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Pregnancy-associated nonspecific immunosuppression: kinetics of the generation and identification of the active factors.

The effect of gestational age and maternal parity on the development of nonspecific immunosuppressive activity in the sera of pregnant women, which inhibited the in vitro transformation of unrelated lymphocytes by phytohemagglutinin, was examined. Quantitative demonstration of this activity was dependent, in part, on the source of the lymphocytes and on the serum concentration in culture. The immunosuppressive activity became evident as the pregnancy progressed, and in late-pregnancy sera it was mediated by two factors, immunosuppressive factors (ISF) I and II with apparent Mr of 2 X 10(6) and 150,000 daltons. By analysis and comparison of different types of sera fractionated by gel filtration on Sephacryl S-300, it was evident that ISF-I and ISF-II were also present in male and nulliparous nonpregnant female sera, but in inactive forms. Hence the immunosuppressive factors did not appear to be "produced" in pregnancy, but the observed activity was a reflection of the "activation" of preexisting molecules in the serum. An accompanying report (Am J Reprod Immunol Microbiol. 1985; 9:84-90) describes the regulation of the activation event.