Background: 

Abdominal aortic aneurysm (AAA) is a chronic inflammatory disease characterized by loss of the extracellular matrix (ECM) driven by elastin degradation, smooth muscle cell death, fibrosis, and inflammation. Particularly macrophage infiltration has been linked to the progression of AAA. Yet, the mechanism leading to activation of AAA resident macrophages is not well understood. The olfactory receptor 2 (OLFR2) is a G-protein coupled receptor recognizing odorants, that was first discovered in the nasal cavity and is involved in mediating the sense of smelling.

Recent data has shown extra-nasal expression on vascular macrophages, in which it mediated a proinflammatory response that aggravated atherosclerosis. The role of OLFR2 in AAA-associated macrophage responses, however, is unknown.

Methods & Results:
To investigate the dynamics of leukocytes and OLFR2 expression in AAA, we induced AAA by porcine pancreas elastase (PPE) infusion and performed spectral flow cytometry with a 26-marker panel at baseline, day 7 and day 28. Clustering analysis revealed a total of 21 leucocyte clusters. OLFR2 expression peaked at day 7 and was reduced to baseline levels at day 28 particularly in pro-inflammatory monocytes, mixed resident / migratory-like macrophages and pro-inflammatory macrophages while no expression was observed on tissue resident-like macrophages. We further investigated whether the OLFR2 orthologue OR6A2 was expressed in human aortic tissue. OR6A2 expression determined by micro-array analysis increased significantly in the AAA with an underlying thrombus in comparison to control tissue and thrombus-free AAA. We further confirmed OR6A2 expression in macrophages by immunofluorescence stainings of AAA sections.

To assess the function of OLFR2 in AAA, we induced AAA by PPE infusion in OLFR2-deficient (KO) mice and controls (WT). As expected, WT mice showed a large increase in aortic diameter 28 days post PPE-infusion while KO mice were protected from aneurysm development. Histologically, aortae from WT mice showed increased elastin degradation in comparison to KO mice, indicating substantial disease progression. Additionally, two-photon-imaging demonstrated higher collagen content in the KO group compared to WT mice. Immunofluorescence staining for CD68 and alpha-smooth muscle actin revealed significantly higher smooth muscle content with markedly reduced macrophage infiltration in KO mice. Furthermore, reactive oxygen species (ROS) and matrix metalloproteinase 2 and 9 activity assayed by staining for dihydroethidium and in-situ zymography, respectively, were markedly reduced in AAA KO sections.

We next used bulk RNA sequencing to determine transcriptional changes between KO aortae and WT controls at day 7 after aneurysm induction. Gene set enrichment analysis revealed upregulated vascular smooth muscle cell contractility while leucocyte activation was downregulated in KO mice. 

Conclusion:

We show that OLFR2-deficient mice are protected from AAA development, aortic remodeling and have a reduced inflammatory response in the aorta.