Immunodominant extracellular loops of Treponema pallidum FadL outer membrane proteins elicit antibodies with opsonic and growth-inhibitory activities

The global resurgence of syphilis has created a potent stimulus for vaccine development. To identify potentially protective antibodies (Abs) against Treponema pallidum (TPA), we used Pyrococcus furiosus thioredoxin (PfTrx) to display extracellular loops (ECLs) from three TPA outer membrane protein families (outer membrane factors for efflux pumps, eight-stranded {beta}-barrels, and FadLs) to assess their reactivity with immune rabbit serum (IRS). Five ECLs from the FadL orthologs TP0856, TP0858 and TP0865 were immunodominant. Rabbits and mice immunized with these five PfTrx constructs produced ECL-specific Abs that promoted opsonophagocytosis of TPA by rabbit peritoneal and murine bone marrow-derived macrophages at levels comparable to IRS and mouse syphilitic serum. ECL-specific rabbit and mouse Abs also impaired viability, motility, and cellular attachment of spirochetes during in vitro cultivation. The results support the use of ECL-based vaccines and suggest that ECL-specific Abs promote spirochete clearance via Fc receptor-independent as well as Fc receptor-dependent mechanisms. Author SummaryThe resurgence of syphilis emphasizes the critical need for vaccine development against Treponema pallidum (TPA). Research utilizing immune rabbit serum (IRS) suggests that an effective syphilis vaccine should induce "functional" antibodies (Abs) capable of enhancing the opsonophagocytosis of treponemes by activated macrophages. Structural models of TPA outer membrane proteins (OMPs), specifically the extracellular loops (ECLs), guided the identification of potential vaccine candidates. Antigenic analysis with IRS of individual ECLs from three TPA OMP families scaffolded onto Pyrococcus furiosus thioredoxin (PfTrx) revealed five FadL antigenic ECLs. Immunization with immunodominant ECL antigens elicited robust ECL-specific Abs, demonstrating functional opsonic activity in the opsonophagocytosis assays. Furthermore, these Abs effectively inhibited the growth inhibition of in vitro-cultivated TPA in both rabbit and mouse models. Our findings underscore the value of antigenic analysis in identifying promising TPA OMP ECL vaccine targets and highlight the multifaceted protective capacity of ECL Abs against TPA. This approach also extends to identifying potential OMP vaccinogens in other bacterial pathogens, offering valuable insights for broader vaccine development strategies.