Clinical Immunology (2007) 122, 125 — 134
available at www.sciencedirect.com
SHORT ANALYTICAL REVIEW
Autoimmunity versus tolerance: Can dying cells tip
Irene C.B. Viorritto a,b, Nikolay P. Nikolov c, Richard M. Siegel b,c,*
Cell Signaling Section, Genetic Disease Research Branch NHGRI, NIH, Building 49, Room 4A38, Bethesda, MD 20892, USA
Immunology Graduate Group, University of Pennsylvania School of Medicine, USA
Immunoregulation Unit, Autoimmunity Branch, NIAMS, NIH, Building 10, Room 9N238, Bethesda, MD 20892, USA
Received 17 July 2006; accepted with revision 19 July 2006
Available online 5 October 2006
KEYWORDS Abstract Apoptosis is a physiological process of self-destruction for cells that are damaged
Apoptosis; or programmed to die. Apoptosis occurs through a series of regulated events that allow
Autoimmunity; cellular debris to be contained and efficiently phagocytosed without initiating a proinflam-
Phagocytosis; matory immune response. Recent data have linked physiological apoptosis and the uptake of
Dentritic cell; apoptotic cells by macrophages and some subsets of dendritic cells to the maintenance of
Toll-like receptor peripheral immune tolerance. However, when cells die through necrosis, spilling their
intracellular contents, or are infected with various pathogens, activation of antigen-
presenting cells and induction of an immune response can occur. Receptors for extrinsic
pathogen-associated structures, such as membrane bound Toll-like receptors (TLRs) or
intracellular Nod-like receptors (NLRs) can also respond to cross-reactive host molecules from
dying cells and may focus autoimmune responses onto these antigens. Several autoimmune
disorders have been linked to defects in the apoptotic process. Defective apoptosis of immune
cells leads to autoimmunity, as in autoimmune lymphoproliferative syndrome (ALPS)
associated with mutations in the death receptor Fas. Defective clearance of apoptotic cell
debris can also lead to autoantibody production. We will discuss how cell death and apoptotic
cell clearance may affect the finely tuned balance between peripheral immune tolerance and
Published by Elsevier Inc.
Apoptosis, or programmed cell death, is a physiological
* Corresponding author. Bldg 10 Rm 9N238, Bethesda MD 20892 process occurring in cells that are damaged or no longer
301-496-3761, USA. Fax: +1 301 480 3880. useful, including embryogenesis and tissue homeostasis.
E-mail address: firstname.lastname@example.org (R.M. Siegel). During apoptosis the cell is degraded by a series of regulated
1521-6616/$ — see front matter. Published by Elsevier Inc.
126 I.C.B. Vioritto et al.
steps that allow rapid uptake and removal of cellular debris which primes the apoptosome, a cytoplasmic protein com-
by phagocytes without causing inflammation. By contrast, plex of caspase-9 and the upstream activator APAF-1 to
cells that die by necrosis spill their cellular contents including cleave caspase-9 into its active form. Caspase-9 cleaves
molecules that stimulate inflammation and dendritic cell caspases-3 and -7, which in turn cleave specific substrates,
activation that can prime adaptive immune responses against activating a defined cellular program of plasma membrane
self-antigens. Apoptotic cell clearance has an important role blebbing, cytoplasmic and organelle contraction and shrink-
in embryonic development, tissue homeostasis, and the age, nuclear chromatin condensation, DNA and RNA degra-
maintenance of self-tolerance. Signals from the dying cell dation by specific nucleases, and cytoskeletal reorganization
such as the exposure of phosphatidylserine on the exterior (reviewed in [1,2]). One of the earliest changes at the plasma
surface of the plasma membrane, are recognized by multiple membrane is the display of phosphatidylserine (PS), a
receptors on the surface of phagocytic cells, providing a swift membrane lipid that is usually restricted to the inner plasma
and efficient disposal system for dying cells. Defects in membrane leaflet, on the external face of the plasma cell
apoptosis or the uptake of dying cells by macrophages and membrane. PS provides a platform for recruiting phagocytic
dendritic cells have been shown to play a role in several cells to vicinity of dying cells, and the resulting apoptotic
pathological conditions, including cancer, neurodegenerative bodies are rapidly ingested by neighboring cells and resident
diseases, and autoimmunity. We will discuss recent data on tissue macrophages and dendritic cells (DCs), via receptor-
the role of macrophages and dendritic cells in apoptotic cell mediated mechanisms (reviewed in ). Although the
clearance, and describe how the mode of cell death and dead dflippingT of membrane lipids to allow PS exposure is
cell uptake may influence the finely tuned balance between caspase-dependent, PS can become accessible in any cell
maintaining immune tolerance and initiation of an potentially losing membrane integrity and is thus not a signal that only
pathogenic autoimmune or autoinflammatory response. marks apoptotic cells.
Necrosis occurs when cell death is daccidentalT rather
Cell death: general mechanisms than programmed. Necrosis is often associated with me-
chanical tissue damage, or certain infectious agents. It is
Apoptosis, also referred to as programmed cell death, is a characterized by cell swelling, total cellular breakdown,
morphologically identifiable form of cell death characterized early loss of plasma membrane integrity, and release of
by a complex series of processes that adhere to a strict intracellular contents. Whether active signaling pathways
timeline (Fig. 1). The initiating signals for cell death are play a role in necrosis or if it is simply a passive response to
integrated by a number of mechanisms, including interac- external catastrophe is not clear at this time. The early
tions between pro and anti-apoptotic members of the bcl-2 breakdown of the plasma membrane in necrosis facilitates
protein family. If a critical threshold is reached, the spilling of intracellular contents, which can contain immu-
mitochondrial outer membrane becomes permeable to large nostimulatory molecules such as heat shock proteins .
molecules (MOMP). Mitochondria release cytochrome c, Undegraded mammalian DNA and RNA may also be immu-
Figure 1 Steps in apoptosis and apoptotic cell uptake. Critical stages are shown for the apoptotic cell at bottom, with the
phosphatidylserine externalized on the plasma membrane early in apoptosis serving as the main signal for apoptotic cell uptake by a
macrophage at top.
Autoimmunity versus tolerance: Can dying cells tip the balance? 127
Table 1 Comparing cellular modifications in apoptotic and necrotic death
Cellular role Active Passive
Distribution Dispersed Contiguous
Morphology Decreased cell volume Increased cell volume
Cellular membrane Preserved Loss of integrity
Induction Slow (hours) Rapid (seconds—minutes)
Cell removal Rapid Slow
Tissue inflammation Absent Present
nostimulatory through cross-reactive interactions with cel- whereby organelles or long-lived proteins are digested in
lular receptors for pathogen-derived nucleic acids. A double-membrane enclosed lysosome-like structures. In
comparison of some of the features of apoptosis and necrosis other cells, a necrosis-like phenotype with rapid cell lysis
is shown in Table 1. is observed [5,6]. A distinct mechanism of cell death
termed danoikisT has also been described following detach-
ment of adherent cells from extracellular matrix substrates
Alternative cell death pathways: autophagic cell
It has recently become clear that there are other mechan- Engulfment of dying cells and presentation of
isms of programmed cell death distinct from caspase- self-antigens
mediated apoptosis. In certain circumstances, stimulation
of cells in the presence of caspase blockade with agents In order for activation of the adaptive immune response by
that would normally induced apoptosis or other cellular antigens derived from dying cells, efficient presentation
responses results in cell death that has biochemical and and costimulation by antigen-presenting cells is necessary
morphological features resembling autophagy, a process (Fig. 2). Macrophages and particular subsets of dendritic
Figure 2 Influence of apoptotic and other cellular stimuli on activation of antigen-presenting cells. The mode of cell death, kinetics
of uptake and presence of other activation signals determine the cell type that clears dying cells and the immunological outcome.
128 I.C.B. Vioritto et al.
cells are responsible for clearance of dying cells in the peripheral tolerance via by presentation of self-antigen
tissues. Once engulfed, antigens derived from the dead cells (Fig. 2) [15,16]. Additionally, dendritic cell maturation is
are processed and presented at the cell surface associated required for the efficient formation of MHC class II-peptide
with the major histocompatibility complex (MHC). Classical- complexes and trafficking of these complexes to the cell
ly, MHC class II molecules, known to associate with peptides surface in dendritic cells . Nevertheless, peripheral
derived from exogenous antigens, present antigens derived non-mature dendritic cells have been shown to be effective
from engulfed apoptotic cells (reviewed in ). However, at inducing clonal deletion and anergy in autoreactive CD8+
exogenous antigens can also enter the MHC class I presenta- T cells [18,19].
tion pathway for presentation to CD8+ T cells, a process For skin-derived antigens the situation may be different,
called bcross-presentationQ, at which dendritic cells are in that epidermal Langerhans cells were recently shown to
particularly adept . Both classical exogenous antigen present a self-antigen derived peptide and activate naRve
presentation by MHC class II and MHC class I restricted antigen-specific T cells and induce a chronic skin disease
antigen cross-presentation are closely linked to the matura- without any apparent exogenous stimuli . This work
tion of dendritic cells, which requires additional signals suggests that, unlike other DCs, Langerhans cells may
beyond phagocytosis of apoptotic cells. spontaneously mature and initiate immunogenic responses,
Dendritic cell maturation involves changes in cellular and that additional mechanisms must control autoreactiv-
morphology and trafficking of antigens that transform ity to skin-derived self-antigens. Physiologically, tissue
these cells from phagocytes to efficient antigen-presenting macrophages are likely to be the cell type that phagocy-
cells. Immature dendritic cells are present in the circula- tose most apoptotic cells in vivo. After apoptotic cell
tion and a number of organs, and have been shown to be uptake, macrophages produce cytokines such as TGF-B,
over five-fold more efficient than mature DCs at phago- PGE2, and PAF that dampen activation of adaptive immune
cytosing apoptotic cells . Maturation signal(s) repro- cells. Thus, macrophages may actively suppress immune
gram dendritic cells to become highly efficient antigen- responses rather than present antigen in a tolerogenic
presenting cells, a differentiation step that involves manner .
changes in the structure of secretory organelles and Two theories were initially proposed as to how innate
antigen trafficking to increase antigen presentation, immune cells are activated. The first, proposed by the late
upregulation of surface molecules including CD40, CD80, Charles Janeway in 1989 , suggested the existence of
and CD86, as well as MHC class II. Maturation signals also an innate pattern recognition system for common struc-
trigger dendritic cell migration to draining lymph nodes, tures of antigens from bstrangersQ (e.g., bacterial compo-
where antigen presentation to T cells takes place [10—12]. nents). In 1997 the discovery of Toll-like receptors
Mature dendritic cells are uniquely efficient at activating validated Janeway’s theory . At least 13 Toll-like
naRve T cells, making them ideal cells to initiate immune receptors (TLRs) that recognize extracellular bforeignQ
responses to foreign antigens, but also able to activate antigens, such as bacterial cell walls, bacterial DNA motifs,
self-reactive T cells present in the periphery that have and viral DNA and RNA, have been identified in mammals.
escaped negative selection in the thymus. However, as we Examples of this are the activation of DCs via TLR5 in
will discuss, consensus has emerged that antigens from response to bacterial flagellum proteins or TLR3 in response
dying cells are presented in a tolerogenic fashion unless to virally infected cells [24,25]. Importantly, it has been
other signals are provided that cause maturation of found that TLRs can, in some cases, be triggered by cross-
antigen-presenting cells or synthesis of cytokines that reacting host molecules, such as in the case of TLR9-
prime the adaptive immune system. mediated B cell costimulation which can apparently be
triggered by mammalian chromosomal DNA released from
dying cells . A more specific demonstration of the
Immunogenicity versus tolerance to antigens involvement of TLR9 in autoantibody production was made
from dying cells by crossing TLR9À/À mice to a lupus-prone MRL/MRlpr/lpr
background. In the absence of TLR9, but not TLR3, anti-
Since apoptosis occurs physiologically in many tissues, one dsDNA and anti-chromatin antibody production was com-
proposed model for inducing peripheral tolerance is that pletely ablated. However, other lupus-associated autoanti-
immature DCs constantly survey their immunological milieu bodies, such as anti-Smith antigen (anti-Sm), were
through endocytosis and phagocytosis mediated by scaven- maintained or even increased in TLR9À/À mice. Surprisingly,
ger receptors and uptake of apoptotic cell debris. Whether these mice still developed immune complex nephritis,
or not a particular subset of dendritic cells is specialized for suggesting that although anti-dsDNA autoantibodies may
the uptake of apoptotic cells is controversial: in the mouse, predict nephritis in SLE, they are not the exclusive
the CD8+ dlymphoidT subset of dendritic cells has been most pathogenic specificity . TLR7 (or TLR8 in humans),
often implicated [13,14]. Normal recirculation of dendritic which binds ssRNA may regulate production of autoantibo-
cells to the lymph node would then allow antigens to be dies against RNA, which may compensate for the lack of
presented in a tolerogenic fashion. Local tissue antigen dsDNA antibodies in these mice. Another possibility is that
uptake and trafficking by DCs to lymph nodes to present the anti-chromatin antibodies present in TLR9 sufficient
tissue-specific self-antigen under non-immunostimulatory mice may actually be protective by mediating clearance of
conditions without inducing active autoimmunity or full T immune complexes under conditions of high antigenic
cell deletion has been demonstrated in a number of animal burden [28,29].
model systems. This suggests that continual transport of There may also be TLR-independent mechanisms by
apoptotic bselfQ cells to the lymph nodes may relate to which cells can respond to foreign and cross-reactive
Autoimmunity versus tolerance: Can dying cells tip the balance? 129
autoantigens. Three groups recently reported a myeloid cell myocarditis, both CD40, a T-cell-derived activation signal,
activation pathway stimulated by mammalian DNA that is and TLR signals were required .
independent of the TLR adapter molecules MyD88 and TRIF,
but dependent on the adaptor protein IPS-1/VISA/MAVS. Mechanisms of apoptotic cell recognition and
This innate inflammatory response is characterized by
interferon (IFN)-beta induction, as well as TNFA and several
chemokines, and a pattern of gene upregulation unique to A better understanding of how antigens from dying cells may
this novel sensing mechanism [30—32]. Whether TLR-inde- or may not trigger autoimmunity will come from elucidating
pendent sensing of chromatin also plays a role in the the molecular pathways by which phagocytes sense and take
pathogenesis of autoimmunity is not known. up these cells, and how these processes impact the
Matzinger’s group has suggested an alternative concept maturation and antigen-presenting capability of these cells.
of dendritic cell activation in which endogenous ddangerT There has been significant recent progress in identifying the
signals from stressed or dying cells may activate host innate molecules involved in apoptotic cell recognition and uptake.
immune cells in addition to exogenous dstrangerT signals. Most of these systems involve indirect recognition of the PS
Several groups have demonstrated that necrotic cells or on the extracellular surface of the plasma membrane on the
their supernatants can trigger maturation of immunostimu- dying cell through various dbridgeT molecules (Fig. 3).
latory DCs, whereas apoptotic cells did not [10,33]. Opsonization via the classical and lectin complement path-
However, simply pipetting DCs to dissociate cell interac- ways has also been demonstrated to play a role in the
tions or transferring the resting dendritic cells to fresh phagocytosis of apoptotic cells as well as its established role
plates was found to activate dendritic cells, which demon- in clearing many viruses, bacteria and fungi. Strikingly,
strates the difficulties in studying this phenomenon in vitro impairment of many of these pathways in both and genetic
. This suggests that there may be several different human diseases strongly predisposes towards systemic
pathways of triggering the activation of immunostimulatory autoimmunity.
dendritic cells. The molecular identity of endogenous Mer, Tyro3 and Axl are members of a family of receptor
bdangerQ signals has been elusive, because it is often tyrosine kinases that have recently been demonstrated to
unclear whether, for example, a virally infected cell play roles in the regulation of macrophage and NKT cell
secreting an inflammatory cytokine, IFNA is doing so in responses and cytokine production [38,39]. These receptors
response to cellular stress/damage, or because the viral have also been shown to mediate recognition and initiate
RNA has triggered an interferon response through TLRs or phagocytosis of apoptotic cells. Mer mRNA was found to be
other receptors . expressed in innate immune cells (macrophages, DCs, NKs,
Some of the molecules that mediate the bdangerQ signals and NKT cells), but not in B or T cells . Philip Cohen and
may in fact be known triggers of inflammation in other colleagues have demonstrated that Mer is required for
settings. In searching for the identity of factors in necrotic apoptotic cell engulfment in vitro by macrophages, but not
cells that stimulated dendritic cell activation and cross- by dendritic cells. Tyro3 and Axl are not yet as well
presentation to CD8+ T cells, Ken Rock’s group made the understood. Together, these three kinases, along with their
surprising observation that uric acid, a byproduct of nucleic ligands, growth-arrest-specific gene 6 (GAS 6) and protein S,
acid metabolism, can induce dendritic cell maturation. both of which have been shown to associate with PS,
Uric acid significantly increased cytotoxic CD8+ T cell constitute one system of recognition of apoptotic cells.
responses when co-injected with antigen in vivo at CD36 is a scavenger receptor that has been shown to
concentrations high enough to form uric acid crystals, associate with phosphatidylserine (PS) via thrombospondin-1
. Long known as a proinflammatory molecule when (TSP-1), which acts as a bridge molecule between PS on the
crystallized in the joints in gout, uric acid was also apoptotic cell and CD36 on the macrophage. This receptor
recently shown to stimulate the intracellular NLR protein has been suggested to work synergistically with the vitro-
cias1 to activate the dinflammasomeT, a protein complex in nectin receptor, a avh3 integrin, on the surface of macro-
which caspase-1 processes the inflammatory cytokine IL1-h phages to mediate the uptake of apoptotic cells [40—42].
into the mature active form . If a bdangerQ signal was The vitronectin receptor may mediate phagocytosis primar-
all that was necessary to break immune tolerance, one ily in unactivated macrophages whereas CD36 may the major
might expect patients with gout, or those with activating mediator of uptake in activated macrophages. The vitro-
cias1 mutations (associated with a number of familial nectin receptor may also be important in recognition of
autoinflammatory syndromes) to suffer from autoimmunity. apoptotic cells through MFG-E8 (see below).
However, in both these cases the disease remains strictly Milk fat globule-epidermal growth factor (EGF)-factor
confined to symptoms resulting from innate immune 8 (MFG-E8) is another bridge molecule that links PS on the
system activation. In addition, patients with genetic apoptotic cell to the vitronectin receptor. MFG-E8 is a gly-
autoinflammatory diseases in which the inflammasome is coprotein secreted by activated macrophages, especially in
hyperactivated through genetic mutations rarely develop germinal centers of the spleen and lymph nodes, as well as by
autoantibody or lymphocyte-mediated pathology. This immature dendritic cells. MFG-E8 specifically binds to
suggests that while TLR-mediated or other innate signals phosphatidylserine on apoptotic cells, facilitating their
may provide dfuel for the fireT of systemic autoimmunity by engulfment, and plays a critical role in facilitating the
focusing adaptive autoimmune responses on particular clearance of apoptotic B cells in germinal centers [43,44].
antigens, but they seem unlikely to initiate autoimmune C1q and C4 are important early components of the
disease alone. In support of this idea, it was found that to classical pathway of complement and are two of the
activate dendritic cells sufficiently to trigger autoimmune strongest disease susceptibility genes for the development
130 I.C.B. Vioritto et al.
Figure 3 Ligands and receptors involved in apoptotic cell recognition and uptake. The major recognition systems for surface
proteins on dying cells are shown. MBP: Maltose binding protein, MFG-E8: milk fat globule protein E8, PSR: putative
of systemic lupus erythematosus (SLE) known in humans. sequence analysis suggests that the proposed PSR could in
Cell clearance is reported to be dependent on C1q both fact be a transcription factor that promotes upregulation of
through direct recognition of apoptotic cells and activation genes required for phagocytosis of PS+ cells, perhaps
of the classical complement pathway. C4 has also been explaining the original transfection experiments that iden-
suggested as a mediator of apoptotic cell uptake, possibly tified this molecule.
through a different mechanism. Studies suggest that cellular
uptake can be mediated by complement receptors on
macrophages [45,46]. One proposed mechanism of uptake
When uptake goes wrong
via C1q is through interaction with CD91 on the phago-
cyte. Receptors mediating C4-dependent uptake by pha- Under normal circumstances, apoptotic cells are rapidly
gocytes have remained elusive . Mannose-binding cleared by phagocytes . However, if uptake is impaired,
lectin and soluble IgM, which can bind C1q and activate or cell death enhanced, apoptotic material may build up to a
compliment have also been implicated in apoptotic cell point that overwhelms the uptake systems discussed above.
clearance, suggesting the involvement of both the classi- In these circumstances, it has been suggested that self-
cal and mannose-binding lectin branches of the comple- tolerance breaks down due to the priming of autoreactive
ment pathway. lymphocytes by more efficient antigen-presenting cells that
would then take up apoptotic material. If uptake of
apoptotic cells is delayed, secondary necrosis may ensue,
Phosphatidylserine receptor (PSR) resulting in the exposure of proinflammatory intracellular
molecules. Mice deficient in a number of the molecules
PSR was discovered in a screen for genes that conferred PS- necessary for apoptotic cell recognition and uptake manifest
specific binding and uptake of apoptotic cells . It was similar autoimmune phenotypes (Table 2).
proposed to directly recognize PS on apoptotic cells. Triple knockout mice for the related Tyro3/Axl/Mer
However, from gene knockout studies in mice it has become receptors have profound defects in apoptotic cell clearance,
clear that PSR may act more indirectly. PSRÀ/À mice have blindness (due to failure of retinal epithelial cells to clear
defects in mammalian organogenesis, erythropoiesis and T shed outer segment layers resulting in the death of
lymphopoiesis, and some groups reported impaired clear- photoreceptor cells), and neurological abnormalities. Addi-
ance of apoptotic cells in embryonic liver and thymus. tionally, these mice develop splenomegaly and spontaneous
However, this did not induce the upregulation of inflamma- antibodies against nuclear antigens, swollen joints with
tory cytokines [49—51]. Rather than being a cell surface lymphocyte infiltration, as well as IgG deposits in the kidney
receptor, PSR appears to be a nuclear protein, and may not leading to glomerulonephritis, a common complication of
have a bona fide transmembrane domain . Further SLE . Macrophages from merÀ/À mice were reported to
Autoimmunity versus tolerance: Can dying cells tip the balance? 131
Table 2 Phenotypes of mice deficient in apoptotic cell uptake
Molecule Phenotype of knockout mouse
C1q/C4 Systemic autoimmunity with autoantibodies and glomerulonephritis
Increased numbers of apoptotic cells in glomeruli (C1q only)
Delayed clearance of apoptotic cells from the peritoneal cavity
IgM Autoimmunity to nuclear Ags; renal IgG and complement deposition
Mer/Tyro3/Axl and Mer (single KO) Autoimmunity to nuclear Ags; renal IgG and complement deposition
Defect in clearance of apoptotic cells in the thymus and spleen
MFG-E8 Glomerulonephritis, splenomegaly, autoantibodies,
Numerous germinal centers with increased apoptotic B cells
PSR (phosphatidylserine receptor) Abnormal development and neonatal lethality
Accumulation of apoptotic cells in brain and lungs
DNaseI Autoimmunity to chromatin; renal IgG and complement deposition
be ~90% less efficient in phagocytosis of apoptotic cells suggesting that CYP1B1 may have an unexpected role in
compared to macrophages from wild-type mice  and also macrophage phagocytic function that is important for
develop elevated levels of antinuclear. However, the genesis maintenance of immunological self-tolerance . Macro-
of autoimmunity in these mice may also be linked to phages from patients with systemic lupus erythematosus
excessive dendritic cell activation by Toll-like receptors, have also been found to have phagocytic defects [58—60],
as MerÀ/À macrophages have increased expression of although in these human studies, it is difficult to determine
costimulatory surface molecules and increased production if this is a cause or consequence of the disease.
of proinflammatory cytokines such as TNFA in response to Another important mouse model for systemic lupus
LPS. erythematosus (SLE) is the C1q-deficient mouse. Homozy-
MFG-E8À/À mice develop splenomegaly with the forma- gous deficiency in C1q is associated with severe SLE in
tion of numerous germinal centers, and suffer from humans. C1q-deficient mice have a defect in phagocytic
glomerulonephritis due to autoantibody production. Macro- uptake of apoptotic cells by activated macrophages as well
phages from the MFG-E8-deficient mouse demonstrated a as resident peritoneal macrophages. C1q-deficient mice
four-fold decrease in their efficiency of apoptotic cell develop glomerulonephritis with an excess of glomerular
uptake compared to wild-type macrophages. However, apoptotic bodies. Mice deficient in C4, another member of
engulfment of microspheres was not affected in the the classical complement pathway, show a less severe
MFG-E8À/À mice. This mouse model suggests that MFG-E8 disease phenotype than C1qÀ/À mice, and no impairment
has a critical role in clearing apoptotic B cells from of resident peritoneal macrophage phagocytic uptake. C3-
germinal centers and failure to remove these cells can deficient mice do not develop autoimmunity, suggesting a
lead to autoimmunity. Notably, it was found that MFG-E8 hierarchical role for classical compliment proteins in
is expressed in immature DCs such as Langerhans cells but apoptotic cell clearance . Similarly, patients with C4
not in thymic macrophages and thus not involved in deficiencies generally have less severe SLE, and those with
clearance of apoptotic thymocytes, suggesting that differ- C3 and C2 deficiencies are often non-symptomatic. Addi-
ent tissues may use different recognition systems for the tionally, low levels of mannose-binding lectin (which plays
uptake of apoptotic cells . This group has also shown the same role as C1q in the lectin pathway) have also been
that MFG-E8 carrying a mutation in the second EGF associated with SLE. In conjunction with these findings,
domain, designated D89E, inhibited the phagocytosis of deficiencies in molecules that can bind C1q and activate the
apoptotic cells both in vitro and in vivo by a variety of classical pathway, such as soluble IgM, have also been linked
phagocytes. This mutation masks PS on apoptotic cells. to increased incidence of autoimmunity in humans and
Injection of mutated MFG-E8 into mice induced autoanti- mouse models .
body production. This effect was exacerbated by co- Supporting the hypothesis that ordered degradation of
injecting apoptotic thymocytes. The authors suggest that DNA reduces its immunogenicity, mice deficient in DNAse I
the perturbation of apoptotic cell clearance and the also develop systemic autoimmunity. Additionally, data
consistent presentation of self-antigens may be enough implicating apoptotic cell debris as immunogens in SLE have
to disrupt peripheral immune tolerance . come from DNAse I-deficient mice. DNAse IÀ/À mice develop
Mice deficient in even seemingly unrelated molecules can anti-chromatin antibodies similar to those in human SLE
develop autoimmunity if apoptotic cell uptake is impaired. patients, as well as glomerulonephritis due to the deposition
For example, cytochrome P450 1B1 (CYP1B1) enzyme- of immune complexes in glomeruli . Two human SLE
deficient mice were found to have evidence of systemic patients were also reported to have heterozygous nonsense
autoimmunity, manifested by antinuclear and anti-DNA mutations in exon 2 of the gene encoding DNAse I .
antibodies as well as immune complex deposition in the DNAse I acts together with C1q to efficiently degrade
kidney and glomerulonephritis. CYP1B1 is highly expressed necrotic cell-derived chromatin . A significant reduction
in the mononuclear phagocyte lineage. Peritoneal macro- of DNAse I activity in sera from SLE patients and patients
phages from CYP1B1-deficient mice were impaired in the with rheumatoid arthritis (RA) compared with sera from
phagocytosis of apoptotic, necrotic and opsonized cells, healthy donors has been observed. However, only SLE sera
132 I.C.B. Vioritto et al.
showed a strongly reduced capacity to degrade necrotic tolerance to autoimmunity so much as how the dying cell is
cell-derived chromatin . Administration of DNAse has perceived; through beat meQ signals from the state of the
been studied as a therapeutic modality in patients with SLE, dying cell, the signals and debris it releases, and how long it
and was well tolerated, but did not produce clinical persists before phagocytosis occurs.
improvement in a small phase I study . Unlike other
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