Abstract
Specific secondary cytotoxic reactivity (as measured by ^51^Cr‐release assay) against SV 40‐induced tumor‐associated antigens was generated in vitro in spleen cells of tumor‐free (BALB/c × C57 BL/6)F~1~ (CBF~1~) mice immunized against a syngeneic SV 40‐induced tumor of BALB/c origin (mKSA), following in vitro sensitization for 5 days with the relevant antigens in mixed lymphocyte‐tumor cell cultures. In contrast, spleen cells of CBF~1~ mice bearing the SV 40‐induced tumor demonstrated suppressed specific secondary cytotoxic reactivity following incubation with the corresponding antigens. Spleens from tumor‐bearing mice contained 4 times the number of mononuclear cells and 3 times the percentage of macrophages, as compared to spleens of normal mice. The percentage of B cells was also elevated in spleens of tumor‐bearing mice. There was a slight reduction in the percentage of T cells. The cytotoxic reactivity of spleen cells of tumor‐bearing mice was restored following removal of macrophages by either rayon adherence columns or iron and magnet, or incubation on plastic petri dishes. No such effect was seen with spleen cells of tumor‐free or normal mice. Spleen cells of tumor‐bearing mice inhibited the in vitro generation of secondary cytotoxic reactivity of spleen cells of tumor‐free mice, sensitized in vitro with SV 40‐induced tumor cells by mixing experiments. The suppressor cells were found to be macrophages by the 3 techniques for removal of macrophages described above. The addition of indomethacin (1–10 μg/ml), a noncompetitive irreversible prostaglandin synthesis inhibitor, to cultures of responding spleen cells from tumor‐bearing mice and stimulating SV 40‐induced tumor cells resulted in marked augmentation of spleen cells responsiveness. With higher indomethacin concentrations (100 μg/ml), no enhancement was seen. The augmenting effect was noted only when indomethacin was present during the initial 24 h of the 5‐day culture. Indomethacin at 1–10 μg/ml had no effect on cytotoxic reactivity of spleen cells of tumor‐free mice, whereas at higher concentrations (100 μg/ml) it had a strong suppressive effect. Preincubation of spleen cells of tumor‐bearing mice with indomethacin for 3 days abrogated their ability to suppress the generation of secondary cytotoxic reactivity of spleen cells of tumor‐free mice. It is hypothesized that indomethacin‐sensitive suppressor macrophages regulate the immune responsiveness in tumor‐bearing mice.