Tick hemocytes have a pleiotropic role in microbial infection and arthropod fitness.

Uncovering the complexity of systems in non-model organisms is critical for understanding arthropod immunology. Prior efforts have mostly focused on Dipteran insects, which only account for a subset of existing arthropod species in nature. Here we use and develop advanced techniques to describe immune cells (hemocytes) from the clinically relevant tick Ixodes scapularis at a single-cell resolution. We observe molecular alterations in hemocytes upon feeding and infection with either the Lyme disease spirochete Borrelia burgdorferi or the rickettsial agent Anaplasma phagocytophilum. We reveal hemocyte clusters exhibiting defined signatures related to immunity, metabolism, and proliferation. Depletion of phagocytic hemocytes affects hemocytin and astakine levels, two I. scapularis hemocyte markers, impacting blood-feeding, molting behavior, and bacterial acquisition. Mechanistically, astakine alters hemocyte proliferation, whereas hemocytin affects the c-Jun N-terminal kinase (JNK) signaling pathway in I. scapularis. Altogether, we discover a role for tick hemocytes in immunophysiology and provide a valuable resource for comparative biology in arthropods.

Genetic manipulation of an Ixodes scapularis cell line.

Although genetic manipulation is one of the hallmarks of model organisms, its applicability to non-model species has remained difficult due to our limited understanding of their fundamental biology. For instance, manipulation of a cell line originated from the black-legged tick Ixodes scapularis, an arthropod that serves as a vector for several human pathogens, has yet to be established. Here, we demonstrate the successful genetic modification of the commonly used tick ISE6 line through ectopic expression and clustered regularly interspaced palindromic repeats [(CRISPR)/CRISPR-associated protein 9 (Cas9)] genome editing. We performed ectopic expression using nucleofection and attained CRISPR-Cas9 editing via homology-dependent recombination. Targeting the E3 ubiquitin ligase x-linked inhibitor of apoptosis (xiap) and its substrate p47 led to an alteration in molecular signaling within the immune deficiency network and increased infection of the rickettsial agent Anaplasma phagocytophilum in I. scapularis ISE6 cells. Collectively, our findings complement techniques for the genetic engineering of I. scapularis ticks, which currently limit efficient and scalable molecular genetic screens in vivo.IMPORTANCEGenetic engineering in arachnids has lagged compared to insects, largely because of substantial differences in their biology. This study unveils the implementation of ectopic expression and CRISPR-Cas9 gene editing in a tick cell line. We introduced fluorescently tagged proteins in ISE6 cells and edited its genome via homology-dependent recombination. We ablated the expression of xiap and p47, two signaling molecules present in the immune deficiency (IMD) pathway of Ixodes scapularis. Impairment of the tick IMD pathway, an analogous network of the tumor necrosis factor receptor in mammals, led to enhanced infection of the rickettsial agent Anaplasma phagocytophilum. Altogether, our findings provide a critical technical resource to the scientific community to enable a deeper understanding of biological circuits in the black-legged tick I. scapularis.

The role of Rab27 in tick extracellular vesicle biogenesis and pathogen infection.

BACKGROUND: The blacklegged tick, Ixodes scapularis, transmits most vector-borne diseases in the US. It vectors seven pathogens of public health relevance, including the emerging human pathogen Anaplasma phagocytophilum. Nevertheless, it remains critically understudied compared to other arthropod vectors. Ixodes scapularis releases a variety of molecules that assist in the modulation of host responses. Recently, it was found that extracellular vesicles (EVs) carry several of these molecules and may impact microbial transmission to the mammalian host. EV biogenesis has been studied in mammalian systems and is relatively well understood, but the molecular players important for the formation and secretion of EVs in arthropods of public health relevance remain elusive. RabGTPases are among the major molecular players in mammalian EV biogenesis. They influence membrane identity and vesicle budding, uncoating, and motility. METHODS: Using BLAST, an in silico pathway for EV biogenesis in ticks was re-constructed. We identified Rab27 for further study. EVs were collected from ISE6 tick cells after knocking down rab27 to examine its role in tick EV biogenesis. Ixodes scapularis nymphs were injected with small interfering RNAs to knock down rab27 and then fed on naïve and A. phagocytophilum-infected mice to explore the importance of rab27 in tick feeding and bacterial acquisition. RESULTS: Our BLAST analysis identified several of the proteins involved in EV biogenesis in ticks, including Rab27. We show that silencing rab27 in I. scapularis impacts tick fitness. Additionally, ticks acquire less A. phagocytophilum after rab27 silencing. Experiments in the tick ISE6 cell line show that silencing of rab27 causes a distinct range profile of tick EVs, indicating that Rab27 is needed to regulate EV biogenesis. CONCLUSIONS: Rab27 is needed for successful tick feeding and may be important for acquiring A. phagocytophilum during a blood meal. Additionally, silencing rab27 in tick cells results in a shift of extracellular vesicle size. Overall, we have observed that Rab27 plays a key role in tick EV biogenesis and the tripartite interactions among the vector, the mammalian host, and a microbe it encounters.

Rab27 in tick extracellular vesicle biogenesis and infection.

Background: The blacklegged tick, Ixodes scapularis, transmits most vector-borne diseases in the United States. It vectors seven pathogens of public health relevance, including the emerging human pathogen Anaplasma phagocytophilum. Nevertheless, it remains critically understudied when compared to other arthropod vectors. I. scapularis releases a variety of molecules that assist in the modulation of host responses. Recently, it was found that extracellular vesicles (EVs) carry several of these molecules and may impact microbial transmission to the mammalian host. EV biogenesis has been studied in mammalian systems and is relatively well understood, but the molecular players important for the formation and secretion of EVs in arthropods of public health relevance remain elusive. RabGTPases are among the major molecular players in mammalian EV biogenesis. They influence membrane identity and vesicle budding, uncoating, and motility. Methods: Using BLAST, an in-silico pathway for EV biogenesis in ticks was re-constructed. We identified Rab27 for further study. EVs were collected from ISE6 tick cells after knocking down rab27 to examine its role in tick EV biogenesis. I. scapularis nymphs were injected with small interfering RNAs to knock down rab27 then fed on naïve and A. phagocytophilum infected mice to explore the importance of rab27 in tick feeding and bacterial acquisition. Results: Our BLAST analysis identified several of the proteins involved in EV biogenesis in ticks, including Rab27. We show that silencing rab27 in I. scapularis impacts tick fitness. Additionally, ticks acquire less A. phagocytophilum after rab27 silencing. Experiments in the tick ISE6 cell line show that silencing of rab27 causes a distinct range profile of tick EVs, indicating that Rab27 is needed to regulate EV biogenesis. Conclusions: Rab27 is needed for successful tick feeding and may be important for acquiring A. phagocytophilum during a blood meal. Additionally, silencing rab27 in tick cells results in a shift of extracellular vesicle size. Overall, we have observed that Rab27 plays a key role in tick EV biogenesis and the tripartite interactions among the vector, the mammalian host, and a microbe it encounters.

Tick extracellular vesicles impair epidermal homeostasis through immune-epithelial networks during hematophagy.

Hematophagous ectoparasites, such as ticks, rely on impaired wound healing for skin attachment and blood feeding. Wound healing has been extensively studied through the lens of inflammatory disorders and cancer, but limited attention has been given to arthropod-borne diseases. Here, we used orthogonal approaches combining single-cell RNA sequencing (scRNAseq), flow cytometry, murine genetics, and intravital microscopy to demonstrate how tick extracellular vesicles (EVs) disrupt networks involved in tissue repair. Impairment of EVs through silencing of the SNARE protein vamp33 negatively impacted ectoparasite feeding and survival in three medically relevant tick species, including Ixodes scapularis. Furthermore, I. scapularis EVs affected epidermal γδ T cell frequencies and co-receptor expression, which are essential for keratinocyte function. ScRNAseq analysis of the skin epidermis in wildtype animals exposed to vamp33-deficient ticks revealed a unique cluster of keratinocytes with an overrepresentation of pathways connected to wound healing. This biological circuit was further implicated in arthropod fitness when tick EVs inhibited epithelial proliferation through the disruption of phosphoinositide 3-kinase activity and keratinocyte growth factor levels. Collectively, we uncovered a tick-targeted impairment of tissue repair via the resident γδ T cell-keratinocyte axis, which contributes to ectoparasite feeding.

Tick hemocytes have pleiotropic roles in microbial infection and arthropod fitness.

Uncovering the complexity of systems in non-model organisms is critical for understanding arthropod immunology. Prior efforts have mostly focused on Dipteran insects, which only account for a subset of existing arthropod species in nature. Here, we describe immune cells or hemocytes from the clinically relevant tick Ixodes scapularis using bulk and single cell RNA sequencing combined with depletion via clodronate liposomes, RNA interference, Clustered Regularly Interspaced Short Palindromic Repeats activation (CRISPRa) and RNA-fluorescence in situ hybridization (FISH). We observe molecular alterations in hemocytes upon tick infestation of mammals and infection with either the Lyme disease spirochete Borrelia burgdorferi or the rickettsial agent Anaplasma phagocytophilum. We predict distinct hemocyte lineages and reveal clusters exhibiting defined signatures for immunity, metabolism, and proliferation during hematophagy. Furthermore, we perform a mechanistic characterization of two I. scapularis hemocyte markers: hemocytin and astakine. Depletion of phagocytic hemocytes affects hemocytin and astakine levels, which impacts blood feeding and molting behavior of ticks. Hemocytin specifically affects the c-Jun N-terminal kinase (JNK) signaling pathway, whereas astakine alters hemocyte proliferation in I. scapularis. Altogether, we uncover the heterogeneity and pleiotropic roles of hemocytes in ticks and provide a valuable resource for comparative biology in arthropods.

Metabolic disruption impacts tick fitness and microbial relationships.

Arthropod-borne microbes rely on the metabolic state of a host to cycle between evolutionarily distant species. For instance, arthropod tolerance to infection may be due to redistribution of metabolic resources, often leading to microbial transmission to mammals. Conversely, metabolic alterations aids in pathogen elimination in humans, who do not ordinarily harbor arthropod-borne microbes. To ascertain the effect of metabolism on interspecies relationships, we engineered a system to evaluate glycolysis and oxidative phosphorylation in the tick Ixodes scapularis. Using a metabolic flux assay, we determined that the rickettsial bacterium Anaplasma phagocytophilum and the Lyme disease spirochete Borrelia burgdorferi, which are transstadially transmitted in nature, induced glycolysis in ticks. On the other hand, the endosymbiont Rickettsia buchneri, which is transovarially maintained, had a minimal effect on I. scapularis bioenergetics. Importantly, the metabolite ß-aminoisobutyric acid (BAIBA) was elevated during A. phagocytophilum infection of tick cells following an unbiased metabolomics approach. Thus, we manipulated the expression of genes associated with the catabolism and anabolism of BAIBA in I. scapularis and detected impaired feeding on mammals, reduced bacterial acquisition, and decreased tick survival. Collectively, we reveal the importance of metabolism for tick-microbe relationships and unveil a valuable metabolite for I. scapularis fitness.

Croquemort elicits activation of the immune deficiency pathway in ticks.

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.

A machine learning model identifies patients in need of autoimmune disease testing using electronic health records.

Systemic autoimmune rheumatic diseases (SARDs) can lead to irreversible damage if left untreated, yet these patients often endure long diagnostic journeys before being diagnosed and treated. Machine learning may help overcome the challenges of diagnosing SARDs and inform clinical decision-making. Here, we developed and tested a machine learning model to identify patients who should receive rheumatological evaluation for SARDs using longitudinal electronic health records of 161,584 individuals from two institutions. The model demonstrated high performance for predicting cases of autoantibody-tested individuals in a validation set, an external test set, and an independent cohort with a broader case definition. This approach identified more individuals for autoantibody testing compared with current clinical standards and a greater proportion of autoantibody carriers among those tested. Diagnoses of SARDs and other autoimmune conditions increased with higher model probabilities. The model detected a need for autoantibody testing and rheumatology encounters up to five years before the test date and assessment date, respectively. Altogether, these findings illustrate that the clinical manifestations of a diverse array of autoimmune conditions are detectable in electronic health records using machine learning, which may help systematize and accelerate autoimmune testing.

Cholesterol Contributes to Risk, Severity, and Machine Learning-Driven Diagnosis of Lyme Disease.

BACKGROUND: Lyme disease is the most prevalent vector-borne disease in the US, yet its host factors are poorly understood and diagnostic tests are limited. We evaluated patients in a large health system to uncover cholesterol's role in the susceptibility, severity, and machine learning-based diagnosis of Lyme disease. METHODS: A longitudinal health system cohort comprised 1 019 175 individuals with electronic health record data and 50 329 with linked genetic data. Associations of blood cholesterol level, cholesterol genetic scores comprising common genetic variants, and burden of rare loss-of-function (LoF) variants in cholesterol metabolism genes with Lyme disease were investigated. A portable machine learning model was constructed and tested to predict Lyme disease using routine lipid and clinical measurements. RESULTS: There were 3832 cases of Lyme disease. Increasing cholesterol was associated with greater risk of Lyme disease and hypercholesterolemia was more prevalent in Lyme disease cases than in controls. Cholesterol genetic scores and rare LoF variants in CD36 and LDLR were associated with Lyme disease risk. Serological profiling of cases revealed parallel trajectories of rising cholesterol and immunoglobulin levels over the disease course, including marked increases in individuals with LoF variants and high cholesterol genetic scores. The machine learning model predicted Lyme disease solely using routine lipid panel, blood count, and metabolic measurements. CONCLUSIONS: These results demonstrate the value of large-scale genetic and clinical data to reveal host factors underlying infectious disease biology, risk, and prognosis and the potential for their clinical translation to machine learning diagnostics that do not need specialized assays.

Tick transmission of Borrelia burgdorferi to the murine host is not influenced by environmentally acquired midgut microbiota.

BACKGROUND: Ixodes scapularis is the predominant tick vector of Borrelia burgdorferi, the agent of Lyme disease, in the USA. Molecular interactions between the tick and B. burgdorferi orchestrate the migration of spirochetes from the midgut to the salivary glands-critical steps that precede transmission to the vertebrate host. Over the last decade, research efforts have invoked a potential role for the tick microbiome in modulating tick-pathogen interactions. RESULTS: Using multiple strategies to perturb the microbiome composition of B. burgdorferi-infected nymphal ticks, we observe that changes in the microbiome composition do not significantly influence B. burgdorferi migration from the midgut, invasion of salivary glands, or transmission to the murine host. We also show that within 24 and 48 h of the onset of tick feeding, B. burgdorferi spirochetes are within the peritrophic matrix and epithelial cells of the midgut in preparation for exit from the midgut. CONCLUSIONS: This study highlights two aspects of tick-spirochete interactions: (1) environmental bacteria associated with the tick do not influence spirochete transmission to the mammalian host and (2) the spirochete may utilize an intracellular exit route during migration from the midgut to the salivary glands, a strategy that may allow the spirochete to distance itself from microbiota in the midgut lumen effectively. This may explain in part, the inability of environment-acquired midgut microbiota to significantly influence spirochete transmission. Unraveling a molecular understanding of this exit strategy will be critical to gain new insights into the biology of the spirochete and the tick. Video Abstract.

The genus Anaplasma: drawing back the curtain on tick-pathogen interactions.

Tick-borne illnesses pose a serious concern to human and veterinary health and their prevalence is on the rise. The interactions between ticks and the pathogens they carry are largely undefined. However, the genus Anaplasma, a group of tick-borne bacteria, has been instrumental in uncovering novel paradigms in tick biology. The emergence of sophisticated technologies and the convergence of entomology with microbiology, immunology, metabolism and systems biology has brought tick-Anaplasma interactions to the forefront of vector biology with broader implications for the infectious disease community. Here, we discuss the use of Anaplasma as an instrument for the elucidation of novel principles in arthropod-microbe interactions. We offer an outlook of the primary areas of study, outstanding questions and future research directions.

Lipid hijacking: a unifying theme in vector-borne diseases.

Vector-borne illnesses comprise a significant portion of human maladies, representing 17% of global infections. Transmission of vector-borne pathogens to mammals primarily occurs by hematophagous arthropods. It is speculated that blood may provide a unique environment that aids in the replication and pathogenesis of these microbes. Lipids and their derivatives are one component enriched in blood and are essential for microbial survival. For instance, the malarial parasite Plasmodium falciparum and the Lyme disease spirochete Borrelia burgdorferi, among others, have been shown to scavenge and manipulate host lipids for structural support, metabolism, replication, immune evasion, and disease severity. In this Review, we will explore the importance of lipid hijacking for the growth and persistence of these microbes in both mammalian hosts and arthropod vectors.

S27 of IFNα1 Contributes to Its Low Affinity for IFNAR2 and Weak Antiviral Activity.

Type I interferons (IFNs) signal by forming a high affinity IFN-IFNAR2 dimer, which subsequently recruits IFNAR1 to form a ternary complex that initiates JAK/STAT signaling. Among the 12 IFNα subtypes, IFNα1 has a uniquely low affinity for IFNAR2 (<100 × of the other IFNα subtypes) and commensurately weak antiviral activity, suggesting an undefined function distinct from suppression of viral infections. Also unique in IFNα1 is substitution of a serine for phenylalanine at position 27, a contact point that stabilizes the IFNα:IFNAR2 hydrophobic interface. To determine whether IFNα1-S27 contributes to the low affinity for IFNAR2, we created an IFNα1 mutein, IFNα1-S27F, and compared it to wild-type IFNα1 and IFNα2. Substitution of phenylalanine for serine increased affinity for IFNAR2 ∼4-fold and commensurately enhanced activation of STAT1, STAT3, and STAT5, transcription of a subset of interferon stimulated genes, and restriction of vesicular stomatitis virus infection in vitro. Structural modeling suggests that S27 of IFNα1 disrupts the IFNα:IFNAR2 hydrophobic interface that is otherwise stabilized by F27 and that replacing S27 with phenylalanine partially restores the hydrophobic surface. Disruption of the hydrophobic IFNα:IFNAR2 interface by the unique S27 of IFN α1 contributes to its low affinity and weak antiviral activity.

Message in a vesicle - trans-kingdom intercommunication at the vector-host interface.

Vector-borne diseases cause over 700,000 deaths annually and represent 17% of all infectious illnesses worldwide. This public health menace highlights the importance of understanding how arthropod vectors, microbes and their mammalian hosts interact. Currently, an emphasis of the scientific enterprise is at the vector-host interface where human pathogens are acquired and transmitted. At this spatial junction, arthropod effector molecules are secreted, enabling microbial pathogenesis and disease. Extracellular vesicles manipulate signaling networks by carrying proteins, lipids, carbohydrates and regulatory nucleic acids. Therefore, they are well positioned to aid in cell-to-cell communication and mediate molecular interactions. This Review briefly discusses exosome and microvesicle biogenesis, their cargo, and the role that nanovesicles play during pathogen spread, host colonization and disease pathogenesis. We then focus on the role of extracellular vesicles in dictating microbial pathogenesis and host immunity during transmission of vector-borne pathogens.

p47 licenses activation of the immune deficiency pathway in the tick Ixodes scapularis.

The E3 ubiquitin ligase X-linked inhibitor of apoptosis (XIAP) acts as a molecular rheostat for the immune deficiency (IMD) pathway of the tick Ixodes scapularis How XIAP activates the IMD pathway in response to microbial infection remains ill defined. Here, we identified the XIAP enzymatic substrate p47 as a positive regulator of the I. scapularis IMD network. XIAP polyubiquitylates p47 in a lysine 63-dependent manner and interacts with the p47 ubiquitin-like (UBX) module. p47 also binds to Kenny (IKKγ/NEMO), the regulatory subunit of the inhibitor of nuclear factor (NF)- κB kinase complex. Replacement of the amino acid lysine to arginine within the p47 linker region completely abrogated molecular interactions with Kenny. Furthermore, mitigation of p47 transcription levels through RNA interference in I. scapularis limited Kenny accumulation, reduced phosphorylation of IKKß (IRD5), and impaired cleavage of the NF-κB molecule Relish. Accordingly, disruption of p47 expression increased microbial colonization by the Lyme disease spirochete Borrelia burgdorferi and the rickettsial agent Anaplasma phagocytophilum Collectively, we highlight the importance of ticks for the elucidation of paradigms in arthropod immunology. Manipulating immune signaling cascades within I. scapularis may lead to innovative approaches to reducing the burden of tick-borne diseases.