ASL-1 × LM(TK)- somatic hybrid cells form both H-2a and H-2k antigen complexes. After forming a localized tumor in syngeneic (A/J × C3H/HeJ)F1 mice, they are rejected. Such mice are resistant to otherwise invariably lethal injections of ASL-1 cells, surviving for prolonged and, in some instances, indefinite periods. To examine the basis of immunity, the capacity of spleen cells from mice rejecting hybrid cells to stimulate the release of 51Cr from labeled ASL-1 cells was investigated. Cells from the spleens of mice rejecting ASL-1 × LM(TK)- cells stimulated the release of 51Cr from labeled ASL-1 cells, but not from Ehrlich ascites or P815 cells. Cells from mice injected with mitomycin-C-treated ASL-1 cells led to the release of 51Cr from labeled ASL-1 cells as well, but the extent of 51Cr release was approximately one-third as occurred in the presence of cells from hybrid cell-injected mice. Cells from noninjected mice or from mice injected with LM(TK)- cells failed to lead to the specific release of 51Cr from ASL-1 cells. The presence of unlabeled ASL-1 cells, but not Ehrlich ascites cells, competitively inhibited the spleen cell-stimulated release of 51Cr from labeled ASL-1 cells. Sera from A/J mice injected with mitomycin-C-treated ASL-1 cells contained antibodies specific for the tumor-associated antigen of ASL-1 cells. The specific release of 51Cr from ASL-1 cells stimulated by spleen cells from hybrid-cell injected F1 mice was inhibited by such antisera, but not by H-2a antisera. Treatment of spleen cells from mice injected with hybrid cells with Thy 1.2 antiserum before incubation with 51Cr-labeled leukemic cells reduced the proportion of 51Cr released by approximately one-half. Spleen cells from animals injected with hybrid cells, as well as to a lesser extent from noninjected mice, which are capable of adhering to plastic Petri dishes were active in the cytotoxic reactions as well. Results similar to these were obtained with 51Cr-labeled RADA-1 cells, a radiation-induced leukemia of A/J mice and spleen cells from (A/J × C3H/HeJ)F1 animals injected with RADA-1 × LM(TK)- hybrid cells.

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This work was supported by grants from the Leukemia Research Foundation and the University of Chicago Cancer Research Center Grant CA-14599.

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