IL-1R1 dependent signals improve clearance of cytosolic virulent mycobacteria in vivo

Mycobacterium tuberculosis infections claim more than a million lives each year and better treatments or vaccines are required. A crucial pathogenicity factor is translocation from the phago-lysosomes to the cytosol upon phagocytosis by macrophages. The translocation from the phago-lysosome into the cytosol is an ESX-1 dependent process as previously shown in vitro . Here we show that in vivo , mycobacteria also translocate to the cytosol but mainly when host immunity is compromised. We observed only low numbers of cytosolic bacilli in mice, armadillo, zebrafish and patient material infected with M. tuberculosis, M. marinum or M. leprae . In contrast, when innate or adaptive immunity was compromised, as in SCID or IL-1R1 deficient mice, a significant number of cytosolic M. tuberculosis bacilli were detected in lungs of infected mice. Taken together, M. tuberculosis infection is controlled by adaptive immune responses as well as IL-1R1-mediated signals that result in clearance of cells containing cytosolic mycobacteria in vivo . Importance For decades, Mycobacterium tuberculosis is one of the deathliest pathogens known. Despite infecting approximately one third of the human population, no effective treatment or vaccine is available. A crucial pathogenicity factor is the subcellular localization, as M. tuberculosis can translocate from the phago-lysosome to the cytosol in macrophages. The situation in vivo is more complicated. In this study we establish that high level cytosolic escape of mycobacteria can indeed occur in vivo , but mainly when host resistance is compromised. The IL-1 pathway is crucial for the control of the number of cytosolic mycobacteria. The establishment that immune signals result in clearance of cells containing cytosolic mycobacteria, connects two important fields: cell-biology and immunology which is vital for the understanding of the pathology of M. tuberculosis . ### Competing Interest Statement The authors have declared no competing interest.