The present study shows that PD-L1 and NGF are co-expressed on spindle cells in the microenvironment. The expression of NGF might be a link of the microenvironment to the TrkA-positive tumor cells. Whether this mechanism is critical for tumor growth and lack of response to anti-PD-1/L1 treatment has to be investigated in further studies.

IMPORTANCE Merkel cell carcinoma (MCC) is a malignant neuroendocrine skin tumor frequently associated with the Merkel cell polyomavirus. Immune checkpoint therapy showed remarkable results, although not all patients are responsive to this therapy. Anti-tropomyosin receptor kinase A (TrkA)-targeted treatment has shown promising results in several tumor entities. OBJECTIVE To determine TrkA expression in MCC as a rationale for potential targeted therapy. DESIGN, SETTING, AND PARTICIPANTS This case series study investigated the MCC specimens of 55 patients treated at the