Both adaptive immunity and IL-1R1 dependent signals improve clearance of cytosolic virulent mycobacteria in vivo. Authors Sanne van der Niet1, Maaike van Zon2, Karin de Punder2,3, Anita Grootemaat1, Sofie Rutten1, Simone Moorlag2, Diane Houben2, Astrid van der Sar4, Wilbert Bitter4, Roland Brosch5, Rogelio Hernandez Pando6, Maria T. Pena7, Eric A. Reits1, Katrin D. Mayer-Barber 8 and Nicole N. van der Wel1* Affiliations 1 Electron Microscopy Centre Amsterdam, Amsterdam University Medical Centre AMC, the Netherlands, 2 Netherlands Cancer Institute, The Netherlands, 3 Charité - Universitätsmedizin Berlin, Germany, 4 Amsterdam University Medical Centre VUMC, The Netherlands, 5 Unit for Integrated Mycobacterial Pathogenomics, CNRS UMR 3525, Paris, France, 6 National Institute of Medical Sciences and Nutrition, Mexico, 7 Department of Health and Human Services, Health Resources and Services Administration, Healthcare Systems Bureau, National Hansen's Disease Programs, Baton Rouge, LA, USA 8 Inflammation and Innate Immunity Unit, Laboratory of Clinical Immunology and Microbiology, National Institute of Allergy and Infectious Diseases (NIAID), National Institutes of Health (NIH), Bethesda, USA. Abstract 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, the cytosolic localization of mycobacteria in vivo is controlled by adaptive immune responses as well as IL-1R1-mediated host resistance to M. tuberculosis.