Mechanism of Immunosuppression of Progesterone on Maternal Lymphocyte Activation during Pregnancy

Lymphocyte reactivity was measured in human mixed lymphocyte (MLC) and mitogen-stimulated cultures in the presence of various steroids. When present during the entire culture period at concentrations between 1 and 20 µg/ml progesterone, estradiol, and testosterone blocked 3H-thymidine (3H-TdR) incorporation. Estriol and 16 α-hydroxy progesterone were without suppressive effects. The kinetics of steroid mediated suppression of thymidine incorporation and DNA synthesis was investigated at various times during lymphocyte activation. When progesterone was present only at the beginning of culture, subsequent DNA content, but not thymidine incorporation was suppressed. However, if added late during the activation process, progesterone reduced thymidine incorporation independent of cellular DNA content. If present during the entire culture period both were suppressed. Cortisol, but not progesterone irreversibly inhibited lymphocyte thymidine incorporation if added during the first 12 hr of culture. A reversible blockade of lymphocyte thymidine uptake could be observed with progesterone even during the final hours of mitogen or MLC cultures. Thus, effects of steroid hormones on cellular DNA synthesis and thymidine incorporation appear to be independent processes. Cell fractionation experiments demonstrated that progesterone blocked the uptake of 3H-TdR into the lymphocyte nucleoside pool thereby reducing its availability for incorporation into DNA. This study is consistent with the hypothesis that lymphocyte proliferation at the maternal fetal-interface during pregnancy may be under specific regulation by progesterone. The relatively high local concentrations of the hormone produced by trophoblast are immunosuppressive whereas the substantially lower peripheral steroid levels are entirely inadequate to suppress systemic immunity. We suggest that at least one important role of progesterone in maintaining pregnancy is its regulation of cellular immunity in the maternal-fetal bed.