Background:

Abdominal aortic aneurysm (AAA) is a chronic inflammatory disease characterized by structural destruction 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 mechanisms leading to activation of AAA resident macrophages are 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 ectopic expression of various olfactory receptors. The OLFR2 was expressed 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 and Results:

To determine whether OLFR2 is also found on macrophages in AAA, we induced AAA by porcine pancreas elastase (PPE)-infusion and performed whole mount aorta immunofluorescence staining of wildtype (WT) mice at baseline, day 7 and day 28. OLFR2 expression was largely detected in macrophages (CD68+) at all time points. We further investigated whether the human OLFR2 orthologue OR6A2 was expressed in human aortic tissue. OR6A2 expression, determined by micro-array analysis, increased significantly in AAA against control tissue. Utilizing immunostainings of human AAA sections, we confirmed OR6A2 expression in macrophages. To assess the functional relevance of OLFR2 in AAA, we induced AAA by PPE-infusion in OLFR2-deficient (KO) mice and controls (WT). The aortic diameter was monitored via ultrasound analysis on baseline, day 7, 14, 21 and 28. While WT mice showed a large increase in aortic diameter (1,908 ± 0,32-fold compared to baseline) 28 days post AAA-induction, KO mice were protected from aneurysm development (1,276 ± 0,132-fold compared to baseline). Histologically, aortae from WT mice showed increased elastin degradation in comparison to KO mice, indicating substantial disease progression. Interestingly, the collagen content, both measured by Masson-Trichrome staining and second harmonics generation in two-photon-imaging, did not differ between both groups. Immunofluorescence staining for CD68 and alpha-smooth muscle actin revealed significantly higher smooth muscle content with markedly reduced macrophage infiltration in KO mice. Additionally, abundance of AAA reactive oxygen species (ROS) assayed by staining for dihydroethidium was significantly reduced in AAA KO sections.

Conclusion:

Here, we show that OLFR2-deficient mice are protected from AAA development, aortic remodeling and show signs of a reduced inflammatory response in the aorta.