OspB antibody prevents Borrelia burgdorferi colonization of Ixodes scapularis

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

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

Interactions between Borrelia burgdorferi and its hosts across the enzootic cycle

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

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.

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.

Novel targets and strategies to combat borreliosis

Lyme borreliosis is a bacterial infection that can be spread to humans by infected ticks and may severely affect many organs and tissues. Nearly four decades have elapsed since the discovery of the disease agent called Borrelia burgdorferi. Although there is a plethora of knowledge on the infectious agent and thousands of scientific publications, an effective way on how to combat and prevent Lyme borreliosis has not been found yet. There is no vaccine for humans available, and only one active vaccine program in clinical development is currently running. A spirited search for possible disease interventions is of high public interest as surveillance data indicates that the number of cases of Lyme borreliosis is steadily increasing in Europe and North America. This review provides a condensed digest of the history of vaccine development up to new promising vaccine candidates and strategies that are targeted against Lyme borreliosis, including elements of the tick vector, the reservoir hosts, and the Borrelia pathogen itself.

The Emerging Role of Microbial Biofilm in Lyme Neuroborreliosis

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

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

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

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

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

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.

Lyme disease/borreliosis as a systemic disease

Lyme disease/borreliosis (LD) is a well-known arthropod-transmitted entity in the northern hemisphere. The incidence of LD is reportedly rising throughout the world, although better diagnostic facilities may be contributory. The disease distribution is expanding in Europe, with its presence being now documented at higher altitudes and latitudes. Borrelia burgdorferi sensu lato is the most important genospecies leading to LD, although newer ones continue to be discovered. The variations in clinical spectrum with genospecies involved are an interesting feature. The alteration in gene expression while the organism cycles between two very different hosts is intriguing and has been described. The disease presents in three stages-namely, the early localized, early disseminated, and late stage. Erythema chronicum migrans is the pathognomic early lesion, and its diagnosis is purely clinical; however, laboratory diagnosis is essential for later manifestations. Two-tier serologic testing using an enzyme-linked immunosorbent assay (ELISA) as the first tier and immunoglobulin M (IgM) and IgG immunoblot as the second, if ELISA is positive or equivocal, is the mainstay of diagnosis. Doxycycline is the cornerstone of treatment, whereas parenteral therapy, mainly with ceftriaxone, is indicated in a few specific scenarios.

Insights into the biology of Borrelia burgdorferi gained through the application of molecular genetics

Borrelia burgdorferi, the vector-borne bacterium that causes Lyme disease, was first identified in 1982. It is known that much of the pathology associated with Lyme borreliosis is due to the spirochete's ability to infect, colonize, disseminate, and survive within the vertebrate host. Early studies aimed at defining the biological contributions of individual genes during infection and transmission were hindered by the lack of adequate tools and techniques for molecular genetic analysis of the spirochete. The development of genetic manipulation techniques, paired with elucidation and annotation of the B. burgdorferi genome sequence, has led to major advancements in our understanding of the virulence factors and the molecular events associated with Lyme disease. Since the dawn of this genetic era of Lyme research, genes required for vector or host adaptation have garnered significant attention and highlighted the central role that these components play in the enzootic cycle of this pathogen. This chapter covers the progress made in the Borrelia field since the application of mutagenesis techniques and how they have allowed researchers to begin ascribing roles to individual genes. Understanding the complex process of adaptation and survival as the spirochete cycles between the tick vector and vertebrate host will lead to the development of more effective diagnostic tools as well as identification of novel therapeutic and vaccine targets. In this chapter, the Borrelia genes are presented in the context of their general biological roles in global gene regulation, motility, cell processes, immune evasion, and colonization/dissemination.

Complement evasion by Borrelia burgdorferi: it takes three to tango

The complement system is one of the major innate defense mechanisms Borrelia burgdorferi sensu lato has to overcome to establish an infection of mammalian hosts and to cause Lyme borreliosis in humans. Borrelia prevents complement-mediated killing during host colonization through (i) recruitment of host complement regulators by Borrelia, (ii) evasion mechanisms by Borrelia itself, and (iii) exploitation of tick proteins by Borrelia. These interactions with complement can be host species-specific. This review provides an overview of interactions between Borrelia, tick, and host leading to evasion of complement-mediated killing.

The role of Borrelia burgdorferi outer surface proteins

Human pathogenic spirochetes causing Lyme disease belong to the Borrelia burgdorferi sensu lato complex. Borrelia burgdorferi organisms are extracellular pathogens transmitted to humans through the bite of Ixodes spp. ticks. These spirochetes are unique in that they can cause chronic infection and persist in the infected human, even though a robust humoral and cellular immune response is produced by the infected host. How this extracellular pathogen is able to evade the host immune response for such long periods of time is currently unclear. To gain a better understanding of how this organism persists in the infected human, many laboratories have focused on identifying and characterizing outer surface proteins of B. burgdorferi. As the interface between B. burgdorferi and its human host is its outer surface, proteins localized to the outer membrane must play an important role in dissemination, virulence, tissue tropism, and immune evasion. Over the last two decades, numerous outer surface proteins from B. burgdorferi have been identified, and more recent studies have begun to elucidate the functional role(s) of many borrelial outer surface proteins. This review summarizes the outer surface proteins identified in B. burgdorferi to date and provides detailed insight into the functions of many of these proteins as they relate to the unique parasitic strategy of this spirochetal pathogen.

Characterization of multiprotein complexes of the Borrelia burgdorferi outer membrane vesicles

Among bacterial cell envelopes, the Borrelia burgdorferi outer membrane (OM) is structurally unique in that the identities of many protein complexes remain unknown; however, their characterization is the first step toward our understanding of membrane protein interactions and potential functions. Here, we used two-dimensional blue native/SDS-PAGE/mass spectrometric analysis for a global characterization of protein-protein interactions as well as to identify protein complexes in OM vesicles isolated from multiple infectious sensu stricto isolates of B. burgdorferi. Although we uncovered the existence of at least 10 distinct OM complexes harboring several unique subunits, the complexome is dominated by the frequent occurrence of a limited diversity of membrane proteins, most notably P13, outer surface protein (Osp) A, -B, -C, and -D and Lp6.6. The occurrence of these complexes and specificity of subunit interaction were further supported by independent two-dimensional immunoblotting and coimmunoprecipitation assays as well as by mutagenesis studies, where targeted depletion of a subunit member (P66) selectively abolished a specific complex. Although a comparable profile of the OM complexome was detected in two major infectious isolates, such as B31 and 297, certain complexes are likely to occur in an isolate-specific manner. Further assessment of protein complexes in multiple Osp-deficient isolates showed loss of several protein complexes but revealed the existence of additional complex/subunits that are undetectable in wild-type cells. Together, these observations uncovered borrelial antigens involved in membrane protein interactions. The study also suggests that the assembly process of OM complexes is specific and that the core or stabilizing subunits vary between complexes. Further characterization of these protein complexes including elucidation of their biological significance may shed new light on the mechanism of pathogen persistence and the development of preventative measures against the infection.

Lyme borreliosis vaccination: the facts, the challenge, the future

Lyme disease, or Lyme borreliosis, the most prevalent arthropod-borne disease in the Western world, is caused by spirochetes belonging to the Borrelia burgdorferi sensu lato group and is predominantly transmitted through Ixodes ticks. There is currently no vaccine available to prevent Lyme borreliosis in humans. Borrelia outer membrane proteins are reviewed which have been investigated as vaccine candidates. In addition, several tick proteins are discussed, on which anti-tick vaccines have been based, or are interesting future candidates, to prevent transmission of the spirochete from the tick vector to the mammalian host. Finally, novel vaccination strategies to prevent Lyme borreliosis are proposed, based on multiple Borrelia antigens, tick antigens or a combination of both Borrelia as well as tick antigens.

Reviewing molecular adaptations of Lyme borreliosis spirochetes in the context of reproductive fitness in natural transmission cycles

Lyme borreliosis (LB) is caused by a group of pathogenic spirochetes – most often Borrelia burgdorferi, B. afzelii, and B. garinii – that are vectored by hard ticks in the Ixodes ricinus-persulcatus complex, which feed on a variety of mammals, birds, and lizards. Although LB is one of the best-studied vector-borne zoonoses, the annual incidence in North America and Europe leads other vector-borne diseases and continues to increase. What factors make the LB system so successful, and how can researchers hope to reduce disease risk – either through vaccinating humans or reducing the risk of contacting infected ticks in nature? Discoveries of molecular interactions involved in the transmission of LB spirochetes have accelerated recently, revealing complex interactions among the spirochete-tick-vertebrate triad. These interactions involve multiple, and often redundant, pathways that reflect the evolution of general and specific mechanisms by which the spirochetes survive and reproduce. Previous reviews have focused on the molecular interactions or population biology of the system. Here molecular interactions among the LB spirochete, its vector, and vertebrate hosts are reviewed in the context of natural maintenance cycles, which represent the ecological and evolutionary contexts that shape these interactions. This holistic system approach may help researchers develop additional testable hypotheses about transmission processes, interpret laboratory results, and guide development of future LB control measures and management.

Molecular characterization of Ehrlichia interactions with tick cells and macrophages

Several tick-transmitted Anaplasmataceae family rickettsiales of the genera Ehrlichia and Anaplasma have been discovered in recent years. Some species are classified as pathogens causing emerging diseases with growing health concern for people. They include human monocytic ehrlichiosis, human granulocytic ewingii ehrlichiosis and human granulocytic anaplasmosis which are caused by Ehrlichia chaffeensis, E. ewingii and Anaplasma phagocytophilum, respectively. Despite the complex cellular environments and defense systems of arthropod and vertebrate hosts, rickettsials have evolved strategies to evade host clearance and persist in both vertebrate and tick host environments. For example, E. chaffeensis growing in vertebrate macrophages has distinct patterns of global host cell-specific protein expression and differs considerably in morphology compared with its growth in tick cells. Immunological studies suggest that host cell-specific differences in Ehrlichia gene expression aid the pathogen, extending its survival. Bacteria from tick cells persist longer when injected into mice compared with mammalian macrophage-grown bacteria, and the host response is also significantly different. This review presents the current understanding of tick-Ehrlichia interactions and implications for future.

Total, membrane, and immunogenic proteomes of macrophage- and tick cell-derived Ehrlichia chaffeensis evaluated by liquid chromatography-tandem mass spectrometry and MALDI-TOF methods

Ehrlichia chaffeensis, a tick-transmitted rickettsial, is the causative agent of human monocytic ehrlichiosis. To examine protein expression patterns, we analyzed total, membrane, and immunogenic proteomes of E. chaffeensis originating from macrophage and tick cell cultures. Total proteins resolved by one-dimensional gel electrophoresis and subjected to liquid chromatography-electrospray ionization ion trap mass spectrometry allowed identification of 134 and 116 proteins from macrophage- and tick cell-derived E. chaffeensis, respectively. Because a majority of immunogenic proteins remained in the membrane fraction, individually picked total and immunogenic membrane proteins were also surveyed by liquid chromatography-tandem mass spectrometry and matrix-assisted laser desorption ionization-time of flight methods. The analysis aided the identification of 48 additional proteins. In all, 278 genes of the E. chaffeensis genome were verified as functional genes. They included genes for DNA and protein metabolism, energy metabolism and transport, membrane proteins, hypothetical proteins, and many novel proteins of unknown function. The data reported in this study suggest that the membrane of E. chaffeensis is very complex, having many expressed proteins. This study represents the first and the most comprehensive analysis of E. chaffeensis-expressed proteins. This also is the first study confirming the expression of nearly one-fourth of all predicted genes of the E. chaffeensis genome, validating that they are functionally active genes, and demonstrating that classic shotgun proteomic approaches are feasible for tick-transmitted intraphagosomal bacteria. The identity of novel expressed proteins reported in this study, including the large selection of membrane and immunogenic proteins, will be valuable in elucidating pathogenic mechanisms and developing effective prevention and control methods.

Biology of infection with Borrelia burgdorferi

The spirochete Borrelia burgdorferi is a tick-borne obligate parasite whose normal reservoir is a variety of small mammals. Although infection of these natural hosts does not lead to disease, infection of humans can result in Lyme disease as a consequence of the human immunopathologic response to B burgdorferi. Consistent with the pathogenesis of Lyme disease, bacterial products that allow B burgdorferi to replicate and survive seem to be primarily what is required for the bacterium to cause disease in a susceptible host. This article describes the basic biology of B burgdorferi and reviews some of the bacterial components required for infection of and survival in the mammalian and tick hosts.

Analysis of the RpoS regulon in Borrelia burgdorferi in response to mammalian host signals provides insight into RpoS function during the enzootic cycle

Borrelia burgdorferi (Bb) adapts to its arthropod and mammalian hosts by altering its transcriptional and antigenic profiles in response to environmental signals associated with each of these milieus. In studies presented here, we provide evidence to suggest that mammalian host signals are important for modulating and maintaining both the positive and negative aspects of mammalian host adaptation mediated by the alternative sigma factor RpoS in Bb. Although considerable overlap was observed between genes induced by RpoS during growth within the mammalian host and following temperature-shift, comparative microarray analyses demonstrated unequivocally that RpoS-mediated repression requires mammalian host-specific signals. A substantial portion of the in vivo RpoS regulon was uniquely upregulated within dialysis membrane chambers, further underscoring the importance of host-derived environmental stimuli for differential gene expression in Bb. Expression profiling of genes within the RpoS regulon by quantitative reverse transcription polymerase chain reaction (qRT-PCR) revealed a level of complexity to RpoS-dependent gene regulation beyond that observed by microarray, including a broad range of expression levels and the presence of genes whose expression is only partially dependent on RpoS. Analysis of Bb-infected ticks by qRT-PCR established that expression of rpoS is induced during the nymphal blood meal but not within unfed nymphs or engorged larvae. Together, these data have led us to postulate that RpoS acts as a gatekeeper for the reciprocal regulation of genes involved in the establishment of infection within the mammalian host and the maintenance of spirochetes within the arthropod vector.

Role of outer surface protein D in the Borrelia burgdorferi life cycle

Borrelia burgdorferi preferentially induces selected genes in mice or ticks, and studies suggest that ospD is down-regulated in response to host-specific signals. We now directly show that ospD expression is generally elevated within Ixodes scapularis compared with mice. We then assessed the importance of OspD throughout the spirochete life cycle by generating OspD-deficient B. burgdorferi and examining the mutant in the murine model of tick-transmitted Lyme borreliosis. The lack of OspD did not influence B. burgdorferi infectivity in mice or the acquisition of spirochetes by I. scapularis. OspD adhered to tick gut extracts in vitro, and the OspD-deficient B. burgdorferi strain had a threefold decrease in colonization of the tick gut in vivo. This decrease, however, did not alter subsequent spirochete transmission during a second blood meal. These data suggest that B. burgdorferi can compensate for the lack of OspD in both ticks and mice and that OspD may have a nonessential, secondary, role in B. burgdorferi persistence within I. scapularis.

Outer surface protein B is critical for Borrelia burgdorferi adherence and survival within Ixodes ticks

Survival of Borrelia burgdorferi in ticks and mammals is facilitated, at least in part, by the selective expression of lipoproteins. Outer surface protein (Osp) A participates in spirochete adherence to the tick gut. As ospB is expressed on a bicistronic operon with ospA, we have now investigated the role of OspB by generating an OspB-deficient B. burgdorferi and examining its phenotype throughout the spirochete life cycle. Similar to wild-type isolates, the OspB-deficient B. burgdorferi were able to readily infect and persist in mice. OspB-deficient B. burgdorferi were capable of migrating to the feeding ticks but had an impaired ability to adhere to the tick gut and survive within the vector. Furthermore, the OspB-deficient B. burgdorferi bound poorly to tick gut extracts. The complementation of the OspB-deficient spirochete in trans, with a wild-type copy of ospB gene, restored its ability to bind tick gut. Taken together, these data suggest that OspB has an important role within Ixodes scapularis and that B. burgdorferi relies upon multiple genes to efficiently persist in ticks.

Lyme disease is the most common vector-borne disease in North America and Europe. The causative agent Borrelia burgdorferi is a bacterium that is maintained in an enzoonotic cycle between Ixodes ticks and a large range of mammals. Accidental encounters of infected Ixodes ticks with humans results in the transmission of B. burgdorferi and subsequent Lyme disease. Given that global control efforts have met with limited success, the need for developing novel interventions to combat this infection has become all the more vital. A better understanding of how B. burgdorferi interacts with its vector might lead to new ideas for combating the Lyme disease. B. burgdorferi upregulates outer surface protein (Osp) A and B during entry into ticks, and OspA contributes to the colonization of bacterium within the vector gut. We now demonstrate that OspB also facilitates the colonization and survival of B. burgdorferi in ticks. This work provides the basis for future studies as to how this protein facilitates interaction of B. burgdorferi to the tick gut and thus ultimately a basis for the development of novel strategies to interrupt the spirochete life cycle.

Differential clearance and immune responses to tick cell-derived versus macrophage culture-derived Ehrlichia chaffeensis in mice

Human monocytic ehrlichiosis is caused by a tick-transmitted rickettsia, Ehrlichia chaffeensis. We recently reported that E. chaffeensis grown in tick cells expresses different proteins than bacteria grown in macrophages. Therefore, we tested the hypothesis that immune responses against E. chaffeensis would be different if the mice are challenged with bacteria grown in macrophages or tick cells. We assessed the E. chaffeensis clearance from the peritoneum, spleen, and liver by C57BL/6J mice using a TaqMan-based real-time reverse transcription-PCR assay. Macrophage-grown E. chaffeensis was cleared in 2 weeks from the peritoneum, whereas the pathogen from tick cells persisted for nine additional days and included three relapses of increasing bacterial load separated by three-day intervals. Tick cell-grown bacteria also persisted in the livers and spleens with higher bacterial loads compared to macrophage-grown bacteria and fluctuated over a period of 35 days. Three-day periodic cycles were detected in T-cell CD62L/CD44 ratios in the spleen and bone marrow in response to infections with both tick cell- and macrophage-grown bacteria and were accompanied by similar periodic cycles of spleen cell cytokine secretions and nitric oxide and interleukin-6 by peritoneal macrophages. The E. chaffeensis-specific immunoglobulin G response was considerably higher and steadily increased in mice infected with the tick cell-derived E. chaffeensis compared to DH82-grown bacteria. In addition, antigens detected by the immunoglobulins were significantly different between mice infected with the E. chaffeensis originating from tick cells or macrophages. The differences in the immune response to tick cell-grown bacteria compared to macrophage-grown bacteria reflected a delay in the shift of gene expression from the tick cell-specific Omp 14 gene to the macrophage-specific Omp 19 gene. These data suggest that the host response to E. chaffeensis depends on the source of the bacteria and that this experimental model requires the most natural inoculum possible to allow for a realistic understanding of host resistance.

Unique macrophage and tick cell-specific protein expression from the p28/p30-outer membrane protein multigene locus in Ehrlichia chaffeensis and Ehrlichia canis

Ehrlichia chaffeensis and Ehrlichia canis are tick-transmitted rickettsial pathogens that cause human and canine monocytic ehrlichiosis respectively. We tested the hypothesis that these pathogens express unique proteins in response to their growth in vertebrate and tick host cells and that this differential expression is similar in closely related Ehrlichia species. Evaluation of nine E. chaffeensis isolates and one E. canis isolate demonstrated that protein expression was host cell-dependent. The differentially expressed proteins included those from the p28/30-Omp multigene locus. E. chaffeensis and E. canis proteins expressed in infected macrophages were primarily the products of the p28-Omp 19 and 20 genes or their orthologues. In cultured tick cells, E. canis expressed only the p30-10 protein, an orthologue of the E. chaffeensis p28-Omp 14 protein which is the only protein expressed by E. chaffeensis propagated in cultured tick cells. The expressed Omp proteins were post-translationally modified to generate multiple molecular forms. E. chaffeensis gene expression from the p28/30-Omp locus was similar in tick cell lines derived from both vector (Amblyomma americanum) and non-vector (Ixodes scapularis) ticks. Differential expression of proteins within the p28/p30-Omp locus may therefore be vital for adaptation of Ehrlichia species to their dual host life cycle.

Borrelia burgdorferi–Traveling incognito?

We outline in this review how Borrelia burgdorferi, the causative agent of Lyme disease, moves from the tick to the vertebrate host, and what molecules are potentially involved in this challenging commute. The survival strategies utilized by the spirochete during transmission and the initial stages of infection are discussed.

Differential infectivity of the Lyme disease spirochete Borrelia burgdorferi derived from Ixodes scapularis salivary glands and midgut

Blood fed nymphal Ixodes scapularis Say infected with Borrelia burgdorferi were dissected to obtain salivary gland and midgut extracts. Extracts were inoculated into C3H/HeJ mice, and ear, heart, and bladder were cultured to determine comparative infectivity. Aliquots of extracts were then analyzed by quantitative polymerase chain reaction to determine the number of spirochetes inoculated into mice. A comparative median infectious dose (ID50) was determined for both salivary gland and midgut extract inoculations. Our data demonstrated a statistically significant difference (P < 0.002) in the ID50 derived from salivary gland (average = 18) versus midgut (average = 251) extracts needed to infect susceptible mice. A rationale for the differential infectivity of salivary and midgut derived spirochetes is discussed.

Chemokines and Toll-like receptors in Lyme disease pathogenesis

Lyme disease is a tick-transmitted inflammatory disorder, caused by the spirochete Borrelia burgdorferi (Bb). Recent discoveries cast new light on Bb dissemination and the ensuing pathogenesis of inflammation. Although the strong proinflammatory Bb lipoproteins have been implicated in the induction of inflammation, they do not seem to act exclusively through Toll-like receptor (TLR) engagement. In fact, mice that are deficient for MyD88, a component of the TLR signaling pathway, manifest similar or increased recruitment of cells into Bb-infected tissues. By contrast, the absence of the chemokine receptor CXCR2 results in reduced inflammation. Overall, these findings highlight the complexity of Lyme disease pathogenesis and identify chemokine pathways as novel therapeutic targets for the control of Bb-induced inflammation.

Structural investigation of Borrelia burgdorferi OspB, a bactericidal Fab target

Certain antibody Fab fragments directed against the C terminus of outer surface protein B (OspB), a major lipoprotein of the Lyme disease spirochete, Borrelia burgdorferi, have the unusual property of being bactericidal even in the absence of complement. We report here x-ray crystal structures of a C-terminal fragment of B. burgdorferi OspB, which spans residues 152-296, alone at 2.0-A resolution, and in a complex with the bactericidal Fab H6831 at 2.6-A resolution. The H6831 epitope is topologically analogous to the LA-2 epitope of OspA and is centered around OspB Lys-253, a residue essential for H6831 recognition. A beta-sheet present in the free OspB fragment is either disordered or removed by proteolysis in the H6831-bound complex. Other conformational changes between free and H6831-bound structures are minor and appear to be related to this loss. In both crystal structures, OspB C-terminal fragments form artificial dimers connected by intermolecular beta-sheets. OspB structure, stability, and possible mechanisms of killing by H6831 and other bactericidal Fabs are discussed in light of the structural data.

Ehrlichia chaffeensis expresses macrophage- and tick cell-specific 28-kilodalton outer membrane proteins

Ehrlichia chaffeensis, a tick-transmitted rickettsial agent, causes human monocyte/macrophage-tropic ehrlichiosis. In this study, proteomic approaches were used to demonstrate host cell-specific antigenic expression by E. chaffeensis. The differentially expressed antigens include those from the 28-kDa outer membrane protein (p28-Omp) multigene locus. The proteins expressed in infected macrophages are the products of p28-Omp19 and p28-Omp20 genes, whereas in tick cells, the protein expressed is the p28-Omp14 gene product. The differentially expressed proteins are posttranslationally modified by phosphorylation and glycosylation to generate multiple expressed forms. Host cell-specific protein expression is not influenced by growth temperatures and is reversible. Host cell-specific protein expression coupled with posttranslational modifications may be a hallmark for the pathogen's adaptation to a dual-host life cycle and its persistence.