Irgm1 promotes M1 but not M2 macrophage polarization in atherosclerosis pathogenesis and development

Atherosclerosis is a chronic inflammatory vascular disease related to macrophages uptake of low-density lipoprotein and their subsequent transformation into foam cells. M1 (inflammatory)/M2 (anti-inflammatory) balance was suggested to impact disease progression. In this study, we investigated whether the immunity related GTPase (Irgm1) regulates macrophage polarization during atherosclerosis development.

We used apolipoprotein E (ApoE) knockout and Irgm1 haplodeficient mice and induced atherosclerosis with high-cholesterol diet for the indicated months. Atherosclerotic arteries were collected from patients undergoing vascular surgery, to determine the lesional expression of Irgm1 and distribution of M1/M2 populations.

Our results showed that IRGM/Irgm1 expression was increased in atherosclerotic artery samples (1.7-fold, p=0.0045) compared with non-atherosclerotic arteries, which was consistent with findings in the murine experimental atherosclerosis model (1.9-fold, p=0.0002). IRGM/Irgm1 expression was mostly found in lesional M1 macrophages. Haplodeficiency of Irgm1 in ApoE(-/-) mice resulted in reduced infiltrating M1 macrophages in atheroma (94%, p=0.0002) and delayed development of atherosclerotic plaques. In vitro experiments also confirmed that Irgm1 haplodeficiency reduced iNOS expression of polarized M1 macrophages (81%, p=0.0034), with negligible impact on the M2 phenotype. Moreover, we found that Irgm1 haplodeficiency in mice significantly reduced expression level of M1 function-related transcription factors, interferon regulatory factor (Irf) 5 and Irf8, but not Irf4, an M2-related transcription factor.

This study shows that Irgm1/IRGM participates in the polarization of M1 macrophage and promotes development of atheroma in murine experimental atherosclerosis.