Expression of genes encoding two major Theileria annulata merozoite surface antigens in Escherichia coli and a Salmonella typhimurium aroA vaccine strain

Humoral and cellular immunity in response to an in silico-designed multi-epitope recombinant protein of Theileria annulata

Tropical theileriosis is a lymphoproliferative disease caused by Theileria annulata and is transmitted by Ixodid ticks of the genus Hyalomma. It causes significant losses in livestock, especially in exotic cattle. The existing methods for controlling it, chemotherapeutic agents and a vaccine based on an attenuated schizont stage parasite, have several limitations. A promising solution to control this disease is the use of molecular vaccines based on potential immunogenic proteins of T. annulata. For this purpose, we selected five antigenic sequences of T. annulata, i.e. SPAG-1, Tams, TaSP, spm2, and Ta9. These were subjected to epitope prediction for cytotoxic T lymphocytes, B-cells, and helper T lymphocytes. CTL and B-cell epitopes with a higher score whereas those of HTL with a lower score, were selected for the construct. A single protein was constructed using specific linkers and evaluated for high antigenicity and low allergenicity. The construct was acidic, hydrophobic, and thermostable in nature. Secondary and tertiary structures of this construct were drawn using the PSIPRED and RaptorX servers, respectively. A Ramachandran plot showed a high percentage of residues in this construct in favorable, allowed, and general regions. Molecular docking studies suggested that the complex was stable and our construct could potentially be a good candidate for immunization trials. Furthermore, we successfully cloned it into the pET-28a plasmid and transformed it into the BL21 strain. A restriction analysis was performed to confirm the transformation of our plasmid. After expression and purification, recombinant protein of 49 kDa was confirmed by western blotting. An ELISA detected increased specific antibody levels in the sera of the immunized animals compared with the control group, and flow cytometric analysis showed a stronger cell-mediated immune response. We believe our multi-epitope recombinant protein has the potential for the large-scale application for disease prevention globally in the bovine population. This study will act as a model for similar parasitic challenges.

The protection afforded to cattle immunized with Theileria annulata infected cell line is enhanced by subunit vaccine candidate TaSP

Tropical theileriosis constraints the development of the dairy industry in the Sudan and vaccination using live attenuated schizont vaccines is considered a promising measure for its control. The present study was carried out to investigate the ability of recombinant T. annulata surface protein (TaSP) to improve the efficacy of the attenuated Atbara cell line in protecting calves against field challenge. To this end, 23 cross-bred (Friesian × Kenana) calves were divided into four groups. Animals in group 1 (n = 5) were left unvaccinated. Group 2 (n = 6) received the Atbara cell line, animals in group 3 (n = 6) were immunized with three doses of TaSP on days 21, 49 and 77, while animals in group 4 (n = 6) received the cell line vaccine on day 0 and three doses of TaSP in Freund's incomplete adjuvant at days 21, 49 and 77. Twenty-eight days after the last TaSP boost, all groups were challenged by exposing them to natural field tick infestation in a region known to be endemic for tropical theileriosis. No thermal reactions, piroplasms or schizonts were observed in the immunized animals following immunization. Upon challenge, all animals showed a range of symptoms of clinical theileriosis with variable degrees of severity. The application of TaSP alone appeared to have no effect in terms of protection. The efficacy of the cell line alone was lower than the 100% level of protection against mortality observed in the group that received the combined cell line vaccine and TaSP, suggesting a synergistic effect of this combination.

Sequence heterogeneity in the equi merozoite antigen gene (ema-1) of Theileria equi and development of an ema-1-specific TaqMan MGB assay for the detection of T. equi

Although a quantitative real-time PCR assay (qPCR) assay for the detection of Theileria equi has been developed and evaluated, it is possible that additional, as yet undetected 18S rRNA gene sequence variants may exist. A qPCR assay targeting a different gene, used in conjunction with the T. equi 18S rRNA qPCR assay, could assist in the detection of all T. equi genotypes in field samples. A T. equi ema-1-specific qPCR (Ueti et al., 2003) was tested on 107 South African field samples, 90 of which tested positive for T. equi antibody using the immuno-fluorescent antibody test (IFAT). The qPCR assay performed poorly, as T. equi was detected in only 67 of the 90 IFAT-positive field samples at quantification cycle (C(q)) values ranging from 27 to 39.95. Furthermore, a high C(q) value of 36.18 was obtained from DNA extracted from a South African in vitro-cultured T. equi WL isolate [1.38% parasitized erythrocytes (PE)] when a low C(q) value (indicative of a high T. equi DNA concentration) was expected. Approximately 600 bp of the ema-1 gene from 38 South African samples were sequenced and BLASTN analysis confirmed all sequences to be merozoite surface protein genes, with an identity of 87.1-100% to previously published T. equi ema-1 gene sequences. Alignment of the sequences revealed extensive sequence variations in the target regions of the primers and probes (Ueti et al., 2003), explaining the poor performance of the qPCR assay. Based on these observations, we developed a new TaqMan minor-groove binder (MGB) probe-based qPCR assay, targeting a more conserved region of the ema-1 gene. This assay was shown to be efficient and specific, and the detection limit, defined as the concentration at which 95% of T. equi-positive samples are detected, was determined to be 1.4 x 10(-4)% PE. The two ema-1 assays were compared by testing 41 South African field samples in parallel. The results suggested that the new assay was more sensitive than the original assay, as T. equi was detected in more samples and at lower C(q) values when the new assay was used. Phylogenetic analyses of the 18S rRNA gene sequences and ema-1 amino acid sequences from the same samples showed inconsistencies between the clades, indicating that the T. equi 18S rRNA genetic groups previously identified in South Africa may not represent distinct T. equi lineages. It is possible that the different T. equi ema-1 genotypes could be related to antigenic variability and pathogenicity and may be associated with clinical differences in equine piroplasmosis cases, but this remains to be elucidated.

Stochastic induction of Theileria annulata merogony in vitro by chloramphenicol

Theileria annulata inhabits the cytoplasm of bovine leukocytes where it can be found as a multinucleated schizont. The schizont is the pathogenic stage of the life cycle and by interfering with host signalling pathways, it induces unlimited host cell proliferation and protection against apoptosis. In the infected animal, the schizont differentiates to the merozoite life cycle stage in a process called merogony. This takes place within the host leukocyte, resulting in the production of merozoites that are subsequently released by leukocyte lysis. In established cultures of T. annulata-transformed cells, merogony does not spontaneously occur, but the process can be activated by a shift in temperature. In this study we show that chloramphenicol induces schizont differentiation in proliferating T. annulata-transformed cells. We demonstrate that chloramphenicol-induced merogony is inherently asynchronous and has a quantitative basis. The process is accompanied by the down-regulation of schizont-specific surface proteins, de novo expression of merozoite-specific markers such as Tamr1 and Tams1 and the morphological hallmarks of merogony. Chloramphenicol-induced parasite differentiation was found to be associated with diminished proliferation potential and extensive morphological changes of the host cell, including increased numbers of pseudopodia. Significantly, chloramphenicol treatment can accelerate merogony induced by elevated temperature, supporting postulation that the differentiation event is a stochastic process that can be manipulated to alter the outcome of parasitic infection.

DNA vaccine for rabies: relevance of the trans-membrane domain of the glycoprotein in generating an antibody response

Various studies have demonstrated the potential of immunization with DNA vaccines encoding the rabies virus glycoprotein (RV-G) to elicit humoral responses. In the present study, we have designed four constructs using a VR1020 vector, wherein the RV-G ectodomain has been cloned without the signal sequence (SS) and the trans-membrane domain (TD) (rGVR), without the SS but with the TD (rGVRt), with the SS but without the TD (rGVRs) and with the SS and the TD (rGVRst), under the control of a cytomegalovirus (CMV) promoter, and downstream of the tissue plasminogen activator (TPA) signal sequence. In addition, RV-G has been expressed as a His6 tag fusion protein, both in Escherichia coli as well as in baculovirus expression systems. Using a prime-boost strategy, BALB/cJ mice administered with the rGVRt construct either in saline (intramuscularly) or adsorbed onto gold microcarriers (delivered intradermally by gene gun) generated the highest rabies virus neutralizing antibody (RVNA) titers. Inclusion of the SS, in addition to the TD (rGVRst), led to a significant decrease in RVNA titers, compared to the rGVRt construct. The DNA vaccine construct lacking both the SS and the TD domain and the vaccine having only the SS generated lower antibody responses, compared to the rGVRt construct. After priming with DNA vaccine, boosting with both E. coli- as well as baculovirus-expressed rRV-G led to an increase in the RVNA titers. The present results demonstrate that a DNA vaccine encoding the full-length sequence of the ectodomain plus TD of the mature native RV-G is capable of expressing an 'ideal' immunogen to produce RVNA titers.

A role for tertiary structure in the generation of antigenic diversity and molecular association of the Tams1 polypeptide in Theileria annulata

The major merozoite-piroplasm surface antigen (mMPSA) of Theileria annulata, Tams1, is known to be antigenically diverse. The possession of variable N-linked glycosylation sites and removal of monoclonal antibody 5E1 reactivity by mild periodate treatment suggested, previously, that divergent epitopes may be conferred by secondary modification. This study has shown that monoclonal antibody 5E1 and polyspecific antisera raised against the native protein react against divergent amino acid epitopes that are dependent on a molecular conformation that is sensitive to periodate. Therefore, no experimental evidence exists to confirm the sequence prediction that Tams1 undergoes N-linked glycosylation. Data is also presented indicating that the conformation of the antigen results in presentation of divergent regions on the external surface of the molecule, while conserved regions are more likely to be internal and hidden. In addition, non-reducing SDS-PAGE analysis demonstrated that Tams1 can undergo molecular association to form homo-dimers, trimers and multimers. The potential influence of tertiary structure and inter-molecular association on Tams1 diversity and function is discussed.

Characterization of attenuated Theileria annulata vaccines from Spain and the Sudan

Theileriosis caused by Theileria annulata can be effectively prevented by vaccination with attenuated, cultured schizonts. Although these attenuated vaccines have been applied for a long time, not much is known about the fate of the vaccine strain in the field. Here, two experimental Spanish vaccine strains originating in Cádiz and Cáceres, and one Sudanese strain are studied to address the development of a carrier status and the infectivity for Hyalomma ticks. Moreover, the heterogeneity of the merozoite surface protein, Tams1, was analyzed in search for an attenuation marker. Using the sensitive reverse line blot (RLB) hybridization, the development of a low level carrier status was demonstrated in the Cáceres and Sudanese line vaccinated calves. Although no signal was detected in the Cádiz line vaccinated calves, seroconversion against the schizont stage was observed, as it was in all other calves. The experimental transmission of T. annulata by Hyalomma ticks to naïve calves was unsuccessful for all cell line inoculated calves. Tams1 heterogeneity indicated a clonal selection of parasites during the process of attenuation, but the Tams1 sequence itself has no connection with the attenuation status. In conclusion, a carrier status develops in attenuated schizont culture vaccinated calves, but is not infective for Hyalomma ticks. Based on these data, the risk for spread of the vaccine strains in the field may be very low.

Refolding, structural transition and spermatozoa-binding of recombinant bonnet monkey (Macaca radiata) zona pellucida glycoprotein-C expressed in Escherichia coli

An internal cDNA fragment (978 bp) corresponding to bonnet monkey (Macaca radiata) zona pellucida glycoprotein-C (bmZPC), excluding the N-terminal signal sequence and the C-terminal transmembrane-like domain, was cloned in pQE-30 vector and the protein expressed as inclusion bodies in Escherichia coli. Recombinant bmZPC (r-bmZPC) was solubilized from purified inclusion bodies in the absence of a high concentration of chaotropic agents and was subsequently refolded. Use of a low concentration of urea (2 M) during solubilization of r-bmZPC helped to minimize the extent of protein aggregation during refolding of the recombinant protein, and retain the existing native-like secondary structure that was essential for proper folding. Purified r-bmZPC appeared as a dominant band of 43 kDa on SDS/PAGE and Western blot. Although it lacked carbohydrate moieties, the purified and refolded r-bmZPC bound to the head region of bonnet monkey spermatozoa, confirming the existence of a native-like conformation. CD revealed a maximum at 200 nm and a single broad minimum extending from 209 to 216 nm, indicating the presence of both alpha-helical and beta-sheet conformations in the refolded r-bmZPC. Two different phases of transition were observed by urea-gradient electrophoresis, suggesting the existence of multiple intermediate stages during the unfolding of r-bmZPC. The availability of refolded r-bmZPC will help in elucidating its role during the complex cascade of events during fertilization.

Development of an indirect Tams1 enzyme-linked immunosorbent assay for diagnosis of Theileria annulata infection in cattle

An enzyme-linked immunosorbent assay (ELISA) was developed based on a recombinant major Theileria annulata merozoite surface antigen, Tams1. Four different recombinant proteins derived from two different Tams1 alleles, both in two different truncated forms, were tested for their performance in the ELISA. Furthermore, antigen concentration, various buffers, washing protocol, and the choice of anti-total-immunoglobulin G (IgG), anti-IgG1, or anti-IgG2 as second antibody were evaluated. The performance of the resulting ELISA was analyzed by measuring the coefficient of variation (CV). A total of 22 sera were analyzed over the measurement range, resulting in a CV of ca. 10%, whereas 30% variation is the maximum acceptable. The cutoff value was determined by the two-graph receiver operating characteristic (TG-ROC), using the indirect fluorescent antibody test (IFAT) as a reference. It was shown that up to 3 months postinfection (p.i.) IFAT is more sensitive and specific, whereas beyond 3 months p.i. ELISA performed as well as IFAT. The cutoff was determined at maximal sensitivity, based on the TG-ROC after 3 months p.i. Nine calves experimentally infected with four different T. annulata stocks remained positive in the ELISA for at least 1 year p.i. Finally, limited cross-reaction was found only with T. parva antisera, but not with any other Theileria or Babesia species. Since the T. parva endemic area hardly overlaps with T. annulata, the Tams1 ELISA has the potential to become a useful tool in the epidemiology of tropical theileriosis.

Immunity and vaccine development in the bovine theilerioses

There are three economically important bovine Theileria species: Theileria annulata, which causes tropical theileriosis and occurs across north Africa and most of central Asia; Theileria parva, which causes East Coast fever and is found in East and Central Africa; and Theileria sergenti, which is predominantly a problem in Japan and Korea. Theileria annulata preferentially infects macrophages in vivo. It is controlled largely by means of live, attenuated vaccines, which are produced by prolonged tissue culture of the schizont-infected cells. The immunity induced in animals, which have either recovered from an infection or have been vaccinated (with an attenuated vaccine), is broad, solid and cell mediated. It is considered that the main effector cells are cytostatic macrophages that produce nitric oxide. Subsidiary roles for bovine leucocyte antigen (BoLA)-restricted, transiently appearing, cytotoxic T cells, and possibly also natural killer (NK) cells, have been identified. Cytokines such as tumour necrosis factor alpha (TNF-alpha) may have important roles, particularly in the induction of pathology. Matrix metalloproteinases have been implicated in the metastatic behaviour of schizont-infected cells. The nature of the protective schizont target antigens remains unknown. Attempts to develop a subunit vaccine have focused upon a sporozoite antigen (SPAG-1) and a merozoite antigen (Tams1). Both SPAG-1 and Tams1 have given partial protection using different delivery systems and adjuvants, but further vaccine development will probably require identification of a range of other antigens, especially from the schizont stage. Theileria parva has a tropism for T cells. Vaccination is currently by the 'infection and treatment' method, which involves challenging with a controlled dose of sporozoite stabilate and the simultaneous administration of long-acting tetracyclines. The immunity thus induced is mediated by BoLA-restricted cytotoxic T cells, which recognize polymorphic schizont antigens. These antigens have not been characterized at the molecular level. However, the polymorphic nature of the target antigens underlies the fact that the immunity is very strain specific--a situation that distinguishes T. parva from T. annulata. Interestingly, it is not possible to produce an attenuated vaccine to T. parva, as T. parva requires up to two orders of magnitude more schizonts in order to achieve transfer to the new host. A suggested reason for this is that the macrophage targets of T. annulata are phagocytes and thus the schizont has a natural, efficient route of entry whilst the preferred host of T. parva is the non-phagocytic T cell. Analysis of the cytotoxic T-cell response has revealed evidence of BoLA haplotype dominance plus competition between parasite epitopes. Subunit vaccination using a recombinant sporozoite antigen (p67) has proved very promising, with levels of protection of the order of 70% being achieved. A proportion of the protected calves exhibits complete sterile immunity. Interestingly, the basis for this immunity is not clear, since there is no correlation between the titre of antibodies that inhibit sporozoite penetration of lymphocytes and protection. Similarly, there is no significant T-cell response that distinguishes the protected and susceptible animals. These data are very encouraging, but other components, particularly those derived from the schizont, need to be identified and characterized. The mild Theileria species of Japan and Korea (termed T. sergenti in the literature) cause fever and severe chronic anaemia. The schizont stage of the life cycle is very rare and the host cell type is not known. The pathology is associated with chronic piroplasm infection. Immunity can be induced by immunizing with crude piroplasm extracts. Serological analysis of immune sera reveals that the immunodominant antigen is a polypeptide of 30-33 kDa, which corresponds to the protective T. annulata polypeptide Tams1. (ABSTRACT T

Different vaccine strategies used to protect against Theileria annulata

SPAG-1, a sporozoite surface antigen of T. annulata, has previously been shown to elicit partial protection when used, as an hepatitis B core antigen fusion, to immunize cattle. The objective of this study was to try and improve the protective capacity of this antigen by enlisting different vaccine strategies. Cattle were immunized with SPAG-1, as a fusion protein with a His6 tag, either incorporated into ISCOMs, with or without the merozoite antigens TAMS 1-1 and 1-2, or with RWL as adjuvant three times at monthly intervals. Another group of cattle were immunized with p67, the T. parva sporozoite antigen, in RWL to assess whether any cross-protection could be induced. The animals were then challenged with an estimated LD50 of T. annulata sporozoites, and their ability to resist the infection was investigated. Serum responses and T-cell proliferative responses were analyzed throughout the trial. Post-challenge analyses included lymph node biopsies and blood smears to check for the presence of parasites, routine hematological parameters, and observation for clinical manifestations of the disease. The results of this trial will be discussed.

Selection for antigenic diversity of Tams1, the major merozoite antigen of Theileria annulata

Tams1, the major merozoite/piroplasm surface antigen of Theileria annulata has the potential to be a component of a diagnostic ELISA test and be included in a recombinant subunit vaccine. However, the observation that this antigen displays diversity could constrain these applications. In this paper we have extensively characterized Tams1 diversity at the DNA level, using a PCR/sequencing strategy. Up to 44 alleles have been cloned and sequenced. The comparison of these alleles has identified regions of sequence conservation, variability and hyper-variability. Computer analysis of these alleles has indicated that positive selection may operate on certain regions of Tams1. Expression and Western blot analysis of selected alleles has indicated that sequence diversity is reflected in altered antigenicity and a continuum of relatedness and antibody cross recognition may exist. The possible function of the sequence conservation and polymorphism within Tams1 is discussed in relation to protein structure, host cell invasion and immune evasion.

Tightly controlled two-stage expression vectors employing the Flp/FRT-mediated inversion of cloned genes

We have developed a tightly controlled, two-stage expression system. It is based on a single plasmid that carries the TetR repressor/Ptet promoter/Otet operator for the first-stage control, and the Flp recombinase/ FRT sites for the second-stage control. The gene to be expressed (GENE) is cloned in an inverted orientation (with respect to the stationary promoter) into a multiple-cloning site (MCS) located between two convergent FRT1 and FRT2 sites. In the OFF stage, no inadvertent transcription can enter the 5' end of cloned GENE because of four rrnBT1 terminators, located just outside the FRT1-MCS-FRT2 cassette and because the FRT2 construct was deprived of any promoter function. When using the lacZ reporter, it was shown that in their OFF stage our two-stage expression plasmids exhibit a significantly lower basal expression than the repressed single-stage tetR/PtetOtet-lacZ vectors. To enter the ON stage, the tetR/PtetOtet module is induced by adding autoclaved chlortetracycline (cTc), leading to synthesis of the Flp recombinase, which in turn, inverts the FRT1-MCS-FRT2 module together with the cloned GENE. This results in the massive GENE expression from one (pInvMS) or two (pImpMS) stationary promoters.

Genetic and biochemical analysis of erythrocyte-stage surface antigens belonging to a family of highly conserved proteins of Babesia equi and Theileria species

Erythrocyte-stage Babesia equi expresses a 34-kDa immunodominant antigen recognized by antibody from persistently infected horses worldwide. This erythrocyte-stage surface protein, equi merozoite antigen-1 (EMA-1) is encoded by a single copy gene, and was previously shown to share 33% amino acid identity with similar sized proteins of Theileria sergenti and T. buffeli. A mean homology of 31% amino acid identity extends to similar sized proteins of T. parva, T. annulata and T. mutans. Genomic and cDNA copies of a second B. equi gene, ema2 were cloned. The single copy ema2 gene encodes a 30-kDa protein (EMA-2) that shares 52% amino acid identity with EMA-1. EMA-2 also shares a mean amino acid identity of 31% with proteins of similar molecular mass from Theileria species. EMA-1 and EMA-2 each contain a glycosylphosphatidylinositol anchor. These unique erythrocyte-stage surface proteins of B. equi and Theileria species lack antigenic repeats, and excluding the signal peptide, contain one or no cysteines. Consistent with the hypothesis that this family of proteins interacts with the erythrocyte surface, the T. species proteins possess a basic isoelectric point. The B. equi proteins have acidic isoelectric points, but 24-mer peptides within them have strongly basic net charges.

Induction of protective immunity to Theileria annulata using two major merozoite surface antigens presented by different delivery systems

Allelic forms (Tams1-1 and Tams1-2) of the major merozoite surface antigen gene of Theileria annulata have recently been expressed in Escherichia coli and in Salmonella typhimurium aroA vaccine strain SL3261. To test the potential of subunit vaccines against T. annulata infection, we immunized four groups of three calves with either recombinant (re-) (Tams1-1 and Tams1-2) proteins or naked DNA encoding these antigens. Group I was immunized intramuscularly with both re-proteins incorporated into immunostimulating complexes (ISCOMs). Group II was inoculated intramuscularly with naked plasmid DNA encoding Tams1-1 and Tams1-2. Groups III and IV received S. typhimurium SL3261 [pSTams1-1][pIP5] and SL3261 [pSTams1-2] [pIP5] subcutaneously and orally, respectively. A final group of three animals (Group V) served as an unimmunized control group. Four weeks after the last immunization all calves were challenged with a T. annulata stabilate generated from blood of an infected animal with 30% piroplasm parasitaemia. All calves vaccinated with ISCOMs proved to be protected from T. annulata infection and had generated antibodies against both re-(Tams1-1 and Tams1-2) at the time of challenge. In two of these animals the antibody had a surface binding profile by IFAT. Two of three calves immunized with naked DNA also proved to be protected, but none of the animals had generated any detectable antibodies against the recombinants. Salmonella-based delivery of the recombinants did not induce any protection; two of six animals died of theileriosis and there was no difference between subcutaneous or oral administration. These preliminary results show that re-(Tams1-1 and/or Tams1-2) may elicit protective immune responses in cattle, depending on the antigen delivery system.

[Cloning and sequencing of p33 in a Korean isolate of Theileria sergenti]

The gene encoding the 33 kDa piroplasm surface protein of Theileria sergenti isolated in Korea was cloned and the nucleotide sequence was determined by dideoxy chain termination method. The cloned gene corresponds to 869 bp encoding an open reading frame 283 amino acids. Comparison of the sequence between Korean and Japanese isolates showed 99.4% homology rate in the nucleotide sequence and 98.9% homology rate in the amino acid sequence.

Vaccines against protozoal diseases of veterinary importance

Protozoan parasites are important animal and human pathogens. At present, most of these infections are controlled by chemotherapy. In addition, vaccines are available for some of these diseases. There is, however, still an urgent need for the development of vaccines against protozoal diseases, since the current array of available vaccines is very limited. This review describes the different approaches that have been taken to develop such vaccines and discusses the difficulties that hampered vaccine development. Many of the problems are related to the complex life cycle of these parasites and the virtual lack of mass in vitro culture systems. We also give an overview of the commercial and non-commercial vaccines that do exist at present. Finally, we describe the future directions of this interesting field. New techniques and strategies include parasite cultivation methods and recombinant-DNA techniques, such as vector vaccines and DNA-vaccines. Moreover, these approaches are complemented by the development of sophisticated adjuvants; the coupling of immunoprotective molecules to entities with adjuvant activity or the use of cytokines, e.g. IL-12. Through these innovations new vaccines against protozoal diseases will become available in the near future.