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

Advances in the Epidemiological Surveillance of Tick-Borne Pathogens

Ticks are obligate blood-feeding ectoparasites of mammals, birds, and reptiles, which are globally important vectors of pathogens that impact both human and animal health [...].

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.

The GP-45 Protein, a Highly Variable Antigen from Babesia bigemina, Contains Conserved B-Cell Epitopes in Geographically Distant Isolates

In B. bigemina, the 45 kilodaltons glycoprotein (GP-45) is the most studied. GP-45 is exposed on the surface of the B. bigemina merozoite, it is believed to play a role in the invasion of erythrocytes, and it is characterized by a high genetic and antigenic polymorphism. The objective of this study was to determine if GP-45 contains conserved B-cell epitopes, and if they would induce neutralizing antibodies. The comparative analysis of nucleotide and amino acids sequences revealed a high percentage of similarity between field isolates. Antibodies against peptides containing conserved B-cell epitopes of GP-45 were generated. Antibodies present in the sera of mice immunized with GP-45 peptides specifically recognize B. bigemina by the IFAT. More than 95% of cattle naturally infected with B. bigemina contained antibodies against conserved GP-45 peptides tested by ELISA. Finally, sera from rabbits immunized with GP-45 peptides were evaluated in vitro neutralization tests and it was shown that they reduced the percentage of parasitemia compared to sera from rabbits immunized with adjuvant. GP-45 from geographically distant isolates of B. bigemina contains conserved B-cell epitopes that induce neutralizing antibodies suggesting that this gene and its product play a critical role in the survival of the parasite under field conditions.

Major Surface Antigens in Zoonotic Babesia

Human babesiosis results from a combination of tick tropism for humans, susceptibility of a host to sustain Babesia development, and contact with infected ticks. Climate modifications and increasing diagnostics have led to an expanded number of Babesia species responsible for human babesiosis, although, to date, most cases have been attributed to B. microti and B. divergens. These two species have been extensively studied, and in this review, we mostly focus on the antigens involved in host–parasite interactions. We present features of the major antigens, so-called Bd37 in B. divergens and BmSA1/GPI12 in B. microti, and highlight the roles of these antigens in both host cell invasion and immune response. A comparison of these antigens with the major antigens found in some other Apicomplexa species emphasizes the importance of glycosylphosphatidylinositol-anchored proteins in host–parasite relationships. GPI-anchor cleavage, which is a property of such antigens, leads to soluble and membrane-bound forms of these proteins, with potentially differential recognition by the host immune system. This mechanism is discussed as the structural basis for the protein-embedded immune escape mechanism. In conclusion, the potential consequences of such a mechanism on the management of both human and animal babesiosis is examined.

Comparative evaluation of Babesia bigemina truncated C-terminal rhoptry associated protein-1 and 200 kDa merozoite protein in indirect enzyme-linked immunosorbent assay

Babesia bigemina is an intra-erythrocytic apicomplexan protozoon which causes an acute as well as chronic disease in cattle and is transmitted by ixodid ticks throughout the world. Due to low sensitivity of microscopy for detection of the parasite, there is a need for developing effective diagnostic tests that can be used to identify carrier animals in endemic areas. In the present study, C-terminal fragment of rhoptry associated protein-1 (RAP-1/CT) and 200 kDa merozoite protein (P200/CT) of B. bigemina were cloned into pET-32a(+) expression vector and expressed in Escherichia coli as thioredoxin-fusion proteins for use in an indirect ELISA. The rRAP-1/CT and rP200/CT showed no cross reactivity with plasma from cattle infected with other common parasites namely Theileria annulata, Trypanosoma evansi, Cryptosporidium parvum and Anaplasma marginale in the standardized ELISA. Examination of 116 blood samples collected from cattle suspected for haemoprotozoan infections revealed that 17 (14.6%), 46 (39.6%), 52 (44.8%) and 53 (45.7%) were positive for B. bigemina by microscopy, nested PCR, rRAP-1/CT based and rP200/CT based indirect ELISA, respectively. The diagnostic sensitivities of rRAP-1/CT and rP200/CT indirect ELISAs were 97.8% and 91.3%, while the diagnostic specificities were 90% and 84.3%, respectively, when nested PCR was taken as a reference test. An almost perfect agreement (Kappa value -0.859) between rRAP-1/CT ELISA and nested PCR results, and a substantial agreement (Kappa value -0.737) between rP200/CT ELISA and nested PCR were noticed. The findings of the present study suggest that rRAP-1/CT is a better diagnostic candidate antigen than rP200/CT for diagnosis of B. bigemina infection and it may be used in an ELISA for surveillance or diagnosis of B. bigemina infection in bovines.

Molecular tools-advances, opportunities and prospects for the control of parasites of veterinary importance

The recent advancement in genome sequencing facilities, proteomics, transcriptomics, and metabolomics of eukaryotes have opened door for employment of molecular diagnostic techniques for early detection of parasites and determining target molecules for formulating control strategies. It further leads to the introduction of several purified vaccines in the field of veterinary parasitology. Earlier, the conventional diagnostic methods was entirely based upon morphological taxonomy for diagnosis of parasites but nowadays improved molecular techniques help in phylogenetic study and open an another area of molecular taxonomy of parasites with high precision. Control measures based upon targeting endosymbionts in parasites like Dirofilaria immitis is also under exploration in veterinary parasitology. Metagenomics have added an inside story of parasites bionomics which have created havoc in human and animals population since centuries. Omics era is playing a key role in opening the new approaches on parasite biology. Various newer generations of safer vaccines like edible vaccines and subunit vaccines and diagnostic techniques based upon purified immunologically active epitopes have become commercially available against the parasites (helminths, protozoa and arthropod borne diseases). Nowadays, a transgenic and gene knock out studies using RNA interference and CRISPR are also helping in understanding the functions of genes and screening of target genes, which are not available before the advent of molecular tools. Molecular techniques had paramount impact on increasing the sensitivity of diagnostic tools, epidemiological studies and more importantly in controlling these diseases. This review is about the advancements in veterinary parasitology and their impact on the control of these pathogens.

Characterization of GASA-1, a new vaccine candidate antigen of Babesia bovis

Surface proteins bound to the cell membrane by glycosylphosphatidylinositol (GPI) anchors are considered essential for the survival of pathogenic protozoans. In the case of the tick-transmitted hemoparasite Babesia bovis, the most virulent causative agent of bovine babesiosis, the GPI-anchored proteome was recently unraveled by an in silico approach. In this work, one of the identified proteins, GASA-1 (GPI-Anchored Surface Antigen-1), was thoroughly characterized. GASA-1 is 179 aa long and has the characteristic features of a GPI-anchored protein, including a signal peptide, a hydrophilic core and a hydrophobic tail that harbors a GPI anchor signal. Transcriptomic analysis shows that it is expressed in pathogenic and attenuated B. bovis strains. Notably, the gasa-1 gene has syntenic counterparts in B. bigemina and B. ovata, which also encode GPI-anchored proteins. This is highly unusual since all piroplasmid GPI-anchored proteins described so far have been found to be species-specific. Sequencing of gasa-1 alleles from B. bovis geographical isolates originating from Argentina, USA, Brazil, Mexico and Australia showed over 98 % identity in both nucleotide and amino acid sequences. A recombinant form of GASA-1 (rGASA-1) was generated in E. coli and anti-rGASA-1 antibodies were raised in mice. Fixed and live immunofluorescence assays showed that GASA-1 is expressed in in vitro cultured B. bovis merozoites and surface-exposed. Moreover, incubation of B. bovis in vitro cultures with anti-GASA-1 antibodies partially, but significantly, reduced erythrocyte invasion, indicating that this protein bears neutralization-sensitive antibody epitopes. Splenocytes of rGASA-1-inoculated mice showed a specific proliferative response when exposed to the recombinant protein, indicating that GASA-1 bears T-cell epitopes. Finally, sera from a group of B. bovis-infected cattle reacted with the recombinant protein, demonstrating that GASA-1 is expressed during natural infection of bovines with B. bovis, and suggesting that it is immunodominant. The high degree of conservation among B. bovis isolates and the presence of syntenic genes in other Babesia species suggest a relevant role of GASA-1 and GASA-1-like proteins for parasite survival, especially considering that, due to their surface location, they are exposed to the selection pressure of the host immune system. The highlighted features of GASA-1 make it an interesting candidate for the development of vaccines against bovine babesiosis.

An Overview of Current Knowledge on in vitro Babesia Cultivation for Production of Live Attenuated Vaccines for Bovine Babesiosis in Mexico

The instrumentation of the in vitro culture system has allowed researchers to learn more about the metabolic and growth behavior of Babesia spp. The various applications for in vitro cultivation of Babesia include obtaining attenuated strains for vaccination or pre-munition, the selection of pure lines with different degrees of virulence, studies on biological cloning, ultrastructure, antigen production for diagnostics, drug sensitivity assessments, and different aspects of parasite biology. Although there are different types of vaccines that have been tested against bovine babesiosis, so far, the only procedure that has offered favorable results in terms of protection and safety has been the use of live attenuated vaccines. In countries, such as Australia, Argentina, Brazil, Uruguay and Israel, this type of vaccine has been produced and used. The alternative to live vaccines other than splenectomized calf-derived biological material, has been the in vitro cultivation of Babesia bovis and B. bigemina. The development of in vitro culture of Babesia spp. strains in a defined medium has been the basis for the initiation of a source of parasites and exoantigens for a variety of studies on the biochemistry and immunology of babesiosis. The use of live immunogens from attenuated strains derived from in vitro culture is highlighted, which has been proposed as an alternative to control bovine babesiosis. In several studies performed in Mexico, this type of immunogen applied to susceptible cattle has shown the induction of protection against the experimental heterologous strain challenge with both, Babesia-infected blood and animal exposure to confrontations on tick vector-infested farms. The combination of transfection technologies and the in vitro culture system as integrated methodologies would eventually give rise to the generation of genetically modified live vaccines. However, a greater challenge faced now by researchers is the large-scale cultivation of Babesia parasites for mass production and vaccine distribution.

Validation of an indirect ELISA using recombinant proteins as antigen to identify animals exposed to Babesia bigemina

The objective of this study was to instrument a serological assay for the epidemiological diagnosis of bovine babesiosis in Mexico, using the Babesia bigemina recombinant protein RAP-1 (rRAP-1α) as antigen. rRAP-1α, r12d3 and rGP45 were the three recombinant antigens initially tested. Based on the highest titres obtained in the indirect ELISA (iELISA) with the positive control serum, using similar antigen concentrations, rRAP-1α was selected for further use. The diagnostic sensitivity and specificity rates estimated for the iELISA with rRAP-1α as antigen were 89.9% and 86.5%, respectively, while for the Indirect Fluorescent Antibody Test (IFAT), the gold standard assay, the sensitivity was 86.66% and the specificity was 95%. The ĸ agreement value determined was 0.52, indicating a moderate agreement between the iELISA and IFAT assays. The instrumented iELISA with rRAP-1α as antigen shows an excellent specificity rate and an acceptable sensitivity that allows for the detection of antibodies to B. bigemina in cattle naturally exposed to the vector tick Rhipicephalus microplus. By using the iELISA-rRAP-1α, along with an iELISA with recombinant Merozoite Surface Antigen (rMSA-1) for antibody determination against Babesia bovis in the serum samples collected from cattle at 'La Posta' experimental station in Mexico, a seroprevalence of 20.3% was estimated for B. bigemina and 19.4% for B. bovis, while 36.89% of samples were positive for both Babesia species. The iELISA test promises to be a safe and low-cost type of diagnosis available to cattle producers in Mexico and would facilitate the definition of herd immunity status to implement measures of control adapted for the prevention of bovine babesiosis outbreaks.

Babesiosis Vaccines: Lessons Learned, Challenges Ahead, and Future Glimpses

The incidence and prevalence of babesiosis in animals and humans is increasing, yet prevention, control, or treatment measures remain limited and ineffective. Despite a growing body of new knowledge of the biology, pathogenicity, and virulence of Babesia parasites, there is still no well-defined, adequately effective and easily deployable vaccine. While numerous published studies suggest that the development of such anti-Babesia vaccines should be feasible, many others identify significant challenges that need to be overcome in order to succeed. Here, we review historic and recent attempts in babesiosis vaccine discovery to avoid past pitfalls, learn new lessons, and provide a roadmap to guide the development of next-generation babesiosis vaccines.

Strong conservation of rhoptry-associated-protein-1 (RAP-1) locus organization and sequence among Babesia isolates infecting sheep from China (Babesia motasi-like phylogenetic group)

Rhoptry-associated-protein 1 (RAP-1) is considered as a potential vaccine candidate due to its involvement in red blood cell invasion by parasites in the genus Babesia. We examined its value as a vaccine candidate by studying RAP-1 conservation in isolates of Babesia sp. BQ1 Ningxian, Babesia sp. Tianzhu and Babesia sp. Hebei, responsible for ovine babesiosis in different regions of China. The rap-1 locus in these isolates has very similar features to those described for Babesia sp. BQ1 Lintan, another Chinese isolate also in the B. motasi-like phylogenetic group, namely the presence of three types of rap-1 genes (rap-1a, rap-1b and rap-1c), multiple conserved rap-1b copies (5) interspaced with more or less variable rap-1a copies (6), and the 3' localization of one rap-1c. The isolates Babesia sp. Tianzhu, Babesia sp. BQ1 Lintan and Ningxian were almost identical (average nucleotide identity of 99.9%) over a putative locus of about 31 Kb, including the intergenic regions. Babesia sp. Hebei showed a similar locus organization but differed in the rap-1 locus sequence, for each gene and intergenic region, with an average nucleotide identity of 78%. Our results are in agreement with 18S rDNA phylogenetic studies performed on these isolates. However, in extremely closely related isolates the rap-1 locus seems more conserved (99.9%) than the 18S rDNA (98.7%), whereas in still closely related isolates the identities are much lower (78%) compared with the 18S rDNA (97.7%). The particularities of the rap-1 locus in terms of evolution, phylogeny, diagnosis and vaccine development are discussed.

Molecular cloning and characterization of Babesia orientalis rhoptry-associated protein 1

The rhoptry-associated protein 1 (RAP-1) gene of Babesia orientalis was obtained from a cDNA expression library by immunoscreening with B. orientalis-infected water buffalo sera. The nucleotide sequence of the cDNA was 1732 bp with an open reading frame (ORF) of 1434 bp, encoding a polypeptide of 478 amino acid residues with a predicted size of 52.5 kDa. The ORF was cloned into a pGEX-KG plasmid and subsequently expressed as a GST-fusion protein. The recombinant RAP-1 of B. orientalis (rBoRAP-1) was purified and evaluated as an antigen using Western blotting. The native BoRAP-1 was recognized by the antibodies raised in rabbits against rBoRAP-1. Strong immunofluorescence signals were observed in erythrocytes infected with B. orientalis. Phylogentic analysis revealed that B. orientalis fell into a Babesia clade and most closely related to Babesia bovis and Babesia ovis, which was similar to the previous reported trees based on 18S rRNA and HSP70 genes. The present study suggests that the BoRAP-1 might be a potential diagnostic antigen, and the RAP-1 genes can aid in the classification of Babesia and Theileria species.

Vaccines against bovine babesiosis: where we are now and possible roads ahead

SUMMARY Bovine babesiosis caused by the tick-transmitted haemoprotozoans Babesia bovis, Babesia bigemina and Babesia divergens commonly results in substantial cattle morbidity and mortality in vast world areas. Although existing live vaccines confer protection, they have considerable disadvantages. Therefore, particularly in countries where large numbers of cattle are at risk, important research is directed towards improved vaccination strategies. Here a comprehensive overview of currently used live vaccines and of the status quo of experimental vaccine trials is presented. In addition, pertinent research fields potentially contributing to the development of novel non-live and/or live vaccines are discussed, including parasite antigens involved in host cell invasion and in pathogen-tick interactions, as well as the protective immunity against infection. The mining of available parasite genomes is continuously enlarging the array of potential vaccine candidates and, additionally, the recent development of a transfection tool for Babesia can significantly contribute to vaccine design. However, the complication and high cost of vaccination trials hinder Babesia vaccine research, and have so far seriously limited the systematic examination of antigen candidates and prevented an in-depth testing of formulations using different immunomodulators and antigen delivery systems.

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.

Molecular characterizations of three distinctBabesia gibsonirhoptry-associated protein-1s (RAP-1s)

Three cDNAs encoding rhoptry-associated protein 1 (RAP-1) homologues were found in theBabesia gibsoniEST database. Based on similarities to BgRAP-1a, which was identified previously by serological screening of a cDNA merozoite library, the two new genes were designatedBgRAP-1b(33·7%) andBgRAP-1c(57%). Mice antiserum raised against each recombinant protein reacted specifically withB. gibsoniparasites as determined by Western blotting, which showed native molecular sizes of the BgRAP-1a (51 kDa), BgRAP-1b (53 kDa) and BgRAP-1c (47 kDa) consistent with predictable molecular weights. Immunofluoresence using these antibodies revealed localization of all BgRAP-1s within the matrix of merozoites; however, BgRAP-1a appeared to diverge from the other two when it was found secreted into the cytoplasm of infected erythrocytes. Apical localization of all 3 BgRAP-1s during the extracellular stage of the parasite combined with their ability to bind a canine erythrocyte membrane fraction was suggestive of a role for these proteins in erythrocyte attachment. Lastly, the ability of these recombinant proteins to be used as diagnostic reagents was tested by ELISA and the sensitivities of BgRAP-1a and BgRAP-1c were found increased through N-terminal truncation. Taken together, our data suggest divergent roles for the 3 BgRAP-1s in the merozoite stage ofB. gibsoni.

Validation of a competitive enzyme-linked immunosorbent assay for detection of Babesia bigemina antibodies in cattle

A competitive enzyme-linked immunosorbent assay (cELISA) based on a broadly conserved, species-specific, B-cell epitope within the C terminus of Babesia bigemina rhoptry-associated protein 1a was validated for international use. Receiver operating characteristic analysis revealed 16% inhibition as the threshold for a negative result, with an associated specificity of 98.3% and sensitivity of 94.7%. Increasing the threshold to 21% increased the specificity to 100% but modestly decreased the sensitivity to 87.2%. By using 21% inhibition, the positive predictive values ranged from 90.7% (10% prevalence) to 100% (95% prevalence) and the negative predictive values ranged from 97.0% (10% prevalence) to 48.2% (95% prevalence). The assay was able to detect serum antibody as early as 7 days after intravenous inoculation. The cELISA was distributed to five different laboratories along with a reference set of 100 defined bovine serum samples, including known positive, known negative, and field samples. The pairwise concordance among the five laboratories ranged from 100% to 97%, and all kappa values were above 0.8, indicating a high degree of reliability. Overall, the cELISA appears to have the attributes necessary for international application.

Babesia gibsoni rhoptry-associated protein 1 and its potential use as a diagnostic antigen

A cDNA encoding the rhoptry-associated protein 1 (RAP-1) homologue was obtained by immunoscreening an expression library prepared from Babesia gibsoni merozoite mRNA. The complete nucleotide sequence of the gene was 1740bp. Computer analysis suggested that the sequence contains an open reading frame of 1425bp encoding an expected protein with a molecular weight of 52kDa. Based on the sequence similarity, this putative protein was designated as the B. gibsoni RAP-1 (BgRAP-1). The BgRAP-1 gene was expressed in the Escherichia coli BL21 strain, and the recombinant BgRAP-1 was used as the antigen in the enzyme-linked immunosorbent assay (ELISA). The results can differentiate between the B. gibsoni-infected dog sera and the Babesia canis infected dog sera or the normal dog sera. Furthermore, the antibody response against the recombinant protein was maintained during the chronic stage of infection, indicating that the recombinant BgRAP-1 protein might be a useful diagnostic antigen for the detection of antibodies to B. gibsoni infection in dogs.

Dynamics of anemia progression and recovery in Babesia bigemina infection is unrelated to initiating parasite burden

To be informative, immunization-and-challenge experiments in support of vaccine development rely on host responses that enable distinctions to be made in the responses of immunized and non-immunized animals to infectious challenge. It is therefore important that animals be challenged with standardized infectious doses that allow such distinctions to be made. We report here the results of a challenge titration experiment in which cattle were challenged with Babesia bigemina, at dosages ranging over six orders of magnitude. No significant dose-dependent differences were observed in the maximum fever attained, duration of fever, minimum hematocrit reached, kinetics with which anemia developed or was resolved, or animal weight gain. Significant differences were noted only in the length of time post-infection required to initiate fever, reach maximum fever, and attain maximum reduction in hematocrit. These results suggest that, in the absence of further supporting evidence, it is not possible to conclude any direct anti-parasite effects from reductions in maximum fever or hematocrit drop during B. bigemina immunization-and-challenge experiments. However, lengthening of the time to reduction in hematocrit may be a useful indicator of overt suppression of the challenge inoculum.

Babesiosis of cattle

Tick fever or cattle fever (babesiosis) is economically the most important arthropod-borne disease of cattle worldwide with vast areas of Australia, Africa, South and Central America and the United States continuously under threat. Tick fever was the first disease for which transmission by an arthropod to a mammal was implicated at the turn of the twentieth century and is the first disease to be eradicated from a continent (North America). This review describes the biology of Babesia spp. in the host and the tick, the scale of the problem to the cattle industry, the various components of control programmes, epidemiology, pathogenesis, immunity, vaccination and future research. The emphasis is on Babesia bovis and Babesia bigemina.

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.

Erythrocyte invasion by Babesia parasites: Current advances in the elucidation of the molecular interactions between the protozoan ligands and host receptors in the invasion stage

During an asexual growth cycle of Babesia parasites in a natural host, the extracellular merozoites invade (i.e., attach to, penetrate, and internalize) the host erythrocytes (RBC) via multiple adhesive interactions of several protozoan ligands with the target receptors on the host cell surface. After internalizing the host RBC, they asexually multiply, egress from the RBC by rupturing the host cells, and then invade the new RBC again. In the invasion stage, several surface-coating molecules of merozoites might be involved in the initial attachment to the RBC, while proteins secreted from apical organelles (rhoptry, microneme, and spherical body) are proposed to play roles mainly in erythrocyte penetration or internalization. On the other hand, several components located on the surface of the RBC, such as sialic acid residues, protease-sensitive proteins, or sulphated glycosaminoglycans, are identified or suspected as the host receptors of erythrocyte invasion by Babesia parasites. The detailed molecular interactions between Babesia merozoites and the host RBC are incompletely understood. In this review, these identified or suspected molecules (protozoan ligands/erythrocyte receptors) are described by especially focusing on Babesia bovis.

Expression of C-terminal truncated and full-length Babesia bigemina rhoptry-associated protein 1 and their potential use in enzyme-linked immunosorbent assay

Recombinant antigen-based enzyme-linked immunosorbent assay (ELISA) was developed for the serological diagnosis of Babesia bigemina infection by using a full-length B. bigemina rhoptry-associated protein 1 (rRAP-1) and the truncated C-terminal RAP-1 (rRAP-1/CT). While the rRAP-1 showed cross reactivity between B. bigemina- and Babesia bovis-infected bovine sera, the rRAP-1/CT was highly specific to B. bigemina-infected bovine sera and proved useful in the detection of sequential sera collected from an experimentally infected cow during the acute and latent infection. The high yield of soluble rRAP-1/CT and its diagnostic specificity demonstrate its potential in the diagnosis of B. bigemina infection. Its usefulness for epidemiological investigation is currently being evaluated.

Genetic basis for GPI-anchor merozoite surface antigen polymorphism of Babesia and resulting antigenic diversity

Glycosyl-phosphatidylinositol anchor merozoite surface antigens (GPI-anchor MSA) are proposed to act in the invasion process of infective merozoites of Babesia into host erythrocytes. Because of their essential function in the survival of Babesia parasites, they constitute good candidates for the development of vaccines against babesiosis and they have been extensively analyzed. These include Babesia bovis variable MSA (VMSA) and Babesia bigemina gp45/gp55 proteins of the agents of bovine babesiosis from tropical and subtropical countries, and the Babesia divergens Bd37 and Babesia canis Bc28 proteins of the main agents of bovine and canine babesiosis in Europe, respectively. However, these are very polymorphic antigens and Babesia parasites have evolved molecular mechanisms that enable these antigens to evade the host immune system as a survival strategy. This review focuses on the genetic basis of GPI-anchor MSA polymorphism and the antigenic diversity of B-cell epitopes that might be generated in each of these Babesia species. The picture is incomplete and no Babesia genome sequence is yet available. However, the available sequences suggest that two distinct, non cross-reactive GPI-anchor MSA (i.e., with unique B-cell epitopes) may be required by all Babesia species for invasion, and that these two distinct GPI-anchor MSA would be encoded by a multigene family. Furthermore, the data are consistent with the ability of biological clones from Babesia to use these multigene families for the expression of GPI-anchor MSA, either conserved (B. canis and B. bovis) or polymorphic (B. divergens and B. bigemina) in their amino acid sequence. Moreover, as a consequence for successful parasitism, the data suggest that both conserved and polymorphic GPI-anchor MSA would present unique B-cell epitopes.

Stimulation of T-helper cell gamma interferon and immunoglobulin G responses specific for Babesia bovis rhoptry-associated protein 1 (RAP-1) or a RAP-1 protein lacking the carboxy-terminal repeat region is insufficient to provide protective immunity against virulent B. bovis challenge

Rhoptry-associated protein 1 (RAP-1) is a targeted vaccine antigen for Babesia bovis and Babesia bigemina infections of cattle. The 60-kDa B. bovis RAP-1 is recognized by antibodies and T lymphocytes from cattle that recovered from infection and were immune to subsequent challenge. Immunization with native or recombinant protein was reported to reduce parasitemias in challenged animals. We recently reported that the NT domain of B. bovis RAP-1 contained immunodominant T-cell epitopes, whereas the repeat-rich CT domain was less immunostimulatory for T lymphocytes from cattle immune to B. bovis. The present study was therefore designed to test the hypothesis that the NT region of RAP-1, used as a vaccine with interleukin-12 and RIBI (catalog no. R-730; RIBI Immunochem Research, Inc., Hamilton, Mont. [now Corixa, Seattle, Wash.]) adjuvant to induce a type 1 response, would prime calves for antibody and T-helper cell responses comparable to or greater than those induced by full-length RAP-1 containing the C-terminal repeats. Furthermore, a type 1 immune response to RAP-1 was hypothesized to induce protection against challenge. Following four inoculations of either recombinant full-length RAP-1 or RAP-1 NT protein, RAP-1-specific immunoglobulin G (IgG) titers, T-lymphocyte proliferation, and gamma interferon production were similar. Similar numbers of NT region peptides were recognized. However, in spite of the presence of strong RAP-1-specific IgG and CD4(+)-T-lymphocyte responses that were recalled upon challenge, neither antigen stimulated a protective immune response. We conclude that successful priming of calves with recombinant RAP-1 and adjuvants that elicit strong Th1 cell and IgG responses is insufficient to protect calves against virulent B. bovis challenge.

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 2 proteins are expressed on the merozoite and sporozoite surface, and specific antibodies inhibit attachment and invasion of erythrocytes

The Babesia bovis merozoite surface antigen 2 (MSA-2) locus encodes four proteins, MSA-2a(1), -2a(2), -2b, and -2c. With the use of specific antibodies, each MSA-2 protein was shown to be expressed on the surface of live extracellular merozoites and coexpression on single merozoites was confirmed. Individual antisera against MSA-2a, MSA-2b, and MSA-2c significantly inhibited merozoite invasion of bovine erythrocytes. As tick-derived sporozoites also directly invade erythrocytes, expression of each MSA-2 protein on the sporozoite surface was examined and verified. Finally, statistically significant inhibition of sporozoite binding to the erythrocytes was demonstrated by using antisera specific for MSA-2a, MSA-2b, and MSA-2c. These results indicate an important role for MSA-2 proteins in the initial binding and invasion of host erythrocytes and support the hypothesis that sporozoites and merozoites use common surface molecules in erythrocyte invasion.

Genetic and antigenic analysis of Babesia bigemina isolates from five geographical regions of Brazil

A molecular epidemiological study was performed with Babesia bigemina isolates from five geographical regions of Brazil. The genetic analysis was done with random amplification of polymorphic DNA (RAPD), repetitive extragenic palindromic elements-polymerase chain reaction (REP-PCR) and enterobacterial repetitive intergenic consensus sequences-polymerase chain reaction (ERIC-PCR) that showed genetic polymorphism between these isolates and generated fingerprinting. In RAPD, ILO872 and ILO876 primers were able to detect at least one fingerprinting for each B. bigemina isolate. The amplification of B. bigemina DNA fragments by REP-PCR and ERIC-PCR gave evidence for the presence in this haemoprotozoan of the sequences described previously in microorganisms of the bacterial kingdom. For the first time it was demonstrated that both techniques can be used for genetic analysis of a protozoan parasite, although the ERIC-PCR was more discriminatory than REP-PCR. The dendogram with similarity coefficient among isolates showed two clusters and one subcluster. The Northeastern and Mid-Western isolates showed the greatest genetic diversity, while the Southeastern and Southern isolates were the closest. The antigenic analysis was done through indirect fluorescent antibody technique and Western blotting using a panel of monoclonal antibodies directed against epitopes on the merozoite membrane surface, rhoptries and membrane of infected erythrocytes. As expected, the merozoite variable surface antigens, major surface antigen (MSA)-1 and MSA-2 showed antigenic diversity. However, B cell epitopes on rhoptries and infected erythrocytes were conserved among all isolates studied. In this study it was possible to identify variable and conserved antigens, which had already been described as potential immunogens. Considering that an attenuated Babesia clone used as immunogen selected populations capable of evading the immunity induced by this vaccine, it is necessary to evaluate more deeply the cross-protection conferred by genetically more distant Brazilian B. bigemina isolates and make an evaluation of the polymorphism degree of variable antigens such as MSA-1 and MSA-2.

Immunodominant epitopes in Babesia bovis rhoptry-associated protein 1 that elicit memory CD4(+)-T-lymphocyte responses in B. bovis-immune individuals are located in the amino-terminal domain

Babesia bovis rhoptry-associated protein 1 (RAP-1), which confers partial protection against B. bovis challenge, is recognized by antibodies and T lymphocytes from cattle that have recovered from infection and are immune to subsequent challenge. RAP-1 is a 60-kDa protein with an N-terminal (NT) region that contains four cysteine residues conserved among all Babesia RAP-1 family members and a C-terminal (CT) region that contains multiple, degenerate, tandem 23-amino-acid (aa) repeats. To define the location of CD4(+)-T-cell epitopes for vaccine development using a recombinant protein or minigene construct, a series of truncated recombinant RAP-1 proteins and peptides were tested for stimulation of T-cell lines derived from B. bovis-immune cattle. CD4(+)-T-cell lines from three B. bovis-immune cattle with different DRB3 haplotypes responded to the NT region of RAP-1, whereas T cells from only one animal responded weakly to the CT region. T-cell lines from the three individuals recognized two to six NT-region peptides spanning aa 134 to 316 and representing at least four dominant epitopes. Using RAP-1-specific CD4(+)-T-cell clones, two NT-region epitopes, EYLVNKVLYMATMNYKT (aa 187 to 203) and EAPWYKRWIKKFR (aa 295 to 307), and one CT-region repeat epitope, FREAPQATKHFL, which is present twice at aa positions 391 to 402 and 414 to 425, were identified. Several peptides representing degenerate repeats of the agonist CT-region peptide FREAPQATKHFL neither stimulated responses of T-cell clones specific for this peptide nor inhibited responses to the agonist peptide. Upon stimulation with specific antigen, T-cell clones specific for NT or CT epitopes produced gamma interferon. The presence of T-helper-cell epitopes in the NT domain of RAP-1, which is highly conserved among otherwise antigenically different strains of B. bovis, supports the inclusion of this region in vaccine constructs to be tested in cattle.

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.

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.

The role of molecular biology in veterinary parasitology

The tools of molecular biology are increasingly relevant to veterinary parasitology. The sequencing of the complete genomes of Caenorhabditis elegans and other helminths and protozoa is allowing great advances in studying the biology, and improving diagnosis and control of parasites. Unique DNA sequences provide very high levels of specificity for the diagnosis and identification of parasite species and strains, and PCR allows extremely high levels of sensitivity. New techniques, such as the use of uniquely designed molecular beacons and DNA microarrays will eventually allow rapid screening for specific parasite genotypes and assist in diagnostic and epidemiological studies of veterinary parasites. The ability to use genome data to clone and sequence genes which when expressed will provide antigens for vaccine screening and receptors and enzymes for mechanism-based chemotherapy screening will increase our options for parasite control. In addition, DNA vaccines can have desirable characteristics, such as sustained stimulation of the host immune system compared with protein based vaccines. One of the greatest threats to parasite control has been the development of drug resistance in parasites. Our knowledge of the basis of drug resistance and our ability to monitor its development with highly sensitive and specific DNA-based assays for 'resistance'-alleles will help maintain the effectiveness of existing antiparasitic drugs and provide hope that we can maintain control of parasitic disease outbreaks.

Molecular basis for variable expression of merozoite surface antigen gp45 among American isolates of Babesia bigemina

Immunization with the merozoite surface glycoprotein gp45 induces protection against challenge using the homologous Babesia bigemina strain. However, gp45 B-cell epitopes are highly polymorphic among B. bigemina strains isolated from different geographical locations within North and South America. The molecular basis for this polymorphism was investigated using the JG-29 biological clone of a Mexico strain of B. bigemina and comparison with the Puerto Rico, St. Croix, and Texcoco strains. The molecular size and antibody reactivity of gp45 expressed by the JG-29 clone were identical to those of the parental Mexico strain. gp45 cDNA and the genomic locus encompassing gp45 were cloned and sequenced from JG-29. The locus sequence and Southern blot data were consistent with a single gp45 copy in the JG-29 genome. The JG-29 cDNA expressed the full-length protein recognized by the gp45-specific monoclonal antibody 14/1.3.2. The genomes of the Puerto Rico and St. Croix strains of B. bigemina were shown to lack a closely related gp45-like gene by PCR using multiple primer sets and by Southern blots using both full-length and region-specific gp45 probes. This genomic difference was confirmed using unpassaged isolates from a 1999 disease outbreak in Puerto Rico. In contrast, the Texcoco strain retains a gp45 gene, encoding an open reading frame identical to that of JG-29. However, the Texcoco gp45 gene is not transcribed. These two mechanisms, lack of a closely related gp45-like gene and failure to transcribe gp45, result in generation of antigenic polymorphism among B. bigemina strains, and the latter mechanism is unique compared to prior mechanisms of antigenic polymorphism identified in babesial parasites.

Monoclonal antibody binding to a surface-exposed epitope on Cowdria ruminantium that is conserved among eight strains

Monoclonal antibodies (MAb) binding to Cowdria ruminantium elementary bodies (EB) were identified by enzyme-linked immunosorbent assay, and surface binding of one MAb (446.15) to intact EB was determined by immunofluorescence, immunogold labeling, and transmission electron microscopy. MAb 446.15 bound an antigen of approximately 43 kDa in immunoblots of eight geographically distinct strains. The MAb did not react with Ehrlichia canis antigens or uninfected bovine endothelial cell lysate and may be useful in diagnostic assays and vaccine development.

Babesiosis

Babesiosis is an emerging, tick-transmitted, zoonotic disease caused by hematotropic parasites of the genus Babesia. Babesial parasites (and those of the closely related genus Theileria) are some of the most ubiquitous and widespread blood parasites in the world, second only to the trypanosomes, and consequently have considerable worldwide economic, medical, and veterinary impact. The parasites are intraerythrocytic and are commonly called piroplasms due to the pear-shaped forms found within infected red blood cells. The piroplasms are transmitted by ixodid ticks and are capable of infecting a wide variety of vertebrate hosts which are competent in maintaining the transmission cycle. Studies involving animal hosts other than humans have contributed significantly to our understanding of the disease process, including possible pathogenic mechanisms of the parasite and immunological responses of the host. To date, there are several species of Babesia that can infect humans, Babesia microti being the most prevalent. Infections with Babesia species generally follow regional distributions; cases in the United States are caused primarily by B. microti, whereas cases in Europe are usually caused by Babesia divergens. The spectrum of disease manifestation is broad, ranging from a silent infection to a fulminant, malaria-like disease, resulting in severe hemolysis and occasionally in death. Recent advances have resulted in the development of several diagnostic tests which have increased the level of sensitivity in detection, thereby facilitating diagnosis, expediting appropriate patient management, and resulting in a more accurate epidemiological description.

Babesiosis

Babesiosis is an emerging, tick-transmitted, zoonotic disease caused by hematotropic parasites of the genus Babesia. Babesial parasites (and those of the closely related genus Theileria) are some of the most ubiquitous and widespread blood parasites in the world, second only to the trypanosomes, and consequently have considerable worldwide economic, medical, and veterinary impact. The parasites are intraerythrocytic and are commonly called piroplasms due to the pear-shaped forms found within infected red blood cells. The piroplasms are transmitted by ixodid ticks and are capable of infecting a wide variety of vertebrate hosts which are competent in maintaining the transmission cycle. Studies involving animal hosts other than humans have contributed significantly to our understanding of the disease process, including possible pathogenic mechanisms of the parasite and immunological responses of the host. To date, there are several species of Babesia that can infect humans, Babesia microti being the most prevalent. Infections with Babesia species generally follow regional distributions; cases in the United States are caused primarily by B. microti, whereas cases in Europe are usually caused by Babesia divergens. The spectrum of disease manifestation is broad, ranging from a silent infection to a fulminant, malaria-like disease, resulting in severe hemolysis and occasionally in death. Recent advances have resulted in the development of several diagnostic tests which have increased the level of sensitivity in detection, thereby facilitating diagnosis, expediting appropriate patient management, and resulting in a more accurate epidemiological description.

Phosphatidylcholine formation is the predominant lipid biosynthetic event in the hemoparasite Babesia bovis

This work examines the lipid composition and metabolism of bovine red blood cells infected by apicomplexan Babesia parasites, organisms closely related to Plasmodium sp. We found that erythrocytes infected with Babesia bovis (i-RBC) accumulate lipids and show striking increases in phosphatidylcholine, phosphatidic acid, diacylglycerol and cholesteryl esters as compared to uninfected erythrocytes cultured under the same conditions (n-RBC). A similar pattern was observed in cultures of erythrocytes infected with Babesia bigemina. The lipid profile of purified B. bovis merozoites showed that phosphatidylcholine is the most abundant phospholipid in this parasite (31.8% +/- 2.8 of total phospholipid), markedly differing from bovine n-RBC, in which it is only a minor component (4.8% +/- 0.6). B. bovis cultures incorporate radiolabeled choline into complex lipids, especially phosphatidylcholine, with minor amounts recovered in sphingomyelin and lysophosphatidylcholine. When [14C] stearate was used as precursor, the labeling pattern again gave the highest incorporation into phosphatidylcholine, with lesser incorporation in sphingomyelin, phosphatidylinositol, phosphatidylethanolamine and phosphatidic acid. Diacylglycerol and small amounts of cholesteryl esters were the only labeled neutral lipids found. B. bovis also incorporates [3H] myo-inositol into phosphatidylinositol. Parallel incubations with n-RBC as a control yielded no incorporation into either polar or neutral lipids with any precursor. These results indicate that the lipid changes observed in i-RBC can be explained on the basis of the lipid biosynthetic activities of the babesial parasite. Gas chromatography-mass spectrometry (GC-MS) analysis of fatty acid methyl esters from phospholipids of i-RBC and n-RBC showed the same qualitative composition in both. However, i-RBC had higher ratios of saturated to unsaturated fatty acids and B. bovis cultures did not desaturate [14C] stearate. Cholesterol was the only sterol detected by GC-MS. Phospholipase A2 treatment of i-RBC and n-RBC revealed no enhanced hemolytic effects in i-RBC, suggesting that the erythrocyte membrane phospholipid composition is essentially unaltered by the parasite. Labeling of i-RBC or n-RBC with [125I] Bolton-Hunter resulted in an enhanced phosphatidylserine labeling in i-RBC. This study provides the first data on B. bovis lipid constitution and biosynthesis. They show that phosphatidylcholine formation is the main biosynthetic process in these cells. The striking differences in the contents of phosphatidylcholine between host erythrocytes and the parasite suggests that it may be a useful target for both chemotherapy and immunoprophylaxis against bovine babesiosis.

A common high molecular weight antigen of Babesia bovis isolates from Mexico

Cattle from an area of Mexico endemic with Babesia bovis infections have a dominant antibody response to a 152kDa antigen of the Tamaulipas strain of B. bovis. A mAb termed PB/5, showing a specific reactivity to this 152kDa antigen in Western blots, was identified. The mAb which reacted with the blunt end of B. bovis in an indirect fluorescent antibody test also reacted to a 152kDa antigen in two other isolates (Nuevo Leon and Yucatan), and a 175kDa antigen in the Huasteca B. bovis isolate from Mexico. Polyclonal monospecific sera from a calf inoculated with mAb-affinity purified 152kDa antigen (Tamaulipas strain) identified B. bovis by the indirect fluorescent antibody test and two antigens of B. bovis (65kDa and 152kDa) in Western blot. Since the epitope reacting to the mAb PB/5 is conserved, this antigen provides a basis for developing a diagnostic test or an immunogen.

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.

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 CD4(+) T-lymphocyte clones specific for rhoptry-associated protein 1 of Babesia bigemina stimulate enhanced immunoglobulin G1 (IgG1) and IgG2 synthesis

Optimal protective immunity against babesial infection is postulated to require both complement-fixing and opsonizing antibodies in addition to gamma interferon (IFN-gamma)-mediated macrophage activation. The rhoptry-associated protein 1 (RAP-1) of Babesia bigemina induces partial protective immunity and is a candidate vaccine antigen. Previous studies demonstrated that cattle immunized with native protein that were subsequently protected against challenge had a strong IFN-gamma and weaker interleukin-4 (IL-4) response in immune lymph node lymphocytes that reflected the cytokine profile of the majority of CD4(+) T-cell clones obtained from peripheral blood. RAP-1-specific T helper (Th) cell clones that coexpress IFN-gamma and IL-4 are typical of numerous parasite-specific clones examined. However, the function of such cells as helper cells to enhance immunoglobulin secretion by bovine B cells has not been reported. In cattle, both immunoglobulin G1 (IgG1) and IgG2 can fix complement, but IgG2 is the superior opsonizing subclass. Therefore, studies were undertaken to ascertain the functional relevance of RAP-1-specific, CD4(+) Th0 cells as helper cells to enhance IgG1 and/or IgG2 production by autologous B lymphocytes. For comparison, Th0 clones specific for the metazoan parasite Fasciola hepatica that expressed relatively more IL-4 than the B. bigemina-specific Th cells were similarly assayed. B. bigemina RAP-1-specific clones could enhance production of both IgG1 and IgG2 by autologous B cells, whereas Th cell clones specific for F. hepatica enhanced predominantly IgG1 production. The capacity to enhance IgG2 production was associated with production of IFN-gamma by Th cells cocultured with B cells, antigen, and IL-2. The in vitro helper T-cell activity of these T-cell clones was representative of the in vivo serologic responses, which were composed of a mixed IgG1-IgG2 response in B. bigemina RAP-1 immune cattle and a biased IgG1 response in F. hepatica-immune cattle.

Sequence and functional analysis of the intergenic regions separating babesial rhoptry-associated protein-1 (rap-1) genes

The rhoptry-associated protein 1 (RAP-1) expressed by all babesial parasites is encoded by tandemly arranged genes separated by discrete intergenic (IG) regions. We hypothesize that these IG regions regulate rap-1 gene expression. In Babesia bovis two identical rap-1 gene copies are separated by a 1.0-kb noncoding region which is also exactly conserved 5' to the rap-1 gene 1. In contrast, the complex B. bigemina rap-1 locus contains at least 5 polymorphic rap-1a genes separated by uncharacterized 3.38-kb regions. A genomic clone encoding the 3' sequence of rap-1 gene copy 1, the 1 kb IG region, and the 5' sequence of gene copy 2 was obtained by PCR amplification of DNA from the Mo7 biological clone of B. bovis and sequenced. This was follow by amplification and sequence analysis of the 3.38-kb region separating two B. bigemina rap-1a genes, revealing the presence of two different IG regions denominated IG-1 (0.7 kb) and IG-2 (1.3 kb), flanking a newly identified rap-1b orf. Sequence analysis and comparison among babesial rap-1 IG regions from B. bovis, B. bigemina, B. canis, and B. ovis revealed conservation of at least three putative regulatory boxes consistently positioned 5' of the start of the rap-1 orfs. To determine whether rap-1 IG regions contained a functional promoter, the entire 1-kb IG region from B. bovis was cloned into pCAT, a promoterless plasmid containing the cat gene. The IG region in the 5' --> 3' orientation strongly promoted transcription in vitro by homologous B. bovis RNA polymerases. The presence of conserved regions 5' to each rap-1 gene copy and among other babesial rap-1 IG regions and the in vitro promoter function in the 5' --> 3' orientation support a role for the IG region in rap-1 gene regulation.

In vivo binding of immunoglobulin M to the surfaces of Babesia bigemina-infected erythrocytes

Babesia bigemina infection of mature bovine erythrocytes results in new proteins specifically exposed on the parasitized cell surface. Monoclonal antibody (MAb) 64/32 binds a protein, designated p94, on B. bigemina-infected erythrocytes but not on either uninfected or B. bovis-parasitized erythrocytes. However, p94 was not encoded by B. bigemina and was not a parasite-modified erythrocyte membrane protein. In contrast, we showed that p94 could be eluted from the infected erythrocyte surface and was identified as specifically bound immunoglobulin M (IgM) heavy chain for the following reasons: (i) MAb 64/32 bound a reduced molecule of 94 kDa in both infected erythrocyte lysates and normal bovine serum; (ii) MAb 64/32 bound a 94-kDa molecule in reduced preparations of purified IgM; (iii) an anti-bovine mu heavy-chain MAb, BIg73, reacted specifically with the surface of infected erythrocytes and bound the 94-kDa molecule in lysates of infected erythrocytes, normal bovine serum, and purified IgM; and (iv) immunoprecipitation of infected erythrocyte lysates with MAb 64/32 depleted the 94-kDa antigen bound by anti-mu MAb BIg73 and vice versa. Binding of IgM to the infected erythrocyte surface was detected in vivo early in acute parasitemia and occurred during both the trophozoite and merozoite stages of intraerythrocytic parasitism. The common feature of IgM binding to the parasitized erythrocyte surface among otherwise genetically and antigenically distinct B. bigemina strains is suggestive of an advantageous role in parasite survival in vivo.

Structure, sequence, and transcriptional analysis of the Babesia bovis rap-1 multigene locus

The complexity of multigene families encoding rhoptry proteins and the generation of new variants in these families are constraints to development of vaccines incorporating rhoptry proteins. For example, the Babesia bigemina rhoptry associated protein (rap)-1 locus is composed of tandemly arranged genes including four polymorphic rap-1a genes and two classes of divergent genes, rap-1b and rap-1c. B. bigemina rap-1 polymorphism reflects recombination and gene conversion and results in multiple RAP-1 proteins with unique B- and T-cell epitopes. Is this complex locus structure and recombination a required feature of the rap-1 gene family among Babesia species? We addressed this question by analysis of the rap-1 locus in B. bovis. Sequence analysis of an 11 kb genomic clone representing the B. burn rap-1 locus revealed only two identical and continuous rap-1a gene copies, rap 1a-1 and rap-1a-2, located in a similar head to tail orientation. Using the conserved ig gene as a marker for the 3' boundary of the rap-1 locus, we conclude that divergent rap-1b and rap-1c genes, present in B. bigemina, are not similarly cis-linked to the B. bovis rap-1 locus. Analysis of the rap-1a genes 1 and 2 from each of multiple B. bovis strains from North and South America demonstrated RAP-1 size conservation with very limited amino acid sequence variation. The results suggest that the simple two gene arrangement in the B. bovis rap-1 gene family was generated by gene duplication and, in contrast to the B. bigemina rap-1 locus, both genes evolved together using homogenization mechanisms with point mutation as the single mechanism for gene variation. Three discontinuous non-rap-1 genes are closely cis-linked to the B. bovis rap-1 locus and the presence of multiple introns in these genes may limit rap-1 gene variation due to unequal crossing over. The different mechanisms likely involved in the evolution of the rap-1 family in B. bigemina versus B. bovis are reflected in the marked structural and antigenic polymorphism in the B. bigemina RAP-1 molecules as compared with the essentially monomorphic RAP-1 in B. bovis.

Helper T-cell epitopes encoded by the Babesia bigemina rap-1 gene family in the constant and variant domains are conserved among parasite strains

Among important candidates for babesial vaccines are apical complex proteins, including rhoptry-associated protein 1 (RAP-1) from Babesia bovis and B. bigemina, which have been shown to induce partial immunity. Four variant B. bigemina rap-1 transcripts identified in a clone of the Mexico strain have highly conserved sequence in the central region but vary in sequence at the amino and carboxy termini (NT and CT) of the predicted proteins, resulting in different combinations of NT and CT domains in the individual gene products. Cattle were immunized with native protein consisting of the RAP-alpha1 variant, which contains NT-1 and CT-1 domains, and T-cell responses were characterized. We previously reported the identification of two T helper (Th) cell epitopes in B. bigemina RAP-1alpha1 protein (I. Hötzel, W. C. Brown, T. F. McElwain, S. D. Rodriguez, and G. H. Palmer, Mol. Biochem. Parasitol. 81:89-99, 1996). One epitope mapped to the constant domain of RAP-1 (amino acids [aa] 144 to 187), and one mapped to the CT-1 variable domain (aa 386 to 480). Th1-like clones responding to these epitopes proliferated differentially to different strains of B. bigemina, raising the possibilities that the T-cell epitopes may vary antigenically and that CT-1 may be differentially expressed with respect to the other RAP-1 CT domains in the different strains. In this report, we definitively map the T-cell epitope identified in the constant domain of RAP-1 to aa 159 to 187 (FVVSLLKKNVVRDPESNDVENFASQYFYM) and show that the predicted amino acid sequence is completely conserved among seven strains. The T-cell epitope in the CT-1 domain was mapped to aa 436 to 465 (VNSEKVDADDAGNAETQQLPDAENEVRADD), which is also completely conserved among eight strains of B. bigemina. We further show that the RAP-1alpha1-immunized cattle were protected against homologous B. bigemina challenge, thus suggesting an association between protective immunity and the helper T-cell response against the two epitopes. The immunogenic and highly conserved nature of these T-cell epitopes and their ability to stimulate functionally relevant Th cells that express gamma interferon support their inclusion in a vaccine.

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.