Structural conservation of neurotropism-associated VspA within the variable Borrelia Vsp-OspC lipoprotein family

Borreliae Part 2: Borrelia Relapsing Fever Group and Unclassified Borrelia

Borreliae of the relapsing fever group (RFG) are heterogenous and can be divided mainly into three groups according to vectors, namely the soft-tick-borne relapsing fever (STBRF) Borreliae, the hard-tick-borne relapsing fever (HTBRF) Borreliae, the louse-borne relapsing fever (LBRF) Borreliae, and the avian relapsing fever ones. With respect to the geographical distribution, the STBRF Borreliae are further subdivided into Old World and New World strains. Except for the Avian relapsing fever group Borreliae, which cause avian spirochetosis, all the others share infectivity in humans. They are indeed the etiological agent of both endemic and epidemic forms of relapsing fever, causing high spirochaetemia and fever. Vectors are primarily soft ticks of Ornithodoros spp. in the STBRF group; hard ticks, notably Ixodes sp., Amblyomma sp., Dermacentor sp., and Rhipicephalus sp., in the HTBRF group; and the louse pediculus humanus humanus in the TBRF one. A recent hypothesis was supported for a common ancestor of RFG Borreliae, transmitted at the beginning by hard-body ticks. Accordingly, STBRF Borreliae switched to use soft-bodied ticks as a vector, which was followed by the use of lice by Borrelia recurrentis. There are also new candidate species of Borreliae, at present unclassified, which are also described in this review.

Strain-specific joint invasion and colonization by Lyme disease spirochetes is promoted by outer surface protein C

Lyme disease, caused by Borrelia burgdorferi, B. afzelii and B. garinii, is a chronic, multi-systemic infection and the spectrum of tissues affected can vary with the Lyme disease strain. For example, whereas B. garinii infection is associated with neurologic manifestations, B. burgdorferi infection is associated with arthritis. The basis for tissue tropism is poorly understood, but has been long hypothesized to involve strain-specific interactions with host components in the target tissue. OspC (outer surface protein C) is a highly variable outer surface protein required for infectivity, and sequence differences in OspC are associated with variation in tissue invasiveness, but whether OspC directly influences tropism is unknown. We found that OspC binds to the extracellular matrix (ECM) components fibronectin and/or dermatan sulfate in an OspC variant-dependent manner. Murine infection by isogenic B. burgdorferi strains differing only in their ospC coding region revealed that two OspC variants capable of binding dermatan sulfate promoted colonization of all tissues tested, including joints. However, an isogenic strain producing OspC from B. garinii strain PBr, which binds fibronectin but not dermatan sulfate, colonized the skin, heart and bladder, but not joints. Moreover, a strain producing an OspC altered to recognize neither fibronectin nor dermatan sulfate displayed dramatically reduced levels of tissue colonization that were indistinguishable from a strain entirely deficient in OspC. Finally, intravital microscopy revealed that this OspC mutant, in contrast to a strain producing wild type OspC, was defective in promoting joint invasion by B. burgdorferi in living mice. We conclude that OspC functions as an ECM-binding adhesin that is required for joint invasion, and that variation in OspC sequence contributes to strain-specific differences in tissue tropism displayed among Lyme disease spirochetes.

Infection by different Lyme disease bacteria is associated with different manifestations, such as cardiac, neurologic, or, in the case of B. burgdorferi, the major cause of Lyme disease in the U.S., joint disease. The basis for these differences is unknown, but likely involve strain-specific interactions with host components in the target tissue. The sequence of the outer surface lipoprotein OspC varies with the strains, and we found that this variation influences the spectrum of host extracellular matrix components recognized. Infection of mice with strains that are identical except for ospC revealed that OspC variants that differ in binding spectrum promote infection of different tissues. A strain producing OspC invaded and colonized the joint in living animals, but an altered OspC protein incapable of binding tissue components did not. Thus, tissue-binding by OspC is critical for infection and joint invasion, and OspC variation directly influences tissue tropism.

Identification of immunoreactive linear epitopes of Borrelia miyamotoi

Borrelia miyamotoi is an emerging tick-borne spirochete transmitted by ixodid ticks. Current serologic assays for B. miyamotoi are impacted by genetic similarities to other Borrelia and limited understanding of optimal antigenic targets. In this study, we employed the TBD-Serochip, a peptide array platform, to identify new linear targets for serologic detection of B. miyamotoi. We examined a wide range of suspected B. miyamotoi antigens and identified 352 IgM and 91 IgG reactive peptides, with the majority mapping to variable membrane proteins. These included peptides within conserved fragments of variable membrane proteins that may have greater potential for differential diagnosis. We also identified reactive regions on FlaB, and demonstrate crossreactivity of B. burgdorferi s.l. C6 with a B. miyamotoi C6-like peptide. The panel of linear peptides identified in this study can be used to enhance serodiagnosis of B. miyamotoi.

Blood treatment of Lyme borreliae demonstrates the mechanism of CspZ-mediated complement evasion to promote systemic infection in vertebrate hosts

Lyme disease, caused by the spirochete Borrelia burgdorferi, is the most common vector-borne disease in the United States and Europe. The spirochetes are transmitted from mammalian and avian reservoir hosts to humans via ticks. Following tick bites, spirochetes colonize the host skin and then disseminate haematogenously to various organs, a process that requires this pathogen to evade host complement, an innate immune defence system. CspZ, a spirochete surface protein, facilitates resistance to complement-mediated killing in vitro by binding to the complement regulator, factor H (FH). Low expression levels of CspZ in spirochetes cultivated in vitro or during initiation of infection in vivo have been a major hurdle in delineating the role of this protein in pathogenesis. Here, we show that treatment of B. burgdorferi with human blood induces CspZ production and enhances resistance to complement. By contrast, a cspZ-deficient mutant and a strain that expressed an FH-nonbinding CspZ variant were impaired in their ability to cause bacteraemia and colonize tissues of mice or quail; virulence of these mutants was however restored in complement C3-deficient mice. These novel findings suggest that FH binding to CspZ facilitates B. burgdorferi complement evasion in vivo and promotes systemic infection in vertebrate hosts.

Relapsing Fevers: Neglected Tick-Borne Diseases

Relapsing fever still remains a neglected disease and little is known on its reservoir, tick vector and physiopathology in the vertebrate host. The disease occurs in temperate as well as tropical countries. Relapsing fever borreliae are spirochaetes, members of the Borreliaceae family which also contain Lyme disease spirochaetes. They are mainly transmitted by Ornithodoros soft ticks, but some species are vectored by ixodid ticks. Traditionally a Borrelia species is associated with a specific vector in a particular geographical area. However, new species are regularly described, and taxonomical uncertainties deserve further investigations to better understand Borrelia vector/host adaptation. The medical importance of Borrelia miyamotoi, transmitted by Ixodes spp., has recently spawned new interest in this bacterial group. In this review, recent data on tick-host-pathogen interactions for tick-borne relapsing fevers is presented, with special focus on B. miyamotoi.

Enhanced Protective Immunogenicity of Homodimeric Borrelia burgdorferi Outer Surface Protein C

Lyme borreliosis is caused by tick-transmitted spirochetes of the Borrelia burgdorferi sensu lato group and is the most common vector-borne disease in the United States and Europe. Outer surface protein C (OspC) is a 23-kDa outer surface lipoprotein expressed during spirochete transmission from the tick to the vertebrate host. In a previous study, we found that immunization with a recombinant disulfide-bridged dimeric form of OspC (D-OspC) stimulates increased antibody responses relative to immunization with commonly employed monomeric OspC. Here, we report that mice immunized with dimeric OspC proteins also exhibited enhanced protection against infection with the cognate B. burgdorferi strain. Mice were protected by four immunizations containing as little as 100 ng of dimeric OspC, suggesting that this form of the protein can induce protective immunity within a dose range reasonable for a human or veterinary vaccine. In contrast, monomeric OspC was only partially protective at much higher doses. IgG subclass analysis revealed that D-OspC-immunized animals mainly possessed anti-OspC-IgG1. In contrast, infected animals develop anti-OspC restricted to the IgG3 isotype. A subset of antibodies generated by dimeric OspC immunization did not recognize the monomeric variant, indicating that unique epitopes exist on the dimeric form. Moreover, monoclonal antibodies that recognized only dimeric OspC protected mice from B. burgdorferi challenge, whereas another monoclonal that recognized both immunogens was not protective. These studies suggest that this dimeric OspC presents distinctive epitopes that generate antibodies protective against B. burgdorferi infection and could be a useful vaccine component.

Spirochetal Lipoproteins in Pathogenesis and Immunity

Lipoproteins are lipid-modified proteins that dominate the spirochetal proteome. While found in all bacteria, spirochetal lipoproteins have unique features and play critical roles in spirochete biology. For this reason, considerable effort has been devoted to determining how the lipoproteome is generated. Essential features of the structural elements of lipoproteins are now understood with greater clarity, enabling greater confidence in identification of lipoproteins from genomic sequences. The journey from the ribosome to the outer membrane, and in some cases, to the cellular surface has been defined, including secretion, lipidation, sorting, and export across the outer membrane. Given their abundance and importance, it is not surprising that spirochetes have developed a number of strategies for regulating the spatiotemporal expression of lipoproteins. In some cases, lipoprotein expression is tied to various environmental cues, while in other cases, it is linked to growth rate. This regulation enables spirochetes to express certain lipoproteins at high levels in one phase of the spirochete lifecycle, while dramatically downregulating the same lipoproteins in other phases. The mammalian host has developed specialized mechanisms for recognizing lipoproteins and triggering an immune response. Evasion of that immune response is essential for spirochete persistence. For this reason, spirochetes have developed mechanisms for altering lipoproteins. Lipoproteins recognized by antibodies formed during infection are key serodiagnostic antigens. In addition, lipoprotein vaccines have been developed for generating an immune response to control or prevent a spirochete infection. This chapter summarizes our current understanding of lipoproteins in interactions of spirochetes with their hosts.

Assessment of the potential contribution of the highly conserved C-terminal motif (C10) of Borrelia burgdorferi outer surface protein C in transmission and infectivity

OspC is produced by all species of the Borrelia burgdorferi sensu lato complex and is required for infectivity in mammals. To test the hypothesis that the conserved C-terminal motif (C10) of OspC is required for function in vivo, a mutant B. burgdorferi strain (B31::ospCΔC10) was created in which ospC was replaced with an ospC gene lacking the C10 motif. The ability of the mutant to infect mice was investigated using tick transmission and needle inoculation. Infectivity was assessed by cultivation, qRT-PCR, and measurement of IgG antibody responses. B31::ospCΔC10 retained the ability to infect mice by both needle and tick challenge and was competent to survive in ticks after exposure to the blood meal. To determine whether recombinant OspC protein lacking the C-terminal 10 amino acid residues (rOspCΔC10) can bind plasminogen, the only known mammalian-derived ligand for OspC, binding analyses were performed. Deletion of the C10 motif resulted in a statistically significant decrease in plasminogen binding. Although deletion of the C10 motif influenced plasminogen binding, it can be concluded that the C10 motif is not required for OspC to carry out its critical in vivo functions in tick to mouse transmission.

Complement regulator-acquiring surface proteins of Borrelia burgdorferi: Structure, function and regulation of gene expression

Borrelia burgdorferi, the etiological agent of Lyme disease, exploits an array of strategies to establish infection and to overcome host innate and adaptive immune responses. One key borrelial immune escape mechanism involves the inactivation of host complement attack through acquisition of human immune regulators factor H (CFH), factor H-like protein 1 (FHL1), factor H-related protein 1 (CFHR1), CFHR2, and/or CFHR5. Binding of these host proteins is primarily mediated by bacterial surface-exposed proteins that have been collectively referred to as complement regulator-acquiring surface proteins, or CRASPs. Different strains of B. burgdorferi produce as many as 5 different CRASP molecules that comprise 3 distinct, genetically unrelated groups. Depending on bacterial genetic composition, different combinations of these proteins can be found on the borrelial outer surface. The 3 groups differ in their gene location, gene regulatory mechanisms, expression patterns during the tick-mammal infection cycle, protein sequence and structure as well as binding affinity for complement regulators and other serum proteins. These attributes influence the proteins' abilities to contribute to complement resistance of this emerging human pathogen. In this review, we focus on the current knowledge on structure, function, and gene regulation of these B. burgdorferi infection-associated proteins.

Probing the Borrelia burgdorferi surface lipoprotein secretion pathway using a conditionally folding protein domain

Surface lipoproteins of Borrelia spirochetes are important virulence determinants in the transmission and pathogenesis of Lyme disease and relapsing fever. To further define the conformational secretion requirements and to identify potential lipoprotein translocation intermediates associated with the bacterial outer membrane (OM), we generated constructs in which Borrelia burgdorferi outer surface lipoprotein A (OspA) was fused to calmodulin (CaM), a conserved eukaryotic protein undergoing calcium-dependent folding. Protein localization assays showed that constructs in which CaM was fused to full-length wild-type (wt) OspA or to an intact OspA N-terminal "tether" peptide retained their competence for OM translocation even in the presence of calcium. In contrast, constructs in which CaM was fused to truncated or mutant OspA N-terminal tether peptides were targeted to the periplasmic leaflet of the OM in the presence of calcium but could be flipped to the bacterial surface upon calcium chelation. This indicated that in the absence of an intact tether peptide, unfolding of the CaM moiety was required in order to facilitate OM traversal. Together, these data further support a periplasmic tether peptide-mediated mechanism to prevent premature folding of B. burgdorferi surface lipoproteins. The specific shift in the OM topology of sequence-identical lipopeptides due to a single-variable change in environmental conditions also indicates that surface-bound Borrelia lipoproteins can localize transiently to the periplasmic leaflet of the OM.

Determination of Borrelia surface lipoprotein anchor topology by surface proteolysis

We used a surface trypsinolysis assay to probe accessibility of the membrane-proximal N-terminal tether peptides of Borrelia surface lipoproteins OspA and Vsp1. Our findings with both wild-type and mutant proteins are only compatible with the anchoring of these surface lipoproteins in the outer leaflet of the outer spirochetal membrane.

Disulfide-mediated oligomer formation in Borrelia burgdorferi outer surface protein C, a critical virulence factor and potential Lyme disease vaccine candidate

Borrelia burgdorferi OspC is an outer membrane lipoprotein required for the establishment of infection in mammals. Due to its universal distribution among B. burgdorferi sensu lato strains and high antigenicity, it is being explored for the development of a next-generation Lyme disease vaccine. An understanding of the surface presentation of OspC will facilitate efforts to maximize its potential as a vaccine candidate. OspC forms homodimers at the cell surface, and it has been hypothesized that it may also form oligomeric arrays. Here, we employ site-directed mutagenesis to test the hypothesis that interdimeric disulfide bonds at cysteine 130 (C130) mediate oligomerization. B. burgdorferi B31 ospC was replaced with a C130A substitution mutant to yield strain B31::ospC(C130A). Recombinant protein was also generated. Disulfide-bond-dependent oligomer formation was demonstrated and determined to be dependent on C130. Oligomerization was not required for in vivo function, as B31::ospC(C130A) retained infectivity and disseminated normally. The total IgG response and the induced isotype pattern were similar between mice infected with untransformed B31 and those infected with the B31::ospC(C130A) strain. These data indicate that the immune response to OspC is not significantly altered by formation of OspC oligomers, a finding that has significant implications in Lyme disease vaccine design.

Surface localization determinants of Borrelia OspC/Vsp family lipoproteins

The dimeric OspC/Vsp family surface lipoproteins of Borrelia spirochetes are crucial to the transmission and persistence of Lyme borreliosis and tick-borne relapsing fever. However, the requirements for their proper surface display remained undefined. In previous studies, we showed that localization of Borrelia burgdorferi monomeric surface lipoprotein OspA was dependent on residues in the N-terminal "tether" peptide. Here, site-directed mutagenesis of the B. burgdorferi OspC tether revealed two distinct regions affecting either release from the inner membrane or translocation through the outer membrane. Determinants of both of these steps appear consolidated within a single region of the Borrelia turicatae Vsp1 tether. Periplasmic OspC mutants still were able to form dimers. Their localization defect could be rescued by the addition of an apparently structure-destabilizing C-terminal epitope tag but not by coexpression with wild-type OspC. Furthermore, disruption of intermolecular Vsp1 salt bridges blocked dimerization but not surface localization of the resulting Vsp1 monomers. Together, these results suggest that Borrelia OspC/Vsp1 surface lipoproteins traverse the periplasm and the outer membrane as unfolded monomeric intermediates and assemble into their functional multimeric folds only upon reaching the spirochetal surface.

Interaction of variable bacterial outer membrane lipoproteins with brain endothelium

Background

Previously we reported that the variable outer membrane lipoprotein Vsp1 from the relapsing fever spirochete Borrelia turicatae disseminates from blood to brain better than the closely related Vsp2 [1]. Here we studied the interaction between Vsp1 and Vsp2 with brain endothelium in more detail.

Methodology/Principal Findings

We compared Vsp1 to Vsp2 using human brain microvascular endothelial cell (HBMEC) association assays with aminoacid radiolabeled Vsp-expressing clones of recombinant Borrelia burgdorferi and lanthanide-labeled purified lipidated Vsp1 (LVsp1) and Vsp2 (LVsp2) and inoculations of the lanthanide-labeled proteins into mice. The results showed that heterologous expression of LVsp1 or LVsp2 in B. burgdorferi increased its association with HBMEC to a similar degree. Purified lanthanide-labeled lipidated Vsp1 (LVsp1) and LVsp2 by themselves were capable of associating with HBMEC. The association of LVsp1 with brain endothelium was time-dependent, saturable, and required the lipidation. The association of Vsp1 with HBMEC was inhibited by incubation at lower temperature or with excess unlabeled LVsp1 or LVsp2 but not with excess rVsp1 or mouse albumin or an anti Vsp1 monoclonal antibody. The association of LVsp2 with HBMEC and its movement from blood to brain parenchyma significantly increased in the presence of LVsp1.

Conclusions/Significance

Variable bacterial outer membrane lipoproteins interact with brain endothelium differently; the lipidation and variable features at the protein dome region are key modulators of this interaction.

Bacterial lipoproteins can disseminate from the periphery to inflame the brain

The current view is that bacteria need to enter the brain to cause inflammation. However, in mice infected with the spirochete Borrelia turicatae, we observed widespread cerebral inflammation despite a paucity of spirochetes in the brain parenchyma at times of high bacteremia. Here we studied the possibility that bacterial lipoproteins may be capable of disseminating from the periphery across the blood-brain barrier to inflame the brain. For this we injected normal and infected mice intraperitoneally with lanthanide-labeled variable outer membrane lipoproteins of B. turicatae and measured their localization in blood, various peripheral organs, and whole and capillary-depleted brain protein extracts at various times. Lanthanide-labeled nonlipidated lipoproteins of B. turicatae and mouse albumin were used as controls. Brain inflammation was measured by TaqMan RT-PCR amplification of genes known to be up-regulated in response to borrelial infection. The results showed that the two lipoproteins we studied, LVsp1 and LVsp2, were capable of inflaming the brain after intraperitoneal injection to different degrees: LVsp1 was better than LVsp2 and Bt1 spirochetes at moving from blood to brain. The dissemination of LVsp1 from the periphery to the brain occurred under normal conditions and significantly increased with infection. In contrast, LVsp2 disseminated better to peripheral organs. We conclude that some bacterial lipoproteins can disseminate from the periphery to inflame the brain.

Translocation of Borrelia burgdorferi surface lipoprotein OspA through the outer membrane requires an unfolded conformation and can initiate at the C-terminus

Borrelia burgdorferi surface lipoproteins are essential to the pathogenesis of Lyme borreliosis, but the mechanisms responsible for their localization are only beginning to emerge. We have previously demonstrated the critical nature of the amino-terminal 'tether' domain of the mature lipoprotein for sorting a fluorescent reporter to the Borrelia cell surface. Here, we show that individual deletion of four contiguous residues within the tether of major surface lipoprotein OspA results in its inefficient translocation across the Borrelia outer membrane. Intriguingly, C-terminal epitope tags of these N-terminal deletion mutants were selectively surface-exposed. Fold-destabilizing C-terminal point mutations and deletions did not block OspA secretion, but rather restored one of the otherwise periplasmic tether mutants to the bacterial surface. Together, these data indicate that disturbance of a confined tether feature leads to premature folding of OspA in the periplasm and thereby prevents secretion through the outer membrane. Furthermore, they suggest that OspA emerges tail-first on the bacterial surface, yet independent of a specific C-terminal targeting peptide sequence.

Identification of residues within ligand-binding domain 1 (LBD1) of the Borrelia burgdorferi OspC protein required for function in the mammalian environment

Borrelia burgdorferi outer surface protein C (ospC) is required for the establishment of infection in mammals. However, its precise function remains controversial. The biologically active form of OspC appears to be a homodimer. Alpha helix 1 and 1' of the apposing monomers form a solvent-accessible pocket at the dimeric interface that presents a putative ligand-binding domain (LBD1). Here we employ site-directed and allelic-exchange mutagenesis to test the hypothesis that LBD1 is a determinant of OspC function in the mammalian environment. Substitution of residues K60, E61 and E63 which line LBD1 resulted in the loss of infectivity or influenced dissemination. Analyses of the corresponding recombinant proteins demonstrated that the loss of function was not due to structural perturbation, impaired dimer formation or the loss of plasminogen binding. This study is the first to assess the involvement of individual residues and domains of OspC in its in vivo function. The data support the hypothesis that OspC interacts with a mammalian derived ligand that is critical for survival during early infection. These results shed new light on the structure-functions relationships of OspC and challenge existing hypotheses regarding OspC function in mammals.

Structure-function investigation of vsp serotypes of the spirochete Borrelia hermsii

Background

Relapsing fever (RF) spirochetes are notable for multiphasic antigenic variation of polymorphic outer membrane lipoproteins, a phenomenon responsible for immune evasion. An additional role in tissue localization is suggested by the finding that isogenic serotypes 1 (Bt1) and 2 (Bt2) of the RF spirochete Borrelia turicatae, which differ only in the Vsp they express, exhibit marked differences in clinical disease severity and tissue localization during infection.

Methodology/Principal Findings

Here we used known vsp DNA sequences encoding for B. turicatae and Borrelia hermsii Vsp proteins with variable regions and then studied whether there are differences in disease expression and tissue localization of their corresponding serotypes during mouse infection. For sequence and structural comparisons we focused exclusively on amino acid residues predicted to project away from the spirochetes surface, referred to as the Vsp dome. Disease severity and tissue localization were studied during persistent infection with individual or mixed serotypes in SCID mice. The results showed that all Vsp domes clustered into 3 main trunks, with the domes for B. turicatae Vsp1 (BtVsp1) and BtVsp2 clustering into separate ones. B. hermsii serotypes whose Vsp domes clustered with the BtVsp1 dome were less virulent but localized to the brain more. The BtVsp2 dome was the oddball among all and Bt2 was the only serotype that caused severe arthritis.

Conclusion/Significance

These findings indicate that there is significant variability in Vsp dome structure, disease severity, and tissue localization among serotypes of B. hermsii.

Microbial genome dynamics in CNS pathogenesis

The balancing act between microbes and their host in commensal and disease states needs to be deciphered in order to fully treat and combat infectious diseases. The elucidation of microbial genome dynamics in each instance is therefore required. In this context, the major bacterial meningitis pathogens are Neisseria meningitidis, Haemophilus influenzae and Streptococcus pneumoniae. In prokaryotic CNS pathogenesis both the intact organism as well as its released components can elicit disease, often resulting in neurological sequelae, neurodegeneration or fatal outcome. The study of microbial virulence in CNS disease is expected to generate findings that yield new information on the general mechanisms of brain edema and excitatory neuronal disturbances due to meningitis, with significant potential for discoveries that can directly influence and inspire new strategies for prevention and treatment of this serious disease.

Functional characterization of BbCRASP-2, a distinct outer membrane protein of Borrelia burgdorferi that binds host complement regulators factor H and FHL-1

Borrelia burgdorferi, the aetiological agent of Lyme disease, employs sophisticated means to survive in diverse mammalian hosts. Recent studies demonstrated that acquisition of complement regulators factor H and factor H-like protein-1 (FHL-1) allows spirochetes to resist complement-mediated killing. Serum-resistant B. burgdorferi express up to five distinct complement regulator-acquiring surface proteins (CRASPs) that bind factor H and/or FHL-1. In this study we have identified and characterized one of those B. burgdorferi proteins, named BbCRASP-2. BbCRASP-2 is distinct from the four previously identified factor H/FHL-1-binding CRASPs of B. burgdorferi strains. The single copy of the gene encoding BbCRASP-2, cspZ, is located on the linear plasmid lp28-3. BbCRASP-2 is highly divergent from the factor H/FHL-1-binding protein BbCRASP-1 and from members of the factor H-binding Erp (OspE/F-related) protein family. Peptide mapping analysis revealed that the factor H/FHL-1 binding site is discontinuous and it was found that C-terminal truncations abrogate factor H and FHL-1 binding. The predominant BbCRASP-2 binding site of both host complement regulators was mapped to the short consensus repeat 7 (SCR 7). Factor H and FHL-1 bound to BbCRASP-2 maintain cofactor activity for factor I-mediated C3b inactivation and accelerate the decay of the C3 convertase. Expression of BbCRASP-2 in serum-sensitive B. burgdorferi mutant B313 increased resistance to complement-mediated lysis. The characterization of BbCRASP-2 now provides a complete picture of the three diverse complement regulator-binding protein families of B. burgdorferi yielding new insights into the pathogenesis of Lyme disease.

Development of an OspC-based tetravalent, recombinant, chimeric vaccinogen that elicits bactericidal antibody against diverse Lyme disease spirochete strains

Lyme disease is the most common arthropod-borne disease in North America and Europe. At present, there is no commercially available vaccine for use in humans. Outer surface protein C (OspC) has antigenic and expression characteristics that make it an attractive vaccine candidate; however, sequence heterogeneity has impeded its use as a vaccinogen. Sequence analyses have identified 21 well defined OspC phyletic groups or "types" (designated A-U). In this report we have mapped the linear epitopes presented by OspC types B, K, and D during human and murine infection and exploited these epitopes (along with the previously identified type A OspC linear epitopes) in the development of a recombinant, tetravalent, chimeric vaccinogen. The construct was found to be highly immunogenic in mice and the induced antibodies surface labeled in vitro cultivated spirochetes. Importantly, vaccination induced complement-dependent bactericidal antibodies against strains expressing each of the OspC types that were incorporated into the construct. These results suggest that an effective and broadly protective polyvalent OspC-based Lyme disease vaccine can be produced as a recombinant, chimeric protein.

Crystal structure of neurotropism-associated variable surface protein 1 (Vsp1) of Borrelia turicatae

Vsp surface lipoproteins are serotype-defining antigens of relapsing fever spirochetes that undergo multiphasic antigenic variation to allow bacterial persistence in spite of an immune response. Two isogenic serotypes of Borrelia turicatae strain Oz1 differ in their Vsp sequences and in disease manifestations in infected mice: Vsp1 is associated with the selection of a neurological niche, while Vsp2 is associated with blood and skin infection. We report here crystal structures of the Vsp1 dimer at 2.7 and 2.2 A. The structures confirm that relapsing fever Vsp proteins share a common helical fold with OspCs of Lyme disease-causing Borrelia. The fold features an inner stem formed by highly conserved N and C termini and an outer "dome" formed by the variable central residues. Both Vsp1 and OspC structures possess small water-filled cavities, or pockets, that are lined largely by variable residues and are thus highly variable in shape. These features appear to signify tolerance of the Vsp-OspC fold for imperfect packing of residues at its antigenic surface. Structural comparison of Vsp1 with a homology model for Vsp2 suggests that observed differences in disease manifestation may arise in part from distinct differences in electrostatic surface properties; additional predicted positively charged surface patches on Vsp2 compared to Vsp1 may be sufficient to explain the relative propensity of Vsp2 to bind to acidic glycosaminoglycans.

Hepatitis B virus capsid-like particles can display the complete, dimeric outer surface protein C and stimulate production of protective antibody responses against Borrelia burgdorferi infection

Hepatitis B virus capsid-like particles (CLPs), icosahedral assemblies formed by 90 or 120 core protein dimers, hold promise as immune-enhancing vaccine carriers for heterologous antigens. Insertions into the immunodominant c/e1 B cell epitope, a surface-exposed loop, are especially immunogenic. However, display of whole proteins, desirable to induce multispecific and possibly neutralizing antibody responses, can be restrained by an unsuitable structure of the foreign protein and by its propensity to undergo homomeric interactions. Here we analyzed CLP formation by core fusions with two distinct variants of the dimeric outer surface lipoprotein C (OspC) of the Lyme disease agent Borrelia burgdorferi. Although the topology of the termini in the OspC dimer does not match that of the insertion sites in the carrier dimer, both fusions, coreOspCa and coreOspCb, efficiently formed stable CLPs. Electron cryomicroscopy clearly revealed the surface disposition of the OspC domains, possibly with OspC dimerization occurring across different core protein dimers. In mice, both CLP preparations induced high-titered antibody responses against the homologous OspC variant, but with substantial cross-reactivity against the other variant. Importantly, both conferred protection to mice challenged with B. burgdorferi. These data show the principal applicability of hepatitis B virus CLPs for the display of dimeric proteins, demonstrate the presence in OspC of hitherto uncharacterized epitopes, and suggest that OspC, despite its genetic variability, may be a valid vaccine candidate.

Borrelia burgdorferi–Traveling incognito?

We outline in this review how Borrelia burgdorferi, the causative agent of Lyme disease, moves from the tick to the vertebrate host, and what molecules are potentially involved in this challenging commute. The survival strategies utilized by the spirochete during transmission and the initial stages of infection are discussed.

Borrelia burgdorferi lipoproteins are secreted to the outer surface by default

Borrelia spirochaetes are unique among diderm bacteria in their abundance of surface-displayed lipoproteins, some of which play important roles in the pathogenesis of Lyme disease and relapsing fever. To identify the lipoprotein-sorting signals in Borrelia burgdorferi, we generated chimeras between the outer surface lipoprotein OspA, the periplasmic oligopeptide-binding lipoprotein OppAIV and mRFP1, a monomeric red fluorescent reporter protein. Localization of OspA and OppAIV point mutants showed that Borrelia lipoproteins do not follow the '+2' sorting rule which targets lipoproteins to the cytoplasmic or outer membrane of Gram-negative bacteria via the Lol pathway. Fusions of mRFP1 to short N-terminal lipopeptides of OspA, and surprisingly OppAIV, were targeted to the spirochaetal surface. Mutagenesis of the OspA N-terminus defined less than five N-terminal amino acids as the minimal secretion-facilitating signal. With the exception of negative charges, which can act as partial subsurface retention signals in certain peptide contexts, lipoprotein secretion occurs independent of N-terminal sequence. Together, these data indicate that Borrelia lipoproteins are targeted to the bacterial surface by default, but can be retained in the periplasm by sequence-specific signals.

Borrelia burgdorferi sensu stricto invasiveness is correlated with OspC-plasminogen affinity

Borrelia burgdorferi sensu lato, the causative agent of Lyme borreliosis, is transmitted through tick bite. Lyme borreliosis evolves in two stages: a primary red skin lesion called erythema migrans; later on, invasive bacteria disseminate to distant sites inducing secondary manifestations (neuropathies, arthritis, carditis, late skin disorders). It has been previously suggested that the ospC gene could be associated with invasiveness in humans depending on its sequence. Here, we confirm the pattern of invasiveness, according to B. burgdorferi sensu stricto (B. b. ss) ospC group, using the mouse as an experimental host of B. b. ss. As it has been shown that the host plasminogen activation system is used by B. burgdorferi to disseminate throughout the host, we studied the interaction of plasminogen with OspC proteins from invasive and non-invasive groups of B. b. ss. Using two methods, ELISA and surface plasmon resonance, we demonstrate that indeed OspC is a plasminogen-binding protein. Moreover, significant differences in binding affinity for plasminogen are correlated with different invasiveness patterns in mice. These results suggest that the correlation between ospC polymorphism and Borrelia invasiveness in humans is linked, at least in part, to differences in OspC affinity for plasminogen.

Research opportunities on human neuroborreliosis

A workshop, sponsored by the National Institutes of Health, was convened in September 2001 to evaluate the current knowledge in neurological Lyme disease. The meeting was centered into discussion of both clinical and basic aspects of the disease. Participants included researchers from the fields of infectious diseases, neurology, rheumatology, autoimmunity and basic immunology, largely but not exclusively focused on Lyme disease. This report summarizes the presentations made at the meeting.

Cross-species surface display of functional spirochetal lipoproteins by recombinant Borrelia burgdorferi

Surface-exposed lipoproteins of relapsing fever (RF) and Lyme borreliosis Borrelia spirochetes mediate certain interactions of the bacteria with their arthropod and vertebrate hosts. RF spirochetes such as Borrelia hermsii serially evade the host's antibody response by multiphasic antigenic variation of Vsp and Vlp proteins. Furthermore, the expression of Vsp1 and Vsp2 by Borrelia turicatae is associated with neurotropism and higher blood densities, respectively. In contrast to RF Borrelia species, the Lyme borreliosis spirochete Borrelia burgdorferi is amenable to genetic manipulation. To facilitate structure-function analyses of RF surface lipoproteins, we used recombinant plasmids to introduce full-length vsp1 and vsp2 as well as two representative vlp genes into B. burgdorferi cells. Recombinant B. burgdorferi cells constitutively expressed the proteins under the control of the B. burgdorferi flaB promoter. Antibody and protease accessibility assays indicated proper surface exposure and folding. Expression of Vsp1 and Vsp2 conferred glycosaminoglycan binding to recombinant B. burgdorferi cells that was similar to that observed with purified recombinant proteins and B. turicatae expressing native Vsp. These data demonstrate that the lipoprotein modification and export mechanisms in the genus Borrelia are conserved. They also validate the use of recombinant B. burgdorferi in studies of surface lipoprotein structure-function and the biogenesis of spirochete membranes.

Outer-surface protein C of the Lyme disease spirochete: a protein induced in ticks for infection of mammals

Environmentally responsive synthesis of surface proteins represents a hallmark of the infectious cycle of the Lyme disease agent, Borrelia burgdorferi. Here we created and analyzed a B. burgdorferi mutant lacking outer-surface protein C (OspC), an abundant Osp that spirochetes normally synthesize in the tick vector during the blood meal and down-regulate after transmission to the mammal. We demonstrate that B. burgdorferi strictly requires OspC to infect mice but not to localize or migrate appropriately in the tick. The induction of a spirochetal virulence factor preceding the time and host in which it is required demonstrates a developmental sequence for transmission of this arthropod-borne pathogen.

Simultaneous coexpression of Borrelia burgdorferi Erp proteins occurs through a specific, erp locus-directed regulatory mechanism

An individual Borrelia burgdorferi bacterium can encode as many as 13 different Erp (OspE/F-related) proteins from mono-and bicistronic loci that are carried on up to 10 separate plasmids. We demonstrate through multilabel immunofluorescence analyses that individual bacteria simultaneously coexpress their entire Erp protein repertoire. While it has been proposed that B. burgdorferi controls expression of Erp and other plasmid-encoded proteins through changes in DNA topology, we observed regulated Erp expression in the absence of detectable differences in DNA supercoiling. Likewise, inhibition of DNA gyrase had no detectable effect on Erp expression. Furthermore, expression of loci physically adjacent to erp loci was observed to be independently regulated. It is concluded that Erp expression is regulated by a mechanism(s) directed at erp loci and not by a global, plasmid-wide mechanism.

Crystal structure of Lyme disease variable surface antigen VlsE of Borrelia burgdorferi

VlsE is an outer surface lipoprotein of Borrelia burgdorferi that undergoes antigenic variation through an elaborate gene conversion mechanism and is thought to play a major role in the immune response to the Lyme disease borellia. The crystal structure of recombinant variant protein VlsE1 at 2.3-A resolution reveals that the six variable regions form loop structures that constitute most of the membrane distal surface of VlsE, covering the predominantly alpha-helical, invariant regions of the protein. The surface localization of the variable amino acid segments appears to protect the conserved regions from interaction with antibodies and hence may contribute to immune evasion.

Antigen polymorphism in Borrelia hermsii, a clonal pathogenic bacterium

The relapsing fever spirochete, Borrelia hermsii, escapes immune selection by alternating expression of surface lipoprotein alleles. The switch results from a duplicative transposition of one of several surface lipoprotein-encoding nucleotide sequences into the singular expression site. These nucleotide sequences constitute a large gene family whose diversity originated, in some cases, before the major divergences of Borrelia species. We have examined the B. hermsii vsp subfamily of alleles, which are carried on linear plasmids within each cell and maintained in several diverse copies as an antigenic archive. Each encodes a distinct serotype-specific protein. We sequenced more than 90% of the alleles within a single strain-B. hermsii strain HS1. A preponderance of allelic mosaicism suggests that intragenic recombination, coupled with selection imposed by host immune response, has driven diversification of the archived ensemble of vsp alleles. The recombinational diversification of vsp alleles generates change in the associated serotypes of the magnitude (30-40% amino acid differentiation) necessary for overcoming cross-reactivity of neutralizing antibodies. We conclude that evolution of vsp has occurred by punctuated occurrence of allelic differentiation, rather than by gradual selection of incremental point mutations that do not meet the threshold for antigenic diversity.

Borrelia burgdorferi RevA antigen is a surface-exposed outer membrane protein whose expression is regulated in response to environmental temperature and pH

Borrelia burgdorferi, the causative agent of Lyme disease, produces RevA protein during the early stages of mammalian infection. B. burgdorferi apparently uses temperature as a cue to its location, producing proteins required for infection of warm-blooded animals at temperatures corresponding to host body temperature, but does not produce such virulence factors at cooler, ambient temperatures. We have observed that B. burgdorferi regulates expression of RevA in response to temperature, with the protein being synthesized by bacteria cultivated at 34 degrees C but not by those grown at 23 degrees C. Tissues encountered by B. burgdorferi during its infectious cycle vary in their pH values, and the level of RevA expression was also found to be dependent upon pH of the culture medium. The cellular localization of RevA was also analyzed. Borrelial inner and outer membranes were purified by isopycnic centrifugation, and membrane fractions were conclusively identified by immunoblot analysis using antibodies raised against the integral inner membrane protein MotB and outer membrane-associated Erp lipoproteins. Immunoblot analyses indicated that RevA is located in the B. burgdorferi outer membrane. These analyses also demonstrated that an earlier report (H. A. Bledsoe et al., Infect. Immun. 176:7447-7455, 1994) had misidentified such B. burgdorferi membrane fractions. RevA was further demonstrated to be exposed to the external environment, where it could facilitate interactions with host tissues.

Surface exposure and protease insensitivity of Borrelia burgdorferi Erp (OspEF-related) lipoproteins

Borrelia burgdorferi can encode numerous lipoproteins of the Erp family. Although initially described as outer surface proteins, the technique used in that earlier study has since been demonstrated to disrupt bacterial membranes and allow labelling of subsurface proteins. Data are now presented from additional analyses indicating that Erp proteins are indeed surface exposed in the outer membrane. Surface localization of these infection-associated proteins indicates the potential for interactions of Erp proteins with vertebrate tissues. Some Erp proteins were resistant to in situ digestion by certain proteases, suggesting that those proteins fold in manners which hide protease cleavage sites, or that they interact with other protective membrane components. Additionally, cultivation of B. burgdorferi in the presence of antibodies directed against Erp proteins inhibited bacterial growth.

Crystal structure of Lyme disease antigen outer surface protein C from Borrelia burgdorferi

The outer surface protein C (OspC) is one of the major host-induced antigens of Borrelia burgdorferi, the causative agent of Lyme disease. We have solved the crystal structure of recombinant OspC to a resolution of 2.5 A. OspC, a largely alpha-helical protein, is a dimer with a characteristic central four-helical bundle formed by association of the two longest helices from each subunit. OspC is very different from OspA and similar to the extracellular domain of the bacterial aspartate receptor and the variant surface glycoprotein from Trypanosoma brucei. Most of the surface-exposed residues of OspC are highly variable among different OspC isolates. The membrane proximal halves of the two long alpha-helices are the only conserved regions that are solvent accessible. As vaccination with recombinant OspC has been shown to elicit a protective immune response in mice, these regions are candidates for peptide-based vaccines.

In vitro and in vivo neutralization of the relapsing fever agent Borrelia hermsii with serotype-specific immunoglobulin M antibodies

The antigenic variation of the relapsing fever agent Borrelia hermsii is associated with changes in the expression of the Vlp and Vsp outer membrane lipoproteins. To investigate whether these serotype-defining proteins are the target of a neutralizing and protective antibody response, monoclonal antibodies were produced from spleens of infected mice just after clearance of serotype 7 cells from the blood. Two immunoglobulin M monoclonal antibodies, H7-7 and H7-12, were studied in detail. Both antibodies specifically agglutinated serotype 7 cells and inhibited their growth in vitro. Administered to mice before or after infection, both antibodies provided protection against infection or substantially reduced the number of spirochetes in the blood of mice after infection. Whereas antibody H7-12 bound to Vlp7 in Western blotting, enzyme-linked immunosorbent assay, and immunoprecipitation assays, as well as to whole cells in other immunoassays, antibody H7-7 only bound to wet, intact cells of serotype 7. Antibody H7-7 selected against cells expressing Vlp7 in vitro and in vivo, an indication that Vlp7 was a conformation-sensitive antigen for the antibody. Vaccination of mice with recombinant Vlp7 with adjuvant elicited antibodies that bound to fixed whole cells of serotype 7 and to Vlp7 in Western blots, but these antibodies did not inhibit the growth of serotype 7 in vitro and did not provide protection against an infectious challenge with serotype 7. The study established that a Vlp protein was the target of a neutralizing antibody response, and it also indicated that the conformation and/or the native topology of Vlp were important for eliciting that immunity.