Prophylactic treatment with progesterone decreases murine miscarriage by suppressing the immunostimulatory activity of macrophages.

Miscarriage and preterm birth (PB) remain major challenges in obstetric care and are often associated with excessive inflammation at the feto-maternal interface. Although the role of progesterone (P4) in maintaining pregnancy is well known, its anti-inflammatory effects in immune-mediated pregnancy complications remain poorly understood. In this study, we investigated the impact of prophylactic P4 administration on miscarriage using a mouse model induced by α- galactosylceramide (αGC), a potent activator of invariant natural killer T (iNKT) cells. Prophylactic, but not therapeutic, P4 administration significantly reduced miscarriage rates. Flow cytometry analysis revealed that P4 suppressed the activity of iNKT cells and the production of inflammatory cytokines by these cells in the myometrium. Moreover, P4 reduced the immunostimulatory activity of antigen-presenting cells, particularly macrophages, by downregulating co-stimulatory molecules and interleukin (IL)-12 production. Immunohistochemistry and flow cytometry results demonstrated that the progesterone receptor (PR) was predominantly expressed on myometrial macrophages. Ex vivo experiments further confirmed that P4 directly modulates macrophage function, decreasing IL-12 and increasing IL-10 production. These findings suggest that prophylactic P4 administration mitigates immune activation at the feto-maternal interface by targeting macrophages, thereby suppressing iNKT cell-mediated inflammation and preventing miscarriage. This study highlights the importance of innate immune modulation in reproductive immunology and the potential of P4 as a prophylactic agent for preventing inflammation-associated miscarriage and PB.