Date of Award


Document Type


Degree Name

Doctor of Philosophy (PhD)


Biological Sciences

First Advisor

Kenneth L. White


The role of conceptus-derived immunosuppressive factors in protecting the fetal allograft was investigated by first examining the immunosuppressive activity derived from ovine, caprine and hybrid conceptus trophoblast tissue. Medium conditioned by 20 day-old conceptus trophoblast tissue from each species suppressed sheep and goat lymphocyte proliferation (P $$.05) between the immunosuppressive effects of the three media on either sheep or goat lymphocytes. In subsequent studies, horse conceptus-conditioned medium (HCCM) derived from embryos at days 9, 15 and 26 of gestation, suppressed mitogen-stimulated horse lymphocytes (P $<$.001). HCCM maintained its suppressive activity following heating and repeated freeze-thawing. Using microconcentrator centrifugation, it was determined that the suppressor factor(s) was $>$100,000 MW. HCCM was tested in donkey and goat lymphocyte cultures and proved capable of suppressing proliferation of xenogeneic lymphocytes (P $<$.01). However, the suppressive capacity of HCCM in caprine lymphocyte cultures was less (P $<$.05) than that in equine cultures. Trophoblast tissue of 21 day-old horse conceptuses was cultured in the presence and absence of indomethacin. Both immunosuppressive activity and the concentration of prostaglandin E$\sb2$ (PGE$\sb2$) in the medium of each culture were measured. While PGE$\sb2$ production was greatly reduced in cultures containing indomethacin, trophoblast-derived immunosuppressive activity was not affected (P $>$.05). The final study involved partial characterization of the HCCM suppressor factor's (HCCM-SF) mechanism of action. By testing HCCM-SF in cultures stimulated with mitogens that selectively activate B and/or T lymphocytes, and by sorting HCCM-SF-treated and non-treated cells labeled with fluorescent antibodies, it was determined that HCCM-SF acts on T lymphocytes. The temporal effect of HCCM-SF was also investigated and, compared to lymphocytes treated at time 0, proliferation was greater (P $<$.01) in cultures treated with HCCM-SF 24 or 48 hours after the start of stimulation. Furthermore, the removal of HCCM-SF from lymphocyte cultures resulted in complete recovery of cell responsiveness. The addition of recombinant human IL-2 to the lymphocyte cultures did not overcome the suppressive effect of HCCM-SF (P $$.05). HCCM-SF may be one factor responsible for inhibiting cell-mediated fetal allograft rejection during pregnancy.