Immunization with full-length TprC variants induces a broad response to surface-exposed epitopes of the Treponema pallidum repeat protein family and is partially protective in the rabbit model of syphilis

Abstract

An effective vaccine against syphilis could aid current control measures to reduce the incidence of infection. Protective immunity from the syphilis agent, Treponema pallidum subsp. pallidum (T. pallidum), is associated with pathogen clearance by phagocytosis, supporting that immunization with an effective vaccine candidate should elicit opsonic antibodies to key epitopes at the host-pathogen interface. The T. pallidum repeat (Tpr) proteins are putative β-barrel outer membrane porins with ten predicted extracellular loops. Here, we immunized three groups of eight rabbits with either a combination of three recombinant variants of the full-length TprC antigen, the TprD₂ protein, or the conserved NH₂-terminal region of TprK, with the latter antigen already known to induce incomplete protection in immunized rabbits. Compared to unimmunized controls, rabbits immunized with the three TprC variants or the TprK fragment exhibited attenuated primary chancres, reduced treponemal burden at the challenge sites, and limited pathogen dissemination to lymph nodes. Immunization with TprD₂, alone did not produce comparable results. Strong humoral and cellular responses against TprC and TprK were elicited by immunization, and functional analyses supported the induction of opsonizing antibodies. Epitope mapping performed using TprC- and TprK-specific synthetic peptides and phage immunoprecipitation-sequencing identified a subset of highly reactive sequences and demonstrated immunity to predicted surface-exposed epitopes across multiple Tpr paralogs, which explained the significant, albeit incomplete protection measured post-challenge. These data advance TprC and TprK as syphilis vaccine candidates and highlight several correlates of their protection that deserve further examination.