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

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.

Identification and functional analysis of a galactosyltransferase capable of cholesterol glycolipid formation in the Lyme disease spirochete Borrelia burgdorferi

Borrelia burgdorferi (Bb), the etiological agent of Lyme disease, produces a series of simple glycolipids where diacylglycerol and cholesterol serve as the precursor. The cholesterol-based glycolipids, cholesteryl 6-O-acyl-β-D-galactopyranoside (ACGal) and cholesteryl-β-D-galactopyranoside (CGal) are immunogenic and proposed to contribute to the pathogenesis of Lyme disease. Detailed studies of CGal and ACGal in Bb have been hampered by a lack of knowledge of their underlying biosynthetic processes. The genome of Bb encodes four putative glycosyltransferases, and only one of these, BB0572, was predicted to be an inverting family 2 glycosyltransferase (GT2 enzyme) capable of using UDP-galactose as a substrate and forming a β-glycosidic bond. Comparison of the 42 kDa BB0572 amino acid sequence from Bb with other Borrelia spp demonstrates that this protein is highly conserved. To establish BB0572 as the galactosyltransferase capable of cholesterol glycolipid formation in Bb, the protein was produced as a recombinant product in Escherichia coli and tested in a cell-free assay with 14C-cholesterol and UDP-galactose as the substrates. This experiment resulted in a radiolabeled lipid that migrated with the cholesterol glycolipid standard of CGal when evaluated by thin layer chromatography. Additionally, mutation in the predicted active site of BB0572 resulted in a recombinant protein that was unable to catalyze the formation of the cholesterol glycolipid. These data characterize BB0572 as a putative cholesterol galactosyltransferase. This provides the first step in understanding how Bb cholesterol glycolipids are formed and will allow investigations into their involvement in pathogen transmission and disease development.

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.

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.

Borrelia burgdorferi adhere to blood vessels in the dura mater and are associated with increased meningeal T cells during murine disseminated borreliosis

Borrelia burgdorferi, the causative agent of Lyme disease, is a vector-borne bacterial infection that is transmitted through the bite of an infected tick. If not treated with antibiotics during the early stages of infection, disseminated infection can spread to the central nervous system (CNS). In non-human primates (NHPs) it has been demonstrated that the leptomeninges are among the tissues colonized by B. burgdorferi spirochetes. Although the NHP model parallels aspects of human borreliosis, a small rodent model would be ideal to study the trafficking of spirochetes and immune cells into the CNS. Here we show that during early and late disseminated infection, B. burgdorferi infects the meninges of intradermally infected mice, and is associated with concurrent increases in meningeal T cells. We found that the dura mater was consistently culture positive for spirochetes in transcardially perfused mice, independent of the strain of B. burgdorferi used. Within the dura mater, spirochetes were preferentially located in vascular regions, but were also present in perivascular, and extravascular regions, as late as 75 days post-infection. At the same end-point, we observed significant increases in the number of CD3+ T cells within the pia and dura mater, as compared to controls. Flow cytometric analysis of leukocytes isolated from the dura mater revealed that CD3+ cell populations were comprised of both CD4 and CD8 T cells. Overall, our data demonstrate that similarly to infection in peripheral tissues, spirochetes adhere to the dura mater during disseminated infection, and are associated with increases in the number of meningeal T cells. Collectively, our results demonstrate that there are aspects of B. burgdorferi meningeal infection that can be modelled in laboratory mice, suggesting that mice may be useful for elucidating mechanisms of meningeal pathogenesis by B. burgdorferi.

Cis-acting DNA elements flanking the variable major protein expression site of Borrelia hermsii are required for murine persistence

In Borrelia hermsii, antigenic variation occurs as a result of a nonreciprocal gene conversion event that places one of ~60 silent variable major protein genes downstream of a single, transcriptionally active promoter. The upstream homology sequence (UHS) and downstream homology sequence (DHS) are two putative cis‐acting DNA elements that have been predicted to serve as crossover points for homologous recombination. In this report, a targeted deletion/in cis complementation technique was used to directly evaluate the role for these elements in antigenic switching. The results demonstrate that deletion of the expression site results in an inability of the pathogen to relapse in immunocompetent mice, and that the utilized technique was successful in producing complemented mutants that are capable of antigenic switching. Additional complemented clones with mutations in the UHS and DHS of the expressed locus were then generated and evaluated for their ability to relapse in immunocompetent mice. Mutation of the UHS and inverted repeat sequence within the DHS rendered these mutants incapable of relapsing. Overall, the results establish the requirement of the inverted repeat of the DHS for antigenic switching, and support the importance of the UHS for B. hermsii persistence in the mammalian host.

Investigating the potential role of non-vls genes on linear plasmid 28-1 in virulence and persistence by Borrelia burgdorferi

Background

The lp28-1 plasmid is required for persistent infection by the Lyme disease spirochete, Borrelia burgdorferi. Mutational studies on this plasmid have shown that the vls locus is important for antigenic variation of the VlsE lipoprotein that leads to immune evasion and persistence. However, it is still unknown whether the vls system is the only genetic locus on this plasmid necessary for long-term infection, and thus the potential role of non-vls genes on lp28-1 in virulence and persistence is yet to be fully determined. Despite extensive mutational analyses, two lp28-1 regions containing the ORFs bbf19 - bbf22 and bbf27 – bbf30 have not yet been mutated in their entirety.

Results

In this study, we set out to establish if these unstudied regions of lp28-1 play a role in spirochete persistence. Results show that the generated mutants were fully infectious in immunocompetent mice, and were able to persist for 91 days following infection. Following this finding, ospC expression by these mutants was determined, as it has been reported that spirochetes lacking lp28-1 fail to downregulate expression of this lipoprotein leading to immune clearance. Data presented here failed to show a definitive difference in ospC expression levels during host infection when the mutants were compared to the wild type.

Conclusions

Overall, the results strongly suggest that non-vls genes residing on lp28-1 do not play a role in spirochete persistence during infection of the mammalian host, and that the regions under study are likely not involved in the regulation of ospC expression. In conjunction with previous studies involving mutation of non-vls loci on lp28-1, these findings suggest that the vls locus is likely the sole genetic element on this plasmid responsible for immune evasion and persistence exhibited by the Lyme disease pathogen.

Evaluation of the Importance of VlsE Antigenic Variation for the Enzootic Cycle of Borrelia burgdorferi

Efficient acquisition and transmission of Borrelia burgdorferi by the tick vector, and the ability to persistently infect both vector and host, are important elements for the life cycle of the Lyme disease pathogen. Previous work has provided strong evidence implicating the significance of the vls locus for B. burgdorferi persistence. However, studies involving vls mutant clones have thus far only utilized in vitro-grown or host-adapted spirochetes and laboratory strains of mice. Additionally, the effects of vls mutation on tick acquisition and transmission has not yet been tested. Thus, the importance of VlsE antigenic variation for persistent infection of the natural reservoir host, and for the B. burgdorferi enzootic life cycle in general, has not been examined to date. In the current work, Ixodes scapularis and Peromyscus maniculatus were infected with different vls mutant clones to study the importance of the vls locus for the enzootic cycle of the Lyme disease pathogen. The findings highlight the significance of the vls system for long-term infection of the natural reservoir host, and show that VlsE antigenic variability is advantageous for efficient tick acquisition of B. burgdorferi from the mammalian reservoir. The data also indicate that the adaptation state of infecting spirochetes influences B. burgdorferi avoidance from host antibodies, which may be in part due to its respective VlsE expression levels. Overall, the current findings provide the most direct evidence on the importance of VlsE for the enzootic cycle of Lyme disease spirochetes, and underscore the significance of VlsE antigenic variation for maintaining B. burgdorferi in nature.