In situ analysis of the mononuclear cell infiltrate in primary malignant melanoma of the skin.

Monoclonal antibodies, directed against functionally different lymphocyte subsets, were applied on frozen sections of primary malignant melanomas and benign nevi. Positive reaction was identified by means of an immunoperoxidase method. It was found that lymphocytic infiltrate underneath and in between malignant melanoma is composed of approximately equal numbers of OKT4 positive helper and OKT8 positive suppressor/cytotoxic T cells. The majority of these lymphocytes also expressed HLA-Dr antigen, indicating an activated state. In addition HLA-Dr, OKT6 positive dendritic cells were present in the infiltrate and between the melanoma cells. Finally, melanoma cells expressed demonstrable amounts of HLA A, B, C antigens, whereas benign nevi did not. It is concluded that all ingredients for a successful immune reaction against primary malignant melanoma are on hand. This finding is in agreement with the relatively frequent occurrence of partial or even complete regression of primary malignant melanoma of the skin.