Efferocytosis

Efferocytosis is the process by which other cells recognize, engulf, and digest cells undergoing apoptosis. It is a specialized form of phagocytosis performed by professional phagocytes, particularly macrophages, and by neighboring tissue cells capable of engulfment. Efferocytosis couples the physical removal of dying cells to changes in the engulfing cell’s behavior, helping maintain tissue homeostasis and regulate inflammation. It includes the processing of ingested material, not merely the uptake of a cell corpse. (nature.com)

Biological role and cellular participants

Apoptosis and efferocytosis are distinct but closely connected processes: apoptosis dismantles a cell, whereas efferocytosis removes its remains. Rapid clearance limits the release of potentially harmful or immunogenic intracellular material into surrounding tissue. Thus, the consequences of cell death depend partly on whether dying cells are efficiently recognized and processed by their neighbors. (pubmed.ncbi.nlm.nih.gov)

Clearance is not restricted to cells of the immune system. For example, during mammary-gland involution after lactation, both epithelial cells and macrophages participate in removing apoptotic cells. In developing nervous tissue, microglia—the resident macrophages of the central nervous system—engulf apoptotic neurons. These examples illustrate how the cellular machinery of clearance contributes to tissue remodeling as well as immune regulation. (pubmed.ncbi.nlm.nih.gov)

Recruitment and recognition

Efferocytosis is commonly analyzed as a sequence of recruitment, recognition, internalization, and degradation. These stages are experimentally distinguishable, although they overlap in living tissues. (nature.com)

“Find-me” signals

Dying cells can actively advertise their location rather than simply await contact with a phagocyte. Experiments have shown that apoptotic cells release small amounts of ATP and uridine triphosphate, which attract phagocytes through the P2Y2 nucleotide receptor. These extracellular nucleotides function as “find-me” signals, connecting the execution of apoptosis to the recruitment of cells capable of clearance. (nature.com)

“Eat-me” signals

A central recognition signal is phosphatidylserine, a phospholipid exposed on the outer surface of the cell membrane during apoptosis. The protein XKR8 contributes to this exposure: apoptotic caspases activate XKR8, enabling phospholipid scrambling. In experimental systems, loss of XKR8 impairs phosphatidylserine exposure and reduces subsequent engulfment by phagocytes. (pubmed.ncbi.nlm.nih.gov)

Phosphatidylserine can be connected to phagocyte receptors through soluble bridging molecules. Two well-characterized systems are:

• MFG-E8–integrin recognition. MFG-E8 binds phosphatidylserine on an apoptotic cell and engages integrins on the phagocyte. Its bridging function promotes attachment and engulfment. (pubmed.ncbi.nlm.nih.gov)
• Gas6 or protein S–MerTK recognition. Gas6 and protein S bind phosphatidylserine-bearing apoptotic cells and connect them to MerTK, a receptor tyrosine kinase. MerTK can participate in both tethering the corpse to the macrophage surface and triggering internalization; these functions differ in their requirement for kinase activity. (pubmed.ncbi.nlm.nih.gov)

Recognition also depends on inhibitory signals. CD47 is an anti-phagocytic surface molecule that can oppose clearance. Research on atherosclerotic lesions has shown that increased CD47 expression can accompany defective removal of diseased vascular cells. Efferocytosis therefore reflects the balance of signals promoting and restraining uptake, rather than the action of a single molecular label. (nature.com)

Engulfment and degradation

After recognition, the phagocyte reorganizes its cytoskeleton and membrane to internalize the target. In the MFG-E8 pathway, signaling through integrins activates the DOCK180–Rac1 machinery, linking extracellular recognition to the cellular movements required for engulfment. (sciencedirect.com)

The internalized corpse occupies a membrane-bound phagosome, sometimes called an efferophagosome. This compartment must mature, acidify, and interact with lysosomes so that its contents can be broken down. Experimental disruption of the ion channel TRPM7 markedly impairs phagosomal acidification and digestion, even when initial uptake is only modestly affected. Successful engulfment therefore does not necessarily imply successful clearance. (nature.com)

This distinction is particularly important when a phagocyte encounters multiple dying cells. Processing one corpse must be coordinated with the capacity to engulf the next. Studies of macrophages have identified rapid changes in gene expression that support phagosome acidification, corpse processing, and successive rounds of efferocytosis. (nature.com)

Metabolic and inflammatory responses

Engulfing an entire cell imposes a substantial processing burden and changes the phagocyte’s metabolism. In experimental models, efferocytosis induces a membrane-transporter program that increases glucose uptake and glycolysis. The resulting lactate release can also influence neighboring cells. These findings show that clearance is an active metabolic adaptation, not merely the disposal of inert material. (nature.com)

Efferocytosis can actively suppress inflammatory responses. A foundational study using human macrophages found that ingestion of apoptotic neutrophils reduced production of several pro-inflammatory mediators while increasing transforming growth factor beta and other suppressive mediators. Blocking these mediators restored some inflammatory cytokine production, demonstrating that the effect involved active signaling rather than simply removal of inflammatory cells. (pubmed.ncbi.nlm.nih.gov)

Metabolism and inflammatory regulation are also interconnected. Research in macrophages has identified a tryptophan-metabolism pathway involving IDO1, kynurenine, and the aryl hydrocarbon receptor that promotes interleukin-10 production and further efferocytosis. Its contribution to tissue resolution has been demonstrated in several mouse models. Such mechanisms should not be interpreted as a universal response shared identically by every phagocyte or tissue. (nature.com)

Impaired clearance and disease research

Experimental defects in efferocytosis have helped establish its importance in immune regulation. Mice lacking MFG-E8 show inefficient engulfment of apoptotic lymphocytes in germinal centers and can develop autoantibodies and inflammatory kidney disease. This provides evidence that failure to remove apoptotic material can contribute to autoimmunity, but it does not establish defective clearance as the sole cause of human autoimmune disease. (mousemine.org)

In atherosclerosis, inefficient clearance is associated with the accumulation of cellular debris and necrotic material within plaques. A study combining human plaque observations with mouse experiments found that cleavage of macrophage MerTK was associated with impaired clearance. Mice expressing a cleavage-resistant form of MerTK in myeloid cells showed improved efferocytosis and smaller plaque necrotic cores. These findings identify a mechanism of clearance failure while distinguishing human associations from experimentally tested causation in animals. (pmc.ncbi.nlm.nih.gov)

Experimental measurement and limitations

Efferocytosis assays commonly expose phagocytes to fluorescently labeled apoptotic cells and assess their fate through imaging or flow cytometry. Different probes can measure corpse association, internalization, and acidification. Tandem fluorescent reporters provide additional ways to follow the transition from uptake to processing over time. (nature.com)

The measured endpoint matters. Surface-bound material can be mistaken for ingested material, and fluorescence changes may reflect acidification, degradation, or both. A decrease in unengulfed corpses alone does not identify which molecular stage has changed. Studies that separately measure binding, uptake, acidification, and cargo breakdown provide a more precise account of efferocytic function than a single uptake measurement. (nature.com)

References

1. Efferocytosis requires periphagosomal Ca2+-signaling and TRPM7-mediated electrical activitynature.com
2. Efferocytosis induces a novel SLC program to promote glucose uptake and lactate releasenature.com
3. Identification of a factor that links apoptotic cells to phagocytes.pubmed.ncbi.nlm.nih.gov
4. Impaired involution of mammary glands in the absence of milk fat globule EGF factor 8.pubmed.ncbi.nlm.nih.gov
5. Rapid unleashing of macrophage efferocytic capacity via transcriptional pause releasenature.com
6. epHero – a tandem-fluorescent probe to track the fate of apoptotic cells during efferocytosisnature.com
7. Nucleotides released by apoptotic cells act as a find-me signal to promote phagocytic clearancenature.com
8. Xk-related protein 8 and CED-8 promote phosphatidylserine exposure in apoptotic cellspubmed.ncbi.nlm.nih.gov
9. Mer receptor tyrosine kinase mediates both tethering and phagocytosis of apoptotic cellspubmed.ncbi.nlm.nih.gov
10. CD47-blocking antibodies restore phagocytosis and prevent atherosclerosisnature.com
11. Macrophages that have ingested apoptotic cells in vitro inhibit proinflammatory cytokine production through autocrine/paracrine mechanisms involving TGF-beta, PGE2, and PAFpubmed.ncbi.nlm.nih.gov