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A pigmentation‐associated, differentiation antigen of human melanoma defined by a precipitating antibody in human serum

Antibodies in the serum of melanoma patient AU precipitate an antigen from 125I‐labelled extracts of cultured autologous melanoma cells. The antigen, which is probably not a cell surface component, is present in other pigmented melanomas but not in non‐pigmented melanomas or other tumor cell types, and the amount of antigen is correlated with the degree of pigmentation. These conclusions are based on absorption experiments with 11 pigmented melanomas, 8 non‐pigmented melanomas, 3 astrocytomas, 12 carcinomas of various histological types, 1 leukemia, 2 EB‐virus‐transformed B lymphocyte lines, and human erythrocytes. The antigen was also detected in cultured human melanocytes. It has a molecular weight of 70,000, an isoelectric point of pH 5.3, and it binds to concanavalin A‐Sepharose. Ninety‐six sera from other melanoma patients were examined and none of them precipitated this antigen. As described previously, the serum from patient AU also has antibodies to a unique (Class 1) tumor antigen found only on AU melanoma cells. The pigmentation‐associated, differentiation antigen and the unique antigen are clearly different in their distribution, but some relationship between these unusual antibody responses is possible.