Deficiency of ITGAM Attenuates Experimental Abdominal Aortic Aneurysm in Mice

Background Integrin αM (CD11b), which is encoded by the Integrin Subunit Alpha M (ITGAM) gene, is not only a surface marker of monocytes but also an essential adhesion molecule. In this study, we investigated the effect of CD11b on experimental abdominal aortic aneurysm and the potential underlying mechanisms. Methods and Results The incidence of abdominal aortic aneurysm was not significantly lower in ITGAM(-/-) mice than in control mice. Nevertheless, knockout of CD11b reduced the maximum abdominal aortic diameter, macrophage infiltration, matrix metalloproteinase-9 expression, and elastin and collagen degradation. Additionally, lower expression of IL-6 was found in both the peripheral blood and abdominal aortas of ITGAM(-/-) mice, indicating a biological correlation between CD11b and the inflammatory response in abdominal aortic aneurysm. In vitro, the number of ITGAM(-/-) bone marrow-derived macrophages (BMDMs) that adhered to endothelial cells was significantly lower than the number of wild-type BMDMs. Moreover, the CD11b monoclonal antibody and CD11b agonist leukadherin-1 decreased and increased the number of adherent wild-type BMDMs, respectively. Through RNA sequencing, genes associated with leukocyte transendothelial migration were found to be downregulated in ITGAM(-/-) BMDMs. Furthermore, immunoprecipitation-mass spectrometry analysis predicted that the Akt pathway might be responsible for the impaired transmigratory ability of ITGAM(-/-) BMDMs. The reduced activation of Akt was then confirmed, and the Akt agonist SC79 partially rescued the transendothelial migratory function of ITGAM(-/-) BMDMs. Conclusions CD11b might promote the development and progression of abdominal aortic aneurysm by mediating the endothelial cells adhesion and transendothelial migration of circulating monocytes/macrophages.