Visualizing the dynamics of tuberculosis pathology using molecular imaging

Abstract

Nearly 140 years after Robert Koch discovered Mycobacterium tuberculosis, tuberculosis (TB) remains a global threat and a deadly human pathogen. M. tuberculosis is notable for complex host-pathogen interactions that lead to poorly understood disease states ranging from latent infection to active disease. Additionally, multiple pathologies with a distinct local milieu (bacterial burden, antibiotic exposure, and host response) can coexist simultaneously within the same subject and change independently over time. Current tools cannot optimally measure these distinct pathologies or the spatiotemporal changes. Next-generation molecular imaging affords unparalleled opportunities to visualize infection by providing holistic, 3D spatial characterization and noninvasive, temporal monitoring within the same subject. This rapidly evolving technology could powerfully augment TB research by advancing fundamental knowledge and accelerating the development of novel diagnostics, biomarkers, and therapeutics.

Figure 1. Spatial and temporal heterogeneity in TB lesions.

(A) Coronal CT section from a TB patient with newly diagnosed cavitary TB demonstrating pathologically distinct TB lesions — granulomas (blue), pneumonia-like disease (blue), or cavities (red) — compared with unaffected lung (yellow). These different lesions demonstrate distinct pathological characteristics. (B) Radiolabeled ¹¹C-rifampin PET/CT demonstrates spatially compartmentalized rifampin exposures in the pathologically distinct TB lesions within the same patient, with low cavity wall rifampin exposures. The ¹¹C-rifampin AUC is shown as a heatmap overlay in the selected transverse section. A and B were adapted with permission from Nature Medicine (13). (C) MRI (T2 flair) demonstrates heterogeneous brain inflammation (arrow) in a patient with TB meningitis with the corresponding spatially heterogeneous ¹¹C-rifampin AUC exposures. Adapted with permission from Science Translational Medicine (78). (D) Longitudinal ¹⁸F-FDG PET/CT in a cavitary TB patient over 6 months of standard treatment. Increased ¹⁸F-FDG uptake (compared with month 1) is noted at 6 months into treatment, coincident with treatment failure. D was adapted with permission from EJNMMI Research (108). (E) Fluorescence microscopy allows longitudinal imaging of the brain (dashed line) and eye (solid line) of a zebrafish larva infected i.v. with approximately 100 CFU of fluorescent Mycobacterium marinum:tdTomato. Infection began in the hindbrain ventricle (arrowheads). (F) Ex vivo H&E staining of a large rabbit necrotic TB granuloma (left), matrix-assisted laser desorption/ionization mass spectrometry imaging (MALDI-MSI) ion map of moxifloxacin in the same section (middle), and coregistration and overlay of moxifloxacin ion image with H&E staining (in grayscale) demonstrating accumulation in macrophage-rich regions (right). Scale bar: 2 mm. Images in F are courtesy of Drs. Landry Blanc and Véronique Dartois.

Figure 2. Molecular imaging in the development of new treatments, biomarkers, and vaccines for TB.

Molecular imaging can be implemented in the various stages of clinical trials, from early-stage studies to patient screening, randomization, and monitoring of treatment response and outcomes (e.g., relapse), to improve the efficiency and accuracy of TB clinical trials. Additionally, the full spectrum of molecular imaging, including optical imaging and ex vivo techniques, can be employed in preclinical studies. The use of molecular imaging across species allows for crosstalk between preclinical and clinical studies and important collaborative, translational research.

Figure 3. Commonly used TB model systems and their characteristics and utilization in TB imaging research.

In vitro granulomas, zebrafish, mice, rabbits, and nonhuman primates are the most common model systems used to recapitulate and study TB. Optical imaging in the zebrafish has been used to study the dynamics of granuloma formation. Mouse and rabbit models have been used to validate multiple imaging tools (PET/SPECT), some of which have been translated into the clinic. ¹⁸F-FDG PET has also been used extensively in nonhuman primate models. ^AAlthough nonhuman primates develop TB meningitis, the characterization of this model has not been reported.