Effects of mutagens on the immunogenicity of murine tumor cells: immunological and biochemical evidence for altered cell surface antigens.

Eb lymphoma cells were subjected to treatment in vitro with the alkylating mutagen N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) and then cloned by limiting dilution. When tested in vivo for tumorigenicity in groups of syngeneic DBA/2 mice, 6 from 18 clones were found to be strongly reduced (tum- phenotype). The other clones showed only moderate or no change in tumorigenicity compared to the untreated control. All clones were able to grow in 400-rad-irradiated mice. Mice in which MNNG clones had regressed were able to generate tumor-specific cytolytic T-lymphocytes in vitro. Limiting dilution analysis indicated that 3 of 4 MNNG clones analyzed in detail displayed additional antigenic determinants that were detected by cytolytic T-lymphocytes. These data thus provided evidence for increased immunogenicity of some of the MNNG clones. Membrane proteins of MNNG clones and original Eb cells were compared biochemically after metabolic labeling with [35S]methionine, TX114 solubilization, and electrophoretic separation. Two-dimensional gel maps revealed a general quantitative decrease in the expression of membrane proteins in MNNG clones. In addition, several proteins were only found in MNNG clones but not in untreated cells. Two membrane proteins of molecular weight 22,000 and 38,000 were greatly increased in expression in all MNNG clones but could be detected at a low level in the original Eb cells. MNNG is known to be a strong mutagenic agent, but it can also interfere with DNA methylation and cause transcriptional activation of genes. We suggest that amplified cell surface structures may be the consequence of such transcriptional activation and could be involved in altered immunogenicity.