Increase in H-2 antigen expression and immunogenicity of BL6 melanoma cells treated with N-methyl-N'-nitronitrosoguanidine.

Treatment of the BL6 melanoma cells in vitro with N-methyl-N'-nitronitrosoguanidine dramatically increased their expression of H-2Kb and H-2Db antigens as well as beta 2-microglobulin but not Class 2 major histocompatibility complex antigens. The treated tumor cells also became immunogenic and were rejected in 70% of syngeneic C57BL/6 recipients, whereas these tumor cells produced progressively growing tumors in 100% of irradiated (550 R) or nude mice. In contrast to the effects of N-methyl-N'-nitronitrosoguanidine treatment, no influence of H-2 antigen expression or tumorigenicity was found when BL6 melanoma cells were treated with 5-azacytidine, phorbol myristate acetate, 5-bromodeoxyuridine, theophylline, or 6-thioguanine. H-2 antigen expression and the tumorigenic properties of 48 individual clones derived from BL6T2 melanoma line and 15 clones from the original BL6 melanoma were investigated. No H-2 antigens were found on the cell surface of the parental BL6 clones, whereas all tum- clones from the BL6T2 line expressed high levels of H-2 antigens. Although four of six tested tum+ clones had high levels of H-2b antigen expression similar to that of tum- clones, they were nonimmunogenic. These data indicate that an increase in major histocompatibility complex antigen expression is essential but not sufficient for the immunogenicity of tumor cells. This conclusion was also supported by the results of interferon treatment of BL6 melanoma cells: this induced an increase in the expression of beta 2-microglobulin and Class 1 H-2b antigens but not an increase in their immunogenicity. Detection of tumor-associated transplantation antigens on the melanoma cells also appeared to be dependent on the level of expression of H-2 antigens. Although tum+ clones grew in normal mice, immune mice were able to prevent the growth of tum+ clones with high levels of H-2 antigens. However, immune mice only partially inhibited the growth of the parental BL6 melanoma or tum+ clones which have low expression of H-2 antigens.