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Characterization and intracellular trafficking pattern of vacuoles containing Chlamydia pneumoniae in human epithelial cells

Chlamydiae are obligate intracellular pathogens that reside within a membrane‐bound vacuole throughout their developmental cycle. In this study, the intraphagosomal pH of Chlamydia pneumoniae (Cpn) was qualitatively assessed, and the intracellular fate of the pathogen‐containing vacuole and its interaction with endocytic organelles in human epithelial cells were analysed using conventional immunofluorescence and confocal microscopy. The pH‐sensitive probes acridine orange (AO), LysoTracker (LyT) and DAMP did not accumulate in the bacterial inclusion. In addition, exposure of cells to bafilomycin A1 (BafA1), a potent acidification inhibitor, did not inhibit or delay chlamydial growth. The chlamydial compartment was not accessible to the fluid‐phase tracer Texas Red (TR)‐dextran and did not exhibit any level of staining for the late endosomal marker cation‐independent mannose‐6‐phosphate receptor (Ci‐M6PR) or for the lysosomal‐associated membrane proteins (LAMP‐1 and ‐2) and CD63. In addition, transferrin receptor (TfR)‐enriched vesicles were observed close to Cpn vacuoles, potentially indicating a specific translocation of these organelles through the cytoplasm to the vicinity of the vacuole. We conclude that Cpn, like other chlamydial spp., circumvents the host endocytic pathway and inhabits a non‐acidic vacuole, which is dissociated from late endosomes and lysosomes, but selectively accumulates early endosomes.