Beta 2 microglobulin expression in normal melanocytes, nevocellular nevi, and malignant melanomas

Expression of beta 2 microglobulin (beta 2M), a light chain of class 1 HLA antigen, was studied in normal melanocytes and in benign and malignant melanocytic tumors by use of immunohistochemical methods. By immunoelectron microscopy, normal melanocytes were shown to express beta 2M on the cell surface. In lentigo maligna melanomas and acral lentiginous melanomas, the mean percentages of beta 2M-positive tumor cells were significantly lower in thick (greater than 1.50 mm) primary lesions and metastases than in thin (less than or equal to 1.50 mm) primary lesions. The evidence suggests that melanocyte-derived melanoma clones with a low grade of malignancy preserve class 1 HLA expression, and that the clones with a high grade of malignancy tend to lose the antigen expression. Nevus cells in common nevi have little or no expression of beta 2M. In halo nevi, however, beta 2M were detected on nevus cells in the lesions associated with inflammatory infiltration. Immunohistochemical analyses of the cellular composition of the inflammatory cells in halo nevi demonstrated the presence of cytotoxic T cells together with helper/inducer T cells, Langerhans cells, and macrophages. It appears that nevus cells of halo nevi are destroyed by cytotoxic T cells and that class 1 HLA antigens expressed on nevus cells play an important role in the target cell recognition and lysis by specific cytotoxic T cells.