Synergism between immune complexes and serum from tumor-bearing mice in the suppression of mitogen responses.

Immune complexes either adhering to the cells, or added to cultures, synergized with an inhibitor present in serum to suppress the proliferative response of lymphocytes to specific antigens or to B cell or T cell mitogens. The suppressive effect was dependent on the concentration of both the complex and the serum and was substantially higher when the serum was taken from tumor-bearing, rather than normal mice. Immune complexes alone were not suppressive, but appeared to activate or increase the effectiveness of an inhibitor present in serum. Complexes of syngeneic mouse serum or specifically purified rabbit or sheep antibody, as well as heat aggregated mouse or rabbit Ig, were all effective, whereas heat-aggregated F(ab')2 fragments of rabbit Ig were not, suggesting that any aggregate hearing exposed Fc fragments could mediate synergy. Purified T lymphocytes were equally as sensitive to inhibition as the whole spleen cell population, implying that the suppression acted directly on responding cells rather than via an accessory cell.