Attachment of Borrelia burgdorferi within Ixodes scapularis mediated by outer surface protein A

Significance of Detecting Serum Antibodies to Outer Surface Protein A of Lyme Disease Borreliae in PCR-Confirmed Blood Infections

Background/Objectives: Lyme disease is caused by some species of tick-borne bacteria of the genus Borrelia, termed Lyme disease Borreliae (LDB). Borrelia burgdorferi is the LDB species principally responsible for Lyme disease in the US. The outer surface protein A (OspA) of LDB attaches the bacteria to the gut of Ixodes tick vectors. OspA expression is downregulated when B. burgdorferi is transmitted from ticks to mammalian hosts. Vaccination with OspA elicits antibody-mediated protective immunity in animals and humans against LDB infection. The possible presence of serum antibodies against OspA in persons with PCR-confirmed LDB infections in blood was investigated in this study. Methods: Ninety-one archived sera from patients with LDB infections in blood demonstrated by a sensitive PCR assay were tested for reactivity with OspA from multiple LDB species in line immunoblots. Results: In total, 14 of the 91 sera (15.4%) had either IgG or IgM antibodies to OspA from one or more LDB species. Conclusions: The results show for the first time that serum antibodies to OspA are formed when LDB are present in human blood. However, the factors that governed the expression of OspA by LDB in patients could not be ascertained. It will be useful to determine whether the observed levels of serum antibodies to OspA in infected persons can protect against subsequent tick-borne infection and whether OspA used in conjunction with other LDB antigens can improve the serological diagnosis of Lyme disease.

Immunogenicity and safety of an 18-month booster dose of the VLA15 Lyme borreliosis vaccine candidate after primary immunisation in healthy adults in the USA: results of the booster phase of a randomised, controlled, phase 2 trial

BACKGROUND

Incidence rates of Lyme borreliosis, a tickborne disease attributed to infection by Borrelia species, are increasing, and limitations to existing treatments potentiate the possibility of severe outcomes. Nevertheless, there are no licensed vaccines for Lyme borreliosis prevention in humans. This study investigated the immunogenicity and safety of a booster dose of VLA15, an investigational outer surface protein A (OspA)-based Lyme borreliosis vaccine that has previously shown safety and immunogenicity when administered as a primary vaccination series, following a primary VLA15 vaccination series.

METHODS

We report the results of the booster phase of a randomised, observer-blinded, placebo-controlled, multicentre, phase 2 study that enrolled healthy adults aged 18-65 years from five US clinical study centres to receive 135 μg or 180 μg VLA15 or placebo at months 0, 2, and 6 in the main study phase. Participants who received 180 μg VLA15 in the main study phase and did not have relevant protocol deviations were eligible for the booster phase (months 18-30). Participants were randomly reassigned (2:1) to receive an intramuscular injection of a VLA15 booster or placebo 1 year after the completion of primary vaccination (month 18) via a randomisation list generated by an unmasked statistician with a block size of six. Individuals involved in data safety monitoring, rerandomisation, vaccine handling, and vaccine accountability were unmasked; the study sponsor and statisticians were only unmasked after analysis of data up to 1 month after booster administration. All other individuals remained masked throughout the booster phase. The outcomes for the booster phase were the immunogenicity (evaluated in the booster per-protocol population) and safety (evaluated for all participants who received the booster) of the booster dose up to month 30. The study is registered at ClinicalTrials.gov (NCT03970733) and is completed.

FINDINGS

Between Feb 4 and March 23, 2021, 58 participants (28 men and 30 women) were screened, randomly assigned, and received VLA15 (n=39) or placebo (n=19). One participant in the placebo group was lost to follow-up. The IgG geometric mean titres for each OspA serotype (serotypes 1-6) in the VLA15 group peaked at 1 month after the booster dose (1277·0 U/mL [95% CI 861·8-1892·3] to 2194·5 U/mL [1566·8-3073·7] vs 23·6 U/mL [18·1-30·8] to 36·8 U/mL [26·4-51·3] in the placebo group [p<0·0001 for all serotypes]), remained elevated at month 24 (137·4 U/mL [95·8-196·9] to 265·8 U/mL [202·9-348·2] vs 22·3 U/mL [17·7-28·0] to 29·1 U/mL [20·8-40·6] in the placebo group; p<0·0001 for all serotypes), and declined by month 30 (54·1 U/mL [38·6-75·7] to 101·6 U/mL [77·6-133·1] vs 21·9 U/mL [18·0-26·6] to 24·9 U/mL [19·0-32·6] in the placebo group; p<0·0001 for all serotypes except serotype 1 [p=0·0006]). Solicited local adverse events were reported more frequently in the VLA15 group (35 [92%, 95% CI 79-97] of 38 participants) than the placebo group (six [32%, 15-54] of 19 participants; p<0·0001) after booster vaccination. There was no significant difference in the frequency of solicited systemic adverse events between groups (20 [59%, 42-74] of 34 participants in the VLA15 group vs six [38%, 18-61] of 16 participants in the placebo group). Related unsolicited adverse events (none severe) were reported by two (5%, 1-17) of 39 participants in the VLA15 group and none (0%, 0-17) of 19 participants in the placebo group. There were no severe solicited local or systemic adverse events or deaths during the study.

INTERPRETATION

A booster dose of VLA15 is safe and induces substantial anamnestic immune responses against all six OspA serotypes. As with previously investigated OspA-based Lyme borreliosis vaccines, waning immune responses were observed with VLA15, and annual boosters might therefore be required.

FUNDING

Valneva.

Pathogens transmitted by Ixodes ricinus

Ixodes ricinus is the most important tick vector in central and western Europe and one of the most researched parasites. However, in the published literature on the tick and the pathogens it transmits, conjecture about specific transmission cycles and the clinical significance of certain microbes is not always clearly separated from confirmed facts. This article aims to present up-to-date, evidence-based information about the well-researched human pathogens tick-borne encephalitis virus, louping-ill virus, Anaplasma phagocytophilum, Borrelia burgdorferi sensu lato and several Babesia species, with a focus on their development in the tick, transmission dynamics and the reservoir hosts that support their circulation in the environment. Borrelia miyamotoi, Neoehrlichia mikurensis, Rickettsia helvetica and Rickettsia monacensis, which are much less common causes of disease but may affect immunocompromised patients, are also briefly discussed. Finally, the possible role of I. ricinus in the transmission of Coxiella burnetii, Francisella tularensis, Bartonella spp. and Spiroplasma ixodetis is reviewed.

Hitchhiker's Guide to Borrelia burgdorferi

Don't Panic. In the nearly 50 years since the discovery of Lyme disease, Borrelia burgdorferi has emerged as an unlikely workhorse of microbiology. Interest in studying host-pathogen interactions fueled significant progress in making the fastidious microbe approachable in laboratory settings, including the development of culture methods, animal models, and genetic tools. By developing these systems, insight has been gained into how the microbe is able to survive its enzootic cycle and cause human disease. Here, we discuss the discovery of B. burgdorferi and its development as a model organism before diving into the critical lessons we have learned about B. burgdorferi biology at pivotal stages of its lifecycle: gene expression changes during the tick blood meal, colonization of a new vertebrate host, and developing a long-lasting infection in that vertebrate until a new tick feeds. Our goal is to highlight the advancements that have facilitated B. burgdorferi research and identify gaps in our current understanding of the microbe.

Optimisation of dose level and vaccination schedule for the VLA15 Lyme borreliosis vaccine candidate among healthy adults: two randomised, observer-blind, placebo-controlled, multicentre, phase 2 studies

BACKGROUND

Rising Lyme borreliosis incidence rates, potential for severe outcomes, and limitations in accurate and timely diagnosis for treatment initiation suggest the need for a preventive vaccine; however, no vaccine is currently available for human use. We performed two studies in adults to optimise the dose level and vaccination schedule for VLA15, an investigational Lyme borreliosis vaccine targeting outer surface protein A (OspA) serotypes 1-6, which are associated with the most common pathogenic Borrelia species in Europe and North America.

METHODS

Both randomised, observer-blind, placebo-controlled, multicentre phase 2 studies included participants aged 18-65 years without recent history of Lyme borreliosis or tick bites. Study one was conducted at nine clinical research and study centre sites in the USA (n=6), Germany (n=2), and Belgium (n=1); study two was conducted at five of the study one US sites. Based on a randomisation list created by an unmasked statistician for each study, participants were randomly assigned via an electronic case report form randomisation module to receive 90 μg (study one only), 135 μg, or 180 μg VLA15 or placebo by intramuscular injection at months 0, 1, and 2 (study one) or 0, 2, and 6 (study two). Study one began with a run-in phase to confirm safety, after which the Data Safety Monitoring Board recommended the removal of the 90 μg group and continuation of the study. In the study one run-in phase, randomisation was stratified by study site, whereas in the study one main phase and in study two, randomisation was stratified by study site, age group, and baseline B burgdorferi (sensu lato) serostatus. All individuals were masked, other than staff involved in randomisation, vaccine preparation or administration, or safety data monitoring. The primary endpoint for both studies was OspA-specific IgG geometric mean titres (GMTs) at 1 month after the third vaccination and was evaluated in the per-protocol population. Safety endpoints were evaluated in the safety population: all participants who received at least one vaccination. Both studies are registered at ClinicalTrials.gov (study one NCT03769194 and study two NCT03970733) and are completed.

FINDINGS

For study one, 573 participants were screened and randomly assigned to treatment groups between Dec 21, 2018, and Sept, 26, 2019. For study two, 248 participants were screened and randomly assigned between June 26 and Sept 3, 2019. In study one, 29 participants were assigned to receive 90 μg VLA15, 215 to 135 μg, 205 to 180 μg, and 124 to placebo. In study two, 97 participants were assigned to receive 135 μg VLA15, 100 to 180 μg, and 51 to placebo. At 1 month after the third vaccination (ie, month 3), OspA-specific IgG GMTs in study one ranged from 74·3 (serotype 1; 95% CI 46·4-119·0) to 267·4 units per mL (serotype 3; 194·8-367·1) for 90 μg VLA15, 101·9 (serotype 1; 87·1-119·4) to 283·2 units per mL (serotype 3; 248·2-323·1) for 135 μg, and 115·8 (serotype 1; 98·8-135·7) to 308·6 units per mL (serotype 3; 266·8-356·8) for 180 μg. In study two, ranges at 1 month after the third vaccination (ie, month 7) were 278·5 (serotype 1; 214·9-361·0) to 545·2 units per mL (serotype 2; 431·8-688·4) for 135 μg VLA15 and 274·7 (serotype 1; 209·4-360·4) to 596·8 units per mL (serotype 3; 471·9-754·8) for 180 μg. Relative to placebo, the VLA15 groups had more frequent reports of solicited local adverse events (study one: 94%, 95% CI 91-96 vs 26%, 19-34; study two: 96%, 93-98 vs 35%, 24-49 after any vaccination) and solicited systemic adverse events (study one: 69%, 65-73 vs 43%, 34-52; study two: 74%, 67-80 vs 51%, 38-64); most were mild or moderate. In study one, unsolicited adverse events were reported by 52% (48-57) of participants in the VLA15 groups and 52% (43-60) of those in the placebo groups; for study two these were 65% (58-71) and 69% (55-80), respectively. Percentages of participants reporting serious unsolicited adverse events (study one: 2%, 1-4; study two: 4%, 2-7) and adverse events of special interest (study one: 1%, 0-2; study two: 1%, 0-3) were low across all groups. A single severe, possibly related unsolicited adverse event was reported (worsening of pre-existing ventricular extrasystoles, which resolved after change of relevant concomitant medication); no related serious adverse events or deaths were reported.

INTERPRETATION

VLA15 was safe, well tolerated, and elicited robust antibody responses to all six OspA serotypes. These findings support further clinical development of VLA15 using the 180 μg dose and 0-2-6-month schedule, which was associated with the greatest immune responses.

FUNDING

Valneva.

The Development of a Rabies Virus-Vectored Vaccine against Borrelia burgdorferi, Targeting BBI39

Lyme disease (LD) is the most common tick-borne illness in the United States (U.S.), Europe, and Asia. Borrelia burgdorferi, a spirochete bacterium transmitted by the tick vector Ixodes scapularis, causes LD in the U.S. If untreated, Lyme arthritis, heart block, and meningitis can occur. Given the absence of a human Lyme disease vaccine, we developed a vaccine using the rabies virus (RABV) vaccine vector BNSP333 and an outer surface borrelial protein, BBI39. BBI39 was previously utilized as a recombinant protein vaccine and was protective in challenge experiments; therefore, we decided to utilize this protective antigen in a rabies virus-vectored vaccine against Borrelia burgdorferi. To incorporate BBI39 into the RABV virion, we generated a chimeric BBI39 antigen, BBI39RVG, by fusing BBI39 with the final amino acids of the RABV glycoprotein by molecular cloning and viral recovery with reverse transcription genetics. Here, we have demonstrated that the BBI39RVG antigen was incorporated into the RABV virion via immunofluorescence and Western blot analysis. Mice vaccinated with our BPL inactivated RABV-BBI39RVG (BNSP333-BBI39RVG) vaccine induced high amounts of BBI39-specific antibodies, which were maintained long-term, up to eight months post-vaccination. The BBI39 antibodies neutralized Borrelia in vaccinated mice when challenged with Borrelia burgdorferi by either syringe injection or infected ticks and they reduced the Lyme disease pathology of arthritis in infected mouse joints. Overall, the RABV-based LD vaccine induced more and longer-term antibodies compared to the recombinant protein vaccine. This resulted in lower borrelial RNA in RABV-based vaccinated mice compared to recombinant protein vaccinated mice. The results of this study indicate the successful use of BBI39 as a vaccine antigen and RABV as a vaccine vector for LD.

Pathogenicity and virulence of Borrelia burgdorferi

Infection with Borrelia burgdorferi often triggers pathophysiologic perturbations that are further augmented by the inflammatory responses of the host, resulting in the severe clinical conditions of Lyme disease. While our apprehension of the spatial and temporal integration of the virulence determinants during the enzootic cycle of B. burgdorferi is constantly being improved, there is still much to be discovered. Many of the novel virulence strategies discussed in this review are undetermined. Lyme disease spirochaetes must surmount numerous molecular and mechanical obstacles in order to establish a disseminated infection in a vertebrate host. These barriers include borrelial relocation from the midgut of the feeding tick to its body cavity and further to the salivary glands, deposition to the skin, haematogenous dissemination, extravasation from blood circulation system, evasion of the host immune responses, localization to protective niches, and establishment of local as well as distal infection in multiple tissues and organs. Here, the various well-defined but also possible novel strategies and virulence mechanisms used by B. burgdorferi to evade obstacles laid out by the tick vector and usually the mammalian host during colonization and infection are reviewed.

Adiponectin in the mammalian host influences ticks' acquisition of the Lyme disease pathogen Borrelia

Arthropod-borne pathogens cause some of the most important human and animal infectious diseases. Many vectors acquire or transmit pathogens through the process of blood feeding. Here, we report adiponectin, the most abundant adipocyte-derived hormone circulating in human blood, directly or indirectly inhibits acquisition of the Lyme disease agent, Borrelia burgdorferi, by Ixodes scapularis ticks. Rather than altering tick feeding or spirochete viability, adiponectin or its associated factors induces host histamine release when the tick feeds, which leads to vascular leakage, infiltration of neutrophils and macrophages, and inflammation at the bite site. Consistent with this, adiponectin-deficient mice have diminished pro-inflammatory responses, including interleukin (IL)-12 and IL-1β, following a tick bite, compared with wild-type animals. All these factors mediated by adiponectin or associated factors influence B. burgdorferi survival at the tick bite site. These results suggest a host adipocyte-derived hormone modulates pathogen acquisition by a blood-feeding arthropod.

Dome1-JAK-STAT signaling between parasite and host integrates vector immunity and development

Ancestral signaling pathways serve critical roles in metazoan development, physiology, and immunity. We report an evolutionary interspecies communication pathway involving a central Ixodes scapularis tick receptor termed Dome1, which acquired a mammalian cytokine receptor motif exhibiting high affinity for interferon-gamma (IFN-γ). Host-derived IFN-γ facilitates Dome1-mediated activation of the Ixodes JAK-STAT pathway. This accelerates tick blood meal acquisition and development while upregulating antimicrobial components. The Dome1-JAK-STAT pathway, which exists in most Ixodid tick genomes, regulates the regeneration and proliferation of gut cells-including stem cells-and dictates metamorphosis through the Hedgehog and Notch-Delta networks, ultimately affecting Ixodes vectorial competence. We highlight the evolutionary dependence of I. scapularis on mammalian hosts through cross-species signaling mechanisms that dually influence arthropod immunity and development.

Human B Cell Epitope Map of the Lyme Disease Vaccine Antigen, OspA

The Lyme disease (LD) vaccine formerly approved for use in the United States consisted of recombinant outer surface protein A (OspA) from Borrelia burgdorferi sensu stricto (ss), the bacterial genospecies responsible for the vast majority of LD in North America. OspA is an ∼30 kDa lipoprotein made up of 21 antiparallel β-strands and a C-terminal α-helix. In clinical trials, protection against LD following vaccination correlated with serum antibody titers against a single epitope near the C-terminus of OspA, as defined by the mouse monoclonal antibody (MAb), LA-2. However, the breadth of the human antibody response to OspA following vaccination remains undefined even as next-generation multivalent OspA-based vaccines are under development. In this report, we employed hydrogen exchange-mass spectrometry (HX-MS) to localize the epitopes recognized by a unique panel of OspA human MAbs, including four shown to passively protect mice against experimental B. burgdorferi infection and one isolated from a patient with antibiotic refractory Lyme arthritis. The epitopes grouped into three spatially distinct bins that, together, encompass more than half the surface-exposed area of OspA. The bins corresponded to OspA β-strands 8-10 (bin 1), 11-13 (bin 2), and 16-20 plus the C-terminal α-helix (bin 3). Bin 3 was further divided into sub-bins relative to LA-2's epitope. MAbs with complement-dependent borreliacidal activity, as well as B. burgdorferi transmission-blocking activity in the mouse model were found within each bin. Therefore, the resulting B cell epitope map encompasses functionally important targets on OspA that likely contribute to immunity to B. burgdorferi.

What do we know about the microbiome of I. ricinus?

I. ricinus is an obligate hematophagous parasitic arthropod that is responsible for the transmission of a wide range of zoonotic pathogens including spirochetes of the genus Borrelia, Rickettsia spp., C. burnetii, Anaplasma phagocytophilum and Francisella tularensis, which are part the tick´s microbiome. Most of the studies focus on "pathogens" and only very few elucidate the role of "non-pathogenic" symbiotic microorganisms in I. ricinus. While most of the members of the microbiome are leading an intracellular lifestyle, they are able to complement tick´s nutrition and stress response having a great impact on tick´s survival and transmission of pathogens. The composition of the tick´s microbiome is not consistent and can be tied to the environment, tick species, developmental stage, or specific organ or tissue. Ovarian tissue harbors a stable microbiome consisting mainly but not exclusively of endosymbiotic bacteria, while the microbiome of the digestive system is rather unstable, and together with salivary glands, is mostly comprised of pathogens. The most prevalent endosymbionts found in ticks are Rickettsia spp., Ricketsiella spp., Coxiella-like and Francisella-like endosymbionts, Spiroplasma spp. and Candidatus Midichloria spp. Since microorganisms can modify ticks' behavior, such as mobility, feeding or saliva production, which results in increased survival rates, we aimed to elucidate the potential, tight relationship, and interaction between bacteria of the I. ricinus microbiome. Here we show that endosymbionts including Coxiella-like spp., can provide I. ricinus with different types of vitamin B (B2, B6, B7, B9) essential for eukaryotic organisms. Furthermore, we hypothesize that survival of Wolbachia spp., or the bacterial pathogen A. phagocytophilum can be supported by the tick itself since coinfection with symbiotic Spiroplasma ixodetis provides I. ricinus with complete metabolic pathway of folate biosynthesis necessary for DNA synthesis and cell division. Manipulation of tick´s endosymbiotic microbiome could present a perspective way of I. ricinus control and regulation of spread of emerging bacterial pathogens.

Past, present, and future of Lyme disease vaccines: antigen engineering approaches and mechanistic insights

INTRODUCTION

Transmitted by ticks, Lyme disease is the most common vector-borne disease in the Northern hemisphere. Despite the geographical expansion of human Lyme disease cases, no effective preventive strategies are currently available. Developing an efficacious and safe vaccine is therefore urgently needed. Efforts have previously been taken to identify vaccine targets in the causative pathogen (Borrelia burgdorferi sensu lato) and arthropod vector (Ixodes spp.). However, progress was impeded due to a lack of consumer confidence caused by the myth of undesired off-target responses, low immune responses, a limited breadth of immune reactivity, as well as by the complexities of the vaccine process development.

AREA COVERED

In this review, we summarize the antigen engineering approaches that have been applied to overcome those challenges and the underlying mechanisms that can be exploited to improve both safety and efficacy of future Lyme disease vaccines.

EXPERT OPINION

Over the past two decades, several new genetically redesigned Lyme disease vaccine candidates have shown success in both preclinical and clinical settings and built a solid foundation for further development. These studies have greatly informed the protective mechanisms of reducing Lyme disease burdens and ending the endemic of this disease.

Agglutination of Borreliella burgdorferi by Transmission-Blocking OspA Monoclonal Antibodies and Monovalent Fab Fragments

Lyme disease vaccines based on recombinant Outer surface protein A (OspA) elicit protective antibodies that interfere with tick-to-host transmission of the disease-causing spirochete Borreliella burgdorferi. Another hallmark of OspA antisera and certain OspA monoclonal antibodies (MAbs) is their capacity to induce B. burgdorferi agglutination in vitro, a phenomenon first reported more than 30 years ago but never studied in molecular detail. In this report, we demonstrate that transmission-blocking OspA MAbs, individually and in combination, promote dose-dependent and epitope-specific agglutination of B. burgdorferi. Agglutination occurred within minutes and persisted for hours. Spirochetes in the core of the aggregates exhibited evidence of outer membrane (OM) stress, revealed by propidium iodide uptake. The most potent agglutinator was the mouse MAb LA-2, which targets the OspA C terminus (β-strands 18 to 20). Human MAb 319-44, which also targets the OspA C terminus (β-strand 20), and 857-2, which targets the OspA central β-sheet (strands 8 to 10), were less potent agglutinators, while MAb 221-7, which targets β-strands 10 to 11, had little to no measurable agglutinating activity, even though its affinity for OspA exceeded that of LA-2. Remarkably, monovalent Fab fragments derived from LA-2, and to a lesser degree 319-44, retained the capacity to induce B. burgdorferi aggregation and OM stress, a particularly intriguing observation considering that "LA-2-like" Fabs have been shown to experimentally entrap B. burgdorferi within infected ticks and prevent transmission during feeding to a mammalian host. It is therefore tempting to speculate that B. burgdorferi aggregation triggered by OspA-specific antibodies in vitro may in fact reflect an important biological activity in vivo.

Host blood meal identity modifies vector gene expression and competency

A vector's susceptibility and ability to transmit a pathogen—termed vector competency—determines disease outcomes, yet the ecological factors influencing tick vector competency remain largely unknown. Ixodes pacificus, the tick vector of Borrelia burgdorferi (Bb) in the western U.S., feeds on rodents, birds, and lizards. Rodents and birds are reservoirs for Bb and infect juvenile ticks, while lizards are refractory to Bb and cannot infect feeding ticks. Additionally, the lizard bloodmeal contains borreliacidal properties, clearing previously infected feeding ticks of their Bb infection. Despite I. pacificus feeding on a range of hosts, it is undetermined how the host identity of the larval bloodmeal affects future nymphal vector competency. We experimentally evaluate the influence of larval host bloodmeal on Bb acquisition by nymphal I. pacificus. Larval I. pacificus were fed on either lizards or mice and after molting, nymphs were fed on Bb‐infected mice. We found that lizard‐fed larvae were significantly more likely to become infected with Bb during their next bloodmeal than mouse‐fed larvae. We also conducted the first RNA‐seq analysis on whole‐bodied I. pacificus and found significant upregulation of tick antioxidants and antimicrobial peptides in the lizard‐fed group. Our results indicate that the lizard bloodmeal significantly alters vector competency and gene regulation in ticks, highlighting the importance of host bloodmeal identity in vector‐borne disease transmission and upends prior notions about the role of lizards in Lyme disease community ecology.

Amplification of the Specific Conformational Fluctuation of Proteins by Site-Specific Mutagenesis and Hydrostatic Pressure

Conformational fluctuation, namely, protein interconversion between different conformations, is crucial to protein function. Outer surface protein A (OspA), comprising N- and C-terminal globular domains linked by a central β-sheet, is expressed on the surface of Borrelia burgdorferi, the causative agent of Lyme disease, and recognizes the TROSPA receptor in the tick gut. Solution nuclear magnetic resonance studies have shown that the central β-sheet and C-terminal domain containing TROSPA recognition sites are less stable than the N-terminal domain, revealing an intermediate conformation between the basic folded and completely unfolded proteins. We previously suggested that exposure of receptor-binding sites following denaturation of the C-terminal domain is advantageous for OspA binding to the receptor. Here, we observed amplification of a specific protein fluctuation by pressure perturbation and site-specific mutagenesis. The salt-bridge-destabilized mutant E160D and the cavity-enlarged mutant I243A favored the intermediate. The proportion of the intermediate accounted for almost 100% in E160D at 250 MPa. Strategies using a suitably chosen point mutation with high pressure are generally applicable for amplification of specific conformational fluctuation and potentially improve our understanding of the intermediate conformations of proteins. Knowledge of various conformations, including OspA intermediates, may be useful for designing a vaccine for Lyme disease.

The year that shaped the outcome of the OspA vaccine for human Lyme disease

The expansion of Lyme borreliosis endemic areas and the corresponding increase of disease incidence have opened the possibility for greater acceptance of a vaccine. In this perspective article, we discuss the discovery of outer surface protein A (OspA) of B. burgdorferi, and the subsequent pre-clinical testing and clinical trials of a recombinant OspA vaccine for human Lyme disease. We also discuss in detail the open public hearings of the FDA Lyme disease vaccine advisory panel held in 1998 where concerns of molecular mimicry induced autoimmunity to native OspA were raised, the limitations of those studies, and the current modifications of recombinant OspA to develop a multivalent subunit vaccine for Lyme disease.

How relevant are in vitro culture models for study of tick-pathogen interactions?

Although tick-borne infectious diseases threaten human and animal health worldwide, with constantly increasing incidence, little knowledge is available regarding vector-pathogen interactions and pathogen transmission. In vivo laboratory study of these subjects using live, intact ticks is expensive, labor-intensive, and challenging from the points of view of biosafety and ethics. Several in vitro models have been developed, including over 70 continuous cell lines derived from multiple tick species and a variety of tick organ culture systems, facilitating many research activities. However, some limitations have to be considered in the translation of the results from the in vitro environment to the in vivo situation of live, intact ticks, and vertebrate hosts. In this review, we describe the available in vitro models and selected results from their application to the study of tick-borne viruses, bacteria, and protozoa, where possible comparing these results to studies in live, intact ticks. Finally, we highlight the strengths and weaknesses of in vitro tick culture models and their essential role in tick-borne pathogen research.

Insights into the mechanism regulating the differential expression of the P28-OMP outer membrane proteins in obligatory intracellular pathogen Ehrlichia chaffeensis

Ehrlichia chaffeensis causes human monocytic ehrlichiosis (HME), which is one of the most prevalent, life-threatening emerging infectious zoonoses. The life cycle of E. chaffeensis includes ticks and mammals, in which E. chaffeensis proteins are expressed differentially contributing to bacterial survival and infection. Among the E. chaffeensis P28-OMP outer membrane proteins, OMP-1B and P28 are predominantly expressed in tick cells and mammalian macrophages, respectively. The mechanisms regulating this differential expression have not been comprehensively studied. Here, we demonstrate that the transcriptional regulators EcxR and Tr1 regulate the differential expression of omp-1B and p28 in E. chaffeensis. Recombinant E. chaffeensis Tr1 bound to the promoters of omp-1B and p28, and transactivated omp-1B and p28 promoter-EGFP fusion constructs in Escherichia coli. The consensus sequence of Tr1 binding motifs was A^C/_TTATA as determined with DNase I footprint assay. Tr1 showed a higher affinity towards the p28 promoter than the omp-1B promoter as determined with surface plasmon resonance. EcxR activated the tr1 expression in response to a temperature decrease. At 37°C low level of Tr1 activated the p28 expression. At 25°C high level of Tr1 activated the omp-1B expression, while repressing the p28 expression by binding to an additional site upstream of the p28 gene. Our data provide insights into a novel mechanism mediated by Tr1 regulating E. chaffeensis differential gene expression, which may aid in the development of new therapeutics for HME.

Comparative analysis of antibody responses to outer surface protein (Osp)A and OspC in dogs vaccinated with Lyme disease vaccines

Lyme disease (LD), the most common tick-borne disease of canines and humans in N. America, is caused by the spirochete Borreliella burgdorferi. Subunit and bacterin vaccines are available for the prevention of LD in dogs. LD bacterin vaccines, which are comprised of cell lysates of two strains of B. burgdorferi, contain over 1000 different proteins and cellular constituents. In contrast, subunit vaccines are defined in composition and consist of either outer surface protein (Osp)A or OspA and an OspC chimeritope. In this study, we comparatively assessed antibody responses to OspA and OspC induced by vaccination with all canine bacterin and subunit LD vaccines that are commercially available in North America. Dogs were administered a two-dose series of the vaccine to which they were assigned (3 weeks apart): Subunit-AC, Subunit-A, Bacterin-1, and Bacterin-2. Antibody titers to OspA and OspC were determined by ELISA and the ability of each vaccine to elicit antibodies that recognize diverse OspC proteins (referred to as OspC types) assessed by immunoblot. While all of the vaccines elicited similar OspA antibody responses, only Subunit-AC triggered a robust and broadly cross-reactive antibody response to divergent OspC proteins. The data presented within provide new information regarding vaccination-induced antibody responses to key tick and mammalian phase antigens by both subunit and bacterin LD canine vaccine formulations.

The need to unravel the twisted nature of the Borrelia burgdorferi sensu lato complex across Europe

Lyme borreliosis is a vector-borne infection caused by bacteria under the Borrelia burgdorferi sensu lato complex, both in Europe and North America. Differential gene expression at different times throughout its infectious cycle allows the spirochete to survive very diverse environments within different mammalian hosts as well as the tick vector. To date, the vast majority of data about spirochetal proteins and their functions are from genetic studies carried out on North American strains of a single species, i.e. B. burgdorferi sensu stricto. The whole-genome sequences recently obtained for several European species/strains make it feasible to adapt and use genetic techniques to study inherent differences between them. This review highlights the crucial need to undertake independent studies of genospecies within Europe, given their varying genetic content and pathogenic potential, and differences in clinical manifestation.

Interactions between Borrelia burgdorferi and its hosts across the enzootic cycle

The bacterial pathogen Borrelia burgdorferi is the causative agent of Lyme disease and is transmitted to humans through an Ixodes tick vector. B. burgdorferi is able to survive in both mammalian and tick hosts through careful modulation of its gene expression. This allows B. burgdorferi to adapt to the environmental and nutritional changes that occur when it is transmitted between the two hosts. Distinct interactions between the spirochete and its host occur at every step of the enzootic cycle and dictate the ability of the spirochete to survive until the next stage of the cycle. Studying the interface between B. burgdorferi, the Ixodes tick vector and the natural mammalian reservoirs has been made significantly more feasible through the complete genome sequences of the organisms and the advent of high throughput screening technologies. Ultimately, a thorough investigation of the interplay between the two domains (and two phyla within one domain) is necessary in order to completely understand how the pathogen is transmitted.

Human and Veterinary Vaccines for Lyme Disease

Lyme disease (LD) is an emerging zoonotic infection that is increasing in incidence in North America, Europe, and Asia. With the development of safe and efficacious vaccines, LD can potentially be prevented. Vaccination offers a cost-effective and safe approach for decreasing the risk of infection. While LD vaccines have been widely used in veterinary medicine, they are not available as a preventive tool for humans. Central to the development of effective vaccines is an understanding of the enzootic cycle of LD, differential gene expression of Borrelia burgdorferi in response to environmental variables, and the genetic and antigenic diversity of the unique bacteria that cause this debilitating disease. Here we review these areas as they pertain to past and present efforts to develop human, veterinary, and reservoir targeting LD vaccines. In addition, we offer a brief overview of additional preventative measures that should employed in conjunction with vaccination.

Interactions Between Ticks and Lyme Disease Spirochetes

Borrelia burgdorferi sensu lato causes Lyme borreliosis in a variety of animals and humans. These atypical bacterial pathogens are maintained in a complex enzootic life cycle that primarily involves a vertebrate host and Ixodes spp. ticks. In the Northeastern United States, I. scapularis is the main vector, while wild rodents serve as the mammalian reservoir host. As B. burgdorferi is transmitted only by I. scapularis and closely related ticks, the spirochete-tick interactions are thought to be highly specific. Various borrelial and arthropod proteins that directly or indirectly contribute to the natural cycle of B. burgdorferi infection have been identified. Discrete molecular interactions between spirochetes and tick components also have been discovered, which often play critical roles in pathogen persistence and transmission by the arthropod vector. This review will focus on the past discoveries and future challenges that are relevant to our understanding of the molecular interactions between B. burgdorferi and Ixodes ticks. This information will not only impact scientific advancements in the research of tick- transmitted infections but will also contribute to the development of novel preventive measures that interfere with the B. burgdorferi life cycle.

The BB0345 Hypothetical Protein of Borrelia burgdorferi Is Essential for Mammalian Infection

During the natural enzootic life cycle of Borrelia burgdorferi (also known as Borreliella burgdorferi), the bacteria must sense conditions within the vertebrate and arthropod and appropriately regulate expression of genes necessary to persist within these distinct environments. bb0345 of B. burgdorferi encodes a hypothetical protein of unknown function that is predicted to contain an N-terminal helix-turn-helix (HTH) domain. Because HTH domains can mediate protein-DNA interactions, we hypothesized that BB0345 might represent a previously unidentified borrelial transcriptional regulator with the ability to regulate events critical for the B. burgdorferi enzootic cycle. To study the role of BB0345 within mammals, we generated a bb0345 mutant and assessed its virulence potential in immunocompetent mice. The bb0345 mutant was able to initiate localized infection and disseminate to distal tissues but was cleared from all sites by 14 days postinfection. In vitro growth curve analyses revealed that the bb0345 mutant grew similar to wild-type bacteria in standard Barbour-Stoenner-Kelley II (BSK-II) medium; however, the mutant was not able to grow in dilute BSK-II medium or dialysis membrane chambers (DMCs) implanted in rats. Proteinase K accessibility assays and whole-cell partitioning indicated that BB0345 was intracellular and partially membrane associated. Comparison of protein production profiles between the wild-type parent and the bb0345 mutant revealed no major differences, suggesting BB0345 may not be a global transcriptional regulator. Taken together, these data show that BB0345 is essential for B. burgdorferi survival in the mammalian host, potentially by aiding the spirochete with a physiological function that is required by the bacterium during infection.

Lipid hijacking: a unifying theme in vector-borne diseases

Vector-borne illnesses comprise a significant portion of human maladies, representing 17% of global infections. Transmission of vector-borne pathogens to mammals primarily occurs by hematophagous arthropods. It is speculated that blood may provide a unique environment that aids in the replication and pathogenesis of these microbes. Lipids and their derivatives are one component enriched in blood and are essential for microbial survival. For instance, the malarial parasite Plasmodium falciparum and the Lyme disease spirochete Borrelia burgdorferi, among others, have been shown to scavenge and manipulate host lipids for structural support, metabolism, replication, immune evasion, and disease severity. In this Review, we will explore the importance of lipid hijacking for the growth and persistence of these microbes in both mammalian hosts and arthropod vectors.

Borrelia burgdorferi spatiotemporal regulation of transcriptional regulator bosR and decorin binding protein during murine infection

Lyme disease, caused by Borrelia burgdorferi, is an inflammatory multistage infection, consisting of localized, disseminated, and persistent disease stages, impacting several organ systems through poorly defined gene regulation mechanisms. The purpose of this study is to further characterize the spatiotemporal transcriptional regulation of B. burgdorferi during mammalian infection of borrelial oxidative stress regulator (bosR) and decorin binding protein (dbpBA) by utilizing bioluminescent B. burgdorferi reporter strains and in vivo imaging. Fluctuating borrelial load was also monitored and used for normalization to evaluate expression levels. bosR transcription is driven by two promoters, P_bb0648 and P_bosR, and we focused on the native promoter. bosR expression is low relative to the robustly expressed dbpBA throughout infection. In distal tissues, bosR was the highest in the heart during in the first week whereas dbpBA was readily detectable at all time points with each tissue displaying a distinct expression pattern. This data suggests bosR may have a role in heart colonization and the induction of dbpBA indicates a RpoS independent transcriptional regulation occurring in the mammalian cycle of pathogenesis. These finding demonstrate that B. burgdorferi engages unknown genetic mechanisms to uniquely respond to mammalian tissue environments and/or changing host response over time.

Interactions between Borrelia burgdorferi and ticks

Borrelia burgdorferi is the causative agent of Lyme disease and is transmitted to vertebrate hosts by Ixodes spp. ticks. The spirochaete relies heavily on its arthropod host for basic metabolic functions and has developed complex interactions with ticks to successfully colonize, persist and, at the optimal time, exit the tick. For example, proteins shield spirochaetes from immune factors in the bloodmeal and facilitate the transition between vertebrate and arthropod environments. On infection, B. burgdorferi induces selected tick proteins that modulate the vector gut microbiota towards an environment that favours colonization by the spirochaete. Additionally, the recent sequencing of the Ixodes scapularis genome and characterization of tick immune defence pathways, such as the JAK–STAT, immune deficiency and cross-species interferon-γ pathways, have advanced our understanding of factors that are important for B. burgdorferi persistence in the tick. In this Review, we summarize interactions between B. burgdorferi and I. scapularis during infection, as well as interactions with tick gut and salivary gland proteins important for establishing infection and transmission to the vertebrate host.

Borrelia burgdorferi has a complex life cycle with several different hosts, causing Lyme disease when it infects humans. In this Review, Fikrig and colleagues discuss how B. burgdorferi infects and interacts with its tick vector to ensure onward transmission.

Dynamic aspects of pressure and temperature-stabilized intermediates of outer surface protein A

Structural characterization of alternatively folded and partially disordered protein conformations remains challenging. Outer surface protein A (OspA) is a pivotal protein in Borrelia infection, which is the etiological agent of Lyme disease. OspA exists in equilibrium with intermediate conformations, in which the central and the C-terminal regions of the protein have lower stabilities than the N-terminal. Here, we characterize pressure- and temperature-stabilized intermediates of OspA by nuclear magnetic resonance spectroscopy combined with paramagnetic relaxation enhancement (PRE). We found that although the C-terminal region of the intermediate was partially disordered, it retains weak specific contact with the N-terminal region, owing to a twist of the central β-sheet and increased flexibility in the polypeptide chain. The disordered C-terminal region of the pressure-stabilized intermediate was more compact than that of the temperature-stabilized form. Further, molecular dynamics simulation demonstrated that temperature-induced disordering of the β-sheet was initiated at the C-terminal region and continued through to the central region. An ensemble of simulation snapshots qualitatively described the PRE data from the intermediate and indicated that the intermediate structures of OspA may expose tick receptor-binding sites more readily than does the basic folded conformation.

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

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

Salp15, a Multifunctional Protein From Tick Saliva With Potential Pharmaceutical Effects

Ixodes ticks are the main vectors for a number of zoonotic diseases, including Lyme disease. Ticks secrete saliva directly into a mammalian host while feeding on the host's blood. This action serves to modulate host immunity and coagulation, thus allowing ticks to attach and feed upon their host. One of the most extensively studied components of tick saliva is Salp15. Research has shown that this protein binds specifically to CD4 molecules on the surface of T lymphocytes, interferes with TCR-mediated signaling transduction, inhibits CD4+ T cell activation and proliferation, and impedes the secretion of interleukin 2 (IL-2). Salp15 also binds specifically to dendritic cell dendritic cell-specific intercellular adhesion molecule-3-grabbing non-integrin (DC-SIGN) to up-regulate the expression of CD73 in regulatory T cells. Collectively, these findings render this salivary protein a potential candidate for a range of therapeutic applications. Here, we discuss our current understanding of Salp15 and the mechanisms that might be used to treat disease.

The RpoS Gatekeeper in Borrelia burgdorferi: An Invariant Regulatory Scheme That Promotes Spirochete Persistence in Reservoir Hosts and Niche Diversity

Maintenance of Borrelia burgdorferi within its enzootic cycle requires a complex regulatory pathway involving the alternative σ factors RpoN and RpoS and two ancillary trans-acting factors, BosR and Rrp2. Activation of this pathway occurs within ticks during the nymphal blood meal when RpoS, the effector σ factor, transcribes genes required for tick transmission and mammalian infection. RpoS also exerts a 'gatekeeper' function by repressing σ⁷⁰-dependent tick phase genes (e.g., ospA, lp6.6). Herein, we undertook a broad examination of RpoS functionality throughout the enzootic cycle, beginning with modeling to confirm that this alternative σ factor is a 'genuine' RpoS homolog. Using a novel dual color reporter system, we established at the single spirochete level that ospA is expressed in nymphal midguts throughout transmission and is not downregulated until spirochetes have been transmitted to a naïve host. Although it is well established that rpoS/RpoS is expressed throughout infection, its requirement for persistent infection has not been demonstrated. Plasmid retention studies using a trans-complemented ΔrpoS mutant demonstrated that (i) RpoS is required for maximal fitness throughout the mammalian phase and (ii) RpoS represses tick phase genes until spirochetes are acquired by a naïve vector. By transposon mutant screening, we established that bba34/oppA5, the only OppA oligopeptide-binding protein controlled by RpoS, is a bona fide persistence gene. Lastly, comparison of the strain 297 and B31 RpoS DMC regulons identified two cohorts of RpoS-regulated genes. The first consists of highly conserved syntenic genes that are similarly regulated by RpoS in both strains and likely required for maintenance of B. burgdorferi sensu stricto strains in the wild. The second includes RpoS-regulated plasmid-encoded variable surface lipoproteins ospC, dbpA and members of the ospE/ospF/elp, mlp, revA, and Pfam54 paralogous gene families, all of which have evolved via inter- and intra-strain recombination. Thus, while the RpoN/RpoS pathway regulates a 'core' group of orthologous genes, diversity within RpoS regulons of different strains could be an important determinant of reservoir host range as well as spirochete virulence.

The CXXC Motifs Are Essential for the Function of BosR in Borrelia burgdorferi

BosR, a Fur family member, is essential for the pathogenesis of the Lyme disease pathogen, Borrelia burgdorferi. Unlike typical Fur proteins in which DNA binding represses gene expression, binding of BosR to the rpoS promoter directly activates rpoS transcription in B. burgdorferi. However, virtually nothing is known concerning potential structural features and amino acid residues of BosR that are important for protein function and virulence regulation in B. burgdorferi. Particularly, it remains unknown what structural motifs or residues of BosR coordinate Zn, although previous analyses have indicated that the function of BosR may depend on Zn. To address these information gaps, we herein introduced mutations into four conserved cysteine residues in two putative CXXC motifs of BosR. Our data showed that the ability of BosR to bind Zn was dramatically reduced when the CXXC motifs were mutated. Moreover, we found that the two CXXC motifs contributed to the ability of BosR to form dimers. By using a trans-complementation genetic approach, we additionally demonstrated that both CXXC motifs of BosR were essential for in vivo gene expression regulation. Mutation of any of the four cysteines abolished the transcriptional activation of rpoS. In contrast to wild type BosR, each mutant protein was incapable of binding the rpoS promoter in electrophoretic mobility shift assays. The combined data strongly support that the two CXXC motifs and four cysteines constitute the structural site essential for Zn-coordination, protein dimerization, and the unique regulatory activity of BosR.

The Emerging Role of Microbial Biofilm in Lyme Neuroborreliosis

Lyme borreliosis (LB) is the most common tick-borne disease caused by the spirochete Borrelia burgdorferi in North America and Borrelia afzelii or Borrelia garinii in Europe and Asia, respectively. The infection affects multiple organ systems, including the skin, joints, and the nervous system. Lyme neuroborreliosis (LNB) is the most dangerous manifestation of Lyme disease, occurring in 10-15% of infected individuals. During the course of the infection, bacteria migrate through the host tissues altering the coagulation and fibrinolysis pathways and the immune response, reaching the central nervous system (CNS) within 2 weeks after the bite of an infected tick. The early treatment with oral antimicrobials is effective in the majority of patients with LNB. Nevertheless, persistent forms of LNB are relatively common, despite targeted antibiotic therapy. It has been observed that the antibiotic resistance and the reoccurrence of Lyme disease are associated with biofilm-like aggregates in B. burgdorferi, B. afzelii, and B. garinii, both in vitro and in vivo, allowing Borrelia spp. to resist to adverse environmental conditions. Indeed, the increased tolerance to antibiotics described in the persisting forms of Borrelia spp., is strongly reminiscent of biofilm growing bacteria, suggesting a possible role of biofilm aggregates in the development of the different manifestations of Lyme disease including LNB.

A bite so sweet: the glycobiology interface of tick-host-pathogen interactions

Vector-borne diseases constitute 17% of all infectious diseases in the world; among the blood-feeding arthropods, ticks transmit the highest number of pathogens. Understanding the interactions between the tick vector, the mammalian host and the pathogens circulating between them is the basis for the successful development of vaccines against ticks or the tick-transmitted pathogens as well as for the development of specific treatments against tick-borne infections. A lot of effort has been put into transcriptomic and proteomic analyses; however, the protein-carbohydrate interactions and the overall glycobiology of ticks and tick-borne pathogens has not been given the importance or priority deserved. Novel (bio)analytical techniques and their availability have immensely increased the possibilities in glycobiology research and thus novel information in the glycobiology of ticks and tick-borne pathogens is being generated at a faster pace each year. This review brings a comprehensive summary of the knowledge on both the glycosylated proteins and the glycan-binding proteins of the ticks as well as the tick-transmitted pathogens, with emphasis on the interactions allowing the infection of both the ticks and the hosts by various bacteria and tick-borne encephalitis virus.

Investigating disease severity in an animal model of concurrent babesiosis and Lyme disease

The incidence of babesiosis, Lyme disease and other tick-borne diseases has increased steadily in Europe and North America during the last five decades. Babesia microti is transmitted by species of Ixodes, the same ticks that transmit the Lyme disease-causing spirochete, Borrelia burgdorferi. B. microti can also be transmitted through transfusion of blood products and is the most common transfusion-transmitted infection in the U.S.A. Ixodes ticks are commonly infected with both B. microti and B. burgdorferi, and are competent vectors for transmitting them together into hosts. Few studies have examined the effects of coinfections on humans and they had somewhat contradictory results. One study linked coinfection with B. microti to a greater number of symptoms of overall disease in patients, while another report indicated that B. burgdorferi infection either did not affect babesiosis symptoms or decreased its severity. Mouse models of infection that manifest pathological effects similar to those observed in human babesiosis and Lyme disease offer a unique opportunity to thoroughly investigate the effects of coinfection on the host. Lyme disease has been studied using the susceptible C3H mouse infection model, which can also be used to examine B. microti infection to understand pathological mechanisms of human diseases, both during a single infection and during coinfections. We observed that high B. microti parasitaemia leads to low haemoglobin levels in infected mice, reflecting the anaemia observed in human babesiosis. Similar to humans, B. microti coinfection appears to enhance the severity of Lyme disease-like symptoms in mice. Coinfected mice have lower peak B. microti parasitaemia compared to mice infected with B. microti alone, which may reflect attenuation of babesiosis symptoms reported in some human coinfections. These findings suggest that B. burgdorferi coinfection attenuates parasite growth while B. microti presence exacerbates Lyme disease-like symptoms in mice.

mRNA transcript distribution bias between Borrelia burgdorferi bacteria and their outer membrane vesicles

Borrelia burgdorferi, a bacterium in the spirochete phylum, is the causative agent of Lyme disease. Borrelia burgdorferi has a linear chromosome with a number of circular and linear plasmids. Bacteria, including B. burgdorferi, release spherical outer membrane vesicles (OMVs) that are known to carry secretory products including metabolites, nucleic acids and proteins. Herein, we provide the first comparative transcriptomic analysis of the vesicles released from B. burgdorferi. We identified a total of ∼1200 unique transcripts with at least one mapped read from the bacterial cell and its OMVs. We compared the spectrum of transcripts between bacterial cell and its OMVs, and found a biased distribution based on the source of transcripts, i.e. plasmid-encoded transcripts are more likely to be enriched in the OMVs. We validated the distribution for some of the transcripts by qPCR. This analysis provides the first evidence that some of the B. burgdorferi transcripts are preferentially packaged in OMV, which further suggest that the bacteria might use its OMVs for bacteria-bacteria or bacteria-host communications. This report also suggests a possible involvement of Borrelia-derived OMVs in the development of Lyme disease in both early and post disease syndromes.

An Overlapping Region between the Two Terminal Folding Units of the Outer Surface Protein A (OspA) Controls Its Folding Behavior

Although many naturally occurring proteins consist of multiple domains, most studies on protein folding to date deal with single-domain proteins or isolated domains of multi-domain proteins. Studies of multi-domain protein folding are required for further advancing our understanding of protein folding mechanisms. Borrelia outer surface protein A (OspA) is a β-rich two-domain protein, in which two globular domains are connected by a rigid and stable single-layer β-sheet. Thus, OspA is particularly suited as a model system for studying the interplays of domains in protein folding. Here, we studied the equilibria and kinetics of the urea-induced folding-unfolding reactions of OspA probed with tryptophan fluorescence and ultraviolet circular dichroism. Global analysis of the experimental data revealed compelling lines of evidence for accumulation of an on-pathway intermediate during kinetic refolding and for the identity between the kinetic intermediate and a previously described equilibrium unfolding intermediate. The results suggest that the intermediate has the fully native structure in the N-terminal domain and the single layer β-sheet, with the C-terminal domain still unfolded. The observation of the productive on-pathway folding intermediate clearly indicates substantial interactions between the two domains mediated by the single-layer β-sheet. We propose that a rigid and stable intervening region between two domains creates an overlap between two folding units and can energetically couple their folding reactions.

Borrelia burgdorferi surface protein Lmp1 facilitates pathogen dissemination through ticks as studied by an artificial membrane feeding system

In its natural infection cycle, the pathogen of Lyme borreliosis transits between a tick vector and a mammalian host. As relatively a minor fraction of spirochetes transits between the host and the vector precluding their reliable detection at early infection, artificial membrane feeders emerged as useful tools to study roles of spirochete proteins in pathogen entry, persistence, and exit through ticks. Here we report the development of a modified membrane feeder to study the role of a Borrelia burgdorferi surface protein called Lmp1 in spirochete transitions between the murine host and ticks. We show that our membrane feeder supports the blood meal engorgement process where ticks can acquire spirochetes from the feeder containing extremely low levels of pathogens (10² cells/ml of blood). Our data revealed that in comparison to wild-type spirochetes, lmp1 deletion mutants are significantly impaired for acquisition in naïve ticks as well as transmission from infected ticks. Taking together, our data suggest that Lmp1 plays an essential role in spirochete transitions between hosts and the vector. These studies also underscore the usefulness of artificial membrane feeding system as a valuable tool to study the role of B. burgdorferi gene-products in pathogen persistence in and passage through vector ticks.

Two Photon Intravital Microscopy of Lyme Borrelia in Mice

Two-photon intravital microscopy is a powerful tool that allows visualization of cells in intact tissues in a live animal in real time. In recent years, this advanced technology has been applied to understand pathogen-host interactions using fluorescently labeled bacteria. In particular, infectious fluorescent transformants of the Lyme disease spirochete Borrelia burgdorferi, an Ixodes tick-transmitted pathogen, have been imaged by two-photon intravital microscopy to study bacterial motility and interactions of the pathogen with feeding ticks and host tissues. Here, we describe the techniques and equipment used to image mammalian-adapted spirochetes in the skin of living mice in vivo and in joints ex vivo using two-photon intravital microscopy.

Vaccination of horses with Lyme vaccines for dogs induces short-lasting antibody responses

Borrelia burgdorferi can induce Lyme disease. Approved Lyme vaccines for horses are currently not available. In an effort to protect horses, veterinarians are using Lyme vaccines licensed for dogs. However, data to assess the response of horses to, or determine the efficacy of this off-label vaccine use are missing. Here, antibodies against outer surface protein A (OspA), OspC, and OspF were quantified in diagnostic serum submissions from horses with a history of vaccination with canine Lyme vaccines. The results suggested that many horses respond with low and often short-lasting antibody responses. Subsequently, four experimental vaccination trials were performed. First, we investigated antibody responses to three canine vaccines in B. burgdorferi-naïve horses. One killed bacterin vaccine induced antibodies against OspC. OspA antibodies were low for all three vaccines and lasted less than 16weeks. The second trial tested the impact of the vaccine dose using the OspA/OspC inducing bacterin vaccine in horses. A 2mL dose produced higher OspA and OspC antibody values than a 1mL dose. However, the antibody response again quickly declined, independent of dose. Third, the horses were vaccinated with 2 doses of a recombinant OspA vaccine. Previous vaccination and/or environmental exposure enhanced the magnitude and longevity of the OspA antibody response to about 20weeks. Last, the influence of intramuscular versus subcutaneous vaccine administration was investigated for the recombinant OspA vaccine. OspA antibody responses were not influenced by injection route. The current work highlights that commercial Lyme vaccines for dogs induce only transient antibody responses in horses which can also be of low magnitude. Protection from infection with B. burgdorferi should not be automatically assumed after vaccinating horses with Lyme vaccines for dogs.

The abundance of the Lyme disease pathogen Borrelia afzelii declines over time in the tick vector Ixodes ricinus

Background

The population dynamics of vector-borne pathogens inside the arthropod vector can have important consequences for vector-to-host transmission. Tick-borne spirochete bacteria of the Borrelia burgdorferi (sensu lato) species complex cause Lyme borreliosis in humans and spend long periods of time (>12 months) in their Ixodes tick vectors. To date, few studies have investigated the dynamics of Borrelia spirochete populations in unfed Ixodes nymphal ticks.

Methods

Larval ticks from our laboratory colony of I. ricinus were experimentally infected with B. afzelii, and killed at 1 month and 4 months after the larva-to-nymph moult. The spirochete load was also compared between engorged larval ticks and unfed nymphs (from the same cohort) and between unfed nymphs and unfed adult ticks (from the same cohort). The spirochete load of B. afzelii in each tick was estimated using qPCR.

Results

The mean spirochete load in the 1-month-old nymphs (~14,000 spirochetes) was seven times higher than the 4-month-old nymphs (~2000 spirochetes). Thus, the nymphal spirochete load declined by 80% over a period of 3 months. An engorged larval tick acquired ~100 spirochetes, and this population was 20 times larger in a young, unfed nymph. The spirochete load also appeared to decline in adult ticks. Comparison between wild and laboratory populations found that lab ticks were more susceptible to acquiring B. afzelii.

Conclusion

The spirochete load of B. afzelii declines dramatically over time in domesticated I. ricinus nymphs under laboratory conditions. Future studies should investigate whether temporal declines in spirochete load occur in wild Ixodes ticks under natural conditions and whether these declines influence the tick-to-host transmission of Borrelia.

Electronic supplementary material

The online version of this article (doi:10.1186/s13071-017-2187-4) contains supplementary material, which is available to authorized users.

Physiologic and Genetic Factors Influencing the Zoonotic Cycle of Borrelia burgdorferi

Borrelia burgdorferi is a symbiont of ticks of the Ixodes ricinus complex. These ticks serve as vectors to disseminate the spirochete to a variety of susceptible vertebrate hosts, which, in turn, act as reservoirs for naïve ticks to become infected, perpetuating the infectious life cycle of B. burgdorferi. The pivotal role of ticks in this life cycle and tick-spirochete interactions are the focus of this chapter. Here, we describe the challenging physiological environment that spirochetes encounter within Ixodes ticks, and the genetic factors that B. burgdorferi uses to successfully infect, persist, and be transmitted from the vector.

Evidence that BosR (BB0647) Is a Positive Autoregulator in Borrelia burgdorferi

Borrelia burgdorferi survives in nature through a complex tick-mammalian life cycle. During its transit between ticks and mammalian hosts, B. burgdorferi must dramatically alter its outer surface profile in order to interact with and adapt to these two diverse niches. It has been established that the regulator BosR (BB0647) in B. burgdorferi plays important roles in modulating borrelial host adaptation. However, to date, how bosR expression itself is controlled in B. burgdorferi remains largely unknown. Previously, it has been shown that DNA sequences upstream of BosR harbor multiple sites for the binding of recombinant BosR, suggesting that BosR may influence its own expression in B. burgdorferi However, direct experimental evidence supporting this putative autoregulation of BosR has been lacking. Here, we investigated the expression of bosR throughout the tick-mammal life cycle of B. burgdorferi via quantitative reverse transcription (RT)-PCR analyses. Our data indicated that bosR is expressed not only during mouse infection, but also during the tick acquisition, intermolt, and transmission phases. Further investigation revealed that bosR expression in B. burgdorferi is influenced by environmental stimuli, such as temperature shift and pH change. By employing luciferase reporter assays, we also identified two promoters potentially driving bosR transcription. Our study offers strong support for the long-postulated function of BosR as an autoregulator in B. burgdorferi.

Functional Equivalence of OspA and OspB, but Not OspC, in Tick Colonization by Borrelia burgdorferi

Borrelia burgdorferi, a Lyme disease agent, makes different major outer surface lipoproteins at different stages of its mouse-tick infectious cycle. Outer surface protein A (OspA) coats the spirochetes from the time they enter ticks until they are transmitted to a mammal. OspA is required for normal tick colonization and has been shown to bind a tick midgut protein, indicating that OspA may serve as a tick midgut adhesin. Tick colonization by spirochetes lacking OspA is increased when the infecting blood meal is derived from mice that do not produce antibody, indicating that OspA may protect the spirochetes from host antibody, which will not recognize tick-specific proteins such as OspA. To further study the importance of OspA during tick colonization, we constructed a form of B. burgdorferi in which the ospA open reading frame, on lp54, was replaced with the ospC gene or the ospB gene, encoding a mammal-specific or tick-specific lipoprotein, respectively. These fusions yielded a strain that produces OspC within a tick (from the fusion gene) and during early mammalian infection (from the normal ospC locus) and a strain that produces OspB in place of OspA within ticks. Here we show that the related, tick-specific protein OspB can fully substitute for OspA, whereas the unrelated, mammal-specific protein OspC cannot. These data were derived from three different methods of infecting ticks, and they confirm and extend previous studies indicating that OspA both protects spirochetes within ticks from mammalian antibody and serves an additional role during tick colonization.

Statins reduce spirochetal burden and modulate immune responses in the C3H/HeN mouse model of Lyme disease

Lyme disease (LD) is a systemic disorder caused by Borrelia burgdorferi. Lyme spirochetes encode for a functional 3-hydroxy-3-methyl-glutaryl-coenzyme A reductase (HMGR EC 1.1.1.88) serving as a rate limiting enzyme of the mevalonate pathway that contribute to components critical for cell wall biogenesis. Statins have been shown to inhibit B. burgdorferi in vitro. Using a mouse model of Lyme disease, we found that statins contribute to reducing bacterial burden and altering the murine immune response to favor clearance of spirochetes.

Transcriptional Profiling the 150 kb Linear Megaplasmid of Borrelia turicatae Suggests a Role in Vector Colonization and Initiating Mammalian Infection

Adaptation is key for survival as vector-borne pathogens transmit between the arthropod and vertebrate, and temperature change is an environmental signal inducing alterations in gene expression of tick-borne spirochetes. While plasmids are often associated with adaptation, complex genomes of relapsing fever spirochetes have hindered progress in understanding the mechanisms of vector colonization and transmission. We utilized recent advances in genome sequencing to generate the most complete version of the Borrelia turicatae 150 kb linear megaplasmid (lp150). Additionally, a transcriptional analysis of open reading frames (ORFs) in lp150 was conducted and identified regions that were up-regulated during in vitro cultivation at tick-like growth temperatures (22°C), relative to bacteria grown at 35°C and infected murine blood. Evaluation of the 3’ end of lp150 identified a cluster of ORFs that code for putative surface lipoproteins. With a microbe’s surface proteome serving important roles in pathogenesis, we confirmed the ORFs expression in vitro and in the tick compared to spirochetes infecting murine blood. Transcriptional evaluation of lp150 indicates the plasmid likely has essential roles in vector colonization and/or initiating mammalian infection. These results also provide a much needed transcriptional framework to delineate the molecular mechanisms utilized by relapsing fever spirochetes during their enzootic cycle.

The lipid raft proteome of Borrelia burgdorferi

Eukaryotic lipid rafts are membrane microdomains that have significant amounts of cholesterol and a selective set of proteins that have been associated with multiple biological functions. The Lyme disease agent, Borrelia burgdorferi, is one of an increasing number of bacterial pathogens that incorporates cholesterol onto its membrane, and form cholesterol glycolipid domains that possess all the hallmarks of eukaryotic lipid rafts. In this study, we isolated lipid rafts from cultured B. burgdorferi as a detergent resistant membrane (DRM) fraction on density gradients, and characterized those molecules that partitioned exclusively or are highly enriched in these domains. Cholesterol glycolipids, the previously known raft-associated lipoproteins OspA and OpsB, and cholera toxin partitioned into the lipid rafts fraction indicating compatibility with components of the DRM. The proteome of lipid rafts was analyzed by a combination of LC-MS/MS or MudPIT. Identified proteins were analyzed in silico for parameters that included localization, isoelectric point, molecular mass and biological function. The proteome provided a consistent pattern of lipoproteins, proteases and their substrates, sensing molecules and prokaryotic homologs of eukaryotic lipid rafts. This study provides the first analysis of a prokaryotic lipid raft and has relevance for the biology of Borrelia, other pathogenic bacteria, as well as for the evolution of these structures. All MS data have been deposited in the ProteomeXchange with identifier PXD002365 (http://proteomecentral.proteomexchange.org/dataset/PXD002365).

Interaction of spirochetes with the host fibrinolytic system and potential roles in pathogenesis

The pathogenic spirochetes Borrelia burgdorferi, B. hermsii, B. recurrentis, Treponema denticola and Leptospira spp. are the etiologic agents of Lyme disease, relapsing fever, periodontitis and leptospirosis, respectively. Lyme borreliosis is a multi-systemic disorder and the most prevalent tick-borne disease in the northern hemisphere. Tick-borne relapsing fever is persistent in endemic areas worldwide, representing a significant burden in some African regions. Periodontal disease, a chronic inflammatory disorder that often leads to tooth loss, is caused by several potential pathogens found in the oral cavity including T. denticola. Leptospirosis is considered the most widespread zoonosis, and the predominant human disease in tropical, undeveloped regions. What these diseases have in common is that they are a significant burden to healthcare costs in the absence of prophylactic measures. This review addresses the interaction of these spirochetes with the fibrinolytic system, plasminogen (Plg) binding to the surface of bacteria and the generation of plasmin (Pla) on their surface. The consequences on host-pathogen interactions when the spirochetes are endowed with this proteolytic activity are discussed on the basis of the results reported in the literature. Spirochetes equipped with Pla activity have been shown to degrade extracellular matrix (ECM) components, in addition to digesting fibrin, facilitating bacterial invasion and dissemination. Pla generation triggers the induction of matrix metalloproteases (MMPs) in a cascade of events that enhances the proteolytic capacity of the spirochetes. These activities in concert with the interference exerted by the Plg/Pla on the complement system - helping the bacteria to evade the immune system - should illuminate our understanding of the mechanisms involved in host infection.