Correlates Of Immune Protection In The Rabbit Model Of Syphilis Vaccination

Background An effective syphilis vaccine will be integral to efforts to eradicate this disease. Promising vaccine candidates are surface antigens of the syphilis spirochete, Treponema pallidum subsp. pallidum (T. pallidum). These antigens can be targeted by vaccination-induced opsonic antibodies and mediate pathogen immune clearance. Defining correlates of protection will aid in the identification of the best vaccine candidates. Here, we sought to investigate whether immunization with variants of the T. pallidum Repeat C (TprC) protein and the conserved NH2-terminus of the TprK induced protection and whether splenocyte proliferation and IFN-γ production correlated with protection. Methods Rabbits were immunized with either a cocktail of three recombinant, full-length TprC variants, or the NH2-terminus of the TprK protein with a RIBI-like adjuvant. Animals were challenged with T. pallidum intradermally (10 sites; 10^5 bacteria/site). Treponemal burden and progression to ulceration were monitored. To assess for immunogen-specific splenocytes, pools of synthetic peptides corresponding to each immunogen were used to stimulate splenocytes collected ex-vivo in proliferation assays. Supernatants from stimulated splenocytes were used to quantify IFN-γ responses by ELISA. Results Immunizations protected animals significantly albeit not completely. At day 35 post-challenge only 14.1% and 15.5% of lesions ulcerated in immunized rabbits compared to the 95% of lesions in unimmunized rabbits. At day 21, there was a 99.3% and 98.7% reduction in treponemal burden averaged across all challenge sites in TprC- and TprK-immunized rabbits compared to unimmunized animals, respectively. Lymphocyte proliferation and IFN-γ production correlated to reduction in both percent of ulcerated lesions and treponemal burden. Conclusions Lymphocyte proliferation and IFN-γ release assays may serve as surrogates to assess for antigen-specific T-cell responses. TprK and TprC immunizations are able to stimulate cellular immunity in a TH2 environment, which is key to development of an effective syphilis vaccine.