Even if the angiotensin II perfusion model reproduces the ma

Even if the angiotensin II perfusion model reproduces the main features of human aneurysm including ECM degradation, leukocyte extravasation, and concomitant presence of atherosclerosis, it is associated with medial dissection and intramural hematoma. In addition, the AAA usually develops in the suprarenal or the ascending aorta, whereas the most frequent localization in humans involves the infrarenal aortic region. The topical application of elastase associated with the systemic neutralization of TGF-β has the advantage of inducing AAA in the infrarenal region of the abdominal Protionamide synthesis and is associated with a high percentage of AAA rupture within 14 days. This model produces features of human-like AAA, including ECM degradation, immune cell accumulation in the aorta, loss of VSMCs, intraluminal thrombus formation, and adventitial neoangiogenesis. In this model, AAA development was precisely characterized by serial in vivo ultrasound imaging and synchrotron-based ultrahigh-resolution ex vivo imaging at different time points (day 0, 7, and 14 for ultrasound and day 0, 3, 5, 7, 10, and 14 for synchrotron imaging). Ultrasound analysis revealed that at day 7, the aortic dilation was increased in both the elastase and elastase + anti-TGF-β groups compared with controls. Between day 7 and day 14, the aortic dilation significantly progressed in the elastase + anti-TGF-β group but not in the elastase alone group. Hence, differences between these two groups were more marked between day 7 and day 14. In addition, synchrotron-based ultrahigh-resolution ex vivo imaging in the elastase + anti-TGF-β group revealed more marked changes in aortic perimeters and adventitial thickness after day 7. Based on these findings, we hypothesized that changes of macrophage phenotype in the aortic tissue would be preferentially observed from day 7 to day 14. Note, however, that whereas the model offers the opportunity to study aortic rupture, the survival rate of mice receiving elastase + anti-TGF-β is estimated at between 50% and 80% at day 14. Analysis performed at day 14 may thus potentially lead to select animals that better resist rupture, underlining the real interest in performing kinetic experiments and analysis at different time points in this model. Our results showed that in elastase-induced AAA, TGF-β neutralization finely tunes macrophage phenotype. Note that several studies have already addressed the role of M1 markers, such as IL-6 and IL-1β, or M2 markers, including IL-10 and TGF-β.4, 9, 10, 11 The expression and the role of ARG1 in AAA have been so far poorly investigated, and this is the reason that we focused on this M2 marker. We investigated ARG1 protein expression in human infrarenal AAA tissue and showed the presence of ARG1-positive cells located mainly in the adventitia, whereas no staining was observed in healthy aortic tissue. ARG1 is an enzyme that converts arginine to produce urea and ornithine. Ornithine can serve as a precursor for the production of proline, an amino acid that can be used for the synthesis of proline-rich proteins, such as collagen. Collagen represents a main component of the ECM, which is vastly degraded during AAA development. Several studies have demonstrated that arginine supplementation increases collagen deposition in rodent and human and favors wound healing.23, 24 In addition, ARG1 promotes VSMC proliferation, and in vitro studies revealed that ARG1 overexpression in smooth muscle cells suppressed lipopolysaccharide-induced tumor necrosis factor α release and inhibited monocyte chemotaxis and migration.25, 26 Hence, ARG1 could potentially play a role in pathways relevant for AAA disease, such as ECM remodeling, VSMC homeostasis, and regulation of inflammatory response. Whereas ARG1 expression increased with AAA progression, further experimental approaches using ARG1-deficient mice are required to determine whether the increase of ARG1 has a pathogenic or a protective compensatory role in AAA development. Last, AAA is frequently associated with atherosclerosis, and several studies have addressed the role of ARG1 in the disease. A first study revealed a higher expression of ARG1 in macrophages in rabbits with genetically determined low atherosclerotic response compared with those with high atherosclerotic response, suggesting its role in atherosclerosis resistance. Besides macrophages, ARG1 can also be detected in endothelial cells and VSMCs. Intriguingly, a study revealed that even though ARG1-specific deletion in hematopoietic cells increased foam cell formation, no significant effect was identified on atherosclerosis, including plaque size and stability as well as plaque macrophage content in the aortic root. To determine whether the global increase of ARG1 or the increase of the macrophage subtype that expresses ARG1 plays a role in AAA, it would be useful to use mice that specifically lack ARG1 in macrophages. Taken together, these results highlight a complex role of ARG1 that may differ according to cell subsets. Further studies using genetic models of ARG1 deletion would be of interest to better understand its role during AAA formation and progression.