| Summary: | Summary: During pregnancy, maternal regulatory T cells (Tregs) are important in establishing immune tolerance to invading fetal extravillous trophoblasts (EVTs). CD25HIFOXP3+ Tregs are found at high levels in decidual tissues and have been shown to suppress fetus-specific and nonspecific responses. However, limited data are available on additional decidual Treg types and the mechanisms by which they are induced. This study investigated three distinct decidual CD4+ Treg types in healthy pregnancies with a regulatory phenotype and the ability to suppress T cell responses: CD25HIFOXP3+, PD1HIIL-10+, and TIGIT+FOXP3dim. Moreover, co-culture of HLA-G+ EVTs or decidual macrophages with blood CD4+ T cells directly increased the proportions of CD25HIFOXP3+ Tregs compared to T cells cultured alone. EVTs also increased PD1HI Tregs that could be inhibited by HLA-C and CD3 antibodies, suggesting an antigen-specific induction. The presence of distinct Treg types may allow for the modulation of a variety of inflammatory responses in the placenta. : Salvany-Celades et al. refine the view of regulatory T cells (Treg) at the maternal-fetal interface by presenting phenotypic and functional data of three decidual Treg populations in human pregnancy. Extravillous trophoblasts and decidual macrophages directly increase Treg populations, and antigen specificity for HLA-C may be involved. Keywords: HLA-C, HLA-G, pregnancy, decidua, placenta, trophoblast, IL-10, immune tolerance, Trl cell, FOXP3
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