Suppression of Lymphocyte Activation by a Soluble Factor Released From the Human Placental Chorionic Membrane: Chemical Analysis and Functional Characterization

Abstract

A heat-resistant factor that markedly and reversibly inhibits human lymphocyte activation was discovered in culture medium of human placental chorion. The chorionic factor inhibits lymphocyte proliferation in response to polyclonal mitogens and in the mixed leukocyte response. The inhibitory action is most effective if the factor is added during the first 24 h of lymphocyte culture and is reversible. The chorionic factor is sensitive to proteinase K, pepsin and bovine pancreatic protease; its activity is completely lost after papain digestion and following treatment with trichloracetic acid (TCA). The factor prevents the expression of IL-2 receptors and class II MHC glycoproteins (HLA-DR) on phytohemagglutinin-stimulated PBMC but does not affect the expression of MHC class I molecules. It inhibits the replication of IL-2-dependent CTLD cells but is without effect on the growth of various human or murine cell lines or acute leukemic cells. Human placental chorion is thus capable of releasing in vitro a nontoxic heat-resistant factor with protein characteristics that reversibly inhibits processes associated with the early stages of lymphoid cell triggering. This factor may play a role in an immunoregulatory mechanism that prevents maternally mediated immune rejection of the conceptus.