| The etiologic agent of epidemic typhus, Rickettsia prowazekii , is an obligate intracellular pathogen that grows directly in the cytosol of the host cell. Due to its highly infectious nature, potential to be spread by aerosol, environmental stability and the high morbidity and fatality associated with this infection, this bacterium is considered a potential bioterror threat by the Centers for Disease Control and Prevention. Examination of the immune avoidance mechanisms of R. prowazekii warrant increased attention, especially interactions with the frontline mediator of cytosolic immunity, autophagy.;These studies characterized the interactions of R. prowazekii with the primary marker of autophagosomes, LC3. Western blot analyses revealed an increased LC3 conversion following infection of the mouse fibroblast L929 cell line with the avirulent R. prowazekii strain Madrid E. This increase was bacterial protein synthesis dependent. Interestingly, this phenotype occurred following infection of L929 cells with the avirulent but not with the virulent strains of R. prowazekii. Microscopic examination revealed that LC3 colocalizes with the avirulent strain during infection. Further characterization revealed that colocalization peaks around 24 hours post infection and is also bacterial protein synthesis dependent. However, this colocalization was not correlated with the presence of intact autophagosomes, as none were observed during electron microscopic analyses.;Finally, the exogenous induction of autophagy during infection resulted in increased LC3 conversion. However induction did not have an effect on rickettsial numbers per cell, indicating rickettsiae were not cleared from the cell and underwent at least one round of replication during long-term (24 hour) exposure to autophagic induction.;In conclusion, this study identified strain-dependent, virulence-associated differences in host LC3 conversion and colocalization following infection with R. prowazekii. Although recognition of the avirulent strain of R. prowazekii by the host autophagic response does not result in the clearance of the avirulent strain from nonphagocytic cells, autophagic recognition may play a role in limiting the pathogenesis of this strain in an animal model. In addition, the ability of R. prowazekii strain Madrid E to survive induced autophagy in non-phagocytic L929 cells indicates that the autophagic response is only part of the anti-rickettsial immunity occurring in response to this infection. |
