Induction of proliferative responses of T cells from Babesia bovis-immune cattle with a recombinant 77-kilodalton merozoite protein (Bb-1)

Humoral and Cell-Mediated Immune Response Validation in Calves after a Live Attenuated Vaccine of Babesia bigemina

The current vaccines to control bovine Babesia bigemina (B. bigemina) infection are not fully protective and vaccination failures incur heavy losses to the cattle industry around the world. Using modified micro-aerophilous stationary phase, we developed a culture-derived attenuated live vaccine against B. bigemina and tested a single subcutaneous inoculation of 2 × 10⁸ infected erythrocytes in calves. The protection was measured after a lethal intravenous challenge with 5 × 10⁸ virulent calf-derived B. bigemina. Our results demonstrated that a single shot of attenuated vaccine was capable of inducing robust humoral and cell-mediated immune responses in calves. We found a significant increase in the IgG antibody titers post-challenge and a strong proliferation of both CD4+ and CD8+ T cells contributing towards the protection. Our vaccine provided complete protection and parasitic clearance, which was followed for more than 100 days post-challenge. This immunity against babesiosis was directly linked to strong humoral responses; however, the parasitic clearance was attributed to significant T cells effector responses in vaccinated calves as compared to the infected control calves. We anticipate that these results will be helpful in the development of more efficient culture-derived vaccines against Babesia infections, thus reducing significant global economic losses to farmers and the cattle industry.

Cocktail Babesia bovis antigens for global detection of Babesia bovis infection in cattle

The diagnostic performance of a cocktail formula consisting of two Babesia (B.) bovis recombinant proteins, including spherical body protein 1 (BbSBP-1) and spherical body protein 4 (BbSBP-4), was evaluated in the present study for the global detection of B. bovis infection in cattle and for the differentiation between B. bovis and B. bigemina infections. The efficacy and the practicality of the rBbSBP-1 and rBbSBP-4 cocktail formula for differentiation between the infection caused by both parasites were assessed using indirect enzyme-linked immunosorbent assay (iELISA) with serum samples collected from cattle experimentally infected by B. bovis (n = 33) or B. bigemina (n = 30). Cocktail antigen exhibited the highest optical density (OD) values with B. bovis-infected sera and the lowest OD values with normal bovine sera or B. bigemina-infected sera in comparison with the single antigen. A total of 581 field serum samples collected from four countries with known B. bovis endemicity: Ghana (n = 154), Egypt (n = 162), Thailand (n = 96), and South Africa (n = 169) were screened also in the current study using iELISA and the results were compared to those of indirect fluorescent antibody test (IFAT) as a reference. A cocktail formula (rBbSBP-1 and rBbSBP-4) exhibited the highest concordance rate (89.90%) and kappa value (0.73). The obtained results revealed the reliability of the rBbSBP-1 and rBbSBP-4 cocktail antigen for the detection of specific antibodies to B. bovis in cattle and demonstrated the usefulness of cocktail antigen for differentiation between B. bovis and B. bigemina infections compared with the single antigen in cattle, which will be useful for epidemiological surveys and control of bovine babesiosis.

Spherical body protein 4 is a new serological antigen for global detection of Babesia bovis infection in cattle

Five Babesia bovis recombinant proteins, including merozoite surface antigen 2c (BbMSA-2c), C-terminal rhoptry-associated protein 1 (BbRAP-1/CT), truncated thrombospondin-related anonymous protein (BbTRAP-T), spherical body protein 1 (BbSBP-1), and spherical body protein 4 (BbSBP-4), were evaluated as diagnostic antigens to detect the infection in cattle. The recombinant proteins were highly antigenic when tested with experimentally B. bovis-infected bovine serum in Western blot analysis. Furthermore, five antisera that had been raised against each of the recombinant proteins reacted specifically with the corresponding authentic protein, as determined in Western blot analysis. Next, enzyme-linked immunosorbent assays (ELISAs) using these recombinant proteins were evaluated for diagnostic use, and the sensitivity and specificity of each protein were demonstrated with a series of serum samples from experimentally B. bovis-infected cattle. Furthermore, a total of 669 field serum samples collected from cattle in regions of B. bovis endemicity in seven countries were tested with the ELISAs, and the results were compared to those of an indirect fluorescent antibody test (IFAT), as a reference. Among five recombinant antigens, recombinant BbSBP-4 (rBbSBP-4) had the highest concordance rate (85.3%) and kappa value (0.705), indicating its reliability in the detection of specific antibodies to B. bovis in cattle, even in different geographical regions. Overall, we have successfully developed an ELISA based on rBbSBP-4 as a new serological antigen for a practical and sensitive test which will be applicable for epidemiologic survey and control programs in the future.

Prospects for recombinant vaccines against Babesia bovis and related parasites

Babesial parasites infect cattle in tropical and temperate regions of the world and cause significant morbidity and mortality. Discovery of protective antigens that could be used in a killed vaccine has been slow and to date there are few promising vaccine candidates for cattle Babesia. This review describes mechanisms of protective innate and adaptive immune responses to babesial parasites and different strategies to identify potentially protective protein antigens of B. bovis, B. bigemina, and B. divergens. Successful parasites often cause persistent infection, and this paper also discusses how B. bovis evades and regulates the immune response to promote survival of parasite and host. Development of successful non-living recombinant vaccines will depend on increased understanding of protective immune mechanisms and availability of parasite genomes.

Expressed sequence tag (EST) analysis of the erythrocytic stages of Babesia bovis

Expressed sequence tags (ESTs) provide an efficient way to identify large numbers of genes expressed in a specific stage of the life cycle of an organism. Here we analysed approximately 13,000 ESTs derived from the erythrocytic stage of the apicomplexan parasite Babesia bovis. The ESTs were clustered in order to obtain information on the expression level of a gene and to increase sequence length and reliability. A total of 3522 clusters were obtained and annotated using BLAST algorithms. The clusters were estimated to represent approximately 2600 genes of which in total approximately 2.1 Mbp sequence information was obtained. Expression levels of the genes, as determined by the numbers of ESTs contained within a cluster, were compared to those of their closest homologs in the erythrocytic stage of Plasmodium falciparum and Toxoplasma gondii tachyzoites. Pathways that are represented relatively abundant in B. bovis are, amongst others, the purine salvage pathway (displaying characteristics not identified before in apicomplexans), isoprenoid biosynthesis in the apicoplast and many genes encoding mitochondrial proteins. Especially remarkable in the latter group are the F-type ATPases - which are hardly expressed in P. falciparum and T. gondii - and two highly expressed glycerol-3-phosphate dehydrogenases creating a shuttle possibly controlling the cytoplasmic NADH/NAD+ -ratio. A comparison of known antigenic proteins from Australian and American strains of B. bovis with the Israel strain used here identifies considerable sequence variation in the rhoptry associated protein-1 (RAP-1), merozoite surface proteins of the variable merozoite surface antigen (VMSA) family and spherical body proteins. Analysis of the EST clusters representing the variable erythocyte surface antigen family reveals many variant transcripts of which a few are dominant. Two putative pseudogenes also seem to be transcribed at high levels.

A novel 78-kDa fatty acyl-CoA synthetase (ACS1) of Babesia bovis stimulates memory CD4+ T lymphocyte responses in B. bovis-immune cattle

Antigen-specific CD4+ T lymphocyte responses contribute to protective immunity against Babesia bovis, however the antigens that induce these responses remain largely unknown. A proteomic approach was used to identify novel B. bovis antigens recognized by memory CD4+ T cells from immune cattle. Fractions obtained from merozoites separated by continuous-flow electrophoresis (CFE) that contained proteins ranging from 20 to 83 kDa were previously shown to stimulate memory CD4+ lymphocyte responses in B. bovis-immune cattle. Expression library screening with rabbit antiserum raised against an immunostimulatory CFE fraction identified a clone encoding a predicted 78 kDa protein. BLAST analysis revealed sequence identity of this B. bovis protein with Plasmodium falciparum fatty acyl coenzyme A synthetase (ACS) family members (PfACS1-PfACS11), and the protein was designated B. bovis acyl-CoA synthetase 1 (ACS1). Southern blot analysis indicated that B. bovis ACS1 is encoded by a single gene, although BLAST analysis of the preliminary B. bovis genome sequence identified two additional family members, ACS2 and ACS3. Peripheral blood lymphocytes and CD4+ T cell lines from B. bovis-immune cattle proliferated significantly against recombinant ACS1 protein, consistent with its predicted involvement in protective immunity. However, immune sera from cattle recovered from B. bovis infection did not react with ACS1, indicating that epitopes may be conformationally dependent.

Use of helper T cells to identify potential vaccine antigens of Babesia bovis

Babesia bovis is an economically important hemoprotozoon parasite o f cattle that is prevalent in many, tropical and subtropical regions o f the world. Effective vaccines against this tick-transmitted parasite consist o f live organisms attenuated by passage through splenectomized calves. However, the nature o f acquired resistance to challenge infection with heterologous isolates of this parasite has not been clearly defined. Unsuccessful attempts to select protective antigens have relied upon the use of antibodies to identify immunodominant proteins. In this review, Wendy Brown and Allison Rice-Ficht discuss the limitations of this approach and the rationale behind using helper T cells to select potential vaccine antigens.

Conservation of Babesia bovis small heat shock protein (Hsp20) among strains and definition of T helper cell epitopes recognized by cattle with diverse major histocompatibility complex class II haplotypes

Babesia bovis small heat shock protein (Hsp20) is recognized by CD4+ T lymphocytes from cattle that have recovered from infection and are immune to challenge. This candidate vaccine antigen is related to a protective antigen of Toxoplasma gondii, Hsp30/bag1, and both are members of the alpha-crystallin family of proteins that can serve as molecular chaperones. In the present study, immunofluorescence microscopy determined that Hsp20 is expressed intracellularly in all merozoites. Importantly, Hsp20 is also expressed by tick larval stages, including sporozoites, so that natural tick-transmitted infection could boost a vaccine-induced response. The predicted amino acid sequence of Hsp20 from merozoites is completely conserved among different B. bovis strains. To define the location of CD4+ T-cell epitopes for inclusion in a multiepitope peptide or minigene vaccine construct, truncated recombinant Hsp20 proteins and overlapping peptides were tested for their ability to stimulate T cells from immune cattle. Both amino-terminal (amino acids [aa] 1 to 105) and carboxy-terminal (aa 48 to 177) regions were immunogenic for the majority of cattle in the study, stimulating strong proliferation and IFN-gamma production. T-cell lines from all individuals with distinct DRB3 haplotypes responded to aa 11 to 62 of Hsp20, which contained one or more immunodominant epitopes for each animal. One epitope, DEQTGLPIKS (aa 17 to 26), was identified by T-cell clones. The presence of strain-conserved T helper cell epitopes in aa 11 to 62 of the ubiquitously expressed Hsp20 that are presented by major histocompatibility complex class II molecules represented broadly in the Holstein breed supports the inclusion of this region in vaccine constructs to be tested in cattle.

A novel 20-kilodalton protein conserved in Babesia bovis and B. bigemina stimulates memory CD4(+) T lymphocyte responses in B. bovis-immune cattle

Acquired immunity against the hemoprotozoan parasite Babesia bovis is believed to depend on activation of antigen-specific CD4(+) T lymphocytes and IFN-gamma production. A strategy was employed to identify potentially protective antigens from B. bovis based on memory CD4(+) T lymphocyte recognition of fractionated merozoite proteins. Fractions of merozoites separated by continuous flow electrophoresis (CFE) that contained proteins of approximately 20 kDa were shown previously to stimulate memory CD4(+) lymphocyte responses in B. bovis-immune cattle with different MHC class II haplotypes. Expression library screening with rabbit antiserum raised against an immunostimulatory 20-kDa CFE fraction identified a 20-kDa protein (Bbo20) that contains a B lymphocyte epitope conserved in geographically distant B. bovis strains. An homologous 20-kDa protein that has 86.4% identity with Bbo20 and contains the conserved B cell epitope was identified in B. bigemina (Bbg20). Southern blot analysis indicated that both Babesia proteins are encoded by a single gene. Antibody against recombinant Bbo20 protein identified the antigen in CFE fractions shown previously to stimulate memory T lymphocyte responses in immune cattle. To verify Bbo20 as an immunostimulatory T lymphocyte antigen, CD4(+) T cell lines were propagated from B. bovis-immune cattle with merozoite antigen and shown to proliferate significantly against recombinant Bbo20 protein. Furthermore, Bbo20-specific CD4(+) T cell clones proliferated in response to several B. bovis strains and produced IFN-gamma. BLAST analysis revealed significant similarity of the Bbo20 and Bbg20 amino acid sequences with the hsp20/alpha-crystallin family.

Designing blood-stage vaccines against Babesia bovis and B. bigemina

The tick-transmitted apicomplexan parasites Babesia bovis and B. bigemina cause significant disease in cattle in many tropical and temperate areas of the world. These parasites present a challenge for vaccine development, and yet provide a system for studying the pathogenesis, mechanisms of protective immunity and regulation of host immune responses associated with intraerythrocytic protozoan parasites in a non-rodent species. In this article, Wendy Brown and Guy Palmer review strategies for identifying candidate vaccine antigens of B. bovis and B. bigemina and for priming immune responses to evoke strain crossprotective immunity.

Bovine T cell responses to recombinant thioredoxin of Fasciola hepatica

Fasciolosis is an economically significant disease of ruminants, caused by infection with the digenetic trematodes, Fasciola hepatica and F. gigantica. Some vaccination trials using irradiated metacercariae or isolated proteins have been shown to afford significant protection. However, the mechanisms of specific immunity against this pathogen have not been elucidated. We have identified thioredoxin, a tegument antigen of F. hepatica, among several proteins that are common to both the juvenile and adult fluke within the mammalian host and have undertaken studies to characterize bovine T cell responses to recombinant thioredoxin protein (FH 2020). Peripheral blood mononuclear cells from immune cattle proliferated specifically to crude F. hepatica antigenic extract but not to FH 2020. However, after repeated stimulation of lymphocytes by alternating crude extract and FH 2020, FH 2020-specific proliferation by T cell lines was observed. T cell clones were subsequently generated and found to respond specifically but weakly to both crude antigen and FH 2020. Thioredoxin appears to be only weakly antigenic for bovine T cells and is, therefore, an unpromising candidate for inducing resistance to F. hepatica.

Babesia bovis: common protein fractions recognized by oligoclonal B. bovis-specific CD4+ T cell lines from genetically diverse cattle

CD4+ helper T cells are believed to be important for inducing protective immunity against Babesia bovis through the production of cytokines, including IFN-gamma, that will provide help to B lymphocytes for IgG production and activate macrophages to become parasiticidal. To provide maximum protection in an outbred population, an effective vaccine against B. bovis should contain antigens that would elicit an IFN-gamma response and would be recognized by cattle with diverse genetic backgrounds. To identify potentially protective "universal" T helper (Th) cell antigens, fractions of homogenized B. bovis merozoites were tested for the ability to stimulate proliferation of oligoclonal CD4+, IFN-gamma-producing T cell lines derived from four immune animals previously shown to differ in major histocompatibility complex class II expression. Homogenized B. bovis merozoites were separated by denaturing continuous flow electrophoresis (CFE) on 15, 10, and 7.5% polyacrylamide gels into fractions containing proteins ranging from <14.5 to approximately 95 kDa. Eighteen of 280 CFE fractions elicited anamnestic proliferative responses in all T cell lines tested. Nine of these cross-stimulatory fractions contained proteins of <14.5 to 24.5 kDa, and the remaining ones contained proteins with estimated molecular weights of 30, 31.5, 44.5, 49, 49.5, 54, 62, 72, and 82 kDa. Immunoblot analysis showed that four cross-stimulatory fractions contained a predicted known B. bovis antigen of similar molecular size. Previous studies had demonstrated that fractionated merozoite proteins stimulatory for CD4+ Th cell clones had apparent molecular weights similar to those present in 7 of the 18 stimulatory fractions. In the present study, two Th cell clones responded to cross-stimulatory CFE fractions, underscoring the potential to use both oligoclonal and monoclonal Th cell lines to identify commonly recognized polypeptides as potential vaccine antigens.

PCR methods for the discrimination of Babesia bovis isolates

Three different polymerase chain reaction assays for the typing of isolates of Babesia bovis have been developed and compared with a hybridisation based method. Primers were designed within conserved regions flanking the variable length tandem repeats of the Bv80 and BvVA1 genes. For the long array of repeats in BvVA1, up to 7.5 kb, a modified long template PCR method was developed. The assays were compared using ten independent isolates of Babesia bovis. Using the BvVA1 and Bv80 PCR assays, 13 and 10 genotypes could be discriminated, respectively, with some isolates containing several genotypes. Combining the two PCR assays, 17 genotypes were identified within the ten Babesia bovis isolates. Whilst simpler and requiring less DNA, the BvVA1 PCR analysis exhibited significant bias towards some genotypes of the BvVA1 repeats. Further discrimination of BvVA1 PCR products was achieved using AccI digests producing population specific ladders. Genomic DNA fingerprints were also generated by PCR of DNA using an arbitrary primer (randomly amplified polymorphic DNA, RAPD) revealing polymorphic genotypes that were isolate specific. No amplification of host DNA resulted from any of the three PCR procedures. Babesia bigemina DNA was not amplified by the Bv80 or BvVA1 primers. Applications demonstrating changes in composition of populations of Babesia bovis parasites during attenuation and prolonged culture maintenance are described.

Babesia bovis rhoptry-associated protein 1 is immunodominant for T helper cells of immune cattle and contains T-cell epitopes conserved among geographically distant B. bovis strains

The ability of rhoptry-associated protein 1 (RAP-1) of Babesia bovis and Babesia bigemina to confer partial protective immunity in cattle has stimulated interest in characterizing both B-cell and T-cell epitopes of these proteins. It was previously shown that B. bovis RAP-1 associates with the merozoite surface as well as rhoptries and expresses B-cell epitopes conserved among otherwise antigenically different B. bovis strains. An amino-terminal 307-amino-acid domain of the molecule that is highly conserved in the B. bigemina RAP-1 homolog did not contain cross-reactive B-cell epitopes. The studies reported here demonstrate that B. bovis RAP-1 is strongly immunogenic for T helper (Th) cells from B. bovis-immune cattle and that like B-cell epitopes, Th-cell epitopes are conserved in different B. bovis strains but not in B. bigemina RAP-1. Lymphocytes from cattle immune to challenge with the Mexico strain of B. bovis proliferated against recombinant B. bovis RAP-1 protein derived from the Mexico strain. T-cell lines established by stimulating lymphocytes with recombinant RAP-1 protein responded against B. bovis, but not B. bigemina, merozoites. T-cell lines established by repeated stimulation of lymphocytes with B. bovis membrane antigen proliferated strongly against RAP-1, demonstrating the immunodominant nature of this protein. RAP-1-specific CD4+ T cell clones recognized Mexico, Texas, Australia, and Israel strains of B. bovis but neither B. bigemina merozoites nor recombinant B. bigemina RAP- 1. Analysis of cytokine mRNA in RAP-1-specific Th cell clones revealed strong expression of gamma interferon but little or no expression of interleukin-2 (IL-2), IL-4, or IL-10. Gamma interferon production was confirmed by enzyme-linked imunosorbent assay. These results indicate the potential to use selected B. bovis RAP-1 peptides as immunogens to prime for strong, anamnestic, strain-cross-reactive type 1 immune responses upon exposure to B. bovis.

A Babesia bovis 225-kilodalton spherical-body protein: localization to the cytoplasmic face of infected erythrocytes after merozoite invasion

A 225-kDa Babesia bovis protein occurs on the cytoplasmic side of infected-erythrocyte membranes. Here it is demonstrated that the 225-kDa protein localizes to spherical-body organelles of merozoites. Organelles consistent in size and shape with spherical bodies were isolated between 1.17 and 1.21 g/cm(3) in a sucrose density gradient. Organelles consistent with rhoptries and micronemes were also present in fractions from 1.17 to 1.19 g/cm(3). Antisera generated by immunizing mice with the fraction (1.20 to 1.21 g/cm(3)) most enriched for spherical bodies reacted predominantly with spherical bodies in B. bovis merozoites. A monoclonal antibody generated from this immunization (70/97.14) recognized an epitope that occurs in the repeat region of the 225-kDa protein (now referred to as SBP2). Monoclonal antibody 70/97.14 bound to merozoite spherical bodies, vesicles in infected-host cytoplasm, and the cytoplasmic face of the infected-erythrocyte membrane. These results indicate that spherical-body proteins become associated with the host membrane via transport through the erythrocyte cytoplasm after intracellular invasion.

CD4+ T-helper lymphocyte responses against Babesia bigemina rhoptry-associated protein I

A multigene family of 58- to 60-kDa proteins, which are designated rhoptry-associated protein 1 (RAP-1) and which come from the parasites Babesia bigemina and Babesia bovis, is a target for vaccine development. The presence of multiple gene copies and conserved sequences and epitopes of RAP-1 implies that these proteins are functionally important for the survival of these parasites. Furthermore, it was previously shown that B. bigemina RAP-1 induced partial protection against challenge infection. However, the lack of correlation between protective immunity to B. bigemina infection and antibody titers against a merozoite surface-exposed, neutralization-sensitive epitope of B. bigemina RAP-1 indicated the potential importance of RAP-1-specific T helper (Th) cells in the observed protection. To begin to understand the mechanism of RAP-1-induced protective immunity, RAP-1-specific T-cell responses were characterized in cattle. Vigorous and sustained proliferative responses of peripheral blood mononuclear cells from native RAP-1-immunized cattle were observed. The anamnestic response in immunized cattle was specific for B. bigemina RAP-1 and predominantly comprised CD4+ T cells, which upon cloning expressed type 1 cytokine mRNA profiles and high levels of gamma interferon protein. The T cells responded to both native and recombinant forms of RAP-1, indicating the potential to use recombinant protein or epitopes derived therefrom as a vaccine that could evoke specific recall responses after exposure to natural infection. The differential responses of peripheral blood mononuclear cells and seven Th-cell clones derived from RAP-1-immunized cattle to different Central American strains of B. bigemina indicated the presence of at least one conserved and one variable Th-cell epitope. The lack of response to B. bovis RAP-1 indicated that a strictly conserved 14-amino-acid peptide shared by the two babesial species was not immunogenic for Th cells in these experiments. However, the Th-cell epitope conserved among strains of B. bigemina may be a useful component of a RAP-1 subunit vaccine.

Identification of Babesia bovis merozoite antigens separated by continuous-flow electrophoresis that stimulate proliferation of helper T-cell clones derived from B. bovis-immune cattle

To characterize Babesia bovis merozoite antigens that stimulate anamnestic T helper (Th)-cell responses from B. bovis-immune cattle, B. bovis-specific Th-cell lines and clones, previously assigned to different antigenic groups (W. C. Brown, S. Zhao, A. C. Rice-Ficht, K. S. Logan, and V. M. Woods, Infect. Immun. 60:4364-4372, 1992), were tested in proliferation assays against fractionated merozoite antigens. The antigenic groups were determined by the patterns of response of Th clones to different parasite isolates and soluble or membrane forms of merozoite antigen. Soluble antigen fractionated by anion-exchange chromatography or gel filtration by using fast-performance liquid chromatography resolved two or three antigenic peaks, respectively. To enable fractionation of membrane-associated proteins and to resolve more precisely the proteins present in homogenized merozoites, a novel technique of continuous-flow electrophoresis was employed. Merozoite membranes or whole merozoites were homogenized and solubilized in sodium dodecyl sulfate-sample buffer, electrophoresed under reducing conditions on 15% or 10% acrylamide gels, eluted, and collected as fractions. Individual fractions were analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and tested for the ability to stimulate Babesia-specific CD4+ T-cell lines and clones. CD4+ Th-cell lines from two cattle displayed differential patterns of reactivity and detected numerous peaks of antigenic activity, ranging from < 14 to 76 kDa. Th-cell clones previously categorized into different antigenic groups detected antigenic peaks unique for clones representative of a given group. Antigens of 29, 51 to 52, and 85 to 95 kDa (group I), 40 kDa (group III), 20 kDa (group IV), 58 to 60 kDa (group VI), and 38, 45, and 83 kDa (group VII) were identified in the stimulatory fractions. Immunization of rabbits with selected fractions produced a panel of antisera that reacted specifically on Western blots (immunoblots) with merozoite antigens of similar sizes, leading to the tentative identification of candidate antigens of B. bovis merozoites with molecular masses of 20, 40, 44, 51 to 52 or 95, and 58 to 60 kDa that stimulate proliferation of Th clones representative of five different antigenic groups. These antisera may be useful for isolating recombinant proteins that are immunogenic for Th cells of immune cattle and therefore potentially useful for vaccine development.

Identification of candidate vaccine antigens of bovine hemoparasites Theileria parva and Babesia bovis by use of helper T cell clones

Current vaccines for bovine hemoparasites utilize live attenuated organisms or virulent organisms administered concurrently with antiparasitic drugs. Although such vaccines can be effective, for most hemoparasites the mechanisms of acquired resistance to challenge infection with heterologous parasite isolates have not been clearly defined. Selection of potentially protective antigens has traditionally made use of antibodies to identify immunodominant proteins. However, numerous studies have indicated that induction of high antibody titers neither predicts the ability of an antigen to confer protective immunity nor correlates with protection. Because successful parasites have evolved antibody evasion tactics, alternative strategies to identify protective immunogens should be used. Through the elaboration of cytokines, T helper 1-(Th1)-like T cells and macrophages mediate protective immunity against many intracellular parasites, and therefore most likely play an important role in protective immunity against bovine hemoparasites. CD4+ T cell clones specific for soluble or membrane antigens of either Theileria parva schizonts or Babesia bovis merozoites were therefore employed to identify parasite antigens that elicit strong Th cell responses in vitro. Soluble cytosolic parasite antigen was fractionated by gel filtration, anion exchange chromatography or hydroxylapatite chromatography, or a combination thereof, and fractions were tested for the ability to induce proliferation of Th cell clones. This procedure enabled the identification of stimulatory fractions containing T. parva proteins of approximately 10 and 24 kDa. Antisera raised against the purified 24 kDa band reacted with a native schizont protein of approximately 30 kDa. Babesia bovis-specific Th cell clones tested against fractionated soluble Babesia bovis merozoite antigen revealed the presence of at least five distinct antigenic epitopes. Proteins separated by gel filtration revealed four patterns of reactivity, and proteins separated by anion exchange revealed two patterns of reactivity when selected T cell clones were assayed for stimulation by antigenic fractions. Studies using a continuous-flow electrophoresis apparatus have indicated the feasibility of identifying T cell-stimulatory proteins from parasite membranes as well as from the cytosolic fraction of B. bovis merozoites. The Th cell clones reactive with these different hemoparasites expressed either unrestricted or Th1 cytokine profiles, and were generally characterized by the production of high levels of IFN-gamma. A comprehensive study of T cell and macrophage responses to defined parasite antigens will help elucidate the reasons for vaccine failure or success, and provide clues to the mechanisms of acquired immunity that are needed for vaccine development.

Molecular basis for vaccine development against anaplasmosis and babesiosis

Immunization of livestock against the erythroparasitic pathogens Anaplasma marginale, Babesia bigemina, and Babesia bovis with safe and effective killed vaccines is not yet feasible on a practical basis. However, the immune protection afforded by recovery from natural infection and premunition indicates that microbial epitopes capable of inducing immunity exist and that the bovine immune system can be primed appropriately. Induction of protection by immunization with killed parasite fractions, enriched for polypeptides with surface exposed epitopes, supports a focus on surface epitopes, including apical complex organellar epitopes in Babesia, for vaccine development. Cloning, sequencing, and expression of genes encoding these key surface polypeptides has allowed examination of polypeptide function and detailed analysis of epitope conservation in light of genetic polymorphism. In this paper, the characterization of these polypeptides at the epitope level and their roles in inducing protective immunity are reviewed. Definition of these epitopes, in combination with improved understanding of immune mechanisms, provides the basis for development of effective recombinant vaccines against anaplasmosis and babesiosis.

Immunization of cattle with recombinant Babesia bovis merozoite surface antigen-1

Cattle immunized with a recombinant merozoite surface antigen-1 molecule (MSA-1) produced high-titered antibody that reacted with the surface of the parasite and neutralized merozoite infectivity in vitro. However, recombinant MSA-1 immunization did not confer protection against challenge with virulent Babesia bovis. These results indicate that antibody-mediated neutralization of merozoite infectivity in vitro, at least for MSA-1-specific antibody, does not reflect in vivo protective immunity to babesiosis.

Identification of two Th1 cell epitopes on the Babesia bovis-encoded 77-kilodalton merozoite protein (Bb-1) by use of truncated recombinant fusion proteins

Previous studies have demonstrated the serologic and T-cell immunogenicity for cattle of a recombinant form of the apical complex-associated 77-kDa merozite protein of Babesia bovis, designated Bb-1. The present study characterizes the immunogenic epitopes of the Bb-1 protein. A series of recombinant truncated fusion proteins spanning the majority of the Bb-1 protein were expressed in Escherichia coli, and their reactivities with bovine peripheral blood mononuclear cells and T-cell clones derived from B. bovis-immune cattle and with rabbit antibodies were determined. Lymphocytes from two immune cattle were preferentially stimulated by the N-terminal half of the Bb-1 protein (amino acids 23 to 266, termed Bb-1A), localizing the T-cell epitopes to the Bb-1A portion of the molecule. CD4+ T-cell clones derived by stimulation with the intact Bb-1 fusion protein were used to identify two T-cell epitopes in the Bb-1A protein, consisting of amino acids SVVLLSAFSGN VWANEAEVSQVVK and FSDVDKTKSTEKT (residues 23 to 46 and 82 to 94). In contrast, rabbit antiserum raised against the intact fusion protein reacted only with the C-terminal half of the protein (amino acids 267 to 499, termed Bb-1B), which contained 28 tandem repeats of the tetrapeptide PAEK or PAET. Biological assays and Northern (RNA) blot analyses for cytokines revealed that following activation with concanavalin A, T-cell clones reactive against the two Bb-1A epitopes produced interleukin-2, gamma interferon, and tumor necrosis factors beta and alpha, but not interleukin-4, suggesting that the Bb-1 antigen preferentially stimulates the Th1 subset of CD4+ T cells in cattle. The studies described here report for the first time the characterization, by cytokine production, of the Th1 subset of bovine T cells and show that, as in mice, protozoal antigens can induce Th1 cells in ruminants. This first demonstration of B. bovis-encoded Th1 cell epitopes provides a rationale for incorporation of all or part of the Bb-1 protein into a recombinant vaccine.

Bovine helper T cell clones recognize five distinct epitopes on Babesia bovis merozoite antigens

Helper T cell clones from two Babesia bovis-immune cattle were characterized for use in identification of potentially protective immunogens of B. bovis merozoites. Proliferation assays with 11 CD4+ clones revealed a differential pattern of response to soluble cytosolic antigen, membrane-enriched antigen, detergent extracts of the membrane-enriched antigen, soluble culture supernatant exoantigen, and different geographical isolates of B. bovis as well as Babesia bigemina parasites. When the data were combined, the clones could be grouped according to five different patterns of response. One group recognized only the membrane-enriched fraction of New World and Australian parasites. Four remaining groups recognized antigens found in the cytosolic as well as the membrane-enriched fraction, and clones representative of each group were used to identify cytosolic antigens fractionated by anion-exchange chromatography with the use of fast-performance liquid chromatography. One clone (C97.3C3), which responded to all B. bovis isolates and to B. bigemina, recognized a single peak of activity that eluted with 0.25 M NaCl and contained protein bands of 70 and 75 kDa. The remaining clones were stimulated by a second antigenic peak that eluted between 0.35 and 0.45 M NaCl and contained protein bands of 42, 47, 56, and 84 kDa. The majority of the clones produced interferon, whereas tumor necrosis factor alpha/tumor necrosis factor beta production was less frequent. These studies provide the basis for using helper T cell clones to identify potentially protective immunogens of B. bovis and delineate a minimum of five helper T cell epitopes recognized by two immune cattle.

Babesia bovis: Bovine helper T cell lines reactive with soluble and membrane antigens of merozoites

Babesia bovis-specific T cell lines were established from cattle infected with either tick-derived or cultured parasites by stimulation of peripheral blood mononuclear cells with a crude parasite membrane fraction. Induction and enrichment of CD4+ T cells occurred over time. All cell lines responded vigorously and in a dose-dependent, MHC-restricted manner to intact merozoites, and to soluble and membrane fractions derived from merozoites by homogenization and high-speed centrifugation. Solubilization of the membrane fraction with nondenaturing zwitterionic or nonionic detergents yielded antigenic extracts which also stimulated the T cells. However, a differential response was observed, in that cell lines from one animal proliferated vigorously to the detergent extracts of the membrane fraction, whereas cell lines from a second animal proliferated only weakly to these extracts. SDS-PAGE analysis revealed common protein bands of 90 and 22 kDa in the various immunogenic fractions. Cell lines from the animal infected with cultured parasites also responded to parasite culture supernatant "exoantigens" and to the related parasite, Babesia bigemina. We conclude that antigens present in merozoite membranes and soluble parasite extracts preferentially stimulate CD4+ T cells from cattle immune to Babesia bovis. The differential pattern of response of T cell lines from different cattle suggests that more than one protein or epitope is immunodominant for T cells.