Immune responses of cattle to biochemically modified antigens from the midgut of the cattle tick, Boophilus microplus

Cocktail Anti-Tick Vaccines: The Unforeseen Constraints and Approaches toward Enhanced Efficacies

Ticks are second to mosquitoes as vectors of disease. Ticks affect livestock industries in Asia, Africa and Australia at ~ $1.13 billion USD per annum. For instance, 80% of the global cattle population is at risk of infestation by the Rhipicephalus microplus species-complex, which in 2016 was estimated to cause $22–30 billion USD annual losses. Although the management of tick populations mainly relies on the application of acaricides, this raises concerns due to tick resistance and accumulation of chemical residues in milk, meat, and the environment. To counteract acaricide-resistant tick populations, immunological tick control is regarded among the most promising sustainable strategies. Indeed, immense efforts have been devoted toward identifying tick vaccine antigens. Until now, Bm86-based vaccines have been the most effective under field conditions, but they have shown mixed success worldwide. Currently, of the two Bm86 vaccines commercialized in the 1990s (Gavac^TM in Cuba and TickGARD^PLUS™ in Australia), only Gavac™ is available. There is thus growing consensus that combining antigens could broaden the protection range and enhance the efficacies of tick vaccines. Yet, the anticipated outcomes have not been achieved under field conditions. Therefore, this review demystifies the potential limitations and proposes ways of sustaining enhanced cocktail tick vaccine efficacy.

Metazoan Parasite Vaccines: Present Status and Future Prospects

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

A systematic, functional genomics, and reverse vaccinology approach to the identification of vaccine candidates in the cattle tick, Rhipicephalus microplus

In the post-genomic era, reverse vaccinology is proving promising in the development of vaccines against bacterial and viral diseases, with limited application in ectoparasite vaccine design. In this study, we present a systematic approach using a combination of functional genomics (DNA microarrays) techniques and a pipeline incorporating in silico prediction of subcellular localization and protective antigenicity using VaxiJen for the identification of novel anti-tick vaccine candidates. A total of 791 candidates were identified using this approach, of which 176 are membrane-associated and 86 secreted soluble proteins. A preliminary analysis on the antigenicity of selected membrane proteins using anti-gut antisera yielded candidates with an IgG binding capacity greater than previously identified epitopes of Bm86. Subsequent vaccination trials using recombinant proteins will not only validate this approach, but will also improve subsequent reverse vaccinology approaches for the identification of novel anti-tick vaccine candidates.

Vaccine Efficacy of Bm86 Ortholog of H. a. anatolicum, rHaa86 Expressed in Prokaryotic Expression System

The use of tick vaccine in controlling ticks and tick borne diseases has been proved effective in integrated tick management format. For the control of H. a. anatolicum, Bm86 ortholog of H. a. anatolicum was cloned and expressed as fusion protein in E. coli as E. coli-pETHaa86. The molecular weight of the rHaa86 was 97 kDa with a 19 kDa fusion tag of thioredoxin protein. The expressed protein was characterized immunologically and vaccine efficacy was evaluated. After 120 hours of challenge, only 26% tick could successfully fed on immunized animals. Besides significant reduction in feeding percentages, a significant reduction of 49.6 mg; P < .01 in the weight of fed females in comparison to the females fed on control animals was recorded. Following oviposition, a significant reduction of 68.1 mg; P < .05 in the egg masses of ticks fed on immunized animals in comparison to the ticks fed on control animals was noted. The reduction of number of females, mean weight of eggs, adult females and efficacy of immunogen were 73.8%, 31.3%, 15.8%, and 82.3%, respectively. The results indicated the possibility of development of rHaa86 based vaccine as a component of integrated control of tick species.

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.

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.

Skin test and tick immune status in susceptible and resistant cattle in Brazil

A cutaneous hypersensitivity test (CHT) was used to correlate host resistance to ticks and type of reaction elicited to unfed larval extract-ULE of the cattle tick Boophilus microplus in European and Indian cattle. Twenty calves were separated into four groups of five animals each: naïve or preinfested Indian or European cattle. CHT was induced by intradermal inoculation of 0.1 ml of ULE cattle tick B. microplus (50 micrograms protein) in the calf ear. Ear thickness was measured using calipers before and 10 min, 1, 2, 6, 18, 24, 48, 72, 96, and 144 h postinoculation (PI). Preinfested European calves showed only an immediate type reaction with maximum response (75% increase in ear thickness) at 10 min PI. On the other hand, preinfested Indian calves presented an immediate response with maximum reaction (70% increase in ear thickness) between 10 min and one hour PI, and a delayed type reaction at 72 h PI (60% increase in ear thickness). These results point out the crucial role of the cellular immune response of cattle in the expression of resistance to cattle tick B. microplus. Skin test might be useful in the ranking of cattle according to the susceptibility/resistance to ticks.

Purification of a diagnostic, secreted cysteine protease-like protein from the hookworm Ancylostoma caninum

The enteric infection of humans with the canine hookworm Ancylostoma caninum varies in its clinical presentation, ranging from asymptomatic to eosinophilic gastroenteritis requiring surgical intervention. Infections are not patent, but can be diagnosed immunologically by detecting antibodies to an immunodominant secreted hookworm protein termed Ac68. To characterise Ac68, we purified the native protein from A. caninum excretory/secretory products using size exclusion followed by anion exchange chromatography. The epitopes in the purified protein recognised by human infection sera were shown to be proteins and not carbohydrates. The N-terminal amino acid sequence of the purified Ac68 was determined and six of the 11 residues obtained were shared with a previously characterised cysteine protease of A. caninum, AcCP1.

Arthropod vaccines

Antigens located in the midgut of the tick are hidden from the host's immune system. Egg production of ticks can be reduced when ticks are fed on animals vaccinated with midgut antigens of the tick, and a subunit vaccine formulated with the recombinant antigen Bm86 is now available that can reduce the number of ticks infesting cattle grazing on pasture. Midgut antigens used in vaccines against insects that transmit pathogenic organisms to humans have not been as effective in reducing insect fecundity and an alternative approach may be necessary. Transmission-blocking vaccines directed at interfering with the vector-pathogen interaction could result in loss of vector competence and block the spread of disease-causing organisms.

Novel vaccines for ectoparasites

Novel vaccines against ectoparasites have the potential to be cost-effective new technology for pest control that avoids some of the real and perceived problems with insecticide and acaricide usage. Nevertheless, their development is in its infancy. A vaccine against the cattle tick Boophilus microplus, the world's first vaccine against an ectoparasite, is in field use in Australia. Considerable effort had gone into the development of a vaccine against the sheep blowfly Lucilia cuprina, while other vaccines are at an earlier stage of development. The identification of critical antigens and their production as effective recombinant proteins remains the greatest hurdle. Characteristics of the few known antigens and the mode of action of the protective immune response are discussed. Development of further vaccines will depend on recognition of likely antigenic targets. The efficacy of such vaccines will depend on the characteristics of the target species, in particular its digestive biology and the way in which the novel vaccine impacts on the parasite population.

Uniformity of protective antigens among isolates of the cattle tick, Boophilus microplus

Gut membrane antigens were extracted from ten isolates of the cattle tick Boophilus microplus; the antigen extracts were probed with bovine antisera and three murine monoclonal antibodies (mAbs) in Western blots and dot-ELISA. The antisera had been obtained from cattle which were vaccinated with larval and gut extracts of B. microplus, and which were subsequently protected (84% and 94% respectively) against challenge with B. microplus. One of the mAbs (QU13) has been demonstrated to precipitate protective antigens form the midgut of B. microplus. Gut antigens from all ten isolates displayed similar reactivity profiles against bovine antisera and also against mAbs in Western blots. The end-point titres of antigens in dot-ELISA showed four-fold variation between isolates against bovine antisera, and also against mAb QU13. Larval membrane antigen extracted from N-strain B. microplus reacted with QU13 in dot-ELISA, indicating that protective antigens are common to both larval and adult stages of B. microplus. It was concluded that protective antigens recognized by QU13 and antigens recognized by sera from protected cattle were conserved between the ten isolates examined, and between life-cycle stages.

A study of the vaccinal value of various extracts of concealed antigens and salivary gland extracts against Ornithodoros erraticus and Ornithodoros moubata

On pig farms, elimination of the argasid ticks acting as reservoirs and vectors for African swine fever greatly favours the eradication of this disease. The elimination of Ornithodoros erraticus involves many problems, most of which could be easily solved by the development of an anti-O. erraticus vaccine. With a view to developing this vaccine, we have tested the protective value of the immune response induced in swine by seven 'concealed' antigens and one soluble salivary gland extract. The latter extract was also prepared from Ornithodoros moubata specimens and tested against this tick. Our results indicate that the immune response elicited by the concealed antigens has no protective value against O. erraticus. The immune response induced by the salivary gland extracts against adults of O. erraticus and O. moubata was apparent in a reduced ingestion of blood (40-60%; P < 0.01) (except in males of O. erraticus) and in a significant decrease (40-60%; P < 0.01) in fecundity in 100% of the females of both species. The good results obtained with salivary antigens, which in situations of natural contact have no protective value, are attributed to the fact that when these antigens are injected with adjuvants, the immune system recognizes certain salivary components (probably those which enable the parasite to feed) which it does not recognize under natural conditions of exposure.

Acquired resistance in dogs to repeated infestation with Ixodes scapularis (Acari: Ixodidae) reduces tick viability and reproductive success

The purpose of this study was to determine whether dogs develop acquired resistance to adult Ixodes scapularis infestation in an experimental model. Five dogs were each infested with ten mating pairs of ticks every week for 7 consecutive weeks, another five dogs were each infested with ten mating pairs once every 2 weeks for 10 weeks and four dogs served as controls not exposed to ticks. All ticks were allowed to feed to repletion and were collected only after dropping from the host. Several variables were measured to determine the extent of blood feeding success. Regression analysis indicated that the engorgement success, survival and mean tick engorgement weight declined with repeated infestation in both groups of dogs (p < 0.05). Tick oviposition as well as the F1 viability declined with each successive infestation in both groups. These results suggest that repeated infestation with I. scapularis elicits a protective immune response against tick feeding and could serve as a limiting factor in the spread and transmission of Borrelia burgdorferi.

Targeting ticks for control of selected hemoparasitic diseases of cattle

Development in and transmission of hemoparasites by tick vectors are phenomena closely synchronized with the tick feeding cycle. In all known life cycles, initial infection of tick tissues occurs in midgut epithelial cells and transmission is effected as ticks feed after parasites have developed and multiplied in salivary glands. Many factors reviewed affect development and transmission of hemoparasites by ticks including age of ticks, artificial temperature, climate and/or season, tick stage or sex, hemoparasite variation, concurrent infection of ticks with other pathogens, host cell susceptibility, transovarial transmission, effect of hemoparasites on tick biology, and the effect of infecting parasitemia level in cattle on infection rates in ticks. Four hemoparasites of cattle, Anaplasma marginale, Cowdria ruminantium, Theileria parva, and Babesia spp., are all dependent on ticks for biological transmission. Babesia is transmitted transovarially whereas the other three are transmitted transstadially. Mechanical transfer of infective blood via fomites and mouthparts of biting arthropods is also a major means of transmission for Anaplasma marginale but not of the others. Potential control methods for hemoparasites that target parasites as they are developing in their respective tick hosts include tick control, vaccines (against ticks and parasites), and drugs (against ticks and parasites). Successful application of control strategies will be dependent upon thorough understanding of parasite developmental cycles, biology of the tick vectors and the immune response of cattle to ticks and to hemoparasites. The most effective control measures will be those that are targeted against both ticks and the hemoparasites they vector.

Antigens identified by monoclonal antibodies in tissue sections of Boophilus microplus

A panel of monoclonal antibodies (mAbs) raised against midgut antigens of Boophilus microplus were used to probe various stages and organs of the tick. One of the monoclonal antibodies in this panel (QU13) has previously been shown to recognize protective antigens. Of the 18 mAbs tested, all except two (QU5 and QU12) reacted with sections of adult midgut and Malpighian tubules using an avidin-biotin alkaline phosphatase method for immunostaining. MAbs QU1, QU2, QU3, QU4, QU12, QU13, and QU18 reacted specifically with the lumenal surfaces of type III acini of the salivary gland. These seven mAbs also stained the midgut in larval sections indicating that the antigens recognized were not stage specific. However, none of the seven mAbs tested recognised antigens in either the adult ovary or the developing egg. Antigens which were immunogold labelled by mAbs QU1, QU4, QU11, QU13, and QU15 in electron microscopy were located either on or near the surface of the microvilli of digestive cells from the midgut of the adult tick. We conclude that common antigens are present on the lumenal surfaces of the adult midgut, type III acini of the salivary gland, and the Malpighian tubules and that these antigens are also located in the larval gut.

Anti-idiotypic antibodies as surrogate antigens for vaccinating against the cattle tick, Boophilus microplus

QU13, a monoclonal antibody (mAb) raised against midgut (GM) antigens from Boophilus microplus and shown to recognise antigens which protect cattle from tick challenge was used to immunise cattle and rabbits to produce anti-idiotypic antibodies (AIA). Polyclonal antisera against mAb QU13 were produced in rabbits and cattle. AIA were purified from these antisera by affinity chromatography procedures. These purified AIA were found to block mAb QU13 binding to GM in enzyme-linked immunosorbent assay (ELISA). AIA purified from bovine antiserum elicited an immune response in cattle to antigens extracted by detergent from the midgut of B. microplus (TXGM) after the fourth vaccination (P = 0.06) compared with the bovine immunoglobulin (Ig) control. The mean antibody level in the group of experimental cattle vaccinated with AIA purified from rabbit antisera was significantly higher (P < 0.03) than that of bovine Ig control cattle after the fourth vaccination and an anamnestic response (P < 0.11) occurred in the rabbit AIA vaccinated group of cattle when a single booster dose of 300 micrograms of TXGM was given after the first tick challenge. The positive control group of cattle vaccinated with TXGM were significantly protected (P < 0.05) against tick infestation after the booster dose of 300 micrograms of TXGM. The AIA vaccinated groups of cattle were not protected against challenge with 20,000 larval ticks either before or after the booster injection of 300 micrograms of TXGM.

Vaccines against blood-sucking arthropods

This paper provides selected personal insights on the development of vaccines against blood-sucking arthropods, with particular emphasis on vaccines against ticks. The emergence of novel or concealed antigens of haematophagous ectoparasites as candidate vaccine antigens is reviewed and the effect of feeding by the parasite on the expression of protective antigens is considered. The distribution of protective antigens through life cycle stages, the stage of the life cycle targeted by protective responses, and the nature of these responses, are commented on briefly. Concealed antigens of the gut, including the peritrophic membrane, and other internal organs, are evaluated for the role they play in induction of immunity artificially. Some of the work carried out to purify and characterise protective antigens of tick guts is described. A commentary is developed on vaccines that combine both "concealed" and "exposed" antigens. Some of the problems associated with the infestation and challenge of vaccinated hosts in the field are identified and the delivery of parasite antigens as vaccines that are both protective and "user-friendly" is emphasised as a major problem to be solved.

Evaluation of pepsinogen, gastrin and antibody response in diagnosing ostertagiasis

Ostertagia ostertagi is widely distributed and is one of the most important parasites affecting young bovine livestock. There is, therefore, a substantial need for sensitive and specific parameters in support of diagnosis of ostertagiasis, especially for subclinical disease related to production losses. In this review, the value and application of pepsinogen, gastrin and antibody response as diagnostic tools are discussed. These three parameters are useful and comparable for confirming clinical disease in calves during their first grazing season. However, their value for detecting subclinical parasitism is questionable. Differences in the course of gastrin and pepsinogen late in the grazing season can be correlated with larval inhibition and the possibility of ostertagiasis Type II. Relatively few serological methods have been developed for the immunodiagnosis of Ostertagia and until now the indirect antibody-detecting enzyme-linked immunosorbent assay (ELISA) has been the method of choice. Antibody measuring methods have several disadvantages, most notably a lack of sensitivity and specificity, which limits their use in longitudinal epidemiological studies. Considering the necessity of cost effectiveness and ease of use, it is anticipated that additional work will result in the enhancement and quality of current immunodiagnostic methods.

Antigenic differences between the life cycle stages of Cooperia oncophora

The antigenic differences among the life cycle stages of Cooperia oncophora were studied by SDS-gel electrophoresis and Western blotting. Sodium dodecylsulphate polyacrylamide gel electrophoresis of the different life cycle stages of C oncophora revealed a complex protein pattern with a decreasing number of protein bands towards the adult stages. Several bands of the fourth stage larvae were in common with both the third stage and the adult nematode. Western blotting with sera from C oncophora monoinfected calves showed that the antigens of the fourth stage larvae were recognised predominantly and the presence of stage specific antigens in all stages. Strong cross reactivity was demonstrated when serum from Ostertagia ostertagi infected calves was used.