Abstract SY38-02: Postpartum tissue remodeling drives breast cancer metastasis

Abstract

Abstract Women diagnosed with breast cancer within 10 years of a completed pregnancy are at ~2-3 fold increased risk for developing metastatic disease (1,2). This increased risk for metastasis is independent of tumor stage and biologic subtype, implicating a tumor-extrinsic biology unique to the postpartum woman (3,4). Further, 30-50% of all young women diagnosed with breast cancer have had a completed pregnancy within the last 10 years (4), making a postpartum diagnosis an independent poor prognostic indicator for a significant number of patients. One postpartum event that may drive tumor progression in rodents (5-8) and women (9) is weaning-induced mammary gland involution, a physiologic tissue remodeling process that shares numerous attributes with protumorigenic wound healing. We have developed rodent models designed to mimic this highly metastatic subset of young women's breast cancer and find that weaning-induced mammary gland involution supports breast cancer growth, dissemination, and escape to secondary organs in a COX-2 dependent manner (6,10,11). Here we extend these studies by investigating the mammary gland from a mucosal organ perspective, as identifying links with mucosal biology and its associated immune suppression may provide insight into the poor prognosis of postpartum breast cancers. While the mammary gland is not classically considered a mucosal organ, bacteria routinely interface with luminal epithelium during lactation, necessitating barrier function. Similar barrier defenses are anticipated during weaning-induced mammary gland involution, as milk stasis increases risk for mastitis. One key immunologic hallmark of a mucosal organ is the presence of a distinct CD4+ helper T-cell subset, Th17 cells. Th17 cells stimulate epithelial cell junction integrity and epithelial secretion of mucins and defensins, mechanisms that barricade against bacterial activation of antitumor Th1 immune cells. Within the mammary gland we find a baseline mucosal program of Th17 T cells, which is elevated within lactating and involuting mammary glands. Mucosal features are further expanded during involution to include tolerogenic dendritic cell phenotypes, barrier-supportive antimicrobials, alternatively activated Th2 CD4+ cells, and immunosuppressive Treg CD4+T cells. Further, during involution, we find suppression of mammary-derived antigen-dependent CD4+ T-cell activation, data consistent with immune tolerance. We also find antigen-independent accumulation of memory Th17- Treg CD4+ T cells, data consistent with weaning-induced mammary inflammation. Overall, these data elucidate strong mucosal immune programs within the lactating gland and immune-suppressive programs within the involuting gland. Tumor cells within the involution immunologic microenvironment are anticipated to avoid immune detection and readily escape the mammary gland, a mechanism that may contribute to increased metastasis observed in postpartum breast cancer patients. Importantly, to successfully metastasize, a tumor cell must not only escape from the primary tumor, but also seed and grow within the secondary site. Thus, an additional potential mechanism by which a postpartum diagnosis may confer increased risk of metastasis is through involution-specific changes at metastatic sites. We report evidence for such a metastatic advantage within the liver of postpartum hosts. Using both intracardiac and portal vein liver metastasis models, we find that the post-weaning liver supports increased seeding of murine mammary tumor cells compared to livers of nulliparous control mice (12). Relevance to women is suggested by data obtained from a cohort of young women's breast cancer patients, where liver metastatic tropism is observed specifically in postpartum patients. In rodents, the biology driving this metastatic advantage is weaning-induced liver involution, a previously unrecognized tissue remodeling process. Post weaning, we find a 50% reduction in liver volume, hepatocyte cell death, deposition of fibrillar collagen and tenascin-c, increased matrix metalloproteinase activity, and influxes of immune cell populations with immune-suppressive phenotypes (12,13). These tissue-level changes are consistent with the establishment of a prometastatic niche during liver involution. Combined, our studies are supportive of the mammary gland being a hormone-responsive, immune-suppressed mucosal organ, and suggest the liver as a hormone-responsive organ that is functionally coordinated with the mammary gland during the cycle of pregnancy, lactation, and weaning. Further, we identify stromal remodeling in the mammary gland and liver as a mediator of breast cancer metastasis in the postpartum window. These findings shed light on how normal reproductive physiology, specifically mucosal biology interfaced with tissue remodeling, alters site-specific metastasis. These findings provide the framework necessary to investigate postpartum involution as a target for the prevention of breast cancer metastasis in young women.