We used inhibitors of receptor-mediated endocytosis to study the mechanisms of infectivity, especially the uptake mechanism, of Chlamydia trachomatis for cultured cells. The effect of
methylamine and
monodansylcadaverine on the different stages of the chlamydial growth cycle in McCoy cells was examined. There was a dose-related decrease in the number of chlamydial inclusions in the presence of these agents.
Monodansylcadaverine also decreased the chlamydia-dependent uptake of radioactive
amino acids. The agents did not affect the attachment of chlamydiae to the cells, but they increased the
protease-removable fraction of cell-bound chlamydiae. The
amines reduced the number of inclusions when added at different times during the first 24 h of
infection. However, this effect was influenced by host cell density, so that the effect of the
amines at the early infectious phase was nullified in confluent monolayers, whereas, during later phases, the effect was comparatively independent of host cell density. This indicates that the
amines have different modes of action at different infectious stages. The effect of the
amines was reversible, and they had no effect on the infectivity of pretreated chlamydial elementary bodies. These experiments suggest that
methylamine and
monodansylcadaverine inhibit both the internalization of chlamydiae into McCoy cells and their intracellular development. These results are consistent with the hypothesis that chlamydiae utilize a constitutive cellular process, such as receptor-mediated endocytosis, to enter cells.