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. Genotypic and 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, such as the OspE/F-like leader peptide (Elp) protein family, which inhibits classical complement activation by binding complement C1s. 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. Using this system, we identified interactions with classical complement proteins and mapped binding regions, then validated interactions using recombinant proteins and B. burgdorferi surface display. We also discovered a novel potential interaction between Elp proteins and the mammalian basement membrane protein perlecan, thus revealing a bifunctional nature of Elps. Our findings indicate 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.