Anaplasma phagocytophilum Infection Subverts Carbohydrate Metabolic Pathways in the Tick Vector, Ixodes scapularis

Differential interactions of Rickettsia species with tick microbiota in Rh. sanguineus and Rh. turanicus

Tick-borne rickettsioses, caused by Gram-negative bacteria of the Rickettsia genus, pose a growing global threat, with various arthropod vectors contributing to their transmission. Understanding the complex interactions within tick microbiota, including the role of Rickettsia species, is crucial for elucidating the dynamics of rickettsial diseases. Here, we investigate the taxonomic profiles and co-occurrence networks of Rickettsia in Rh. sanguineus sensus lato (s.l.) and Rh. turanicus ticks, revealing significant differences in community composition and local connectivity of Rickettsia species. While the microbiota of both tick species share common taxa, distinct differences in relative abundance and network topology suggest unique ecological niches. Moreover, robustness analysis demonstrates varying resilience to perturbations, indicating different strategies for network organization. Our findings also highlight metabolic differences between tick species, suggesting potential implications for Rickettsia interactions. Overall, this study provides insights into the intricate microbial landscape within ticks, shedding light on the functional redundancy and metabolic pathways associated with Rickettsia, thus advancing our understanding of tick-borne diseases.

Bacterial reprogramming of tick metabolism impacts vector fitness and susceptibility to infection

Arthropod-borne pathogens are responsible for hundreds of millions of infections in humans each year. The blacklegged tick, Ixodes scapularis, is the predominant arthropod vector in the United States and is responsible for transmitting several human pathogens, including the Lyme disease spirochete Borrelia burgdorferi and the obligate intracellular rickettsial bacterium Anaplasma phagocytophilum, which causes human granulocytic anaplasmosis. However, tick metabolic response to microbes and whether metabolite allocation occurs upon infection remain unknown. Here we investigated metabolic reprogramming in the tick ectoparasite I. scapularis and determined that the rickettsial bacterium A. phagocytophilum and the spirochete B. burgdorferi induced glycolysis in tick cells. Surprisingly, the endosymbiont Rickettsia buchneri had a minimal effect on bioenergetics. An unbiased metabolomics approach following A. phagocytophilum infection of tick cells showed alterations in carbohydrate, lipid, nucleotide and protein metabolism, including elevated levels of the pleiotropic metabolite β-aminoisobutyric acid. We manipulated the expression of genes associated with β-aminoisobutyric acid metabolism in I. scapularis, resulting in feeding impairment, diminished survival and reduced bacterial acquisition post haematophagy. Collectively, we discovered that metabolic reprogramming affects interspecies relationships and fitness in the clinically relevant tick I. scapularis.

Nutritional symbiosis in ticks: singularities of the genus Ixodes

Symbiosis with intracellular bacteria is essential for the nutrition of ticks, particularly through the biosynthesis of B vitamins. Yet, ticks of the genus Ixodes, which include major vectors of human pathogens, lack the nutritional symbionts usually found in other tick genera. This paradox raises questions about the mechanisms that Ixodes ticks use to prevent nutritional deficiencies. Nonetheless, Ixodes ticks commonly harbor other symbionts belonging to the order Rickettsiales. Although these obligate intracellular bacteria are primarily known as human pathogens, Rickettsiales symbionts often dominate the Ixodes microbial community without causing diseases. They also significantly influence Ixodes physiology, synthesize key B vitamins, and are crucial for immatures. These findings underscore unique associations between Rickettsiales and Ixodes ticks distinct from other tick genera.

Comprehensive review on glucose 6 phosphate dehydrogenase: A critical immunometabolic and redox switch in insects

Mounting an active immune response is energy intensive and demands the reallocation of nutrients to maintain the body's resistance and tolerance against infections. Central to this metabolic adaptation is Glucose-6-phosphate dehydrogenase (G6PDH), a housekeeping enzyme involve in pentose phosphate pathway (PPP). PPP play an essential role in generating ribose, which is critical for nicotinamide adenine dinucleotide phosphate (NADPH). It is vital for physiological and cellular processes such as generating nucleotides, fatty acids and reducing oxidative stress. The G6PDH is extremely conserved enzyme across species in PP shunt. The deficiency of enzymes leads to serious consequences on organism, particularly on adaptation and development. Acute deficiency can lead to impaired cell development, halted embryonic growth, reduce sensitivity to insulin, hypertension and increase inflammation. Historically, research focusing on G6PDH and PPP have primarily targeted diseases on mammalian. However, our review has investigated the unique functions of the G6PDH enzyme in insects and greatly improved mechanistic understanding of its operations. This review explore how G6PDH in insects plays a crucial role in managing the redox balance and immune related metabolism. This study aims to investigate the enzyme's role in different metabolic adaptations.

Evidence of Incomplete Feeding Behaviors among South Carolina Tick Populations

Dynamic environmental conditions, such as climate change and host availability, have greatly influenced the expansion of medically relevant tick vectors into new regions throughout the southeastern United States of America. As tick populations migrate into new areas, it has been suggested they can exhibit a phenomenon known as incomplete feeding. With this phenomenon, tick vectors feed on more than one host at each life stage, thus increasing the likelihood of pathogen transmission. Although this behavior is not well understood, it presents an important threat to human health. Here we present evidence of incomplete feeding behaviors in multiple tick species in South Carolina. Engorged, blood-fed female ticks were collected from feral dogs at animal shelters across South Carolina in 2022. All ticks were tested for human blood meals using rapid stain identification blood tests. Approximately one third (33.78%) of all ticks tested positive for a human blood meal, with various patterns seen across species, geographic location, and collection month. The results of this pilot study follow the current national trend of increasing rates of tick-borne disease incidence in the southeastern United States of America and warrant further investigation into the relationship between seasonality, geographic distribution, species, and incomplete feeding among tick populations in South Carolina.

Mining gene expression data for rational identification of novel drug targets and vaccine candidates against the cattle tick, Rhipicephalus microplus

Control of complex parasites via vaccination remains challenging, with the current combination of vaccines and small drugs remaining the choice for an integrated control strategy. Studies conducted to date, are providing evidence that multicomponent vaccines will be needed for the development of protective vaccines against endo- and ectoparasites, though multicomponent vaccines require an in-depth understanding of parasite biology which remains insufficient for ticks. With the rapid development and spread of acaricide resistance in ticks, new targets for acaricide development also remains to be identified, along with novel targets that can be exploited for the design of lead compounds. In this study, we analysed the differential gene expression of Rhipicephalus microplus ticks that were fed on cattle vaccinated with a multi-component vaccine (Bm86 and 3 putative Bm86-binding proteins). The data was scrutinised for the identification of vaccine targets, small drug targets and novel pathways that can be evaluated in future studies. Limitations associated with targeting novel proteins for vaccine and/or drug design is also discussed and placed into the context of challenges arising when targeting large protein families and intracellular localised proteins. Lastly, this study provide insight into how Bm86-based vaccines may reduce successful uptake and digestion of the bloodmeal and overall tick fecundity.

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 β-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.

Down-Regulation of Lipid Metabolism in the Hepatopancreas of Shrimp Litopenaeus vannamei upon Light and Heavy Infection of Enterocytozoon hepatopenaei: A Comparative Proteomic Study

Enterocytozoon hepatopenaei (EHP) is the pathogen of hepatopancreatic microsporidiosis (HPM) in shrimp. The diseased shrimp Litopenaeus vannamei exhibits a slow growth syndrome, which causes severe economic losses. Herein, 4D label-free quantitative proteomics was employed to analyze the hepatopancreas of L. vannamei with a light (EHPptp2 < 103 copies/50 ng hpDNA, L group) and heavy (EHPptp2 > 104 copies/50 ng hpDNA, H group) load of EHP to better understand the pathogenesis of HPM. Exactly 786 (L group) and 1056 (H group) differentially expressed proteins (DEPs) versus the EHP-free (C group) control were mainly clustered to lipid metabolism, amino acid metabolism, and energy production processing. Compared with the L group, the H group exhibited down-regulation significantly in lipid metabolism, especially in the elongation and degradation of fatty acid, biosynthesis of unsaturated fatty acid, metabolism of α-linolenic acid, sphingolipid, and glycerolipid, as well as juvenile hormone (JH) degradation. Expression pattern analysis showed that the degree of infection was positively correlated with metabolic change. About 479 EHP proteins were detected in infected shrimps, including 95 predicted transporters. These findings suggest that EHP infection induced the consumption of storage lipids and the entire down-regulation of lipid metabolism and the coupling energy production, in addition to the hormone metabolism disorder. These were ultimately responsible for the stunted growth.

Redox Imbalance and Its Metabolic Consequences in Tick-Borne Diseases

One of the growing global health problems are vector-borne diseases, including tick-borne diseases. The most common tick-borne diseases include Lyme disease, tick-borne encephalitis, human granulocytic anaplasmosis, and babesiosis. Taking into account the metabolic effects in the patient's body, tick-borne diseases are a significant problem from an epidemiological and clinical point of view. Inflammation and oxidative stress are key elements in the pathogenesis of infectious diseases, including tick-borne diseases. In consequence, this leads to oxidative modifications of the structure and function of phospholipids and proteins and results in qualitative and quantitative changes at the level of lipid mediators arising in both reactive oxygen species (ROS) and ROS enzyme-dependent reactions. These types of metabolic modifications affect the functioning of the cells and the host organism. Therefore, links between the severity of the disease state and redox imbalance and the level of phospholipid metabolites are being searched, hoping to find unambiguous diagnostic biomarkers. Assessment of molecular effects of oxidative stress may also enable the monitoring of the disease process and treatment efficacy.

A proteomics informed by transcriptomics insight into the proteome of Ornithodoros erraticus adult tick saliva

BACKGROUND

The argasid tick Ornithodoros erraticus is the main vector of tick-borne human relapsing fever (TBRF) and African swine fever (ASF) in the Mediterranean Basin. The prevention and control of these diseases would greatly benefit from the elimination of O. erraticus populations, and anti-tick vaccines are envisaged as an effective and sustainable alternative to chemical acaricide usage for tick control. Ornithodoros erraticus saliva contains bioactive proteins that play essential functions in tick feeding and host defence modulation, which may contribute to host infection by tick-borne pathogens. Hence, these proteins could be candidate antigen targets for the development of vaccines aimed at the control and prevention of O. erraticus infestations and the diseases this tick transmits. The objective of the present work was to obtain and characterise the proteome of the saliva of O. erraticus adult ticks as a means to identify and select novel salivary antigen targets.

METHODS

A proteomics informed by transcriptomics (PIT) approach was applied to analyse samples of female and male saliva separately using the previously obtained O. erraticus sialotranscriptome as a reference database and two different mass spectrometry techniques, namely liquid chromatography-tandem mass spectrometry (LC-MS/MS) in data-dependent acquisition mode and sequential window acquisition of all theoretical fragment ion spectra MS (SWATH-MS).

RESULTS

Up to 264 and 263 proteins were identified by LC-MS/MS in the saliva of O. erraticus female and male ticks, respectively, totalling 387 non-redundant proteins. Of these, 224 were further quantified by SWATH-MS in the saliva of both male and female ticks. Quantified proteins were classified into 23 functional categories and their abundance compared between sexes. Heme/iron-binding proteins, protease inhibitors, proteases, lipocalins and immune-related proteins were the categories most abundantly expressed in females, while glycolytic enzymes, protease inhibitors and lipocalins were the most abundantly expressed in males. Ninety-seven proteins were differentially expressed between the sexes, of which 37 and 60 were overexpressed in females and males, respectively.

CONCLUSIONS

The PIT approach demonstrated its usefulness for proteomics studies of O. erraticus, a non-model organism without genomic sequences available, allowing the publication of the first comprehensive proteome of the saliva of O. erraticus reported to date. These findings confirm important quantitative differences between sexes in the O. erraticus saliva proteome, unveil novel salivary proteins and functions at the tick-host feeding interface and improve our understanding of the physiology of feeding in O. erraticus ticks. The integration of O. erraticus sialoproteomic and sialotranscriptomic data will drive a more rational selection of salivary candidates as antigen targets for the development of vaccines aimed at the control of O. erraticus infestations and the diseases it transmits.

Ecological and evolutionary perspectives on tick-borne pathogen co-infections

Tick-borne pathogen co-infections are common in nature. Co-infecting pathogens interact with each other and the tick microbiome, which influences individual pathogen fitness, and ultimately shapes virulence, infectivity, and transmission. In this review, we discuss how tick-borne pathogens are an ideal framework to study the evolutionary dynamics of co-infections. We highlight the importance of inter-species and intra-species interactions in vector-borne pathogen ecology and evolution. We also propose experimental evolution in tick cell lines as a method to directly test the impact of co-infections on pathogen evolution. Experimental evolution can simulate in real-time the long periods of time involved in within-vector pathogen interactions in nature, a major practical obstacle to cracking the influence of co-infections on pathogen evolution and ecology.

Proteomics informed by transcriptomics for a qualitative and quantitative analysis of the sialoproteome of adult Ornithodoros moubata ticks

Background

The argasid tick Ornithodoros moubata is the main vector in mainland Africa of African swine fever virus and the spirochete Borrelia duttoni, which causes human relapsing fever. The elimination of populations of O. moubata would contribute to the prevention and control of these two serious diseases. Anti-tick vaccines are an eco-friendly and sustainable means of eliminating tick populations. Tick saliva forms part of the tick-host interface, and knowledge of its composition is key to the identification and selection of vaccine candidate antigens. The aim of the present work is to increase the body of data on the composition of the saliva proteome of adult O. moubata ticks, particularly of females, since in-depth knowledge of the O. moubata sialome will allow the identification and selection of novel salivary antigens as targets for tick vaccines.

Methods

We analysed samples of female and male saliva using two different mass spectrometry (MS) approaches: data-dependent acquisition liquid chromatography-tandem MS (LC–MS/MS) and sequential window acquisition of all theoretical fragment ion spectra–MS (SWATH-MS). To maximise the number of proteins identified, a proteomics informed by transcriptomics analysis was applied using the O. moubata salivary transcriptomic dataset previously obtained by RNA-Seq.

Results

SWATH-MS proved to be superior to LC–MS/MS for the study of female saliva, since it identified 61.2% more proteins than the latter, the reproducibility of results was enhanced with its use, and it provided a quantitative picture of salivary components. In total, we identified 299 non-redundant proteins in the saliva of O. moubata, and quantified the expression of 165 of these in both male and female saliva, among which 13 were significantly overexpressed in females and 40 in males. These results indicate important quantitative differences in the saliva proteome between the sexes.

Conclusions

This work expands our knowledge of the O. moubata sialome, particularly that of females, by increasing the number of identified novel salivary proteins, which have different functions at the tick–host feeding interface. This new knowledge taken together with information on the O. moubata sialotranscriptome will allow a more rational selection of salivary candidates as antigen targets for tick vaccine development.

Graphical Abstract

Supplementary Information

The online version contains supplementary material available at 10.1186/s13071-021-04892-2.

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.

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.

Enlisting the Ixodes scapularis Embryonic ISE6 Cell Line to Investigate the Neuronal Basis of Tick-Pathogen Interactions

Neuropeptides are small signaling molecules expressed in the tick central nervous system, i.e., the synganglion. The neuronal-like Ixodes scapularis embryonic cell line, ISE6, is an effective tool frequently used for examining tick–pathogen interactions. We detected 37 neuropeptide transcripts in the I. scapularis ISE6 cell line using in silico methods, and six of these neuropeptide genes were used for experimental validation. Among these six neuropeptide genes, the tachykinin-related peptide (TRP) of ISE6 cells varied in transcript expression depending on the infection strain of the tick-borne pathogen, Anaplasma phagocytophilum. The immunocytochemistry of TRP revealed cytoplasmic expression in a prominent ISE6 cell subpopulation. The presence of TRP was also confirmed in A. phagocytophilum-infected ISE6 cells. The in situ hybridization and immunohistochemistry of TRP of I. scapularis synganglion revealed expression in distinct neuronal cells. In addition, TRP immunoreaction was detected in axons exiting the synganglion via peripheral nerves as well as in hemal nerve-associated lateral segmental organs. The characterization of a complete Ixodes neuropeptidome in ISE6 cells may serve as an effective in vitro tool to study how tick-borne pathogens interact with synganglion components that are vital to tick physiology. Therefore, our current study is a potential stepping stone for in vivo experiments to further examine the neuronal basis of tick–pathogen interactions.

Tick-virus interactions: Current understanding and future perspectives

Ticks are the primary vector of arboviruses in temperate climates worldwide. They are both the vector of these pathogens to humans and an integral component of the viral sylvatic cycle. Understanding the tick-pathogen interaction provides information about the natural maintenance of these pathogens and informs the development of countermeasures against human infection. In this review, we discuss currently available information on tick-viral interactions within the broader scope of general tick immunology. While the tick immune response to several pathogens has been studied extensively, minimal work centres on responses to viral infection. This is largely due to the high pathogenicity of tick-borne viruses; this necessitates high-containment laboratories or low-pathogenicity substitute viruses. This has biased most research towards tick-borne flaviviruses. More work is required to fully understand the role of tick-virus interaction in sylvatic cycling and transmission of diverse tick-borne viruses.

Transcriptome analysis of Ehrlichia ruminantium in the ruminant host at the tick bite site and in the tick vector salivary glands

Heartwater is a non-contagious tick-borne disease of domestic and wild ruminants. Data regarding the complex processes involved during pathogen-vector-host interaction during Ehrlichia ruminantium infection is lacking and could be improved with knowledge associated with gene expression changes in both the pathogen and the host. Thus, in the current study, we aimed to identify E. ruminantium genes that are up-regulated when the pathogen enters the host and before the disease is established. Identification of such genes/proteins may aid in future vaccine development strategies against heartwater. RNA-sequencing was used to identify E. ruminantium genes that were exclusively expressed at the tick bite site in sheep skin biopsies (SB) and in adult tick salivary glands (SG). RNA was extracted from pooled samples of the SB or SG collected at different time points during tick attachment and prior to disease manifestation. Ribosomal RNA (rRNA) was removed and the samples were sequenced. Several E. ruminantium genes were highly expressed in all the samples while others were exclusively expressed in each. It was concluded that E. ruminantium genes that were exclusively expressed in the SB or both SB and SG when compared to the transcriptome datasets from bovine elementary bodies (BovEBs) from cell culture may be considered as early antigenic targets of host immunity. In silico immunogenic epitope prediction analysis and preliminary characterization of selected genes in vitro using ELIspot assay showed that they could possibly be ideal targets for future vaccine development against heartwater, however, further epitope characterization is still required.

Tick salivary gland transcriptomics and proteomics

'Omics' technologies have facilitated the identification of hundreds to thousands of tick molecules that mediate tick feeding and play a role in the transmission of tick-borne diseases. Deep sequencing methodologies have played a key role in this knowledge accumulation, profoundly facilitating the study of the biology of disease vectors lacking reference genomes. For example, the nucleotide sequences of the entire set of tick salivary effectors, the so-called tick 'sialome', now contain at least one order of magnitude more transcript sequences compared to similar projects based on Sanger sequencing. Tick feeding is a complex and dynamic process, and while the dynamic 'sialome' is thought to mediate tick feeding success, exactly how transcriptome dynamics relate to tick-host-pathogen interactions is still largely unknown. The identification and, importantly, the functional analysis of the tick 'sialome' is expected to shed light on this 'black box'. This information will be crucial for developing strategies to block pathogen transmission, not only for anti-tick vaccine development but also the discovery and development of new, pharmacologically active compounds for human diseases.

De novo assembly and analysis of the transcriptome of the Dermacentor marginatus genes differentially expressed after blood-feeding and long-term starvation

Background

The ixodid tick Dermacentor marginatus is a vector of many pathogens wide spread in Eurasia. Studies of gene sequence on many tick species have greatly increased the information on tick protective antigen which might have the potential to function as effective vaccine candidates or drug targets for eco-friendly acaricide development. In the current study, RNA-seq was applied to identify D. marginatus sequences and analyze differentially expressed unigenes.

Methods

To obtain a broader picture of gene sequences and changes in expression level, RNA-seq was performed to obtain the whole-body transcriptome data of D. marginatus adult female ticks after engorgement and long-term starvation. Subsequently, the real-time quantitative PCR (RT-qPCR) was applied to validate the RNA-seq data.

Results

RNA-seq produced 30,251 unigenes, of which 32% were annotated. Gene expression was compared among groups that differed by status as newly molted, starved and engorged female adult ticks. Nearly one third of the unigenes in each group were differentially expressed compared to the other two groups, and the most numerous were genes encoding proteins involved in catalytic and binding activities and apoptosis. Selected up-regulated differentially expressed genes in each group were associated to protein, lipids, carbohydrate and chitin metabolism. Blood-feeding and long-term starvation also caused genes differentially expressed in the defense response and antioxidant response. RT-qPCR results indicated 6 differentially expressed transcripts showed similar trends in expression changes with RNA-seq results confirming that the gene expression profiles in transcriptome data is in consistent with RT-qPCR validation.

Conclusions

Obtaining the sequence information of D. marginatus and characterizing the expression pattern of the genes involved in blood-feeding and during starvation would be helpful in understanding molecular physiology of D. marginatus and provides data for anti-tick vaccine and drug development for controlling the tick.

Immunometabolism in Arthropod Vectors: Redefining Interspecies Relationships

Metabolism influences biochemical networks, and arthropod vectors are endowed with an immune system that affects microbial acquisition, persistence, and transmission to humans and other animals. Here, we aim to persuade the scientific community to expand their interests in immunometabolism beyond mammalian hosts and towards arthropod vectors. Immunometabolism investigates the interplay of metabolism and immunology. We provide a conceptual framework for investigators from diverse disciplines and indicate that relationships between microbes, mammalian hosts and their hematophagous arthropods may result in cost-effective (mutualism) or energetically expensive (parasitism) interactions. We argue that disparate resource allocations between species may partially explain why some microbes act as pathogens when infecting humans and behave as mutualistic or commensal organisms when colonizing arthropod vectors.

Transcriptomic analysis of chicken immune response to infection of different doses of Newcastle disease vaccine

Newcastle disease virus (NDV) is a contagious poultry paramyxovirus, leading to substantial economic losses to the poultry industry. Here, RNA-seq was carried out to investigate the altered expression of immune-related genes in chicken thymus within 96 h in response to NDV infection. In NDV-infected chicken thymus tissues, comparative transcriptome analysis revealed 1386 differentially expressed genes (DEGs) at 24 h with 989 up- and 397 down-regulated genes, 728 DEGs at 48 h with 567 up- and 161 down-regulated genes, 1514 DEGs at 72 h with 1016 up- and 498 down-regulated genes, and 1196 DEGs at 96 h with 522 up- and 674 down-regulated genes, respectively. Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis showed that these candidate targets mainly participate in biological processes or biochemical, metabolic and signal transduction processes. Notably, there is large enrichment in biological processes, cell components and metabolic processes, which may be related to NDV pathogenicity. In addition, the expression of five immune-related DEGs identified by RNA-seq was validated by quantitative real-time polymerase chain reaction (qRT-PCR). Our results indicated that the expression levels of AvBD5, IL16, IL22 and IL18R1 were obviously up-regulated, and Il-18 expression was also changed, but not significantly, which play key roles in the defense against NDV. Overall, we identified several candidate targets that may be involved in the regulation of NDV infection, which provide new insights into the complicated regulatory mechanisms of virus-host interactions, and explore new strategies for protecting chickens against the virus.

Large-Scale Comparative Analyses of Tick Genomes Elucidate Their Genetic Diversity and Vector Capacities

Among arthropod vectors, ticks transmit the most diverse human and animal pathogens, leading to an increasing number of new challenges worldwide. Here we sequenced and assembled high-quality genomes of six ixodid tick species and further resequenced 678 tick specimens to understand three key aspects of ticks: genetic diversity, population structure, and pathogen distribution. We explored the genetic basis common to ticks, including heme and hemoglobin digestion, iron metabolism, and reactive oxygen species, and unveiled for the first time that genetic structure and pathogen composition in different tick species are mainly shaped by ecological and geographic factors. We further identified species-specific determinants associated with different host ranges, life cycles, and distributions. The findings of this study are an invaluable resource for research and control of ticks and tick-borne diseases.

Pathogens Manipulating Tick Behavior-Through a Glass, Darkly

Pathogens can manipulate the phenotypic traits of their hosts and vectors, maximizing their own fitness. Among the phenotypic traits that can be modified, manipulating vector behavior represents one of the most fascinating facets. How pathogens infection affects behavioral traits of key insect vectors has been extensively investigated. Major examples include Plasmodium, Leishmania and Trypanosoma spp. manipulating the behavior of mosquitoes, sand flies and kissing bugs, respectively. However, research on how pathogens can modify tick behavior is patchy. This review focuses on current knowledge about the behavioral changes triggered by Anaplasma, Borrelia, Babesia, Bartonella, Rickettsia and tick-borne encephalitis virus (TBEV) infection in tick vectors, analyzing their potential adaptive significance. As a general trend, being infected by Borrelia and TBEV boosts tick mobility (both questing and walking activity). Borrelia and Anaplasma infection magnifies Ixodes desiccation resistance, triggering physiological changes (Borrelia: higher fat reserves; Anaplasma: synthesis of heat shock proteins). Anaplasma infection also improves cold resistance in infected ticks through synthesis of an antifreeze glycoprotein. Being infected by Anaplasma, Borrelia and Babesia leads to increased tick survival. Borrelia, Babesia and Bartonella infection facilitates blood engorgement. In the last section, current challenges for future studies are outlined.

Rickettsial pathogen uses arthropod tryptophan pathway metabolites to evade reactive oxygen species in tick cells

Reactive oxygen species (ROS) that are induced upon pathogen infection plays an important role in host defence. The rickettsial pathogen Anaplasma phagocytophilum, which is primarily transmitted by Ixodes scapularis ticks in the United States, has evolved many strategies to escape ROS and survive in mammalian cells. However, little is known on the role of ROS in A. phagocytophilum infection in ticks. Our results show that A. phagocytophilum and hemin induce activation of l-tryptophan pathway in tick cells. Xanthurenic acid (XA), a tryptophan metabolite, supports A. phagocytophilum growth in tick cells through inhibition of tryptophan dioxygenase (TDO) activity leading to reduced l-kynurenine levels that subsequently affects build-up of ROS. However, hemin supports A. phagocytophilum growth in tick cells by inducing TDO activity leading to increased l-kynurenine levels and ROS production. Our data reveal that XA and kynurenic acid (KA) chelate hemin. Furthermore, treatment of tick cells with 3-hydroxyl l-kynurenine limits A. phagocytophilum growth in tick cells. RNAi-mediated knockdown of kynurenine aminotransferase expression results in increased ROS production and reduced A. phagocytophilum burden in tick cells. Collectively, these results suggest that l-tryptophan pathway metabolites influence A. phagocytophilum survival by affecting build up of ROS levels in tick cells.

Quantitative Proteomics Identifies Metabolic Pathways Affected by Babesia Infection and Blood Feeding in the Sialoproteome of the Vector Rhipicephalus bursa

The negative impact of ticks and tick-borne diseases on animals and human health is driving research to discover novel targets affecting both vectors and pathogens. The salivary glands are involved in feeding and pathogen transmission, thus are considered as a compelling target to focus research. In this study, proteomics approach was used to characterize Rhipicephalusbursa sialoproteome in response to Babesiaovis infection and blood feeding. Two potential tick protective antigens were identified and its influence in tick biological parameters and pathogen infection was evaluated. Results demonstrate that the R. bursa sialoproteome is highly affected by feeding but infection is well tolerated by tick cells. The combination of both stimuli shifts the previous scenario and a more evident pathogen manipulation can be suggested. Knockdown of ub2n led to a significative increase of infection in tick salivary glands but a brusque decrease in the progeny, revealing its importance in the cellular response to pathogen infection, which is worth pursuing in future studies. Additionally, an impact in the recovery rate of adults (62%), the egg production efficiency (45.75%), and the hatching rate (88.57 %) was detected. Building knowledge on vector and/or pathogen interplay bridges the identification of protective antigens and the development of novel control strategies.

The redox metabolic pathways function to limit Anaplasma phagocytophilum infection and multiplication while preserving fitness in tick vector cells

Aerobic organisms evolved conserved mechanisms controlling the generation of reactive oxygen species (ROS) to maintain redox homeostasis signaling and modulate signal transduction, gene expression and cellular functional responses under physiological conditions. The production of ROS by mitochondria is essential in the oxidative stress associated with different pathologies and in response to pathogen infection. Anaplasma phagocytophilum is an intracellular pathogen transmitted by Ixodes scapularis ticks and causing human granulocytic anaplasmosis. Bacteria multiply in vertebrate neutrophils and infect first tick midgut cells and subsequently hemocytes and salivary glands from where transmission occurs. Previous results demonstrated that A. phagocytophilum does not induce the production of ROS as part of its survival strategy in human neutrophils. However, little is known about the role of ROS during pathogen infection in ticks. In this study, the role of tick oxidative stress during A. phagocytophilum infection was characterized through the function of different pathways involved in ROS production. The results showed that tick cells increase mitochondrial ROS production to limit A. phagocytophilum infection, while pathogen inhibits alternative ROS production pathways and apoptosis to preserve cell fitness and facilitate infection. The inhibition of NADPH oxidase-mediated ROS production by pathogen infection appears to occur in both neutrophils and tick cells, thus supporting that A. phagocytophilum uses common mechanisms for infection of ticks and vertebrate hosts. However, differences in ROS response to A. phagocytophilum infection between human and tick cells may reflect host-specific cell tropism that evolved during pathogen life cycle.

Pyruvate produced by Brugia spp. via glycolysis is essential for maintaining the mutualistic association between the parasite and its endosymbiont, Wolbachia

Human parasitic nematodes are the causative agents of lymphatic filariasis (elephantiasis) and onchocerciasis (river blindness), diseases that are endemic to more than 80 countries and that consistently rank in the top ten for the highest number of years lived with disability. These filarial nematodes have evolved an obligate mutualistic association with an intracellular bacterium, Wolbachia, a symbiont that is essential for the successful development, reproduction, and survival of adult filarial worms. Elimination of the bacteria causes adult worms to die, making Wolbachia a primary target for developing new interventional tools to combat filariases. To further explore Wolbachia as a promising indirect macrofilaricidal drug target, the essential cellular processes that define the symbiotic Wolbachia-host interactions need to be identified. Genomic analyses revealed that while filarial nematodes encode all the enzymes necessary for glycolysis, Wolbachia does not encode the genes for three glycolytic enzymes: hexokinase, 6-phosphofructokinase, and pyruvate kinase. These enzymes are necessary for converting glucose into pyruvate. Wolbachia, however, has the full complement of genes required for gluconeogenesis starting with pyruvate, and for energy metabolism via the tricarboxylic acid cycle. Therefore, we hypothesized that Wolbachia might depend on host glycolysis to maintain a mutualistic association with their parasitic host. We did conditional experiments in vitro that confirmed that glycolysis and its end-product, pyruvate, sustain this symbiotic relationship. Analysis of alternative sources of pyruvate within the worm indicated that the filarial lactate dehydrogenase could also regulate the local intracellular concentration of pyruvate in proximity to Wolbachia and thus help control bacterial growth via molecular interactions with the bacteria. Lastly, we have shown that the parasite's pyruvate kinase, the enzyme that performs the last step in glycolysis, could be a potential novel anti-filarial drug target. Establishing that glycolysis is an essential component of symbiosis in filarial worms could have a broader impact on research focused on other intracellular bacteria-host interactions where the role of glycolysis in supporting intracellular survival of bacteria has been reported.

Anaplasma phagocytophilum modifies tick cell microRNA expression and upregulates isc-mir-79 to facilitate infection by targeting the Roundabout protein 2 pathway

The microRNAs (miRNAs) are a class of small noncoding RNAs that have important regulatory roles in multicellular organisms including innate and adaptive immune pathways to control bacterial, parasite and viral infections, and pathogens could modify host miRNA profile to facilitate infection and multiplication. Therefore, understanding the function of host miRNAs in response to pathogen infection is relevant to characterize host-pathogen molecular interactions and to provide new targets for effective new interventions for the control infectious diseases. The objective of this study was to characterize the dynamics and functional significance of the miRNA response of the tick vector Ixodes scapularis in response to Anaplasma phagocytophilum infection, the causative agent of human and animal granulocytic anaplasmosis. To address this objective, the composition of tick miRNAs, functional annotation, and expression profiling was characterized using high throughout RNA sequencing in uninfected and A. phagocytophilum-infected I. scapularis ISE6 tick cells, a model for tick hemocytes involved in pathogen infection. The results provided new evidences on the role of tick miRNA during pathogen infection, and showed that A. phagocytophilum modifies I. scapularis tick cell miRNA profile and upregulates isc-mir-79 to facilitate infection by targeting the Roundabout protein 2 (Robo2) pathway. Furthermore, these results suggested new targets for interventions to control pathogen infection in ticks.

Transcriptome and Proteome Response of Rhipicephalus annulatus Tick Vector to Babesia bigemina Infection

A system biology approach was used to gain insight into tick biology and interactions between vector and pathogen. Rhipicephalus annulatus is one of the main vectors of Babesia bigemina which has a massive impact on animal health. It is vital to obtain more information about this relationship, to better understand tick and pathogen biology, pathogen transmission dynamics, and new potential control approaches. In ticks, salivary glands (SGs) play a key role during pathogen infection and transmission. RNA sequencing obtained from uninfected and B. bigemina infected SGs obtained from fed female ticks resulted in 6823 and 6475 unigenes, respectively. From these, 360 unigenes were found to be differentially expressed (p < 0.05). Reversed phase liquid chromatography-mass spectrometry identified a total of 3679 tick proteins. Among them 406 were differently represented in response to Babesia infection. The omics data obtained suggested that Babesia infection lead to a reduction in the levels of mRNA and proteins (n = 237 transcripts, n = 212 proteins) when compared to uninfected controls. Integrated transcriptomics and proteomics datasets suggested a key role for stress response and apoptosis pathways in response to infection. Thus, six genes coding for GP80, death-associated protein kinase (DAPK-1), bax inhibitor-1 related (BI-1), heat shock protein (HSP), heat shock transcription factor (PHSTF), and queuine trna-ribosyltransferase (QtRibosyl) were selected and RNA interference (RNAi) performed. Gene silencing was obtained for all genes except phstf. Knockdown of gp80, dapk-1, and bi-1 led to a significant increase in Babesia infection levels while hsp and QtRibosyl knockdown resulted in a non-significant decrease of infection levels when compared to the respective controls. Gene knockdown did not affect tick survival, but engorged female weight and egg production were affected in the gp80, dapk-1, and QtRibosyl-silenced groups in comparison to controls. These results advanced our understanding of tick-Babesia molecular interactions, and suggested new tick antigens as putative targets for vaccination to control tick infestations and pathogen infection/transmission.

Tick-Pathogen Interactions: The Metabolic Perspective

The first tick genome published in 2016 provided an invaluable tool for studying the molecular basis of tick-pathogen interactions. Metabolism is a key element in host-pathogen interactions. However, our knowledge of tick-pathogen metabolic interactions is very limited. Recently, a systems biology approach, using omics datasets, has revealed that tick-borne pathogen infection induces transcriptional reprograming affecting several metabolic pathways in ticks, facilitating infection, multiplication, and transmission. Results suggest that the response of tick cells to tick-borne pathogens is associated with tolerance to infection. Here we review our current understanding of the modulation of tick metabolism by tick-borne pathogens, with a focus on the model intracellular bacterium Anaplasma phagocytophilum.

Nutrition and Bipartite Metabolism of Intracellular Pathogens

The host is a nutrient-rich niche for microbial pathogens, but one that comes with obstacles and challenges. Many intracellular pathogens like Legionella pneumophila, Coxiella burnetii, Listeria monocytogenes, and Chlamydia trachomatis have developed bipartite metabolism within their hosts. This style of metabolic regulation enables pathogen sensing of specific nutrients to engage them into catabolic and anabolic processes, and contributes to temporal and spatial pathogen phenotypic modulation. Not only have intracellular pathogens adapted their metabolism to the host, they have also acquired idiosyncratic strategies to exploit host nutritional supplies and intercept metabolites. Francisella tularensis and Anaplasma phagocytophilum alter host autophagy, Shigella flexneri intercepts all host pyruvate, while L. pneumophila induces host protein degradation and blocks protein translation. Strategies of pathogen manipulation of host nutrients could serve as therapeutic targets.

Tick galactosyltransferases are involved in α-Gal synthesis and play a role during Anaplasma phagocytophilum infection and Ixodes scapularis tick vector development

The carbohydrate Galα1-3Galβ1-(3)4GlcNAc-R (α-Gal) is produced in all mammals except for humans, apes and old world monkeys that lost the ability to synthetize this carbohydrate. Therefore, humans can produce high antibody titers against α-Gal. Anti-α-Gal IgE antibodies have been associated with tick-induced allergy (i.e. α-Gal syndrome) and anti-α-Gal IgG/IgM antibodies may be involved in protection against malaria, leishmaniasis and Chagas disease. The α-Gal on tick salivary proteins plays an important role in the etiology of the α-Gal syndrome. However, whether ticks are able to produce endogenous α-Gal remains currently unknown. In this study, the Ixodes scapularis genome was searched for galactosyltransferases and three genes were identified as potentially involved in the synthesis of α-Gal. Heterologous gene expression in α-Gal-negative cells and gene knockdown in ticks confirmed that these genes were involved in α-Gal synthesis and are essential for tick feeding. Furthermore, these genes were shown to play an important role in tick-pathogen interactions. Results suggested that tick cells increased α-Gal levels in response to Anaplasma phagocytophilum infection to control bacterial infection. These results provided the molecular basis of endogenous α-Gal production in ticks and suggested that tick galactosyltransferases are involved in vector development, tick-pathogen interactions and possibly the etiology of α-Gal syndrome in humans.

Use of Graph Theory to Characterize Human and Arthropod Vector Cell Protein Response to Infection With Anaplasma phagocytophilum

One of the major challenges in modern biology is the use of large omics datasets for the characterization of complex processes such as cell response to infection. These challenges are even bigger when analyses need to be performed for comparison of different species including model and non-model organisms. To address these challenges, the graph theory was applied to characterize the tick vector and human cell protein response to infection with Anaplasma phagocytophilum, the causative agent of human granulocytic anaplasmosis. A network of interacting proteins and cell processes clustered in biological pathways, and ranked with indexes representing the topology of the proteome was prepared. The results demonstrated that networks of functionally interacting proteins represented in both infected and uninfected cells can describe the complete set of host cell processes and metabolic pathways, providing a deeper view of the comparative host cell response to pathogen infection. The results demonstrated that changes in the tick proteome were driven by modifications in protein representation in response to A. phagocytophilum infection. Pathogen infection had a higher impact on tick than human proteome. Since most proteins were linked to several cell processes, the changes in protein representation affected simultaneously different biological pathways. The method allowed discerning cell processes that were affected by pathogen infection from those that remained unaffected. The results supported that human neutrophils but not tick cells limit pathogen infection through differential representation of ras-related proteins. This methodological approach could be applied to other host-pathogen models to identify host derived key proteins in response to infection that may be used to develop novel control strategies for arthropod-borne pathogens.

De novo assembly and analysis of midgut transcriptome of the argasid tick Ornithodoros erraticus and identification of genes differentially expressed after blood feeding

Ticks are hematophagous vectors of great medical and veterinary importance because they transmit numerous pathogenic microorganisms to humans and animals. The argasid Ornithodoros erraticus is the main vector of tick-borne human relapsing fever and African swine fever in the Mediterranean Basin. Tick enterocytes express bioactive molecules that perform key functions in blood digestion, feeding, toxic waste processing and pathogen transmission. To explore new strategies for tick control, in this work we have obtained and compared the midgut transcriptomes of O. erraticus female ticks before and after a blood meal and identified the genes whose expression is differentially regulated after feeding. The transcript sequences were annotated, functionally and structurally characterised and their expression levels compared between both physiological conditions (unfed females and fed females at 2 days post-engorgement). Up to 29,025 transcripts were assembled, and 9290 of them corresponded to differentially expressed genes (DEGs) after feeding. Of these, 4656 genes were upregulated and nearly the same number of genes was downregulated in fed females compared to unfed females. BLASTN and BLASTX analyses of the 29,025 transcripts allowed the annotation of 9072 transcripts/proteins. Among them, the most numerous were those with catalytic and binding activities and those involved in diverse metabolic pathways and cellular processes. The analyses of functional groups of upregulated DEGs potentially related to the digestion of proteins, carbohydrates and lipids, and the genes involved in the defence response and response to oxidative stress, confirm that these processes are narrowly regulated in ticks, highlighting their complexity and importance in tick biology. The expression patterns of six genes throughout the blood digestion period revealed significant differences between these patterns, strongly suggesting that the transcriptome composition is highly dynamic and subjected to important variation along the trophogonic cycle. This may guide future studies aimed at improving the understanding of the molecular physiology of tick digestion and digestion-related processes. The current work provides a more robust and comprehensive understanding of the argasid tick digestive system.

Arthropod transcriptional activator protein-1 (AP-1) aids tick-rickettsial pathogen survival in the cold

Ixodes scapularis ticks transmit several pathogens to humans including rickettsial bacterium, Anaplasma phagocytophilum. Here, we report that A. phagocytophilum uses tick transcriptional activator protein-1 (AP-1) as a molecular switch in the regulation of arthropod antifreeze gene, iafgp. RNAi-mediated silencing of ap-1 expression significantly affected iafgp gene expression and A. phagocytophilum burden in ticks upon acquisition from the murine host. Gel shift assays provide evidence that both the bacterium and AP-1 influences iafgp promoter and expression. The luciferase assays revealed that a region of approximately 700 bp upstream of the antifreeze gene is sufficient for AP-1 binding to promote iafgp gene expression. Furthermore, survival assays revealed that AP-1-deficient ticks were more susceptible to cold in comparison to the mock controls. In addition, this study also indicates arthropod AP-1 as a global regulator for some of the tick genes critical for A. phagocytophilum survival in the vector. In summary, our study defines a novel mode of arthropod signaling for the survival of both rickettsial pathogen and its medically important vector in the cold.

The Tick Cell Biobank: A global resource for in vitro research on ticks, other arthropods and the pathogens they transmit

Tick cell lines are increasingly used in many fields of tick and tick-borne disease research. The Tick Cell Biobank was established in 2009 to facilitate the development and uptake of these unique and valuable resources. As well as serving as a repository for existing and new ixodid and argasid tick cell lines, the Tick Cell Biobank supplies cell lines and training in their maintenance to scientists worldwide and generates novel cultures from tick species not already represented in the collection. Now part of the Institute of Infection and Global Health at the University of Liverpool, the Tick Cell Biobank has embarked on a new phase of activity particularly targeted at research on problems caused by ticks, other arthropods and the diseases they transmit in less-developed, lower- and middle-income countries. We are carrying out genotypic and phenotypic characterisation of selected cell lines derived from tropical tick species. We continue to expand the culture collection, currently comprising 63 cell lines derived from 18 ixodid and argasid tick species and one each from the sand fly Lutzomyia longipalpis and the biting midge Culicoides sonorensis, and are actively engaging with collaborators to obtain starting material for primary cell cultures from other midge species, mites, tsetse flies and bees. Outposts of the Tick Cell Biobank will be set up in Malaysia, Kenya and Brazil to facilitate uptake and exploitation of cell lines and associated training by scientists in these and neighbouring countries. Thus the Tick Cell Biobank will continue to underpin many areas of global research into biology and control of ticks, other arthropods and vector-borne viral, bacterial and protozoan pathogens.

Rhipicephalus bursa Sialotranscriptomic Response to Blood Feeding and Babesia ovis Infection: Identification of Candidate Protective Antigens

Ticks are among the most prevalent blood-feeding arthropods, and they act as vectors and reservoirs for numerous pathogens. Sialotranscriptomic characterizations of tick responses to blood feeding and pathogen infections can offer new insights into the molecular interplay occurring at the tick-host-pathogen interface. In the present study, we aimed to identify and characterize Rhipicephalus bursa salivary gland (SG) genes that were differentially expressed in response to blood feeding and Babesia ovis infection. Our experimental approach consisted of RNA sequencing of SG from three different tick samples, fed-infected, fed-uninfected, and unfed-uninfected, for characterization and inter-comparison. Overall, 7,272 expressed sequence tags (ESTs) were constructed from unfed-uninfected, 13,819 ESTs from fed-uninfected, and 15,292 ESTs from fed-infected ticks. Two catalogs of transcripts that were differentially expressed in response to blood feeding and B. ovis infection were produced. Four genes coding for a putative vitellogenin-3, lachesin, a glycine rich protein, and a secreted cement protein were selected for RNA interference functional studies. A reduction of 92, 65, and 51% was observed in vitellogenin-3, secreted cement, and lachesin mRNA levels in SG, respectively. The vitellogenin-3 knockdown led to increased tick mortality, with 77% of ticks dying post-infestation. The reduction of the secreted cement protein-mRNA levels resulted in 46% of ticks being incapable of correctly attaching to the host and significantly lower female weights post-feeding in comparison to the control group. The lachesin knockdown resulted in a 70% reduction of the levels associated with B. ovis infection in R. bursa SG and 70% mortality. These results improved our understanding of the role of tick SG genes in Babesia infection/proliferation and tick feeding. Moreover, lachesin, vitellogenin-3, and secreted cement proteins were validated as candidate protective antigens for the development of novel tick and tick-borne disease control measures.

Metazoan Parasite Vaccines: Present Status and Future Prospects

Eukaryotic parasites and pathogens continue to cause some of the most detrimental and difficult to treat diseases (or disease states) in both humans and animals, while also continuously expanding into non-endemic countries. Combined with the ever growing number of reports on drug-resistance and the lack of effective treatment programs for many metazoan diseases, the impact that these organisms will have on quality of life remain a global challenge. Vaccination as an effective prophylactic treatment has been demonstrated for well over 200 years for bacterial and viral diseases. From the earliest variolation procedures to the cutting edge technologies employed today, many protective preparations have been successfully developed for use in both medical and veterinary applications. In spite of the successes of these applications in the discovery of subunit vaccines against prokaryotic pathogens, not many targets have been successfully developed into vaccines directed against metazoan parasites. With the current increase in -omics technologies and metadata for eukaryotic parasites, target discovery for vaccine development can be expedited. However, a good understanding of the host/vector/pathogen interface is needed to understand the underlying biological, biochemical and immunological components that will confer a protective response in the host animal. Therefore, systems biology is rapidly coming of age in the pursuit of effective parasite vaccines. Despite the difficulties, a number of approaches have been developed and applied to parasitic helminths and arthropods. This review will focus on key aspects of vaccine development that require attention in the battle against these metazoan parasites, as well as successes in the field of vaccine development for helminthiases and ectoparasites. Lastly, we propose future direction of applying successes in pursuit of next generation vaccines.

A Roadmap for Tick-Borne Flavivirus Research in the "Omics" Era

Tick-borne flaviviruses (TBFs) affect human health globally. Human vaccines provide protection against some TBFs, and antivirals are available, yet TBF-specific control strategies are limited. Advances in genomics offer hope to understand the viral complement transmitted by ticks, and to develop disruptive, data-driven technologies for virus detection, treatment, and control. The genome assemblies of Ixodes scapularis, the North American tick vector of the TBF, Powassan virus, and other tick vectors, are providing insights into tick biology and pathogen transmission and serve as nucleation points for expanded genomic research. Systems biology has yielded insights to the response of tick cells to viral infection at the transcript and protein level, and new protein targets for vaccines to limit virus transmission. Reverse vaccinology approaches have moved candidate tick antigenic epitopes into vaccine development pipelines. Traditional drug and in silico screening have identified candidate antivirals, and target-based approaches have been developed to identify novel acaricides. Yet, additional genomic resources are required to expand TBF research. Priorities include genome assemblies for tick vectors, “omic” studies involving high consequence pathogens and vectors, and emphasizing viral metagenomics, tick-virus metabolomics, and structural genomics of TBF and tick proteins. Also required are resources for forward genetics, including the development of tick strains with quantifiable traits, genetic markers and linkage maps. Here we review the current state of genomic research on ticks and tick-borne viruses with an emphasis on TBFs. We outline an ambitious 10-year roadmap for research in the “omics era,” and explore key milestones needed to accomplish the goal of delivering three new vaccines, antivirals and acaricides for TBF control by 2030.

Ixodes scapularis Tick Cells Control Anaplasma phagocytophilum Infection by Increasing the Synthesis of Phosphoenolpyruvate from Tyrosine

The obligate intracellular pathogen, Anaplasma phagocytophilum, is the causative agent of life-threatening diseases in humans and animals. A. phagocytophilum is an emerging tick-borne pathogen in the United States, Europe, Africa and Asia, with increasing numbers of infected people and animals every year. It is increasingly recognized that intracellular pathogens modify host cell metabolic pathways to increase infection and transmission in both vertebrate and invertebrate hosts. Recent reports have shown that amino acids are central to the host–pathogen metabolic interaction. In this study, a genome-wide search for components of amino acid metabolic pathways was performed in Ixodes scapularis, the main tick vector of A. phagocytophilum in the United States, for which the genome was recently published. The enzymes involved in the synthesis and degradation pathways of the twenty amino acids were identified. Then, the available transcriptomics and proteomics data was used to characterize the mRNA and protein levels of I. scapularis amino acid metabolic pathway components in response to A. phagocytophilum infection of tick tissues and ISE6 tick cells. Our analysis was focused on the interplay between carbohydrate and amino acid metabolism during A. phagocytophilum infection in ISE6 cells. The results showed that tick cells increase the synthesis of phosphoenolpyruvate (PEP) from tyrosine to control A. phagocytophilum infection. Metabolic pathway analysis suggested that this is achieved by (i) increasing the transcript and protein levels of mitochondrial phosphoenolpyruvate carboxykinase (PEPCK-M), (ii) shunting tyrosine into the tricarboxylic acid (TCA) cycle to increase fumarate and oxaloacetate which will be converted into PEP by PEPCK-M, and (iii) blocking all the pathways that use PEP downstream gluconeogenesis (i.e., de novo serine synthesis pathway (SSP), glyceroneogenesis and gluconeogenesis). While sequestering host PEP may be critical for this bacterium because it cannot actively carry out glycolysis to produce PEP, excess of this metabolite may be toxic for A. phagocytophilum. The present work provides a more comprehensive view of the major amino acid metabolic pathways involved in the response to pathogen infection in ticks, and provides the basis for further studies to develop novel strategies for the control of granulocytic anaplasmosis.

Vaccinomics Approach to the Identification of Candidate Protective Antigens for the Control of Tick Vector Infestations and Anaplasma phagocytophilum Infection

Anaplasma phagocytophilum is an emerging tick-borne pathogen causing human granulocytic anaplasmosis (HGA), tick-borne fever (TBF) in small ruminants, and other forms of anaplasmosis in different domestic and wild animals. The main vectors of this pathogen are Ixodes tick species, particularly I. scapularis in the United States and I. ricinus in Europe. One of the main limitations for the development of effective vaccines for the prevention and control of A. phagocytophilum infection and transmission is the identification of effective tick protective antigens. The objective of this study was to apply a vaccinomics approach to I. scapularis-A. phagocytophilum interactions for the identification and characterization of candidate tick protective antigens for the control of vector infestations and A. phagocytophilum infection. The vaccinomics pipeline included the use of quantitative transcriptomics and proteomics data from uninfected and A. phagocytophilum-infected I. scapularis ticks for the selection of candidate protective antigens based on the variation in tick mRNA and protein levels in response to infection, their putative biological function, and the effect of antibodies against these proteins on tick cell apoptosis and pathogen infection. The characterization of selected candidate tick protective antigens included the identification and characterization of I. ricinus homologs, functional characterization by different methodologies including RNA interference, immunofluorescence, gene expression profiling, and artificial tick feeding on rabbit antibodies against the recombinant antigens to select the candidates for vaccination trials. The vaccinomics pipeline developed in this study resulted in the identification of two candidate tick protective antigens that could be selected for future vaccination trials. The results showed that I. scapularis lipocalin (ISCW005600) and lectin pathway inhibitor (AAY66632) and I. ricinus homologs constitute candidate protective antigens for the control of vector infestations and A. phagocytophilum infection. Both antigens are involved in the tick evasion of host defense response and pathogen infection and transmission, but targeting different immune response pathways. The vaccinomics pipeline proposed here could be used to continue the identification and characterization of candidate tick protective antigens for the development of effective vaccines for the prevention and control of HGA, TBF, and other forms of anaplasmosis caused by A. phagocytophilum.

A proteomic insight into the midgut proteome of Ornithodoros moubata females reveals novel information on blood digestion in argasid ticks

Background

The argasid tick Ornithodoros moubata is the main African vector of the human relapsing fever agent Borrelia duttoni and the African swine fever virus. Together with saliva, the tick midgut forms part of the host-tick-pathogen interface, and numerous midgut proteins play key functions in the blood digestion-related process and the infection and transmission of pathogens. This work explores the composition of the midgut proteome of unfed and fed O. moubata females with the aim to complete the biological information already obtained from the midgut transcriptome and provide a more robust and comprehensive perspective of this biological system.

Methods

Midgut tissues taken from females before feeding and 48 h after feeding were subjected to LC/MS-MS analysis. After functional characterization and classification of the proteins identified, the differences in the proteome between unfed and fed females were analysed and discussed. Additionally, a detailed analysis of particular groups of proteins that are involved in the processes of nutrient digestion and responses to the oxidative stress was carried out.

Results

1491 non-redundant tick proteins were identified: 1132 of them in the midgut of unfed ticks, 1138 in the midgut of fed ticks, and up to 779 shared by both physiological conditions. Overall, the comparative analysis of the midgut proteomes of O. moubata females before and after feeding did not reveal great differences in the number or class of proteins expressed, enzymatic composition or functional classification.

Conclusions

The hemoglobinolytic system in ixodids and argasids is very similar in spite of the fact that they display very different feeding and reproductive strategies. Although the main source of nutrients in ticks are proteins, lipids and carbohydrates also constitute significant nutritional sources and play an important part in the process of blood digestion. The genes and proteins involved in intracellular transport mechanisms, defensive responses, detoxifying responses and stress responses seem to be closely regulated, highlighting the complexity and importance of these processes in tick biology, which in turn assigns them a great interest as targets for therapeutic and immunological interventions.

Electronic supplementary material

The online version of this article (doi:10.1186/s13071-017-2300-8) contains supplementary material, which is available to authorized users.

Anaplasma phagocytophilum MSP4 and HSP70 Proteins Are Involved in Interactions with Host Cells during Pathogen Infection

Anaplasma phagocytophilum transmembrane and surface proteins play a role during infection and multiplication in host neutrophils and tick vector cells. Recently, A. phagocytophilum Major surface protein 4 (MSP4) and Heat shock protein 70 (HSP70) were shown to be localized on the bacterial membrane, with a possible role during pathogen infection in ticks. In this study, we hypothesized that A. phagocytophilum MSP4 and HSP70 have similar functions in tick-pathogen and host-pathogen interactions. To address this hypothesis, herein we characterized the role of these bacterial proteins in interaction and infection of vertebrate host cells. The results showed that A. phagocytophilum MSP4 and HSP70 are involved in host-pathogen interactions, with a role for HSP70 during pathogen infection. The analysis of the potential protective capacity of MSP4 and MSP4-HSP70 antigens in immunized sheep showed that MSP4-HSP70 was only partially protective against pathogen infection. This limited protection may be associated with several factors, including the recognition of non-protective epitopes by IgG in immunized lambs. Nevertheless, these antigens may be combined with other candidate protective antigens for the development of vaccines for the control of human and animal granulocytic anaplasmosis. Focusing on the characterization of host protective immune mechanisms and protein-protein interactions at the host-pathogen interface may lead to the discovery and design of new effective protective antigens.

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.