541 Characterization of myeloid cell subsets in the tumor microenvironment of merkel cell carcinoma

PD-1 pathway blockade has changed the landscape for advanced Merkel cell carcinoma (MCC) as over 50% of patients initially respond to therapy. However, for patients who do not respond (or later recur), there is a need for additional therapies. In the US, ?80% of MCC tumors are caused by the Merkel cell polyomavirus, while ?20% are caused only by UV-induced mutations. MCC tumors are thus highly immunogenic because they express either non-self viral antigens or numerous UV-induced neoantigens. Historically, studies have been focused on adaptive immunity, and little is known about innate immunity in MCC which may play an important role in immune evasion.