CCR2 is the only known functional receptor for MCP-1 in macrophage migration [18
]. Deficiency of this receptor on bone-marrow-derived cells reduces atherosclerosis in hypercholesterolaemic mice [19
]. One study has also demonstrated that CCR2 deficiency in chimaeric mice developed by bone marrow transplantation attenuated AngII-induced atherosclerosis and AAAs [5
]. However, bone marrow transplantation can lead to differences in the development of atherosclerosis and AAAs [25
]. No previous studies have demonstrated the effects of whole body CCR2 deficiency on the development of AngII-induced vascular diseases. In the present study, it was demonstrated that whole-body CCR2 deficiency attenuated both AngII-induced atherosclerosis and AAAs.
Ascending aortic dilatation, with subsequent rupture, is the most life-threatening manifestation of Marfan's disease [32
]. Previous studies in mice expressing a fibrillin-1 mutant have demonstrated a similar pathology to that generated during AngII infusion in the present study [12
]. These similarities include dilatation that is localized to the ascending aorta, medial thickening and elastin fragmentation. Thus, the results of the present study are complementary to the previously described effect of losartan in fibrillin-1 mutant mice being due to antagonism of the effects of AngII.
Many publications have demonstrated that AngII infusion leads to aneurysmal formation in the abdominal aorta [1
]. In the present study, we demonstrated that AngII infusions led to large lumen dilatations of the ascending aorta, as has been described previously for the abdominal aorta [37
]. However, there are marked differences in the pathological characteristics of aneurysmal formation in these two regions. AAAs form rapidly as a consequence of a highly localized transmural elastin disruption that colocalizes with focal medial macrophage accumulation [9
]. Adventitial thrombi form adjacent to medial ruptures that promote an intense inflammatory response with the recruitment of macrophages. During subsequent remodelling, aneurysmal tissue contains abundant macrophages, with the accumulation of both T- and B-lymphocytes [9
]. In contrast, ascending aortas exhibit extensive elastin fragmentation following infusion of AngII, but not transmural as seen in AAAs. In addition, the distance between the elastin layers progressively increases towards the adventitial side of ascending aortas. Regions of greatest elastin disruption and intra-laminar expansion were associated with macrophage accumulation. Furthermore, unlike AAAs in which macrophages have been detected at focal sites in the aortic media at an early phase of the disease, AngII promoted macrophage accumulation throughout the medial layers of ascending aortas. At the anterior aspect of ascending aortas, a different pathology was observed in which there was a focal thinning compared with the surrounding hypertrophic areas. Furthermore, we did not detect thrombotic material at this anterior region. Thus, the highly contrasting pathologies between the aneurysms in abdominal aortas compared with ascending aortas are indicative of different mechanisms by which AngII generates these diseases.
AngII-induced pathology in ascending aortas extended proximal to the subclavian artery, while the descending portion of the thoracic aorta was spared. This localization strongly resembles the distribution of dilatation observed in mice expressing a mutation of fibrillin-1 [12
]. The basis for specificity of the dilatation is presumably due to regional differences that exist throughout the aorta. The aorta has considerable functional diversity of smooth muscle cells that may be based on developmental origin [31
]. Using mice expressing Cre under the control of Wnt1 in combination with a floxed Rosa26 reporter, it has been demonstrated that the region from the ascending aorta to just distal of the subclavian artery are populated by smooth muscle cells of neural crest origin [38
]. This distribution is strikingly similar to the region of ascending aortic aneurysms promoted by AngII that are shown in . It remains to be determined what property of these cells enable AngII to promote this localized pathology.
Previous studies using fibrillin-1 mutant transgenic mice demonstrated that while losartan and propranolol administration resulted in comparable haemodynamic effects, only losartan administration reduced ascending aortic aneurysms [12
]. Thus, blood pressure did not appear to contribute to aortic aneurysms in this mouse model of Marfan's syndrome. In the present study, the infusion of 1000 ng·kg−1
of body weight·min−1
of AngII was associated with an increase in systolic blood pressure. However, the AngII-induced increases in systolic blood pressure were not influenced by whole-body CCR2 deficiency, despite the attenuated size of the ascending aortic aneurysms. Thus, in a similar manner to previously described for AngII-induced atherosclerosis [2
] and AAAs [39
], the increase in blood pressure from AngII infusion is not responsible for the development of ascending aortic aneurysms.
Ascending aortas from AngII-infused mice had considerable accumulation of macrophages throughout the media that have a preponderance on the adventitial side of the vessel. Infusion of AngII into C57BL/6 mice has previously been shown to promote macrophage accumulation predominantly in the adventitia [17
]. However, that study was performed in the descending thoracic aorta, which did not exhibit dilatation. Thus, CCR2 may be the major stimulus for the recruitment of macrophages to the aortic adventitia. Therefore, the combined results of Bush et al. [17
] and the present study demonstrate that another mediator contributes to migration of macrophages from the adventitia into the media. The identification of this putative mediator is unknown.
It is of interest that AngII infusion generates three distinct vascular pathologies within the aorta that have specific locations and pathological characteristics. A unified finding in the AngII-induced vascular diseases was macrophage accumulation. However, even within this unified finding, there are many interesting differences. For AngII-induced atherosclerosis, macrophages are only recruited to the intima and are rarely present in the media. In AngII-induced AAAs, there are initial small focal regions of macrophage accumulation in the media. In contrast, we describe macrophage accumulation in AngII-induced ascending aortic aneurysms occurring throughout the circumference of the artery and predominantly on the adventitial side of the aorta. It will be a fascinating challenge to determine why CCR2 deficiency attenuates macrophage accumulation in these aortic regions.
In summary, the present study is the first description of AngII infusion leading to a highly localized pathology of dilatation and medial remodelling and destruction that is localized to the ascending aortic region. A role of AngII in ascending aortic aneurysms of Marfan's patients is consistent with the benefits of ACE (angiotensin-converting enzyme) inhibition on aortic dilatation in these patients [40
]. Further evidence will be derived from a currently ongoing clinical trial that is assessing the effects of losartan in Marfan's patients [42
]. The pathology of AngII-induced ascending aortic aneurysms is distinct from that described previously for AAAs and will require further study to fully elucidate the basis for the disparate pathological characteristics of aneurysms that are localized to ascending compared with abdominal aortic regions.