PROJECT SUMMARY Dilation & curettage (D&C) is one of the most common surgical procedures performed on women throughout the world. Intrauterine adhesions (iUA), or Asherman, develop in about 40% of women who undergo D&C in the postpartum (6 week period after birth), but is very rare after D&C in nonpregnant women (<1%) for reasons that are not well understood. It is a debilitating condition characterized by intrauterine fibrosis and scarring. Patients with iUA suffer from infertility, recurrent pregnancy loss and a broad range of dangerous pregnancy complications (e.g. preterm birth). While endometrial mesenchymal stem/progenitor cells (eMPCs) and macrophage immune cell populations are crucial for endometrial repair and postpartum uterine remodeling, the role of these cells and underlying molecular mechanisms in this postpartum susceptibility of the endometrium to fibrosis and abnormal repair are unknown. This application is specifically focused on defining the role that macrophages play in abnormal uterine repair, given the known important role of macrophage activation as a driver of fibrosis in other organs. The central hypothesis is that macrophages in the postpartum uterus are polarized to M2 phenotype, resulting in a profibrotic response to uterine injury as compared to M1 macrophages of the nonpregnant uterus, ultimately leading to fibrosis and scar formation. The approach is to use our novel postpartum mouse uterine injury model which recapitulates the susceptibility of the human postpartum uterus to injury. Using this model, we will characterize the macrophage populations and polarization dynamics in the postpartum and nonpregnant uterus in response to injury (Aim 1). We will directly examine the role of macrophage polarization in inducing profibrotic transformation of endometrial stromal fibroblasts using in vitro co-cultures (Aim 2) to gain detailed insights into the cellular and molecular differences that predispose the postpartum endometrium to form iUA. The proposed aims are conceptually innovative and together will have a broad impact on the field by filling a substantial gap in our fundamental knowledge of endometrial biology and infertility pathogenesis. Ultimately, the knowledge gained from this proposal will not only be invaluable to our understanding of many more subtle conditions of abnormal endometrial repair, but provide unique insights into the body’s physiological anti-fibrotic wound healing mechanisms leading to a deeper understanding of the pathogenesis of fatal idiopathic fibrotic diseases in other organs.