Human chorionic gonadotropin: a hormone with immunological and angiogenic properties.

The success of implantation depends on a receptive endometrium, a normal blastocyst and synchronized cross-talk at the maternal-fetal interface. The progression of pregnancy then requires immunological tolerance which allows conceptus survival. A cascade of cytokines mediates this dialogue and is crucial in the cross-talk between the immune and endocrine systems. The first known human embryo-derived signal is chorionic gonadotropin (hCG) by which the embryo profoundly influences immunological tolerance and angiogenesis at the maternal-fetal interface. hCG levels coincide with the development of trophoblast tolerance. Indeed, it increases the number of uterine natural killer cells that play a key role in the establishment of pregnancy. hCG also intervenes in the development of local immune tolerance through the cellular system of apoptosis via Fas/Fas-Ligand. It modulates the Th1/Th2 balance and acts on complement C3 and C4A/B factors modulating decidual immunity. The transient tolerance evident during gestation is at least partially achieved via the presence of regulatory T cells which are attracted by hCG at the fetal-maternal interface. Finally, hCG treatment of activated dendritic cells results in an up-regulation of MHC class II, IL-10 and IDO expression, reducing the ability to stimulate T cell proliferation. Successful implantation requires an extensive endometrial angiogenesis in the implantation site. Recent data demonstrate angiogenic effects of hCG via its interaction with endometrial and endothelial LH/hCG receptors. Our review focuses on these functions of hCG, giving new insight into the endocrine-immune dialogue that exists between the conceptus and immune cells within the receptive endometrium at the time of implantation.