Background The bacterium that causes syphilis, Treponema pallidum subsp. pallidum (Tp), elicits cellular and humoral immune responses to numerous antigens during infection. We have identified three recombinant peptide antigens that, when used separately for immunization, show promise in the attenuation of chancre development or dissemination to distant tissues. Here, we report protection induced by a three-antigen cocktail emulsified in either of two custom adjuvants containing Natural or Synthetic TLR4 agonists + a natural Mincle agonist.

Methods Three purified recombinant peptides [TprK (aa37-273), Tp0751 (aa24-237), and Tpr Subfamily I (23-351)] were emulsified in either adjuvant and used to immunize groups of 8 rabbits. The immunized rabbits and 8 Unimmunized controls were challenged intradermally with 10^{^5} Tp/site at 10 sites. Lesion development was recorded daily. Treponemal burden was measured by darkfield (DF) microscopy and qPCR of lesion aspirates, and dissemination to distant tissues was evaluated by rabbit infectivity test (RIT).

Results Compared to Unimmunized, treponemal burden by DF in lesion aspirates at Day 19 was lower in both Natural (P=0.001) and Synthetic (P=0.004) groups; by qPCR, treponemal burden was lower in the Natural group (P=0.008). At Days 19 and 30, the proportion of lesions ulcerating was lower in the Natural group, compared to Unimmunized (P=0.0001 [d.19] and P=0.0002, [d.30]). At day 30, the proportion of lesions ulcerating in the Natural group was lower than in the Synthetic (P=0.04) group. Mean lesion volume was smaller in immunized groups versus Unimmunized on days 10–25. RIT indicated the lowest number of disseminated Tp in rabbit tissues from the Natural group, followed by the Synthetic group, then the Unimmunized group (P=0.0247).

Conclusion Immunization with the three-antigen cocktail significantly attenuates syphilis infection: enhancing Tp 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.