Induction of suppressor cell activity by human pregnancy serum

Third trimester pregnancy serum caused a dose-dependent inhibition of the one-way allogeneic mixed leukocyte reaction (MLR) through an effect that occurred during the first 48 hours of culture. Pregnancy serum also inhibited the mitogenic responsiveness of normal mononuclear leukocytes to concanavalin A (Con A) while the responses to phytohemagglutinin (PHA) and pokeweed mitogen (PWM) were less affected. Preincubation of normal peripheral blood mononuclear cells (PBMC) for 48 hours with 10% pregnancy serum enhanced a suppressor activity transferable with cells. These pregnancy serum-induced effector cells suppressed the MLR only when they were autologous to the responder population (p less than 0.05). The same suppressor cell preparation inhibited the proliferative responses of autologous PBMC to Con A (p less than 0.001) and PWM (p less than 0.05). These data suggest that one or more factors in pregnancy serum can induce or enhance suppressor cell activity in vitro. A comparable increase in suppressor cell activity in vivo may be responsible for blocking maternal rejection of the fetus and for the observed improvement in clinical activity of rheumatoid arthritis during pregnancy.