Coxiella burnetii phase I and II variants replicate with similar kinetics in degradative phagolysosome-like compartments of human macrophages.
Coxiella burnetii infects mononuclear phagocytes, where it directs biogenesis of a vacuolar niche termed the parasitophorous vacuole (PV). Owing to its lumenal pH (approximately 5) and fusion with endolysosomal vesicles, the PV is considered phagolysosome-like. However, the degradative properties of the mature PV are unknown, and there are conflicting reports on the maturation state and growth permissiveness of PV harboring virulent phase I or avirulent phase II C. burnetii variants in human mononuclear phagocytes. Here, we employed infection of primary human monocyte-derived macrophages (HMDMs) and THP-1 cells as host cells to directly compare the PV maturation kinetics and pathogen growth in cells infected with the Nine Mile phase I variant (NMI) or phase II variant (NMII) of C. burnetii. In both cell types, phase variants replicated with similar kinetics, achieving roughly 2 to 3 log units of growth before they reached stationary phase. HMDMs infected by either phase variant secreted similar amounts of the proinflammatory cytokines interleukin-6 and tumor necrosis factor alpha. In infected THP-1 cells, equal percentages of NMI and NMII PVs decorate with the early endosomal marker Rab5, the late endosomal/lysosomal markers Rab7 and CD63, and the lysosomal marker cathepsin D at early (8 h) and late (72 h) time points postinfection (p.i.). Mature PVs (2 to 4 days p.i.) harboring NMI or NMII contained proteolytically active cathepsins and quickly degraded Escherichia coli. These data suggest that C. burnetii does not actively inhibit phagolysosome function as a survival mechanism. Instead, NMI and NMII resist degradation to replicate in indistinguishable digestive PVs that fully mature through the endolysosomal pathway.
Coxiella Pathogenesis Section, Rocky Mountain Laboratories, National Institute of Allergy and Infectious Diseases, National Institutes of Health, Hamilton, Montana 59840, USA.
Dale
D
Howe
Jeffrey G
JG
Shannon
Seth
S
Winfree
David W
DW
Dorward
Robert A
RA
Heinzen
D002403:Cathepsins / Q000378:metabolism; D002478:Cells, Cultured; D016997:Coxiella burnetii / Q000276:immunology / Q000472:pathogenicity*; D004926:Escherichia coli / Q000276:immunology; D006801:Humans; D015850:Interleukin-6 / Q000378:metabolism; D008264:Macrophages / Q000276:immunology / Q000382:microbiology*; D010588:Phagosomes / Q000276:immunology / Q000382:microbiology*; D014409:Tumor Necrosis Factor-alpha / Q000378:metabolism
D015850:Interleukin-6; D014409:Tumor Necrosis Factor-alpha; D002403:Cathepsins