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Ixodes scapularis, commonly known as the deer tick, transmits a wide array of human and animal pathogens including Borrelia burgdorferi. Despite substantial advances in our understanding of immunity in model arthropods, including other disease vectors, precisely how I. scapularis immunity functions and influences persistence of invading pathogens remains largely unknown. This review provides a comprehensive analysis of the recently sequenced I. scapularis genome for the occurrence of immune-related genes and related pathways. We will also discuss the potential influence of immunity-related genes on the persistence of tick-borne pathogens with an emphasis on the Lyme disease pathogen B. burgdorferi. Further enhancement of our knowledge of tick immune responses is critical to understanding the molecular basis of the persistence of tick-borne pathogens and development of novel interventions against the relevant infections. [ABSTRACT FROM AUTHOR]

Borrelia burgdorferi, the bacterial pathogen of Lyme borreliosis, differentially expresses select genes in vivo, likely contributing to microbial persistence and disease. Expression analysis of spirochete genes encoding potential membrane proteins showed that surface-located membrane protein 1 (lmp1) transcripts were expressed at high levels in the infected murine heart, especially during early stages of infection. Mice and humans with diagnosed Lyme borreliosis also developed antibodies against Lmp1. Deletion of lmp1 severely impaired the pathogen's ability to persist in diverse murine tissues including the heart, and to induce disease, which was restored upon chromosomal complementation of the mutant with the lmp1 gene. Lmp1 performs an immune-related rather than a metabolic function, as its deletion did not affect microbial persistence in immunodeficient mice, but significantly decreased spirochete resistance to the borreliacidal effects of anti-B. burgdorferi sera in a complement-independent manner. These data demonstrate the existence of a virulence factor that helps the pathogen evade host-acquired immune defense and establish persistent infection in mammals. [ABSTRACT FROM AUTHOR]

Anap1asma phagocytophila persists within neutrophils and prevents the respiratory burst by inhibiting gp91[sup phox]. Mutations in gp91[sup phox] result in chronic granulomatous disease (CGD), which is diagnosed by use of the nitroblue tetrazolium (NBT) and Fc-Oxyburst assays that examine whether cells produce O[sub 2, sup -]. This study assessed whether the NBT and Fc-Oxyburst assays could detect a respiratory burst during A. phagocytophila infection. O[sub 2, sup -] production was inhibited in HL-60 cells and neutrophils infected with A. phagocytophila. In a mouse model of A. phagocytophila infection, 15% ± 4% (mean ± SD) of polymorphonuclear leukocytes from infected mice had an ineffective respiratory burst compared with 1% ± 1% (mean ± SD) of the neutrophils from uninfected animals. A population of neutrophils that did not produce O[sub 2, sup -] was also detected in 2 patients with A. phagocytophila infection. These data demonstrate respiratory burst inhibition by A. phagocytophila in vivo and on an individual cell basis by use of assays designed to evaluate CGD. [ABSTRACT FROM AUTHOR]

Confocal immunofluorescence image of gut tissues in a partially‐fed Ixodes scapularis nymphal tick. An acellular membrane called peritrophic membrane (green) separates the gut epithelial cells (red) from underlying luminal areas (black). For further details, readers are referred to the article by Yang et al. on p. e13275 of this issue. [ABSTRACT FROM AUTHOR]

Borrelia burgdorferi is the causative agent of Lyme disease that persists in a complex enzootic life cycle, involving Ixodes ticks and vertebrate hosts. The microbe invades ticks and vertebrate hosts in spite of active immune surveillance and potent microbicidal responses, and establishes long‐term infection utilising mechanisms that are yet to be unravelled. The pathogen can cause multi‐system disorders when transmitted to susceptible mammalian hosts, including in humans. In the past decades, several studies identified a limited number of B. burgdorferi gene‐products critical for pathogen persistence, transmission between the vectors and the host, and host–pathogen interactions. This review will focus on the interactions between B. burgdorferi proteins, as well as between microbial proteins and host components, protein and non‐protein components, highlighting their roles in pathogen persistence in the mammalian host. A better understanding of the contributions of protein interactions in the microbial virulence and persistence of B. burgdorferi would support development of novel therapeutics against the infection. [ABSTRACT FROM AUTHOR]

Ticks have an unparalleled ability to parasitize diverse land vertebrates. Their natural persistence and vector competence are supported by the evolution of sophisticated hematophagy and remarkable host immune-evasion activities. We analyze the immunomodulatory roles of tick saliva which facilitates their acquisition of a blood meal from natural hosts and allows pathogen transmission. We also discuss the contrasting immunological events of tick–host associations in non-reservoir or incidental hosts, in which the development of acquired tick resistance can deter tick attachment. A critical appraisal of the intricate immunobiology of tick–host associations can plant new seeds of innovative research and contribute to the development of novel preventive strategies against ticks and tick-transmitted infections. Ticks can repeatedly parasitize their permissive reservoir hosts. Recent studies suggest that the tick bite sites reflect a broad sessile lesion without signs of pain or itchiness, despite intense inflammation. These observations reinforce the 'immune evasion theory', where the tick sialome can efficiently engage and evade immune responses in permissive hosts. Many non-natural or incidental hosts, potentially including humans, can develop acquired tick resistance against repeated tick bites. New research has uncovered key factors that drive these tick rejections in resistant hosts, suggesting diversity in the architectural, sensory, and neurological aspects of tick bite lesions and the types of host responses. Although research has yet to identify the precise tick proteins that trigger the host responses associated with tick rejection in incidental hosts, new studies suggest that repeated host exposure to tick saliva, specifically its glycoproteins or particular saliva fractions, can independently drive partial immunity against tick bites. [ABSTRACT FROM AUTHOR]

Background: Ixodes scapularis organic anion transporting polypeptides (OATPs) play important roles in tick-rickettsial pathogen interactions. In this report, we characterized the role of these conserved molecules in ticks infected with either Lyme disease agent Borrelia burgdorferi or tick-borne Langat virus (LGTV), a pathogen closely related to tick-borne encephalitis virus (TBEV). Results: Quantitative real-time polymerase chain reaction analysis revealed no significant changes in oatps gene expression upon infection with B. burgdorferi in unfed ticks. Synchronous infection of unfed nymphal ticks with LGTV in vitro revealed no significant changes in oatps gene expression. However, expression of specific oatps was significantly downregulated upon LGTV infection of tick cells in vitro. Treatment of tick cells with OATP inhibitor significantly reduced LGTV loads, kynurenine amino transferase (kat), a gene involved in the production of tryptophan metabolite xanthurenic acid (XA), levels and expression of several oatps in tick cells. Furthermore, bioinformatics characterization of OATPs from some of the medically important vectors including ticks, mosquitoes and lice revealed the presence of several glycosylation, phosphorylation and myristoylation sites. Conclusions: This study provides additional evidence on the role of arthropod OATPs in vector-intracellular pathogen interactions. [ABSTRACT FROM AUTHOR]

Ticks are important vectors of pathogenic viruses, bacteria, and protozoans to humans, wildlife, and domestic animals. Due to their life cycles, ticks face significant challenges related to water homeostasis. When blood‐feeding, they must excrete water and ions, but when off‐host (for stretches lasting several months), they must conserve water to avoid desiccation. Aquaporins (AQPs), a family of membrane‐bound water channels, are key players in osmoregulation in many animals but remain poorly characterized in ticks. Here, we bioinformatically identified AQP‐like genes from the deer tick Ixodes scapularis and used phylogenetic approaches to map the evolution of the aquaporin gene family in arthropods. Most arachnid AQP‐like sequences (including those of I. scapularis) formed a monophyletic group clustered within aquaglycerolporins (GLPs) from bacteria to vertebrates. This gene family is absent from insects, revealing divergent evolutionary paths for AQPs in different hematophagous arthropods. Next, we sequenced the full‐length cDNA of I. scapularis aquaporin 1 (IsAQP1) and expressed it heterologously in Xenopus oocytes to functionally characterize its permeability to water and solutes. Additionally, we examined IsAQP1 expression across different life stages and adult female organs. We found IsAQP1 is an efficient water channel with high expression in salivary glands prior to feeding, suggesting it plays a role in osmoregulation before or during blood feeding. Its functional properties are unique: unlike most GLPs, IsAQP1 has low glycerol permeability, and unlike most AQPs, it is insensitive to mercury. Together, our results suggest IsAQP1 plays an important role in tick water balance physiology and that it may hold promise as a target of novel vector control efforts. [ABSTRACT FROM AUTHOR]

Leptospira interrogans is the agent for leptospirosis, an important zoonosis in humans and animals across the globe. Surface proteins of invading pathogens, such as L. interrogans, are thought to be responsible for successful microbial persistence in vivo via interaction with specific host components. In particular, a number of invasive infectious agents exploit host proteolytic pathways, such as one involving plasminogen (Pg), which aid in efficient pathogen dissemination within the host. Here we show that L. interrogans serovar Lai binds host Pg and that the leptospiral gene product LA1951, annotated as enolase, is involved in this interaction. Interestingly, unlike in related pathogenic Spirochetes, such as Borrelia burgdorferi, LA1951 is not readily detectable in the L. interrogans outer membrane. We show that the antigen is indeed secreted extracellularly; however, it can reassociate with the pathogen surface, where it displays Pg-binding and measurable enzymatic activity. Hamsters infected with L. interrogans also develop readily detectable antibody responses against enolase. Taken together, our results suggest that the L. interrogans enolase has evolved to play a role in pathogen interaction with host molecules, which may contribute to the pathogenesis of leptospirosis. [ABSTRACT FROM AUTHOR]

The immune deficiency (IMD) pathway directs host defense in arthropods upon bacterial infection. In Pancrustacea, peptidoglycan recognition proteins sense microbial moieties and initiate nuclear factor-κB-driven immune responses. Proteins that elicit the IMD pathway in non-insect arthropods remain elusive. Here, we show that an Ixodes scapularis homolog of croquemort (Crq), a CD36-like protein, promotes activation of the tick IMD pathway. Crq exhibits plasma membrane localization and binds the lipid agonist 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoglycerol. Crq regulates the IMD and jun N-terminal kinase signaling cascades and limits the acquisition of the Lyme disease spirochete B. burgdorferi. Additionally, nymphs silenced for crq display impaired feeding and delayed molting to adulthood due to a deficiency in ecdysteroid synthesis. Collectively, we establish a distinct mechanism for arthropod immunity outside of insects and crustaceans [ABSTRACT FROM AUTHOR]

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. Tick-transmitted bacterial pathogens have evolved a plethora of immune evasion strategies that support their successful persistence through a complex enzootic infection cycle involving a variety of vertebrate hosts and arthropods. A range of major immune evasion strategies employed by widely-studied bacterial pathogens – such as the avoidance of host recognition or immune offense via the modulation of surface components, inhibition of complement, blocking of antimicrobial molecules, and interference with cellular defense effector pathways or cytokine production – are also reflected by major tick-borne pathogens like Borrelia burgdorferi sensu lato, relapsing fever spirochetes, and several intracellular bacterial pathogens from the genera Anaplasma , Francisella , and Ehrlichia. The immune evasion strategies of tick-borne pathogens are relatively more well-studied in mammalian hosts, as compared to vector ticks. A better understanding of the basic mechanisms of microbial immune evasion in both mammals and arthropods, focusing on contributions toward pathogen fitness, could contribute to the development of novel control measures against tick-borne infections. [ABSTRACT FROM AUTHOR]

Lyme disease is a tick‐borne disease prevalent in North America, Europe, and Asia. Despite the accumulated knowledge from epidemiological, in vitro, and in animal studies, the understanding of dissemination of vector‐borne pathogens, such as Borrelia burgdorferi (Bb), remains incomplete with several important knowledge gaps, especially related to invasion and intravasation into circulation. To elucidate the mechanistic details of these processes a tissue‐engineered human dermal microvessel model is developed. Fluorescently labeled Bb are injected into the extracellular matrix (ECM) to mimic tick inoculation. High resolution, confocal imaging is performed to visualize the sub‐acute phase of infection. From analysis of migration paths no evidence to support adhesin‐mediated interactions between Bb and ECM components is found, suggesting that collagen fibers serve as inert obstacles to migration. Intravasation occurs at cell–cell junctions and is relatively fast, consistent with Bb swimming in ECM. In addition, it is found that Bb alone can induce endothelium activation, resulting in increased immune cell adhesion but no changes in global or local permeability. Together these results provide new insight into the minimum requirements for Bb dissemination and highlight how tissue‐engineered models are complementary to animal models in visualizing dynamic processes associated with vector‐borne pathogens. [ABSTRACT FROM AUTHOR]

Lyme disease is caused by transmission of the spirochete Borrelia burgdorferi from ticks to humans. Although much is known about B. burgdorferi replication, the routes and mechanisms by which it disseminates within the tick remain unclear. To better understand this process, we imaged live, infectious B. burgdorferi expressing a stably integrated, constitutively expressed GFP reporter. Using isolated tick midguts and salivary glands, we observed B. burgdorferi progress through the feeding tick via what we believe to be a novel, biphasic mode of dissemination. In the first phase, replicating spirochetes, positioned at varying depths throughout the midgut at the onset of feeding, formed networks of nonmotile organisms that advanced toward the basolateral surface of the epithelium while adhering to differentiating, hypertrophying, and detaching epithelial cells. In the second phase of dissemination, the nonmotile spirochetes transitioned into motile organisms that penetrated the basement membrane and entered the hemocoel, then migrated to and entered the salivary glands. We designated the first phase of dissemination "adherence-mediated migration" and provided evidence that it involves the inhibition of spirochete motility by one or more diffusible factors elaborated by the feeding tick midgut. Our studies, which we believe are the first to relate the transmission dynamics of spirochetes to the complex morphological and developmental changes that the midgut and salivary glands undergo during engorgement, challenge the conventional viewpoint that dissemination of Lyme disease-causing spirochetes within ticks is exclusively motility driven. [ABSTRACT FROM AUTHOR]

Outer surface lipoprotein C (OspC) is a key virulence factor of Borrelia burgdorferi, ospC is differentially regulated during borrelial transmission from ticks to rodents, and such regulation is essential for maintaining the spirochete in its natural enzootic cycle. Recently, we showed that the expression of ospC in B. burgdorferi is governed by a novel alternative sigma factor regulatory network, the RpoN-RpoS pathway. However, the precise mechanism by which the RpoN-RpoS pathway controls ospC expression has been unclear. In particular, there has been uncertainty regarding whether ospC is controlled directly by RpoS (σs) or indirectly through a transactivator (induced by RpoS). Using deletion analyses and genetic complementation in an OspC-deficient mutant of B. burgdorferi, we analyzed the cis element(s) required for the expression of ospC in its native borrelial background. Two highly conserved upstream inverted repeat elements, previously implicated in ospC regulation, were not required for ospC expression in B. burgdorferi. Using similar approaches, a minimal promoter that contained a canonical -35/-10 sequence necessary and sufficient for σs-dependent regulation of ospC was identified. Further, targeted mutagenesis of a C at position -15 within the extended -10 region of ospC, which is postulated to function like the strategic C residue important for Eσs binding in Escherichia coli, abolished ospC expression. The minimal ospC promoter also was responsive to coumermycin AD further supporting its σs character. The combined data constitute a body of evidence that the RpoN-RpoS regulatory network controls ospC expression by direct binding of σs to a σs-dependent promoter of ospC. The implication of our findings to understanding how B. burgdorferi differentially regulates ospC and other ospC-like genes via the RpoN-RpoS regulatory pathway is discussed. [ABSTRACT FROM AUTHOR]

Ticks are regarded as one of the most ancient, unique, and highly evolved ectoparasites. They can parasitize diverse vertebrates and transmit a number of widespread infections. Once acquired from infected hosts, many tick‐borne pathogens, like Borrelia burgdorferi, are confined within the tick gut lumen and are surrounded by discrete gut barriers. Such barriers include the peritrophic membrane (PM) and the dityrosine network (DTN), which are in close contact with resident microbiota and invading pathogens, influencing their survival within the vector. Herein, we review our current state of knowledge about tick–microbe interactions involving the PM and DTN structures. As a model, we will focus on Ixodes ticks, their microbiome, and the pathogen of Lyme disease. We will address the most salient findings on the structural and physiological roles of these Ixodes gut barriers on microbial interactions, with a comparison to analogous functions in other model vectors, such as mosquitoes. We will distill how this information could be leveraged towards a better understanding of the basic mechanisms of gut biology and tick–microbial interactions, which could contribute to potential therapeutic strategies in response to ticks and tick‐borne infections. [ABSTRACT FROM AUTHOR]

Ixodes scapularis and Ixodes pacificus are the predominant vectors of multiple human pathogens, including Borrelia burgdorferi , one of the causative agents of Lyme disease in North America. Differences in the habitats and host preferences of these closely related tick species present an opportunity to examine key aspects of the tick microbiome. While advances in sequencing technologies have accelerated a descriptive understanding of the tick microbiome, molecular and mechanistic insights into the tick microbiome are only beginning to emerge. Progress is stymied by technical difficulties in manipulating the microbiome and by biological variables related to the life cycle of Ixodid ticks. This review highlights these challenges and examines avenues to understand the significance of the tick microbiome in tick biology. Ixodes scapularis and Ixodes pacificus harbor a simple primary microbiome composed predominantly of the endosymbiont Rickettsia buchneri. The tick microbiome also includes microbiota acquired from the environment and represents a transient microbiome. The tick microbiome composition is in a state of flux and is likely influenced by multiple biotic and abiotic factors. The microbiome composition is regulated by immune responses of the tick at the vector–host and environment–vector interfaces. Understanding the interactions between the tick microbiota, tick-borne pathogens and tick immune responses will reveal new insights in tick biology. [ABSTRACT FROM AUTHOR]