The Francisella tularensis subsp. novicida-containing phagosome (FCP) matures into a late endosome-like stage that acquires the late endosomal marker LAMP-2 but does not fuse to lysosomes, for the first few hours after bacterial entry. This modulation in phagosome biogenesis is followed by disruption of the phagosome and bacterial escape into the cytoplasm where they replicate. Here we examined the role of the Francisella pathogenicity island (FPI) protein IgIC and its regulator MgIA in the intracellular fate of F. tularensis subsp. novicida within human macrophages. We show that F. tularensis mgIA and igIC mutant strains are defective for survival and replication within U937 macrophages and human monocyte-derived macrophages (hMDMs). The defect in intracellular replication of both mutants is associated with a defect in disruption of the phagosome and failure to escape into the cytoplasm. Approximately, 80-90% of the mgIA and igIC mutants containing phagosomes acquire the late endosomal/lysosomal marker LAMP-2 similar to the wild-type (WT) strain. Phagosomes harbouring the mgIA or igIC mutants acquire the lysosomal enzyme Cathepsin D, which is excluded from the phagosomes harbouring the WT strain. In hMDMs in which the lysosomes are preloaded with BSA-gold or Texas Red Ovalbumin, phagosomes harbouring the mgIA or the igIC mutants acquire both lysosomal tracers. We conclude that the FPI protein IgIC and its regulator MgIA are essential for modulating phagosome biogenesis and subsequent bacterial escape into the cytoplasm. Therefore, acquisition of the FPI, within which igIC is contained, is essential for the pathogenic evolution of F. tularensis to evade lysosomal fusion within human macrophages and cause tularemia. This is the first example of specific virulence factors of F. tularensis that are essential for evasion of fusion of the FCP to lysosomes.
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