Efferocytosis

Efferocytosis (coined in 2003 by the immunologists Aimée M. deCathelineau and Peter M. Henson from the Latin prefix effero-, meaning "to carry to the grave" or "to bury") is the specialized, non-inflammatory process by which professional phagocytes (macrophages, immature dendritic cells, and microglia) and tissue-resident non-professional phagocytes (epithelial cells, fibroblasts, and Sertoli cells) recognize, engulf, and digest host cells undergoing programmed cell death (apoptosis). Every day, a healthy adult human turns over approximately 100 to 150 billion cells (~1–2 million cells per second, or ~80 grams of cellular mass daily) across bone marrow hematopoiesis, thymic T-cell selection, intestinal epithelial shedding, and senescent erythrocyte clearance. Yet when a healthy tissue section is examined under a microscope, virtually zero dead or dying cells are visible! This extraordinary clearance efficiency is the result of efferocytosis, which executes a four-step molecular choreography ("Find-Me" → "Eat-Me" vs. "Don't-Eat-Me" → Cytoskeletal Engulfment → Anti-Inflammatory Resolution) that disposes of apoptotic corpses before their plasma membranes rupture into secondary necrosis—actively suppressing autoimmune lupus-like antinuclear antibody production and resolving acute inflammation.

How does a patrolling tissue macrophage locate a single cell undergoing early apoptosis within a dense three-dimensional organ parenchyma? During the execution phase of apoptosis, activated Executioner Caspase-3 and Caspase-7 proteolytically cleave the C-terminal autoinhibitory tail of the plasma-membrane channel Pannexin-1 (PANX1) (Chekeni & Ravichandran, 2010). Opening of cleaved Pannexin-1 channels and vesicular export release four chemically distinct classes of soluble "Find-Me" chemoattractants that establish a local chemotactic gradient around the dying cell:

1. Extracellular Nucleotides (ATP and UTP): Released through cleaved Pannexin-1 channels at nanomolar-to-micromolar levels (far lower than the millimolar DAMP release of necrotic lysis) and sensed by macrophage purinergic P2Y₂ G-protein-coupled receptors.
2. Sphingosine-1-Phosphate (S1P): Generated by upregulation of sphingosine kinase 1/2 (SphK1/2) in dying cells and sensed by macrophage S1PR1–S1PR5 receptors, which simultaneously prime macrophage survival and anti-inflammatory polarization.
3. Lysophosphatidylcholine (LPC): Produced when caspase-3 cleaves and activates calcium-independent phospholipase A₂ (iPLA₂), which hydrolyzes membrane phosphatidylcholine into LPC sensed by the macrophage receptor G2A (GPR132).
4. Fractalkine (CX₃CL1): Shed as a 60 kDa soluble chemokine fragment from apoptotic lymphocytes (such as germinal-center B cells) and microglia-Surveilled neurons, recruiting phagocytes via CX₃CR1.

Once a macrophage arrives at the dying cell, it must distinguish the apoptotic corpse from the healthy living cells right beside it. This life-or-death decision is governed by the balance between externalized "Eat-Me" signals and downregulated "Don't-Eat-Me" checkpoints:

The Caspase-Regulated Phosphatidylserine (PS) Asymmetric Switch

In a healthy living cell, the negatively charged aminophospholipid phosphatidylserine (PS) is confined almost 100% to the inner (cytoplasmic) leaflet of the plasma membrane lipid bilayer by ATP-dependent P4-ATPase flippases (ATP11A and ATP11C, complexed with CDC50A), which continuously pump any strayed PS molecules from the outer leaflet back to the inner leaflet.

When Caspase-3 and Caspase-7 are activated during apoptosis, they perform a synchronized dual proteolytic strike discovered by Shigekazu Nagata and colleagues (2010–2014):

1. Inactivation of Flippases (ATP11A / ATP11C): Caspase-3 cleaves the cytoplasmic caspase-recognition sites of ATP11A and ATP11C, permanently shutting down inward PS pumping.
2. Activation of the Phospholipid Scramblase Xkr8: Simultaneously, Caspase-3 cleaves the C-terminal tail of the transmembrane scramblase Xkr8 (X-linked Kell blood group precursor-related family, member 8), inducing Xkr8 dimerization and bidirectional lipid scrambling that floods the outer (exoplasmic) leaflet with > 280-fold higher surface density of phosphatidylserine within minutes—without disrupting plasma membrane integrity!

Direct and Bridging Receptors for Externalized Phosphatidylserine

When a macrophage swallows an apoptotic cell of roughly its own size, it suddenly doubles its total cellular content of cholesterol, fatty acids, amino acids, and nucleic acids! Efferocytosis resolves this massive metabolic burden while actively turning off tissue inflammation through four integrated pathways:

1. LC3-Associated Phagocytosis (LAP) and DNase II Degradation: Unlike classical macroautophagy (which forms double-membrane autophagosomes around starved organelles), efferosomes recruit the class III PI3-kinase Rubicon–VPS34–Beclin-1 complex to conjugate LC3-II directly onto the single-membrane efferosome (Douglas R. Green). LAP accelerates fusion with lysosomes containing DNase II (an acid endonuclease required to digest apoptotic chromosomal DNA). Mice lacking DNase II or MFG-E8 accumulate undigested DNA inside bloated macrophages, triggering cGAS–STING and TLR9 activation, type I interferon storms, lethal fetal anemia, and systemic lupus erythematosus (SLE).
2. Mitochondrial Fission (Drp1) for Continual "Serial Efferocytosis": To swallow multiple apoptotic cells in succession during acute tissue injury, the first engulfed corpse triggers Drp1-mediated mitochondrial fission in the macrophage (Wang & Ira Tabas, 2017). Mitochondrial fission prevents mitochondrial Ca²⁺ overload via MCU, freeing cytosolic Ca²⁺ to drive vesicular membrane recycling so the macrophage can immediately engulf a second and third corpse.
3. Fatty Acid β-Oxidation and LXR/PPARγ Cholesterol Efflux: Fatty acids liberated from digested apoptotic membranes fuel macrophage mitochondrial fatty-acid β-oxidation, generating NAD⁺ that activates Sirtuin-1 (SIRT1) and the transcription factor PBX1 to drive transcription of the anti-inflammatory cytokine Interleukin-10 (IL-10). Simultaneously, apoptotic sterols and oxysterols activate the nuclear receptors LXRα/β (Liver X Receptors) and PPARγ/δ, which upregulate both MerTK (positive feedback for more efferocytosis) and the ATP-binding cassette transporter ABCA1—pumping excess cholesterol out of the macrophage onto HDL apolipoprotein A-I while trans-repressing NF-κB, TNF-α, IL-1β, and IL-12.
4. Specialized Pro-Resolving Mediators (SPMs): Efferocytic macrophages switch their eicosanoid lipid synthesis away from pro-inflammatory leukotriene B₄ (LTB₄) and prostaglandin E₂ toward Specialized Pro-Resolving Mediators—lipoxin A₄ (LXA₄), resolvins (RvD1, RvE1), protectins, and maresins (Charles N. Serhan)—which halt further neutrophil infiltration and stimulate tissue repair (TGF-β1, VEGF, IGF-1).

When efferocytosis fails—or is hijacked by pathogens and tumors—three major human diseases ensue:

• Atherosclerotic Plaque Necrotic Cores: In advanced coronary atherosclerosis, foam-cell macrophages inside the arterial intima undergo ER-stress apoptosis. Initially, neighboring macrophages clear these dying foam cells via MerTK; however, oxidative stress activates the metalloprotease ADAM17, which cleaves the ectodomain of MerTK off the macrophage surface while upregulated CD47 on dying smooth muscle cells blocks engulfment (Ira Tabas; Nicholas Leeper). Uncleared apoptotic foam cells undergo secondary necrosis, coalescing into the lipid-rich, thrombogenic necrotic core of unstable vulnerable plaques that rupture to cause myocardial infarction.
• Systemic Lupus Erythematosus (SLE): Genetic deficiencies in early classical complement opsonins (C1q [> 90% penetrance for SLE], C4, C2), MFG-E8, or MerTK impair the clearance of apoptotic cells in germinal centers and UV-exposed skin. When apoptotic blebs (which concentrate ribonucleoproteins Ro/SSA, La/SSB, Sm, and nucleosomal dsDNA on their outer surface) undergo secondary necrosis, they license autoreactive B and T cells to produce pathogenic anti-dsDNA and antinuclear antibodies (ANAs).
• Cancer Immune Evasion via CD47 Overexpression and Anti-CD47 Checkpoint Blockade: Conversely, leukemias, lymphomas, and solid tumors hijack efferocytic checkpoints in two ways: (1) viable cancer cells overexpress the "Don't-Eat-Me" ligand CD47 by 3-fold to 10-fold to engage macrophage SIRPα and escape opsonophagocytosis (target of therapeutic anti-CD47 / SIRPα blockade developed by Irving L. Weissman); and (2) when tumor cells die, efferocytosis by tumor-associated macrophages (TAMs) via MerTK/TIM-4 polarizes the macrophages into an immunosuppressive M2-like phenotype secreting TGF-β and IL-10, shielding the tumor from cytotoxic CD8⁺ T cells.

1. deCathelineau, A. M., & Henson, P. M. (2003). The final step in programmed cell death: Phagocytes carry apoptotic cells to the grave. Essays in Biochemistry, 39, 105–117. https://doi.org/10.1042/bse0390105
2. Hanayama, R., Tanaka, M., Miwa, K., Shinohara, A., Iwamatsu, A., & Nagata, S. (2002). Identification of a factor that links apoptotic cells to phagocytes. Nature, 417(6885), 182–187. https://doi.org/10.1038/417182a
3. Suzuki, J., Denning, D. P., Imanishi, E., Horvitz, H. R., & Nagata, S. (2013). Xk-related protein 8 and CED-8 promote phosphatidylserine exposure in apoptotic cells. Science, 341(6144), 403–406. https://doi.org/10.1126/science.1236758
4. Boada-Romero, E., Martinez, J., Heckmann, B. L., & Green, D. R. (2020). The clearance of dead cells by efferocytosis. Nature Reviews Molecular Cell Biology, 21(7), 398–414. https://doi.org/10.1038/s41580-020-0232-1