DOWN THE RABBIT HOLE: UNRAVELING THE PATHOGENESIS OF PULMONARY CAVITATION DURING MYCOBACTERIUM TUBERCULOSIS INFECTION
Ihms, Elizabeth Ann
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Tuberculosis infects an estimated one-third of the world’s population, and is responsible for more deaths than any other single infectious agent. The continued success of Mycobacterium tuberculosis (MTB) in the post-antibiotic area can be attributed principally to pulmonary cavities: pathologic air spaces surrounded by scar tissue that have replaced healthy lung tissue. Cavities are the primary source of bacterial transmission during infection and contribute significantly to antibiotic resistance and treatment failure, yet the pathogenesis of cavitation is poorly understood. Proposed contributing factors include mechanical stress and enzymatic tissue destruction. To investigate these phenomena, we use a novel repetitive aerosol infection protocol in rabbits to produce a reliable model of tuberculous cavitation in which cavities are monitored by serial computed tomography. Using this model, we demonstrate that pharmacologic inhibition of collagenases does not reduce cavitation, contrary to their speculated role as drivers of cavitation. Using high-resolution 4D cavity maps to track cavity dynamics over time, we show that mechanical stress contributes significantly to cavity formation and persistence dynamics. We also establish that central necrosis of the granuloma is a necessary precursor lesion, but is not sufficient in itself to cause cavitation. Finally, we examine the role of necrosis during infection and cavitation in C3HeB/FeJ mice – specifically, we probe the involvement of the RIP-kinase mediated programmed necrosis pathway. We demonstrate robust necroptosis activation in infected macrophages within and around granulomas in mice – the first in vivo demonstration of necroptosis induction during MTB infection. However, pharmacologic inhibition of RIP1 – the key decision checkpoint in the necroptosis pathway - does not alter outcomes in this model, suggesting alternative activation by one of several RIP1 bypass pathways. In this thesis, we establish two optimized animal models for investigating the pathogenesis of cavitation, and show that these models are well-suited for screening of novel therapeutics. It is our hope that these models will be used in the future not only to further our understanding of the disease, but also to advance novel host-directed therapies to improve patient outcomes and decrease the worldwide burden of tuberculosis.