Altered murine tissue colonization by Borrelia burgdorferi following targeted deletion of linear plasmid 17-carried genes

The Consistent Tick-Vertebrate Infectious Cycle of the Lyme Disease Spirochete Enables Borrelia burgdorferi To Control Protein Expression by Monitoring Its Physiological Status

The Lyme disease spirochete, Borrelia burgdorferi, persists in nature by alternatingly cycling between ticks and vertebrates. During each stage of the infectious cycle, B. burgdorferi produces surface proteins that are necessary for interactions with the tick or vertebrate tissues it encounters while also repressing the synthesis of unnecessary proteins. Among these are the Erp surface proteins, which are produced during vertebrate infection for interactions with host plasmin, laminin, glycosaminoglycans, and components of the complement system. Erp proteins are not expressed during tick colonization but are induced when the tick begins to ingest blood from a vertebrate host, a time when the bacteria undergo rapid growth and division. Using the erp genes as a model of borrelial gene regulation, our research group has identified three novel DNA-binding proteins that interact with DNA to control erp transcription. At least two of those regulators are, in turn, affected by DnaA, the master regulator of chromosome replication. Our data indicate that B. burgdorferi has evolved to detect the change from slow to rapid replication during tick feeding as a signal to begin expression of Erp and other vertebrate-specific proteins. The majority of other known regulatory factors of B. burgdorferi also respond to metabolic cues. These observations lead to a model in which the Lyme spirochete recognizes unique environmental conditions encountered during the infectious cycle to "know" where they are and adapt accordingly.

Potential Regulatory Role in Mammalian Host Adaptation for a Small Intergenic Region of Lp17 in the Lyme Disease Spirochete

The bacterial agent of Lyme disease, Borrelia burgdorferi, relies on an intricate gene regulatory network to transit between the disparate Ixodes tick vector and mammalian host environments. We recently reported that a B. burgdorferi mutant lacking a transcriptionally active intergenic region of lp17 displayed attenuated murine tissue colonization and pathogenesis due to altered expression of multiple antigens. In this study, a more detailed characterization of the putative regulatory factor encoded by the intergenic region was pursued. In cis complemented strains featuring mutations aimed at eliminating potential protein translation were capable of full tissue colonization, suggesting that the functional product encoded by the intergenic region is not a protein as previously predicted. In trans complementation of the intergenic region resulted in elevated transcription of the sequence compared to wild type and was found to completely abolish infectivity in both immunocompetent "and immunodeficient mice. Quantitative analysis of transcription of the intergenic region by wild-type B. burgdorferi showed it to be highly induced during murine infection relative to in vitro culture. Lastly, targeted deletion of this intergenic region resulted in significant changes to the transcriptome, including genes with potential roles in transmission and host adaptation. The findings reported herein strongly suggest that this segment of lp17 serves a potentially critical role in the regulation of genes required for adaptation and persistence of the pathogen in a mammalian host.

Deletion of a Genetic Region of lp17 Affects Plasmid Copy Number in Borrelia burgdorferi

Borrelia burgdorferi, the Lyme disease pathogen, is maintained in its enzootic life cycle through complex gene regulatory pathways encoded on its uniquely fragmented genome. This genome consists of over 20 plasmids, and the regulatory mechanisms of plasmid maintenance and replication are largely unknown. The bbd21 gene, encoded on lp17 and a member of the paralogous family 32 proteins, was originally proposed to be a putative parA orthologue involved with plasmid partitioning; however, this function has not been confirmed to date. To determine the role of bbd21 in B. burgdorferi, we utilized targeted gene deletion and discovered bbd21 and bbd22 are co-transcribed. The effects of bbd21 and bbd22 deletion on plasmid copy number and mammalian infectivity were assessed. By qPCR, lp17 copy number did not differ amongst strains during mid-exponential and stationary growth phases. However, after in vitro passaging, the mutant strain demonstrated an 8-fold increase in lp17 copies, suggesting a cumulative defect in plasmid copy number regulation. Additionally, we compared lp17 copy number between in vitro and mammalian host-adapted conditions. Our findings showed 1) lp17 copy number was significantly different between these growth conditions for both the wild type and bbd21-bbd22 deletion mutant and 2) under mammalian host-adapted cultivation, the absence of bbd21-bbd22 resulted in significantly decreased copies of lp17. Murine infection studies using culture and qPCR demonstrated bbd21-bbd22 deletion resulted in a tissue colonization defect, particularly in the heart. Lastly, we showed bbd21 transcription appears to be independent of direct rpoS regulation based on similar expression levels in wild type and ΔrpoS. Altogether, our findings indicate the bbd21-bbd22 genetic region is involved with regulation of lp17 plasmid copy number. Furthermore, we propose the possibility that lp17 plasmid copy number is important for microbial pathogenesis by the Lyme disease spirochete.

The Borrelia burgdorferi VlsE Lipoprotein Prevents Antibody Binding to an Arthritis-Related Surface Antigen

Arp is an immunogenic protein of the Lyme disease spirochete Borrelia burgdorferi and contributes to joint inflammation during infection. Despite Arp eliciting a strong humoral response, antibodies fail to clear the infection. Given previous evidence of immune avoidance mediated by the antigenically variable lipoprotein of B. burgdorferi, VlsE, we use passive immunization assays to examine whether VlsE protects the pathogen from anti-Arp antibodies. The results show that spirochetes are only able to successfully infect passively immunized mice when VlsE is expressed. Subsequent immunofluorescence assays reveal that VlsE prevents binding of Arp-specific antibodies, thereby providing an explanation for the failure of Arp antisera to clear the infection. The results also show that the shielding effect of VlsE is not universal for all B. burgdorferi cell-surface antigens. The findings reported here represent a direct demonstration of VlsE-mediated protection of a specific B. burgdorferi surface antigen through a possible epitope-shielding mechanism.

Lone and Bankhead report that the antigenically variable VlsE protein of the Lyme disease agent Borrelia burgdorferi can prevent antibody binding to a surface antigen of the pathogen. They show that protection is likely via an epitope-shielding mechanism, thus expanding the current role of VlsE in immune evasion.

High conservation combined with high plasticity: genomics and evolution of Borrelia bavariensis

Background

Borrelia bavariensis is one of the agents of Lyme Borreliosis (or Lyme disease) in Eurasia. The genome of the Borrelia burgdorferi sensu lato species complex, that includes B. bavariensis, is known to be very complex and fragmented making the assembly of whole genomes with next-generation sequencing data a challenge.

Results

We present a genome reconstruction for 33 B. bavariensis isolates from Eurasia based on long-read (Pacific Bioscience, for three isolates) and short-read (Illumina) data. We show that the combination of both sequencing techniques allows proper genome reconstruction of all plasmids in most cases but use of a very close reference is necessary when only short-read sequencing data is available. B. bavariensis genomes combine a high degree of genetic conservation with high plasticity: all isolates share the main chromosome and five plasmids, but the repertoire of other plasmids is highly variable. In addition to plasmid losses and gains through horizontal transfer, we also observe several fusions between plasmids. Although European isolates of B. bavariensis have little diversity in genome content, there is some geographic structure to this variation. In contrast, each Asian isolate has a unique plasmid repertoire and we observe no geographically based differences between Japanese and Russian isolates. Comparing the genomes of Asian and European populations of B. bavariensis suggests that some genes which are markedly different between the two populations may be good candidates for adaptation to the tick vector, (Ixodes ricinus in Europe and I. persulcatus in Asia).

Conclusions

We present the characterization of genomes of a large sample of B. bavariensis isolates and show that their plasmid content is highly variable. This study opens the way for genomic studies seeking to understand host and vector adaptation as well as human pathogenicity in Eurasian Lyme Borreliosis agents.

A small intergenic region of lp17 is required for evasion of adaptive immunity and induction of pathology by the Lyme disease spirochete

The causative agent of Lyme disease, Borrelia burgdorferi, harbours a single linear chromosome and upwards of 23 linear and circular plasmids. Only a minority of these plasmids, including linear plasmid 17, are maintained with near-absolute fidelity during extended in vitro passage, and characterisation of any putative virulence determinants they encode has only recently begun. In this work, a mutant lacking a ~4.7 kb fragment of lp17 was studied. Colonisation of murine tissues by this lp17 mutant was significantly impaired, as was the ability to induce carditis and arthritis. The deficiency in tissue colonisation was alleviated in severe combined immunodeficient (SCID) mice, implicating a role for this plasmid region in adaptive immune evasion. Through genetic complementation, the mutant phenotype could be fully attributed to a 317 bp intergenic region that corresponds to the discontinued bbd07 ORF and upstream sequence. The intergenic region was found to be transcriptionally active, and mutant spirochetes lacking this region exhibited an overall difference in the antigenic profile during infection of an immunocompetent murine host. Overall, this study is the first to provide evidence for the involvement of lp17 in colonisation of joint and heart tissues, along with the associated pathologies caused by the Lyme disease spirochete.

Use of in vivo Expression Technology for the Identification of Putative Host Adaptation Factors of the Lyme Disease Spirochete

The causative agent of Lyme disease, Borrelia burgdorferi, is an obligate parasite that requires either a tick vector or a mammalian host for survival. Identification of the bacterial genes that are specifically expressed during infection of the mammalian host could provide targets for novel therapeutics and vaccines. In vivo expression technology (IVET) is a reporter-based promoter trap system that utilizes selectable markers to identify promoters of bacterial host-specific genes. Using previously characterized genes for in vivo and in vitro selection, this study utilized an IVET system that allows for selection of B. burgdorferi sequences that act as active promoters only during murine infection. This promoter trap system was able to successfully distinguish active promoter sequences both in vivo and in vitro from control sequences and a library of cloned B. burgdorferi genomic fragments. However, a bottleneck effect during the experimental mouse infection limited the utility for genome-wide promoter screening. Overall, IVET was demonstrated as a tool for the identification of in vivo-induced promoter elements of B. burgdorferi, and the observed infection bottleneck apparent using a polyclonal infection pool provides insight into the dynamics of experimental infection with B. burgdorferi.

Borrelia burgdorferi Pathogenesis and the Immune Response

Borrelia burgdorferi is the tick-borne etiologic agent of Lyme disease. The spirochete must negotiate numerous barriers in order to establish a disseminated infection in a mammalian host. These barriers include migration from the feeding tick midgut to the salivary glands, deposition in skin, manipulation or evasion of the localized host immune response, adhesion to and extravasation through an endothelial barrier, hematogenous dissemination, and establishment of infection in distal tissue sites. Borrelia burgdorferi proteins that mediate many of these processes and the nature of the host response to infection are described.

Bacterial heterogeneity is a requirement for host superinfection by the Lyme disease spirochete

In nature, mixed Borrelia burgdorferi infections are common and possibly can be acquired by either superinfection or coinfection. Superinfection by heterologous B. burgdorferi strains has been established experimentally, although the ability of homologous B. burgdorferi clones to superinfect a host has not been studied in detail. Information regarding any potential immune barriers to secondary infection also currently is unavailable. In the present study, the ability to superinfect various mouse models by homologous wild-type clones was examined and compared to superinfection by heterologous strains. To assess the ability of homologous B. burgdorferi clones to successfully superinfect a mouse host, primary- and secondary-infecting spirochetes were recovered via in vitro cultivation of collected blood or tissue samples. This was accomplished by generating two different antibiotic-resistant versions of the wild-type B31-A3 clone in order to distinguish superinfecting B. burgdorferi from primary-infecting spirochetes. The data demonstrate an inability of homologous B. burgdorferi to superinfect immunocompetent mice as opposed to heterologous strains. Attempts to superinfect different types of immunodeficient mice with homologous B. burgdorferi indicate that the murine innate immune system represents a major barrier to intrastrain superinfection. Consequently, the possibility of innate immunity as a driving force for B. burgdorferi heterogeneity during the enzootic cycle is discussed.

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.

Regulatory protein BBD18 of the lyme disease spirochete: essential role during tick acquisition?

The Lyme disease spirochete Borrelia burgdorferi senses and responds to environmental cues as it transits between the tick vector and vertebrate host. Failure to properly adapt can block transmission of the spirochete and persistence in either vector or host. We previously identified BBD18, a novel plasmid-encoded protein of B. burgdorferi, as a putative repressor of the host-essential factor OspC. In this study, we investigate the in vivo role of BBD18 as a regulatory protein, using an experimental mouse-tick model system that closely resembles the natural infectious cycle of B. burgdorferi. We show that spirochetes that have been engineered to constitutively produce BBD18 can colonize and persist in ticks but do not infect mice when introduced by either tick bite or needle inoculation. Conversely, spirochetes lacking BBD18 can persistently infect mice but are not acquired by feeding ticks. Through site-directed mutagenesis, we have demonstrated that abrogation of spirochete infection in mice by overexpression of BBD18 occurs only with bbd18 alleles that can suppress OspC synthesis. Finally, we demonstrate that BBD18-mediated regulation does not utilize a previously described ospC operator sequence required by B. burgdorferi for persistence in immunocompetent mice. These data lead us to conclude that BBD18 does not represent the putative repressor utilized by B. burgdorferi for the specific downregulation of OspC in the mammalian host. Rather, we suggest that BBD18 exhibits features more consistent with those of a global regulatory protein whose critical role occurs during spirochete acquisition by feeding ticks.

Lyme disease, caused by Borrelia burgdorferi, is the most common arthropod-borne disease in North America. B. burgdorferi is transmitted to humans and other vertebrate hosts by ticks as they take a blood meal. Transmission between vectors and hosts requires the bacterium to sense changes in the environment and adapt. However, the mechanisms involved in this process are not well understood. By determining how B. burgdorferi cycles between two very different environments, we can potentially establish novel ways to interfere with transmission and limit infection of this vector-borne pathogen. We are studying a regulatory protein called BBD18 that we recently described. We found that too much BBD18 interferes with the spirochete’s ability to establish infection in mice, whereas too little BBD18 appears to prevent colonization in ticks. Our study provides new insight into key elements of the infectious cycle of the Lyme disease spirochete.

Borrelia chilensis, a new member of the Borrelia burgdorferi sensu lato complex that extends the range of this genospecies in the Southern Hemisphere

Borrelia burgdorferi sensu lato (s.l.), transmitted by Ixodes spp. ticks, is the causative agent of Lyme disease. Although Ixodes spp. ticks are distributed in both Northern and Southern Hemispheres, evidence for the presence of B. burgdorferi s.l. in South America apart from Uruguay is lacking. We now report the presence of culturable spirochetes with flat-wave morphology and borrelial DNA in endemic Ixodes stilesi ticks collected in Chile from environmental vegetation and long-tailed rice rats (Oligoryzomys longicaudatus). Cultured spirochetes and borrelial DNA in ticks were characterized by multilocus sequence typing and by sequencing five other loci (16S and 23S ribosomal genes, 5S-23S intergenic spacer, flaB, ospC). Phylogenetic analysis placed this spirochete as a new genospecies within the Lyme borreliosis group. Its plasmid profile determined by polymerase chain reaction and pulsed-field gel electrophoresis differed from that of B. burgdorferi B31A3. We propose naming this new South American member of the Lyme borreliosis group B. chilensis VA1 in honor of its country of origin.

Presence of Arp specifically contributes to joint tissue edema associated with early-onset Lyme arthritis

Antiserum to the Borrelia burgdorferi arthritis-related protein, Arp, has been shown to prevent or reduce arthritis in immunodeficient mice. To directly investigate the requirement for this lipoprotein in the generation of Lyme arthritis, we utilized targeted deletion to generate a B. burgdorferi clone that lacked only the arp gene locus. Infection of Lyme disease-susceptible C3H/HeN mice with the arp deletion mutant demonstrated significantly reduced tibiotarsal joint swelling during the first 6 weeks of infection compared to a wild-type control. The severity of joint swelling was restored to wild-type levels in mice infected with an arp mutant clone complemented in cis. Interestingly, the reduced swelling of joint tissues exhibited by mice infected with the arp deletion mutant did not directly correspond to reduced underlying arthritis. Histopathology data at 2 weeks postinfection showed some reduction in arthritis severity caused by the arp mutant clone; however, by 8 weeks, no significant difference was observed between joint tissues infected by the wild-type or arp mutant clones. The spirochete load in the joint tissues of mice infected with the arp mutant was found to be greater than that exhibited by the wild-type control. Our findings demonstrate that this lipoprotein contributes to the generation of early-onset joint swelling and suggests that arp expression has a negative secondary effect on total spirochete numbers in joint tissues.