Background A protective vaccine against Treponema pallidum subsp. pallidum (T. pallidum) would facilitate syphilis control at a global scale. We have identified three T. pallidum antigens that, when used separately as immunogens, either attenuate chancre development at the challenge site or pathogen dissemination. Here, we report protection induced by a vaccine desing composed of these three antigens + two custom adjuvants containing Natural or Synthetic TLR4 agonists and a natural Mincle agonist.

Methods Recombinant TprK (aa 37–273), Tp0751 (aa 24–237), and Subfamily I Tpr (aa 23–351) peptides were emulsified in either adjuvant and used to immunize groups of eight rabbits. Immunized animals were challenged intradermally with 10^{^5} T. pallidum/site at 10 sites, along with eight unimmunized control animals. Lesion development was recorded daily and treponemal burden measured by darkfield (DF) microscopy and qPCR of lesion aspirates and biopsies. Dissemination to distant tissues was evaluated by qPCR and rabbit infectivity test (RIT).

Results Compared to unimmunized, treponemal burden by DF in lesion aspirates at Day 19 was significantly lower in animals that received both Natural and Synthetic adjuvant groups. By qPCR, treponemal burden was significantly lower in the Natural group. At day 19 and 30, respectively, the proportion of lesions ulcerating was significantly lower in the Natural group, compared to Unimmunized. At day 30, the proportion of lesions ulcerating in the Natural group was significantly lower than in the Synthetic group. Mean lesion volume was smaller in immunized groups versus unimmunized on days 10–25 post-challenge. RIT indicated the lowest number of disseminated T. pallidum in rabbit tissues from the Natural group, followed by the Synthetic group, then the unimmunized group.

Conclusion Immunization with the three-antigen cocktail significantly attenuates syphilis infection: enhancing T. pallidum clearance, promoting lesion healing, and reducing dissemination. In rabbits, Natural adjuvant was more effective than Synthetic adjuvant in inducing protective immunity.

Disclosure No significant relationships.