Modulation of biogenesis of the Francisella tularensis subsp. novicida‐containing phagosome in quiescent human macrophages and its maturation into a phagolysosome upon activation by IFN‐γ

Summary Francisella tularensis is a highly virulent facultative intracellular pathogen that has been categorized as a class A bioterrorism agent, and is classified into four subsp, tularensis, holarctica, mediasiatica and novicida. Although the ability of F. tularensis subsp. novicida to cause tularemia in mice is similar to the virulent subsp. tularensis and holarctica, it is attenuated in humans. It is not known whether attenuation of F. tularensis subsp. novicida in humans is resulting from a different route of trafficking within human macrophages, compared with the tularensis or holarctica subsp. Here we show that in quiescent human monocytes‐derived macrophages (hMDMs), the F. 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. This modulation of phagosome biogenesis by F. tularensis is followed by disruption of the phagosome at 4–12 h and subsequent bacterial escape into cytoplasm where the organism replicates. In IFN‐γ‐activated hMDMs, intracellular replication of F. tularensis is completely inhibited, and is associated with failure of the organism to escape from the phagosome into the cytoplasm for up to 24 h after infection. In IFN‐γ‐activated hMDMs, the FCPs acquire the lysosomal enzymes Cathepsin D, which is excluded in quiescent hMDMs. When the lysosomes of IFN‐γ‐activated hMDMs are preload with Texas Red Ovalbumin or BSA‐gold, the FCPs acquire both lysosomal tracers. In contrast, both lysosomal tracers are excluded from the FCPs within quiescent hMDMs. We conclude that although F. tularensis subsp. novicida is attenuated in humans, it modulates biogenesis of its phagosome into a late endosome‐like compartment followed by bacterial escape into the cytoplasm within quiescent hMDMs, similar to the virulent subsp. tularensis. In IFN‐γ‐activated hMDMs, the organism fails to escape into the cytoplasm and its phagosome fuses to lysosomes, similar to inert particles.