Experimental approaches to inform design of Host-Directed
Therapies
Several experimental approaches experimentally modelling (parts of) Mtb
infection are of relevance to inform design of HDTs. Such experiments
can provide quantitative understanding about components of
drug-host-pathogen interactions (Figure 2 ), which can be
combined and integrated through the use of QSP modelling. Key aspects
include the immune modulatory effect of therapeutic agents on immune
cells that in turn lead to changes in Mtb inhibition-dynamics of immune
cells, immune system evasion strategies induced by Mtb, and direct
pharmacodynamic effects of antimicrobial agents used in combination with
HDTs. Parametrization of QSP models requires quantitative data of both
the time course of effects (i.e., rates) and the concentration-effect
relationships for therapeutic agents studied; as well as the ability to
perform time course measurements. Data to be measured during such
experiments both include endpoints such as Mtb disease burden, cell
counts of lymphocytes, and regulatory proteins and biomarkers that can
explain observed treatment response[70]. Here, we discuss key in
vitro and in vivo preclinical models that can be of specific relevance
for characterization of HDTs using a QSP modelling approach.
In Vitro Macrophage Infection Models and Advanced Cell
Culture
Systems
Human-derived macrophage and peripheral blood mononuclear cell
(PBMC) cultures are extensively used to screen for the activity of
antimicrobials but also identify potential compounds with HDT
potential[27,71–73]. The in vitro setting allows conducting
experiments in high-throughput setting including the use of reporter
cell lines to screen for specific effects at the molecular level, e.g.,
autophagy induction. This approach thus readily allows characterization
of the time course and dose response relationship of compounds.
Several advanced cell culture systems have been increasingly used to
study Mtb host-pathogen interactions and for screening of compounds
including HDTs, such as based on 3D cell cultures and
organoids[71,74], and the development of a lung-on-chip
system[75]. Similar to simpler cell culture systems, the
longitudinal measurement of cytokines, chemokines, and bacterial load is
possible, and has been used to study HDT interventions[74]. The use
of lung organoids and lung-on-chip and their overall use in drug
discovery and development is yet to be advanced further.
The in vitro hollow fibre infection model (HFIM) is commonly used to
study the direct effects of antimicrobial agents on Mtb, but it also
readily allows to include co-cultures with macrophages to better reflect
aforementioned in vitro intracellular infection systems. In the HFIM,
Mtb is cultured in a closed chemostat system with continuous flow of
medium, while it allows simulation of concentration-time profiles that
occur in patients. The corresponding changes in bacterial load over time
can be quantified, allowing characterization of underlying PK/PD
relationships of antibiotic and/or HDTs in combination with PK/PD
modelling[76].
Whilst all in vitro cell culture-based approaches are attractive for
purposes of screening and quantitative characterization of key
mechanisms and phenotypic response profiles, these systems remain a
simplified model system that does not include all aspects related to the
host immune response. The use of QSP modelling could facilitate
translation of such in vitro responses based on human host cells towards
expected in vivo response.
Zebrafish Infection
Models
Adult zebrafish Mycobacterium marinum (Mm) infection models have
gained increasing attention as a preclinical Mtb infection
model[77–79]. Zebrafish embryos and larvae are of interest due to
their optical transparency and thus allowing the use of advanced imaging
methods. Zebrafish possess an innate immune system that is highly
similar to that of mammals; therefore, it has been used in many studies
for the analysis of cellular and systemic responses to
infection[78,80,81]. Because infection with various mycobacteria,
including Mtb leads rapidly to the formation of granuloma structures
that are highly similar to those observed in human tuberculosis
patients, it has been a successful model to study the progression of
tuberculosis and the effects of drug treatment[82,83]. It also
enables pharmacological screening of drugs, to treat mycobacterial
infection at a high throughput level with an emphasis on the measurement
of drug uptake characteristics[84]. Knockdown and overexpression
experiments in zebrafish combined with QSP modelling would especially
provide insights into contribution of certain component to overall
immune response and anti-TB effects. A recent study provided the proof
of concept that use of zebrafish larvae combined with translational
PK/PD modelling can accurately predict effects of anti-TB drugs in
humans[85]. Thus, zebrafish is a promising experimental TB model
that can be used to generate the data required for QSP models to
evaluate HDT strategies.
Vertebrate Infection
Models
Rodent infection models using mice, rabbits, and guinea pigs are
commonly used as infection model for Mtb[77]. Mice have been used in
TB research from a long time due to the small size, availability of
humanized and genetically modified strains, and cost-benefits over other
vertebrate in vivo models, such as rabbits, guinea pigs, and NHPs. Even
though rodent infection models such as mice incorporate a full immune
system, differences between the human immune response remain[86] and
lead to translational challenges[71]. QSP models could help address
some of those translational challenges. Mice infected with ultra-low
dose aerosol Mtb showed heterogeneous disease progression and granuloma
formation similar to humans. Analysis of the transcriptomics data
obtained from the ultra-low dose Mtb infected mice and the controls
enabled predictions of risk of progression to active TB disease
following Mtb infection in humans.[87] QSP models can incorporate
various doses of Mtb inoculum and findings from the transcriptomics data
analysis, and can enable translational predictions of treatment
outcome[88–90].
NHPs have been widely used in immunology research and TB vaccine
studies. NHP-Mtb infection models are of interest to generate
HDT-relevant data due to their similarities to humans in basic
physiology, immunology, and disease pathology. The use of these models
has been however limited in TB treatment research due to the
requirements of scientific and financial resources as well as safety
issues due to highly infectious and contagious nature of Mtb.[91]
QSP models can fill in the gaps between in vitro, zebrafish, smaller
vertebrates such as mice, and humans to overcome the limitations of NHP
models. In general, QSP models can link results from various
experimental infection models to enable predictions in humans.