with MCC.
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