Progesterone-mediated immunomodulation in ruminant early pregnancy: mechanisms and implications for bovine fertility.

Progesterone, traditionally recognized for maintaining the gestational environment, also plays a pivotal immunomodulatory role essential for successful pregnancy in bovine species. This review synthesizes current understanding of progesterone-mediated immune regulation during early bovine pregnancy, focusing on its molecular mechanisms. These include genomic pathways involving nuclear progesterone receptors, non-genomic signaling through membrane progesterone receptors and progesterone receptor membrane component 1, and immunoregulatory effects mediated by the progesterone-induced blocking factor (PIBF). Progesterone acts both locally at the maternal-fetal interface and systemically, shaping immune tolerance and protecting the developing conceptus. Locally, it promotes an anti-inflammatory microenvironment by expanding regulatory T cells, reducing the cytotoxicity of natural killer cells, and suppressing dendritic cell activation. It downregulates pro-inflammatory cytokines such as tumor necrosis factor-alpha and interferon-gamma, while upregulating anti-inflammatory cytokines including interleukin-4 and interleukin-10. Additionally, progesterone inhibits the nuclear factor kappa B signaling pathway, thereby attenuating inflammatory responses. PIBF contributes by enhancing T helper 2 cytokine profiles and reducing cytotoxic activity. Emerging evidence also suggests that progesterone modulates immune function in peripheral organs, including the thymus, spleen, lymph nodes, and bone marrow, indicating systemic immunoregulation. However, species-specific differences in receptor expression, signaling cascades, and immune cell responsiveness remain poorly understood. Addressing these gaps is essential for optimizing reproductive management in cattle. In conclusion, a comprehensive understanding of progesterone's immunological roles may lead to improved fertility outcomes and advance knowledge of maternal-fetal immune tolerance in ruminants.