Borrelia burgdorferi surface-localized proteins expressed during persistent murine infection are conserved among diverse Borrelia spp

Borrelia burgdorferi lacking all cp32 prophage plasmids retains full infectivity in mice

The causative agent of Lyme disease, Borrelia burgdorferi, contains a unique, segmented genome comprising multiple linear and circular plasmids. To date, the genomes of over 63 sequenced Lyme disease Borrelia carry one or more 32 kbp circular plasmids (cp32) or cp32-like elements. The cp32 plasmids are endogenous prophages and encode, among other elements, a family of surface exposed lipoproteins termed OspEF-related proteins. These lipoproteins are synthesized during mammalian infection and are considered important components of the spirochete's adaptive response to the vertebrate host. Here, we detail the construction and infectivity of the first described B. burgdorferi strain lacking all cp32 plasmids. Despite their universal presence, our findings indicate that B. burgdorferi does not require any cp32 plasmids to complete the experimental mouse-tick-mouse infectious cycle and a total lack of cp32s does not impair spirochete infectivity.

Analysis of the BadR regulon in Borrelia burgdorferi

BACKGROUND

Borrelia burgdorferi, the causative agent of Lyme disease, relies on tightly coordinated gene expression to quickly adapt and survive in the tick vector and mammalian host. BadR, an ROK (repressor, open reading frame, kinase) family transcriptional regulator, binds directly to B. burgdorferi promoter DNA, however, many questions concerning the role for BadR in gene regulation remain unanswered. In particular, there are conflicting reports concerning what genes are regulated by BadR in B. burgdorferi. Furthermore, previous studies have suggested important roles for BadR in unfed ticks, but the BadR regulon has not been defined under such conditions. Additionally, although BadR regulates rpoS expression in a growth phase-dependent manner, it remains unknown whether BadR regulates other genes during different growth phases.

RESULTS

To address these questions, we cultivated a B. burgdorferi badR mutant and wild-type strain under various conditions and analyzed the transcriptome using RNA-sequencing. When spirochetes were grown at 37 °C and collected at the mid-logarithmic and stationary phase of growth, 211 and 272 genes were differentially expressed in the badR mutant, respectively. A total of 79 genes were differentially expressed when spirochetes were grown at 23 °C. A vast majority of genes identified in this study encode proteins of unknown function.

CONCLUSIONS

Complex transcriptional regulation mechanisms coordinate the expression of genes required for the survival of B. burgdorferi throughout its tick-mammal enzootic lifecycle. As part of this process, BadR functions as a global regulatory protein and regulates B. burgdorferi virulence gene expression. Combined, this work supports a role for BadR in global B. burgdorferi gene regulation by modulating expression of different sets of genes at different stages of the enzootic lifecycle. We anticipate that investigating the function of genes in the BadR regulon will lead to the identification of novel virulence factors for therapeutic and vaccine development.

Antibody profiling of a Borreliella burgdorferi (Lyme disease) C6 antibody positive, symptomatic Rottweiler and her pups

Lyme disease (LD) is a tick-transmitted disease caused by Borreliella burgdorferi (Bb). Temporal studies of maternal antibody (Ab) profiles in Bb infected pregnant dogs and their pups have not been conducted. In this study, Ab profiles of a client-owned Bb C6 Ab positive Rottweiler and her nine pups were assessed. The dam presented with lameness 12 days prior to parturition and was C6 Ab positive with a Quant C6 Ab concentration of 237U/mL. Treatment with amoxicillin was initiated and 11 days later nine pups were delivered. Screening of the sera from the dam and pups against Bb cell lysates and a panel of antigens revealed similar immunoreactivity profiles. While antigen-specific IgG and IgM reactivity persisted in the dam for at least 7 months, a rapid decline in IgG specific for BBA36, BBK53, BB0238, BBA73 and outer surface protein (Osp) E in the pups occurred between days 29 and 52 post-parturition. In contrast, Ab specific for DbpA and the diagnostic antigens VlsE (C6) and OspF, remained elevated in the pups. Sera from the dam displayed potent complement-dependent bactericidal activity against Bb. Sera from the pups was also bactericidal but primarily through a complement-independent mechanism. Lastly, single dose vaccination of the dam at day 51 post-parturition with a LD subunit vaccine consisting of OspA and an OspC chimeritope triggered a broad anti-OspC Ab response indicative of an anamnestic response. Although this study focused on a single case, these findings add to our knowledge of maternal Ab profiles and will aid the interpretation of serological assays in pups delivered by a Bb C6 Ab positive dog.

Structural analysis of the outer surface proteins from Borrelia burgdorferi paralogous gene family 54 that are thought to be the key players in the pathogenesis of Lyme disease

Lyme disease is a tick-borne infection caused by Borrelia burgdorferi sensu lato complex spirochetes. Through a complex enzootic cycle, the bacteria transfer between two different hosts: Ixodes ticks and mammalian organisms. At the start of the tick blood meal, the spirochetes located in the tick gut upregulate the expression of several genes, mainly coding for outer surface proteins. Outer surface proteins belonging to the paralogous gene family 54 (PFam54) have been shown to be the most upregulated among the other borrelial proteins and the results clearly point to the potential importance of these proteins in the pathogenesis of Lyme disease. The significance of PFam54 proteins is confirmed by the fact that of all ten PFam54 proteins, BBA64 and BBA66 are necessary for the transfer of B. burgdorferi from infected Ixodes ticks to mammalian hosts. To enhance the understanding of the pathogenesis of Lyme disease and to promote the development of novel therapies against Lyme disease, we solved the crystal structure of the PFam54 member BBA65. Additionally, we report the structure of the B. burgdorferi BBA64 orthologous protein from B. spielmanii. Together with the previously determined crystal structures of five PFam54 members and several related proteins, we performed a comprehensive structural analysis for this important group of proteins. In addition to revealing the molecular aspects of the proteins, the structural data analysis suggests that the gene families PFam54 and PFam60, which have long been referred to as separate paralogous families, should be merged into one and designated as PFam54_60.

The RpoS Gatekeeper in Borrelia burgdorferi: An Invariant Regulatory Scheme That Promotes Spirochete Persistence in Reservoir Hosts and Niche Diversity

Maintenance of Borrelia burgdorferi within its enzootic cycle requires a complex regulatory pathway involving the alternative σ factors RpoN and RpoS and two ancillary trans-acting factors, BosR and Rrp2. Activation of this pathway occurs within ticks during the nymphal blood meal when RpoS, the effector σ factor, transcribes genes required for tick transmission and mammalian infection. RpoS also exerts a 'gatekeeper' function by repressing σ⁷⁰-dependent tick phase genes (e.g., ospA, lp6.6). Herein, we undertook a broad examination of RpoS functionality throughout the enzootic cycle, beginning with modeling to confirm that this alternative σ factor is a 'genuine' RpoS homolog. Using a novel dual color reporter system, we established at the single spirochete level that ospA is expressed in nymphal midguts throughout transmission and is not downregulated until spirochetes have been transmitted to a naïve host. Although it is well established that rpoS/RpoS is expressed throughout infection, its requirement for persistent infection has not been demonstrated. Plasmid retention studies using a trans-complemented ΔrpoS mutant demonstrated that (i) RpoS is required for maximal fitness throughout the mammalian phase and (ii) RpoS represses tick phase genes until spirochetes are acquired by a naïve vector. By transposon mutant screening, we established that bba34/oppA5, the only OppA oligopeptide-binding protein controlled by RpoS, is a bona fide persistence gene. Lastly, comparison of the strain 297 and B31 RpoS DMC regulons identified two cohorts of RpoS-regulated genes. The first consists of highly conserved syntenic genes that are similarly regulated by RpoS in both strains and likely required for maintenance of B. burgdorferi sensu stricto strains in the wild. The second includes RpoS-regulated plasmid-encoded variable surface lipoproteins ospC, dbpA and members of the ospE/ospF/elp, mlp, revA, and Pfam54 paralogous gene families, all of which have evolved via inter- and intra-strain recombination. Thus, while the RpoN/RpoS pathway regulates a 'core' group of orthologous genes, diversity within RpoS regulons of different strains could be an important determinant of reservoir host range as well as spirochete virulence.

Evaluation of in vivo expressed Borrelia burgdorferi antigens for improved IgM serodiagnosis of early Lyme disease

Improved serologic tests are needed for accurate diagnosis and proper treatment of early stage Lyme disease. We evaluated the 3 antigens currently used for 2-tiered IgM immunoblot testing (FlaB, OspC, and BmpA) in combination with 3 additional antigens (BBA65, BBA70, and BBA73) and measured the sensitivity and specificity against a serum repository of positive and negative controls. Using 3 statistical methods for positivity cutoff determinations and scoring criteria, we found increased sensitivities for early Lyme disease when 2 of 6 antigens were positive as compared with the 2 of 3 antigen IgM criteria currently used for second-tier immunoblot scoring. Specificities for negative controls were comparable or superior to using 2 of 3 antigens. These results indicate that IgM sensitivity and specificity of serological testing for Lyme disease in the early stages of illness can be improved by employing antigens that target the initial host antibody responses.

Identification of Surface Epitopes Associated with Protection against Highly Immune-Evasive VlsE-Expressing Lyme Disease Spirochetes

The tick-borne pathogen Borrelia burgdorferi is responsible for approximately 300,000 Lyme disease (LD) cases per year in the United States. Recent increases in the number of LD cases, in addition to the spread of the tick vector and a lack of a vaccine, highlight an urgent need for designing and developing an efficacious LD vaccine. Identification of protective epitopes that could be used to develop a second-generation (subunit) vaccine is therefore imperative. Despite the antigenicity of several lipoproteins and integral outer membrane proteins (OMPs) on the B. burgdorferi surface, the spirochetes successfully evade antibodies primarily due to the VlsE-mediated antigenic variation. VlsE is thought to sterically block antibody access to protective epitopes of B. burgdorferi However, it is highly unlikely that VlsE shields the entire surface epitome. Thus, identification of subdominant epitope targets that induce protection when they are made dominant is necessary to generate an efficacious vaccine. Toward the identification, we repeatedly immunized immunocompetent mice with live-attenuated VlsE-deleted B. burgdorferi and then challenged the animals with the VlsE-expressing (host-adapted) wild type. Passive immunization and Western blotting data suggested that the protection of 50% of repeatedly immunized animals against the highly immune-evasive B. burgdorferi was antibody mediated. Comparison of serum antibody repertoires identified in protected and nonprotected animals permitted the identification of several putative epitopes significantly associated with the protection. Most linear putative epitopes were conserved between the main pathogenic Borrelia genospecies and found within known subdominant regions of OMPs. Currently, we are performing immunization studies to test whether the identified protection-associated epitopes are protective for mice.

Human neuroglial cells internalize Borrelia burgdorferi by coiling phagocytosis mediated by Daam1

Borrelia burgdorferi, the agent of Lyme borreliosis, can elude hosts’ innate and adaptive immunity as part of the course of infection. The ability of B. burgdorferi to invade or be internalized by host cells in vitro has been proposed as a mechanism for the pathogen to evade immune responses or antimicrobials. We have previously shown that B. burgdorferi can be internalized by human neuroglial cells. In this study we demonstrate that these cells take up B. burgdorferi via coiling phagocytosis mediated by the formin, Daam1, a process similarly described for human macrophages. Following coincubation with glial cells, B. burgdorferi was enwrapped by Daam1-enriched coiling pseudopods. Coiling of B. burgdorferi was significantly reduced when neuroglial cells were pretreated with anti-Daam1 antibody indicating the requirement for Daam1 for borrelial phagocytosis. Confocal microscopy showed Daam1 colocalizing to the B. burgdorferi surface suggesting interaction with borrelial membrane protein(s). Using the yeast 2-hybrid system for identifying protein-protein binding, we found that the B. burgdorferi surface lipoprotein, BBA66, bound the FH2 subunit domain of Daam1. Recombinant proteins were used to validate binding by ELISA, pull-down, and co-immunoprecipitation. Evidence for native Daam1 and BBA66 interaction was suggested by colocalization of the proteins in the course of borrelial capture by the Daam1-enriched pseudopodia. Additionally, we found a striking reduction in coiling for a BBA66-deficient mutant strain compared to BBA66-expressing strains. These results show that coiling phagocytosis is a mechanism for borrelial internalization by neuroglial cells mediated by Daam1.

Immunization of mice with Borrelia burgdorferi lp54 gene encoded recombinant proteins does not provide protection against tick transmitted infectious challenge

The Borrelia burgdorferi outer surface membrane proteins BBA65, BBA66, BBA69, BBA70, and BBA73 were tested for their ability to confer protection against B. burgdorferi infection challenge. Mice were immunized with recombinant forms of the proteins singly or in combinations. Following initial protein inoculation and booster injections, seroconversion was confirmed prior to B. burgdorferi challenge by tick bite. Despite mice having high antibody titers for each antigen, no significant protections against the challenge infections were observed. These results demonstrate that these recombinant proteins were not protective and reflects the challenges confronted to identify effective novel vaccine candidates for Lyme disease.

Comprehensive Spatial Analysis of the Borrelia burgdorferi Lipoproteome Reveals a Compartmentalization Bias toward the Bacterial Surface

The Lyme disease spirochete Borrelia burgdorferi is unique among bacteria in its large number of lipoproteins that are encoded by a small, exceptionally fragmented, and predominantly linear genome. Peripherally anchored in either the inner or outer membrane and facing either the periplasm or the external environment, these lipoproteins assume varied roles. A prominent subset of lipoproteins functioning as the apparent linchpins of the enzootic tick-vertebrate infection cycle have been explored as vaccine targets. Yet, most of the B. burgdorferi lipoproteome has remained uncharacterized. Here, we comprehensively and conclusively localize the B. burgdorferi lipoproteome by applying established protein localization assays to a newly generated epitope-tagged lipoprotein expression library and by validating the obtained individual protein localization results using a sensitive global mass spectrometry approach. The derived consensus localization data indicate that 86 of the 125 analyzed lipoproteins encoded by B. burgdorferi are secreted to the bacterial surface. Thirty-one of the remaining 39 periplasmic lipoproteins are retained in the inner membrane, with only 8 lipoproteins being anchored in the periplasmic leaflet of the outer membrane. The localization of 10 lipoproteins was further defined or revised, and 52 surface and 23 periplasmic lipoproteins were newly localized. Cross-referencing prior studies revealed that the borrelial surface lipoproteome contributing to the host-pathogen interface is encoded predominantly by plasmids. Conversely, periplasmic lipoproteins are encoded mainly by chromosomal loci. These studies close a gap in our understanding of the functional lipoproteome of an important human pathogen and set the stage for more in-depth studies of thus-far-neglected spirochetal lipoproteins.IMPORTANCE The small and exceptionally fragmented genome of the Lyme disease spirochete Borrelia burgdorferi encodes over 120 lipoproteins. Studies in the field have predominantly focused on a relatively small number of surface lipoproteins that play important roles in the transmission and pathogenesis of this global human pathogen. Yet, a comprehensive spatial assessment of the entire borrelial lipoproteome has been missing. The current study newly identifies 52 surface and 23 periplasmic lipoproteins. Overall, two-thirds of the B. burgdorferi lipoproteins localize to the surface, while outer membrane lipoproteins facing the periplasm are rare. This analysis underscores the dominant contribution of lipoproteins to the spirochete's rather complex and adaptable host-pathogen interface, and it encourages further functional exploration of its lipoproteome.

Antibody Response to Lyme Disease Spirochetes in the Context of VlsE-Mediated Immune Evasion

Lyme disease (LD), the most prevalent tick-borne illness in North America, is caused by Borrelia burgdorferi The long-term survival of B. burgdorferi spirochetes in the mammalian host is achieved though VlsE-mediated antigenic variation. It is mathematically predicted that a highly variable surface antigen prolongs bacterial infection sufficiently to exhaust the immune response directed toward invariant surface antigens. If the prediction is correct, it is expected that the antibody response to B. burgdorferi invariant antigens will become nonprotective as B. burgdorferi infection progresses. To test this assumption, changes in the protective efficacy of the immune response to B. burgdorferi surface antigens were monitored via a superinfection model over the course of 70 days. B. burgdorferi-infected mice were subjected to secondary challenge by heterologous B. burgdorferi at different time points postinfection (p.i.). When the infected mice were superinfected with a VlsE-deficient clone (ΔVlsE) at day 28 p.i., the active anti-B. burgdorferi immune response did not prevent ΔVlsE-induced spirochetemia. In contrast, most mice blocked culture-detectable spirochetemia induced by wild-type B. burgdorferi (WT), indicating that VlsE was likely the primary target of the antibody response. As the B. burgdorferi infection further progressed, however, reversed outcomes were observed. At day 70 p.i. the host immune response to non-VlsE antigens became sufficiently potent to clear spirochetemia induced by ΔVlsE and yet failed to prevent WT-induced spirochetemia. To test if any significant changes in the anti-B. burgdorferi antibody repertoire accounted for the observed outcomes, global profiles of antibody specificities were determined. However, comparison of mimotopes revealed no major difference between day 28 and day 70 antibody repertoires.

The Journey of Lipoproteins Through the Cell: One Birthplace, Multiple Destinations

Bacterial lipoproteins are a very diverse group of proteins characterized by the presence of an N-terminal lipid moiety that serves as a membrane anchor. Lipoproteins have a wide variety of crucial functions, ranging from envelope biogenesis to stress response. In Gram-negative bacteria, lipoproteins can be targeted to various destinations in the cell, including the periplasmic side of the cytoplasmic or outer membrane, the cell surface or the external milieu. The sorting mechanisms have been studied in detail in Escherichia coli, but exceptions to the rules established in this model bacterium exist in other bacteria. In this chapter, we will present the current knowledge on lipoprotein sorting in the cell. Our particular focus will be on the surface-exposed lipoproteins that appear to be much more common than previously assumed. We will discuss the different targeting strategies, provide numerous examples of surface-exposed lipoproteins and discuss the techniques used to assess their surface exposure.

Evaluation of Selected Borrelia burgdorferi lp54 Plasmid-Encoded Gene Products Expressed during Mammalian Infection as Antigens To Improve Serodiagnostic Testing for Early Lyme Disease

Laboratory testing for the diagnosis of Lyme disease is performed primarily by serologic assays and is accurate for detection beyond the acute stage of the infection. Serodiagnostic assays to detect the early stages of infection, however, are limited in their sensitivity, and improvement is warranted. We analyzed a series of Borrelia burgdorferi proteins known to be induced within feeding ticks and/or during mammalian infection for their utility as serodiagnostic markers against a comprehensive panel of Lyme disease patient serum samples. The antigens were assayed for IgM and IgG reactivity in line immunoblots and separately by enzyme-linked immunosorbent assay (ELISA), with a focus on reactivity against early Lyme disease with erythema migrans (EM), early disseminated Lyme neuroborreliosis, and early Lyme carditis patient serum samples. By IgM immunoblotting, we found that recombinant proteins BBA65, BBA70, and BBA73 reacted with early Lyme EM samples at levels comparable to those of the OspC antigen used in the current IgM blotting criteria. Additionally, these proteins reacted with serum samples from patients with early neuroborreliosis and early carditis, suggesting value in detecting early stages of this disease progression. We also found serological reactivity against recombinant proteins BBA69 and BBA73 with early-Lyme-disease samples using IgG immunoblotting and ELISA. Significantly, some samples that had been scored negative by the Centers for Disease Control and Prevention-recommended 2-tiered testing algorithm demonstrated positive reactivity to one or more of the antigens by IgM/IgG immunoblot and ELISA. These results suggest that incorporating additional in vivo-expressed antigens into the current IgM/IgG immunoblotting tier in a recombinant protein platform assay may improve the performance of early-Lyme-disease serologic testing.

Characterization of the β-barrel assembly machine accessory lipoproteins from Borrelia burgdorferi

Background

Like all diderm bacteria studied to date, Borrelia burgdorferi possesses a β-barrel assembly machine (BAM) complex. The bacterial BAM complexes characterized thus far consist of an essential integral outer membrane protein designated BamA and one or more accessory proteins. The accessory proteins are typically lipid-modified proteins anchored to the inner leaflet of the outer membrane through their lipid moieties. We previously identified and characterized the B. burgdorferi BamA protein in detail and more recently identified two lipoproteins encoded by open reading frames bb0324 and bb0028 that associate with the borrelial BamA protein. The role(s) of the BAM accessory lipoproteins in B. burgdorferi is currently unknown.

Results

Structural modeling of B. burgdorferi BB0028 revealed a distinct β-propeller fold similar to the known structure for the E. coli BAM accessory lipoprotein BamB. Additionally, the structural model for BB0324 was highly similar to the known structure of BamD, which is consistent with the prior finding that BB0324 contains tetratricopeptide repeat regions similar to other BamD orthologs. Consistent with BB0028 and BB0324 being BAM accessory lipoproteins, mutants lacking expression of each protein were found to exhibit altered membrane permeability and enhanced sensitivity to various antimicrobials. Additionally, BB0028 mutants also exhibited significantly impaired in vitro growth. Finally, immunoprecipitation experiments revealed that BB0028 and BB0324 each interact specifically and independently with BamA to form the BAM complex in B. burgdorferi.

Conclusions

Combined structural studies, functional assays, and co-immunoprecipitation experiments confirmed that BB0028 and BB0324 are the respective BamB and BamD orthologs in B. burgdorferi, and are important in membrane integrity and/or outer membrane protein localization. The borrelial BamB and BamD proteins both interact specifically and independently with BamA to form a tripartite BAM complex in B. burgdorferi. A working model has been developed to further analyze outer membrane biogenesis and outer membrane protein transport in this pathogenic spirochete.

Complete genome sequence of Borrelia afzelii K78 and comparative genome analysis

The main Borrelia species causing Lyme borreliosis in Europe and Asia are Borrelia afzelii, B. garinii, B. burgdorferi and B. bavariensis. This is in contrast to the United States, where infections are exclusively caused by B. burgdorferi. Until to date the genome sequences of four B. afzelii strains, of which only two include the numerous plasmids, are available. In order to further assess the genetic diversity of B. afzelii, the most common species in Europe, responsible for the large variety of clinical manifestations of Lyme borreliosis, we have determined the full genome sequence of the B. afzelii strain K78, a clinical isolate from Austria. The K78 genome contains a linear chromosome (905,949 bp) and 13 plasmids (8 linear and 5 circular) together presenting 1,309 open reading frames of which 496 are located on plasmids. With the exception of lp28-8, all linear replicons in their full length including their telomeres have been sequenced. The comparison with the genomes of the four other B. afzelii strains, ACA-1, PKo, HLJ01 and Tom3107, as well as the one of B. burgdorferi strain B31, confirmed a high degree of conservation within the linear chromosome of B. afzelii, whereas plasmid encoded genes showed a much larger diversity. Since some plasmids present in B. burgdorferi are missing in the B. afzelii genomes, the corresponding virulence factors of B. burgdorferi are found in B. afzelii on other unrelated plasmids. In addition, we have identified a species specific region in the circular plasmid, cp26, which could be used for species determination. Different non-coding RNAs have been located on the B. afzelii K78 genome, which have not previously been annotated in any of the published Borrelia genomes.

Insights into the biology of Borrelia burgdorferi gained through the application of molecular genetics

Borrelia burgdorferi, the vector-borne bacterium that causes Lyme disease, was first identified in 1982. It is known that much of the pathology associated with Lyme borreliosis is due to the spirochete's ability to infect, colonize, disseminate, and survive within the vertebrate host. Early studies aimed at defining the biological contributions of individual genes during infection and transmission were hindered by the lack of adequate tools and techniques for molecular genetic analysis of the spirochete. The development of genetic manipulation techniques, paired with elucidation and annotation of the B. burgdorferi genome sequence, has led to major advancements in our understanding of the virulence factors and the molecular events associated with Lyme disease. Since the dawn of this genetic era of Lyme research, genes required for vector or host adaptation have garnered significant attention and highlighted the central role that these components play in the enzootic cycle of this pathogen. This chapter covers the progress made in the Borrelia field since the application of mutagenesis techniques and how they have allowed researchers to begin ascribing roles to individual genes. Understanding the complex process of adaptation and survival as the spirochete cycles between the tick vector and vertebrate host will lead to the development of more effective diagnostic tools as well as identification of novel therapeutic and vaccine targets. In this chapter, the Borrelia genes are presented in the context of their general biological roles in global gene regulation, motility, cell processes, immune evasion, and colonization/dissemination.

Evaluation of the Borrelia burgdorferi BBA64 protein as a protective immunogen in mice

The Borrelia burgdorferi bba64 gene product is a surface-localized lipoprotein synthesized within mammalian and tick hosts and is involved in vector transmission of disease. These properties suggest that BBA64 may be a vaccine candidate against Lyme borreliosis. In this study, protective immunity against B. burgdorferi challenge was assessed in mice immunized with the BBA64 protein. Mice developed a high-titer antibody response following immunization with soluble recombinant BBA64 but were not protected when challenged by needle inoculation of culture-grown spirochetes. Likewise, mice passively immunized with an anti-BBA64 monoclonal antibody were not protected against needle-inoculated organisms. BBA64-immunized mice were subjected to B. burgdorferi challenge by the natural route of a tick bite, but these trials did not demonstrate significant protective immunity in either outbred or inbred strains of mice. Lipidated recombinant BBA64 produced in Escherichia coli was assessed for possible improved elicitation of a protective immune response. Although inoculation with this antigen produced a high-titer antibody response, the lipidated BBA64 also was unsuccessful in protecting mice from B. burgdorferi challenge by tick bites. Anti-BBA64 antibodies raised in rats eradicated the organisms, as evidenced by in vitro borreliacidal assays, thus demonstrating the potential for BBA64 to be effective as a protective immunogen. However, passive immunization with the same monospecific rat anti-BBA64 polyclonal serum failed to provide protection against tick bite-administered challenge. These results reveal the challenges faced in not only identifying B. burgdorferi proteins with potential protective capability but also in producing recombinant antigens conducive to preventive therapies against Lyme borreliosis.

Crystal structure of the infectious phenotype-associated outer surface protein BBA66 from the Lyme disease agent Borrelia burgdorferi

Borrelia burgdorferi, the causative agent of Lyme disease is transmitted to the mammalian host organisms by infected Ixodes ticks. Transfer of the spirochaetal bacteria from Ixodes ticks to the warm-blooded mammalian organism provides a challenge for the bacteria to adapt and survive in the different environmental conditions. B. burgdorferi has managed to differentially express genes in response to the encountered changes such as temperature and pH variance or metabolic rate to survive in both environments. In recent years, much interest has been turned on genes that are upregulated during the borrelial transfer to mammalian organisms as this could reveal the proteins important in the pathogenesis of Lyme disease. BBA66 is one of the upregulated outer surface proteins thought to be important in the pathogenesis of B. burgdorferi as it has been found out that BBA66 is necessary during the transmission and propagation phase to initiate Lyme disease. As there is still little known about the pathogenesis of B. burgdorferi, we have solved the crystal structure of the outer surface protein BBA66 at 2.25Å resolution. A monomer of BBA66 consists of 6 α-helices packed in a globular domain, and the overall folding is similar to the homologous proteins BBA64, BBA73, and CspA. Structure-based sequence alignment with the homologous protein BBA64 revealed that the conserved residues are mainly located inwards the core region of the protein and thus may be required to maintain the overall fold of the protein. Unlike the other homologous proteins, BBA66 has an atypically long disordered region at the N terminus thought to act as a "tether" between the structural domain and the cell surface.

BBA70 of Borrelia burgdorferi is a novel plasminogen-binding protein

The Lyme disease spirochete Borrelia burgdorferi lacks endogenous, surface-exposed proteases. In order to efficiently disseminate throughout the host and penetrate tissue barriers, borreliae rely on recruitment of host proteases, such as plasmin(ogen). Here we report the identification of a novel plasminogen-binding protein, BBA70. Binding of plasminogen is dose-dependent and is affected by ionic strength. The BBA70-plasminogen interaction is mediated by lysine residues, primarily located in a putative C-terminal α-helix of BBA70. These lysine residues appear to interact with the lysine-binding sites in plasminogen kringle domain 4 because a deletion mutant of plasminogen lacking that domain was unable to bind to BBA70. Bound to BBA70, plasminogen activated by urokinase-type plasminogen activator was able to degrade both a synthetic chromogenic substrate and the natural substrate fibrinogen. Furthermore, BBA70-bound plasmin was able to degrade the central complement proteins C3b and C5 and inhibited the bacteriolytic effects of complement. Consistent with these functional activities, BBA70 is located on the borrelial outer surface. Additionally, serological evidence demonstrated that BBA70 is produced during mammalian infection. Taken together, recruitment and activation of plasminogen could play a beneficial role in dissemination of B. burgdorferi in the human host and may possibly aid the spirochete in escaping the defense mechanisms of innate immunity.

Borrelia burgdorferi bba66 gene inactivation results in attenuated mouse infection by tick transmission

The impact of the Borrelia burgdorferi surface-localized immunogenic lipoprotein BBA66 on vector and host infection was evaluated by inactivating the encoding gene, bba66, and characterizing the mutant phenotype throughout the natural mouse-tick-mouse cycle. The BBA66-deficient mutant isolate, Bb(ΔA66), remained infectious in mice by needle inoculation of cultured organisms, but differences in spirochete burden and pathology in the tibiotarsal joint were observed relative to the parental wild-type (WT) strain. Ixodes scapularis larvae successfully acquired Bb(ΔA66) following feeding on infected mice, and the organisms persisted in these ticks through the molt to nymphs. A series of tick transmission experiments (n = 7) demonstrated that the ability of Bb(ΔA66)-infected nymphs to infect laboratory mice was significantly impaired compared to that of mice fed upon by WT-infected ticks. trans-complementation of Bb(ΔA66) with an intact copy of bba66 restored the WT infectious phenotype in mice via tick transmission. These results suggest a role for BBA66 in facilitating B. burgdorferi dissemination and transmission from the tick vector to the mammalian host as part of the disease process for Lyme borreliosis.

The role of Borrelia burgdorferi outer surface proteins

Human pathogenic spirochetes causing Lyme disease belong to the Borrelia burgdorferi sensu lato complex. Borrelia burgdorferi organisms are extracellular pathogens transmitted to humans through the bite of Ixodes spp. ticks. These spirochetes are unique in that they can cause chronic infection and persist in the infected human, even though a robust humoral and cellular immune response is produced by the infected host. How this extracellular pathogen is able to evade the host immune response for such long periods of time is currently unclear. To gain a better understanding of how this organism persists in the infected human, many laboratories have focused on identifying and characterizing outer surface proteins of B. burgdorferi. As the interface between B. burgdorferi and its human host is its outer surface, proteins localized to the outer membrane must play an important role in dissemination, virulence, tissue tropism, and immune evasion. Over the last two decades, numerous outer surface proteins from B. burgdorferi have been identified, and more recent studies have begun to elucidate the functional role(s) of many borrelial outer surface proteins. This review summarizes the outer surface proteins identified in B. burgdorferi to date and provides detailed insight into the functions of many of these proteins as they relate to the unique parasitic strategy of this spirochetal pathogen.

Proteome analysis of Borrelia burgdorferi response to environmental change

We examined global changes in protein expression in the B31 strain of Borrelia burgdorferi, in response to two environmental cues (pH and temperature) chosen for their reported similarity to those encountered at different stages of the organism's life cycle. Multidimensional nano-liquid chromatographic separations coupled with tandem mass spectrometry were used to examine the array of proteins (i.e., the proteome) of B. burgdorferi for different pH and temperature culture conditions. Changes in pH and temperature elicited in vitro adaptations of this spirochete known to cause Lyme disease and led to alterations in protein expression that are associated with increased microbial pathogenesis. We identified 1,031 proteins that represent 59% of the annotated genome of B. burgdorferi and elucidated a core proteome of 414 proteins that were present in all environmental conditions investigated. Observed changes in protein abundances indicated varied replicon usage, as well as proteome functional distributions between the in vitro cell culture conditions. Surprisingly, the pH and temperature conditions that mimicked B. burgdorferi residing in the gut of a fed tick showed a marked reduction in protein diversity. Additionally, the results provide us with leading candidates for exploring how B. burgdorferi adapts to and is able to survive in a wide variety of environmental conditions and lay a foundation for planned in situ studies of B. burgdorferi isolated from the tick midgut and infected animals.

The bba64 gene of Borrelia burgdorferi, the Lyme disease agent, is critical for mammalian infection via tick bite transmission

The spirochetal agent of Lyme disease, Borrelia burgdorferi, is transmitted by bites of Ixodes ticks to mammalian reservoir hosts and humans. The mechanism(s) by which the organism is trafficked from vector to host is poorly understood. In this study, we demonstrate that a B. burgdorferi mutant strain deficient in the synthesis of the bba64 gene product was incapable of infecting mice via tick bite even though the mutant was (i) infectious in mice when introduced by needle inoculation, (ii) acquired by larval ticks feeding on infected mice, and (iii) able to persist through tick molting stages. This finding of a B. burgdorferi gene required for pathogen transfer and/or survival from the tick to the susceptible host represents an important breakthrough toward understanding transmission mechanisms involved for the Lyme disease agent.

Methods of identifying membrane proteins in spirochetes

Bacterial membrane proteins serve vital functions such as nutrient acquisition, sensation of the environment, and in gene regulation, secretion, and attachment. Proteins on the cell surface are instrumental in host-pathogen interactions and many serve as immunogens that confer protection as targets for neutralizing antibodies. Integral membrane and lipidated proteins possess hydrophobic domains or lipid anchors that interact with the lipid bilayer of cellular membranes, allowing the investigator to use hydrophobicity as a means for enrichment. The spirochete Borrelia burgdorferi is a Gram negative-like microorganism that produces many integral and lipidated proteins, several of which have proven important during infection and transmission of the bacterium from the tick vector to the mammalian host. Protocols described in this unit for enriching membrane proteins have been extensively used by investigators in the study of B. burgdorferi, but can be easily adapted to identify and characterize membrane-associated and surface-exposed proteins associated with other bacteria.

Passage through Ixodes scapularis ticks enhances the virulence of a weakly pathogenic isolate of Borrelia burgdorferi

Lyme disease is the most common tick-borne illness in the United States. In this paper we explore the contribution of Ixodes scapularis ticks to the pathogenicity of Borrelia burgdorferi in mice. Previously we demonstrated that an isolate of B. burgdorferi sensu stricto (designated N40), passaged 75 times in vitro (N40-75), was infectious but was no longer able to cause arthritis and carditis in C3H mice. We now show that N40-75 spirochetes can readily colonize I. scapularis and multiply during tick engorgement. Remarkably, tick-transmitted N40-75 spirochetes cause disease in mice. N40-75 spirochetes isolated from these animals also retained their pathogenicity when subsequently administered to mice via syringe inoculation. Array analysis revealed that several genes associated with virulence, including bba25, bba65, bba66, bbj09, and bbk32, had higher expression levels in the tick-passaged N40-75 spirochete. These data suggest that transmission of a high-passage attenuated isolate of B. burgdorferi by the arthropod vector results in the generation of spirochetes that have enhanced pathogenesis in mice.

Fast, adaptive evolution at a bacterial host-resistance locus: the PFam54 gene array in Borrelia burgdorferi

Microbial pathogens have evolved sophisticated mechanisms for evasion of host innate and adaptive immunities. PFam54 is the largest paralogous gene family in the genomes of Borrelia burgdorferi, the Lyme disease bacterium. One member of PFam54, the complement-regulator acquiring surface proteins 1 (BbCrasp-1), is able to abort the alternative pathway of complement activation via binding human complement-regulator factor H (FH). The gene coding for BbCRASP-1 exists in a tandem array of PFam54 genes in the B. burgdorferi genome, a result apparently of repeated gene duplications. To help elucidate the functions of the large number of PFam54 genes, we performed phylogenomic and structural analyses of the PFam54 gene array from ten B. burgdorferi genomes. Analyses based on gene tree, genome synteny, and structural models revealed rapid adaptive evolution of this array through gene duplication, gene loss, and functional diversification. Individual PFam54 genes, however, do not show high intra-population sequence polymorphisms as genes providing evasion from adaptive immunity generally do. PFam54 members able to bind human FH are not monophyletic, suggesting that human FH affinity, however strong, is an incidental rather than main function of these PFam54 proteins. The large number of PFam54 genes existing in any single B. burgdorferi genome may target different innate-immunity proteins of a single host species or the same immune protein of a variety of host species. Genetic variability of the PFam54 gene array suggests that universally present PFam54 lineages such as BBA64, BBA65, BBA66, and BBA73 may be better candidates for the development of broad-spectrum vaccines or drugs than strain-restricted lineages such as BbCRASP-1.

Borrelia burgdorferi bba74 is expressed exclusively during tick feeding and is regulated by both arthropod- and mammalian host-specific signals

Although BBA74 initially was described as a 28-kDa virulence-associated outer-membrane-spanning protein with porin-like function, subsequent studies revealed that it is periplasmic and downregulated in mammalian host-adapted spirochetes. To further elucidate the role of this protein in the Borrelia burgdorferi tick-mammal cycle, we conducted a thorough examination of its expression profile in comparison with the profiles of three well-characterized, differentially expressed borrelial genes (ospA, ospC, and ospE) and their proteins. In vitro, transcripts for bba74 were expressed at 23 degrees C and further enhanced by a temperature shift (37 degrees C), whereas BBA74 protein diminished at elevated temperatures; in contrast, neither transcript nor protein was expressed by spirochetes grown in dialysis membrane chambers (DMCs). Primer extension of wild-type B. burgdorferi grown in vitro, in conjunction with expression analysis of DMC-cultivated wild-type and rpoS mutant spirochetes, revealed that, like ospA, bba74 is transcribed by sigma(70) and is subject to RpoS-mediated repression within the mammalian host. A series of experiments utilizing wild-type and rpoS mutant spirochetes was conducted to determine the transcriptional and translational profiles of bba74 during the tick-mouse cycle. Results from these studies revealed (i) that bba74 is transcribed by sigma(70) exclusively during the larval and nymphal blood meals and (ii) that transcription of bba74 is bracketed by RpoS-independent and -dependent forms of repression that are induced by arthropod- and mammalian host-specific signals, respectively. Although loss of BBA74 does not impair the ability of B. burgdorferi to complete its infectious life cycle, the temporal compartmentalization of this gene's transcription suggests that BBA74 facilitates fitness of the spirochete within a narrow window of its tick phase. A reexamination of the paradigm for reciprocal regulation of ospA and ospC, performed herein, revealed that the heterogeneous expression of OspA and OspC displayed by spirochete populations during the nymphal blood meal results from the intricate sequence of transcriptional and translational changes that ensue as B. burgdorferi transitions between its arthropod vector and mammalian host.

Borrelia burgdorferi expression of the bba64, bba65, bba66, and bba73 genes in tissues during persistent infection in mice

Borrelia burgdorferi, the etiological agent of Lyme disease in humans, is vectored between mammalian hosts in nature by Ixodes ticks. The organism adapts to diverse environments encountered throughout the enzootic cycle by differentially expressing essential gene products to survive the specialized conditions, whether in ticks or warm-blooded hosts. However, little is known regarding the identity and/or function of B. burgdorferi genes expressed during colonization of tissues during mammalian infection. Experimental evidence has shown that a group of genes (formerly classified as paralogous gene family 54) contiguously localized on the 54-kilobase linear plasmid of B. burgdorferi, are among the most highly regulated by in vitro conditions resembling mammalian infection. In this study, we employed quantitative reverse transcription-PCR to measure temporal gene expression of a subset of this B. burgdorferi gene family (bba64, bba65, bba66, and bba73) in tissues during chronic murine infection. The goal was to gain insight into the role of these genes in infectivity and pathogenesis by identifying when the genes are induced and whether they are expressed in specific target tissues. B. burgdorferi bba64, bba65, bba66, and bba73 expression was measured from infected mouse tissues relative to expression in in vitro culture conditions at specific times post-infection. bba64 expression was highly upregulated in bladder, heart, and spleen tissues throughout the infection period, contrasting with the sharp downregulation previously observed in ear tissues. bba65, bba66, and bba73 demonstrated upregulated differential expression in various tissues over 1 year post-infection. These results suggest an essential role for these genes in borrelial survival, persistence, and/or pathogenesis.

Deletion of BBA64, BBA65, and BBA66 loci does not alter the infectivity of Borrelia burgdorferi in the murine model of Lyme disease

Borrelia burgdorferi, the causative agent of Lyme disease, alters its gene expression in response to highly disparate environmental signals encountered in its tick vector versus vertebrate hosts. Whole-genome transcriptional profile analysis of B. burgdorferi, propagated in vitro under mammalian-host-specific conditions, revealed significant upregulation of several linear plasmid 54 (lp54)-encoded open reading frames (ORFs). Among these ORFs, BBA64, BBA65, and BBA66 have been shown to be upregulated in response to multiple mammalian-host-specific signals. Recently, we determined that there was no significant difference in the ability of BBA64(-) mutant to infect C3H/HeN mice compared to its isogenic control strains, suggesting that B. burgdorferi might utilize multiple, functionally related determinants to establish infection. We further generated BBA65(-) and BBA66(-) single mutants in a noninfectious, lp25(-) clonal isolate of B. burgdorferi strain B31 (ML23) and complemented them with the minimal region of lp25 (BBE22) required for restoring the infectivity. In addition, we generated a BBA64(-) BBA65(-) BBA66(-) triple mutant using an infectious, clonal isolate of B. burgdorferi strain B31 (5A11) that has all of the infection-associated plasmids. There were no significant differences in the ability to isolate viable spirochetes from different tissues of C3H/HeN mice infected via intradermal needle inoculation with either the individual single mutants or the triple mutant compared to their respective isogenic parental strains at days 21 and 62 postinfection. These observations suggest that B. burgdorferi can establish infection in the absence of expression of BBA64, BBA65, and BBA66 in the murine model of Lyme disease.