Pregnancy is closely associated with maternal-fetal immunity (Billingham et al., 1953). Activated immune cells in peripheral blood and maternal-fetal interface are recognized as an immune response against the semi-allogeneic fetus (Mor et al., 2017). Maternal immune tolerance to the fetus takes on great significance to a successful pregnancy. Contrarily, the rejection of the fetus by maternal immunity may result in recurrent pregnancy loss (RPL), embryo implantation failure, PE, GDM, premature birth, and many other pregnancy complications (Zhang and Wei, 2021, Krop et al., 2020).

CD4+CD25highFoxp3+ Tregs are critical to suppress immune reactivity to paternal- (fetal-specific) and auto-antigens, which can be detrimental to the host (Sakaguchi, 2004). Tregs exhibit strong local tropism and migration abilities. They quickly react than Teffs and serve as the first responders, infiltrating into tissues stimulated by persistently expressed self-antigens (Gratz et al., 2014). The decreased immune regulatory function of Tregs is significantly associated with a wide variety of pregnancy complications (Wang et al., 2022a, Green et al., 2021). Research has suggested that CD45RA+ naïve Tregs (nTregs) are transformed into memory Tregs (mTregs) expressing CD45RO+ after antigen stimulation (Rosenblum et al., 2016).

mTregs are stable in vitro, whereas they exhibit stronger immunosuppressive capacity than nTregs and exert a dominant effect on immunosuppression in the total Tregs pool (Miyara et al., 2009, Granne et al., 2022). In the mice model, Tregs were quickly recruited to uterus-draining lymph nodes and activated after the embryo implantation, and expressed the memory table of CD44HighCD62Llow. The abnormal number and immune regulation of mTregs induce the unbalanced immune response in pregnant women, which is correlated with the onset of GDM and PE (Schober et al., 2014, Wagner et al., 2016).

The expression levels of cell surface markers, e.g., cytotoxic T lymphocyte antigen 4 (CTLA-4/CD152), programmed cell death receptor-1 (PD-1/CD279), human leucocyte antigen-G (HLA-G), and HLA-DR, contribute to the regulation of the Treg function (Zheng et al., 2009). In mice study, the short-term treatment of CTLA-4-Ig can block the interaction between CD28 and CD80/CD86, and significantly decreased mTregs cell cycle and quantity (Panda et al., 2022). In our previous studies, the reduced number of of PD-1, HLA-G and CCR6 expressing mTregs was associated with the incidence of RPL by reducing stability, proliferation frequency, immunosuppressive intensity, or chemotaxis (Wang et al., 2022b).

Although mTregs serve as a critical immune effector in maintaining healthy pregnancy, the specific changes of mTregs in pregnancy and various complications remain unclear, especially the expression of surface markers. This study investigated if the proportion of mTreg subsets in the peripheral blood is different between healthy and complicated pregnancies (GDM and PE), and the expression of PD-1, HLA-G, and HLA-DR on mTreg subsets to seek for possible immunopathological mechanisms.