The TprK protein of Treponema pallidum is periplasmic and is not a target of opsonic antibody or protective immunity

The finding that Treponema pallidum, the syphilis spirochete, contains 12 o rthologs of the Treponema denticola outer membrane major sheath protein has engendered speculation that members of this T. pallidum repeat (Tpr) famil y may be similarly surface exposed. In this regard, the TprK protein was re ported to he a target of opsonic antibody and protective immunity and subje ct to immunologically driven sequence variation. Despite these findings, re sults front our previous analyses of treponemal outer membranes in concert with computer-based predictions for TprK prompted us to examine the cellula r location of this protein. TprK-alkaline phosphatase fusions expressed in Escherichia coli demonstrate that TprK contains a signal peptide. However, opsonophagocytosis assays failed to indicate surface exposure of TprK. More over, results from three independent methodologies, i.e., (a) indirect immu nofluorescence analysis of agarose-encapsulated organisms, (b) proteinase K treatment of intact spirochetes, and (c) Triton X-114 phase partitioning o f T. pallidum conclusively demonstrated that native TprK is entirely peripl asmic. Consistent with this location, immunization with the recombinant pro tein failed to induct either protective immunity or select for TprK variant s in the rabbit model experimental syphilis. These findings challenge the n otion that TprK will be a component of an efficacious syphilis vaccine.