Legionella pneumophila Excludes Autophagy Adaptors from the Ubiquitin-Labeled Vacuole in Which It Resides.

Xenophagy targets intracellular pathogens for destruction by the host autophagy pathway. Ubiquitin chains are conjugated to xenophagic targets and recruit multiple autophagy adaptors. The intracellular pathogen Legionella pneumophila resides in a vacuole that is ubiquitinated; however, this pathogen avoids xenophagic detection. Here, the mechanisms by which L. pneumophila can prevent the host xenophagy pathway from targeting the vacuole in which it resides were examined. Ubiquitin-labeled vacuoles containing L. pneumophila failed to recruit autophagy adaptors by a process that was independent of RavZ function. Coinfection studies were conducted using a strain of Listeria monocytogenes that served as a robust xenophagic target. Legionella pneumophila infection blocked xenophagic targeting of L. monocytogenes by a RavZ-dependent mechanism. Importantly, when coinfection studies were conducted with a RavZ-deficient strain of L. pneumophila, L. monocytogenes was targeted by the host xenophagy system but vacuoles containing L. pneumophila avoided targeting. Enhanced adaptor recruitment to the vacuole was observed by using a strain of L. pneumophila in which all of the effector proteins in the SidE family were deleted; however, this strain was still not targeted by the host autophagy pathway. Thus, there are at least two pathways by which L. pneumophila can disrupt xenophagic targeting of the vacuole in which it resides. One mechanism involves global disruption of the host autophagy machinery by the effector protein RavZ. A second cis-acting mechanism prevents the binding of autophagy adaptors to the ubiquitin-decorated surface of the L. pneumophila-containing vacuole.

Manipulation of Host Cell Organelles by Intracellular Pathogens.

In this article, we explore the unique adaptations of intracellular bacterial pathogens that manipulate conserved cellular pathways, organelles, and cargo to convert the phagosome into a pathogen-containing vacuole (PCV). The phagosome is a degradative organelle that rapidly acidifies as it delivers cargo to the lysosome to destroy microbes and cellular debris. However, to avoid this fate, intracellular bacterial pathogens hijack the key molecular modulators of intracellular traffic: small GTPases, phospholipids, SNAREs, and their associated effectors. Following uptake, pathogens that reside in the phagosome either remain associated with the endocytic pathway or rapidly diverge from the preprogrammed route to the lysosome. Both groups rely on effector-mediated mechanisms to meet the common challenges of intracellular life, such as nutrient acquisition, vacuole expansion, and evasion of the host immune response. Mycobacteria, Salmonella, and Coxiella serve as a lens through which we explore regulators of the canonical endocytic route and pathogens that seek to subvert it. On the other hand, pathogens such as Chlamydia, Legionella, and Brucella disconnect from the canonical endocytic route. This bifurcation is linked to extensive hijacking of the secretory pathway and repurposing of the PCV into specialized compartments that resemble organelles in the secretory network. Finally, each pathogen devises specific strategies to counteract host immune responses, such as autophagy, which aim to destroy these aberrant organelles. Collectively, each unique intracellular niche and the pathogens that construct them reflect the outcome of an aggressive and ongoing molecular arms race at the host-pathogen interface. Improving our understanding of these well-adapted pathogens can help us refine our knowledge of conserved cell biological processes.