Aberrant macrophage activation and failed regeneration of pulmonary epithelium promote tuberculosis progression uniquely in lung tissue
Pulmonary tuberculosis (PTB) represents 85% of the disease burden caused by Mycobacterium tuberculosis (Mtb) and promotes aerosol transmission infecting about a quarter of people globally. Most Mtb infections are effectively limited within primary granulomatous lesions. Containment failures lead to hematogenous spread and the formation of post-primary destructive PTB lesions. Factors that favor Mtb survival and replication in the lungs after hematogenous spread despite systemic immunity represent appealing targets for host-directed TB therapies, but are currently unknown. We developed a novel mouse model that mimics progression of chronic post-primary PTB in humans: wherein PTB lesions form after hematogenous spread from a remote primary lesion in immunocompetent but TB-susceptible B6.Sst1S mice. The B6.Sst1S mice developed PTB lesions featuring granulomatous pneumonia, bronchogenic expansion and broncho-occlusion closely resembling post-primary PTB in humans. Using spatial transcriptomic and fluorescent multiplexed immunochemistry, we demonstrated the expansion of myeloid cell populations with the appearance of alternatively activated macrophages, dissolution of initial lymphoid follicles, and accumulation of de-differentiated lung epithelial cells in the advanced PTB lesions. To determine whether lung parenchymal cells or lung oxygenation were necessary for the pulmonary TB progression, we implanted lung and spleen fragments subcutaneously to serve as potential targets for hematogenous spread. The lung (but not spleen) implants displayed characteristic organized granulomas with necrosis and Mtb replication demonstrating that deleterious interactions of aberrantly activated macrophages with the inflammation-injured lung resident cells, and possibly hypoxia, not oxygenation, are critical determinants of PTB progression in immunocompetent hosts. Necrotic TB lesions also developed in subcutaneous implants of human lung tissue in mice with human immune system after respiratory infection. These animal models may serve to further dissect the lung-specific mechanisms of host susceptibility to virulent Mtb and for testing therapeutic interventions targeting these mechanisms.