Seth LeCates

Introduction: Syphilis is a chronic venereal disease caused by the spirochete T. pallidum subspecies pallidum.⁶ Syphilis is more common in developing countries, but developed countries have seen a significant increase in cases.⁶ During sexual contact, T. pallidum transmits via exudate by penetrating skin abrasions, open wounds, or mucous membranes.⁶ Bacterial multiplication leads to an inflammatory response which can elicit multiple symptomology.⁶ Tp0751 protein has been identified as an important component in the vascular dissemination mechanism of T. pallidum, and research has focused on studying this protein to better understand the bacterial pathogenesis of syphilis and immunization strategies using Tp0751 as a treatment target.^3,5 Methods: Researchers identified Tp0751 as a vascular adhesin candidate and determined whether it could withstand host vasculature by testing Tp0751-expressing B. burgdorferi (Bb-Tp0751) against HUVECs (Human Umbilical Vein Endothelial Cells) under shear stress.² To analyze its binding characteristics with ECM components, the three-dimensional structure of Tp0751 was determined by subjecting recombinant Tp0751_78A protein to crystallography and SDS-Page analysis.¹ The affinity of Tp0751 for the LamR receptor was evaluated by incubating HUVEC proteins with recombinant Tp0751 followed by column elution and binding assays.³ Disruption of VE-Cadherin intercellular junctions by Tp0751 was assessed using time-dependent immunofluorescent microscopy followed by a solute flux permeability assay.⁴ In an animal study, experimental rabbits were immunized with Tp0751 and then infected with T. pallidum to evaluate the effects of Tp0751 vaccination on disease severity.⁵  Results: Bb-Tp0751 adhered to endothelial cells under shear conditions.² Crystallography & database analysis showed that the three-dimensional architecture of Tp0751_78A possessed a lipocalin fold.¹ Incubation and column elution of Tp0751 with HUVEC proteins displayed Tp-0751-LamR interactions.³ Binding assays confirmed this interaction.³  Disruption and permeability experiments revealed that Tp0751 modifies the architecture of VE-Cadherin junctions without disrupting endothelial permeability.⁴ Animals immunized with Tp0751 displayed decreases in bacterial burden & dissemination.⁵  Conclusions: Tp0751 is a multifunctional outer-membrane T. pallidum protein with a major role in bacterial dissemination.^2,4,6 Upon infection of host vasculature by the spirochete, Tp0751 binds to ECM components (fibrinogen, fibronectin & laminin) via its lipocalin fold, allowing T. pallidum to adhere to endothelia.^1,2 Binding of Tp0751 to endothelial cell receptor markers (i.e., LamR) helps the bacterium localize to specific organ sites (i.e., CNS).^3,7 Tp0751 disrupts VE-cadherin junctions permitting extravasation through endothelial barriers—a method similar to that used by leukocytes.⁴ Targeting Tp0751 in therapeutics reduces vascular dissemination and disease severity.⁵

1.  Parker ML, Houston S, Pětrošová H, et al. The Structure of Treponema pallidum Tp0751 (Pallilysin) Reveals a Non-canonical Lipocalin Fold That Mediates Adhesion to Extracellular Matrix Components and Interactions with Host Cells. PLoS Pathog. 2016;12(9):e1005919. Published 2016 Sep 28. doi:10.1371/journal.ppat.1005919
2. Kao WA, Pětrošová H, Ebady R, et al. Identification of Tp0751 (Pallilysin) as a Treponema pallidum Vascular Adhesin by Heterologous Expression in the Lyme disease Spirochete. Sci Rep. 2017;7(1):1538. Published 2017 May 8. doi:10.1038/s41598-017-01589-4
3. Lithgow KV, Church B, Gomez A, et al. Identification of the Neuroinvasive Pathogen Host Target, LamR, as an Endothelial Receptor for the Treponema pallidum Adhesin Tp0751. mSphere. 2020;5(2):e00195-20. Published 2020 Apr 1. doi:10.1128/mSphere.00195-20
4. Lithgow KV, Tsao E, Schovanek E, Gomez A, Swayne LA, Cameron CE. Treponema pallidum Disrupts VE-Cadherin Intercellular Junctions and Traverses Endothelial Barriers Using a Cholesterol-Dependent Mechanism. Front Microbiol. 2021;12:691731. Published 2021 Jul 20. doi:10.3389/fmicb.2021.691731
5. Lithgow KV, Hof R, Wetherell C, Phillips D, Houston S, Cameron CE. A defined syphilis vaccine candidate inhibits dissemination of Treponema pallidum subspecies pallidum. Nat Commun. 2017;8:14273. Published 2017 Feb 1. doi:10.1038/ncomms14273
6. Peeling RW, Mabey D, Kamb ML, Chen XS, Radolf JD, Benzaken AS. Syphilis. Nat Rev Dis Primers. 2017 Oct 12;3:17073. doi: 10.1038/nrdp.2017.73. Review. PubMed PMID: 29022569; PubMed Central PMCID: PMC5809176.
7. Orihuela CJ, Mahdavi J, Thornton J, et al. Laminin receptor initiates bacterial contact with the blood brain barrier in experimental meningitis models. J Clin Invest. 2009;119(6):1638-1646. doi:10.1172/JCI36759