Experimental animal models of central nervous system tuberculosis: A historical review

Central Nervous System Tuberculosis in a Murine Model: Neurotropic Strains or a New Pathway of Infection?

Tuberculosis (TB) of the central nervous system (CNS) is a lethal and incapacitating disease. Several studies have been performed to understand the mechanism of bacterial arrival to CNS, however, it remains unclear. Although the interaction of the host, the pathogen, and the environment trigger the course of the disease, in TB the characteristics of these factors seem to be more relevant in the genesis of the clinical features of each patient. We previously tested three mycobacterial clinical isolates with distinctive genotypes obtained from the cerebrospinal fluid of patients with meningeal TB and showed that these strains disseminated extensively to the brain after intratracheal inoculation and pulmonary infection in BALB/c mice. In this present study, BALB/c mice were infected through the intranasal route. One of these strains reaches the olfactory bulb at the early stage of the infection and infects the brain before the lungs, but the histological study of the nasal mucosa did not show any alteration. This observation suggests that some mycobacteria strains can arrive directly at the brain, apparently toward the olfactory nerve after infecting the nasal mucosa, and guides us to study in more detail during mycobacteria infection the nasal mucosa, the associated connective tissue, and nervous structures of the cribriform plate, which connect the nasal cavity with the olfactory bulb.

Mycobacterium tuberculosis Infection Induces BCSFB Disruption but No BBB Disruption In Vivo: Implications in the Pathophysiology of Tuberculous Meningitis

Central nervous system (CNS) tuberculosis is the most lethal and devastating form among the diseases caused by Mycobacterium tuberculosis. The mechanisms by which M. tuberculosis bacilli enter the CNS are still unclear. However, the BBB and the BCSFB have been proposed as possible routes of access into the brain. We previously reported that certain strains of M. tuberculosis possess an enhanced ability to cause secondary CNS infection in a mouse model of progressive pulmonary tuberculosis. Here, we evaluated the morphostructural and molecular integrity of CNS barriers. For this purpose, we analyzed through transmission electron microscopy the ultrastructure of brain parenchymal microvessels and choroid plexus epithelium from animals infected with two mycobacterial strains. Additionally, we determined the expression of junctional proteins and cytokines by immunological techniques. The results showed that the presence of M. tuberculosis induced disruption of the BCSFB but no disruption of the BBB, and that the severity of such damage was related to the strain used, suggesting that variations in the ability to cause CNS disease among distinct strains of bacteria may also be linked to their capacity to cause direct or indirect disruption of these barriers. Understanding the pathophysiological mechanisms involved in CNS tuberculosis may facilitate the establishment of new biomarkers and therapeutic targets.

Nos2-/- mice infected with M. tuberculosis develop neurobehavioral changes and immunopathology mimicking human central nervous system tuberculosis

BACKGROUND

Understanding the pathophysiology of central nervous system tuberculosis (CNS-TB) is hampered by the lack of a good pre-clinical model that mirrors the human CNS-TB infection. We developed a murine CNS-TB model that demonstrates neurobehavioral changes with similar immunopathology with human CNS-TB.

METHODS

We injected two Mycobacterium tuberculosis (M.tb) strains, H37Rv and CDC1551, respectively, into two mouse strains, C3HeB/FeJ and Nos2-/- mice, either into the third ventricle or intravenous. We compared the neurological symptoms, histopathological changes and levels of adhesion molecules, chemokines, and inflammatory cytokines in the brain induced by the infections through different routes in different strains.

RESULTS

Intra-cerebroventricular infection of Nos2-/- mice with M.tb led to development of neurological signs and more severe brain granulomas compared to C3HeB/FeJ mice. Compared with CDC1551 M.tb, H37Rv M.tb infection resulted in a higher neurobehavioral score and earlier mortality. Intra-cerebroventricular infection caused necrotic neutrophil-dominated pyogranulomas in the brain relative to intravenous infection which resulted in disseminated granulomas and mycobacteraemia. Histologically, intra-cerebroventricular infection of Nos2-/- mice with M.tb resembled human CNS-TB brain biopsy specimens. H37Rv intra-cerebroventricular infected mice demonstrated higher brain concentrations of inflammatory cytokines, chemokines and adhesion molecule ICAM-1 than H37Rv intravenous-infected mice.

CONCLUSIONS

Intra-cerebroventricular infection of Nos2-/- mice with H37Rv creates a murine CNS-TB model that resembled human CNS-TB immunopathology, exhibiting the worst neurobehavioral score with a high and early mortality reflecting disease severity and its associated neurological morbidity. Our murine CNS-TB model serves as a pre-clinical platform to dissect host-pathogen interactions and evaluate therapeutic agents for CNS-TB.

Phenotype of Peripheral NK Cells in Latent, Active, and Meningeal Tuberculosis

The mechanisms underlying the immunopathology of tuberculous meningitis (TBM), the most severe clinical form of extrapulmonary tuberculosis (TB), are not understood. It is currently believed that the spread of Mycobacterium tuberculosis (Mtb) from the lung is an early event that occurs before the establishment of adaptive immunity. Hence, several innate immune mechanisms may participate in the containment of Mtb infection and prevent extrapulmonary disease manifestations. Natural killer (NK) cells participate in defensive processes that distinguish latent TB infection (LTBI) from active pulmonary TB (PTB). However, their role in TBM is unknown. Here, we performed a cross-sectional analysis of circulating NK cellCID="C008" value="s" phenotype in a prospective cohort of TBM patients (n = 10) using flow cytometry. Also, we addressed the responses of memory-like NK cell subpopulations to the contact with Mtb antigens in vitro. Finally, we determined plasma levels of soluble NKG2D receptor ligands in our cohort of TBM patients by enzyme-linked immunosorbent assay (ELISA). Our comparative groups consisted of individuals with LTBI (n = 11) and PTB (n = 27) patients. We found that NK cells from TBM patients showed lower absolute frequencies, higher CD69 expression, and poor expansion of the CD45RO⁺ memory-like subpopulation upon Mtb exposure in vitro compared to LTBI individuals. In addition, a reduction in the frequency of CD56^brightCD16⁻ NK cells characterized TBM patients but not LTBI or PTB subjects. Our study expands on earlier reports about the role of NK cells in TBM showing a reduced frequency of cytokine-producing cells compared to LTBI and PTB.

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.

Tuberculous meningitis: a roadmap for advancing basic and translational research

Tuberculous meningitis is a serious, life-threatening disease affecting vulnerable populations, including HIV-infected individuals and young children. The US National Institutes of Health convened a workshop to identify knowledge gaps in the molecular and immunopathogenic mechanisms of tuberculous meningitis and to develop a roadmap for basic and translational research that could guide clinical studies.