PGE2-independent immunosuppressive activity of horse trophoblast tissue

It has been proposed that PGE2 is an important immunosuppressant acting at the fetal-maternal interface during pregnancy. We have previously shown that horse conceptus-conditioned medium suppresses lymphocyte proliferation. This experiment was designed to determine if horse conceptus-derived immunosuppressive activity could be attributed to PGE2 production by the trophoblast tissue. Trophoblast tissue from 21-day-old conceptuses was cut into equal sections and cultured in the presence or absence of the prostaglandin inhibitor, indomethacin. Following culture, immunosuppressive activity and the concentration of PGE2 were determined for each sample of both horse-trophoblast conditioned medium (HTCM) and indomethacin-treated HTCM (I-HTCM). Suppressive activity was identified in lymphocyte proliferation assays via reduced [3H]thymidine uptake by pokeweed mitogen stimulated horse lymphocytes. A radioimmunoassay was used to quantify PGE2. While PGE2 production was greatly reduced in cultures containing indomethacin, trophoblast-derived immunosuppressive activity was not affected. These data indicate that PGE2 is not the primary immunosuppressant produced by horse trophoblast tissue.