Targeting inflammation in atherosclerosis - from experimental insights to the clinic

Abstract

Atherosclerosis, a dominant and growing cause of death and disability worldwide, involves inflammation from its inception to the emergence of complications. Targeting inflammatory pathways could therefore provide a promising new avenue to prevent and treat atherosclerosis. Indeed, clinical studies have now demonstrated unequivocally that modulation of inflammation can forestall the clinical complications of atherosclerosis. This progress pinpoints the need for preclinical investigations to refine strategies for combatting inflammation in the human disease. In this Review, we consider a gamut of attractive possibilities for modifying inflammation in atherosclerosis, including targeting pivotal inflammatory pathways such as the inflammasomes, inhibiting cytokines, manipulating adaptive immunity and promoting pro-resolution mechanisms. Along with lifestyle measures, pharmacological interventions to mute inflammation could complement traditional targets, such as lipids and hypertension, to make new inroads into the management of atherosclerotic risk.

Fig. 1. Integration of inflammatory processes during atherosclerosis development.

a | Overview of inflammatory processes. At the early stages of atherosclerosis, activated platelets secrete chemokines (such as C-C motif chemokine 5 (CCL5)) that promote adhesion of monocytes and neutrophils. Neutrophils themselves secrete chemotactic granule proteins (including cathelicidin, cathepsin G and CCL2), thus paving the way for arterial monocyte infiltration. The chemokine milieu is supplemented by chemokines secreted by activated smooth muscle cells (SMCs), such as CCL2 and CCL5. In progressing atherosclerotic lesions, medial SMCs migrate towards the developing fibrous cap where they undergo clonal expansion. SMC lipid loading triggers phenotype switching towards SMCs that express α-smooth muscle actin (αSMA⁺ SMCs), macrophage-like SMCs and smooth muscle foam cells. Heightened lipid loading of SMCs induces SMC apoptosis and — if not cleared quickly — necrosis. SMCs also undergo cell death after interaction with histone H4 presented in neutrophil extracellular traps (NETs). NET-associated cytotoxicity is observed during plaque erosion when NETs released at sites of disturbed flow induce endothelial cell desquamation. In systemic infections with Gram-negative organisms, which produce lipopolysaccharide (LPS), NET-associated histones promote the adhesion of monocytes, hence contributing to accelerated plaque growth under these conditions. Monocyte-derived macrophages ingest modified lipids and, in response, secrete inflammatory chemokines and cytokines. Excessive lipid uptake triggers macrophage proliferation or even cell death. b–d | Core inflammatory processes fuelled by SMCs (part b), macrophages (part c) and neutrophils (part d). b | Cholesterol uptake induces cell death in SMCs. SMC death, in turn, reduces the amount of extracellular matrix that is produced, which further fuels SMC death. Senescent SMCs release pro-inflammatory cytokines and matrix-degrading enzymes, including matrix metalloproteinases (MMPs). c | Priming and activation of the NACHT, LRR and PYD domains-containing protein 3 (NLRP3) inflammasome. Priming by cholesterol crystals, modified lipids such as oxidized low-density lipoproteins (oxLDL) or impaired cholesterol efflux triggers the nuclear factor-κB (NF-κB) signalling pathway, promoting the transcription of NLRP3 and pro-IL-1β. Assembly of the NLRP3 inflammasome induces activation of caspase 1, which cleaves pro-IL-1β into mature IL-1β. d | Release of NETs is licensed by cholesterol crystals, LPS, modified lipids and chemokines such as CCL7. NETs exert cytotoxicity by means of NET-resident histones, prime the NLRP3 inflammasome in macrophages and induce coagulation by cleavage of factor XII and tissue factor pathway inhibitor (TFPI), and by direct platelet activation.

Fig. 2. Preclinical strategies to limit cardiovascular inflammation and stimulate its resolution.

a | Reducing monocyte recruitment. Silencing of C-C chemokine receptor 2 (CCR2) or timed inhibition of CCR2 signalling reduces monocyte adhesion. Overriding chemokine receptor signalling, for example with the small molecule Ac2-26, reduces integrin activation and monocyte arrest. Heterodimers of C-C motif chemokine 5 (CCL5) and C-X-C motif chemokine 4 (CXCL4) as well as of CCL5 and neutrophil defensin 1 (DEFA1) promote monocyte adhesion. Small peptides that disrupt these interactions reduce monocyte adhesion during cardiovascular inflammation. b | Inhibiting neutrophil extracellular traps (NETs). Inhibition of protein-arginine deiminase type 4 (PAD4) halts NET release. DNase I cleaves DNA strands in NETs. Neutralization of gasdermin D prevents NET discharge. Neutralization of NET-resident proteins by antibodies or small-molecule inhibitors reduces NET-driven inflammation. c | Examples of strategies to inhibit immune checkpoints. Neutralization of CD80/86 can reduce T cell and dendritic cell responses. The CD40–CD40 ligand (CD40L) interaction activates macrophages via intracellular TNF receptor-associated factor 6 (TRAF6) signalling, a cascade that can be inhibited with small molecules. d | Increasing inflammation resolution. Putrescine improves the ability of macrophages to engulf dead cells. Resolving N-formyl peptide receptor 2 (FPR2) agonists (Ac2-26, lipoxin A4, resolvin D1) and chemokine-like receptor 1 (CMKLR1) agonists (chemerin C15, resolvin E1) lower the production of inflammatory cytokines and improve the efferocytosis capacity. NF-κB, nuclear factor-κB.

Fig. 3. Improved cardiovascular therapy in time and space.

a | Organ-specific oscillation of genes regulating expression of drug targets that are relevant to cardiovascular inflammation in humans. Group of selected oscillating gene targets in human tissues reported in CircaDB. Logarithmic peak of expression level from RNA sequencing data is shown in different colours over 24-hour clocks. The time of gene expression peaks vary between tissues; the night is presented as grey shadow. b | Examples of nanodelivery methods that can direct functionality to lesional cells. Vascular endothelial growth factor C (VEGF-C) conjugated to an antibody recognizing the extra domain A of fibronectin is presented to smooth muscle cells (SMCs) in the fibrous cap (example 1). These cells respond by improving cholesterol efflux and lowering cell death. Ac2-26 incorporated in collagen IV-targeting nanoparticles reprogrammes macrophages towards a resolving phenotype with improved ability to clear dead cells, lower release of matrix-degrading proteases and increase the secretion of anti-inflammatory cytokines (example 2). Nanoparticles that mimic high-density lipoprotein (HDL), loaded with small-molecule inhibitors of TNF receptor-associated factor 6 (TRAF6), lower inflammatory macrophage responses in atherosclerotic lesions (example 3). Single-walled carbon nanotubes loaded with a chemical inhibitor of the antiphagocytic CD47–SIRPα axis target lesional macrophages and improve their ability to clear dead cells (example 4). CCL2, C-C motif chemokine 2; FPR2, N-formyl peptide receptor 2; ICAM1, intercellular adhesion molecule 1; IL-1R1, IL-1 receptor type 1; IRAK1, IL-1 receptor-associated kinase 1; LDLR, low-density lipoprotein receptor; MALT1, mucosa-associated lymphoid tissue lymphoma translocation protein 1; MMPs, matrix metalloproteinases; NF-κB, nuclear factor-κB; PDE4D, cAMP-specific 3′,5′-cyclic phosphodiesterase 4D; PTGS2, prostaglandin G/H synthase 2.