Immunosuppressive effects of rabbit blastocoelic fluid and embryo culture medium

In order to understand the mechanism of the feto-maternal immune relationship, we assayed the immunosuppression activities of fresh blastocoelic fluid and decomplemented peripheral serum collected from day-9 pregnant white New Zealand rabbits and of rabbit embryo culture medium (ECM). Because the viability of the human lymphocytes was not affected by either of these biological fluids and since they were easy to obtain in sufficient quantities, they were used uniformly in all the experiments. Immunosuppressive effect was calculated by the relative inhibition of proliferation of Con A-stimulated lymphocytes. The immunosuppressive effect of blastocoelic fluid of the 9-day pregnant rabbits was significantly higher than that of autologous decomplemented serum (P less than 0.001). The inhibition by the serum was non-specific because sera from non-pregnant animals as well as sera from different stages of pregnancy and pseudo-pregnancy showed the same level of inhibition. The ECM of 6.5-7-day-old embryo showed a pronounced immunosuppressive effect. When embryos of 1,3 and 5 days were cultured and their culture media were assayed only with 5-day-old embryo the effect had begun to appear, but it was far less than that of 7-day-old embryo (P less than 0.02). The suppressive activity of both the blastocoelic fluid and ECM was not due to cytotoxic effect, since this fluid supported the in vitro growth of single-cell rabbit embryos up to the stage of blastocyst. These results suggest that the immunologic tolerance of the embryo might be due to the immunosuppressors secreted by the embryo and that there might be a localized effect at the implantation site rather than a maternal systemic immunosuppressive effect.