SUMMARY: Previous observations on the highly infectious LG V strain 434 of and the guinea pig inclusion conjunctivitis (GPIC) strain of (which requires centrifugation of inocula with host cell monolayers for maximum infectivity) indicated that infectivity differences were expressed, not at entry, but at an intracellular stage affecting multiplication. Centrifugation increased the potential of internalized chlamydiae to undergo productive infection. Here, analysis of the intracellular fate of chlamydiae by ultrastructural methods indicates that strain GPIC exhibits two patterns of behaviour depending on the mode of inoculation. Strain GPIC showed limited entry, with 47% of intracellular organisms becoming associated with thorotrast-labelled lysosomes, following static incubation with monolayers. In contrast, with centrifugation, entry was not limited and association with lysosomes was reduced to 12%; strain 434 behaved similarly but independently of the mode of inoculation. The different results for strain GPIC correlated with distinct entry mechanisms. Entry during static incubation was unimpaired either by treatment with cytochalasin D or by temperature reduction to 20°C, suggesting that it was pinocytic. Entry during centrifugation was markedly impaired by both treatments, suggesting that it was phagocytic. The data lead to two novel conclusions: first, that chlamydiae can apparently enter cells by both pinocytic and phagocytic mechanisms; second, that the entry mechanism influences intracellular fate. It is suggested that entry mechanism is linked to selection of the vesicle membrane forming around the internalizing chlamydiae. This, in turn, may influence both intracellular translocation and subsequent inhibition or promotion of multiplication of the internalized parasite.


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