Merkel cell carcinoma showing regression after biopsy: Evaluation of programmed cell death 1-positive cells.

We present a cases of Merkel cell carcinoma (MCC) with Merkel cell polyomavirus that showed complete regression after biopsy. The exact mechanism of regression in MCC has remained unclear. It has been reported that apoptosis caused by T-cell immunity was implicated in the regression, and programmed cell death 1 (PD-1), an inhibitory receptor, was expressed in approximately half of tumor-infiltrating T cells in MCC. However, the contribution of PD-1-positive cells for the regression of MCC has not been evaluated. We examined the rate of PD-1-positive cells among the peritumoral mononuclear cells, which showed that the percentage of PD-1-positive cells in the case was significantly lower compared with in MCC without regression. We propose that PD-1-positive cells suppress tumor immunity for MCC, and that reduction of PD-1-positive cells may be associated with tumor regression.