Abstract: Mycobacteria such as M. tuberculosis and M. leprae are considered to be prototypical intracellular bacilli that have evolved strategies to enable growth in the intracellular phagosomes of the host cell. By contrast, we show that lysosomes rapidly fuse with the virulent M. tuberculosis and M. leprae— containing phagosomes of human monocyte-derived dendritic cells and macrophages. After 2 days, M. tuberculosis progressively translocate from phagolysosomes into the cytosol where they replicate. Cytosolic entry is also observed for M. leprae but not for the vaccine strain, M. bovis BCG, or killed mycobacteria, and is dependent upon secretion of the mycobacterial gene products CFP-IO and ESAT-6 of the RDI region. The present invention further provides means and methods for using these findings in therapeutic and immunogenic compositions.