Introduction
Mycobacterium tuberculosis (Mtb) infections are associated with
approximately 1.5 - 2 million deaths annually worldwide[1]. The
current first-line treatments for tuberculosis (TB) disease include a
combination of antibiotics (rifampicin, isoniazid, pyrazinamide, and
ethambutol) for at least six months[2]. However, the ongoing
emergence of multidrug resistant Mtb threatens the effectiveness of the
treatment with conventional antibiotics[3]. Host-directed therapy
(HDT) strategies targeting the host immune response against Mtb to
complement conventional antibiotic treatment strategies have received
increasing attention[4–11] to enhance treatment outcomes, shorter
treatment durations, and avoidance of resistance development.
HDTs target interactions between the host immune response and the Mtb
pathogen. The host immune response
to Mtb infection is reliant on the cumulative activities of various
defence mechanisms such as macrophage activation, phagocytosis,
autophagy, antigen presentation, and cytokine and T-lymphocytes
production. Mtb has several mechanisms to modulate the host response
enabling evasion of immune system-mediated clearance[1,11].
Pharmacological targeting of specific host-pathogen interaction
mechanisms reflects an important approach for HDTs. Understanding the
multiscale nature of host-pathogen interactions is essential to identify
relevant drug targets for HDTs, and design appropriate combination
treatments and dosing schedules.
A major challenge in the discovery and development of HDTs for TB is the
prediction of treatment responses associated with specific
pharmacological modulation of an immune response-associated target due
to complex systems-level host-drug-pathogen interactions[4,6,12].
The translation of systems-level responses to HDT strategies from
preclinical models to patients is challenged by inter-species
differences in immune responses to Mtb pathogen. Mathematical modelling,
and in particular the use of quantitative systems pharmacology (QSP)
modelling can serve as a valuable tool to identify relevant HDT targets,
and to inform subsequent design of combination drug treatment strategies
and dosing schedules[13–18]. The utility of quantitative modelling
to design improved treatment strategies for TB have already been
demonstrated extensively for conventional antibiotic
therapies[17–19]. For design of HDTs, however, QSP approaches
remain have not yet been developed.
Here, we discuss the utility of QSP modelling strategies to support
discovery and development of HDT strategies. We summarize high-potential
host-pathogen interactions of relevance for HDTs. We then provide an
overview of several relevant infection models to characterize
host-pathogen interactions of Mtb. Based on this, we discuss how QSP
models can be developed with a focus on required model components and
the integration with experimental and clinical data, for application in
target selection, inter-species translation and for clinical study
design and treatment optimization.
Host -Pathogen Interactions as basis for Host-Directed Therapy
Strategies
Several host-pathogen interactions of Mtb involved in its pathogenesis
and immune system evasion offer potential targets for design of
HDTs[11] (Figure 1 ), and are of relevance to capture in QSP
modelling approaches.
Induction of Autophagy
Autophagy involves the formation of a double-membrane phagophore,
elongation of the phagophore, autophagosome maturation, and fusion with
lysosomes for degradation of the selected cellular material, and
requires a complex interplay between various protein complexes.
Autophagy plays an essential role in controlling Mtb
infections[20–23] and has been studied extensively as potential HDT
strategy for Mtb[4,11,20,24]. Currently, two therapeutic targets,
mammalian target of rapamycin (mTOR) and intracellular cholesterol, are
being studied to induce autophagy.
mTOR Inhibitors
Mammalian target of rapamycin complex 1 (mTORC1) plays a role in
regulation of autophagy by two mechanisms, (1) inhibition of unc-51-like
kinase 1 (ULK1) and transcription Factor EB (TFEB)
phosphorylation[24] and (2) activation of glycolysis[25]. Mtb
activates mTORC1 and thus inhibits autophagy.
Metformin is the most evaluated mTOR inhibitor as potential HDT
treatment for Mtb infections. Metformin inhibited the growth of
intracellular MDR Mtb strains in vitro[26]. Adjunctive treatment of
metformin with isoniazid induced phagosome-lysosome fusion, enhanced the
immune response, and reduced intracellular growth of Mtb in
mice[26]. Study of transcriptional changes in healthy human
volunteers following metformin dosing reported that metformin alters
mTOR signalling, induces autophagy, and enhances the host response to
Mtb[27]. Multiple reports suggest that metformin adjunctive therapy
in diabetic TB patients improved TB therapy success rate and lowered
mortality rate[26,28,29].
Everolimus, an mTOR inhibitor, showed significant potential against Mtb
as an HDT. In a human granuloma model, everolimus treatment alone or in
combination with isoniazid or pyrazinamide showed significant reduction
in Mtb load as compared to the controls.[30] Adjunctive everolimus
treatment with rifabutin-substituted standard TB therapy improved lung
functions as measured by forced expiratory volume (FEV1) when compared
to a control group in a randomized clinical trial[31]. A recent
study identified that protein kinase inhibitor ibrutinib as a potential
HDT against Mtb. Ibrutinib therapy alone significantly promoted
auto-lysosome fusion in vitro, inhibited the mTOR pathway in vitro, and
reduced Mtb load in mice[32]. Overall, induction of autophagy via
mTOR inhibitors, especially in combination with conventional Mtb
therapy, holds a potential as an adjunctive HDT strategy for treatment
of TB.
HMG-CoA Inhibitors
Autophagy is also dependent on intracellular cholesterol. Key proteins,
1A/1B-light chain 3 (LC3) and lysosomal associated membrane protein 3
(LAMP3), and Ca2+ are essential for autophagosome
maturation and autophagosome-lysosome fusion. LC3, LAMP3, and
Ca2+ are inhibited by intracellular
cholesterol[5,33], and thus cholesterol inhibits autophagy and
promotes Mtb survival.
The HMG-CoA reductase pathway has been associated with intracellular
cholesterol reduction, autophagy induction and improved Mtb clearance.
Therapy with HMG-CoA inhibitors, such as simvastatin, pravastatin, and
fluvastatin, as adjunctive therapy to conventional anti-TB drugs
improved bacterial clearance by the host and improved the efficacy of
first-line TB drugs by promoting phagosome maturation and autophagy in
macrophage cell cultures and in mice models.[7,34–36] In vitro
screening and experiments in mice for eight HMG-CoA inhibitors
discovered that pravastatin, simvastatin, and fluvastatin had the most
favourable anti-TB activity and pravastatin showed the least toxicity
and drug-drug interactions when used as an adjunctive to standard
anti-TB treatment[7,33]. On the other hand, a population-based
cohort analysis of data from newly diagnosed TB patients recognized no
statistically significant difference in hazard ratio between patients
who were using statins (as a lipid lowering treatment) in addition to
standard TB treatment as compared to patients who did not use
statins[37]. Several retrospective clinical studies have identified
that chronic use of statins reduced the risk of developing TB; however,
to our knowledge, no studies have evaluated statins as a treatment in
active TB patients alone or in combination with conventional anti-TB
therapy[38]. As such, prospective clinical studies assessing the use
of statins, especially pravastatin, at different doses as adjunctive to
standard TB therapy may be needed.
Regulation of Host
Epigenetics
Infection with Mtb alters some host gene functions important for the
ensuring immune response. Two key pathways involved in Mtb-induced host
epigenetic alterations are histone deacetylases1 (HDAC1) and
TLR3-BMP-miR27a pathway both of which can be pharmacologically exploited
for HDTs[39–41].
HDAC Inhibitors
Upregulation of HDAC1 was noted in macrophages containing live Mtb and
HDAC1 recruitment suppressed the expression of IL-12B that plays a vital
role in initiating type 1 T cell immunity to Mtb. HDAC1 is also known to
modulate autophagy associated genes[42,43]. Treatment with a
broad-spectrum HDAC inhibitor (Trichostatin A) decreased bacterial
growth in both M1 and M2 macrophage cell cultures, while selective HDAC
inhibitors (TMP195, and TMP269) reduced bacterial growth in M2
macrophage cell cultures. Vorinostat, an HDAC inhibitor, promoted immune
response by human macrophage cell cultures[44]. In zebrafish embryos
infected with Mm, HDAC inhibition significantly reduced microbial
burden[40]. Additionally, HDAC inhibition significantly inhibited
Mtb growth in lungs and showed increased production of key cytokines in
mice[45].
Abl Tyrosine Kinase
Inhibitors
Abl tyrosine kinase is involved in entry and survival of Mtb within
macrophages through TLR3-BMP-miR27a pathway. Abl tyrosine kinase also
inhibits expression of vATPase pump-relevant genes, and thus inhibits
acidification of autolysosomes. Pharmacological inhibition of Abl
tyrosine kinase using imatinib improved containment of Mtb within
macrophages, induced expressions of iNOS, increased acidification of
phagosomes, and decreased bacterial load in human macrophage cell
cultures and in mice[11,41]. A clinical study assessing effects of
imatinib alone and in combination with conventional anti-TB drugs in
drug-resistant- and HIV co-infected- TB patients[46] is currently
ongoing.
Modulation of Cytokine
Response
The kinetics of the key cytokines, such as interferon gamma (IFN-γ) ,
tumour necrosis alpha (TNF-α), interleukin (IL)-1β, IL-10, IL-4, IL-12,
and IL-2, during the course of Mtb infections have been well studied in
vitro and in vivo[47–51]. IFN-γ is one of the most important
players to the host immune response and its main role is activation of
macrophages. IFN-γ also induces
infected macrophage apoptosis via induction of more than 200
pro-apoptotic genes (i.e. Fas/Fas ligand, cathepsin, protein kinase R,
etc.)[52,53]. Activated macrophage produce reactive nitrogen
intermediates (RNIs) and pro-inflammatory cytokines, TNF-α and IL-1β,
that possess microbicidal properties against Mtb. Resident macrophages
also produce RNIs, TNF-α, and, IL-1β; however activated
macrophage-mediated production is much more efficient[54–56].
Excessive production of pro-inflammatory cytokines, however, can lead to
tissue damage[57]. Anti-inflammatory cytokines, IL-10 and IL-4, are
also induced upon macrophage phagocytosis and balance pro-inflammatory
cytokine levels by macrophage deactivation[57]. However, excessive
production of anti-inflammatory cytokines may result in limiting the
host immune systems’ microbicidal activities[58]. Thus, the fine
balance between the pro- and anti-inflammatory cytokines may determine
the overall outcome of the Mtb infection.
Adjunctive treatment with IFN-y have been evaluated in various clinical
studies; however, different patient conditions, routes of administration
(intravenous vs. subcutaneous) and dosing regimen resulted in varying
outcomes[59]. Adjunctive treatment with aerosolized IFN-y showed
benefits in reducing cavitary lesions and induced negative sputum
conversion in TB patients in clinical studies[60,61].
Anti-inflammatory agents such as cyclooxygenase (COX) 1/2 inhibitors,
corticosteroids, 5-lipoxygenase inhibitor (Zileuton), phosphodiesterase
(PDE) inhibitors, and matrix metalloproteinases (MMP) inhibitors have
been shown to reduce Mtb burden in vitro or in preclinical
species[4]. However, treatments with corticosteroids and celecoxib
(COX1 inhibitor) in combination with conventional anti-TB drugs did not
show significant benefits of these additional HDT in human
subjects[4,62]. Adjunctive treatments with PDE inhibitors and MMP
inhibitors have not been evaluated yet in human subjects to our
knowledge. Retrospective analysis of existing data where TB patients
took approved anti-inflammatory drugs, especially PDE inhibitors, as
concomitant medications for other conditions and their impact on TB
outcome can be a valuable approach.
Enhancing T-cell Mediated Host
Response
The overall innate immune reaction play an important role in the
initiation of adaptive immune response by antigen presentation,
cytokines, and costimulatory signals[55]. Two to three weeks after
the initial infection, antigen-presenting cells (APCs) that drain into
regional lymph nodes initiate adaptive T-lymphocytes mediated immune
response. Upon antigen presentation, the APCs, through antigen
presentation via major histocompatibility molecules (MHC)-I and II ,
prime CD8+ T cells (cytotoxic T cells) and CD4+ T cells, respectively to
initiate adaptive immune response.[63–65] Both activated CD4+ and
CD8+ T cells secrete IFN‑γ, IL-2, IL-17A, and IL-10. The production of
CD4+ mediated IFN-γ is further stimulated by activated macrophages,
whereas the production of CD8+ mediated IFN-γ is driven by
concentrations of IL-12 and correlates with bacterial load[50].
Mature dendritic cells secrete IL-12p70 which helps increasing
recruitment of additional CD4+ T cells[66,67]. IL-2 play a role in
further proliferation of T cells[57]. CD8+ cells have direct
microbicidal capabilities through perforin, granzymes, and granulysin or
induce apoptosis through Fas/Fas ligand interaction[68].
Adjunctive cytokine supplementation with IL-12 and IL-2 have been
evaluated in clinical studies, but did not result in significant
benefits[4,59]. However, recombinant human IL-2 supplementation
showed significant improvements in negative sputum culture conversion
rates and in enhanced X-ray resolution in MDR TB patients[69].
Therefore, the use of recombinant IL-2 supplementation as HDT strategy
for TB should be further evaluated.