Interleukin-33 Signaling Controls the Development of Iron-Recycling Macrophages.

Splenic red pulp macrophages (RPMs) contribute to erythrocyte homeostasis and are required for iron recycling. Heme induces the expression of SPIC transcription factor in monocyte-derived macrophages and promotes their differentiation into RPM precursors, pre-RPMs. However, the requirements for differentiation into mature RPMs remain unknown. Here, we have demonstrated that interleukin (IL)-33 associated with erythrocytes and co-cooperated with heme to promote the generation of mature RPMs through activation of the MyD88 adaptor protein and ERK1/2 kinases downstream of the IL-33 receptor, IL1RL1. IL-33- and IL1RL1-deficient mice showed defective iron recycling and increased splenic iron deposition. Gene expression and chromatin accessibility studies revealed a role for GATA transcription factors downstream of IL-33 signaling during the development of pre-RPMs that retained full potential to differentiate into RPMs. Thus, IL-33 instructs the development of RPMs as a response to physiological erythrocyte damage with important implications to iron recycling and iron homeostasis.

NR4A1 Deletion in Marginal Zone B Cells Exacerbates Atherosclerosis in Mice-Brief Report.

OBJECTIVE: NR4A orphan receptors have been well studied in vascular and myeloid cells where they play important roles in the regulation of inflammation in atherosclerosis. NR4A1 (nerve growth factor IB) is among the most highly induced transcription factors in B cells following BCR (B-cell receptor) stimulation. Given that B cells substantially contribute to the development of atherosclerosis, we examined whether NR4A1 regulates B-cell function during atherogenesis. Approach and Results: We found that feeding Ldlr-/- mice a Western diet substantially increased Nr4a1 expression in marginal zone B (MZB) cells compared with follicular B cells. We then generated Ldlr-/- mice with complete B- or specific MZB-cell deletion of Nr4a1. Complete B-cell deletion of Nr4a1 led to increased atherosclerosis, which was accompanied by increased T follicular helper cell-germinal center axis response, as well as increased serum total cholesterol and triglycerides levels. Interestingly, specific MZB-cell deletion of Nr4a1 increased atherosclerosis in association with an increased T follicular helper-germinal center response but without any impact on serum cholesterol or triglyceride levels. Nr4a1-/- MZB cells showed decreased PDL1 (programmed death ligand-1) expression, which may have contributed to the enhanced T follicular helper response. CONCLUSIONS: Our findings reveal a previously unsuspected role for NR4A1 in the atheroprotective role of MZB cells.

Necrotic Cell Sensor Clec4e Promotes a Proatherogenic Macrophage Phenotype Through Activation of the Unfolded Protein Response.

BACKGROUND: Atherosclerotic lesion expansion is characterized by the development of a lipid-rich necrotic core known to be associated with the occurrence of complications. Abnormal lipid handling, inflammation, and alteration of cell survival or proliferation contribute to necrotic core formation, but the molecular mechanisms involved in this process are not properly understood. C-type lectin receptor 4e (Clec4e) recognizes the cord factor of Mycobacterium tuberculosis but also senses molecular patterns released by necrotic cells and drives inflammation. METHODS: We hypothesized that activation of Clec4e signaling by necrosis is causally involved in atherogenesis. We addressed the impact of Clec4e activation on macrophage functions in vitro and on the development of atherosclerosis using low-density lipoprotein receptor-deficient (Ldlr-/-) mice in vivo. RESULTS: We show that Clec4e is expressed within human and mouse atherosclerotic lesions and is activated by necrotic lesion extracts. Clec4e signaling in macrophages inhibits cholesterol efflux and induces a Syk-mediated endoplasmic reticulum stress response, leading to the induction of proinflammatory mediators and growth factors. Chop and Ire1a deficiencies significantly limit Clec4e-dependent effects, whereas Atf3 deficiency aggravates Clec4e-mediated inflammation and alteration of cholesterol efflux. Repopulation of Ldlr-/- mice with Clec4e-/- bone marrow reduces lipid accumulation, endoplasmic reticulum stress, and macrophage inflammation and proliferation within the developing arterial lesions and significantly limits atherosclerosis. CONCLUSIONS: Our results identify a nonredundant role for Clec4e in coordinating major biological pathways involved in atherosclerosis and suggest that it may play similar roles in other chronic inflammatory diseases.

Vascular smooth muscle cells in atherosclerosis.

Vascular smooth muscle cells (VSMCs) are a major cell type present at all stages of an atherosclerotic plaque. According to the 'response to injury' and 'vulnerable plaque' hypotheses, contractile VSMCs recruited from the media undergo phenotypic conversion to proliferative synthetic cells that generate extracellular matrix to form the fibrous cap and hence stabilize plaques. However, lineage-tracing studies have highlighted flaws in the interpretation of former studies, revealing that these studies had underestimated both the content and functions of VSMCs in plaques and have thus challenged our view on the role of VSMCs in atherosclerosis. VSMCs are more plastic than previously recognized and can adopt alternative phenotypes, including phenotypes resembling foam cells, macrophages, mesenchymal stem cells and osteochondrogenic cells, which could contribute both positively and negatively to disease progression. In this Review, we present the evidence for VSMC plasticity and summarize the roles of VSMCs and VSMC-derived cells in atherosclerotic plaque development and progression. Correct attribution and spatiotemporal resolution of clinically beneficial and detrimental processes will underpin the success of any therapeutic intervention aimed at VSMCs and their derivatives.

Deletion of chromosome 9p21 noncoding cardiovascular risk interval in mice alters Smad2 signaling and promotes vascular aneurysm.

BACKGROUND: Vascular aneurysm is an abnormal local dilatation of an artery that can lead to vessel rupture and sudden death. The only treatment involves surgical or endovascular repair or exclusion. There is currently no approved medical therapy for this condition. Recent data established a strong association between genetic variants in the 9p21 chromosomal region in humans and the presence of cardiovascular diseases, including aneurysms. However, the mechanisms linking this 9p21 DNA variant to cardiovascular risk are still unknown. METHODS AND RESULTS: Here, we show that deletion of the orthologous 70-kb noncoding interval on mouse chromosome 4 (chr4(Δ70kb/Δ70kb) mice) is associated with reduced aortic expression of cyclin-dependent kinase inhibitor genes p19Arf and p15Inkb. Vascular smooth muscle cells from chr4(Δ70kb/Δ70kb) mice show reduced transforming growth factor-ß-dependent canonical Smad2 signaling but increased cyclin-dependent kinase-dependent Smad2 phosphorylation at linker sites, a phenotype previously associated with tumor growth and consistent with the mechanistic link between reduced canonical transforming growth factor-ß signaling and susceptibility to vascular diseases. We also show that targeted deletion of the 9p21 risk interval promotes susceptibility to aneurysm development and rupture when mice are subjected to a validated model of aneurysm formation. The vascular disease of chr4(Δ70kb/Δ70kb) mice is prevented by treatment with a cyclin-dependent kinase inhibitor. CONCLUSIONS: The results establish a direct mechanistic link between 9p21 noncoding risk interval and susceptibility to aneurysm and may have important implications for the understanding and treatment of vascular diseases.