Cellular mechanisms for maintenance of feto-maternal tolerance during pregnancy

Abstract

Pregnancy is an immunological paradox, since a fetus carrying paternal antigens is a semiallogeneic transplant that should be rejected by the mother’s body. However, the fetus is completely protected from immune attack, thus suggesting some complex mechanisms of feto-maternal interaction. Hormonal, autocrine and paracrine immune signals and neuronal pathways play an important role in the development and maintenance of pregnancy. Pregnancy is considered a dynamic and actively modulated immunological process at each stage of pregnancy, including embryo implantation, placentation, fetal development, and delivery, being represented by a unique immune status. Studying the mechanisms of maintenance of pregnancy is vital to address the problems of miscarriage of unknown etiology. Successful pregnancy is closely related to the ability of the maternal immune system to properly adapt for each distinct stage of gestation. This review considers the main cell populations, such as regulatory subtypes of T and B cells, T helper cells, decidual natural killers, myeloid suppressors, erythroid nucleated cells which provide feto-maternal tolerance via various intercellular and humoral mechanisms. Maternal immune cells in the placenta do not attack fetal cells (trophoblasts) due to the tolerogenic microenvironment created by regulatory T cells and other immune cells. During pregnancy, each subpopulation of T helper cells plays a key role in promotion of fetal development through the production of angiogenic factors, providing immune surveillance and suppressing aberrant effector cell responses against a semi-allogeneic fetus. Accumulation of myeloid suppressor cells is especially relevant, when the immune tolerance is required for survival. Decidual NK cells closely interact with trophoblast cells and secrete cytokines that promote growth, mediate differentiation, trophoblast invasion, and remodeling of the spiral arteries. The favorable tolerogenic state in utero predisposes the newborn to severe infections, especially those caused by intracellular pathogens. Hence, the fetal tolerance may differ from other types of tolerance due to the presence of various immunosuppressive cells, such as erythroid suppressor cells in newborns. In the course of pregnancy, the properties of these cells change dynamically in order to meet the demands that arise during pregnancy in a timely manner. Understanding the immunological changes induced by pregnancy may not only reveal new therapeutic strategies to improve pregnancy outcomes, but also highlight new aspects of how the immune tolerance works being applicable in other physiological and pathological contexts.