Heterologous Surface Display Reveals Conserved Complement Inhibition and Functional Diversification of Borrelia burgdorferi Elp Proteins

Lyme disease is a tick-borne spirochetosis with diverse clinical manifestations. Phenotypic variation among Borrelia burgdorferi strains correlates with variable manifestations of Lyme disease in humans; this diversity is attributed in part to variation in surface-exposed lipoproteins, which are targets of the human antibody response and contribute to tissue adhesion, immune evasion, and other host interactions. Many B. burgdorferi lipoproteins are encoded as multi-copy gene families including the OspE/F- like leader peptide (Elp) protein family. To characterize Elp allelic variants, we adapted the Pseudomonas syringae ice nucleation protein (INP) system to present B. burgdorferi lipoproteins on the surface of Escherichia coli. We identified interactions with classical complement proteins and mapped binding regions in the E. coli system. We validated interactions using recombinant proteins and B. burgdorferi surface display. By assessing potential interactions with extracellular matrix components, we identified a novel interaction between Elp proteins and perlecan, a component of mammalian basement membranes, and revealed the bifunctional nature of Elps. Our findings reveal that Elps have undergone functional diversification while maintaining classical complement inhibition mediated by potent and conserved C1s binding and demonstrate that E. coli surface display offers an efficient, cost-effective, and relatively high throughput approach to characterize B. burgdorferi lipoproteins.