Semi-allogeneic hybrid clones were derived by fusion of the TEPC-15 plasmacytoma (H-2d) and mouse L cells of C3H (H-2k) origin. Three representative clones were chosen to study the relationship between the expression of different membrane antigens and their immunogenicities leading to protection of recipient mice from the parent TEPC-15 plasmacytoma. The level of surface tumor-specific transplantation antigens (TSTA) was measured by radioimmunoprecipitation with syngeneic anti-TSTA and by inhibition of anti-TSTA binding to the TEPC-15 tumor cells. The most immunogenic hybrid clone (LTC-1) expressed the highest level of TSTA and the weakly immunogenic (LTC-2) and nonimmunogenic (LTC-4) hybrid clones exhibited relatively low levels of TSTA on the surface. Moreover, the strongly immunogenic LTC-1 hybrid cells, but not the parent tumor cells, were effective in priming recipient spleen cells to generate TEPC-15 tumor-specific cytotoxic cells upon subsequent in vitro exposure to the TSTA-bearing cells. Therefore, the level of TSTA on the semi-allogeneic hybrid clones may play an important role in enhancing the immunogenicity of TSTA.
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