Protection against Boophilus annulatus infestations in cattle vaccinated with the B. microplus Bm86-containing vaccine Gavac. off

Inclusion of Anti-Tick Vaccines into an Integrated Tick Management Program in Mexico: A Public Policy Challenge

Acaricides are the most widely used method to control the cattle tick Rhipicephalus microplus. However, its use increases production costs, contaminates food and the environment, and directly affects animal and human health. The intensive use of chemical control has resulted in the selection of genes associated with resistance to acaricides, and consumers are increasingly less tolerant of food contamination. This scenario has increased the interest of different research groups around the world for anti-tick vaccine development, in order to reduce the environmental impact, the presence of residues in food, and the harmful effects on animal and human health. There is enough evidence that vaccination with tick antigens induces protection against tick infestations, reducing tick populations and acaricide treatments. Despite the need for an anti-tick vaccine in Mexico, vaccination against ticks has been limited to one vaccine that is used in some regions. The aim of this review is to contribute to the discussion on tick control issues and provide a reference for readers interested in the importance of using anti-tick vaccines encouraging concerted action on the part of Mexican animal health authorities, livestock organizations, cattle producers, and academics. Therefore, it is suggested that an anti-tick vaccine should be included as a part of an integrated tick management program in Mexico.

More than Three Decades of Bm86: What We Know and Where to Go

Tick and tick-borne disease control have been a serious research focus for many decades. In a global climate of increasing acaricide resistance, host immunity against tick infestation has become a much-needed complementary strategy to common chemical control. From the earliest acquired resistance studies in small animal models to proof of concept in large production animals, it was the isolation, characterization, and final recombinant protein production of the midgut antigen Bm86 from the Australian cattle tick strain of Rhipicephalus (Boophilus) microplus (later reinstated as R. (B.) australis) that established tick subunit vaccines as a viable alternative in tick and tick-borne disease control. In the past 37 years, this antigen has spawned numerous tick subunit vaccines (either Bm86-based or novel), and though we are still describing its molecular structure and function, this antigen remains the gold standard for all tick vaccines. In this paper, advances in tick vaccine development over the past three decades are discussed alongside the development of biotechnology, where existing gaps and future directives in the field are highlighted.

Universal Tick Vaccines: Candidates and Remaining Challenges

Recent advancements in molecular biology, particularly regarding massively parallel sequencing technologies, have enabled scientists to gain more insight into the physiology of ticks. While there has been progress in identifying tick proteins and the pathways they are involved in, the specificities of tick-host interaction at the molecular level are not yet fully understood. Indeed, the development of effective commercial tick vaccines has been slower than expected. While omics studies have pointed to some potential vaccine immunogens, selecting suitable antigens for a multi-antigenic vaccine is very complex due to the participation of redundant molecules in biological pathways. The expansion of ticks and their pathogens into new territories and exposure to new hosts makes it necessary to evaluate vaccine efficacy in unusual and non-domestic host species. This situation makes ticks and tick-borne diseases an increasing threat to animal and human health globally, demanding an urgent availability of vaccines against multiple tick species and their pathogens. This review discusses the challenges and advancements in the search for universal tick vaccines, including promising new antigen candidates, and indicates future directions in this crucial research field.

Transcriptome Profiling of Rhipicephalus annulatus Reveals Differential Gene Expression of Metabolic Detoxifying Enzymes in Response to Acaricide Treatment

Ticks are hematophagous ectoparasites of economic consequence by virtue of being carriers of infectious diseases that affect livestock and other sectors of the agricultural industry. A widely prevalent tick species, Rhipicephalus (Boophilus) annulatus, has been recognized as a prime vector of tick-borne diseases in South Indian regions. Over time, the use of chemical acaricides for tick control has promoted the evolution of resistance to these widely used compounds through metabolic detoxification. Identifying the genes related to this detoxification is extremely important, as it could help detect valid insecticide targets and develop novel strategies for effective insect control. We performed an RNA-sequencing analysis of acaricide-treated and untreated R. (B.) annulatus and mapped the detoxification genes expressed due to acaricide exposure. Our results provided high-quality RNA-sequenced data of untreated and amitraz-treated R. (B.) annulatus, and then the data were assembled into contigs and clustered into 50,591 and 71,711 uni-gene sequences, respectively. The expression levels of the detoxification genes across different developmental stages of R. (B.) annulatu identified 16,635 transcripts as upregulated and 15,539 transcripts as downregulated. The annotations of the differentially expressed genes (DEGs) revealed the significant expression of 70 detoxification genes in response to the amitraz treatment. The qRT-PCR revealed significant differences in the gene expression levels across different life stages of R. (B.) annulatus.

Putative target sites in synganglion for novel ixodid tick control strategies

Acaricide resistance is a global problem that has impacts worldwide. Tick populations with broad resistance to all commercially available acaricides have been reported. Since resistance selection in ticks and their role in pathogen transmission to animals and humans result in important economic and public health burden, it is essential to develop new strategies for their control (i.e., novel chemical compounds, vaccines, biological control). The synganglion is the tick central nervous system and it is responsible for synthesizing and releasing signaling molecules with different physiological functions. Synganglion proteins are the targets of the majority of available acaricides. In this review we provide an overview of the mode-of-action and resistance mechanisms against neurotoxic acaricides in ticks, as well as putative target sites in synganglion, as a supporting tool to identify new target proteins and to develop new strategies for tick control.

Co-Immunization Efficacy of Recombinant Antigens against Rhipicephalus microplus and Hyalomma anatolicum Tick Infestations

The immunoprophylactic management of ticks is the most effective option to control tick infestations and counter spread the acaricide resistance problem worldwide. Several researchers reported an inconsistent efficacy of the single antigen-based immunization of hosts against different tick species. In the present study, to develop a multi-target immunization protocol, proteins from Rhipicephalus microplus BM86 and Hyalomma anatolicum subolesin (SUB) and tropomyosin (TPM) were targeted to evaluate the cross-protective potential. The sequence identities of the BM86, SUB, and TPM coding genes amongst Indian tick isolates of targeted species were 95.6–99.8%, 98.7–99.6%, and 98.9–99.9%, respectively, while at the predicted amino acid level, the identities were 93.2 to 99.5, 97.6 to 99.4, and 98.2 to 99.3%. The targeted genes were expressed in the eukaryotic expression system, pKLAC2-Kluyveromyces lactis, and 100 µg each of purified recombinant protein (Bm86-89 kDa, SUB-21 kDa, and TPM-36 kDa) mixed with adjuvant was injected individually through the intramuscular route at different sites of the body on days 0, 30, and 60 to immunize cross-bred cattle. Post-immunization, a statistically significant (p < 0.001) antibody response (IgG, IgG1, and IgG2) in comparison to the control, starting from 15 to 140 days, against each antigen was recorded. Following multi-antigen immunization, the animals were challenged twice with the larvae of R. microplus and H. anatolicum and theadults of H. anatolicum, and a significant vaccine efficacy of 87.2% and 86.2% against H. anatolicum larvae and adults, respectively, and 86.7% against R. microplus was obtained. The current study provides significant support to develop a multi-antigen vaccine against cattle tick species.

The Bm86 Discovery: A Revolution in the Development of Anti-Tick Vaccines

The presence in nature of species with genetic resistance to ticks, or with acquired resistance after repeated tick infestations, has encouraged the scientific community to consider vaccination as an alternative to the unsustainable chemical control of ticks. After numerous attempts to artificially immunize hosts with tick extracts, the purification and characterization of the Bm86 antigen by Willadsen et al. in 1989 constituted a revolutionary step forward in the development of vaccines against ticks. Previously, innovative studies that had used tick gut extracts for the immunization of cattle against Rhipicepahalus microplus (previously named Boophilus microplus) ticks, with amazingly successful results, demonstrated the feasibility of using antigens other than salivary-gland-derived molecules to induce a strong anti-tick immunity. However, the practical application of an anti-tick vaccine required the isolation, identification, and purification of the responsible antigen, which was finally defined as the Bm86 protein. More than thirty years later, the only commercially available anti-tick vaccines are still based on this antigen, and all our current knowledge about the field application of immunological control based on vaccination against ticks has been obtained through the use of these vaccines.

Ticks and antibodies: May parasite density and tick evasion influence the outcomes following immunization protocols?

Ticks are a major concern to human health and livestock worldwide, being responsible for economic losses that go beyond billions of US dollars per year. This scenario instigates the development of vaccines against these ectoparasites, emphasized by the fact that the main method of controlling ticks still relies on the use of acaricides, what increases costs and may affect the environment as well as human and animal health. The first commercial vaccines against ectoparasites were produced against the tick Rhipicephalus microplus and their efficacy were based on antibodies. Many additional attempts have been conducted to produce protective immune responses against ticks by immunization with specific antigens and the antibody response has usually been the main target of evaluation. But some controversy still populates the roles possibly performed by humoral responses in tick-mammalian host relationships. This review focuses on the analysis of specific aspects concerning antibodies and ticks, especially the influence of parasite density and evasion/modulation. The immunization trials already described against R. microplus were also compiled and analyzed based on the characteristics of the molecules tested, protocols of immunization and tick challenge. Within these issues, it is discussed if or when antibody levels can be directly correlated with the development of tick resistance, and whether anti-tick protective immune responses generated by infestations may become ineffective under a different tick density. Also, higher titers of antibodies can be correlated with protection or susceptibility to tick infestations, what may be altered following continuous or repeated infestations and differ greatly comparing hosts with distinct genetic backgrounds. Regarding evasion, ticks present a sophisticated mechanism for dealing with antibodies, including Immunoglobulin Binding Proteins (IGBPs), that capture, transport and inject them back into the host, while keeping their properties within the parasite. The comparison of immunization protocols shows a total of 22 molecules already tested in cattle vaccination trials against R. microplus, with the predominance of concealed and dual antigens as well as marked differences in tick challenge schemes. The presence of an antibody evasion apparatus and variable levels of tick resistance when facing different densities of parasites are concerns that should be considered when testing vaccine candidates. Ultimately, more refinement may be necessary to effectively design a cocktail vaccine with tick molecules, which may be needed to be altered and combined in non-competing immune contexts to be universally secure and protective.

From Bench to Field: A Guide to Formulating and Evaluating Anti-Tick Vaccines Delving beyond Efficacy to Effectiveness

Ticks are ubiquitous blood-sucking ectoparasites capable of transmitting a wide range of pathogens such as bacteria, viruses, protozoa, and fungi to animals and humans. Although the use of chemicals (acaricides) is the predominant method of tick-control, there are increasing incidents of acaricide tick resistance. Furthermore, there are concerns over accumulation of acaricide residues in meat, milk and in the environment. Therefore, alternative methods of tick-control have been proposed, of which anti-tick cattle vaccination is regarded as sustainable and user-friendly. Over the years, tremendous progress has been made in identifying and evaluating novel candidate tick vaccines, yet none of them have reached the global market. Until now, Bm86-based vaccines (Gavac™ in Cuba and TickGARD^PLUS™ Australia-ceased in 2010) are still the only globally commercialized anti-tick vaccines. In contrast to Bm86, often, the novel candidate anti-tick vaccines show a lower protection efficacy. Why is this so? In response, herein, the potential bottlenecks to formulating efficacious anti-tick vaccines are examined. Aside from Bm86, the effectiveness of other anti-tick vaccines is rarely assessed. So, how can the researchers assess anti-tick vaccine effectiveness before field application? The approaches that are currently used to determine anti-tick vaccine efficacy are re-examined in this review. In addition, a model is proposed to aid in assessing anti-tick vaccine effectiveness. Finally, based on the principles for the development of general veterinary vaccines, a pipeline is proposed to guide in the development of anti-tick vaccines.

Experimental Infection of Calves with Transfected Attenuated Babesia bovis Expressing the Rhipicephalus microplus Bm86 Antigen and eGFP Marker: Preliminary Studies towards a Dual Anti-Tick/Babesia Vaccine

Bovine babesiosis, caused by Babesia bovis and B. bigemina, is a major tick-borne disease of cattle with global economic impact. The disease can be prevented using integrated control measures including attenuated Babesia vaccines, babesicidal drugs, and tick control approaches. Vaccination of cattle with the Rhipicephalus microplus Bm86-based recombinant vaccine reduces the fitness of R. microplus and R. annulatus, but several booster inoculations are required to maintain protection. Herein, we generated a stable transfected strain of B. bovis expressing an enhanced GFP (eGFP) and a chimeric version of Bm86 (B. bovis/Bm86/eGFP). The eGFP was expressed in the parasite cytoplasm, whereas Bm86 was displayed on the surface of merozoites. Three splenectomized calves experimentally infected with B. bovis/Bm86/eGFP showed mild signs of acute disease and developed long-lasting antibody responses to B. bovis and native Bm86. No evidence of sequestration of parasites in the cerebral capillaries was found upon postmortem analysis, confirming attenuation of the strain. This is the first report of transfected B. bovis expressing the tick antigen Bm86 on the merozoite surface that elicits an antibody response to native Bm86. These results represent a proof of concept for a novel live, attenuated, tagged dual-vaccine approach to attempt simultaneous control of babesiosis and tick infestation.

Prediction, mapping and validation of tick glutathione S-transferase B-cell epitopes

In search of ways to address the increasing incidence of global acaricide resistance, tick control through vaccination is regarded as a sustainable alternative approach. Recently, a novel cocktail antigen tick-vaccine was developed based on the recombinant glutathione S-transferase (rGST) anti-sera cross-reaction to glutathione S-transferases of Rhipicephalus appendiculatus (GST-Ra), Amblyomma variegatum (GST-Av), Haemaphysalis longicornis (GST-Hl), Rhipicephalus decoloratus (GST-Rd) and Rhipicephalus microplus (GST-Rm). Therefore, the current study aimed to predict the shared B-cell epitopes within the GST sequences of these tick species. Prediction of B-cell epitopes and proteasomal cleavage sites were performed using immunoinformatics algorithms. The conserved epitopes predicted within the sequences were mapped on the homodimers of the respective tick GSTs, and the corresponding peptides were independently used for rabbit immunization experiments. Based on the dot blot assay, the immunogenicity of the peptides and their potential to be recognized by corresponding rGST anti-sera raised by rabbit immunization in a previous work were investigated. This study revealed that the predicted conserved B-cell epitopes within the five tick GST sequences were localized on the surface of the respective GST homodimers. The epitopes of GST-Ra, GST-Rd, GST-Av, and GST-Hl were also shown to contain a seven residue-long peptide sequence with no proteasomal cleavage sites, whereas proteasomal digestion of GST-Rm was predicted to yield a 4-residue fragment. Given that a few proteasomal cleavage sites were found within the conserved epitope sequences of the four GSTs, the sequences could also contain a T-cell epitope. Finally, the peptide and rGST anti-sera reacted against the corresponding peptide, confirming their immunogenicity. These data support the claim that the rGSTs, used in the previous study, contain conserved B-cell epitopes, which elucidates why the rGST anti-sera cross-reacted to non-homologous tick GSTs. Taken together, the data suggest that the B-cell epitopes predicted in this study could be useful for constituting epitope-based GST tick vaccines.

Rhipicephalus microplus cystatin as a potential cross-protective tick vaccine against Rhipicephalus appendiculatus

Rhipicephalus appendiculatus, the brown ear tick, is an important disease vector of livestock in eastern, central and southern Africa. Rhipicephalus appendiculatus acaricide resistance requires the search for alternative methods for its control. Cystatins constitute a superfamily of cysteine peptidase inhibitors vital for tick blood feeding and development. These inhibitors were proposed as antigens in anti-tick vaccines. In this work, we applied structural and biochemical approaches to characterize a new cystatin named R. appendiculatus cystatin 2a (Racys2a). Structural modeling showed that this new protein possesses characteristic type 2 cystatin motifs, besides conservation of other structural patterns along the protein. Peptidase inhibitory assays with recombinant Racys2a showed modulation of tick and host cathepsins involved in blood digestion and immune system responses, respectively. A heterologous tick challenge with R. appendiculatus in rabbits immunized with recombinant Rhipicephalus microplus cystatin 2c (rBmcys2c) was performed to determine cross-reactivity. Histological staining showed that rBmcys2c vaccination caused damage to the gut, salivary gland and ovary tissues in R. appendiculatus. Furthermore, cystatin vaccine reduced the number of fully engorged adult females in 11.5 %. Consequently, strategies to increase the protection rate are necessary, including the selection of two or more antigens to compose a vaccine cocktail.

Cross protection induced by combined Subolesin-based DNA and protein immunizations against adult Haemaphysalis longicornis

Vaccination against ticks is an environmentally friendly alternative control method compared to chemical acaricide applications. Subolesin is a conserved protein in ticks, which can provide protection against some tick species. In this study, we evaluated the capacity of cocktail vaccination with Subolesin and ribosomal acidic protein 0 (P0) peptide against adults of Haemaphysalis longicornis. Priming with DNA vaccine expressing subolesin, followed by boosters of a single antigen (rRhSub) or a chimeric polypeptide (rRhSub/P0), provided cross protection. This treatment resulted in significant mortality, reduced blood ingestion and reduced reproduction in H. longicornis adults. Vaccination efficacies of 79.3% and 86.6% are reported in groups supplemented with rRhSub and rRhSub/P0, respectively. Conserved antigens, such as subolesin, formulated as DNA vaccine and enhanced with chimeric polypeptides, could be used as an anti-tick vaccine application, especially for control of infestation involving several tick species.

Evaluation of vaccine candidates purified from the adult ticks of Ornithodoros savignyi (Acari: Argasidae) and Hyalomma dromedarii (Acari: Ixodidae) against tick infestations

Ticks cause anemia, toxicosis, growth delay, and transmit infectious diseases in animals and humans. The current study aimed to evaluate the immunoprophylactic properties of two vaccine candidates to develop vaccine against tick infestations. These two vaccine candidates were specific fraction from the adults of the soft tick Ornithodoros savignyi and cross-reactive fraction from the adults of the hard tick Hyalomma dromedarii. Both specific and cross-reactive fractions were isolated by Cyanogen Bromide-activated Sepharose-4B affinity column chromatography. Both candidates proved their cross-reactivity by enzyme linked immunosorbent assay and Western blot. Characterization of the two vaccines by SDS-PAGE showed that the O. savignyi specific fraction consists of four bands; 97, 85, 66 and 11.5 kDa compared with nine bands associated with its crude antigen (196-11.5 kDa). The H. dromedarii cross-reactive vaccine candidate consists of three bands; 97, 66 and 45 kDa compared to eight bands of its crude antigen (196-21 kDa). Two common bands of 97 and 66 kDa between two candidates showed immunogenic cross-reactivity with the developed antisera of both infestations by Western blot. Immunization of rabbits intramuscularly with two doses of the fractions separately (40 µg/kg) led to immunological and parasitological changes. Immunologically; the level of immunoglobulins in vaccinated rabbits increased significantly compared with control infested non-vaccinated rabbits. These immunoglobulins are probably responsible for the protective effect of both candidates. Parasitologically, immunized rabbits showed protection against infestation by adult ticks as proved by significant feeding rejection percentage and significant reduction in egg and engorgement weights of H. dromedarii. While insignificant protection was observed against O. savignyi ticks infestation in feeding rejection and reduction in engorgement weight. In conclusion, this study suggests promising immunoprophylactic potentials of the purified fractions against tick infestations in rabbits through induction of IgG responses. The protective effect of both vaccine candidates deserves further evaluation in other hosts and against other tick infestations.

Constituting a glutathione S-transferase-cocktail vaccine against tick infestation

Cocktail vaccines are proposed as an attractive way to increase protection efficacy against specific tick species. Furthermore, such vaccines made with different tick antigens have the potential of cross-protecting against a broad range of tick species. However, there are still limitations to the selection of immunogen candidates. Acknowledging that glutathione S-transferases (GSTs) have been exploited as vaccines against ticks and other parasites, this study aimed to analyze a GST-cocktail vaccine as a potential broad-spectrum tick vaccine. To constitute the GST-cocktail vaccine, five tick species of economic importance for livestock industry were studied (Rhipicephalus appendiculatus, Rhipicephalus decoloratus, Rhipicephalus microplus, Amblyomma variegatum, and Haemaphysalis longicornis). Tick GST ORF sequences were cloned, and the recombinant GSTs were produced in Escherichia coli. rGSTs were purified and inoculated into rabbits, and the immunological response was characterized. The humoral response against rGST-Rd and rGST-Av showed a stronger cross-reactivity against heterologous rGSTs compared to rGST-Hl, rGST-Ra, and rGST-Rm. Therefore, rGST-Rd and rGST-Av were selected for constituting an experimental rGST-cocktail vaccine. Vaccination experiment in rabbits showed that rGST-cocktail caused 35% reduction in female numbers in a Rhipicephalus sanguineus infestation. This study brings forward an approach to selecting immunogens for cocktail vaccines, and the results highlight rGST-Rd and rGST-Av as potentially useful tools for the development of a broad-spectrum tick vaccine.

Characterization of a glycine-rich protein from Rhipicephalus microplus: tissue expression, gene silencing and immune recognition

Salivary molecules, as glycine-rich proteins (GRPs), are essential to tick attachment and feeding on the host and are suggested to be involved in the host's immune system evasion, therefore representing natural candidates in the search for protective vaccine antigens. This work shows the molecular characterization of a GRP from Rhipicephalus microplus (RmGRP). The cDNA and putative amino acid sequences were analysed, as well as the transcription level in tick tissues/developmental stages, showing the highest levels of gene expression in 1-day-old larvae and salivary glands of fully engorged females. RmGRP gene silencing resulted in a lower hatching rate of larvae from treated females. In addition, recombinant RmGRP (rRmGRP) was recognized by sera from naturally and experimentally infested bovines, displaying considerable differences among the individuals tested. rRmGRP was recognized by anti-saliva and anti-salivary glands sera, while anti-rRmGRP serum recognized RmGRP in saliva and salivary glands, indicating its secretion into the host. The data collected indicate that RmGRP may present roles other than in the tick–host relationship, especially in embryo development. In addition, the high expression in adult females, antigenicity and presence of shared characteristics with other tick protective GRPs turns RmGRP a potential candidate to compose an anti-tick vaccine cocktail.

Molecular and structural characterization of novel cystatins from the taiga tick Ixodes persulcatus

Cystatins are cysteine peptidase inhibitors that in ticks mediate processes such as blood feeding and digestion. The ixodid tick Ixodes persulcatus is endemic to the Eurasia, where it is the principal vector of Lyme borreliosis. To date, no I. persulcatus cystatin has been characterized. In the present work, we describe three novel cystatins from I. persulcatus, named JpIpcys2a, JpIpcys2b and JpIpcys2c. In addition, the potential of tick cystatins as cross-protective antigens was evaluated by vaccination of hamsters using BrBmcys2c, a cystatin from Rhipicephalus microplus, against I. persulcatus infestation. Sequence analysis showed that motifs that are characteristic of cystatins type 2 are fully conserved in JpIpcys2b, while mutations are present in both JpIpcys2a and JpIpcys2c. Protein-protein docking simulations further revealed that JpIpcys2a, JpIpcys2b and JpIpcys2c showed conserved binding sites to human cathepsins L, all of them covering the active site cleft. Cystatin transcripts were detected in different I. persulcatus tissues and instars, showing their ubiquitous expression during I. persulcatus development. Serological analysis showed that although hamsters immunized with BrBmcys2c developed a humoral immune response, this response was not adequate to protect against a heterologous challenge with I. persulcatus adult ticks. The lack of cross-protection provided by BrBmcys2c immunization is perhaps linked to the fact that cystatins cluster into multigene protein families that are expressed differentially and exhibit functional redundancy. How to target such small proteins that are secreted in low quantities remains a challenge in the development of suitable anti-tick vaccine antigens.

Extensive polymorphism of Ra86 genes in field populations of Rhipicephalus appendiculatus from Kenya

Commercial vaccines based on recombinant forms of the Bm86 tick gut antigen are used to control the southern cattle tick, Rhipicephalus microplus, a 1-host species, in Australia and Latin America. We describe herein sequence polymorphism in genes encoding Ra86 homologues of Bm86 in the brown ear tick, Rhipicephalus appendiculatus, isolated from four Kenyan field populations and one laboratory colony. Sequencing of 19 Ra86 sequences defined two alleles differentiated by indels, encoding 693 amino acids (aa) and 654 aa respectively, from the Muguga laboratory reference strain. Ra86 sequences were also determined from gut cDNA from four field populations of R. appendiculatus collected in different livestock production systems in Kenya. Analysis of approximately 20 Ra86 sequences from each of the four field sites in central and Western Kenya; Makuyu, Kiambu, Kakamega and Uasin Gishu, revealed three additional size types differentiated by 39-49 amino acid indels resulting in a total of 5 indel-defined genotypes. The 693 aa type 5 was isolated only from the laboratory tick stock; genotypes 1, 2 and 3 were identified in ticks from the four Kenyan field sites and appeared to be derivatives of the shorter RA86 genotype found in Muguga laboratory stock genotype 4. By contrast no large indels have yet been observed between R. microplus sequences from Australia, South America or Africa. Evidence that selection contributes to the observed sequence variation was provided by analysis of ratio of synonymous and non-synonymous substitutions and application of the selective neutrality and neutral evolution tests to the primary data. Phylogenetic analysis clustered sequences from all Ra86 size types and Bm86, into four major clades based on amino acid substitutions, but there was no evidence that these groupings correlated with geographical separation of R. appendiculatus populations.

A review of reverse vaccinology approaches for the development of vaccines against ticks and tick borne diseases

The field of reverse vaccinology developed as an outcome of the genome sequence revolution. Following the introduction of live vaccinations in the western world by Edward Jenner in 1798 and the coining of the phrase 'vaccine', in 1881 Pasteur developed a rational design for vaccines. Pasteur proposed that in order to make a vaccine that one should 'isolate, inactivate and inject the microorganism' and these basic rules of vaccinology were largely followed for the next 100 years leading to the elimination of several highly infectious diseases. However, new technologies were needed to conquer many pathogens which could not be eliminated using these traditional technologies. Thus increasingly, computers were used to mine genome sequences to rationally design recombinant vaccines. Several vaccines for bacterial and viral diseases (i.e. meningococcus and HIV) have been developed, however the on-going challenge for parasite vaccines has been due to their comparatively larger genomes. Understanding the immune response is important in reverse vaccinology studies as this knowledge will influence how the genome mining is to be conducted. Vaccine candidates for anaplasmosis, cowdriosis, theileriosis, leishmaniasis, malaria, schistosomiasis, and the cattle tick have been identified using reverse vaccinology approaches. Some challenges for parasite vaccine development include the ability to address antigenic variability as well the understanding of the complex interplay between antibody, mucosal and/or T cell immune responses. To understand the complex parasite interactions with the livestock host, there is the limitation where algorithms for epitope mining using the human genome cannot directly be adapted for bovine, for example the prediction of peptide binding to major histocompatibility complex motifs. As the number of genomes for both hosts and parasites increase, the development of new algorithms for pan-genomic mining will continue to impact the future of parasite and ricketsial (and other tick borne pathogens) disease vaccine development.

Targeted silencing of the Aquaporin 2 gene of Rhipicephalus (Boophilus) microplus reduces tick fitness

BACKGROUND

Ticks are blood-feeding arthropods that can affect human and animal health both directly by blood-feeding and indirectly by transmitting pathogens. The cattle tick Rhipicephalus (Boophilus) microplus is one of the most economically important ectoparasites of bovines worldwide and it is responsible for the transmission of the protozoan Babesia bovis, the etiological agent of bovine babesiosis. Aquaporins (AQPs) are water channel proteins implicated in physiological mechanisms of osmoregulation. Members of the AQP family are critical for blood-feeding arthropods considering the extreme osmoregulatory changes that occur during their feeding. We investigated the pattern of expression of a newly identified AQP2 gene of R. microplus (RmAQP2) in different tick tissues and stages. We also examined in vivo the biological implications of silencing expression of RmAQP2 silencing during tick feeding on either uninfected or B. bovis-infected cattle.

METHODS

In silico gene analyses were performed by multiple alignments of amino acid sequences and topology prediction. Levels of RmAQP2 transcripts in different tick tissues and stages were analyzed by reverse transcriptase quantitative PCR. Patterns of expression of RmAQP2 protein were investigated by immunoblots. Gene silencing was performed by RNA interference and in vivo functional analyses carried out by feeding ticks on either uninfected or B. bovis-infected cattle.

RESULTS

RmAQP2 transcripts were found in unfed larvae, engorged nymphs, and salivary glands and guts of partially engorged females; however, of all tick tissues and stages examined, RmAQP2 protein was found only in salivary glands of partially engorged females. RmAQP2 silencing significantly reduced tick fitness and completely abrogated protein expression. The effect of RmAQP2 silencing on fitness was more pronounced in females fed on a B. bovis-infected calf than in ticks fed on an uninfected calf and none of their larval progeny survived.

CONCLUSIONS

Collectively, considering the gene expression and tick fitness data, we conclude that RmAQP2 is critical for tick blood feeding and may be a suitable candidate target for the development of novel strategies to control R. microplus and tick-borne parasites.

Transmembrane proteins--Mining the cattle tick transcriptome

Managing the spread and load of pathogen-transmitting ticks is an important task worldwide. The cattle tick, Rhipicephalus microplus, not only impacts the economy through losses in dairy and meat production, but also raises concerns for human health in regards to the potential of certain transmitted pathogens becoming zoonotic. However, novel strategies to control R. microplus are hindered by lack of understanding tick biology and the discovery of suitable vaccine or acaricide targets. The importance of transmembrane proteins as vaccine targets are well known, as is the case in tick vaccines with Bm86 as antigen. In this study, we describe the localization and functional annotation of 878 putative transmembrane proteins. Thirty proteins could be confirmed in the R. microplus gut using LC-MS/MS analysis and their roles in tick biology are discussed. To the best of our knowledge, 19 targets have not been reported before in any proteomics study in various tick species and the possibility of using the identified proteins as targets for tick control are discussed. Although tissue expression of identified putative proteins through expansive proteomics is necessary, this study demonstrates the possibility of using bioinformatics for the identification of targets for further evaluation in tick control strategies.

Conserved Amblyomma americanum tick Serpin19, an inhibitor of blood clotting factors Xa and XIa, trypsin and plasmin, has anti-haemostatic functions

Tick saliva serine protease inhibitors (serpins) facilitate tick blood meal feeding through inhibition of protease mediators of host defense pathways. We previously identified a highly conserved Amblyomma americanum serpin 19 that is characterised by its reactive center loop being 100% conserved in ixodid ticks. In this study, biochemical characterisation reveals that the ubiquitously transcribed A. americanum serpin 19 is an anti-coagulant protein, inhibiting the activity of five of the eight serine protease blood clotting factors. Pichia pastoris-expressed recombinant (r) A. americanum serpin 19 inhibits the enzyme activity of trypsin, plasmin and blood clotting factors (f) Xa and XIa, with stoichiometry of inhibition estimated at 5.1, 9.4, 23.8 and 28, respectively. Similar to typical inhibitory serpins, recombinant A. americanum serpin 19 forms irreversible complexes with trypsin, fXa and fXIa. At a higher molar excess of recombinant A. americanum serpin 19, fXIIa is inhibited by 82.5%, and thrombin (fIIa), fIXa, chymotrypsin and tryptase are inhibited moderately by 14-29%. In anti-hemostatic functional assays, recombinant A. americanum serpin 19 inhibits thrombin but not ADP and cathepsin G activated platelet aggregation, delays clotting in recalcification and thrombin time assays by up to 250s, and up to 40s in the activated partial thromboplastin time assay. Given A. americanum serpin 19 high cross-tick species conservation, and specific reactivity of recombinant A. americanum serpin 19 with antibodies to A. americanum tick saliva proteins, we conclude that recombinant A. americanum serpin 19 is a potential candidate for development of a universal tick vaccine.

Rhipicephalus (Boophilus) microplus aquaporin as an effective vaccine antigen to protect against cattle tick infestations

Background

Vaccination as a control method against the cattle tick, Rhipicephalus (Boophilus) microplus has been practiced since the introduction of two products in the mid-1990s. There is a need for a vaccine that could provide effective control of R. microplus in a more consistent fashion than existing products. During our transcriptome studies of R. microplus, several gene coding regions were discovered to encode proteins with significant amino acid similarity to aquaporins.

Methods

A cDNA encoding an aquaporin from the cattle tick, Rhipicephalus microplus, was isolated from transcriptomic studies conducted on gut tissues dissected from fully engorged adult female R. microplus.

Results

Bioinformatic analysis indicates this aquaporin, designated RmAQP1, shows greatest amino acid similarity to the human aquaporin 7 family. Members of this family of water-conducting channels can also facilitate the transport of glycerol in addition to water. The efficacy of this aquaporin as an antigen against the cattle tick was explored in cattle vaccine trials conducted in Brazil. A cDNA encoding a significant portion of RmAQP1 was expressed as a recombinant protein in Pichia pastoris, purified under native conditions using a polyhistidine C-terminus tag and nickel affinity chromatography, emulsified with Montanide adjuvant, and cattle vaccinated intramuscularly. The recombinant protein provided 75% and 68% efficacy in two cattle pen trials conducted in Campo Grande, Brazil on groups of 6 one year old Holstein calves.

Conclusion

The effectiveness of this vaccine in reducing the numbers of adult female ticks shows this aquaporin antigen holds promise as an active ingredient in cattle vaccines targeted against infestations of R. microplus.

Electronic supplementary material

The online version of this article (doi:10.1186/s13071-014-0475-9) contains supplementary material, which is available to authorized users.

Immunoglobulin G binding protein (IGBP) from Rhipicephalus haemaphysaloides: identification, expression, and binding specificity

As the second most important human ectoparasite, ranked only after mosquitoes, the tick threatens the development of husbandry and even the health of humans worldwide. Immunoglobulin G binding proteins (IGBPs) are considered to be the major factors used by ticks to evade the host immune system and the damage caused by host antibodies. In this study, an IGBP-MB homologue was identified in the tick Rhipicephalus haemaphysaloides, which was predominantly detected in the salivary glands and hemolymph of male ticks. Recombinant IGBP (rIGBP/His) displayed significant binding activity to IgGs from rabbits and pigs, and bound to the F(ab)'2 but not the Fc fragment of rabbit IgG. Although the silencing of IGBP expression in ticks had no obvious effect on their blood-feeding and subsequent oviposition, antibodies raised to rIGBP/GST reduced the replete body weight (218.9 ± 20 mg in the control group vs. 142.5 ± 43.3 mg in the test group, P < 0.05 by Student's t test) and increased the mortality of the ticks. This study extends our understanding of the immunoevasive function of IGBPs and is a step towards the development of a vaccine against ticks.

Calculation of the efficacy of vaccines against tick infestations on cattle

Cattle ticks are responsible for great economic losses in cattle farming worldwide, and their main control method, chemicals, has been showing problems, whether resulting from the development of resistant strains of ticks or environmental contamination. Research studies directed toward developing vaccines against ticks are emerging. One way to evaluate those vaccines is to calculate the percentage of efficacy. The aim of this study was to analyze scientific publications archived in PubMed that used this method of assessment and discuss the main factors that may affect its calculation. Thus, 25 articles addressing this subject were selected. The percentage of efficacy was usually calculated in one of two ways, with one considering the reduced fertility of eggs and the other not. The latter method may underestimate the vaccine efficacy, and the most complete formula for calculating the efficacy reflects how much the vaccine actually affects the infestation. In our view, the use of the complete formula for calculating the percentage of efficacy is broader and more representative of the vaccine effect on the tick population.

Hd86 mRNA expression profile in Hyalomma scupense life stages, could it contribute to explain anti-tick vaccine effect discrepancy between adult and immature instars?

Bm86 midgut protein has been used in order to control ticks of the Hyalomma genus. Previous studies demonstrated the inefficacity of this antigen in the control of Hyalomma scupense, whereas recombinant Hd86 antigen, the Bm86 ortholog in H. scupense produced in Pichia pastoris, was protective against larval H. scupense tick stage infestations but ineffective in the control of the adult stage. One possible explanation for this result is the variation in Hd86 expression levels between these two developmental stages. To test this hypothesis, Hd86 mRNA levels were characterized in H. scupense developmental stages. The expression profile of Hd86 demonstrated a significant variation between tick life stages and showed a significant reduction in the number of transcripts during feeding and, particularly after molting to adults. The most interesting result was noted after molting of engorged nymphs in unfed adults where the expression levels decreased significantly by 12.78 (10.77-17.39) (p<0.001) and 9.25 (5.77-15.72)-fold (p<0.001) in unfed males and unfed females, respectively. Comparing unfed nymphs to unfed adult ticks, the Hd86 expression levels decreased by 13.82 (5.39-24.45) (p=0.035) and 9.93 (2.87-22.08)-fold (p=0.038) in males and females respectively. Lower Hd86 mRNA levels in adult ticks should result in lower protein levels and thus less antibody-antigen interactions necessary for vaccine efficacy in ticks fed on vaccinated animals. Thus, the observed differences in Hd86 expression profile between immature and adult stages might explain, in part, the discrepancy of the Hd86 vaccine efficacy against these two life stages of H. scupense.

Deorphanization and target validation of cross-tick species conserved novel Amblyomma americanum tick saliva protein

We previously identified a cross-tick species conserved tick feeding stimuli responsive Amblyomma americanum (Aam) AV422 gene. This study demonstrates that AamAV422 belongs to a novel group of arthropod proteins that is characterized by 14 cysteine amino acid residues: C(23)-X7/9-C(33)-X23/24-C(58)-X8-C(67)-X7-C(75)-X23-C(99)-X15-C(115)-X10-C(126)-X24/25/33-C(150)C(151)-X7-C(159)-X8-C(168)-X23/24-C(192)-X9/10-C(202) predicted to form seven disulfide bonds. We show that AamAV422 protein is a ubiquitously expressed protein that is injected into the host within the first 24h of the tick attaching onto the host as revealed by Western blotting analyses of recombinant (r)AamAV422, tick saliva and dissected tick organ protein extracts using antibodies to 24 and 48 h tick saliva proteins. Native AamAV422 is apparently involved with mediating tick anti-hemostasis and anti-complement functions in that rAamAV422 delayed plasma clotting time in a dose responsive manner by up to ≈ 160 s, prevented platelet aggregation by up to ≈ 16% and caused ≈ 24% reduction in production of terminal complement complexes. Target validation analysis revealed that rAamAV422 is a potential candidate for a cocktail or multivalent tick vaccine preparation in that RNA interference (RNAi)-mediated silencing of AamAV422 mRNA caused a statistically significant (≈ 44%) reduction in tick engorgement weights, which is proxy for amounts of ingested blood. We speculate that AamAV422 is a potential target antigen for development of the highly desired universal tick vaccine in that consistent with high conservation among ticks, antibodies to 24h Ixodes scapularis tick saliva proteins specifically bound rAamAV422. We discuss data in this study in the context of advancing the biology of tick feeding physiology and discovery of potential target antigens for tick vaccine development.

Efficacy of Hyalomma scupense (Hd86) antigen against Hyalomma excavatum and H. scupense tick infestations in cattle

The Rhipicephalus microplus recombinant Bm86-based tick vaccines have shown their efficacy for the control of several Hyalomma cattle ticks genera, namely H. dromedarii and H. anatolicum. However, H. scupense species, the most important tick in North Africa has never been studied. Vaccination trials using either a recombinant Bm86-based vaccine or a recombinant Hd86-based vaccine (the Bm86 ortholog in H. scupense) were conducted in cattle against immature and adult H. scupense ticks and adult H. excavatum ticks. The results showed a 59.19% reduction in the number of scupense nymphs engorging on Hd86 vaccinated cattle. However, cattle vaccination with Bm86 or Hd86 did not have an effect on H. scupense or H. excavatum adult ticks infestations. These results showed that Hd86 vaccines are selectively effective against H. scupense immature instars and emphasize on an integrated anti-tick vaccine control in North Africa.

Comparative efficacy of rHaa86 and rBm86 against Hyalomma anatolicum anatolicum and Rhipicephalus (Boophilus) microplus

Hyalomma anatolicum anatolicum and Rhipicephalus (Boophilus) microplus are the most economically important tick species in India and other tropical and subtropical regions of the world and transmit pathogens causing animal and human diseases. We demonstrated that vaccination of animal by rHaa86 could be used for the control of both H. a. anatolicum and R. (B.) microplus infestations. By comparing the efficacy of rHaa86 and rBm86, it was observed that vaccine based on rHaa86 will be more effective in controlling homologous challenge infestations (68·7% against larvae and 45·8% against adults). The results of this trial demonstrated that species-specific antigens are the better choice for vaccine development and could serve as an effective tool for the integrated control of H. a. anatolicum.

Exploring the use of an anti-tick vaccine as a tool for the integrated eradication of the cattle fever tick, Rhipicephalus (Boophilus) annulatus

Bovine babesiosis, also known as cattle fever, is a tick-borne protozoal disease foreign to the United States. It was eradicated by eliminating the vector species, Rhipicephalus (Boophilus) annulatus and Rhipicephalus (Boophilus) microplus, through the efforts of the Cattle Fever Tick Eradication Program (CFTEP), with the exception of a permanent quarantine zone (PQZ) in south Texas along the border with Mexico. Keeping the U.S. free of cattle fever ticks in a sustainable manner is a critical national agricultural biosecurity issue. The efficacy of a Bm86-based anti-tick vaccine commercialized outside of the U.S. was evaluated against a strain of R. annulatus originated from an outbreak in Texas. Vaccination controlled 99.9 and 91.4% of the ticks 8 weeks and 5.5 months after the initial vaccination, respectively. Computer modeling of habitat suitability within the PQZ typically at risk of re-infestation with R. annulatus from Mexico predicted that at a level of control greater than 40%, eradication would be maintained indefinitely. Efficacy and computer modeling data indicate that the integration of vaccination using a Bm86-based anti-tick vaccine with standard eradication practices within the northwestern half of the PQZ could incentivize producers to maintain cattle on pasture thereby avoiding the need to vacate infested premises. Implementing this epidemiologically proactive strategy offers the opportunity to prevent R. annulatus outbreaks in the U.S., which would represent a significant shift in the way the CFTEP operates.

Cattle tick vaccines: many candidate antigens, but will a commercially viable product emerge?

The cattle tick, Rhipicephalus microplus, is arguably the world's most economically important external parasite of cattle. Sustainable cattle tick control strategies are required to maximise the productivity of cattle in both large production operations and small family farms. Commercially available synthetic acaricides are commonly used in control and eradication programs, but indiscriminate practices in their application have resulted in the rapid evolution of resistance among populations in tropical and subtropical regions where the invasive R. microplus thrives. The need for novel technologies that could be used alone or in combination with commercially available synthetic acaricides is driving a resurgence of cattle tick vaccine discovery research efforts by various groups globally. The aim is to deliver a next-generation vaccine that has an improved efficacy profile over the existing Bm86-based cattle tick vaccine product. We present a short review of these projects and offer our opinion on what constitutes a good target antigen and vaccine, and what might influence the market success of candidate vaccines. The previous experience with Bm86-based vaccines offers perspective on marketing and producer acceptance aspects that a next-generation cattle tick vaccine product must meet for successful commercialisation.

Vaccine potential of recombinant antigens of Theileria annulata and Hyalomma anatolicum anatolicum against vector and parasite

In an attempt to develop vaccine against Hyalomma anatolicum anatolicum and Theileria annulata, three antigens were expressed in prokaryotic expression system and protective potentiality of the antigens was evaluated in cross bred calves. Two groups (grs. 1 and 4) of male cross-bred (Bos indicus × Bos taurus) calves were immunized with rHaa86, a Bm86 ortholog of H. a. anatolicum, while one group of calves (gr. 2) were immunized with cocktails of two antigens viz., surface antigens of T. annulata (rSPAG1, rTaSP). One group each was kept as negative controls (grs. 3 and 5). The animals of groups 1, 2 and 3 were challenged with T. annulata infected H. a. anatolicum adults while the animals of groups 1, 3, 4 and 5 were challenged with uninfected adult ticks. A significantly high (p<0.05) antibody responses to all the three antigens were detected in immunized calves, but the immune response was comparatively higher with rHaa86 followed by rTaSP and rSPAG1. Upon challenge with T. annulata infected ticks, animals of all groups showed symptoms of the disease but there was 50% survival of calves of group 1 while all non immunized control calves (group 3) and rSPAG1+rTaSP immunized calves died. The rHaa86 antigen was found efficacious to protect calves against more than 71.4-75.5% of the challenge infestation. The experiment has given a significant clue towards the development of rHaa86 based vaccine against both H. a. anatolicum and T. annulata.

Efficacy of rBm86 against Rhipicephalus (Boophilus) microplus (IVRI-I line) and Hyalomma anatolicum anatolicum (IVRI-II line) infestations on bovine calves

With an aim to evaluate the protective potentiality of rBm86 against Rhipicephalus (Boophilus) microplus Indian Veterinary Research Institute (IVRI)-I line and Hyalomma anatolicum anatolicum IVRI-II line infestations on crossbred (Bos indicus × Bos taurus) calves, 20 animals of 3 months of age were randomly divided in to four equal groups and maintained in tick-proof conditions. Animals of groups 1 and 2 were immunized with 2 ml of rBm86 (100 μg)-based vaccine (procured from Revetmex S.A. de C.V, Mexico City, Mexico) thrice at 30 days interval. Animals of groups 3 and 4 were kept as negative control and inoculated with PBS only. Each animal of group 1 and 3 was challenged with 7-day-old 50 unfed adults of H. anatolicum anatolicum (1:1, male and female), and each animal of groups 2 and 4 was challenged with 6-8-day-old R. (B.) microplus larvae obtained from 50 mg of eggs, on 17th day of the last immunization. The efficacy of rBm86 against tick infestations was determined as percentage reduction in number of adults dropped (DT%), engorged body weight (DR%), egg masses (DO%), and immunogen efficacy (E%). The calculated data were 11.8, 10.8, 15.0, and 25.1 %, respectively, for DT, DR, DO, and E% against H. anatolicum anatolicum infestation, while in the case of R. (B.) microplus infestation, the corresponding data were 6.4, 11.24, 40.7, and 44.5 %, respectively. The results indicated partial effectiveness of rBm86 antigen(s) in imparting protection against homologous and heterologous challenge infestations of Indian ticks. The results indicated identification of more effective antigen(s) for the development of vaccine against economically important tick species in India.

Hd86, the Bm86 tick protein ortholog in Hyalomma scupense (syn. H. detritum): expression in Pichia pastoris and analysis of nucleotides and amino acids sequences variations prior to vaccination trials

The genus Hyalomma includes the most frequent tick species infesting livestock in North Africa, one of these species, Hyalomma scupense (syn. H. detritum) is particularly important due to its role in the transmission of tropical theileriosis to cattle (Theileria annulata infection). We have cloned and characterized the orthologs of the Bm86 gene from H. scupense strains collected over Tunisia in 2006 and 2009. The recombinant protein rHd86 was expressed in Pichia pastoris for vaccination purpose using a transcript from the 2006 strain. The rHd86 was then purified from the yeast culture supernatant by a filtration and a size exclusion process. It was recognized by specific anti-Bm86 antisera. An important extent of inter-specific diversity ranging from 35 to 40% was recorded between Hd86 and Bm86/Bm95 proteins whilst a very limited level of intra-specific diversity (1.7%) occurred between the Hd86 vaccine candidate protein and its homologues from H. scupense strains collected in 2009. These results emphasise the need for assessing the efficacy against H. scupense and others important cattle Hyalomma species in Tunisia of our Hd86 vaccine candidate alongside with a Bm86 vaccine.

Bm86 midgut protein sequence variation in South Texas cattle fever ticks

Background

Cattle fever ticks, Rhipicephalus (Boophilus) microplus and R. (B.) annulatus, vector bovine and equine babesiosis, and have significantly expanded beyond the permanent quarantine zone established in South Texas. Currently, there are no vaccines approved for use within the United States for controlling these vectors. Vaccines developed in Australia and Cuba based on the midgut antigen Bm86 have variable efficacy against cattle fever ticks. A possible explanation for this variation in vaccine efficacy is amino acid sequence divergence between the recombinant Bm86 vaccine component and native Bm86 expressed in ticks from different geographical regions of the world.

Results

There was 91.8% amino acid sequence identity in Bm86 among R. microplus and R. annulatus sequenced from South Texas infestations. When South Texas isolates were compared to the Australian Yeerongpilly and Cuban Camcord vaccine strains, there was 89.8% and 90.0% identity, respectively. Most of the sequence divergence was focused in one region of the protein, amino acids 206-298. Hydrophilicity profiles revealed that two short regions of Bm86 (amino acids 206-210 and 560-570) appear to be more hydrophilic in South Texas isolates compared to vaccine strains. Only one amino acid difference was found between South Texas and vaccine strains within two previously described B-cell epitopes. A total of 4 amino acid differences were observed within three peptides previously shown to induce protective immune responses in cattle.

Conclusions

Sequence differences between South Texas isolates and Yeerongpilly and Camcord strains are spread throughout the entire Bm86 sequence, suggesting that geographic variation does exist. Differences within previously described B-cell epitopes between South Texas isolates and vaccine strains are minimal; however, short regions of hydrophilic amino acids found unique to South Texas isolates suggest that additional unique surface exposed peptides could be targeted.