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Eur. J. Immunol. 2016. 46: 269–280 HIGHLIGHTS
DOI: 10.1002/eji.201545839 269
Review
AIM2 inflammasome in infection, cancer,
and autoimmunity: Role in DNA sensing, inflammation,
and innate immunity
Si Ming Man, Rajendra Karki and Thirumala-Devi Kanneganti
Department of Immunology, St. Jude Children’s Research Hospital, Memphis, TN, USA
Recognition of DNA by the cell is an important immunological signature that marks
the initiation of an innate immune response. AIM2 is a cytoplasmic sensor that rec-
ognizes dsDNA of microbial or host origin. Upon binding to DNA, AIM2 assembles a
multiprotein complex called the inflammasome, which drives pyroptosis and proteolytic
cleavage of the proinflammatory cytokines pro-IL-1β and pro-IL-18. Release of microbial
DNA into the cytoplasm during infection by Francisella, Listeria, Mycobacterium, mouse
cytomegalovirus, vaccinia virus, Aspergillus, and Plasmodium species leads to activation
of the AIM2 inflammasome. In contrast, inappropriate recognition of cytoplasmic self-
DNA by AIM2 contributes to the development of psoriasis, dermatitis, arthritis, and other
autoimmune and inflammatory diseases. Inflammasome-independent functions of AIM2
have also been described, including the regulation of the intestinal stem cell proliferation
and the gut microbiota ecology in the control of colorectal cancer. In this review we pro-
vide an overview of the latest research on AIM2 inflammasome and its role in infection,
cancer, and autoimmunity.
Keywords: AIM2 inflammasome
r
Autoimmunity
r
Bacterial/viral infection
r
Cancer
r
DNA sensing
r
Gut microbiota
Introduction
DNA recognition by innate immune receptors triggers a myriad of
immunological responses that are both beneficial and detrimen-
tal to the host. The discovery of Toll-like receptor 9 (TLR9) as a
membrane-associated sensor of bacterial CpG DNA provides evi-
dence for the existence of host receptors that specifically mediate
immune responses to DNA [1]. Translocation of microbial or mam-
malian DNA into the cytoplasm of host cells further induces tran-
scription of genes encoding type I interferon (IFN) molecules and
inflammation independently of TLR9 [2, 3]. Recent advances in
the field h ave identified multiple cytoplasmic DNA sensors that are
responsible for transcriptional activity, including cyclic-GMP-AMP
synthase (cGAS), STING, DDX41, Ku70, LRRFIP1, DNA-dependent
activator of IFN-regulatory factors (IRFs; DAI, also known as
Correspondence: Dr. Thirumala-Devi Kanneganti
e-mail: Thirumala-Devi.Kanneganti@STJUDE.ORG
ZBP1), and IFI16 (reviewed elsewhere [4–6]). Of these DNA sen-
sors, cGAS binds to double-stranded DNA (dsDNA), resulting in a
conformational change in cGAS that allows it to convert ATP and
GTP to a cyclic dinucleotide cyclic-GMP-AMP [7]. Cyclic-GMP-
AMP then binds and activates STING to induce transcription of
genes encoding type I IFN and proinflammatory cytokines via the
transcription factors IRF3 and NF-κB, respectively, (reviewed in
[7]). The molecular basis underlying recognition of DNA by the
other aforementioned cytoplasmic DNA sensors is less understood.
DNA introduced into the cytoplasm also induces IL-1β secre-
tion and pyroptosis [8, 9], and these responses are dependent on
the activity of a cytoplasmic caspase-1-containing complex known
as the inflammasome [10]. In 2009, four groups independently
identified AIM2 as the sensor that triggers inflammasome acti-
vation, pyroptosis, and release of IL-1β and IL-18 in response to
intracellularly delivered dsDNA [11–14].
AIM2 consists of a C-terminal HIN-200 domain, which binds
directly to dsDNA, and an N-terminal pyrin domain (PYD), which
interacts with the PYD of the bipartite PYD-CARD-containing
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270 Si Ming Man et al. Eur. J. Immunol. 2016. 46: 269–280
Figure 1. The molecular basis for the activation of the AIM2 inflam-
masome. The DNA sensor AIM2 is composed of an N-terminal pyrin
domain and a C-terminal HIN-200 domain. The pyrin and HIN-200
domain of AIM2 form an intramolecular complex and are maintained in
an autoinhibitory state. Cytoplasmic dsDNA induces activation of AIM2.
The HIN-200 domain interacts with dsDNA in a sequence-independent
manner, by binding to the sugar-phosphate backbone of dsDNA. The
pyrin domain of AIM2 binds to the pyrin domain of ASC. CARD of ASC
binds the CARD of procaspase-1, forming a macromolecular complex
known as the AIM2 inflammasome. Activated caspase-1 drives cleavage
of pro-IL-1β and pro-IL-18. Caspase-1 also cleaves the substrate gasder-
min D. The N-terminal fragment of gasdermin D induces pyroptosis,
allowing mature IL-1β and IL-18 to be released from the cell.
inflammasome adaptor protein ASC (apoptosis-associated speck-
like protein containing a carboxy-terminal CARD) [10]. The CARD
of ASC binds the CARD of procaspase-1, forming a macromolec-
ular complex fulfilling the basic structural elements of an inflam-
masome (Fig. 1) [10]. AIM2, IFI16, and other pyrin and HIN
domain-containing proteins form the AIM2-like receptor family
[15–17].
AIM2 recognizes dsDNA in a sequence-independent manner;
however, the DNA sequence must be at least 80 base pairs in length
[18]. Elucidation of the crystal structure of AIM2 provided insights
into the activation mechanism of this DNA-sensing inflammasome.
The PYD and HIN-200 domain of AIM2 form an intramolecular
complex and are maintained in an autoinhibitory state during
homeostasis (Fig. 1) [18, 19]. Binding of dsDNA to the HIN-200
domain displaces PYD from the intramolecular complex, liberating
PYD for interaction with ASC [18]. The sugar-phosphate backbone
of dsDNA interacts with the positively charged HIN-200 domain
via electrostatic attraction, allowing sequence-independent recog-
nition of DNA by AIM2. The PYD of AIM2 can also self-oligomerize
to induce the activation of the AIM2 inflammasome [20, 21]. Acti-
vation of AIM2 or a related inflammasome sensor NLRP3 initiates
polymerization of the PYD of ASC [22, 23], ultimately forming
a single and visually distinct inflammasome speck that is readily
observed in primary macrophages and DCs [24–29]. Recent work
has even suggested that the role of PYD of AIM2 is not to maintain
autoinhibition, but to oligomerize and drive filament formation
[30].
Activation of the AIM2 inflammasome and other canonical
inflammasomes results in a type of inflammatory cell death called
pyroptosis [10], which is mediated, in part, by the inflamma-
tory caspase substrate gasdermin D [31, 32] (Fig. 1). Activa-
tion of the AIM2 inflammasome is tightly regulated by the cell
and requires phosphorylation and linear ubiquitination of ASC
[33, 34]. Autophagy can mediate degradation of the AIM2 inflam-
masome to terminate inflammatory responses [35]. Furthermore,
several proteins produced in the cell have the ability to inhibit acti-
vation of the AIM2 inflammasome [14, 36–43]. These include the
PYD-containing proteins POP1 [39] and POP3 [36] in human cells
and the bipartite protein containing two HIN domains, p202, in
mouse cells [14, 40–43]. Here, we summarize the latest research
on the regulation of AIM2 inflammasome, and its role in pathogen
recognition after infection, cancer, and autoimmunity.
AIM2 in the response to infection
Bacterial infection
During infection of a host cell, microbial DNA and other microbe-
associated molecular patterns are released into the cytoplasm,
where they are recognized by cytoplasmic DNA sensors, i.e. cGAS,
STING, or AIM2. AIM2 resides in the cytoplasm and has been
shown to provide immunosurveillance to the pathogenic bacte-
ria Francisella tularensis Live Vaccine Strain (LVS), F. tularensis
subspecies novicida (F. novicida), Listeria monocytogenes, Strep-
tococcus pneumonia, Mycobacterium species, Porphyromonas gingi-
valis, Staphylococcus aureus, Brucella abortus, and Chlamydia muri-
darum (Table 1) [26, 44–61]. F. novicida and F. tularensis LVS are
the only bacterial pathogens known to exclusively activate the
AIM2 inflammasome in mouse macrophages and DCs, whereas
other bacteria have been shown to activate more than one inflam-
masome sensor in mouse and human cells [44–46]. In the human
THP-1 macrophage-like cell line, both AIM2 and NLRP3 contribute
to the activation of the inflammasome in response to F. novicida
and F. tularensis LVS [48]. Interestingly, caspase-8 is recruited to
the AIM2 inflammasome to drive apoptosis in Francisella-infected
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Table 1. Bacteria recognized by the AIM2 inflammasome
Bacteria Cells Mice
Francisella tularensis LVS
or F. tularensis subspecies
novicida (F. novicida)
r
Reduced caspase-1 activation, IL-1β
and IL-18, and pyroptosis in Aim2
−/−
mouse BMDMs or BMDCs
[26–28, 44–47].
r
Reduced IL-1β in shRNA-silenced
Aim2 THP1 human monocytic cell line
[48].
r
Increased overall susceptibility,
reduced serum IL-18 levels 1d p.i. and
increased bacterial burden 3d p.i.
[27, 44, 45].
Listeria monocytogenes
r
Reduced caspase-1 cleavage, IL-1β
release, and pyroptosis in
Aim2-silenced mouse BMDMs [49–52].
r
N/A
Streptococcus pneumoniae
r
Reduced caspase-1 cleavage, IL-1β
and IL-18 release, and pyroptosis in
Aim2-silenced peritoneal mouse
macrophages [53].
r
N/A
Mycobacterium tuberculosis
r
Reduced IL-1β and IL-18 release in
Aim2
−/−
and Aim2-silenced THP-1
human monocytic cell line [54, 55].
r
Increased overall susceptibility,
increased bacterial burden in the
lungs and liver 4 weeks p.i. [56].
r
Reduced caspase-1 cleavage and
increased infiltration of inflammatory
cells in lungs [56].
r
Reduced IL-1β in BALF and IL-18 in
serum at 3 weeks p.i. [56].
r
Reduced IFN-γ production by CD4
+
T cells [56].
Mycobacterium bovis
r
Reduced caspase-1 cleavage, IL-1β
release, and pyroptosis in Aim2
-silenced mouse macrophage J774A.1
[57].
r
N/A
Porphyromonas gingivalis
r
Reduced caspase-1 cleavage, IL-1β,
and pyroptosis in siRNA-silenced
Aim2 THP-1 human monocytic cell
line [58].
r
N/A
Legionella pneumophila
SdhA
r
Reduced caspase-1 cleavage and IL-1β
release in Aim2
−/−
mouse BMDMs [78].
r
N/A
Staphylococcus aureus
r
N/A
r
Increased susceptibility upon
intracranial infection [59].
r
Reduced IL-1β, IL-6, CCL2, and CXCL10
in wound abscess [59].
Brucella abortus
r
Reduced caspase-1 cleavage, IL-1β,
and cell death in Aim2
−/−
mouse
BMDMs [60].
r
Increased bacterial burden 4 weeks p.i
[60].
Chlamydia muridarum
r
Reduced IL-1β and IL-18 in Aim2
−/−
mouse BMDMs [61].
r
N/A
BALF: bronchoalveolar lavage fluid; BMDCs: bone marrow-derived dendritic cells; BMDMs: bone marrow-derived macrophages; N/A: information
not available; p.i.: postinfection.
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272 Si Ming Man et al. Eur. J. Immunol. 2016. 46: 269–280
Figure 2. Regulation of the activation of the AIM2 inflammasome. The AIM2 inflammasome is activated by a number of microbial pathogens
and dsDNA ligands, including the DNA virus MCMV, the cytosolic bacterium F. novicida, and the dsDNA ligand poly(dA:dT). MCMV infection or
transfection of poly(dA:dT) leads to “canonical” activation of the AIM2 inflammasome, which does not require the type I IFN pathway. F. novicida
infection activates the AIM2 inflammasome via a “noncanonical” pathway owing to its requirement for type I IFN, analogous to the non-canonical
NLRP3 inflammasome pathway. Intracellular F. novicida releases DNA into the cytoplasm to activate the DNA sensors cGAS, STING, and IFI204,
which drive transcription of genes encoding type I IFN molecules. It remains unclear why the released DNA is unable to activate AIM2 at this stage,
since AIM2 is constitutively expressed in the cell. Type I IFN provides a feedback loop to induce expression of the transcription factor IRF1, which
upregulates expression of the IFN-inducible GTPases, including GBP2 and GBP5. GBP2 and GBP5 are recruited to bacterial structures, however,
whether they directly target the bacterial membrane or the membrane of intact Francisella-containing vacuole is unclear. Nevertheless, GBPs
mediate bacterial killing, resulting in abundant release of bacterial DNA for recognition by AIM2. Assembly of the AIM2 inflammasome induces
caspase-1-dependent cleavage of pro-IL-1β and pro-IL-18. Caspase-1 also drives cleavage of the substrate gasdermin D to induce pyroptosis.
cells in the absence of caspase-1 [47, 62], suggesting a complex
interplay between members of the caspase family.
Activation of the AIM2 inflammasome by F. novicida and
F. tularensis LVS requires the ability of the bacteria to escape the
vacuole into the host cytoplasm, a process mediated by a range
of bacterial virulence factors, including the transcriptional regu-
lator MglA and proteins encoded by the Francisella-pathogenicity
island [26–28, 63, 64]. The Francisella-pathogenicity island is a
genomic region that contains a cluster of 16–19 genes encoding
virulence factors of the bacterium [65]. Similarly, L. monocyto-
genes must escape the vacuole and undergo bacteriolysis in order
to induce the activation of the AIM2 inflammasome [49–52, 66].
Type I IFN potentiates the activity of the AIM2 inflammasome dur-
ing bacterial infection [26–28, 67, 68]. In response to F. novicida
infection, the DNA sensors cGAS, IFI204, and STING cooperate
to detect small amounts of DNA released by the bacteria to drive
production of type I IFN in mouse macrophages [27, 46, 69]. Type
I IFN is then released to the outside of the cell, where it binds to
the type I IFN receptor (IFNR) in an autocrine manner, activat-
ing the IFN-stimulated gene factor 3 [70]. IFN-stimulated gene
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Eur. J. Immunol. 2016. 46: 269–280 HIGHLIGHTS 273
factor 3 is comprised of the transcription factors STAT1, STAT2,
and IRF9, which drives the transcription of IFN-stimulated genes
(ISGs) [70]. A study has now demonstrated that, during infection
with F. novicida, signaling via type I IFN induces the expression
of the transcription factor IRF1, where IRF1 further drives expres-
sion of IFN-inducible GTPases called guanylate-binding proteins
(GBPs) [27]. Induction of both IRF1 and GBPs are necessary to
fully engage the AIM2 inflammasome by F. novicida infection
(Fig. 2) [27, 28].
GBPs are clustered over two locations in the genome of mice.
Genes encoding GBP1, GBP2, GBP3, GBP5, and GBP7 are located
on chromosome 3, whereas genes encoding GBP4, GBP6, GBP8,
GBP9, GBP10, and GBP11 are found on chromosome 5 [71]. Of
these, GBP2 and GBP5 are recruited to cytoplasmic F. novicida
bacteria to drive bacterial killing, exposing abundant amounts of
bacterial DNA for detection by AIM2 [27, 28]. GBP2 and GBP5
function in a nonredundant manner, and reconstitution of either
GBP in type I IFN receptor 1-deficient macrophages cannot rescue
inflammasome activation, indicating that type I IFN signaling acti-
vates GBPs, possibly via expression other IFN-inducible proteins
[27, 28]. Overall, mice lacking AIM2, caspase-1, IRF1, or GBPs
have been shown to secrete reduced levels of IL-18 in response
to F. novicida infection and are all hypersusceptible to F. novicida
infection compared with wild-type mice [27, 28, 44–46]. In agree-
ment with this observation, antibody-mediated neutralization of
IL-1β and IL-18 in WT mice increases susceptibility to F. novicida
infection [64].
The precise mechanism of bacterial killing mediated by GBPs
remains unknown. Recent studies have shown that the antimicro-
bial activity of the gp91 subunit of NADPH oxidase (also known
as NOX2) and inducible nitric oxide synthase are not required for
mediating activation of the AIM2 inflammasome [28, 72]. How-
ever, the pharmacological inhibition of reactive oxygen species
(ROS) and mitochondrial ROS partially reduces caspase-1 acti-
vation, and therefore, the release of IL-1β driven by F. novicida
infection [28, 72]. ROS inhibition impairs the expression of IL-1β
and TNF, arguing that further evidence is required to convinc-
ingly link ROS-mediated bacterial killing and activation of the
AIM2 inflammasome [73]. It also remains a mystery as to why
DNA molecules that are released to activate cGAS, STING, and
IFI204 are unable to activate AIM2 at this stage. One possibility is
that the concentration of DNA that is sufficient to activate cGAS,
STING, and IFI204 is lower than the concentration required to
activate AIM2. AIM2 is constitutively expressed in the cell, but its
expression is also induced by IFN [11, 37, 38]. However, trans-
fection of dsDNA into the cytoplasm can directly activate AIM2
independently of IFN [27, 28, 73], ruling out a requirement for
“priming” in activation of the AIM2 inflammasome.
Bacteria have evolved virulence determinants to prevent
release of DNA and other bacterial ligands and avoid cytoplasmic
detection and clearance by inflammasomes. Francisella tularensis
subspecies tularensis SchuS4 have been shown to induce low levels
of inflammasome activation and IL-1β secretion in primary mouse
bone marrow-derived macrophages, possibly due to enhanced
resistance to H
2
O
2
to protect itself from bacteriolysis, or other
virulence factors that confer evasion of the immune system [72].
The putative lipid II flippase, MviN, and RipA, a protein used for
intracellular replication, of F. tularensis LVS are both required to
dampen AIM2 inflammasome responses [74, 75]. Further studies
have shown that F. tularensis LVS or F. novicida mutants lack-
ing MviN, RipA, and several membrane-associated proteins or
proteins involved in O-antigen or LPS biosynthesis are hypersus-
ceptible to intracellular lysis and DNA release in macrophages,
providing a rationale for why these mutants induce elevated
activation of the AIM2 inflammasome [63]. Genes encoding the
5-formyltetrahydrofolate cycloligase within the folate metabolic
pathway and pseudouridine synthase in F. tularensis LVS have also
been demonstrated to influence the magnitude of AIM2 inflam-
masome activation [76]. A more recent study identified a clus-
tered, regularly interspaced, short palindromic repeats-CRISPR
associated (CRISPR-Cas) system used by F. novicida to strengthen
the integrity of its bacterial membrane, leading to reduced DNA
release in the cytoplasm [77]. Another example is found in
Legionella pneumophila, which encodes an effector protein SdhA,
shown to prevent rupture of the Legionella-containing vacuole and
thereby minimizing the amount of bacterial DNA released into the
cytoplasm [78].
There is limited evidence so far to support the existence of
mechanisms used by bacteria to directly inhibit or evade activation
of the AIM2 inflammasome. However, F. tularensis LVS and the
virulent SchuS4 strain do suppress TLR2-dependent responses to
reduce the level of pro-IL-1β available for cleavage by the inflam-
masome [79]. Overall, the AIM2 inflammasome is an effective
antimicrobial machinery against certain bacterial pathogens.
Viral infection
Inflammasome responses play an essential role in the host pro-
tection against viral infection [80, 81]. Genetic materials from
DNA viruses that enter the cytoplasm can be detected by AIM2,
for instance mouse cytomegalovirus (MCMV), vaccinia virus, and
human papillomaviruses (Table 2) [11, 44, 82]. MCMV and vac-
cinia viruses robustly induce inflammasome responses in mouse
macrophages in an AIM2-dependent manner (Fig. 2) [11, 27, 44].
Further, Aim2
−/−
mice infected with MCMV have an impaired abil-
ity to secrete IL-18, carry a higher viral titre, and have reduced lev-
els of IFN-γ-producing NK cells compared with infected WT mice
[44]. Human papillomaviruses have also been shown to drive IL-1β
and IL-18 release in human keratinocytes in an AIM2-dependent
manner [82].
To date, there is no strong evidence in the literature to indi-
cate that DNA viruses other than MCMV and vaccinia virus
activate the AIM2 inflammasome. A recent study suggests that
in the human glomerular mesangial cell line infected with
hepatitis B virus, siRNA-mediated silencing of the gene encoding
AIM2 leads to a reduced expression of IL-1β, IL-18, and caspase-
1 [83]. Whether AIM2 directly mediates the recognition of viral
DNA derived from hepatitis B virus in immune or nonimmune cells
has not been established. Comparison of the expression of AIM2 in
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274 Si Ming Man et al. Eur. J. Immunol. 2016. 46: 269–280
Table 2. Viruses recognized by the AIM2 inflammasome
Viruses Cells Mice
MCMV (DNA virus)
r
Reduced caspase-1 cleavage
and IL-1β release in Aim2
−/−
mouse peritoneal
macrophages BMDMs and
BMDCs [27, 44].
r
Increased viral titers 36 h p.i. [44].
r
Reduced serum IL-18 level 36 h p.i. [44].
r
Reduced IFN-γ production [44].
Vaccinia virus (DNA virus)
r
Reduced caspase-1 cleavage
and IL-1β release in Aim2
−/−
mouse peritoneal
macrophages and BMDCs [44].
r
N/A
Human papillomaviruses (DNA virus)
r
Reduced IL-1β and IL-18
release in Aim2-silenced
human keratinocytes [82].
r
N/A
Hepatitis B virus (DNA virus)
r
Reduced gene expression of
IL-1β, IL-18, and caspase-1 in
Aim2-silenced human
glomerular mesangial cell line
[83].
r
N/A
Chikungunya virus (RNA virus)
r
Reduced IL-1β release in
Aim2-silenced human primary
dermal fibroblasts [92].
r
N/A
West Nile virus (RNA virus)
r
Reduced IL-1β release in
Aim2-silenced primary human
dermal fibroblasts [92].
r
N/A
BMDCs: bone marrow-derived dendritic cells; BMDMs: bone marrow-derived macrophages; N/A: information not available; p.i.: postinfection.
patients with acute and chronic hepatitis B revealed that those at
the acute stage expressed higher levels of AIM2 in peripheral blood
mononuclear cells [84]. A subsequent study reported that 89.4%
of the liver tissues collected from individuals with chronic hepatitis
B virus infection were positive for AIM2 expression by immuno-
histochemistry, compared with only 8.7% of those with chronic
hepatitis C infection [85]. Expression of the gene encoding AIM2
has been reported to be significantly higher in kidney tissues of
patients with hepatitis B virus-associated glomerulonephritis com-
pared with patients with chronic glomerulonephritis [83].
In all cases, it is probable that viral DNA binds directly to
AIM2 to trigger inflammasome activation, but the precise molecu-
lar mechanism that leads to exposure of the viral DNA for sensing
by AIM2 is not entirely clear. Unlike F. novicida infection, type I
IFN signaling, IRF1, and GBPs are dispensable for the activation of
the AIM2 inflammasome by either MCMV infection or transfected
dsDNA [27]. However, AIM2 does not respond to all DNA viruses.
For example, adenovirus and herpesviruses HSV-1 and MHV-68
activate the NLRP3 or putative IFI16 inflammasome, rather than
the AIM2 inflammasome in mouse BM-derived macrophages or
mouse thioglycollate-elicited macrophages [8, 44, 80, 81]. During
HSV-1 infection of human macrophages, the capsid that encapsu-
lates the viral DNA is degraded by the proteasome, which releases
DNA for recognition by IFI16 in the cytoplasm [86], suggesting
that the inability of AIM2 to sense viral DNA is probably not due
to a lack of viral DNA release in the cytoplasm. It might be possi-
ble that certain DNA viruses can strategically inhibit the ability of
AIM2 to interact with their DNA. Alternatively, the DNA of certain
viruses, such as Hepatitis B virus and other Hepadnaviruses, could
be transcribed into RNA templates, which may serve as activators
for NLRP3 [87–89]. Indeed, a precedent exists for indirect sensing
of DNA by the RNA sensor RIG-I [90, 91]. RNA polymerase III tran-
scribes AT-rich DNA into dsRNA transcripts carrying an uncapped
5
-triphosphate moiety, which has been shown to activate RIG-I
[90, 91]. Conversely, a study has reported a role for AIM2 in driv-
ing IL-1β secretion in response to the RNA viruses [92]. Silencing
of genes encoding AIM2 and caspase-1 reduces proteolytic cleav-
age and release of IL-1β in human dermal fibroblasts infected with
the RNA viruses Chikungunya virus or West Nile virus [92]. How
AIM2 might sense RNA viruses is still unclear, and further charac-
terization of the molecular mechanism involved in the activation
of the AIM2 inflammasome by viruses is required.
Other pathogens
In addition to bacteria and viruses, AIM2 has been shown to
mediate pathogen recognition of, and host defense to, the fun-
gal pathogen Aspergillus fumigatus and the protozoan Plasmodium
berghei (Table 3) [93, 94]. AIM2 and NLRP3 function in a redun-
dant fashion to confer inflammasome activation against A. fumi-
gatus infection in mouse BM-derived DCs and mice [93] Further-
more, mice lacking both AIM2 and NLRP3, ASC or caspase-1, and
infected with A. fumigatus, are more susceptible than infected
WT mice [93]. The requirement for dual sensing of pathogens
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Table 3. Fungi and parasites recognized by the AIM2 inflammasome
Cells Mice
Fungi
Aspergillus fumigatus Slight reduction in IL-1β and IL-18
in Aim2
−/−
mouse BMDCs owing
to redundant roles with NLRP3
[93].
Not susceptible owing to
redundant roles with NLRP3 [93].
Protozoa
Plasmodium berghei Slight reduction in IL-1β and
pyroptosis in Aim2
−/−
mouse
BMDMs infected with iRBCs,
synthetic and natural hemozoin
owing to redundant roles with
NLRP3 [94].
Decreased neutrophils recruitment
in peritoneal cavity owing to
redundant roles with NLRP3,
after 15 h p.i. [94].
BMDCs: bone marrow-derived dendritic cells; iRBCs: infected red blood cells.
by both AIM2 and NLRP3 has also been observed in mouse
BM-derived macrophages stimulated with Plasmodium berghei-
infected red blood cells or synthetic and natural hemozoins [94].
AIM2 also directly recognizes A. fumigatus genomic DNA intro-
duced into the cytoplasm by a transfection agent [93] or P. falci-
parum genomic DNA transported into the cytoplasm by hemozoins
[94]. It would be interesting to identify additional pathogens that
can activate the AIM2 inflammasome.
AIM2 role in cancer biology
AIM2 has also been shown to suppress the development of can-
cer [95, 96]. The gene encoding AIM2 was originally isolated
from human melanoma cells [97]. Reduced expression and fre-
quent frameshift microsatellite instability of the AIM2 have been
observed in tumor tissues from patients with colorectal cancer
[98–101]. Colorectal cancer patients whose tissues have reduced
AIM2 expression have a poorer prognosis compared with those
with a higher level of AIM2 expression [98]. Reduced expression
of AIM2 has also been reported in prostate cancer [102], whereas
increased expression has been detected in nasopharyngeal carci-
noma tumors [103, 104], oral squamous cell carcinoma [105],
and lung adenocarcinoma [106]. The differential expression of
AIM2 in a range of tumor tissues suggests that it may have unique
roles in different types of cancer.
The mechanism for AIM2 in the regulation of tumorigenesis
has been described in a mouse model of colitis-associated col-
orectal cancer [95, 96]. Two groups have recently demonstrated
that AIM2 operates independently of the inflammasome to prevent
colorectal cancer [95, 96]. Both studies found that Aim2
−/−
mice
developed severe colitis, polyps, and higher tumor burden upon
administration of AOM and DSS [95, 96]. Although differential
production of the major inflammatory mediators, including TNF
and IL-6, was not observed between WT and Aim2
−/−
mice, pro-
liferation of enterocytes was more pronounced in Aim2
−/−
mice
[95, 96]. Indeed, murine fibroblasts and colon cancer cell lines
expressing an AIM2-encoding construct have been shown to have
an impaired ability to undergo proliferation [40, 107]. Overex-
pression of AIM2 in colon cancer cell lines also induces cell cycle
arrest, and the transformed cells exhibit a delayed transition from
the late S-phase to the G2/M phase [107], suggesting that AIM2
plays a proliferation-inhibitory role in these cancer cell lines.
Furthermore, in another mouse model of intestinal cancer,
Aim2
−/−
mice carrying aberrant activating β-catenin mutations
failed to prevent the expansion of cancer-associated stem cells
in the small and large intestine [95]. Similarly, Aim2
−/−
mice
harboring the heterozygous mutation in the adenomatous poly-
posis coli gene developed more tumors than Aim2
+/+
mice car-
rying the mutant adenomatous polyposis coli gene [96]. Intesti-
nal stem cells lacking AIM2 proliferated more than WT intestinal
stem cells in organoid culture, and this proliferation was associ-
ated with increased activation of the kinase AKT [95, 96]. Wilson
and colleagues [108, 109] identified DNA-PK, a kinase that can
phosphorylate and activate AKT, as a binding partner of AIM2,
whereby AIM2 suppresses the activation of DNA-PK- and DNA-PK-
dependent phosphorylation of AKT at the serine residue 473 [96].
Indeed, treatment of Aim2
−/−
mice with an AKT inhibitor reduced
tumor burden in Aim2
−/−
mice with colitis [96].
Our laboratory performed further analysis and found that
Aim2
−/−
mice harbored a microbial ecology different to that of
WT mice [95]. Reciprocal exchange of the microbiota between
Aim2
−/−
and WT mice by means of cohousing substantially
reduced tumorigenesis in Aim2
−/−
mice and increased tumorige-
nesis in WT mice [95]. Collectively, these studies provide insights
into the function of AIM2 in colorectal cancer, and highlight that
potential therapies that inhibit the AKT pathway can be further
investigated for treatment of cancer associated with AIM2 muta-
tions [40, 95, 96, 107].
AIM2 in inflammatory, autoimmune,
and other pathological conditions
Given that the host DNA is normally sequestered in the nucleus or
mitochondria, the accumulation of host DNA in the cytosol, due to
impaired degradation or clearance or excess uptake of extracellu-
lar DNA from dying neighboring cells, could induce inflammation.
C
2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eji-journal.eu
276 Si Ming Man et al. Eur. J. Immunol. 2016. 46: 269–280
For instance, accumulated DNA can serve as an endogenous dan-
ger signal, and has been shown to trigger AIM2-dependent release
of IL-1β in skin cells, contributing to the pathogenesis of psoriasis
[38]. Scavenging of DNA by the antimicrobial cathelicidin pep-
tide LL-37 produced by the inflamed skin of psoriasis patients
prevents overt activation of the AIM2 inflammasome and IL-1β
release [38]. Increased AIM2 expression has been observed in
patients with acute and chronic skin conditions, including pso-
riasis, atopic dermatitis, venous ulcera, contact dermatitis, and
experimental wounds in humans [38, 110]. In addition, expres-
sion of the gene encoding AIM2 is elevated in immune cells of
male patients with systemic lupus erythematosus (SLE) and both
increases and decreases in AIM2 expression have been observed
in female patients with SLE [111, 112]. Further, DNA methylation
of the gene encoding AIM2 is reduced in patients with SLE com-
pared with their healthy siblings [113], suggesting that differential
expression or epigenetic changes could be linked to development
of the disease.
Increased expression of AIM2 has been reported in patients
with inflammatory bowel diseases and liver inflammation [114–
116]. For example, elevated expression of AIM2 has been detected
in ascitic fluid macrophages collected from cirrhotic patients,
compared with PBMCs from the same patients, or with CD14
+
macrophages from peripheral blood mononuclear cells of healthy
individuals [115]. Increased expression of AIM2 and NLRP3 and
elevated activation of caspase-1 and maturation of IL-1β have been
found in liver tissues of mice with steatohepatitis [116].
Moreover, there is evidence to suggest that AIM2 is involved in
inflammation and cell death of the brain. Cell-free DNA fragments
are more frequently detected in the cerebrospinal fluid (CSF) of
patients with traumatic brain injury than in CSF from nontrauma
patients [117]. When human embryonic cortical neurons, which
express AIM2 inflammasome components and have been shown
to be capable of IL-1β release and cell death upon AIM2 activa-
tion with poly(dA:dT) transfection [117], were exposed to the
CSF of traumatic brain injury patients, they exhibited increased
AIM2 expression and caspase-1 activation compared with embry-
onic cortical neurons that had been exposed to the CSF of non-
trauma patients [117]. A further study has shown that mice lacking
AIM2 are more protected than WT mice to ischemic brain injury
[118]. Upon induction of focal cerebral ischemia, less activation
of microglial cells and recruitment of leukocytes were found in
Aim2
−/−
mice compared with WT mice [118].
The inability to degrade self-DNA also contributes to the patho-
genesis of autoimmune polyarthritis. Mice lacking the lysoso-
mal endonuclease DNAse II (Dnase II
−/−
mice) are embryoni-
cally lethal, owing to an impaired ability to degrade self-DNA
by macrophages [119]. Genomic deletion of type I IFN recep-
tor (IFNAR) rescued Dnase II
−/−
mice from embryonic lethal-
ity [120]; however, the mice lacking both IFNAR and DNAse II
(Dnase II
−/−
Ifnar
−/−
mice) would eventually develop polyarthri-
tis [121]. Intriguingly, genomic deletion of AIM2 was shown to
prevent inflammasome activation, inflammatory cytokine produc-
tion, macrophage infiltration in the joint, and the development of
arthritis in Dnase II
−/−
Ifnar
−/−
mice [122, 123]. Genomic dele-
tion of STING was also shown to protect Dnase II
−/−
Ifnar
−/−
mice
from joint inflammation [122], indicating that multiple DNA sen-
sors might contribute to the inappropriate DNA recognition driv-
ing clinical manifestation. Overall, aberrant activation of AIM2
from self-DNA is a key driver of inflammatory and autoimmune
diseases.
Conclusions
A wide range of microbial pathogens is sensed by AIM2 in mam-
malian cells. Recognition of DNA from pathogens by AIM2 leads to
protective inflammasome-mediated host responses. Recent stud-
ies have demonstrated that AIM2 inflammasome also plays impor-
tant roles in nonmicrobial diseases, highlighting the multifaceted
nature of AIM2 beyond immunity to infectious diseases. In colorec-
tal cancer, AIM2 orchestrates inflammasome-independent func-
tions by suppressing stem cell proliferation, and contributes to
maintenance of a healthy gut microbiota. The role of AIM2 inflam-
masome in other types of cancer should be further explored.
Moreover, inappropriate recognition of self-DNA by AIM2 trig-
gers detrimental inflammatory responses, leading to superficial
and systemic inflammation. Inhibiting AIM2 inflammasome activ-
ity using synthetic inhibitors, such as suppressive oligodeoxynu-
cleotides [124], or harnessing the power of endogenous AIM2
inhibitors, such as pyrin-containing proteins [36, 39] or antimi-
crobial cathelicidin peptides [38], could be investigated for their
potential to control unwanted inflammation. An additional line of
research could focus on understanding the complementary rela-
tionship between AIM2 and a plethora of other DNA sensors in
the context of different cell types, tissues, and organs. A holis-
tic understanding of the biology of AIM2 could lead to improved
immunosurveillance in the fight against infectious diseases and
cancer, while avoiding debilitating inflammatory and autoimmune
diseases.
Acknowledgments: Research studies from the lab are supported
by the US National Institutes of Health (AR056296, CA163507,
and AI101935 to T.-D.K.), the American Lebanese Syrian Asso-
ciated Charities (to T.-D.K.), and the R.G. Menzies Early Career
Fellowship from the National Health and Medical Research Coun-
cil of Australia (to S.M.M.).
Conflict of interest: The authors declare no financial or commer-
cial conflict of interest.
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Abbreviations: ASC: apoptosis-associated speck-like protein containing
a carboxy-terminal caspase activation and recruitment domain · cGAS:
cyclic-GMP-AMP synthase · CSF: cerebrospinal fluid · GBP: guanylate-
binding protein · IRF: interferon-regulatory factor · LVS: live vaccine
strain · MCMV: mouse cytomegalovirus · SLE: systemic lupus erythe-
matosus
Full correspondence: Dr. Thirumala-Devi Kanneganti, Department of
Immunology, St. Jude Children’s Research Hospital MS #351, 262
Danny Thomas Place Memphis TN 38105-3678, USA
Fax: +1-(901) 595-5766
e-mail: Thirumala-Devi.Kanneganti@STJUDE.ORG
Received: 28/9/2015
Revised: 13/11/2015
Accepted: 26/11/2015
Accepted article online: 2/12/2015
C
2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eji-journal.eu
