Immunological and molecular polymorphisms of OspC, an immunodominant major outer surface protein of Borrelia burgdorferi

Structure of a Human Monoclonal Antibody in Complex with Outer Surface Protein C of the Lyme Disease Spirochete, Borreliella burgdorferi

Lyme disease is a tick-borne, multisystem infection caused by the spirochete Borreliella burgdorferi. Although Abs have been implicated in the resolution of Lyme disease, the specific B cell epitopes targeted during human infections remain largely unknown. In this study, we characterized and defined the structural epitope of a patient-derived bactericidal monoclonal IgG (B11) against outer surface protein C (OspC), a homodimeric lipoprotein necessary for B. burgdorferi tick-mediated transmission and early-stage colonization of vertebrate hosts. High-resolution epitope mapping was accomplished through hydrogen deuterium exchange-mass spectrometry and X-ray crystallography. Structural analysis of B11 Fab-OspCA complexes revealed the B11 Fabs associated in a 1:1 stoichiometry with the lateral faces of OspCA homodimers such that the Abs are essentially positioned perpendicular to the spirochete's outer surface. B11's primary contacts reside within the membrane-proximal regions of α-helices 1 and 6 and adjacent loops 5 and 6 in one OspCA monomer. In addition, B11 spans the OspCA dimer interface, engaging opposing α-helix 1', α-helix 2', and loop 2-3' in the second OspCA monomer. The B11-OspCA structure is reminiscent of the recently solved mouse transmission blocking monoclonal IgG B5 in complex with OspCA, indicating a mode of engagement with OspC that is conserved across species. In conclusion, we provide a detailed insight into the interaction between a functional human Ab and an immunodominant Lyme disease Ag long considered an important vaccine candidate.

Structure of a human monoclonal antibody in complex with Outer surface protein C (OspC) of the Lyme disease spirochete, Borreliella burgdorferi

Lyme disease is a tick-borne, multisystem infection caused by the spirochete, Borreliella burgdorferi . Although antibodies have been implicated in the resolution of Lyme disease, the specific B cell epitopes targeted during human infections remain largely unknown. In this study, we characterized and defined the structural epitope of a patient-derived bactericidal monoclonal IgG ("B11") against Outer surface protein C (OspC), a homodimeric lipoprotein necessary for B. burgdorferi tick-mediated transmission and early-stage colonization of vertebrate hosts. High-resolution epitope mapping was accomplished through hydrogen deuterium exchange-mass spectrometry (HDX-MS) and X-ray crystallography. Structural analysis of B11 Fab-OspC A complexes revealed the B11 Fabs associated in a 1:1 stoichiometry with the lateral faces of OspC A homodimers such that the antibodies are essentially positioned perpendicular to the spirochete's outer surface. B11's primary contacts reside within the membrane proximal regions of α-helices 1 and 6 and adjacent loops 5 and 6 in one OspC A monomer. In addition, B11 spans the OspC A dimer interface, engaging opposing α-helix 1', α-helix 2', and loop 2-3' in the second OspC A monomer. The B11-OspC A structure is reminiscent of the recently solved mouse transmission blocking monoclonal IgG B5 in complex with OspC A , indicating a mode of engagement with OspC that is conserved across species. In conclusion, we provide the first detailed insight into the interaction between a functional human antibody and an immunodominant Lyme disease antigen long considered an important vaccine target.

Characterization and genomic analysis of the Lyme disease spirochete bacteriophage ϕBB-1

Lyme disease is a tick-borne infection caused by the spirochete Borrelia (Borreliella) burgdorferi. Borrelia species have highly fragmented genomes composed of a linear chromosome and a constellation of linear and circular plasmids some of which are required throughout the enzootic cycle. Included in this plasmid repertoire by almost all Lyme disease spirochetes are the 32-kb circular plasmid cp32 prophages that are capable of lytic replication to produce infectious virions called ϕBB-1. While the B. burgdorferi genome contains evidence of horizontal transfer, the mechanisms of gene transfer between strains remain unclear. While we know that ϕBB-1 transduces cp32 and shuttle vector DNA during in vitro cultivation, the extent of ϕBB-1 DNA transfer is not clear. Herein, we use proteomics and long-read sequencing to further characterize ϕBB-1 virions. Our studies identified the cp32 pac region and revealed that ϕBB-1 packages linear cp32s via a headful mechanism with preferential packaging of plasmids containing the cp32 pac region. Additionally, we find ϕBB-1 packages fragments of the linear chromosome and full-length plasmids including lp54, cp26, and others. Furthermore, sequencing of ϕBB-1 packaged DNA allowed us to resolve the covalently closed hairpin telomeres for the linear B. burgdorferi chromosome and most linear plasmids in strain CA-11.2A. Collectively, our results shed light on the biology of the ubiquitous ϕBB-1 phage and further implicates ϕBB-1 in the generalized transduction of diverse genes and the maintenance of genetic diversity in Lyme disease spirochetes.

Diversity and host specificity of Borrelia burgdorferi's outer surface protein C (ospC) alleles in synanthropic mammals, with a notable ospC allele U absence from mixed infections

Interactions among pathogen genotypes that vary in host specificity may affect overall transmission dynamics in multi-host systems. Borrelia burgdorferi, a bacterium that causes Lyme disease, is typically transmitted among wildlife by Ixodes ticks. Despite the existence of many alleles of B. burgdorferi's sensu stricto outer surface protein C (ospC) gene, most human infections are caused by a small number of ospC alleles ["human infectious alleles" (HIAs)], suggesting variation in host specificity associated with ospC. To characterize the wildlife host association of B. burgdorferi's ospC alleles, we used metagenomics to sequence ospC alleles from 68 infected individuals belonging to eight mammalian species trapped at three sites in suburban New Brunswick, New Jersey (USA). We found that multiple allele ("mixed") infections were common. HIAs were most common in mice (Peromyscus spp.) and only one HIA was detected at a site where mice were rarely captured. ospC allele U was exclusively found in chipmunks (Tamias striatus), and although a significant number of different alleles were observed in chipmunks, including HIAs, allele U never co-occurred with other alleles in mixed infections. Our results suggest that allele U may be excluding other alleles, thereby reducing the capacity of chipmunks to act as reservoirs for HIAs.

Characterization and genomic analysis of the Lyme disease spirochete bacteriophage ϕBB-1

Lyme disease is a tick-borne infection caused by the spirochete Borrelia (Borreliella) burgdorferi. Borrelia species have highly fragmented genomes composed of a linear chromosome and a constellation of linear and circular plasmids some of which are required throughout the enzootic cycle. Included in this plasmid repertoire by almost all Lyme disease spirochetes are the 32-kb circular plasmid cp32 prophages that are capable of lytic replication to produce infectious virions called ϕBB-1. While the B. burgdorferi genome contains evidence of horizontal transfer, the mechanisms of gene transfer between strains remain unclear. While we know that ϕBB-1 transduces cp32 and shuttle vector DNA during in vitro cultivation, the extent of ϕBB-1 DNA transfer is not clear. Herein, we use proteomics and long-read sequencing to further characterize ϕBB-1 virions. Our studies identified the cp32 pac region and revealed that ϕBB-1 packages linear cp32s via a headful mechanism with preferentially packaging of plasmids containing the cp32 pac region. Additionally, we find ϕBB-1 packages fragments of the linear chromosome and full-length plasmids including lp54, cp26, and others. Furthermore, sequencing of ϕBB-1 packaged DNA allowed us to resolve the covalently closed hairpin telomeres for the linear B. burgdorferi chromosome and most linear plasmids in strain CA-11.2A. Collectively, our results shed light on the biology of the ubiquitous ϕBB-1 phage and further implicates ϕBB-1 in the generalized transduction of diverse genes and the maintenance of genetic diversity in Lyme disease spirochetes.

Coupled induction of prophage and virulence factors during tick transmission of the Lyme disease spirochete

The alternative sigma factor RpoS plays a central role in the critical host-adaptive response of the Lyme disease spirochete, Borrelia burgdorferi. We previously identified bbd18 as a negative regulator of RpoS but could not inactivate bbd18 in wild-type spirochetes. In the current study we employed an inducible bbd18 gene to demonstrate the essential nature of BBD18 for viability of wild-type spirochetes in vitro and at a unique point in vivo. Transcriptomic analyses of BBD18-depleted cells demonstrated global induction of RpoS-dependent genes prior to lysis, with the absolute requirement for BBD18, both in vitro and in vivo, circumvented by deletion of rpoS. The increased expression of plasmid prophage genes and the presence of phage particles in the supernatants of lysing cultures indicate that RpoS regulates phage lysis-lysogeny decisions. Through this work we identify a mechanistic link between endogenous prophages and the RpoS-dependent adaptive response of the Lyme disease spirochete.

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.

Phylogenomic Diversity Elucidates Mechanistic Insights into Lyme Borreliae-Host Association

Host association-the selective adaptation of pathogens to specific host species-evolves through constant interactions between host and pathogens, leaving a lot yet to be discovered on immunological mechanisms and genomic determinants. The causative agents of Lyme disease (LD) are spirochete bacteria composed of multiple species of the Borrelia burgdorferi sensu lato complex, including B. burgdorferi (Bb), the main LD pathogen in North America-a useful model for the study of mechanisms underlying host-pathogen association. Host adaptation requires pathogens' ability to evade host immune responses, such as complement, the first-line innate immune defense mechanism. We tested the hypothesis that different host-adapted phenotypes among Bb strains are linked to polymorphic loci that confer complement evasion traits in a host-specific manner. We first examined the survivability of 20 Bb strains in sera in vitro and/or bloodstream and tissues in vivo from rodent and avian LD models. Three groups of complement-dependent host-association phenotypes emerged. We analyzed complement-evasion genes, identified a priori among all strains and sequenced and compared genomes for individual strains representing each phenotype. The evolutionary history of ospC loci is correlated with host-specific complement-evasion phenotypes, while comparative genomics suggests that several gene families and loci are potentially involved in host association. This multidisciplinary work provides novel insights into the functional evolution of host-adapted phenotypes, building a foundation for further investigation of the immunological and genomic determinants of host association. IMPORTANCE Host association is the phenotype that is commonly found in many pathogens that preferential survive in particular hosts. The Lyme disease (LD)-causing agent, B. burgdorferi (Bb), is an ideal model to study host association, as Bb is mainly maintained in nature through rodent and avian hosts. A widespread yet untested concept posits that host association in Bb strains is linked to Bb functional genetic variation conferring evasion to complement, an innate defense mechanism in vertebrate sera. Here, we tested this concept by grouping 20 Bb strains into three complement-dependent host-association phenotypes based on their survivability in sera and/or bloodstream and distal tissues in rodent and avian LD models. Phylogenomic analysis of these strains further correlated several gene families and loci, including ospC, with host-specific complement-evasion phenotypes. Such multifaceted studies thus pave the road to further identify the determinants of host association, providing mechanistic insights into host-pathogen interaction.

Maximum antigen diversification in a lyme bacterial population and evolutionary strategies to overcome pathogen diversity

Natural populations of pathogens and their hosts are engaged in an arms race in which the pathogens diversify to escape host immunity while the hosts evolve novel immunity. This co-evolutionary process poses a fundamental challenge to the development of broadly effective vaccines and diagnostics against a diversifying pathogen. Based on surveys of natural allele frequencies and experimental immunization of mice, we show high antigenic specificities of natural variants of the outer surface protein C (OspC), a dominant antigen of a Lyme Disease-causing bacterium (Borrelia burgdorferi). To overcome the challenge of OspC antigenic diversity to clinical development of preventive measures, we implemented a number of evolution-informed strategies to broaden OspC antigenic reactivity. In particular, the centroid algorithm-a genetic algorithm to generate sequences that minimize amino-acid differences with natural variants-generated synthetic OspC analogs with the greatest promise as diagnostic and vaccine candidates against diverse Lyme pathogen strains co-existing in the Northeast United States. Mechanistically, we propose a model of maximum antigen diversification (MAD) mediated by amino-acid variations distributed across the hypervariable regions on the OspC molecule. Under the MAD hypothesis, evolutionary centroids display broad cross-reactivity by occupying the central void in the antigenic space excavated by diversifying natural variants. In contrast to vaccine designs based on concatenated epitopes, the evolutionary algorithms generate analogs of natural antigens and are automated. The novel centroid algorithm and the evolutionary antigen designs based on consensus and ancestral sequences have broad implications for combating diversifying pathogens driven by pathogen-host co-evolution.

Borreliae Part 1: Borrelia Lyme Group and Echidna-Reptile Group

Simple Summary

Borreliae are spirochaetes, which represent a heterogeneous phylum within bacteria. Spirochaetes are indeed distinguished from other bacteria for their spiral shape, which also characterizes Borreliae. This review describes briefly the organization of the phylum Spirocheteales with a digression about its pathogenicity and historical information about bacteria isolation and characterization. Among spirochaetes, Borrelia genus is here divided into three groups, namely the Lyme group (LG), the Echidna-Reptile group (REPG) and the Relapsing Fever group (RFG). Borreliae Part 1 deals with Lyme group and Echidna-Reptile group Borreliae, while the subject of Borreliae Part 2 is Relapsing Fever group and unclassified Borreliae. Lyme group Borreliae is organized here in sections describing ecology, namely tick vectors and animal hosts, epidemiology, microbiology, and Borrelia genome organization and antigen characterization. Furthermore, the main clinical manifestations in Lyme borreliosis are also described. Although included in the Lyme group due to their particular clinical features, Borrelia causing Baggio Yoshinari syndrome and Borrelia mayonii are described in dedicated paragraphs. The Borrelia Echidna-Reptile group has been recently characterized including spirochaetes that apparently are not pathogenic to humans, but infect reptiles and amphibians. The paragraph dedicated to this group of Borreliae describes their vectors, hosts, geographical distribution and their characteristics.

Abstract

Borreliae are divided into three groups, namely the Lyme group (LG), the Echidna-Reptile group (REPG) and the Relapsing Fever group (RFG). Currently, only Borrelia of the Lyme and RF groups (not all) cause infection in humans. Borreliae of the Echidna-Reptile group represent a new monophyletic group of spirochaetes, which infect amphibians and reptiles. In addition to a general description of the phylum Spirochaetales, including a brief historical digression on spirochaetosis, in the present review Borreliae of Lyme and Echidna-Reptile groups are described, discussing the ecology with vectors and hosts as well as microbiological features and molecular characterization. Furthermore, differences between LG and RFG are discussed with respect to the clinical manifestations. In humans, LG Borreliae are organotropic and cause erythema migrans in the early phase of the disease, while RFG Borreliae give high spirochaetemia with fever, without the development of erythema migrans. With respect of LG Borreliae, recently Borrelia mayonii, with intermediate characteristics between LG and RFG, has been identified. As part of the LG, it gives erythema migrans but also high spirochaetemia with fever. Hard ticks are vectors for both LG and REPG groups, but in LG they are mostly Ixodes sp. ticks, while in REPG vectors do not belong to that genus.

Laboratory Diagnosis of Lyme Borreliosis

Lyme borreliosis is caused by a growing list of related, yet distinct, spirochetes with complex biology and sophisticated immune evasion mechanisms. It may result in a range of clinical manifestations involving different organ systems, and can lead to persistent sequelae in a subset of cases. The pathogenesis of Lyme borreliosis is incompletely understood, and laboratory diagnosis, the focus of this review, requires considerable understanding to interpret the results correctly. Direct detection of the infectious agent is usually not possible or practical, necessitating a continued reliance on serologic testing. Still, some important advances have been made in the area of diagnostics, and there are many promising ideas for future assay development. This review summarizes the state of the art in laboratory diagnostics for Lyme borreliosis, provides guidance in test selection and interpretation, and highlights future directions.

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.

Strain-specific joint invasion and colonization by Lyme disease spirochetes is promoted by outer surface protein C

Lyme disease, caused by Borrelia burgdorferi, B. afzelii and B. garinii, is a chronic, multi-systemic infection and the spectrum of tissues affected can vary with the Lyme disease strain. For example, whereas B. garinii infection is associated with neurologic manifestations, B. burgdorferi infection is associated with arthritis. The basis for tissue tropism is poorly understood, but has been long hypothesized to involve strain-specific interactions with host components in the target tissue. OspC (outer surface protein C) is a highly variable outer surface protein required for infectivity, and sequence differences in OspC are associated with variation in tissue invasiveness, but whether OspC directly influences tropism is unknown. We found that OspC binds to the extracellular matrix (ECM) components fibronectin and/or dermatan sulfate in an OspC variant-dependent manner. Murine infection by isogenic B. burgdorferi strains differing only in their ospC coding region revealed that two OspC variants capable of binding dermatan sulfate promoted colonization of all tissues tested, including joints. However, an isogenic strain producing OspC from B. garinii strain PBr, which binds fibronectin but not dermatan sulfate, colonized the skin, heart and bladder, but not joints. Moreover, a strain producing an OspC altered to recognize neither fibronectin nor dermatan sulfate displayed dramatically reduced levels of tissue colonization that were indistinguishable from a strain entirely deficient in OspC. Finally, intravital microscopy revealed that this OspC mutant, in contrast to a strain producing wild type OspC, was defective in promoting joint invasion by B. burgdorferi in living mice. We conclude that OspC functions as an ECM-binding adhesin that is required for joint invasion, and that variation in OspC sequence contributes to strain-specific differences in tissue tropism displayed among Lyme disease spirochetes.

Infection by different Lyme disease bacteria is associated with different manifestations, such as cardiac, neurologic, or, in the case of B. burgdorferi, the major cause of Lyme disease in the U.S., joint disease. The basis for these differences is unknown, but likely involve strain-specific interactions with host components in the target tissue. The sequence of the outer surface lipoprotein OspC varies with the strains, and we found that this variation influences the spectrum of host extracellular matrix components recognized. Infection of mice with strains that are identical except for ospC revealed that OspC variants that differ in binding spectrum promote infection of different tissues. A strain producing OspC invaded and colonized the joint in living animals, but an altered OspC protein incapable of binding tissue components did not. Thus, tissue-binding by OspC is critical for infection and joint invasion, and OspC variation directly influences tissue tropism.

A Fully Automated Multiplex Assay for Diagnosis of Lyme Disease with High Specificity and Improved Early Sensitivity

Lyme borreliosis is a tick-borne disease caused by the Borrelia burgdorferi sensu lato complex. Bio-Rad Laboratories has developed a fully automated multiplex bead-based assay for the detection of IgM and IgG antibodies to B. burgdorferi The BioPlex 2200 Lyme Total assay exhibits an improved rate of seropositivity in patients with early Lyme infection. Asymptomatic subjects from endemic and nonendemic origins demonstrated a seroreactivity rate of approximately 4% that was similar to other commercial assays evaluated in this study. Coupled to this result was the observation that the Lyme Total assay retained a high first-tier specificity of 96% while demonstrating a relatively high sensitivity of 91% among a well-characterized CDC Premarketing Lyme serum panel. The Lyme Total assay also performs well under a modified two-tier algorithm (sensitivity, 84.4 to 88.9%; specificity, 98.4 to 99.5%). Furthermore, the new assay is able to readily detect early Lyme infection in patient samples from outside North America.

Diversity of the Lyme Disease Spirochetes and its Influence on Immune Responses to Infection and Vaccination

The Lyme disease spirochetes are a highly diverse group of bacteria with unique biological properties. Their ability to cycle between ticks and mammals requires that they adapt to variable and constantly changing environmental conditions. Outer surface protein C is an essential virulence determinant that has received considerable attention in vaccine and diagnostic assay development. Knowledge of OspC diversity, its antigenic determinants, and its production patterns throughout the enzootic cycle, as well as in the laboratory setting, is essential for understanding immune responses induced by infection or vaccination.

Outer surface protein polymorphisms linked to host-spirochete association in Lyme borreliae

Lyme borreliosis is caused by multiple species of the spirochete bacteria Borrelia burgdorferi sensu lato. The spirochetes are transmitted by ticks to vertebrate hosts, including small- and medium-sized mammals, birds, reptiles, and humans. Strain-to-strain variation in host-specific infectivity has been documented, but the molecular basis that drives this differentiation is still unclear. Spirochetes possess the ability to evade host immune responses and colonize host tissues to establish infection in vertebrate hosts. In turn, hosts have developed distinct levels of immune responses when invaded by different species/strains of Lyme borreliae. Similarly, the ability of Lyme borreliae to colonize host tissues varies among different spirochete species/strains. One potential mechanism that drives this strain-to-strain variation of immune evasion and colonization is the polymorphic outer surface proteins produced by Lyme borreliae. In this review, we summarize research on strain-to-strain variation in host competence and discuss the evidence that supports the role of spirochete-produced protein polymorphisms in driving this variation in host specialization. Such information will provide greater insights into the adaptive mechanisms driving host and Lyme borreliae association, which will lead to the development of interventions to block pathogen spread and eventually reduce Lyme borreliosis health burden.

Evaluation of in vivo expressed Borrelia burgdorferi antigens for improved IgM serodiagnosis of early Lyme disease

Improved serologic tests are needed for accurate diagnosis and proper treatment of early stage Lyme disease. We evaluated the 3 antigens currently used for 2-tiered IgM immunoblot testing (FlaB, OspC, and BmpA) in combination with 3 additional antigens (BBA65, BBA70, and BBA73) and measured the sensitivity and specificity against a serum repository of positive and negative controls. Using 3 statistical methods for positivity cutoff determinations and scoring criteria, we found increased sensitivities for early Lyme disease when 2 of 6 antigens were positive as compared with the 2 of 3 antigen IgM criteria currently used for second-tier immunoblot scoring. Specificities for negative controls were comparable or superior to using 2 of 3 antigens. These results indicate that IgM sensitivity and specificity of serological testing for Lyme disease in the early stages of illness can be improved by employing antigens that target the initial host antibody responses.

A Serological Survey of Borrelia burgdorferi Infection in Sheep in Northeast China Regions Through Outer Surface Protein C-Based Enzyme-Linked Immunosorbent Assay

Borrelia burgdorferi as a causative agent of Lyme disease is transmitted by Ixodes spp. ticks to humans and animals. Sheep is considered a natural reservoir for B. burgdorferi and plays a pivotal role in disease transmission and the expansion of natural foci. An epidemiological investigation of B. burgdorferi in sheep is essential for prevention and control of Lyme disease. In this study, we developed a recombinant outer surface protein C (OspC)-based ELISA for serological study of B. burgdorferi in sheep with a specificity and sensitivity of 84.4% and 86.2%, respectively. A total of 972 collected serum samples from the Northeast China regions in 2015 and 2016 were determined with positive rates of 5.8% and 12.2%, respectively. Thus, specific pathogen-free sheep were infected with B. burgdorferi SZ strain to study on the secretion of specificity antibody against OspC. It revealed that specific antibody was detected on day 5 postinoculation and sustained in a high level for ∼28 days, the peak occurred at ∼13 days. Taken together, the result indicated that the established ELISA is capable for clinical diagnosis and epidemiological study on B. burgdorferi in sheep at the early stage of infection and detecting the specific antibody during the secretion period.

Reservoir and vector evolutionary pressures shaped the adaptation of Borrelia

The life cycle of spirochetes of the genus Borrelia includes complex networks of vertebrates and ticks. The tripartite association of Borrelia-vertebrate-tick has proved ecologically successful for these bacteria, which have become some of the most prominent tick-borne pathogens in the northern hemisphere. To keep evolutionary pace with its double-host life history, Borrelia must adapt to the evolutionary pressures exerted by both sets of hosts. In this review, we attempt to reconcile functional, phylogenetic, and ecological perspectives to propose a coherent scenario of Borrelia evolution. Available empirical information supports that the association of Borrelia with ticks is very old. The major split between the tick families Argasidae-Ixodidae (dated some 230-290 Mya) resulted in most relapsing fever (Rf) species being restricted to Argasidae and few associated with Ixodidae. A further key event produced the diversification of the Lyme borreliosis (Lb) species: the radiation of ticks of the genus Ixodes from the primitive stock of Ixodidae (around 217 Mya). The ecological interactions of Borrelia demonstrate that Argasidae-transmitted Rf species remain restricted to small niches of one tick species and few vertebrates. The evolutionary pressures on this group are consequently low, and speciation processes seem to be driven by geographical isolation. In contrast to Rf, Lb species circulate in nested networks of dozens of tick species and hundreds of vertebrate species. This greater variety confers a remarkably variable pool of evolutionary pressures, resulting in large speciation of the Lb group, where different species adapt to circulate through different groups of vertebrates. Available data, based on ospA and multilocus sequence typing (including eight concatenated in-house genes) phylogenetic trees, suggest that ticks could constitute a secondary bottleneck that contributes to Lb specialization. Both sets of adaptive pressures contribute to the resilience of highly adaptable meta-populations of bacteria.

Infection history of the blood-meal host dictates pathogenic potential of the Lyme disease spirochete within the feeding tick vector

Lyme disease in humans is caused by several genospecies of the Borrelia burgdorferi sensu lato (s.l.) complex of spirochetal bacteria, including B. burgdorferi, B. afzelii and B. garinii. These bacteria exist in nature as obligate parasites in an enzootic cycle between small vertebrate hosts and Ixodid tick vectors, with humans representing incidental hosts. During the natural enzootic cycle, infected ticks in endemic areas feed not only upon naïve hosts, but also upon seropositive infected hosts. In the current study, we considered this environmental parameter and assessed the impact of the immune status of the blood-meal host on the phenotype of the Lyme disease spirochete within the tick vector. We found that blood from a seropositive host profoundly attenuates the infectivity (>104 fold) of homologous spirochetes within the tick vector without killing them. This dramatic neutralization of vector-borne spirochetes was not observed, however, when ticks and blood-meal hosts carried heterologous B. burgdorferi s.l. strains, or when mice lacking humoral immunity replaced wild-type mice as blood-meal hosts in similar experiments. Mechanistically, serum-mediated neutralization does not block induction of host-adapted OspC+ spirochetes during tick feeding, nor require tick midgut components. Significantly, this study demonstrates that strain-specific antibodies elicited by B. burgdorferi s.l. infection neutralize homologous bacteria within feeding ticks, before the Lyme disease spirochetes enter a host. The blood meal ingested from an infected host thereby prevents super-infection by homologous spirochetes, while facilitating transmission of heterologous B. burgdorferi s.l. strains. This finding suggests that Lyme disease spirochete diversity is stably maintained within endemic populations in local geographic regions through frequency-dependent selection of rare alleles of dominant polymorphic surface antigens.

Primordial origin and diversification of plasmids in Lyme disease agent bacteria

BACKGROUND

With approximately one-third of their genomes consisting of linear and circular plasmids, the Lyme disease agent cluster of species has the most complex genomes among known bacteria. We report here a comparative analysis of plasmids in eleven Borreliella (also known as Borrelia burgdorferi sensu lato) species.

RESULTS

We sequenced the complete genomes of two B. afzelii, two B. garinii, and individual B. spielmanii, B. bissettiae, B. valaisiana and B. finlandensis isolates. These individual isolates carry between seven and sixteen plasmids, and together harbor 99 plasmids. We report here a comparative analysis of these plasmids, along with 70 additional Borreliella plasmids available in the public sequence databases. We identify only one new putative plasmid compatibility type (the 30th) among these 169 plasmid sequences, suggesting that all or nearly all such types have now been discovered. We find that the linear plasmids in the non-B. burgdorferi species have undergone the same kinds of apparently random, chaotic rearrangements mediated by non-homologous recombination that we previously discovered in B. burgdorferi. These rearrangements occurred independently in the different species lineages, and they, along with an expanded chromosomal phylogeny reported here, allow the identification of several whole plasmid transfer events among these species. Phylogenetic analyses of the plasmid partition genes show that a majority of the plasmid compatibility types arose early, most likely before separation of the Lyme agent Borreliella and relapsing fever Borrelia clades, and this, with occasional cross species plasmid transfers, has resulted in few if any species-specific or geographic region-specific Borreliella plasmid types.

CONCLUSIONS

The primordial origin and persistent maintenance of the Borreliella plasmid types support their functional indispensability as well as evolutionary roles in facilitating genome diversity. The improved resolution of Borreliella plasmid phylogeny based on conserved partition-gene clusters will lead to better determination of gene orthology which is essential for prediction of biological function, and it will provide a basis for inferring detailed evolutionary mechanisms of Borreliella genomic variability including homologous gene and plasmid exchanges as well as non-homologous rearrangements.

Borrelia burgdorferi outer surface protein C (OspC) binds complement component C4b and confers bloodstream survival

Borrelia burgdorferi (Bb) is the causative agent of Lyme disease in the United States, a disease that can result in carditis, and chronic and debilitating arthritis and/or neurologic symptoms if left untreated. Bb survives in the midgut of the Ixodes scapularis tick, or within tissues of immunocompetent hosts. In the early stages of infection, the bacteria are present in the bloodstream where they must resist clearance by the innate immune system of the host. We have found a novel role for outer surface protein C (OspC) from B. burgdorferi and B. garinii in interactions with the complement component C4b and bloodstream survival in vivo. Our data show that OspC inhibits the classical and lectin complement pathways and competes with complement protein C2 for C4b binding. Resistance to complement is important for maintenance of the lifecycle of Bb, enabling survival of the pathogen within the host as well as in the midgut of a feeding tick when ospC expression is induced.

Epitope mapping of Borrelia burgdorferi OspC protein in homodimeric fold

In current work, we used recombinant OspC protein derived from B. afzelii strain BRZ31 in the native homodimeric fold for mice immunization and following selection process to produce three mouse monoclonal antibodies able to bind to variable parts of up to five different OspC proteins. Applying the combination of mass spectrometry assisted epitope mapping and affinity based theoretical prediction we have localized regions responsible for antigen-antibody interactions and approximate epitopes' amino acid composition. Two mAbs (3F4 and 2A9) binds to linear epitopes located in previously described immunogenic regions in the exposed part of OspC protein. The third mAb (2D1) recognises highly conserved discontinuous epitope close to the ligand binding domain 1.

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.

Antibody Response to Lyme Disease Spirochetes in the Context of VlsE-Mediated Immune Evasion

Lyme disease (LD), the most prevalent tick-borne illness in North America, is caused by Borrelia burgdorferi The long-term survival of B. burgdorferi spirochetes in the mammalian host is achieved though VlsE-mediated antigenic variation. It is mathematically predicted that a highly variable surface antigen prolongs bacterial infection sufficiently to exhaust the immune response directed toward invariant surface antigens. If the prediction is correct, it is expected that the antibody response to B. burgdorferi invariant antigens will become nonprotective as B. burgdorferi infection progresses. To test this assumption, changes in the protective efficacy of the immune response to B. burgdorferi surface antigens were monitored via a superinfection model over the course of 70 days. B. burgdorferi-infected mice were subjected to secondary challenge by heterologous B. burgdorferi at different time points postinfection (p.i.). When the infected mice were superinfected with a VlsE-deficient clone (ΔVlsE) at day 28 p.i., the active anti-B. burgdorferi immune response did not prevent ΔVlsE-induced spirochetemia. In contrast, most mice blocked culture-detectable spirochetemia induced by wild-type B. burgdorferi (WT), indicating that VlsE was likely the primary target of the antibody response. As the B. burgdorferi infection further progressed, however, reversed outcomes were observed. At day 70 p.i. the host immune response to non-VlsE antigens became sufficiently potent to clear spirochetemia induced by ΔVlsE and yet failed to prevent WT-induced spirochetemia. To test if any significant changes in the anti-B. burgdorferi antibody repertoire accounted for the observed outcomes, global profiles of antibody specificities were determined. However, comparison of mimotopes revealed no major difference between day 28 and day 70 antibody repertoires.

Live Attenuated Borrelia burgdorferi Targeted Mutants in an Infectious Strain Background Protect Mice from Challenge Infection

Borrelia burgdorferi, B. garinii, and B. afzelii are all agents of Lyme disease in different geographic locations. If left untreated, Lyme disease can cause significant and long-term morbidity, which may continue after appropriate antibiotic therapy has been administered and live bacteria are no longer detectable. The increasing incidence and geographic spread of Lyme disease are renewing interest in the vaccination of at-risk populations. We took the approach of vaccinating mice with two targeted mutant strains of B. burgdorferi that, unlike the parental strain, are avirulent in mice. Mice vaccinated with both strains were protected against a challenge with the parental strain and a heterologous B. burgdorferi strain by either needle inoculation or tick bite. In ticks, the homologous strain was eliminated but the heterologous strain was not, suggesting that the vaccines generated a response to antigens that are produced by the bacteria both early in mammalian infection and in the tick. Partial protection against B. garinii infection was also conferred. Protection was antibody mediated, and reactivity to a variety of proteins was observed. These experiments suggest that live attenuated B. burgdorferi strains may be informative regarding the identification of protective antigens produced by the bacteria and recognized by the mouse immune system in vivo Further work may illuminate new candidates that are effective and safe for the development of Lyme disease vaccines.

The Position of His-Tag in Recombinant OspC and Application of Various Adjuvants Affects the Intensity and Quality of Specific Antibody Response after Immunization of Experimental Mice

Lyme disease, Borrelia burgdorferi-caused infection, if not recognized and appropriately treated by antibiotics, may lead to chronic complications, thus stressing the need for protective vaccine development. The immune protection is mediated by phagocytic cells and by Borrelia-specific complement-activating antibodies, associated with the Th1 immune response. Surface antigen OspC is involved in Borrelia spreading through the host body. Previously we reported that recombinant histidine tagged (His-tag) OspC (rOspC) could be attached onto liposome surfaces by metallochelation. Here we report that levels of OspC-specific antibodies vary substantially depending upon whether rOspC possesses an N' or C' terminal His-tag. This is the case in mice immunized: (a) with rOspC proteoliposomes containing adjuvants MPLA or non-pyrogenic MDP analogue MT06; (b) with free rOspC and Montanide PET GEL A; (c) with free rOspC and alum; or (d) with adjuvant-free rOspC. Stronger responses are noted with all N'-terminal His-tag rOspC formulations. OspC-specific Th1-type antibodies predominate post-immunization with rOspC proteoliposomes formulated with MPLA or MT06 adjuvants. Further analyses confirmed that the structural features of soluble N' and C' terminal His-tag rOspC and respective rOspC proteoliposomes are similar including their thermal stabilities at physiological temperatures. On the other hand, a change in the position of the rOspC His-tag from N' to C' terminal appears to affect substantially the immunogenicity of rOspC arguably due to steric hindrance of OspC epitopes by the C' terminal His-tag itself and not due to differences in overall conformations induced by changes in the His-tag position in rOspC variants.

Gene Regulation During the Enzootic Cycle of the Lyme Disease Spirochete

Borrelia burgdorferi, the spirochete that causes Lyme disease, exists in an enzootic cycle, alternating between a tick vector and a vertebrate host. To adapt to and survive the environmental changes associated with its enzootic cycle, including nutrient availability, B. burgdorferi uses three different systems to regulate the expression of genes: RpoN-RpoS, histidine kinase (Hk)1/response regulator 1 (Rrp1), and Rel_Bbu. The RpoN-RpoS alternative sigma factor cascade activates genes required for transmission from the tick to the vertebrate, maintenance of the vertebrate infection, and persistence in the tick. Rel_Bbu controls the levels of the alarmones guanosine pentaphosphate and guanosine tetraphosphate, which are necessary for surviving the nutrient-deficient conditions in the midgut of the tick following absorption of the blood meal and the subsequent molt. The Hk1/Rrp1 two-component system produces cyclic dimeric guanosine monophosphate that regulates the genes required for the transitions between the tick and vertebrate as well as protective responses to the blood meal.

Vaccination with the variable tick protein of the relapsing fever spirochete Borrelia hermsii protects mice from infection by tick-bite

Background

Tick-borne relapsing fevers of humans are caused by spirochetes that must adapt to both warm-blooded vertebrates and cold-blooded ticks. In western North America, most human cases of relapsing fever are caused by Borrelia hermsii, which cycles in nature between its tick vector Ornithodoros hermsi and small mammals such as tree squirrels and chipmunks. These spirochetes alter their outer surface by switching off one of the bloodstream-associated variable major proteins (Vmps) they produce in mammals, and replacing it with the variable tick protein (Vtp) following their acquisition by ticks. Based on this reversion to Vtp in ticks, we produced experimental vaccines comprised on this protein and tested them in mice challenged by infected ticks.

Methods

The vtp gene from two isolates of B. hermsii that encoded antigenically distinct types of proteins were cloned, expressed, and the recombinant Vtp proteins were purified and used to vaccinate mice. Ornithodoros hermsi ticks that were infected with one of the two strains of B. hermsii from which the vtp gene originated were used to challenge mice that received one of the two Vtp vaccines or only adjuvant. Mice were then followed for infection and seroconversion.

Results

The Vtp vaccines produced protective immune responses in mice challenged with O. hermsi ticks infected with B. hermsii. However, polymorphism in Vtp resulted in mice being protected only from the spirochete strain that produced the same Vtp used in the vaccine; mice challenged with spirochetes producing the antigenically different Vtp than the vaccine succumbed to infection.

Conclusions

We demonstrate that by having knowledge of the phenotypic changes made by B. hermsii as the spirochetes are acquired by ticks from infected mammals, an effective vaccine was developed that protected mice when challenged with infected ticks. However, the Vtp vaccines only protected mice from infection when challenged with that strain producing the identical Vtp. A vaccine containing multiple Vtp types may have promise as an oral vaccine for wild mammals if applied to geographic settings such as small islands where the mammal diversity is low and the Vtp types in the B. hermsii population are defined.

Seroprevalence of Borrelia burgdorferi in occupationally exposed persons in the Belgrade area, Serbia

Lyme disease (LD) is a natural focal zoonotic disease caused by Borrelia burgdorferi, which is mainly transmitted through infected Ixodes ricinus tick bites. The presence and abundance of ticks in various habitats, the infectivity rate, as well as prolonged human exposure to ticks are factors that may affect the infection risk as well as the incidence of LD. In recent years, 20% to 25% of ticks infected with different borrelial species, as well as about 5,300 citizens with LD, have been registered in the Belgrade area. Many of the patients reported tick bites in city’s grassy areas. The aim of this study was to assess the seroprevalence of B. burgdorferi in high-risk groups (forestry workers and soldiers) in the Belgrade area, and to compare the results with healthy blood donors. A two-step algorithm consisting of ELISA and Western blot tests was used in the study. Immunoreactivity profiles were also compared between the groups. The results obtained showed the seroprevalence to be 11.76% in the group of forestry workers, 17.14% in the group of soldiers infected by tick bites and 8.57% in the population of healthy blood donors. The highest IgM reactivity was detected against the OspC protein, while IgG antibodies showed high reactivity against VlsE, p19, p41, OspC, OspA and p17. Further investigations in this field are necessary in humans and animals in order to improve protective and preventive measures against LD.

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

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

A conserved region from biofilm associated protein as a biomarker for detection of Acinetobacter baumannii

Acinetobacter baumannii is the leading cause of nosocominal infection within the family Moraxellaceae. Due to multiple antibiotic resistances of the bacteria, the treatment is very difficult, hence specific and economical test for early diagnosis of infection is needed. Development of such a test requires targeting specific cell surface antigens. Bacterial ability of biofilm formation grants major contribution in antimicrobial resistance and other environmental stresses such as nutrient limitation and dehydration. Biofilm associated protein (Bap), a specific cell surface protein, is directly involved in A. baumannii biofilm formation. The goal of this study is diagnosis of A. baumannii infection exploiting specific target from Bap. A selected subunit of Bap was cloned, expressed and purified. Mice were divided into three groups. Group one was immunized with recombinant Bap subunit, mice in group two were infected with A. baumannii (positive control) and mice in groups three served as negative control. Immunization with Bap subunit resulted in high antibody titers. Animals in control group that received same amount of adjuvant and PBS showed no Bap-specific antibodies. Sensitivity and specificity of the antibodies raised were determined by ELISA and Western blotting. Recombinant Bap subunit was tested by ELISA using sera obtained from A. baumannii infected patients and healthy individuals. This conserved and immunodominant region of Bap could serve as an appropriate target for diagnosis A. baumannii infection.

Strain-specific variation of the decorin-binding adhesin DbpA influences the tissue tropism of the lyme disease spirochete

Lyme disease spirochetes demonstrate strain- and species-specific differences in tissue tropism. For example, the three major Lyme disease spirochete species, Borrelia burgdorferi sensu stricto, B. garinii, and B. afzelii, are each most commonly associated with overlapping but distinct spectra of clinical manifestations. Borrelia burgdorferi sensu stricto, the most common Lyme spirochete in the U.S., is closely associated with arthritis. The attachment of microbial pathogens to cells or to the extracellular matrix of target tissues may promote colonization and disease, and the Lyme disease spirochete encodes several surface proteins, including the decorin- and dermatan sulfate-binding adhesin DbpA, which vary among strains and have been postulated to contribute to strain-specific differences in tissue tropism. DbpA variants differ in their ability to bind to its host ligands and to cultured mammalian cells. To directly test whether variation in dbpA influences tissue tropism, we analyzed murine infection by isogenic B. burgdorferi strains that encode different dbpA alleles. Compared to dbpA alleles of B. afzelii strain VS461 or B. burgdorferi strain N40-D10/E9, dbpA of B. garinii strain PBr conferred the greatest decorin- and dermatan sulfate-binding activity, promoted the greatest colonization at the inoculation site and heart, and caused the most severe carditis. The dbpA of strain N40-D10/E9 conferred the weakest decorin- and GAG-binding activity, but the most robust joint colonization and was the only dbpA allele capable of conferring significant joint disease. Thus, dbpA mediates colonization and disease by the Lyme disease spirochete in an allele-dependent manner and may contribute to the etiology of distinct clinical manifestations associated with different Lyme disease strains. This study provides important support for the long-postulated model that strain-specific variations of Borrelia surface proteins influence tissue tropism.

Lyme disease, the most common vector-borne disease in the United States, is caused by a bacterium, Borrelia burgdorferi. This bacterium infects the skin at the site of the tick bite and then can spread to other tissues, such as the heart, joints or nervous system, causing carditis, arthritis or neurologic disease. To colonize human tissues, the pathogen produces surface proteins that promote bacterial attachment to these sites. For example, DbpA binds to decorin, a component of human tissue. Different Lyme disease strains differ in the particular tissues they colonize and the disease they cause, but we do not understand why. Different strains also make distinct versions of DbpA that bind decorin differently, so variation of DbpA might contribute to strain-to-strain variation in clinical manifestations. To test this, we infected mice with Lyme disease strains that were identical except for the particular DbpA variant they produced. We found that the strains colonized different tissues and caused different diseases, such as arthritis or carditis. These results provide the first solid evidence that variation of an outer surface protein, in this case DbpA, influences what tissues are most affected during Lyme disease.

Comparative population genomics of the Borrelia burgdorferi species complex reveals high degree of genetic isolation among species and underscores benefits and constraints to studying intra-specific epidemiological processes

Lyme borreliosis, one of the most frequently contracted zoonotic diseases in the Northern Hemisphere, is caused by bacteria belonging to different genetic groups within the Borrelia burgdorferi species complex, which are transmitted by ticks among various wildlife reservoirs, such as small mammals and birds. These features make the Borrelia burgdorferi species complex an attractive biological model that can be used to study the diversification and the epidemiology of endemic bacterial pathogens. We investigated the potential of population genomic approaches to study these processes. Sixty-three strains belonging to three species within the Borrelia burgdorferi complex were isolated from questing ticks in Alsace (France), a region where Lyme disease is highly endemic. We first aimed to characterize the degree of genetic isolation among the species sampled. Phylogenetic and coalescent-based analyses revealed clear delineations: there was a ∼50 fold difference between intra-specific and inter-specific recombination rates. We then investigated whether the population genomic data contained information of epidemiological relevance. In phylogenies inferred using most of the genome, conspecific strains did not cluster in clades. These results raise questions about the relevance of different strategies when investigating pathogen epidemiology. For instance, here, both classical analytic approaches and phylodynamic simulations suggested that population sizes and migration rates were higher in B. garinii populations, which are normally associated with birds, than in B. burgdorferi s.s. populations. The phylogenetic analyses of the infection-related ospC gene and its flanking region provided additional support for this finding. Traces of recombination among the B. burgdorferi s.s. lineages and lineages associated with small mammals were found, suggesting that they shared the same hosts. Altogether, these results provide baseline evidence that can be used to formulate hypotheses regarding the host range of B. burgdorferi lineages based on population genomic data.

BB0347, from the lyme disease spirochete Borrelia burgdorferi, is surface exposed and interacts with the CS1 heparin-binding domain of human fibronectin

The causative agent of Lyme disease, Borrelia burgdorferi, codes for several known fibronectin-binding proteins. Fibronectin a common the target of diverse bacterial pathogens, and has been shown to be essential in allowing for the development of certain disease states. Another borrelial protein, BB0347, has sequence similarity with these other known fibronectin-binding proteins, and may be important in Lyme disease pathogenesis. Herein, we perform an initial characterization of BB0347 via the use of molecular and biochemical techniques. We found that BB0347 is expressed, produced, and presented on the outer surface of intact B. burgdorferi. We also demonstrate that BB0347 has the potential to be important in Lyme disease progression, and have begun to characterize the nature of the interaction between human fibronectin and this bacterial protein. Further work is needed to define the role of this protein in the borrelial infection process.

Identification of Borrelia burgdorferi ospC genotypes in canine tissue following tick infestation: implications for Lyme disease vaccine and diagnostic assay design

In endemic regions, Lyme disease is a potential health threat to dogs. Canine Lyme disease manifests with arthritis-induced lameness, anorexia, fever, lethargy, lymphadenopathy and, in some cases, fatal glomerulonephritis. A recent study revealed that the regional mean for the percentage of seropositive dogs in the north-east of the USA is 11.6%. The outer surface protein C (OspC) of Lyme disease spirochetes is an important virulence factor required for the establishment of infection in mammals. It is a leading candidate in human and canine Lyme disease vaccine development efforts. Over 30 distinct ospC phyletic types have been defined. It has been hypothesized that ospC genotype may influence mammalian host range. In this study, Ixodes scapularis ticks collected from the field in Rhode Island were assessed for infection with B. burgdorferi. Ticks were fed on purpose bred beagles to repletion and infection of the dogs was assessed through serology and PCR. Tissue biopsies (n=2) were collected from each dog 49 days post-tick infestation (dpi) and the ospC genotype of the infecting strains determined by direct PCR of DNA extracted from tissue or by PCR after cultivation of spirochetes from biopsy samples. The dominant ospC types associated with B. burgdorferi canine infections differed from those associated with human infection, indicating a relationship between ospC sequence and preferred host range. Knowledge of the most common ospC genotypes associated specifically with infection of dogs will facilitate the rational design of OspC-based canine Lyme disease vaccines and diagnostic assays.

Inferring epitopes of a polymorphic antigen amidst broadly cross-reactive antibodies using protein microarrays: a study of OspC proteins of Borrelia burgdorferi

Epitope mapping studies aim to identify the binding sites of antibody-antigen interactions to enhance the development of vaccines, diagnostics and immunotherapeutic compounds. However, mapping is a laborious process employing time- and resource-consuming ‘wet bench’ techniques or epitope prediction software that are still in their infancy. For polymorphic antigens, another challenge is characterizing cross-reactivity between epitopes, teasing out distinctions between broadly cross-reactive responses, limited cross-reactions among variants and the truly type-specific responses. A refined understanding of cross-reactive antibody binding could guide the selection of the most informative subsets of variants for diagnostics and multivalent subunit vaccines. We explored the antibody binding reactivity of sera from human patients and Peromyscus leucopus rodents infected with Borrelia burgdorferi to the polymorphic outer surface protein C (OspC), an attractive candidate antigen for vaccine and improved diagnostics for Lyme disease. We constructed a protein microarray displaying 23 natural variants of OspC and quantified the degree of cross-reactive antibody binding between all pairs of variants, using Pearson correlation calculated on the reactivity values using three independent transforms of the raw data: (1) logarithmic, (2) rank, and (3) binary indicators. We observed that the global amino acid sequence identity between OspC pairs was a poor predictor of cross-reactive antibody binding. Then we asked if specific regions of the protein would better explain the observed cross-reactive binding and performed in silico screening of the linear sequence and 3-dimensional structure of OspC. This analysis pointed to residues 179 through 188 the fifth C-terminal helix of the structure as a major determinant of type-specific cross-reactive antibody binding. We developed bioinformatics methods to systematically analyze the relationship between local sequence/structure variation and cross-reactive antibody binding patterns among variants of a polymorphic antigen, and this method can be applied to other polymorphic antigens for which immune response data is available for multiple variants.

Variable VlsE is critical for host reinfection by the Lyme disease spirochete

Many pathogens make use of antigenic variation as a way to evade the host immune response. A key mechanism for immune evasion and persistent infection by the Lyme disease spirochete, Borrelia burgdorferi, is antigenic variation of the VlsE surface protein. Recombination results in changes in the VlsE surface protein that prevent recognition by VlsE-specific antibodies in the infected host. Despite the presence of a substantial number of additional proteins residing on the bacterial surface, VlsE is the only known antigen that exhibits ongoing variation of its surface epitopes. This suggests that B. burgdorferi may utilize a VlsE-mediated system for immune avoidance of its surface antigens. To address this, the requirement of VlsE for host reinfection by the Lyme disease pathogen was investigated. Host-adapted wild type and VlsE mutant spirochetes were used to reinfect immunocompetent mice that had naturally cleared an infection with a VlsE-deficient clone. Our results demonstrate that variable VlsE is necessary for reinfection by B. burgdorferi, and this ability is directly related to evasion of the host antibody response. Moreover, the data presented here raise the possibility that VlsE prevents recognition of B. burgdorferi surface antigens from host antibodies. Overall, our findings represent a significant advance in our knowledge of immune evasion by B. burgdorferi, and provide insight to the possible mechanisms involved in VlsE-mediated immune avoidance.

LipL32 Is a Subsurface Lipoprotein of Leptospira interrogans: presentation of new data and reevaluation of previous studies

The agents of leptospirosis, a zoonosis with worldwide distribution, are pathogenic spirochetes belonging to the genus Leptospira. The leptospiral life cycle involves transmission via fresh water and colonization of the renal tubules of their reservoir hosts. Infection of accidental hosts, including humans, may result in life-threatening sequelae. Bacterial outer membrane proteins (OMPs), particularly those with surface-exposed regions, play crucial roles in pathogen virulence mechanisms and adaptation to environmental conditions, including those found in the mammalian host. Therefore, elucidation and characterization of the surface-exposed OMPs of Leptospira spp. is of great interest in the leptospirosis field. A thorough, multi-pronged approach for assessing surface exposure of leptospiral OMPs is essential. Herein, we present evidence for a sub-surface location for most or all of the major leptospiral lipoprotein, LipL32, based on surface immunofluorescence utilizing three different types of antibodies and four different permeabilization methods, as well as surface proteolysis of intact and lysed leptospires. We reevaluate prior evidence presented in support of LipL32 surface-exposure and present a novel perspective on a protein whose location has been misleading researchers, due in large part to its extraordinary abundance in leptospiral cells.

Highlights on molecular identification of closely related species

The term "complex" emerged in the literature at the beginning of the genomic era associated to taxonomy and grouping organisms that belong to different species but exhibited similar patterns according to their morphological, physiological and/or other phenotypic features. DNA-DNA hybridization values ~70% and high identity on 16S rRNA gene sequences were recommended for species delineation. Electrophoretic methods showed in some cases to be useful for species identification and population structure but the reproducibility was questionable. Later, the implementation of polyphasic approaches involving phenotypic and molecular methods brought new insights into the analysis of population structure and phylogeny of several "species complexes", allowing the identification of new closely related species. Likewise, the introduction of multilocus sequence typing and sequencing analysis of several genes offered an evolutionary perspective to the term "species complex". Several centres worldwide have recently released increasing genetic information on distinct microbial species. A brief review will be presented to highlight the definition of "species complex" for selected microorganisms, mainly the prokaryotic Acinetobacter calcoaceticus -Acinetobacter baumannii, Borrelia burgdorferi sensu lato, Burkholderia cepacia, Mycobacterium tuberculosis and Nocardia asteroides complexes, and the eukaryotic Aspergillus fumigatus, Leishmania donovani and Saccharomyces sensu stricto complexes. The members of these complexes may show distinct epidemiology, pathogenicity and susceptibility, turning critical their correct identification. Dynamics of prokaryotic and eukaryotic genomes can be very distinct and the term "species complex" should be carefully extended.

Genetics of Borrelia burgdorferi

The spirochetes in the Borrelia burgdorferi sensu lato genospecies group cycle in nature between tick vectors and vertebrate hosts. The current assemblage of B. burgdorferi sensu lato, of which three species cause Lyme disease in humans, originated from a rapid species radiation that occurred near the origin of the clade. All of these species share a unique genome structure that is highly segmented and predominantly composed of linear replicons. One of the circular plasmids is a prophage that exists as several isoforms in each cell and can be transduced to other cells, likely contributing to an otherwise relatively anemic level of horizontal gene transfer, which nevertheless appears to be adequate to permit strong natural selection and adaptation in populations of B. burgdorferi. Although the molecular genetic toolbox is meager, several antibiotic-resistant mutants have been isolated, and the resistance alleles, as well as some exogenous genes, have been fashioned into markers to dissect gene function. Genetic studies have probed the role of the outer membrane lipoprotein OspC, which is maintained in nature by multiple niche polymorphisms and negative frequency-dependent selection. One of the most intriguing genetic systems in B. burgdorferi is vls recombination, which generates antigenic variation during infection of mammalian hosts.

Borrelia afzelii ospC genotype diversity in Ixodes ricinus questing ticks and ticks from rodents in two Lyme borreliosis endemic areas: contribution of co-feeding ticks

In Europe, the Lyme borreliosis (LB) agents like Borrelia burgdorferi sensu stricto (ss), B. afzelii, and B. garinii are maintained in nature by enzoonotic transmission cycles between vertebrate hosts and Ixodes ricinus ticks. The outer surface protein C is a highly antigenic protein expressed by spirochaetes during transmission from ticks to mammals as well as during dissemination in the vertebrate hosts. Previous studies based on analysis of ospC gene sequences have led to the classification of ospC genotypes into ospC groups. The aim of this study was to analyse and compare ospC group distribution among isolates of the rodent-associated genospecies, B. afzelii, at 3 levels (questing ticks, ticks feeding on rodents, and xenodiagnostic ticks). Isolates were obtained during a study carried out in 2 LB endemic areas located on the Swiss Plateau [Portes-Rouges (PR) and Staatswald (SW)], where rodents were differently infested by co-feeding ticks (Pérez et al., unpublished data). Overall, we identified 10 different ospC groups with different distributions among isolates from questing ticks, ticks that detached from rodents, and xenodiagnostic ticks at the 2 sites. We observed a higher ospC diversity among isolates from ticks that fed on rodents at SW, and mixed infections with 2 ospC groups were also more frequent among isolates from ticks that fed on rodents at SW (n=18) than at PR (n=1). At both sites, B. afzelii isolates obtained from larvae that were feeding on the rodents simultaneously with nymphs displayed a higher diversity of ospC groups (mean number of ospC groups: 2.25 for PR and 1.75 for SW) than isolates from larvae feeding without nymphs (mean number of ospC groups: 1.17 for PR and 1 for SW). We suggest that co-feeding transmission of Borrelia, previously described in laboratory models, contributes in nature in promoting and maintaining ospC diversity within local tick populations.

Acylated cholesteryl galactosides are ubiquitous glycolipid antigens among Borrelia burgdorferi sensu lato

Lyme disease (LD) is the most common tick-borne disease in the Northern hemisphere. It is caused by Borrelia burgdorferi sensu lato, in particular, B. burgdorferi sensu stricto, Borrelia garinii, and Borrelia afzelii. However, other genospecies have been implicated as causative factors of LD as well. Borrelia burgdorferi exhibits numerous immunogenic lipoproteins, but due to strong heterogeneity, the use of these proteins for serodiagnosis and vaccination is hampered. We and others have identified acylated cholesteryl galactosides (ACGal) as a novel glycolipid present in B. burgdorferi sensu stricto, B. afzelii, and B. garinii. ACGal is a strong antigen and the majority of patients display anti-ACGal antibodies in the chronic stages of LD. However, it is unknown whether ACGal is present in other presumably pathogenic B. burgdorferi genospecies. Therefore, we performed an analysis of the total lipid extracts of a wide spectrum of genospecies of B. burgdorferi sensu lato using thin-layer chromatography as well as Western blot and dot-blot assays. We show that ACGal is present in substantial quantities in all B. burgdorferi genospecies tested. Therefore, this molecule might improve the serological detection of rarely pathogenic genospecies, and may be used as a protective vaccine regardless of the prevailing genospecies.

Interpretation criteria for standardized Western blot for the predominant species of Borrelia burgdorferi sensu lato in China

OBJECTIVE

Western blotting (WB; immunoblotting) is a widely used tool for the serodiagnosis of Lyme borreliosis (LB), but so far, no generally accepted criteria for its performance and interpretation have been established in China. The present study was designed to determine the criteria for standardized Western blot for the predominant species of Borrelia burgdorferi sensu lato in China, in which WB was produced with strain PD₉₁ as the representative strain attributed to predominant genospecies Borrelia garinii of Borrelia burgdorferi sensu lato.

METHODS

Approximately 13 bands between 14 and 100 kD were differentiated for strain PD₉₁ by using Gel-Pro analysis software. In a study with 631 serum samples (taken from 127 patients with Lyme borreliosis and 504 controls), all observed bands were documented. To establish criteria for a positive WB result for strain PD₉₁, receiver operating characteristic (ROC) curves were used.

RESULTS

The following interpretation criteria were recommended: for IgG, at least one band of P83/100, P58, P39, P30, OspC, P17, P66, and OspA; for IgM, at least one band of P83/100, P58, OspA, P30, OspC, P17 or P41. In addition, syphilis, leptospirosis and other related diseases should be excluded when the positive band is P41 in IgM. For IgG criteria, the sensitivity is 73.2%, the specificity is 99.4% and Youden index is 0.726; for IgM criteria, the sensitivity is 50.6%, the specificity is 93.1% and Youden index is 0.437.

CONCLUSION

Standardization of WB assays is necessary for comparison of results from different laboratories. Moreover, the criteria of other genospecies of Borrelia burgdorferi sensu lato should be determined in the future to complete the criteria of WB for the diagnosis of the Lyme disease in China.

Chemoenzymatic synthesis of a glycolipid library and elucidation of the antigenic epitope for construction of a vaccine against Lyme disease

Lyme disease (LD) is the most common tick-borne disease in Europe, North America, and Asia. The etiologic agents of LD are spirochetes of the group Borrelia burgdorferi sensu lato, which possess a lipid content of 25-30% of the dry weight. The major glycolipid cholesteryl 6-O-acyl-beta-D-galactopyranoside (ACGal), present in B. burgdorferi sensu stricto, B. afzelii, and B. garinii, is a specific and highly prevalent antigen frequently recognized by antibodies in late-stage LD. Here we report a convenient route for the chemical synthesis of ACGal by employing a combination of chemical synthesis steps with enzymatic transformations. This synthesized molecule was compared with bacterial extracts by immunoblots with patient sera, confirming the preserved antigenicity. Next, a glycolipid library derived from the native molecules with variations in the fatty acyl moiety and derivatives in which the cholesterol has been replaced was designed and synthesized. The chemical structures were confirmed by 1D and 2D NMR spectroscopy and mass spectrometry. The native and synthetic glycolipids were utilized in immunoblots to determine the epitope recognized by antibodies in patient sera. By this method we could demonstrate that galactose, cholesterol, and a fatty acid with a minimal chain length of four carbon atoms comprises the essential structure for recognition by antibodies. Finally, this finding allowed the synthesis of a functionalized ACGal with an omega-mercapto group at the fatty acid and a facile protection and deprotection strategy. This antigenic hapten can be conjugated to a carrier protein to effect immunization against Lyme disease.

BBA52 facilitates Borrelia burgdorferi transmission from feeding ticks to murine hosts

Borrelia burgdorferi, the pathogen of Lyme borreliosis, persists in nature through a tick-rodent transmission cycle. A selective assessment of the microbial transcriptome, limited to gene-encoding putative membrane proteins, reveals that bba52 transcription in vivo is strictly confined to the vector-specific portion of the microbial life cycle, with the highest levels of expression noted in feeding ticks and with swift down-regulation noted in mice. bba52 deletion did not affect murine disease as assessed by the genesis of arthritis and carditis or long-term persistence of pathogens in mice or ticks. However, bba52 deficiency did impair microbial transitions between hosts and vector, defects that could be fully rescued when bba52 expression was genetically restored to the original genomic locus. These studies establish that BBA52 facilitates vector-host transitions by the pathogen and therefore is a potential antigenic target for interference with transmission of B. burgdorferi from ticks to mammalian hosts.