Suppression of in vitro primary immune response by L1210 cells and their culture supernatant: Evidence for cytotoxic effects

L1210 cells and their culture supernatants were found to inhibit the generation of PFC in the in vitro primary immune response of spleen cells to SRBC. As few as 1% of L1210 cells and 1% of culture fluid were inhibitory. Inhibition of DNA or protein synthesis of L1210 cells did not abolish their immunosuppressive activity, excluding exhaustion of culture medium as a possible mechanism of inhibition of PFC. Heating of the supernatant completely abrogated the suppressive effect and resulted in a marked increase of PFC. Daily evaluation of cell viability in the cultures revealed that, in the presence of L1210 and supernatants, the fraction of surviving cells is markedly reduced. We conclude that a direct cytotoxic effect on splenic lymphocytes and macrophages is the predominant immunosuppressive mechanism of L1210 cells and their culture supernatants.