The predominant phagocyte in established human syphilitic lesions is the mononuclear phagocyte. As these leucocytes may be important in immunity to Treponema pallidum, the initial interaction between human phagocytes and pathogenic T. pallidum (Nichols strain) was studied in vitro. Motile, virulent T. pallidum attach to the surface membrane of phagocytes but are not ingested by these cells. Heated, non-motile treponemes are not cell-associated but are observed free in the extracellular medium. Attachment is polar, at one or both terminal portions of the treponeme, and is neither serum- nor complement-dependent. Ingestion of virulent treponemes was not observed by phase-contrast or by electron microscopy in the presence of normal human or rabbit serum and complement. Techniques were chosen to preserve both the fragile surface constituents of the treponeme and the phagocytic function of mononuclear cells. Unfixed preparations were observed by phase-contrast microscopical examination during incubation to differentiate motile from non-motile organisms and fixed preparations were used for quantitation of attachment. This model should be useful for studying humoral and cell-mediated immunity in syphilis.
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