HLA-G in human early pregnancy: control of uterine immune cell activation and likely vascular remodeling.

Despite a number of controversies, the functional importance of human leukocyte antigen G (HLA-G) in early human pregnancy is now sustained by a large amount of sound data. Membrane-bound and soluble HLA-G isoforms, either as β2-microglobulin-free or -associated as monomers or dimers, are expressed by different trophoblast subpopulations, the only fetal-derived cells that are directly in contact with maternal cells (maternal-fetal interfaces). Trophoblast HLA-G is the specific ligand of multiple cellular receptors present in maternal immune and non-immune cells, including CD8, leukocyte immunoglobulin-like receptor (LILR) B1, LILRB2, killer cell immunoglobulin-like receptor (KIR) 2DL4, and possibly CD160. Trophoblast HLA-G specific engagement of these cellular receptors triggers either inhibitory or activating signals in decidual CD8 + T cells, CD4 + T cells, natural killer (NK) cells, macrophages, dendritic cells, or endothelial cells. Such HLA-G-receptor specific interactions first contribute to limit potentially harmful maternal anti-paternal immune response by impairment of decidual NK cell cytotoxicity, inhibition of CD4 + and CD8 + T-cell and B-cell proliferation, and induction of apoptosis of activated CD8 + T cells. Second, these HLA-G specific interactions contribute to stimulate placental development through secretion of angiogenic factors by decidual NK cells and macrophages, and to provide a protective effect for the outcome of pregnancy by the secretion of interleukin (IL)-4 by decidual trophoblast antigen-specific CD4 + T cells.