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BACKGROUND: Myocardial infarction (MI) triggers myelopoiesis resulting in heightened production of neutrophils. However, the mechanisms that sustain their production and recruitment to the injured heart are unclear. METHODS: Using a mouse model of the permanent ligation of the left anterior descending (LAD) artery and flow cytometry, we first characterized the temporal and spatial effects of MI on different myeloid cell types. We next performed global transcriptome analysis of different cardiac cell types within the infarct to identify the drivers of acute inflammatory response and the underlying signaling pathways. Utilizing a combination of genetic and pharmacological strategies, we identified the sequalae of events that led to MI-induced myelopoiesis. Cardiac function was assessed by echocardiography. The association of early indices of neutrophilia with major adverse cardiovascular events (MACE) was studied in a cohort of acute MI patients. RESULTS: Induction of MI resulted in a rapid recruitment of neutrophils to the infarct, where they release specific alarmins, S100A8 and S100A9. These alarmins bind to the Toll Like Receptor (TLR) 4 and prime the Nod Like Receptor (NLR) family Pyrin Domain-Containing 3 (Nlrp3) inflammasome in naïve neutrophils and promote interleukin 1 (IL-1β) secretion. The released IL-1β interact with its receptor (Interleukin 1 Receptor Type 1, IL1R1) on hematopoietic stem and progenitor cells in the bone marrow (BM), and stimulate granulopoiesis in a cell-autonomous manner. Genetic or pharmacological strategies aimed at disruption of S100A8/A9 and its downstream signaling cascade suppress MI-induced granulopoiesis and improve cardiac function. Furthermore, in patients with acute coronary syndrome (ACS), higher neutrophil count on admission and post-revascularization correlates positively with major adverse cardiovascular disease (CVD) outcomes. CONCLUSIONS: Our study provides novel evidence for the primary role of neutrophil-derived alarmins (S100A8/A9) in dictating the nature of the ensuing inflammatory response following myocardial injury. Therapeutic strategies aimed at disruption of S100A8/A9 signaling or its downstream mediators (e.g. Nlrp3, IL-1β) in neutrophils suppress granulopoiesis and may improve cardiac function in ACS patients.

Monocyte-derived macrophages are critical in the development of atherosclerosis and can adopt a wide range of functional phenotypes depending on their surrounding milieu. High-density lipoproteins (HDLs) have many cardio-protective properties including potent anti-inflammatory effects. We investigated the effects of HDL on human macrophage phenotype and the mechanisms by which these occur.Human blood monocytes were differentiated into macrophages in the presence or absence of HDL and were then induced to either an inflammatory macrophage (M1) or anti-inflammatory macrophage (M2) phenotype using LPS and IFN-γ or IL-4, respectively.HDL inhibited the induction of macrophages to an M1-phenotype, as evidenced by a decrease in the expression of M1-specific cell surface markers CD192 and CD64, as well as M1-associated inflammatory genes TNF-α, IL-6 and MCP-1 (CCL2). HDL also inhibited M1 function by reducing the production of ROS. In contrast, HDL had no effect on macrophage induction to the M2-phenotype. Similarly, methyl-β-cyclodextrin, a non-specific cholesterol acceptor also suppressed the induction of M1 suggesting that cholesterol efflux is important in this process. Furthermore, HDL decreased membrane caveolin-1 in M1 macrophages. We confirmed that caveolin-1 is required for HDL to inhibit M1 induction as bone marrow-derived macrophages from caveolin-1 knockout mice continued to polarize into M1-phenotype despite the presence of HDL. Moreover, HDL decreased ERK1/2 and STAT3 phosphorylation in M1 macrophages.We concluded that HDL reduces the induction of macrophages to the inflammatory M1-phenotype via redistribution of caveolin-1, preventing the activation of ERK1/2 and STAT3.

Background and PurposeAngiotensin AT(2) receptors are upregulated in disease states such as atherosclerosis and blockade of the AT(2) receptors exacerbates plaque formation. Direct stimulation of these receptors is anti-atherogenic but the mechanisms and pathways involved remain unknown. We examined the effect of direct AT(2) receptor stimulation with Compound 21 (C21) on the leukocyte adhesion cascade in vitro, right through to plaque formation in vivo. Experimental ApproachEffects of C21 on TNF-induced inflammation were assessed in human umbilical vein endothelial cells (HUVECs), activation of monocytes, polarisation of monocyte-derived macrophages and in intact mouse aortae. Key ResultsC21 attenuated TNF-induced: monocyte adhesion to cultured HUVECs, adhesion molecule expression and abolished TNF-induced ROS production. TNF-induced NFB translocation from the cytoplasm to the nucleus, essential for cytokine production, was prevented by C21. C21 did not influence monocyte activation or macrophage polarisation but did reduce TNF and IL-6 mRNA expression in M1 macrophages. The anti-inflammatory effects of C21 were abolished by an AT(2) receptor antagonist confirming that the effects of C21 were AT(2) receptor-mediated. Also, leukocyte adhesion and cytokine gene expression, induced by high-fat diet (HFD), was attenuated in ApoE(-/-) mice treated with C21. Plaque size and stability were improved with C21 treatment with increased smooth muscle cell composition and decreased lipid size, compared with HFD-saline treated mice. Conclusion and ImplicationsC21 prevented TNF-induced and HFD-induced vascular inflammation in vitro and in vivo. Our data provide strong evidence that the anti-atherosclerotic actions of C21 were due to vascular anti-inflammatory effects, mediated by AT(2) receptors. Linked ArticlesThis article is part of a themed section on Inflammation: maladies, models, mechanisms and molecules. To view the other articles in this section visit