Evidence that immunization with TP0751, a bipartite Treponema pallidum lipoprotein with an intrinsically disordered region and lipocalin fold, fails to protect in the rabbit model of experimental syphilis

Abstract

Deconvolution of syphilis pathogenesis and selection of candidate syphilis vaccinogens requires detailed knowledge of the molecular architecture of the Treponema pallidum outer membrane (OM). The T. pallidum OM contains a low density of integral OM proteins, while the spirochete's many lipoprotein immunogens are periplasmic. TP0751, a lipoprotein with a lipocalin fold, is reportedly a surface-exposed protease/adhesin and protective antigen. The rapid expansion of calycin/lipocalin structures in the RCSB PDB database prompted a comprehensive reassessment of TP0751. Small angle X-ray scattering analysis of full-length protein revealed a bipartite topology consisting of an N-terminal, intrinsically disordered region (IDR) and the previously characterized C-terminal lipocalin domain. A DALI server query using the lipocalin domain yielded 97 hits, 52 belonging to the calycin superfamily, including 15 bacterial lipocalins, but no Gram-negative surface proteins. Surprisingly, Tpp17 (TP0435) was identified as a structural ortholog of TP0751. In silico docking predicted that TP0751 can bind diverse ligands along the rim of its eight-stranded β-barrel; high affinity binding of one predicted ligand, heme, to the lipocalin domain was demonstrated. qRT-PCR and immunoblotting revealed very low expression of TP0751 compared to other T. pallidum lipoproteins. Immunoblot analysis of immune rabbit serum failed to detect TP0751 antibodies, while only one of five patients with secondary syphilis mounted a discernible TP0751specific antibody response. In opsonophagocytosis assays, neither TP0751 nor Tpp17 antibodies promoted uptake of T. pallidum by rabbit peritoneal macrophages. Rabbits

Structural Modeling of the Treponema pallidum Outer Membrane Protein Repertoire: a Road Map for Deconvolution of Syphilis Pathogenesis and Development of a Syphilis Vaccine

“…Because T. pallidum is an extracellular bacterium (6,8), an effective vaccine must elicit antibodies against antigens on the bacterial surface (105,145). Years of investigation of the molecular architecture of the T. pallidum outer membrane have shown that the principal candidates for syphilis vaccine design are contained within the spirochete's OMPeome (6,11,108,137). The structural models described herein provide investigators, for the first time, with a platform for a rational, systematic investigation of the combination of OMPs needed to elicit long-lived protection.…”

Leptolysin, a Leptospira secreted metalloprotease of the pappalysin family with broad-spectrum activity

“…In association with T. pallidum Tp0750, which contains a Von Willebrand factor type A domain, it has been reported to promote fibrinolysis and ECM degradation ( Houston et al., 2012 ; Houston et al., 2014 ). However, leptolysin is secreted extracellularly whereas pallilysin was recently shown to reside within the periplasmic space ( Luthra et al., 2020 ).…”

Extracellular Loops of the Treponema pallidum FadL Orthologs TP0856 and TP0858 Elicit IgG Antibodies and IgG+-Specific B-Cells in the Rabbit Model of Experimental Syphilis

“…TP0856 and TP0858 were expressed in E. coli Overexpress C41(DE3) ( 5 ). Cells were centrifuged at 6,000 × g for 15 min at 4°C, resuspended in buffer (50 mM Tris [pH 7.5], 50 mM NaCl) containing 100 μg of lysozyme and protease inhibitors, lysed by sonication, and then centrifuged at 20,000 × g for 30 min at 4°C.…”