The mammalian lysosome is central to cellular homeostasis, acting as a control centre for nutrient recycling and destruction of foreign matter. Distinguishing features of the lysosome include an acidic pH and the presence of degradative enzymed known as acid hydrolases. The only known human pathogens to replicate inside a lysosome-derived niche are the Gram-negative bacterium Coxiella burnetii and protozoan parasites of the Leishmania species.
Here, we used activity-based probes to examine the activity of lysosomal proteases known as cathepsins during infection of macrophages with C. burnetii or Leishmania mexicana. This protease profiling revealed distinct changes to the host proteolytic capacity driven by each pathogen. Infection with C. burnetii led to a decrease in the activity of several cathepsins, while infection with L. mexicana largely increased cathepsin activity. The disparate host responses induced by these pathogens suggest divergent methods of avoiding lysosomal degradation.
Our team utilised SILAC-mass spectrometry to examine host proteome changes during C. burnetii infection. Using this method, we observed and experimentally validated a decrease in the abundance of several cathepsins during C. burnetii infection, though RT-qPCR demonstrated that cathepsin transcripts were not decreased. Furthermore, infection with a C. burnetii secretion system mutant revealed that cathepsin loss is dependent on a functional type IV secretion system, suggesting that a translocated bacterial effector protein may be destroying cathepsins to prevent bacterial degradation inside the lysosome.
Lastly, we developed a coinfection model to examine lysosomal proteolysis in cells infected with both C. burnetii and L. mexicana. Coinfection with L. mexicana led to a small rescue of cathepsins in C. burnetii-infected cells. The impact of cathepsin retention during coinfection is yet to be elucidated. These data suggest that C. burnetii and L. mexicana are actively modifying the lysosomal compartment in divergent ways. This likely reflects a mechanism of avoiding degradation in the lysosomal niche, shedding light on how these pathogens survive in such a harsh environment.