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Background-Neutrophils may be an important source of matrix metalloproteinase-2 (MMP-2) and matrix metalloproteinase-9 (MMP-9), two matrix-degrading enzymes thought to be critical in the formation of an abdominal aortic aneurysm (AAA). The purpose of this investigation was to test the hypothesis that neutrophil depletion would limit experimental AAA formation by altering one or both of these enzymes. Methods and Results-Control, rabbit serum-treated (RS; nϭ27) or anti-neutrophil-antibody-treated (anti-PMN; nϭ25) C57BL/6 mice underwent aortic elastase perfusion to induce experimental aneurysms. Anti-PMN-treated mice became neutropenic (mean, 349 cells/L), experiencing an 84% decrease in the circulating absolute neutrophil count (PϽ0.001) before elastase perfusion. Fourteen days after elastase perfusion, control mice exhibited a mean aortic diameter (AD) increase of 104Ϯ14% (PϽ0.0001), and 67% developed AAAs, whereas anti-PMN-treated mice exhibited a mean AD increase of 42Ϯ33%, with 8% developing AAAs. The control group also had increased tissue neutrophils (20.3 versus 8.6 cells per 5 high-powered fields [HPFs]; Pϭ0.02) and macrophages (6.1 versus 2.1 cells per 5 HPFs, Pϭ0.005) as compared with anti-PMN-treated mice. There were no differences in monocyte chemotactic protein-1 or macrophage inflammatory protein-1␣ chemokine levels between groups by enzyme-linked immunosorbent assay. Neutrophil collagenase (MMP-8) expression was detected only in the 14-day control mice, with increased MMP-8 protein levels by Western blotting (Pϭ0.017), and MMP-8 -positive neutrophils were seen almost exclusively in this group. Conversely, there were no statistical differences in MMP-2 or MMP-9 mRNA expression, protein levels, enzyme activity, or immunostaining patterns between groups. When C57BL/6 wild-type (nϭ15) and MMP-8 -deficient mice (nϭ17) were subjected to elastase perfusion, however, ADs at 14 days were no different in size (134Ϯ7.9% versus 154Ϯ9.9%; Pϭ0.603), which suggests that MMP-8 serves only as a marker for the presence of neutrophils and is not critical for AAA formation. Conclusions-Circulating

Abdominal aortic aneurysms (AAAs) represent a complex degenerative disorder involving chronic aortic wall inflammation and destructive remodeling of structural connective tissue. Studies using human AAA tissues have helped identify a variety of molecular mediators and matrix-degrading proteinases, which contribute to aneurysm disease, thereby providing a sound foundation for understanding AAAs; however, these human tissue specimens represent only the "end stage" of a long and progressive disease process. Further progress in understanding the pathophysiology of AAAs is therefore dependent in part on the development and application of effective animal models that recapitulate key aspects of the disease. Based on original studies in rats, transient perfusion of the abdominal aorta with porcine pancreatic elastase has provided a reproducible and robust model of AAAs. More recent applications of this model to mice have also opened new avenues for investigation. In this review, we summarize investigations using the elastase-induced mouse model of AAAs including results in animals with targeted deletion of specific genes and more general differences in mice on different genetic backgrounds. These studies have helped us identify genes that are essential to the development of AAAs (such as MMP9, IL6, and AT1R) and to reveal other genes that may be dispensable in aneurysm formation. Investigations on mice from different genetic backgrounds are also beginning to offer a novel approach to evaluate the genetic basis for susceptibility to aneurysm development.

Background-Mast cell chymase may participate in the pathogenesis of human abdominal aortic aneurysm (AAA), yet a direct contribution of this serine protease to AAA formation remains unknown. Methods and Results-Human AAA lesions had high numbers of chymase-immunoreactive mast cells. Serum chymase level correlated with AAA growth rate (Pϭ0.009) in a prospective clinical study. In experimental AAA produced by aortic elastase perfusion in wild-type (WT) mice or those deficient in the chymase ortholog mouse mast cell protease-4