The mechanism by which Chlamydia trachomatis is endocytosed by host cells is unclear. Studies of the kinetics of chlamydial attachment and uptake in the susceptible HeLa229 cell line showed that chlamydial endocytosis was rapid and saturable but limited by the slow rate of chlamydial attachment. To overcome this limitation and to investigate the mechanism of endocytosis, chlamydiae were centrifuged onto the host cell surface in the cold to promote attachment. Endocytosis of the adherent chlamydiae was initiated synchronously by rapid warming to 36 °C. Electron micrographs of chlamydial uptake 5 min after onset showed that chlamydial ingestion involves movement of the host cell membrane, leading to interiorization in tight, endocytic vacuoles which were not clathrin coated. Chlamydial ingestion was not inhibited by monodansylcadaverine or amantadine, inhibitors of receptor-mediated endocytosis and chlamydiae failed to displace [H]sucrose from micropinocytic vesicles. Chlamydial endocytosis was markedly inhibited by cytochalasin D, an inhibitor of host cell microfilament function, and by vincristine or vinblastine, inhibitors of host cell microtubules. Hyperimmune rabbit antibody prevented the ingestion of adherent chlamydiae, suggesting that endocytosis requires the circumferential binding of chlamydial and host cell surface ligands. These findings were incompatible with the suggestion that chlamydiae enter cells by taking advantage of the classic mechanism of receptor-mediated endocytosis into clathrin-coated vesicles, used by the host cell for the internalization of δ-lipoprotein and other macromolecules, but were consistent with the hypothesis that chlamydiae enter cells by a microfilament-dependent zipper mechanism.


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