Immune evasion of Borrelia miyamotoi: CbiA, a novel outer surface protein exhibiting complement binding and inactivating properties

Florian Röttgerding,Joppe W Hovius,Alex Wagemakers,Peter Kraiczy,Joris Koetsveld,Volker Fingerle,Reinhard Wallich,Peter F Zipfel,Michael Kirschfink

doi:10.1038/s41598-017-00412-4

Abstract

Borrelia (B.) miyamotoi, an emerging tick-borne relapsing fever spirochete, resists complement-mediated killing. To decipher the molecular principles of immune evasion, we sought to identify determinants contributing to complement resistance. Employing bioinformatics, we identified a gene encoding for a putative Factor H-binding protein, termed CbiA (complement binding and inhibitory protein A). Functional analyses revealed that CbiA interacted with complement regulator Factor H (FH), C3, C3b, C4b, C5, and C9. Upon binding to CbiA, FH retained its cofactor activity for Factor I-mediated inactivation of C3b. The Factor H-binding site within CbiA was mapped to domain 20 whereby the C-terminus of CbiA was involved in FH binding. Additionally, CbiA directly inhibited the activation of the classical pathway and the assembly of the terminal complement complex. Of importance, CbiA displayed inhibitory activity when ectopically produced in serum-sensitive B. garinii G1, rendering this surrogate strain resistant to human serum. In addition, long-term in vitro cultivation lead to an incremental loss of the cbiA gene accompanied by an increase in serum susceptibility. In conclusion, our data revealed a dual strategy of B. miyamotoi to efficiently evade complement via CbiA, which possesses complement binding and inhibitory activities.

Highlights

The genus Borrelia (B.) comprises a large number of host-associated spirochetes, all of which are transmitted by hematophagous arthropods and are the causative agents of relapsing fever as well as Lyme disease
Including Borrelia and Leptospira, recruitment of fluid phase complement regulators Factor H (FH) and C4BP plays a pivotal role in inactivating complement at the central point, namely the activation of C340, 41
It has been shown that distinct outer surface proteins, e.g. BBK32, CspA, BGA66, and BGA71 of Borrelia interfere with certain complement components to inhibit CP activation by binding to C1r or to terminate assembly of the bacteriolytic complement complex by the interaction with C7, C8, and C929, 31, 32, 42

Summary

Introduction

The genus Borrelia (B.) comprises a large number of host-associated spirochetes, all of which are transmitted by hematophagous arthropods and are the causative agents of relapsing fever as well as Lyme disease. As an emerging human pathogen B. miyamotoi was first discovered in 1995 in Hokkaido (Japan)[1] and is transmitted, in contrast to other TBRF spirochetes, by hard-bodied ticks, e.g. Ixodes (I.) scapularis and I. pacificus in North America, I. ricinus in Europe, and I. persulcatus in Asia. Thereafter more clinical cases of B. miyamotoi-caused infections have been described in Europe[5], Japan[6] and North America[7,8,9] leading to the implementation of a new entity termed hard tick-borne relapsing fever (HTBRF)[10]. The complement cascade is sequentially activated by three distinct pathways, the classical (CP), alternative (AP), and the lectin pathway (LP), thereby generating cleavage products that display multiple effector functions These three pathways converge at the level of C3 by generating highly reactive C3b molecules, which function as opsonins and mark intruding microorganisms for phagocytosis. FH is organized into 20 individually folding complement control protein (CCP) domains of which the N-terminally located CCP domains 1–4 are responsible for decay-accelerating and cofactor activity[18, 19]

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