Induction by progesterone of immunosuppressive activity in uterine secretions of ovariectomized ewes.

Immune responses in the uterus are depressed when concentrations of progesterone in the blood are elevated. We tested whether this action of progesterone involves induction of secretion of immunosuppressive molecules into the uterine lumen. Uterine fluid from ovariectomized ewes treated with progesterone for 60 days inhibited [3H]thymidine (TdR) incorporation by phytohemagglutinin-stimulated lymphocytes more than uterine flushings from vehicle-treated ewes. Uterine fluid from progesterone-treated ewes also inhibited TdR uptake by other mitogen-stimulated lymphocytes and mixed lymphocyte cultures, but had no effect on the growth of three nonlymphocyte cell lines. A group of ewes immunized with ovalbumin mixed with uterine fluid from progesterone-treated ewes produced an antibody titer that was lower than the titer for ewes immunized with ovalbumin without uterine fluid. Analysis of the physical properties of the active molecules in uterine fluid from progesterone-treated ewes indicated that activity could be separated into two fractions: a large mol wt (Mr; greater than 5000) fraction that was sensitive to pronase and a low Mr (less than 1000) fraction that was more resistant to pronase. There was more inhibitory activity in the high mol wt fraction. Further analysis of this fraction indicated that most activity was of basic isoelectric point (pI greater than 8.2). The most active fraction of basic material eluted at the void volume of a Sepharose CL-6B column (Mr, 4 x 10(6)). In conclusion, progesterone caused accumulation of factors in the uterine lumen that inhibited lymphocyte function in vitro and antibody formation in vivo. These molecules may play an important role in regulating immune responses in the uterus during pregnancy.