Studies of natural killer cells in pregnancy. II. The immunoregulatory effect of pregnancy substances.

This study was performed utilizing a standard 51Cr-release cytotoxicity and a single cell assay against K-562 NK-sensitive target cells. Pregnancy sera and amniotic fluids, were found to decrease human natural killer (NK) activity. Pregnancy sera were able to inhibit overall NK activity of partly purified peripheral blood lymphocytes in a dose dependent fashion. However, the exposure of effector cells to concentrations higher than 20% did not further increase the serum's suppressive activity. Sera taken from preeclamptic pregnancies had a similar inhibitory effect as those from normal pregnancies. Amniotic fluid was found to be even more suppressive than pregnancy serum. Using a single cell cytotoxicity assay we determined that neither serum nor amniotic fluid affected potentially cytotoxic target binding cells ("pre"-NK). The relative number of conjugates with dead target was, however, significantly depressed under the influence of pregnancy serum and to an even greater extent by amniotic fluid. Both pregnancy substances were also able to decrease the number of conjugates with dead target in an interferon-augmented population of lymphocytes. Our investigation indicates that the suppressive effect of amniotic fluid and pregnancy serum on NK activity is based on a blocking effect exerted at the killing capability of the active NK cells.