Structural basis for complement evasion by Lyme disease pathogen Borrelia burgdorferi

Abstract

Borrelia burgdorferi spirochetes that cause Lyme borreliosis survive for a long time in human serum because they successfully evade the complement system, an important arm of innate immunity. The outer surface protein E (OspE) of B. burgdorferi is needed for this because it recruits complement regulator factor H (FH) onto the bacterial surface to evade complement-mediated cell lysis. To understand this process at the molecular level, we used a structural approach. First, we solved the solution structure of OspE by NMR, revealing a fold that has not been seen before in proteins involved in complement regulation. Next, we solved the x-ray structure of the complex between OspE and the FH C-terminal domains 19 and 20 (FH19-20) at 2.83 Å resolution. The structure shows that OspE binds FH19-20 in a way similar to, but not identical with, that used by endothelial cells to bind FH via glycosaminoglycans. The observed interaction of OspE with FH19-20 allows the full function of FH in down-regulation of complement activation on the bacteria. This reveals the molecular basis for how B. burgdorferi evades innate immunity and suggests how OspE could be used as a potential vaccine antigen.

Keywords: Bb-CRASP; Borrelia; Complement; Immune Evasion; Microbial Pathogenesis; NMR; Outer Surface Protein E; Protein Complex Structure; Protein Structure; X-ray Crystallography.

FIGURE 1.

The NMR structure of OspE. a, schematic representation of the mean NMR structure of OspE (residues 21–171, dashed line indicating the flexible N terminus). b, overlay of 20 energy-minimized NMR structures of OspE with the backbone of residues 42–171 being superimposed. Two orientations are shown rotated 90° from each other.

FIGURE 2.

OspE·FH19-20 complex formation and the x-ray crystal structure. a, gel filtration analysis of OspE (magenta), FH19-20 (green), and OspE·FH19-20 complex (blue). b, a detail of the electron density map (2F_O − F_C) of the OspE·FH19-20 complex in stereo. Carbon atoms of FH19-20 are shown in gray, and those of OspE are shown in yellow. c, crystal structure of OspE (yellow schematic) in complex with FH19-20 (gray surface model, interacting atoms in yellow). d, interface between FH domain 20 (gray schematic) and OspE (yellow surface model) from two directions about 180° apart with the interacting residues shown in a ball-and-stick representation and the main interacting residues annotated.

FIGURE 3.

The interaction site between OspE and FH19-20. a, chemical shift perturbation of OspE residues in an HSQC titration NMR experiment upon the addition of FH19-20 (residues not assigned indicated with asterisks). b, residues with larger that 0.4 ppm (dotted line) perturbation considered to interact with FH19-20 are colored red in the schematic representation of OspE, whereas residues not observed in the OspE-FH19-20 complex are colored black. The β-strands involved are indicated (β1–β4). c, surface representation of FH domain 20 with the OspE-binding residues found in the crystal structure highlighted in green (left), the heparin-binding residues highlighted in blue (middle), and the endothelial cell-binding residues highlighted in orange (right). The key heparin and endothelial cell binding residues have been annotated.

FIGURE 4.

Sequence alignment of the FH binding region of OspE with the homologous parts of the Erps. Sequence alignment between OspE from the N40 strain of B. burgdorferi (used in this study) and Erp paralog proteins encoded by other FH-binding B. burgdorferi strains (B31, BL206, 297, and Sh-2-82) and single strains of B. afzelii and B. garinii is shown. Residues of OspE forming hydrogen bonds with FH19-20 are annotated, their alignments are shown with boxes, and identical residues are highlighted in boldface type.

FIGURE 5.

Superimposition based models of OspE in complex with FH19-20 and C3d or C3b. a, superimposition of the OspE·FH19-20 structure with the FH19-20·C3d structure (20), indicating no steric clashes in the OspE·FH19-20·C3d complex. b, model of the spatial organization of the OspE·FH19-20 complex bound to C3b (65) on the target surface as based on superimposition of the OspE·FH19-20·C3d complex model with the structure of C3b (containing the C3d domain). The location of the thioester site is indicated in orange.