Silent Corpses, Dangerous Plaques: How Defective Cell Clearance Shapes Atherosclerosis Progression and Rupture

The Molecular Choreography of Efferocytosis

Apoptotic cell clearance, or efferocytosis, is a tightly coordinated process ensuring that dying cells do not transition into pro-inflammatory necrotic debris. The central players are macrophages, which deploy an arsenal of receptors and bridging molecules to recognize phosphatidylserine and other “eat-me” signals exposed on apoptotic membranes. This recognition is not simply a matter of adhesion but involves stereospecific tethering followed by cytoskeletal remodeling that culminates in engulfment. When intact, these pathways allow atherosclerotic lesions to remain quiescent, suppressing unchecked inflammation and preserving tissue integrity.

The internalization of apoptotic bodies requires dynamic remodeling of the macrophage cytoskeleton. Activation of Rac1 and Cdc42 initiates actin polymerization for phagocytic cup formation, while RhoA-mediated contraction completes engulfment. Mitochondrial fission further supplies vesicular trafficking capacity, enabling macrophages to efficiently recycle membrane material and sustain continuous uptake. Deficiencies in these processes, whether through genetic alterations or inflammatory signaling, cause severe delays in apoptotic corpse clearance. In vascular lesions, such inefficiency produces an immediate surplus of cell debris.

Efferocytosis does not end at engulfment; degradation is mediated by LC3-associated phagocytosis. Lysosomal fusion and hydrolytic breakdown of engulfed cells liberate nucleic acids, proteins, and lipids into macrophage compartments. Nuclear receptors such as PPAR and LXR detect this cargo, activating cholesterol efflux pathways to avoid lipid overload. When degradation pathways fail, macrophages accumulate undigested DNA and cholesterol, adopting a pro-inflammatory phenotype instead of resolving the injury. The result is a shift from repair toward amplification of tissue damage.

In early lesions, this choreography largely holds. Monocyte-derived macrophages clear apoptotic foam cells with relative efficiency, producing anti-inflammatory cytokines like IL-10 and TGF-β. However, as atherosclerosis advances, this finely tuned machinery unravels. Oxidative stress, chronic cytokine exposure, and receptor cleavage progressively compromise the phagocytic process, setting the stage for necrotic core expansion. These early disruptions link the mechanistic collapse of efferocytosis directly to plaque vulnerability.

How Atherosclerotic Plaques Overwhelm Clearance

Atherosclerosis begins with subendothelial retention of apolipoprotein B-containing lipoproteins. Oxidized derivatives of these lipoproteins incite endothelial activation, drawing monocytes that differentiate into macrophages. Initially, efferocytosis mitigates damage by swiftly removing apoptotic cells, preventing debris accumulation. However, the relentless lipid influx, oxidative stress, and cytokine-rich milieu gradually impair recognition and engulfment pathways. Over time, the protective cycle transitions to a destructive one.

The buildup of apoptotic cells that are not cleared transitions them into secondary necrosis. Necrotic debris spills cytotoxic enzymes, nuclear antigens, and modified lipids into the local environment. This debris transforms lesions into chronic inflammatory zones, characterized by persistent immune recruitment and compromised tissue repair. These zones of necrosis, once established, are no longer amenable to the same homeostatic repair mechanisms that functioned in earlier stages.

Key mediators actively sabotage efferocytosis within plaques. For instance, lesional cells aberrantly express CD47, a “don’t eat-me” signal that inactivates phagocytic machinery. Similarly, reductions in calreticulin and other “eat-me” signals render apoptotic bodies poor substrates for macrophage clearance. Autoantibodies generated against oxidized phospholipids further mask recognition motifs, creating competitive inhibition for efferocytic receptors. Collectively, these processes create an environment where macrophages, even when present, cannot efficiently execute clearance.

This failure has anatomical consequences. The necrotic core, a hallmark of advanced atherosclerotic plaques, arises as uncleared apoptotic cells collapse into disordered debris. The necrotic core expands, destabilizing the fibrous cap and heightening the risk of rupture. A ruptured plaque exposes thrombogenic material to the circulation, precipitating myocardial infarction or stroke. Thus, defective efferocytosis is not simply an immunological inconvenience but a direct mechanistic driver of acute cardiovascular events.

Molecular Defects Undermining Efferocytosis

Receptor cleavage is a critical mechanism disabling efferocytosis. The receptor MerTK, central to apoptotic cell recognition, undergoes proteolytic cleavage by ADAM17 in inflamed lesions. Soluble fragments of MerTK accumulate, competitively inhibiting Gas6-mediated signaling and further blocking efferocytosis. Mice engineered with cleavage-resistant MerTK variants demonstrate reduced necrotic core formation, proving this mechanism is causal rather than correlative. Loss of receptor integrity represents a direct biochemical blockade of clearance.

Bridging molecules, essential for linking apoptotic phosphatidylserine to macrophage receptors, are also depleted in advanced plaques. Declining levels of MFG-E8 and C1q compromise tethering, while oxidative modifications generate competitive ligands that misdirect receptor binding. Inflammation suppresses transcription of these molecules, creating a deficit exactly when demand for clearance surges. The absence of these bridges converts apoptotic bodies into inert particles that macrophages can no longer recognize effectively.

Toll-like receptor signaling adds another layer of interference. Oxidized lipids activate TLR4 pathways, increasing TNF-α and IL-1β production while simultaneously suppressing TGF-β and IL-10. This shift in cytokine balance reduces receptor expression and biases macrophages toward inflammatory lipid uptake rather than corpse engulfment. Over time, macrophages transform into foam cells unable to perform their clearance duties, essentially weaponizing their dysfunction to expand plaque burden.

MicroRNAs regulate efferocytosis in atherosclerosis as well. For example, miR-21 enhances MerTK expression and promotes clearance, whereas loss of miR-21 leads to enlarged necrotic cores. Conversely, miR-33 exerts suppressive effects, and its inhibition improves efferocytosis in experimental models. These regulatory RNAs highlight that efferocytosis is not merely receptor-ligand biology but also an epigenetically tuned process susceptible to transcriptional reprogramming under chronic inflammatory stress.

Resolution Failure and the Persistence of Inflammation

Efferocytosis is intrinsically tied to inflammation resolution. Clearance of apoptotic cells not only prevents necrosis but also generates specialized pro-resolving mediators (SPMs), including resolvins, protectins, and lipoxins. Activation of MerTK and related receptors drives production of these lipid mediators, which suppress pro-inflammatory cytokine signaling while amplifying anti-inflammatory outputs. When efferocytosis fails, this feedback loop collapses, removing a critical brake on chronic inflammation.

Stable plaques display a favorable ratio of SPMs to leukotrienes, creating an environment biased toward resolution. Vulnerable plaques, by contrast, exhibit reduced SPM production and increased leukotriene-driven inflammation. This biochemical imbalance correlates with larger necrotic cores and thinner fibrous caps. In experimental systems, supplementation with SPMs such as resolvin D1 rescues efferocytosis efficiency and reduces lesion necrosis, demonstrating causality in the resolution-efficiency axis.

Macrophages deficient in clearance lose their capacity to switch into a pro-resolving phenotype. Instead, they perpetuate cycles of TNF-α and IL-1β production, ensuring continued immune recruitment. The absence of apoptotic cell-derived signals that normally reinforce tolerance compounds this effect. As a result, non-resolving inflammation becomes self-sustaining within the lesion, embedding efferocytosis failure into the natural history of atherosclerosis.

Resolution failure has systemic implications. Beyond the local vascular environment, defective efferocytosis feeds into systemic immune activation, contributing to broader cardiovascular risk. The persistence of low-grade inflammation increases vulnerability not only to plaque rupture but also to thrombotic complications. This reveals why defective efferocytosis is not a microscopic curiosity but a systemic determinant of cardiovascular disease progression.

Therapeutic Frontiers in Restoring Clearance

Understanding efferocytosis failure has opened avenues for targeted therapy. Antibodies blocking CD47 signaling restore macrophage recognition of apoptotic cells, reducing necrotic core size in experimental models. However, off-target clearance of red blood cells creates anemia, highlighting the delicate balance of therapeutic design. Strategies aimed at selectively modulating CD47 pathways in plaques are actively under investigation.

Preventing receptor cleavage is another approach. Inhibiting ADAM17-mediated degradation of MerTK preserves macrophage clearance function and enhances lesion stability. Gene-engineered cleavage-resistant receptors provide proof-of-principle, though translating such strategies into human therapy remains challenging. Nonetheless, this approach underscores the importance of preserving receptor integrity as a therapeutic axis.

Pro-resolving mediator supplementation offers a complementary strategy. Administration of resolvins or annexin A1 derivatives restores resolution, reduces necrosis, and enhances clearance. Unlike global immunosuppressive therapies, these mediators fine-tune macrophage behavior without compromising host defense. Their success in preclinical models raises the possibility of developing resolution-targeted drugs that directly counter efferocytosis failure.

Combination therapies may prove most effective. Targeting CD47, protecting MerTK, and enhancing pro-resolving mediator availability could synergistically re-establish clearance capacity. Integrating these approaches with traditional lipid-lowering therapies would address both the cause and consequence of plaque development. By focusing not only on cholesterol but also on the clearance of cellular corpses, cardiovascular medicine may finally bridge the gap between plaque burden reduction and plaque stabilization.

Study DOI: https://doi.org/10.3389/fcvm.2017.00086

Engr. Dex Marco Tiu Guibelondo, B.Sc. Pharm, R.Ph., B.Sc. CpE

Editor-in-Chief, PharmaFEATURES