Treponema pallidum subspecies pallidum (T. pallidum) is growing in incidence in high-income countries like the United States and remains endemic and highly prevalent in low-income countries, primarily sub-Saharan Africa, and South America. Despite being treatable, syphilis is associated with significant fetal and perinatal mortality in low-income settings, due to congenital transmission of the infection. Improving our understanding of syphilis pathogenesis, immunology, and T. pallidum biology, could result in novel measures to curtail syphilis spread, including improved diagnostics, novel therapeutics, and a preventive vaccine. We are exploring the use of a chimeric antigen, composed of a scaffolding/carrier protein of T. pallidum and a series of protective epitopes from other T. pallidum antigens previously described and patented by our laboratory, to be used as a recombinant vaccine. The first step to using this protein as a vaccine scaffolding is determining the most immunogenic sequences of the protein to be used for replacement. Specifically, we used sera from 63 patients with syphilis at different stages, and sera longitudinally collected from rabbits infected with either the Nichols or SS14 isolates of T. pallidum, which represent the model strains for the two known clades of this pathogen. Recognized amino acid sequences were then mapped to the experimentally determined Tp17 structure. Reactive epitopes in both serum groups mapped predominantly to the α-helix preceding the Tp17 soluble β-barrel and to the loops of the barrel. We are currently using the same Enzyme-Linked Immunosorbent Assay (ELISA) to assess reactivity levels in control sera of naive human patients. These results identify sequences of the Tp17 antigen that could be replaced by protective epitopes. Our work provides the basis for future research on the use of this scaffolding to develop a syphilis vaccine able to confer protection from this this serious infection.

Syphilis remains a serious global health concern, underscoring the need for better control strategies. If left untreated, the syphilis agent, Treponema pallidum subsp. pallidum (T. pallidum), can persist for decades due to its ability to evade the host immune response. Antigenic variation of the surface-exposed outer membrane protein TprK is believed to mediate persistence. TprK contains seven discrete variable (V) regions. TprK antigenic variation occurs via non-reciprocal gene conversion between the variable regions in the tprK expression site and 53 donor cassettes (DCs). We previously engineered a T. pallidum strain impaired of antigenic variation (SS14-DC^KO) by eliminating 51 of the 53 DCs. Rabbits infected with the DC^KO strain developed an attenuated infection phenotype with a reduced burden of T. pallidum cells compared to wild type (WT). Therefore, we hypothesized that if immunosuppressed rabbits are infected with SS14-DC^KO, the disease would undergo similar progression to rabbits infected with the WT strain. In this study, two rabbit groups (n=8) were either immunosuppressed with Depomedrol or untreated. Four rabbits in each group were infected intradermally with SS14-WT or SS14-DC^KO strain on clipped backs. Sera was isolated weekly to measure antibody titers using VDRL and TPPA. In addition, DNA was isolated from lesion biopsies to perform TprK profiling. Based on TprK profiling data, synthetic peptides for the most abundant V5, V6, and V7 variants were used to measure antibody reactivity by ELISA. Antibody titers measured by TPPA and VDRL were similar between immunosuppressed rabbits regardless of infecting strain. TprK profiling shows that the most abundant V sequence at the inoculum decreases over time in lesions as humoral reactivity to these peptides increases. Overall, this research demonstrates the role that TprK plays in the persistence of T. pallidum during syphilis infection and the need for novel control strategies.