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.