Lipoproteome screening of the Lyme disease agent identifies novel inhibitors of antibody-mediated complement killing

Spirochetal pathogens such as the causative agent of Lyme disease, Borrelia burgdorferi sensu lato, encode an abundance of lipoproteins; however, due in part to their evolutionary distance from more well-studied bacteria such as Proteobacteria and Firmicutes, very few spirochetal lipoproteins have assigned functions. Indeed, B. burgdorferi devotes almost 8% of its genome to lipoprotein genes and interacts with its environment primarily through the production of at least eighty surface-exposed lipoproteins throughout its tick vector-vertebrate host lifecycle (57). Several B. burgdorferi lipoproteins have been shown to serve diverse roles, such as cellular adherence or immune evasion, but the functions for most B. burgdorferi surface lipoproteins remain unknown. In this study, we developed a B. burgdorferi lipoproteome screening platform utilizing intact spirochetes that enables the identification of previously unrecognized host interactions. As spirochetal survival in the bloodstream is essential for dissemination, we targeted our screen to C1, the first component of the classical (antibody-mediated) complement pathway. We identified two high-affinity C1 interactions by the paralogous lipoproteins, ErpB and ErpQ. Using biochemical, microbiological, and biophysical approaches, we demonstrated that ErpB and ErpQ inhibit the activated forms of the C1 proteases, C1r and C1s, and represent a new mechanistic class of C1 inhibitors that protect the spirochete from antibody-mediated complement killing by allosteric regulation. In addition to identifying a novel mode of complement inhibition, our study establishes a lipoproteome screening methodology as a discovery platform for identifying direct host-pathogen interactions that are central to the pathogenesis of spirochetes, such as the Lyme disease agent.