Pathological and immunological profiles of rat tuberculosis

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.

Animal models for COVID-19 and tuberculosis

Respiratory infections cause tremendous morbidity and mortality worldwide. Amongst these diseases, tuberculosis (TB), a bacterial illness caused by Mycobacterium tuberculosis which often affects the lung, and coronavirus disease 2019 (COVID-19) caused by the Severe Acute Respiratory Syndrome Coronavirus type 2 (SARS-CoV-2), stand out as major drivers of epidemics of global concern. Despite their unrelated etiology and distinct pathology, these infections affect the same vital organ and share immunopathogenesis traits and an imperative demand to model the diseases at their various progression stages and localizations. Due to the clinical spectrum and heterogeneity of both diseases experimental infections were pursued in a variety of animal models. We summarize mammalian models employed in TB and COVID-19 experimental investigations, highlighting the diversity of rodent models and species peculiarities for each infection. We discuss the utility of non-human primates for translational research and emphasize on the benefits of non-conventional experimental models such as livestock. We epitomize advances facilitated by animal models with regard to understanding disease pathophysiology and immune responses. Finally, we highlight research areas necessitating optimized models and advocate that research of pulmonary infectious diseases could benefit from cross-fertilization between studies of apparently unrelated diseases, such as TB and COVID-19.

One Size Fits All? Not in In Vivo Modeling of Tuberculosis Chemotherapeutics

Tuberculosis (TB) remains a global health problem despite almost universal efforts to provide patients with highly effective chemotherapy, in part, because many infected individuals are not diagnosed and treated, others do not complete treatment, and a small proportion harbor Mycobacterium tuberculosis (Mtb) strains that have become resistant to drugs in the standard regimen. Development and approval of new drugs for TB have accelerated in the last 10 years, but more drugs are needed due to both Mtb's development of resistance and the desire to shorten therapy to 4 months or less. The drug development process needs predictive animal models that recapitulate the complex pathology and bacterial burden distribution of human disease. The human host response to pulmonary infection with Mtb is granulomatous inflammation usually resulting in contained lesions and limited bacterial replication. In those who develop progressive or active disease, regions of necrosis and cavitation can develop leading to lasting lung damage and possible death. This review describes the major vertebrate animal models used in evaluating compound activity against Mtb and the disease presentation that develops. Each of the models, including the zebrafish, various mice, guinea pigs, rabbits, and non-human primates provides data on number of Mtb bacteria and pathology resolution. The models where individual lesions can be dissected from the tissue or sampled can also provide data on lesion-specific bacterial loads and lesion-specific drug concentrations. With the inclusion of medical imaging, a compound's effect on resolution of pathology within individual lesions and animals can also be determined over time. Incorporation of measurement of drug exposure and drug distribution within animals and their tissues is important for choosing the best compounds to push toward the clinic and to the development of better regimens. We review the practical aspects of each model and the advantages and limitations of each in order to promote choosing a rational combination of them for a compound's 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.

The current state of animal models and genomic approaches towards identifying and validating molecular determinants of Mycobacterium tuberculosis infection and tuberculosis disease

Animal models are important in understanding both the pathogenesis of and immunity to tuberculosis (TB). Unfortunately, we are beginning to understand that no animal model perfectly recapitulates the human TB syndrome, which encompasses numerous different stages. Furthermore, Mycobacterium tuberculosis infection is a very heterogeneous event at both the levels of pathogenesis and immunity. This review seeks to establish the current understanding of TB pathogenesis and immunity, as validated in the animal models of TB in active use today. We especially focus on the use of modern genomic approaches in these models to determine the mechanism and the role of specific molecular pathways. Animal models have significantly enhanced our understanding of TB. Incorporation of contemporary technologies such as single cell transcriptomics, high-parameter flow cytometric immune profiling, proteomics, proteomic flow cytometry and immunocytometry into the animal models in use will further enhance our understanding of TB and facilitate the development of treatment and vaccination strategies.

Animal Models of Tuberculosis Vaccine Research: An Important Component in the Fight against Tuberculosis

Tuberculosis (TB), an infectious disease caused by Mycobacterium tuberculosis, is one of the top ten infectious diseases worldwide, and is the leading cause of morbidity from a single infectious agent. M. tuberculosis can cause infection in several species of animals in addition to humans as the natural hosts. Although animal models of TB disease cannot completely simulate the occurrence and development of human TB, they play an important role in studying the pathogenesis, immune responses, and pathological changes as well as for vaccine research. This review summarizes the commonly employed animal models, including mouse, guinea pig, rabbit, rat, goat, cattle, and nonhuman primates, and their characteristics as used in TB vaccine research, and provides a basis for selecting appropriate animal models according to specific research needs. Furthermore, some of the newest animal models used for TB vaccine research (such as humanized animal models, zebrafish, Drosophila, and amoeba) are introduced, and their characteristics and research progress are discussed.

Total IgM and Anti-Phosphatidylcholine IgM Antibody Secretion Continue After Clearance of Mycobacterium bovis Bacillus Calmette-Guerin Pleural Infection

The cellular immune response to Mycobacterium tuberculosis infection has been well characterized, while the humoral antibody response remains underexplored. We aimed to examine the total and anti-phospholipid IgM levels in the pleural lavage from mice with Mycobacterium bovis BCG extrapulmonary infection. We found that the levels of total and anti-phosphatidylcholine IgM antibodies remained significantly higher in infected mice as compared to non-infected mice up to day 90 after BCG infection, while the anti-cardiolipin IgM antibody levels decreased with bacteria clearance. Our findings suggest that IgM antibodies are secreted and their composition vary during early and late immune response to BCG pleurisy.

Dormancy models for Mycobacterium tuberculosis: A minireview

Dormancy models for Mycobacterium tuberculosis play important roles in understanding various aspects of tuberculosis pathogenesis and in the testing of novel therapeutic regimens. By simulating the latent tuberculosis infection, in which the bacteria exist in a non-replicative state, the models demonstrate reduced susceptibility to antimycobacterial agents. This minireview outlines the models available for simulating latent tuberculosis both in vitro and in several animal species. Additionally, this minireview discusses the advantages and disadvantages of these models for investigating the bacterial subpopulations and susceptibilities to sterilization by various antituberculosis drugs.

Comprehensive analysis of methods used for the evaluation of compounds against Mycobacterium tuberculosis

In drug development, there are typically a series of preclinical studies that must be completed with new compounds or regimens before use in humans. A sequence of in vitro assays followed by in vivo testing in validated animal models to assess the activity against Mycobacterium tuberculosis, pharmacology and toxicity is generally used for advancing compounds against tuberculosis in a preclinical stage. A plethora of different assay systems and conditions are used to study the effect of drug candidates on the growth of M. tuberculosis, making it difficult to compare data from one laboratory to another. The Bill and Melinda Gates Foundation recognized the scientific gap to delineate the spectrum of variables in experimental protocols, identify which of these are biologically significant, and converge towards a rationally derived standard set of optimized assays for evaluating compounds. The goals of this document are to recommend protocols and hence accelerate the process of TB drug discovery and testing. Data gathered from preclinical in vitro and in vivo assays during personal visits to laboratories and an electronic survey of methodologies sent to investigators is reported. Comments, opinions, experiences as well as final recommendations from those currently engaged in such preclinical studies for TB drug testing are being presented. Certain in vitro assays and mouse efficacy models were re-evaluated in the laboratory as head-to-head experiments and a summary is provided on the results obtained. It is our hope that this information will be a valuable resource for investigators in the field to move forward in an efficient way and that key variables of assays are included to ensure accuracy of results which can then be used for designing human clinical trials. This document then concludes with remaining questions and critical gaps that are in need of further validation and experimentation.

Experimental tuberculosis in the Wistar rat: a model for protective immunity and control of infection

Background

Despite the availability of many animal models for tuberculosis (TB) research, there still exists a need for better understanding of the quiescent stage of disease observed in many humans. Here, we explored the use of the Wistar rat model for the study of protective immunity and control of Mycobacterium tuberculosis (Mtb) infection.

Methodology/Principal Findings

The kinetics of bacillary growth, evaluated by the colony stimulating assay (CFU) and the extent of lung pathology in Mtb infected Wistar rats were dependent on the virulence of the strains and the size of the infecting inoculums. Bacillary growth control was associated with induction of T helper type 1 (Th1) activation, the magnitude of which was also Mtb strain and dose dependent. Histopathology analysis of the infected lungs demonstrated the formation of well organized granulomas comprising epithelioid cells, multinucleated giant cells and foamy macrophages surrounded by large numbers of lymphocytes. The late stage subclinical form of disease was reactivated by immunosuppression leading to increased lung CFU.

Conclusion

The Wistar rat is a valuable model for better understanding host-pathogen interactions that result in control of Mtb infection and potentially establishment of latent TB. These properties together with the ease of manipulation, relatively low cost and well established use of rats in toxicology and pharmacokinetic analyses make the rat a good animal model for TB drug discovery.

Aerosol infection model of tuberculosis in wistar rats

We explored suitability of a rat tuberculosis aerosol infection model for investigating the pharmacodynamics of new antimycobacterial agents. Infection of rats via the aerosol route led to a reproducible course of M. tuberculosis infection in the lungs. The pulmonary bacterial load increased logarithmically during the first six weeks, thereafter, the infection stabilized for the next 12 weeks. We observed macroscopically visible granulomas in the lungs with demonstrable acid-fast bacilli and associated histopathology. Rifampicin (RIF) at a dose range of 30 to 270 mg/kg exhibited a sharp dose response while isoniazid (INH) at a dose range of 10 to 90 mg/kg and ethambutol (EMB) at 100 to 1000 mg/kg showed shallow dose responses. Pyrazinamide (PZA) had no dose response between 300 and 1000 mg/kg dose range. In a separate time kill study at fixed drug doses (RIF 90 mg/kg, INH 30 mg/kg, EMB 300 mg/kg, and PZA 300 mg/kg) the bactericidal effect of all the four drugs increased with longer duration of treatment from two weeks to four weeks. The observed infection profile and therapeutic outcomes in this rat model suggest that it can be used as an additional, pharmacologically relevant efficacy model to develop novel antitubercular compounds at the interface of discovery and development.

Experimental tuberculosis: the role of comparative pathology in the discovery of improved tuberculosis treatment strategies

The use of laboratory animals is critical to the discovery and in vivo pre-clinical testing of new drugs and drug combinations for use in humans. M. tuberculosis infection of mice, rats, guinea pigs, rabbits and non-human primates are the most commonly used animal models of human tuberculosis. While granulomatous inflammation characterizes the most fundamental host response to M. tuberculosis aerosol infection in humans and animals, there are important species differences in pulmonary and extra-pulmonary lesion morphology which may influence responses to drug therapy. Lesions that progress to necrosis or cavitation are common, unfavorable host responses in naturally occurring tuberculosis of humans, but are not seen consistently in experimental infections in most animal model species. The importance of these unique lesion morphologies is that they represent irreversible tissue damage that can harbor persistent bacilli which are difficult to treat with standard therapies. Understanding the differences in host response to experimental tuberculosis infections may aid in selecting the most appropriate animal models to test drugs that have been rationally designed to have specific mechanisms of action in vivo. A better understanding of lesion pathogenesis across species may also aid in the identification of novel therapeutic targets or strategies that can be used alone or in combination with more conventional tuberculosis treatments in humans.

Retinoic acid therapy attenuates the severity of tuberculosis while altering lymphocyte and macrophage numbers and cytokine expression in rats infected with Mycobacterium tuberculosis

Because retinoic acid (RA) exerts a stimulatory effect on macrophages and tubercle bacilli target alveolar macrophages, the therapeutic potential of RA was examined in rats with tuberculosis. In the main study, 15 rats were randomized to treatment with oil (control) or RA, 100 microg/100 g body weight per dose, given 3 times weekly for 3 and 5 wk after infection with Mycobacterium tuberculosis strain H37Rv. There was a significant difference in the severity of tuberculosis histopathology between control and RA-treated rats, and oral administration of RA decreased the number of colony-forming units (CFU) in both lung and spleen at 3 and 5 wk after H37Rv infection (P < 0.005). CD4-positive and CD8-positive T cells, natural killer cells, and CD163-positive macrophages increased (P < 0.05) in the infected lung tissues of RA-treated rats. Expression of IFNgamma and inducible nitric oxide synthetase messenger RNA (mRNA) was higher in the infected lung tissues of RA-treated rats than in control rats. Alveolar macrophages from rats treated in vivo with RA and infected in vitro with M. tuberculosis showed significantly higher expression of TNFalpha and IL-1beta mRNA than macrophages in control rats. To our knowledge, this is the first reported study to demonstrate that orally administered RA significantly inhibits the in vivo growth of M. tuberculosis and the development of tuberculosis.

The American cotton rat: A novel model for pulmonary tuberculosis

Several animal models are used to study Mycobacterium tuberculosis (MTB) infections, but none is a fully ideal model of human disease. The American cotton rat is an excellent model for the study of several human viral and bacterial respiratory infectious diseases, but until now has not been reported to be a model with MTB infection. Preliminary experiments were designed in which two species of cotton rats (Sigmodon hispidus and Sigmodon fulviventer) received respiratory challenges with M. tuberculosis via either intranasal or aerosol inoculation. Granulomatous disease, often with central necrosis, developed in the lungs, spleen, and lymph nodes of infected animals. The number of MTB bacilli in the lungs increased logarithmically until reaching a plateau in the second month after aerosol inoculation. There were differences in response to infection between the two species, with S. fulviventer demonstrating greater mortality than S. hispidus. Cytokine gene expression analysis by reverse transcriptase polymerase chain reaction (RT-PCR) was performed on both normal appearing and granulomatous lung tissue from infected animals. Many cytokine genes were more highly expressed in the focal areas of inflammation. Cotton rats provide another valuable tool in future research with tuberculosis.

Nude rat (F344/N-rnu) tuberculosis

As many mononuclear cells from Mycobacterium tuberculosis-infected lung tissues are not available for fluorescence-activated cell sorter (FACS) analysis and the tuberculin test is not feasible in a mouse tuberculosis model, we attempted to develop a rat tuberculosis model. We have previously reported that rat tuberculosis is associated with granulomas that lack central necrosis. In order to develop a better animal model of tuberculosis in immunocompromised humans (tuberculosis associated with HIV infection or tuberculosis of the elderly), we infected F344/N-rnu nude rats with M. tuberculosis via the airborne route. The animals developed pulmonary granulomas with central necrosis encapsulated by dense collagen fibres, closely resembling those of human tuberculosis. The nude rats died of disseminated tuberculosis by the 85th day after aerosol infection, while F344 wild-type rats did not. Interestingly, T-cells that were reactive with anti-CD4 antibody and anti-CD8 antibody, indicating the presence of remnant thymus, were observed in the infected lung tissues of the nude rats. Therefore, T-cell precursors may be present in nude rats. The nude rat tuberculosis model mimics tuberculosis in immunocompromised humans and may provide a suitable model for immunological studies in vivo.

Newly designed primer sets available for evaluating various cytokines and iNOS mRNA expression in guinea pig lung tissues by RT-PCR

Guinea pigs are often used as an animal model of human tuberculosis (TB). However, there are few methods available for pursuing the immunological processes involved in guinea pig TB. In this study, we developed for the first time systematic reverse transcription (RT)-PCR for evaluation of guinea pig mRNA expression. RT-PCR primer sets were newly designed for detection of cytokines and inducible nitric oxide synthase (iNOS) mRNA in guinea pig TB. Interferon (IFN)-gamma, tumor necrosis factor (TNF)-alpha, transforming growth factor (TGF)-beta, interleukin (IL)-1beta, IL-2, IL-10, IL-12p40, granulocyte-macrophage-colony stimulating factor (GM-CSF) and iNOS mRNA expression were detected significantly and reproducibly when these primer sets were used. The data by real-time PCR were comparable with those of RT-PCR. We showed that these RT-PCR primer sets could be used to examine mRNA expression semi-quantitatively in guinea pig tissues, and conclude that these newly designed primer sets for conventional RT-PCR will be useful for studying the immunological processes in guinea pig tuberculosis experiments to investigate and evaluate efficacy of new vaccines or anti-mycobacterial drugs.

The use of animal models to guide rational vaccine design

Although there are several varieties of animal models of tuberculosis, the mouse and the guinea pig are by far the most validated and useful. These provide information about vaccine-induced protection, immunogenicity, toxicity, and immunopathological effects. There is still much to be learned, however, in terms of rational vaccine design, especially in the context of therapeutic or anti-latent vaccine formulations and animal models of these situations.