4. NOD2 and Mycobacterial infection
Different from other bacteria, mycobacteria produces a specific enzyme
called N-acetyl muramic acid
hydroxylase (NamH), which can convert the basic unit of the mycobacteria
cell wall, N-acetyl MDP, to N-glycolyl MDP, with stronger
nod2-stimulating activity[88]. Mtb -infected
monocyte-derived macrophage (MDM)
upregulate Nod2 transcripts up to 10-fold, mRNAs up to 2-3-fold and
proteins up to 10-fold[89,90]. In this section, we will summaries
the contribution of NOD2-mediated innate and adaptive immunity in
response to mycobacteria (Figure 3) .
In human and mouse-derived cells infected with mycobacteria, NOD2
produces a large number of immune effectors through the innate immune
response, including TNF, IL-1β, IL-6, IL-12, IL-32, type I IFNs, nitric
oxide (NO). TNF- and
IL-1R-deficient mice fail to
control mycobacteria and BCG, with increased bacterial burden,
exacerbated lung inflammation, reduced IL-23, and rapid
death[91–94]. Mtb -infected BMDM activates IRF5 and induces
IFNα/β transcription via Nod2-RIP2-TBK1 signaling pathway, and promotes
cell bactericidal effect[95]. However, NOD2-mediated type I IFNs is
controversial in its capacity to clear mycobacteria. It has been
suggested that type I IFNs encourage cell-to-cell spread of mycobacteria
by secreting CXCL chemokines and necrotic apoptosis, but it is not yet
known how type I IFNs leads to necroptosis[96,97]. Whereas some in
vivo and in vitro experiments in mice suggest that type I IFNs enhances
the expression of Inducible Nitric Oxide Synthase (iNOS) and eliminates
mycobacteria through NO production[98,99]. Genetic disorders of type
I IFNs immune function underlie severe mycobacterial
infections[100–102]. Therefore, future studies should determine
whether IFN is beneficial or detrimental under certain circumstances.
Mtb escapes the immune system by inhibiting autophagosome and
lysosome fusion[103]. During infection, NOD2-dependent autophagy
serves as an antimicrobial mechanism for clearing intracellular
pathogens and Mesenchymal Stem
Cells (MSC), which often excrete anti-tuberculosis mediators, leading to
prolonged intracellular viability and maintenance of Mtb[104,105]. The autophagy proteins IRGM, LC3, and ATG16L1 were
recruited into Mtb vesicles in a Rip2/p38-dependent manner and
prompted the conversion of LC3-I
to LC3-II, which serves as an
indication of autophagosome formation[105,106,74]. The Signaling
pathways of NOD-2 and TLR-4
(N2T4) also co-localizes Mtb in
the lysosomes and induced autophagy to reduce the survival of Mtb hidden
in host MSCs[104].
The BCG Vaccine is the longest used global vaccine and the only approved
vaccine against Mtb . Recently, the role of trained immunity has
been identified as the mechanism of BCG-mediated immunity againstMtb , which relies on NOD2[107]. Stimulation of NOD2-deficient
macrophages with BCG did not show an increase in cytokine production
after heterologous stimulation, suggesting that NOD2 is essential in the
establishment of trained immunity[76]. Comparison of PBMC isolated
before and after 3 months of BCG vaccination in volunteers revealed that
the vaccine caused epigenetic modification (H3K4me3) reprogramming of
innate immune cells induced by NOD2. When cells were re-exposed to
mycobacteria and non-mycobacteria stimuli IFN-γ, TNF-α, and IL-1β
expression were significantly up-regulated, as well as an increase in
the expression of CD11b and TLR4 activation markers[77,108]. The
mortality rate of children infected with M. tuberculosisfollowing BCG vaccination at birth is almost three times lower than that
of unvaccinated children[109]. Moreover, the trained immunity is not
dependent on adaptive immunity, as reduced Mtb loads could be
consistently observed 1 week to 1 month following advance vaccination of
T-cell-deficient mice with BCG, highlighting the role of macrophages in
BCG-induced killing of Mtb [110]. The NOD2 signaling pathway
for the treatment of infectious diseases through innate immune memory
deserves further from investigation in future studies, not only in the
field of immune training, but also in immune tolerance[111]. It is
unknown whether NOD2-mediated immune tolerance plays a role in chronic
infectious disease caused by mycobacteria. Understanding the effective
disease tolerance mechanisms present in 90-95% of asymptomaticMtb -infected patients could help in designing prevention and
treatment of active Mycobacteria.
DCs are a bridge between innate and adaptive immunity.
N2T4enhances the efficacy of DCs and the potency to kill Mtb in vitro
and in vivo, mainly through the following aspects: (1) the secretion of
effectors IL-6, IL-12, IFN-γ, TNF-α, NO, (2) the induced of autophagy,
(3) increasing CCR7, which is known to improve migration of immune cells
to the lymph nodes, (4) enlarging the number of effector memory CD4+ and
CD8+ T-cells. N2T4 also lowed the dose
of isoniazid (one-fifth of the recommended concentration), and
significant reduced in lung bacterial load.
N2T4-targeted
DC cells may be a suitable immunotherapeutic approach to evoke host
immunity to Mtb [112–114]. The role of NOD2 on adaptive
immunity was also confirmed by in vivo experiments in mice, where
Nod2-/- mice infected with Mtb and BCG produced
less Th1-type cytokines and reduced recruitment of
CD8+ and CD4+ T cells In the later
phase of the infection, with a higher mortality rate in mice[115].
Although Th2-type cytokines limit tissue damage in the periphery of
granulomas and facilitate tissue repair during Mtbinfection[111,116], there is no direct evidence indicating whether
NOD2-dependent IL-4, IL-5 plays a role in it.
Gene variants in the NOD2-mediated signaling pathway associated with
susceptibility to Mycobacterium leprae infection in a large-scale
leprosy GWAS in a Chinese population[4]. It has later been
successively validated in Vietnam, India, and ethnically mixed Amazonian
populations[117–119]. A study shows that human NOD2 recognizes
structurally unique MDPs from Mycobacterium leprae [120]. Most
interestingly, however, M. leprae , the bacterium most associated
with the NOD2 polymorphism, is the only one known not to produce an
N-glycolyl MDP structure that enables NOD2 to be more readily
recognized[121]. The role of NOD2 in the immune response in leprosy
has also been supported based on cell function experiments. Compared to
TLR2/1 ligands, MDP acting on NOD2-activated monocytes is more likely to
induce rapid differentiation of monocytes into CD1+ DCs and to
accomplish MHC I-associated antigen presentation by a IL-32-dependent
mechanism[122], which was confirmed in leprosy lesions. Due to the
harsh culture conditions of M. leprae and the absence of in vivo
or in vitro leprosy models, the mechanism of action of NOD2 in M.
leprae infection is unclear.