Cig2-mediated modifications of the Coxiella-containing vacuole

摘要

Coxiella burnetii is a unique intracellular bacterial pathogen that replicates in the lysosome-like Coxiella-containing vacuole (CCV). C. burnetii uses a Dot/Icm type IV secretion system (T4SS) to translocate over 130 bacterial proteins, called effectors, into the host cell cytosol. A functional T4SS is required for CCV biogenesis and bacterial replication; however, the functions of most C. burnetii effector proteins remain unknown.;Genetic and cell biological analyses revealed that the T4SS effector, Cig2, promotes constitutive fusion of autophagosomes with the CCV, effectively locking the CCV in an autolysosomal state. Protein markers of autophagosomes including microtubule-associated protein 1A/1B-light chain 3 (LC3), nuclear dot protein 52 kDa (NDP52), and sequestosome 1 (p62/SQSTM1), localized to the lumen of the CCV in a Cig2 dependent mechanism and were not present until the vacuole had matured. Furthermore, expression of autophagy receptor proteins that participate in formation of autophagosomes was required for LC3 delivery to the CCV. The autolysosomal nature of the vacuole promoted homotypic fusion of CCVs, suggesting that the autolysosomal state of the CCV increases fusogenicity of this compartment. These data not only suggest a novel role for the effector Cig2, but clarify that the Coxiella-containing vacuole is not targeted by LC3-asscoaited phagocytosis (LAP) or canonical xenophagy, but rather that mature autophagosomes fuse with the pathogen containing vacuole resulting in increased fusogenicity of the CCV. There was no difference in replication of cig2 mutants and wild type C. burnetii in tissue culture cell lines or the wax moth, Galleria mellonella . However, G. mellonella infected with cig2 mutants died more slowly than those infected with wild type C. burnetii. Together these data demonstrate that Cig2 promotes maintenance of an autolysosomal environment within the CCV that decreases tolerance of the host toward C. burnetii infection. Indeed, a preliminary RNA-seq analysis supports this finding as expression of Cig2 altered the expression of innate immune genes in the host. Thus, Cig2 allows C. burnetii to subvert the autophagic pathway by promoting autophagosome fusion with the pathogen containing vacuole and modifies the host response to infection.