Cellular recognition of mycobacteria
Accepted Article
Receptor-mediated recognition of mycobacterial pathogens 1
Kate E. Killick1*, Clíona Ní Cheallaigh2, Cliona O’Farrelly2, Karsten Hokamp3, David E.
MacHugh1,4 and James Harris5
1
Animal Genomics Laboratory, UCD School of Agriculture and Food Science, University
College Dublin, Belfield, Dublin 4, Ireland.
2
School of Biochemistry and Immunology, Trinity Biomedical Sciences Institute, Trinity
College Dublin, Dublin 2, Ireland.
3
4
Smurfit Institute of Genetics, Trinity College, Dublin 2, Ireland
UCD Conway Institute of Biomolecular and Biomedical Research, University College
Dublin, Belfield, Dublin 4, Ireland
5
Faculty of Medicine, Nursing and Health Sciences, Southern Clinical School, Monash
Medical Centre, Monash University, Clayton, Victoria, Australia.
*
Corresponding author:
killickk@tcd.ie
Phone number:
+353-1-7166256
Fax:
+353-1-7161103
This article has been accepted for publication and undergone full peer review but has not been through the
copyediting, typesetting, pagination and proofreading process, which may lead to differences between this
version and the Version of Record. Please cite this article as doi: 10.1111/cmi.12161
1
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Accepted Article
Cellular recognition of mycobacteria
Summary
Mycobacteria are a genus of bacteria that range from the non-pathogenic
Mycobacterium smegmatis to Mycobacterium tuberculosis, the causative agent of
tuberculosis in humans. Mycobacteria primarily infect host tissues through inhalation or
ingestion. They are phagocytosed by host macrophages and dendritic cells. Here, conserved
pathogen-associated molecular patterns (PAMPs) on the surface of mycobacteria are
recognised by phagocytic pattern recognition receptors (PRRs). Several families of PRRs
have been shown to non-opsonically recognise mycobacterial PAMPs, including membranebound C-type lectin receptors, membrane-bound and cytosolic Toll-like receptors and
cytosolic NOD-like receptors. Recently, a possible role for intracellular cytosolic PRRs in the
recognition of mycobacterial pathogens has been proposed. Here, we discuss current ideas on
receptor-mediated recognition of mycobacterial pathogens by macrophages and dendritic
cells.
Introduction
Mycobacterium is a genus of Actinobacteria that includes more than 50 different
species, ranging in virulence from the non-pathogenic M. smegmatis to the causative agent of
tuberculosis (TB) in humans, M. tuberculosis, (Table 1). Infection with mycobacteria most
commonly occurs through inhalation or ingestion of bacilli. The bacilli are phagocytosed by
host macrophages and dendritic cells (DCs) at the site of infection, for example by alveolar
macrophages in M. tuberculosis-infected lungs or by intestinal macrophages in animals
infected with M. avium subsp. paratuberculosis (MAP). Mycobacteria have evolved a range
of mechanisms to circumvent phagosome maturation, preventing lysosomal degradation, and
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Cellular recognition of mycobacteria
Accepted Article
are therefore able to both survive and replicate inside the host phagosome (Ni Cheallaigh et
al., 2012, Welin et al., 2012).
In order for mycobacteria to be phagocytosed by macrophages and DCs, the pathogen
is first recognised by PRRs on the host cell (Gordon, 2002, Medzhitov et al., 2000). This is
achieved through the recognition of highly conserved molecular structures found on the
surface of pathogens, often critical for microbial survival, termed pathogen-associated
molecular patterns [PAMPs] (Akira et al., 2006). Several families of PRRs exist, all of which
are capable of recognising a different repertoire of PAMPs; including membrane-bound and
cytosolic Toll-like receptors (TLRs) and cytosolic NOD-like receptors (NLRs) and RIG-I like
receptors [RLRs] (Jo, 2008). This review examines some of the receptors that are involved in
the non-opsonic recognition of mycobacteria by macrophages and DCs.
C-type lectin receptors
C-type lectin receptors are a family of membrane-bound calcium-dependent receptors
that recognise carbohydrate-rich molecules. Four C-type lectin receptors have been closely
associated with responses to mycobacteria: the mannose receptor, DC-SIGN, Mincle and
Dectin-1 (Figure 1).
Mannose receptors
Mannose receptors (MRs), encoded by the mannose receptor C type 1 and 2 (MRC1
and MRC2) genes, recognise lipoarabinomannan (LAM) and mannosylated LAM (ManLAM)
glycolipids on the cell wall of mycobacteria and other microbes (Martinez-Pomares, 2012).
Early work demonstrated a role for MRs in the recognition of virulent strains of M.
tuberculosis (Schlesinger et al., 1996). More recent research has found that stimulation of
human monocyte-derived macrophages (MDM) with either virulent M. tuberculosis or
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Cellular recognition of mycobacteria
Accepted Article
ManLAM resulted in upregulation of peroxisome proliferator-activated receptor γ (PPARγ)
with a simultaneous increase in production of IL-8 and prostoglandin E2, as well as an
increase in COX-2 expression. This was independent of NF-κB activation and TLR2
expression, which indicates the employment of an MR-specific signalling pathway.
Interestingly, infection with BCG induced less PPARγ and was mediated via NF-κB
activation. This indicates that MR-mediated PPARγ expression and the downstream
signalling it initiates are specific to virulent M. tuberculosis (Rajaram et al., 2010).
Macrophage MRs may be targeted by mycobacteria to enhance survival within host
macrophages. Binding of ManLAM by MRs has been shown to limit phagosome-lysosome
fusion, favouring mycobacterial persistence within macrophages while blocking MRs during
infection with virulent M. tuberculosis increased phagosome-lysosome fusion (Kang et al.,
2005). While recognition of ManLAM by MRs may aid mycobacterial survival within the
host macrophage, it is not clear whether differential binding via MRs is responsible for
differences in virulence between strains. It has been shown that mannose capping of LAM
was not crucial for mycobacterial survival in macrophages (Afonso-Barroso et al., 2012);
murine bone marrow-derived macrophages (BMDM) were infected with mutant forms of M.
bovis BCG and H37Rv M. tuberculosis lacking the mannose cap of LAM. No difference in
the ability of these mutant strains to survive and proliferate in macrophages compared to the
parental strains of M. bovis BCG and M. tuberculosis was observed (Afonso-Barroso et al.,
2012). It also remains to be determined whether the differential signalling induced by
virulent and non-virulent strains of mycobacteria (Rajaram et al., 2010) are responsible for
differences in phagosome maturation. MRs have also been shown to be involved in the
uptake of MAP (Souza et al., 2007). However, polymorphisms in MRC1 have been
associated with susceptibility to TB and leprosy, suggesting that MR may have an important
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Cellular recognition of mycobacteria
Accepted Article
functional role in the immune response to mycobacteria (Azad et al., 2012, Wang et al.,
2012, Zhang et al., 2013).
DC-SIGN
Dendritic cell-specific intercellular adhesion molecule-3-grabbing non-integrin (DC-
SIGN), encoded by the CD209 gene, is a type II transmembrane C-type lectin receptor found
on the surface of immature DCs (Tailleux et al., 2003). DC-SIGN-specific antibodies have
been shown to block DC interactions with M. bovis BCG. The receptor also interacts with
ManLAM, found on the surface of M. tuberculosis, but not with non-mannosylated
arabinofuranosyl-terminated LAM (AraLAM), found on the surface of M. smegmatis
(Geijtenbeek et al., 2003). It has yet to be determined whether mannose capping of LAM
affects the interaction between mycobacteria and DCs.
DC-SIGN appears to play a role in mycobacterial-induced immune suppression, as it
mediates ManLAM-induced production of the anti-inflammatory cytokine IL-10 in DCs and
inhibits LPS-induced DC maturation/activation (Wu et al., 2011). Interestingly, cyanovirin-N
(CV-N), a mannose binding lectin, has been found to compete with both MRs and DC-SIGN
to bind LAM and inhibits the recognition of M. tuberculosis by DCs but not macrophages,
indicating that DCs are of lesser importance in the recognition of M. tuberculosis than
macrophages. Despite this, CV-N did not inhibit M. tuberculosis infection in a mouse model,
suggesting that mannose-dependent C-type lectins, such as DC-SIGN and MRs, may not be
essential during the early stages of murine M. tuberculosis infection. These findings
demonstrate the functional redundancy of C-type lectin PRRs in the recognition of
mycobacterial pathogens (Driessen et al., 2012).
Interestingly, despite these findings, it has been hypothesised that a variant of the
CD209 gene contributes to susceptibility to TB infection (Vannberg et al., 2008, Zheng et al.,
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Cellular recognition of mycobacteria
Accepted Article
2011). However, a meta-analysis of the CD209 336A/G polymorphism and TB susceptibility
that integrated ten independent association studies found no evidence for a genetic
association at this locus (Miao et al., 2012). The apparent discrepancies between these studies
could be due to differences in genotype distributions of CD209 variants in Asian or African
populations compared to other human populations. Consequently, population-specific
associations of susceptibility variants with TB infection could exist. It may be possible to
clarify this by studying larger samples of geographically distinct human populations.
Mincle
The C-type lectin receptor Mincle, encoded by the C-type lectin domain family 4,
member E gene (CLEC4E), is expressed on the surface of macrophages (Matsumoto et al.,
1999). In 2009, Ishikawa and colleagues demonstrated Mincle to be an essential receptor for
trehalose-6, 6-dimycolate (TDM), the mycobacterial cord factor (Ishikawa et al., 2009).
Recently, Mincle has been shown to play a role in CARD9-dependent signalling and
inflammasome activation of IL-1β and subsequent induction of a TH17 response (Shenderov
et al., 2013). Upon exposure to M. bovis BCG, alveolar macrophages from Mincle-deficient
mice demonstrated significantly reduced proinflammatory cytokine profile, decreased
leukocyte infiltration and increased pulmonary and extrapulmonary bacterial load compared
to wild type mice (Behler et al., 2012). However, in another study, both low and high dose
infection with H37Rv a similar granulomatous, TH1 and TH17 response in Mincle deficient
mice to that in wild-type controls. As well as suggesting that Mincle may play a different role
in the response to different strains of mycobacteria, these results also indicate that the
receptor is dispensible in an in vivo model of M. tuberculosis infection. It is probable that
other receptors can compensate for Mincle deficiency and that other PRRs recognise TDM
(Heitmann et al., 2013). Indeed, macrophage C-type lectin (MCL), thought to have arisen
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Cellular recognition of mycobacteria
Accepted Article
from a gene duplication of Mincle, has been reported to recognise TDM (Miyake et al.,
2013), as has the scavenger receptor macrophage receptor with a collagenous structure
(MARCO) (Bowdish et al., 2009), discussed below.
Dectin-1
Dectin-1, encoded by the C-type lectin domain family 7, member A gene (CLEC7A),
has been shown to play a role in the recognition of the non-tuberculous M. abscessus (Shin et
al., 2008). A more recent study using human MDM has demonstrated that Dectin-1 is
required for activation of the NLRP3 inflammasome in response to infection with M.
abscessus (Lee et al., 2012). Dectin-1 has also been shown to play a role in infection with M.
tuberculosis. A study by Lee and colleagues demonstrated that M. tuberculosis H37Rv
induced the expression of Dectin-1 in A549 alveolar epithelial cells in a TLR2-dependent
manner (Lee et al., 2009). Moreover, Dectin-1 is required for TLR2-mediated production of
TNF-α by murine BMDM in response to infection with the attenuated mycobacterial strains
M. bovis BCG and M. tuberculosis H37Ra, as well as the avirulent M. smegmatis and M.
phlei, although this was not the case for the virulent M. avium and M. tuberculosis H37Rv
strains (Yadav et al., 2006). However, a role for Dectin-1 in M. tuberculosis-induced
production of IL-12p40 in splenic DCs, a cytokine subunit previously shown to be important
in granuloma formation , has also been proposed (Rothfuchs et al., 2007).
Interestingly, Dectin-1 activation by M. tuberculosis has been shown to promote a
mixed TH1/TH17 response; driving the secretion of IL-1, IL-23, TNF-α and IL-6 by
monocyte-derived DC, which in turn drives IFN-γ and IL-17 secretion by CD+ T cells in
vitro(Zenaro et al., 2009). Co-stimulation of MR or DC-SIGN, on the other hand, inhibits the
secretion of these cytokines and drives a predominantly TH1 phenotype in CD4+ Cells
(Zenaro et al., 2009). TH1-type responses, typified by the secretion of IFN-γ, TNF-α and IL7
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Cellular recognition of mycobacteria
Accepted Article
12 are essential for protective immunity to M. tuberculosis (Harris et al., 2010). The role of
IL-17 in M. tuberculosis infection is not clear, although studies suggest that high numbers of
IFN-γ/IL-17-producing T lymphocytes correspond with more severe disease and low
responders (Jurado et al., 2012). However, IL-17 may have an important role to play in
protective responses following vaccination (Chatterjee et al., 2011, Gopal et al., 2012). Thus,
targeting Dectin-1 during vaccination may represent a therapeutically advantagous approach.
However, a study by Marakalala and colleagues detected no differences in pulmonary
cytokine expression between wild type and Clec7a-/- mice infected with M. tuberculosis
H37Rv (Marakalala et al., 2011), suggesting that Dectin-1 may be functionally redundant for
proinflammatory cytokine production during M. tuberculosis infection in vivo.
Scavenger receptors
Scavenger receptors (SRs) are expressed on the cell surface of mammalian monocytes
and macrophages and recognise a wide range of ligands. SR sub-group A consists of SR-AI,
SR-AII and MARCO, while SR sub-group B consists of SR-B1 and CD36 [Figure 1] (Peiser
et al., 2002). It has been demonstrated that SR-A-deficient mice infected with M. tuberculosis
H37Rv survive significantly longer than their wild type cohorts (Sever-Chroneos et al.,
2011), suggesting that recognition by these receptors may favour subversion of the immune
response by the bacilli. Evidence exists for a role of the SR-B1 as a macrophage receptor
involved in BCG recognition. However, this was shown to be only a minor role in in vivo,
possibly due to the ability of other PRRs to compensate for the SR-B1 function (Schafer et
al., 2009). Similarly, mice deficient in CD36 were found to be less susceptible to infection
with M. bovis BCG, while Cd36-/- macrophages demonstrated increased intracellular killing
of M. tuberculosis H37Rv and M. marinum in vitro (Hawkes et al., 2010). The mechanism
through which CD36 deficiency confers greater protection against infection is not clear, but it
does not involve differences in phagocytic uptake, macrophage apoptosis, TNF-α or IL-10
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Cellular recognition of mycobacteria
Accepted Article
production, or the generation of reactive oxygen or nitrogen intermediates (Hawkes et al.,
2010).
MARCO has been found to be involved, at least in part, in the recognition of TDM
(Bowdish et al., 2009). However, mice deficient in MARCO show no differences in response
to acute or chronic infection with M. tuberculosis compared to WT controls (Court et al.,
2010). Interestingly, in this same study, no difference in response to infection were observed
in CD36-/-, SRA-/-, or MR-/- mice, in contrast to the studies discussed above. The reasons for
these differences are not clear, but there may be some functional redundancy within the
different classes of SR. However, a polymorphism in MARCO has been shown to be
associated with susceptibility to TB infection in a Gambian population (Bowdish et al.,
2013).
Nod-like receptors
There are two human NOD genes, NOD1 and NOD2, both of which detect
peptidoglycan, a structural component essential to the bacterial cell wall (Kanneganti et al.,
2007). NOD-2 also senses muramyl dipeptide (MDP), which is a common cell wall
component of Gram-negative and Gram-positive bacteria (Girardin et al., 2003). A role for
NOD-2 in the recognition of M. tuberculosis was first demonstrated using BMDM and DCs
from Nod2-/- mice. NOD-2 was found to be required for optimal production of
proinflammatory cytokines in response to M. tuberculosis in vitro (Gandotra et al., 2007),
suggesting that NOD-2 is at least one of the receptors required for sensing of the bacillus.
Moreover, Nod2-/- mice have been shown to display decreased immunopathology and T cell
recruitment to the lungs upon infection with M. bovis BCG, coupled with higher bacterial
burdens and increased mortality. In this model, M. bovis BCG-infected alveolar macrophages
from Nod2-/- mice released significantly less TNF-α than wild type cells (Divangahi et al.,
2008). Similarly, Brooks and colleagues studied the NOD-2 receptor in both human MDM
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Cellular recognition of mycobacteria
Accepted Article
and alveolar macrophages in response to infection with M. tuberculosis. Survival of the
bacteria within Nod2-/- macrophages was higher than in controls, suggesting that NOD-2
plays an important role in the recognition and control of mycobacterial infection (Brooks et
al., 2011).
In addition, activation of NOD-2 with MDP in human alveolar macrophages infected
with M. tuberculosis has been shown to increase intracellular control of bacterial growth and
lead to the recruitment of proteins linked with autophagy to the bacteria-containing
phagosome/autophagosome (Juarez et al., 2012). It is well established that autophagy
represents an important mechanism for the intracellular killing of mycobacteria by
macrophages (Castillo et al., 2012, Gutierrez et al., 2004). These data highlight the
possibility of a PRR-dependent mechanism for the activation of autophagy, unlike previous
studies that have shown that autophagic killing of mycobacteria is regulated by cytokines—
particularly IFN-γ (Gutierrez et al., 2004, Harris et al., 2007). However, the role of
macrophage-derived cytokines, such as TNF-α and IL-1, which can also induce autophagy,
was not assessed in this study (Harris, 2011, Harris, 2013).
In a case-control study of an African-American population in the United States, three
common nonsynonymous polymorphisms in the NOD2 gene were significantly associated
with a genetic susceptibly to TB infection (Austin et al., 2008). An association has also been
found between TB susceptibility and the NOD2 synonymous Arg5878Arg SNP in the
Chinese Han population (Zhao et al., 2012). This association was further validated in a larger
Chinese population (Pan et al., 2012). Genetic associations between allelic variants of NOD2
and susceptibility to infection have also been reported for other mycobacterial species. For
example, a highly significant association (P < 0.0001) with MAP infection for genotypes at a
nonsynonymous SNP in the bovine NOD2 gene has been reported in a mixed population of
domestic cattle (Pinedo et al., 2009) and in Holstein-Friesian cattle (Ruiz-Larranaga et al.,
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Cellular recognition of mycobacteria
Accepted Article
2010). In addition, it has been proposed that several NOD2 polymorphisms are associated
with susceptibility to leprosy, an infectious disease of the peripheral nerves and skin, caused
by the mycobacterial pathogen M. leprae (Berrington et al., 2010) [Figure 1].
RIG-I-like receptors and cytosolic DNA sensors
Retinoic acid-induced gene (RIG)-I-like receptors (RLRs) are a family of PRRs located
in the cytoplasm. The involvement of RLRs in the detection of intracellular bacteria has been
previously demonstrated (Chiliveru et al., 2010, Monroe et al., 2009). Moreover a RIG-I
signalling pathway was over-represented in a genome-wide transcriptomics study of M.
bovis-infected bovine MDM (Magee et al., 2012). A murine cytosolic DNA sensor, IFI204,
has also been shown to recognise M. tuberculosis and activates the STING/Tbk1/Irf3 axis,
resulting in IFN-β secretion (Manzanillo et al., 2012). This pathway is activated by multiple
receptors, including NLRs, TLRs and RLRs (Barber, 2011). The STING/Tbk1 axis has been
shown to be required for targeting of M. tuberculosis to the autophagosome (Watson et al.,
2012). Interestingly, Irf3-/- mice are resistant to infection with M. tuberculosis, suggesting
that cytosolic DNA receptors may be specifically targeted by the bacteria to promote their
intracellular survival (Manzanillo et al., 2012). A role for intracellular PRRs in the
recognition of mycobacterial pathogens may also tally with recent evidence that M.
tuberculosis has the potential to escape from the phagosome (Harriff et al., 2012, Watson et
al., 2012). Further research in this area may reveal the full extent to which intracellular PRRs
are involved in the sensing of mycobacterial pathogens.
Toll-like receptors
Toll-like receptors (TLRs) are a conserved family of ten human (TLRs 1-10) and
twelve murine (TLRs 1-9 and TLRs 11, 12, 13) PRRs that have a fundamental role in nonopsonic phagocytosis and recognition of intracellular ligands (Kawai et al., 2010). TLRs 1, 2,
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Cellular recognition of mycobacteria
Accepted Article
4, 5 and 6 are located on the cell surface, while TLRs 3, 7, 8 and 9 are located intracellularly,
mainly on the endoplasmic reticulum (ER) membrane. TLR-10 is an orphan member of the
human TLRs, expressed on B cells and plasmacytoid DCs and non-functional in mice (Hasan
et al., 2005).
TLRs have been shown to play a role in the cellular response to mycobacterial
pathogens (Basu et al., 2012). Interestingly, whereas live M. tuberculosis activates NF-κB via
TLR-2 and TLR-4, heat-killed M. tuberculosis only activates through TLR-2. Expression of a
dominant-negative TLR-2 construct in RAW 264.7 macrophages consistently blocked M.
tuberculosis-induced NF-κB activation by 70–80%, whereas expression of a dominantnegative TLR-4 construct blocked NF-κB activation by 30–40%. Co-expression of both TLR2 and TLR-4 dominant-negative constructs resulted in almost complete absence of NF-κB
activation (Means et al., 2001). Moreover, a TLR-4 inhibitor inhibited mycobacteria-induced
TNF-α production and apoptosis in both RAW 264.7 cells and primary human alveolar
macrophages (Means et al., 2001). A role for TLR-2 and TLR-4 in the induction of apoptosis
has subsequently been confirmed by other groups (Lopez et al., 2003, Sanchez et al., 2010).
TLR-2 is also involved in the production of IL-1β following infection by M. tuberculosis
(Kleinnijenhuis et al., 2009).
In human cells, stimulation of TLR-2 triggers a pathway that involves up-regulation of
the vitamin D receptor and the vitamin D-activating enzyme CYP2R1, transcription of
cathelicidin and beta 4 defensin, and culminates in the co-localisation of cathelicidin and
mycobacteria and intracellular killing of bacteria (Liu et al., 2006). Similarly, autophagy in
response to the mycobacterial lipoprotein LpqH is TLR1/2- and vitamin D receptordependent (Shin et al., 2010). TLR-2 may also be involved in mycobacterial inhibition of
host immune responses; Noss and colleagues reported that inhibition of MHC-II antigen
processing by the 19 kDa mycobacterial lipoprotein was TLR2-dependent (Noss et al., 2001).
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Cellular recognition of mycobacteria
Accepted Article
Further work demonstrated that this was mediated via TLR2-dependent inhibition of IFN-γregulated HLA-DR protein and mRNA expression (Gehring et al., 2003). It has subsequently
been shown that exposure of the murine J774 macrophage cell line to the TLR2 ligands
19 kDa mycobacterial lipoprotein or zymosan, but not the TLR4 ligand LPS, inhibits IFN-γinduced killing of M. bovis BCG in a TLR2-dependent manner (Arko-Mensah et al., 2007).
In addition, mycobacterial CpG motifs have been found to be recognised by TLR-9 (Carvalho
et al., 2011), and classical TLR-9 signalling has been shown to be blocked by MAP in bovine
monocytes (Arsenault et al., 2013).
Genetic associations between TLR gene polymorphisms and TB susceptibility has been
reviewed previously (Azad et al., 2012). Briefly, TLR2 gene polymorphisms and TB,
particularly extrapulmonary TB, have been reported for a range of different human
populations in Africa and Asia (Chen et al., 2010, Motsinger-Reif et al., 2010). No studies
have yet shown an association between TLR4 polymorphisms and TB in HIV-negative
patients; however, a link between a TLR4 SNP and TB susceptibility in HIV-infected
individuals has been reported (Pulido et al., 2010). Two studies have also reported an
association between a TLR9 polymorphism and TB (Velez et al., 2010, Kobayashi et al.,
2012). A study on a Columbian population found no association between TB susceptibility
and polymorphisms in either TLR2, TLR4 or TLR9. However, this study may have been
underpowered to detect minor associations (Sanchez et al., 2012). In addition, SNPs within
the TIR domain containing adaptor protein gene (TIRAP, or Mal), which encodes a signalling
adaptor protein downstream of TLR-2 and TLR-4, have also been associated with TB
susceptibility in some studies (Selvaraj et al., 2010, Zheng et al., 2011).
The importance of TLRs in infection with M. tuberculosis in the murine model,
particularly the key role of TLR-2 has previously been demonstrated (Bafica et al., 2005,
Drennan et al., 2004, Mayer-Barber et al., 2010). However, these results do vary by model:
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Cellular recognition of mycobacteria
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Hölscher and colleagues report that Tlr2/4/9 triple knockout mice infected with 100 cfu
H37Rv have normal production of TNF-α, IL-12p40, normal CD4+ and CD8+ reponses to
stimulation with M. tuberculosis peptides, and no difference in bacterial burden at 150 days
compared to wild type mice (Holscher et al., 2008). These conflicting results may be due to
different H37Rv exposure times. MyD88 is a signalling adaptor protein downstream of all
TLRs, with the exception of TLR-3; it is also involved in other signalling pathways including
IFN-γ and IL-18 (Schneider et al., 2010, Sun et al., 2006). In addition, Myd88-/- mice show a
markedly increased susceptibility to infection with M. tuberculosis, which is thought to
primarily reflect its role in IL-1β signalling (Holscher et al., 2008, Mayer-Barber et al.,
2010).
With regards to other mycobacterial infections, genetic variants of the TLR1 gene have
been identified as a major determinant of leprosy susceptibilty (Wong et al., 2010). DNA
sequence polymorphisms in TLR2 and TLR4 have also been associated with susceptibility to
M. leprae infection (Hart et al., 2012). In addition, susceptibility to MAP infection in cattle
has been associated with polymorphisms in the bovine TLR1, TLR2 and TLR4 genes [Figure
1] (Mucha et al., 2009).
Conclusion
Host macrophage and DC recognition of mycobacterial pathogens is a complex
process: different populations of myeloid immune cells use different receptors or
combinations of receptors and act in conjunction with other molecules to successfully
identify and phagocytose mycobacteria, while the bacteria themselves may preferentially
target specific receptors to manipulate the host response and promote their own survival.
Understanding these processes will help us to better understand the co-evolution of host
immune genes and mycobacterial genomes. The development of effective vaccines against
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Cellular recognition of mycobacteria
Accepted Article
mycobacterial diseases will be paramount to the future of human and animal health.
Deciphering the mycobacterial-specific mechanisms for evading macrophage and DC
recognition will be be key to this, including a greater knowledge of the relationship between
different PRRs and mycobacterial disease prevalence across global populations. Future
studies targeting different combinations of receptors may lead to a better appreciation of the
complex interactions between phagocyte and mycobacterium. Ultimately, increasing our
understanding of innate immune recognition of mycobacterial pathogens will aid in our abilty
to treat and prevent mycobacterial diseases.
Conflict of interest
The authors declare that they have no conflict of interest.
Acknowledgements
Work that led to this review was supported by Investigator Grants from Science Foundation
Ireland (Nos: SFI/01/F.1/B028 and SFI/08/IN.1/B2038), a Research Stimulus Grant from the
Department of Agriculture, Food and the Marine (No: RSF 06 405) and a European Union
Framework 7 Project Grant (No: KBBE-211602-MACROSYS). KEK is supported by the
Irish Research Council (IRC) funded Bioinformatics and Systems Biology thematic PhD
programme. COF is supported by a Research Stimulus Grant from the Department of
Agriculture, Fisheries and Food (2006 06/340). JH was supported by Science Foundation
Ireland (SFI) as part of the Immunology Research Centre, SFI Strategic Research Cluster
(07/SRC/B1144). CNC is supported by the Health Research Board, Ireland and the Health
15
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Cellular recognition of mycobacteria
Accepted Article
Service Executive, Ireland, under the National SpR Academic Fellowship Programme. We
thank Prof. Stephen Gordon (UCD) and Dr. David Magee (UCD) for valuable comments.
16
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Cellular recognition of mycobacteria
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Accepted Article
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Accepted Article
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Accepted Article
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Cellular recognition of mycobacteria
Tables
Table 1: Overview of important mycobacterial species
Table 2: Pattern recognition receptors (PRRs) and mycobacterial infections
Figure legends
Figure 1: Pattern recognition receptors (PRRs) involved in the non-opsonic recognition
of mycobacterial pathogens. PRRs and their cellular locations are shown. Gene symbols are
indicated in grey text and PRR genes with sequence variants associated with resistance to
various mycobacterial infections are also indicated. These infections are tuberculosis caused
by M. tuberculosis, leprosy caused by M. leprae and Johnne’s disease in cattle caused by M.
avium subsp. paratuberculosis.
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Cellular recognition of mycobacteria
Table 1: Overview of important mycobacterial species
Mycobacterial group
Mycobacterial species
Disease
Host species
Primary tissue/cell
infected
M. tuberculosis
M.
tuberculosis
H37Rv
Virulent strains
Tuberculosis
M.
tuberculosis
Erdman
Avirulent strains
M.
tuberculosis
H37Ra
Human
Non-pathogenic
Human (high
incidence in HIVpositive patients)
M. africanum
Mycobacterium
tuberculosis complex
Human (other hosts
are unknown)
M. canettii
M. caprae
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Tuberculosis
32
Humans, cattle, goats
Alveolar macrophage
Accepted Article
Cellular recognition of mycobacteria
Mycobacterium avium
complex (MAC)
M. leprae
Nontuberculous
mycobacteria (NTM)
M. pinnipedii
Seals (Pinnipeds)
M. microti
Voles
M. bovis
Bovine tuberculosis
Alveolar macrophage
Cattle, humans and
other mammals
Avirulent strains
M. bovis
BCG
Vaccine against
tuberculosis
M. avium
Opportunistic infections
M. intracellulare
Circulating monocytes
Opportunistic infections
Found in soil and
water but can infect
humans, particularly
AIDS patients
Infection mainly occurs via
the gastrointestinal tract
but occasionally occurs via
the lungs
M. avium subspecies paratuberculosis
Johne’s disease
Cattle
Intestinal macrophages
M. leprae
Leprosy
Human
Macrophages and
Schwann cells
M. marinum
Skin infections in
people persistently
exposed to saltwater or
fresh water,
disseminated infection
in immunecompromised patients
Found in soil and
water but can infect
humans
Macrophages
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33
Accepted Article
Cellular recognition of mycobacteria
M. abscessus
Chronic lung disease in
immune-compromised
patients
M. smegmatsis
Can cause primary
lesions in immunecompromised patients
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34
Macrophages
Soil, water and plants
Macrophages
Accepted Article
Cellular recognition of mycobacteria
Table 2: Pattern recognition receptors (PRRs) and mycobacterial infections
Gene
symbol/s
Ligand/s
MRC1
MRC2
Lipoarabinomannon (LAM)
Mannosylated LAM
(ManLAM)
DC-SIGN
CD209
ManLAM,
DNaK,
Glyceraldehyde-3 phosphate
dehydrogenase (GAPDH),
IprG
Mincle
CLEC4E
Protein/s
Mannose
receptors
Trehalose-6, 6-dimycolate
(TDM)
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PRR/PAMP
interactions
reported for
other microbes
Cellular
location
Gene variation and susceptibility/resistance
Cell surface
Association between MRC1 polymorphism and
susceptibily to TB and leprosy
Yes
Cell surface
Association between CD209 polymorphism and
susceptibility to TB
None reported
Cell surface
No associations reported
None reported
35
Accepted Article
Cellular recognition of mycobacteria
Dectin-1
CLEC7A
Scavenger
receptors:SRAI
MSR1
SR-AII
MARCO
MARCO
SCARB1
SR-B1
CD36
Oligosaccharide ligands
Cell surface
No associations reported
Yes
Wide variety of ligands
Cell surface
Polymorphism in MARCO has been shown to be
associated with susceptibility to TB
Yes
Cytoplasm
Conflicting evidence for an association between
CARD15 polymorphisms and susceptibility to TB
Association between CARD15 polymorphisms and
susceptibility to leprosy in humans and to MAP infection
in cattle
Association between CARD15 polymorphisms and
leprosy
Yes
CD36
NOD2
TLR1
CARD15
Muramyl dipeptide
TLR1
Peptidoglycan and
lipoproteins (forms a
heterodimer with TLR2)
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Association between TLR1 polymorphisms and
susceptility to leprosy
Yes
Cell surface
TLR1 polymorphisms have also been associated with
susceptibility to MAP infection in cattle
36
Accepted Article
Cellular recognition of mycobacteria
TLR2
TLR4
TLR2
TLR4
Lipoproteins and
mycoplasma
Lipopolysaccharide (LPS)
Association between TLR2 polymorphisms and
susceptibility to TB (particulary extrapulmonary TB),
leprosy and MAP infection in cattle
Cell surface
Association between TLR4 polymorphisms and TB
susceptibility in HIV-positive patients (no association
shown for HIV-negative patients)
Cell surface
Yes
Yes
TLR4 polymorphisms associated with susceptibility to
leprosy and MAP infections in cattle
TLR9
TLR9
CpG motifs on bacterial and
viral DNA
Cytoplasm
Association between TLR9 polymorphisms and TB
susceptibility
RIG-I
DDX58
Viral double-stranded RNA
Cytoplasm
No associations reported
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37
Yes
Yes
Accepted Article
Extracellular
TLR-1 TLR-2 TLR-4 TLR-9
TLR1 TLR2
TLR4 TLR9
Mannose
receptor,
C type 1 Dectin-1 Mincle DC-SIGN
MRC1
Toll-like receptors
CLEC7A CLEC4E
RIG-I
NOD2
NOD2
Intracellular
M. leprae
SR-A1
SR-A2
MARCO
MSR1
MSR1
MARCO SCARB1
C-type lectin receptors
DDX58
M. tuberculosis
CD209
M. avium subsp.
paratuberculosis
SR-B1
CD36
CD36
Scavenger receptors
Leucin-rich repeat region
Carbohydrate
recognition domain
Cystein-rich domain
Collagenous domain
Cytoplasmic TIR domain
CARD domain
Type II fibronectin domain
Helicase domain
Immunoreceptor ITAM domain
NACHT domain
23-amino acid tandem
repeat region
Alpha helical
coiled domain