Structural determination of the complement inhibitory domain of Borrelia burgdorferi BBK32 provides insight into classical pathway complement evasion by Lyme disease spirochetes

The carboxy-terminal domain of the BBK32 protein from Borrelia burgdorferi sensu stricto, termed BBK32-C, binds and inhibits the initiating serine protease of the human classical complement pathway, C1r. In this study we investigated the function of BBK32 orthologues of the Lyme-associated Borrelia burgdorferi sensu lato complex, designated BAD16 from B. afzelii strain PGau and BGD19 from B. garinii strain IP90. Our data show that B. afzelii BAD16-C exhibits BBK32-C-like activities in all assays tested, including high-affinity binding to purified C1r protease and C1 complex, and potent inhibition of the classical complement pathway. Recombinant B. garinii BGD19-C also bound C1 and C1r with high-affinity yet exhibited significantly reduced in vitro complement inhibitory activities relative to BBK32-C or BAD16-C. Interestingly, natively produced BGD19 weakly recognized C1r relative to BBK32 and BAD16 and, unlike these proteins, BGD19 did not confer significant protection from serum killing. Site-directed mutagenesis was performed to convert BBK32-C to resemble BGD19-C at three residue positions that are identical between BBK32 and BAD16 but different in BGD19. The resulting chimeric protein was designated BXK32-C and this BBK32-C variant mimicked the properties observed for BGD19-C. To query the disparate complement inhibitory activities of BBK32 orthologues, the crystal structure of BBK32-C was solved to 1.7Ã… limiting resolution. BBK32-C adopts an anti-parallel four-helix bundle fold with a fifth alpha-helix protruding from the helical core. The structure revealed that the three residues targeted in the BXK32-C chimera are surface-exposed, further supporting their potential relevance in C1r binding and inhibition. Additional binding assays showed that BBK32-C only recognized C1r fragments containing the serine protease domain. The structure-function studies reported here improve our understanding of how BBK32 recognizes and inhibits C1r and provide new insight into complement evasion mechanisms of Lyme-associated spirochetes of the B. burgdorferi sensu lato complex

Analysis of Mechanisms Associated with Loss of Infectivity of Clonal Populations of Borrelia burgdorferi B31MI

Numerous studies have provided suggestive evidence that the loss of plasmids correlates with the loss of infectivity of the Lyme disease spirochetes. In this study we have further investigated this correlation. Clonal populations were obtained from the skin of a mouse infected for 3 months with a clonal population of Borrelia burgdorferi B31MI. The complete plasmid compositions of these populations were determined using a combination of PCR and Southern hybridization. The infectivities of clones differing in plasmid composition were tested using the C3H-HeJ murine model for Lyme disease. While several clones were found to be noninfectious, a correlation between the loss of a specific plasmid and loss of infectivity in the clones analyzed in this report was not observed. While it is clear from recent studies that the loss of some specific plasmids results in attenuated virulence, this study demonstrates that additional mechanisms also contribute to the loss of infectivity

A Structural Basis for Inhibition of the Complement Initiator Protease C1r by Lyme Disease Spirochetes

Complement evasion is a hallmark of extracellular microbial pathogens such as Borreliella burgdorferi, the causative agent of Lyme disease. Lyme disease spirochetes express nearly a dozen outer surface lipoproteins that bind complement components and interfere with their native activities. Among these, BBK32 is unique in its selective inhibition of the classical pathway. BBK32 blocks activation of this pathway by selectively binding and inhibiting the C1r serine protease of the first component of complement, C1. To understand the structural basis for BBK32-mediated C1r inhibition, we performed crystallography and size exclusion chromatography-coupled small angle x-ray scattering experiments, which revealed a molecular model of BBK32-C in complex with activated human C1r. Structure-guided site-directed mutagenesis was combined with surface plasmon resonance binding experiments and assays of complement function to validate the predicted molecular interface. Analysis of the structures shows that BBK32 inhibits activated forms of C1r by occluding substrate interaction subsites (i.e., S1 and S1’) and reveals a surprising role for the C1r B loop for full inhibitory activity of BBK32. The studies reported here provide a structural basis for classical pathway-specific inhibition by a human pathogen

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

Citation: Garcia, B. L., Zhi, H., Wager, B., Hook, M., & Skare, J. T. (2016). Borrelia burgdorferi BBK32 Inhibits the Classical Pathway by Blocking Activation of the C1 Complement Complex. Plos Pathogens, 12(1), 28. doi:10.1371/journal.ppat.1005404Pathogens 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

The Classical Complement Pathway Is Required to Control Borrelia burgdorferi Levels During Experimental Infection

Activation of the classical complement pathway occurs to varying degrees within strains of the Borrelia burgdorferi sensu lato complex, which contain a group of pathogenic spirochetes that cause tick-borne Lyme borreliosis, including the agent of Lyme disease in the United States, B. burgdorferi. Despite this information, details related to the control of B. burgdorferi by the classical pathway are not clear. To address this question, we infected C1qα−/− mice, which cannot assemble the C1 complex and thus fail to activate the classical pathway, with B. burgdorferi sensu stricto strain B31. Using bioluminescent in vivo imaging, we found that C1qα−/− mice harbored more B. burgdorferi following 10 days of infection relative to their isogenic C57BL/6 parent. Quantitative PCR (qPCR) demonstrated that C1qα−/− mice harbored significantly more B. burgdorferi than parent mice did within lymph nodes, skin, heart, and joints. The increased B. burgdorferi load in C1qα−/− mice was observed at 21 and 28 days of infection, consistent with the classical pathway promoting complement-dependent, antibody-mediated killing following the development of a B. burgdorferi-specific humoral immune response. In addition, circulating borrelial-specific IgM was higher in C1qα−/− mice relative to their parent mouse strain and did not decrease at 21 and 28 days post-infection, indicating that IgG class switching was delayed in C1qα−/− mice. At day 28, both Borrelia-specific IgG1 and IgG3 levels were higher in infected C1qα−/− mice, but that these antibodies were not sufficient to control borrelial infection in the absence of the classical pathway. Furthermore, the lack of C1q also altered the balance of the Th1/Th2 response, as both circulating Th1 (MIP-1α, IL-2, IL-12, and TNFα), Th2 (IL-4, IL-10, and MCP-1), and Th17 (IL-17) cytokines were elevated in infected C1qα−/− mice. These data imply that C1q and the classical pathway play important roles in controlling borrelial infection via antibody and complement-dependent killing, as well as altering both antibody maturation processes and the T cell response following exposure to infectious B. burgdorferi

Invasion of Eukaryotic Cells by Borrelia burgdorferi Requires β1 Integrins and Src Kinase Activity ▿ ‡

Lyme disease, caused by the bacterium Borrelia burgdorferi, is the most widespread tick-borne infection in the northern hemisphere that results in a multistage disorder with concomitant pathology, including arthritis. During late-stage experimental infection in mice, B. burgdorferi evades the adaptive immune response despite the presence of borrelia-specific bactericidal antibodies. In this study we asked whether B. burgdorferi could invade fibroblasts or endothelial cells as a mechanism to model the avoidance from humorally based clearance. A variation of the gentamicin protection assay, coupled with the detection of borrelial transcripts following gentamicin treatment, indicated that a portion of B. burgdorferi cells were protected in the short term from antibiotic killing due to their ability to invade cultured mammalian cells. Long-term coculture of B. burgdorferi with primary human fibroblasts provided additional support for intracellular protection. Furthermore, decreased invasion of B. burgdorferi in murine fibroblasts that do not synthesize the β1 integrin subunit was observed, indicating that β1-containing integrins are required for optimal borrelial invasion. However, β1-dependent invasion did not require either the α5β1 integrin or the borrelial fibronectin-binding protein BBK32. The internalization of B. burgdorferi was inhibited by cytochalasin D and PP2, suggesting that B. burgdorferi invasion required the reorganization of actin filaments and Src family kinases (SFK), respectively. Taken together, these results suggest that B. burgdorferi can invade and retain viability in nonphagocytic cells in a process that may, in part, help to explain the phenotype observed in untreated experimental infection

Characterization of a Conditional bosR Mutant in Borrelia burgdorferi▿

Borrelia burgdorferi, the etiological agent of Lyme disease, adapts to unique host environments as a consequence of its complex life cycle that spans both arthropod and mammalian species. In this regard, B. burgdorferi must adapt to various environmental signals, pHs, temperatures, and O2 and CO2 levels to establish infectious foci. We hypothesize that the BosR protein functions as a global regulator that is required for both borrelial oxidative homeostasis and pathogenesis. To assess the role of BosR in B. burgdorferi, we constructed an IPTG (isopropyl-β-d-thiogalactopyranoside)-regulated bosR strain. The selective decrease of bosR resulted in a change in growth when cells were cultured either anaerobically or microaerobically; however, a distinct growth defect was observed for anaerobically grown B. burgdorferi relative to the growth attenuation observed for microaerobically grown B. burgdorferi. B. burgdorferi cells in which BosR levels were reduced were more sensitive to hydrogen peroxide and produced lower levels of NapA (Dps) and SodA, proteins involved in the oxidative stress response. In addition, the levels of OspC and DbpA were also induced coincident with increased BosR levels, suggesting that BosR interfaces with the RpoS regulatory cascade, which is known to modulate virulence gene expression in B. burgdorferi. Taken together, these results indicate that BosR is involved in the resistance of B. burgdorferi to oxidative stressors and affects the expression of genes, either directly or indirectly, whose products are important in borrelial pathogenesis

BBK32 inhibits the enzymatic cleavage of C1s proenzyme by C1r enzyme.

<p>(A) The ability of BBK32 to inhibit the <i>in vitro</i> proteolytic cleavage of C1s proenzyme by 50 nM of activated C1r enzyme was assessed by monitoring for the presence of C1s proenzyme (single 86-kDa chain; asterisk) or activated C1s enzyme (58-kDa chain 1 and 28-kDa chain 2; arrows) by SDS-PAGE. Overnight reactions were incubated at 37°C in HBS-Ca²⁺ in the presence of various concentrations of BBK32-FL (gel 1), BBK32-C (gel 2), or BBK32-N (gel 3). In the absence of BBK32, C1r enzyme converts 100% of C1s proenzyme to C1s enzyme under the conditions used (see “No BBK32” lanes on gels 1–3). BBK32-FL and BBK32-C inhibited C1r activity in a dose-dependent manner, whereas, BBK32-N failed to inhibit C1r up to a final concentration of 10 μM. All C1r activity assays were conducted a minimum of three times and representative gel images are shown. (B) Densitometry was performed and the normalized peak intensity of the band corresponding to C1s proenzyme was plotted against the concentration of BBK32 present in each reaction. IC₅₀ values were calculated and are reported in <a href="http://www.plospathogens.org/article/info:doi/10.1371/journal.ppat.1005404#ppat.1005404.t002" target="_blank">Table 2</a> along with statistics for individual fits. (C) The ability of 1 μM BBK32 proteins to inhibit the cleavage of complement C4 by previously activated and purified C1s enzyme was evaluated <i>in vitro</i> by monitoring the conversion of the C4 α-chain to the C4 α’-chain by SDS-PAGE. C1s enzyme activity could not be detected under the conditions used for any BBK32 proteins, indicating that the BBK32 inhibitory activity is specific for C1r. C1s activity assays were performed in duplicate and a gel image from a representative experiment is shown.</p

BBK32 inhibits the autocatalysis of C1r within the C1 complex.

<p>C1 (40 nM) was incubated at 37°C for 2 hours in the presence or absence of 5 μM BBK32 or OspC proteins. Reactions were co-immunoprecipitated using a C1q monoclonal antibody previously adsorbed to protein G beads, and bound fractions were subjected to Western immunoblot analysis. (A) Reactions were probed with C1r polyclonal antibody. Previously purified activated C1r enzyme is loaded as a reference. Activated C1r is detected in buffer only reactions as judged by the presence of C1r enzyme chain 1 (57 kDa) and chain 2 (35 kDa), denoted by arrowheads. Reactions incubated with BBK32-FL or BBK32-C lack processed C1r and contained only the 92 kDa C1r proenzyme form (denoted by an asterisk). BBK32-N and a negative control protein (OspC) reactions contain processed C1r at levels indistinguishable from the buffer only reaction. (B) Reactions were probed with a C1s polyclonal antibody and previously purified activated C1s is loaded for reference. As was observed for C1r, reactions incubated with BBK32-FL or BBK32-C lack activated C1s and contain only C1s proenzyme (asterisk). Buffer only, BBK32-N, and OspC reactions contain equal amounts of activated C1s as judged by the presence of C1s enzyme chain 1 (58 kDa) and chain 2 (28 kDa) (denoted with arrowheads). (C) Detection with C1q polyclonal antibody indicates equivalent amounts of C1q are pulled down in all reactions. (D) Detection with BBK32 polyclonal antibody demonstrates that BBK32-FL or BBK32-C but not BBK32-N are pulled down with the C1 complex. The last three rightmost lanes contain 100 ng of each form of BBK32 protein as a reference.</p