Estriol effect on proliferative activity of regulatory thymocyte subpopulations

Estriol (E3) is a steroid hormone synthesized by fetoplacental unit which effectively regulates reproductive tissues and functioning of immune cells. A changing ratio in regulatory lymphocyte subsets, i.e., IL-17-producing cells (Th17) and regulatory T cells (Treg) is one of key mechanisms for maintaining immunological tolerance during pregnancy. Invariant natural killer T cells (iNKT), which have both cytotoxic and immunoregulatory activity, also play an important role during the gestation period. Differentiation of these lymphocyte subtypes begins in thymus gland. The aim of our work was to study the in vitro effect of E3 upon composition of thymic regulatory cells subpopulations (Th17, Treg, iNKT). Thymocytes were cultured for 72 h with E3 at a concentration of 20 ng/ml, which corresponds to its maximum level in peripheral blood during pregnancy, with CD3/CD28-activated particles, then followed by flow cytometry-based assessment of regulatory cells subpopulations and Ki-67 expression. Thymic nTreg cells were defined as percentage of CD4+CD25+FoxP3+ cells; Th17 cells were defined as CD4+IL-17A+RORγt+ cells; iNKT cells were determined as percentage of CD3hiVa24Ja18+ cells in thymocyte culture. Treg, Th17, and iNKT proliferative potential was estimated by Ki-67 expression. E3 inhibited differentiation of CD3/CD28- stimulated thymocytes towards nTreg and enhanced iNKT formation. The hormone did not significantly affect Ki-67+ nTreg and iNKT cells numbers, however, with a trend for decrease. E3 did not influence Th17 numbers but inhibited its proliferative response in this subset. Hence, E3 changes its classical effect to the opposite direction in the presence of an antigen, protecting mother’s body from potential danger. Estriol was shown to enhance iNKT formation and inhibited nTreg formation from thymocytes. At the same time, E3 showed downregulation of thymic Th17 cells proliferation, thus compensating the decreased nTreg number, and preventing premature birth risk. Since pregnancy is known to be associated with steroid-induced thymus atrophy, we have suggested a possible mechanism underlying this process, involving the pregnancy hormone E3.