A serine proteinase inhibitor (serpin) from ixodid tick Haemaphysalis longicornis; cloning and preliminary assessment of its suitability as a candidate for a tick vaccine

Effect of vaccination with a recombinant metalloprotease from Haemaphysalis longicornis

We report the cloning, expression and characterization of an Haemaphysalis longicornis metalloprotease (named HLMP1). The gene encodes a predicted 550 aminoacid protein with similarity to metalloproteases of the reprolysin family. The protein sequence contains a signal sequence, the zinc-binding motif (HEXXHXXGXXH) common to metalloproteases and a cysteine-rich region. Reverse transcription-PCR expression analysis indicates the presence of mRNA in the salivary gland of larva, nymph and adult ticks. Rabbit repeatedly infested with H. longicornis recognized rHLMP1, suggesting that the immune-response against HLMP1 is naturally induced through the feeding of ticks. Vaccination of rabbit with rHLMP1 produced protective immunity against ticks, resulting in 15.6 and 14.6% mortality in nymph and adult ticks, respectively. This work provides information to understand the tick's defense system, and offers new insights to develop strategies to block this defense system with an anti-tick vaccine based on a metalloprotease.

Ixodes scapularis tick serine proteinase inhibitor (serpin) gene family; annotation and transcriptional analysis

BACKGROUND

Serine proteinase inhibitors (Serpins) are a large superfamily of structurally related, but functionally diverse proteins that control essential proteolytic pathways in most branches of life. Given their importance in the biology of many organisms, the concept that ticks might utilize serpins to evade host defenses and immunizing against or disrupting their functions as targets for tick control is an appealing option.

RESULTS

A sequence homology search strategy has allowed us to identify at least 45 tick serpin genes in the Ixodes scapularis genome that are structurally segregated into 32 intronless and 13 intron-containing genes. Nine of the intron-containing serpins occur in a cluster of 11 genes that span 170 kb of DNA sequence. Based on consensus amino acid residues in the reactive center loop (RCL) and signal peptide scanning, 93% are putatively inhibitory while 82% are putatively extracellular. Among the 11 different amino acid residues that are predicted at the P1 sites, 16 sequences possess basic amino acid (R/K) residues. Temporal and spatial expression analyses revealed that 40 of the 45 serpins are differentially expressed in salivary glands (SG) and/or midguts (MG) of unfed and partially fed ticks. Ten of the 38 serpin genes were expressed from six to 24 hrs of feeding while six and fives genes each are predominantly or exclusively expressed in either MG and SG respectively.

CONCLUSION

Given the diversity among tick species, sizes of tick serpin families are likely to be variable. However this study provides insight on the potential sizes of serpin protein families in ticks. Ticks must overcome inflammation, complement activation and blood coagulation to complete feeding. Since these pathways are regulated by serpins that have basic residues at their P1 sites, we speculate that I. scapularis may utilize some of the serpins reported in this study to manipulate host defense. We have discussed our data in the context of advances on the molecular physiology of I. scapularis. Although the paper is descriptive, this study provides the first step toward a comprehensive understanding of serpins in tick physiology.

The immunosuppresive tick salivary protein, Salp15

The interaction between Ixodid ticks and their mammalian hosts is a complex relationship. While the mammalian host tries to avoid the completion of the feeding process, the tick has devised strategies to counteract these attempts. Tick saliva contains a vast array of pharmacological activities that presumably aid the tick to evade host responses, including anticomplement, oxidative and innate and adaptive immune responses. The characterization of these activities has gained momentum in the last several years. One of the best studied activities present in tick saliva corresponds to the antigen known as Salp15, which binds specifically to the T-cell coreceptor CD4 resulting in the specific repression of CD4+ T-cell activation. We discuss here the current state of our knowledge of the mode of action of this salivary protein.

Exploring the mialome of ticks: an annotated catalogue of midgut transcripts from the hard tick, Dermacentor variabilis (Acari: Ixodidae)

Background

Ticks are obligate blood feeders. The midgut is the first major region of the body where blood and microbes ingested with the blood meal come in contact with the tick's internal tissues. Little is known about protein expression in the digestive tract of ticks. In this study, for analysis of global gene expression during tick attachment and feeding, we generated and sequenced 1,679 random transcripts (ESTs) from cDNA libraries from the midguts of female ticks at varying stages of feeding.

Results

Sequence analysis of the 1,679 ESTs resulted in the identification of 835 distinct transcripts, from these, a total of 82 transcripts were identified as proteins putatively directly involved in blood meal digestion, including enzymes involved in oxidative stress reduction/antimicrobial activity/detoxification, peptidase inhibitors, protein digestion (cysteine-, aspartic-, serine-, and metallo-peptidases), cell, protein and lipid binding including mucins and iron/heme metabolism and transport. A lectin-like protein with a high match to lectins in other tick species, allergen-like proteins and surface antigens important in pathogen recognition and/or antimicrobial activity were also found. Furthermore, midguts collected from the 6-day-fed ticks expressed twice as many transcripts involved in bloodmeal processing as midguts from unfed/2-day-fed ticks.

Conclusion

This tissue-specific transcriptome analysis provides an opportunity to examine the global expression of transcripts in the tick midgut and to compare the gut response to host attachment versus blood feeding and digestion. In contrast to those in salivary glands of other Ixodid ticks, most proteins in the D. variabilis midgut cDNA library were intracellular. Of the total ESTs associated with a function, an unusually large number of transcripts were associated with peptidases, cell, lipid and protein binding, and oxidative stress or detoxification. Presumably, this is consistent with their role in intracellular processing of the blood meal and response to microbial infections. The presence of many proteins with similar functions is consistent with the hypothesis that gene duplication contributed to the successful adaptation of ticks to hematophagy. Furthermore, these transcripts may be useful to scientists investigating the role of the tick midgut in blood-meal digestion, antimicrobial activity or the transmission of tick-borne pathogens.

Molecular and expression analysis of a family of the Amblyomma americanum tick Lospins

Serine proteinase inhibitors (serpins) are a family of structurally similar but functionally diverse proteins that regulate several important proteolytic cascades in most branches of life. We have characterized 17 Amblyomma americanum serpin cDNAs here named as `Lospins' (L; an acronym for Lone Star tick serpin) that possess three β-sheets, eight α-helices and a reactive center loop consistent with the consensus serpin superfamily secondary structures. Visual inspection of deduced amino acid sequences revealed two patterns of basic residues: (i) 86DKSRVLKAYKRL97 in L5 and L13–16 and (ii) 158VRDKTRGKI166 in all Lospins, which are similar to consensus glycosaminoglycan (GAG) binding sites (XBnXmBX, where X and B are non-basic and basic residues, n=1 or 2 and m=1, 2 or 3). On three-dimensional models, the two putative GAG binding sites mapped onto α-helices D and F, respectively, with calculation of electrostatic surface potentials revealing basic patches on L5 and L13–16 models that are comparable to the heparin-binding site on antithrombin. RT-PCR expression analysis of 15 selected genes showed that the majority (11/15) of the Lospins were ubiquitously expressed in the midgut, ovary and salivary glands. On a neighbor-joining phylogeny guide tree, 15 serpins from other ticks and 17 Lospins from this study, a total of 32 tick serpin sequences, segregated into five groups with Lospins in groups A and D being conserved across tick species. The discovery of Lospins in this study sets the framework for future studies to understand the role of serpins in tick physiology.

Tick anti-hemostatics: targets for future vaccines and therapeutics

For ticks, a significant obstacle in obtaining a blood meal is counteracting the hemostatic system of the host. To this end, ticks have developed a broad array of anti-hemostatics, which is reflected in the presence of structurally related tick proteins with different functions. Disruption of blood flow which blocks successful tick feeding makes anti-hemostatics attractive targets for anti-tick vaccines. Moreover, the limited number of drugs currently available for a range of important cardio-vascular diseases makes ticks a potential source of novel therapeutics. This review aims to summarize the key features of tick anti-hemostatics, their structures, mode of action and possible future application as vaccines and novel therapeutic agents.

Attachment Duration Required forRhipicephalus appendiculatusto TransmitTheileria parvato the Host

Theileria parva, the agent of East Coast fever (ECF), is transmitted to the host during the blood meal feeding of Rhipicephalus appendiculatus ticks. In order to investigate the relationship between the attachment duration of R. appendiculatus and the transmission of T. parva, infected adult ticks were allowed to attach to naive mice for variable lengths of time. Attached ticks and host animal's back skin biopsies from the tick attachment site were collected daily, starting from 24 hours post-tick attachment, and used for seminested polymerase chain reaction (PCR) detection of T. parva. T. parva-infected ticks started to transmit the parasites from 72 hours post-tick attachment. As expected, the transmission of T. parva from ticks to mouse skin increased with duration of tick attachment. Transmission of the parasites was 77.7%, 100%, 85.5%, and 100% on day 4, 5, 6, and 7 post-tick attachment, respectively, as could be detected from mice skin biopsies taken from T. parva-infected ticks' attachment sites. These results have important implications for our understanding of early events in the transmission of T. parva and would help in the development of effective pharmacologic substances and/or vaccines against ticks.

Vaccination against ectoparasites

Ectoparasites of livestock are of great economic and social importance but their effective control remains difficult. The feasibility of vaccination as a novel control measure was established over a decade ago with the commercial release of a recombinant vaccine against the cattle tick Boophilus microplus. Since then, research has continued on ticks and other ectoparasites. While some ectoparasite species will undoubtedly be refractory to immunological control, for others there has been a steady accumulation of knowledge of partially protective antigens, now accelerating through the application of genomic technologies. Nevertheless, progress towards usable, commercially available vaccines has been limited by a number of factors. The number of highly effective antigens is still very small. Although some classes of antigen have been investigated in more detail than others, we have no systematic knowledge of what distinguishes an effective antigen. Much hope has been placed on the potential of multi-antigen mixtures to deliver the efficacy required of a successful vaccine but with little experimental evidence. The application of current knowledge across parasite and host species needs to be explored but little has been done. In most cases, the path to commercial delivery is uncertain. Although many constraints and challenges remain, the need for vaccines and our capacity to develop them can only increase.

Molecular characterization of Rhipicephalus (Boophilus) microplus Bm86 homologue from Haemaphysalis longicornis ticks

One sequence in the EST database of a midgut cDNA library prepared from semi-engorged female Haemaphysalis longicornis ticks has been found to be a homologue of the Bm86 gene of Rhipicephalus (Boophilus) microplus ticks. The full-length sequence containing a 1785 bp open reading fragment (ORF) was obtained and designated as the Hl86 gene. The predicted amino acid sequence of the Hl86 gene shows a 37% identity to the Bm86 gene. Hl86 is predicted to be a GPI-anchored membrane-bound glycoprotein with a 19-amino acid signal sequence and a 22-amino acid hydrophobic region adjacent to the carboxyl terminus. The most important feature that Hl86 has in common with Bm86 is the repeated pattern of 6 cysteine residues forming epidermal growth factor (EGF)-like domains. RT-PCR analysis showed that Hl86 mRNA transcripts are expressed in all the life cycles of H. longicornis, and the expression was found in the midgut of the adult tick. The Hl86 was expressed in Escherichia coli as a gene10 fusion protein. Mouse anti-recombinant Hl86 serum recognized an 86 kDa protein band in the midgut lysate of semi-engorged ticks in Western blot analysis and showed a strong reaction on the luminal surface of midgut cells in an indirect immunofluorescent antibody test (IFAT). Silencing of the Hl86 gene by RNAi led to a significant reduction in the engorged tick body weight. This is the first report of cloning and characterization of the Bm86 homologue in different genera and species of ixodid and argasid ticks since Bm86 was first reported in 1989.

Protective immunity against Ixodes ricinus induced by a salivary serpin

Iris is a specific elastase inhibitor expressed in the salivary glands of the hard tick Ixodes ricinus. It belongs to the superfamily of serpins and interferes with both haemostasis and the immune response of the host. In this study, we first show that Iris is expressed in nymphs but not in the female midgut nor in males. We also show that Iris is present in the saliva. To examine its potency as anti-tick vaccine candidate, we set up three models of I. ricinus infestation on immunized animals: nymphs on mice, and adults and nymphs on rabbits. We report the rise of neutralizing antibodies following immunization of rabbits and mice. This comes with a significant protective immunity against ticks in rabbits only, resulting in a 30% mortality rate and a diminution of weight gain in both nymphs and adults and a prolongation of blood feeding time in adults. This is the first report on an anti-tick vaccine trial on I. ricinus using a protein able to interact with both host immunity and haemostasis, as a vaccinating antigen.

A thrombin inhibitor from the gut of Boophilus microplus ticks

A thrombin inhibitor was identified for the first time in the gut of the cattle tick Boophilus microplus. Here we present the partial purification and characterization of this new molecule, which was purified from the gut extract by three chromatographic steps: ion-exchange, gel filtration and affinity chromatography in a thrombin-Sepharose resin. In SDS-PAGE the inhibitor showed an apparent molecular mass of circa 26 kDa, which is different from the two thrombin inhibitors present in the saliva of this tick. The new inhibitor delays bovine plasma clotting time and inhibits both thrombin induced fibrinogen clotting and thrombin induced platelet aggregation. However, it does not interfere with thrombin amidolytic activity upon a small substrate (H-D-Phe-Pip-Arg-para-nitroanilide), which does not require binding to thrombin exosites. Therefore, the inhibitor does not block thrombin active site, although it must interfere with one of the thrombin exosites. B. microplus gut thrombin inhibitor (BmGTI) is also capable of enhancing activated protein C (APC) activity upon its specific substrate (H-D-Glu-Pro-Arg-para-nitroanilide), an activity never described before among B. microplus molecules.

Anti-tick vaccines

There is now abundant evidence that vaccination with defined protein antigens is able to induce significant immunity to tick infestation. In a limited number of cases, this immunity has been duplicated by vaccination with recombinant antigens, a critical step on the pathway to commercial vaccine production. The existence of two commercial vaccines has allowed a number of field studies showing that the existing products can make an important contribution to an integrated approach to the control of ticks in the field. Under most circumstances however, the use of a tick vaccine as the single, stand alone control technology is likely to require more efficacious vaccines than those currently available. Increases in efficacy are most likely to come through the discovery of additional, effective vaccine antigens. The number of antigens with demonstrated effect is increasing, though only slowly, while the number of potential antigens that remain to be evaluated is increasing more quickly. There is limited, though convincing, evidence that some of these antigens will show effective cross-species protection, though in a poorly understood and unpredictable way. The groundwork has been laid; the potential of the field is still to be effectively exploited.

Identification of a glycine-rich protein from the tick Rhipicephalus haemaphysaloides and evaluation of its vaccine potential against tick feeding

A cDNA coding a glycine-rich protein was identified from the Rhipicephalus haemaphysaloides tick. The cDNA named here as RH50 was 1,823 bp, including a single open reading frame (ORF) of 1,518 nucleotides. The ORF encodes a polypeptide of 506 amino acid residues with a size of 50 kDa, as calculated by a computer. The predicted amino acid sequence of RH50 showed a low homology to sequences of some known extracellular matrix-like proteins. The native protein was identified in both the fed tick salivary gland lysates and extracts of cement material using the serum against the recombinant protein. Reverse transcription polymerase chain reaction results showed that RH50 mRNA was only transcribed in partially fed tick salivary glands, not in unfed tick salivary glands or partially fed tick midgut, fat body, or ovary. The differential expression of RH50 protein in fed tick salivary glands was confirmed by immunofluorescence. The low attachment rate both in the adult and nymphal tick, and the high mortality of immature ticks (nymph) feeding on recombinant RH50-immunized rabbits were found. These results show that the RH50 protein could be a useful candidate for anti-tick vaccine development.

Anti-hemostatic effects of a serpin from the saliva of the tick Ixodes ricinus

Serpins (serine protease inhibitors) are a large family of structurally related proteins found in a wide variety of organisms, including hematophagous arthropods. Protein analyses revealed that Iris, previously described as an immunomodulator secreted in the tick saliva, is related to the leukocyte elastase inhibitor and possesses serpin motifs, including the reactive center loop (RCL), which is involved in the interaction between serpins and serine proteases. Only serine proteases were inhibited by purified recombinant Iris (rIris), whereas mutants L339A and A332P were found devoid of any protease inhibitory activity. The highest Ka was observed with human leukocyte-elastase, suggesting that elastase-like proteases are the natural targets of Iris. In addition, mutation M340R completely changed both Iris substrate specificity and affinity. This likely identified Met-340 as amino acid P1 in the RCL. The effects of rIris and its mutants were also tested on primary hemostasis, blood clotting, and fibrinolysis. rIris increased platelet adhesion, the contact phase-activated pathway of coagulation, and fibrinolysis times in a dose-dependent manner, whereas rIris mutant L339A affected only platelet adhesion. Taken together, these results indicate that Iris disrupts coagulation and fibrinolysis via the anti-proteolytic RCL domain. One or more other domains could be responsible for primary hemostasis inhibition. To our knowledge, this is the first ectoparasite serpin that interferes with both hemostasis and the immune response.

Exposed and concealed antigens as vaccine targets for controlling ticks and tick-borne diseases

Tick vaccines derived from Bm86, a midgut membrane-bound protein of the cattle tick, Boophilus microplus, are currently the only commercially available ectoparasite vaccines. Despite its introduction to the market in 1994, and the recognized need for alternatives to chemical pesticides, progress in developing effective antitick vaccines (and ectoparasite vaccines in general) is slow. The primary rate-limiting step is the identification of suitable antigenic targets for vaccine development. Two sources of candidate vaccine antigens have been identified: 'exposed' antigens that are secreted in tick saliva during attachment and feeding on a host and 'concealed' antigens that are normally hidden from the host. Recently, a third group of antigens has been distinguished that combines the properties of both exposed and concealed antigens. This latter group offers the prospect of a broad-spectrum vaccine effective against both adults and immature stages of a wide variety of tick species. It also shows transmission-blocking and protective activity against a tick-borne pathogen. With the proliferation of molecular techniques and their application to vaccine development, there are high hopes for new and effective antitick vaccines that also control tick-borne diseases.

Two serine protease inhibitors (serpins) that induce a bovine protective immune response against Rhipicephalus appendiculatus ticks

We have previously undertaken preliminary characterization of two Rhipicephalus appendiculutus serine protease inhibitors (RAS-1 and -2) as anti-tick vaccine candidates. In this study, to clarify this hypothesis, we generated and further characterized recombinant RAS-1 and -2 (rRAS-1 and -2) and tested their potency as a cocktail anti-tick vaccine in cattle. RT-PCR analysis showed that RAS-1 and -2 mRNA transcripts are expressed during all life cycle stages of ticks, independent of sex. As judged by SDS-PAGE rRAS-1 and -2 migrated as a molecular weight of around 64 and 60 kDa protein, respectively, considering that the expression vector produced a recombinant protein fused with 18-22 kDa TRX protein. RAS-1 and -2 were found not to be secreted into the bite site as determined by the reactivity of anti-tick saliva sera to rRAS-1 and -2, suggesting that both proteins are concealed antigens. Vaccination of cattle with a combination of rRAS-1 and -2 conferred significant protective immunity against ticks, resulting in 61.4% reduction in nymph engorgement rate, and in 28 and 43% increased mortality rate in adult female and male ticks, respectively. This is the first report on an anti-tick vaccine trial using a combination of two different serpins derived from R. appendiculatus, and using cattle as a natural host.

A Novel Gene Encoding a Thrombin Inhibitory Protein in a cDNA Library from Haemaphysalis longicornis Salivary Gland

A novel thrombin inhibitory protein coding gene was identified from a cDNA library derived from salivary gland of partially-fed Haemaphysalis longicornis (hard tick). The gene encoded a 93-amino acid protein, designated chimadanin, which had a signal peptide sequence and was predicted to be a secretory protein. It showed no similarity to any other previously identified proteins or conserved domain sequences. The gene was expressed during blood feeding and suggested to be expressed mainly in the salivary gland. The predicted mature region of chimadanin was expressed in Escherichia coli and characteristics of the recombinant chimadanin were determined. The activated partial thromboplastin time and the prothrombin time in sheep plasma were significantly prolonged by chimadanin in a dose dependent manner. Amidolytic activity of thrombin was also inhibited by chimadanin in a dose dependent manner and it suggested that chimadanin was an anticoagulant with thrombin inhibitory activity. This newly identified thrombin inhibitor may play an important role in tick blood feeding.

A serine protease inhibitor (serpin) from Haemaphysalis longicornis as an anti-tick vaccine

The application of anti-tick vaccine has been shown to be the most promising alternative tick control strategy compared to the current use of acaricides that suffer from a number of limitations. The success of this strategy is dependent on the cloning, and characterization of tick molecules involved in the mediation of tick central physiological roles. Rapid amplification of the cDNA ends (RACE) and primers designed based on a conserved serpin amino acid motif (NAVYFKG) were used to clone a cDNA with high similarity in the reactive center loop (RCL) to representative serpin, heparin cofactor II. We have named this novel gene as Haemaphysalis longicornis serpin-2 (HLS2). RT-PCR analysis showed that HLS2 mRNA transcripts are not expressed in salivary glands but in hemolymph by feeding ticks. HLS2 was not introduced into the bite site as measured by Western blot analysis. The activated partial thromboplastin time (APTT) and the thrombin inhibitory assay using recombinant HLS2 (rHLS2) demonstrated prolonged coagulation time and inhibition of thrombin activity. These results suggested that HLS2 is present only in hemolymph of the feeding ticks and the function of HLS2 is homeostasis in tick physiological compartment. Vaccination of rabbits with rHLS2 conferred protective immunity against ticks, resulting in 44.6 and 43.0% mortality in nymphal and adult ticks, respectively. These results show that rHLS2 could be an important candidate as a component of a cocktail anti-tick vaccine.