Immunohistochemical distinction of epithelioid histiocytic proliferations from epithelioid melanocytic nevi

Histiocytic tumors can be confused with melanocytic nevi and malignant mela noma and vice versa. To explore the use of immunohistochemistry for this di agnostic problem, we examined the expression of S-100 protein, gp100 (the a ntigen recognized by HMB-45), tyrosinase (T311), Melan-A (A103), Factor XII Ia (FXIIIa), and CD68 in 10 juvenile xanthogranulomas (JXGs), five epitheli oid histiocytomas (EHs), and 15 melanocytic nevi composed of large epitheli oid cells. All epithelioid melanocytic nevi were immunoreactive for Melan-A , tyrosinase, and S-100 protein in most melanocytes. Four nevi were complet ely negative with HMB-45. Nine nevi had only a minor HMB-45-positive compon ent in the superficial dermis. Two nevi were diffusely HMB-45-positive. Mel anocytes in all nevi were completely negative for FXIIIa. Thirteen nevi wer e completely negative for CD68. Two nevi contained rare cells with weak sta ining for CD68. All 15 histiocytic proliferations were completely negative for Melan-A, tyrosinase, and gp100. They lacked expression of S-100 protein or had at most 10% immunopositive cells. In JXGs, most cells were strongly reactive for CD68, although only a few were positive for FXIIIa. In EHs, 4 0% to 60% of cells were immunoreactive for FXIIIa, and only 20% to 30% were positive for CD68. Our results demonstrate that Melan-A and tyrosinase are sensitive and specific markers to distinguish epithelioid melanocytic nevi from epithelioid histiocytic tumors.