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J. Cell Biol. Vol. 189 No. 7 1059–1070
Apoptosis plays an essential role in the development and mainte-
nance of all mammalian tissues. The apoptotic program ensures
that damaged, aged, or excess cells are deleted in a regulated
manner that is not harmful to the host. Beyond the cell intrinsic
apoptotic program initiated after a variety of insults, an integral
second step in apoptosis is the removal of the cell corpse (Kerr
et al., 1972). Indeed, the physical removal and subsequent deg-
radation of the corpse via phagocytosis represents the final act
necessary for the successful removal of a cell fated to die. Recent
advances in our understanding of apoptotic cell clearance have
led to the identification of molecules and signaling pathways that
orchestrate this process (Lauber et al., 2004; Ravichandran and
Lorenz, 2007; Erwig and Henson, 2008).
The efficiency of the phagocytic clearance of apoptotic
cells appears enormous when one considers that despite the
loss of >109 cells per day, the incidence of histologically de-
tectable apoptotic cells is rare in normal tissues (Mochizuki
et al., 1996; Scott et al., 2001; Schrijvers et al., 2005; Yang
et al., 2006; Elliott et al., 2009). The engulfment of apoptotic
cells is performed by both professional phagocytes (such as
Correspondence to Kodi S. Ravichandran: Ravi@virginia.edu; or Michael
R. Elliott: firstname.lastname@example.org
Abbreviations used in this paper: CF, cystic fibrosis; CFTR, cystic fibrosis trans-
membrane conductance regulator; COPD, chronic obstructive pulmonary dis-
ease; LDL, low-density lipoprotein; PtdSer, phosphatidylserine.
macrophages and dendritic cells) and by nonprofessional
“neighboring” phagocytes (such as epithelial cells, endothelial
cells, and fibroblasts). Current evidence suggests that the steps
involved in the phagocytic clearance of apoptotic cells are similar
between professional and nonprofessional phagocytes (Fig. 1),
although the kinetics may differ, with professional phagocytes
exhibiting higher rates and capacity for phagocytosis (Parnaik
et al., 2000).
Based on work from many laboratories over the past decade,
several broadly defined steps have been identified in the recogni-
tion and removal of apoptotic cells by phagocytes. Each step ap-
pears to be tightly regulated by signaling events to ensure swift
and efficient clearance (Fig. 1). At the early stage of apoptosis,
the dying cells release “find-me” signals that are sensed by motile
phagocytes, which help attract these phagocytes to the proximity
of the dying cell. Several soluble chemoattractant find-me signals
released during apoptosis have been recently defined, including
triphosphate nucleotides (ATP/UTP), lysophosphatidylchloline
(lysoPC), and the chemokine CX3CL1 (Lauber et al., 2003;
Truman et al., 2008; Elliott et al., 2009; Muñoz et al., 2010). Once
in the proximity of the dying cell, the physical contact between
the apoptotic cell and the phagocyte is mediated via ligands on
apoptotic cells (referred to as “eat-me” signals) and engulfment
receptors on phagocytes that can recognize these eat-me markers.
Among the array of identified eat-me molecules (Ravichandran
and Lorenz, 2007), the exposure of phosphatidylserine (PtdSer) on
the outer leaflet of the apoptotic cell plasma membrane appears to
be a key eat-me marker (Fadok et al., 1992; Vandivier et al., 2006).
Phagocyte recognition of PtdSer is mediated directly via one or
more PtdSer recognition receptors, including Bai1, Tim-4, and
Stabilin-2 (Kobayashi et al., 2007; Park et al., 2007, 2008, 2009;
Miyanishi et al., 2007; Nakayama et al., 2009), or by soluble
bridging molecules that bind PtdSer on the apoptotic cell and a
receptor on the phagocyte (MFG-E8/v3/5, Gas6/MER; Savill
et al., 1990; Scott et al., 2001; Hanayama et al., 2004). Engagement
of the PtdSer receptors initiates signaling events within the phago-
cytes that lead to activation of the small GTPase Rac, and subse-
quent cytoskeletal reorganization of the phagocyte membrane to
allow corpse internalization (Albert et al., 2000; Gumienny et al.,
2001). From studies in Caenorhabditis elegans and Drosophila
Recent advances in defining the molecular signaling path-
ways that regulate the phagocytosis of apoptotic cells
have improved our understanding of this complex and
evolutionarily conserved process. Studies in mice and
humans suggest that the prompt removal of dying cells is
crucial for immune tolerance and tissue homeostasis.
Failed or defective clearance has emerged as an impor-
tant contributing factor to a range of disease processes.
This review addresses how specific molecular alterations
of engulfment pathways are linked to pathogenic states.
A better understanding of the apoptotic cell clearance
process in healthy and diseased states could offer new
Clearance of apoptotic cells: implications in health
Michael R. Elliott and Kodi S. Ravichandran
Center for Cell Clearance and the Department of Microbiology, University of Virginia, Charlottesville, VA 22908
© 2010 Elliott and Ravichandran This article is distributed under the terms of an Attribution–
Noncommercial–Share Alike–No Mirror Sites license for the first six months after the pub-
lication date (see http://www.rupress.org/terms). After six months it is available under a
Creative Commons License (Attribution–Noncommercial–Share Alike 3.0 Unported license,
as described at http://creativecommons.org/licenses/by-nc-sa/3.0/).
T H E J O U R N A L O F C E L L B I O L O G Y
JCB • VOLUME 189 • NUMBER 7 • 2010 1060
ELMO, and Dock180 appear to be widely expressed (Hasegawa
et al., 1996; Gumienny et al., 2001), whereas the expression
of many of the surface molecules responsible for recognition
of apoptotic cells varies widely among different tissues and cell
types (Ferrero et al., 1990; Graham et al., 1994; Falkowski et al.,
2003; Miyanishi et al., 2007; Park et al., 2007). Thus, because of
the redundancy in the engulfment machinery among cell types, it
is critical to know the expression pattern of identified phagocytic
receptors when considering apoptotic cell clearance in a specific
tissue or by a particular cell type. Interestingly, many of the disease
states linked to failed clearance have been associated with aberra-
tions in the recognition or eat-me step of clearance (Table I). This
observation might reflect an investigator-induced bias toward
phagocyte–corpse interactions, or it may be the result of selective
expression of phagocytic receptors that reduces the redundancy
of uptake mechanisms, and thus is more likely to reveal failures
Regardless of the specific molecules mediating uptake,
the ability to efficiently clear apoptotic cells is strongly linked
to the homeostatic maintenance of healthy tissues in mammals.
This is thought to be the result of two key features of the clear-
ance process. The first is the obvious function of phagocytes
melanogaster, and in vitro mammalian cell experiments, two key
evolutionarily conserved Rac-dependent apoptotic cell engulf-
ment pathways have been identified (Fig. 1; Reddien and Horvitz,
2004; Kinchen, 2010). In addition to receptors that can directly
signal after engaging eat-me signals, there are also contributions
from other “tethering” receptors (e.g., CD14 and CD31) that help
the binding/specific recognition between the apoptotic cell and
the phagocyte (Brown et al., 2002; Devitt et al., 2003). Once
inside the phagosome, the ingested apoptotic cargo is processed
via a phagolysosomal pathway that shares both overlapping and
unique features with the endocytic machinery (Erwig et al., 2006;
Kinchen et al., 2008; Yu et al., 2008; Kinchen and Ravichandran,
2010; Bohdanowicz and Grinstein, 2010). Because of this over-
lap, it is difficult to distinguish disease states related specifically
to aberrant signaling in the phagosomal pathways from those in-
volving endocytosis dysfunction. Indeed, a role for endocytosis in
human disease has been well established (Mosesson et al., 2008;
Ballabio and Gieselmann, 2009). Thus, we will focus on diseases
related to engulfment signaling upstream of corpse degradation.
Although the clearance of apoptotic cells occurs throughout
the body, the specific molecular pathways can vary by tissue. For
example the intracellular engulfment signaling molecules Rac,
Figure 1. Stages of apoptotic cell engulfment and associated cell signaling events that regulate each stage. The four stages of apoptotic cell clearance
are shown, with some of the specific key signaling players identified. The “find-me” step occurs when apoptotic cells release soluble chemoattractants that
promote chemotaxis of phagocytes via corresponding receptors on the phagocyte. The broken line from LPC to G2A indicates uncertainty of direct ligand–
receptor interaction. The “eat-me” stage is characterized by the appearance of ligands on the surface of the dying cell that mark it as a target to be engulfed
by phagocytes bearing appropriate DAMP or PtdSer recognition receptors. The “engulfment” stage occurs when signaling downstream of the apoptotic cell
recognition receptors stimulates Rac-dependent cytoskeletal rearrangement and formation of the phagocytic cup around the target and subsequent internal-
ization. Once fully internalized, the cell corpse undergoes “processing” through the phagolysosomal pathway that results in the degradation and reprocessing
of the dead cell material. DAMP, damage-associated molecular patterns; LPC, lysophosphatidylcholine; MBL, mannose-binding lectin; PS, phosphatidylserine.
Apoptotic cell clearance and disease • Elliott and Ravichandran
The release of intracellular contents from necrotic cells is thought
to provoke an inflammatory response, particularly toward intra-
cellular antigens and DNA released from the dying cells. This
may provide the immunogenic impetus for the onset of some
autoimmune disorders in humans, including systemic lupus
erythematosus and rheumatoid arthritis (Gaipl et al., 2004).
Early experiments in mice showed that the administration of
excess syngeneic apoptotic cells or the masking of PtdSer on
apoptotic cells via annexin V (to block PtdSer-mediated uptake)
produces hallmarks of autoimmunity, such as autoantibody
production and IgG deposition in the glomeruli (Mevorach
et al., 1998; Asano et al., 2004). More recently, several genetic
mouse models bearing defects in PtdSer-mediated recognition
have further confirmed that the failure to efficiently clear apop-
totic cells can result in autoimmunity (Botto et al., 1998; Scott
et al., 2001; Cohen et al., 2002; Hanayama et al., 2004; Lacy-
Hulbert et al., 2007; Rodriguez-Manzanet et al., 2010). Nuclear
antigens, particularly DNA and DNA–protein complexes (e.g.,
high mobility group box 1–containing nucleosomes), appear
especially crucial in human systemic lupus erythematosus and
rheumatoid arthritis (Taniguchi et al., 2003). Studies in knock-
out mice demonstrated that to maintain self-tolerance, DNase-
mediated degradation of apoptotic cell-derived DNA in the
phagosome is necessary (Napirei et al., 2000; Krieser et al.,
2002; Kawane et al., 2003). There is now a solid link between
the inefficient engulfment of apoptotic cells and autoimmunity
in humans (Ren et al., 2003; Gaipl et al., 2004).
An additional means for controlling the immune response
to apoptotic cells is through the active production of anti-
inflammatory mediators by phagocytes. The PtdSer-dependent
as “garbage collectors,” mediating the physical removal of
the dying cells. Such clearance sequesters the dying cell and
prevents the release of potentially toxic or immunogenic intra-
cellular contents from the dying cell into the local environment.
This is a key distinction from necrotic cell death, where the un-
regulated release of dead cell material can cause very strong
inflammatory responses (such as ischemic injury). The second
homeostatic function of the clearance process is the production
of anti-inflammatory mediators by phagocytes that suppress in-
flammation and facilitate the “immunologically silent” clear-
ance of apoptotic cells.
The purpose of this review is to examine the current body
of knowledge linking apoptotic cell clearance to disease patho-
genesis. We will discuss several families of disease states that
appear to have as a contributing factor some level of impaired
cell clearance. We will also attempt to highlight how components
of the engulfment signaling pathways may function in myriad
Failed clearance, altered immune tolerance,
Autoimmune disorders represent the best-characterized rela-
tionship between apoptotic cell clearance and disease pathogen-
esis (Table I; Savill et al., 2002; Gaipl et al., 2004; Erwig and
Henson, 2007; Nagata et al., 2010). The self-contained, regulated
nature of apoptotic cell death preserves membrane integrity and
prevents the release of potentially inflammatory and immuno-
genic intracellular contents. However, if the apoptotic cells
are not promptly cleared, the membrane integrity is lost over
time, and apoptotic cells can progress to secondary necrosis.
Table I. A survey of disease states associated with defects in engulfment-related genes
Le et al., 2001
Cardona et al., 2006
Combadière et al., 2003
AI, cancer, neuropathy, atherosclerosisH/MGal et al., 2000; Scott et al., 2001; Cohen et al., 2002;
Keating et al., 2006; Nandrot et al., 2007; Ait-Oufella
et al., 2008; Thorp et al., 2008
Hanayama et al., 2004; Ait-Oufella et al., 2007;
Nandrot et al., 2007
Botto et al., 1998; Fonseca et al., 2004; Bhatia et al., 2007
Weng et al., 2003; Lacy-Hulbert et al., 2007
Rodriguez-Manzanet et al., 2010
Lutgens et al., 2008
MFG-E8AI, atherosclerosis, neuropathyM
AI, atherosclerosis, neuropathy
HShimazaki et al., 2005; Leak et al., 2009;
Pezzolesi et al., 2009a
Qingchun et al., 2008; Chen et al., 2009
Chen et al., 2009
A-Gonzalez et al., 2009
Mukundan et al., 2009
Kawane et al., 2003
Genes are grouped by known roles in engulfment (find-me, eat-me, engulfment, and post-engulfment). AI, autoimmune phenotype; H, human; M, mouse.
aThere is evidence of genetic linkage but no direct causal relationship was established.
JCB • VOLUME 189 • NUMBER 7 • 2010 1062
At least two factors in CF sputum have been shown to disrupt
apoptotic cell engulfment, including elevated levels of neutrophil-
derived elastase, which may cleave eat-me signals (Vandivier et al.,
2002), and pyocyanin, a toxic by-product of Pseudomonas
aeruginosa, a common infectious pathogen found in the lungs
of about half of all CF patients (Bianchi et al., 2008). Finally,
the inflammation associated with lung disease appears to create
a cytokine milieu (notably increased TNF) that may suppress
apoptotic cell engulfment (Borges et al., 2009), perhaps by hin-
dering the differentiation of monocytes to macrophages, thus
exacerbating these clearance defects.
Intrinsic defects in macrophages in the context of the dis-
eased lung also appear to contribute to the reduced clearance
seen in these respiratory diseases. Alveolar macrophages from
COPD, CF, and asthma patients show a decreased ability to en-
gulf apoptotic cells in vitro (Hodge et al., 2003, 2007; Huynh
et al., 2005; Vandivier et al., 2009). To date, there are no re-
ported links to specific engulfment pathways that are defective
in these lung diseases, although decreased expression of at least
two collectins (mannose-binding lectin and surfactant protein-D)
in COPD patients suggests a possible role for decreased pat-
tern recognition receptor (PRR)/C1q receptor–mediated up-
take (Hodge et al., 2008). Intriguingly, Vandivier et al. (2009)
recently found that cystic fibrosis transmembrane conductance
regulator (CFTR)-deficient epithelial cells are defective in the
phagocytosis of apoptotic cells, whereas CFTR-deficient al-
veolar macrophages show no engulfment defect. These findings
suggest that a persistent disease state in the lung (i.e., COPD)
and/or genetic anomalies may drive engulfment defects, and
thus point to a prominent role for engulfment in the establish-
ment and progression of disease. Moreover, the relative contri-
butions of macrophages and the epithelial cells for apoptotic cell
clearance, as well as the anti-inflammatory cytokines generated
(or lack thereof), need to be determined in the context of lung in-
flammation. Future genetic studies that target engulfment genes
in particular phagocyte populations may reveal some important
information on the onset and progression of lung inflammation.
An interesting feature of defective apoptotic cell clearance
in the diseased lung is the potential role of the small GTPase
RhoA. During engulfment, activation of the small GTPase Rac in
the phagocyte is crucial for actin rearrangement during corpse
internalization (Fig. 1). In contrast, RhoA antagonizes Rac in
this process, and increased levels of RhoA-GTP potently im-
pair engulfment (Leverrier and Ridley, 2001; Tosello-Trampont
et al., 2003; Nakaya et al., 2006). Independent studies have
shown that CFTR deficiency in lung epithelial cells results in
higher basal levels of activated RhoA (Kreiselmeier et al., 2003;
Vandivier et al., 2009). Studies using in vitro treated lung epi-
thelial cells similarly show increased basal levels of RhoA-GTP
in response to cigarette smoke (Richens et al., 2009). Pharma-
cological inhibitors of RhoA activity, particularly statins, en-
hance apoptotic cell engulfment in vitro and in vivo, and thus
suggest that elevated RhoA-GTP levels may play a signifi-
cant role in the impaired clearance observed in diseased lungs
(Morimoto et al., 2006). Although the molecular events leading
to increased levels of RhoA-GTP levels are poorly understood,
cigarette smoke exerts a similar effect (activation of RhoA) and
recognition of apoptotic cells by a phagocyte elicits the re-
lease of anti-inflammatory mediators such as IL-10, TGF, and
prostaglandins in vitro (Voll et al., 1997; Fadok et al., 1998;
McDonald et al., 1999; Ogden et al., 2005). Moreover, this
recognition actively suppresses inflammatory cytokine release
in vitro, particularly those elicited via Toll-like receptors (TLRs;
Voll et al., 1997; Fadok et al., 1998). This immunosuppressive
response extends in vivo, as studies in mice have shown that
the systemic administration of apoptotic cells induces a toler-
izing effect on the immune response in rodent allograft models
(Sun et al., 2004; Wang et al., 2009). Recently, key insights into
the signaling events that regulate the release of these immune
modulators have been gained. PtdSer-dependent engagement of
apoptotic cells induces in phagocytes the p38 MAPK-dependent
transcriptional regulation of IL-10, as well as translational con-
trol of TGF in the phagocyte (Chung et al., 2007; Xiao et al.,
2008). The ability of apoptotic cells to suppress TLR-dependent
release of IL-6, IL-8, and TNF has also been shown to be regu-
lated at the transcript level (Cvetanovic and Ucker, 2004). Thus,
in addition to the physical removal of dying cells, the “tickling”
of phagocytic receptors generates signals that lead to regulation
of anti-inflammatory mediators and in turn, the elicitation of
an immunosuppressive environment during removal of apoptotic
cells. Even under normal healthy conditions, there is a turnover
of >200 billion cells per day in many tissues throughout our
body, and therefore interruptions to the finely tuned clearance
system can lead to inflammation, tissue destruction, and the
onset of disease.
Respiratory diseases and impaired
Intriguingly, increased levels of apoptotic cells are seen in the
sputum and lung tissue of several serious respiratory diseases,
including chronic obstructive pulmonary disease (COPD), cystic
fibrosis (CF), and asthma (Henson and Tuder, 2008). Because
aberrant lung inflammation is a common feature of these dis-
eases, one possibility is that uncleared apoptotic cells progress-
ing to secondary necrosis may contribute to lung inflammation.
But a common underlying question is whether or not these “un-
cleared” apoptotic cells represent increased rates of apoptosis
or defects in apoptotic cell clearance. In the past few years,
several studies have established considerable links between re-
spiratory disease and inefficient apoptotic cell clearance in the
lung (Vandivier et al., 2002; Hodge et al., 2003; Huynh et al.,
2005). Although the focus of these studies has primarily been
on the phagocytic activity of lung resident macrophages (alveo-
lar macrophages), it will be interesting to determine the relative
contribution of healthy lung epithelial cells in the clearance of
neighboring apoptotic cells.
The environment of the diseased lung contributes to poor
apoptotic cell clearance. Cigarette smoking, the leading cause
of COPD, is correlated with increased apoptotic cell debris in
the lung (Hodge et al., 2005), and cigarette smoke impairs the
uptake of apoptotic cells by alveolar macrophages in vitro
(Kirkham et al., 2004; Hodge et al., 2007). Sputum from CF pa-
tients, when added to normal alveolar macrophages, inhibits their
ability to engulf apoptotic targets in vitro (Vandivier et al., 2002).
1063Apoptotic cell clearance and disease • Elliott and Ravichandran
and apoptotic cell engulfment. We and others have found that
macrophages engulfing apoptotic cells up-regulate the key lipid
transporter ABCA1, and this leads to enhanced cholesterol
efflux from the phagocytes (Gerbod-Giannone et al., 2006;
Kiss et al., 2006a). This cholesterol efflux requires PtdSer-
dependent recognition and signaling within the phagocytes (Kiss
et al., 2006a). These findings reveal that a phagocyte taking up
an apoptotic cell has the ability to regulate and normalize the
level of cellular material. Another intracellular engulfment sig-
naling protein, GULP1, has been shown to promote cholesterol
efflux, and GULP1 functions downstream of the LDL-receptor
related protein 1 (LRP1), which is also linked to engulfment of
apoptotic cells (Su et al., 2002; Gardai et al., 2005; Kiss et al.,
2006b). Nuclear receptors, a family of transcriptional regulators
that control the response to cellular lipids (Hong and Tontonoz,
2008), have been implicated in this response, as antagonists
blocked this efflux (Gerbod-Giannone et al., 2006; Kiss et al.,
2006a). As further evidence of the interplay between engulfment
and lipid metabolism, mice deficient in the LXR/ or PPAR
nuclear receptors showed decreased expression of engulfment
genes, with impaired engulfment of apoptotic cells by macro-
phages in vitro and in vivo (A-Gonzalez et al., 2009; Mukundan
et al., 2009). These mice also showed aberrant expression of
inflammatory mediators and eventually develop hallmarks of
autoimmunity. Because uncleared dead cells are a fundamental
issue in atherogenesis, it would seem that the ability to modu-
late apoptotic cell clearance in this environment could serve as
a useful and novel tool to prevent or treat disease.
Cell clearance defects in
Over a decade ago, several studies identified excess apoptotic
cells associated with chronic neurodegenerative diseases, includ-
ing in patients with Parkinson’s, Alzheimer’s, and Huntington’s
disease, and in aging brains (Su et al., 1994; Thomas et al., 1995;
Zhang et al., 1995; Mochizuki et al., 1996). Microglia are one of
the primary phagocytes for apoptotic cells and debris in the brain
(Witting et al., 2000; Magnus et al., 2002; Stolzing and Grune,
2004; Garden and Möller, 2006). Considered to be of myeloid
lineage, these highly motile cells provide necessary surveillance
to respond to cell death associated with acute injury and stroke
(Davalos et al., 2005; Garden and Möller, 2006). Upon the initia-
tion of neuronal cell death, microglia migrate to the site of injury
and mediate the inflammatory response (Davalos et al., 2005;
Koizumi et al., 2007). Recently, engulfment signaling pathways
have been implicated in glial function during chronic neurologi-
cal diseases. Although the discussion in the following paragraph
focuses on microglial cells, it is important to keep in mind that
other cell types in the brain such as astrocytes can also engulf
apoptotic cells (Chang et al., 2000; Magnus et al., 2002; Park
et al., 2007) and thus may play a role in clearance and disease
in the brain.
To date, MFG-E8 is the engulfment-related molecule
best linked to clearance of apoptotic cells in the brain. Cultured
astrocytes and microglia produce MFG-E8, and MFG-E8 can
promote the phagocytosis of apoptotic neurons by microglia
in vitro (Boddaert et al., 2007; Fuller and Van Eldik, 2008).
may in part explain the defective engulfment seen in COPD
(Richens et al., 2009). There is currently no definite linkage be-
tween lung disease and specific engulfment receptors, and the
high rate of cell death in the lung due to inhaled toxins could
provide valuable insights into clearance mechanisms through the
use of genetically modified mice.
Macrophages play a prominent role in the development of
atherosclerotic plaques, and their function in clearing apoptotic
cells appears to be a key to the pathogenesis of this widespread
and life-threatening disease. At the onset of plaque formation,
monocytes in the blood adhere to intimal smooth muscle cells and
differentiate almost exclusively to macrophages. These macro-
phages then take up low-density lipoprotein (LDL) via scaven-
ger receptors and, once they are cholesterol-laden, are known as
“foam cells.” These foam cells eventually undergo apoptosis, yet
early atherosclerotic lesions display few uncleared apoptotic
cells, which suggests efficient clearance (Tabas, 2005). As leuko-
cytes continue to infiltrate the lesion and release inflammatory
mediators, cell death increases (Schrijvers et al., 2005). Indeed,
late plaques feature much higher levels of free, uncleared apop-
totic cells, and eventually a necrotic core forms and becomes un-
stable, leading to possible lesions that can cause thrombosis
In recent years, the role of apoptotic cell clearance has
begun to be appreciated in atherogenesis. Through the use of
atherosclerosis mouse models—ApoE/ and Ldlr/—genetic
studies of engulfment molecules have demonstrated the role of
cell clearance in atherosclerosis (Table I). Mice deficient in the
apoptotic cell-bridging molecules MFG-E8 (Ait-Oufella et al.,
2007) and C1q (Bhatia et al., 2007) develop accelerated athero-
genesis and display increased plaque-bound apoptotic cells on
ApoE/ and Ldlr/ genetic backgrounds, respectively. Like-
wise, mice deficient in transglutamase 2 (TG2), a cross-linking
enzyme that promotes engulfment via v3/5 (Lorand and
Graham, 2003; Szondy et al., 2003), also enhances atheroscle-
rotic plaque formation in Ldlr/-deficient mice (Boisvert et al.,
2006), but not in ApoE-deficient mice (Williams et al., 2010).
In addition, the receptor tyrosine kinase MER, which recog-
nizes apoptotic cells via the PtdSer-binding Gas6 bridging mol-
ecule, functions in vivo to inhibit plaque formation and can
promote apoptotic cell clearance in atherosclerosis models
(Ait-Oufella et al., 2008; Thorp et al., 2008). Paradoxically,
Gas6 deficiency on the ApoE/ background leads to the forma-
tion of more stable plaques with smaller necrotic cores, fewer
macrophages, and increased TGF levels (Lutgens et al., 2008),
which suggests possible additional nonengulfment related anti-
atherogenic roles for MER. These studies suggest divergent
roles for the receptor–ligand interactions in atherogenesis,
which may be due to nonengulfment functions of both proteins
or the lack of our full understanding of cell death/cell clearance
in an atherosclerotic plaque.
Lipid handling by macrophages plays an important role in
atherosclerosis, and so it is interesting that there is considerable
overlap in the cellular mechanisms that regulate lipid metabolism
JCB • VOLUME 189 • NUMBER 7 • 2010 1064
MFG-E8 in mouse models of solid tumors also enhances anti-
tumor activity (Jinushi et al., 2008; Jinushi et al., 2009). These
findings suggest that interfering with PtdSer uptake promotes
dendritic cell-mediated antitumor activity by favoring inflam-
matory uptake mechanisms. Still, despite what appears to be a
plausible scenario wherein apoptotic cell clearance could have a
profound impact on carcinogenesis, there is only limited genetic
evidence to implicate specific engulfment signaling pathways in
this process. Indeed, the expression of several key engulfment
players, including MER (Linger et al., 2008) and v5 (Burvenich
et al., 2008), is up-regulated in neoplastic cells, but the impor-
tance of this observation is unclear.
With the recent discovery of several “find-me” factors
released by apoptotic cells that act to promote recruitment of
phagocytes to apoptotic cells, new insights have been gained in
our understanding of connections between cell clearance and
tumorigenesis. Several insightful studies from the laboratory of
C.D. Gregory (Ogden et al., 2005; Truman et al., 2008) have fo-
cused on how macrophages sense and subsequently engulf apop-
totic Burkitt lymphoma cells and how these signaling events may
impact disease progression. These neoplastic B cells express high
levels of fractalkine on their surface that is cleaved during apop-
tosis and subsequently functions as a potent chemoattractant for
macrophages (Truman et al., 2008). Recruitment of macrophages
to splenic follicles is impaired in fractalkine receptor-deficient
mice, an observation consistent with a role for fractalkine as a key
mediator of macrophage recruitment to germinal centers (Truman
et al., 2008). Within the germinal center environment, high levels
of IL-10 (likely produced by the engulfing macrophages) appear
to suppress tumor immunity, whereas the release of B cell sur-
vival factors by engulfing macrophages is thought to promote
tumor growth (Ogden et al., 2005).
Additionally, we have recently found that apoptotic cells
release nucleotide triphosphates (ATP/UTP) early during the
apoptotic process (within 2–4 h), and that these nucleotides act as
chemoattractants for monocytes and macrophages in vitro and
in vivo (Elliott et al., 2009). The amount of ATP released by apop-
totic cells under these conditions, which promotes silent clear-
ance, represents a very small percentage of the total intracellular
pool of nucleotides (<2%; Elliott et al., 2009). In contrast, a few
other recent studies have demonstrated that ATP is released
by tumor cells undergoing apoptosis in response to chemo-
therapeutics, with considerably higher amounts of ATP release
(10–100 fold greater) seen at later times after induction (12–24 h;
Ghiringhelli et al., 2009; Martins et al., 2009; Aymeric et al., 2010).
This apoptotic cell-derived ATP stimulates activation of the
NLRP3 inflammasome in dendritic cells via the P2X7 receptor
(Ghiringhelli et al., 2009). This heightened activation state appears
necessary to drive IL-1 secretion and subsequent priming of
CD8+ T cells for IFN production and antitumor responses. These
studies highlight an emerging role for factors released by apoptotic
cells in shaping the immune response in normal and tumor environ-
ments. This has led to the concept of “immunogenic” versus “non-
immunogenic” cell death, and the idea that immunogenic cell
death may be beneficial in antitumor therapies (Green et al., 2009;
Locher et al., 2009). Thus, whether apoptotic cell clearance has a
beneficial or detrimental effect in the context of tumor progression
There is also a correlative relationship between MFG-E8 and
Alzheimer’s disease, as suppressed levels of MFG-E8 are as-
sociated with the disease in humans and mice (Boddaert et al.,
2007; Fuller and Van Eldik, 2008). Additional evidence of en-
gulfment signaling in the brain comes from studies of microglial
chemoattractants. Dying neurons release find-me cues, namely
extracellular nucleotides as well as CX3CL1 (fractalkine or neuro-
tactin) that promote chemotaxis of microglia via the P2Y and
CX3CR1 receptors, respectively (Harrison et al., 1998; Koizumi
et al., 2007). Interestingly, both fractalkine and UDP appear
to enhance glial cell engulfment: fractalkine by enhancing
microglial secretion of MFG-E8, and UDP through an as yet un-
known mechanism (Koizumi et al., 2007; Fuller and Van Eldik,
2008). The role of fractalkine signaling has been studied in the
context of amyotrophic lateral sclerosis and Parkinson’s disease
using CX3CR1-deficient mice. In these disease models, loss of
fractalkine signaling resulted in increased numbers of dying
neurons, which suggests a potential role for fractalkine as an
important find-me signal in the maintenance of brain homeosta-
sis (Cardona et al., 2006). A key unexplored area of clearance
in the central nervous system is the immune response generated
by microglial cells or astrocytes during engulfment (i.e., the
release of anti-inflammatory mediators) and how that impacts
homeostasis and disease. Finally, in the developed brain, cell
turnover is thought to be quite low with the exception of restricted
regions where adult neurogenesis takes place (Kempermann
et al., 2004; Zhao et al., 2008; Taupin, 2009). Defining how
apoptotic cell clearance impacts other developmental processes
in the brain related to cell turnover, including adult neuro-
genesis, will require additional studies with appropriate neuro-
Tumorigenesis and cell clearance
Because apoptotic cell clearance typically generates an immuno-
suppressive environment, its role in the development and pro-
gression of cancer is enigmatic. As has been reviewed elsewhere
(Coussens and Werb, 2002; Condeelis and Pollard, 2006; Solinas
et al., 2009), chronic inflammation is a key factor in tumorigene-
sis. Thus, the efficient clearance of dying cells, and the associated
production of anti-inflammatory mediators, would be predicted
to be beneficial in limiting tumorigenesis. However, within a
tumor environment where rapid cell proliferation and apoptosis are
ongoing, phagocyte-mediated clearance can exert an unwanted
immunosuppressive effect. This is particularly the case upon the
administration of antitumor chemotherapeutics, most of which
act by inducing apoptosis of tumor cells. In this setting, efficient
engulfment and the characteristic release of anti-inflammatory
mediators, particularly TGF, upon encounter with eat-me sig-
nals during this process appear to suppress the antitumor immune
response. Indeed, in several rodent tumor models, treatment with
monoclonal antibodies to block PtdSer-mediated uptake retards
the growth of tumors (Huang et al., 2005; Ran et al., 2005; He
et al., 2009). Similarly, vaccination of mice with UV-irradiated
lymphoma cells coated with annexin V to mask PtdSer provides
significant tumor protection against subsequent challenge with
living tumor cells, presumably by initiating an antitumor inflam-
matory response (Bondanza et al., 2004). Antibody depletion of
1065 Apoptotic cell clearance and disease • Elliott and Ravichandran
The small GTPase RhoG acts upstream of ELMO1, and active
RhoG-GTP interacts with ELMO1, and thereby recruits the
ELMO–Dock180 complex to the membrane to promote Rac acti-
vation, membrane ruffling, and engulfment (Katoh and Negishi,
2003; deBakker et al., 2004). IPGB1 mimics the activity of RhoG-
GTP, and the Rac-generated ruffles serve as a site of entry for
S. flexneri (Handa et al., 2007). Similarly, Yersinia enterocolitica
virulence factors Invasin and YopE also modulate Rac1 activity
at the level of RhoG, and appear to do so in an ELMO–Dock180-
dependent manner in cultured cells (Roppenser et al., 2009).
However, neither of these Y. enterocolitica virulence factors have
been reported to directly interact with ELMO–Dock180, and the
role of this module was inferred by expression of a dominant-
negative mutant of ELMO1 that did not further alter Rac activa-
tion in the presence of YopE (Roppenser et al., 2009).
Usurping the engulfment machinery is not exclusive to
bacteria, and in fact can be used by viruses to promote patho-
genesis. Janardhan et al. (2004) found that the Nef gene product
of HIV-1 is able to complex with the ELMO2–Dock2 module in
T cells to promote Rac activation. Further, we have found that
Nef interacts with Dock2 in Jurkat T cells and promotes the
activation of a key cytoskeletal Rac effector, p21-activated
kinase (PAK; unpublished data). The outcome of this inter-
action appears to be dysregulated Rac activation, which is
or anticancer therapies will depend on gaining a better under-
standing of the role of factors released by apoptotic tumor cells.
in microbial pathogenesis
An emerging facet of engulfment signaling is how these path-
ways can be usurped by microbial pathogens. It has been known
for some time that bacteria can hijack or mimic host signaling
pathways to aid in pathogenic steps, including cell entry and
immune evasion (Stebbins and Galán, 2001). This is achieved by
delivery of bacterial effector proteins into the host cell that mimic
a range of cellular activities. As key regulators of the cytoskeleton
and numerous other cellular processes, small G proteins, particu-
larly the Rho family (e.g., RhoA, Rac, and Cdc42), are frequent
targets for these clever effector mechanisms (Mattoo et al., 2007).
The signaling machinery that controls phagocyte morphology
during apoptotic cell engulfment relies on these GTPases as well,
and thus it is not surprising that several bacteria target these path-
ways. In particular, the RhoG–ELMO–Dock–Rac pathway has
been found to be such a target (Fig. 2). The invasive pathogen
Shigella flexneri utilizes a type III secretion system to inject ef-
fectors to promote entry into epithelial cells, including IPGB1
(Handa et al., 2007). IPGB1 promotes membrane ruffling via
Rac activation in a mechanism that requires binding to ELMO1.
Figure 2. Pathogens usurp the ELMO–Dock–Rac engulfment module. Examples of mechanisms whereby microbial pathogens use the ELMO–Dock–Rac
module to alter the host cellular response. The area above the broken line shows mechanism of enhanced S. flexneri invasion via IPGB1 interaction with
ELMO, leading to enhanced Rac activation and membrane ruffles that serve as entry points for the bacteria. The area below the broken line shows that
HIV-1 uses Nef interaction with the ELMO–Dock2 complex to disrupt CXCR4-dependent chemotaxis in CD4+ T cells.
JCB • VOLUME 189 • NUMBER 7 • 2010 1066
topic (using in vivo models) portend potentially therapeutic ben-
efits by targeting the components of the engulfment machinery.
We thank members of the Ravichandran laboratory for helpful comments
during preparation of this manuscript.
This work was supported by a post-doctoral fellowship from the Ameri-
can Cancer Society (to M.R. Elliott), and grants from the National Institute of
General Medical Sciences/National Institutes of Health (to K.S. Ravichandran).
K.S. Ravichandran is a William Benter Senior Fellow of the American
Submitted: 20 April 2010
Accepted: 7 June 2010
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Engulfment genes and other types of
Several recent studies have discovered associations with human
disease and genetic mutations of components of the engulfment
signaling machinery. For example, several point mutations in the
intronic regions of Elmo1 have been linked to diabetic nephropathy
and diabetes (Shimazaki et al., 2005; Pezzolesi et al., 2009a,b).
ELMO has also been shown to promote the invasive phenotype
of glioblastoma cells in concert with Dock180 and Rac (Jarzynka
et al., 2007). Although the role of MFG-E8 in cell clearance and
self-tolerance in mice is well-established, there is now evidence
that improper splicing of this gene in humans, which results in
the production of a PtdSer-binding mutant protein, can be seen in
some systemic lupus erythematosus patients (Yamaguchi et al.,
2010). Finally, several mutations in engulfment-related genes have
been seen in human diseases, including Alzheimer’s disease,
schizophrenia, and multiple types of cancer (Table I). It will be
important to determine the contribution of these genes in these
diseases and understand how they relate to engulfment- and
nonengulfment-related cell signaling. As such, these studies point
to the importance of the signaling molecules relevant for apoptotic
cell engulfment, or the respective signaling pathways in disease,
and may help unravel a few of the complex disease pathologies.
The past decade has seen an impressive expansion of our knowl-
edge regarding the fundamentals of apoptotic cell clearance.
Despite the complexity and what appears to be redundancy of
this process, several key themes emerge, with relevance for dis-
ease onset and progression. First, the presence of excess apoptotic
cells, particularly in a disease state, is not simply a sign of disease
but is likely to have a role in pathogenesis. With the tools cur-
rently available, it is difficult to distinguish whether excess apop-
totic cells observed in vivo are the result of normal cell death with
failed clearance, or an increase in the rate of cell death. However,
most tissues appear to have mechanisms in place to support very
efficient clearance of apoptotic cells, and uncleared apoptotic
cells thus likely represent, at least to some extent, a failure of
clearance. A key question related to this idea is the relative contri-
bution of the actual physical removal of the dying cell versus the
anti-inflammatory signaling generated by this event in homeo-
stasis and disease. This question will require a more thorough
understanding of the signaling events that regulate these two
closely related, but experimentally distinguishable, steps of
engulfment. Finally, it is also important to carefully consider the
impact of apoptotic cell clearance on progression of particular dis-
ease states, as in most cases apoptotic cell clearance is a benefi-
cial event. Yet, as in the case of tumorigenesis, it may be that
certain disease conditions are exacerbated by the clearance of
apoptotic cells. The widespread role of apoptotic cell clearance
in many tissues and the recent flood of information on this
Apoptotic cell clearance and disease • Elliott and Ravichandran
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