Immune cells at the maternal-fetal interphase: Role in implantation and establishment of tolerance

Abstract

Reproduction is essential to the survival of all species; hence, elaborate mechanisms ensure the persistence of pregnancy and the establishment of the fetus. For the establishment of fetus which is a semi allogeneic graft, immunological mechanisms which favor tolerance are preferred. A successful pregnancy is an outcome of interactions between the hormones produced by the placenta and the immune cells present in the decidua. The maternal-fetal interface (MFI) shows a predominance of cells involved in providing innate immunity, with uterine NK cells representing 70% of all immune cells. Human leucocyte antigen G (HLA-G), a non-classical major histocompatibility complex, is expressed by the trophoblast cells. The interaction between HLA-G present on the trophoblast and the receptors on NK cells is important for maintenance of immune tolerance. Macrophages, another important innate immune cells represent about 20%, while the adaptive T-cells account for <20% of all immune cells at the MFI. Different subpopulations of T-cells have also been identified at MFI. There is a growing evidence for the presence of unconventional γδ T lymphocytes at the MFI. The T helper cells can be divided into Th1 and Th2. Macrophages can be grouped into subtypes M1 and M2. This subdivision is according to their activation status and the type of cytokines secreted by them. Th1 lymphocytes and M1 macrophages are pro-inflammatory while Th2 lymphocytes and M2 macrophages are anti-inflammatory. As fetus grows an equilibrium between pro-and anti-inflammatory immune mechanisms is maintained and that results in establishment of sustained pregnancy. Here, I have reviewed the characteristic features and the role of all these cells in implantation of the fetus and establishment of tolerance. Understanding the functions of immune cells and interactions among these cells can give a better insight of pregnancy-related complications.