Abstract
We previously reported that Mycobacterium tuberculosis triggers macrophage necrosis in vitro at a threshold intracellular load of ∼25 bacilli. This suggests a model for tuberculosis where bacilli invading lung macrophages at low multiplicity of infection proliferate to burst size and spread to naïve phagocytes for repeated cycles of replication and cytolysis. The current study evaluated that model in vivo, an environment significantly more complex than in vitro culture. In the lungs of mice infected with M. tuberculosis by aerosol we observed three distinct mononuclear leukocyte populations (CD11b− CD11c+/hi, CD11b+/lo CD11clo/−, CD11b+/hi CD11c+/hi) and neutrophils hosting bacilli. Four weeks after aerosol challenge, CD11b+/hi CD11c+/hi mononuclear cells and neutrophils were the predominant hosts for M. tuberculosis while CD11b+/lo CD11clo/− cells assumed that role by ten weeks. Alveolar macrophages (CD11b− CD11c+/hi) were a minority infected cell type at both time points. The burst size model predicts that individual lung phagocytes would harbor a range of bacillary loads with most containing few bacilli, a smaller proportion containing many bacilli, and few or none exceeding a burst size load. Bacterial load per cell was enumerated in lung monocytic cells and neutrophils at time points after aerosol challenge of wild type and interferon-γ null mice. The resulting data fulfilled those predictions, suggesting a median in vivo burst size in the range of 20 to 40 bacilli for monocytic cells. Most heavily burdened monocytic cells were nonviable, with morphological features similar to those observed after high multiplicity challenge in vitro: nuclear condensation without fragmentation and disintegration of cell membranes without apoptotic vesicle formation. Neutrophils had a narrow range and lower peak bacillary burden than monocytic cells and some exhibited cell death with release of extracellular neutrophil traps. Our studies suggest that burst size cytolysis is a major cause of infection-induced mononuclear cell death in tuberculosis.
Highlights
Natural infection with Mycobacterium tuberculosis (Mtb) occurs by inhalation, followed by invasion of resident alveolar macrophages that provide the major initial replication niche for the pathogen
In previous studies we showed that upon reaching a threshold intracellular number, virulent M. tuberculosis kills macrophages by necrosis and escapes for spreading infection
Alveolar macrophages are the overwhelming majority of leukocytes in the normal alveolar space and they are considered the primary leukocyte type initially infected by inhaled Mtb in vivo
Summary
Natural infection with Mycobacterium tuberculosis (Mtb) occurs by inhalation, followed by invasion of resident alveolar macrophages that provide the major initial replication niche for the pathogen. Macrophages infected with Mtb in vitro may die with primarily apoptotic or necrotic features [1]; the cell death mode most relevant to TB disease in vivo remains undefined. A widely held paradigm is that macrophage apoptosis promotes host defense in TB while necrosis favors spreading infection. Mtb triggers a primarily necrotic death dependent on bacterial genes regulated by the PhoPR 2-component system [3]. Our in vitro studies and data from other groups suggest that virulent Mtb strains suppress apoptosis of host macrophages [4,5,6,7,8] and grow to a threshold burden [2,9] whereupon necrosis is triggered as an exit mechanism analogous to the burst size of lytic viruses
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