The Biological Basis for Cardiac Repair After Myocardial Infarction: From Inflammation to Fibrosis

Abstract

In adult mammals, massive sudden loss of cardiomyocytes after infarction overwhelms the limited regenerative capacity of the myocardium, resulting in the formation of a collagen-based scar. Necrotic cells release danger signals, activating innate immune pathways and triggering an intense inflammatory response. Stimulation of toll-like receptor signaling and complement activation induces expression of proinflammatory cytokines (such as interleukin-1 and tumor necrosis factor-α) and chemokines (such as monocyte chemoattractant protein-1/ chemokine (C-C motif) ligand 2 [CCL2]). Inflammatory signals promote adhesive interactions between leukocytes and endothelial cells, leading to extravasation of neutrophils and monocytes. As infiltrating leukocytes clear the infarct from dead cells, mediators repressing inflammation are released, and anti-inflammatory mononuclear cell subsets predominate. Suppression of the inflammatory response is associated with activation of reparative cells. Fibroblasts proliferate, undergo myofibroblast transdifferentiation, and deposit large amounts of extracellular matrix proteins maintaining the structural integrity of the infarcted ventricle. The renin-angiotensin-aldosterone system and members of the transforming growth factor-β family play an important role in activation of infarct myofibroblasts. Maturation of the scar follows, as a network of cross-linked collagenous matrix is formed and granulation tissue cells become apoptotic. This review discusses the cellular effectors and molecular signals regulating the inflammatory and reparative response after myocardial infarction. Dysregulation of immune pathways, impaired suppression of postinfarction inflammation, perturbed spatial containment of the inflammatory response, and overactive fibrosis may cause adverse remodeling in patients with infarction contributing to the pathogenesis of heart failure. Therapeutic modulation of the inflammatory and reparative response may hold promise for the prevention of postinfarction heart failure.

Keywords: chemokines; cytokines; fibrosis; immune cells; inflammation; myocardial infarction; myocytes, cardiac.

© 2016 American Heart Association, Inc.

Figures

Figure 1

Biphasic nature of cardiac repair after myocardial infarction (MI). Early after MI, tissue injury and necrosis initiates the inflammatory phase, consisting of intense sterile inflammation, and the dynamic recruitment of several immune cell subtypes including neutrophils, monocyte/macrophages, dendritic cells, and lymphocytes. After ~4 days in murine models, this transitions to a reparative and proliferative phase, with shift of immune cell polarity toward immunomodulation and resolution, myofibroblast proliferation, collagen deposition and scar formation, and neovascularization, thereby resulting in wound healing. Neurohormonal activation and mechanical stress are other factors that influence this healing process. ROS, reactive oxygen species.

Figure 2

TLR and NLR signaling. DAMPs released from necrotic and damaged cells and extracellular matrix bind to the TLRs, which are comprised of an extracellular leucine rich repeat (LRR) domain, a transmembrane (TM) domain, and a conserved cytoplasmic Toll/IL-1R (TIR) domain, which serves as docking site for other TIR-containing cytoplasmic adaptor proteins (top left box). The binding of DAMPs, and often co-receptors such as CD14 and MD2, to TLR4 engages the adaptor MyD88 via the adaptor TIRAP. MyD88 recruits IRAK4 and the IRAK1/2 and TRAF6 complex, which activates TAK1. TAK1, a MAPKKK, then activates the MAPK cascade and also phosphorylates IκB kinase (IKK), that leads to NF-κB p65 and p50 nuclear translocation and the transcription of a panel of inflammatory genes, including cytokines, chemokines, cell adhesion molecules, and complement factor B. Further, after endocytosis, TLR4 signals in a MyD88-independent manner via the cytoplasmic adaptor TRIF, in turn recruited through the bridging adaptor TRAM. This pathway results in non-canonical IKK and NF-κB activation, as well as the induction of type I IFN via TBK1 and the transcription factor IRF3. Except for TLR3, the other TLRs also signal, directly or indirectly as depicted, via MyD88. Endolysosomal TLR3 activates IRF3 signaling via TRIF and TBK1, whereas endolysosomal TLR7/8 and TLR9 also induce type I IFN via TRAF3 and IRF7 in a MyD88-dependent manner (not pictured). In addition to TLRs, intracellular NLRs respond to a variety of PAMPs via the formation of multiprotein inflammasomes, comprised of an activated NLR protein, the adaptor protein ASC, and procaspase-1. One proposed model of NLRP3 inflammasome activation results from stimulation of the P2X7 ion channel by extracellular ATP, resulting in K+ efflux, recruitment of the pannexin-1 pore, DAMP entry into cytosol, and activation of NLRP3. This activates caspase-1, which then converts pro-IL18 and pro-IL1β to active IL-18 and IL-1β. As discussed in the text, cell type-specific innate immune signaling can result in distinctive, and sometimes divergent, in vivo physiological responses during acute MI. Schema adapted from references 13, 14, 18, 76, and 83. (Illustration credit: Ben Smith).

Figure 3

Neutrophil extravasation in the infarcted myocardium is dependent on activation of adhesive interactions between leukocytes and endothelial cells (EC). Pro-inflammatory mediators induce expression of selectins on the endothelial surface, leading to tethering and rolling of circulating neutrophils. Rolling neutrophils sense chemokines bound to glycosaminoglycans on the endothelial surface and exhibit integrin activation. Interactions between leukocyte integrins and their endothelial ligands result in firm adhesion of the neutrophils. Subsequently neutrophils crawl towards endothelial junctions and transmigrate between pericytes, through basement membrane regions with low expression of matrix proteins. Extravasated neutrophils release proteases (both serine proteases and MMPs), and reactive oxygen species (ROS), thus contributing to clearance of the wound. Neutrophils may also modulate inflammatory responses, both by secreting cytokines, and by regulating cytokine activity through release of proteases. Excessive or prolonged neutrophil actions may promote matrix degradation. Because neutrophils are predominantly localized in the infarct border zone, it has been suggested that they may adhere to viable cardiomyocytes, exerting cytotoxic effects. However, the significance of leukocyte-mediated cardiomyocyte injury remains debated.

Figure 4

Cellular effectors and molecular signals that repress and resolve inflammation following myocardial infarction leading to the transition from the inflammatory to the proliferative phase of cardiac repair. Recruitment of anti-inflammatory monocyte (Mo) subsets (1), T cell subpopulations, such as regulatory T cells (Tregs) (2) and invariant Natural Killer T cells (iNKT) (3) contributes to repression of the post-infarction inflammatory response. Moreover, members of the TGF-β family (such as TGF-β1 and GDF-15) inhibit neutrophil transmigration by attenuating expression of adhesion molecules by endothelial cells (EC) (4). Recruitment of pericytes (P) by microvascular ECs is mediated through PDGFRβ actions and may also contribute to suppression of post-infarction inflammation (5). Macrophages (Ma) acquire an anti-inflammatory phenotype, secreting TGF-β, IL-10 and pro-resolving lipid mediators, upon ingestion of apoptotic neutrophils (aN) (6). Dendritic cells are also activated following infarction and secrete anti-inflammatory cytokines (7). Cardiomyocytes (CM) in the border zone may contribute to suppression and spatial containment of the post-infarction inflammatory response by secreting mediators that promote an anti-inflammatory macrophage phenotype (such as Reg-3β) (8). Fibroblasts (F) also exhibit dynamic phenotypic alterations that mark the transition from the inflammatory to the proliferative phase (9). During the inflammatory phase, inflammatory cytokines (such as IL-1β and TNF-α) and activation of TLR-dependent signaling by matrix fragments may activate a pro-inflammatory fibroblast phenotype. Stimulation of fibroblasts with IL-1β induces MMP expression and chemokine synthesis, while reducing α-SMA levels. During the proliferative phase, activation of TGF-β-dependent cascades stimulates a matrix-preserving myofibroblast (MF) phenotype.

Figure 5

Early and late inflammation after myocardial infarction (MI). The early inflammatory phase after MI (~4 d in mice) is characterized by robust innate and adaptive immune cell infiltration and tissue digestion. This is subsequently followed a phase of resolution, myofibroblast proliferation, and wound repair (lasting ~10–14 d in mice), during which immune cells are polarized toward an anti-inflammatory state. However, larger infarcts with more pronounced inflammatory activation exhibit progressive ventricular dilatation and heart failure (HF) over the long-term, together with persistent inflammation and tissue immune cell infiltration. Chronic inflammation may represent incomplete inflammation resolution during the reparative phase and subsequent amplification with time, or a second wave of resurgent immune activation in response to poorly defined factors.