Tissue Distribution, Molecular Profile, and Shedding of a Cytoplasmic Antigen Identified by the Monoclonal Antibody 465.12S to Human Melanoma Cells1

The mouse immunoglobulin G2 monoclonal antibody (MoAb) 465.12S reacts with a cytoplasmic antigen present in human melanoma cells but not detectable in melanocytes. Indirect immunofluorescent staining of a large number of surgically removed normal adult and fetal tissues with the MoAb 465.12S detected the cytoplasmic antigen in epithelial cells from several organs. The intensity of staining was greater in adult tissues than in the corresponding fetal tissues. Furthermore, the MoAb 465.12S stained nearly all of the surgically removed tumors tested but did not stain many of the normal tissues from which they originated. In almost all cases, the intensity and frequency of staining was greater for tumor cells than for corresponding normal tissues. From cultured carcinoma and melanoma cells, the MoAb 465.12S immunoprecipitated four glycopolypeptides with molecular weights of 94,000, 75,000, 70,000, and 25,000. Incorporation of 3H-labeled sugars into the various components of the cytoplasmic antigen revealed that the M.W. 75,000 component was unusual in that it contained only glucosamine and mannose. The antigenic determinant defined by the MoAb 465.12S appears to be protein rather than carbo hydrate in nature since it is heat sensitive and is expressed on the antigens synthesized by cells in presence of tunicamycin. Analysis of the spent culture medium of carcinoma and mela noma cell lines revealed that the cytoplasmic antigen is readily shed by these cells and consists of a major M.W. 94,000 and a minor M.W. 72,000 component. Treatment of cultured mel anoma cells with the antibiotic tunicamycin showed that glycosylation of the cytoplasmic antigen is required for its shed ding and/or stability in the spent culture medium.