Abstract
In pregnancy, the semi-allogeneic fetus needs to be tolerated by the mother's immune system. Regulatory T cells (Tregs) play a prominent role in this process. Novel technologies allow for in-depth phenotyping of previously unidentified immune cell subsets, which has resulted in the appreciation of a vast heterogeneity of Treg subsets. Similar to other immunological events, there appears to be great diversity within the Treg population during pregnancy, both at the maternal-fetal interface as in the peripheral blood. Different Treg subsets have distinct phenotypes and various ways of functioning. Furthermore, the frequency of individual Treg subsets varies throughout gestation and is altered in aberrant pregnancies. This suggests that distinct Treg subsets play a role at different time points of gestation and that their role in maintaining healthy pregnancy is crucial, as reflected for instance by their reduced frequency in women with recurrent pregnancy loss. Since pregnancy is essential for the existence of mankind, multiple immune regulatory mechanisms and cell types are likely at play to assure successful pregnancy. Therefore, it is important to understand the complete microenvironment of the decidua, preferably in the context of the whole immune cell repertoire of the pregnant woman. So far, most studies have focused on a single mechanism or cell type, which often is the FoxP3 positive regulatory T cell when studying immune regulation. In this review, we instead focus on the contribution of FoxP3 negative Treg subsets to the decidual microenvironment and their possible role in pregnancy complications. Their phenotype, function, and effect in pregnancy are discussed.
Highlights
Reviewed by: Sayaka Tsuda, University of Toyama, Japan Lianjun Zhang, Suzhou Institute of Systems Medicine (ISM), China
Since direct contact between maternal and fetal cells occurs at the maternal-fetal interface in the placenta, it is thought that maternal immune cells in the placenta do not attack the fetal cells because of the tolerogenic microenvironment created by regulatory T cells (Tregs) and other immune cells
The maternal peripheral blood comes into contact with the SCTs lining the fetal villi. From the moment these maternal-fetal interfaces have been established, it is of utmost importance for maternal immune cells to keep the balance between tolerizing the semi-allogeneic fetus, and at Abbreviations: Tregs, regulatory T cells; tTreg, thymic derived regulatory T cell; pTreg, periphery induced regulatory T cell; EVTs, cytotrophoblasts (CTBs), syncytiotrophoblasts (SCTs), and extravillous trophoblasts; HLA, human leukocyte antigen; antigen-presenting cells (APCs), antigen presenting cells; KIR, killer-cell immunoglobulin-like receptor; TCR, T cell receptor; IDO, indoleamine 2,3-dioxygenase; dNK, decidual NK; RPL, recurrent pregnancy loss; PE, pre-eclampsia; Single nucleotide polymorphisms (SNPs), single nucleotide polymorphisms; NK, natural killer; ILCs, innate lymphoid cell; DCs, dendritic cells; DC-10, tolerogenic DCs; mTOR, mammalian target of rapamycin; NO, nitric oxide; TGF-β, transforming growth factor-beta; IFN-γ, interferon gamma
Summary
The most striking feature of pregnancy is that a semi-allogeneic fetus is tolerated by the maternal immune system. This is in sharp contrast with solid organ transplantation, where an allograft will be rejected by the patient’s immune system unless the patient takes immunosuppressive drugs. Since direct contact between maternal and fetal cells occurs at the maternal-fetal interface in the placenta, it is thought that maternal immune cells in the placenta do not attack the fetal cells (trophoblasts) because of the tolerogenic microenvironment created by regulatory T cells (Tregs) and other immune cells
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