Macrophages Are Essential for Maintenance of Corpus Luteum Function in Early Pregnancy.

Abstract

Macrophages are abundant within the ovary, and have been identified in the corpus luteum (CL) of most species studied, including in rodents and human. Through their secretory products, macrophages are thought to be involved in ovarian tissue remodelling, including luteinization, and in regulating steroidogenesis. Macrophages co-cultured with granulosa or luteal cells act to stimulate progesterone secretion in vitro. To determine the impact of macrophage ablation during early pregnancy, we utilised CD11b-DTR transgenic mice to elicit transient systemic ablation of macrophages by administration of diphtheria toxin (DT). The effects of macrophage ablation during the pre-implantation phase of pregnancy were evaluated. Ablation of macrophages on day 1 pc or day 4 pc caused complete pregnancy loss in all DT-treated CD11b-DTR mice, while DT-treated wild-type mice or PBS-treated CD11b-DTR mice maintained viable pregnancies. Macrophage ablation on day 3 pc significantly reduced serum progesterone (P4) levels to 40% of control levels when measured 24 h later. Administration of exogenous P4 on each of day 4-7 pc prevented fetal loss in DT-treated CD11b-DTR mice, and pregnancy progressed with viable pups at late gestation, while no pregnancies remained viable in DT-treated mice administered vehicle only. Immunohistochemical evaluation with anti-CD31 mAb showed that macrophage ablation in the CD11b-DTR mice resulted in disappearance of endothelial cells from within the CL. This suggests that macrophages are essential for support of endothelial cells within the CL, either by provision of trophic support and/or by macrophage transdifferentiation into endothelial cells. In conclusion, these data indicate a critical role for macrophages in corpus luteum development and steroidogenic function in early pregnancy. Given that various nutritional and immune stressors can profoundly affect macrophage numbers and behaviour, it seems reasonable to speculate that macrophage-regulated CL development is a vulnerable event and may contribute to some forms of unexplained infertility. (platform)