Abstract
Macrophages are major cellular inhabitants of cycling and pregnant mammalian uteri. Their densities and patterns of tissue distribution in this organ fluctuate in concert with levels of circulating female sex steroid hormones, estrogens and progesterone, and their production of various effector molecules also may be hormonally regulated. Hormonal control may be achieved by direct binding to receptors or by indirect pathways where hormones modulate production of various autocrine and paracrine cytokines and growth factors that then target to resident macrophages and influence their secretory profiles. In this paper, we marshall evidence supporting the concept that progesterone acts as a powerful negative regulator of these versatile cells, reducing their migration into the uterus and impairing their ability to produce potent effector molecules such as nitric oxide that could interfere with the success of pregnancy.