Animal models of tuberculosis

The Human Pathogen Mycobacterium tuberculosis and the Fish Pathogen Mycobacterium marinum Trigger a Core Set of Late Innate Immune Response Genes in Zebrafish Larvae

Zebrafish is a natural host of various Mycobacterium species and a surrogate model organism for tuberculosis research. Mycobacterium marinum is evolutionarily one of the closest non-tuberculous species related to M. tuberculosis and shares the majority of virulence genes. Although zebrafish is not a natural host of the human pathogen, we have previously demonstrated successful robotic infection of zebrafish embryos with M. tuberculosis and performed drug treatment of the infected larvae. In the present study, we examined for how long M. tuberculosis can be propagated in zebrafish larvae and tested a time series of infected larvae to study the transcriptional response via Illumina RNA deep sequencing (RNAseq). Bacterial aggregates carrying fluorescently labeled M. tuberculosis could be detected up to 9 days post-infection. The infected larvae showed a clear and specific transcriptional immune response with a high similarity to the inflammatory response of zebrafish larvae infected with the surrogate species M. marinum. We conclude that M. tuberculosis can be propagated in zebrafish larvae for at least one week after infection and provide further evidence that M. marinum is a good surrogate model for M. tuberculosis. The generated extensive transcriptome data sets will be of great use to add translational value to zebrafish as a model for infection of tuberculosis using the M. marinum infection system. In addition, we identify new marker genes such as dusp8 and CD180 that are induced by M. tuberculosis infection in zebrafish and in human macrophages at later stages of infection that can be further investigated.

Mathematical modeling suggests heterogeneous replication of Mycobacterium tuberculosis in rabbits

Tuberculosis (TB), the disease caused by Mycobacterium tuberculosis (Mtb), remains a major health problem with 10.6 million cases of the disease and 1.6 million deaths in 2021. It is well understood that pulmonary TB is due to Mtb growth in the lung but quantitative estimates of rates of Mtb replication and death in lungs of patients or animals such as monkeys or rabbits remain largely unknown. We performed experiments with rabbits infected with a novel, virulent clinical Mtb isolate of the Beijing lineage, HN878, carrying an unstable plasmid pBP10. In our in vitro experiments we found that pBP10 is more stable in HN878 strain than in a more commonly used laboratory-adapted Mtb strain H37Rv (the segregation coefficient being s = 0.10 in HN878 vs. s = 0.18 in H37Rv). Interestingly, the kinetics of plasmid-bearing bacteria in lungs of Mtb-infected rabbits did not follow an expected monotonic decline; the percent of plasmid-bearing cells increased between 28 and 56 days post-infection and remained stable between 84 and 112 days post-infection despite a large increase in bacterial numbers in the lung at late time points. Mathematical modeling suggested that such a non-monotonic change in the percent of plasmid-bearing cells can be explained if the lung Mtb population consists of several (at least 2) sub-populations with different replication/death kinetics: one major population expanding early and being controlled/eliminated, while another, a smaller population expanding at later times causing a counterintuitive increase in the percent of plasmid-bearing cells. Importantly, a model with one kinetically homogeneous Mtb population could not explain the data including when the model was run stochastically. Given that in rabbits HN878 strain forms well circumscribed granulomas, our results suggest independent bacterial dynamics in subsets of such granulomas. Our model predictions can be tested in future experiments in which HN878-pBP10 dynamics in individual granulomas is followed over time.

VEGF-delivering PEG hydrogels promote vascularization in the porcine subcutaneous space

For cell therapies, the subcutaneous space is an attractive transplant site due to its large surface area and accessibility for implantation, monitoring, biopsy, and retrieval. However, its poor vascularization has catalyzed research to induce blood vessel formation within the site to enhance cell revascularization and survival. Most studies focus on the subcutaneous space of rodents, which does not recapitulate important anatomical features and vascularization responses of humans. Herein, we evaluate biomaterial-driven vascularization in the porcine subcutaneous space. Additionally, we report the first use of cost-effective fluorescent microspheres to quantify perfusion in the porcine subcutaneous space. We investigate the vascularization-inducing efficacy of vascular endothelial growth factor (VEGF)-delivering synthetic hydrogels based on 4-arm poly(ethylene) glycol macromers with terminal maleimides (PEG-4MAL). We compare three groups: a non-degradable hydrogel with a VEGF-releasing PEG-4MAL gel coating (Core+VEGF gel); an uncoated, non-degradable hydrogel (Core-only); and naïve tissue. After 2 weeks, Core+VEGF gel has significantly higher tissue perfusion, blood vessel area, blood vessel density, and number of vessels compared to both Core-only and naïve tissue. Furthermore, healthy vital signs during surgery and post-procedure metrics demonstrate the safety of hydrogel delivery. We demonstrate that VEGF-delivering synthetic hydrogels induce robust vascularization and perfusion in the porcine subcutaneous space.

Animals in Respiratory Research

The respiratory barrier, a thin epithelial barrier that separates the interior of the human body from the environment, is easily damaged by toxicants, and chronic respiratory diseases are common. It also allows the permeation of drugs for topical treatment. Animal experimentation is used to train medical technicians, evaluate toxicants, and develop inhaled formulations. Species differences in the architecture of the respiratory tract explain why some species are better at predicting human toxicity than others. Some species are useful as disease models. This review describes the anatomical differences between the human and mammalian lungs and lists the characteristics of currently used mammalian models for the most relevant chronic respiratory diseases (asthma, chronic obstructive pulmonary disease, cystic fibrosis, pulmonary hypertension, pulmonary fibrosis, and tuberculosis). The generation of animal models is not easy because they do not develop these diseases spontaneously. Mouse models are common, but other species are more appropriate for some diseases. Zebrafish and fruit flies can help study immunological aspects. It is expected that combinations of in silico, in vitro, and in vivo (mammalian and invertebrate) models will be used in the future for drug development.

Prediction of Tuberculosis From Lung Tissue Images of Diversity Outbred Mice Using Jump Knowledge Based Cell Graph Neural Network

Tuberculosis (TB), primarily affecting the lungs, is caused by the bacterium Mycobacterium tuberculosis and poses a significant health risk. Detecting acid-fast bacilli (AFB) in stained samples is critical for TB diagnosis. Whole Slide (WS) Imaging allows for digitally examining these stained samples. However, current deep-learning approaches to analyzing large-sized whole slide images (WSIs) often employ patch-wise analysis, potentially missing the complex spatial patterns observed in the granuloma essential for accurate TB classification. To address this limitation, we propose an approach that models cell characteristics and interactions as a graph, capturing both cell-level information and the overall tissue micro-architecture. This method differs from the strategies in related cell graph-based works that rely on edge thresholds based on sparsity/density in cell graph construction, emphasizing a biologically informed threshold determination instead. We introduce a cell graph-based jumping knowledge neural network (CG-JKNN) that operates on the cell graphs where the edge thresholds are selected based on the length of the mycobacteria's cords and the activated macrophage nucleus's size to reflect the actual biological interactions observed in the tissue. The primary process involves training a Convolutional Neural Network (CNN) to segment AFBs and macrophage nuclei, followed by converting large (42831*41159 pixels) lung histology images into cell graphs where an activated macrophage nucleus/AFB represents each node within the graph and their interactions are denoted as edges. To enhance the interpretability of our model, we employ Integrated Gradients and Shapely Additive Explanations (SHAP). Our analysis incorporated a combination of 33 graph metrics and 20 cell morphology features. In terms of traditional machine learning models, Extreme Gradient Boosting (XGBoost) was the best performer, achieving an F1 score of 0.9813 and an Area under the Precision-Recall Curve (AUPRC) of 0.9848 on the test set. Among graph-based models, our CG-JKNN was the top performer, attaining an F1 score of 0.9549 and an AUPRC of 0.9846 on the held-out test set. The integration of graph-based and morphological features proved highly effective, with CG-JKNN and XGBoost showing promising results in classifying instances into AFB and activated macrophage nucleus. The features identified as significant by our models closely align with the criteria used by pathologists in practice, highlighting the clinical applicability of our approach. Future work will explore knowledge distillation techniques and graph-level classification into distinct TB progression categories.

Comparative pathology of experimental pulmonary tuberculosis in animal models

Research in human tuberculosis (TB) is limited by the availability of human tissues from patients, which is often altered by therapy and treatment. Thus, the use of animal models is a key tool in increasing our understanding of the pathogenesis, disease progression and preclinical evaluation of new therapies and vaccines. The granuloma is the hallmark lesion of pulmonary tuberculosis, regardless of the species or animal model used. Although animal models may not fully replicate all the histopathological characteristics observed in natural, human TB disease, each one brings its own attributes which enable researchers to answer specific questions regarding TB immunopathogenesis. This review delves into the pulmonary pathology induced by Mycobacterium tuberculosis complex (MTBC) bacteria in different animal models (non-human primates, rodents, guinea pigs, rabbits, cattle, goats, and others) and compares how they relate to the pulmonary disease described in humans. Although the described models have demonstrated some histopathological features in common with human pulmonary TB, these data should be considered carefully in the context of this disease. Further research is necessary to establish the most appropriate model for the study of TB, and to carry out a standard characterisation and score of pulmonary lesions.

Initial immune response after exposure to Mycobacterium tuberculosis or to SARS-COV-2: similarities and differences

Tuberculosis (TB), caused by Mycobacterium tuberculosis (Mtb) and Coronavirus disease-2019 (COVID-19), whose etiologic agent is severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2), are currently the two deadliest infectious diseases in humans, which together have caused about more than 11 million deaths worldwide in the past 3 years. TB and COVID-19 share several aspects including the droplet- and aerosol-borne transmissibility, the lungs as primary target, some symptoms, and diagnostic tools. However, these two infectious diseases differ in other aspects as their incubation period, immune cells involved, persistence and the immunopathological response. In this review, we highlight the similarities and differences between TB and COVID-19 focusing on the innate and adaptive immune response induced after the exposure to Mtb and SARS-CoV-2 and the pathological pathways linking the two infections. Moreover, we provide a brief overview of the immune response in case of TB-COVID-19 co-infection highlighting the similarities and differences of each individual infection. A comprehensive understanding of the immune response involved in TB and COVID-19 is of utmost importance for the design of effective therapeutic strategies and vaccines for both diseases.

Application of Precision-Cut Lung Slices as an In Vitro Model for Research of Inflammatory Respiratory Diseases

The leading cause of many respiratory diseases is an ongoing and progressive inflammatory response. Traditionally, inflammatory lung diseases were studied primarily through animal models, cell cultures, and organoids. These technologies have certain limitations, despite their great contributions to the study of respiratory diseases. Precision-cut lung slices (PCLS) are thin, uniform tissue slices made from human or animal lung tissue and are widely used extensively both nationally and internationally as an in vitro organotypic model. Human lung slices bridge the gap between in vivo and in vitro models, and they can replicate the living lung environment well while preserving the lungs' basic structures, such as their primitive cells and trachea. However, there is no perfect model that can completely replace the structure of the human lung, and there is still a long way to go in the research of lung slice technology. This review details and analyzes the strengths and weaknesses of precision lung slices as an in vitro model for exploring respiratory diseases associated with inflammation, as well as recent advances in this field.

Disruption of MenT2 toxin impairs the growth of Mycobacterium tuberculosis in guinea pigs

Toxin-antitoxin (TA) systems are abundantly present in the genomes of various bacterial pathogens. TA systems have been implicated in either plasmid maintenance or protection against phage infection, stress adaptation or disease pathogenesis. The genome of Mycobacterium tuberculosis encodes for more than 90 TA systems and 4 of these belong to the type IV subfamily (MenAT family). The toxins and antitoxins belonging to type IV TA systems share sequence homology with the AbiEii family of nucleotidyl transferases and the AbiEi family of putative transcriptional regulators, respectively. Here, we have performed experiments to understand the role of MenT2, a toxin from the type IV TA system, in mycobacterial physiology and disease pathogenesis. The ectopic expression of MenT2 using inducible vectors does not inhibit bacterial growth in liquid cultures. Bioinformatic and molecular modelling analysis suggested that the M. tuberculosis genome has an alternative start site upstream of the annotated menT2 gene. The overexpression of the reannotated MenT2 resulted in moderate growth inhibition of Mycobacterium smegmatis. We show that both menT2 and menA2 transcript levels are increased when M. tuberculosis is exposed to nitrosative stress, in vitro. When compared to the survival of the wild-type and the complemented strain, the ΔmenT2 mutant strain of M. tuberculosis was more resistant to being killed by nitrosative stress. However, the survival of both the ΔmenT2 mutant and the wild-type strain was similar in macrophages and when exposed to other stress conditions. Here, we show that MenT2 is required for the establishment of disease in guinea pigs. Gross pathology and histopathology analysis of lung tissues from guinea pigs infected with the ∆menT2 strain revealed significantly reduced tissue damage and inflammation. In summary, these results provide new insights into the role of MenT2 in mycobacterial pathogenesis.

A comparative study of a rabbit spinal tuberculosis model constructed by local direct infection via the posterior lateral approach

The present study aims to establish a method of constructing a New Zealand rabbit spinal tuberculosis model by direct local infusion of M. tuberculosis H37Rv strain into the intervertebral disc space through the posterior lateral approach. Sixty-six New Zealand rabbits were pretreated with complete Freund's adjuvant and randomly divided into 4 group: the posterolateral approach model group (Group A, 25), ventral transverse process approach model group (Group B, 25), control group (Group C, 10), and blank group (Group D, 6). In Groups A and B, the bone holes were filled with gelatin sponge after drilling, and the local area was directly infused with 0.1 ml of M. tuberculosis H37Rv strain suspension. In Group C, the gelatin sponge was filled through the posterolateral approach and the local area was infused with 0.1 ml of normal saline suspension. In Group D, No specific treatment was performed. The general conditions of the experimental rabbits in each group were compared to those of a control group; the degree of vertebral body exposure, incision length, and complications of the two methods were compared; and the tuberculosis models were evaluated by imaging, histopathology, and bacterial culture. In Group A, the lateral side of the vertebral body was well exposed, the damage was mild, and no peritoneal rupture or gastrointestinal complications were observed. In Group B, the ventral side of the vertebral body and the intervertebral disc were exposed, and abdominal complications were more likely to occur. The survival rates of the experimental rabbits at 8 weeks after surgery were 92.0% in Group A, 88.00% in Group B, 90.0% in Group C, and 100% in Group D. MRI examinations showed that in Group A, the positive rate of radiographic bone findings was 86.9% at 4 weeks after surgery and 100% at 8 weeks after surgery; in Group B, the positive rate of radiographic bone findings was 78.2% at 4 weeks after surgery and 95.4% at 8 weeks after surgery. There was no significant difference between Groups A and B in the radiographic bone findings rate detected by the same imaging method at the same time point (P > 0.05). Eight weeks after surgery, bone destruction, paravertebral abscess, and caseous necrosis occurred in the vertebral bodies of surviving rabbits in Groups A and B. The BacT/ALERT 3D rapid culture system was used to culture the pus in the lesion, and the results showed that the positive rate of tuberculosis was 52.17% in Group A and 54.54% in Group B, and the difference was not statistically significant (P > 0.05). After pretreatment with complete Freund's adjuvant, direct infusion of the H37Rv strain of M. tuberculosis into the intervertebral disc space of New Zealand rabbits via the posterolateral approach and the ventral transverse process approach can successfully establish rabbit spinal tuberculosis models.

Mice infected with Mycobacterium tuberculosis are resistant to acute disease caused by secondary infection with SARS-CoV-2

Mycobacterium tuberculosis (Mtb) and SARS-CoV-2 (CoV2) are the leading causes of death due to infectious disease. Although Mtb and CoV2 both cause serious and sometimes fatal respiratory infections, the effect of Mtb infection and its associated immune response on secondary infection with CoV2 is unknown. To address this question we applied two mouse models of COVID19, using mice which were chronically infected with Mtb. In both model systems, Mtb-infected mice were resistant to the pathological consequences of secondary CoV2 infection, and CoV2 infection did not affect Mtb burdens. Single cell RNA sequencing of coinfected and monoinfected lungs demonstrated the resistance of Mtb-infected mice is associated with expansion of T and B cell subsets upon viral challenge. Collectively, these data demonstrate that Mtb infection conditions the lung environment in a manner that is not conducive to CoV2 survival.

Mycobacterium tuberculosis (Mtb) and SARS-CoV-2 (CoV2) are distinct organisms which both cause lung disease. We report the surprising observation that Mtb-infected mice are resistant to secondary infection with CoV2, with no impact on Mtb burden and resistance associating with lung T and B cell expansion.

Evaluating the effects of Cyclosporine A immunosuppression on Mycobacterial infection by inhaling of Cyclosporine A administrated BALB/c mice with live Bacillus Calmette Guérin

Cyclosporine A (CsA) is an immunosuppressive drug used in organ transplantation and treatment of autoimmune diseases. Effects of CsA on determining the direction of the immune response and pathogenesis of infections by altering immune responses particulary T cells functions have always been questionable. We evaluated the effect of different doses of CsA on course of infection in BALB/c mice infected with live Bacillus Calmette Guérin (BCG) (as an example of Mycobacterial infections). Four groups of mice (n = 5) receiving 5, 25, 125, and 0 mg/kg of CsA, three times a week, were infected with BCG aerosolly. Before BCG inhalation and 40-/60- days post-infection, cell proliferation and CD4⁺CD25⁺ cell percentage were evaluated in splenocytes of mice after culture and stimulation with PHA or BCG lysate. The histopathological alterations and bacterial burden were assessed in lung tissue. Cells showed a dose-dependent decrease in proliferation and the percentage of CD4⁺ CD25⁺ cells. After BCG infection, in presence of dose 125 mg/kg, there were some exceptions. The number of bacteria and histopathological lesions and inflammation in lung tissues increased in a dose-dependent manner. CsA immunosuppressed BCG infected mice can be used as a safe model for studying Mycobacterium species pathogenesis and related cellular immune responses.

M. tuberculosis CRISPR/Cas proteins are secreted virulence factors that trigger cellular immune responses

The role of prokaryotic CRISPR/Cas system proteins as a defensive shield against invasive nucleic acids has been studied extensively. Non-canonical roles in pathogenesis involving intracellular targeting of certain virulence-associated endogenous mRNA have also been reported for some Type I and Type II CRISPR/Cas proteins, but no such roles have yet been established for Type III system proteins. Here, we demonstrate that M. tuberculosis (Type III-A system) CRISPR/Cas proteins Csm1, Csm3, Csm5, Csm6, and Cas6 are secreted and induce host immune responses. Using cell and animal experiments, we show that Cas6, in particular, provokes IFN-γ release from PBMCs from active tuberculosis (TB) patients, and its deletion markedly attenuates virulence in a murine M. tuberculosis challenge model. Recombinant MTBCas6 induces apoptosis of macrophages and lung fibroblasts, and interacts with the surface of cells in a caspase and TLR-2 independent manner. Transcriptomic and signal pathway studies using THP-1 macrophages stimulated with MTBCas6 indicated that MTBCas6 upregulates expression of genes associated with the NF-κB pathway leading to higher levels of IL-6, IL-1β, and TNF-α release, cytokines known to activate immune system cells in response to M. tuberculosis infection. Our findings suggest that, in addition to their intracellular shielding role, M. tuberculosis CRISPR/Cas proteins have non-canonical extracellular roles, functioning like a virulent sword, and activating host immune responses.

Mesenchymal Stem Cells and Tuberculosis: Clinical Challenges and Opportunities

Tuberculosis (TB) is one of the communicable diseases caused by Mycobacterium tuberculosis (Mtb) infection, affecting nearly one-third of the world's population. However, because the pathogenesis of TB is still not fully understood and the development of anti-TB drug is slow, TB remains a global public health problem. In recent years, with the gradual discovery and confirmation of the immunomodulatory properties of mesenchymal stem cells (MSCs), more and more studies, including our team's research, have shown that MSCs seem to be closely related to the growth status of Mtb and the occurrence and development of TB, which is expected to bring new hope for the clinical treatment of TB. This article reviews the relationship between MSCs and the occurrence and development of TB and the potential application of MSCs in the treatment of TB.

Disease Models: Lung Models for Testing Drugs Against Inflammation and Infection

Lung diseases have increasingly attracted interest in the past years. The all-known fear of failing treatments against severe pulmonary infections and plans of the pharmaceutical industry to limit research on anti-infectives to a minimum due to cost reasons makes infections of the lung nowadays a "hot topic." Inhalable antibiotics show promising efficacy while limiting adverse systemic effects to a minimum. Moreover, in times of increased life expectancy in developed countries, the treatment of chronic maladies implicating inflammatory diseases, like bronchial asthma or chronic obstructive pulmonary disease, becomes more and more exigent and still lacks proper treatment.In this chapter, we address in vitro models as well as necessary in vivo models to help develop new drugs for the treatment of various severe pulmonary diseases with a strong focus on infectious diseases. By first presenting the essential hands-on techniques for the setup of in vitro models, we intend to combine these with already successful and interesting model approaches to serve as some guideline for the development of future models. The overall goal is to maximize time and cost-efficacy and to minimize attrition as well as animal trials when developing novel anti-infective therapeutics.

Lipid nanoparticles with improved biopharmaceutical attributes for tuberculosis treatment

Tuberculosis is a topic of relevance worldwide because of the social and biological factors that triggered the disease and the economic burden on the health-care systems that imply its therapeutic treatment. Challenges to handle these issues include, among others, research on technological breakthroughs modifying the drug regimens to facilitate therapy adherence, avoid mycobacterium drug resistance, and minimize toxic side-effects. Lipid nanoparticles arise as a promising strategy in this respect as deduced from the reported scientific data. They are prepared from biodegradable and biocompatible starting materials and compared to the use of the free drugs, the entrapment of active molecules into the carriers might lead to both dose reduction and controlled delivery. Moreover, the target to the lung, the organ mainly affected by the disease, could be possible if the particle surface is modified. Although conclusive statements cannot be made considering the limited number of available research works, looking into what has been achieved up to now definitively encourages to continue investigations in this regard.

The potential of microdialysis to estimate rifampicin concentrations in the lung of guinea pigs

Optimised pre-clinical models are required for TB drug development to better predict the pharmacokinetics of anti-tuberculosis (anti-TB) drugs to shorten the time taken for novel drugs and combinations to be approved for clinical trial. Microdialysis can be used to measure unbound drug concentrations in awake freely moving animals in order to describe the pharmacokinetics of drugs in the organs as a continuous sampling technique. The aim of this work was to develop and optimise the microdialysis methodology in guinea pigs to better understand the pharmacokinetics of rifampicin in the lung. In vitro experiments were performed before progressing into in vivo studies because the recovery (concentration of the drug in the tissue fluid related to that in the collected dialysate) of rifampicin was dependent on a variety of experimental conditions. Mass spectrometry of the dialysate was used to determine the impact of flow rate, perfusion fluid and the molecular weight cut-off and membrane length of probes on the recovery of rifampicin at physiologically relevant concentrations. Following determination of probe efficiency and identification of a correlation between rifampicin concentrations in the lung and skeletal muscle, experiments were conducted to measure rifampicin in the sacrospinalis of guinea pigs using microdialysis. Lung concentrations of rifampicin were estimated from the rifampicin concentrations measured in the sacrospinalis. These studies suggest the potential usefulness of the microdialysis methodology to determine drug concentrations of selected anti-TB drugs to support new TB drug development.

Addressing Latent Tuberculosis: New Advances in Mimicking the Disease, Discovering Key Targets, and Designing Hit Compounds

Despite being discovered and isolated more than one hundred years ago, tuberculosis (TB) remains a global public health concern arch. Our inability to eradicate this bacillus is strongly related with the growing resistance, low compliance to current drugs, and the capacity of the bacteria to coexist in a state of asymptomatic latency. This last state can be sustained for years or even decades, waiting for a breach in the immune system to become active again. Furthermore, most current therapies are not efficacious against this state, failing to completely clear the infection. Over the years, a series of experimental methods have been developed to mimic the latent state, currently used in drug discovery, both in vitro and in vivo. Most of these methods focus in one specific latency inducing factor, with only a few taking into consideration the complexity of the granuloma and the genomic and proteomic consequences of each physiological factor. A series of targets specifically involved in latency have been studied over the years with promising scaffolds being discovered and explored. Taking in account that solving the latency problem is one of the keys to eradicate the disease, herein we compile current therapies and diagnosis techniques, methods to mimic latency and new targets and compounds in the pipeline of drug discovery.

Neonatal and infant immunity for tuberculosis vaccine development: importance of age-matched animal models

Neonatal and infant immunity differs from that of adults in both the innate and adaptive arms, which are critical contributors to immune-mediated clearance of infection and memory responses elicited during vaccination. The tuberculosis (TB) research community has openly admitted to a vacuum of knowledge about neonatal and infant immune responses to Mycobacterium tuberculosis (Mtb) infection, especially in the functional and phenotypic attributes of memory T cell responses elicited by the only available vaccine for TB, the Bacillus Calmette-Guérin (BCG) vaccine. Although BCG vaccination has variable efficacy in preventing pulmonary TB during adolescence and adulthood, 80% of endemic TB countries still administer BCG at birth because it has a good safety profile and protects children from severe forms of TB. As such, new vaccines must work in conjunction with BCG at birth and, thus, it is essential to understand how BCG shapes the immune system during the first months of life. However, many aspects of the neonatal and infant immune response elicited by vaccination with BCG remain unknown, as only a handful of studies have followed BCG responses in infants. Furthermore, most animal models currently used to study TB vaccine candidates rely on adult-aged animals. This presents unique challenges when transitioning to human trials in neonates or infants. In this Review, we focus on vaccine development in the field of TB and compare the relative utility of animal models used thus far to study neonatal and infant immunity. We encourage the development of neonatal animal models for TB, especially the use of pigs.

The rough colony morphotype of Mycobacterium avium exhibits high virulence in human macrophages and mice

Introduction. The incidence of Mycobacterium avium complex (MAC) pulmonary disease (MAC PD), a refractory chronic respiratory tract infection, is increasing worldwide. MAC has three predominant colony morphotypes: smooth opaque (SmO), smooth transparent (SmT) and rough (Rg).Aim. To determine whether colony morphotypes can predict the prognosis of MAC PD, we evaluated the virulence of SmO, SmT and Rg in mice and in human macrophages.Methodology. We compared the characteristics of mice and human macrophages infected with the SmO, SmT, or Rg morphotypes of M. avium subsp. hominissuis 104. C57BL/6 mice and human macrophages derived from peripheral mononuclear cells were used in these experiments.Results. In comparison to SmO- or SmT-infected mice, Rg-infected mice revealed severe pathologically confirmed pneumonia, increased lung weight and increased lung bacterial burden. Rg-infected macrophages revealed significant cytotoxicity, increased bacterial burden, secretion of proinflammatory cytokines (TNF-α and IL-6) and chemokines (CCL5 and CCL3), and formation of cell clusters. Rg formed larger bacterial aggregates than SmO and SmT. Cytotoxicity, bacterial burden and secretion of IL-6, CCL5 and CCL3 were induced strongly by Rg infection, and were decreased by disaggregation of the bacteria.Conclusion. M. avium Rg, which is associated with bacterial aggregation, has the highest virulence among the predominant colony morphotypes.

Human mesenchymal stem cell based intracellular dormancy model of Mycobacterium tuberculosis

Understanding the biology of the tuberculosis pathogen during dormant asymptomatic infection, called latent tuberculosis is crucial to decipher a resilient therapeutic strategy for the disease. Recent discoveries exhibiting presence of pathogen's DNA and bacilli in mesenchymal stem cells (MSCs) of human and mouse despite completion of antitubercular therapy, indicates that these specific cells could be one of the niches for dormant Mycobacterium tuberculosis in humans. To determine if in vitro infection of human MSCs could recapitulate the in vivo characteristics of dormant M. tuberculosis, we examined survival, phenotype, and drug susceptibility of the pathogen in MSCs. When a very low multiplicity of infection (1:1) was used, M. tuberculosis could survive in human bone marrow derived MSCs for more than 22 days without any growth. At this low level of infection, the pathogen did not cause any noticeable host cell death. During the later phase of infection, MSC-residing M. tuberculosis exhibited increased expression of HspX (a 16-kDa alpha-crystallin homolog) with a concurrent increase in tolerance to the frontline antitubercular drugs Rifampin and isoniazid. These results present a human MSC-based intracelllular model of M. tuberculosis infection to dissect the mechanisms through which the pathogen acquires and maintains dormancy in the host.

Repetitive Aerosol Exposure Promotes Cavitary Tuberculosis and Enables Screening for Targeted Inhibitors of Extensive Lung Destruction

Background

Cavitation is a serious consequence of tuberculosis. We tested the hypothesis that repetitive exposure to the same total bacterial burden of Mycobacterium tuberculosis drives greater lung destruction than a single exposure. We also tested whether inhibition of endogenous matrix metalloproteinase-1 (MMP-1) may inhibit cavitation during tuberculosis.

Methods

Over a 3-week interval, we infected rabbits with either 5 aerosols of 500 colony-forming units (CFU) of M. tuberculosis or a single aerosol of 2500 CFU plus 4 sham aerosols. We administered the MMP-1 inhibitor cipemastat (100 mg/kg daily) during weeks 5-10 to a subset of the animals.

Results

Repetitive aerosol infection produced greater lung inflammation and more cavities than a single aerosol infection of the same bacterial burden (75% of animals vs 25%). Necropsies confirmed greater lung pathology in repetitively exposed animals. For cipemastat-treated animals, there was no significant difference in cavity counts, cavity volume, or disease severity compared to controls.

Conclusions

Our data show that repetitive aerosol exposure with M. tuberculosis drives greater lung damage and cavitation than a single exposure. This suggests that human lung destruction due to tuberculosis may be exacerbated in settings where individuals are repeatedly exposed. MMP-1 inhibition with cipemastat did not prevent the development of cavitation in our model.

Precision mouse models with expanded tropism for human pathogens

A major limitation of current humanized mouse models is that they primarily enable the analysis of human-specific pathogens that infect hematopoietic cells. However, most human pathogens target other cell types, including epithelial, endothelial and mesenchymal cells. Here, we show that implantation of human lung tissue, which contains up to 40 cell types, including nonhematopoietic cells, into immunodeficient mice (lung-only mice) resulted in the development of a highly vascularized lung implant. We demonstrate that emerging and clinically relevant human pathogens such as Middle East respiratory syndrome coronavirus, Zika virus, respiratory syncytial virus and cytomegalovirus replicate in vivo in these lung implants. When incorporated into bone marrow/liver/thymus humanized mice, lung implants are repopulated with autologous human hematopoietic cells. We show robust antigen-specific humoral and T-cell responses following cytomegalovirus infection that control virus replication. Lung-only mice and bone marrow/liver/thymus-lung humanized mice substantially increase the number of human pathogens that can be studied in vivo, facilitating the in vivo testing of therapeutics.

Implantation of lung tissue into humanized mice enables in vivo study of the human immune response to pathogens.

Structures of soluble rabbit neprilysin complexed with phosphoramidon or thiorphan

Neutral endopeptidase (neprilysin; NEP) is a proteinase that cleaves a wide variety of peptides and has been implicated in Alzheimer's disease, cardiovascular conditions, arthritis and other inflammatory diseases. The structure of the soluble extracellular domain (residues 55-750) of rabbit neprilysin was solved both in its native form at 2.1 Å resolution, and bound to the inhibitors phosphoramidon and thiorphan at 2.8 and 3.0 Å resolution, respectively. Consistent with the extracellular domain of human neprilysin, the structure reveals a large central cavity which contains the active site and the location for inhibitor binding.

Humanized Mouse Model Mimicking Pathology of Human Tuberculosis for in vivo Evaluation of Drug Regimens

Human immune system mice are highly valuable for in vivo dissection of human immune responses. Although they were employed for analyzing tuberculosis (TB) disease, there is little data on the spatial organization and cellular composition of human immune cells in TB granuloma pathology in this model. We demonstrate that human immune system mice, generated by transplanted human fetal liver derived hematopoietic stem cells develop a continuum of pulmonary lesions upon Mycobacterium tuberculosis aerosol infection. In particular, caseous necrotic granulomas, which contribute to prolonged TB treatment time, developed, and had cellular phenotypic spatial-organization similar to TB patients. By comparing two recommended drug regimens, we confirmed observations made in clinical settings: Adding Moxifloxacin to a classical chemotherapy regimen had no beneficial effects on bacterial eradication. We consider this model instrumental for deeper understanding of human specific features of TB pathogenesis and of particular value for the pre-clinical drug development pipeline.

Animal models of tuberculosis: Lesson learnt

Tuberculosis (TB) remains a leading cause of death globally among infectious diseases that has killed more numbers of people than any other infectious diseases. Animal models have become the lynchpin for mimicking human infectious diseases. Research on TB could be facilitated by animal challenge models such as the guinea pig, mice, rabbit and non-human primates. No single model presents all aspects of disease pathogenesis due to considerable differences in disease resistance/susceptibility between these models. Availability of a wide range of animal strains, Mycobacterium tuberculosis strains, route of infection and doses affect the disease progression and intervention outcome. Different animal models have contributed significantly to the drug and vaccine development, identification of biomarkers, understanding of TB immunopathogenesis and host genetic influence on infection. In this review, the commonly used animal models in TB research are discussed along with their advantages and limitations.

Classification and reporting of severity experienced by animals used in scientific procedures: FELASA/ECLAM/ESLAV Working Group report

Directive 2010/63/EU introduced requirements for the classification of the severity of procedures to be applied during the project authorisation process to use animals in scientific procedures and also to report actual severity experienced by each animal used in such procedures. These requirements offer opportunities during the design, conduct and reporting of procedures to consider the adverse effects of procedures and how these can be reduced to minimize the welfare consequences for the animals. Better recording and reporting of adverse effects should also help in highlighting priorities for refinement of future similar procedures and benchmarking good practice. Reporting of actual severity should help inform the public of the relative severity of different areas of scientific research and, over time, may show trends regarding refinement. Consistency of assignment of severity categories across Member States is a key requirement, particularly if re-use is considered, or the safeguard clause is to be invoked. The examples of severity classification given in Annex VIII are limited in number, and have little descriptive power to aid assignment. Additionally, the examples given often relate to the procedure and do not attempt to assess the outcome, such as adverse effects that may occur. The aim of this report is to deliver guidance on the assignment of severity, both prospectively and at the end of a procedure. A number of animal models, in current use, have been used to illustrate the severity assessment process from inception of the project, through monitoring during the course of the procedure to the final assessment of actual severity at the end of the procedure (Appendix 1).

Tuberculosis vaccines: Opportunities and challenges

Tuberculosis (TB) is a serious disease around the world. Bacillus Calmette-Guérin (BCG) is the only TB vaccine licensed for use in human beings, and is effective in protecting infants and children against severe miliary and meningeal TB. However, BCG's protective efficacy is variable in adults. Novel TB vaccine candidates being developed include whole-cell vaccines (recombinant BCG (rBCG), attenuated Mycobacterium tuberculosis, killed M. tuberculosis or Mycobacterium vaccae), adjuvanted protein subunit vaccines, viral vector-delivered subunit vaccines, plasmid DNA vaccines, RNA-based vaccines etc. At least 12 novel TB vaccine candidates are now in clinical trials, including killed M. vaccae, rBCG ΔureC::hly, adjuvanted fusion proteins M72 and H56 and viral vectored MVA85A. Unfortunately, in TB, there are no correlates of vaccine-induced protection, although cell-mediated immune responses such as interferon-gamma (IFN-γ) production are widely used to assess vaccine's immunogenicity. Recent studies suggested that central memory T cells and local secreted IgA correlated with protection against TB disease. Clinical TB vaccine efficacy trials should invest in identifying correlates of protection, and evaluate new TB biomarkers emerging from human and animal studies. Accumulating new knowledge on M. tuberculosis antigens and immune profiles correlating with protection or disease risk will be of great help in designing next generation of TB vaccines.

Experimental study of tuberculosis: From animal models to complex cell systems and organoids

Tuberculosis (TB) is a devastating disease to mankind that has killed more people than any other infectious disease. Despite many efforts and successes from the scientific and health communities, the prospect of TB elimination remains distant. On the one hand, sustainable public health programs with affordable and broad implementation of anti-TB measures are needed. On the other hand, achieving TB elimination requires critical advances in three areas: vaccination, diagnosis, and treatment. It is also well accepted that succeeding in advancing these areas requires a deeper knowledge of host—pathogen interactions during infection, and for that, better experimental models are needed. Here, we review the potential and limitations of different experimental approaches used in TB research, focusing on animal and human-based cell culture models. We highlight the most recent advances in developing in vitro 3D models and introduce the potential of lung organoids as a new tool to study Mycobacterium tuberculosis infection.

Tuberculosis (TB) is the number 1 killer in the world due to a bacterial infection. The study of this disease through clinical and epidemiological data and through the use of different experimental models has provided important knowledge on the role of the immune response generated during infection. This is critical for the development of novel vaccines and therapeutic strategies. However, in spite of the advances made, it is well accepted that better models are needed to study TB. This review discusses the different models used to study TB, highlighting the advantages and disadvantages of the available animal and cellular models and introducing recently developed state-of-the-art approaches based on human-based cell culture systems. These new advances are integrated in a road map for future study of TB, converging for the potential of lung organoids in TB research.

Mouse and Guinea Pig Models of Tuberculosis

This article describes the nature of the host response to Mycobacterium tuberculosis in the mouse and guinea pig models of infection. It describes the great wealth of information obtained from the mouse model, reflecting the general availability of immunological reagents, as well as genetic manipulations of the mouse strains themselves. This has led to a good understanding of the nature of the T-cell response to the infection, as well as an appreciation of the complexity of the response involving multiple cytokine- and chemokine-mediated systems. As described here and elsewhere, we have a growing understanding of how multiple CD4-positive T-cell subsets are involved, including regulatory T cells, TH17 cells, as well as the subsequent emergence of effector and central memory T-cell subsets. While, in contrast, our understanding of the host response in the guinea pig model is less advanced, considerable strides have been made in the past decade in terms of defining the basis of the immune response, as well as a better understanding of the immunopathologic process. This model has long been the gold standard for vaccine testing, and more recently is being revisited as a model for testing new drug regimens (bedaquiline being the latest example).

bioA mutant of Mycobacterium tuberculosis shows severe growth defect and imparts protection against tuberculosis in guinea pigs

Owing to the devastation caused by tuberculosis along with the unsatisfactory performance of the Bacillus Calmette–Guérin (BCG) vaccine, a more efficient vaccine than BCG is required for the global control of tuberculosis. A number of studies have demonstrated an essential role of biotin biosynthesis in the growth and survival of several microorganisms, including mycobacteria, through deletion of the genes involved in de novo biotin biosynthesis. In this study, we demonstrate that a bioA mutant of Mycobacterium tuberculosis (MtbΔbioA) is highly attenuated in the guinea pig model of tuberculosis when administered aerogenically as well as intradermally. Immunization with MtbΔbioA conferred significant protection in guinea pigs against an aerosol challenge with virulent M. tuberculosis, when compared with the unvaccinated animals. Booster immunization with MtbΔbioA offered no advantage over a single immunization. These experiments demonstrate the vaccinogenic potential of the attenuated M. tuberculosis bioA mutant against tuberculosis.

Immune Responses to Bacillus Calmette-Guérin Vaccination: Why Do They Fail to Protect against Mycobacterium tuberculosis?

Mycobacterium tuberculosis (M.tb), the causative agent of tuberculosis (TB), is the current leading cause of death due to a single infectious organism. Although curable, the broad emergence of multi-, extensive-, extreme-, and total-drug resistant strains of M.tb has hindered eradication efforts of this pathogen. Furthermore, computational models predict a quarter of the world’s population is infected with M.tb in a latent state, effectively serving as the largest reservoir for any human pathogen with the ability to cause significant morbidity and mortality. The World Health Organization has prioritized new strategies for improved vaccination programs; however, the lack of understanding of mycobacterial immunity has made it difficult to develop new successful vaccines. Currently, Mycobacterium bovis bacillus Calmette–Guérin (BCG) is the only vaccine approved for use to prevent TB. BCG is highly efficacious at preventing meningeal and miliary TB, but is at best 60% effective against the development of pulmonary TB in adults and wanes as we age. In this review, we provide a detailed summary on the innate immune response of macrophages, dendritic cells, and neutrophils in response to BCG vaccination. Additionally, we discuss adaptive immune responses generated by BCG vaccination, emphasizing their specific contributions to mycobacterial immunity. The success of future vaccines against TB will directly depend on our understanding of mycobacterial immunity.

Generation and characterization of induced pluripotent stem cells from guinea pig fetal fibroblasts

Induced pluripotent stem cells (iPS) represent an important tool to develop disease-modeling assays, drug testing assays and cell-based replacement therapies. The application of iPS in these fields requires the development of suitable animal models. Of the suitable species, guinea pigs are particularly important and offer significant advantages. Successful iPS generation has been accomplished in a number of species; however, it has not been reported in the guinea pig. The present study successfully generated iPS from guinea pigs (giPS) using single polycistronic virus transduction with mouse octamer-binding transcription factor 4 (Oct4), sex determining region Y-box 2 (Sox2), Kruppel-like factor 4 and c-Myc. The giPS cell lines were cultured in media containing leukemia inhibitory factor and guinea pig fibroblast cells were used as feeder cells. These cultures were expanded under feeder-free culture conditions using ESGRO Complete Plus Clonal Grade medium containing 15% fetal bovine serum on gelatin-coated dishes. The resultant cells had a normal karyotype, exhibited alkaline phosphatase activity and expressed the pluripotency markers Oct4, Sox2 and Nanog. The cells differentiated in vivo to form teratomas that contained all three germ layers of the tissue cells. The generation of giPS may facilitate future studies investigating the mechanisms underlying innate immunity, particularly for tuberculosis. These experiments provide proof of principle that iPS technology may be adapted to use the guinea pig as a model of human diseases.

A novel benzothiazinethione analogue SKLB-TB1001 displays potent antimycobacterial activities in a series of murine models

New chemotherapeutic compounds and regimens are needed to combat multidrug-resistant Mycobacterium tuberculosis. Here, we used a series of murine models to assess an antitubercular lead compound SKLB-TB1001. In the Mycobacterium bovis bacillus Calmette-Guérin and the acute M. tuberculosis H37Rv infection mouse models, SKLB-TB1001 significantly attenuated the mycobacterial load in lungs and spleens. The colony forming unit counts and histological examination of lungs from H37Rv infected mice revealed that the benzothiazinethione analogue SKLB-TB1001 as a higher dose level was as effective as isoniazid. Moreover, in a multidrug-resistant (MDR)-TB mouse model, SKLB-TB1001 showed significant activity in a dose-dependent manner and was more effective than streptomycin. These results suggested that SKLB-TB1001 could be an antitubercular drug candidate worth further investigation.

Tuberculosis Susceptibility and Vaccine Protection Are Independently Controlled by Host Genotype

The outcome of Mycobacterium tuberculosis infection and the immunological response to the bacillus Calmette-Guerin (BCG) vaccine are highly variable in humans. Deciphering the relative importance of host genetics, environment, and vaccine preparation for the efficacy of BCG has proven difficult in natural populations. We developed a model system that captures the breadth of immunological responses observed in outbred individual mice, which can be used to understand the contribution of host genetics to vaccine efficacy. This system employs a panel of highly diverse inbred mouse strains, consisting of the founders and recombinant progeny of the "Collaborative Cross" project. Unlike natural populations, the structure of this panel allows the serial evaluation of genetically identical individuals and the quantification of genotype-specific effects of interventions such as vaccination. When analyzed in the aggregate, our panel resembled natural populations in several important respects: the animals displayed a broad range of susceptibility to M. tuberculosis, differed in their immunological responses to infection, and were not durably protected by BCG vaccination. However, when analyzed at the genotype level, we found that these phenotypic differences were heritable. M. tuberculosis susceptibility varied between lines, from extreme sensitivity to progressive M. tuberculosis clearance. Similarly, only a minority of the genotypes was protected by vaccination. The efficacy of BCG was genetically separable from susceptibility to M. tuberculosis, and the lack of efficacy in the aggregate analysis was driven by nonresponsive lines that mounted a qualitatively distinct response to infection. These observations support an important role for host genetic diversity in determining BCG efficacy and provide a new resource to rationally develop more broadly efficacious vaccines.

IMPORTANCE

Tuberculosis (TB) remains an urgent global health crisis, and the efficacy of the currently used TB vaccine, M. bovis BCG, is highly variable. The design of more broadly efficacious vaccines depends on understanding the factors that limit the protection imparted by BCG. While these complex factors are difficult to disentangle in natural populations, we used a model population of mice to understand the role of host genetic composition in BCG efficacy. We found that the ability of BCG to protect mice with different genotypes was remarkably variable. The efficacy of BCG did not depend on the intrinsic susceptibility of the animal but, instead, correlated with qualitative differences in the immune responses to the pathogen. These studies suggest that host genetic polymorphism is a critical determinant of vaccine efficacy and provide a model system to develop interventions that will be useful in genetically diverse populations.

Development of immune-biomarkers of pulmonary tuberculosis in a rabbit model

Tuberculosis (TB) causes extensive morbidity and mortality worldwide with approximately 10 million new cases of active disease emerging mostly from a pool of two billion individuals latently infected with Mycobacterium tuberculosis (M. tb) every year. The underlying host immune responses that drive M. tb infection to active disease or latency are not well understood. We propose that identification and characterization of host immune biomarkers will be helpful to better understand the mechanisms that drive this process, and may, in addition, lead to the development of better diagnostic tools for TB. We have previously reported the profiles of plasma immune biomarkers in pulmonary TB patients in endemic countries, and in M. tb-infected nonhuman primates. However, biomarker profiling for a cost-effective and user-friendly animal model relevant to human disease, such as rabbit, has not been developed. One challenge in the analysis of circulating cytokines/chemokines for rabbit model of TB is the limited availability of validated immune-reagents. Here we report the use of a commercially available multiplex microbead human cytokine/chemokine panels as development platform for rabbit immune reagents. The results demonstrate their utility to determine circulating analytes and define their profiles related to TB in the rabbit model. In addition, we report the profiles of circulating anti-M. tb antibodies in the plasma of rabbits with active pulmonary TB. These studies show that the pattern of expression of circulating immune biomarkers correlate with TB pathology in rabbits, and are similar to those defined in pulmonary TB patients.

Mathematical modeling and systems pharmacology of tuberculosis: Isoniazid as a case study

Tuberculosis (TB) treatment needs to be optimized as it is currently long and associated with increasing drug resistance. The antimycobacterial effect of isoniazid (INH) is characterized by a biphasic kill curve, whose causes are still debated. In this work, we developed a complete mathematical model describing the time-course of TB infection and its treatment by INH in human lung. This model was based on a pharmacokinetic model, a pharmacodynamic model and a pathophysiological model. It was used to simulate the antibacterial effect of INH during the first days of therapy. This full model adequately reproduced some qualitative and quantitative properties of the early bactericidal activity of INH observed in TB patients. The kill curves simulated with the model reproduced the biphasic killing effect of INH and the predicted declines in extracellular bacteria were comparable to clinical data. A sensitivity analysis provided interesting insights regarding the biphasic kill curve. The first phase appeared to be essentially driven by the drug effect. In the second phase, while drug pharmacology was the major determinant of the antibacterial effect, a slight influence of the dynamics of infected macrophages was also observed. This work permits to formulate hypotheses for optimizing the efficacy of TB drug candidates and confirms the utility of mathematical modeling to generate new assumptions for TB research.

Current perspective in tuberculosis vaccine development for high TB endemic regions

Tuberculosis (TB) continues to be a global epidemic, despite of the availability of Bacillus Calmette Guerin (BCG) vaccine for more than six decades. In an effort to eradicate TB, vaccinologist around the world have made considerable efforts to develop improved vaccine candidates, based on the understanding of BCG failure in developing world and immune response thought to be protective against TB. The present review represents a current perspective on TB vaccination research, including additional research strategies needed for increasing the efficacy of BCG, and for the development of new effective vaccines for high TB endemic regions.

Immunometabolism within the tuberculosis granuloma: amino acids, hypoxia, and cellular respiration

Tuberculosis (TB) granulomas are compact, organized agglomerations of infected and uninfected macrophages, T cells, neutrophils, and other immune cells. Within the granuloma, several unique metabolic adaptations occur to modify the behavior of immune cells, potentially favoring bacterial persistence balanced with protection against immunopathology. These include the induction of arginase-1 in macrophages to temper nitric oxide (NO) production and block T cell proliferation, inhibition of oxygen-requiring NO production in hypoxic regions, and induction of tryptophan-degrading enzymes that modify T cell proliferation and function. The spatial and time-dependent organization of granulomas further influences immunometabolism, for example through lactate production by activated macrophages, which can induce arginase-1. Although complex, the metabolic changes in and around TB granulomas can be potentially modified by host-directed therapies. While elimination of the TB bacilli is often the goal of any anti-TB therapy, host-directed approaches must also account for the possibility of immunopathologic damage to the lung.

Understanding anti-tuberculosis drug efficacy: rethinking bacterial populations and how we model them

Tuberculosis still remains a global health emergency, claiming 1.5 million lives in 2013. The bacterium responsible for this disease, Mycobacterium tuberculosis (M.tb), has successfully survived within hostile host environments, adapting to immune defence mechanisms, for centuries. This has resulted in a disease that is challenging to treat, requiring lengthy chemotherapy with multi-drug regimens. One explanation for this difficulty in eliminating M.tb bacilli in vivo is the disparate action of antimicrobials on heterogeneous populations of M.tb, where mycobacterial physiological state may influence drug efficacy. In order to develop improved drug combinations that effectively target diverse mycobacterial phenotypes, it is important to understand how such subpopulations of M.tb are formed during human infection. We review here the in vitro and in vivo systems used to model M.tb subpopulations that may persist during drug therapy, and offer aspirations for future research in this field.

Mycobacteria infect different cell types in the human lung and cause species dependent cellular changes in infected cells

Background

Mycobacterial infections remain a significant cause of morbidity and mortality worldwide. Due to limitations of the currently available model systems, there are still comparably large gaps in the knowledge about the pathogenesis of these chronic inflammatory diseases in particular with regard to the human host. Therefore, we aimed to characterize the initial phase of mycobacterial infections utilizing a human ex vivo lung tissue culture model designated STST (Short-Term Stimulation of Tissues).

Methods

Human lung tissues from 65 donors with a size of 0.5–1 cm³ were infected each with two strains of three different mycobacterial species (M. tuberculosis, M. avium, and M. abscessus), respectively. In order to preserve both morphology and nucleic acids, the HOPE® fixation technique was used. The infected tissues were analyzed using histo- and molecular-pathological methods. Immunohistochemistry was applied to identify the infected cell types.

Results

Morphologic comparisons between ex vivo incubated and non-incubated lung specimens revealed no noticeable differences. Viability of ex vivo stimulated tissues demonstrated by TUNEL-assay was acceptable. Serial sections verified sufficient diffusion of the infectious agents deep into the tissues. Infection was confirmed by Ziel Neelsen-staining and PCR to detect mycobacterial DNA. We observed the infection of different cell types, including macrophages, neutrophils, monocytes, and pneumocytes-II, which were critically dependent on the mycobacterial species used. Furthermore, different forms of nuclear alterations (karyopyknosis, karyorrhexis, karyolysis) resulting in cell death were detected in the infected cells, again with characteristic species-dependent differences.

Conclusion

We show the application of a human ex vivo tissue culture model for mycobacterial infections. The immediate primary infection of a set of different cell types and the characteristic morphologic changes observed in these infected human tissues significantly adds to the current understanding of the initial phase of human pulmonary tuberculosis. Further studies are ongoing to elucidate the molecular mechanisms involved in the early onset of mycobacterial infections in the human lung.

Adjunctive Phosphodiesterase-4 Inhibitor Therapy Improves Antibiotic Response to Pulmonary Tuberculosis in a Rabbit Model

Objectives

Adjunctive host-directed therapy is emerging as a new potential approach to improve the outcome of conventional antimicrobial treatment for tuberculosis (TB). We tested the ability of a phosphodiesterase-4 inhibitor (PDE4i) CC-11050, co-administered with the first-line anti-TB drug isoniazid (INH), to accelerate bacillary killing and reduce chronic inflammation in the lungs of rabbits with experimental Mycobacterium tuberculosis (Mtb) infection.

Methods

A rabbit model of pulmonary TB that recapitulates the pathologic manifestations seen in humans was used. Rabbits were infected with virulent Mtb by aerosol exposure and treated for eight weeks with INH with or without CC-11050, starting at four weeks post infection. The effect of CC-11050 treatment on disease severity, pathology, bacillary load, T cell proliferation and global lung transcriptome profiles were analyzed.

Results

Significant improvement in bacillary clearance and reduced lung pathology and fibrosis were noted in the rabbits treated for eight weeks with INH + CC-11050, compared to those treated with INH or CC-11050 only. In addition, expression of host genes associated with tissue remodeling, tumor necrosis factor alpha (TNF-α) regulation, macrophage activation and lung inflammation networks was dampened in CC-11050-treated, compared to the untreated rabbits.

Conclusions

Adjunctive CC-11050 therapy significantly improves the response of rabbits with experimental pulmonary TB to INH treatment. We propose that CC-11050 may be a promising candidate for host directed therapy of patients with pulmonary TB, reducing the duration and improving clinical outcome of antibiotic treatment.

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CC-11050 is an anti-inflammatory molecule targeting host phosphodiesterase-4.

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CC-11050 plus isoniazid therapy significantly reduced bacillary load and pathology in a rabbit model pulmonary tuberculosis.

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CC-11050 can be a promising candidate for adjunctive host directed therapy of patients with active pulmonary tuberculosis.

In 2013, tuberculosis (TB) caused by Mycobacterium tuberculosis (Mtb) killed 1.5 million people worldwide. Current antibiotic therapy for tuberculosis is ineffective in eliminating the infecting bacilli and/or disease pathology such as lung fibrosis. Therefore, alternate approaches are urgently needed to control the TB epidemic. In this study, using a rabbit model of pulmonary TB, which closely mimics the human disease, we tested the hypothesis that reducing the host inflammatory response during Mtb infection would improve the outcome of antibiotic treatment; we show that adjunctive phosphodiesterase-4 inhibition therapy with isoniazid improves bacterial clearance and lung pathology.

Respiratory nanoparticle-based vaccines and challenges associated with animal models and translation

Vaccine development has had a huge impact on human health. However, there is a significant need to develop efficacious vaccines for several existing as well as emerging respiratory infectious diseases. Several challenges need to be overcome to develop efficacious vaccines with translational potential. This review focuses on two aspects to overcome some barriers - 1) the development of nanoparticle-based vaccines, and 2) the choice of suitable animal models for respiratory infectious diseases that will allow for translation. Nanoparticle-based vaccines, including subunit vaccines involving synthetic and/or natural polymeric adjuvants and carriers, as well as those based on virus-like particles offer several key advantages to help overcome the barriers to effective vaccine development. These include the ability to deliver combinations of antigens, target the vaccine formulation to specific immune cells, enable cross-protection against divergent strains, act as adjuvants or immunomodulators, allow for sustained release of antigen, enable single dose delivery, and potentially obviate the cold chain. While mouse models have provided several important insights into the mechanisms of infectious diseases, they are often a limiting step in translation of new vaccines to the clinic. An overview of different animal models involved in vaccine research for respiratory infections, with advantages and disadvantages of each model, is discussed. Taken together, advances in nanotechnology, combined with the right animal models for evaluating vaccine efficacy, has the potential to revolutionize vaccine development for respiratory infections.

Immunology studies in non-human primate models of tuberculosis

Non-human primates, primarily macaques, have been used to study tuberculosis for decades. However, in the last 15 years, this model has been refined substantially to allow careful investigations of the immune response and host-pathogen interactions in Mycobacterium tuberculosis infection. Low-dose challenge with fully virulent strains in cynomolgus macaques result in the full clinical spectrum seen in humans, including latent and active infection. Reagents from humans are usually cross-reactive with macaques, further facilitating the use of this model system to study tuberculosis. Finally, macaques develop the spectrum of granuloma types seen in humans, providing a unique opportunity to investigate bacterial and host factors at the local (lung and lymph node) level. Here, we review the past decade of immunology and pathology studies in macaque models of tuberculosis.

A 3D Human Lung Tissue Model for Functional Studies on Mycobacterium tuberculosis Infection

Tuberculosis (TB) still holds a major threat to the health of people worldwide, and there is a need for cost-efficient but reliable models to help us understand the disease mechanisms and advance the discoveries of new treatment options. In vitro cell cultures of monolayers or co-cultures lack the three-dimensional (3D) environment and tissue responses. Herein, we describe an innovative in vitro model of a human lung tissue, which holds promise to be an effective tool for studying the complex events that occur during infection with Mycobacterium tuberculosis (M. tuberculosis). The 3D tissue model consists of tissue-specific epithelial cells and fibroblasts, which are cultured in a matrix of collagen on top of a porous membrane. Upon air exposure, the epithelial cells stratify and secrete mucus at the apical side. By introducing human primary macrophages infected with M. tuberculosis to the tissue model, we have shown that immune cells migrate into the infected-tissue and form early stages of TB granuloma. These structures recapitulate the distinct feature of human TB, the granuloma, which is fundamentally different or not commonly observed in widely used experimental animal models. This organotypic culture method enables the 3D visualization and robust quantitative analysis that provides pivotal information on spatial and temporal features of host cell-pathogen interactions. Taken together, the lung tissue model provides a physiologically relevant tissue micro-environment for studies on TB. Thus, the lung tissue model has potential implications for both basic mechanistic and applied studies. Importantly, the model allows addition or manipulation of individual cell types, which thereby widens its use for modelling a variety of infectious diseases that affect the lungs.

Preclinical safety and efficacy models for pulmonary drug delivery of antimicrobials with focus on in vitro models

New pharmaceutical formulations must be proven as safe and effective before entering clinical trials. Also in the context of pulmonary drug delivery, preclinical models allow testing of novel antimicrobials, reducing risks and costs during their development. Such models allow reducing the complexity of the human lung, but still need to reflect relevant (patho-) physiological features. This review focuses on preclinical pulmonary models, mainly in vitro models, to assess drug safety and efficacy of antimicrobials. Furthermore, approaches to investigate common infectious diseases of the respiratory tract, are emphasized. Pneumonia, tuberculosis and infections occurring due to cystic fibrosis are in focus of this review. We conclude that especially in vitro models offer the chance of an efficient and detailed analysis of new antimicrobials, but also draw attention to the advantages and limitations of such currently available models and critically discuss the necessary steps for their future development.

Establishment of a neonatal rhesus macaque model to study Mycobacterium tuberculosis infection

Mycobacterium tuberculosis (Mtb) is the causative agent of human tuberculosis (TB) with an estimated 8.8 million new TB cases and 1.4 million deaths annually. Tuberculosis is the leading cause of death in AIDS patients worldwide but very little is known about early TB infection or TB/HIV co-infection in infants. A clinically relevant newborn animal model to study TB infection is urgently needed. We have successfully established an aerosol newborn/infant model in neonatal nonhuman primates (NHPs) that mimics clinical and bacteriological characteristics of Mtb infection as seen in human newborns/infants. Further, this model will allow the establishment of a TB coinfection model of pediatric AIDS. Aerosol versus intra broncho-alveolar Mtb infection was studied. Interestingly, 42 days post infection specific lesions were detected suggestive of the classic Ghon focus in human children. Concurrently, specific cellular immune responses developed 4-6 weeks after Mtb infection. Using the enzyme-linked immunospot (ELISPOT) assays, we found that IL-12 production correlated with early Mtb infection lesions seen by routine thoracic radiographs. Overall, this work represents the first example of early Mtb infection of newborn macaques. This study gives us a unique opportunity to further characterize immunopathogenesis and establish a TB/SIV co-infection model for pediatric AIDS.