Inhibition of lymphocyte aggregation by progesterone

Investigations were carried out on the influence of progesterone and murine trophoblast culture supernatants on the capacity of lymphocytes to form aggregates (clusters), an important feature in immunologic recognition and response. Both progesterone and trophoblast supernatants inhibited this cluster formation in a dose-dependent way. The effect of trophoblast supernatants appeared to be mediated mainly by progesterone since they lost their inhibitory effect on the cluster formation after treatment with anti-progesterone serum (APS). Preparations with I1-2 activity of rat and mouse origin could either prevent or restore the suppressive effect of both progesterone and trophoblast supernatants on lymphocyte aggregation. The interference with lymphocyte interaction by trophoblast may represent one of the mechanisms by which the fetal allograft is protected against maternal recognition.