Spontaneous regression of Merkel cell carcinoma is associated with clonal T-cell expansion

Merkel cell carcinoma (MCC) is a rare and aggressive neuroendocrine skin tumor. MCC is immunogenic and highly responsive to immune checkpoint inhibitors. There are at least 45 reported cases of spontaneous regression of both local and metastatic MCC following biopsies, excisions, fine needle aspirations, and in 1 case after allergic contact dermatitis. Spontaneous tumor regression has also been observed in other tumor types, however the mechanism is poorly understood. There has been speculation that trauma-induced antigen shedding and subsequent recruitment of a host immune response drives tumor regression.