Background Immunological analysis of primary and secondary syphilis in rabbits and humans suggests that T helper cells mount a vigorous interferon-γ-dominated immune response (T_H1) to facilitate macrophage-mediated clearance of T. pallidum. In this study, we used quantitative reverse transcriptase (qRT)-PCR to evaluate post-challenge rabbit cytokine profiles of primary lesions in unimmunized rabbits and rabbits immunized with a tri-antigen vaccine cocktail.

Methods Groups of 8 male New Zealand White rabbits were immunized with a trivalent recombinant antigen cocktail, (N-term of TprK + N- term of Tpr Subfamily I + Tp 0751, emulsified in either of two custom adjuvants containing Natural or Synthetic TLR4 agonists + a natural Mincle agonist). Unimmunized control and immunized animals were intradermally challenged with 10⁵ T. pallidum (Nichols) at each of 10 sites. Lesion biopsies were collected at days 2 and 21 post-challenge. Expression of IFN-γ, TGF-β, p40 IL-12/23, IL-4, IL-2, TNF-α, IL-10, IL-17A, IL-17F, and IL-22 was quantified by qRT-PCR using plasmids containing the target rabbit cytokines sequences, and expression levels were normalized to rabbit HPRT.

Results At day 2, transcripts for IFN-γ, IL-2, IL-17A, IL-17F, and TNF-α were significantly upregulated in both immunized groups (P<0.01) compared to controls, a finding consistent with the development of clinical delayed type hypersensitivity and the induction of a T_H1-type immune response at challenge sites. At day 21, the level of IFN-γ was lower in the Natural adjuvant group (P<0.02), compared to controls, consistent with enhanced treponemal clearance in that group.

Conclusion In the development of a syphilis vaccine, it is important to determine correlates of protection to allow for assessment of the induction of a protective immune response. Our results demonstrate a robust immunization-induced T_H1 and T_H17 proinflammatory response in immunized rabbit groups, which suggests an effective level of resistance conferred by immunization with the trivalent protein vaccine.

Disclosure No significant relationships.