In vitro inhibition of multiplication of Babesia bigemina by using monoclonal antibodies

Comparison of high throughput RNA sequences between Babesia bigemina and Babesia bovis revealed consistent differential gene expression that is required for the Babesia life cycle in the vertebrate and invertebrate hosts

Bovine babesiosis caused by Babesia bigemina and Babesia bovis is an economically important disease that affects cattle worldwide. Both B. bigemina and B. bovis are transovarially transmitted by Rhipicephalus ticks. However, little is known regarding parasite gene expression during infection of the tick vector or mammalian host, which has limited the development of effective control strategies to alleviate the losses to the cattle industry. To understand Babesia gene regulation during tick and mammalian host infection, we performed high throughput RNA-sequencing using samples collected from calves and Rhipicephalus microplus ticks infected with B. bigemina. We evaluated gene expression between B. bigemina blood-stages and kinetes and compared them with previous B. bovis RNA-seq data. The results revealed similar patterns of gene regulation between these two tick-borne transovarially transmitted Babesia parasites. Like B. bovis, the transcription of several B. bigemina genes in kinetes exceeded a 1,000-fold change while a few of these genes had a >20,000-fold increase. To identify genes that may have important roles in B. bigemina and B. bovis transovarial transmission, we searched for genes upregulated in B. bigemina kinetes in the genomic datasets of B. bovis and non-transovarially transmitted parasites, Theileria spp. and Babesia microti. Using this approach, we identify genes that may be potential markers for transovarial transmission by B. bigemina and B. bovis. The findings presented herein demonstrate common Babesia genes linked to infection of the vector or mammalian host and may contribute to elucidating strategies used by the parasite to complete their life cycle.

RON2, a novel gene in Babesia bigemina, contains conserved, immunodominant B-cell epitopes that induce antibodies that block merozoite invasion

Bovine babesiosis is the most important protozoan disease transmitted by ticks. In Plasmodium falciparum, another Apicomplexa protozoan, the interaction of rhoptry neck protein 2 (RON2) with apical membrane antigen-1 (AMA-1) has been described to have a key role in the invasion process. To date, RON2 has not been described in Babesia bigemina, the causal agent of bovine babesiosis in the Americas. In this work, we found a ron2 gene in the B. bigemina genome. RON2 encodes a protein that is 1351 amino acids long, has an identity of 64% (98% coverage) with RON2 of B. bovis and contains the CLAG domain, a conserved domain in Apicomplexa. B. bigemina ron2 is a single copy gene and it is transcribed and expressed in blood stages as determined by RT-PCR, Western blot, and confocal microscopy. Serum samples from B. bigemina-infected bovines were screened for the presence of RON2-specific antibodies, showing the recognition of conserved B-cell epitopes. Importantly, in vitro neutralization assays showed an inhibitory effect of RON2-specific antibodies on the red blood cell invasion by B. bigemina. Therefore, RON2 is a novel antigen in B. bigemina and contains conserved B-cell epitopes, which induce antibodies that inhibit merozoite invasion.

Variable and Variant Protein Multigene Families in Babesia bovis Persistence

Cattle infected with Babesia bovis face a bifurcated fate: Either die of the severe acute infection, or survive and carry for many years a highly persistent but generally asymptomatic infection. In this review, the author describes known and potential contributions of three variable or highly variant multigene-encoded families of proteins to persistence in the bovine host, and the mechanisms by which variability arises among these families. Ramifications arising from this variability are discussed.

Expression analysis and biological characterization of Babesia sp. BQ1 (Lintan) (Babesia motasi-like) rhoptry-associated protein 1 and its potential use in serodiagnosis via ELISA

Background

In China, ovine babesiosis is one of the most important tick-borne haemoparasitic diseases of small ruminants. It has a significant economic impact, and several Babesia motasi-like isolates have been recently shown to be responsible for ovine babesiosis in this country.

Methods

Full-length and C-terminal-truncated forms of the rap-1a61-1 gene of Babesia sp. BQ1 (Lintan) were cloned into the pET-30a plasmid and subsequently expressed as His-fusion proteins. The resulting recombinant RAP-1a proteins (rRAP-1a61-1 and rRAP-1a61-1/CT) were purified and evaluated as diagnostic antigens using Western blot analysis and ELISA. The native Babesia sp. BQ1 (Lintan) RAP-1 protein was recognized using Western blots and IFAT by antibodies that were raised in rabbits against rRAP-1a61-1/CT. The specificity, sensitivity and positive threshold values for rRAP-1a61-1/CT in ELISA were evaluated.

Results

Cross-reactivity was observed between rRAP-1a61-1/CT and positive sera for Babesia sp. BQ1 (Lintan), Babesia sp. BQ1 (Ningxian) and Babesia sp. Tianzhu isolates obtained from infected sheep. At one week post-inoculation, a significant increase was observed in the amount of antibodies produced against RAP-1a, and high levels of antibodies against RAP-1a were observed for 3 months (at 84 days p.i.). A total of 3198 serum samples were collected from small ruminants in 54 different regions in 23 provinces of China. These samples were tested using ELISA based on the rRAP-1a61-1/CT protein. The results indicated that the average positive rate was 36.02 %.

Conclusions

The present study suggests that rRAP-1a61-1/CT might be a potential diagnostic antigen for detecting several isolates of B. motasi-like parasites infection.

Rhoptry-associated protein (rap-1) genes in the sheep pathogen Babesia sp. Xinjiang: Multiple transcribed copies differing by 3' end repeated sequences

Sheep babesiosis occurs mainly in tropical and subtropical areas. The sheep parasite Babesia sp. Xinjiang is widespread in China, and our goal is to characterize rap-1 (rhoptry-associated protein 1) gene diversity and expression as a first step of a long term goal aiming at developing a recombinant subunit vaccine. Seven different rap-1a genes were amplified in Babesia sp. Xinjiang, using degenerate primers designed from conserved motifs. Rap-1b and rap-1c gene types could not be identified. In all seven rap-1a genes, the 5' regions exhibited identical sequences over 936 nt, and the 3' regions differed at 28 positions over 147 nt, defining two types of genes designated α and β. The remaining 3' part varied from 72 to 360 nt in length, depending on the gene. This region consists of a succession of two to ten 36 nt repeats, which explains the size differences. Even if the nucleotide sequences varied, 6 repeats encoded the same stretch of amino acids. Transcription of at least four α and two β genes was demonstrated by standard RT-PCR.

Sequence and organization of the rhoptry-associated-protein-1 (rap-1) locus for the sheep hemoprotozoan Babesia sp. BQ1 Lintan (B. motasi phylogenetic group)

Babesiosis is a frequent infection of animals worldwide by tick borne pathogen Babesia, and several species are responsible for ovine babesiosis. Recently, several Babesia motasi-like isolates were described in sheep in China. In this study, we sequenced the multigenic rap-1 gene locus of one of these isolates, Babesia sp. BQ1 Lintan. The RAP-1 proteins are involved in the process of red blood cells invasion and thus represent a potential target for vaccine development. A complex composition and organization of the rap-1 locus was discovered with: (1) the presence of 3 different types of rap-1 sequences (rap-1a, rap-1b and rap-1c); (2) the presence of multiple copies of rap-1a and rap-1b; (3) polymorphism among the rap-1a copies, with two classes (named rap-1a61 and rap-1a67) having a similarity of 95.7%, each class represented by two close variants; (4) polymorphism between rap-1a61-1 and rap-1a61-2 limited to three nucleotide positions; (5) a difference of eight nucleotides between rap-1a67-1 and rap-1a67-2 from position 1270 to the putative stop site of rap-1a67-1 which might produce two putative proteins of slightly different sizes; (6) the ratio of rap-1a copies corresponding to one rap-1a67, one rap-1a61-1 and one rap-1a61-2; (7) the presence of three different intergenic regions separating rap-1a, rap-1b and rap-1c; (8) interspacing of the rap-1a copies with rap-1b copies; and (9) the terminal position of rap-1c in the locus. A 31kb locus composed of 6 rap-1a sequences interspaced with 5 rap-1b sequences and with a terminal rap-1c copy was hypothesized. A strikingly similar sequence composition (rap-1a, rap-1b and rap-1c), as well as strong gene identities and similar locus organization with B. bigemina were found and highlight the conservation of synteny at this locus in this phylogenetic clade.

Expression analysis of heat shock protein 20 and rhoptry-associated protein 1a in sexual stages and kinetes of Babesia bigemina

Heat shock protein 20 (HSP-20) and rhoptry-associated protein 1a (RAP-1a) are two proteins considered as candidates to be included in vaccines or diagnostics methods for the control of bovine babesiosis. It has been hypothesized that both genes have a basic function in the cellular physiology of erythrocyte-infecting stages; it is not known if they have a functional role in tick stages. The objective of this work was to analyze whether hsp-20 and rap-1a are expressed in sexual stages and kinetes of Babesia bigemina. Purified RNA from sexual stages and kinetes was analyzed by reverse transcriptase (RT)-PCR with specific primers for hsp-20 or rap-1a. Expression analysis was carried out using an indirect immunofluorescence test with specific antibodies against HSP-20 and RAP-1a. Results obtained by RT-PCR showed amplicons for hsp-20 and rap-1a in sexual stages and kinetes. Positive signals were also detected when sexual stages and kinetes were analyzed with specific antibodies for HSP-20 and RAP-1a. The results obtained here confirm the hypothesis that the genes hsp-20 and rap-1a from B. bigemina are expressed in sexual stages and kinetes and stress the importance of these proteins in the cellular physiology of tick stages.

Babesia bigemina: Sporozoite Isolation fromBoophilus microplusNymphs and Initial Immunomolecular Characterization

It has been hypothesized that babesial sporozoites express specific antigens that induce protective immunologic responses in cattle. However, they remain uncharacterized, partly for lack of research on the sporozoite stage of Babesia spp. This field suffers from complete knowledge of parasite development in the tick salivary gland; limited amounts of sporozoites from ticks, and a lack of protocols for induction and purification of sporozoites. In this work, Boophilus microplus larvae infected with B. bigemina were fed on susceptible cattle. Nymphs were collected and macerates were separated by a Percoll density gradient. Microscopic analysis of Giemsa-stained smears showed a larger number of sporozoites from nymphs fed for 9 days. Percoll-purified sporozoites were observed in large numbers in groups or individually and free of tick cells. RT-PCR analysis of total RNA extracted from purified sporozoites indicated transcription of the rhoptry associate protein 1 (rap-1) genes: rap-1a, rap-b, rap-1c, as well as the heat shock protein 20 (hsp-20) gene. Purified sporozoites were cultured in vitro analyzed for RAP-1a expression using an immunocytochemistry assay. Erythrocyte-attached sporozoites reacted with a specific RAP-1a monoclonal antibody. This is the first report of Babesia bigemina sporozoite antigens. Moreover, purified sporozoites will allow the characterization of stage-specific antigens involved in immunologic protection.

Identification of a Coronin-Like Protein inBabesiaSpecies

The present study was designed to immunochemically identify a coronin-like protein in Babesia bovis, B. bigemina, B. divergens, and B. canis. A 2-kbp cDNA insert of B. bovis carried by plasmid BvN9 was sequenced by the dideoxichain-termination method on both strands. The cDNA insert contained a 1719-bp long open reading frame coding for a deduced protein sequence of 61.7 kDa. Sequence analysis using the PSI-BLAST program revealed about 30% protein sequence identity with a coronin-like protein of Plasmodium falciparum. The encoding sequence of the cDNA insert lacking 70 amino acids at the N-terminal was subcloned in frame into pGEX 4T-3 to produce a recombinant glutathione S-transferase (GST)-pBv fusion protein. Polyclonal antibodies prepared in rabbits immunized with the purified GST-fusion protein recognized a Babesia-specific component of approximately 60 kDa by immunoprecipitation with [35S]methionine-labeled parasites. However, two molecules with relative sizes of 60 and 70 kDa were recognized in Babesia-infected erythrocyte extracts by immunobloting analysis. The 70-kDa component was apparently of host erythrocyte origin. In an indirect fluorescent antibody test, the rabbit serum strongly reacted with the merozoite stage of the four Babesia species, but also, although weakly, with the host erythrocyte. A cosedimentation assay performed with GST-pBv fusion protein and exogenous actin from rabbit liver showed that the GST-pBv fusion protein, but not the GST protein, was associated to actin. From these results, we conclude that the protein present in the four Babesia species analyzed here may be considered as a novel coronin-like, actin-binding protein.

Development of a microtitre-based spectrophotometric method to monitor Babesia divergens growth in vitro

Babesia divergens multiplication cycle involves erythrocyte invasion, intracellular division, and erythrocyte lysis with the simultaneous liberation of hemoglobin. We have decided to set up a spectrophotometric protocol based on hemoglobin concentration in the culture supernatants to monitor B. divergens in vitro growth. After the selection of 405 nm as the most appropriate endpoint hemoglobin wavelength in our conditions (hemoglobin concentration in the supernatant), cultures were standardized [1 x 10(9) red blood cell (RBC)/ml, 1-2.5 x 10(5) infected red blood cell (iRBC)/ml] to allow their monitoring over 3 days. The protocol was then compared to the most commonly used growth measurement methods: parasitemia counting and [(3)H]hypoxanthine incorporation. An excellent correlation was demonstrated between A(405) of the culture supernatant and parasitemia of the iRBC, whatever the RBC concentration used in the medium. This correlation was also evidenced between A(405) and [(3)H]hypoxanthine incorporation for [(3)H]hypoxanthine concentrations lower than 4 microCi/ml. Our assays also highlighted the inhibitory effect of [(3)H]hypoxanthine on B. divergens growth even when used at low concentrations (0.8 microCi/ml) and for a short incorporation duration (24 h). This effect was confirmed by both A(405) and parasitemia counting. In conclusion, A(405) measurement of B. divergens culture supernatant represents a simple, rapid, safe, and reliable way to measure the in vitro growth of this parasite. Generation times of three different B. divergens strains were then determined by the protocol described here and varied between 8 h 36 min and 13 h 8 min.

Organization, transcription, and expression of rhoptry associated protein genes in the Babesia bigemina rap-1 locus

The Babesia bigemina rap-1 gene locus contains five tandemly arranged copies of rap-1a genes. However, the size of the locus, as defined by conserved, unrelated orfs at the 5' and 3' ends, suggests that additional genes may be present. In this study, we identified all additional genes in the locus and characterized their pattern of expression in merozoites. The rap-1a genes are separated by 3.38-kbp intergenic (IG) regions, each of which contains an identical copy of a related gene designated rap-1b. One additional copy of rap-1b and one copy of another related gene designated rap-1c is present in the 3' end of the locus. Common sequence features that define the Babesia rap-1 family are present in rap-1b and rap-1c, but otherwise these genes average only 27% identity to rap-1a. Homologues of the rap-1b and rap-1c genes identified in diverse B. bigemina strains have a high degree of predicted amino acid sequence conservation (averaging >90%), with the largest number of changes in the carboxyl end of RAP-1c. We tested whether all rap-1 genes in the locus are co-transcribed in merozoites using RT-PCR, Northern blots, and quantitative real-time PCR. Rap-1a genes produce the most abundant transcripts of the family, while rap-1b transcripts are the least abundant despite the large number of gene copies. Similar patterns of transcription were observed whether merozoites were obtained from in vitro cultures or in vivo infection. Immunoblot analysis of merozoites revealed the expected RAP-1a expression but failed to detect expressed RAP-1b and RAP-1c, indicating that expression of the rap-1 genes is regulated both at the transcriptional and translational levels.

Babesia bovis merozoite surface antigen 1 and rhoptry-associated protein 1 are expressed in sporozoites, and specific antibodies inhibit sporozoite attachment to erythrocytes

We examined Babesia bovis sporozoites for the expression of two molecules, merozoite surface antigen 1 (MSA-1) and rhoptry-associated protein 1 (RAP-1), that are postulated to be involved in the invasion of host erythrocytes. Both MSA-1 and RAP-1 were transcribed and expressed in infectious sporozoites. Importantly, monospecific MSA-1 and RAP-1 antisera each inhibited sporozoite invasion of erythrocytes in vitro. This is the first identification of antigens expressed in Babesia sp. sporozoites and establishes that, at least in part, sporozoites and merozoites share common targets of antibody mediated inhibition of erythrocyte invasion.

Inhibitory effect of monoclonal antibodies on the growth of Babesia caballi

Monoclonal antibodies (mAbs) were produced against Babesia caballi (USDA strain) to define a species-specific antigen for use in diagnosis and vaccine development. Eight positive clones of B. caballi mAbs determined by indirect immunofluorescent antibody test were selected for purification and further characterisation. Confocal laser microscopy showed that the antigens recognised by the mAbs were located on the surface/cytoplasm, central part, and/or anterior end of B. caballi parasites, with five different reactive patterns. These mAbs seemed to be species-specific, since they did not cross-react with Babesia equi-infected erythrocytes or uninfected erythrocytes. In Western blotting analysis, 18, 20, 34, 36, 48, and 155 kDa proteins of B. caballi merozoites were recognised by six different mAbs. When added to in vitro cultures, four of the mAbs significantly inhibited the in vitro growth of B. caballi parasites. These results provide a rationale for evaluating antigens for the development of diagnostic methods or vaccines.

Monoclonal antibody against Babesia equi: characterization and potential application of antigen for serodiagnosis

Monoclonal antibody (MAb) BEG3 was produced against Babesia equi parasites to define a species-specific antigen for diagnostic use. The MAb reacted with single, paired, and Maltese cross forms of B. equi, and no reaction was observed with this MAb on acetone-fixed Babesia caballi, Babesia ovata, or Babesia microti parasites in the indirect immunofluorescent antibody test. Confocal laser and immunoelectron microscopic studies showed that the antigen which was recognized by this MAb was located on the surface of B. equi parasites. This MAb recognized a 19-kDa protein of B. equi antigen and did not react with B. caballi antigen or normal horse erythrocytes in immunoblot analysis. This MAb also significantly inhibited the in vitro growth of the B. equi parasite. Preliminary studies using partially purified antigen, which was separated by high-pressure liquid chromatography and recognized by the MAb, suggested that it is a suitable antigen for enzyme-linked immunosorbent assay detection of anti-B. equi antibodies in naturally infected horse sera.

Genetic variation in the dimorphic regions of RAP-1 genes and rap-1 loci of Babesia bigemina

The rhoptry-associated protein-1 (RAP-1) of Babesia bigemina induces protective immune responses in cattle. RAP-1 has two regions of sequence dimorphism at the carboxy and amino terminal ends, respectively. Neutralization-sensitive, surface-exposed B-cell epitopes are present in the amino terminal variant type 1 (NT-1), and CD4+ T-cell epitopes in the carboxy terminal variant type 1 (CT-1). Importantly, antibodies recognizing NT-1 epitopes do not cross react with NT-2 and CD4+ T-cells recognizing epitopes in CT-1 do not cross react with CT-2, suggesting that variation in dimorphic regions of RAP-1 is immunologically significant. We evaluated rap-1 locus structure and the extent of sequence variation in the dimorphic regions of rap-1 genes from geographically diverse strains of B. bigemina. All strains contained NT-1 and NT-2 the encoding sequences were highly conserved, with at least 99%, nucleotide identity among strains. However, the Puerto Rico strain encoded a hybrid NT-1/NT-2 sequence which appears to have originated by a gene conversion event. The 3' ends of rap-1 genes, which include the carboxy terminal variants, are conserved among strains. A new and conserved CT variant (CT-3), with a region of sequence identity to CT-2 and a sequence not related to either CT-1 or CT-2, was identified in all strains of B. bigemina. All but one strain encode both NTs and the three CT variants. The S1A strain, an attenuated strain from Argentina, does not encode CT-2. While NT-1 is associated only with CT-1, NT-2 can be associated with all three CT variants in RAP-1. Within the genome, rap-1 genes are arranged in tandem repeats but with different gene copy number and arrangements among strains. Collectively, the data suggest that gene conversion and unequal recombination events contribute to overall rap-1 sequence conservation among gene variants and strains but may also generate new rap-1 variants.

Immunization with Babesia bigemina rhoptry-associated protein 1 induces a type 1 cytokine response

Rhoptry-associated protein-1 (RAP-1) homologues of Babesia bigemina and Babesia bovis are promising candidates for inclusion in subunit vaccines against these hemoprotozoan parasites. Partial protection against challenge infection has been achieved with native forms of these antigens, but the mechanism of immunity has not been thoroughly defined. We previously demonstrated that a panel of antigen-specific T helper cell clones derived from B. bigemina RAP-1-immunized cattle expressed relatively high levels of interferon-gamma (IFN-gamma) protein and transcript and low levels of interleukin-4 (IL-4), indicative of a type 1 immune response. In the current study we present evidence that subcutaneous immunization with native B. bigemina RAP-1 protein in RIBI adjuvant induces a predominant type 1 immune response in vivo, characterized by relatively high levels of IFN-gamma and IL-2 and low levels of IL-4 and IL-10 mRNA in the draining prescapular lymph node. Ex vivo restimulation of draining lymph node lymphocytes with specific antigen resulted in proliferation and enhanced expression of IL-2 and IFN-gamma, whereas IL-4 and IL-10 transcript levels remained relatively low. These findings show that our previously described cytokine profiles of antigen-specific cloned T cell lines are representative of autologous in vivo responses and confirm that type 1 recall responses to B. bigemina RAP-1 can be evoked in immunized animals by native parasite antigen.

Prospects for subunit vaccines against tick-borne diseases

Tick-borne parasites are a serious impediment to the improvement of live-stock production in the developing world. The major parasites affecting cattle include Theileria parva, T. annulata, Babesia bigemina, B. bovis, Anaplasma marginale and Cowdria ruminantium. The control of these infections is dependent on the use of acaricides to decrease transmission by the tick vectors, and immunization of susceptible animals with live vaccines. The use of acaricide is hampered by the development of resistance, and live vaccines require cold chain facilities, which are generally unreliable in developing countries. There is therefore a need for improved vaccines that can circumvent these problems. There is a subunit vaccine being developed for T. parva based on the major surface antigen of the sporozoite (p67). A similar antigen, SPAG 1, has been identified as a candidate for T. annulata. Although several candidate antigens have been identified for Babesia spp., progress towards development of a subunit vaccine based on these antigens has been hampered by polymorphism among isolates and between species, and lack of knowledge of the immune effector mechanisms responsible for protection. The search for protective antigens of A. marginale has focused on outer membrane proteins; immunization with a variety of these antigens alone or in combination, has yielded promising results. As with Babesia, further definition of immune effector mechanisms is needed to optimize immunization strategies. The work on identifying the protective antigens of C. ruminantium is in its embryonic stages; however, two antigens have been identified and are currently being evaluated. There is high expectancy for subunit vaccines for all these diseases; however there is need for further work to elucidate the immune mechanisms in order to select appropriate antigen delivery systems.

Dimorphic sequences of rap-1 genes encode B and CD4+ T helper lymphocyte epitopes in the Babesia bigemina rhoptry associated protein-1

The rhoptry-associated protein-1 (RAP-1) of Babesia bigemina induces protective immune responses in cattle and contains neutralization-sensitive B cell epitopes. RAP-1 variants containing blocks of sequence dimorphism in the amino and carboxy terminal ends are encoded by four nonallelic genes in B. bigemina. Epitopes recognized by RAP-1 specific monoclonal antibodies (MAbs) and bovine CD4+ T cell clones were mapped to determine whether these epitopes are localized in the amino and carboxy terminal dimorphic regions. Four B cell epitopes, including a neutralization-sensitive epitope, required both the amino terminal variant type 1 (NT-1) and non-dimorphic sequences for conformation. Intrachain disulfide bonds were required for at least one of these epitopes, since reduction and alkylation of cysteine residues abolished MAb binding. A fifth B cell epitope was mapped to the carboxy terminal variant type 1 (CT-1). As expected, the neutralizing MAb and two other MAbs requiring NT-1 for epitope binding recognized only the two RAP-1 variants with the NT-1 sequence, while the MAb binding an epitope in CT-1 did not bind RAP-1 variants with CT-2. In contrast, the fourth MAb requiring NT-1 for binding recognized all rap-1 gene products, indicating that dimorphic residues are not part of the epitope recognized by this MAb. Bovine CD4+ T cell clones characterized previously as responding in a strain dependent fashion recognized at least one epitope in CT-1, and did not cross-react with CT-2. A second group of bovine CD4+ T cell clones that responded to multiple parasite strains recognized an epitope in a non-dimorphic region of RAP-1. These data indicate that dimorphic regions of RAP-1 encode unique B and T helper lymphocyte epitopes and may be required for enhanced protective immune responses in cattle.

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.

Conservation of merozoite membrane and apical complex B cell epitopes among Babesia bigemina and Babesia bovis strains isolated in Brazil

Babesia merozoite polypeptides bear surface exposed and neutralization-sensitive B cell epitopes and have been shown to induce partial protection against experimental challenge. Variation in these epitopes has been examined in a limited number of strains. In this study, utilizing strains of Babesia bovis and Babesia begemina from Matto Grosso do Sul in Brazil, we examined the conservation of epitopes bound by monoclonal antibodies developed against Mexico strains of B. bovis and B. bigemina. Apical complex B-cell epitopes, previously shown to be species-specific but common among otherwise antigenically distinct strains, were also conserved between clones of the Mexico strains and the Matto Grosso do Sul strains of each Babesia species. Mexico strain polypeptides bearing these epitopes were recognized by sera from cattle infected with the Matto Grosso do Sul strains. Two distinct epitopes on the B. bovis neutralization-sensitive merozoite surface antigen-1 (MSA-1) were also conserved between the Mexico Mo7 clone and the Matto Grosso do Sul strain, in contrast to previous studies which demonstrated variability among strains. Sera from cattle with B. bovis infections naturally acquired in Matto Grosso do Sul bound Mexico Mo7 MSA-1, demonstrating that conserved MSA-1 epitopes were recognized by the bovine immune system. Similarly, merozoite surface epitopes on the B. bigemina 45 kDa and 55 kDa glycoproteins were conserved on the Matto Grosso do Sul strain of B. bigemina.

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.

Antibody response to a Babesia bigemina rhoptry-associated protein 1 surface-exposed and neutralization-sensitive epitope in immune cattle

Protective immunity against Babesia bigemina is hypothesized to involve antibodies directed against merozoite surface-exposed epitopes. Levels of antibody against a rhoptry-associated protein 1 (RAP-1) B-lymphocyte epitope, defined by surface-reactive and inhibitory monoclonal antibodies, in immune cattle sera were determined. All cattle produced antibodies to the epitope; however, there was limited correlation between immune protection induced by infection or RAP-1 immunization and the level of antibody to the neutralization-sensitive B-lymphocyte epitope examined.

Interstrain conservation of babesial RAP-1 surface-exposed B-cell epitopes despite rap-1 genomic polymorphism

Members of the babesial rhoptry-associated protein 1 (RAP-1) family express surface-exposed B-cell epitopes and are candidate antigens for vaccine development. The relationship between rap-1 genomic polymorphism and surface-exposed B-cell epitope expression was analyzed by comparison of biological clones of Mexico strain Babesia bigemina and Babesia bovis with strains isolated in Argentina. Despite genomic polymorphism between strains, including sequences located within the open reading frame, defined RAP-1 B-cell surface epitopes and RAP-1 molecular size were conserved in both B. bovis and B. bigemina.

Molecular variation and diversity in candidate vaccine antigens from Babesia

Recombinant vaccines are being developed against a number of species of protozoan parasites in the genus Babesia. Protozoan parasites are notorious for their diversity of strains and their ability to express families of equivalent, but antigenically distinct, surface proteins. In order to reduce the likelihood of evasion of the immune response induced by a recombinant vaccine, ideal components should be essential proteins encoded by single copy genes. The proteins should also have a limited ability to tolerate polymorphism in amino acid sequence, especially in critical epitopes. While little is known about the function of the candidate protective antigens, there is now considerable information concerning the variation of a number of candidate vaccine antigens from several species of Babesia. Four of the well studied antigens are all members of multi-gene families. The members of the VMSA gene family of Babesia bovis are also highly polymorphic in sequence. The members of the Bv60/p58 family of rhoptry protein homologues exhibit more limited polymorphism within a single species of Babesia. However, comparison of the sequences of the equivalent proteins and the organisation of the corresponding genes from B. bovis, Babesia bigemina, Babesia canis and Babesia ovis suggests that members of this family have the potential to acquire and to tolerate substantial polymorphism in amino acid sequence. The choice of protein, and particular region of the protein, suitable for incorporation in a recombinant vaccine may require extensive analysis of the genetic systems encoding the candidate antigens.

Screening of a Babesia bigemina cDNA library with monoclonal antibodies directed to surface antigens

A Babesia bigemina cDNA library prepared in lambda ZAP bacteriophage vector was immunoscreened to detect clones expressing surface-exposed epitopes of B. bigemina. A nonradioactive indirect plaque-lift immunoassay was used to detect the positive clones. The primary antibody consisted of a pooled sample of six monoclonal antibodies (mAb) specific for B. bigemina that recognizes various parasite surface antigens of different molecular mass. Screening of approximately 300,000 plaque-forming units from the lambda ZAP cDNA expression library resulted in the identification of five positive clones. The five recombinant clones were immunoscreened individually with each of the six mAb. All five independently obtained clones consisted of lambda ZAP recombinants expressing B. bigemina components recognized by mAb C2F3G3 and B1B3C4. Restriction enzyme digests of rescued recombinant phagemids showed that only four clones contained B. bigemina cDNA. One clone (lambda ZAP Bbi1) contained an insert of approximately 0.6 kBp whereas the other three clones (lambda ZAP Bbi2, lambda ZAP Bbi3, and lambda ZAP Bbi5) carried a cDNA insert of approximately 1.7 kBp. Immunoblotting of protein extracts from recombinants lambda ZAP Bbi2, lambda ZAP Bbi3, and lambda ZAP Bbi5 with mAb C2F3G3 and B1B3C4 demonstrated the expression of a recombinant B. bigemina polypeptide of 55 kDa in E. coli.