Blocking activity of mouse pregnancy serum was investigated using a well defined one-way mixed lymphocyte culture and a cell-mediated microcytotoxicity test. Allogeneic (C57BL (H-2b) × CBA/Ca (H-2k)) and syngeneic (C57BL (H-2b) × C57BL (H-2b)) pregnancy and postpartum sera were found to stimulate rather than inhibit the mixed lymphocyte response. In contrast, the same sera were able to abrogate target cell killing by hyperimmune lymphocytes in a cell-mediated microcytotoxicity test. Although inhibitory activity was present in both allogeneic and syngeneic 18-day pregnancy sera, indicating a nonspecific effect, only allogeneic postpartum sera had significant blocking activity when compared with either the equivalent syngeneic group or the normal serum control. The specificity implied by this gained support from preliminary third-party experiments. Blocking activity was found in primiparous as well as multiparous sera.SUMMARY
These results provide further evidence for the existence of maternal serum-blocking factors which may operate to protect the conceptus against maternal cell-mediated immunity during pregnancy, and indicate that they may act at the efferent rather than the afferent phase of the immune response.