A tick salivary protein targets cathepsin G and chymase and inhibits host inflammation and platelet aggregation

The effect of feeding on different hosts on the egg proteins in Haemaphysalis qinghaiensis tick

The majority of ixodid ticks display host-specificity to varying extents. Feeding on different hosts affects their development and reproduction. Consequences can be analyzed at the level of the egg, as it is the initial stage of tick development. Tick egg proteins are abundant and diverse, providing nutrients for embryonic development. However, studies on tick egg profiles are scarce. In this study, we aimed to analyze whether feeding Haemaphysalis qinghaiensis ticks on the yaks (Bos grunniens) and domestic sheep (Ovis aries) has an impact on the variety and variability of the egg proteome. Detached engorged females were used to lay eggs, which were then collected, dewaxed, and subjected to protein extraction. The extracted egg proteins were enzymatically digested using Filter-Aided Sample Preparation (FASP), and the unique peptides were separated and detected by Liquid Chromatography-tandem Mass Spectrometry (LC-MS/MS). The MS data were searched against the previously constructed whole tick transcriptome library of H. qinghaiensis, and the UniProt database for the identification of tick-derived egg proteins. The analysis revealed 49 and 53 high-confidence proteins identified in eggs collected from B. grunniens (EggBg) and O. aries (EggOa), respectively. Of these, 46 high-confidence proteins were common to both egg types, while three were unique to EggBg and seven to EggOa. All the identified proteins mainly belonged to enzymes, enzyme inhibitors, transporters, and proteins with unknown functions. The differential abundance analysis showed that nine proteins were significantly more present in EggBg, while six were significantly more present in EggOa. Overall, enzymes were the most diverse group, while vitellogenin (Vg) was the most abundant. Blood meal uptake on different hosts has a certain effect on the egg proteome composition and the abundance of some proteins, but it may also lead to compensation of protein roles.

A tick saliva serpin, IxsS17 inhibits host innate immune system proteases and enhances host colonization by Lyme disease agent

Lyme disease (LD) caused by Borrelia burgdorferi is among the most important human vector borne diseases for which there is no effective prevention method. Identification of tick saliva transmission factors of the LD agent is needed before the highly advocated tick antigen-based vaccine could be developed. We previously reported the highly conserved Ixodes scapularis (Ixs) tick saliva serpin (S) 17 (IxsS17) was highly secreted by B. burgdorferi infected nymphs. Here, we show that IxsS17 promote tick feeding and enhances B. burgdorferi colonization of the host. We show that IxsS17 is not part of a redundant system, and its functional domain reactive center loop (RCL) is 100% conserved in all tick species. Yeast expressed recombinant (r) IxsS17 inhibits effector proteases of inflammation, blood clotting, and complement innate immune systems. Interestingly, differential precipitation analysis revealed novel functional insights that IxsS17 interacts with both effector proteases and regulatory protease inhibitors. For instance, rIxsS17 interacted with blood clotting proteases, fXII, fX, fXII, plasmin, and plasma kallikrein alongside blood clotting regulatory serpins (antithrombin III and heparin cofactor II). Similarly, rIxsS17 interacted with both complement system serine proteases, C1s, C2, and factor I and the regulatory serpin, plasma protease C1 inhibitor. Consistently, we validated that rIxsS17 dose dependently blocked deposition of the complement membrane attack complex via the lectin complement pathway and protected complement sensitive B. burgdorferi from complement-mediated killing. Likewise, co-inoculating C3H/HeN mice with rIxsS17 and B. burgdorferi significantly enhanced colonization of mouse heart and skin organs in a reverse dose dependent manner. Taken together, our data suggests an important role for IxsS17 in tick feeding and B. burgdorferi colonization of the host.

Ixodes scapularis nymph saliva protein blocks host inflammation and complement-mediated killing of Lyme disease agent, Borrelia burgdorferi

Tick serine protease inhibitors (serpins) play crucial roles in tick feeding and pathogen transmission. We demonstrate that Ixodes scapularis (Ixs) nymph tick saliva serpin (S) 41 (IxsS41), secreted by Borrelia burgdorferi (Bb)-infected ticks at high abundance, is involved in regulating tick evasion of host innate immunity and promoting host colonization by Bb. Recombinant (r) proteins were expressed in Pichia pastoris, and substrate hydrolysis assays were used to determine. Ex vivo (complement and hemostasis function related) and in vivo (paw edema and effect on Bb colonization of C3H/HeN mice organs) assays were conducted to validate function. We demonstrate that rIxsS41 inhibits chymase and cathepsin G, pro-inflammatory proteases that are released by mast cells and neutrophils, the first immune cells at the tick feeding site. Importantly, stoichiometry of inhibition analysis revealed that 2.2 and 2.8 molecules of rIxsS41 are needed to 100% inhibit 1 molecule of chymase and cathepsin G, respectively, suggesting that findings here are likely events at the tick feeding site. Furthermore, chymase-mediated paw edema, induced by the mast cell degranulator, compound 48/80 (C48/80), was blocked by rIxsS41. Likewise, rIxsS41 reduced membrane attack complex (MAC) deposition via the alternative and lectin complement activation pathways and dose-dependently protected Bb from complement killing. Additionally, co-inoculating C3H/HeN mice with Bb together with rIxsS41 or with a mixture (rIxsS41 and C48/80). Findings in this study suggest that IxsS41 markedly contributes to tick feeding and host colonization by Bb. Therefore, we conclude that IxsS41 is a potential candidate for an anti-tick vaccine to prevent transmission of the Lyme disease agent.

Transcriptome analysis of Haemaphysalis flava female using Illumina HiSeq 4000 sequencing: de novo assembly, functional annotation and discovery of SSR markers

BACKGROUND

Ticks are ectoparasites capable of directly damaging their hosts and transmitting vector-borne diseases. The ixodid tick Haemaphysalis flava has a broad distribution that extends from East to South Asia. This tick is a reservoir of severe fever with thrombocytopenia syndrome virus (SFTSV) that causes severe hemorrhagic disease, with cases reported from China, Japan and South Korea. Recently, the distribution of H. flava in South Korea was found to overlap with the occurrence of SFTSV.

METHODS

This study was undertaken to discover the molecular resources of H. flava female ticks using the Illumina HiSeq 4000 system, the Trinity de novo sequence assembler and annotation against public databases. The locally curated Protostome database (PANM-DB) was used to screen the putative adaptation-related transcripts classified to gene families, such as angiotensin-converting enzyme, aquaporin, adenylate cyclase, AMP-activated protein kinase, glutamate receptors, heat shock proteins, molecular chaperones, insulin receptor, mitogen-activated protein kinase and solute carrier family proteins. Also, the repeats and simple sequence repeats (SSRs) were screened from the unigenes using RepeatMasker (v4.0.6) and MISA (v1.0) software tools, followed by the designing of SSRs flanking primers using BatchPrimer 3 (v1.0) software.

RESULTS

The transcriptome produced a total of 69,822 unigenes, of which 46,175 annotated to the homologous proteins in the PANM-DB. The unigenes were also mapped to the EuKaryotic Orthologous Groups (KOG), Kyoto Encyclopedia of Genes and Genomes (KEGG) and Gene Ontology (GO) specializations. Promiscuous presence of protein kinase, zinc finger (C2H2-type), reverse transcriptase, and RNA recognition motif domains was observed in the unigenes. A total of 3480 SSRs were screened, of which 1907 and 1274 were found as tri- and dinucleotide repeats, respectively. A list of primer sequences flanking the SSR motifs was detailed for validation of polymorphism in H. flava and the related tick species.

CONCLUSIONS

The reference transcriptome information on H. flava female ticks will be useful for an enriched understanding of tick biology, its competency to act as a vector and the study of species diversity related to disease transmission.

Guianensin, a Simulium guianense salivary protein, has broad anti-hemostatic and anti-inflammatory properties

Background

Salivary glands from blood-feeding arthropods secrete several molecules that inhibit mammalian hemostasis and facilitate blood feeding and pathogen transmission. The salivary functions from Simulium guianense, the main vector of Onchocerciasis in South America, remain largely understudied. Here, we have characterized a salivary protease inhibitor (Guianensin) from the blackfly Simulium guianense.

Materials and methods

A combination of bioinformatic and biophysical analyses, recombinant protein production, in vitro and in vivo experiments were utilized to characterize the molecula mechanism of action of Guianensin. Kinetics of Guianensin interaction with proteases involved in vertebrate inflammation and coagulation were carried out by surface plasmon resonance and isothermal titration calorimetry. Plasma recalcification and coagulometry and tail bleeding assays were performed to understand the role of Guianensin in coagulation.

Results

Guianensin was identified in the sialotranscriptome of adult S. guianense flies and belongs to the Kunitz domain of protease inhibitors. It targets various serine proteases involved in hemostasis and inflammation. Binding to these enzymes is highly specific to the catalytic site and is not detectable for their zymogens, the catalytic site-blocked human coagulation factor Xa (FXa), or thrombin. Accordingly, Guianensin significantly increased both PT (Prothrombin time) and aPTT (Activated partial thromboplastin time) in human plasma and consequently increased blood clotting time ex vivo. Guianensin also inhibited prothrombinase activity on endothelial cells. We show that Guianensin acts as a potent anti-inflammatory molecule on FXa-induced paw edema formation in mice.

Conclusion

The information generated by this work highlights the biological functionality of Guianensin as an antithrombotic and anti-inflammatory protein that may play significant roles in blood feeding and pathogen transmission.

Iripin-1, a new anti-inflammatory tick serpin, inhibits leukocyte recruitment in vivo while altering the levels of chemokines and adhesion molecules

Serpins are widely distributed and functionally diverse inhibitors of serine proteases. Ticks secrete serpins with anti-coagulation, anti-inflammatory, and immunomodulatory activities via their saliva into the feeding cavity to modulate host's hemostatic and immune reaction initiated by the insertion of tick's mouthparts into skin. The suppression of the host's immune response not only allows ticks to feed on a host for several days but also creates favorable conditions for the transmission of tick-borne pathogens. Herein we present the functional and structural characterization of Iripin-1 (Ixodes ricinus serpin-1), whose expression was detected in the salivary glands of the tick Ixodes ricinus, a European vector of tick-borne encephalitis and Lyme disease. Of 16 selected serine proteases, Iripin-1 inhibited primarily trypsin and further exhibited weaker inhibitory activity against kallikrein, matriptase, and plasmin. In the mouse model of acute peritonitis, Iripin-1 enhanced the production of the anti-inflammatory cytokine IL-10 and chemokines involved in neutrophil and monocyte recruitment, including MCP-1/CCL2, a potent histamine-releasing factor. Despite increased chemokine levels, the migration of neutrophils and monocytes to inflamed peritoneal cavities was significantly attenuated following Iripin-1 administration. Based on the results of in vitro experiments, immune cell recruitment might be inhibited due to Iripin-1-mediated reduction of the expression of chemokine receptors in neutrophils and adhesion molecules in endothelial cells. Decreased activity of serine proteases in the presence of Iripin-1 could further impede cell migration to the site of inflammation. Finally, we determined the tertiary structure of native Iripin-1 at 2.10 Å resolution by employing the X-ray crystallography technique. In conclusion, our data indicate that Iripin-1 facilitates I. ricinus feeding by attenuating the host's inflammatory response at the tick attachment site.

Tick Salivary Kunitz-Type Inhibitors: Targeting Host Hemostasis and Immunity to Mediate Successful Blood Feeding

Kunitz domain-containing proteins are ubiquitous serine protease inhibitors with promising therapeutic potential. They target key proteases involved in major cellular processes such as inflammation or hemostasis through competitive inhibition in a substrate-like manner. Protease inhibitors from the Kunitz superfamily have a low molecular weight (18-24 kDa) and are characterized by the presence of one or more Kunitz motifs consisting of α-helices and antiparallel β-sheets stabilized by three disulfide bonds. Kunitz-type inhibitors are an important fraction of the protease inhibitors found in tick saliva. Their roles in inhibiting and/or suppressing host homeostatic responses continue to be shown to be additive or synergistic with other protease inhibitors such as cystatins or serpins, ultimately mediating successful blood feeding for the tick. In this review, we discuss the biochemical features of tick salivary Kunitz-type protease inhibitors. We focus on their various effects on host hemostasis and immunity at the molecular and cellular level and their potential therapeutic applications. In doing so, we highlight that their pharmacological properties can be exploited for the development of novel therapies and vaccines.

Mast Cell and Basophil Granule Proteases - In Vivo Targets and Function

Proteases are stored in very large amounts within abundant cytoplasmic granules of mast cells (MCs), and in lower amounts in basophils. These proteases are stored in their active form in complex with negatively charged proteoglycans, such as heparin and chondroitin sulfate, ready for rapid release upon MC and basophil activation. The absolute majority of these proteases belong to the large family of chymotrypsin related serine proteases. Three such enzymes are found in human MCs, a chymotryptic enzyme, the chymase, a tryptic enzyme, the tryptase and cathepsin G. Cathepsin G has in primates both chymase and tryptase activity. MCs also express a MC specific exopeptidase, carboxypeptidase A3 (CPA3). The targets and thereby the functions of these enzymes have for many years been the major question of the field. However, the fact that some of these enzymes have a relatively broad specificity has made it difficult to obtain reliable information about the biologically most important targets for these enzymes. Under optimal conditions they may cleave a relatively large number of potential targets. Three of these enzymes, the chymase, the tryptase and CPA3, have been shown to inactivate several venoms from snakes, scorpions, bees and Gila monster. The chymase has also been shown to cleave several connective tissue components and thereby to be an important player in connective tissue homeostasis. This enzyme can also generate angiotensin II (Ang II) by cleavage of Ang I and have thereby a role in blood pressure regulation. It also display anticoagulant activity by cleaving fibrinogen and thrombin. A regulatory function on excessive TH2 immunity has also been observed for both the chymase and the tryptase by cleavage of a highly selective set of cytokines and chemokines. The chymase also appear to have a protective role against ectoparasites such as ticks, mosquitos and leeches by the cleavage of their anticoagulant proteins. We here review the data that has accumulated concerning the potential in vivo functions of these enzymes and we discuss how this information sheds new light on the role of MCs and basophils in health and disease.

Serpins in Tick Physiology and Tick-Host Interaction

Tick saliva has been extensively studied in the context of tick-host interactions because it is involved in host homeostasis modulation and microbial pathogen transmission to the host. Accumulated knowledge about the tick saliva composition at the molecular level has revealed that serine protease inhibitors play a key role in the tick-host interaction. Serpins are one highly expressed group of protease inhibitors in tick salivary glands, their expression can be induced during tick blood-feeding, and they have many biological functions at the tick-host interface. Indeed, tick serpins have an important role in inhibiting host hemostatic processes and in the modulation of the innate and adaptive immune responses of their vertebrate hosts. Tick serpins have also been studied as potential candidates for therapeutic use and vaccine development. In this review, we critically summarize the current state of knowledge about the biological role of tick serpins in shaping tick-host interactions with emphasis on the mechanisms by which they modulate host immunity. Their potential use in drug and vaccine development is also discussed.

Host Immune Responses to Salivary Components - A Critical Facet of Tick-Host Interactions

Tick sialome is comprised of a rich cocktail of bioactive molecules that function as a tool to disarm host immunity, assist blood-feeding, and play a vibrant role in pathogen transmission. The adaptation of the tick's blood-feeding behavior has lead to the evolution of bioactive molecules in its saliva to assist them to overwhelm hosts' defense mechanisms. During a blood meal, a tick secretes different salivary molecules including vasodilators, platelet aggregation inhibitors, anticoagulants, anti-inflammatory proteins, and inhibitors of complement activation; the salivary repertoire changes to meet various needs such as tick attachment, feeding, and modulation or impairment of the local dynamic and vigorous host responses. For instance, the tick's salivary immunomodulatory and cement proteins facilitate the tick's attachment to the host to enhance prolonged blood-feeding and to modulate the host's innate and adaptive immune responses. Recent advances implemented in the field of "omics" have substantially assisted our understanding of host immune modulation and immune inhibition against the molecular dynamics of tick salivary molecules in a crosstalk between the tick-host interface. A deep understanding of the tick salivary molecules, their substantial roles in multifactorial immunological cascades, variations in secretion, and host immune responses against these molecules is necessary to control these parasites. In this article, we reviewed updated knowledge about the molecular mechanisms underlying host responses to diverse elements in tick saliva throughout tick invasion, as well as host defense strategies. In conclusion, understanding the mechanisms involved in the complex interactions between the tick salivary components and host responses is essential to decipher the host defense mechanisms against the tick evasion strategies at tick-host interface which is promising in the development of effective anti-tick vaccines and drug therapeutics.

rDromaserpin: A Novel Anti-Hemostatic Serpin, from the Salivary Glands of the Hard Tick Hyalomma dromedarii

Hemostatic disorders are caused either by platelet-related dysfunctions, defective blood coagulation, or by a combination of both, leading to an increased susceptibility to cardiovascular diseases (CVD) and other related illnesses. The unique specificity of anticoagulants from hematophagous arthropods, such as ticks, suggests that tick saliva holds great promise for discovering new treatments for these life-threatening diseases. In this study, we combined in silico and in vitro analyses to characterize the first recombinant serpin, herein called Dromaserpin, from the sialotranscriptome of the Hyalomma dromedarii tick. Our in silico data described Dromaserpin as a secreted protein of ~43 kDa with high similarities to previously characterized inhibitory serpins. The recombinant protein (rDromaserpin) was obtained as a well-structured monomer, which was tested using global blood coagulation and platelet aggregation assays. With this approach, we confirmed rDromaserpin anticoagulant activity as it significantly delayed plasma clotting in activated partial thromboplastin time and thrombin time assays. The profiling of proteolytic activity shows its capacity to inhibit thrombin in the micromolar range (0.2 to 1 μM) and in the presence of heparin this inhibition was clearly increased. It was also able to inhibit Kallikrein, FXIa and slightly FXIIa, with no significant effect on other factors. In addition, the rDromaserpin inhibited thrombin-induced platelet aggregation. Taken together, our data suggest that rDromaserpin deserves to be further investigated as a potential candidate for developing therapeutic compounds targeting disorders related to blood clotting and/or platelet aggregation.

Mast Cells and Basophils in the Defense against Ectoparasites: Efficient Degradation of Parasite Anticoagulants by the Connective Tissue Mast Cell Chymases

Ticks, lice, flees, mosquitos, leeches and vampire bats need to prevent the host's blood coagulation during their feeding process. This is primarily achieved by injecting potent anticoagulant proteins. Basophils frequently accumulate at the site of tick feeding. However, this occurs only after the second encounter with the parasite involving an adaptive immune response and IgE. To study the potential role of basophils and mast cells in the defense against ticks and other ectoparasites, we produced anticoagulant proteins from three blood-feeding animals; tick, mosquito, and leech. We tested these anticoagulant proteins for their sensitivity to inactivation by a panel of hematopoietic serine proteases. The majority of the connective tissue mast cell proteases tested, originating from humans, dogs, rats, hamsters, and opossums, efficiently cleaved these anticoagulant proteins. Interestingly, the mucosal mast cell proteases that contain closely similar cleavage specificity, had little effect on these anticoagulant proteins. Ticks have been shown to produce serpins, serine protease inhibitors, upon a blood meal that efficiently inhibit the human mast cell chymase and cathepsin G, indicating that ticks have developed a strategy to inactivate these proteases. We show here that one of these tick serpins (IRS-2) shows broad activity against the majority of the mast cell chymotryptic enzymes and the neutrophil proteases from human to opossum. However, it had no effect on the mast cell tryptases or the basophil specific protease mMCP-8. The production of anticoagulants, proteases and anti-proteases by the parasite and the host presents a fascinating example of an arms race between the blood-feeding animals and the mammalian immune system with an apparent and potent role of the connective tissue mast cell chymases in the host defense.

Immunomodulatory Proteins in Tick Saliva From a Structural Perspective

To feed successfully, ticks must bypass or suppress the host’s defense mechanisms, particularly the immune system. To accomplish this, ticks secrete specialized immunomodulatory proteins into their saliva, just like many other blood-sucking parasites. However, the strategy of ticks is rather unique compared to their counterparts. Ticks’ tendency for gene duplication has led to a diverse arsenal of dozens of closely related proteins from several classes to modulate the immune system’s response. Among these are chemokine-binding proteins, complement pathways inhibitors, ion channels modulators, and numerous poorly characterized proteins whose functions are yet to be uncovered. Studying tick immunomodulatory proteins would not only help to elucidate tick-host relationships but would also provide a rich pool of potential candidates for the development of immunomodulatory intervention drugs and potentially new vaccines. In the present review, we will attempt to summarize novel findings on the salivary immunomodulatory proteins of ticks, focusing on biomolecular targets, structure-activity relationships, and the perspective of their development into therapeutics.

Structural and biochemical characterization of the novel serpin Iripin-5 from Ixodes ricinus

Iripin-5 is the main Ixodes ricinus salivary serpin, which acts as a modulator of host defence mechanisms by impairing neutrophil migration, suppressing nitric oxide production by macrophages and altering complement functions. Iripin-5 influences host immunity and shows high expression in the salivary glands. Here, the crystal structure of Iripin-5 in the most thermodynamically stable state of serpins is described. In the reactive-centre loop, the main substrate-recognition site of Iripin-5 is likely to be represented by Arg342, which implies the targeting of trypsin-like proteases. Furthermore, a computational structural analysis of selected Iripin-5-protease complexes together with interface analysis revealed the most probable residues of Iripin-5 involved in complex formation.

Ixodes ricinus Salivary Serpin Iripin-8 Inhibits the Intrinsic Pathway of Coagulation and Complement

Tick saliva is a rich source of antihemostatic, anti-inflammatory, and immunomodulatory molecules that actively help the tick to finish its blood meal. Moreover, these molecules facilitate the transmission of tick-borne pathogens. Here we present the functional and structural characterization of Iripin-8, a salivary serpin from the tick Ixodes ricinus, a European vector of tick-borne encephalitis and Lyme disease. Iripin-8 displayed blood-meal-induced mRNA expression that peaked in nymphs and the salivary glands of adult females. Iripin-8 inhibited multiple proteases involved in blood coagulation and blocked the intrinsic and common pathways of the coagulation cascade in vitro. Moreover, Iripin-8 inhibited erythrocyte lysis by complement, and Iripin-8 knockdown by RNA interference in tick nymphs delayed the feeding time. Finally, we resolved the crystal structure of Iripin-8 at 1.89 Å resolution to reveal an unusually long and rigid reactive center loop that is conserved in several tick species. The P1 Arg residue is held in place distant from the serpin body by a conserved poly-Pro element on the P′ side. Several PEG molecules bind to Iripin-8, including one in a deep cavity, perhaps indicating the presence of a small-molecule binding site. This is the first crystal structure of a tick serpin in the native state, and Iripin-8 is a tick serpin with a conserved reactive center loop that possesses antihemostatic activity that may mediate interference with host innate immunity.

Transcriptomic Analysis of Salivary Glands of Ornithodoros brasiliensis Aragão, 1923, the Agent of a Neotropical Tick-Toxicosis Syndrome in Humans

Tick salivary glands produce and secrete a variety of compounds that modulate host responses and ensure a successful blood meal. Despite great progress made in the identification of ticks salivary compounds in recent years, there is still a paucity of information concerning salivary molecules of Neotropical argasid ticks. Among this group of ticks, considering the number of human cases of parasitism, including severe syndromes and hospitalization, Ornithodoros brasiliensis can be considered one of the major Neotropical argasid species with impact in public health. Here, we describe the transcriptome analysis of O. brasiliensis salivary glands (ObSG). The transcriptome yielded ~14,957 putative contigs. A total of 368 contigs were attributed to secreted proteins (SP), which represent approximately 2.5% of transcripts but ~53% expression coverage transcripts per million. Lipocalins are the major protein family among the most expressed SP, accounting for ~16% of the secretory transcripts and 51% of secretory protein abundance. The most expressed transcript is an ortholog of TSGP4 (tick salivary gland protein 4), a lipocalin first identified in Ornithodoros kalahariensis that functions as a leukotriene C₄ scavenger. A total of 55 lipocalin transcripts were identified in ObSG. Other transcripts potentially involved in tick-host interaction included as: basic/acid tail secretory proteins (second most abundant expressed group), serine protease inhibitors (including Kunitz inhibitors), 5' nucleotidases (tick apyrases), phospholipase A₂, 7 disulfide bond domain, cystatins, and tick antimicrobial peptides. Another abundant group of proteins in ObSG is metalloproteases. Analysis of these major protein groups suggests that several duplication events after speciation were responsible for the abundance of redundant compounds in tick salivary glands. A full mitochondrial genome could be assembled from the transcriptome data and confirmed the close genetic identity of the tick strain sampled in the current study, to a tick strain previously implicated in tick toxicoses. This study provides novel information on the molecular composition of ObSG, a Brazilian endemic tick associated with several human cases of parasitism. These results could be helpful in the understanding of clinical findings observed in bitten patients, and also, could provide more information on the evolution of Neotropical argasids.

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.

Tick host immunity: vector immunomodulation and acquired tick resistance

Ticks have an unparalleled ability to parasitize diverse land vertebrates. Their natural persistence and vector competence are supported by the evolution of sophisticated hematophagy and remarkable host immune-evasion activities. We analyze the immunomodulatory roles of tick saliva which facilitates their acquisition of a blood meal from natural hosts and allows pathogen transmission. We also discuss the contrasting immunological events of tick-host associations in non-reservoir or incidental hosts, in which the development of acquired tick resistance can deter tick attachment. A critical appraisal of the intricate immunobiology of tick-host associations can plant new seeds of innovative research and contribute to the development of novel preventive strategies against ticks and tick-transmitted infections.

Virulence potential of Rickettsia amblyommatis for spotted fever pathogenesis in mice

Rickettsia amblyommatis belongs to the spotted fever group of Rickettsia and infects Amblyomma americanum (Lone Star ticks) for transmission to offspring and mammals. Historically, the geographic range of A. americanum was restricted to the southeastern USA. However, recent tick surveys identified the progressive northward invasion of A. americanum, contributing to the increased number of patients with febrile illnesses of unknown etiology after a tick bite in the northeastern USA. While serological evidence strongly suggests that patients are infected with R. amblyommatis, the virulence potential of R. amblyommatis is not well established. Here, we performed a bioinformatic analysis of three genome sequences of R. amblyommatis and identified the presence of multiple putative virulence genes whose products are implicated for spotted fever pathogenesis. Similar to other pathogenic spotted fever rickettsiae, R. amblyommatis replicated intracellularly within the cytoplasm of tissue culture cells. Interestingly, R. amblyommatis displayed defective attachment to microvascular endothelial cells. The attachment defect and slow growth rate of R. amblyommatis required relatively high intravenous infectious doses to produce dose-dependent morbidity and mortality in C3H mice. In summary, our results corroborate clinical evidence that R. amblyommatis can cause mild disease manifestation in some patients.

Molecular communication between the monogenea and fish immune system

Monogeneans parasitise mainly the outer structures of fish, such as the gills, fins, and skin, that is, tissues covered with a mucous layer. While attached by sclerotised structures to host's surface, monogeneans feed on its blood or epidermal cells and mucus. Besides being a rich source of nutrients, these tissues also contain humoral immune factors and immune cells, which are ready to launch defence mechanisms against the tegument or gastrointestinal tract of these invaders. The exploitation of hosts' resources by the Monogenea must, therefore, be accompanied by suppressive and immunomodulatory mechanisms which protect the parasites against attacks by host immune system. Elimination of hosts' cytotoxic molecules and evasion of host immune response is often mediated by proteins secreted by the parasites. The aim of this review is to summarise existing knowledge on fish immune responses against monogeneans. Results gleaned from experimental infections illustrate the various interactions between parasites and the innate and adaptive immune system of the fish. The involvement of monogenean molecules (mainly inhibitors of peptidases) in molecular communication with host immune system is discussed.

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.

A Comprehensive Phylogenetic Analysis of the Serpin Superfamily

Serine protease inhibitors (serpins) are found in all kingdoms of life and play essential roles in multiple physiological processes. Owing to the diversity of the superfamily, phylogenetic analysis is challenging and prokaryotic serpins have been speculated to have been acquired from Metazoa through horizontal gene transfer due to their unexpectedly high homology. Here, we have leveraged a structural alignment of diverse serpins to generate a comprehensive 6,000-sequence phylogeny that encompasses serpins from all kingdoms of life. We show that in addition to a central “hub” of highly conserved serpins, there has been extensive diversification of the superfamily into many novel functional clades. Our analysis indicates that the hub proteins are ancient and are similar because of convergent evolution, rather than the alternative hypothesis of horizontal gene transfer. This work clarifies longstanding questions in the evolution of serpins and provides new directions for research in the field of serpin biology.

Borrelia burgdorferi infection modifies protein content in saliva of Ixodes scapularis nymphs

BACKGROUND

Lyme disease (LD) caused by Borrelia burgdorferi is the most prevalent tick-borne disease. There is evidence that vaccines based on tick proteins that promote tick transmission of B. burgdorferi could prevent LD. As Ixodes scapularis nymph tick bites are responsible for most LD cases, this study sought to identify nymph tick saliva proteins associated with B. burgdorferi transmission using LC-MS/MS. Tick saliva was collected using a non-invasive method of stimulating ticks (uninfected and infected: unfed, and every 12 h during feeding through 72 h, and fully-fed) to salivate into 2% pilocarpine-PBS for protein identification using LC-MS/MS.

RESULTS

We identified a combined 747 tick saliva proteins of uninfected and B. burgdorferi infected ticks that were classified into 25 functional categories: housekeeping-like (48%), unknown function (18%), protease inhibitors (9%), immune-related (6%), proteases (8%), extracellular matrix (7%), and small categories that account for <5% each. Notably, B. burgdorferi infected ticks secreted high number of saliva proteins (n=645) than uninfected ticks (n=376). Counter-intuitively, antimicrobial peptides, which function to block bacterial infection at tick feeding site were suppressed 23-85 folds in B. burgdorferi infected ticks. Similar to glycolysis enzymes being enhanced in mammalian cells exposed to B. burgdorferi : eight of the 10-glycolysis pathway enzymes were secreted at high abundance by B. burgdorferi infected ticks. Of significance, rabbits exposed to B. burgdorferi infected ticks acquired potent immunity that caused 40-60% mortality of B. burgdorferi infected ticks during the second infestation compared to 15-28% for the uninfected. This might be explained by ELISA data that show that high expression levels of immunogenic proteins in B. burgdorferi infected ticks.

CONCLUSION

Data here suggest that B. burgdorferi infection modified protein content in tick saliva to promote its survival at the tick feeding site. For instance, enzymes; copper/zinc superoxide dismutase that led to production of H2O2 that is toxic to B. burgdorferi were suppressed, while, catalase and thioredoxin that neutralize H2O2, and pyruvate kinase which yields pyruvate that protects Bb from H2O2 killing were enhanced. We conclude data here is an important resource for discovery of effective antigens for a vaccine to prevent LD.

Iripin-3, a New Salivary Protein Isolated From Ixodes ricinus Ticks, Displays Immunomodulatory and Anti-Hemostatic Properties In Vitro

Tick saliva is a rich source of pharmacologically and immunologically active molecules. These salivary components are indispensable for successful blood feeding on vertebrate hosts and are believed to facilitate the transmission of tick-borne pathogens. Here we present the functional and structural characterization of Iripin-3, a protein expressed in the salivary glands of the tick Ixodes ricinus, a European vector of tick-borne encephalitis and Lyme disease. Belonging to the serpin superfamily of protease inhibitors, Iripin-3 strongly inhibited the proteolytic activity of serine proteases kallikrein and matriptase. In an in vitro setup, Iripin-3 was capable of modulating the adaptive immune response as evidenced by reduced survival of mouse splenocytes, impaired proliferation of CD4+ T lymphocytes, suppression of the T helper type 1 immune response, and induction of regulatory T cell differentiation. Apart from altering acquired immunity, Iripin-3 also inhibited the extrinsic blood coagulation pathway and reduced the production of pro-inflammatory cytokine interleukin-6 by lipopolysaccharide-stimulated bone marrow-derived macrophages. In addition to its functional characterization, we present the crystal structure of cleaved Iripin-3 at 1.95 Å resolution. Iripin-3 proved to be a pluripotent salivary serpin with immunomodulatory and anti-hemostatic properties that could facilitate tick feeding via the suppression of host anti-tick defenses. Physiological relevance of Iripin-3 activities observed in vitro needs to be supported by appropriate in vivo experiments.

Changing the Recipe: Pathogen Directed Changes in Tick Saliva Components

Ticks are obligate hematophagous parasites and are important vectors of a wide variety of pathogens. These pathogens include spirochetes in the genus Borrelia that cause Lyme disease, rickettsial pathogens, and tick-borne encephalitis virus, among others. Due to their prolonged feeding period of up to two weeks, hard ticks must counteract vertebrate host defense reactions in order to survive and reproduce. To overcome host defense mechanisms, ticks have evolved a large number of pharmacologically active molecules that are secreted in their saliva, which inhibits or modulates host immune defenses and wound healing responses upon injection into the bite site. These bioactive molecules in tick saliva can create a privileged environment in the host’s skin that tick-borne pathogens take advantage of. In fact, evidence is accumulating that tick-transmitted pathogens manipulate tick saliva composition to enhance their own survival, transmission, and evasion of host defenses. We review what is known about specific and functionally characterized tick saliva molecules in the context of tick infection with the genus Borrelia, the intracellular pathogen Anaplasma phagocytophilum, and tick-borne encephalitis virus. Additionally, we review studies analyzing sialome-level responses to pathogen challenge.

Insights into the Role of Tick Salivary Protease Inhibitors during Ectoparasite-Host Crosstalk

Protease inhibitors (PIs) are ubiquitous regulatory proteins present in all kingdoms. They play crucial tasks in controlling biological processes directed by proteases which, if not tightly regulated, can damage the host organism. PIs can be classified according to their targeted proteases or their mechanism of action. The functions of many PIs have now been characterized and are showing clinical relevance for the treatment of human diseases such as arthritis, hepatitis, cancer, AIDS, and cardiovascular diseases, amongst others. Other PIs have potential use in agriculture as insecticides, anti-fungal, and antibacterial agents. PIs from tick salivary glands are special due to their pharmacological properties and their high specificity, selectivity, and affinity to their target proteases at the tick-host interface. In this review, we discuss the structure and function of PIs in general and those PI superfamilies abundant in tick salivary glands to illustrate their possible practical applications. In doing so, we describe tick salivary PIs that are showing promise as drug candidates, highlighting the most promising ones tested in vivo and which are now progressing to preclinical and clinical trials.

De novo assembled salivary gland transcriptome and expression pattern analyses for Rhipicephalus evertsi evertsi Neuman, 1897 male and female ticks

Ticks secrete proteins in their saliva that change over the course of feeding to modulate the host inflammation, immune responses, haemostasis or may cause paralysis. RNA next generation sequencing technologies can reveal the complex dynamics of tick salivary glands as generated from various tick life stages and/or males and females. The current study represents 15,115 Illumina sequenced contigs of the salivary gland transcriptome from male and female Rhipicephalus evertsi evertsi ticks of early, mid and late feeding stages from 1320 separate assemblies using three short read assemblers. The housekeeping functional class contributed to the majority of the composition of the transcriptome (80%) but with lower expression (51%), while the secretory protein functional class represented only 14% of the transcriptome but 46% of the total coverage. Six percent had an unknown status contributing 3% of the overall expression in the salivary glands. Platelet aggregation inhibitors, blood clotting inhibitors and immune-modulators orthologous to the ancestral tick lineages were confirmed in the transcriptome and their differential expression during feeding in both genders observed. This transcriptome contributes data of importance to salivary gland biology and blood feeding physiology of non-model organisms.

Acquired tick resistance: The trail is hot

Acquired tick resistance is a phenomenon wherein the host elicits an immune response against tick salivary components upon repeated tick infestations. The immune responses, potentially directed against critical salivary components, thwart tick feeding, and the animal becomes resistant to subsequent tick infestations. The development of tick resistance is frequently observed when ticks feed on non-natural hosts, but not on natural hosts. The molecular mechanisms that lead to the development of tick resistance are not fully understood, and both host and tick factors are invoked in this phenomenon. Advances in molecular tools to address the host and the tick are beginning to reveal new insights into this phenomenon and to uncover a deeper understanding of the fundamental biology of tick-host interactions. This review will focus on the expanding understanding of acquired tick resistance and highlight the impact of this understanding on anti-tick vaccine development efforts.

Dendritic Cells as a Disputed Fortress on the Tick-Host Battlefield

From seminal publications in the early 1970s, the world learned that dendritic cells (DCs) are powerful and versatile antigen-presenting cells. It took a few years until the first studies expanded our understanding of the pivotal role of these immune 'soldiers' against ticks. Advances in biochemistry, molecular biology, and bioinformatics have shed light on the identification of key salivary molecules that modulate the biology of DCs in favor of tick parasitism. Here, we present a critical overview of the discoveries accumulated on the tick-host battlefield from a DC perspective. Moreover, the clinical significance of DC-targeted tick salivary components is discussed, not only as facilitators of the transmission of tick-borne pathogens or vaccine candidates, but also as potential immunobiologics to treat immune-mediated diseases.

Tick Salivary Compounds for Targeted Immunomodulatory Therapy

Immunodeficiency disorders and autoimmune diseases are common, but a lack of effective targeted drugs and the side-effects of existing drugs have stimulated interest in finding therapeutic alternatives. Naturally derived substances are a recognized source of novel drugs, and tick saliva is increasingly recognized as a rich source of bioactive molecules with specific functions. Ticks use their saliva to overcome the innate and adaptive host immune systems. Their saliva is a rich cocktail of molecules including proteins, peptides, lipid derivatives, and recently discovered non-coding RNAs that inhibit or modulate vertebrate immune reactions. A number of tick saliva and/or salivary gland molecules have been characterized and shown to be promising candidates for drug development for vertebrate immune diseases. However, further validation of these molecules at the molecular, cellular, and organism levels is now required to progress lead candidates to clinical testing. In this paper, we review the data on the immuno-pharmacological aspects of tick salivary compounds characterized in vitro and/or in vivo and present recent findings on non-coding RNAs that might be exploitable as immunomodulatory therapies.

Amblyomma americanum serpin 41 (AAS41) inhibits inflammation by targeting chymase and chymotrypsin

Ticks inject serine protease inhibitors (serpins) into their feeding sites to evade serine protease-mediated host defenses against tick-feeding. This study describes two highly identitical (97%) but functionally different Amblyomma americanum tick saliva serpins (AAS41 and 46) that are secreted at the inception of tick-feeding. We show that AAS41, which encodes a leucine at the P1 site inhibits inflammation system proteases: chymase (SI = 3.23, Ka = 5.6 ± 3.7X103M-1 s-1) and α-chymotrypsin (SI = 3.18, Ka = 1.6 ± 4.1X104M-1 s-1), while AAS46, which encodes threonine has no inhibitory activity. Similary, rAAS41 inhibits rMCP-1 purified from rat peritonuem derived mast cells. Consistently, rAAS41 inhibits chymase-mediated inflammation induced by compound 48/80 in rat paw edema and vascular permeability models. Native AAS41/46 proteins are among tick saliva immunogens that provoke anti-tick immunity in repeatedly infested animals as revealed by specific reactivity with tick immune sera. Of significance, native AAS41/46 play critical tick-feeding functions in that RNAi-mediated silencing caused ticks to ingest significantly less blood. Importantly, monospecific antibodies to rAAS41 blocked inhibitory functions of rAAS41, suggesting potential for design of vaccine antigens that provokes immunity to neutralize functions of this protein at the tick-feeding site. We discuss our findings with reference to tick-feeding physiology and discovery of effective tick vaccine antigens.

Rhipicephalus microplus serpins interfere with host immune responses by specifically modulating mast cells and lymphocytes

Rhipicephalus microplus ticks feed on a bovine host for three weeks. At the attachment site, inflammatory and immune responses are triggered resulting in the recruitment of cells and production of a set of immunological mediators. To oppose the host's immune responses, ticks inoculate bioactive salivary molecules capable of interfering with these defense mechanisms. Serpins are among the most frequent molecules present in tick saliva and have been shown to negatively affect the host's anti-tick immunity. R. microplus has at least eighteen full-length serpins (RmS) and eleven are transcribed during blood feeding. Among them, RmS-3, RmS-6, and RmS-17 are present in the saliva of engorged females. Here, the effect of these serpins on the immune responses was evaluated in cells involved in innate/inflammatory (mast cells and macrophages) and adaptive (T cells) immunity. RmS-3 modulated mast cells due to its inhibitory activity on peritoneal rat chymase and on vascular permeability in acute inflammation. In addition, both RmS-6 and RmS-17 inhibited vascular permeability. Of the three serpins studied, neither affected activation nor inflammatory cytokine production by murine macrophages. On the other hand, RmS-3 and RmS-17 presented an inhibitory effect on the metabolic activity of lymphocytes, with the latter being the most potent, while RmS-6 had no effect on it. This activity was associated with a decrease in lymphocyte proliferation, but not with induction of cell death. The present study highlights the powerful modulatory role of tick salivary serpins in the host's immune system and inspire the discovery of targets for the treatment of inflammatory/immune disorders.

A family of serine protease inhibitors (serpins) and its expression profiles in the ovaries of Rhipicephalus haemaphysaloides

Serpins are evolutionarily conserved serine protease inhibitors found in many organisms. In arthropods, serpins are involved in feeding, development, oviposition, anti-coagulation and innate immune responses. We characterized of 11 serpins in the tick Rhipicephalus haemaphysaloides. These serpins have orthologous genes in other ticks, as indicated by phylogenetic analysis. Analysis of the reactive center loop and hinge regions of the protein sequences indicated that RHS7 encodes proteins that may lack proteinase inhibitor activity. All R. haemaphysaloides serpins had high amino acid sequence identities to Rhipicephalus microplus serpins. Tissue and temporal transcriptional profiling of eight R. haemaphysaloides serpins located in the ovaries demonstrated that they are transcribed during feeding and oviposition. These suggested their participation in the regulation of tick physiology. Immune serum from rabbits repeatedly infested with larvae, nymphs and adults of R. haemaphysaloides can recognize multiple recombinant serpins, respectively. After gene silencing, the blood feeding to repletion time was significantly longer and the 24 h attachment rate was significantly lower in the RHS3 and RHS7 knock down groups. The RHS9 and RHS11 silenced ticks had significant reduction in repletion time and egg-laying rate. Egg hatchability was significantly decreased in RHS4, RHS5 and RHS9 silenced ticks. All groups had significant reductions in engorged body weight. This study increases information on the serpins of R. haemaphysaloides and suggests that some RHSs are potential targets for development of tick vaccines.

A serpin from the tick Rhipicephalus haemaphysaloides: Involvement in vitellogenesis

Tick serpins are involved in enzyme activity, food digestion, blood-feeding, immune response and anticoagulation. Little is known about the potential roles of serpins in tick reproduction. RHS8, a serpin from the tick Rhipicephalus haemaphysaloides, has an open reading frame 1212 bp long and encodes a protein that has 404 amino acids and a predicted molecular weight of 45 kDa. RHS8 exhibits 89.58 % amino acid identity with RmS15 in Rhipicephalus microplus. RHS8 was expressed primarily in larvae and nymphs. RHS8 mRNA expression in the ovaries, fat bodies and salivary glands were up-regulated from feeding to ovipositing ticks. RNAi results showed that RHS8 dsRNA-injected ticks had a lower body weight, longer feeding time, fewer eggs laid and lower egg hatchability. Tick reproduction, such as egg laying and hatching, was disrupted by RNAi. Compared with the control group, ovaries of the RHS8 interference group were light brown color, indicating a reduction in yolk granule accumulation. Western blot results showed that the expression of RHVg3 and RHVg4 proteins in ovaries was reduced in the RHS8 dsRNA-injected group. These results indicate that RHS8 is related to tick reproduction and its interference affects vitellogenesis.

Identification of proteins from the secretory/excretory products (SEPs) of the branchiuran ectoparasite Argulus foliaceus (Linnaeus, 1758) reveals unique secreted proteins amongst haematophagous ecdysozoa

BACKGROUND

It is hypothesised that being a blood-feeding ectoparasite, Argulus foliaceus (Linnaeus, 1758), uses similar mechanisms for digestion and host immune evasion to those used by other haematophagous ecdysozoa, including caligid copepods (e.g. sea louse). We recently described and characterised glands associated with the feeding appendages of A. foliaceus using histological techniques. The work described in the present study is the first undertaken with the objective of identifying and partially characterising the components secreted from these glands using a proteomic approach.

METHODS

Argulus foliaceus parasites were sampled from the skin of rainbow trout (Oncorhynchus mykiss), from Loch Fad on the Isle of Bute, Scotland, UK. The proteins from A. foliaceus secretory/excretory products (SEPs) were collected from the supernatant of artificial freshwater conditioned with active adult parasites (n = 5-9 per ml; n = 560 total). Proteins within the SEPs were identified and characterised using LC-ESI-MS/MS analysis. Data are available via ProteomeXchange with identifier PXD016226.

RESULTS

Data mining of a protein database translated from an A. foliaceus dataset using ProteinScape allowed identification of 27 predicted protein sequences from the A. foliaceus SEPs, each protein matching the criteria of 2 peptides with at least 4 contiguous amino acids. Nine proteins had no matching sequence through OmicsBox (Blast2GO) analysis searches suggesting that Argulus spp. may additionally have unique proteins present in their SEPs. SignalP 5.0 software, identified 13 proteins with a signal sequence suggestive of signal peptides and supportive of secreted proteins being identified. Notably, the functional characteristics of identified A. foliaceus proteins/domains have also been described from the salivary glands and saliva of other blood-feeding arthropods such as ticks. Identified proteins included: transporters, peroxidases, metalloproteases, proteases and serine protease inhibitors which are known to play roles in parasite immune evasion/induction (e.g. astacin), immunomodulation (e.g. serpin) and digestion (e.g. trypsin).

CONCLUSIONS

To our knowledge, the present study represents the first proteomic analysis undertaken for SEPs from any branchiuran fish louse. Here we reveal possible functional roles of A. foliaceus SEPs in digestion and immunomodulation, with a number of protein families shared with other haematophagous ectoparasites. A number of apparently unique secreted proteins were identified compared to other haematophagous ecdysozoa.

A survey of proteins in midgut contents of the tick, Haemaphysalis flava, by proteome and transcriptome analysis

Tick blood meals are stored and digested in their midguts. Blood digestion is complex, and many proteins are involved. Study of the tick-derived proteins in the midgut content may aid in the discovery of active molecules that would be useful for anti-tick vaccines. We analyzed the midgut content proteomes of partially engorged female Haemaphysalis flava, fully engorged female H. flava, and hedgehog serum using liquid chromatography tandem-mass spectrometry and label-free quantitation. In this study, high-confidence protein profiling of tick midgut content was determined. Based on the search against our in-house transcriptome database, the 28 high-confidence proteins were identified. Of these, 17 were identified as tick-derived, and the rest were of unspecified origin (proteins that could not be differentiated as host-derived or tick-derived proteins). The function of these midgut content proteins identified here may involve nutrient transportation, anti-coagulation, erythrocyte lysis, detoxification, lipid metabolism, and immunization. The presence of hemoglobin suggested that the red blood cells were lysed in the gut lumen. The midgut contents contain a large amount of fibrinogen and it has the ability to clot immediately. The midgut contained mostly host-derived proteins, and these host proteins provide rich nutrients for tick development and reproduction. However, some intracellular proteins were also identified, suggesting the possibility of shedding of the midgut epithelium and ingestion of saliva during feeding. This finding advances our understanding of the digestive mechanism and will be useful in the screening of vaccine antigens.

Novel targets and strategies to combat borreliosis

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

The immunosuppressive functions of two novel tick serpins, HlSerpin-a and HlSerpin-b, from Haemaphysalis longicornis

Serpins are evolutionarily conserved serine protease inhibitors that are widely distributed in animals, plants and microbes. In this study, we reported the cloning and functional characterizations of two novel serpin genes, HlSerpin-a and HlSerpin-b, from the hard tick Haemaphysalis longicornis of China. Recombinant HlSerpin-a and HlSerpin-b displayed protease inhibitory activities against multiple mammalian proteases. Similar to other tick serpins, HlSerpin-a and HlSerpin-b suppressed the expression of inflammatory cytokines such as TNF-α, interleukin (IL)-6 and IL-1β from lipopolysaccharide-stimulated mouse bone-marrow-derived macrophages (BMDMs) or mouse bone-marrow-derived dendritic cells (BMDCs). The minimum active region (reaction centre loop) of HlSerpin-a, named SA-RCL, showed similar biological activities as HlSerpin-a in the protease inhibition and immune suppression assays. The immunosuppressive activities of full-length HlSerpin-a and SA-RCL are impaired in Cathepsin G or Cathepsin B knockout mouse macrophages, suggesting that the immunomodulation functions of SA and SA-RCL are dependent on their protease inhibitory activity. Finally, we showed that both full-length HlSerpins and SA-RCL can relieve the joint swelling and inflammatory response in collagen-induced mouse arthritis models. These results suggested that HlSerpin-a and HlSerpin-b are two functional arthropod serpins, and the minimal reactive peptide SA-RCL is a potential candidate for drug development against inflammatory diseases.

Immunomodulatory effects of Rhipicephalus haemaphysaloides serpin RHS2 on host immune responses

Background

Rhipicephalus haemaphysaloides is a widespread tick species in China and other South East Asian countries, where it is the vector of many pathogens. The objective of this study was to study the role of serpin (serine protease inhibitor) during the tick-host interaction.

Methods

The differentiation of bone marrow-derived dendritic cells (BMDC) was induced in vitro, and the effect of RHS2 on the maturation of DCs was evaluated. The effects of RHS2 on T cell activation and cytotoxic T lymphocytes’ (CTLs) activity were analyzed by flow cytometry. Antibody subtypes after immunization of mice with RHS2 and OVA were determined.

Results

RHS2 can inhibit the differentiation of bone marrow-derived cells into DCs and promote their differentiation into macrophages. RHS2 can inhibit the maturation of DCs and the expression of CD80, CD86 and MHCII. The number of CD3⁺CD4⁺ and CD3⁺CD8⁺ T cells secreting IFN-γ, IL-2 and TNF-α was decreased, and the number of CD3⁺CD4⁺ T cells secreting IL-4 was increased, indicating that RHS2 can inhibit the activation of CD4 T cells and CD8 T cells, leading to inhibition of Th1 immune response. RHS2 inhibits the elimination of target cells by cytotoxic T lymphocytes. After immunization of mice with RHS2 and OVA, serum IgG2b was significantly reduced and IgM was increased.

Conclusions

The results show that RHS2 has an inhibitory effect on the host immune response. Ticks have evolved various ways to circumvent adaptive immunity. Their serpin inhibits BMDC differentiation to reduce immune responses.

Deciphering Biological Processes at the Tick-Host Interface Opens New Strategies for Treatment of Human Diseases

Ticks are obligatory blood-feeding ectoparasites, causing blood loss and skin damage in their hosts. In addition, ticks also transmit a number of various pathogenic microorganisms that cause serious diseases in humans and animals. Ticks evolved a wide array of salivary bioactive compounds that, upon injection into the host skin, inhibit or modulate host reactions such as hemostasis, inflammation and wound healing. Modulation of the tick attachment site in the host skin involves mainly molecules which affect physiological processes orchestrated by cytokines, chemokines and growth factors. Suppressing host defense reactions is crucial for tick survival and reproduction. Furthermore, pharmacologically active compounds in tick saliva have a promising therapeutic potential for treatment of some human diseases connected with disorders in hemostasis and immune system. These disorders are often associated to alterations in signaling pathways and dysregulation or overexpression of specific cytokines which, in turn, affect mechanisms of angiogenesis, cell motility and cytoskeletal regulation. Moreover, tick salivary molecules were found to exert cytotoxic and cytolytic effects on various tumor cells and have anti-angiogenic properties. Elucidation of the mode of action of tick bioactive molecules on the regulation of cell processes in their mammalian hosts could provide new tools for understanding the complex changes leading to immune disorders and cancer. Tick bioactive molecules may also be exploited as new pharmacological inhibitors of the signaling pathways of cytokines and thus help alleviate patient discomfort and increase patient survival. We review the current knowledge about tick salivary peptides and proteins that have been identified and functionally characterized in in vitro and/or in vivo models and their therapeutic perspective.

Brown Spider (Loxosceles) Venom Toxins as Potential Biotools for the Development of Novel Therapeutics

Brown spider envenomation results in dermonecrosis with gravitational spreading characterized by a marked inflammatory reaction and with lower prevalence of systemic manifestations such as renal failure and hematological disturbances. Several toxins make up the venom of these species, and they are mainly peptides and proteins ranging from 5–40 kDa. The venoms have three major families of toxins: phospholipases-D, astacin-like metalloproteases, and the inhibitor cystine knot (ICK) peptides. Serine proteases, serpins, hyaluronidases, venom allergens, and a translationally controlled tumor protein (TCTP) are also present. Toxins hold essential biological properties that enable interactions with a range of distinct molecular targets. Therefore, the application of toxins as research tools and clinical products motivates repurposing their uses of interest. This review aims to discuss possibilities for brown spider venom toxins as putative models for designing molecules likely for therapeutics based on the status quo of brown spider venoms. Herein, we explore new possibilities for the venom components in the context of their biochemical and biological features, likewise their cellular targets, three-dimensional structures, and mechanisms of action.

Dynamic and structural insights into tick serpin from Ixodes ricinus

Ixodid ticks have a crucial impact on people and domestic animals worldwide. These parasites also pose a serious threat to livestock. To date, vaccination of hosts against ticks is a safer, more sustainable alternative to chemical control of ticks and the disease agents they transmit. Because of their roles in tick physiology, serpins (serine protease inhibitors) from tick saliva are among the candidates for anti-tick vaccines. Inhibitory serpins employ a suicide inhibition mechanism to inhibit proteases, where the serpin reactive centre loop (RCL) is cleaved, by the targeted protease, and then inserted into the main β-sheet of the serpin. This causes a massive conformational change called the 'stressed to relaxed' (S→R) transition, leading to the breakdown of serpin into two regions (core domain and cleaved polypeptide). Recently, the first tick serpin crystal structure from Ixodes ricinus in R-state was reported. We thus employed molecular dynamics simulations to better understand serpin structure and dynamics in atomic detail. Overall, R-state serpin showed high rigidity, especially the core domain. The most flexible region is the terminal of the cleaved polypeptide, due to its high-water exposure, while the rest of the cleaved polypeptide is stably trapped behind the core domain. T363, D367 and N375 are found to play a vital role in protein-protein attachment. This finding can be used to explain the high stability of the R-state serpin at the atomic level and provides insight into this tick serpin which will be useful for rational anti-tick vaccine development. AbbreviationsMDMolecular DynamicsRCLReactive centre loopCommunicated by Ramaswamy H. Sarma.

Chemical Equilibrium at the Tick-Host Feeding Interface:A Critical Examination of Biological Relevance in Hematophagous Behavior

Ticks secrete hundreds to thousands of proteins into the feeding site, that presumably all play important functions in the modulation of host defense mechanisms. The current review considers the assumption that tick proteins have functional relevance during feeding. The feeding site may be described as a closed system and could be treated as an ideal equilibrium system, thereby allowing modeling of tick-host interactions in an equilibrium state. In this equilibrium state, the concentration of host and tick proteins and their affinities will determine functional relevance at the tick-host interface. Using this approach, many characterized tick proteins may have functional relevant concentrations and affinities at the feeding site. Conversely, the feeding site is not an ideal closed system, but is dynamic and changing, leading to possible overestimation of tick protein concentration at the feeding site and consequently an overestimation of functional relevance. Ticks have evolved different possible strategies to deal with this dynamic environment and overcome the barrier that equilibrium kinetics poses to tick feeding. Even so, cognisance of the limitations that equilibrium binding place on deductions of functional relevance should serve as an important incentive to determine both the concentration and affinity of tick proteins proposed to be functional at the feeding site.

Cathepsin L-a novel cysteine protease from Haemaphysalis flava Neumann, 1897

Ixodid ticks are ectoparasites responsible for the transmission of a large number of bacterial, viral, and protozoan pathogens to animals and humans. As long-term blood-pool feeders, the digestion of host blood is critical to their development as well as to the establishment of the sexual cycle of hemoparasites such as Babesia parasites, the agents of human and animal babesiosis. Previous studies have demonstrated that cysteine proteases are involved in blood digestion, embryogenesis, and pathogen transmission in other species of ticks, but their characteristics and functions are still unidentified in Haemaphysalis flava. Here, we describe the characterization of a cysteine protease HfCL from H. flava. We show that HfCL belongs to the L-like papain family of proteases, exhibits high expression in nymphs and adults, and localizes to both the midgut and salivary glands. Biochemical assays using purified recombinant enzyme reveal that rHfCL can hydrolyze the fluorogenic substrate Z-phe-Arg-MCA with optimal activity detected at pH 6. Furthermore, the short-term growth assay indicates that rHfCL can inhibit the intraerythrocytic development of Babesia microti and Babesia gibsoni in vitro.

Identification and partial characterization of a novel serpin from Eudiplozoon nipponicum (Monogenea, Polyopisthocotylea)

Background: Serpins are a superfamily of serine peptidase inhibitors that participate in the regulation of many physiological and cell peptidase-mediated processes in all organisms (e.g. in blood clotting, complement activation, fibrinolysis, inflammation, and programmed cell death). It was postulated that in the blood-feeding members of the monogenean family Diplozoidae, serpins could play an important role in the prevention of thrombus formation, activation of complement, inflammation in the host, and/or in the endogenous regulation of protein degradation.

Results: In silico analysis showed that the DNA and primary protein structures of serpin from Eudiplozoon nipponicum (EnSerp1) are similar to other members of the serpin superfamily. The inhibitory potential of EnSerp1 on four physiologically-relevant serine peptidases (trypsin, factor Xa, kallikrein, and plasmin) was demonstrated and its presence in the worm’s excretory-secretory products (ESPs) was confirmed.

Conclusion: EnSerp1 influences the activity of peptidases that play a role in blood coagulation, fibrinolysis, and complement activation. This inhibitory potential, together with the serpin’s presence in ESPs, suggests that it is likely involved in host-parasite interactions and could be one of the molecules involved in the control of feeding and prevention of inflammatory responses.

Preclinical evaluation of Amblyomin-X, a Kunitz-type protease inhibitor with antitumor activity

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Amblyomin-X is a recombinant protein with selective antitumor activity, causing tumor reduction in vivo.

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Acute and subchronic toxicity studies shows low toxicity in healthy mice, with reversible adverse effects.

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Amblyomin-X toxicity profile was defined.

Amblyomin-X, a Kunitz-type protease inhibitor, is a recombinant protein that selectively induces apoptosis in tumor cells and promotes tumor reduction in vivo in melanoma animal models. Furthermore, Amblyomin-X was able to drastically reduce lung metastasis in a mice orthotopic kidney tumor model. Due to its antitumor activity, Amblyomin-X potential to become a new drug is currently under investigation, therefore the aim of the present study was to perform preclinical assays to evaluate Amblyomin-X toxicity in healthy mice. Exploratory toxicity assays have shown that treatment with 512 mg/kg of Amblyomin-X lead to animal mortality, therefore two groups of treatment were evaluated in the present work: in the acute toxicity assay, animals were injected once with doses ranging from 4 to 256 mg/kg of Amblyomin-X, while in the subacute toxicity assay, animals were injected with 0.25, 0.57 and 1 mg/kg of Amblyomin-X daily, during 28 days. Following this treatment regimens, Amblyomin-X did not cause any mortality; moreover, toxicity signs were discrete, reversible and observed only at the higher doses, thus establishing a safety profile for administration in mice, which can be further used to determine the dose translation of this novel drug candidate for treatment in other species.

Wonders of tick saliva

Saliva of ticks is arguably the most complex saliva of any animal. This is particularly the case for ixodid species that feed for many days firmly attached to the same skin site of their obliging host. Sequencing and spectrometry technologies combined with bioinformatics are enumerating ingredients in the saliva cocktail. The dynamic and expanding saliva recipe is helping decipher the wonderous activities of tick saliva, revealing how ticks stealthily hide from their hosts while satisfying their gluttony and sharing their individual resources. This review takes a tick perspective on the composition and functions of tick saliva, covering water balance, gasket and holdfast, control of host responses, dynamics, individuality, mate guarding, saliva-assisted transmission, and redundancy. It highlights areas sometimes overlooked - feeding aggregation and sharing of sialomes, and the contribution of salivary gland storage granules - and questions whether the huge diversity of tick saliva molecules is 'redundant' or more a reflection on the enormous adaptability wonderous saliva confers on ticks.

Peptidase inhibitors in tick physiology

Peptidase inhibitors regulate a wide range of physiological processes involved in the interaction between hematophagous parasites and their hosts, including tissue remodeling, the immune response and blood coagulation. In tick physiology, peptidase inhibitors have a crucial role in adaptation to improve parasitism mechanisms, facilitating blood feeding by interfering with defense-related host peptidases. Recently, a larger number of studies on this topic led to the description of several new tick inhibitors displaying interesting novel features, for example a role in pathogen transmission to the host. A comprehensive review discussing these emerging concepts can therefore shed light on peptidase inhibitor functions, their relevance to tick physiology and their potential applications. Here, we summarize and examine the general characteristics, functional diversity and action of tick peptidase inhibitors with known physiological roles in the tick-host-pathogen interaction.

De novo assembly and annotation of Hyalomma dromedarii tick (Acari: Ixodidae) sialotranscriptome with regard to gender differences in gene expression

BACKGROUND

Hard ticks are hematophagous ectoparasites characterized by their long-term feeding. The saliva that they secrete during their blood meal is their crucial weapon against host-defense systems including hemostasis, inflammation and immunity. The anti-hemostatic, anti-inflammatory and immune-modulatory activities carried out by tick saliva molecules warrant their pharmacological investigation. The Hyalomma dromedarii Koch, 1844 tick is a common parasite of camels and probably the best adapted to deserts of all hard ticks. Like other hard ticks, the salivary glands of this tick may provide a rich source of many compounds whose biological activities interact directly with host system pathways. Female H. dromedarii ticks feed longer than males, thereby taking in more blood. To investigate the differences in feeding behavior as reflected in salivary compounds, we performed de novo assembly and annotation of H. dromedarii sialotranscriptome paying particular attention to variations in gender gene expression.

RESULTS

The quality-filtered Illumina sequencing reads deriving from a cDNA library of salivary glands led to the assembly of 15,342 transcripts. We deduced that the secreted proteins included: metalloproteases, glycine-rich proteins, mucins, anticoagulants of the mandanin family and lipocalins, among others. Expression analysis revealed differences in the expression of transcripts between male and female H. dromedarii that might explain the blood-feeding strategies employed by both genders.

CONCLUSIONS

The annotated sialome of H. dromedarii helps understand the interaction of tick-host molecules during blood-feeding and can lead to the discovery of new pharmacologically active proteins of ticks of the genus Hyalomma.