Early Changes by ¹⁸ Fluorodeoxyglucose Positron Emission Tomography Coregistered with Computed Tomography Predict Outcome after Mycobacterium tuberculosis Infection in Cynomolgus Macaques

Abstract

Cynomolgus macaques infected with low-dose Mycobacterium tuberculosis develop both active tuberculosis and latent infection similar to those of humans, providing an opportunity to study the clinically silent early events in infection.18 Fluorodeoxyglucose radiotracer with positron emission tomography coregistered with computed tomography (FDG PET/CT) provides a noninvasive method to measure disease progression. We sought to determine temporal patterns of granuloma evolution that distinguished active-disease and latent outcomes. Macaques (n ‫؍‬ 10) were infected with low-dose M. tuberculosis with FDG PET/CT performed during infection. At 24 weeks postinfection, animals were classified as having active disease (n ‫؍‬ 3) or latent infection (n ‫؍‬ 6), with one "percolator" monkey. Imaging characteristics (e.g., lesion number, metabolic activity, size, mineralization, and distribution of lesions) were compared among active and latent groups. As early as 3 weeks postinfection, more pulmonary granulomas were observed in animals that would later develop active disease than in those that would develop latent infection. Over time, new lesions developed in active-disease animals but not in latent animals. Granulomas and mediastinal lymph nodes from active-disease but not latent animals consistently increased in metabolic activity at early time points. The presence of fewer lesions at 3 weeks and the lack of new lesion development in animals with latent infection suggest that innate and rapid adaptive responses are critical to preventing active tuberculosis. A greater emphasis on innate responses and/or rapid recruitment of adaptive responses, especially in the airway, should be emphasized in newer vaccine strategies.

Rhesus Macaques Are More Susceptible to Progressive Tuberculosis than Cynomolgus Macaques: a Quantitative Comparison

“…Here we assessed the formation of new granulomas that appeared between 4 and 8 weeks postinfection in each macaque ( Fig. 2A), using axial orientation for each scan (31). Four weeks postinfection is the most reliable time point for identifying initial granulomas, since by that time, all initial granulomas are large enough (Ն1 mm) to be seen by CT.…”

“…Dissemination of infection can be defined as the formation of new granulomas after initial establishment of infection, and early dissemination is an indicator of eventual development of active disease (31). Dissemination can be monitored by PET-CT (31,33).…”

“…All animals and infection date, dose, and necropsy date are listed in Table S1 in the supplemental material. Some of the data reported here on a subset of the macaques have been reported in previous publications (as control animals for certain studies), as noted in Table S1 (31,32,44). These studies were conducted in an AAALAC-approved biosafety level 3 (BSL-3) facility within the Regional Biocontainment Laboratory at the University of Pittsburgh.…”

“…Infection doses for the animals in this study ranged from 3 to 20 CFU for CCM, 7 to 18 CFU for rhesus macaques, and 3 to 41 CFU for MCM. Infection was confirmed by PET-CT scan, tuberculin skin testing, and ELISPOTs for M. tuberculosis-specific antigens as previously described (6,31).…”

“…We use serial positron emission tomography and computed tomography (PET-CT), which combines a functional map (PET) of the thoracic cavity, depending on the PET probe used, with a spatial map of anatomic structures (CT) (29). Using this imaging strategy in our macaques, we can assess drug efficacy in vivo (29,30) and study critical events during infection (26,(31)(32)(33). Comparative genomic studies have been performed for rhesus and cynomolgus macaques and showed that, while abundant heterogeneity exists in each population, there is an extraordinarily high degree of similarity in both species to human genes (34), and there is great similarity in the T cell responses to M. tuberculosis epitopes between both cynomolgus and rhesus macaques and humans (35).…”