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

Immune parameters to p67C antigen adjuvanted with ISA206VG correlate with protection against East Coast fever

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Three doses of p67C antigen generated stronger immune responses than two doses.

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Antibody titers and CD4⁺ T-cell proliferation correlated with protection against ECF.

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The number of doses could not be reduced from three to two without compromising the protection.

East Coast fever (ECF) is a lymphoproliferative disease caused by the tick-transmitted protozoan parasite Theileria parva. ECF is one of the most serious cattle tick-borne diseases in Sub-Saharan Africa. We have previously demonstrated that three doses of the C-terminal part of the sporozoite protein p67 (p67C) adjuvanted with ISA206VG confers partial protection against ECF at a herd level. We have tested the efficacy of two doses of this experimental vaccine, as reducing the vaccination regimen would facilitate its deployment in the field. We reconfirm that three antigen doses gave a significant level of protection to severe disease (46%, ECF score < 6) when compared with the control group, while two doses did not (23%). Animals receiving three doses of p67C developed higher antibody titers and CD4⁺ T-cell proliferation indices, than those which received two doses. A new panel of immune parameters were tested in order to identify factors correlating with protection: CD4⁺ proliferation index, total IgG, IgG1, IgG2 and IgM half maximal titers and neutralization capacity of the sera with and without complement. We show that some of the cellular and humoral immune responses provide preliminary correlates of protection.

Establishment of a mouse-tick infection model for Theileria orientalis and analysis of its transcriptome

Oriental theileriosis caused by Theileria orientalis is an economically significant disease in cattle farming. The lack of laboratory animal models and in vitro culture systems is a major obstacle in the drive to better understand the biology of this parasite. Notably, research on the sporozoite stage of T. orientalis has rarely been undertaken, although such investigations are of paramount importance for vaccine development based on blocking sporozoite invasion of its host animals. In the present study, we established a mouse-tick infection model for propagating T. orientalis in mice and for producing the sporozoite stage in tick salivary glands. Splenectomized severe combined immunodeficient mice transfused with bovine erythrocytes were infected with T. orientalis. The larval ticks of Haemaphysalis longicornis were then fed on the T. orientalis-infected mice. The piroplasm and sporozoite stages were microscopically observed in the mouse blood and nymphal salivary glands, respectively. The transcriptomics data generated from the piroplasm and sporozoite stages revealed a stage-specific expression pattern for the parasite genes. The mouse-tick infection model and the transcriptomics data it has provided will contribute to a better understanding of T. orientalis biology and will also provide much needed information for the design of effective control measures targeting oriental theileriosis.

Assessment and optimization of Theileria parva sporozoite full-length p67 antigen expression in mammalian cells

Delivery of various forms of recombinant Theileria parva sporozoite antigen (p67) has been shown to elicit antibody responses in cattle capable of providing protection against East Coast fever, the clinical disease caused by T. parva. Previous formulations of full-length and shorter recombinant versions of p67 derived from bacteria, insect, and mammalian cell systems are expressed in non-native and highly unstable forms. The stable expression of full-length recombinant p67 in mammalian cells has never been described and has remained especially elusive. In this study, p67 was expressed in human-derived cells as a full-length, membrane-linked protein and as a secreted form by omission of the putative transmembrane domain. The recombinant protein expressed in this system yielded primarily two products based on Western immunoblot analysis, including one at the expected size of 67 kDa, and one with a higher than expected molecular weight. Through treatment with PNGase F, our data indicate that the larger product of this mammalian cell-expressed recombinant p67 cannot be attributed to glycosylation. By increasing the denaturing conditions, we determined that the larger sized mammalian cell-expressed recombinant p67 product is likely a dimeric aggregate of the protein. Both forms of this recombinant p67 reacted with a monoclonal antibody to the p67 molecule, which reacts with the native sporozoite. Additionally, through this work we developed multiple mammalian cell lines, including both human and bovine-derived cell lines, transduced by a lentiviral vector, that are constitutively able to express a stable, secreted form of p67 for use in immunization, diagnostics, or in vitro assays. The recombinant p67 developed in this system is immunogenic in goats and cattle based on ELISA and flow cytometric analysis. The development of a mammalian cell system that expresses full-length p67 in a stable form as described here is expected to optimize p67-based immunization.

East Coast fever, caused by the tick-borne protozoan parasite Theileria parva, is a disease that results in significant bovine morbidity, mortality, and production losses in regions of sub-Saharan Africa. Susceptible cattle develop clinical signs within a 7–14 days of exposure, which often progress to severe pulmonary edema and death. Control of East Coast fever in affected regions of Africa is largely prohibited by the lack of an affordable and efficacious vaccine. Furthermore, pastoralist farmers in affected regions of Africa often lack resources to prevent losses due to East Coast fever, so these production losses play a significant role in food security and protein availability. Experimental immunization of cattle with a recombinant T. parva-derived antigen, p67, has shown promise in preventing East Coast fever, but this antigen is extremely difficult to produce in full-length in sufficient quantities, and results of immunization studies using truncated recombinant p67 products are highly inconsistent. In this study, p67 antigen production was optimized and produced for use in future immunization studies. Optimization of p67-based immunization strategies is an important step forward in the development of a sustainable, next-generation vaccine against T. parva, which is urgently needed to minimize losses associated with East Coast fever.

Evolution and genetic diversity of Theileria

Theileria parasites infect a wide range of domestic and wild ruminants worldwide, causing diseases with varying degrees of severity. A broad classification, based on the parasite's ability to transform the leukocytes of host animals, divides Theileria into two groups, consisting of transforming and non-transforming species. The evolution of transforming Theileria has been accompanied by drastic changes in its genetic makeup, such as acquisition or expansion of gene families, which are thought to play critical roles in the transformation of host cells. Genetic variation among Theileria parasites is sometimes linked with host specificity and virulence in the parasites. Immunity against Theileria parasites primarily involves cell-mediated immune responses in the host. Immunodominance and major histocompatibility complex class I phenotype-specificity result in a host immunity that is tightly focused and strain-specific. Immune escape in Theileria is facilitated by genetic diversity in its antigenic determinants, which potentially results in a loss of T cell receptor recognition in its host. In the recent past, several reviews have focused on genetic diversity in the transforming species, Theileriaparva and Theileriaannulata. In contrast, genetic diversity in Theileriaorientalis, a benign non-transforming parasite, which occasionally causes disease outbreaks in cattle, has not been extensively examined. In this review, therefore, we provide an outline of the evolution of Theileria, which includes T. orientalis, and discuss the possible mechanisms generating genetic diversity among parasite populations. Additionally, we discuss the potential implications of a genetically diverse parasite population in the context of Theileria vaccine development.

Antigen delivery systems for veterinary vaccine development. Viral-vector based delivery systems

The recent advances in molecular genetics, pathogenesis and immunology have provided an optimal framework for developing novel approaches in the rational design of vaccines effective against viral epizootic diseases. This paper reviews most of the viral-vector based antigen delivery systems (ADSs) recently developed for vaccine testing in veterinary species, including attenuated virus and DNA and RNA viral vectors. Besides their usefulness in vaccinology, these ADSs constitute invaluable tools to researchers for understanding the nature of protective responses in different species, opening the possibility of modulating or potentiating relevant immune mechanisms involved in protection.

Current status of vaccine development against Theileria parasites

The tick-borne protozoan parasites Theileria parva and Theileria annulata cause economically important diseases of cattle in tropical and sub-tropical regions. Because of shortcomings in disease control measures based on therapy and tick control, there is a demand for effective vaccines against these diseases. Vaccines using live parasites have been available for over two decades, but despite their undoubted efficacy they have not been used on a large scale. Lack of infrastructure for vaccine production and distribution, as well as concerns about the introduction of vaccine parasite strains into local tick populations have curtailed the use of these vaccines. More recently, research has focused on the development of subunit vaccines. Studies of immune responses to different stages of the parasites have yielded immunological probes that have been used to identify candidate vaccine antigens. Immunisation of cattle with antigens expressed in the sporozoite, schizont or merozoite stages has resulted in varying degrees of protection against challenge. Although the levels of protection achieved have not been sufficient to allow exploitation for vaccination, there are clearly further lines of investigation, relating to both the choice of antigens and the antigen delivery systems employed, that need to be pursued to fully explore the potential of the candidate vaccines. Improved knowledge of the molecular biology and immunology of the parasites gained during the course of these studies has also opened up opportunities to refine and improve the quality of live vaccines.

Vaccination as a control strategy against the coccidial parasitesEimeria,ToxoplasmaandNeospora

The protozoan parasitesEimeriaspp.Toxoplasma gondiiandNeospora caninumare significant causes of disease in livestock worldwide andT. gondiiis also an important human pathogen. Drugs have been used with varying success to help control aspects of these diseases and commercial vaccines are available for all three groups of parasites. However, there are issues with increasing development of resistance to many of the anti-coccidial drugs used to help control avian eimeriosis and public concerns about the use of drugs in food animals. In addition there are no drugs available that can act against the tissue cyst stage of eitherT. gondiiorN. caninumand thus cure animals or people of infection. All three groups of parasites multiply within the cells of their host species and therefore cell mediated immune mechanisms are thought to be an important component of host protective immunity. Successful vaccination strategies for bothEimeriaandToxoplasmahave relied on using a live vaccination approach using attenuated parasites which allows correct processing and presentation of antigen to the host immune system to stimulate appropriate cell mediated immune responses. However, live vaccines can have problems with safety, short shelf-life and large-scale production; therefore there is continued interest in devising new vaccines using defined recombinant antigens. The major challenges in devising novel vaccines are to select relevant antigens and then present them to the immune system in an appropriate manner to enable the induction of protective immune responses. With all three groups of parasites, vaccine preparations comprising antigens from the different life cycle stages may also be advantageous. In the case ofEimeriaparasites there are also problems with strain-specific immunity therefore a cocktail of antigens from different parasite strains may be required. Improving our knowledge of the different parasite transmission routes, host-parasite relationships, disease pathogenesis and determining the various roles of the host immune response being at times host-protective, parasite protective and in causing immunopathology will help to tailor a vaccination strategy against a particular disease target. This paper discusses current vaccination strategies to help combat infections withEimeria,ToxoplasmaandNeosporaand recent research looking towards developing new vaccine targets and approaches.

Novel baculovirus-derived p67 subunit vaccines efficacious against East Coast fever in cattle

Two novel baculovirus-derived recombinant Theileria parva p67 constructs were tested for their vaccine potential against East Coast fever. Boran calves were immunized with a his-GFP-p67 fusion protein (GFP:p67deltaSS) or with GP64:p67C, a protein fusion between a C-terminal domain of p67 and the baculovirus envelope protein GP64. Both GFP:p67deltaSS and GP64:p67C induced antibodies with high ELISA titers that neutralized T. parva sporozoites with high efficiency. Upon challenge, a correlation was observed between the in vitro neutralizing capacity and the reduction in severe ECF for individual animals. A protection level upto 85% was obtained. This level of protection was achieved with only two inoculations of 100 microg per dose, which is a major improvement over previous recombinant p67 products.

Cytokines as adjuvants for avian vaccines

The worldwide trend towards a reduced reliance on in-feed antibiotics has increased the pressure to develop alternative strategies to manage infectious diseases in poultry. With this in mind, there is a great emphasis on vaccine use and the enhancement of existing vaccines to provide long-term protection. Currently existing adjuvants for poultry can have deleterious side-effects, such as inflammation, resulting in the down-grading of meat quality and a subsequent reduction in profits. Therefore, to enhance the use of vaccination, alternative adjuvants must be developed. The use of recombinant cytokines as adjuvants in poultry is attracting considerable attention, and their potential role as such has been addressed by several studies. The recent identification of a number of chicken cytokine genes has provided the possibility to study their effectiveness in enhancing the immune response during infection and vaccination. This review focuses on the recent studies involving the assessment of cytokines as vaccine adjuvants.

Improved protection conferred by vaccination with a recombinant vaccinia virus that incorporates a foreign antigen into the extracellular enveloped virion

Recombinant poxviruses have shown promise as vaccine vectors. We hypothesized that improved cellular immune responses could be developed to a foreign antigen by incorporating it as part of the extracellular enveloped virion (EEV). We therefore constructed a recombinant vaccinia virus that replaced the cytoplasmic domain of the B5R protein with a test antigen, HIV-1 Gag. Mice immunized with the virus expressing Gag fused to B5R had significantly better primary CD4 T-cell responses than recombinant virus expressing HIV-Gag from the TK-locus. The CD8 T-cell responses were less different between the two groups. Importantly, although we saw differences in the immune response to the test antigen, the vaccinia virus-specific immune responses were similar with both constructs. When groups of vaccinated mice were challenged 30 days later with a recombinant Listeria monocytogenes that expresses HIV-Gag, mice inoculated with the virus that expresses the B5R-Gag fusion protein had lower colony counts of Listeria in the liver and spleen than mice vaccinated with the standard recombinant. Thus, vaccinia virus expressing foreign antigen incorporated into EEV may be a better vaccine strategy than standard recombinant vaccinia virus.

Improved immunogenicity of novel baculovirus-derived Theileria parva p67 subunit antigens

East Coast fever (ECF) in cattle is caused by the tick-borne protozoan parasite Theileria parva. The major sporozoite surface antigen of T. parva (p67) is an important candidate for inclusion in a subunit vaccine. Recently, we reported the expression and production of different parts of p67 as fusions to either GFP or to the baculovirus GP64 envelope glycoprotein in insect cells, which resulted in stable proteins recognized by a monoclonal specific for native p67. The immunogenicity of these fusion proteins was examined in out-bred mice and cattle. In mice, the full length p67 molecule without its signal peptide and transmembrane region, but fused to GFP (GFP:p67deltaSS) was the best immunogen followed by the C-terminus of p67 fused to GP64 (GP64:p67C). These two immunogens also provoked a high level of sero-conversion in cattle when formulated in a water-in-oil or saponin-derived adjuvant with only 100 microg of protein and a single booster. The vaccine-elicited antibodies efficiently inhibited the infectivity of T. parva sporozoites in in vitro neutralization assays. This study demonstrated that these new baculovirus-derived p67 vaccines were highly immunogenic, and that in combination with a suitable adjuvant, they have a clear potential to induce protective immunity in cattle.

Interleukin-2 directly induces activation and proliferation of chicken T cells in vivo

Cytokines, as immune activators, have been investigated in mammalian systems as natural adjuvants and therapeutics. In particular, interleukin-2 (IL-2) has been studied widely as a vaccine adjuvant and immuno-enhancer because of its role in activating T cell proliferation. We show here that the first nonmammalian IL-2 gene cloned, chicken IL-2 (ChIL-2), exhibits similar biologic activities to those of mammalian IL-2. To assess the activities of ChIL-2 in vivo, we injected birds with recombinant ChIL-2 (rChIL-2) protein. rChIL-2 treatment induced peripheral blood lymphocytes to express cell surface IL-2 receptors (IL-2R) within 48 h and resulted in an increase in the proportion of peripheral blood CD4+ and CD8+ T cells. Using bromodeoxyuridine (BrdU) incorporation as a measurement of cell proliferation, we showed the increase in T cell populations to be due to cell proliferation. The ability of ChIL-2 to cause both activation and proliferation of T cells in vivo indicates that it has the potential to be used as an immune activator.

Vaccine development against Neospora caninum infection

Neospora caninum is a recognized protozoan parasite of a wide range of mammalian hosts, and was reported for the first time in 1988. The isolation of its oocysts in dog's faeces in 1998 led to its establishment as a parasitic species undergoing typical coccidian life cycle. Infection with N. caninum causes paralysis and death in young livestock and companion animals, and is associated with abortions and stillbirth in cattle, and neurologic disease in calves. Considering the economic and agricultural importance of neosporosis, there is the urgent need to develop biological control measures aimed at preventing its transmission, infection, as well as reducing severity of the disease. In this paper, we have reviewed the progress made to date on the parasite-host immunology and on vaccine development including its prospects, and discussed possible strategies in the formulation of vaccine(s) against neosporosis.

Advances and prospects for subunit vaccines against protozoa of veterinary importance

Protozoa are responsible for considerable morbidity and mortality in domestic and companion animals. Preventing infection may involve deliberate exposure to virulent or attenuated parasites so that immunity to natural infection is established early in life. This is the basis for vaccines against theilerosis and avian coccidiosis. Vaccination may not be effective or practical with diseases, such as cryptosporidiosis, that primarily afflict the immune-compromised or individuals with an incompletely developed immune system. Strategies for combating these diseases often rely on passive immunotherapy using serum or colostrums containing antibodies to parasite surface proteins. Subunit vaccines offer an attractive alternative to virulent or attenuated parasites for several reasons. These include the use of bacteria or lower eukaryotes to produce recombinant proteins in batch culture, the relative stability of recombinant proteins compared to live parasites, and the flexibility to incorporate only those antigens that elicit "protective" immune responses. Although subunit vaccines offer many theoretical advantages, our lack of understanding of immune mechanisms to primary and secondary infection and the capacity of many protozoa to evade host immunity remain obstacles to developing effective vaccines. This review examines the progress made on developing recombinant proteins of Eimeria, Giardia, Cryptosporidium, Toxoplasma, Neospora, Trypanosoma, Babesia, and Theileria and attempts to use these antigens for vaccinating animals against the associated diseases.

Prevention of vertical transmission of Neospora caninum in BALB/c mice by recombinant vaccinia virus carrying NcSRS2 gene

Neospora caninum infection is the major cause of bovine abortion. To develop a vaccine against N. caninum infection, recombinant vaccinia viruses carrying NcSRS2 and NcSAG1 genes (vv/Nc-p43 and vv/Nc-p36, respectively) were constructed and were tested in a mouse model. Vaccination of dams with vv/Nc-p43 appeared to confer effective protection against vertical transmission to offspring, though that with vv/Nc-p36 only provided partial protection. Moreover, the vv/Nc-p43 vaccination provoked cellular immune responses and antibody production against N. caninum. In conclusion, it is expected that vv/Nc-p43 can be used as an effective live vaccine to prevent vertical transmission of N. caninum in natural hosts.

Protective efficacy of vaccination by recombinant vaccinia virus against Neospora caninum infection

The recombinant vaccinia viruses expressing the surface protein of Neospora caninum tachyzoite, NcSAG1 or NcSRS2, were constructed. The vaccination with these recombinant viruses could protect effectively the parasite invasion in a mouse model system. The vaccine efficacy of NcSRS2 was higher than that of NcSAG1. The present study indicated that a high level of IgG1 Ab production to parasite is important for clearance of parasite at the early stage of infection and that T cell response has a crucial role for protection against the intracellular infection at the late stage of infection. The recombinant vaccinia viruses might be applicable as vaccine against N. caninum infection in a natural host.

Reciprocal cross-protection induced by sporozoite antigens SPAG-1 from Theileria annulata and p67 from Theileria parva

Theileria annulata and Theileria parva both possess a major surface antigen on the sporozoite stage of the life-cycle, called SPAG-1 and p67, respectively. In each case, these antigens are vaccine candidates and have been shown to induce a degree of homologous protection in earlier work. These antigens share sequence homology and are serologically cross-reactive. Here, we confirm that these antigens confer protection against homologous species challenge. More importantly, they mutually confer a degree of cross-species protection raising the prospect of a common vaccine in the future.

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

Cloning and expression of a 48-kilodalton Babesia caballi merozoite rhoptry protein and potential use of the recombinant antigen in an enzyme-linked immunosorbent assay

A cDNA expression library prepared from Babesia caballi merozoite mRNA was screened with a monoclonal antibody BC11D against the rhoptry protein of B. caballi merozoite. A cDNA encoding a 48-kDa protein of B. caballi was cloned and designated BC48. The complete nucleotide sequence of the BC48 gene had 1,828 bp and was shown to contain no intron. Southern blotting analysis indicated that the BC48 gene contained more than two copies in the B. caballi genome. Computer analysis suggested that this sequence contained an open reading frame of 1,374 bp with a coding capacity of approximately 52 kDa. The recombinant protein expressed by the vaccinia virus vector in horse cells had an apparent molecular mass of 48 kDa, which was the same as that of the native B. caballi 48-kDa protein. Moreover, recombinant proteins expressed by the pGEX4T expression vector in Escherichia coli as glutathione S-transferase fusion proteins were used for antigen in an enzyme-linked immunosorbent assay (ELISA). The ELISA was able to differentiate very clearly between B. caballi-infected horse sera and B. equi-infected horse sera or noninfected normal horse sera. These results suggest that this simple and highly sensitive test might be applicable to the detection of B. caballi-infected horses in the field.

Antibodies produced by mice immunized with recombinant vaccinia viruses expressing two different types of a major Theileria sergenti surface antigen (p32) react with the native surface antigen

A 32 kDa major surface antigen, p32, of Theileria sergenti at the piroplasm stage is the main target of the host immune response. The immunogenic property of the p32 varies in some strains among the population of Theileria sergenti in Japan where the Chitose type and the Ikeda type are the most common varieties. We have constructed vaccinia virus recombinants vv/p32C and vv/p32I which harbor the Chitose and Ikeda types of p32 gene, respectively. It was found that vv/p32C and vv/p32I produced type-specific p32 which did not cross react with the monoclonal antibodies (mAbs) against the other type of p32. When mice were immunized with vv/p32C and vv/p32I, antibodies against p32 were detectable 2 weeks after the immunization, and these antibodies reacted with the native surface antigen in purified T. sergenti merozoite.