Borrelia burgdorferi BBK32 Inhibits the Classical Pathway by Blocking Activation of the C1 Complement Complex

Abstract

Pathogens that traffic in blood, lymphatics, or interstitial fluids must adopt strategies to evade innate immune defenses, notably the complement system. Through recruitment of host regulators of complement to their surface, many pathogens are able to escape complement-mediated attack. The Lyme disease spirochete, Borrelia burgdorferi, produces a number of surface proteins that bind to factor H related molecules, which function as the dominant negative regulator of the alternative pathway of complement. Relatively less is known about how B. burgdorferi evades the classical pathway of complement despite the observation that some sensu lato strains are sensitive to classical pathway activation. Here we report that the borrelial lipoprotein BBK32 potently and specifically inhibits the classical pathway by binding with high affinity to the initiating C1 complex of complement. In addition, B. burgdorferi cells that produce BBK32 on their surface bind to both C1 and C1r and a serum sensitive derivative of B. burgdorferi is protected from killing via the classical pathway in a BBK32-dependent manner. Subsequent biochemical and biophysical approaches localized the anti-complement activity of BBK32 to its globular C-terminal domain. Mechanistic studies reveal that BBK32 acts by entrapping C1 in its zymogen form by binding and inhibiting the C1 subcomponent, C1r, which serves as the initiating serine protease of the classical pathway. To our knowledge this is the first report of a spirochetal protein acting as a direct inhibitor of the classical pathway and is the only example of a biomolecule capable of specifically and noncovalently inhibiting C1/C1r. By identifying a unique mode of complement evasion this study greatly enhances our understanding of how pathogens subvert and potentially manipulate host innate immune systems.

Borrelia burgdorferiouter surface protein C (OspC) binds complement component C4b and confers bloodstream survival

“…Recently, it was shown that the fibronectin and glycosaminoglycan binding protein, BBK32, is able to bind to the complement component C1 cleavage product, C1r, and inhibit its activation (Garcia, Zhi, Wager, Höök, & Skare, ). This binding activity on the surface of B. burgdorferi was found to inhibit in vitro complement activation through the classical pathway, in vitro (Garcia et al, ). Interestingly, two studies of mice deficient in complement component C3 suggested that complement‐mediated defence diminishes the bacterial burden in a variety of tissues, particularly at 2 weeks after infection (Lawrenz et al, ).…”

Complement and Bacterial Infections: From Molecular Mechanisms to Therapeutic Applications

“…There are several ways by which bacterial pathogens block the complement system; these include capsule production [49,50], modification of LPS DOI: 10.1159/000491439 [51,52], recruitment of human complement regulators to the bacterial surface (C4BP, factor H, and FHL-1) and production of proteases that cleave complement components [46]. Furthermore, bacteria produce specific complement inhibitory molecules that block specific steps in the complement cascade; for example, bacteria can frustrate the activation of C1s [53,54], block C3 and C5 convertases [20,55], block C5 cleavage [56,57], prevent C5aR activation [58], or inhibit MAC formation [59]. For more detailed information on these evasion mechanisms, we refer to an extensive review on this topic [46].…”

Spirochetal Lipoproteins and Immune Evasion

“…The multi-stage process of complement activation presents spirochetes (such as B. burgdorferi ) with the opportunity to attack at multiple phases. For example, B. burgdorferi binds and inhibits the C1 initiation complex and accelerates C3b inactivation (91, 125). Furthermore, B. burgdorferi can bind either Factor H or FHL-1, two important complement regulators which upon being bound by CRASP-2 and CRASP-1 ( B. burgdorferi membrane-bound lipoproteins), respectively, are inactivated and inhibit formation of complement system activation products (126, 127).…”