Astragalus root increases Treg and Th17 involvement in embryo implantation and pregnancy maintenance by decreasing CTLA-4+ Tregs.

Maintenance of pregnancy is highly dependent on the maternal immune system. High levels of regulatory T cells (Tregs) accumulate in the maternal placenta to suppress immunoreactivity against fetal antigens. We assessed whether Astragalus root (AsR) and AsR-containing Kampo medicines modulate immunoreactivity and thereby increase mouse litter size. AsR-exposed murine splenocytes exhibited significantly increased IL-2 secretion. In AsR-exposed mice, total Tregs were significantly increased, whereas cytotoxic T lymphocyte antigen 4 (CTLA-4)-positive Tregs were decreased in AsR-exposed mice. Tregs express IL-2 receptor subunit alpha and are activated by IL-2. CTLA-4 interacts with B7 expressed in antigen-presenting cells (APCs) with high affinity, and CTLA-4/B7 signaling plays a critical role in inhibiting APC activity, thereby suppressing CD4⁺ T cell proliferation and activation. The decrease in CTLA-4⁺ Tregs in AsR-exposed mice is thought to induce an increase in CD4⁺ T cells, leading to increased IL-2 secretion from CD4⁺ T cells followed by Treg activation. Th17 cells prevent trophoblast apoptosis, resulting in trophoblast invasion into the decidua. AsR increases Th17 cells, thereby inducing dose-dependent increases in litter size. Although Keishikaogito (KO)- and Ogikenchuto (OK)-exposed mice exhibited increased IL-2 secretion and splenic Tregs, KO also increased CTLA-4⁺ Tregs. Therefore, KO promoted immunosuppression by increasing CTLA-4⁺ Tregs, which induced a decrease in Th17 and exerted little effect on litter size. Therefore, an increase in both Tregs and Th17 cells can be considered necessary for embryo implantation and pregnancy maintenance.