As in human disease, macrophages (MØ) are central players in the development and progression of experimental atherosclerosis. In this study we have evaluated the phenotype of MØ associated with progression of atherosclerosis in the apolipoprotein E (ApoE) knockout (KO) mouse model.We found that bone marrow-derived MØ submitted to M1 and M2 polarization specifically expressed arginase (Arg) II and Arg I, respectively. This distinct arginase expression was used to evaluate the frequency and distribution of M1 and M2 MØ in cross-sections of atherosclerotic plaques of ApoE KO mice. Early lesions were infiltrated by Arg I+ (M2) MØ. This type of MØ favored the proliferation of smooth muscle cells, in vitro. Arg II+ (M1) MØ appeared and prevailed in lesions of aged ApoE KO mice and lesion progression was correlated with the dominance of M1 over the M2 MØ phenotype. In order to address whether the M2->M1 switch could be due to a phenotypic switch of the infiltrated cells, we performed in vitro repolarization experiments. We found that fully polarized MØ retained their plasticity since they could revert their phenotype. The analysis of the distribution of Arg I- and Arg II-expressing MØ also argued against a recent recruitment of M1 MØ in the lesion. The combined data therefore suggest that the M2->M1 switch observed in vivo is due to a conversion of cells already present in the lesion. Our study suggests that interventional tools able to revert the MØ infiltrate towards the M2 phenotype may exert an atheroprotective action.

Background-The atheromodulating activity of B cells during the development of atherosclerosis is well documented, but the mechanisms by which these cells are regulated have not been investigated. Methods and Results-Here, we analyzed the contribution of Qa-1-restricted CD8 + regulatory T cells to the control of the T follicular helper-germinal center B-cell axis during atherogenesis. Genetic disruption of CD8 + regulatory T cell function in atherosclerosis-prone apolipoprotein E knockout mice resulted in overactivation of this axis in secondary lymphoid organs, led to the increased development of tertiary lymphoid organs in the aorta, and enhanced disease development. In contrast, restoring control of the T follicular helper-germinal center B-cell axis by blocking the ICOS-ICOSL pathway reduced the development of atherosclerosis and the formation of tertiary lymphoid organs. Moreover, analyses of human atherosclerotic aneurysmal arteries by flow cytometry, gene expression analysis, and immunofluorescence confirmed the presence of T follicular helper cells within tertiary lymphoid organs. Conclusions-This study is the first to demonstrate that the T follicular helper-germinal center B-cell axis is proatherogenicand that CD8 + regulatory T cells control the germinal center reaction in both secondary and tertiary lymphoid organs. Therefore, disrupting this axis represents an innovative therapeutic approach.