Aberrant immune activation in the tuberculosis granuloma: a pivotal role in necrosis

Boston University Medical Campus, Boston MA

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Abstract

Necrosis of TB granulomas in the lung is a highly evolved virulence strategy, pivotal for M.tb. transmission. Understanding this process is essential for rational design of desperately needed vaccines and therapeutics for TB. M.tb elicits IFN? secretion by triggering the innate cytosolic surveillance pathway (CSP), and it is increasingly clear that this type I IFN (IFN-I) response plays detrimental role in host control of TB. We have determined that the sst1 genetic locus which governs necrosis in TB granulomas plays a strong role in coupling macrophage stress signaling with the IFN-I response. We propose that pathogen triggering of the CSP initiates, while an aberrant host response perpetuates, the IFN-I-driven hyperinflammation leading to necrosis. In this proposal we will evaluate TB pathogenesis as a tissue response gone awry rather than the traditional cellular focus on macrophages alone. Our aims are: 1. To dissect mechanisms linking IFN-I responses with integrated stress response (ISR) signaling in susceptible macrophages stimulated with TNF? in vitro, we will identify IFN-I- dependent mechanisms of stress leading to the pre-apoptotic state (PAS) observed in sst1S cells, determine how PAS affects macrophage interactions with M.tb, and whether PAS inhibitors can improve M.tb control and/or macrophage survival. 2. To characterize the IFN-I pathway contribution to macrophage stress signaling and necrosis within lung TB granulomas in vivo, we will identify early biomarkers for necrosis in TB granulomas and determine which IFN-I-dependent pathway plays a dominant role in granuloma necrotization by introducing the IFNAR1 and IRF3 knockout mutations in the B6-sst1S mice. 3. To determine the role of M.tb regulated cyclic dinucleotide (CDN) levels in regulating host macrophage stress signaling, death and intragranulomatous necrosis, we will use a panel of M.tb mutant strains that induce different levels of c-di-AMP and cGAMP to determine how they control macrophage IFN-I and cell death pathways in vitro and granuloma necrosis in vivo. 4. To selectively target mechanisms of granuloma necrosis in vivo using small molecule inhibitors to prevent immunopathology without immune suppression, we will capitalize on our findings to develop novel necrotic granuloma-directed therapies which block the aberrant host responses that drive necrosis in susceptible hosts. We envision that those therapies will correct IFN-I hyperactivity and necrotic inflammation in TB granulomas without suppressing systemic anti-tuberculosis immunity. This approach is especially important for treating multi-drug resistant forms of M.tb, where reducing lung damage, while preserving host immunity, is essential.