D6.1, a murine antimelanoma monoclonal antibody from congenitally athymic nude mice immunized with purified melanoma tumor-associated antigen.

An IgM monoclonal antibody, D6.1, has been obtained by fusion of P3 X 63Ag8.653 with spleen cells from congenitally athymic nu/nu mice immunized with partially purified human melanoma TAA bound to insolubilized lens culinaris lectin. The D6.1-defined antigen is present on the cell membrane of melanoma cells maintained in tissue culture but is not detected in spent culture media. Eighteen of twenty-three melanoma cell lines tested were found to react with D6.1, in contrast to only 17% of nonmelanoma tumor cell lines. Among those reactive nonmelanoma tumor lines were 3/3 colon adenocarcinomas tested. Melanoma cells, in general, express greater quantities of this antigen than do reactive nonmelanomas. Similar data were obtained using immunoperoxidase techniques on frozen sections of fresh tumor. All melanomas tested (8/8) stained intensely, although there was tumor cell heterogeneity. D6.1 reacted, but generally less intensely, with 42% (18/42) of the nonmelanoma tumors tested, including 7/8 colon carcinomas, but with smaller percentages of all other tumor types. D6.1 lightly stained normal stroma in many tissues and reacted strongly with macrophages but not with epithelial tissues. By HPLC filtration, the D6.1-defined TAA from detergent-solubilized melanoma cell membranes was determined to have an apparent molecular weight of 150,000 to 280,000 daltons. D6.1 does not precipitate TAA from exogenously (125I) or endogenously (35S-methionine, 33H-leucine, 33H-glucosamine) labeled melanoma cells.