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

Comparative genomics of the Western Hemisphere soft tick-borne relapsing fever borreliae highlights extensive plasmid diversity

BACKGROUND

Tick-borne relapsing fever (TBRF) is a globally prevalent, yet under-studied vector-borne disease transmitted by soft and hard bodied ticks. While soft TBRF (sTBRF) spirochetes have been described for over a century, our understanding of the molecular mechanisms facilitating vector and host adaptation is poorly understood. This is due to the complexity of their small (~ 1.5 Mb) but fragmented genomes that typically consist of a linear chromosome and both linear and circular plasmids. A majority of sTBRF spirochete genomes' plasmid sequences are either missing or are deposited as unassembled sequences. Consequently, our goal was to generate complete, plasmid-resolved genomes for a comparative analysis of sTBRF species of the Western Hemisphere.

RESULTS

Utilizing a Borrelia specific pipeline, genomes of sTBRF spirochetes from the Western Hemisphere were sequenced and assembled using a combination of short- and long-read sequencing technologies. Included in the analysis were the two recently isolated species from Central and South America, Borrelia puertoricensis n. sp. and Borrelia venezuelensis, respectively. Plasmid analyses identified diverse sequences that clustered plasmids into 30 families; however, only three families were conserved and syntenic across all species. We also compared two species, B. venezuelensis and Borrelia turicatae, which were isolated ~ 6,800 km apart and from different tick vector species but were previously reported to be genetically similar.

CONCLUSIONS

To truly understand the biological differences observed between species of TBRF spirochetes, complete chromosome and plasmid sequences are needed. This comparative genomic analysis highlights high chromosomal synteny across the species yet diverse plasmid composition. This was particularly true for B. turicatae and B. venezuelensis, which had high average nucleotide identity yet extensive plasmid diversity. These findings are foundational for future endeavors to evaluate the role of plasmids in vector and host adaptation.

Borreliosis Transmission from Ticks Associated with Desert Tortoise Burrows: Examples of Tick-Borne Relapsing Fever in the Mojave Desert

Ticks transmit pathogens and parasitize wildlife in turn causing zoonotic diseases in many ecosystems. Argasid ticks, such as Ornithodoros spp., harbor and transmit Borrelia spp., resulting in tick-borne relapsing fever (TBRF) in people. In the western United States, TBRF is typically associated with the bite of an infected Ornithodoros hermsi tick found in habitats at high elevations (>1500 ft). This report describes the first TBRF cases in people in the Mojave Desert (Clark County, NV). Individuals documented in these case studies were exposed to Ornithodoros ticks during excavation of soil burrows associated with Mojave Desert tortoises (Gopherus agassizii), with bacteria from one of the human case's blood sample genetically matching to Borrelia turicatae as determined by quantitative PCR and sequencing. Our findings should serve as a precaution to individuals working with tortoises or animal burrows, or those in contact with Ornithodoros ticks in this region.

Lost in a sagebrush sea: comparative genetic assessment of an isolated montane population of Tamias amoenus

The montane sky islands of the Great Basin are characterized by unique, isolated habitats and communities that likely are vulnerable to extirpation with environmental change. A subspecies of yellow pine chipmunk, the Humboldt yellow pine chipmunk (Tamias amoenus celeris), is associated with the whitebark and limber pine forests of the Pine Forest Range (PFR) in Nevada. We sampled T. amoenus and least chipmunks (T. minimus) from the isolated PFR and compared genetic diversity between these populations and more “mainland” populations, including other subspecies of chipmunks. Given the high frequency of hybridization in Tamias, we tested for hybridization between T. amoenus and T. minimus in the PFR. We examined phylogenetic relationships, population divergence and diversity, and screened populations for a common pathogen, Borrelia hermsii, to gain insight into population health. We found T. amoenus of the PFR are closely related to T. amoenus in the Warner Mountains and Sierra Nevada, but maintain substantively lower genetic variation. Microsatellite analyses show PFR T. amoenus are highly genetically differentiated from other populations. In contrast, PFR T. minimus had higher genetic diversity that was comparable to the other T. minimus population we sampled. Pathogen screening revealed that T. amoenus carried higher pathogen loads than T. minimus in the PFR, although the prevalence of infection was similar to other Tamias populations. Our assessment of habitat associations suggests that the Humboldt yellow pine chipmunk almost entirely is restricted to the conifer systems of the PFR, while least chipmunks are prevalent in the other forests. Our work highlights the need for continued conservation and research efforts to identify how response to environmental change can be facilitated in isolated species and habitats.

An immunocompromised mouse model to infect Ixodes scapularis ticks with the relapsing fever spirochete, Borrelia miyamotoi

The hard tick-borne relapsing fever spirochete, Borrelia miyamotoi, has recently gained attention as a cause of human illness, but fundamental aspects of its enzootic maintenance are still poorly understood. Challenges to experimental studies with B. miyamotoi-infected vector ticks include low prevalence of infection in field-collected ticks and seemingly inefficient horizontal transmission from infected immunocompetent rodents to feeding ticks. To reliably produce large numbers of B. miyamotoi-infected ticks in support of experimental studies, we developed an animal model where immunocompromised Mus musculus SCID mice were used as a source of B. miyamotoi-infection for larval and nymphal Ixodes scapularis ticks. Following needle inoculation with 1 × 105 spirochetes, the SCID mice developed a high spirochetemia (greater than 1 × 107 copies of B. miyamotoi purB per mL of blood) that persisted for at least 30 d after inoculation. In comparison, immunocompetent M. musculus CD-1 mice developed transient infections, detectable for only 2-8 d within the first 16 d after needle inoculation, with a brief, lower peak spirochetemia (8.5 × 104 - 5.6 × 105purB copies per mL of blood). All larval or nymphal ticks fed on infected SCID mice acquired B. miyamotoi, but frequent loss of infection during the molt led to the proportion infected ticks of the resulting nymphal or adult stages declining to 22-29%. The ticks that remained infected after the molt had well-disseminated infections which then persisted through successive life stages, including transmission to larval offspring.

Relapsing Fevers: Neglected Tick-Borne Diseases

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

Tick-Borne Relapsing Fever Outbreak Among a High School Football Team at an Outdoor Education Camping Trip, Arizona, 2014

During August 2014, five high school students who had attended an outdoor education camp were hospitalized with a febrile illness, prompting further investigation. Ten total cases of tick-borne relapsing fever (TBRF) were identified-six cases confirmed by culture or visualization of spirochetes on blood smear and four probable cases with compatible symptoms (attack rate: 23%). All patients had slept in the campsite's only cabin. Before the camp, a professional pest control company had rodent proofed the cabin, but no acaricides had been applied. Cabin inspection after the camp found rodents and Ornithodoros ticks, the vector of TBRF. Blood samples from a chipmunk trapped near the cabin and from patients contained Borrelia hermsii with identical gene sequences (100% over 630 base pairs). Health departments in TBRF endemic areas should consider educating cabin owners and pest control companies to apply acaricides during or following rodent proofing, because ticks that lack rodents for a blood meal might feed on humans.

Characterization of a DNA Adenine Methyltransferase Gene of Borrelia hermsii and Its Dispensability for Murine Infection and Persistence

DNA methyltransferases have been implicated in the regulation of virulence genes in a number of pathogens. Relapsing fever Borrelia species harbor a conserved, putative DNA methyltransferase gene on their chromosome, while no such ortholog can be found in the annotated genome of the Lyme disease agent, Borrelia burgdorferi. In the relapsing fever species Borrelia hermsii, the locus bh0463A encodes this putative DNA adenine methyltransferase (dam). To verify the function of the BH0463A protein product as a Dam, the gene was cloned into a Dam-deficient strain of Escherichia coli. Restriction fragment analysis subsequently demonstrated that complementation of this E. coli mutant with bh0463A restored adenine methylation, verifying bh0463A as a Dam. The requirement of bh0463A for B. hermsii viability, infectivity, and persistence was then investigated by genetically disrupting the gene. The dam^- mutant was capable of infecting immunocompetent mice, and the mean level of spirochetemia in immunocompetent mice was not significantly different from wild type B. hermsii. Collectively, the data indicate that dam is dispensable for B. hermsii viability, infectivity, and persistence.

Variable Major Proteins as Targets for Specific Antibodies against Borrelia miyamotoi

Borrelia miyamotoi is a relapsing fever spirochete in Ixodes ticks that has been recently identified as a human pathogen causing hard tick-borne relapsing fever (HTBRF) across the Northern Hemisphere. No validated serologic test exists, and current serologic assays have low sensitivity in early HTBRF. To examine the humoral immune response against B. miyamotoi, we infected C3H/HeN mice with B. miyamotoi strain LB-2001 expressing variable small protein 1 (Vsp1) and demonstrated that spirochetemia was cleared after 3 d, coinciding with anti-Vsp1 IgM production. Clearance was also observed after passive transfer of immune sera to infected SCID mice. Next, we showed that anti-Vsp1 IgG eliminates Vsp1-expressing B. miyamotoi, selecting for spirochetes expressing a variable large protein (VlpC2) resistant to anti-Vsp1. The viability of Asian isolate B. miyamotoi HT31, expressing Vlp15/16 and Vlp18, was also unaffected by anti-Vsp1. Finally, in nine HTBRF patients, we demonstrated IgM reactivity to Vsp1 in two and against Vlp15/16 in four ∼1 wk after these patients tested positive for B. miyamotoi by PCR. Our data show that B. miyamotoi is able to express various variable major proteins (VMPs) to evade humoral immunity and that VMPs are antigenic in humans. We propose that serologic tests based on VMPs are of additional value in diagnosing HTBRF.

Relapsing Fever Borreliae: A Global Review

Relapsing fever borreliae were notorious and feared infectious agents that earned their place in history through their devastating impact as causes of both epidemic and endemic infection. They are now considered more as an oddity, and their burden of infection is largely overshadowed by other infections such as malaria, which presents in a similar clinical way. Despite this, they remain the most common bacterial infection in some developing countries. Transmitted by soft ticks or lice, these fascinating spirochetes have evolved a myriad of mechanisms to survive within their diverse environments.

Relapsing fever group Borrelia in Southern California rodents

Wild rodent reservoir host species were surveyed prospectively for infection with Borrelia hermsii, the causative agent of tick-borne relapsing fever in the western United States. Trapping occurred during the summer of 2009-2012 at field sites surrounding Big Bear Lake, CA, a region where human infection has been reported for many years. Using quantitative polymerase chain reaction (qPCR), we tested 207 rodents from 11 species and found chipmunks (Tamias spp.) and a woodrat (Neotoma macrotis) infected. Chipmunks represented the majority of captures at these sites. Sixteen of the 207 (7.7%; CI = 4.6-12.4) animals were qPCR-positive for Borrelia spp. associated with relapsing fever, and of those, we obtained bacterial DNA sequences from eight. The phylogram made from these sequences depict a clear association with B. hermsii genomic group I. In addition, we identified an infection with Borrelia coriaceae in a Tamias merriami, a potentially nonpathogenic member of the tick-borne relapsing fever group. Our findings support the hypothesis that chipmunk species play an important role in the maintenance of Borrelia species that cause tick-borne relapsing fever in the western United States, and therefore the risk of infection to people.

Cotransmission of divergent relapsing fever spirochetes by artificially infected Ornithodoros hermsi

The soft tick Ornithodoros hermsi, which ranges in specific arboreal zones of western North America, acts as a vector for the relapsing fever spirochete Borrelia hermsii. Two genomic groups (genomic group I [GGI] and GGII) of B. hermsii are differentiated by multilocus sequence typing yet are codistributed in much of the vector's range. To test whether the tick vector can be infected via immersion, noninfected, colony-derived O. hermsi larvae were exposed to reduced-humidity conditions before immersion in culture suspensions of several GGI and GGII isolates. We tested for spirochetes in ticks by immunofluorescence microscopy and in mouse blood by quantitative PCR of the vtp locus to differentiate spirochete genotypes. The immersed larval ticks were capable of spirochete transmission to mice at the first nymphal feeding. Tick infection with mixed cultures of isolates DAH (vtp-6) (GGI) and MTW-2 (vtp-5) (GGII) resulted in ticks that caused spirochetemias in mice consisting of MTW-2 or both DAH and MTW-2. These findings show that this soft tick species can acquire B. hermsii by immersion in spirochete suspensions, that GGI and GGII isolates can coinfect the tick vector by this method, and that these spirochetes can be cotransmitted to a rodent host.

IL-10 Prevents apoptosis of brain endothelium during bacteremia

IL-10-deficient mice infected with the relapsing fever bacterium Borrelia turicatae rapidly succumb to a brain hemorrhage if they are unable to clear peak bacteremia. In this study, we investigated the protective role of IL-10 during relapsing-remitting bacteremia and explored the molecular events involved in the protection of brain endothelium by IL-10. Brain endothelial injury was measured with cytotoxicity and diverse apoptotic assays, whereas the signaling pathway analysis was done by quantitative PCR array. The results showed that severe endothelial cell injury leading to hemorrhage in the brain and other organs occurred in IL-10-deficient mice during relapsing-remitting infection. Human brain microvascular endothelial cells (HBMEC) produced abundant proinflammatory mediators upon exposure to whole bacteria or purified bacterial lipoprotein but did not produce any detectable IL-10. Whole bacteria and purified outer membrane lipoprotein rapidly killed HBMEC by apoptosis in a time- and concentration-dependent manner. Exogenous IL-10 protected HBMEC from apoptosis. HBMEC apoptosis during exposure to a low number of bacteria was associated with downregulation of TNF and TNFAIP3 and upregulation of BAX. In contrast, HBMEC apoptosis during exposure to high concentrations of purified outer membrane lipoprotein was associated with marked upregulation of FAS, FAS ligand, and the adaptor molecules RIPK1 and CFLAR. Exogenous IL-10 reversed all the apoptotic signaling changes induced by whole bacteria or its purified lipoprotein. The results indicate that prominent brain endothelial cell apoptosis occurs during relapsing-remitting bacteremia in the absence of IL-10 and point to a prominent role for bacterial lipoprotein-mediated activation of FAS and caspase-3 in this process.

Interaction of variable bacterial outer membrane lipoproteins with brain endothelium

Background

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

Methodology/Principal Findings

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

Conclusions/Significance

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

Bacterial lipoproteins can disseminate from the periphery to inflame the brain

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