Mechanisms of failed apoptotic cell clearance by phagocyte subsets in cardiovascular disease

Abstract

Recent evidence in humans indicate that defective phagocytic clearance of dying cells is linked to progression of advanced atherosclerotic lesions, the precursor to atherothrombosis, ischemic heart disease, and leading cause of death in the industrialized world. During atherogenesis, apoptotic cell turnover in the vascular wall is counterbalanced by neighboring phagocytes with high clearance efficiency, thereby limiting cellularity and maintaining lesion integrity. However, as lesions mature, phagocytic removal of apoptotic cells (efferocytosis) becomes defective, leading to secondary necrosis, expansion of plaque necrotic cores, and susceptibility to rupture. Recent genetic causation studies in experimental rodents have implicated key molecular regulators of efferocytosis in atherosclerotic progression. These include MER tyrosine kinase (MERTK), milk fat globule-EGF factor 8 (MFGE8), and complement C1q. At the cellular level, atheromata are infiltrated by a heterogenous population of professional phagocytes, comprised of monocytes, differentiated macrophages, and CD11c(+) dendritic-like cells. Each cell type is characterized by disparate clearance efficiencies and varying activities of key phagocytic signaling molecules. It is in this context that we outline a working model whereby plaque necrosis and destabilization is jointly promoted by (1) direct inhibition of core phagocytic signaling pathways and (2) expansion of phagocyte subsets with poor clearance capacity. Towards identifying targets for promoting efficient apoptotic cell clearance and resolving inflammation in atherosclerosis and during ischemic heart disease and post myocardial infarction, this review will discuss potential in vivo suppressors of efferocytosis at each stage of clearance and how these putative interventional targets may differentially affect uptake at the level of vascular phagocyte subsets.

Fig. 1

Efferocytosis in plaque. Accumulation of apoptotic cells accompanies advanced lesional maturation and necrotic core expansion in murine plaque. In the left image, an atherosclerotic lesion is populated by circular nuclei (blue; hoechst) and F4/80 + phagocytes within the intima (green). The circumscribed area is shown at higher magnification to the right where TUNEL positive apoptotic cells can be found in association (A) or free (F) from neighboring phagocytes. Bar = 10 µm (Color figure online)

Fig. 2

Key players during efferocytosis in CVD. Efferocytosis requires the interplay between numerous cellular and extracellular ligands. At the level of the apoptotic cells (AC), presentation of “eat-me” signals [such as phosphatidylserine (ps)] and down-regulation of “don’t-eat” me ligands (such as CD47, CD31, and PAI-1) prepare apoptotic cell ligands for engulfment by macrophage (MU) and immature and mature dendritic (IDC and DC) phagocyte subsets. Neutrophils (PMN) may act as phagocytes and/or apoptotic cells. Interactions between apoptotic target and phagocyte effector often require ‘‘bridging’’ molecules such as Gas6/protein S, lactadherin (MFGE8), complement (C1q, C3b), calreticulin (cal), annexin (anxII), and immunoglobulin (IgM) to facilitate target-effector juxtaposition. Numerous apoptotic cell receptors [such as scavenger receptor A (SRA), TAMs, integrins, LRP, and CD36] and signaling (CrkII-Dock180-Rac1, Rho/actin) and metabolic (NR nuclear receptors) molecules participate during engulfment and modulate downstream inflammatory signaling pathways. “Necrotic” indicates secondary necrotic cells. Arrows indicate differentiation pathways from immature monocyte and dendritic cell precursors. ACAMP apoptotic cell-associated molecular pattern, AC apoptotic cell, MO: monocyte, IDC immature dendritic cell, DC dendritic cell, PAI-1 plasminogen activator inhibitor-1, PMN polymorphonuclear leukocyte or neutrophil, PS phosphatidylserine, TAM Tyro3, Axl tyrosine kinase, MerTK

Fig. 3

Schematic of phagocyte heterogeneity in plaque. Myeloid derived cells, including newly recruited monocyte (Mo) subsets, differentially activated macrophages (M1 and M2 MΦs) (that are symbolized by differentially striped patterns), and immature (iDC) or possible mature CD11C⁺ dendritic-like cells (mDC) in plaque. Arrows indicate differentiation pathways from immature monocyte and dendritic cell precursors. Endo endothelial cell, VSMC vascular smooth muscle cell, NC necrotic core, AC apoptotic cell, oxLDL oxidized LDL