Sterilization of granulomas is common in active and latent tuberculosis despite within-host variability in bacterial killing

Understanding how Mycobacterium tuberculosis is controlled by the body, leading to active disease in only a small fraction of infected individuals, is important for developing medical interventions to prevent and manage disease. Lin et al . now show that infected macaques with active tuberculosis have some sterile granulomas, suggesting immune-mediated control at certain sites of infection. Insight into the mechanisms underlying the heterogeneity of mycobacterial killing may inform vaccine development. Over 30% of the world's population is infected with Mycobacterium tuberculosis (Mtb), yet only ∼5–10% will develop clinical disease 1 . Despite considerable effort, researchers understand little about what distinguishes individuals whose infection progresses to active tuberculosis (TB) from those whose infection remains latent for decades. The variable course of disease is recapitulated in cynomolgus macaques infected with Mtb 2 . Active disease occurs in ∼45% of infected macaques and is defined by clinical, microbiologic and immunologic signs, whereas the remaining infected animals are clinically asymptomatic 2 , 3 . Here, we use individually marked Mtb isolates and quantitative measures of culturable and cumulative bacterial burden to show that most lung lesions are probably founded by a single bacterium and reach similar maximum burdens. Despite this observation, the fate of individual lesions varies substantially within the same host. Notably, in active disease, the host sterilizes some lesions even while others progress. Our data suggest that lesional heterogeneity arises, in part, through differential killing of bacteria after the onset of adaptive immunity. Thus, individual lesions follow diverse and overlapping trajectories, suggesting that critical responses occur at a lesional level to ultimately determine the clinical outcome of infection. Defining the local factors that dictate outcome will be useful in developing effective interventions to prevent active TB.