Effects of alloantibodies on lymphocyte proliferation in vitro

Inhibition of MLC and antigen-induced proliferation by alloantibodies was studied. A serum, obtained from a multipara 3 yr after her sixth pregnancy, inhibited the capacity of the immunizer's lymphocytes to stimulate and to respond in unilateral MLC. This alloantiserum was shown to inhibit antigen-induced proliferation as well as MLC of lymphocytes from individuals unrelated to the immunizer. The inhibitory titer was much higher than the upper limit of complement dependent cytotoxicity. The inhibitory factor was found in the 7S fraction of the alloantiserum; it could be fully absorbed on leukocytes but only partially on platelets, whereas both eluates from platelets and leukocytes were inhibitory. Inhibition was still observed when the alloantiserum was added up to 3 days after the beginning of the culture, but in some instances, a potentiating affect was noticed. Lymphocytes coated with alloantibodies and then cultured with antigen did not proliferate, but replacement of the culture medium after 1 or 2 days of incubation, reversed this inhibition. In contrast, inhibition of antigen-induced transformation by heterologous antibodies was not reversible. Reversibility of the blocking effect was associated with a high rate of shedding of antibodies bound to membrane-associated structures.