A surface-exposed region of a novel outer membrane protein (P66) of Borrelia spp. is variable in size and sequence

Borrelia burgdorferi BB0346 is an Essential, Structurally Variant LolA Homolog that is Primarily Required for Homeostatic Localization of Periplasmic Lipoproteins

In diderm bacteria, the Lol pathway canonically mediates the periplasmic transport of lipoproteins from the inner membrane (IM) to the outer membrane (OM) and therefore plays an essential role in bacterial envelope homeostasis. After extrusion of modified lipoproteins from the IM via the LolCDE complex, the periplasmic chaperone LolA carries lipoproteins through the periplasm and transfers them to the OM lipoprotein insertase LolB, itself a lipoprotein with a LolA-like fold. Yet, LolB homologs appear restricted to γ-proteobacteria and are missing from spirochetes like the tick-borne Lyme disease pathogen Borrelia burgdorferi, suggesting a different hand-off mechanism at the OM. Here, we solved the crystal structure of the B. burgdorferi LolA homolog BB0346 (LolABb) at 1.9 Å resolution. We identified multiple structural deviations in comparative analyses to other solved LolA structures, particularly a unique LolB-like protruding loop domain. LolABb failed to complement an Escherichia coli lolA knockout, even after codon optimization, signal I peptide adaptation, and a C-terminal chimerization which had allowed for complementation with an α-proteobacterial LolA. Analysis of a conditional B. burgdorferi lolA knockout strain indicated that LolABb was essential for growth. Intriguingly, protein localization assays indicated that initial depletion of LolABb led to an emerging mislocalization of both IM and periplasmic OM lipoproteins, but not surface lipoproteins. Together, these findings further support the presence of two separate primary secretion pathways for periplasmic and surface OM lipoproteins in B. burgdorferi and suggest that the distinct structural features of LolABb allow it to function in a unique LolB-deficient lipoprotein sorting system.

A Borrelia burgdorferi LptD homolog is required for flipping of surface lipoproteins through the spirochetal outer membrane

Borrelia spirochetes are unique among diderm bacteria in their lack of lipopolysaccharide (LPS) in the outer membrane (OM) and their abundance of surface-exposed lipoproteins with major roles in transmission, virulence, and pathogenesis. Despite their importance, little is known about how surface lipoproteins are translocated through the periplasm and the OM. Here, we characterized Borrelia burgdorferi BB0838, a distant homolog of the OM LPS assembly protein LptD. Using a CRISPR interference approach, we showed that BB0838 is required for cell growth and envelope stability. Upon BB0838 knockdown, surface lipoprotein OspA was retained in the inner leaflet of the OM, as determined by its inaccessibility to in situ proteolysis but its presence in OM vesicles. The topology of the OM porin/adhesin P66 remained unaffected. Quantitative mass spectrometry of the B. burgdorferi membrane-associated proteome confirmed the selective periplasmic retention of surface lipoproteins under BB0838 knockdown conditions. Additional analysis identified a single in situ protease-accessible BB0838 peptide that mapped to a predicted β-barrel surface loop. Alphafold Multimer modeled a B. burgdorferi LptB2 FGCAD complex spanning the periplasm. Together, this suggests that BB0838/LptDBb facilitates the essential terminal step in spirochetal surface lipoprotein secretion, using an orthologous OM component of a pathway that secretes LPS in proteobacteria.

Allelic Variants of P66 Gene in Borrelia bavariensis Isolates from Patients with Ixodid Tick-Borne Borreliosis

Protein P66 is one of the crucial virulence factors of Borrelia, inducing the production of specific antibodies in patients with ixodid tick-borne borreliosis (ITBB). Various species of Borrelia are characterized by genetic variability of the surface-exposed loop of P66. However, little is known about this variability in Borrelia bavariensis. Here we describe the variability of the nucleotide sequences of P66 gene locus in isolates of B. bavariensis. Analysis of nucleotide sequences of P66 in 27 isolates of B. bavariensis from ITBB patients revealed three allelic variants of this gene. The alignment score of amino acid sequences in the isolates showed amino acid replacements in various positions confirming the presence of three allelic variants. Two of them are characteristic only for some isolates of B. bavariensis of the Eurasian gene pool from various parts of the geographic ranges of B. bavariensis from various samples. At least three allelic variants of P66 B. bavariensis have been identified, which have different amino acid expression, occur with different frequency in ITBB patients and, presumably, can have different effects on the course of the infection.

Identification of amino acid domains of Borrelia burgdorferi P66 that are surface exposed and important for localization, oligomerization, and porin function of the protein

P66, a bifunctional integral outer membrane protein, is necessary for Borrelia burgdorferi to establish initial infection and to disseminate in mice. The integrin binding function of P66 facilitates extravasation and dissemination, but the role of its porin function during murine infection has not been investigated. A limitation to studying P66 porin function during mammalian infection has been the lack of structural information for P66. In this study, we experimentally characterized specific domains of P66 with regard to structure and function. First, we aligned the amino acid sequences of P66 from Lyme disease-causing Borrelia and relapsing fever-causing Borrelia to identify conserved and unique domains between these disease-causing clades. Then, we examined whether specific domains of P66 are exposed on the surface of the bacteria by introducing c-Myc epitope tags into each domain of interest. The c-Myc epitope tag inserted C-terminally to E33 (highly conserved domain), to T187 (integrin binding region domain and a non-conserved domain), and to E334 (non-conserved domain) were all detected on the surface of Borrelia burgdorferi. The c-Myc epitope tag inserted C-terminally to E33 and D303 in conserved domains disrupted P66 oligomerization and porin function. In a murine model of infection, the E33 and D303 mutants exhibited decreased infectivity and dissemination. Taken together, these results suggest the importance of these conserved domains, and potentially P66 porin function, in vivo.

Pathogenesis of Relapsing Fever

Relapsing fever (RF) is caused by several species of Borrelia; all, except two species, are transmitted to humans by soft (argasid) ticks. The species B. recurrentis is transmitted from one human to another by the body louse, while B. miyamotoi is vectored by hard-bodied ixodid tick species. RF Borrelia have several pathogenic features that facilitate invasion and dissemination in the infected host. In this article we discuss the dynamics of vector acquisition and subsequent transmission of RF Borrelia to their vertebrate hosts. We also review taxonomic challenges for RF Borrelia as new species have been isolated throughout the globe. Moreover, aspects of pathogenesis including symptomology, neurotropism, erythrocyte and platelet adhesion are discussed. We expound on RF Borrelia evasion strategies for innate and adaptive immunity, focusing on the most fundamental pathogenetic attributes, multiphasic antigenic variation. Lastly, we review new and emerging species of RF Borrelia and discuss future directions for this global disease.

Broadly Protective Multivalent OspA Vaccine against Lyme Borreliosis, Developed Based on Surface Shaping of the C-Terminal Fragment

The development of vaccines for prevention of diseases caused by pathogenic species can encounter major obstacles if high sequence diversity is observed between individual strains. Therefore, development might be restricted either to conserved antigens, which are often rare, or to multivalent vaccines, which renders the production more costly and cumbersome. In light of this complexity, we applied a structure-based surface shaping approach for the development of a Lyme borreliosis (LB) vaccine suitable for the United States and Europe. The surface of the C-terminal fragment of outer surface protein A (OspA) was divided into distinct regions, based primarily on binding sites of monoclonal antibodies (MAbs). In order to target the six clinically most relevant OspA serotypes (ST) in a single protein, exposed amino acids of the individual regions were exchanged to corresponding amino acids of a chosen OspA serotype. Six chimeric proteins were constructed, and, based on their immunogenicity, four of these chimeras were tested in mouse challenge models. Significant protection could be demonstrated for all four proteins following challenge with infected ticks (OspA ST1, OspA ST2, and OspA ST4) or with in vitro-grown spirochetes (OspA ST1 and OspA ST5). Two of the chimeric proteins were linked to form a fusion protein, which provided significant protection against in vitro-grown spirochetes (OspA ST1) and infected ticks (OspA ST2). This article presents the proof-of-concept study for a multivalent OspA vaccine targeting a wide range of pathogenic LB Borrelia species with a single recombinant antigen for prevention of Lyme borreliosis.

Multilocus sequence analysis of Borrelia burgdorferi sensu lato isolates from Western Siberia, Russia and Northern Mongolia

Lyme borreliosis (LB) is the most frequently recorded tick-transmitted disease in Eurasia. Tomsk Province, Western Siberia in Russia and Selenge Aimag in Northern Mongolia are leading regions in the LB incidence rate in these countries. Spirochaetes of the Borrelia burgdorferi sensu lato (s.l.) complex isolated from Ixodes ticks from Tomsk Province (n = 56) and Ixodes persulcatus ticks from Selenge Aimag (n = 5) were genetically characterized using Multi Locus Sequence Typing (MLST), analysis of the 5S23S rRNA intergenic spacer (IGS) amplicons, and p83/100 gene sequencing. According to MLST, B. afzelii (n = 26), B. bavariensis (n = 23), B. garinii (n = 11), and B. valaisiana (n = 1) isolates were detected in Tomsk Province, while B. afzelii and B. bavariensis isolates were identified in Selenge Aimag. Of the 32 revealed sequence types (ST), 21 STs were new and 14 of the new STs belonged to B. afzelii. Several STs of B. afzelii, B. garinii and B. valaisiana identified in this study clustered with European STs found in I. ricinus ticks. Analysis of the 5S23S IGS demonstrated that the studied Borrelia strains showed RFLP pattern characteristic for the following 5S23S IGS types: VS461 (B. afselii), NT29 (B. bavariensis), 20047 (B. bavariensis and B. garinii), VS116 (B. valaisiana), and three new groups (B. afzelii and B. bavariensis). Notably, this is the first report of Asian B. bavariensis possessing a 5S23S IGS RFLP pattern identical to 20047, and analysis of the 5S23S IGS did not provide correct determination of Borrelia species occurring in Asia. Genotyping of Borrelia strains using the clpA, pepX, and p83/100 genes demonstrated the same result as genotyping based on MLST; and further investigations are required to confirm that these three genetic loci could be used for determination of bacterial species from the B. burgdorferi s.l. complex because data based on single loci may be misleading.

Borrelia burgdorferi HtrA: evidence for twofold proteolysis of outer membrane protein p66

In prokaryotes, members of the High Temperature Requirement A (HtrA) family of serine proteases function in the periplasm to degrade damaged or improperly folded membrane proteins. Borrelia burgdorferi, the agent of Lyme disease, codes for a single HtrA homolog. Two-dimensional electrophoresis analysis of B. burgdorferi B31A3 and a strain that overexpresses HtrA (A3HtrAOE) identified a downregulated protein in A3HtrAOE with a mass, pI and MALDI-TOF spectrum consistent with outer membrane protein p66. P66 and HtrA from cellular lysates partitioned into detergent-resistant membranes, which contain cholesterol-glycolipid-rich membrane regions known as lipid rafts, suggesting that HtrA and p66 may reside together in lipid rafts also. This agrees with previous work from our laboratory, which showed that HtrA and p66 are constituents of B. burgdorferi outer membrane vesicles. HtrA degraded p66 in vitro and A3HtrAOE expressed reduced levels of p66 in vivo. Fluorescence confocal microscopy revealed that HtrA and p66 colocalize in the membrane. The association of HtrA and p66 establishes that they could interact efficiently and their protease/substrate relationship provides functional relevance to this interaction. A3HtrAOE also showed reduced levels of p66 transcript in comparison with wild-type B31A3, indicating that HtrA-mediated regulation of p66 may occur at multiple levels.

The BBA33 lipoprotein binds collagen and impacts Borrelia burgdorferi pathogenesis

Borrelia burgdorferi, the etiologic agent of Lyme disease, adapts to the mammalian hosts by differentially expressing several genes in the BosR and Rrp2-RpoN-RpoS dependent pathways, resulting in a distinct protein profile relative to that seen for survival in the Ixodes spp. tick. Previous studies indicate that a putative lipoprotein, BBA33, is produced in an RpoS-dependent manner under conditions that mimic the mammalian component of the borrelial lifecycle. However, the significance and function for BBA33 is not known. Given its linkage to the BosR/Rrp2-RpoN-RpoS regulatory cascade, we hypothesized that BBA33 facilitates B. burgdorferi infection in the mammalian host. The deletion of bba33 eliminated B. burgdorferi infectivity in C3H mice, which was rescued by genetic complementation with intact bba33. With regard to function, a combinatorial peptide approach, coupled with subsequent in vitro binding assays, indicated that BBA33 binds to collagen type VI and, to a lesser extent, collagen type IV. Whole cell binding assays demonstrated BBA33-dependent binding to human collagen type VI. Taken together, these results suggest that BBA33 interacts with collagenous structures and may function as an adhesin in a process that is required to prevent bacterial clearance.

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.

Structural modeling and physicochemical characterization provide evidence that P66 forms a β-barrel in the Borrelia burgdorferi outer membrane

The Borrelia burgdorferi outer membrane (OM) contains numerous surface-exposed lipoproteins but a relatively low density of integral OM proteins (OMPs). Few membrane-spanning OMPs of B. burgdorferi have been definitively identified, and none are well characterized structurally. Here, we provide evidence that the borrelial OMP P66, a known adhesin with pore-forming activity, forms a β-barrel in the B. burgdorferi OM. Multiple computer-based algorithms predict that P66 forms a β-barrel with either 22 or 24 transmembrane domains. According to our predicted P66 topology, a lysine residue (K487) known to be sensitive to trypsin cleavage is located within a surface-exposed loop. When we aligned the mature P66 amino acid sequences from B. burgdorferi and B. garinii, we found that K487 was present only in the B. burgdorferi P66 protein sequence. When intact cells from each strain were treated with trypsin, only B. burgdorferi P66 was trypsin sensitive, indicating that K487 is surface exposed, as predicted. Consistent with this observation, when we inserted a c-Myc tag adjacent to K487 and utilized surface localization immunofluorescence, we detected the loop containing K487 on the surface of B. burgdorferi. P66 was examined by both Triton X-114 phase partitioning and circular dichroism, confirming that the protein is amphiphilic and contains extensive (48%) β-sheets, respectively. Moreover, P66 also was able to incorporate into liposomes and form channels in large unilamellar vesicles. Finally, blue native PAGE (BN-PAGE) revealed that under nondenaturing conditions, P66 is found in large complexes of ∼400 kDa and ∼600 kDa. Outer surface lipoprotein A (OspA) and OspB both coimmunoprecipitate with P66, demonstrating that P66 associates with OspA and OspB in B. burgdorferi. The combined computer-based structural analyses and supporting physicochemical properties of P66 provide a working model to further examine the porin and integrin-binding activities of this OMP as they relate to B. burgdorferi physiology and Lyme disease pathogenesis.

Specificity and role of the Borrelia burgdorferi CtpA protease in outer membrane protein processing

To further characterize the function of the Borrelia burgdorferi C-terminal protease CtpA, we used site-directed mutagenesis to alter the putative CtpA cleavage site of one of its known substrates, the outer membrane (OM) porin P13. These mutations resulted in only partial blockage of P13 processing. Ectopic expression of a C-terminally truncated P13 in B. burgdorferi indicated that the C-terminal peptide functions as a safeguard against misfolding or mislocalization prior to its proteolytic removal by CtpA. In a parallel study of Borrelia burgdorferi lipoprotein sorting mechanisms, we observed a lower-molecular-weight variant of surface lipoprotein OspC that was particularly prominent with OspC mutants that mislocalized to the periplasm or contained C-terminal epitope tags. Further investigation revealed that the variant resulted from C-terminal proteolysis by CtpA. Together, these findings indicate that CtpA rather promiscuously targets polypeptides that lack structurally constrained C termini, as proteolysis appears to occur independently of a specific peptide recognition sequence. Low-level processing of surface lipoproteins such as OspC suggests the presence of a CtpA-dependent quality control mechanism that may sense proper translocation of integral outer membrane proteins and surface lipoproteins by detecting the release of C-terminal peptides.

The central region of the msp gene of Treponema denticola has sequence heterogeneity among clinical samples, obtained from patients with periodontitis

Background

Treponema denticola is an oral spirochete involved in the pathogenesis and progression of periodontal disease. Of its virulence factors, the major surface protein (MSP) plays a role in the interaction between the treponeme and host. To understand the possible evolution of this protein, we analyzed the sequence of the msp gene in 17 T. denticola positive clinical samples.

Methods

Nucleotide and amino acid sequence of MSP have been determined by PCR amplification and sequencing in seventeen T. denticola clinical specimens to evaluate the genetic variability and the philogenetic relationship of the T. denticola msp gene among the different amplified sequence of positive samples. In silico antigenic analysis was performed on each MSP sequences to determined possible antigenic variation.

Results

The msp sequences showed two highly conserved 5' and 3' ends and a central region that varies substantially. Phylogenetic analysis categorized the 17 specimens into 2 principal groups, suggesting a low rate of evolutionary variability and an elevated degree of conservation of msp in clinically derived genetic material. Analysis of the predicted antigenic variability between isolates, demonstrated that the major differences lay between amino acids 200 and 300.

Conclusion

These findings showed for the first time, the nucleotide and amino acids variation of the msp gene in infecting T. denticola, in vivo. This data suggested that the antigenic variability found in to the MSP molecule, may be an important factor involved in immune evasion by T. denticola.

Interaction of Borrelia burgdorferi Hbb with the p66 promoter

Borrelia burgdorferi, an agent of Lyme disease, encodes the β₃-chain integrin ligand P66. P66 is expressed by B. burgdorferi in the mammal, in laboratory media, and as the bacteria are acquired or transmitted by the tick, but is not expressed by the bacterium in unfed ticks. Attempts to reveal factors influencing expression revealed that P66 was expressed in all in vitro conditions investigated. Candidate regulators identified in a search of the B. burgdorferi genome for homologs to other bacterial transcription factors were cloned and introduced into E. coli carrying a p66 promoter-signal sequence-phoA (alkaline phosphatase, or AP) fusion. Three candidate transcription factors—two that decreased AP activity (Hbb and BB0527), and one that increased AP activity (BBA23)—were identified. BBA23 and BB0527 did not bind to the p66 promoter at physiologically relevant concentrations. In contrast, several promoter fragments, including p66, were bound by Hbb (BB0232), with slightly different affinities. Consistent with results from other laboratories, Hbb appears to recognize multiple DNA sequences. Changes in the expression of p66 and bb0232 in the tick at various points with respect to feeding on mice, along with the results of the reporter experiment in the surrogate host E. coli, are consistent with Hbb/BB0232 being involved in regulating p66 expression.

Identification of conserved antigens for early serodiagnosis of relapsing fever Borrelia

Borrelia hermsii is a blood-borne pathogen transmitted by the argasid tick Ornithodoros hermsi. Since spirochaete clearance in mice is associated with an IgM-mediated response, an immunoproteomic analysis was used to identify proteins reactive with IgM. We report that IgM from both mice and human patients infected with B. hermsii not only reacted with the previously identified variable membrane proteins but also identified candidate antigens including heat-shock proteins, an adhesin protein, ABC transporter proteins, flagellar proteins, housekeeping proteins, an immune evasion protein, and proteins with unknown function. Furthermore, IgM reactivity to recombinant glycerophosphodiester phosphodiesterase was detected during early spirochaete infection and prior to a detectable IgG response. Lastly, a conserved hypothetical protein was produced in Escherichia coli and tested with immune serum against B. hermsii and Borrelia recurrentis. These results identify a much larger set of immunoreactive proteins, and could help in the early serodiagnosis of this tick-borne infection.

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.

Isolation and molecular characterization of a Brazilian strain of Borrelia anserina, the agent of fowl spirochaetosis

Borrelia anserina the agent of fowl spirochaetosis, has a worldwide distribution, where it is transmitted by Argas spp. ticks. The present study reports the first molecular characterization and in vitro isolation of an avian spirochaete strain from Brazil, presumably identified as B. anserina originated from naturally infected Argas miniatus ticks. DNA fragments of the rrs and flab genes were amplified by PCR and sequenced to determine phylogenetic similarities. The resulting sequences were 99.8% (483 of 484) and 98.7% (754 of 764) similar to GenBank corresponding sequences of B. anserina rrs and flaB genes, respectively. By neighbor-joining phylogenetic analysis, the flaB sequence of the Brazilian strain clustered in a monophyletic group with the sequence of B. anserina under 100% bootstrap support. The isolate was successfully isolated in BSK medium, with seven passages performed. The spirochaete strain isolated in the present study was genetically identified as B. anserina labeled as strain PL.

Dual binding specificity of a Borrelia hermsii-associated complement regulator-acquiring surface protein for factor H and plasminogen discloses a putative virulence factor of relapsing fever spirochetes

Tick-borne relapsing fever in North America is primarily caused by the spirochete Borrelia hermsii. The pathogen employs multiple strategies, including the acquisition of complement regulators and antigenic variation, to escape innate and humoral immunity. In this study we identified in B. hermsii a novel member of the complement regulator-acquiring surface protein (CRASP) family, designated BhCRASP-1, that binds the complement regulators factor H (FH) and FH-related protein 1 (FHR-1) but not FH-like protein 1 (FHL-1). BhCRASP-1 specifically interacts with the short consensus repeat 20 of FH, thereby maintaining FH-associated cofactor activity for factor I-mediated C3b inactivation. Furthermore, ectopic expression of BhCRASP- 1 converted the serum-sensitive Borrelia burgdorferi B313 strain into an intermediate complement-resistant strain. Finally, we report for the first time that BhCRASP-1 binds plasminogen/plasmin in addition to FH via, however, distinct nonoverlapping domains. The fact that surface-bound plasmin retains its proteolytic activity suggest that the dual binding specificity of BhCRASP-1 for FH and plasminogen/plasmin contributes to both the dissemination/invasion of B. hermsii and its resistance to innate immunity.

Lyme borreliosis: insights into tick-/host-borrelia relations

Lyme borreliosis (LB) is a serious infectious disease of humans and some domestic animals in temperate regions of the Northern Hemisphere. It is caused by certain spirochetes in the Borrelia burgdorferi sensu lato (s.l.) species complex. The complex consists of 11 species (genospecies). Borrelia burgdorferi sensu stricto (s.s.), Borrelia garinii and Borrelia afzelii are the major agents of human disease. Borrelia burgdorferi s.l. species are transmitted mainly by ticks belonging to the Ixodes ricinus species complex plus a few additional species not currently assigned to the complex. B. burgdorferi infections may produce an acute or chronic disease with a wide array of clinical symptoms such as erythema migrans (EM), carditis, arthritis, neuroborreliosis, and acrodermatitis chronica atrophicans (ACA). Differences in LB spirochetes 'genospecies' and strains/isolates determine the occurrence and severity of this multi-system disease. Accurate and reliable identification of the LB spirochetes in ticks as well as knowledge of their prevalence are essential for prevention against the disease and development of an effective vaccine. An overview of the knowledge of molecular factors with emphasis on potential protein-carbohydrate interactions in the tick-borrelia system is the main focus of this review.

Horizontally acquired genes for purine salvage in Borrelia spp. causing relapsing fever

Unlike Borrelia burgdorferi, the relapsing fever agent Borrelia hermsii and the related Borrelia miyamotoi had purA and purB genes of the purine salvage pathway. These were located among the rRNA genes. Phylogenetic analysis indicated that these genes had a different evolutionary history than those of orthologs in other spirochetes.

Binding properties and adhesion-mediating regions of the major sheath protein of Treponema denticola ATCC 35405

There is growing evidence that a number of oral Treponema species, in particular Treponema denticola, are associated with the progression of human periodontal disease. The major sheath (or surface) protein (Msp) of T. denticola is implicated in adhesion of bacteria to host cells and tissue proteins and is likely to be an important virulence factor. However, the binding regions of the Msp are not known. We have purified from Escherichia coli recombinant Msp (rMsp) polypeptides corresponding to the following: full-length Msp (rMsp) minus 13 N-terminal amino acid (aa) residues, an amino-terminal fragment (rN-Msp, 189 aa residues), a 57-aa residue segment from the central region (rV-Msp), and a C-terminal fragment (rC-Msp, 272 aa residues). rMsp (530 aa residues) bound to immobilized fibronectin, keratin, laminin, collagen type I, fibrinogen, hyaluronic acid, and heparin. The N- and V-region polypeptides, but not rC-Msp, also bound to these substrates. Binding of rMsp to fibronectin was targeted to the N-terminal heparin I/fibrin I domain. Antibodies to the N-region or V-region polypeptides, but not antibodies to the rC-Msp fragment, blocked adhesion of T. denticola ATCC 35405 cells to a range of host protein molecules. These results suggest that the N-terminal half of Msp carries epitopes that are surface exposed and that are involved in mediating adhesion. Binding of rMsp onto the cell surface of low-level fibronectin-binding Treponema isolates conferred a 10-fold increase in fibronectin binding. This confirms that Msp functions autonomously as an adhesin and raises the possibility that phenotypic complementation of virulence functions might occur within mixed populations of Treponema species.

An ecological approach to preventing human infection: vaccinating wild mouse reservoirs intervenes in the Lyme disease cycle

Many pathogens, such as the agents of West Nile encephalitis and plague, are maintained in nature by animal reservoirs and transmitted to humans by arthropod vectors. Efforts to reduce disease incidence usually rely on vector control or immunization of humans. Lyme disease, for which no human vaccine is currently available, is a commonly reported vector-borne disease in North America and Europe. In a recently developed, ecological approach to disease prevention, we intervened in the natural cycle of the Lyme disease agent (Borrelia burgdorferi) by immunizing wild white-footed mice (Peromyscus leucopus), a reservoir host species, with either a recombinant antigen of the pathogen, outer surface protein A, or a negative control antigen in a repeated field experiment with paired experimental and control grids stratified by site. Outer surface protein A vaccination significantly reduced the prevalence of B. burgdorferi in nymphal blacklegged ticks (Ixodes scapularis) collected at the sites the following year in both experiments. The magnitude of the vaccine's effect at a given site correlated with the tick infection prevalence found on the control grid, which in turn correlated with mouse density. These data, as well as differences in the population structures of B. burgdorferi in sympatric ticks and mice, indicated that nonmouse hosts contributed more to infecting ticks than previously expected. Thus, where nonmouse hosts play a large role in infection dynamics, vaccination should be directed at additional species.

Typing of Borrelia relapsing fever group strains

Partial sequencing of the 16S-23S rDNA intergenic spacer showed two to four genotypes each for Borrelia hermsii and B. turicatae, both relapsing fever agents transmitted by argasid ticks, and for B. miyamotoi and B. lonestari, transmitted by ixodid ticks. Field surveys of Ixodes ticks in Connecticut and Sweden showed limited local diversity for B. miyamotoi.

Sequence typing reveals extensive strain diversity of the Lyme borreliosis agents Borrelia burgdorferi in North America and Borrelia afzelii in Europe

The genetic polymorphism of Borrelia burgdorferi and Borrelia afzelii, two species that cause Lyme borreliosis, was estimated by sequence typing of four loci: the rrs-rrlA intergenic spacer (IGS) and the outer-membrane-protein gene p66 on the chromosome, and the outer-membrane-protein genes ospA and ospC on plasmids. The major sources of DNA for PCR amplification and sequencing were samples of the B. burgdorferi tick vector Ixodes scapularis, collected at a field site in an endemic region of the north-eastern United States, and the B. afzelii vector Ixodes ricinus, collected at a similar site in southern Sweden. The sequences were compared with those of reference strains and skin biopsy isolates, as well as database sequences. For B. burgdorferi, 10-13 alleles for each of the 4 loci, and a total of 9 distinct clonal lineages with linkage of all 4 loci, were found. For B. afzelii, 2 loci, ospC and IGS, were examined, and 11 IGS genotypes, 12 ospC alleles, and a total of 9 linkage groups were identified. The genetic variants of B. burgdorferi and B. afzelii among samples from the field sites accounted for the greater part of the genetic diversity previously reported from larger areas of the north-eastern United States and central and northern Europe. Although ospC alleles of both species had higher nucleotide diversity than other loci, the ospC locus showed evidence of intragenic recombination and was unsuitable for phylogenetic inference. In contrast, there was no detectable recombination at the IGS locus of B. burgdorferi. Moreover, beyond the signature nucleotides that specified 10 IGS genotypes, there were additional nucleotide polymorphisms that defined a total of 24 subtypes. Maximum-likelihood and parsimony cladograms of B. burgdorferi aligned IGS sequences revealed the subtype sequences to be terminal branches of clades, and the existence of at least three monophyletic lineages within B. burgdorferi. It is concluded that B. burgdorferi and B. afzelii have greater genetic diversity than had previously been estimated, and that the IGS locus alone is sufficient for strain typing and phylogenetic studies.

Differential immune response to the variable surface loop antigen of P66 of Borrelia burgdorferi sensu lato species in geographically diverse populations of lyme borreliosis patients

We have studied the immune response to a variable surface-exposed loop region of the P66 outer membrane protein from Borrelia burgdorferi sensu lato by using an enzyme immunoassay. Lyme borreliosis populations found in North America and Sweden were preferentially more seroreactive to P66 from their respective regional species, namely, B. burgdorferi sensu stricto and B. garinii and B. afzelii, respectively.

Isolation and characterization of Borrelia parkeri in Ornithodoros parkeri (Ixodida: Argasidae) collected in Colorado

This study describes the identification of Borrelia parkeri spirochetes in Colorado. Two isolates of B. parkeri (6230 and 6232) were recovered from Ornithodoros parkeri Cooley ticks collected at an inactive prairie dog town in Moffat County. Both isolates were partially characterized by sequencing and subsequent parsimony and neighbor-joining analyses of appropriate regions of the 16S ribosomal RNA, flagellin and P66 genes. Analyses of the 16S gene sequences from the Colorado isolates indicated that they were more closely related to B. parkeri and B. tucatae than to B. hermsii or the other species of Borrelia investigated in this study. Additional analyses of amino acid sequences for flagellin and P66, however, clearly demonstrated that isolates 6230 and 6232 were most closely related to B. parkeri. The possible significance of B. parkeri as an agent of human disease is discussed.

Non-heritable change of a spirochaete's phenotype by decoration of the cell surface with exogenous lipoproteins

Genetic transformation of Borrelia spp. is limited in development and has found application in only one species. For a non-genetic approach for manipulating the phenotype of these spirochaetes, we determined whether exogenous recombinant lipoproteins would incorporate in the cell's outer membrane. Using unlabelled or 125I-labelled Osp proteins, Osp-specific monoclonal antibodies, proteinase K and formaldehyde as reagents, we found that decoration of spirochaetes had the following characteristics. (i) Purified recombinant OspA or OspD lipoproteins associated with Borrelia burgdorferi and B. hermsii cells that lacked abundant lipoproteins of their own. (ii) This decoration of the cells with exogenous OspA did not affect cell's viability. (iii) The decoration was concentration and temperature dependent and stable for at least 24 h. (iv) Like native OspA, the recombinant OspA decorating the cells was accessible to antibodies and proteases and could be cross-linked to the integral outer membrane protein, P66. (v) Decoration of viable B. burgdorferi and B. hermsii with OspA rendered the cells susceptible to killing by OspA-specific antiserum. Such non-genetic alteration of the surface of a bacterium may be used to study functions and properties of lipoproteins in situ.

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

Vsp surface lipoproteins are serotype-defining antigens of relapsing fever spirochetes that undergo multiphasic antigenic variation to avoid the immune response. One of these proteins, VspA of Borrelia turicatae, is also associated with neurotropism in infected mice. Vsp proteins are highly polymorphic in sequence, which may relate to their specific antibody reactivities and host cell interactions. To determine whether sequence variations affect protein structure, we compared B. turicatae VspA with three related proteins: VspB of B. turicatae, Vsp26 of the relapsing fever agent Borrelia hermsii, and OspC of the Lyme disease spirochete Borrelia burgdorferi. Recombinant non-lipidated proteins were purified by affinity or ion exchange chromatography. Circular dichroism spectra revealed similar, highly alpha-helical secondary structures for all four proteins. In vitro assays demonstrated protease-resistant, thermostable Vsp cores starting at a conserved serine at position 34 (Ser(34)). All proteins aggregate as dimers in solution. In situ trypsin treatment and surface protein cross-linking showed that the native lipoproteins also form protease-resistant dimers. These findings indicate that Vsp proteins have a common compact fold and that their established functions are based on localized polymorphisms. Two forms of VspA crystals suitable for structure determination by x-ray diffraction methods have been obtained.

Genetic characteristics of Borrelia coriaceae isolates from the soft tick Ornithodoros coriaceus (Acari: Argasidae)

Two Borrelia isolates (CA434 and CA435) cultured from the soft tick Ornithodoros coriaceus were analyzed by contour-clamped homogeneous electric field gel electrophoresis of unrestricted and ApaI-restricted DNA, standard electrophoresis of BamHI- and HindIII-restricted DNA, Southern hybridization, restriction fragment length polymorphism and sequencing of the 16S rRNA gene, and amplification of the 5S-23S intergenic spacer region. These isolates were compared with Borrelia coriaceae type strain Co53, B. burgdorferi sensu stricto strain CA4, and the relapsing-fever spirochete B. parkeri (undesignated). The 16S rRNA region of CA434 and CA435 differed from that of B. coriaceae type strain Co53 by the presence of 1 base (C) at position 367 (GenBank accession no. U42286). The linear plasmid profile of CA434 was similar to that of Co53, and the ApaI, BamHI, and HindIII restriction fingerprints of the total cellular DNA of CA434 and Co53 were similar. In contrast, CA435 differed somewhat from CA434 and Co53, which demonstrates that B. coriaceae is genetically diverse. Southern hybridization showed that the DNAs of CA434 and CA435 hybridized strongly with the digoxigenin-labeled DNA of Co53. Low homology was found between the DNA of Co53 and that of B. parkeri. The 16S rRNA sequence of B. parkeri was identical to previously published results for B. parkeri strain M3001 (GenBank accession number U42296). CA434 and CA435 represent only the second and third isolates of B. coriaceae obtained from any source since its initial isolation from an O. coriaceus tick in 1985. All three B. coriaceae isolates were derived from adult ticks collected from the same locality in northwestern California. Difficulties encountered in detecting B. coriaceae in, and isolating this spirochete from, the tissues of O. coriaceus are discussed. The lack of concordance between different detection or isolation methods suggests that reliance upon a single technique may grossly underestimate the true prevalence of spirochetal infection in wild-caught O. coriaceus ticks.

Protection elicited by native outer membrane protein Oms66 (p66) against host-adapted Borrelia burgdorferi: conformational nature of bactericidal epitopes

Oms66 is a Borrelia burgdorferi outer membrane porin protein whose role in Lyme disease pathogenesis and immunity has not been well established. Oms66 was solubilized from whole-cell lysates of strain B313 (which is derived from B31 but lacks OspA, -B, -C, and -D) and purified to homogeneity by fast-protein liquid chromatography. Purified native Oms66 (nOms66), which retained the ability to form large channels in a planar lipid bilayer model membrane system, and denatured Oms66 (hOms66) were used to immunize New Zealand White rabbits. The resulting Oms66 antisera were tested in a complement-dependent borreliacidal assay in parallel with basal serum and with serum from rabbits immune to reinfection with B. burgdorferi (IRS). IRS showed high-titer complement-dependent killing of both strains B31 and B313. Sera from animals immunized with nOms66 showed high-titer complement-dependent killing activity against strain B313 but exhibited no killing of B31. By comparison, serum generated from immunizations with hOms66 showed no killing activity against either strain. Following adsorption of antiserum to nOms66 with recombinant Oms66 (rOms66), the serum antibodies no longer bound to rOms66 or to nOms66 that had been denatured with 8 M urea. However, the antibodies still bound to nOms66 and killing activity against B313 was retained, thus suggesting that native, conformational epitopes are targets of this bactericidal activity. Six C3H HeJ mice were immunized with nOms66 and were challenged using "host-adapted" B. burgdorferi B31 by skin implantation of infected mouse ear tissue. Four of the six mice were protected against both localized and disseminated infection. These findings indicate that native Oms66 can elicit potent bactericidal activity and significant protective immunity against host-adapted organisms.

Identification of a candidate glycosaminoglycan-binding adhesin of the Lyme disease spirochete Borrelia burgdorferi

Binding of glycosaminoglycans (GAGs) by Borrelia burgdorferi, the Lyme disease spirochete, has the potential to promote the colonization of diverse tissues. GAG binding by B. burgdorferi is associated with haemagglutination and we have identified a 26 kDa protein, which we have termed Bgp (Borrelia GAG-binding protein), on the basis of its ability to bind to heparin and erythrocytes. Bgp was found in outer membrane fractions of B. burgdorferi and on the surface of intact bacteria, as assayed by labelling with a membrane-impermeable biotinylating agent or anti-Bgp antibodies. Purified recombinant Bgp agglutinated erythrocytes, binds to the same spectrum of GAGs as the B. burgdorferi strain from which the cloned bgp sequence was obtained, and inhibited B. burgdorferi binding to purified GAGs and to cultured mammalian cells. Thus, Bgp is a strong candidate for a GAG-binding adhesin of B. burgdorferi.

Scored antibody reactivity determined by immunoblotting shows an association between clinical manifestations and presence of Borrelia burgdorferi sensu stricto, B. garinii, B. afzelii, and B. Valaisiana in humans

An immunoglobulin G immunoblot was developed with antigenic extracts of Borrelia burgdorferi sensu stricto, B. garinii, B. afzelii, and B. valaisiana genospecies and was reacted with sera from patients with neuroborreliosis, acrodermatitis, and Lyme arthritis. A detailed analysis of the reactivities of the protein bands was performed, and a two-step scoring procedure was selected to determine the preferential reactivity of sera to one particular genospecies. The discriminative potential of 5 proteins (12-kDa, 16-kDa, 18-kDa, OspA, and 66-kDa proteins) was used as a rapid first-step scoring method, followed by scoring of 14 additional protein bands if necessary. The advantage of this procedure is the low percentage of serum samples with inconclusive results for one of the four species (10% for patients with neuroborreliosis, 6% for patients with acrodermatitis chronica atrophicans, and 6% for patients with Lyme arthritis). Among 31 serum samples from patients with neuroborreliosis, 16 were more reactive to B. garinii, 7 were more reactive to B. afzelii, 3 were more reactive to B. valaisiana, and 2 were more reactive to B. burgdorferi sensu stricto. Of 31 serum samples from patients with acrodermatitis, 26 showed a higher level of reactivity to B. afzelii. Of 34 serum samples from patients with Lyme arthritis, 21 were more reactive to B. burgdorferi sensu stricto, 10 were more reactive to B. afzelii, and 1 was more reactive to B. valaisiana. Our results suggest an organotropism of Borrelia species and provide some evidence of a pathogenic potential of B. valaisiana in humans.

Characterization of a candidate Borrelia burgdorferi beta3-chain integrin ligand identified using a phage display library

The spirochaetal agents of Lyme disease, Borrelia burgdorferi (sensu lato) bind to integrins alphaIIbbeta3, alphavbeta3 and alpha5beta1 in purified form and on the surfaces of human cells. Using a phage display library of B. burgdorferi (sensu stricto) DNA, a candidate ligand for beta3-chain integrins was identified. The native B. burgdorferi protein, termed p66, is known to be recognized by human Lyme disease patient sera and to be expressed on the surface of the spirochaete. We show here that recombinant p66 binds specifically to beta3-chain integrins and inhibits attachment of intact B. burgdorferi to the same integrins. When expressed on the surface of Escherichia coli, this protein increases the attachment of E. coli to a transfected cell line that expresses alphavbeta3, but not to the parental cell line, which expresses no beta3-chain integrins. Localization of p66 on the surface of B. burgdorferi, the ability of recombinant forms of the protein to bind to beta3-chain integrins and the fact that p66 and B. burgdorferi bind to beta3-chain integrins in a mutually exclusive manner make p66 an attractive candidate bacterial ligand for integrins alphaIIbbeta3 and alphavbeta3.

The Treponema denticola major sheath protein is predominantly periplasmic and has only limited surface exposure

The recent discovery that the Treponema pallidum genome encodes 12 orthologs of the Treponema denticola major sheath protein (Msp) prompted us to reexamine the cellular location and topology of the T. denticola polypeptide. Experiments initially were conducted to ascertain whether Msp forms an array on or within the T. denticola outer membrane. Transmission electron microscopy (EM) of negatively stained and ultrathin-sectioned organisms failed to identify a typical surface layer, whereas freeze-fracture EM revealed that the T. denticola outer membrane contains heterogeneous transmembrane proteins but no array. In contrast, a lattice-like structure was observed in vesicles released from mildly sonicated treponemes; combined EM and biochemical analyses demonstrated that this structure was the peptidoglycan sacculus. Immunoelectron microscopy (IEM) subsequently was performed to localize Msp in T. denticola. Examination of negatively stained whole mounts identified substantial amounts of Msp in sonicated organisms. IEM of ultrathin-sectioned, intact treponemes also demonstrated that the preponderance of antigen was unassociated with the outer membrane. Lastly, immunofluorescence analysis of treponemes embedded in agarose gel microdroplets revealed that only minor portions of Msp are surface exposed. Taken as a whole, our findings challenge the widely held belief that Msp forms an array within the T. denticola outer membrane and demonstrate, instead, that it is predominantly periplasmic with only limited surface exposure. These findings also have implications for our evolving understanding of the contribution(s) of Msp/Tpr orthologs to treponemal physiology and disease pathogenesis.

Access of antibody or trypsin to an integral outer membrane protein (P66) of Borrelia burgdorferi is hindered by Osp lipoproteins

The outer membrane of Borrelia burgdorferi, the Lyme disease agent, contains lipoproteins anchored by their lipid moieties and integral proteins with membrane-spanning regions. We used the techniques of in situ proteolysis, immunofluorescence, in vitro growth inhibition, and cross-linking with formaldehyde to characterize topological relationships between P66, an integral membrane protein, and selected Osp lipoproteins of B. burgdorferi. Protease treatment of intact spirochetes cleaved P66 and Osp proteins but not the periplasmic flagellin or the BmpA protein of the cytoplasmic membrane. P66 of cells lacking OspA, OspB, and OspC was more susceptible to trypsin cleavage than was P66 of cells with these Osp proteins. A monoclonal antibody against the surface loop of P66 bound, agglutinated, and inhibited the growth of viable spirochetes lacking OspA, OspB, OspC, and OspD but not of the cells that expressed OspA, OspC, and/or OspD. When cells were fixed, the antibody bound to cells that express OspD and OspC but still not to cells with OspA. The close association of OspA and P66 was confirmed by the crosslinking of the two proteins by formaldehyde. These results show that Osp proteins, particularly OspA, limit the access of antibody or trypsin to the surface loop region of P66. The proximity and possible contact between P66 and OspA (or other Osp proteins) may hinder the effectiveness of antibodies to what otherwise would be an appropriate vaccine target.