Linking Human TB Genetic Susceptibility Loci to Granuloma Biology

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Abstract Mycobacterium tuberculosis kills approximately 1.4 million people annually. People exposed to similar initial doses may have very different outcomes, ranging from clearance to active disease to subclinical or latent infection. A central structure of tuberculosis, the granuloma represents a key interface between pathogen and host, and its dynamics contribute to disease outcome. Granuloma macrophages induce expression of an unusual epithelioid program that dictates granuloma structure and infection trajectory. Due to a lack of genetically tractable animal models that fully reproduce key aspects of granuloma biology – namely, epithelioid transformation and necrosis – many aspects of granuloma formation have not been probed. Genome-wide association studies in humans have identified several potential TB susceptibility loci, but the mechanisms by which these genes act, particularly within the granuloma, are not well understood. Here we will link established human genetic susceptibility loci to the biology of mycobacterial granulomas. We will use murine, zebrafish, and explant models of bona fide mycobacterial granulomas to directly probe granuloma structure, epithelioid state, immune and drug access and granuloma dynamics. Using single cell transcriptional profiling and host and bacterial mutants, we have defined a repertoire of diverse cell types and transcriptional states that contribute to the physiological properties of the mycobacterial granuloma. We will define how alteration of host immune pathways identified in genome-wide human genetic studies alter granuloma biology through murine and zebrafish models that recapitulate key properties of human granulomas, including caseation necrosis and epithelioid transformation. We will validate these findings in human TB granuloma specimens. Finally, we will use comprehensive TnSeq profiling using saturating Mtb transposon libraries to define the interplay between bacteria and host within the context of the tuberculous granuloma. Overall, these approaches will lead to a fuller understanding of granuloma biology as well as new opportunities for therapeutic interventions.