The tick salivary protein Salp15 inhibits the killing of serum-sensitive Borrelia burgdorferi sensu lato isolates

Peromyscus leucopus , Mus musculus , and humans have distinct transcriptomic responses to larval Ixodes scapularis bites

Ixodes scapularis ticks are an important vector for at least six tick-borne human pathogens, including the predominant North American Lyme disease spirochete Borrelia burgdorferi . The ability for these ticks to survive in nature is credited, in part, to their ability to feed on a variety of hosts without excessive activation of the proinflammatory branch of the vertebrate immune system. While the ability for nymphal ticks to feed on a variety of hosts has been well-documented, the host-parasite interactions between larval I. scapularis and different vertebrate hosts is relatively unexplored. Here we report on the changes in the vertebrate transcriptome present at the larval tick bite site using the natural I. scapularis host Peromyscus leucopus deermouse, a non-natural rodent host Mus musculus (BALB/c), and humans. We note substantially less evidence of activation of canonical proinflammatory pathways in P. leucopus compared to BALB/c mice and pronounced evidence of inflammation in humans. Pathway enrichment analyses revealed a particularly strong signature of interferon gamma, tumor necrosis factor, and interleukin 1 signaling at the BALB/c and human tick bite site. We also note that bite sites on BALB/c mice and humans, but not deermice, show activation of wound-healing pathways. These data provide molecular evidence of the coevolution between larval I. scapularis and P. leucopus as well as expand our overall understanding of I. scapularis feeding.

Significance

Ixodes scapularis tick bites expose humans to numerous diseases in North America. While larval tick feeding enables pathogens to enter the tick population and eventually spread to humans, how larval ticks interact with mammals has been understudied compared to other tick stages. Here we examined the transcriptomic response of a natural I. scapularis rodent host ( Peromyscus leucopus ), a non-native I. scapularis rodent host ( Mus musculus ), and an incidental host (humans). We find that there are differences in how all three species respond to larval I. scapularis , with the natural host producing the smallest transcriptomic signature of a canonical proinflammatory immune response and the incidental human host producing the most robust signature of inflammation in response to the larval tick. These data expand our understanding of the pressures on ticks in the wild and inform our ability to model these interactions in laboratory settings.

Human Tick-Borne Diseases and Advances in Anti-Tick Vaccine Approaches: A Comprehensive Review

This comprehensive review explores the field of anti-tick vaccines, addressing their significance in combating tick-borne diseases of public health concern. The main objectives are to provide a brief epidemiology of diseases affecting humans and a thorough understanding of tick biology, traditional tick control methods, the development and mechanisms of anti-tick vaccines, their efficacy in field applications, associated challenges, and future prospects. Tick-borne diseases (TBDs) pose a significant and escalating threat to global health and the livestock industries due to the widespread distribution of ticks and the multitude of pathogens they transmit. Traditional tick control methods, such as acaricides and repellents, have limitations, including environmental concerns and the emergence of tick resistance. Anti-tick vaccines offer a promising alternative by targeting specific tick proteins crucial for feeding and pathogen transmission. Developing vaccines with antigens based on these essential proteins is likely to disrupt these processes. Indeed, anti-tick vaccines have shown efficacy in laboratory and field trials successfully implemented in livestock, reducing the prevalence of TBDs. However, some challenges still remain, including vaccine efficacy on different hosts, polymorphisms in ticks of the same species, and the economic considerations of adopting large-scale vaccine strategies. Emerging technologies and approaches hold promise for improving anti-tick vaccine development and expanding their impact on public health and agriculture.

Inhibition of vertebrate complement system by hematophagous arthropods: inhibitory molecules, mechanisms, physiological roles, and applications

In arthropods, hematophagy has arisen several times throughout evolution. This specialized feeding behavior offered a highly nutritious diet obtained during blood feeds. On the other hand, blood-sucking arthropods must overcome problems brought on by blood intake and digestion. Host blood complement acts on the bite site and is still active after ingestion, so complement activation is a potential threat to the host's skin feeding environment and to the arthropod gut enterocytes. During evolution, blood-sucking arthropods have selected, either in their saliva or gut, anticomplement molecules that inactivate host blood complement. This review presents an overview of the complement system and discusses the arthropod's salivary and gut anticomplement molecules studied to date, exploring their mechanism of action and other aspects related to the arthropod-host-pathogen interface. The possible therapeutic applications of arthropod's anticomplement molecules are also discussed.

Differential Resistance of Borrelia burgdorferi Clones to Human Serum-Mediated Killing Does Not Correspond to Their Predicted Invasiveness

Reservoir host associations have been observed among and within Borrelia genospecies, and host complement-mediated killing is a major determinant in these interactions. In North America, only a subset of Borrelia burgdorferi lineages cause the majority of disseminated infections in humans. We hypothesize that differential resistance to human complement-mediated killing may be a major phenotypic determinant of whether a lineage can establish systemic infection. As a corollary, we hypothesize that borreliacidal action may differ among human subjects. To test these hypotheses, we isolated primary B. burgdorferi clones from field-collected ticks and determined whether the killing effects of human serum differed among those clones in vitro and/or whether these effects were consistent among human sera. Clones associated with human invasiveness did not show higher survival in human serum compared to noninvasive clones. These results indicate that differential complement-mediated killing of B. burgdorferi lineages is not a determinant of invasiveness in humans. Only one significant difference in the survivorship of individual clones incubated in different human sera was detected, suggesting that complement-mediated killing of B. burgdorferi is usually similar among humans. Mechanisms other than differential human complement-mediated killing of B. burgdorferi lineages likely explain why only certain lineages cause the majority of disseminated human infections.

Choreography of Lyme Disease Spirochete Adhesins To Promote Vascular Escape

The Lyme disease spirochete Borrelia burgdorferi sensu lato can cause a multitude of clinical manifestations because of its ability to disseminate into any organ system via migration through soft tissue, the lymphatic system, and the circulatory system. The latter is believed to constitute the predominant pathway for dissemination to distal sites from the inoculating tick bite. In spite of its importance, the hematogenous dissemination process remains largely uncharacterized, particularly due to difficulties studying this process in a living host and the lack of an in vitro system that recapitulates animal infection. In the current work, we provide the first information regarding the stage of the vascular transmigration pathway where three important adhesins function during invasion of mouse knee joint peripheral tissue from postcapillary venules. Using intravital imaging coupled with genetic experiments employing sequential double infection, we show a complex temporal choreography of P66, decorin binding proteins (DbpA/B), and outer surface protein C (OspC) at discrete steps along the pathway of vascular escape, underscoring the importance of B. burgdorferi adhesins in hematogenous dissemination in the mouse knee joint and the complexity of vascular transmigration by a disseminating pathogen. IMPORTANCE Lyme disease is caused by the spirochete Borrelia burgdorferi, which is transmitted by a bite from an infected tick. Disease development involves a complex series of host-pathogen interactions as well as dissemination of the infecting organisms to sites distal to the original tick bite. The predominant pathway for this is believed to be hematogenous dissemination. The mechanism by which the spirochetes escape circulation is unknown. Here, using intravital microscopy, where the Lyme spirochete can be observed in a living mouse, we have studied the stage in the vascular escape process where each of three surface adhesins functions to facilitate escape of the spirochete from postcapillary venules to invade mouse knee joint peripheral tissue. A complex pattern of involvement at various locations in the multistage process is described using a unique experimental approach that is applicable to other disseminating pathogens.

Recent Advances in Tick Antigen Discovery and Anti-Tick Vaccine Development

Ticks can seriously affect human and animal health around the globe, causing significant economic losses each year. Chemical acaricides are widely used to control ticks, which negatively impact the environment and result in the emergence of acaricide-resistant tick populations. A vaccine is considered as one of the best alternative approaches to control ticks and tick-borne diseases, as it is less expensive and more effective than chemical controls. Many antigen-based vaccines have been developed as a result of current advances in transcriptomics, genomics, and proteomic techniques. A few of these (e.g., Gavac^® and TickGARD^®) are commercially available and are commonly used in different countries. Furthermore, a significant number of novel antigens are being investigated with the perspective of developing new anti-tick vaccines. However, more research is required to develop new and more efficient antigen-based vaccines, including on assessing the efficiency of various epitopes against different tick species to confirm their cross-reactivity and their high immunogenicity. In this review, we discuss the recent advancements in the development of antigen-based vaccines (traditional and RNA-based) and provide a brief overview of recent discoveries of novel antigens, along with their sources, characteristics, and the methods used to test their efficiency.

Immune evasion strategies of major tick-transmitted bacterial pathogens

Tick-transmitted bacterial pathogens thrive in enzootic infection cycles, colonizing disparate vertebrate and arthropod tissues, often establishing persistent infections. Therefore, the evolution of robust immune evasion strategies is central to their successful persistence or transmission between hosts. To survive in nature, these pathogens must counteract a broad range of microbicidal host responses that can be localized, tissue-specific, or systemic, including a mix of these responses at the host-vector interface. Herein, we review microbial immune evasion strategies focusing on Lyme disease spirochetes and rickettsial or tularemia agents as models for extracellular and intracellular tick-borne pathogens, respectively. A better understanding of these adaptive strategies could enrich our knowledge of the infection biology of relevant tick-borne diseases, contributing to the development of future preventions.

Interference with LTβR signaling by tick saliva facilitates transmission of Lyme disease spirochetes

Lyme spirochetes have coevolved with ticks to optimize transmission to hosts using tick salivary molecules (TSMs) to counteract host defenses. TSMs modulate various molecular events at the tick-host interface. Lymphotoxin-beta receptor (LTβR) is a vital immune receptor and plays protective roles in host immunity against microbial infections. We found that Ltbr knockout mice were more susceptible to Lyme disease spirochetes, suggesting the involvement of LTβR signaling in tick-borne Borrelia infection. Further investigation showed that a 15-kDa TSM protein from Ixodes persulcatus (I. persulcatus salivary protein; IpSAP) functioned as an immunosuppressant to facilitate the transmission and infection of Lyme disease spirochetes. IpSAP directly interacts with LTβR to block its activation, thus inhibiting the downstream signaling and consequently suppressing immunity. IpSAP immunization provided mice with significant protection against I. persulcatus-mediated Borrelia garinii infection. Notably, the immunization showed considerable cross-protection against other Borrelia infections mediated by other ixodid ticks. One of the IpSAP homologs from other ixodid ticks showed similar effects on Lyme spirochete transmission. Together, our findings suggest that LTβR signaling plays an important role in blocking the transmission and pathogenesis of tick-borne Lyme disease spirochetes, and that IpSAP and its homologs are promising candidates for broad-spectrum vaccine development.

Tick feeding modulates the human skin immune landscape to facilitate tick-borne pathogen transmission

During cutaneous tick attachment, the feeding cavity becomes a site of transmission for tick salivary compounds and tick-borne pathogens. However, the immunological consequences of tick feeding for human skin remain unclear. Here, we assessed human skin and blood samples upon tick bite and developed a human skin explant model mimicking Ixodes ricinus bites and tick-borne pathogen infection. Following tick attachment, we observed rapidly occurring patterns of immunomodulation, including increases in neutrophils and cutaneous B and T cells. T cells upregulated tissue residency markers, while lymphocytic cytokine production was impaired. In early stages of Borrelia burgdorferi model infections, we detected strain-specific immune responses and close spatial relationships between macrophages and spirochetes. Preincubation of spirochetes with tick salivary gland extracts hampered accumulation of immune cells and increased spirochete loads. Collectively, we showed that tick feeding exerts profound changes on the skin immune network that interfere with the primary response against tick-borne pathogens.

Deceiving and escaping complement - the evasive journey of the malaria parasite

During its life cycle, Plasmodium, the malaria parasite, is exposed to the human and mosquito complement systems. Early experiments demonstrated that activation of complement can pose a serious threat to parasites, but recent studies revealed complement-evasion mechanisms important for parasite survival. Blood-stage parasites and gametes recruit regulators to neutralize human complement activation, while ookinetes inhibit mosquito complement by disrupting epithelial nitration in response to midgut invasion. Here we provide an in-depth overview of the evasion mechanisms currently known and speculate on the existence of others not yet identified. Finally, we discuss how these mechanisms could provide novel targets for urgently needed malaria vaccines and therapeutics.

Tick Saliva and Salivary Glands: What Do We Know So Far on Their Role in Arthropod Blood Feeding and Pathogen Transmission

Ticks are blood-sucking arthropods that have developed myriad of strategies to get a blood meal from the vertebrate host. They first attach to the host skin, select a bite site for a blood meal, create a feeding niche at the bite site, secrete plethora of molecules in its saliva and then starts feeding. On the other side, host defenses will try to counter-attack and stop tick feeding at the bite site. In this constant battle between ticks and the host, arthropods successfully pacify the host and completes a blood meal and then replete after full engorgement. In this review, we discuss some of the known and emerging roles for arthropod components such as cement, salivary proteins, lipocalins, HSP70s, OATPs, and extracellular vesicles/exosomes in facilitating successful blood feeding from ticks. In addition, we discuss how tick-borne pathogens modulate(s) these components to infect the vertebrate host. Understanding the biology of arthropod blood feeding and molecular interactions at the tick-host interface during pathogen transmission is very important. This information would eventually lead us in the identification of candidates for the development of transmission-blocking vaccines to prevent diseases caused by medically important vector-borne pathogens.

A Structural Basis for Inhibition of the Complement Initiator Protease C1r by Lyme Disease Spirochetes

Complement evasion is a hallmark of extracellular microbial pathogens such as Borrelia burgdorferi, the causative agent of Lyme disease. Lyme disease spirochetes express nearly a dozen outer surface lipoproteins that bind complement components and interfere with their native activities. Among these, BBK32 is unique in its selective inhibition of the classical pathway. BBK32 blocks activation of this pathway by selectively binding and inhibiting the C1r serine protease of the first component of complement, C1. To understand the structural basis for BBK32-mediated C1r inhibition, we performed crystallography and size-exclusion chromatography-coupled small angle X-ray scattering experiments, which revealed a molecular model of BBK32-C in complex with activated human C1r. Structure-guided site-directed mutagenesis was combined with surface plasmon resonance binding experiments and assays of complement function to validate the predicted molecular interface. Analysis of the structures shows that BBK32 inhibits activated forms of C1r by occluding substrate interaction subsites (i.e., S1 and S1') and reveals a surprising role for C1r B loop-interacting residues for full inhibitory activity of BBK32. The studies reported in this article provide for the first time (to our knowledge) a structural basis for classical pathway-specific inhibition by a human pathogen.

Host tropism determination by convergent evolution of immunological evasion in the Lyme disease system

Pathogens possess the ability to adapt and survive in some host species but not in others-an ecological trait known as host tropism. Transmitted through ticks and carried mainly by mammals and birds, the Lyme disease (LD) bacterium is a well-suited model to study such tropism. Three main causative agents of LD, Borrelia burgdorferi, B. afzelii, and B. garinii, vary in host ranges through mechanisms eluding characterization. By feeding ticks infected with different Borrelia species, utilizing feeding chambers and live mice and quail, we found species-level differences in bacterial transmission. These differences localize on the tick blood meal, and specifically complement, a defense in vertebrate blood, and a polymorphic bacterial protein, CspA, which inactivates complement by binding to a host complement inhibitor, Factor H (FH). CspA selectively confers bacterial transmission to vertebrates that produce FH capable of allele-specific recognition. CspA is the only member of the Pfam54 gene family to exhibit host-specific FH-binding. Phylogenetic analyses revealed convergent evolution as the driver of such uniqueness, and that FH-binding likely emerged during the last glacial maximum. Our results identify a determinant of host tropism in Lyme disease infection, thus defining an evolutionary mechanism that shapes host-pathogen associations.

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.

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.

Induced Transient Immune Tolerance in Ticks and Vertebrate Host: A Keystone of Tick-Borne Diseases?

Ticks and tick transmitted infectious agents are increasing global public health threats due to increasing abundance, expanding geographic ranges of vectors and pathogens, and emerging tick-borne infectious agents. Greater understanding of tick, host, and pathogen interactions will contribute to development of novel tick control and disease prevention strategies. Tick-borne pathogens adapt in multiple ways to very different tick and vertebrate host environments and defenses. Ticks effectively pharmacomodulate by its saliva host innate and adaptive immune defenses. In this review, we examine the idea that successful synergy between tick and tick-borne pathogen results in host immune tolerance that facilitates successful tick infection and feeding, creates a favorable site for pathogen introduction, modulates cutaneous and systemic immune defenses to establish infection, and contributes to successful long-term infection. Tick, host, and pathogen elements examined here include interaction of tick innate immunity and microbiome with tick-borne pathogens; tick modulation of host cutaneous defenses prior to pathogen transmission; how tick and pathogen target vertebrate host defenses that lead to different modes of interaction and host infection status (reservoir, incompetent, resistant, clinically ill); tick saliva bioactive molecules as important factors in determining those pathogens for which the tick is a competent vector; and, the need for translational studies to advance this field of study. Gaps in our understanding of these relationships are identified, that if successfully addressed, can advance the development of strategies to successfully disrupt both tick feeding and pathogen transmission.

Tsetse salivary glycoproteins are modified with paucimannosidic N-glycans, are recognised by C-type lectins and bind to trypanosomes

African sleeping sickness is caused by Trypanosoma brucei, a parasite transmitted by the bite of a tsetse fly. Trypanosome infection induces a severe transcriptional downregulation of tsetse genes encoding for salivary proteins, which reduces its anti-hemostatic and anti-clotting properties. To better understand trypanosome transmission and the possible role of glycans in insect bloodfeeding, we characterized the N-glycome of tsetse saliva glycoproteins. Tsetse salivary N-glycans were enzymatically released, tagged with either 2-aminobenzamide (2-AB) or procainamide, and analyzed by HILIC-UHPLC-FLR coupled online with positive-ion ESI-LC-MS/MS. We found that the N-glycan profiles of T. brucei-infected and naïve tsetse salivary glycoproteins are almost identical, consisting mainly (>50%) of highly processed Man₃GlcNAc₂ in addition to several other paucimannose, high mannose, and few hybrid-type N-glycans. In overlay assays, these sugars were differentially recognized by the mannose receptor and DC-SIGN C-type lectins. We also show that salivary glycoproteins bind strongly to the surface of transmissible metacyclic trypanosomes. We suggest that although the repertoire of tsetse salivary N-glycans does not change during a trypanosome infection, the interactions with mannosylated glycoproteins may influence parasite transmission into the vertebrate host.

In addition to helping the ingestion of a bloodmeal, the saliva of vector insects can modulate vertebrate immune responses. However, most research has focused on the salivary proteins, while the sugars (glycans) that modify them remain unexplored. Here we studied N-glycosylation, a common post-translational modification where sugar structures are attached to specific sites of a protein. Insect salivary N-glycans may affect how the saliva is recognized by the host, possibly playing a role during pathogen transmission. In this manuscript, we present the first detailed structural characterization of the salivary N-glycans in the tsetse fly Glossina morsitans, vector of African trypanosomiasis. We found that tsetse fly glycoproteins are mainly modified by simple N-glycans with short mannose modifications, which are recognised by mammalian C-type lectins (mannose receptor and DC-SIGN). Furthermore, we show that salivary glycoproteins bind to the surface of the trypanosomes that are transmitted to the vertebrate host; this opens up interesting questions as to the role of these glycoproteins in the successful establishment of infection by this parasite. Overall, our work represents a novel contribution towards the salivary N-glycome of an important insect vector, and towards the understanding of vector saliva and its complex effects in the vertebrate host.

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.

Systemic immune responses in patients with early localized or early disseminated Borrelia afzelii lyme borreliosis

Introduction

The role of host immune responses in the pathogenesis of borrelial dissemination in early Lyme borreliosis (LB) in the form of multiple erythema migrans (MEM) or LB‐associated symptoms is incompletely understood.

Methods

In this study, fifteen cytokine or chemokine levels, representative of innate, Th1, and Th17 immune responses, were assessed using a bead‐based Luminex multiplex assay in acute sera from 76 adult patients with skin culture‐positive Borrelia afzelii solitary erythema migrans (SEM) and 58 patients with MEM at a single‐center university hospital. Differences between the groups were tested by modeling each cytokine or chemokine concentration by means of left‐censored regression using the classic Tobit model.

Results

Mean serum cytokine or chemokine levels were low. When taking into account the proportion of patients with cytokine or chemokine concentrations below the lowest detectable limit, only levels of CXCL10 (p = .03) and CCL19 (p = .02), representatives of the Th1 immune response, differed between patients with SEM and those with MEM; however, the differences did not reach statistical significance when adjusted for multiple comparisons. In addition, we did not find differences in systemic inflammatory responses when comparing patients with and those without LB‐associated constitutional symptoms.

Conclusion

No significant differences in systemic immune responses represented by selected cytokines or chemokines in serum samples of patients with EM infected with B. afzelii suggest that systemic mediators are not pivotal in the pathogenesis of dissemination of early infection in the form of MEM or LB‐associated symptoms. Localized immune responses in the skin or other pathogenetic mechanisms may be more important in this regard.

Lyme Disease Pathogenesis

Lyme disease Borrelia are obligately parasitic, tick- transmitted, invasive, persistent bacterial pathogens that cause disease in humans and non-reservoir vertebrates primarily through the induction of inflammation. During transmission from the infected tick, the bacteria undergo significant changes in gene expression, resulting in adaptation to the mammalian environment. The organisms multiply and spread locally and induce inflammatory responses that, in humans, result in clinical signs and symptoms. Borrelia virulence involves a multiplicity of mechanisms for dissemination and colonization of multiple tissues and evasion of host immune responses. Most of the tissue damage, which is seen in non-reservoir hosts, appears to result from host inflammatory reactions, despite the low numbers of bacteria in affected sites. This host response to the Lyme disease Borrelia can cause neurologic, cardiovascular, arthritic, and dermatologic manifestations during the disseminated and persistent stages of infection. The mechanisms by which a paucity of organisms (in comparison to many other infectious diseases) can cause varied and in some cases profound inflammation and symptoms remains mysterious but are the subjects of diverse ongoing investigations. In this review, we provide an overview of virulence mechanisms and determinants for which roles have been demonstrated in vivo, primarily in mouse models of infection.

Strong interactions between Salp15 homologues from the tick I. ricinus and distinct types of the outer surface OspC protein from Borrelia

Ticks belonging to the genus Ixodes are parasites feeding on vertebrate blood and vectors for many pathogenic microbes, including Borrelia burgdorferi sensu lato spirochetes, the causative agent of Lyme borreliosis. The tick saliva contains a mixture of bioactive molecules showing a wide range of properties for efficient engorgement. One of the most extensively studied components of tick saliva is a 15-kDa salivary gland protein (Salp15) from Ixodes scapularis. This multifunctional protein suppresses the immune response of hosts through pleiotropic action on a few crucial defense pathways. Salp15 and its homologue from I. ricinus Iric1 have been also shown to bind to Borrelia burgdorferi sensu stricto outer surface protein C (OspC) permitting the spirochetes to evade antibody-mediated killing in the human host. Further studies revealed that Salp15 and Iric1 protected B. burgdorferi s. s. and B. garinii expressing OspC against the complement system. OspC is the most variable protein on the outer surface of Borrelia, which in addition to Salp15 can also bind other ligands, such as plasminogen, fibrinogen, fibronectin or complement factor 4. So far several OspC variants produced by B. burgdorferi s. l. spirochetes were shown to be capable of binding Salp15 or its homologue, but the protection against borreliacidal antibodies has only been proven in the case of B. burgdorferi s. s. The question of Salp15 contribution to Borrelia survival during the infection has been comprehensively studied during the last decades. In contrast, the organization of the OspC-Salp15 complex has been poorly explored. This report describes the binding between three Salp15 homologues from the tick Ixodes ricinus (Iric1, Iric2 and Iric3) and OspC from four B. burgdorferi sensu lato strains in terms of the binding parameters, analyzed with two independent biophysical methods - Microscale thermophoresis (MST) and Biolayer interferometry (BLI). The results of both experiments show a binding constant at the nanomolar level, which indicates very strong interactions. While the Iric1-OspC binding has been reported before, we show in this study that also Iric2 and Iric3 are capable of OspC binding with high affinity. This observation suggests that these two Salp15 homologues might be used by B. burgdorferi s. l. in a way analogous to Iric1. A comparison of the results from the two methods let us propose that N-terminal immobilization of OspC significantly increases the affinity between the two proteins. Finally, our results indicate that the Iric binding site is located in close proximity of the OspC epitopes recognized by human antibodies, which may have important biological and medical implications.

Proteomic as an Exploratory Approach to Develop Vaccines Against Tick-Borne Diseases Using Lyme Borreliosis as a Test Case

Tick-borne diseases affecting humans and animals are on the rise worldwide. Vaccines constitute an effective control measure, but very few are available. We selected Lyme borreliosis, a bacterial infection transmitted by the hard tick Ixodes, to validate a new concept to identify vaccine candidates. This disease is the most common tick-borne disease in the Northern Hemisphere. Although attempts to develop a vaccine exist, none have been successfully marketed. In tick-borne diseases, the skin constitutes a very specific environment encountered by the pathogen during its co-inoculation with tick saliva. In a mouse model, we developed a proteomic approach to identify vaccine candidates in skin biopsies. We identified 30 bacterial proteins after syringe inoculation or tick inoculation of bacteria. Discovery proteomics using mass spectrometry might be used in various tick-borne diseases to identify pathogen proteins with early skin expression. It should help to better develop sub-unit vaccines based on a cocktail of several antigens, associated with effective adjuvant and delivery systems of antigens. In all vector-borne diseases, the skin deserves further investigation to better define its role in the elaboration of protective immunity against pathogens.

Repeat tick exposure elicits distinct immune responses in guinea pigs and mice

Ticks deposit salivary proteins into the skin during a bite to mediate acquisition of a blood meal. Acquired resistance to tick bites has been demonstrated to prevent Borrelia burgdorferi sensu lato (s.l.) transmission. However, the mechanism of resistance, as well as the protective antigens, have remained elusive. To address these unknowns, we utilized a guinea pig model of tick resistance and a mouse model of permissiveness. Guinea pigs developed immunity after multiple Ixodes scapularis tick infestations, characterized by rapid tick detachment and impaired feeding. In comparison, mice tolerated at least 6 infestations with no significant impact on feeding. We analyzed the bite sites by RNA-sequencing and histology, identifying several inflammatory pathways in tick immune animals, such as FcεRI signaling and complement activation, and activation of coagulation pathways that could impair local blood flow. Together, these results identify important pathways altered during tick rejection and potential tick proteins that could serve as vaccine candidates.

TickSialoFam (TSFam): A Database That Helps to Classify Tick Salivary Proteins, a Review on Tick Salivary Protein Function and Evolution, With Considerations on the Tick Sialome Switching Phenomenon

Tick saliva contains a complex mixture of peptides and non-peptides that counteract their hosts' hemostasis, immunity, and tissue-repair reactions. Recent transcriptomic studies have revealed over one thousand different transcripts coding for secreted polypeptides in a single tick species. Not only do these gene products belong to many expanded families, such as the lipocalins, metalloproteases, Antigen-5, cystatins, and apyrases, but also families that are found exclusively in ticks, such as the evasins, Isac, DAP36, and many others. Phylogenetic analysis of the deduced protein sequences indicate that the salivary genes exhibit an increased rate of evolution due to a lower evolutionary constraint and/or positive selection, allowing for a large diversity of tick salivary proteins. Thus, for each new tick species that has its salivary transcriptome sequenced and assembled, a formidable task of annotation of these transcripts awaits. Currently, as of November 2019, there are over 287 thousand coding sequences deposited at the National Center for Biotechnology Information (NCBI) that are derived from tick salivary gland mRNA. Here, from these 287 thousand sequences we identified 45,264 potential secretory proteins which possess a signal peptide and no transmembrane domains on the mature peptide. By using the psiblast tools, position-specific matrices were constructed and assembled into the TickSialoFam (TSF) database. The TSF is a rpsblastable database that can help with the annotation of tick sialotranscriptomes. The TSA database identified 136 tick salivary secreted protein families, as well as 80 families of endosomal-related products, mostly having a protein modification function. As the number of sequences increases, and new annotation details become available, new releases of the TSF database may become available.

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.

New Insights Into CRASP-Mediated Complement Evasion in the Lyme Disease Enzootic Cycle

Lyme disease (LD), which is caused by genospecies of the Borrelia burgdorferi sensu lato complex, is the most common vector-borne disease in the Northern hemisphere. Spirochetes are transmitted by Ixodes ticks and maintained in diverse vertebrate animal hosts. Following tick bite, spirochetes initially establish a localized infection in the skin. However, they may also disseminate hematogenously to several distal sites, including heart, joints, or the CNS. Because they need to survive in diverse microenvironments, from tick vector to mammalian hosts, spirochetes have developed multiple strategies to combat the numerous host defense mechanisms. One of these strategies includes the production of a number of complement-regulator acquiring surface proteins (CRASPs) which encompass CspA, CspZ, and OspE paralogs to blunt the complement pathway. These proteins are capable of preventing complement activation on the spirochete surface by binding to complement regulator Factor H. The genes encoding these CRASPs differ in their expression patterns during the tick-to-host infection cycle, implying that these proteins may exhibit different functions during infection. This review summarizes the recent published reports which investigated the roles that each of these molecules plays in conferring tick-borne transmission and dissemination in vertebrate hosts. These findings offer novel mechanistic insights into LD pathobiology and may facilitate the identification of new targets for preventive strategies against Lyme borreliosis.

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

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

Lyme Disease Frontiers: Reconciling Borrelia Biology and Clinical Conundrums

Lyme disease is a complex tick-borne zoonosis that poses an escalating public health threat in several parts of the world, despite sophisticated healthcare infrastructure and decades of effort to address the problem. Concepts like the true burden of the illness, from incidence rates to longstanding consequences of infection, and optimal case management, also remain shrouded in controversy. At the heart of this multidisciplinary issue are the causative spirochetal pathogens belonging to the Borrelia Lyme complex. Their unusual physiology and versatile lifestyle have challenged microbiologists, and may also hold the key to unlocking mysteries of the disease. The goal of this review is therefore to integrate established and emerging concepts of Borrelia biology and pathogenesis, and position them in the broader context of biomedical research and clinical practice. We begin by considering the conventions around diagnosing and characterizing Lyme disease that have served as a conceptual framework for the discipline. We then explore virulence from the perspective of both host (genetic and environmental predispositions) and pathogen (serotypes, dissemination, and immune modulation), as well as considering antimicrobial strategies (lab methodology, resistance, persistence, and clinical application), and borrelial adaptations of hypothesized medical significance (phenotypic plasticity or pleomorphy).

Differential Tick Salivary Protein Profiles and Human Immune Responses to Lone Star Ticks (Amblyomma americanum) From the Wild vs. a Laboratory Colony

Ticks are a growing concern to human and animal health worldwide and they are leading vectors of arthropod-borne pathogens in the United States. Ticks are pool blood feeders that can attach to the host skin for days to weeks using their saliva to counteract the host defenses. Tick saliva, as in other hematophagous arthropods, contains pharmacological and immunological active compounds, which modulate local and systemic immune responses and induce antibody production. In the present study, we explore differences in the salivary gland extract (SGE) protein content of Amblyomma americanum ticks raised in a laboratory colony (CT) vs. those collected in the field (FT). First, we measured the IgG antibody levels against SGE in healthy volunteers residing in Kansas. ELISA test showed higher IgG antibody levels when using the SGE from CT as antigen. Interestingly, antibody levels against both, CT-SGE and FT-SGE, were high in the warm months (May-June) and decreased in the cold months (September-November). Immunoblot testing revealed a set of different immunogenic bands for each group of ticks and mass spectrometry data revealed differences in at 19 proteins specifically identified in the CT-SGE group and 20 from the FT-SGE group. Our results suggest that differences in the salivary proteins between CT-SGE and FT-SGE may explain the differential immune responses observed in this study.

Anopheles gambiae Lacking AgTRIO Inefficiently Transmits Plasmodium berghei to Mice

Antibodies to AgTRIO, a mosquito salivary protein, partially reduce the initial Plasmodium burden in mice. We therefore silenced AgTRIO in mosquitoes and determined the relative contribution of AgTRIO to the ability of Anopheles gambiae to transmit Plasmodium berghei to mice. RNA interference-mediated silencing of AgTRIO in A. gambiae resulted in a 60% reduction in AgTRIO expression. The decrease in AgTRIO expression did not alter the burden of Plasmodium sporozoites in mosquito salivary glands. When experimentally injected into mice, sporozoites from AgTRIO-silenced mosquitoes colonized the liver less effectively than sporozoites from control mosquitoes. Silencing of AgTRIO did not decrease the infectivity of sporozoites in vitro or influence the expression of genes associated with Plasmodium cell adhesion or traversal activity. AgTRIO decreased the expression of proinflammation cytokines by splenocytes in vitro Moreover, in vivo, AgTRIO decreased the expression of TNF-α when coinjected with sporozoites into the skin and there was more TNF-α expression at the bite site of AgTRIO knockdown mosquitoes than at the bite site of control mosquitoes. AgTRIO therefore influences the local environment in the vertebrate host, which facilitates Plasmodium sporozoite infection in mice.

Counterattacking the tick bite: towards a rational design of anti-tick vaccines targeting pathogen transmission

Hematophagous arthropods are responsible for the transmission of a variety of pathogens that cause disease in humans and animals. Ticks of the Ixodes ricinus complex are vectors for some of the most frequently occurring human tick-borne diseases, particularly Lyme borreliosis and tick-borne encephalitis virus (TBEV). The search for vaccines against these diseases is ongoing. Efforts during the last few decades have primarily focused on understanding the biology of the transmitted viruses, bacteria and protozoans, with the goal of identifying targets for intervention. Successful vaccines have been developed against TBEV and Lyme borreliosis, although the latter is no longer available for humans. More recently, the focus of intervention has shifted back to where it was initially being studied which is the vector. State of the art technologies are being used for the identification of potential vaccine candidates for anti-tick vaccines that could be used either in humans or animals. The study of the interrelationship between ticks and the pathogens they transmit, including mechanisms of acquisition, persistence and transmission have come to the fore, as this knowledge may lead to the identification of critical elements of the pathogens' life-cycle that could be targeted by vaccines. Here, we review the status of our current knowledge on the triangular relationships between ticks, the pathogens they carry and the mammalian hosts, as well as methods that are being used to identify anti-tick vaccine candidates that can prevent the transmission of tick-borne pathogens.

Tick saliva and its role in pathogen transmission

Tick saliva is a complex mixture of peptidic and non-peptidic molecules that aid engorgement. The composition of tick saliva changes as feeding progresses and the tick counters the dynamic host response. Ixodid ticks such as Ixodes ricinus, the most important tick species in Europe, transmit numerous pathogens that cause debilitating diseases, e.g. Lyme borreliosis and tick-borne encephalitis. Tick-borne pathogens are transmitted in tick saliva during blood feeding; however, saliva is not simply a medium enabling pathogen transfer. Instead, tick-borne pathogens exploit saliva-induced modulation of host responses to promote their transmission and infection, so-called saliva-assisted transmission (SAT). Characterization of the saliva factors that facilitate SAT is an active area of current research. Besides providing new insights into how tick-borne pathogens survive in nature, the research is opening new avenues for vaccine development.

Antigen Engineering Approaches for Lyme Disease Vaccines

Increasing rates of Lyme disease necessitate preventive measures such as immunization to mitigate the risk of contracting the disease. At present, there is no human Lyme disease vaccine available on the market. Since the withdrawal of the first and only licensed Lyme disease vaccine based on lipidated recombinant OspA, vaccine and antigen research has aimed to overcome its risks and shortcomings. Replacement of the putative cross-reactive T-cell epitope in OspA via mutation or chimerism addresses the potential risk of autoimmunity. Multivalent approaches in Lyme disease vaccines have been pursued to address sequence heterogeneity of Lyme borreliae antigens and to induce a repertoire of functional antibodies necessary for efficient heterologous protection. This Review summarizes recent antigen engineering strategies that have paved the way for the development of next generation vaccines against Lyme disease, some of which have reached clinical testing. Bioconjugation methods that incorporate antigens to self-assembling nanoparticles for immune response potentiation are also discussed.

The role of Mannose Binding Lectin in the immune response against Borrelia burgdorferi sensu lato

The causative agents of Lyme borreliosis, spirochetes belonging to the Borrelia burgdorferi sensu lato group, have developed several ways to protect themselves against killing by the host complement system. In addition, it has been shown that serum sensitive isolates are (partially) protected by the Ixodes Tick Salivary Lectin Pathway Inhibitor (TSLPI) protein; a salivary gland protein that inhibits the function of Mannose Binding Lectin (MBL). MBL is a C-type lectin that recognizes oligosaccharides on pathogens and activates the complement system via the lectin pathway. MBL deficiency has been linked to a more severe course of several infectious diseases and humans with detectable antibodies against B. burgdorferi are significantly more often MBL deficient compared to humans without antibodies against B. burgdorferi. Here we set out to investigate the role of MBL in the immune response against B. burgdorferi in more detail. We demonstrate that B. burgdorferi N40 needle-infected C57BL/6 MBL deficient mice harbored significantly higher B. burgdorferi numbers in skin tissue during the early course of infection. In line with these findings they also developed higher anti-B. burgdorferi IgG serum antibodies compared to WT controls. In contrast, B. burgdorferi loads in distant tissue such as heart, joints or bladder at later time points were similar for both mouse strains. These in vivo findings were corroborated using a B. burgdorferi N40-infected I. scapularis infestation model. We showed that MBL is capable of binding B. burgdorferi through its carbohydrate recognition domains, but in vitro complement killing assays, peritoneal macrophage and whole blood stimulations, phagocytosis assays and an in vivo migration experiment did not reveal the mechanism by which MBL facilitates early clearance of B. burgdorferi. To conclude, we show a protective role of MBL in the early stages of B. burgdorferi infection, yet the underlying mechanism warrants further investigation.

A mosquito salivary gland protein partially inhibits Plasmodium sporozoite cell traversal and transmission

The key step during the initiation of malaria is for motile Plasmodium parasites to exit the host dermis and infect the liver. During transmission, the parasites in the form of sporozoites, are injected together with mosquito saliva into the skin. However, the contribution of vector saliva to sporozoite activity during the establishment of the initial infection of the liver is poorly understood. Here we identify a vector protein by mass spectrometry, with similarity to the human gamma interferon inducible thiol reductase (GILT), that is associated with saliva sporozoites of infected Anopheles mosquitoes and has a negative impact on the speed and cell traversal activity of Plasmodium. This protein, referred to as mosquito GILT (mosGILT) represents an example of a protein found in mosquito saliva that may negatively influence sporozoite movement in the host and could lead to new approaches to prevent malaria.

Immune escape strategies of Borrelia burgdorferi

The borrelial resurge demonstrates that Borrelia burgdorferi is a persistent health problem. This spirochete is responsible for a global public health concern called Lyme disease. B. burgdorferi faces diverse environmental conditions of its vector and host during its life cycle. To circumvent the host immune system is a prominent feature of B. burgdorferi. To date, numerous studies have reported on the various mechanisms used by this pathogen to evade the host defense mechanisms. This current review attempts to consolidate this information to describe the immunological and molecular methods used by B. burgdorferi for its survival.

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.

The Essential Role of Tick Salivary Glands and Saliva in Tick Feeding and Pathogen Transmission

As long-term pool feeders, ticks have developed myriad strategies to remain discreetly but solidly attached to their hosts for the duration of their blood meal. The critical biological material that dampens host defenses and facilitates the flow of blood-thus assuring adequate feeding-is tick saliva. Saliva exhibits cytolytic, vasodilator, anticoagulant, anti-inflammatory, and immunosuppressive activity. This essential fluid is secreted by the salivary glands, which also mediate several other biological functions, including secretion of cement and hygroscopic components, as well as the watery component of blood as regards hard ticks. When salivary glands are invaded by tick-borne pathogens, pathogens may be transmitted via saliva, which is injected alternately with blood uptake during the tick bite. Both salivary glands and saliva thus play a key role in transmission of pathogenic microorganisms to vertebrate hosts. During their long co-evolution with ticks and vertebrate hosts, microorganisms have indeed developed various strategies to exploit tick salivary molecules to ensure both acquisition by ticks and transmission, local infection and systemic dissemination within the vertebrate host.

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.

Complement Immune Evasion by Spirochetes

The complement system plays an important role in the innate and acquired immune response against pathogens. A sophisticated network of activating and regulating proteins allows the distinction between intact and damaged host and non-host surfaces such as bacteria and other parasites. Non-host structures trigger the alternative pathway which may lead to their elimination by phagocytosis or cell lysis. In addition, complement proteins such as C1q, mannose binding lectin (MBL), and ficolins act as pathogen pattern-recognition molecules. Biological functions such as opsonization, activation of B lymphocytes and production of antibodies, degranulation of mast cells and basophils, and cell lysis that are important for elimination of microorganisms are dependent on complement activation. However, several pathogens including spirochetes have developed several specialized mechanisms to evade the complement system, thereby contributing to survival in the host. In this review, we give a brief overview of complement activation and regulation, and discuss in detail the strategies used by spirochetes from the genera Borrelia, Leptospira, and Treponema to overcome complement activation.

An Ixodes ricinus Tick Salivary Lectin Pathway Inhibitor Protects Borrelia burgdorferi sensu lato from Human Complement

INTRODUCTION

We previously identified tick salivary lectin pathway inhibitor (TSLPI) in Ixodes scapularis, a vector for Borrelia burgdorferi sensu stricto (s.s.) in North America. TSLPI is a salivary protein facilitating B. burgdorferi s.s. transmission and acquisition by inhibiting the host lectin complement pathway through interference with mannose binding lectin (MBL) activity. Since Ixodes ricinus is the predominant vector for Lyme borreliosis in Europe and transmits several complement sensitive B. burgdorferi sensu lato (s.l.) strains, we aimed to identify, describe, and characterize the I. ricinus ortholog of TSLPI.

METHODS

We performed (q)PCRs on I. ricinus salivary gland cDNA to identify a TSLPI ortholog. Next, we generated recombinant (r)TSLPI in a Drosophila expression system and examined inhibition of the MBL complement pathway and complement-mediated killing of B. burgdorferi s.l. in vitro.

RESULTS

We identified a TSLPI ortholog in I. ricinus salivary glands with 93% homology at the RNA and 89% at the protein level compared to I. scapularis TSLPI, which was upregulated during tick feeding. In silico analysis revealed that TSLPI appears to be part of a larger family of Ixodes salivary proteins among which I. persulcatus basic tail salivary proteins and I. scapularis TSLPI and Salp14. I. ricinus rTSLPI inhibited the MBL complement pathway and protected B. burgdorferi s.s. and Borrelia garinii from complement-mediated killing.

CONCLUSION

We have identified a TSLPI ortholog, which protects B. burgdorferi s.l. from complement-mediated killing in I. ricinus, the major vector for tick-borne diseases in Europe.

Occurrence of Borrelia burgdorferi s.l. in different genera of mosquitoes (Culicidae) in Central Europe

Lyme disease or Lyme borreliosis is a vector-borne infectious disease caused by spirochetes of the Borrelia burgdorferi sensu lato complex. Some stages of the borrelial transmission cycle in ticks (transstadial, feeding and co-feeding) can potentially occur also in insects, particularly in mosquitoes. In the present study, adult as well as larval mosquitoes were collected at 42 different geographical locations throughout Germany. This is the first study, in which German mosquitoes were analyzed for the presence of Borrelia spp. Targeting two specific borrelial genes, flaB and ospA encoding for the subunit B of flagellin and the outer surface protein A, the results show that DNA of Borrelia afzelii, Borrelia bavariensis and Borrelia garinii could be detected in ten Culicidae species comprising four distinct genera (Aedes, Culiseta, Culex, and Ochlerotatus). Positive samples also include adult specimens raised in the laboratory from wild-caught larvae indicating that transstadial and/or transovarial transmission might occur within a given mosquito population.

Outer surface protein OspC is an antiphagocytic factor that protects Borrelia burgdorferi from phagocytosis by macrophages

Outer surface protein C (OspC) is one of the major lipoproteins expressed on the surface of Borrelia burgdorferi during tick feeding and the early phase of mammalian infection. OspC is required for B. burgdorferi to establish infection in both immunocompetent and SCID mice and has been proposed to facilitate evasion of innate immune defenses. However, the exact biological function of OspC remains elusive. In this study, we showed that the ospC-deficient spirochete could not establish infection in NOD-scid IL2rγ(null) mice that lack B cells, T cells, NK cells, and lytic complement. The ospC mutant also could not establish infection in anti-Ly6G-treated SCID and C3H/HeN mice (depletion of neutrophils). However, depletion of mononuclear phagocytes at the skin site of inoculation in SCID and C3H/HeN mice allowed the ospC mutant to establish infection in vivo. In phagocyte-depleted mice, the ospC mutant was able to colonize the joints and triggered neutrophilia during dissemination. Furthermore, we found that phagocytosis of green fluorescent protein (GFP)-expressing ospC mutant spirochetes by murine peritoneal macrophages and human THP-1 macrophage-like cells, but not in PMN-HL60, was significantly higher than parental wild-type B. burgdorferi strains, suggesting that OspC has an antiphagocytic property. In addition, overproduction of OspC in spirochetes also decreased the uptake of spirochetes by murine peritoneal macrophages. Together, our findings provide evidence that mononuclear phagocytes play a key role in clearance of the ospC mutant and that OspC promotes spirochetes' evasion of macrophages during early Lyme borreliosis.

Whole-Chain Tick Saliva Proteins Presented on Hepatitis B Virus Capsid-Like Particles Induce High-Titered Antibodies with Neutralizing Potential

Ticks are vectors for various, including pathogenic, microbes. Tick saliva contains multiple anti-host defense factors that enable ticks their bloodmeals yet also facilitate microbe transmission. Lyme disease-causing borreliae profit specifically from the broadly conserved tick histamine release factor (tHRF), and from cysteine-rich glycoproteins represented by Salp15 from Ixodes scapularis and Iric-1 from Ixodes ricinus ticks which they recruit to their outer surface protein C (OspC). Hence these tick proteins are attractive targets for anti-tick vaccines that simultaneously impair borrelia transmission. Main obstacles are the tick proteins´ immunosuppressive activities, and for Salp15 orthologs, the lack of efficient recombinant expression systems. Here, we exploited the immune-enhancing properties of hepatitis B virus core protein (HBc) derived capsid-like particles (CLPs) to generate, in E. coli, nanoparticulate vaccines presenting tHRF and, as surrogates for the barely soluble wild-type proteins, cysteine-free Salp15 and Iric-1 variants. The latter CLPs were exclusively accessible in the less sterically constrained SplitCore system. Mice immunized with tHRF CLPs mounted a strong anti-tHRF antibody response. CLPs presenting cysteine-free Salp15 and Iric-1 induced antibodies to wild-type, including glycosylated, Salp15 and Iric-1. The broadly distributed epitopes included the OspC interaction sites. In vitro, the anti-Salp15 antibodies interfered with OspC binding and enhanced human complement-mediated killing of Salp15 decorated borreliae. A mixture of all three CLPs induced high titered antibodies against all three targets, suggesting the feasibility of combination vaccines. These data warrant in vivo validation of the new candidate vaccines´ protective potential against tick infestation and Borrelia transmission.

Molecular identification and bioinformatics analysis of a potential anti-vector vaccine candidate, 15-kDa salivary gland protein (Salp15), from Ixodes affinis ticks

Salp15, a 15-kDa salivary gland protein plays an important role in tick blood-feeding and transmission of Borrelia burgdorferi, the causative agent of Lyme borreliosis. The comparative studies reveal that Salp15 is a genetically conserved protein across various Ixodes species. In this study, we have identified a Salp15 homolog, designated as Iaff15, from Ixodes affinis ticks that are the principal enzootic vectors of B. burgdorferi sensu stricto in the southeastern part of the United States. Comparison of the annotated amino acid sequences showed that Iaff15 share 81% homology with I. sinensis Salp15 homolog and 64% homology with I. scapularis Salp15. Phylogenetic analysis revealed that Iaff15 come within the same clade with I. sinensis, I. scapularis, and I. pacificus Salp15 homologs. The bioinformatics analysis of the posttranslational modifications prediction revealed that all the Salp15 family members contain glycosylation sites. In addition, Iaff15 carried a higher number of Casein Kinase II phosphorylation sites in comparison to the other Salp15 family members. Collectively, high sequence conservation distributed over the entire amino acids sequence not only suggests an important role for Iaff15 in I. affinis blood feeding and vector-pathogen interactions but may also lead to the development of an anti-vector vaccine against this group of ticks.

An investigation of binding ability of Ixodes persulcatus Schulze Salp15 with Lyme disease spirochetes

Salp15, a 15-kDa tick salivary gland protein, has several suppressive modes of activity against host immunity and plays a critical role in the transmission of Lyme disease spirochetes in Ixodes scapularis and Ixodes ricinus, major vectors of Lyme disease in North America and Western Europe. Salp15 adheres to Borrelia burgdorferi and specifically interacts with its outer surface protein C (OspC), protecting the spirochete from antibody-mediated cytotoxicity and facilitating infection in the mice. Recently, we identified two Salp15 homologues, IperSalp15-1 and IperSalp15-2, in Ixodes persulcatus, a vector for Lyme disease in Japan. Here we describe the function of IperSalp15 in the transmission of Lyme borreliosis. To investigate the function of IperSalp15, recombinant IperSalp15-1 and IperSalp15-2 were prepared in bacterial and insect cells. Both were identified in the sera of tick-immunized hamsters, indicating that these are secretory proteins in exposed host animals. Solid-phase overlay and indirect fluorescence assays showed that IperSalp15 binds to OspC from B. burgdorferi, Borrelia garinii, and Borrelia afzelii. Importantly, this binding likely protected the spirochete from antibody-mediated cytotoxicity in vitro. In addition, IperSalp15 tended to facilitate infection in mice. Thus, further characterization of tick molecules, including IperSalp15, could lead to the development of new strategies to prevent the transmission of tick-borne diseases.

Soluble cysteine-rich tick saliva proteins Salp15 and Iric-1 from E. coli

•

Tick saliva proteins Salp15 and Iric-1 promote tick feeding and pathogen transmission.

•

We established the first bacterial expression system for soluble Salp15 and Iric-1.

•

Using this system we mapped monoclonal antibody epitopes on Salp15 and Iric-1.

•

We defined the interaction sites with Borrelia outer surface protein C (OspC).

•

We elucidated first secondary structure features in Iric-1 by NMR.

Ticks transmit numerous pathogens, including borreliae, which cause Lyme disease. Tick saliva contains a complex mix of anti-host defense factors, including the immunosuppressive cysteine-rich secretory glycoprotein Salp15 from Ixodes scapularis ticks and orthologs like Iric-1 from Ixodesricinus. All tick-borne microbes benefit from the immunosuppression at the tick bite site; in addition, borreliae exploit the binding of Salp15 to their outer surface protein C (OspC) for enhanced transmission. Hence, Salp15 proteins are attractive targets for anti-tick vaccines that also target borreliae. However, recombinant Salp proteins are not accessible in sufficient quantity for either vaccine manufacturing or for structural characterization. As an alternative to low-yield eukaryotic systems, we investigated cytoplasmic expression in Escherichia coli, even though this would not result in glycosylation. His-tagged Salp15 was efficiently expressed but insoluble. Among the various solubility-enhancing protein tags tested, DsbA was superior, yielding milligram amounts of soluble, monomeric Salp15 and Iric-1 fusions. Easily accessible mutants enabled epitope mapping of two monoclonal antibodies that, importantly, cross-react with glycosylated Salp15, and revealed interaction sites with OspC. Free Salp15 and Iric-1 from protease-cleavable fusions, despite limited solubility, allowed the recording of ¹H–¹⁵N 2D NMR spectra, suggesting partial folding of the wild-type proteins but not of Cys-free variants. Fusion to the NMR-compatible GB1 domain sufficiently enhanced solubility to reveal first secondary structure elements in ¹³C/¹⁵N double-labeled Iric-1. Together, E. coli expression of appropriately fused Salp15 proteins may be highly valuable for the molecular characterization of the function and eventually the 3D structure of these medically relevant tick proteins.

Suppressive effects of neutrophil by Salp16-like salivary gland proteins from Ixodes persulcatus Schulze tick

Salp16, a 16-kDa tick salivary gland protein, is known to be the molecule involved in the transmission of Anaplasma phagocytophilum, an obligate intracellular pathogen causing zoonotic anaplasmosis, from its mammalian hosts to Ixodes scapularis. Recently, the presence of A. phagocytophilum was documented in Japan and Ixodes persulcatus was identified as one of its vectors. The purpose of this study was to identify Salp16 genes in I. persulcatus and characterize their function. Two cDNA clones encoding the Salp16-like sequences were obtained from the salivary glands of fed female I. persulcatus ticks and designated Salp16 Iper1 and Iper2. Gene expression analyses showed that the Salp16 Iper genes were expressed specifically in the salivary glands and were up-regulated by blood feeding. These proteins attenuated the oxidative burst of activated bovine neutrophils and inhibited their migration induced by the chemoattractant interleukin-8 (IL-8). These results demonstrate that Salp16 Iper proteins contribute to the establishment of blood feeding as an immunosuppressant of neutrophil, an essential factor in innate host immunity. Further examination of the role of Salp16 Iper in the transmission of pathogens, including A. phagocytophilum, will increase our understanding of the tick-host-pathogen interface.