Transcriptional and translational regulation of the expression of the major outer surface proteins in Lyme disease Borrelia strains

Inducible CRISPRi-Based Operon Silencing and Selective in Trans Gene Complementation in Borrelia burgdorferi

To accelerate genetic studies on the Lyme disease pathogen Borrelia burgdorferi, we developed an enhanced CRISPR interference (CRISPRi) approach for isopropyl-β-d-thiogalactopyranoside (IPTG)-inducible repression of specific B. burgdorferi genes. The entire system is encoded on a compact 11-kb shuttle vector plasmid that allows for inducible expression of both the sgRNA module and a nontoxic codon-optimized dCas9 protein. We validated this CRISPRi system by targeting the genes encoding OspA and OspB, abundant surface lipoproteins coexpressed by a single operon, and FlaB, the major subunit forming the periplasmic flagella. As in other systems, single guide RNAs (sgRNAs) complementary to the nontemplate strand were consistently effective in gene repression, with 4- to 994-fold reductions in targeted transcript levels and concomitant reductions of protein levels. Furthermore, we showed that ospAB knockdowns could be selectively complemented in trans for OspA expression via the insertion of CRISPRi-resistant, synonymously or nonsynonymously mutated protospacer adjacent motif (PAM*) ospA alleles into a unique site within the CRISPRi plasmid. Together, this establishes CRISPRi PAM* as a robust new genetic tool to simplify the study of B. burgdorferi genes, bypassing the need for gene disruptions by allelic exchange and avoiding rare codon toxicity from the heterologous expression of dCas9. IMPORTANCE Borrelia burgdorferi, the spirochetal bacterium causing Lyme disease, is a tick-borne pathogen of global importance. Here, we expand the genetic toolbox for studying B. burgdorferi physiology and pathogenesis by establishing a single plasmid-based, fully inducible, and nontoxic CRISPR interference (CRISPRi) system for transcriptional silencing of B. burgdorferi genes and operons. We also show that alleles of CRISPRi-targeted genes with mutated protospacer-adjacent motif (PAM*) sites are CRISPRi resistant and can be used for simultaneous in trans gene complementation. The CRISPRi PAM* system will streamline the study of essential Borrelia proteins and accelerate investigations into their structure-function relationships.

Strong interactions between Salp15 homologues from the tick I. ricinus and distinct types of the outer surface OspC protein from Borrelia

Ticks belonging to the genus Ixodes are parasites feeding on vertebrate blood and vectors for many pathogenic microbes, including Borrelia burgdorferi sensu lato spirochetes, the causative agent of Lyme borreliosis. The tick saliva contains a mixture of bioactive molecules showing a wide range of properties for efficient engorgement. One of the most extensively studied components of tick saliva is a 15-kDa salivary gland protein (Salp15) from Ixodes scapularis. This multifunctional protein suppresses the immune response of hosts through pleiotropic action on a few crucial defense pathways. Salp15 and its homologue from I. ricinus Iric1 have been also shown to bind to Borrelia burgdorferi sensu stricto outer surface protein C (OspC) permitting the spirochetes to evade antibody-mediated killing in the human host. Further studies revealed that Salp15 and Iric1 protected B. burgdorferi s. s. and B. garinii expressing OspC against the complement system. OspC is the most variable protein on the outer surface of Borrelia, which in addition to Salp15 can also bind other ligands, such as plasminogen, fibrinogen, fibronectin or complement factor 4. So far several OspC variants produced by B. burgdorferi s. l. spirochetes were shown to be capable of binding Salp15 or its homologue, but the protection against borreliacidal antibodies has only been proven in the case of B. burgdorferi s. s. The question of Salp15 contribution to Borrelia survival during the infection has been comprehensively studied during the last decades. In contrast, the organization of the OspC-Salp15 complex has been poorly explored. This report describes the binding between three Salp15 homologues from the tick Ixodes ricinus (Iric1, Iric2 and Iric3) and OspC from four B. burgdorferi sensu lato strains in terms of the binding parameters, analyzed with two independent biophysical methods - Microscale thermophoresis (MST) and Biolayer interferometry (BLI). The results of both experiments show a binding constant at the nanomolar level, which indicates very strong interactions. While the Iric1-OspC binding has been reported before, we show in this study that also Iric2 and Iric3 are capable of OspC binding with high affinity. This observation suggests that these two Salp15 homologues might be used by B. burgdorferi s. l. in a way analogous to Iric1. A comparison of the results from the two methods let us propose that N-terminal immobilization of OspC significantly increases the affinity between the two proteins. Finally, our results indicate that the Iric binding site is located in close proximity of the OspC epitopes recognized by human antibodies, which may have important biological and medical implications.

Borrelia outer surface protein C is capable of human fibrinogen binding

Outer surface protein C (OspC) is one of the most abundant surface lipoproteins produced during early infection by the Borrelia spirochete, the causative agent of Lyme disease. The high sequence variability of the ospC gene results in the production of several and strongly divergent OspC types. One of the known roles of OspC is the recruitment of blood components, including complement regulators, to facilitate the bloodstream survival of Borrelia at an essential stage of host infection. Here, we identify and describe a new interaction between OspC and human fibrinogen. To test the ability of OspC to bind fibrinogen, we developed a microscale thermophoresis assay using four fluorescently labeled types of OspC. We show that OspC binds fibrinogen tightly, with nanomolar K_d , and that the binding depends on the OspC type. The binding assays combined with SAXS studies allowed us to map the OspC-binding site on the fibrinogen molecule. Spectrometric measurements of fibrinogen clotting in the presence of OspC indicate that OspC negatively influences the clot formation process. Taken together, our findings are consistent with the hypothesis that OspC interacts with blood protein partners to facilitate Borrelia spreading by the hematogenous route.

Two Distinct Mechanisms Govern RpoS-Mediated Repression of Tick-Phase Genes during Mammalian Host Adaptation by Borrelia burgdorferi, the Lyme Disease Spirochete

The alternative sigma factor RpoS plays a key role modulating gene expression in Borrelia burgdorferi, the Lyme disease spirochete, by transcribing mammalian host-phase genes and repressing σ⁷⁰-dependent genes required within the arthropod vector. To identify cis regulatory elements involved in RpoS-dependent repression, we analyzed green fluorescent protein (GFP) transcriptional reporters containing portions of the upstream regions of the prototypical tick-phase genes ospAB, the glp operon, and bba74. As RpoS-mediated repression occurs only following mammalian host adaptation, strains containing the reporters were grown in dialysis membrane chambers (DMCs) implanted into the peritoneal cavities of rats. Wild-type spirochetes harboring ospAB- and glp-gfp constructs containing only the minimal (−35/−10) σ⁷⁰ promoter elements had significantly lower expression in DMCs relative to growth in vitro at 37°C; no reduction in expression occurred in a DMC-cultivated RpoS mutant harboring these constructs. In contrast, RpoS-mediated repression of bba74 required a stretch of DNA located between −165 and −82 relative to its transcriptional start site. Electrophoretic mobility shift assays employing extracts of DMC-cultivated B. burgdorferi produced a gel shift, whereas extracts from RpoS mutant spirochetes did not. Collectively, these data demonstrate that RpoS-mediated repression of tick-phase borrelial genes occurs by at least two distinct mechanisms. One (e.g., ospAB and the glp operon) involves primarily sequence elements near the core promoter, while the other (e.g., bba74) involves an RpoS-induced transacting repressor. Our results provide a genetic framework for further dissection of the essential “gatekeeper” role of RpoS throughout the B. burgdorferi enzootic cycle.

Borrelia burgdorferi, the Lyme disease spirochete, modulates gene expression to adapt to the distinctive environments of its mammalian host and arthropod vector during its enzootic cycle. The alternative sigma factor RpoS has been referred to as a “gatekeeper” due to its central role in regulating the reciprocal expression of mammalian host- and tick-phase genes. While RpoS-dependent transcription has been studied extensively, little is known regarding the mechanism(s) of RpoS-mediated repression. We employed a combination of green fluorescent protein transcriptional reporters along with an in vivo model to define cis regulatory sequences responsible for RpoS-mediated repression of prototypical tick-phase genes. Repression of ospAB and the glp operon requires only sequences near their core promoters, whereas modulation of bba74 expression involves a putative RpoS-dependent repressor that binds upstream of the core promoter. Thus, Lyme disease spirochetes employ at least two different RpoS-dependent mechanisms to repress tick-phase genes within the mammal.

Outer Membrane Proteins BB0405 and BB0406 Are Immunogenic, but Only BB0405 Is Required for Borrelia burgdorferi Infection

We recently identified the Borrelia burgdorferi outer membrane protein (OMP) BB0406 and found that the gene encoding this OMP was cotranscribed with the gene encoding the OMP BB0405. Interestingly, BB0405 and BB0406 share 59% similarity and are grouped into the same B. burgdorferi paralogous gene family. Given their overall similarity, it is plausible that both OMPs have similar or overlapping functions in this pathogenic spirochete. BB0405 was recently shown to be required for mammalian infection despite the observations that BB0405 is poorly immunogenic and not recognized during mouse or human infection. BB0405 orthologs have also been shown to bind the complement regulator protein factor H. Therefore, to better elucidate the role of BB0405 and its paralog BB0406 during infection and in serum resistance, we examined both proteins in animal infection, factor H binding, and serum sensitivity assays. Our combined results suggest that BB0405- and BB0406-specific antibodies are borreliacidal and that both OMPs are immunogenic during nonhuman primate infection. Additionally, while BB0405 was found to be required for establishing mouse infection, BB0406 was not found to be essential for infectivity. In contrast to data from previous reports, however, neither OMP was found to bind human factor H or to be required for enhancing serum resistance of B. burgdorferi in vitro.

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.

More specific bands in the IgG western blot in sera from Scottish patients with suspected Lyme borreliosis

Aims

To identify further Western blot bands that may be specific in the diagnosis of Lyme borreliosis.

Methods

The Borrelia burgdorferi antibody profiles of 270 western blot positive patients and 241 western blot negative patients from 2008 were examined.

Results

27 different non-specific bands were detected in both groups. Six of 27 (22%) of the non-specific bands were detected significantly more in the western blot positive patients compared to the western blot negative patients (20 kDa, p<0.0001; 28 kDa, p<0.002; 36 kDa, p<0.002; 37 kDa, p<0.007; 48 kDa, p<0.023; 56 kDa, p<0.028; two-tailed F test).

Conclusion

Results suggest that the 20, 28 and 48 kDa bands should be regarded as specific.

T-cell epitope mapping of the Borrelia garinii outer surface protein A in lyme neuroborreliosis

We studied the T-cell reactivity to overlapping peptides of B. garinii OspA, in order to locate possible immunodominant T-cell epitopes in neuroborreliosis. Cells from cerebrospinal fluid (CSF) and blood from 39 patients with neuroborreliosis and 31 controls were stimulated with 31 overlapping peptides, and interferon-gamma secreting cells were detected by ELISPOT. The peptides OspA(17-36), OspA(49-68), OspA(105-124), OspA(137-156), OspA(193-212) and OspA(233-252) showed the highest frequency of positive responses, being positive in CSF from 38% to 50% of patients with neuroborreliosis. These peptides also elicited higher responses in CSF compared with controls (P = 0.004). CSF cells more often showed positive responses to these peptides than blood cells (P = 0.001), in line with a compartmentalization to the central nervous system. Thus, a set of potential T-cell epitopes were identified in CSF cells from patients with neuroborreliosis. Further studies may reveal whether these epitopes can be used diagnostically and studies involving HLA interactions may show their possible pathogenetic importance.

Genetic diversity of the outer surface protein C gene of southern Borrelia isolates and its possible epidemiological, clinical, and pathogenetic implications

The ospC genes of 20 southern Borrelia strains were sequenced. The strains consisted of B. burgdorferi sensu stricto, B. andersonii, B. bissettii, one undescribed genospecies, MI-8, and one probably new Borrelia species, TXW-1. A high degree of similarity exists between B. burgdorferi sensu stricto and B. bissettii and between B. bissettii and B. andersonii. Lateral transfers of the ospC gene probably occurred between B. burgdorferi sensu stricto and B. bissettii and between B. bissettii and B. andersonii. Internal gene recombination appears to occur among them. The highest degree of genetic diversity among them was observed in the two variable domains (V1 and V2), semivariable domain (SV), and the species-specific epitopes (between amino acids 28 and 31). Differences in ospC sequences among southern strains reflect diversity at the strain and genospecies levels. MI-8, which was recognized as an undescribed genospecies in our previous reports, remains distinguishable in our current analysis of ospC genes and is distinct from B. burgdorferi sensu stricto. Interestingly, another undescribed southern isolate, TXW-1, was not amplified under various PCR conditions. Compared to European B. burgdorferi sensu stricto strains, American B. burgdorferi sensu stricto strains show greater genetic heterogeneity. Southern B. burgdorferi sensu stricto, B. andersonii, and B. bissettii isolates were intermixed with each other in the phylogenetic trees. In the derived trees in our work, at least one southeastern strain of B. burgdorferi, MI-2, most closely aligns with a so-called invasive cluster that possesses many proven human-invasive strains. Transmission experiments show that MI-2 and the strains in this group of southern spirochetes are able to infect mice and hamsters and that the typical vector of Lyme disease, Ixodes scapularis, can acquire the spirochetes from infected mammals. Currently, strain MI-2 appears to be the only southern isolate among the 20 we analyzed that clusters with an OspC invasive group and thus might be invasive for humans.

DNA microarray analysis of differential gene expression in Borrelia burgdorferi, the Lyme disease spirochete

DNA microarrays were used to survey the adaptive genetic responses of Borrelia burgdorferi (Bb) B31, the Lyme disease spirochete, when grown under conditions analogous to those found in unfed ticks (UTs), fed ticks (FTs), or during mammalian host adaptation (Bb in dialysis membrane chambers implanted in rats). Microarrays contained 95.4% of the predicted B31 genes, 150 (8.6%) of which were differentially regulated (changes of > or = 1.8-fold) among the three growth conditions. A substantial proportion (46%) of the differentially regulated genes encoded proteins with predicted export signals (29% from predicted lipoproteins), emphasizing the importance to Bb of modulating its extracellular proteome. For B31 cultivated at the more restrictive UT condition, microarray data provided evidence of a bacterial stringent response and factors that restrict cell division. A large proportion of genes were responsive to the FT growth condition, wherein increased temperature and reduced pH were prominent environmental parameters. A surprising theme, supported by cluster analysis, was that many of the gene expression changes induced during the FT growth condition were transient and largely tempered as B31 adapted to the mammalian host, suggesting that once Bb gains entry and adapts to mammalian tissues, fewer differentially regulated genes are exploited. It therefore would seem that although widely dissimilar, the UT and dialysis membrane chamber growth conditions promote more static patterns of gene expression in Bb. The microarray data thus provide a basis for formulating new testable hypotheses regarding the life cycle of Bb and attaining a more complete understanding of many aspects of Bb's complex parasitic strategies.

Purification and characterization of Borrelia burgdorferi from feeding nymphal ticks (Ixodes scapularis)

Here we describe a protocol for purifying Borrelia burgdorferi from feeding ticks by velocity centrifugation and Percoll density gradient centrifugation. The purified spirochetes were motile and 10- to 20-fold purer than the bacteria in crude tick homogenates. The purified bacteria were present in sufficient quantity for protein and gene expression studies. In comparison to culture-grown bacteria, tick-borne spirochetes had several proteins that were upregulated and a few that were downregulated. When the levels of B. burgdorferi outer surface proteins OspA and OspC were measured, OspC protein and mRNA levels were lower in cultured bacteria than in bacteria purified from ticks. Although differences in OspA mRNA levels were observed between cultured and tick-borne bacteria, no differences were observed at the protein level. These experiments demonstrate that tick-transmitted borreliae display a gene expression and antigen profile different from that of spirochetes cultured in vitro.

P13, an integral membrane protein of Borrelia burgdorferi, is C-terminally processed and contains surface-exposed domains

To elucidate antigens present on the bacterial surface of Borrelia burgdorferi sensu lato that may be involved in pathogenesis, we characterized a protein, P13, with an apparent molecular mass of 13 kDa. The protein was immunogenic and was expressed in large amounts during in vitro cultivation compared to other known antigens. An immunofluorescence assay, immunoelectron microscopy, and protease sensitivity assays indicated that P13 is surface exposed. The deduced sequence of the P13 peptide revealed a possible signal peptidase type I cleavage site, and computer analysis predicted that P13 is an integral membrane protein with three transmembrane-spanning domains. Mass spectrometry, in vitro translation, and N- and C-terminal amino acid sequencing analyses indicated that P13 was posttranslationally processed at both ends and modified by an unknown mechanism. Furthermore, p13 belongs to a gene family with five additional members in B. burgdorferi sensu stricto. The p13 gene is located on the linear chromosome of the bacterium, in contrast to five paralogous genes, which are located on extrachromosomal plasmids. The size of the p13 transcript was consistent with a monocistronic transcript. This new gene family may be involved in functions that are specific for this spirochete and its pathogenesis.

Crystal structure of outer surface protein C (OspC) from the Lyme disease spirochete, Borrelia burgdorferi

Outer surface protein C (OspC) is a major antigen on the surface of the Lyme disease spirochete, Borrelia burgdorferi, when it is being transmitted to humans. Crystal structures of OspC have been determined for strains HB19 and B31 to 1.8 and 2.5 A resolution, respectively. The three-dimensional structure is predominantly helical. This is in contrast to the structure of OspA, a major surface protein mainly present when spirochetes are residing in the midgut of unfed ticks, which is mostly beta-sheet. The surface of OspC that would project away from the spirochete's membrane has a region of strong negative electrostatic potential which may be involved in binding to positively charged host ligands. This feature is present only on OspCs from strains known to cause invasive human disease.

Expression and immunological analysis of the plasmid-borne mlp genes of Borrelia burgdorferi strain B31

A lipoprotein gene family first identified in Borrelia burgdorferi strain 297, designated 2.9 LP and recently renamed mlp, was found on circular and linear plasmids in the genome sequence of B. burgdorferi strain B31-M1. Sequence analyses of the B31 mlp genes and physically linked variant gene families indicated that mlp gene heterogeneity is unique and unrelated to location or linkage to divergent sequences. Evidence of recombination between B31 mlp alleles was also detected. Northern blot analysis of cultured strain B31 indicated that the mlp genes were not expressed at a temperature (23 degrees C) characteristic of that of ticks in the environment. In striking contrast, expression of many mlp genes increased substantially when strain B31 was shifted to 35 degrees C, a temperature change mimicking that occurring in the natural transmission cycle of the spirochete from tick to mammal. Primer extension analysis of the mlp mRNA transcripts suggested that sigma 70-like promoters are involved in mlp expression during temperature shift conditions. Antibodies were made against strain B31 Mlp proteins within the first 4 weeks after experimental mouse infection. Importantly, Lyme disease patients also had serum antibodies reactive with purified recombinant Mlp proteins from strain B31, a result indicating that humans are exposed to Mlp proteins during infection. Taken together, the data indicate that strain B31 mlp genes encode a diverse array of lipoproteins which may participate in early infection processes in the mammalian host.

Decorin-binding protein A (DbpA) of Borrelia burgdorferi is not protective when immunized mice are challenged via tick infestation and correlates with the lack of DbpA expression by B. burgdorferi in ticks

Previous studies showed that decorin-binding protein A (DbpA) of Borrelia burgdorferi was a protective immunogen in the murine model of Lyme borreliosis when mice were challenged (needle inoculated) intradermally with in vitro-cultivated spirochetes. In the present study, DbpA-immunized C3H/HeJ mice were not protected from infection when infested with Ixodes scapularis nymphs harboring virulent B. burgdorferi 297. This lack of protection correlated with the failure to detect DbpA on B. burgdorferi in ticks, suggesting that DbpA is not available as a target for bactericidal antibodies in serum when B. burgdorferi-infected ticks take their blood meal from an immunized host. The failure of DbpA immunization to protect tick-challenged mice contradicts the results of earlier needle inoculation vaccination experiments and suggests that DbpA may not be suitable as a Lyme disease vaccine.

Genetic diversity of ospC in a local population of Borrelia burgdorferi sensu stricto

The outer surface protein, OspC, is highly variable in Borrelia burgdorferi sensu stricto, the agent of Lyme disease. We have shown that even within a single population OspC is highly variable. The variation of ospA and ospC in the 40 infected deer ticks collected from a single site on Shelter Island, New York, was determined using PCR-SSCP. There is very strong apparent linkage disequilibrium between ospA and ospC alleles, even though they are located on separate plasmids. Thirteen discernible SSCP mobility classes for ospC were identified and the DNA sequence for each was determined. These sequences, combined with 40 GenBank sequences, allow us to define 19 major ospC groups. Sequences within a major ospC group are, on average, <1% different from each other, while sequences between major ospC groups are, on average, approximately 20% different. The tick sample contains 11 major ospC groups, GenBank contains 16 groups, with 8 groups found in both samples. Thus, the ospC variation within a local population is almost as great as the variation of a similar-sized sample of the entire species. The Ewens-Watterson-Slatkin test of allele frequency showed significant deviation from the neutral expectation, indicating balancing selection for these major ospC groups. The variation represented by major ospC groups needs to be considered if the OspC protein is to be used as a serodiagnostic antigen or a vaccine.

Differential expression of Borrelia burgdorferi proteins during growth in vitro

In an earlier paper we described the transcriptionally regulated differential levels of expression of two lipoproteins of Borrelia burgdorferi, P35 and P7.5, during growth of the spirochetes in culture from logarithmic phase to stationary phase (K. J. Indest, R. Ramamoorthy, M. Solé, R. D. Gilmore, B. J. B. Johnson, and M. T. Philipp, Infect. Immun. 65:1165-1171, 1997). Here we further assess this phenomenon by investigating whether the expression of other antigens of B. burgdorferi, including some well-characterized ones, are also regulated in a growth-phase-dependent manner in vitro. These studies revealed 13 additional antigens, including OspC, BmpD, and GroEL, that were upregulated 2- to 66-fold and a 28-kDa protein that was downregulated 2- to 10-fold, during the interval between the logarithmic- and stationary-growth phases. Unlike with these in vitro-regulated proteins, the levels of expression of OspA, OspB, P72, flagellin, and BmpA remained unchanged throughout growth of the spirochetes in culture. Furthermore, ospAB, bmpAB, groEL, and fla all exhibited similar mRNA profiles, which is consistent with the constitutive expression of these genes. By contrast, the mRNA and protein profiles of ospC and bmpD indicated regulated expression of these genes. While bmpD exhibited a spike in mRNA expression in early stationary phase, ospC maintained a relatively higher level of mRNA throughout culture. These findings demonstrate that there are additional genes besides P7.5 and P35 whose regulated expression can be investigated in vitro and which may thus serve as models to facilitate the study of regulatory mechanisms in an organism that cycles between an arthropod and a vertebrate host.

Decorin-binding protein of Borrelia burgdorferi is encoded within a two-gene operon and is protective in the murine model of Lyme borreliosis

Isolated outer membranes of Borrelia burgdorferi were used in immunoblotting experiments with sera from immune mice to identify new putative Lyme disease vaccine candidates. One immunoreactive polypeptide migrated on polyacrylamide gels just proximal to outer surface protein C and comigrated with [3H]palmitate-labeled polypeptides. A degenerate oligonucleotide primer based upon internal amino acid sequence information was used to detect the corresponding gene within a B. burgdorferi total genomic library. The relevant open reading frame (ORF) encoded a polypeptide comprised of a 24-amino-acid putative signal peptide terminated by LLISC, a probable consensus sequence for lipoprotein modification, and a mature protein of 163 amino acids. Immunoblots of a recombinant fusion protein corresponding to this ORF supported the idea that the encoded protein was a previously reported decorin-binding protein (DBP) of B. burgdorferi N40 (B. P. Guo, S. J. Norris, L. C. Rosenberg, and M. Höök, Infect. Immun. 63:3467-3472, 1995). However, further DNA sequencing revealed the presence of a second ORF, designated ORF-1, whose termination codon was 119 bp upstream of the dbp gene. ORF-1 also encoded a putative lipoprotein with a mature length of 167 amino acids. Northern blots, Southern blots, and primer extension analyses indicated that ORF-1 and dbp comprised a two-gene operon located on the 49-kb linear plasmid. Both proteins, which were 40% identical and 56% similar, partitioned into Triton X-114 detergent extracts of B. burgdorferi isolated outer membranes. Mice infected with B. burgdorferi produced high titers of antibodies against the ORF-1-encoded protein and DBP during both early and later stages of chronic infection. Both DBP and the ORF-1-encoded protein were sensitive to proteinase K treatment of intact borreliae, suggesting that they were surface exposed. In active immunization experiments, 78% of mice immunized with recombinant DBP were immune to challenge. While it is not clear whether the two lipoproteins encoded by the ORF-1-dbp operon have analogous decorin-binding functions in vivo, the combined studies implicate DBP as a new candidate for a human Lyme disease vaccine.

Analysis of promoters in Borrelia burgdorferi by use of a transiently expressed reporter gene

A transient chloramphenicol acetyltransferase (CAT) expression system was developed for Borrelia burgdorferi. An Escherichia coli vector containing a promoterless Streptococcus agalactiae cat gene was constructed. Promoters for ospA, ospC, and flaB were placed upstream of this cat gene, and CAT assays were performed in E. coli from these stably maintained plasmids. The plasmids with putative promoters ospA and flaB were found to be approximately 20-fold more active than were the plasmids with ospC or no promoter. The level of activity correlated well with the resistance to chloramphenicol that each plasmid provided. Next, the nonreplicative plasmid constructs were transformed by electroporation into B. burgdorferi. CAT assays were performed by both thin-layer chromatography and the fluor diffusion method. Measurement of CAT activity demonstrated that the ospA promoter was again about 20-fold more active than the promoterless cat gene. The flaB and ospC promoters increased the activity seven- and threefold, respectively, over that with the promoterless construct. This simple transient-expression assay was shown to be an effective method to study promoter function in B. burgdorferi in the absence of a well-developed genetic system.

Long-term in vitro cultivation of Borrelia burgdorferi sensu lato strains: influence on plasmid patterns, genome stability and expression of proteins

Low (7th) and high (298th/304th) in vitro passages (cultivated over a period of 3 years) of two human Borrelia burgdorferi sensu lato strains, PKo (B. afzelii) and PBi (B. garinii) were compared by pulse-field gel electrophoresis, Southern blot, sequencing of the ospA gene, SDS-PAGE and Western blot. Digestion of genomic DNA with ApaI, BssHII, KspI, MluI, SmaI and XhoI did not reveal any differences between low and high passages. The loss of two linear plasmids with sizes of 6 and 31 kbp was detected in strain PKo between passages 34-50 and 101-304, respectively, whereas the ospA-carrying plasmid remained unchanged. In contrast, analysis of linear plasmid profiles obtained from low and high passages of B. garinii strain PBi showed no differences. Sequence analysis of the ospA gene demonstrated no difference in the strain PBi and one nucleotide exchange in the strain PKo when low and high passages were compared. The observed transition (G-A) in the third codon position did not alter the amino acid sequence. However, the rate of expression of the outer surface proteins OspA, OspB and OspC of strain PKo during low and high stages of cultivation varied significantly. In summary, our data suggest that the B. burgdorferi sensu lato genome is stable during long-term in vitro cultivation.

Molecular characterization of a 6.6-kilodalton Borrelia burgdorferi outer membrane-associated lipoprotein (lp6.6) which appears to be downregulated during mammalian infection

Isolated outer membranes of Borrelia burgdorferi 297 were utilized to obtain partial amino acid sequence information for a low-molecular-weight, outer membrane-associated polypeptide. Degenerate oligonucleotide primers based upon this information were used to amplify a 100-bp probe for detection of the corresponding full-length gene within a B. burgdorferi total genomic library. The relevant open reading frame (ORF) encoded a polypeptide comprised of a 17-amino-acid putative signal peptide terminated by LFVAC, a probable consensus sequence for lipoprotein modification, and a mature protein of 51 amino acids (predicted molecular mass of 5.8 kDa). The DNA sequences of the corresponding ORFs in B. burgdorferi 297 and B31 were identical; the corresponding ORF in strain N40 differed by only one nucleotide. Assuming conventional processing and acylation, the molecular weight of the lipoprotein, designated lp6.6, is about 6,600. The lp6.6 gene, which was localized to the 49-kb linear plasmid of B. burgdorferi, subsequently was cloned and expressed in Escherichia coli as a fusion protein with glutathione S-transferase. Immunoblot analysis with monoclonal antibody 240.7 revealed that lp6.6 was identical to a low-molecular-weight, highly conserved B. burgdorferi lipoprotein reported previously (L. I. Katona, G. Beck, and G. S. Habicht, Infect. Immun. 60:4995-5003, 1992). Results of indirect immunofluorescence assays, growth inhibition assays, passive immunizations, and active immunizations indicated that this outer membrane-associated antigen is not surface exposed in B. burgdorferi. Particularly interesting was the finding that mice and rhesus monkeys chronically infected with B. burgdorferi failed to develop antibodies against this antigen. We propose that high-level expression of lp6.6 is associated with the arthropod phase of the spirochetal life cycle and that expression of the gene is downregulated during mammalian infection.

Immunity to Lyme disease: protection, pathology and persistence

Persistence of the Lyme disease spirochete, Borrelia burgdorferi, in the presence of an active immune response has been well documented. Evidence from the past year indicates that modulation of surface antigens by the spirochete may be a major mechanism for evading the immune response.

Limited surface exposure of Borrelia burgdorferi outer surface lipoproteins

We used novel immunofluorescence strategies to demonstrate that outer surface proteins (Osps) A, B and C of Borrelia burgdorferi have limited surface exposure, finding that contradicts the prevailing viewpoint that these antigens are exclusively surface exposed. Light labeling was observed when antibodies to OspA or OspB were added to motile organisms, whereas intense fluorescence was observed when the same slides were methanol-fixed and reprobed. Modest labeling also was observed when spirochetes encapsulated in agarose beads (gel microdroplets) were incubated with antibodies to these same two antigens. This contrasted with the intense fluorescence observed when encapsulated spirochetes were probed in the presence of 0.06% Triton X-100, which selectively removed outer membranes. Proteinase K (PK) treatment of encapsulated spirochetes abrogated surface labeling. However, PK-treated spirochetes fluoresced intensely after incubation with antibodies to OspA or OspB in the presence of detergent, confirming the existence of large amounts of subsurface Osp antigens. Modest surface labeling once again was detected when PK-treated spirochetes were reprobed after overnight incubation, a result consistent with the existence of a postulated secretory apparatus that shuttles lipoproteins to the borrelial surface. Last, experiments with the OspC-expressing B. burgdorferi strain 297 revealed that this antigen was barely detectable on spirochetal surfaces even though it was a major constituent of isolated outer mem- branes. We propose a model of B. burgdorferi molecular architecture that helps to explain spirochetal persistence during chronic Lyme disease.

Borrelia burgdorferi supercoiled plasmids encode multicopy tandem open reading frames and a lipoprotein gene family

DNA sequencing and Southern blot analyses of a Borrelia burgdorferi DNA fragment encoding a signal sequence led to the discovery of a genetic locus, designated 2.9, which appears to be present in at least seven copies in virulent B. burgdorferi 297. DNA sequence analysis of these regions revealed that each 2.9 locus contained an operon of four genes (ABCD) and open reading frames designated rep+ (positive strand) and rep- (negative strand) which encoded multiple repeat motifs. Downstream of the rep+ gene(s) in six of the completely cloned and sequenced 2.9 loci also were lipoprotein (LP) genes possessing highly similar signal sequences but encoding variable mature polypeptides. The lipoproteins could he separated into two classes on the basis of hydrophilicity profiles, sequence similarities, and reactivity with specific antibodies. The 2.9 loci were localized to two (20- and 30-kb) supercoiled plasmids in B. burgdorferi 297. Northern (RNA) blot analysis established that the 2.9 ABCD operon was only minimally expressed, whereas the rep- gene(s) and at least three of the seven LP genes were expressed by B. burgdorferi in vitro. A single putative promoter element was identified by RNA primer extension analysis upstream of the ABCD operon, whereas a number of potential promoter regions existed upstream of the LP genes. The combined data indicate that the ABCD operon, rep+ and rep- genes, and LP genes are separately transcribed during in vitro growth. The 2.9 loci possess a repetitiveness, diversity, and complexity not previously described for B. burgdorferi; differential expression of these genes may facilitate the spirochete's ability to survive in diverse host environments.

Diversity of OspA and OspC among cerebrospinal fluid isolates of Borrelia burgdorferi sensu lato from patients with neuroborreliosis in Germany

Neuroborreliosis is the most frequent manifestation of the second stage of Lyme borreliosis in Europe. However, only few isolates from the cerebrospinal fluid (CSF) have been characterized with controversial results. A large panel of 36 CSF isolates isolated over a 10-year period in Munich has now been analyzed for their OspA and OspC type, resulting in at least eight different types, respectively. Representatives of the different types cultivated from CSF in Munich have also been isolated from other geographical regions in Europe from CSF or ticks, suggesting a widespread distribution of pathogenic strains. A certain OspA type (type 4) was frequently observed in adults but rarely in children or ticks. Since OspA and OspC are the most promising candidates for a Borrelia vaccine, the considerable heterogeneity found among CSF isolates has important implications for development of a vaccine in Europe.