Implication of HLA-G in human embryo implantation

The mechanisms underlying the processes of human embryo implantation and maternal immunotolerance to the fetus remain incompletely understood. Growing evidence indicates that one of the nonclassic human leukocyte antigen (HLA) genes, HLA-G, is expressed selectively on the surface of the extravillous throphoblast and plays an important role in the adaptations of the maternal immune system to pregnancy; however the implication of this molecule in the process of human embryo implantation is also plausible. Therefore the aim of the present article is to review the available literature specifically focused on the possible relationship between HLA-G and human embryo implantation. In particular, studies investigating HLA-G expression on human preimplantation embryos and in the endometrium, as well as its levels in embryo culture supernatants and circulating maternal blood, are discussed.