Identification of the predominant proteins in uterine fluids of unilaterally pregnant ewes that inhibit lymphocyte proliferation.

Uterine fluid from unilaterally pregnant ewes contains activity inhibitory to lymphocyte proliferation. The molecules responsible for this activity may thereby regulate uterine immune responses during pregnancy. The purpose of the experiment described here was to identify the major protein in uterine fluid responsible for this activity. When uterine fluid was fractionated by a combination of cation exchange chromatography, gel filtration, and lectin affinity chromatography, the majority of the lymphocyte activity co-migrated with a pair of proteins previously identified as related, serpin-like glycoproteins. Together, this pair of proteins, called the uterine milk proteins (UTM-proteins), are the predominant endometrial secretory protein produced under the influence of progesterone. Preparations of uterine protein highly enriched for the UTM-proteins inhibited lymphocyte proliferation induced by phytohemagglutinin, concanavalin A, and mixed lymphocyte reactions but did not inhibit proliferation induced by pokeweed mitogen. In some experiments, UTM-proteins also reduced viability of cultured lymphocytes. Another previously described lymphocyte-inhibitory factor, megasuppressin, was also observed. Megasuppressin, which eluted at an apparent molecular weight of greater than 4 x 10(6) even after treatment with urea, guanidine-HCl, and beta-mercaptoethanol, was a more potent inhibitor of lymphocyte proliferation than UTM-proteins. Megasuppressin is not very abundant, however, and probably is responsible for only a small fraction of the lymphocyte inhibitory activity in uterine fluid. The majority of lymphocyte-inhibitory activity is caused by the UTM-proteins or by a molecule that co-purifies in trace amounts with UTM-proteins.