Suppression of lymphocyte proliferation by a greater than 30,000 molecular weight factor in horse conceptus-conditioned medium.

In this experiment we have identified and partially characterized the immunosuppressive activity of preimplantation horse conceptus-conditioned medium (HCCM). Horse conceptuses were nonsurgically flushed from mares at Days 9-10 (n = 6), 1516 (n = 3), and 25-26 (n = 3). After incubating the concepwses for 24 h in RPMI-16-40 supplemented with 15% fetal calf serum (FCS) and 1% penicillin/streptomycin, HCCM was obtained from cultures and tested for immunosuppressive activity In lymphocyte proliferation assays. Peripheral blood lymphocytes obtained from randomly selected mares were stimulated with mitogens (pokeweed mitogen [PWM], concanavalm A [Con A], and phytohemagglutinin [PHA]) in cultures supplemented with 0%, 25%, or 50% HCCM. HCCM from all cultures suppressed lymphocyte proliferation induced by all three mitogens (p 30 000 MW fraction. HCCM was further tested interspecifically on donkey and goat lymphocytes stimulated with PWM. HCCM did suppress proliferation of interspecific lymphocytes (p < 0.01); however, the suppressive capacity of HCCM in caprine lymphocyte cultures was less (p < 0.05) than that observed in equine cultures. These data support the hypothesis that the horse conceptus produces an immunoregulatory factor. This factor is extremely stabile and appears to exhibit some degree of species-specificity. The production and immunosuppressive effectiveness of such a factor may play an important role in maintaining the fetal allograft throughout gestation.