The role of syndecan-1 in the resolution of chronic inflammatory responses
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Inflammation is an integral part of the body defense mechanism that occurs in vascularized tissue in response to harmful stimuli that is perceived as being a threat to tissue homeostasis. It is a complex physiological host response that is designed to neutralize and eliminate harmful agents, initiate tissue healing, and orchestrate a return to tissue homeostasis. While inflammation is designed to be an acute event that resolves following the elimination of harmful stimuli and tissue healing, there are instances where inflammation fails to resolve and instead evolves into chronic inflammation. It is now well understood that ongoing inflammation can serve as the underlying cause of many chronic inflammatory diseases, including atherosclerosis. In fact, one of the most pressing issues that is currently faced in the field of inflammation research, one that has also become the focus of numerous ongoing investigations, is how to turn this excessive, unwarranted and undesirable inflammation response off. Once thought to be a passive and simple process, resolution is now understood to be an active and complex process that is orchestrated by various inflammatory mediators, signaling pathways and biophysical processes. The discovery of novel biosynthetic pathways that turn on the pro-resolution signals has lead to a surge in research aimed at taking a closer look at processes that can stimulate the resolution of inflammation. While major advances in the field have resulted in a better understanding of the proactive nature of resolution, many of the mechanisms involved are still unknown. To date, the repertoire of chemokine receptors that participate in macrophage clearance during resolution, for the most part, remain unidentified. Overall, there is a growing appreciation that the discovery of mechanisms involved in the resolution responses can lead to the development of novel therapeutic approaches to resolve many chronic inflammatory diseases. Syndecan-1 (Sdc-1), a member of a family of cell surface proteoglycans, has been previously shown to regulate events relevant to tissue repair and chronic injury responses. Macrophage Sdc-1 expression during inflammation has been reported to be protective in various inflammatory models. Given these observations, we hypothesize that Sdc-1 expression on macrophages is a critical component of an anti-inflammatory, pro resolution program necessary for the successful resolution of inflammatory response. In this dissertation, we report the presence of a unique population of macrophages expressing Sdc-1 that are present within the vascular wall of mice undergoing atherosclerosis. Consistent with previous publications, the presence of Sdc-1 expressing macrophages was found to limit atherosclerosis progression. In addition, Sdc-1 expression on macrophages was associated with anti-inflammatory M2 polarization state and high intrinsic motility. Macrophage Sdc-1 expression was also linked with efferocytosis and enhanced macrophage egress from the site of inflammation to the draining lymphatic network. Moreover, we discovered that the chemokine receptor CXCR4, which was found on Sdc-1 expressing macrophages, was also involved in macrophage egress during inflammation resolution. In summary, while the overall mechanism regulating resolution processes is still unknown, our work has managed to identify two components that are involved in the process: macrophage Sdc-1 and CXCR4. Collectively, these results reinforce the physiological significance of macrophage efferocytosis and macrophage motility as endogenous modulators of the inflammatory response.