Ultra-low dose of Mycobacterium tuberculosis aerosol creates partial infection in mice

Abstract

SUMMARY A murine low-dose (LD) aerosol model is commonly used to test tuberculosis vaccines. Doses of 50-400 CFU (24-hour lung CFU) infect 100% of exposed mice. The LD model measures progression from infection to disease based on organ CFU at defined time points. To mimic natural exposure, we exposed mice to an ultra-low dose (ULD) aerosol. We estimated the presented dose by sampling the aerosol. Female C57BL/6 mice were exposed to Mycobacterium tuberculosis H37Rv aerosol at 1.0, 1.1, 1.6, 5.4, and 11 CFU presented dose, infecting 27%, 36%, 36%, 100%, and 95% of mice, respectively. These data are compatible with a stochastic infection event (Poisson distribution, weighted R2= 0.97) or with a dose-response relationship (sigmoid distribution, weighted R2= 0.97). Based on the later assumption, the ID50 was 1.6 CFU presented dose (95% confidence interval, 1.2 to 2.1). We compared organ CFU after ULD and LD aerosols (5.4 vs. 395 CFU presented dose). Lung burden was 30-fold lower in the ULD model at 4 weeks (3.4 vs. 4.8 logs, p<0.001) and 18 weeks (≤3.6 vs. 5.0 logs, p=0.01). Mice exposed to ULD aerosols as compared to LD aerosols had greater within-group CFU variability. Exposure to ULD aerosols leads to infection in a subset of mice, and to persistently low organ CFU. The ULD aerosol model may resemble human pulmonary tuberculosis more closely than the standard LD model, and may be used to identify host or bacterial factors that modulate the initial infection event.

Latently and uninfected healthcare workers exposed to TB make protective antibodies against Mycobacterium tuberculosis

“…The in vivo mouse protection model that we tested used a "low-dose aerosol" infection method: delivering ∼100-200 viable bacilli per mouse on average to ensure that every animal was infected. Nonetheless, this low-dose challenge represents 2 logs more than the actual infectious dose: although the precise infectious dose of Mtb is not known, it is thought that most sufficiently infectious microdroplets will harbor just one bacillus (36,37). Furthermore, we showed that protection was enhanced in vitro when the Mtb surface was not disrupted by detergents (Fig.…”

Tuberculosis Vaccines and Prevention of Infection

“…Generally, the majority of these studies employed aerosol routes to deliver M. tuberculosis challenge doses that are as low as possible and yet still ensure uniform infection and disease outcome for all animals. Thus, infection doses in experimental animal studies tend to be significantly higher (50 to 3,000 CFU per animal, commonly 10 to 20 for guinea pigs and 100 for mice) than the relatively few bacilli thought to be transmitted during natural exposure (though the precise number has never been measured) (91,(121)(122)(123). In addition, the bacillus is prepared under laboratory conditions with a metabolic state and physical conditions (sputum versus broth culture) that likely differ from those in natural transmission settings (93).…”

“…For example, commercial rabbits are relatively resistant to M. tuberculosis, requiring approximately 300 to 3,000 inhaled bacilli to produce 1 primary pulmonary lesion, and are therefore less well suited for low-dose infection models (89). Another constraint for small-animal studies utilizing aerosol infection chambers is the potential for partial infection of a group of animals, as has been recently described for aerosol chamber infection of mice at doses lower than 10 CFU per infected animal (122). Guinea pigs are very susceptible to M. tuberculosis, generating roughly 1 primary pulmonary lesion per inhaled bacillus, and thus low-dose challenges may be delivered via infection chambers.…”

“…For example, macaque studies utilizing very-low-dose challenges via aerosol or intrabroncheal instillation, similar to those of Barclay et al (131), could be conducted in conjunction with early bronchoalveolar lavage sampling for culture as well as IGRAs to verify infection and to assess the possibility of clearance over time. In addition, a recently described ultra-low-dose challenge mouse model could be used to assess the protective effects of vaccination when mice are infected with Ͻ10 bacilli (122). Although such low doses result in partial infection, the use of mice, which are relatively low in cost, would permit the necessary increase in animals used per experiment.…”

Mathematical model of mycobacterium–host interaction describes physiology of persistence

“…88 • To simplify the innate immunity scenario, we included in our model only Nitric Oxide (N), 89 which was considered as the sole effective weapon against infection growth followed a logistic curve with a fixed growth rate (α 1 ), whose value was taken from 97 published work (Vijay S, 2014;Gil W, 2009;Ray JCJ, 2008). During early stages of infection, 98 which naturally occur at a low dose (Saini D, 2012), bacilli are confined in the infected 99 macrophages. Our model described a population of cells that accounted for both infected and 100 uninfected macrophages.…”