Structural Elucidation of a Protective B cell Epitope on Outer Surface Protein C (OspC) of the Lyme disease spirochete,Borreliella burgdorferi

Abstract

Outer surface protein C (OspC) plays a pivotal role in mediating tick-to-host transmission and infectivity of the Lyme disease spirochete, Borreliella burgdorferi. OspC is a helical-rich homodimer that interacts with tick salivary proteins, as well as components of the mammalian immune system. Several decades ago, it was shown that the OspC-specific monoclonal antibody, B5, was able to passively protect mice from experimental tick-transmitted infection by B. burgdorferi strain B31. However, B5's epitope has never elucidated, despite widespread interest in OspC as a possible Lyme disease vaccine antigen. Here we report the crystal structure of B5 antigen-binding fragments (Fabs) in complex with recombinant OspC type A (OspCA). Each OspC monomer within the homodimer was bound by a single B5 Fab in a side-on orientation, with contact points along OspC's a-helix 1 and a-helix 6, as well as interactions with the variable loop between a-helices 5 and 6. In addition, B5's complementarity-determining region (CDR) H3 bridged the OspC-OspC' homodimer interface, revealing the quaternary nature of the protective epitope. To provide insight into the molecular basis of B5 serotype specificity, we solved the crystal structures of recombinant OspC types B and K and compared them to OspCA. This study represents the first structure of a protective B cell epitope on OspC and will aid in the rational design of OspC-based vaccines and therapeutics for Lyme disease.

Structure of a human monoclonal antibody in complex with Outer surface protein C (OspC) of the Lyme disease spirochete,Borreliella burgdorferi

“…From this information, we speculate that B11 likely recognizes four additional OspC types: C3, I3, J, and M ( Figure S5B ). This contrasts with B5 IgG, which we proposed is restricted to OspC types A, C3 and I3 [38]. Cross reactivity with I3 is not surprising given that is a naturally occurring chimera between OspC F and OspC A with the C-terminus (residues 128-199) derived from type A [44].…”

“…Moderate protection was also observed elsewhere (e.g., peptide 157-162). The B11 IgG HX-MS protection profile is reminiscent of the profile we reported for B5 IgG using slightly different HX-MS conditions [38]. To enable a direct comparison between B11 and B5, we subjected OspC A to HX-MS with B5 IgG under the same conditions as B11.…”

“…Furthermore, the addition of propidium iodide (PI) to the spirochete preparations just prior to flow cytometry serves as indicator of antibody-induced changes to OM permeability [41]. We have postulated that antibody-mediated agglutination explains, at least in part, how OspA and possibly OspC antibodies compromise B. burgdorferi within the tick midgut and limit their transmission to vertebrate hosts [23, 30, 38]. We therefore examined what impact that B11 had on B. burgdorferi agglutination.…”

“…Understanding the molecular basis of OspC type-immunospecificity is a longstanding challenge with relevance to vaccine design [42, 43]. We previously speculated that B5 IgG reactivity is restricted to just three OspC types (A, C3 and I3), based the variability of key paratope-epitope contacts [38]. By the same token, multiple sequence alignment (MSA) of 23 prominent B. burgdorferi OspC types suggest that B11 is similarly restricted.…”

“…Affinity purified IgG 1 B11 was then subjected to buffer exchange into PBS and stored at 4 °C. B11 Fabs were generated as described [38]. Purification and specificity of MAbs B5 [38]and hPB10 [56]have been previously described.…”