Olfactory receptor 2 signaling drives experimental abdominal aortic aneurysm formation

Abstract Background Abdominal aortic aneurysms (AAA) are defined as enlargement above 3cm or 50% of its original diameter. Therapeutic options are limited to surgical and endovascular treatment, while no pharmacological interventions are available. Chronic inflammation of the vessel wall and in particularly macrophage infiltration has been associated with disease progression, leading to extensive tissue remodeling, yet the underlying mechanisms are incompletely understood. A recent study has shown extra-nasal expression of the olfactory receptor 2 (OLFR2) on vascular macrophages, in which activation induced a pro-atherogenic response. The role of macrophage OLFR2 signaling in AAA, however, is unknown. Methods & Results To determine whether the OLFR2 ortholog OR6A2 is expressed in human AAA, we performed in silico micro-array analysis of human AAA tissue. OR6A2 was shown to be specifically upregulated in AAA tissue with thrombus formation compared to healthy control tissue. We confirmed OR6A2 expression on macrophages by immunofluorescence. To determine OLFR2 expression among vascular immune cells in mice, we induced AAA formation by porcine pancreas (PPE) infusion into the infrarenal aorta of C57BL6/J (WT) mice and performed spectral cytometry utilizing a 26-marker panel at baseline, day 7 and day 28. OLFR2 expression peaked at day 7 and decreased to baseline levels at day 28 particularly on pro-inflammatory monocytes, mixed resident / migratory-like macrophages and pro-inflammatory macrophages, while OLFR2 was not detectable on tissue resident-like macrophages. To assess a functional role of OLFR2 in AAA formation, we induced AAA in OLFR2-deficient (KO) mice and WT controls by PPE. Ultrasound analysis of the abdominal aorta revealed protection from AAA formation in KO mice. Histological analysis showed significantly reduced macrophage while increased smooth muscle cell content in aortic sections of KO mice compared to WT controls. This was accompanied by higher collagen content in KO mice. Additionally, plasma levels of the pro-inflammatory cytokines TNF-alpha and Interferon-gamma were significantly reduced in KO mice 28 days post PPE. Bulk transcriptome analysis corroborated these findings and indicated increased pathways associated with vascular smooth muscle cell contractility while decreased leukocyte activation in KO aortas. Conclusion We show that OLFR2 deficiency in mice attenuates abdominal aortic aneurysm development, aortic remodeling and pro-inflammatory response. OLFR2 might thus present a novel target for drug-based AAA therapies.