Supraphysiological estradiol promotes human T follicular helper cell differentiation and favours humoural immunity during in vitro fertilization

The impact of reproductive hormones on T cell immunity; normal and assisted reproductive cycles

During pregnancy, a unique immune milieu is established systemically and locally at the maternal-fetal interface. While preparing for embryonic implantation, endometrial effectors significantly change their proportions and function, which are synchronized with hormonal changes. During assisted reproductive technology cycles, various cytokines, chemokines, and immune factors dynamically change with the altered receptor expressions on the immune effectors. Thus, the hormonal regulation of immune effectors is critical to maintaining the immune milieu. In this review, hormonal effects on T cell subsets are reviewed. Sex hormones affect T cell ontogeny and development, consequently affecting their functions. Like other T cell subsets, CD4+ T helper (Th) cells are modulated by estrogen, where low estrogen concentration promotes Th1-driven cell-mediated immunity in the uterus and in vitro by enhancing IFN-γ production, while a high estrogen level decreases it. The abundance and differentiation of T regulatory (Treg) cells are controlled by estrogen, inducing Treg expansion. Conversely, progesterone maintains immune homeostasis by balancing Th1/Th2 and Th17/Treg immunity, leading to maternal-fetal tolerance. Therefore, the understanding of the hormonal impact on various T cell subsets during the reproductive cycles is critical to improving reproductive outcomes in women with recurrent pregnancy losses, repeated implantation failures, and undergoing assisted reproductive cycles.

Supraphysiological estradiol promotes human T follicular helper cell differentiation and favours humoural immunity during in vitro fertilization

During pregnancy, humoural immunity is essential for protection against many extracellular pathogens; however, autoimmune diseases may be induced or aggravated. T follicular helper (Tfh) cells contribute to humoural immunity. The aim of this study was to test whether Tfh cell function can be manipulated via hormones. Seventy‐four women who underwent in vitro fertilization were recruited and divided into four groups: menstrual period (MP), controlled ovarian hyperstimulation (COH), embryo transfer (ET) and pregnant after embryo transfer (P). A flow cytometry analysis was performed to identify Tfh cells in peripheral blood mononuclear cells (PBMCs). Bioinformatics analysis revealed a possible pathway between Tfh and B cells. Enzyme‐linked immunosorbent assays were used to detect interleukin (IL)‐21 and IL‐6. The quantitative polymerase chain reaction was performed to quantify BCL‐6, BACH2, XBP‐1, IRF‐4 and G protein‐coupled (GP)ER‐1 mRNA expression. Compared with the MP group, the COH, ET and P groups showed more Tfh and B cells, as well as higher IL‐21, IL‐6, BCL‐6 and BACH2 expression. Furthermore, Tfh cell frequency in PBMCs, as well as serum IL‐21 and IL‐6 levels, were all positively correlated with serum estradiol (E₂) levels; the B cell percentage also correlated positively with Tfh cells in PBMCs. Combined with the bioinformatics analysis, XBP‐1, IRF‐4 and GPER‐1 expression was related to E₂ levels, both in vivo and in vitro. We speculate that E₂ augments Tfh cells and favours humoural immunity. This study indicates that Tfh cell regulation may be a novel target in maintaining the maternal‐foetal immune balance.