Abstract 4120741: Estrogen Regulates Abdominal Aortic Aneurysm Tregs via CD45 ⁺ Fibroblast-derived IL-33

Background: Sexual dimorphisms manifest during the progression of abdominal aortic aneurysm (AAA). Regulatory T cells (Tregs) are a subset of T cells characterized by their immunosuppressive and tissue-repairing functions, therefore playing a protective role in AAA. Aims: We aim to elucidate the mechanism underlying sexual dimorphism in Tregs presented in AAA aortas, which is significant for unraveling the pathogenesis of AAA and developing gender-specific therapeutic approaches. Methods and Results: AAA mice were induced by receiving porcine pancreatic elastase. By flow cytometry, we observed that ovariectomy (OVX) increased the ratio of Tregs in female mice, while estrogen led to a decrease in aortic Tregs. Additionally, compared to littermate controls, estrogen receptor α (ERα) KO mice exhibited an increase in Treg ratio. Similar results were observed for the ST2 + Treg ratio. IL-33, as an alarming molecule, can promote the maintenance of ST2 + Tregs in tissues. Western blot revealed lower IL-33 expression levels in female mice than in male mice. By analyzing the IL-33 expression profiles of various cells in AAA tissue, we found that IL-33 mainly originates from fibroblasts. We found two subpopulations of fibroblasts, CD45 + and CD45 - fibroblasts. Immunofluorescence on the aortas verified the presence of cells co-expressing CD45 and PDGFRα in AAA fragments. Furthermore, IL-33 expression only showed gender differences in CD45 + fibroblasts. Adoptive transfer of CD45.1 + mouse bone marrow macrophages into CD45.2 + mice revealed that aortic CD45.1 + cells could regain the PDGFRα phenotype, with a lower proportion of female PDGFRα + cells in CD45.2 - CD45.1 + cells than in male mice. Lyz2(cre/+); Rosa26(tdtomato/+) mice showed the presence of lyz2-tdtomato + cells expressing PDGFRα in the aortic adventitia after AAA induction. This suggests that CD45 + fibroblasts are partially derived from bone marrow macrophages, which can migrate to aneurysms and acquire a PDGFRα phenotype. Conclusions: Female sex hormones negatively regulate CD45 + fibroblast formation and IL-33 secretion between female and male mice during AAA. This resulted in reduced Treg function in female mice compared to male mice.