Mouse models of human TB pathology: roles in the analysis of necrosis and the development of host-directed therapies

Abstract

A key aspect of TB pathogenesis that maintains Mycobacterium tuberculosis in the human population is the ability to cause necrosis in pulmonary lesions. As co-evolution shaped M . tuberculosis (M.tb) and human responses, the complete TB disease profile and lesion manifestation are not fully reproduced by any animal model. However, animal models are absolutely critical to understand how infection with virulent M.tb generates outcomes necessary for the pathogen transmission and evolutionary success. In humans, a wide spectrum of TB outcomes has been recognized based on clinical and epidemiological data. In mice, there is clear genetic basis for susceptibility. Although the spectra of human and mouse TB do not completely overlap, comparison of human TB with mouse lesions across genetically diverse strains firmly establishes points of convergence. By embracing the genetic heterogeneity of the mouse population, we gain tremendous advantage in the quest for suitable in vivo models. Below, we review genetically defined mouse models that recapitulate a key element of M.tb pathogenesis-induction of necrotic TB lesions in the lungs-and discuss how these models may reflect TB stratification and pathogenesis in humans. The approach ensures that roles that mouse models play in basic and translational TB research will continue to increase allowing researchers to address fundamental questions of TB pathogenesis and bacterial physiology in vivo using this well-defined, reproducible, and cost-efficient system. Combination of the new generation mouse models with advanced imaging technologies will also allow rapid and inexpensive assessment of experimental vaccines and therapies prior to testing in larger animals and clinical trials.

Keywords: Animal models; Granuloma; Host-directed therapies; Inbred mice; Mechanisms; Necrosis; Tuberculosis; sst1.

Fig. 1

Manifestation of necrotizing responses and cavitation induced in vivo in the lungs of mice experimentally infected with Mycobacterium tuberculosis. Formalin-fixed, paraffin-embedded lung sections were stained with hematoxylin and eosin (a through d) or stained with a modification of Ziehl-Neelsen’s acid-fast method (f). Death can be observed in individual macrophages as apoptosis recognized by nuclear fragmentation, magnified 1,000 times, encircled (a). Necrosis of an individual macrophage in an alveolar space is recognized by cytoplasmic eosinophilia and a pyknotic, condensed nucleus, magnified 1,000 times (b). Alveolar septae are present but difficult to recognize due to accumulation of dead inflammatory cells and cellular debris within alveolar air spaces, magnified 400 times (c). As the necrotizing process continues, fibrin thrombosis of capillaries is observed as a transition from a capillary with red blood cells to a capillary containing eosinophilic fibrillar material consistent with fibrin (arrows), magnified 400 times (d). Finally, complete destruction of alveolar septae allows necrotic regions to coalesce and undergo liquefaction and removal of necrotic material, thus contributing to cavity formation, magnified 20 times normal (e). Cavities contain variable necrotic debris in which abundant acid-fast bacilli are detected, magnified 400 times (f). (a–d) necrotic lung lesions in supersusceptible DO mouse; (e–f) cavity in the lung of a CBA/J mouse

Fig. 2

The sst1 locus controls necrosis of TB granulomas. a TB granulomas in the lungs of B6.C3H-sst1 mice 6 (left panels) and 12 weeks (right panels) after a low dose aerosol infection with virulent M.tb Erdman. TB granulomas double stained with hematoxylin and eosin (upper panels) and auramine-rhodamine (lower panels). The acid fast fluorescent staining with auramine-rhodamine identifies intracellular (at 6 weeks) and extracellular (at 12 weeks) M.tb. Caseous necrotic center containing M.tb is surrounded by organized wall at 12 weeks. b Total M.tb loads in the lungs of the parental C3HeB/FeJ (C3H) and C57BL/6J (B6) and the sst1-congenic (B6-sst1S) mouse strains. c Effects of the sst1 locus and the host genetic background on lung tissue damage and M.tb lung burdens: the sst1-mediated control of granuloma necrosis (Y-axis) is uncoupled from the bacterial (X-axis)