Abstract 109: Tumor Necrosis Factor Inducible Gene 6 Protein (TSG-6) is Highly Expressed in Human Abdominal Aortic Aneurysms

Abstract

Objective: The formation of an abdominal aortic aneurysm (AAA) is characterized by a dominance of pro-inflammatory forces that result in smooth muscle cell apoptosis, extra-cellular matrix degradation, and progressive diameter expansion. Additional defects in the anti-inflammatory response may also contribute to AAA progression, however have yet to be characterized robustly. Here, we describe the role of the anti-inflammatory cytokine TSG-6 (TNF-stimulated gene-6) in AAA formation. Methods: Blood and aortic tissue samples were collected from patients undergoing elective AAA screening and open surgical AAA repair. Aortic specimens collected were preserved for IHC or immediately assayed after tissue homogenization. Cytokine concentrations in tissue and plasma were assayed by ELISA. All immune cell populations were assayed using FACS analysis. In vitro, macrophage polarization from monocytes were performed with young, healthy donor PBMCs. Results: TSG-6 was found to be abnormally elevated in both the plasma and aorta of patients with AAA compared to healthy and risk-factor matched non-AAA donors. We observed the highest tissue concentration of TSG-6 in the less diseased proximal and distal shoulders compared to the central aspect of the aneurysm. IHC localized the majority of TSG-6 to the tunica media with minor expression in the tunica adventitia of the aortic wall. Higher concentrations of both M1 and M2 macrophages where also observed in the aortic wall, however M1/M2 ratios were unchanged from healthy controls. Additionally, we observed no difference in M1/M2 ratios in the peripheral blood of risk-factor matched non-AAA and AAA patients. Interesting, TSG-6 inhibited the polarization of the anti-inflammatory M2 phenotype in vitro . Conclusions: AAA formation results from an imbalance of inflammatory forces causing aortic wall infiltration of mononuclear cells leading to resultant vessel breakdown. From our results, we suggest TSG-6 is elevated in the AAA patient as a compensatory anti-inflammatory feedback mechanism. However, it’s effects may be abrogated by defects in CD44, its cognate receptor or downstream signaling pathways, future areas for investigation.