Inhibition of the Complement Classical Pathway as an Immune Evasion and Survival Technique Across Borrelia Species

Abstract

Lyme disease (LD), caused by the spirochete Borreliella burgdorferi, is the most common vector-borne disease in the United States, with greater than 476,000 patients diagnosed each year. Tick-Borne Relapsing Fever (TBRF) is caused by Borrelia species that share some virulence factors with LD-causing spirochetes. Both pathogens disseminate and establish infection hematogenously, indicative of their need to contend with host immune components in blood and body fluids. Characterizing immune evasion mechanisms these spirochetes use to establish chronic infection is a crucial aspect of understanding the pathogenesis of LD and TBRF. The classical pathway (CP) of the complement cascade, initiated by antibody-antigen complexes through specific or non-specific recognition, plays an important role in the response to infections via formation of a lethal membrane attack complex (MAC), inflammation at the site of activation, and opsonization of target cells. Regarding LD and TBRF spirochetes, antibodies activating the CP can consist of both natural IgM and targeted antibodies to these spirochetal species. Historical evidence indicates that LD spirochetes generate a robust humoral response but are unable to clear borrelial infection, suggesting that B. burgdorferi can mitigate clearance mediated by the classical pathway. Several proteins in LD and TBRF spirochetes inhibit the complement cascade, ostensibly contributing to their pathogenic potential during mammalian infection. Specifically, the B. burgdorferi protein, BBK32, binds and inhibits C1r of the CP, in addition to its ability to bind to fibronectin and glycosoaminoglycans, believed to promote extravasation. Proteins with sequence and structural similarity to BBK32 in TBRF Borrelia (B. hermsii, B. miyamotoi, and B. turicatae), termed Fibronectin-binding proteins (Fbps), might also have a role in CP inhibition, and form part of a larger family of borrelial C1r inhibitors. In this work, we have determined the mechanism for BBK32’s inhibition of C1r, assessed the conserved nature of this mechanism across borrelial C1r inhibitors, and shown the relevance of BBK32’s C1r-inhibiting activity in the face of a targeted antibody response. These studies help to define C1r-inhibitory tactics used by borrelial pathogens, and contribute to our understanding of borrelial immune evasion.