Expression of melanocyte differentiation antigens and Ki-67 in nodal nevi and comparison of Ki-67 expression with metastatic melanoma

Nodal nevi represent a potential diagnostic pitfall in the analysis of lymph nodes. They may be confused with metastatic melanoma or carcinoma. Although several morphologic guidelines exist for the recognition of nodal nevi, on occasion immunohistochemical studies may be helpful for diagnosis, especially when melanocytes extend into the lymph node parenchyma. To learn more about the immunohistochemical profile of nodal nevi we examined 15 nodal nevi for the expression of S-100 protein, gp100 (HMB-45), Melan-A/MART-1 (A103), and tyrosinase (T311), and we studied the expression of Ki-67 (MIB-1) in nodal nevi and 40 melanoma metastases (35 lymph node and five cutaneous metastases). All nodal nevi were homogeneously immunoreactive for S-100 protein, tyrosinase, and Melan-A/MART-1. Two nodal nevi were focally positive for gp100. Fourteen of 15 nodal nevi were completely negative for Ki-67. One large cellular nodal nevus showed nuclear labeling in <0.2% of melanocytes. All metastases showed MIB-1 labeling. However, the percentage of labeled tumor cells varied widely, ranging from 2% to 80%. These results demonstrate that MIB-1 and HMB-45 are helpful reagents for the distinction of nodal nevi from melanoma. Immunohistochemistry for S-100 protein, Melan-A/MART-1, or tyrosinase facilitates the recognition of melanocytes but does not distinguish between nodal nevus and metastatic melanoma.