Autophagy has been involved as a cellular response against certain bacterial infections. The etiologic agent of Q fever Coxiella burnetii is an intracellular obligate parasite that resides and replicates in large vacuoles with phagolysosomal characteristics. These large replicative vacuoles, generated after 24-48 h post infection, are decorated with the autophagic protein LC3. We have examined at earlier stages of infection, the distribution and roles of the small GTPases Rab5 and Rab7 on C. burnetii trafficking. Our results indicate that Coxiella phagosomes (CPh) acquire these two Rab proteins sequentially during infection. Overexpression of a dominant negative mutant form of Rab5, but not of Rab7, impaired Coxiella entry, whereas both Rab5 and Rab7 dominant negative mutants inhibited vacuole formation. Interestingly, Cph colocalized with the protein LC3 as early as 5 min after infection and this recruitment was dependent on the microorganism protein synthesis. Based on our results we propose that C. burnetii transits through the normal endo/phagocytic pathway but actively interacts with autophagosomes at early times after infection to delay fusion with the lysosomal compartment.
Though interactions of whole pathogens with the autophagy pathway have been described in a number of reports, specific bacterial components involved in autophagy have not been identified so far. We have evidence demonstrating a link of autophagy with extracellular pathogens, like Vibrio cholerae, through a specific secreted molecule termed V. cholerae cytolysin (VCC).
VCC-induced vacuoles colocalized with LC3, indicating the interaction of the large vacuoles with autophagic vesicles. Electron microscopy analysis confirmed that the vacuoles caused by VCC presented hallmarks of autophagosomes. Interestingly, autophagy inhibition resulted in a decreased survival of the cells upon VCC intoxication. In addition, Atg5-/- cells treated with VCC were defectively vacuolated and the survival response against the toxin was dramatically impaired. Our results demonstrate that autophagy also acts as a cellular defense pathway against a secreted bacterial toxin.
