Robust immune responses clear millions of treponemes to resolve lesions of primary and secondary syphilis, but cannot clear the treponemes that lead to debilitating and sometimes fatal tertiary syphilis. It is also known that the rabbit model and humans can be reinfected with heterologous isolates. How some treponemes are able to escape the immune system is unknown. In our labo-ratories rabbits immunized with the Seattle Nichols strain Treponema pallidum repeat protein K (TprK) were previously shown to have attenuated lesion development following challenge. In other isolates, TprK was shown to have seven discrete variable regions, with sequence variation among and within isolates. Using overlapping synthetic 20-aa peptides, we demonstrate that during experimental infection with the Nichols strain, the T cell responses are directed to conserved regions, while the Ab responses are directed primarily to variable regions. Abs from rabbits immunized with recombinant TprK recognized conserved and variable regions, suggesting that the conserved regions are inherently as immunogenic as the variable regions. TprK vari-ability may allow some treponemes to escape recognition from Abs. The variable region heterogeneity may help explain the lack of protection against heterologous isolates. The Journal of Immunology, 2002, 169: 952–957. S yphilis is a spirochetal infection with multiple manifesta-tions of disease. Robust specific immune responses areable to resolve primary and secondary lesions, but are not able to clear all treponemes. Syphilitic lesions are infiltrated by T cells that activate macrophages to phagocytose opsonized trepo-nemes, the major mechanism of treponemal clearance (1–4). Man
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