OspC facilitates Borrelia burgdorferi invasion of Ixodes scapularis salivary glands

Assessment of the hypothetical protein BB0616 in the murine infection of Borrelia burgdorferi

bb0616 of Borrelia burgdorferi, the Lyme disease pathogen, encodes a hypothetical protein of unknown function. In this study, we showed that BB0616 was not surface-exposed or associated with the membrane through localization analyses using proteinase K digestion and cell partitioning assays. The expression of bb0616 was influenced by a reduced pH but not by growth phases, elevated temperatures, or carbon sources during in vitro cultivation. A transcriptional start site for bb0616 was identified by using 5' rapid amplification of cDNA ends, which led to the identification of a functional promoter in the 5' regulatory region upstream of bb0616. By analyzing a bb0616-deficient mutant and its isogenic complemented counterparts, we found that the infectivity potential of the mutant was significantly attenuated. The inactivation of bb0616 displayed no effect on borrelial growth in the medium or resistance to oxidative stress, but the mutant was significantly more susceptible to osmotic stress. In addition, the production of global virulence regulators such as BosR and RpoS as well as virulence-associated outer surface lipoproteins OspC and DbpA was reduced in the mutant. These phenotypes were fully restored when gene mutation was complemented with a wild-type copy of bb0616. Based on these findings, we concluded that the hypothetical protein BB0616 is required for the optimal infectivity of B. burgdorferi, potentially by impacting B. burgdorferi virulence gene expression as well as survival of the spirochete under stressful conditions.

A systemic approach to identify non-abundant immunogenic proteins in Lyme disease pathogens

Borrelia burgdorferi, the pathogen of Lyme disease, differentially produces many outer surface proteins (Osp), some of which represent the most abundant membrane proteins, such as OspA, OspB, and OspC. In cultured bacteria, these proteins can account for a substantial fraction of the total cellular or membrane proteins, posing challenges to the identification and analysis of non-abundant proteins, which could serve as novel pathogen detection markers or as vaccine candidates. Herein, we introduced serial mutations to remove these abundant Osps and generated a B. burgdorferi mutant deficient in OspA, OspB, and OspC in an infectious 297-isolate background, designated as OspABC- mutant. Compared to parental isolate, the mutant did not reflect growth defects in the cultured medium but showed differential mRNA expression of representative tested genes, in addition to gross changes in cellular and membrane protein profiles. The analysis of differentially detectable protein contents of the OspABC- mutant, as compared to the wild type, by two-dimensional gel electrophoresis followed by liquid chromatography-mass spectrometry, identified several spirochete proteins that are dominated by proteins of unknown functions, as well as membrane transporters, chaperons, and metabolic enzymes. We produced recombinant forms of two of these represented proteins, BBA34 and BB0238, and showed that these proteins are detectable during spirochete infection in the tick-borne murine model of Lyme borreliosis and thus serve as potential antigenic markers of the infection.IMPORTANCEThe present manuscript employed a systemic approach to identify non-abundant proteins in cultured Borrelia burgdorferi that are otherwise masked or hidden due to the overwhelming presence of abundant Osps like OspA, OspB, and OspC. As these Osps are either absent or transiently expressed in mammals, we performed a proof-of-concept study in which their removal allowed the analysis of otherwise less abundant antigens in OspABC-deficient mutants and identified several immunogenic proteins, including BBA34 and BB0238. These antigens could serve as novel vaccine candidates and/or genetic markers of Lyme borreliosis, promoting new research in the clinical diagnosis and prevention of Lyme disease.

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.

The Rrp2-RpoN-RpoS pathway plays an important role in the blood-brain barrier transmigration of the Lyme disease pathogen

Lyme disease, caused by Borrelia (or Borreliella) burgdorferi, is a complex multisystemic disorder that includes Lyme neuroborreliosis resulting from the invasion of both the central and peripheral nervous systems. However, factors that enable the pathogen to cross the blood-brain barrier (BBB) and invade the central nervous system (CNS) are still not well understood. The objective of this study was to identify the B. burgdorferi factors required for BBB transmigration. We utilized a transwell BBB model based on human brain-microvascular endothelial cells and focused on investigating the Rrp2-RpoN-RpoS pathway, a central regulatory pathway that is essential for mammalian infection by B. burgdorferi. Our results demonstrated that the Rrp2-RpoN-RpoS pathway is crucial for BBB transmigration. Furthermore, we identified OspC, a major surface lipoprotein controlled by the Rrp2-RpoN-RpoS pathway, as a significant contributor to BBB transmigration. Constitutive production of OspC in a mutant defective in the Rrp2-RpoN-RpoS pathway did not rescue the impairment in BBB transmigration, indicating that this pathway controls additional factors for this process. Two other major surface lipoproteins controlled by this pathway, DbpA/B and BBK32, appeared to be dispensable for BBB transmigration. In addition, both the surface lipoprotein OspA and the Rrp1 pathway, which are required B. burgdorferi colonization in the tick vector, were found not required for BBB transmigration. Collectively, our findings using in vitro transwell assays uncover another potential role of the Rrp2-RpoN-RpoS pathway in BBB transmigration of B. burgdorferi and invasion into the CNS.

Role of the Hypothetical Protein BB0563 during Borrelia burgdorferi Infection in Animals

The alternative sigma factor RpoS in Borrelia burgdorferi, the etiological agent of Lyme disease, has long been postulated to regulate virulence-associated genes other than ospC and dbpA. Here, we demonstrate that bb0563, a gene encoding a hypothetical protein, is regulated by RpoS and contributes to the optimal infectivity of B. burgdorferi. When B. burgdorferi was exposed to environmental stimuli, bb0563 showed similar expression patterns as rpoS, ospC, and dbpA. Expression of bb0563 was significantly downregulated when rpoS was inactivated and was restored in the complemented strain. By using rapid amplification of cDNA ends (RACE) and luciferase reporter assays, a functional promoter was identified in the regulatory region upstream of bb0563. Gene expression from this promoter was drastically decreased in the rpoS mutant. We next investigated the role of bb0563 during animal infection. By using quantitative reverse transcription-PCR (RT-PCR), we found that bb0563 was highly expressed in mouse tissues during infection. We further created a bb0563-deficient mutant in a bioluminescent B. burgdorferi strain and examined infection dynamics using in vivo imaging. Relative to the parental and complemented strains, the mutant showed a delayed infection pattern and bacterial load was reduced. Another bb0563 deletion mutant was also created in the strain 297 background, and quantitative PCR (qPCR) analysis revealed a significantly lower spirochetal burden in tissue samples collected from animals infected with the mutant. In addition, localization studies indicate that BB0563 is not exposed on the cell surface but is associated with outer membrane. Taken together, these results suggest that bb0563 is required for optimal infectivity of B. burgdorferi during experimental infection.

Quantitative analyses of interactions between SpoVG and RNA/DNA

The Borrelia burgdorferi SpoVG protein has previously been found to be a DNA- and RNA-binding protein. To aid in the elucidation of ligand motifs, affinities for numerous RNAs, ssDNAs, and dsDNAs were measured and compared. The loci used in the study were spoVG, glpFKD, erpAB, bb0242, flaB, and ospAB, with particular focus on the untranslated 5' portion of the mRNAs. Performing binding and competition assays yielded that the 5' end of spoVG mRNA had the highest affinity while the lowest observed affinity was to the 5' end of flaB mRNA. Mutagenesis studies of spoVG RNA and ssDNA sequences suggested that the formation of SpoVG-nucleic acid complexes are not entirely dependent on either sequence or structure. Additionally, exchanging uracil for thymine in ssDNAs did not affect protein-nucleic acid complex formation.

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

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

Lipoproteome screening of the Lyme disease agent identifies inhibitors of antibody-mediated complement killing

Spirochetal pathogens encode an abundance of lipoproteins that can provide a critical interface with the host environment. Borrelia burgdorferi, the model species for spirochetal biology, must survive an enzootic life cycle defined by fluctuations between vector (tick) and vertebrate host. While B. burgdorferi expresses over 80 surface lipoproteins—many of which likely contribute to host survival—the B. burgdorferi lipoproteome is poorly characterized. Here, we generated a platform to rapidly identify targets of B. burgdorferi surface lipoproteins and identified two paralogs that confer resistance to antibody-initiated complement killing that may promote survival in immunocompetent hosts. This work expands our understanding of complement evasion mechanisms and points toward a discovery approach for identifying host–pathogen interactions central to spirochete pathogenesis.

Spirochetal pathogens, such as the causative agent of Lyme disease, Borrelia burgdorferi sensu lato, encode an abundance of lipoproteins; however, due in part to their evolutionary distance from more well-studied bacteria, such as Proteobacteria and Firmicutes, few spirochetal lipoproteins have assigned functions. Indeed, B. burgdorferi devotes almost 8% of its genome to lipoprotein genes and interacts with its environment primarily through the production of at least 80 surface-exposed lipoproteins throughout its tick vector–vertebrate host lifecycle. Several B. burgdorferi lipoproteins have been shown to serve roles in cellular adherence or immune evasion, but the functions for most B. burgdorferi surface lipoproteins remain unknown. In this study, we developed a B. burgdorferi lipoproteome screening platform utilizing intact spirochetes that enables the identification of previously unrecognized host interactions. As spirochetal survival in the bloodstream is essential for dissemination, we targeted our screen to C1, the first component of the classical (antibody-initiated) complement pathway. We identified two high-affinity C1 interactions by the paralogous lipoproteins, ElpB and ElpQ (also termed ErpB and ErpQ, respectively). Using biochemical, microbiological, and biophysical approaches, we demonstrate that ElpB and ElpQ bind the activated forms of the C1 proteases, C1r and C1s, and represent a distinct mechanistic class of C1 inhibitors that protect the spirochete from antibody-mediated complement killing. In addition to identifying a mode of complement inhibition, our study establishes a lipoproteome screening methodology as a discovery platform for identifying direct host–pathogen interactions that are central to the pathogenesis of spirochetes, such as the Lyme disease agent.

Lyme Neuroborreliosis: Mechanisms of B. burgdorferi Infection of the Nervous System

Lyme borreliosis is the most prevalent tick-borne disease in the United States, infecting ~476,000 people annually. Borrelia spp. spirochetal bacteria are the causative agents of Lyme disease in humans and are transmitted by Ixodes spp ticks. Clinical manifestations vary depending on which Borrelia genospecies infects the patient and may be a consequence of distinct organotropism between species. In the US, B. burgdorferi sensu stricto is the most commonly reported genospecies and infection can manifest as mild to severe symptoms. Different genotypes of B. burgdorferi sensu stricto may be responsible for causing varying degrees of clinical manifestations. While the majority of Lyme borreliae-infected patients fully recover with antibiotic treatment, approximately 15% of infected individuals experience long-term neurological and psychological symptoms that are unresponsive to antibiotics. Currently, long-term antibiotic treatment remains the only FDA-approved option for those suffering from these chronic effects. Here, we discuss the current knowledge pertaining to B. burgdorferi sensu stricto infection in the central nervous system (CNS), termed Lyme neuroborreliosis (LNB), within North America and specifically the United States. We explore the molecular mechanisms of spirochete entry into the brain and the role B. burgdorferi sensu stricto genotypes play in CNS infectivity. Understanding infectivity can provide therapeutic targets for LNB treatment and offer public health understanding of the B. burgdorferi sensu stricto genotypes that cause long-lasting symptoms.

Borrelia burgdorferi infection modifies protein content in saliva of Ixodes scapularis nymphs

BACKGROUND

Lyme disease (LD) caused by Borrelia burgdorferi is the most prevalent tick-borne disease. There is evidence that vaccines based on tick proteins that promote tick transmission of B. burgdorferi could prevent LD. As Ixodes scapularis nymph tick bites are responsible for most LD cases, this study sought to identify nymph tick saliva proteins associated with B. burgdorferi transmission using LC-MS/MS. Tick saliva was collected using a non-invasive method of stimulating ticks (uninfected and infected: unfed, and every 12 h during feeding through 72 h, and fully-fed) to salivate into 2% pilocarpine-PBS for protein identification using LC-MS/MS.

RESULTS

We identified a combined 747 tick saliva proteins of uninfected and B. burgdorferi infected ticks that were classified into 25 functional categories: housekeeping-like (48%), unknown function (18%), protease inhibitors (9%), immune-related (6%), proteases (8%), extracellular matrix (7%), and small categories that account for <5% each. Notably, B. burgdorferi infected ticks secreted high number of saliva proteins (n=645) than uninfected ticks (n=376). Counter-intuitively, antimicrobial peptides, which function to block bacterial infection at tick feeding site were suppressed 23-85 folds in B. burgdorferi infected ticks. Similar to glycolysis enzymes being enhanced in mammalian cells exposed to B. burgdorferi : eight of the 10-glycolysis pathway enzymes were secreted at high abundance by B. burgdorferi infected ticks. Of significance, rabbits exposed to B. burgdorferi infected ticks acquired potent immunity that caused 40-60% mortality of B. burgdorferi infected ticks during the second infestation compared to 15-28% for the uninfected. This might be explained by ELISA data that show that high expression levels of immunogenic proteins in B. burgdorferi infected ticks.

CONCLUSION

Data here suggest that B. burgdorferi infection modified protein content in tick saliva to promote its survival at the tick feeding site. For instance, enzymes; copper/zinc superoxide dismutase that led to production of H2O2 that is toxic to B. burgdorferi were suppressed, while, catalase and thioredoxin that neutralize H2O2, and pyruvate kinase which yields pyruvate that protects Bb from H2O2 killing were enhanced. We conclude data here is an important resource for discovery of effective antigens for a vaccine to prevent LD.

The Brilliance of Borrelia: Mechanisms of Host Immune Evasion by Lyme Disease-Causing Spirochetes

Lyme disease (LD) has become the most common vector-borne illness in the northern hemisphere. The causative agent, Borrelia burgdorferi sensu lato, is capable of establishing a persistent infection within the host. This is despite the activation of both the innate and adaptive immune responses. B. burgdorferi utilizes several immune evasion tactics ranging from the regulation of surface proteins, tick saliva, antimicrobial peptide resistance, and the disabling of the germinal center. This review aims to cover the various methods by which B. burgdorferi evades detection and destruction by the host immune response, examining both the innate and adaptive responses. By understanding the methods employed by B. burgdorferi to evade the host immune response, we gain a deeper knowledge of B. burgdorferi pathogenesis and Lyme disease, and gain insight into how to create novel, effective treatments.

Lyme Disease in Humans

Lyme disease (Lyme borreliosis) is a tick-borne, zoonosis of adults and children caused by genospecies of the Borrelia burgdorferi sensu lato complex. The ailment, widespread throughout the Northern Hemisphere, continues to increase globally due to multiple environmental factors, coupled with increased incursion of humans into habitats that harbor the spirochete. B. burgdorferi sensu lato is transmitted by ticks from the Ixodes ricinus complex. In North America, B. burgdorferi causes nearly all infections; in Europe, B. afzelii and B. garinii are most associated with human disease. The spirochete's unusual fragmented genome encodes a plethora of differentially expressed outer surface lipoproteins that play a seminal role in the bacterium's ability to sustain itself within its enzootic cycle and cause disease when transmitted to its incidental human host. Tissue damage and symptomatology (i.e., clinical manifestations) result from the inflammatory response elicited by the bacterium and its constituents. The deposition of spirochetes into human dermal tissue generates a local inflammatory response that manifests as erythema migrans (EM), the hallmark skin lesion. If treated appropriately and early, the prognosis is excellent. However, in untreated patients, the disease may present with a wide range of clinical manifestations, most commonly involving the central nervous system, joints, or heart. A small percentage (~10%) of patients may go on to develop a poorly defined fibromyalgia-like illness, post-treatment Lyme disease (PTLD) unresponsive to prolonged antimicrobial therapy. Below we integrate current knowledge regarding the ecologic, epidemiologic, microbiologic, and immunologic facets of Lyme disease into a conceptual framework that sheds light on the disorder that healthcare providers encounter.

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.

Immune Response to Borrelia: Lessons from Lyme Disease Spirochetes

The mammalian host responds to infection with Borrelia spirochetes through a highly orchestrated immune defense involving innate and adaptive effector functions aimed toward limiting pathogen burdens, minimizing tissue injury, and preventing subsequent reinfection. The evolutionary adaptation of Borrelia spirochetes to their reservoir mammalian hosts may allow for its persistence despite this immune defense. This review summarizes our current understanding of the host immune response to B. burgdorferi sensu lato, the most widely studied Borrelia spp. and etiologic agent of Lyme borreliosis. Pertinent literature will be reviewed with emphasis on in vitro, ex vivo and animal studies that influenced our understanding of both the earliest responses to B. burgdorferi as it enters the mammalian host and those that evolve as spirochetes disseminate and establish infection in multiple tissues. Our focus is on the immune response of inbred mice, the most commonly studied animal model of B. burgdorferi infection and surrogate for one of this pathogen's principle natural reservoir hosts, the white-footed deer mouse. Comparison will be made to the immune responses of humans with Lyme borreliosis. Our goal is to provide an understanding of the dynamics of the mammalian immune response during infection with B. burgdorferi and its relation to the outcomes in reservoir (mouse) and non-reservoir (human) hosts.

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.

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.

Complement Evasion by Lyme Disease Spirochetes

The complement system is an ancient arm of the innate immune system that plays important roles in pathogen recognition and elimination. Upon activation by microbes, complement opsonizes bacterial surfaces, recruits professional phagocytes, and causes bacteriolysis. Borreliella species are spirochetal bacteria that are transmitted to vertebrate hosts via infected Ixodes ticks and are the etiologic agents of Lyme disease. Pathogens that traffic in blood and other body fluids, like Borreliella, have evolved means to evade complement. Lyme disease spirochetes interfere with complement by producing a small arsenal of outer-surface lipoproteins that bind host complement components and manipulate their native activities. Here we review the current landscape of complement evasion by Lyme disease spirochetes and provide an update on recent discoveries.

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.

The intergenic small non-coding RNA ittA is required for optimal infectivity and tissue tropism in Borrelia burgdorferi

Post-transcriptional regulation via small regulatory RNAs (sRNAs) has been implicated in diverse regulatory processes in bacteria, including virulence. One class of sRNAs, termed trans-acting sRNAs, can affect the stability and/or the translational efficiency of regulated transcripts. In this study, we utilized a collaborative approach that employed data from infection with the Borrelia burgdorferi Tn library, coupled with Tn-seq, together with borrelial sRNA and total RNA transcriptomes, to identify an intergenic trans-acting sRNA, which we designate here as ittA for infectivity-associated and tissue-tropic sRNA locus A. The genetic inactivation of ittA resulted in a significant attenuation in infectivity, with decreased spirochetal load in ear, heart, skin and joint tissues. In addition, the ittA mutant did not disseminate to peripheral skin sites or heart tissue, suggesting a role for ittA in regulating a tissue-tropic response. RNA-Seq analysis determined that 19 transcripts were differentially expressed in the ittA mutant relative to its genetic parent, including vraA, bba66, ospD and oms28 (bba74). Subsequent proteomic analyses also showed a significant decrease of OspD and Oms28 (BBA74) proteins. To our knowledge this is the first documented intergenic sRNA that alters the infectivity potential of B. burgdorferi.

Mutation of the Carboxy-Terminal Processing Protease in Acinetobacter baumannii Affects Motility, Leads to Loss of Membrane Integrity, and Reduces Virulence

Motility plays an essential role in the host–parasite relationship of pathogenic bacteria, and is often associated with virulence. While many pathogenic bacteria use flagella for locomotion, Acinetobacter baumannii strains do not have flagella, but have other features that aid in their motility. To study the genes involved in motility, transposon mutagenesis was performed to construct A. baumannii mutant strains. Mutant strain MR14 was found to have reduced motility, compared to wild-type ATCC 17978. NCBI BLAST analysis revealed that the Tn10 transposon in the MR14 genome is integrated into the gene that encodes for carboxy-terminal processing protease (Ctp). Additionally, MR14 exhibits a mucoidy, sticky phenotype as the result of increased extracellular DNA (eDNA) caused by bacterial autolysis. Transmission and scanning electron microscopy revealed cytoplasmic content leaving the cell and multiple cell membrane depressions, respectively. MR14 showed higher sensitivity to environmental stressors. Mutation of the ctp gene reduced invasion and adhesion of A. baumannii to airway epithelial cells, potentially due to increased hydrophobicity. In the zebrafish model of infection, MR14 increased the survival rate by 40% compared to the wild-type. Taken together, the ctp gene in A. baumannii has a pivotal role in maintaining membrane integrity, adaptation to environmental stress, and controlling virulence.

Protective Immunity and New Vaccines for Lyme Disease

Lyme disease, caused by some Borrelia burgdorferi sensu lato, is the most common tick-borne illness in the Northern Hemisphere and the number of cases, and geographic spread, continue to grow. Previously identified B. burgdorferi proteins, lipid immunogens, and live mutants lead the design of canonical vaccines aimed at disrupting infection in the host. Discovery of the mechanism of action of the first vaccine catalyzed the development of new strategies to control Lyme disease that bypassed direct vaccination of the human host. Thus, novel prevention concepts center on proteins produced by B. burgdorferi during tick transit and on tick proteins that mediate feeding and pathogen transmission. A burgeoning area of research is tick immunity as it can unlock mechanistic pathways that could be targeted for disruption. Studies that shed light on the mammalian immune pathways engaged during tick-transmitted B. burgdorferi infection would further development of vaccination strategies against Lyme disease.

Characterization of 6S RNA in the Lyme disease spirochete

6S RNA binds to RNA polymerase and regulates gene expression, contributing to bacterial adaptation to environmental stresses. In this study, we examined the role of 6S RNA in murine infectivity and tick persistence of the Lyme disease spirochete Borrelia (Borreliella) burgdorferi. B. burgdorferi 6S RNA (Bb6S RNA) binds to RNA polymerase, is expressed independent of growth phase or nutrient stress in culture, and is processed by RNase Y. We found that rny (bb0504), the gene encoding RNase Y, is essential for B. burgdorferi growth, while ssrS, the gene encoding 6S RNA, is not essential, indicating a broader role for RNase Y activity in the spirochete. Bb6S RNA regulates expression of the ospC and dbpA genes encoding outer surface protein C and decorin binding protein A, respectively, which are lipoproteins important for host infection. The highest levels of Bb6S RNA are found when the spirochete resides in unfed nymphs. ssrS mutants lacking Bb6S RNA were compromised for infectivity by needle inoculation, but injected mice seroconverted, indicating an ability to activate the adaptive immune response. ssrS mutants were successfully acquired by larval ticks and persisted through fed nymphs. Bb6S RNA is one of the first regulatory RNAs identified in B. burgdorferi that controls the expression of lipoproteins involved in host infectivity.

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.

Cross-kingdom analysis of nymphal-stage Ixodes scapularis microbial communities in relation to Borrelia burgdorferi infection and load

The tick microbiota may influence the colonization of Ixodes scapularis by Borrelia burgdorferi, the Lyme disease bacterium. Using conserved and pathogen-specific primers we performed a cross-kingdom analysis of bacterial, fungal, protistan and archaeal communities of I. scapularis nymphs (N = 105) collected from southern Vermont, USA. The bacterial community was dominated by a Rickettsia and several environmental taxa commonly reported in I. scapularis, as well as the human pathogens B. burgdorferi and Anaplasma phagocytophilum, agent of human granulocytic anaplasmosis. With the fungal primer set we detected primarily plant- and litter-associated taxa and >18% of sequences were Malassezia, a fungal genus associated with mammalian skin. Two 18S rRNA gene primer sets, intended to target protistan communities, returned mostly Ixodes DNA as well as the wildlife pathogen Babesia odocoilei (7% of samples), a Gregarines species (14%) and a Spirurida nematode (18%). Data from pathogen-specific and conserved primers were consistent in terms of prevalence and identification. We measured B. burgdorferi presence/absence and load and found that bacterial beta diversity varied based on B. burgdorferi presence/absence. Load was weakly associated with bacterial community composition. We identified taxa associated with B. burgdorferi infection that should be evaluated for their role in vector colonization by pathogens.

Genetic Manipulation of Borrelia Spp

The spirochetes Borrelia (Borreliella) burgdorferi and Borrelia hermsii, the etiologic agents of Lyme disease and relapsing fever, respectively, cycle in nature between an arthropod vector and a vertebrate host. They have extraordinarily unusual genomes that are highly segmented and predominantly linear. The genetic analyses of Lyme disease spirochetes have become increasingly more sophisticated, while the age of genetic investigation in the relapsing fever spirochetes is just dawning. Molecular tools available for B. burgdorferi and related species range from simple selectable markers and gene reporters to state-of-the-art inducible gene expression systems that function in the animal model and high-throughput mutagenesis methodologies, despite nearly overwhelming experimental obstacles. This armamentarium has empowered borreliologists to build a formidable genetic understanding of the cellular physiology of the spirochete and the molecular pathogenesis of Lyme disease.

Tracking of Borrelia afzelii Transmission from Infected Ixodes ricinus Nymphs to Mice

Quantitative and microscopic tracking of Borrelia afzelii transmission from infected Ixodes ricinus nymphs has shown a transmission cycle different from that of Borrelia burgdorferi and Ixodes scapularis. Borrelia afzelii organisms are abundant in the guts of unfed I. ricinus nymphs, and their numbers continuously decrease during feeding.

Quantitative and microscopic tracking of Borrelia afzelii transmission from infected Ixodes ricinus nymphs has shown a transmission cycle different from that of Borrelia burgdorferi and Ixodes scapularis. Borrelia afzelii organisms are abundant in the guts of unfed I. ricinus nymphs, and their numbers continuously decrease during feeding. Borrelia afzelii spirochetes are present in murine skin within 1 day of tick attachment. In contrast, spirochetes were not detectable in salivary glands at any stage of tick feeding. Further experiments demonstrated that tick saliva is not essential for B. afzelii infectivity, the most important requirement for successful host colonization being a change in expression of outer surface proteins that occurs in the tick gut during feeding. Spirochetes in vertebrate mode are then able to survive within the host even in the absence of tick saliva. Taken together, our data suggest that the tick gut is the decisive organ that determines the competence of I. ricinus to vector B. afzelii. We discuss possible transmission mechanisms of B. afzelii spirochetes that should be further tested in order to design effective preventive and therapeutic strategies against Lyme disease.

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.

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.

Borrelia burgdorferi surface-located Lmp1 protein processed into region-specific polypeptides that are critical for microbial persistence

One of the Borrelia burgdorferi virulence determinants, annotated as Lmp1, is a surface-exposed, conserved, and potential multi-domain protein involved in various functions in spirochete infectivity. Lmp1 contributes to host-pathogen interactions and evasion of host adaptive immunity by spirochetes. Here, we show that in diverse B. burgdorferi species, Lmp1 exists as distinct, region-specific, and lower molecular mass polypeptides encompassing 1 or more domains, including independent N-terminal and middle regions and a combined middle and C-terminal region. These polypeptides originate from complex posttranslational maturation events, partly supported by a periplasmic serine protease termed as BbHtrA. Although spirochete persistence in mice is independently supported by domain-specific Lmp1 polypeptides, transmission of B. burgdorferi from ticks to mammals requires essential contributions from both N-terminal and middle regions. Interference with the functions of Lmp1 domains or their complex posttranslational maturation events may aid in development of novel therapeutic strategies to combat infection and transmission of pathogens.

Plasticity in early immune evasion strategies of a bacterial pathogen

Borrelia burgdorferi is one of the few extracellular pathogens capable of establishing persistent infection in mammals. The mechanisms that sustain long-term survival of this bacterium are largely unknown. Here we report a unique innate immune evasion strategy of B. burgdorferi, orchestrated by a surface protein annotated as BBA57, through its modulation of multiple spirochete virulent determinants. BBA57 function is critical for early infection but largely redundant for later stages of spirochetal persistence, either in mammals or in ticks. The protein influences host IFN responses as well as suppresses multiple host microbicidal activities involving serum complement, neutrophils, and antimicrobial peptides. We also discovered a remarkable plasticity in BBA57-mediated spirochete immune evasion strategy because its loss, although resulting in near clearance of pathogens at the inoculum site, triggers nonheritable adaptive changes that exclude detectable nucleotide alterations in the genome but incorporate transcriptional reprograming events. Understanding the malleability in spirochetal immune evasion mechanisms that ensures their host persistence is critical for the development of novel therapeutic and preventive approaches to combat long-term infections like Lyme borreliosis.

Negative Frequency-Dependent Selection Is Frequently Confounding

Persistent genetic variation within populations presents an evolutionary problem, as natural selection and genetic drift tend to erode genetic diversity. Models of balancing selection were developed to account for the maintenance of genetic variation observed in natural populations. Negative frequency-dependent selection is a powerful type of balancing selection that maintains many natural polymorphisms, but it is also commonly misinterpreted. This review aims to clarify the processes underlying negative frequency-dependent selection, describe classes of polymorphisms that can and cannot result from these processes, and discuss the empirical data needed to accurately identify processes that generate or maintain diversity in nature. Finally, the importance of accurately describing the processes affecting genetic diversity within populations as it relates to research progress is considered.

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.

Interaction of primary mast cells with Borrelia burgdorferi (sensu stricto): role in transmission and dissemination in C57BL/6 mice

Background

Borrelia burgdorferi (sensulato), the causative agent of Lyme borreliosis is a bacterium transmitted by hard ticks, Ixodes spp. Bacteria are injected into the host skin during the tick blood meal with tick saliva. There, Borrelia and saliva interact together with skin cells such as keratinocytes, fibroblasts, mast cells and other specific immune cells before disseminating to target organs.

Methods

To study the role of mast cells in the transmission of Lyme borreliosis, we isolated mouse primary mast cells from bone marrow and incubated them in the presence of Borrelia burgdorferi (sensu stricto) and tick salivary gland extract. We further analyzed their potential role in vivo, in a mouse model of deficient in mast cells (Kit^wsh−/− mice).

Results

To our knowledge, we report here for the first time the bacteria ability to induce the inflammatory response of mouse primary mast cells. We show that OspC, a major surface lipoprotein involved in the early transmission of Borrelia, induces the degranulation of primary mast cells but has a limited effect on the overall inflammatory response of these cells. In contrast, whole bacteria have an opposite effect. We also show that mast cell activation is significantly inhibited by tick salivary gland extract. Finally, we demonstrate that mast cells are likely not the only host cells involved in the early transmission and dissemination of Borrelia since the use of mast cell deficient Kit^wsh−/− mice shows a limited impact on these two processes in the context of this mouse genetic background.

Conclusions

The absence of mast cells did not change the replication rate of Borrelia in the skin. However, in the absence of mast cells, Borrelia dissemination to the joints was faster. Mast cells do not control skin bacterial proliferation during primary infection and the establishment of the primary infection, as shown in the C57BL/6 mouse model studied. Nevertheless, the Borrelia induced cytotokine modulation on mast cells might be involved in long term and/or repeated infections and protect from Lyme borreliosis due to the development of a hypersensitivity to tick saliva.

Electronic supplementary material

The online version of this article (doi:10.1186/s13071-017-2243-0) contains supplementary material, which is available to authorized users.

Investigation of ospC Expression Variation among Borrelia burgdorferi Strains

Outer surface protein C (OspC) is the most studied major virulence factor of Borrelia burgdorferi, the causative agent of Lyme disease. The level of OspC varies dramatically among B. burgdorferi strains when cultured in vitro, but little is known about what causes such variation. It has been proposed that the difference in endogenous plasmid contents among strains contribute to variation in OspC phenotype, as B. burgdorferi contains more than 21 endogenous linear (lp) and circular plasmids (cp), and some of which are prone to be lost. In this study, we analyzed several clones isolated from B. burgdorferi strain 297, one of the most commonly used strains for studying ospC expression. By taking advantage of recently published plasmid sequence of strain 297, we developed a multiplex PCR method specifically for rapid plasmid profiling of B. burgdorferi strain 297. We found that some commonly used 297 clones that were thought having a complete plasmid profile, actually lacked some endogenous plasmids. Importantly, the result showed that the difference in plasmid profiles did not contribute to the ospC expression variation among the clones. Furthermore, we found that B. burgdorferi clones expressed different levels of BosR, which in turn led to different levels of RpoS and subsequently, resulted in OspC level variation among B. burgdorferi strains.

Host Immune Evasion by Lyme and Relapsing Fever Borreliae: Findings to Lead Future Studies for Borrelia miyamotoi

The emerging pathogen, Borrelia miyamotoi, is a relapsing fever spirochete vectored by the same species of Ixodes ticks that carry the causative agents of Lyme disease in the US, Europe, and Asia. Symptoms caused by infection with B. miyamotoi are similar to a relapsing fever infection. However, B. miyamotoi has adapted to different vectors and reservoirs, which could result in unique physiology, including immune evasion mechanisms. Lyme Borrelia utilize a combination of Ixodes-produced inhibitors and native proteins [i.e., factor H-binding proteins (FHBPs)/complement regulator-acquiring surface proteins, p43, BBK32, BGA66, BGA71, CD59-like protein] to inhibit complement, while some relapsing fever spirochetes use C4b-binding protein and likely Ornithodoros-produced inhibitors. To evade the humoral response, Borrelia utilize antigenic variation of either outer surface proteins (Osps) and the Vmp-like sequences (Vls) system (Lyme borreliae) or variable membrane proteins (Vmps, relapsing fever borreliae). B. miyamotoi possesses putative FHBPs and antigenic variation of Vmps has been demonstrated. This review summarizes and compares the common mechanisms utilized by Lyme and relapsing fever spirochetes, as well as the current state of understanding immune evasion by B. miyamotoi.

Multifunctional and Redundant Roles of Borrelia burgdorferi Outer Surface Proteins in Tissue Adhesion, Colonization, and Complement Evasion

Borrelia burgdorferi is the causative agent of Lyme disease in the U.S., with at least 25,000 cases reported to the CDC each year. B. burgdorferi is thought to enter and exit the bloodstream to achieve rapid dissemination to distal tissue sites during infection. Travel through the bloodstream requires evasion of immune surveillance and pathogen clearance in the host, a process at which B. burgdorferi is adept. B. burgdorferi encodes greater than 19 adhesive outer surface proteins many of which have been found to bind to host cells or components of the extracellular matrix. Several others bind to host complement regulatory factors, in vitro. Production of many of these adhesive proteins is tightly regulated by environmental cues, and some have been shown to aid in vascular interactions and tissue colonization, as well as survival in the blood, in vivo. Recent work has described multifaceted and redundant roles of B. burgdorferi outer surface proteins in complement component interactions and tissue targeted adhesion and colonization, distinct from their previously identified in vitro binding capabilities. Recent insights into the multifunctional roles of previously well-characterized outer surface proteins such as BBK32, DbpA, CspA, and OspC have changed the way we think about the surface proteome of these organisms during the tick-mammal life cycle. With the combination of new and old in vivo models and in vitro techniques, the field has identified distinct ligand binding domains on BBK32 and DbpA that afford tissue colonization or blood survival to B. burgdorferi. In this review, we describe the multifunctional and redundant roles of many adhesive outer surface proteins of B. burgdorferi in tissue adhesion, colonization, and bloodstream survival that, together, promote the survival of Borrelia spp. throughout maintenance in their multi-host lifestyle.

Borrelia burgdorferi Induces TLR2-Mediated Migration of Activated Dendritic Cells in an Ex Vivo Human Skin Model

Borrelia burgdorferi is transmitted into the skin of the host where it encounters and interacts with two dendritic cell (DC) subsets; Langerhans cells (LCs) and dermal DCs (DDCs). These cells recognize pathogens via pattern recognition receptors, mature and migrate out of the skin into draining lymph nodes, where they orchestrate adaptive immune responses. In order to investigate the response of skin DCs during the early immunopathogenesis of Lyme borreliosis, we injected B. burgdorferi intradermally into full-thickness human skin and studied the migration of DCs out of the skin, the activation profile and phenotype of migrated cells. We found a significant increase in the migration of LCs and DDCs in response to B. burgdorferi. Notably, migration was prevented by blocking TLR2. DCs migrated from skin inoculated with higher numbers of spirochetes expressed significantly higher levels of CD83 and produced pro-inflammatory cytokines. No difference was observed in the expression of HLA-DR, CD86, CD38, or CCR7. To conclude, we have established an ex vivo human skin model to study DC-B. burgdorferi interactions. Using this model, we have demonstrated that B. burgdorferi-induced DC migration is mediated by TLR2. Our findings underscore the utility of this model as a valuable tool to study immunity to spirochetal infections.

In Vivo Imaging Demonstrates That Borrelia burgdorferi ospC Is Uniquely Expressed Temporally and Spatially throughout Experimental Infection

Borrelia burgdorferi is a spirochetal bacterium transmitted by the Ixodes tick that causes Lyme disease in humans due to its ability to evade the host immune response and disseminate to multiple immunoprotective tissues. The pathogen undergoes dynamic genetic alterations important for adaptation from the tick vector to the mammalian host, but little is known regarding the changes at the transcriptional level within the distal tissues they colonize. In this study, B. burgdorferi infection and gene expression of the essential virulence determinant ospC was quantitatively monitored in a spatial and temporal manner utilizing reporter bioluminescent borrelial strains with in vivo and ex vivo imaging. Although expressed from a shuttle vector, the PospC-luc construct exhibited a similar expression pattern relative to native ospC. Bacterial burden in skin, inguinal lymph node, heart, bladder and tibiotarsal joint varied between tissues and fluctuated over the course of infection possibly in response to unique cues of each microenvironment. Expression of ospC, when normalized for changes in bacterial load, presented unique profiles in murine tissues at different time points. The inguinal lymph node was infected with a significant B. burgdorferi burden, but showed minimal ospC expression. B. burgdorferi infected skin and heart induced expression of ospC early during infection while the bladder and tibiotarsal joint continued to display PospC driven luminescence throughout the 21 day time course. Localized skin borrelial burden increased dramatically in the first 96 hours following inoculation, which was not paralleled with an increase in ospC expression, despite the requirement of ospC for dermal colonization. Quantitation of bioluminescence representing ospC expression in individual tissues was validated by qRT-PCR of the native ospC transcript. Taken together, the temporal regulation of ospC expression in distal tissues suggests a role for this virulence determinant beyond early infection.

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.

HtrA, a Temperature- and Stationary Phase-Activated Protease Involved in Maturation of a Key Microbial Virulence Determinant, Facilitates Borrelia burgdorferi Infection in Mammalian Hosts

High-temperature requirement protease A (HtrA) represents a family of serine proteases that play important roles in microbial biology. Unlike the genomes of most organisms, that of Borrelia burgdorferi notably encodes a single HtrA gene product, termed BbHtrA. Previous studies identified a few substrates of BbHtrA; however, their physiological relevance could not be ascertained, as targeted deletion of the gene has not been successful. Here we show that BbhtrA transcripts are induced during spirochete growth either in the stationary phase or at elevated temperature. Successful generation of a BbhtrA deletion mutant and restoration by genetic complementation suggest a nonessential role for this protease in microbial viability; however, its remarkable growth, morphological, and structural defects during cultivation at 37°C confirm a high-temperature requirement for protease activation and function. The BbhtrA-deficient spirochetes were unable to establish infection of mice, as evidenced by assessment of culture, PCR, and serology. We show that transcript abundance as well as proteolytic processing of a borrelial protein required for cell fission and infectivity, BB0323, is impaired in BbhtrA mutants grown at 37°C, which likely contributed to their inability to survive in a mammalian host. Together, these results demonstrate the physiological relevance of a unique temperature-regulated borrelial protease, BbHtrA, which further enlightens our knowledge of intriguing aspects of spirochete biology and infectivity.

Toll-like receptor cascade and gene polymorphism in host-pathogen interaction in Lyme disease

Lyme disease (LD) risk occurs in North America and Europe where the tick vectors of the causal agent Borrelia burgdorferi sensu lato are found. It is associated with local and systemic manifestations, and has persistent posttreatment health complications in some individuals. The innate immune system likely plays a critical role in both host defense against B. burgdorferi and disease severity. Recognition of B. burgdorferi, activation of the innate immune system, production of proinflammatory cytokines, and modulation of the host adaptive responses are all initiated by Toll-like receptors (TLRs). A number of Borrelia outer-surface proteins (eg, OspA and OspB) are recognized by TLRs. Specifically, TLR1 and TLR2 were identified as the receptors most relevant to LD. Several functional single-nucleotide polymorphisms have been identified in TLR genes, and are associated with varying cytokines types and synthesis levels, altered pathogen recognition, and disruption of the downstream signaling cascade. These single-nucleotide polymorphism-related functional alterations are postulated to be linked to disease development and posttreatment persistent illness. Elucidating the role of TLRs in LD may facilitate a better understanding of disease pathogenesis and can provide an insight into novel therapeutic targets during active disease or postinfection and posttreatment stages.

Interaction of the Lyme disease spirochete with its tick vector

Borrelia burgdorferi, the causative agent of Lyme disease (along with closely related genospecies), is in the deeply branching spirochete phylum. The bacterium is maintained in nature in an enzootic cycle that involves transmission from a tick vector to a vertebrate host and acquisition from a vertebrate host to a tick vector. During its arthropod sojourn, B. burgdorferi faces a variety of stresses, including nutrient deprivation. Here, we review some of the spirochetal factors that promote persistence, maintenance and dissemination of B. burgdorferi in the tick, and then focus on the utilization of available carbohydrates as well as the exquisite regulatory systems invoked to adapt to the austere environment between blood meals and to signal species transitions as the bacteria traverse their enzootic cycle. The spirochetes shift their source of carbon and energy from glucose in the vertebrate to glycerol in the tick. Regulation of survival under limiting nutrients requires the classic stringent response in which RelBbu controls the levels of the alarmones guanosine tetraphosphate and guanosine pentaphosphate (collectively termed (p)ppGpp), while regulation at the tick-vertebrate interface as well as regulation of protective responses to the blood meal require the two-component system Hk1/Rrp1 to activate production of the second messenger cyclic-dimeric-GMP (c-di-GMP).

Oral Immunization with OspC Does Not Prevent Tick-Borne Borrelia burgdorferi Infection

Oral vaccination strategies are of interest to prevent transmission of Lyme disease as they can be used to deliver vaccines to humans, pets, and to natural wildlife reservoir hosts of Borrelia burgdorferi. We developed a number of oral vaccines based in E. coli expressing recombinant OspC type K, OspB, BBK32 from B. burgdorferi, and Salp25, Salp15 from Ixodes scapularis. Of the five immunogenic candidates only OspC induced significant levels of antigen-specific IgG and IgA when administered to mice via the oral route. Antibodies to OspC did not prevent dissemination of B. burgdorferi as determined by the presence of spirochetes in ear, heart and bladder tissues four weeks after challenge. Next generation sequencing of genomic DNA from ticks identified multiple phyletic types of B. burgdorferi OspC (A, D, E, F, I, J, K, M, Q, T, X) in nymphs that engorged on vaccinated mice. PCR amplification of OspC types A and K from flat and engorged nymphal ticks, and from heart and bladder tissues collected after challenge confirmed sequencing analysis. Quantification of spirochete growth in a borreliacidal assay shows that both types of spirochetes (A and K) survived in the presence of OspC-K specific serum whereas the spirochetes were killed by OspA specific serum. We show that oral vaccination of C3H-HeN mice with OspC-K induced significant levels of antigen-specific IgG. However, these serologic antibodies did not protect mice from infection with B. burgdorferi expressing homologous or heterologous types of OspC after tick challenge.

Virulence of the Lyme disease spirochete before and after the tick bloodmeal: a quantitative assessment

Background

Borrelia burgdorferi, the tick-transmitted agent of Lyme disease, adapts to different environments as it cycles between an arthropod vector and vertebrate host. Signals encountered during nymphal tick feeding prior to transmission activate a regulon that is controlled by the alternative sigma factors RpoN and RpoS, which are required for mammalian infection. The ingested bloodmeal also provides nutrients that stimulate spirochete replication. Although the influence of tick feeding on spirochete growth and gene expression is well documented, a quantitative assessment of spirochete virulence before and after tick feeding has not been made.

Methods

Homogenates were prepared from unfed and fed infected Ixodes scapularis nymphs that had acquired B. burgdorferi as larvae. Serially diluted tick homogenates were needle-inoculated into mice to determine the infectious dose of tick-derived spirochetes before and after the bloodmeal. Mouse infection was assessed by sero-reactivity with B. burgdorferi whole cell lysates on immunoblots and attempted isolation of spirochetes from mouse tissues. Viable spirochetes in tick-derived inocula were quantified by colony formation in solid media.

Results

We found that an inoculum containing as many as 10⁴B. burgdorferi from unfed ticks is largely non-infectious, while the calculated ID₅₀ for spirochetes from fed ticks is ~30 organisms. Engineered constitutive production of the essential virulence factor OspC by spirochetes within unfed ticks did not confer an infectious phenotype.

Conclusion

Conditional priming of B. burgdorferi during tick feeding induces changes in addition to OspC that are required for infection of the mammalian host.

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.

A Borrelia burgdorferi Surface-Exposed Transmembrane Protein Lacking Detectable Immune Responses Supports Pathogen Persistence and Constitutes a Vaccine Target

Borrelia burgdorferi harbors a limited set of transmembrane surface proteins, most of which constitute key targets of humoral immune responses. Here we show that BB0405, a conserved membrane-spanning protein of unknown function, fails to evoke detectable antibody responses despite its extracellular exposure. bb0405 is a member of an operon and ubiquitously expressed throughout the rodent-tick infection cycle. The gene product serves an essential function in vivo, as bb0405-deletion mutants are unable to transmit from ticks and establish infection in mammalian hosts. Despite the lack of BB0405-specific immunoglobulin M or immunoglobulin G antibodies during natural infection, mice immunized with a recombinant version of the protein elicited high-titer and remarkably long-lasting antibody responses, conferring significant host protection against tick-borne infection. Taken together, these studies highlight the essential role of an apparently immune-invisible borrelial transmembrane protein in facilitating infection and its usefulness as a target of protective host immunity blocking the transmission of B. burgdorferi.

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.

The Fur homologue BosR requires Arg39 to activate rpoS transcription in Borrelia burgdorferi and thereby direct spirochaete infection in mice

Borrelia burgdorferi is the causative agent of Lyme disease. In B. burgdorferi, RpoS controls the expression of virulence genes needed for mammalian infection. The Fur homologue BosR regulates the transcription of rpoS and therefore BosR determines, albeit indirectly, the infection status of the spirochaete. Transcription of rpoS in B. burgdorferi is complex: rpoS can be transcribed either from an RpoD-dependent promoter to yield a long transcript or from an RpoN-dependent promoter to yield a short transcript. This study shows that BosR repressed synthesis of the long transcript while at the same time activating synthesis of the short transcript. How BosR does this is unclear. To address this, spirochaetes were engineered to express either BosR or the naturally occurring variant BosRR39K. Mice became infected by the spirochaetes expressing BosR but not by the spirochaetes expressing BosRR39K. Furthermore, the spirochaetes expressing BosR activated rpoS transcription during growth in culture whereas the spirochaetes expressing BosRR39K did not. Thus, BosR's activation of rpoS transcription somehow involves Arg39. This arginine is highly conserved in other FUR proteins and therefore other FUR proteins may also require this arginine to function.