Resistance of cattle to tsetse-transmitted challenge with Trypanosoma brucei or Trypanosoma congolense after spontaneous recovery from syringe-passaged infections

Groups of cattle were inoculated intravenously with cloned populations of bloodstream forms of Trypanosoma brucei or Trypanosoma congolense. All five steers infected with T. brucei ILTat 2.1 and six of the eight steers infected with T. congolense IL 13-E14 became aparasitemic within 16 and 32 weeks postinfection, respectively. Examination of sera from animals infected with T. brucei by indirect immunofluorescence and neutralization assays revealed the presence of antibodies against all the metacyclic variable antigen types (VATs) of the infecting clone. The neutralizing capacity of the sera increased with the course of infection from 1:10 at 2 months to 1:100 at 3 to 4 months postinfection. The recovered animals were completely immune to challenge by Glossina morsitans subsp. centralis infected with clone IL Tat 2.1, which had initiated the infection, as well as with another clone (IL Tat 2.2) belonging to the same serodeme, but they were susceptible to a tsetse-transmitted heterologous challenge with isolate STIB 367-H. Similar results were obtained with sera from T. congolense IL 13-E14-infected steers. The six steers infected with a different T. congolense ILNat 3.1 clone did not recover spontaneously; however, 2 months postinfection, sera from five of them also contained neutralizing antibodies against ILNat 3.1 metacyclic VATs. These results indicate that some of the bloodstream VATs that arise during the course of a chronic infection possess surface epitopes in their variable surface glycoproteins that are identical to those of the metacyclic VATs. It is suggested that in chronic infection, the infecting trypanosomes could exhaust their VAT repertoire, including those that cross-react with metacyclics, thereby leading to both "self-cure" and subsequent immunity to homologous cyclically transmitted challenge.

Molecular characterization and immunogenicity of neutralization-sensitive Babesia bigemina merozoite surface proteins

Monoclonal antibodies binding to the surface of live Mexico isolate Babesia bigemina merozoites have defined 4 parasite-encoded surface antigens (36, 45, 55, and 58 kDa) that are potential targets for immune-mediated neutralization of merozoites. In this study, we have characterized the post-translational modification, antigenic polymorphism, and immunogenicity of these 4 proteins. Monoclonal antibody immunoaffinity-purified 36- and 55-kDa polypeptides were identical in gel electrophoresis to immunoprecipitated radiolabeled proteins, while the purified 45-kDa protein consisted of 2 closely spaced polypeptides with relative molecular weights of 45 and 43 kDa. The 36-, 45-, and 55-kDa proteins were post-translationally modified by incorporation of [3H]glucosamine and [3H]myristic acid, suggesting they are integral membrane proteins secured by a phosphatidylinositol anchor. Cross-reactivity studies with monoclonal and monospecific polyclonal antibodies revealed marked antigenic polymorphism of these 3 glycoproteins among diverse geographic isolates. In contrast, none of the polypeptides bound by anti-p58 monoclonal antibody were glycosylated or myristilated. Both monoclonal and monospecific polyclonal antibodies recognizing p58 bound to similar molecular weight proteins in 4 additional isolates of B. bigemina from Mexico, Puerto Rico, St. Croix, and Kenya, suggesting widespread conservation of p58 immunogenic epitopes among geographic isolates. Calves immunized with immunoaffinity purified gp45, gp55, or p58 antigens were able to neutralize the infectivity of merozoites as indicated by significant reductions in the peak parasitemia after experimental challenge. Precise definition and appropriate presentation of neutralization sensitive epitopes on gp45, gp55, or p58 may enhance the merozoite neutralizing immune response in immunized cattle.

Characterisation of the gene encoding a candidate vaccine antigen of Theileria parva sporozoites

We have cloned and characterised the gene encoding the 67-kilodalton stage-specific surface antigen, p67, of Theileria parva (Muguga) sporozoites. The gene which is present in a single copy, is divided into 2 exons by an intron 29 bp long and is transcribed into mRNA of about 2500 nucleotides. The gene is present in all stocks of T. parva and there is a related gene in Theileria annulata. The deduced amino acid sequence of 709 residues predicts that p67 is a membrane protein and that it lacks tandemly repeated sequences. Recombinant p67 has been expressed in Escherichia coli as a fusion protein with Sj-26, a glutathione-S-transferase of Schistosoma japonicum. Antibodies to purified recombinant proteins containing residues 9-316 or 397-709 of p67 bind to p67 in immunoblots and neutralise sporozoite infectivity in vitro. Recombinant p67 is, therefore, a candidate antigen for development of an anti-sporozoite vaccine for East Coast fever in cattle.

The entry of Theileria parva sporozoites into bovine lymphocytes: evidence for MHC class I involvement

We have examined the process of Theileria parva sporozoite entry into susceptible bovine lymphocytes and have begun to identify one of the possible molecular interactions involved in the process. The entry process involves a defined series of events and we have used a number of experimental procedures in combination with a method of quantitation to examine various aspects of this process. T. parva sporozoites are nonmotile organisms and the initial sporozoite-lymphocyte interaction is a chance event which can occur at 0-2 degrees C. All subsequent stages in the process are temperature dependent, require the participation of live intact sporozoites and host cells, and involve some cytochalasin-inhibitable rearrangement of the host cell surface membrane or cytoskeleton. Sporozoite entry can be inhibited by antibodies (mAbs) reactive with major histocompatibility complex (MHC) class I molecules (IL-A 19, IL-A 88) and with beta 2 microglobulin (B1G6), whereas mAbs reactive with MHC class II molecules (IL-A 21, J 11), and a common panleucocyte surface antigen, (IL-A 87; a bovine equivalent of CD 11a) have no effect. These results indicate that MHC class I molecules play a role in the process of T. parva sporozoite entry into bovine lymphocytes although as yet the precise role has not been determined. Once internalized within the lymphocyte, a process that takes less than 3 min at 37 degrees C, the sporozoite rapidly escapes from the encapsulating host cell membrane; a process which occurs concurrently with the discharge of the contents of the sporozoite rhoptries and microspheres. The intracytoplasmic parasite is covered by a layer of sporozoite-derived fuzzy material to which host cell microtubules rapidly become associated.

Immunisation of cattle with cysteine proteinases of Trypanosoma congolense: targetting the disease rather than the parasite

In order to test the hypothesis that trypanosome cysteine proteinases (CPs) contribute to pathology of trypanosomosis, cattle were immunised with CP1 and/or CP2, the major CPs of Trypanosoma congolense, and subsequently challenged with T. congolense. Immunisation had no effect on the establishment of infection and the development of acute anaemia. However, immunised cattle, unlike control cattle, maintained or gained weight during infection. Their haematocrit and leukocyte counts showed a tendency to recovery after 2-3 months of infection. Cattle immunised with CP2 mounted early and prominent IgG responses to CPs and to the variable surface glycoprotein following challenge. Thus trypanosome CPs may play a role in anaemia and immunosuppression; conversely, anti-CP antibody may modulate the trypanosome-induced pathology.

Characterization of a polymorphic immunodominant molecule in sporozoites and schizonts of Theileria parva

This study examines several aspects of a polymorphic, immunodominant molecule (PIM) found in the protozoan parasite, Theileria parva. The antigen is present in all T.p. parva stocks examined, and in the related subspecies, T.p.bovis and T.p.lawrencei. It is the predominant antigen recognized by antisera from immune cattle on Western blot analysis of schizont-infected lymphocytes, and is the only antigen which has been shown to react with anti-schizont monoclonal antibodies (MoAbs) on Western blots or in immunoprecipitations. The antigen shows polymorphism in both size and expression of antibody epitopes among the different stocks of T. parva. The antigen is present in sporozoites as well as schizonts.

Identification of a Theileria mutans-specific antigen for use in an antibody and antigen detection ELISA

Purified piroplasms of Theileria mutans were used to immunize BALB/c mice to generate monoclonal antibodies (MoAbs). The MoAbs recognized an antigen of a relative molecular mass of 32 kDa in Western blots. This antigen was also recognized by sera from cattle which had recovered naturally from experimental tick-transmission or infections induced by the blood stages of T. mutans. The MoAbs did not react, in indirect immunofluorescence or enzyme-linked immunosorbent assays (ELISA), with the common haemoparasites of cattle, namely, T. parva, T. annulata, Babesia bigemina, B. bovis, Anaplasma marginale, Trypanosoma congolense, T. vivax or T. brucei. An antigen capture ELISA was established with two of the MoAbs which recognized different epitopes on the 32 kDa molecule. Using this test it was possible to detect circulating antigens or immune complexes in sera collected from cattle during the acute or chronic phases of infection. When the purified 32 kDa protein was used as antigen in a micro-ELISA to detect circulating antibodies in both experimental and field cattle sera, it was found that the titres of antibodies ranged between 1:20 and 1:10,240. Results of this study indicate that the antigen and immune complex capture assays and the antibody detection ELISA can be complementary in the immunodiagnosis of acute and chronic T. mutans infections. Moreover, the tests are useful in the differential diagnosis of the disease and for epidemiological studies.

Immune depression in bovine trypanosomiasis: effects of acute and chronic Trypanosoma congolense and chronic Trypanosoma vivax infections on antibody response to Brucella abortus vaccine

Cattle were vaccinated with Brucella abortus (S19) vaccine during acute (25 days) and chronic (25 weeks) Trypanosoma congolense and chronic Trypanosoma vivax (25 weeks) infections in order to determine the effect of such infections on the antibody response to the vaccine. It was found that the specific antibody responses of IgG1 and IgG2 sub-classes were profoundly depressed (80%) in both the acute and chronic infections with T. congolense. Whereas IgM antibody response was also profoundly depressed (90%) in cattle with the acute infection, it was only 50% depressed in those with chronic infection. There was no depression of IgG1, IgG2, or IgM in cattle infected with T. vivax. These animals, however, had no detectable parasitaemia at the time of vaccination and thereafter. These results suggest that during acute infection with T. congolense depressive mechanisms could be acting on the afferent arm of the immune response, namely, antigen recognition and/or processing.

Monoclonal antibodies that distinguish Trypanosoma congolense, T. vivax and T. brucei

Monoclonal antibodies (MoAbs) were derived against in-vitro-propagated procyclic forms of Trypanosoma congolense, T. vivax, T. brucei brucei and T.b. rhodesiense in order to identify antigens for use in immunodiagnosis of African trypanosomiasis. The antibodies have been tested against procyclic and bloodstream form trypanosomes of 13 T. congolense, six T. vivax six T.b. brucei, four T.b. rhodesiense, five T.b. gambiense and three T. simiae isolates from different geographical areas by indirect immunofluorescent antibody test (IFAT) and enzyme-linked immunosorbent assay (ELISA). The MoAbs raised against T.b. brucei reacted with all the brucei group of trypanosomes but not with T. congolense, T. simiae or T. vivax. Likewise, MoAbs against T. congolense reacted with T. congolense and T. simiae but not with any of the other species, while those against T. vivax reacted with T. vivax only. The antigens recognized by these MoAbs were present in lysates of bloodstream trypanosomes as well as midgut (for T. congolense and T. brucei) and epimastigote forms from infected Glossina morsitans centralis. There was no reactivity of the MoAbs with Theileria parva, Anaplasma marginale, Babesia bigemina or Plasmodium falciparum. These antibodies and the antigens they recognize should, therefore, prove useful in the development of assay systems for immunodiagnosis of African trypanosomiasis.

SfiI and NotI polymorphisms in Theileria stocks detected by pulsed field gel electrophoresis

DNAs of Theileria parva parva, T. p. lawrencei, T. p. bovis and Theileria mutans stocks, from Kenya, Uganda, Zanzibar and Zimbabwe were digested with either SfiI or NotI and analysed using contour-clamped homogeneous electric field (CHEF) and field-inversion gel electrophoresis (FIGE). The SfiI-digested T. parva genomic DNA resolved into approximately 30 fragments while the NotI digestion produced between 4-7 bands. The summation of the sizes of SfiI fragments gave an estimate of 9-10 X 10(6) base pairs for the size of the T. parva genome. Heterogeneity within T. p. parva Muguga, Pemba/Mnarani and Mariakani stocks was detected. All the T. parva stocks analysed showed SfiI and NotI restriction fragment length polymorphisms (RFLP). Hybridisation of 5 SfiI-digested T. parva DNAs with a Plasmodium berghei telomeric repeat probe suggested that most of the polymorphisms and heterogeneity occurred in the telomeric or sub-telomeric regions of the genome. The recognition by the Plasmodium telomeric probe of 7-8 strongly hybridising SfiI bands indicates that the T. parva genome may possess at least 4 chromosomes. The T. mutans genome was cut frequently with the above enzymes resulting in large numbers of fragments predominantly below 50 kb, thus suggesting either a much higher G + C content than T. parva or the presence of highly reiterated G + C-rich regions.

Characterisation of the gene encoding a 104-kilodalton microneme-rhoptry protein of Theileria parva

A neutralizing antiserum, C16, raised against sporozoites of Theileria parva parva was used to screen a lambda gt11 expression library of T. parva parva (Muguga) genomic DNA fragments. Proteins encoded by one phage clone, lambda TpS-17, were reactive with the C16 antiserum. Detailed characterisation of the DNA insert showed it to encode determinants found on four theilerial antigens of approximately 104, 90, 85 and 35 kDa. The sequence encoded by the clone is expressed during sporogony as a single RNA transcript of about 3000 nucleotides. On sequencing a portion of the 5000-bp insert, an open reading frame of 2772 bp was revealed that encoded a 104-kDa protein. Immunoscreening a library of subfragments of the DNA insert with the original antiserum localised sequences encoding the dominant antigenic determinants to an 800-bp stretch of DNA at the 3' end of the open reading frame. Sequence data from three subclones spanning this region show portions of the antigenic domains to be unusually rich in proline residues which are repeated every three amino acids. These repeats often take the form X-S(T)-P or X-K(R)-P. Antibodies directed against each of the three subclones recognize the 104- and 35-kDa antigens and different combinations of the 90- and 85-kDa kDa antigens, suggesting that the smaller proteins are derived from the 104-kDa antigen by limited proteolysis occurring at the carboxyl terminus end of the protein. In immunoelectron micrographs the antigen is associated with the microneme/rhoptry complexes of the sporozoite.

Bovine immune response to African ;trypanosomes: specific antibodies to variable surface glycoproteins of Trypanosoma brucei

Cattle were infected with two different clones of Trypanosoma brucei (MITat 1.2 and ILTat 1.3) and antibody response was followed by radioimmunoassay. In four of the seven animals there were at least two peaks of antibody activity to the infecting clones, with the second peak much higher than the first. Specific antibodies (IgG1 and IgM but not IgG2) were eluted from the immunoabsorbent columns on which the variant surface glycoproteins (VSGs) were coupled. By neutralization of infectivity tests, IgM antibodies from the first peak of antibody activity were more efficient in neutralizing trypanosomes than IgG1 but the reverse was true of the antibodies isolated from the second peak. By absorption with multiple variable antigen types isolated during the course of infection, all the IgM and IgG1 in the first 3 weeks of infection were shown to be specific. It is suggested that polyclonal B cell stimulation leading to dysfunction in the control of IgM and IgG production may not be responsible for the high levels of these immunoglobulins in bovine trypanosomiasis.

Protective immune mechanisms against Theileria parva: evolution of vaccine development strategies

Theileria parva is an intracellular sporozoan parasite that infects and transforms bovine lymphocytes, causing a severe lymphoproliferative disease known as East Coast fever in eastern, central and southern Africa. In this article, Declan McKeever and colleagues summarize the current understanding of immune mechanisms provoked by the parasite with regard to their role in both pathogenesis and protection. In particular, the influence of genomic polymorphism in parasite and host on the development of immunity is discussed, along with the evolution of current vaccine development strategies as a result of immunological research on the disease.

Cloning, characterization, and expression of a 200-kilodalton diagnostic antigen of Babesia bigemina

Current serological tests for Babesia bigemina use semipurified merozoite antigens derived from infected erythrocytes. One of the major drawbacks of these tests is that antigen quality can vary from batch to batch. Since the quality of the antigen contributes to the sensitivity and specificity of serological tests, the use of standardized recombinant antigens should ensure consistency in assay quality. Previously, a 200-kDa merozoite antigen (p200) was identified as a candidate diagnostic antigen for use in a serological assay for the detection of B. bigemina antibodies in infected cattle. In this study, we have cloned, characterized, and expressed p200. A 3.5-kbp cDNA clone encoding p200 was isolated and shown to be almost full length, lacking approximately 300 bp at the 5' end. The predicted amino acid sequence shows that p200 consists of a long, highly charged central repeat region of an uninterrupted alpha helix, indicative of a fibrous protein. Immunoelectron microscopy localized p200 to the merozoite cytoplasm, suggesting that the antigen may be a structural protein involved in forming filament structures within the cytoskeleton. The 3.5-kbp cDNA was expressed in bacteria as a fusion protein with glutathione S-transferase (GST), but the yield was poor. To improve the yield, cDNA fragments encoding antigenic domains of p200 were expressed as fusions with GST. One of these fusion proteins, C1A-GST, is composed of a 7-kDa fragment of the p200 repeat region and contains epitopes that react strongly with sera from cattle experimentally infected with B. bigemina. Recombinant C1A-GST should permit the development of an improved enzyme-linked immunosorbent assay for the detection of antibodies against B. bigemina.

Apparent exhaustion of the variable antigen repertoires of Trypanosoma vivax in infected cattle

Three groups of cattle, each group comprising six animals, were inoculated intravenously with populations of bloodstream forms of Trypanosoma vivax. The first group received T. vivax clone ILDat 1.3 derived from an isolate from Nigeria, while the other two received T. vivax stocks IL 1875 or IL 2133 isolated from Coast Province, Kenya. One animal from the group that was infected with IL 1875 died 8 weeks postinfection. The remaining 17 animals became aparasitemic in 8 to 12 weeks without intervention by drug therapy. The recovered animals developed serodeme-specific immunity against Glossina morsitans subsp. centralis-transmitted challenge. There was complete cross-protection between the two East African T. vivax stocks, although they were isolated from areas 80 to 90 km apart, indicating that they belong to the same serodeme. Antibodies to the homologous metacyclic variable antigen types (VATs) were not detected in sera from recovered animals, suggesting that the immunity displayed by the recovered animals was directed at the bloodstream and not the metacyclic VATs. It is thus suggested that recovery in these animals is due to exhaustion of the repertoire of bloodstream VATs expressed in the animals by the infecting T. vivax clone or stocks.

The African buffalo parasite Theileria. sp. (buffalo) can infect and immortalize cattle leukocytes and encodes divergent orthologues of Theileria parva antigen genes

African Cape buffalo (Syncerus caffer) is the wildlife reservoir of multiple species within the apicomplexan protozoan genus Theileria, including Theileria parva which causes East coast fever in cattle. A parasite, which has not yet been formally named, known as Theileria sp. (buffalo) has been recognized as a potentially distinct species based on rDNA sequence, since 1993. We demonstrate using reverse line blot (RLB) and sequencing of 18S rDNA genes, that in an area where buffalo and cattle co-graze and there is a heavy tick challenge, T. sp. (buffalo) can frequently be isolated in culture from cattle leukocytes. We also show that T. sp. (buffalo), which is genetically very closely related to T. parva, according to 18s rDNA sequence, has a conserved orthologue of the polymorphic immunodominant molecule (PIM) that forms the basis of the diagnostic ELISA used for T. parva serological detection. Closely related orthologues of several CD8 T cell target antigen genes are also shared with T. parva. By contrast, orthologues of the T. parva p104 and the p67 sporozoite surface antigens could not be amplified by PCR from T. sp. (buffalo), using conserved primers designed from the corresponding T. parva sequences. Collectively the data re-emphasise doubts regarding the value of rDNA sequence data alone for defining apicomplexan species in the absence of additional data. ‘Deep 454 pyrosequencing’ of DNA from two Theileria sporozoite stabilates prepared from Rhipicephalus appendiculatus ticks fed on buffalo failed to detect T. sp. (buffalo). This strongly suggests that R. appendiculatus may not be a vector for T. sp. (buffalo). Collectively, the data provides further evidence that T. sp. (buffalo). is a distinct species from T. parva.

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Theileria sp. (buffalo) can infect and immortalize cattle leukocytes.

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Antigen genes of T. sp. (buffalo) vary in level of identity to those of T. parva

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The tick that transmits T. sp. (buffalo) to cattle is not Rhipicephalus appendiculatus

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18s rDNA sequence information alone is insufficient to define species of Theileria

Identification of neutralization and diagnostic epitopes on PIM, the polymorphic immunodominant molecule of Theileria parva

The polymorphic immunodominant molecule (PIM) of Theileria parva is expressed by the schizont and sporozoite stages of the parasite. We have recently cloned the cDNA encoding the PIM antigen from two stocks of the parasite: the cattle-derived T. parva (Muguga) stock and a buffalo-derived stock. The cDNAs were used in transient-transfection assays to assess the reactivity of the antigen with monoclonal antibodies (MAb) previously raised against schizont-infected cells and used for parasite strain identification. We demonstrate that 19 of the 25 MAb are specific for PIM. Antibody reactivities with deletion mutants of a fusion protein containing PIM and Pepscan analysis of the Muguga version of the molecule with 13 of the MAb indicate that there are at least 10 different epitopes throughout the molecule. None of the MAb react with a tetrapeptide repeat present in the central region of the molecule, probably because of an inability of BALB/c mice to produce antibodies to this repeat. In contrast, sera from infected cattle react strongly with the repeat region, suggesting that this region alone may be useful as a diagnostic reagent. Previous studies showed that MAb to PIM inhibit sporozoite infectivity of bovine lymphocytes in vitro, which suggests that the antigen may be useful in immunizing cattle against T. parva infection. Pepscan analysis revealed that sera from infected cattle reacted with peptides recognized by the neutralizing MAb, as did sera from cattle inoculated with a PIM-containing recombinant protein. The latter sera did not, however, neutralize sporozoite infectivity in vitro. These results will be useful in exploiting the strain identification, diagnostic, and immunizing potentials of this family of antigens.

Characterization of Trypanosoma congolense serodemes in stocks isolated from cattle introduced onto a ranch in Kilifi, Kenya

A herd of 20 cattle was introduced on a ranch in Kilifi, Coast Province of Kenya, where they were in contact with Glossina austeni for 6 months. In total, 65 trypanosome isolates were made from these animals. Examination of the isolates revealed that 61 were Trypanosoma congolense and 4 were T. theileri. Out of the 61 T. congolense isolates, 55 were successfully passaged and cloned in mice to provide trypanosome populations for further analyses. The stocks and their clones were inoculated into goats on which teneral G. morsitans centralis were later fed in order to provide metacyclics for use in serodeme analysis. Identification of serodemes was carried out by indirect immunofluorescence and neutralization using antimetacyclic hyperimmune sera prepared in mice against metacyclics of cloned trypanosome populations. So far 4 serodemes have been identified in 8 stocks and 7 clones. Each of the 9 stocks contained a mixture of at least 2 of the 4 serodemes identified. Furthermore, stocks isolated sequentially from individual animals contained the same serodemes despite repeated treatment with a curative dose (6 mg/kg body weight) of Berenil between isolations. From the latter finding, it can be inferred that the 4 serodemes were present on the ranch throughout the study period.

Cloning and characterization of a 150 kDa microsphere antigen of Theileria parva that is immunologically cross-reactive with the polymorphic immunodominant molecule (PIM)

To identify the genes encoding novel immunodominant antigens of Theileria parva a lambda gt11 library of piroplasm genomic DNA was immunoscreened with bovine recovery serum and a gene encoding a 150 kDa antigen (p150) was identified. The predicted polypeptide contains an N-terminal secretory signal sequence and a proline-rich region of repeated amino acid motifs. The repeat region is polymorphic between stocks of T. parva in both copy number and sequence, and analysis of the repeat region from 10 stocks of T. parva revealed 5 p150 variants. A monoclonal antibody (mAb) against the T. parva polymorphic immunodominant molecule (PIM) cross-reacted with the recombinant p150. The p150 has sequence homology with a PIM peptide sequence containing the anti-PIM mAb epitope. Immunoelectron microscopy demonstrated that the p150 antigen, like PIM, is located in the microspheres of the sporozoites and is exocytosed following sporozoite invasion of the host lymphocyte. By immunoelectron microscopy p150 was subsequently transiently detectable on the sporozoite surface and in the lymphocyte cytosol. Immunoblotting showed that p150 is also expressed by the schizont stage, but at much lower levels compared to the sporozoite. These results suggest a major role for p150 in the early events of host-sporozoite interaction.

Delivery of the Theileria parva p67 antigen to cattle using recombinant vaccinia virus: IL-2 enhances protection

To evaluate vaccinia virus as a delivery system for recombinant antigen in cattle, calves were immunized with a recombinant vaccinia virus (rVV) expressing the sporozoite surface antigen (p67) of Theileria parva (V-67) combined with those expressing bovine IL-4 (V-IL4) or IL-2 (V-IL2). The anti-p67 antibody levels detected in calves inoculated with the combination of V-67 and V-IL4 were higher than those produced by animals injected with V-67 alone or V-67 and V-IL2. On challenge with cryopreserved sporozoites, 5 of 7 animals receiving V-67 combined with V-IL2 were protected, while those receiving V-67 in conjunction with V-IL4 behaved like unimmunized control calves. Vaccination with a recombinant virus expressing a chimaeric p67(p583)/IL2 product gave rise to a lower level of protection, whereas V-IL2 provided no immunity. The results of this study demonstrate the potential of rVV as a delivery system for use in vaccination of cattle against Theileria parva infection.