Locoregional Immune Cell Infiltration in Aortic Aneurysm Formation

Aortic aneurysms, accounting for approximately 200 000 deaths annually worldwide, are defined as a pathological dilatation of the aorta, which can occur in either the thoracic or abdominal part of the aorta. The primary and most serious complication is a rupture of the aneurysm. Currently, treatment options are constrained to surgical or endovascular aortic repair, as no pharmacological therapy is available. Several studies have demonstrated that the perivascular adipose tissue (PVAT) surrounding the aorta plays a pivotal role in the development of aneurysms. Interestingly, thoracic PVAT differs in structure and function from abdominal PVAT, which may influence its role in disease processes. While inflammation is a well-described phenomenon in the context of abdominal aortic aneurysm (AAA) development, its importance in the context of thoracic aortic aneurysms (TAA) remains to be elucidated. Additionally, the mechanisms by which these inflammatory cells invade the aorta and the full extent to which these are affected by PVAT is yet to be revealed. 

The objective of the present study was to investigate early locoregional immune cell infiltration at the sites of developing abdominal and thoracic aortic aneurysm (AAA and TAA).

For induction of TAA and AAA, male, 8-12-week-old C57BL/6J wild-type mice were treated with angiotensin II (Ang-II, 1000 ng/kg/min) and β-Aminopropionitrile (BAPN, 0,1% in drinking water) or sodium chloride (NaCl) as a respective control for four or seven days. To identify infiltrating macrophages in the aorta and adjacent PVAT, histological analyses were performed using Mac-2 DAB staining. Furthermore, immune cell populations in PVAT were assessed by fluorescence activated cell sorting (FACS). 

Administration of Ang-II/BAPN resulted in aneurysm formation, with 30% of the treated mice developing TAA, 20% developing AAA, and 42% developing a combination of TAA and AAA. Histological analysis after seven days of Ang-II/BAPN treatment revealed a significant increase in the number of macrophages in the aortic wall, specifically in the adventitia and PVAT, and both the thoracic and abdominal aorta, compared to NaCl treated controls. Notably, even mice that did not show an established aneurysm within seven days of Ang-II/BAPN exhibited a significant increase in the presence of macrophages within the adventitia and PVAT in both the thoracic and abdominal aorta when compared to the control group. Given that the aneurysm had already been established and immune cells were already detected in the aortic wall within seven days of treatment, FACS analysis was conducted after four days of Ang-II/BAPN treatment to characterize an earlier phase of immune response. This analysis revealed a significant increase in myeloid cells, Ly6Chigh and Ly6Clow monocytes and neutrophils, in the PVAT compared to four days of NaCl treated controls. Of note, at this time point increased numbers of infiltrating macrophages were observed particularly in the region of AAA development and this increase was not observed in the respective thoracic region of the same mice which did not show any signs of aneurysm development pointing towards a locoregional inflammatory response.

These findings underscore the pivotal function of inflammation in the initiation and progression of both abdominal and thoracic aneurysms. Further, our results give first hints for a locoregional inflammatory response especially at the site of aneurysm development.