The contribution of macrophages to normal and pathological pregnancies

Macrophages represent one of the major leukocyte subsets in the uterine decidua. Owing to their remarkable phenotypic plasticity, decidual macrophages can participate in diverse activities during pregnancy. At baseline, decidual macrophages are characterized by an immunosuppressive phenotype and M2 polarization, supporting feto-maternal immune tolerance. In early pregnancy, macrophage-derived pro-angiogenic factors prompt vascular remodeling within the uterine wall to ensure appropriate utero-placental circulation. Upon invasion by pathogens, pattern recognition receptors on decidual macrophages help to alter the characteristics of these malleable cells toward an M1, inflammatory phenotype. Similar inflammatory characteristics are seen in those macrophages that accumulate in the lower segment of the uterus to drive cervical ripening. Disturbances in the tight control that balances macrophage function during pregnancy can trigger the development of pregnancy complications. Here, we discuss the physiologic role of uterine macrophages at different stages of pregnancy and describe their relevance in selected pregnancy disorders.