Complete spontaneous regression of primary Merkel cell carcinoma with tumoural infiltration: a systematic review

Merkel cell carcinoma (MCC) is an aggressive tumour that rapidly evolves to metastasis, however, paradoxically, complete spontaneous regression (CSR) of MCC has been reported. CSR may be linked to the presence of Merkel cell polyomavirus (MCPyV) combined with immune response tumour-infiltrating lymphocytes (TILs). To elucidate the mechanism of CSR by studying the profile of TIL infiltration and the role of MCPyV. We describe a clinical case of CSR with MCC and provide the results of a literature review based on PubMeb and Embase databases, from January 1^st 1986 to April 1^st 2010, using the terms: "Merkel cell carcinoma" and "complete spontaneous regression". We found 38 clinical cases of CSR of primary MCC at different stages. The rate of MCPyV positivity was 75%, as found generally in MCC. The peritumoural infiltration was mostly composed by CD3+ T cells, whereas the intratumoural infiltration revealed CD8+ T cells, defined as TILs. To further investigate TILs in CSR of MCC, immunohistochemical staining was performed and compared to three non-regressive MCCs. CD8, IFNɣ and LAG3 expression was higher in biopsy samples with tumour regression, mainly inside the tumour nest. TILs were significantly more abundant in regressive MCC than in non-regressive MCC, in which both IFNɣ and LAG3 levels were low. This review and our clinical case confirms the central role of TILs in regressive MCC associated with IFNɣ and LAG3 secretion, thus underlining the interest in checkpoint inhibitors and adoptive T cell therapy in the treatment of MCC.