Molecular analysis of sequence heterogeneity among genes encoding decorin binding proteins A and B of Borrelia burgdorferi sensu lato

Antibody responses to immunoevasion proteins BBK32 and OspE constitute part of the serological footprint in neuroborreliosis but are insufficient to prevent the disease

Lyme borreliosis, caused by Borrelia burgdorferi sensu lato, is the most common tickborne disease. Its neuronal form, neuroborreliosis, comprises 3 to 38% of borreliosis cases in Europe. Borrelia outer surface proteins and virulence factors, OspE and BBK32, have been previously reported to help cause infection by promoting attachment to human host epithelial cells and evading complement attack. We assessed the serological responses to BBK32 and OspE in 19 individuals diagnosed with neuroborreliosis to see whether antibodies that could both target the bacteria and neutralize the virulence mechanisms on the microbial surface emerge. Results evaluate levels of total protein, IgG and the chemokine CXCL13, a determinant for B-cell recruitment during neuroinflammation, in patients' cerebrospinal fluid samples. Antibody levels against BBK32 and OspE correlated with those against VlsE, a well-characterized diagnostic serological marker of the disease. A dual serological profile of the patients was observed. K-means clustering split the cohort into two discrete groups presenting distinct serological and CNS responses. One group contained young patients with low levels of anti-BBK32 and OspE antibodies. The other group showed stronger responses, possibly following prolonged infections or reinfections. Additionally, we assessed anti-ganglioside antibodies that could cause autoimmunity or complement dysregulation but observed that they did not correlate with neuroborreliosis in our patient cohort. The dual nature of antibody responses against the virulence factors BBK32 and OspE in neuroborreliosis patients may suggest the necessity of repeated exposures for efficient immune responses. Better protection could be achieved if the virulence factors were formulated into vaccines.

Pathogenicity and virulence of Borrelia burgdorferi

Infection with Borrelia burgdorferi often triggers pathophysiologic perturbations that are further augmented by the inflammatory responses of the host, resulting in the severe clinical conditions of Lyme disease. While our apprehension of the spatial and temporal integration of the virulence determinants during the enzootic cycle of B. burgdorferi is constantly being improved, there is still much to be discovered. Many of the novel virulence strategies discussed in this review are undetermined. Lyme disease spirochaetes must surmount numerous molecular and mechanical obstacles in order to establish a disseminated infection in a vertebrate host. These barriers include borrelial relocation from the midgut of the feeding tick to its body cavity and further to the salivary glands, deposition to the skin, haematogenous dissemination, extravasation from blood circulation system, evasion of the host immune responses, localization to protective niches, and establishment of local as well as distal infection in multiple tissues and organs. Here, the various well-defined but also possible novel strategies and virulence mechanisms used by B. burgdorferi to evade obstacles laid out by the tick vector and usually the mammalian host during colonization and infection are reviewed.

Antibody Cross-Reactivity in Serodiagnosis of Lyme Disease

Lyme disease is a tick-borne disease caused by spirochetes belonging to the Borrelia burgdorferi sensu lato complex. The disease is characterized by a varied course; therefore, the basis for diagnosis is laboratory methods. Currently, a two-tiered serological test is recommended, using an ELISA as a screening test and a Western blot as a confirmatory test. This approach was introduced due to the relatively high number of false-positive results obtained when using an ELISA alone. However, even this approach has not entirely solved the problem of false-positive results caused by cross-reactive antibodies. Many highly immunogenic B. burgdorferi s.l. proteins are recognized nonspecifically by antibodies directed against other pathogens. This also applies to antigens, such as OspC, BmpA, VlsE, and FlaB, i.e., those commonly used in serodiagnostic assays. Cross-reactions can be caused by both bacterial (relapsing fever Borrelia, Treponema pallidum) and viral (Epstein-Baar virus, Cytomegalovirus) infections. Additionally, a rheumatoid factor has also been shown to nonspecifically recognize B. burgdorferi s.l. proteins, resulting in false-positive results. Therefore, it is necessary to carefully interpret the results of serodiagnostic tests so as to avoid overdiagnosis of Lyme disease, which causes unnecessary implementations of strong antibiotic therapies and delays in the correct diagnosis.

Phylogenomic Diversity Elucidates Mechanistic Insights into Lyme Borreliae-Host Association

Host association-the selective adaptation of pathogens to specific host species-evolves through constant interactions between host and pathogens, leaving a lot yet to be discovered on immunological mechanisms and genomic determinants. The causative agents of Lyme disease (LD) are spirochete bacteria composed of multiple species of the Borrelia burgdorferi sensu lato complex, including B. burgdorferi (Bb), the main LD pathogen in North America-a useful model for the study of mechanisms underlying host-pathogen association. Host adaptation requires pathogens' ability to evade host immune responses, such as complement, the first-line innate immune defense mechanism. We tested the hypothesis that different host-adapted phenotypes among Bb strains are linked to polymorphic loci that confer complement evasion traits in a host-specific manner. We first examined the survivability of 20 Bb strains in sera in vitro and/or bloodstream and tissues in vivo from rodent and avian LD models. Three groups of complement-dependent host-association phenotypes emerged. We analyzed complement-evasion genes, identified a priori among all strains and sequenced and compared genomes for individual strains representing each phenotype. The evolutionary history of ospC loci is correlated with host-specific complement-evasion phenotypes, while comparative genomics suggests that several gene families and loci are potentially involved in host association. This multidisciplinary work provides novel insights into the functional evolution of host-adapted phenotypes, building a foundation for further investigation of the immunological and genomic determinants of host association. IMPORTANCE Host association is the phenotype that is commonly found in many pathogens that preferential survive in particular hosts. The Lyme disease (LD)-causing agent, B. burgdorferi (Bb), is an ideal model to study host association, as Bb is mainly maintained in nature through rodent and avian hosts. A widespread yet untested concept posits that host association in Bb strains is linked to Bb functional genetic variation conferring evasion to complement, an innate defense mechanism in vertebrate sera. Here, we tested this concept by grouping 20 Bb strains into three complement-dependent host-association phenotypes based on their survivability in sera and/or bloodstream and distal tissues in rodent and avian LD models. Phylogenomic analysis of these strains further correlated several gene families and loci, including ospC, with host-specific complement-evasion phenotypes. Such multifaceted studies thus pave the road to further identify the determinants of host association, providing mechanistic insights into host-pathogen interaction.

Resonance assignment and secondary structure of DbpA protein from the European species, Borrelia afzelii

Decorin binding proteins (Dbps) mediate attachment of spirochetes in host organisms during the early stages of Lyme disease infection. Previously, different binding mechanisms of Dbps to glycosaminoglycans have been elucidated for the pathogenic species Borrelia burgdorferi sensu stricto and B. afzelii. We are investigating various European Borrelia spirochetes and their interactions at the atomic level using NMR. We report preparative scale recombinant expression of uniformly stable isotope enriched B. afzelii DbpA in Escherichia coli, its chromatographic purification, and solution NMR assignments of its backbone and sidechain 1H, 13C, and 15N atoms. This data was used to predict secondary structure propensity, which we compared to the North American B. burgdorferi sensu stricto and European B. garinii DbpA for which solution NMR structures had been determined previously. Backbone dynamics of DbpA from B. afzelii were elucidated from spin relaxation and heteronuclear NOE experiments. NMR-based secondary structure analysis together with the backbone dynamics characterization provided a first look into structural differences of B. afzelii DbpA compared to the North American species and will serve as the basis for further investigation of how these changes affect interactions with host components.

Borrelia burgdorferi BmpA-BBK32 and BmpA-BBA64: New Recombinant Chimeric Proteins with Potential Diagnostic Value

Currently, the diagnosis of Lyme disease is based mostly on two-tiered serologic testing. In the new generation of immunoenzymatic assays, antigens comprise whole-cell lysates of members of the Borrelia burgdorferi sensu lato (s.l.) species complex, with the addition of selected recombinant proteins. Due to the high diversity of members of the B. burgdorferi s.l. genospecies and the low degree of conservation among the amino acid sequences of their proteins, serodiagnostic methods currently in use are not sufficient for the correct diagnosis of borreliosis. Two divalent chimeric proteins (BmpA-BBK32 and BmpA-BBA64) were expressed in Escherichia coli. Following purification by one-step metal-affinity chromatography, preparations were obtained containing milligram levels of chimeric protein exhibiting electrophoretic purity in excess of 98%. Reactivity of the new chimeric proteins with specific human IgG antibodies was preliminarily determined by Western blot. For this purpose, 20 negative sera and 20 positive sera was used. The new chimeric proteins were highly reactive with IgG antibodies contained in the serum of patients suffering from borreliosis. Moreover, no immunoreactivity of chimeric proteins was observed with antibodies in the sera of healthy people. These promising results suggest that new chimeric proteins have the potential to discriminate between positive and negative sera.

Conserved lysine residues in decorin binding proteins of Borrelia garinii are critical in adhesion to human brain microvascular endothelial cells

Lyme borreliosis is a tick-borne disease caused by Borrelia burgdorferi sensu lato spirochetes (Lyme borreliae). When the disease affects the central nervous system, it is referred to as neuroborreliosis. In Europe, neuroborreliosis is most often caused by Borrelia garinii. Although it is known that in the host Lyme borreliae spread from the tick bite site to distant tissues via the blood vasculature, the adherence of Lyme borreliae to human brain microvascular endothelial cells has not been studied before. Decorin binding proteins are adhesins expressed on Lyme borreliae. They mediate the adhesion of Lyme borreliae to decorin and biglycan, and the lysine residues located in the binding site of decorin binding proteins are important to the binding activity. In this study, we show that lysine residues located in the canonical binding site can also be found in decorin binding proteins of Borrelia garinii, and that these lysines contribute to biglycan and decorin binding. Most importantly, we show that the lysine residues are crucial for the binding of Lyme borreliae to decorin and biglycan expressing human brain microvascular endothelial cells, which in turn suggests that they are involved in the pathogenesis of neuroborreliosis.

Lyme Disease in Humans

Lyme disease (Lyme borreliosis) is a tick-borne, zoonosis of adults and children caused by genospecies of the Borrelia burgdorferi sensu lato complex. The ailment, widespread throughout the Northern Hemisphere, continues to increase globally due to multiple environmental factors, coupled with increased incursion of humans into habitats that harbor the spirochete. B. burgdorferi sensu lato is transmitted by ticks from the Ixodes ricinus complex. In North America, B. burgdorferi causes nearly all infections; in Europe, B. afzelii and B. garinii are most associated with human disease. The spirochete's unusual fragmented genome encodes a plethora of differentially expressed outer surface lipoproteins that play a seminal role in the bacterium's ability to sustain itself within its enzootic cycle and cause disease when transmitted to its incidental human host. Tissue damage and symptomatology (i.e., clinical manifestations) result from the inflammatory response elicited by the bacterium and its constituents. The deposition of spirochetes into human dermal tissue generates a local inflammatory response that manifests as erythema migrans (EM), the hallmark skin lesion. If treated appropriately and early, the prognosis is excellent. However, in untreated patients, the disease may present with a wide range of clinical manifestations, most commonly involving the central nervous system, joints, or heart. A small percentage (~10%) of patients may go on to develop a poorly defined fibromyalgia-like illness, post-treatment Lyme disease (PTLD) unresponsive to prolonged antimicrobial therapy. Below we integrate current knowledge regarding the ecologic, epidemiologic, microbiologic, and immunologic facets of Lyme disease into a conceptual framework that sheds light on the disorder that healthcare providers encounter.

Detection of antibodies to decorin-binding protein A (DbpA) and DbpB after infection of dogs with Borrelia burgdorferi by tick challenge

We characterized the antibody response to decorin-binding protein A (DbpA) or DbpB from immune serum samples collected from 27 dogs infected with Borrelia burgdorferi by Ixodes scapularis ticks. Immunoglobulin M (IgM) antibodies to DbpA or DbpB were rarely detected, but high levels of IgG antibodies to DbpA were detected in 16 of 27 of the immune sera collected 1 mo after infection, 20 of 25 of the sera collected after 2 mo, and each of the 23, 17, or 11 serum samples evaluated after 3, 4, or 5 mo, respectively. In addition, IgG antibodies to DbpB were detected in 22 of 27 (p = 0.005) tested dogs after 1 mo, and the frequency of detecting the antibodies thereafter closely mimicked the antibody responses to DbpA. Moreover, antibodies to DbpA or DbpB were not produced by dogs vaccinated with a whole-cell B. burgdorferi bacterin; removing the antibodies to DbpA by adsorption to recombinant DbpA (rDbpA) did not affect the reactivity detected by a rDbpB ELISA. Therefore, the findings from our preliminary study showed that antigenically distinct antibodies to DbpA or DbpB are produced reliably during canine infection with B. burgdorferi, and the response is not confounded by vaccination with a Lyme disease bacterin. Larger studies are warranted to more critically evaluate whether detecting the antibody responses can improve serodiagnostic confirmation of canine Lyme disease.

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.

Outer surface protein polymorphisms linked to host-spirochete association in Lyme borreliae

Lyme borreliosis is caused by multiple species of the spirochete bacteria Borrelia burgdorferi sensu lato. The spirochetes are transmitted by ticks to vertebrate hosts, including small- and medium-sized mammals, birds, reptiles, and humans. Strain-to-strain variation in host-specific infectivity has been documented, but the molecular basis that drives this differentiation is still unclear. Spirochetes possess the ability to evade host immune responses and colonize host tissues to establish infection in vertebrate hosts. In turn, hosts have developed distinct levels of immune responses when invaded by different species/strains of Lyme borreliae. Similarly, the ability of Lyme borreliae to colonize host tissues varies among different spirochete species/strains. One potential mechanism that drives this strain-to-strain variation of immune evasion and colonization is the polymorphic outer surface proteins produced by Lyme borreliae. In this review, we summarize research on strain-to-strain variation in host competence and discuss the evidence that supports the role of spirochete-produced protein polymorphisms in driving this variation in host specialization. Such information will provide greater insights into the adaptive mechanisms driving host and Lyme borreliae association, which will lead to the development of interventions to block pathogen spread and eventually reduce Lyme borreliosis health burden.

Whole genome sequence and comparative analysis of Borrelia burgdorferi MM1

Lyme disease is caused by spirochaetes of the Borrelia burgdorferi sensu lato genospecies. Complete genome assemblies are available for fewer than ten strains of Borrelia burgdorferi sensu stricto, the primary cause of Lyme disease in North America. MM1 is a sensu stricto strain originally isolated in the midwestern United States. Aside from a small number of genes, the complete genome sequence of this strain has not been reported. Here we present the complete genome sequence of MM1 in relation to other sensu stricto strains and in terms of its Multi Locus Sequence Typing. Our results indicate that MM1 is a new sequence type which contains a conserved main chromosome and 15 plasmids. Our results include the first contiguous 28.5 kb assembly of lp28-8, a linear plasmid carrying the vls antigenic variation system, from a Borrelia burgdorferi sensu stricto strain.

Borrelia burgdorferi glycosaminoglycan-binding proteins: a potential target for new therapeutics against Lyme disease

The spirochete bacterium Borrelia burgdorferi sensu lato is the causative agent of Lyme disease, the most common vector-borne disease in Europe and the United States. The spirochetes can be transmitted to humans via ticks, and then spread to different tissues, leading to arthritis, carditis and neuroborreliosis. Although antibiotics have commonly been used to treat infected individuals, some treated patients do not respond to antibiotics and experience persistent, long-term arthritis. Thus, there is a need to investigate alternative therapeutics against Lyme disease. The spirochete bacterium colonization is partly attributed to the binding of the bacterial outer-surface proteins to the glycosaminoglycan (GAG) chains of host proteoglycans. Blocking the binding of these proteins to GAGs is a potential strategy to prevent infection. In this review, we have summarized the recent reports of B. burgdorferi sensu lato GAG-binding proteins and discussed the potential use of synthetic and semi-synthetic compounds, including GAG analogues, to block pathogen interaction with GAGs. Such information should motivate the discovery and development of novel GAG analogues as new therapeutics for Lyme disease. New therapeutic approaches should eventually reduce the burden of Lyme disease and improve human health.

Borrelia burgdorferi Keeps Moving and Carries on: A Review of Borrelial Dissemination and Invasion

Borrelia burgdorferi is the etiological agent of Lyme disease, a multisystemic, multistage, inflammatory infection resulting in patients experiencing cardiac, neurological, and arthritic complications when not treated with antibiotics shortly after exposure. The spirochetal bacterium transmits through the Ixodes vector colonizing the dermis of a mammalian host prior to hematogenous dissemination and invasion of distal tissues all the while combating the immune response as it traverses through its pathogenic lifecycle. The innate immune response controls the borrelial burden in the dermis, but is unable to clear the infection and thereby prevent progression of disease. Dissemination in the mammalian host requires temporal regulation of virulence determinants to allow for vascular interactions, invasion, and colonization of distal tissues. Virulence determinants and/or adhesins are highly heterogenetic among environmental B. burgdorferi strains with particular genotypes being associated with the ability to disseminate to specific tissues and the severity of disease, but fail to generate cross-protective immunity between borrelial strains. The unique motility of B. burgdorferi rendered by the endoflagella serves a vital function for dissemination and protection from immune recognition. Progress has been made toward understanding the chemotactic regulation coordinating the activity of the two polar localized flagellar motors and their role in borrelial virulence, but this regulation is not yet fully understood. Distinct states of motility allow for dynamic interactions between several B. burgdorferi adhesins and host targets that play roles in transendothelial migration. Transmigration across endothelial and blood-brain barriers allows for the invasion of tissues and elicits localized immune responses. The invasive nature of B. burgdorferi is lacking in proactive mechanisms to modulate disease, such as secretion systems and toxins, but recent work has shown degradation of host extracellular matrices by B. burgdorferi contributes to the invasive capabilities of the pathogen. Additionally, B. burgdorferi may use invasion of eukaryotic cells for immune evasion and protection against environmental stresses. This review provides an overview of B. burgdorferi mechanisms for dissemination and invasion in the mammalian host, which are essential for pathogenesis and the development of persistent infection.

Structure of decorin binding protein B from Borrelia burgdorferi and its interactions with glycosaminoglycans

Decorin-binding proteins (DBPs), DBPA and DBPB, are surface lipoproteins on Borrelia burgdorferi, the causative agent of Lyme disease. DBPs bind to the connective tissue proteoglycan decorin and facilitate tissue colonization by the bacterium. Although structural and biochemical properties of DBPA are well understood, little is known about DBPB. In current work, we determined the solution structure of DBPB from strain B31 of B. burgdorferi and characterized its interactions with glycosaminoglycans (GAGs). Our structure shows that DBPB adopts the same topology as DBPA, but possesses a much shorter terminal helix, resulting in a longer unstructured C-terminal tail, which is also rich in basic amino acids. Characterization of DBPB-GAG interactions reveals that, despite similar GAG affinities of DBPA and DBPB, the primary GAG-binding sites in DBPB are different from DBPA. In particular, our results indicate that lysines in the C-terminus of DBPB are vital to DBPB's ability to bind GAGs whereas C-terminal tail for DBPA from strain B31 only plays a minor role in facilitating GAG bindings. Furthermore, the traditional GAG-binding pocket important to DBPA-GAG interactions is only secondary to DBPB's GAG-binding ability.

Host cell heparan sulfate glycosaminoglycans are ligands for OspF-related proteins of the Lyme disease spirochete

Borrelia burgdorferi, the agent of Lyme disease, spreads from the site of the tick bite to tissues such as heart, joints and the nervous tissues. Host glycosaminoglycans, highly modified repeating disaccharides that are present on cell surfaces and in extracellular matrix, are common targets of microbial pathogens during tissue colonization. While several dermatan sulfate-binding B. burgdorferi adhesins have been identified, B. burgdorferi adhesins documented to promote spirochetal binding to heparan sulfate have not yet been identified. OspEF-related proteins (Erps), a large family of plasmid-encoded surface lipoproteins that are produced in the mammalian host, can be divided into the OspF-related, OspEF-leader peptide (Elp) and OspE-related subfamilies. We show here that a member of the OspF-related subfamily, ErpG, binds to heparan sulfate and when produced on the surface of an otherwise non-adherent B. burgdorferi strain, ErpG promotes heparan sulfate-mediated bacterial attachment to the glial but not the endothelial, synovial or respiratory epithelial cells. Six other OspF-related proteins were capable of binding heparan sulfate, whereas representative OspE-related and Elp proteins lacked this activity. These results indicate that OspF-related proteins are heparan sulfate-binding adhesins, at least one of which promotes bacterial attachment to glial cells.

Structural mechanisms underlying sequence-dependent variations in GAG affinities of decorin binding protein A, a Borrelia burgdorferi adhesin

Decorin-binding protein A (DBPA) is an important surface adhesin of the bacterium Borrelia burgdorferi, the causative agent of Lyme disease. DBPA facilitates the bacteria's colonization of human tissue by adhering to glycosaminoglycan (GAG), a sulfated polysaccharide. Interestingly, DBPA sequence variation among different strains of Borrelia spirochetes is high, resulting in significant differences in their GAG affinities. However, the structural mechanisms contributing to these differences are unknown. We determined the solution structures of DBPAs from strain N40 of B. burgdorferi and strain PBr of Borrelia garinii, two DBPA variants whose GAG affinities deviate significantly from strain B31, the best characterized version of DBPA. Our structures revealed that significant differences exist between PBr DBPA and B31/N40 DBPAs. In particular, the C-terminus of PBr DBPA, unlike C-termini from B31 and N40 DBPAs, is positioned away from the GAG-binding pocket and the linker between helices one and two of PBr DBPA is highly structured and retracted from the GAG-binding pocket. The repositioning of the C-terminus allowed the formation of an extra GAG-binding epitope in PBr DBPA and the retracted linker gave GAG ligands more access to the GAG-binding epitopes than other DBPAs. Characterization of GAG ligands' interactions with wild-type (WT) PBr and mutants confirmed the importance of the second major GAG-binding epitope and established the fact that the two epitopes are independent of one another and the new epitope is as important to GAG binding as the traditional epitope.

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.

Decorin binding proteins A and B in the serodiagnosis of Lyme disease in North America

The laboratory diagnosis of Lyme disease is based upon the detection of antibodies generated against Borrelia burgdorferi using a two-tier assay, typically consisting of an enzyme-linked immunosorbent assay (ELISA), followed by a Western blot. This system, put into place to address the nonspecificity associated with standalone first-tier assays, is insensitive for diagnosing early infection, when most people seek care. The use of bacterial lysates or whole-protein antigens as first-tier assay targets contributes to nonspecificity due, in part, to the presence of cross-reactive epitopes that are also found in other bacteria. This precludes their use as sensitive standalone assays. The use of peptides containing linear epitopes that are highly specific for B. burgdorferi offers a method for reducing this cross-reactivity. In the present study, we mapped the linear epitopes of the prominently expressed Borrelia adhesins decorin binding protein A (DbpA) and DbpB. We identified several epitopes in each protein that were highly conserved among North American strains of B. burgdorferi, and we screened peptides containing specific epitopes using serum panels from early and late Lyme disease patients. The individual peptides primarily detected IgM but not IgG, while the proteins efficiently detected both IgM and IgG. While no individual peptide demonstrated better utility for antibody detection than its respective whole protein, an assay containing a combination of a DbpA and a DbpB peptide adequately detected both IgM and IgG, accurately identifying 87.5% (84/96) of the early Lyme disease patients and 80.0% (16/20) of the late Lyme disease patients.

Strain-specific variation of the decorin-binding adhesin DbpA influences the tissue tropism of the lyme disease spirochete

Lyme disease spirochetes demonstrate strain- and species-specific differences in tissue tropism. For example, the three major Lyme disease spirochete species, Borrelia burgdorferi sensu stricto, B. garinii, and B. afzelii, are each most commonly associated with overlapping but distinct spectra of clinical manifestations. Borrelia burgdorferi sensu stricto, the most common Lyme spirochete in the U.S., is closely associated with arthritis. The attachment of microbial pathogens to cells or to the extracellular matrix of target tissues may promote colonization and disease, and the Lyme disease spirochete encodes several surface proteins, including the decorin- and dermatan sulfate-binding adhesin DbpA, which vary among strains and have been postulated to contribute to strain-specific differences in tissue tropism. DbpA variants differ in their ability to bind to its host ligands and to cultured mammalian cells. To directly test whether variation in dbpA influences tissue tropism, we analyzed murine infection by isogenic B. burgdorferi strains that encode different dbpA alleles. Compared to dbpA alleles of B. afzelii strain VS461 or B. burgdorferi strain N40-D10/E9, dbpA of B. garinii strain PBr conferred the greatest decorin- and dermatan sulfate-binding activity, promoted the greatest colonization at the inoculation site and heart, and caused the most severe carditis. The dbpA of strain N40-D10/E9 conferred the weakest decorin- and GAG-binding activity, but the most robust joint colonization and was the only dbpA allele capable of conferring significant joint disease. Thus, dbpA mediates colonization and disease by the Lyme disease spirochete in an allele-dependent manner and may contribute to the etiology of distinct clinical manifestations associated with different Lyme disease strains. This study provides important support for the long-postulated model that strain-specific variations of Borrelia surface proteins influence tissue tropism.

Lyme disease, the most common vector-borne disease in the United States, is caused by a bacterium, Borrelia burgdorferi. This bacterium infects the skin at the site of the tick bite and then can spread to other tissues, such as the heart, joints or nervous system, causing carditis, arthritis or neurologic disease. To colonize human tissues, the pathogen produces surface proteins that promote bacterial attachment to these sites. For example, DbpA binds to decorin, a component of human tissue. Different Lyme disease strains differ in the particular tissues they colonize and the disease they cause, but we do not understand why. Different strains also make distinct versions of DbpA that bind decorin differently, so variation of DbpA might contribute to strain-to-strain variation in clinical manifestations. To test this, we infected mice with Lyme disease strains that were identical except for the particular DbpA variant they produced. We found that the strains colonized different tissues and caused different diseases, such as arthritis or carditis. These results provide the first solid evidence that variation of an outer surface protein, in this case DbpA, influences what tissues are most affected during Lyme disease.

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.

Performance of a multiplexed serological microarray for the detection of antibodies against central nervous system pathogens

Central nervous system (CNS) infections have multiple potential causative agents for which simultaneous pathogen screening can provide a useful tool. This study evaluated a multiplexed microarray for the simultaneous detection of antibodies against CNS pathogens. The performance of selected microarray antigens for the detection of IgG antibodies against herpes simplex virus 1 and 2 (HSV-1 and HSV-2), varicella-zoster virus (VZV), adenovirus, Mycoplasma pneumoniae and Borrelia burgdorferi sensu lato, was evaluated using serum sample panels tested with reference assays used in a routine diagnostic laboratory. The microarray sensitivity for HSV-1, HSV-2, VZV, adenovirus and M. pneumonia ranged from 77% to 100%, and the specificity ranged from 74% to 97%. Very variable sensitivities and specificities were found for borrelial antigens of three different VlsE protein IR(6) peptide variants (IR6p1, IR6p2, IR6p4) and three recombinant decorin binding proteins A (DbpA; DbpAIa, DbpA91, DbpAG40). For single antigens, good specificity was shown for antigens of IR6p4 and DbpAIa (96%), while DbpA91, IR6p1 and IR6p2 were moderately specific (88–92%). The analytical sensitivity of the microarray was dependent on the borrelial IgG concentration of the specimen. The overall performance and technical features of the platform showed that the platform supports both recombinant proteins, whole viruses and peptides as antigens. This study showed diagnostic potential for all six CNS pathogens, including Borrelia burgdorferi sensu lato, using glutaraldehyde based microarray, and further highlighted the importance of careful antigen selection and the requirement for the use of multiple borrelial antigens in order to increase specificity without a major lack of sensitivity.

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To find a suitable microarray platform that supports different kinds of antigens

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To evaluate the feasibility of a multiplexed glutaraldehyde based microarray

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To detect simultaneously different viral and bacterial antibodies

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Performance of the platform proved promising

Dynamics of connective-tissue localization during chronic Borrelia burgdorferi infection

The etiologic agent of Lyme disease, Borrelia burgdorferi, localizes preferentially in the extracellular matrix during persistence. In chronically infected laboratory mice, there is a direct association between B. burgdorferi and the proteoglycan decorin, which suggests that decorin has a role in defining protective niches for persistent spirochetes. In this study, the tissue colocalization of B. burgdorferi with decorin and the dynamics of borrelial decorin tropism were evaluated during chronic infection. Spirochetes were found to colocalize absolutely with decorin, but not collagen I in chronically infected immunocompetent C3H mice. Passive immunization of infected C3H-scid mice with B. burgdorferi-specific immune serum resulted in the localization of spirochetes in decorin-rich microenvironments, with clearance of spirochetes from decorin-poor microenvironments. In passively immunized C3H-scid mice, tissue spirochete burdens were initially reduced, but increased over time as the B. burgdorferi-specific antibody levels waned. Concurrent repopulation of the previously cleared decorin-poor microenvironments was observed with the rising tissue spirochete burden and declining antibody titer. These findings indicate that the specificity of B. burgdorferi tissue localization during chronic infection is determined by decorin, driven by the borrelia-specific antibody response, and fluctuates with the antibody response.

The early dissemination defect attributed to disruption of decorin-binding proteins is abolished in chronic murine Lyme borreliosis

The laboratory mouse model of Lyme disease has revealed that Borrelia burgdorferi differentially expresses numerous outer surface proteins that influence different stages of infection (tick-borne transmission, tissue colonization, dissemination, persistence, and tick acquisition). Deletion of two such outer surface proteins, decorin-binding proteins A and B (DbpA/B), has been documented to decrease infectivity, impede early dissemination, and, possibly, prevent persistence. In this study, DbpA/B-deficient spirochetes were confirmed to exhibit an early dissemination defect in immunocompetent, but not immunodeficient, mice, and the defect was found to resolve with chronicity. Development of disease (arthritis and carditis) was attenuated only in the early stage of infection with DbpA/B-deficient spirochetes in both types of mice. Persistence of the DbpA/B-deficient spirochetes occurred in both immunocompetent and immunodeficient mice in a manner indistinguishable from that of wild-type spirochetes. Dissemination through the lymphatic system was evaluated as an underlying mechanism for the early dissemination defect. At 12 h, 3 days, 7 days, and 14 days postinoculation, DbpA/B-deficient spirochetes were significantly less prevalent and in lower numbers in lymph nodes than wild-type spirochetes. However, in immunodeficient mice, deficiency of DbpA/B did not significantly decrease the prevalence or spirochete numbers in lymph nodes. Complementation of DbpA/B restored a wild-type phenotype. Thus, the results indicated that deficiency of DbpA/B allows the acquired immune response to restrict early dissemination of spirochetes, which appears to be at least partially mediated through the lymphatic system.

Allelic variation of the Lyme disease spirochete adhesin DbpA influences spirochetal binding to decorin, dermatan sulfate, and mammalian cells

After transmission by an infected tick, the Lyme disease spirochete, Borrelia burgdorferi sensu lato, colonizes the mammalian skin and may disseminate systemically. The three major species of Lyme disease spirochete--B. burgdorferi sensu stricto, B. garinii, and B. afzelii--are associated with different chronic disease manifestations. Colonization is likely promoted by the ability to bind to target tissues, and Lyme disease spirochetes utilize multiple adhesive molecules to interact with diverse mammalian components. The allelic variable surface lipoprotein decorin binding protein A (DbpA) promotes bacterial binding to the proteoglycan decorin and to the glycosaminoglycan (GAG) dermatan sulfate. To assess allelic variation of DbpA in GAG-, decorin-, and cell-binding activities, we expressed dbpA alleles derived from diverse Lyme disease spirochetes in B. burgdorferi strain B314, a noninfectious and nonadherent strain that lacks dbpA. Each DbpA allele conferred upon B. burgdorferi strain B314 the ability to bind to cultured kidney epithelial (but not glial or endothelial) cells, as well as to purified decorin and dermatan sulfate. Nevertheless, allelic variation of DbpA was associated with dramatic differences in substrate binding activity. In most cases, decorin and dermatan sulfate binding correlated well, but DbpA of B. afzelii strain VS461 promoted differential binding to decorin and dermatan sulfate, indicating that the two activities are separable. DbpA from a clone of B. burgdorferi strain N40 that can cause disseminated infection in mice displayed relatively low adhesive activity, indicating that robust DbpA-mediated adhesive activity is not required for spread in the mammalian host.

A comparative analysis of molecular markers for the detection and identification of Borrelia spirochaetes in Ixodes ricinus

Borrelia burgdorferi sensu lato, carried by Ixodes ticks, is one of the most significant human pathogens, causing Lyme disease. As there is no standardized PCR method for detection and identification of spirochaete DNA, we carried out a comparative analysis using a set of complementary primers for three regions in the genomic DNA of these bacteria (genes fla and rrs and the non-coding rrs-rrlA region). DNA extracted from 579 Ixodes ricinus ticks was subjected to nested PCR. DNA of the examined spirochaetes was detected in 43 (7.4 %) lysates when the fla gene was used as a molecular marker, in 7 (1.2 %) lysates when using primers complementary to the rrs gene, and in 12 (2.1 %) lysates using primers complementary to the non-coding rrs-rrlA sequence. RFLP analysis based on the fla gene helped identify species from the B. burgdorferi sensu lato complex (B. burgdorferi sensu stricto, Borrelia afzelii, Borrelia garinii, Borrelia valaisiana), detect co-infections, and also identify Borrelia miyamotoi. Therefore, the fla gene is the most sensitive and specific molecular marker for the detection and identification of Borrelia spirochaetes in I. ricinus.

Assessment of decorin-binding protein A to the infectivity of Borrelia burgdorferi in the murine models of needle and tick infection

Background

Decorin-binding proteins (Dbps) A and B of Borrelia burgdorferi, the agent of Lyme disease, are surface-exposed lipoproteins that presumably bind to the extracellular matrix proteoglycan, decorin. B. burgdorferi infects various tissues including the bladder, heart, joints, skin and the central nervous system, and the ability of B. burgdorferi to bind decorin has been hypothesized to be important for this disseminatory pathogenic strategy.

Results

To determine the role of DbpBA in the infectious lifecycle of B. burgdorferi, we created a DbpBA-deficient mutant of B. burgdorferi strain 297 and compared the infectious phenotype of the mutant to the wild-type strain in the experimental murine model of Lyme borreliosis. The mutant strain exhibited a 4-log decrease in infectivity, relative to the wild-type strain, when needle inoculated into mice. Upon complementation of the DbpBA-mutant strain with DbpA, the wild-type level of infectivity was restored. In addition, we demonstrated that the DbpBA-deficient mutant was able to colonize Ixodes scapularis larval ticks after feeding on infected mice and persist within the ticks during the molt to the nymphal state. Moreover, surprisingly, the DbpBA-mutant strain was capable of being transmitted to naïve mice via tick bite, giving rise to infected mice.

Conclusion

These results suggest that DbpBA is not required for the natural tick-transmission process to mammals, despite inferences from needle-inoculation experiments implying a requirement for DbpBA during mammalian infection. The combined findings also send a cautionary note regarding how results from needle-inoculation experiments with mice should be interpreted.

A genome-wide proteome array reveals a limited set of immunogens in natural infections of humans and white-footed mice with Borrelia burgdorferi

Humans and other animals with Lyme borreliosis produce antibodies to a number of components of the agent Borrelia burgdorferi, but a full accounting of the immunogens during natural infections has not been achieved. Employing a protein array produced in vitro from 1,292 DNA fragments representing approximately 80% of the genome, we compared the antibody reactivities of sera from patients with early or later Lyme borreliosis to the antibody reactivities of sera from controls. Overall, approximately 15% of the open reading frame (ORF) products (Orfs) of B. burgdorferi in the array detectably elicited an antibody response in humans with natural infections. Among the immunogens, 103 stood out on the basis of statistical criteria. The majority of these Orfs were also immunogenic with sera obtained from naturally infected Peromyscus leucopus mice, a major reservoir. The high-ranking set included several B. burgdorferi proteins hitherto unrecognized as immunogens, as well as several proteins that have been established as antigens. The high-ranking immunogens were more likely than nonreactive Orfs to have the following characteristics: (i) plasmid-encoded rather than chromosome-encoded proteins, (ii) a predicted lipoprotein, and (iii) a member of a paralogous family of proteins, notably the Bdr and Erp proteins. The newly discovered antigens included Orfs encoded by several ORFs of the lp36 linear plasmid, such as BBK07 and BBK19, and proteins of the flagellar apparatus, such as FliL. These results indicate that the majority of deduced proteins of B. burgdorferi do not elicit antibody responses during infection and that the limited sets of immunogens are similar for two different host species.

Antibodies to recombinant decorin-binding proteins A and B in the cerebrospinal fluid of patients with Lyme neuroborreliosis

Cerebrospinal fluid (CSF) and serum samples from 34 patients with proven neuroborreliosis (NB) and 22 patients with suspected neuroborreliosis (SNB) from Finland were analysed for antibodies to decorin-binding proteins A (DbpA) and B (DbpB). Antibodies to recombinant protein antigens originating from Borrelia burgdorferi sensu stricto, B. afzelii, or B. garinii species were studied by enzyme-linked immunosorbent assay (ELISA). Of the 34 patients with NB, 100% of the CSF and 88% of the serum samples had IgG antibodies to 1 to 3 variants of DbpA and 79% of the CSF and 70% of the serum samples were positive for 1 to 3 DbpB variants. Antibodies to DbpB seemed to be associated with lymphocytic pleocytosis in the CSF and short duration of the disease, whereas antibodies to DbpA in the CSF were observed irrespective of the duration of the disease and lymphocytic pleocytosis. Among the variant antigens, CSF reactivity was mainly with the DbpB from B. garinii, whereas positivity with the DbpA from B. afzelii or B. garinii predominated. The results suggest that CSF antibodies to DbpB might be useful as a marker of active infection whereas antibodies to DbpA seem to persist a long time after acute phases of NB.

[Role of biological assays in the diagnosis of Lyme borreliosis presentations. What are the techniques and which are currently available?]

The biological diagnosis of Borrelia burgdorferi sensu lato infection is usually made by antibody detection in patient sera. Thus, serological testing (Elisa, immunoblotting) is essential for a biological diagnosis. Specific antibody detection is usually done in serum and CSF of patients suspected of Lyme borreliosis. Laboratories must follow European recommendations to validate these assays in routine practice. Antibody detection lacks sensitivity in the early cutaneous phase of the infection. Therefore, serological testing is not recommended for the diagnosis of erythema migrans. The interpretation of serology must take into account the variability of Elisa sensitivity and specificity and the lack of standardization for Western-blotting in Europe. Besides these indirect diagnosis techniques, there is also direct detection of spirochetes by culture or by in vitro DNA amplification but these require adequate samples. These molecular tests must not be performed routinely, but only for specific clinical situations and in specialized laboratories only.

Co-evolution of the outer surface protein C gene (ospC) and intraspecific lineages of Borrelia burgdorferi sensu stricto in the northeastern United States

Clinical and tick isolates of Borrelia burgdorferi sensu stricto, the bacterial agent of Lyme disease, from the northeastern United States were sequenced at 12 loci located on the main chromosome and 7 plasmids (lp54, cp26, cp9, lp17, lp25, lp28-2, and lp38). The outer surface protein C gene (ospC) showed the highest number (12) of major alleles (defined as alleles differing by 5% or more in nucleotide sequence) while other ORFs had only two to four major alleles. A non-recombining chromosomal marker, the rrs-rrlA ribosomal RNA spacer, was used to infer the intraspecific phylogeny among these B. burgdorferi isolates. We were thus able to analyze the multilocus genotypes in the context of a B. burgdorferi intraspecific phylogeny. Except for ospC, sequence variations at plasmid-borne loci showed broad inconsistency with the intraspecific phylogeny, supporting DNA exchanges mediated by plasmid transfers. The multilocus linkage frequently observed in B. burgdorferi populations is due more likely to a "founder effect" than to a lack of recombination. The exceptional phylogenetic consistency of ospC, in conjunction with its selectively maintained high intraspecific diversity, suggested a dominant role ospC plays in the initiation and maintenance of adaptive differentiation in B. burgdorferi.

Molecular analysis of decorin-binding protein A (DbpA) reveals five major groups among European Borrelia burgdorferi sensu lato strains with impact for the development of serological assays and indicates lateral gene transfer of the dbpA gene

The Borrelia (B.) burgdorferi adhesin DbpA (decorin-binding protein A) is a valuable antigen for serodiagnosis of Lyme borreliosis and a promising candidate for a vaccine. To investigate the heterogeneity of DbpA, we aligned DNA sequences of 83 different dbpA genes (37 from the database, where the majority of sequences belong to B. burgdorferi sensu stricto and 46 were newly sequenced). Analysis of 25 sequences from the species B. burgdorferi s.s., 16 from B. afzelii, 40 from B. garinii, and two from the recently described human pathogenic genospecies A14S revealed five distinct DbpA groups. Group I comprises B. burgdorferi s.s. and group II B. afzelii. B. garinii is divided into groups III and IV, whereas A14S strains form group V. Formation of groups is mainly due to insertions of whole sequence sections. Comparison of dbpA sequences with ospC sequences from a subset of 59 strains revealed all kinds of cross-connections indicating processes of lateral gene transfer among strains. The extent of sequence identity within the dbpA genes decreases from the DNA (67%) to the amino acid (AA) level (44%) by about 23%, in contrast ospC sequence identities differed only by about 10%. This might be an indication that DbpA plays an important role in immune escape. Immunoblots using four recombinant DbpAs representing groups I-IV show that DbpA proteins are sensitive and specific antigens and complement one another in their reactivity. Part of the sera showed group-specific reactivity which could also be demonstrated with monoclonal antibodies.

Solving a sticky problem: new genetic approaches to host cell adhesion by the Lyme disease spirochete

The Lyme disease spirochetes, comprised of at least three closely related species, Borrelia burgdorferi, Borrelia garinii and Borrelia afzelii, are fascinating and enigmatic bacterial pathogens. They are maintained by tick-mediated transmission between mammalian hosts, usually small rodents. The ability of these bacteria, which have relatively small genomes, to survive and disseminate in both an immunocompetent mammal and in an arthropod vector suggests that they have evolved elegant and indispensable strategies for interacting with their hosts. Recognition of specific mammalian and tick tissues is likely to be essential for successful completion of the enzootic life cycle but, given the historical difficulties in genetic manipulation of these organisms, characterization of factors promoting cell adhesion has until recently largely been confined to either the manipulation of host cells or the analysis of potential bacterial ligands in the form of recombinant proteins. These studies have led to the identification of several mammalian receptors for Lyme disease spirochetes, including glycosaminoglycans, decorin, fibronectin and integrins, as well as a tick receptor for the bacterium, and also candidate cognate bacterial ligands. Recent advances in our ability to genetically manipulate Lyme disease spirochetes, particularly B. burgdorferi, are now providing us with firm evidence that these ligands indeed do promote bacterial adherence to host cells, and with new insights into the roles of these multifacted Borrelia-host cell interactions during mammalian and arthropod infection.

Improvement of Lyme borreliosis serodiagnosis by a newly developed recombinant immunoglobulin G (IgG) and IgM line immunoblot assay and addition of VlsE and DbpA homologues

We developed and evaluated a recombinant Borrelia line immunoblot assay based on 18 homologues of seven different antigens, i.e., p100, p58, p41i, BmpA, VlsE, OspC, and DbpA. Each recombinant antigen can be detected separately and is distinct even from homologues with identical molecular weights. This blot was compared to the recently described recombinant Borrelia Western immunoblot assay (U. Schulte-Spechtel, G. Lehnert, G. Liegl, V. Fingerle, C. Heimerl, B. J. Johnson, and B. Wilske, J. Clin. Microbiol. 41:1299-1303, 2003). To verify sensitivity and specificity, both blots were evaluated for reactivity with Borrelia-specific immunoglobulin G (IgG) and IgM antibodies with 85 sera from patients with different manifestations of Lyme borreliosis and 110 controls. According to European interpretation criteria for Borrelia Western blots, which define a serum as positive when it recognizes at least two bands, sensitivity increased significantly from 70.6% (Western blot) to 84.7% (line blot) for IgG (P = 0.042) and from 40.0% (Western blot) to 73.8% (line blot) for IgM (P < 0.005). The increased sensitivity for IgG detection is due to the new line blot technique, whereas the improvement in detection of IgM is mainly achieved through incorporation of the additional antigens. Notably, the recombinant VlsE of Borrelia garinii strain PBi displayed the highest sensitivity of all antigens tested for IgG detection and is also one of the most useful antigens for IgM. Due to its excellent sensitivity and specificity combined with ease of evaluation, this line immunoblot assay offers a useful improvement in serodiagnosis of Lyme borreliosis.

Outer membrane proteins of pathogenic spirochetes

Pathogenic spirochetes are the causative agents of several important diseases including syphilis, Lyme disease, leptospirosis, swine dysentery, periodontal disease and some forms of relapsing fever. Spirochetal bacteria possess two membranes and the proteins present in the outer membrane are at the site of interaction with host tissue and the immune system. This review describes the current knowledge in the field of spirochetal outer membrane protein (OMP) biology. What is known concerning biogenesis and structure of OMPs, with particular regard to the atypical signal peptide cleavage sites observed amongst the spirochetes, is discussed. We examine the functions that have been determined for several spirochetal OMPs including those that have been demonstrated to function as adhesins, porins or to have roles in complement resistance. A detailed description of the role of spirochetal OMPs in immunity, including those that stimulate protective immunity or that are involved in antigenic variation, is given. A final section is included which covers experimental considerations in spirochetal outer membrane biology. This section covers contentious issues concerning cellular localization of putative OMPs, including determination of surface exposure. A more detailed knowledge of spirochetal OMP biology will hopefully lead to the design of new vaccines and a better understanding of spirochetal pathogenesis.

Survival strategies of Borrelia burgdorferi, the etiologic agent of Lyme disease

To fight, flee or hide are the imperatives of long-term survival by an infectious microbe. Active immune suppression, induction of immune tolerance, phase and antigenic variation, intracellular seclusion, and incursion into immune privileged sites are examples of survival strategies of persistent pathogens. Here we critically review the supporting evidence for possible stratagems utilized by Borrelia burgdorferi, the spirochete that causes Lyme disease, to persist in the mammalian host.

Decorin-binding sites in the adhesin DbpA from Borrelia burgdorferi: a synthetic peptide approach

Lyme disease is caused by the spirochete Borrelia burgdorferi following transmission from infected Ixodes ticks to human hosts. Following colonization of the skin, spirochetes can disseminate throughout the body, resulting in complications that can include ocular, cardiac, neural, and skeletal disease. We have previously shown that B. burgdorferi expresses two closely related decorin-binding adhesins (DbpA and DbpB) of the MSCRAMM (microbial surface component recognizing adhesive matrix molecule) type that can mediate bacterial attachment to extracellular matrices in the host. Furthermore, three Lys residues in DbpA appear to be critical for the binding of DbpA to decorin. We have now characterized the interaction of DbpA and decorin further by using a synthetic peptide approach. We synthesized a panel of peptides that spanned the DbpA sequence and examined their ability to inhibit the binding of intact DbpA to decorin. From these studies, we identified a decorin-binding peptide that lost this activity if the sequence was either scrambled or if a critical Lys residue was chemically modified. A minimal decorin-binding peptide was identified by examining a set of truncated peptides. One peptide is proposed to contain the primary decorin-binding site in DbpA. By comparing the amino acid sequences of 29 different DbpA homologs from different B. burgdorferi sensu lato isolates, we discovered that the identified decorin-binding sequence was quite variable. Therefore, we synthesized a new panel of peptides containing the putative decorin-binding sequence of the different DbpA homologs. All of these peptides were active in our decorin-binding assay, and consensus decorin binding motifs are discussed.

Immunological and genetic characterization of Borrelia burgdorferi BapA and EppA proteins

A large majority of examined Lyme disease spirochaete isolates were demonstrated to contain one or both of the paralogous genes bapA and eppA. Immunological analyses of serum samples collected from infected patients coupled with comparative sequence analyses indicated that bapA gene sequences are quite stable but the encoded proteins do not provoke a strong immune response in most individuals. Conversely, EppA proteins are much more antigenic but vary widely in sequence between different bacteria. Considerable evidence of insertion, deletion and other mutations within eppA genes was observed. A number of significant recombination events were also found to have occurred in regions flanking bapA genes, while the genes themselves rarely exhibited evidence of mutation, suggesting strong selective pressure to maintain BapA sequences within narrow limits. Data from these and other studies suggest important roles for BapA and EppA during the Borrelia burgdorferi infectious cycle.

Significant improvement of the recombinant Borrelia-specific immunoglobulin G immunoblot test by addition of VlsE and a DbpA homologue derived from Borrelia garinii for diagnosis of early neuroborreliosis

We investigated whether the recombinant Borrelia Western blot test previously described (B. Wilske, C. Habermann, V. Fingerle, B. Hillenbrand, S. Jauris-Heipke, G. Lehnert, I. Pradel, D. Rössler, and U. Schulte-Spechtel, Med. Microbiol. Immunol. 188:139-144, 1999) can be improved by the addition of VlsE and additional DbpA and OspC homologues. By using a panel of sera from 36 neuroborreliosis patients and 67 control patients, the diagnostic sensitivity of the recombinant immunoblot test was significantly increased (86.1% versus 52.7%) without loss of specificity and was higher (86.1% versus 63.8%) than that of the conventional whole-cell lysate immunoblot test (U. Hauser, G. Lehnert, R. Lobentanzer, and B. Wilske, J. Clin. Microbiol. 35:1433-1444, 1997). Improvement was mainly due to the presence of VlsE and DbpA.

DNA microarray assessment of putative Borrelia burgdorferi lipoprotein genes

A DNA microarray containing fragments of 137 Borrelia burgdorferi B31 putative lipoprotein genes was used to examine Lyme disease spirochetes. DNA from B. burgdorferi sensu stricto B31, 297, and N40; Borrelia garinii IP90; and Borrelia afzelii P/Gau was fluorescently labeled and hybridized to the microarray, demonstrating the degree to which the individual putative lipoprotein genes were conserved among the genospecies. These data show that a DNA microarray can globally examine the genes encoding B. burgdorferi lipoproteins.

Species-specific serodiagnosis of Lyme arthritis and neuroborreliosis due to Borrelia burgdorferi sensu stricto, B. afzelii, and B. garinii by using decorin binding protein A

The antigenic potential of decorin binding protein A (DbpA) was evaluated in serodiagnosis of human Lyme borreliosis (LB). The dbpA was cloned and sequenced from the three pathogenic Borrelia species common in Europe. Sequence analysis revealed high interspecies heterogeneity. The identity of the predicted amino acid sequences was 43 to 62% among Borrelia burgdorferi sensu stricto, B. afzelii, and B. garinii. The respective recombinant DbpAs (rDbpAs) were produced and tested as antigens by Western blotting and enzyme-linked immunosorbent assay (ELISA). One hundred percent of patients with neuroborreliosis (NB) and 93% of patients with Lyme arthritis (LA) reacted positively. Sera from the majority of patients reacted with one rDbpA only and had no or low cross-reactivity to other two variant proteins. In patients with culture-positive erythema migrans (EM), the sensitivity of rDbpA immunoglobulin G (IgG) or IgM ELISA was low. The DbpA seems to be a sensitive and specific antigen for the serodiagnosis of LA or NB, but not of EM, provided that variants from all three pathogenic borrelial species are included in the combined set of antigens.

Analysis of a VMP-like sequence (vls) locus in Borrelia garinii and Vls homologues among four Borrelia burgdorferi sensu lato species

The VMP-like sequence (vls) locus that consists of one expressed vlsE gene and 15 silent vls cassettes has been described in Borrelia burgdorferi sensu stricto B31. In the present study, the vls locus from a Borrelia garinii isolate A87SA was analyzed. DNA fragments that contained three complete and five partial vls cassettes were cloned and sequenced. Pulsed-field gel electrophoresis (PFGE) analysis and Southern hybridization of the PFGE blot indicated that the vls locus of B. garinii A87SA, consisting of at least eight vls cassettes, was located on a 21-kb linear plasmid. The size of the three complete vls cassettes varied from 573 to 612 bp. They had 93.8-94.3% identity at the nucleotide level and 84.9-87.3% amino acid identity. The amino acid sequences of the three vls cassettes of B. garinii A87SA exhibited 45.9-50.8% identity to the VlsE sequence of B. burgdorferi B31, and 30.0-33.8% identity to the VMP17 sequence of B. hermsii HS1. Homologues of the vls locus of B. garinii were detected by dot blot hybridization among 24 of the 30 (80.0%) isolates representing four B. burgdorferi sensu lato species distributed widely in Europe. Our findings indicate that B. garinii might possess a similar vls structure to that described in B. burgdorferi sensu stricto. The highly conserved nature of the vls locus among various B. burgdorferi sensu lato species suggests that it may be important in the physiology and pathogenesis of Lyme disease spirochetes.

Conformational nature of the Borrelia burgdorferi decorin binding protein A epitopes that elicit protective antibodies

Decorin binding protein A (DbpA) has been shown by several laboratories to be a protective antigen for the prevention of experimental Borrelia burgdorferi infection in the mouse model of Lyme borreliosis. However, different recombinant forms of the antigen having either lipidated amino termini, approximating the natural secretion and posttranslational processing, or nonprocessed cytosolic forms have elicited disparate levels of protection in the mouse model. We have now used the unique functional properties of this molecule to investigate the structural requirements needed to elicit a protective immune response. Genetic and physicochemical alterations to DbpA showed that the ability to bind to the ligand decorin is indicative of a potent immunogen but is not conclusive. By mutating the two carboxy-terminal nonconserved cysteines of DbpA from B. burgdorferi strain N40, we have determined that the stability afforded by the putative disulfide bond is essential for the generation of protective antibodies. This mutated protein was more sensitive to thermal denaturation and proteolysis, suggesting that it is in a less ordered state. Immunization with DbpA that was thermally denatured and functionally inactivated stimulated an immune response that was not protective and lacked bactericidal antibodies. Antibodies against conformationally altered forms of DbpA also failed to kill heterologous B. garinii and B. afzelii strains. Additionally, nonsecreted recombinant forms of DbpA(N40) were found to be inferior to secreted lipoprotein DbpA(N40) in terms of functional activity and antigenic potency. These data suggest that elicitation of a bactericidal and protective immune response to DbpA requires a properly folded conformation for the production of functional antibodies.

Identification of 11 pH-regulated genes in Borrelia burgdorferi localizing to linear plasmids

When Borrelia burgdorferi is transmitted from the tick vector to the mammalian host, the bacterium experiences alterations in its environment, such as changes in temperature and pH. Previously, we observed numerous alterations in the membrane protein profile when B. burgdorferi B31 was grown at pH 7.0 compared to pH 8.0. Here we identify 11 genes localizing to linear plasmids that are up-regulated at pH 7.0 relative to pH 8.0 in vitro. Seven genes (bba03, bba24, bba64, bba66, bbe31, bbj41/bbi39 [encoding products that are 99% identical], and bbk01) were indirectly identified by proteomic analysis of membrane proteins. Another gene, bba36, was identified by screening a B. burgdorferi B31 genomic library with cross-adsorbed hyperimmune rabbit serum. Two additional genes, bba65 and bba73, were identified by Northern blot analysis. Genes bba64, bba65, bba66, bbj41/bbi39, and bba73 are members of paralogous gene family 54, and bbe31 is a member of the closely related paralogous gene family 60. Gene bba24 is part of a bicistronic operon with bba25 that encodes the well-characterized decorin binding proteins A and B. All 11 genes were transcriptionally regulated, yet the degree of pH regulation varied, with some genes more tightly regulated than others. The regions upstream of these pH-regulated genes appeared to be unrelated, yet many contained dyad repeats ranging from 12 to 25 nucleotides in length that may be involved in the regulation of these genes.

Evidence for vaccine synergy between Borrelia burgdorferi decorin binding protein A and outer surface protein A in the mouse model of lyme borreliosis

Mice immunized with either the predominantly vector-stage lipoprotein outer surface protein A (OspA) or the in vivo-expressed lipoprotein decorin binding protein A (DbpA) are protected against Borrelia burgdorferi challenge. DbpA-OspA combinations protected against 100-fold-higher challenge doses than did either single-antigen vaccine and conferred significant protection against heterologous B. burgdorferi, B. garinii, and B. afzelii isolates, suggesting that there is synergy between these two immunogens.

Borrelia burgdorferi B31 Erp proteins that are dominant immunoblot antigens of animals infected with isolate B31 are recognized by only a subset of human lyme disease patient sera

Sera from animals infected with Borrelia burgdorferi isolates yield intense immunoblot signals from the B31 ErpA/I/N and ErpB/J/O proteins, which have apparent molecular masses of 19 and 60 kDa, respectively. Since B. burgdorferi proteins with those molecular masses are of immunodiagnostic importance, Lyme disease patient sera were used in studies of B31 lysates and recombinant B31 ErpA/I/N and ErpB/J/O proteins. Immunoblot analyses indicated that only a minority of the patients produced antibodies that recognized the tested B31 Erp proteins. Southern blot analyses of Lyme disease spirochetes cultured from 16 of the patients indicated that all these bacteria contain genes related to the B31 erpA/I/N and erpB/J/O genes, although signal strengths indicated only weak similarities in many cases, suggestive of genetic variability of erp genes among these bacteria. These data indicate that Erp proteins are generally not the 19- and 60-kDa antigens observed on serodiagnostic immunoblots.

Molecular typing of Borrelia burgdorferi sensu lato: taxonomic, epidemiological, and clinical implications

Borrelia burgdorferi sensu lato, the spirochete that causes human Lyme borreliosis (LB), is a genetically and phenotypically divergent species. In the past several years, various molecular approaches have been developed and used to determine the phenotypic and genetic heterogeneity within the LB-related spirochetes and their potential association with distinct clinical syndromes. These methods include serotyping, multilocus enzyme electrophoresis, DNA-DNA reassociation analysis, rRNA gene restriction analysis (ribotyping), pulsed-field gel electrophoresis, plasmid fingerprinting, randomly amplified polymorphic DNA fingerprinting analysis, species-specific PCR and PCR-based restriction fragment length polymorphism (RFLP) analysis, and sequence analysis of 16S rRNA and other conserved genes. On the basis of DNA-DNA reassociation analysis, 10 different Borrelia species have been described within the B. burgdorferi sensu lato complex: B. burgdorferi sensu stricto, Borrelia garinii, Borrelia afzelii, Borrelia japonica, Borrelia andersonii, Borrelia valaisiana, Borrelia lusitaniae, Borrelia tanukii, Borrelia turdi, and Borrelia bissettii sp. nov. To date, only B. burgdorferi sensu stricto, B. garinii, and B. afzelii are well known to be responsible for causing human disease. Different Borrelia species have been associated with distinct clinical manifestations of LB. In addition, Borrelia species are differentially distributed worldwide and may be maintained through different transmission cycles in nature. In this paper, the molecular methods used for typing of B. burgdorferi sensu lato are reviewed. The current taxonomic status of B. burgdorferi sensu lato and its epidemiological and clinical implications, especiallly correlation between the variable clinical presentations and the infecting Borrelia species, are discussed in detail.

Adherence of Borrelia burgdorferi. Identification of critical lysine residues in DbpA required for decorin binding

Borrelia burgdorferi, the causative agent of Lyme disease, expresses on its surface two decorin binding adhesins, DbpA and DbpB. Previous studies have demonstrated that vaccination of mice with DbpA provided protection against challenge with heterologous Borrelia strains despite considerable sequence variability among DbpA in these strains. We have now examined the importance of individual amino acid residues in DbpA for decorin binding. We demonstrated that chemical modification of lysine residues resulted in loss of ligand binding activity. Of the 27 lysine residues in native DbpA from strain 297, 6 are present in most and 5 are conserved in all 30 DbpA sequences examined so far. Analysis of recombinant DbpA in which individual lysine residues have been mutated to alanine suggested that three of the conserved residues distributed throughout the DbpA sequence are required for decorin binding. These mutants lost their ability to bind decorin in Western ligand blot assay and bound reduced amounts of decorin in an ELISA. Furthermore, these mutant DbpA proteins did not inhibit the adherence of B. burgdorferi to a decorin substrata, and they did not recognize decorin in an extracellular matrix established by human fibroblast cultures. We conclude that the three lysine residues Lys-82, Lys-163, and Lys-170 are crucial for the binding of DbpA to decorin.

DbpA, but not OspA, is expressed by Borrelia burgdorferi during spirochetemia and is a target for protective antibodies

DbpA is a target for antibodies that protect mice against infection by cultured Borrelia burgdorferi. Infected mice exhibit early and sustained humoral responses to DbpA and DbpB, suggesting that these proteins are expressed in vivo. Many antigens expressed in mammals by B. burgdorferi are repressed in vitro at lower growth temperatures, and we have now extended these observations to include DbpA and DbpB. To confirm that the protective antigen DbpA is expressed in vivo and to address the question of its accessibility to antibodies during infection, we examined B. burgdorferi in blood samples from mice following cutaneous inoculation. B. burgdorferi was visualized by dark-field microscopy in plasma samples from spirochetemic mice, and an indirect immunofluorescence assay showed that these spirochetes were DbpA positive and OspA negative. We developed an ex vivo borreliacidal assay to show that hyperimmune antiserum against DbpA, but not OspA, killed these plasma-derived spirochetes, demonstrating that DbpA is accessible to antibodies during this phase of infection. Blood transferred from spirochetemic donor mice readily established B. burgdorferi infection in naive recipient mice or mice hyperimmunized with OspA, while mice hyperimmunized with DbpA showed significant protection against challenge with host-adapted spirochetes. Antiserum from persistently infected mice had borreliacidal activity against both cultured and plasma-derived spirochetes, and adsorption of this serum with DbpA substantially depleted this killing activity. Our observations show that immunization with DbpA blocks B. burgdorferi dissemination from the site of cutaneous inoculation and suggest that DbpA antibodies may contribute to control of persistent infection.