Coxiella burnetii is an intracellular pathogen that infects alveolar macrophages in humans. The resulting disease is known as Q fever, which can present as a febrile, flu-like illness. Once inside cells, a unique intracellular replicative niche is created. This is the Coxiella-containing vacuole (CCV) which has lysosomal characteristics, including a low internal pH and hydrolytic enzymes. Despite this hostile environment, Coxiella becomes metabolically active in the lysosome-like CCV, and remodels this compartment to support replication. Under the pathogens direction the CCV becomes extremely fusogenic, promiscuously fusing with endosomes, autophagosomes, clathrin-coated vesicles and undergoing homotypic fusion.
Autophagy is the process of degradation of dysfunctional or unnecessary cellular compartments, where an autophagosome forms to carry the cargo to lysosomes for destruction. Autophagy is known to aid the formation of the typical CCV through autophagosome fusion with the CCV. Additionally, we have previously reported that clathrin-mediated traffic plays an important part in the growth of Coxiella and CCV biogenesis.
Here, we present data to show that Coxiella use of autophagosomes during infection requires clathrin. Gene silencing expression of clathrin demonstrated that clathrin is required for LC3, an autophagy marker, to be delivered to the inside of the CCV. The origin of this clathrin may not be from the plasma membrane, as during reduction of clathrin-mediated endocytosis, using a knockout cell line, autophagosomes still fuse with the CCV. We show increased fusion of autophagosomes with the CCV, through starvation of HeLa cells, induces a significant increase in CCV area. In the absence of clathrin however, autophagosomes can no longer contribute to causing the large vacuole phenotype. Ultimately, we provide a link between clathrin and autophagy in the context of Coxiella infection.