As early as 1927, Sampson theorized that endometriosis occurs as a consequence of the mechanical transfer of endometrial tissue to the peritoneum via retrograde menstruation, a process that occurs in most women. Using both in vivo observations and in vitro studies, our laboratory has demonstrated that, in the endometrium of endometriosis, an inflammatory-like microenvironment alters the expression ratio of progesterone receptors (PR-A and PR-B), disrupts the ability of progesterone to up-regulate transforming growth factor [unreadable] (TGF-[unreadable]) expression and down-regulates cell-specific matrix metalloproteinase (MMP) expression. Many investigators now suspect that endometriosis occurs as a predictable consequence of the failure of progesterone to control the proinflammatory activity of immune cells that migrate into the endometrium prior to menstruation. In the normal endometrium, decay accelerating factor (CD55) is upregulated in response to progesterone, suggesting that this complement protective protein may play a critical role in controlling the behavior of immune cells during preparation for pregnancy. Although the relationship of reduced epithelial CD55 expression to the development of endometriosis is unknown, both TGF-[unreadable] and CD55 are known to impact the activation and function of polymorphonuclear neutrophils (PMNs) during inflammation. PMNs are key trigger cells during early inflammation and can significantly increase MMP expression during menstruation. We have noted the rapid accumulation of PMNs at ectopic sites of human cell growth in our chimeric models of endometriosis using immune deficient mice. In this application, we propose that reduced expression of PR-B in endometrial stromal cells of endometriosis patients will inhibit progesterone-dependent expression of TGF-[unreadable]2 and CD55 in vivo, in vitro and at ectopic sites of endometrial cell growth in experimental murine models of endometriosis. We also predict that altered expression of these factors will affect the activated state of PMNs that migrate into ectopic sites of human endometrial cell and tissue growth in mice. Our specific aims are: 1)- to examine whether reduced endometrial responsiveness to progesterone affects the in vivo and in vitro expression of CD55 by epithelial cells in the eutopic endometrium of women with endometriosis;2)- to utilize established in vitro and in vivo co-culture models to examine whether reduced sensitivity to progesterone affects the ability of endometrial stromal cells acquired from endometriosis patients to mediate CD55 expression in adjacent epithelial cells, and 3)- to utilize newly established in vivo models to examine whether the expression of TGF-[unreadable]2 and CD55 correlates with PMN recruitment and function at ectopic sites of human cell and tissue growth in RAG2? (c) mice.