Identification of antibodies induced by immunization with the syphilis vaccine candidate Tp0751.

The continued and increasing prevalence of syphilis worldwide highlights the need for an effective syphilis vaccine to complement public health measures. Previous work demonstrated that immunization of the rabbit animal model with vaccine candidates derived from the T. pallidum endothelial cell adhesin Tp0751 could reduce dissemination of T. pallidum to lymph nodes. In those studies, a proportion of animals exhibited complete inhibition of treponemal dissemination and others exhibited partial or no inhibition of treponemal dissemination, consistent with results expected from an outbred animal model. In the current study we further characterized the Tp0751-specific antibody response in immunized animals that showed inhibition of T. pallidum dissemination. To do this, we generated Tp0751 tetramers to identify Tp0751-specific B cells before and after immunization. Using this approach, we found a robust expansion of Tp0751-specific B cells in the blood and spleens of immunized animals compared to unimmunized control animals. Ten antibodies from Tp0751-immunized rabbits were cloned and binding to specific structural regions of the Tp0751 protein was assessed using epitope mapping assays and structural modeling. Importantly, nine out of the ten antibodies cloned from Tp0751 tetramer-binding B cells were able to significantly inhibit T. pallidum attachment to human endothelial cells in vitro, including antibodies exhibiting weaker binding to Tp0751. Combined, our results provide a proof-of-principle that Tp0751-based subunit vaccines can stimulate strong B cell responses resulting in the production of antibodies able to inhibit T. pallidum attachment to endothelial cells.