Sex Hormones-Mediated Modulation of Immune Checkpoints in Pregnancy and Recurrent Pregnancy Loss

Recurrent pregnancy loss (RPL) is defined as the loss of two or more pregnancies before the 22nd gestational week and affects 10-15% of clinical pregnancies. Despite extensive diagnostics, over 50% of RPL cases remain unexplained, suggesting an important role for immunological mechanisms. Sex hormones (SH) are key regulators of immune responses during pregnancy; however, their influence on immune checkpoint proteins (ICPs) is poorly understood. This study evaluated the effects of progesterone, β-estradiol, and dihydrotestosterone (DHT) on ICP expression on immune cells, including Treg, NK, NKT, TC, Th, and T cells, collected from pregnant women and patients with unexplained RPL (uRPL). Peripheral blood mononuclear cells from 20 pregnant women and 20 uRPL patients were cultured for 48 h with SH. The expression of the first generation of ICPs-PD-1 and TIM-3-and the second-LAG-3, TIGIT, and VISTA-on T, NK, and NKT cells was analyzed by the flow cytometry method. In pregnant women, SH exerted modest effects, with DHT increasing VISTA and LAG-3 expression, while progesterone and estradiol mainly upregulated LAG-3 and TIM-3 on cytotoxic cells. In contrast, uRPL immune cells showed pronounced SH sensitivity, characterized by increased TIM-3 and VISTA expression and reduced TIGIT expression, particularly after DHT stimulation. In conclusion, SH modulates ICP expression in a cell-specific manner, with stronger effects observed in uRPL patients' lymphocytes. These findings highlight a potential role for hormonal and ICP-targeted strategies in RPL management.