The human blastocyst produces a soluble factor(s) that interferes with lymphocyte proliferation.

Animal studies have reported that the blastocyst produces immunosuppressive factors that are critical to successful implantation. The production of such factor(s) by the human blastocyst has not yet been described. To test that hypothesis, the spent media of 67 human embryos developed in vitro was evaluated using either the allogeneic one-way mixed lymphocyte response (MLR) or the concanavalin A (Con-A) stimulated lymphocyte response (SLR). Conditioned embryo media was obtained at in vitro fertilization (IVF) in aliquots of (A) the first 24 hours in vitro, (B) the second 24 hours in vitro, and (C) from 24 hours until development of the blastocyst stage. Control media consisted of Ham's F-10 medium (Gibco, Grand Island, NY) with identical maternal serum concentration to test media. Conditioned media (either A, B, or C) or control media was added to 200-microL wells containing lymphocytes in either the MLR or SLR. A significant suppressive effect of 25% to 60% was observed in seven of nine samples of blastocyst-conditioned media (P less than 0.05). No significant suppressive effect was found in the earlier embryo media (A or B) in either the MLR or the SLR. These data suggest that the human blastocyst produces a factor(s) that interferes with lymphocyte proliferation.