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Pathogens and Disease, 74, 2016, ftw012
doi: 10.1093/femspd/ftw012
Advance Access Publication Date: 22 February 2016
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MINIREVIEW
Neutrophils in tuberculosis—first line of defence or
booster of disease and targets for host-directed
therapy?
Tobias Dallenga1,2 and Ulrich E. Schaible1,2,∗
1
Cellular Microbiology, Priority Program Infections, Research Center Borstel, Parkallee 1-40, 23845 Borstel,
Germany and 2 Thematic Translation Unit Tuberculosis, German Center for Infection Research, Parkallee 1-40,
23845 Borstel, Germany
∗
Corresponding author: Molecular Infection Research, Research Centre Borstel, 23845 Borstel, Germany. Tel: +49-4537-188-6000;
E-mail: [email protected]
One sentence summary: Recent studies reveal neutrophils as drivers of pathology in Mycobacterium tuberculosis infection, rendering them prime
candidates for host-directed therapies adjunct to antibiotic therapy to better control tuberculosis; this exceptional treatment of the subject of the role of
neutrophils in tuberculosis presents a comprehensive and fresh perspective.
Editor: Patrick Brennan
ABSTRACT
Necrotizing granulomas, exacerbating pathogenesis and neutrophil influx at the site of infection are hallmarks of active
pulmonary tuberculosis (TB) in humans. The role of polymorphonuclear neutrophils (PMN) in host defence and TB
pathogenesis has recently attracted broader interest. Association of infiltrating PMN, enhanced mycobacterial load and
disease exacerbation in both, mice susceptible to experimental TB as well as in TB patients, link PMN to exacerbated
pathology. Targeting PMN resulted in smaller lung lesions and reduced mycobacterial burden. Therefore, PMN-associated
molecules represent interesting biomarkers to determine TB severity and treatment success. More importantly, PMN are
putative targets for host-directed therapies (HDT) in TB. Due to the rise of multi- and extensively drug-resistant
Mycobacterium tuberculosis isolates, HDT represent adjunct measures to support antibiotic treatment by ameliorating
pathology and local host defences.
Keywords: neutrophils; tuberculosis; Mycobacterium tuberculosis; polymorphonuclear cells; host-directed therapy
INTRODUCTION
Despite a century of intensive research, tuberculosis (TB) still
represents a major threat to humankind. With 1.5 million deaths
and 9.6 million new infections worldwide (World Health Organization 2015) its causative agent, Mycobacterium tuberculosis
(M. tuberculosis), remains the number one bacterial killer of humans and ranks alongside HIV as a leading cause of death by an
infectious agent. According to the World Health Organization,
increasing incidence of multi- and extensively drug-resistant
M. tuberculosis genotypes massively challenges the global fight
against TB. As long as efficient vaccination strategies are not
available and M. tuberculosis continues to develop antibiotic resistances even to newly introduced compounds (Andries et al.
2014; Parida et al. 2015; Hoffmann et al. 2016), innovative TB treatment strategies are required. Host-directed therapies (HDT) as
adjunct treatment measures to support classical antibiotic therapy and counteract TB pathology need to be explored.
Studies on TB pathogenesis have been performed mainly
in murine infection models, primarily using the C57BL/6 or
BALB/c strains. These mice are rather resistant to M. tuberculosis
Received: 18 December 2015; Accepted: 15 February 2016
C FEMS 2016. All rights reserved. For permissions, please e-mail: [email protected]
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Figure 1. Altered type of neutrophil cell death as a correlate to M. tuberculosis virulence. Mycobacterium tuberculosis (Mtb) enter PMN through phagocytosis (A). During
uptake, the three different types of PMN granules (azurophil, specific and secretory) readily fuse with the phagosomal cup. PMN infected with virulent M. tuberculosis
quickly succumb to necrotic cell death. PMN likely contribute to the formation of caseous material associated with extracellular M. tuberculosis within necrotizing
granuloma and exacerbating tissue destruction. When PMN phagocytose attenuated mycobacteria such as the vaccine strain M. bovis BCG or the RD1 M. tuberculosis mutant (B), default apoptosis is prolonged, which can potentially promote antigen cross-presentation to professional antigen-presenting cells promoting T cell
immunity [71, 72].
infection with a chronic pathogenesis that does not reflect the
one observed in patients with active TB (Keller et al. 2006). In contrast, progressing disease with central necrotic granulomas associated with cell death and massive tissue destruction (primary
TB) or with caseous pneumonia (post-primary TB) characterizes
active human TB (Hunter 2011; Cambier, Falkow and Ramakrishnan 2014; Hunter et al. 2014). Recently, an increasing number of reports emphasize the contribution of polymorphonuclear
(PMN) neutrophils to TB pathogenesis (Kasahara et al. 1998; Aleman et al. 2001; Kisich et al. 2002; Pokkali and Das 2009). PMN
transcriptome signatures have been associated with active human TB and PMN represent the prime mycobacteria-infected
cell population in pulmonary patient samples (Berry et al. 2010;
Eum et al. 2010). Therefore, model systems of M. tuberculosis
infections different from the ones predominantly used are required to better analyse PMN-driven host responses and pathogenesis in vivo.
PMN represent the first line of defence cells upon bacterial infections. These cells quickly enter sites of infection in large numbers by migrating along a chemokine gradient formed by CXCL8
(aka IL-8) or keratinocyte chemoattractant (KC) in humans or
mice, respectively, which is recognized by the chemokine receptors CXCR1 and CXCR2 (Nathan 2006). PMN rapidly phagocytose and subsequently kill bacteria through use of their antimicrobial armamentarium. These ready-to-use antibacterial effectors comprise antimicrobial peptides (AMP), reactive oxygen
species (ROS) and different hydrolytic enzymes, which are stored
in three different types of granules (Fig. 1; Amulic et al. 2012).
Notably, in contrast to phagosome biogenesis in macrophages,
PMN phagosome-lysosome fusion takes place concomitantly
with phagocytosis (Weiss and Schaible 2015). Therefore, granules release their content during phagocytosis in direction of engulfed bacteria reaching both, the phagosomal lumen as well as
the close vicinity of the PMN. Association of PMN with active
TB however indicates that these highly effective defence cells,
present in vast numbers at the site of infection, obviously fail
to eliminate M. tuberculosis. Thus, emerging evidence suggests
that PMN contribute to exacerbation of TB pathology, which is
a prerequisite for horizontal transmission of M. tuberculosis via
coughed out aerosol particles.
Mycobacterium tuberculosis infection models reflecting
PMN-associated TB
In the last decade, it became more obvious that the mouse
models primarily used for experimental M. tuberculosis infection studies do not reflect the pathology of human active
TB with regard to granuloma formation, cellular composition
and M. tuberculosis’s growth kinetics. Ultimately, coughing up
mycobacteria-containing aerosols as a prerequisite for transmission and spread of M. tuberculosis is not observed in any
murine TB model (Helke, Mankowski and Manabe 2006). The
C57BL/6 mouse strain, widely used due to the fact that most
gene knockout and transgenic mutants were generated in or
backcrossed to this mouse haplotype, is considered rather resistant to TB. In C57BL/6 mice, immune responses to M. tuberculosis control mycobacterial numbers for more than 120 days
and are only associated with small numbers of PMN early in
infection. Depletion of PMN in the C57BL/6 TB model did not
significantly alter mycobacterial burden and pathology (Seiler
Dallenga and Schaible
et al. 2000). However, using this mouse strain, Repasy et al. (2015)
demonstrated a correlation between the virulence of the used
M. tuberculosis strain and involvement of PMN in host response.
C57BL/6 mice infected with the M. tuberculosis Erdmann strain
showed higher PMN numbers in the lungs, when compared to
mice infected with H37Rv or the attenuated H37Ra- and avirulent phoPR strains. Higher bacterial replication rates of M. tuberculosis Erdmann correlated with the highest rates of horizontal transmission from macrophages to PMN, indicating a link
between mycobacterial growth, macrophage necrosis and PMN
accumulation. Macrophage burst size necrosis lead to release
of the damage-associated molecular pattern (DAMP) molecule
calprotectin, a heterodimer of S100A8 and S100A9, which subsequently resulted in PMN recruitment. In contrast, infection
with the avirulent M. tuberculosis phoPR mutant did not cause
macrophage necrosis, DAMP release and PMN attraction, but
bacterial clearance.
Gopal et al. (2013) identified IL-17-mediated accumulation
of S100A8/A9-positive PMN as the main factor driving inflammation in Diversity Outbred (DO) mice. While contributing significantly to lung pathology, S100A8/A9-positive PMN neither
changed bacterial burdens nor participated in protective immunity against M. tuberculosis infection. Pulmonary PMN-derived
S100A8/A9 protein levels increased after M. tuberculosis infection, reduced upon antibiotics treatment and increased again
after M. tuberculosis reactivation. Hence, S100A8/A9 could be considered as a biomarker for the grade of severity of TB pathology.
Correlation between the presence of PMN and mycobacterial load was also described for granulomas of M. tuberculosisinfected Cynomolgus macaques by Mattila et al. (2015). In this
study, the numbers of granzyme B-positive granules within PMN,
but not CD8 T cells, corresponded to numbers of M. tuberculosis bacilli. Exposure to M. tuberculosis antigens induced PMN
granzyme B production, whereas perforin expression, essential
for granzyme B-mediated T cell cytotoxicity, was not detectable.
Since isolated granzyme B did neither have a bactericidal nor
bacteriostatic effect on M. tuberculosis, the function of granzyme
B-positive PMN, the most abundant cell type in macaque granulomas, is still unclear. Mattila et al. (2015) discuss that granzyme
B may target extracellular matrix components during TB pathogenesis similar to the matrix metalloprotease-8 (MMP-8) in humans (see below).
After aerosol infection of DO mice, Niazi et al. (2015) differentiated between super-susceptible, susceptible and resistant
groups of mice and compared clinical symptoms, bacterial burden, lung pathology, cellular composition of inflammatory infiltrates and cyto- and chemokine levels between infected DO
and C57 BL/6 mice and uninfected ones. Notably, immune cytokines (IFNγ , IL-2, IL-12 and IL-10) in lungs and sera from these
mice weakly correlated with disease progression. Importantly,
whereas ‘supersusceptibility’ and ‘susceptibility’ of mice correlated with highest and second highest numbers of infiltrating PMN, respectively, and with necrotic lesions, resistant DO
and C57BL/6 mice showed reduced PMN numbers and necrotic
pathology. Ultimately, using a statistical and machine learning approach of chemokine data sets, pulmonary PMN necrosis
and CXCL1 levels were revealed as statistically significant correlates for disease exacerbation in TB. In support for Gopal et al.
(2013), plasma CXCL1 was identified as promising biomarker for
neutrophil-associated lung damage.
Yeremeev et al. (2015) showed that PMN exacerbated experimental TB in I/St mice. First reported in Russian literature in the
1980s, the inbred mouse strain I/StSnEgYCit (I/St) showed high
susceptibility to M. tuberculosis infection as reflected by rapid
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body weight loss and short survival times (Apt et al. 1982; Nikonenko et al. 1986, 1990). Later, genome-wide scans for quantitative trait loci (QTLs), linking this phenotype to body weight after infection, revealed mutations in QTLs on chromosomes 3, 9
and 17 compared to the control inbred mouse strain A/SnYCit
(A/Sn) (Lavebratt et al. 1999; Sanchez et al. 2003). The affected
genes for the QTLs on chromosomes 3 and 9 remain unclear. Discussed candidates lying within those loci are the peroxisomal
membrane protein 1 (Pxmp1), the matrix metalloproteinase-12
(MMP-12) and the IL-10 receptor gene (Lavebratt et al. 1999). The
QTL on chromosome 17 has been identified as the classical class
II H2-Ab1 gene (Logunova et al. 2015). Upon M. tuberculosis infection, this mouse strain forms PMN-containing necrotic lung
lesions. Neutropenia induced by anti-Ly6G treatment resulted
in decreased PMN counts, reduced tissue pathology, lower pulmonary mycobacterial burden, less weight loss and increased
survival time in I/St, but not in C57 BL/6 mice. Furthermore, PMN
depletion significantly increased the frequency of mycobacteriaspecific, IFN-γ -producing T cells, suggesting a PMN suppressor
function of acquired immunity to M. tuberculosis.
Recently, Kimmey et al. (2015) showed a unique autophagyindependent role for autophagy protein 5 (ATG5) in PMNmediated pathology in TB. In this study, Atg5fl/fl -Lysm-cre mice
lacking ATG5 expression in macrophages, inflammatory monocytes, dendritic cells and PMN were aerosol infected with M. tuberculosis. In contrast to both, C57BL/6 and autophagy-impaired
Atg16l1fl/fl -Lysm-cre mice, the Atg5fl/fl -Lysm-cre mutants showed
significant weight loss, increased numbers of pulmonary lesions, mycobacterial burden, as well as enhanced levels of the
pro-inflammatory cytokines IFNγ , TNF-α, IL-1α, IL-1β, IL-6, MIP1α, MIP-2, IL-17, KC and G-CSF. Importantly, at this time point,
these mice exhibited strong pulmonary PMN infiltrates resulting in premature death between 30 and 40 days p.i. In contrast, mouse mutants lacking autophagy-associated genes such
as Ulk1, Ulk2 (autophagy induction), Atg4b (isolation membrane
elongation) or p62 (substrate targeting to autophagosome) or
mice with myeloid cell specific disruptions of Atg14, Atg12,
Atg16, Atg7 and Atg3 were similarly susceptible to M. tuberculosis infection as wild type ones. These findings indicate that
impaired autophagy is not responsible for increased susceptibility of Atg5 knockout mice to M. tuberculosis infection but
an unrelated ATG5-mediated mechanism. This is surprising in
view of a number of in vitro studies, which suggested an important role of autophagy for elimination of mycobacteria by activated macrophages (Gutierrez et al. 2004; Dutta et al. 2012; Watson, Manzanillo and Cox 2012; Wang et al. 2013; Deretic 2014;
Sakowski et al. 2015). Interestingly, treatment of Atg5fl/fl -Lysmcre mice with the PMN-specific antibody to Ly6G (1A8) ameliorated weight loss, inflammatory cytokine levels, pulmonary mycobacterial burden, lung pathology and survival. These results
indicate that susceptibility of Atg5fl/fl -Lysm-cre to M. tuberculosis
is due to uncontrolled recruitment of PMN to infected lungs.
A protective role for PMN has been described for infections
with Salmonella enterica typhimurium (Conlan 1997), Francisella tularensis (Sjostedt, Conlan and North 1994) and Chlamydia trachomatis (Barteneva et al. 1996). Feng et al. (2006) observed a
residual role of PMN in protection to experimental TB in common cytokine receptor γ chain knockout mice, where PMN depletion further enhanced susceptibility to M. tuberculosis infection. Furthermore, Pedrosa et al. (2000) also described a protective role for PMN, independent of their phagocytic capacity, after observing a 10-fold higher M. tuberculosis load in infected
but neutropenic BALB/c mice, suggesting an immunomodulatory function of PMN in TB. Similarly, M. tuberculosis infection
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of murine PMN induced the anti-inflammatory cytokine IL-10 in
vitro, and induction of neutropenia lead to a 2.5-fold reduced mycobacterial burden in infected mice (Zhang et al. 2009). It is however important to note that earlier reports on PMN function in
M. tuberculosis infection induced neutropenia using the Gr-1reactive RB6-8C5 antibody, which depletes not only granulocytes, but also other cells including monocytic precursors of
inflammatory macrophages, dendritic cells and lymphocytes
(Daley et al. 2008). An immunoregulatory role of PMN in mycobacterial infections was also discussed in the context of studies on myeloid-derived suppressor cells (MDSC) in TB. MDSC
are described as immature myeloid cells of both, the neutrophil
and monocyte lineage, with potential to suppress specific T cell
responses primarily via secretion of the anti-inflammatory cytokines, IL-10 and TGF-β. Patients with active TB showed enhanced frequencies of MDSC (du Plessis et al. 2013). In mice,
MDSC were associated with exacerbated pathology and enhanced mycobacteria loads (Knaul et al. 2014; Tsiganov et al.
2014). Of note, even before the term MDSC was created, Bennett, Rao and Mitchell (1978) already described induction of natural suppressor cells by mycobacteria most likely representing MDSC. In a cancer model, the alarmins, S100A9, associated
with PMN accumulation, together with TLR signals promote tissue immigration of MDSC (Vogl et al. 2007; Cheng et al. 2008), a
scenario, which can also be envisaged to happen in active TB,
thereby further compromising M. tuberculosis specific T cell immunity. Taken together, these reports indicate a functional flexibility of PMN in TB and call for further specific studies in relevant
animal models and patients.
Neutrophils as drivers of active pulmonary TB in
humans
Several studies demonstrate the important contribution of PMN
to the pathogenesis in human TB. Eum et al. (2010) identified
PMN as the main immune cell population in sputum and bronchoalveolar lavage (BAL) from patients suffering from active pulmonary TB. More importantly, PMN in sputum, BAL and cavity
caseum carried the main mycobacterial loads.
In vitro, we found that human PMN quickly engulfed vast
numbers of M. tuberculosis, but failed to kill the mycobacteria
(Corleis et al. 2012). Instead, PMN quickly succumbed to necrotic
cell death within 10 h after infection, which was associated with
release of cytosolic components, DNA and M. tuberculosis freed
into the extracellular space. Induction of this type of cell death
was dependent on the M. tuberculosis genomic RD1 virulence
gene cluster and PMN’s own ROS. PMN from chronic granulomatous disease patients, suffering from impaired ROS production
due to mutations in the NADP oxidase gene, did not succumb to
necrotic cell death upon infection with M. tuberculosis.
Berry et al. (2010) identified a specific whole blood mRNA
profile in patients with active TB that was specific to M. tuberculosis infection, correlated with the radiological disease score
and reverted to the profiles of healthy controls after successful antibiotics treatment. This TB profile indicated that immune responses to M. tuberculosis infection are dominated by
a PMN-driven type 1 interferon (α, β, and γ ) inducible gene
signature, suggesting multiple PMN-derived biomarkers for TB
diagnosis.
Gopal et al. (2013) further corroborate these results, showing a
predominance of S100A8/A9-producing PMN within inflammatory lung granulomas of patients with active TB. Importantly,
S100A8/A9 levels in TB patient sera positively correlated with
PMN numbers in peripheral blood, CXCL8 serum levels and lung
pathology. These parameters were reduced to normal after antibiotic treatment, ultimately indicating S100A8/A9 serum levels as potential correlates to monitor lung disease severity in TB
patients, drug efficacy and the risk of long-term and irreversible
lung sequelae.
Reverting the bad guys—PMN as targets for
host-directed therapy
The importance of PMN as correlates for disease progression, exacerbating pathology and, consequently, M. tuberculosis burden,
indicates that PMN-associated signatures are potential biomarkers for point of care (POC) diagnosis of disease exacerbation and
drug efficacy monitoring in TB patients. More importantly, PMN
and their effector functions are also putative candidates for HDT.
The bactericidal but also immunosuppressant, Clofazimine, efficiently reduced pulmonary mycobacterial loads in
M. tuberculosis-infected BALB/c mice, whereas the same treatment was not efficient in infected C3Heb/FeJ mice. These mice
are highly susceptible to M. tuberculosis and form hypoxic, PMNdriven, necrotic granulomas upon infection, whereas BALB/c
mice develop multifocal macrophage-dominated cellular infiltrates devoid of necrotizing granulomas and PMN (Irwin et al.
2014). The immunomodulating properties of Clofazimine include, besides other effects, stimulation of ROS production and
inhibition of PMN motility (van Rensburg et al. 1982; Anderson
et al. 1986; Krajewska, Anderson and O’Sullivan 1993). However,
it is still unclear if Clofazimine was inefficient in C3Heb/FeJ mice
due to its lack of activity under hypoxic conditions found in
granulomas of these mice, but not in those of the BALB/c strain.
Another study in the C3Heb/FeJ mouse strain infected with
M. tuberculosis revealed that treatment with Ibuprofen 3 to 4
weeks after infection, when health status of these mice started
to deteriorate, improved both, pathology and mycobacterial
loads (Vilaplana et al. 2013). Ibuprofen is a nonsteroidal antiinflammatory drug that inhibits cyclooxygenases, involved in
prostaglandin synthesis, strong pro-inflammatory mediators
and neutrophil attractants. After 1 week of Ibuprofen treatment, fewer and smaller lung lesions with intra-alveolar PMNs
(both, viable and apoptotic) were observed in treated mice,
whereas untreated ones showed central caseous necrotic areas.
More surprisingly, treated mice revealed a one-log reduction in
pulmonary mycobacterial burdens and extended survival. This
study shows that repurposing a widely used drug, even suited
for children, ameliorates massive PMN-associated inflammation
upon M. tuberculosis infection. Ultimately, Ibuprofen treatment
demonstrated beneficial effects even though treatment started
at a late time point, probably mimicking the time point when a
TB patient would seek medical attention.
Maiga et al. (2015) administered a type 4 phosphodiesterase
inhibitor (PDE-I4), Roflumilast, clinically approved as Daxas and
Daliresp to treat COPD, to M. tuberculosis H37Rv-infected BALB/c
mice (Giembycz and Field 2010). Roflumilast blocks PMN recruitment and reduces production of pro-inflammatory cytokines,
such as TNF-α. While Roflumilast alone had no effect on mouse
lung mycobacterial burden and animal survival rates, its combination with the anti-M. tuberculosis drug Isoniazid resulted in
a half log reduction of M. tuberculosis counts compared to Isoniazid treatment alone 8 weeks after infection. Roflumilast alone
had no effects on lung TNF-α and IFN-γ levels, but reduced pulmonary IL-1β production. Maiga et al. (2015) hypothesized that
the reduction in IL-1β levels facilitated bacterial replication rates
and thereby reduced mycobacterial entry into persister state,
which augments Isoniazid efficacy. Isoniazid primarily kills
Dallenga and Schaible
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Figure 2. Mycobacterium tuberculosis resides inside neutrophil phagosomes associated with LAMP-1 but devoid of cathelicidin, indicating escape from this antimicrobial
effector. Human PMN were infected with serum-opsonized M. tuberculosis H37Rv-mCherry (red) and immunolabelled for Lamp-1 (green, A, B) or the antimicrobial
cathelicidin CAP18 (green, C, D). Fluorescent picture z-Stacks were recorded using a confocal laser-scanning microscope (A, C) and signals were 3D reconstructed
(B, D) (Imaris software). Blue = DAPI. Representative photographs for M. tuberculosis-containing compartments 3 h after phagocytosis are shown.
dividing mycobacteria but not persister cells (Ahmad et al. 2009;
Fauvart, De Groote and Michiels 2011).
Rekha et al. (2015) examined the underlying mechanism of
the previously reported mycobactericidal activity of 4-phenyl
butyrate (PBA) in combination with Vitamin D3 (VitD3 ) in human monocyte-derived macrophages (MDM), and suggested
that PMN support antimycobacterial defence by secreting the
cathelicidin LL-37. It has been shown that VitD3 increases LL37 production (Coussens, Wilkinson and Martineau 2015). LL-37
mediates induction of autophagy-mediated killing of bacteria,
additionally to its antimicrobial properties (Yuk et al. 2009). PBA
is an aromatic fatty acid that has been used since decades to
treat urea disorders, type-2 diabetes and cancers (Li et al. 2012). It
synergizes with VitD3 -mediated increase of cathelecidin expression (Steinmann et al. 2009; Mily et al. 2013; Raqib et al. 2014). The
study by Rekha et al. (2015) revealed that M. tuberculosis inhibited
LL-37 production in MDM. However, PBA plus VitD3 treatment
synergistically boosted LL-37 levels resulting in reduced intracellular growth of M. tuberculosis. PBA-mediated control of mycobacterial growth by MDM was attributed to LL-37-dependent
induction of autophagy. Importantly, the authors hypothesize
that LL-37 secreted from PMN enhances phagocytosis of M. tuber-
culosis by macrophages and enables killing by autophagy in vivo
(Sørensen et al. 2001; Wan et al. 2014). However, though produced
by mycobacteria-infected PMN, we did not find LL-37 localized
to mycobacterial phagosomes in PMN despite the late endosomal nature of this compartment as indicated by association with
LAMP-1 (Fig. 2).
For a clinical study, groups of sputum smear-positive pulmonary TB patients were treated adjunctively with PBA, VitD3 ,
a combination of both or a placebo control (Raqib et al. 2014).
Four weeks after treatment, the PBA and the PBA + VitD3 groups
showed lower counts of peripheral blood neutrophils. Together
with increased LL-37 production by lymphocytes and peripheral blood mononuclear cells this resulted in improved clinical TB scores in patients of all study groups with a significantly better outcome, when treatment was further supplemented by VitD3 . Whether lower PMN counts, increased LL37 levels in MDM, or the combination of both contributed
to the better outcome, was not further addressed by the
authors.
A different HDT approach targets the tissue disrupting functions of PMN to ameliorate TB pathogenesis. The collagen degrading PMN enzyme MMP-8 is associated with PMN markers in
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sputum from TB patients and collagenase levels correlate with
radiological scores and disease severity (Ong et al. 2015). The
licensed MMP-8 inhibitor doxycycline inhibited collagenase activity of MMP-8 in vitro, which will surely prompt clinical studies
using this drug as accompaniment to antibiotic therapy. Of note,
AMP-activated protein kinase regulates the secretion of MMP-8
by PMN, making this pathway another promising HDT target to
improve TB pathogenesis.
CONCLUSION
Massively infiltrating neutrophils into sites of M. tuberculosis infection during human active pulmonary TB and their contribution to pathology have long been a neglected area of research.
Recently, the negative impact of PMN on TB disease outcome
and their fate to necrotic cell death upon M. tuberculosis infection has placed this otherwise highly effective antimicrobial defence cell in the centre of TB research as both, POC biomarker
and HDT target. Neutrophils provide a short-lived environment
for M. tuberculosis that greatly differs to that of macrophages.
Although their vast influx and subsequently fast cell death are
likely involved in the development of necrotic, caseous cores
of hypoxic granulomas, these lesions also seem to provide a
niche for mycobacterial growth. However, only very few studies so far targeted PMN and PMN-associated mechanisms in TB.
PMN involvement in granuloma formation and generation of hypoxic conditions needs to be carefully considered to advance urgently needed innovative TB treatment strategies. Due to multiand extensively drug-resistant M. tuberculosis genotypes arising worldwide treatment options become scarce. Hence, tailored
HDTs represent measures, which could support antibiotic control of the worldwide TB epidemic. Neutrophils, their profiles
and effectors, are predestined candidates for a rapid, simple and
straightforward POC diagnosis, correlating to TB disease progression, mycobacterial loads, antibiotic treatment efficacy and
post-treatment pathological sequelae. Ultimately, PMN represent interesting targets for immune modulating HDT adjunct to
antimicrobial treatment to limit granuloma pathology.
FUNDING
We thank the Priority Program 1580 ‘Intracellular compartments as places for pathogen—host interactions’ and the Excellence Cluster ‘Inflammation at Interfaces’ of the Deutsche
Forschungsgemeinschaft, for financial support.
Conflict of interest. None declared.
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