Weis, D. D. | bioRxiv Science

bioRxiv2024

Single-domain antibodies reveal unique borreliacidal epitopes on the Lyme disease vaccine antigen, Outer surface protein A (OspA)

Camelid-derived, single-domain antibodies (VHHs) have proven to be extremely powerful tools in defining the antigenic landscape of immunologically heterogeneous surface proteins. In this report, we generated a phage-displayed VHH library directed against the candidate Lyme disease vaccine antigen, Outer surface protein A (OspA). Two alpacas were immunized with recombinant OspA serotype 1 (ST1) from Borrelia burgdorferi sensu stricto strain B31, in combination with the canine vaccine RECOMBITEK(R) Lyme containing lipidated OspA. The phage library was subjected to two rounds of affinity enrichment ("panning") against recombinant OspA, yielding 21 unique VHHs within two epitope bins, as determined through competition ELISAs with a panel of OspA-specific human monoclonal antibodies. Epitope refinement was conducted by hydrogen exchange-mass spectrometry (HX-MS). Six of the monovalent VHHs were expressed as human IgG1-Fc fusion proteins and shown to have functional properties associated with protective human monoclonal antibodies, including B. burgdorferi agglutination, outer membrane damage, and complement-dependent borreliacidal activity. The VHHs displayed unique reactivity profiles with the seven OspA serotypes associated with B. burgdorferi genospecies in the United States and Europe consistent with there being conserved epitopes across OspA serotypes that should be considered when designing and evaluating multivalent Lyme disease vaccines.

immunology↗

bioRxiv2022

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

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, B5s epitope has never been 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 OspCs -helix 1 and -helix 6, as well as interactions with the loop between a-helices 5 and 6. In addition, B5s 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. IMPORTANCEThe spirochete, Borreliella burgdorferi, is the causative agent of Lyme borreliosis, the most common tickborne disease in the United States. The spirochete is transmitted to humans during the course of a tick taking a bloodmeal. After B. burgdorferi is deposited into the skin of a human host, it replicates locally and spreads systemically, often resulting in clinical manifestations involving the central nervous system, joints, and/or heart. Antibodies directed against B. burgdorferis outer surface protein C (OspC) are known to block tick-to-host transmission, as well as dissemination of the spirochete within a mammalian host. In this report, we reveal the first atomic structure of one such antibody in complex with OspC. Our results have implications for the design of a Lyme disease vaccine capable to interfering with multiple stages in B. burgdorferi infection.

immunology↗