Host- and pathogen-regulated exocytosis facilitates dynamic distribution of endolysosomal content during infection by the vacuolar pathogen Coxiella burnetii

Abstract

ABSTRACT Coxiella burnetii is an obligate intracellular bacterial pathogen that causes the zoonotic disease Q fever. Coxiella extensively subverts host vesicle traffic and replicates within lysosome-derived, spacious vacuoles by secreting a suite of effectors through the type IVB secretion system (T4BSS). This study investigates the regulation and role of a fundamental lysosomal process, exocytosis, in the prolonged maintenance of Coxiella vacuolar niche. Biochemical, microscopic, and proteomic approaches indicate that infection promotes release of extracellular vesicles (EVs). Increase in extracellular levels of endolysosomal proteins and surface localization of LAMP1 were identified in both phagocytic and non-phagocytic cells, during later stages of infection. Interestingly, infection-induced exocytosis was dependent on the activity of the bacterial T4BSS. On one hand, knockdown of the lysosomal master transcriptional regulator TFEB significantly reduces basal exocytosis and enhances intracellular Coxiella replication, while deficiency of the exocytosis-associated SNARE, Syntaxin 11, does not reduce basal exocytosis and yet, increases Coxiella replication, suggesting that host exocytosis is not the sole or a dominant determinant of the infection process. On the other hand, modulating the activity of a key host exocytosis regulator- the calcium channel TRPML1, decreases intracellular Coxiella replication suggesting a tightly regulated role of TRPML1. Infection was also associated with the extracellular detection of LAMP1-associated Coxiella, suggesting an encased egress mechanism. Together, this study demonstrates that Coxiella infection activates exocytosis in a temporal, host-, and bacterial factor-dependent manner.