Abstract
One of the main functions of the human placenta is to provide a barrier between the fetal and maternal blood circulations, where gas exchange and transfer of nutrients to the developing fetus take place. Despite being a barrier, there is a multitude of crosstalk between maternal immune cells and fetally derived semi-allogeneic trophoblast cells. Therefore, the maternal immune system has a difficult task to both tolerate the fetus but at the same time also defend the mother and the fetus from infections. Mucosal-associated invariant T (MAIT) cells are an increasingly recognized subset of T cells with anti-microbial functions that get activated in the context of non-polymorphic MR1 molecules, but also in response to inflammation. MAIT cells accumulate at term pregnancy in the maternal blood that flows into the intervillous space inside the placenta. Chemotactic factors produced by the placenta may be involved in recruiting and retaining particular immune cell subsets, including MAIT cells. In this Mini-Review, we describe what is known about MAIT cells during pregnancy and discuss the potential biological functions of MAIT cells at the fetal-maternal interface. Since MAIT cells have anti-microbial and tissue-repairing functions, but lack alloantigen reactivity, they could play an important role in protecting the fetus from bacterial infections and maintaining tissue homeostasis without risks of mediating harmful responses toward semi-allogenic fetal tissues.
Highlights
During pregnancy, the maternal immune system is confronted with foreign antigens derived from the semi-allogenic fetus and placenta
The overall frequencies of peripheral Mucosal-associated invariant T (MAIT) cells were unaltered during the course of healthy pregnancy, in HIV-infected pregnant women, and in women with subsequent preterm birth (PTB), MAIT cells subsets were altered with a higher proportion of CD8+ MAIT cells in first trimester in women with PTB compared to term birth [41]
Several questions remain regarding the function of MAIT cells during pregnancy and the data available so far derive solely from observational studies on human pregnancies
Summary
The maternal immune system is confronted with foreign antigens derived from the semi-allogenic fetus and placenta. Mucosal-associated invariant T (MAIT) cells are one type of immune cell subset that is relatively enriched in intervillous compared to peripheral blood at term pregnancy [21, 22] and MAIT cells are present in decidual tissues [24]. It can be speculated that this atypical chemokine receptor with CC chemokine scavenging function can play a role in regulating the number and position of maternal immune cells at the fetal-maternal interface It remains to be determined if D6 is involved in shaping the immune cell composition with increased accumulation of MAIT cells, effector memory T cells and mature naïve B cells in the intervillous space. MR1+ cells are detected in term decidua and some of these cells are macrophages, as assessed by CD68 expression [21], indicating that decidual macrophages have the potential to present antigens to MAIT cells
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