Soluble parasite antigens (SPA) in supernatants of in vitro cultures of Babesia canis can be used to vaccinate dogs against virulent B. canis infection. The moment that immunity becomes apparent coincides with the appearance of antibodies against SPA in the serum of the vaccinated animals. This so-called vaccination-challenge serum (VC-serum) was used to precipitate antigens from B. cants culture supernatants in agarose gels. This antigen preparation was then used to analyse the reactivity of sera from vaccinated dogs on western blots. Results: showed that the first appearance of antibody reactivity against a protein that migrated at the 39 kDa position in SDS-PAGE gels was associated with the moment vaccinated dogs started to recover from a virulent challenge infection. In addition, pulse-chase experiments revealed that a 39-40 kDa doublet was released into the supernatant of B. canis cultures starting 15 min after the chase. This doublet was specifically precipitated by VC-serum, thus corroborating that the 39-40 kDa doublet in SPA preparations was of parasite origin. Partial amino acid sequencing allowed the discovery of the gene that encoded the 39-40 kDa doublet (canine Babesia antigen; CBA). The full-length gene was cloned and expressed in E. coli. The recombinant CBA protein (rCBA) was recognized by VC-serum, and antibodies against rCBA precipitated the 39 kDa antigen of SPA preparations and of merozoites of B. canis. In addition, anti-rCBA serum reacted with the surface of B. canis merozoites (but not with B. rossi merozoites) in immunofluorescence. Vaccination of dogs with rCBA induced antibodies against rCBA, which recognized B. canis merozoites. Vaccinated dogs were protected against virulent challenge infection by limiting parasite proliferation. As a result, the development of clinical signs was prevented and the animals self-cured. In contrast, six out of seven non-vaccinated control dogs developed relatively high parasitaemia and serious clinical signs associated with poor tissue perfusion. This antigen can be used to replace the SPA antigen in the conventional B. canis vaccines, which eliminates the need for dog blood and serum for vaccine production.

Babesia divergens is the Apicomplexa agent of the bovine babesiosis in Europe: this infection leads to growth and lactation decrease, so that economical losses due to this parasite are sufficient to require the development of a vaccine. The major surface antigen of B. divergens has been described as a 37 kDa protein glycosyl phosphatidyl inositol (GPI)anchored at the surface of the merozoite. The immuno-prophylactic potential of Bd37 has been demonstrated, and we present here the high-resolution solution structure of the 27 kDa structured core of Bd37 (Delta-Bd37) using NMR spectroscopy. A model for the whole protein has been obtained using additional small angle X-ray scattering (SAXS) data. The knowledge of the 3D structure of Bd37 allowed the precise epitope mapping of antibodies on its surface. Interestingly, the geometry of Delta-Bd37 reveals an intriguing similarity with the exocyst subunit Exo84p C-terminal region, an eukaryotic protein that has a direct implication in vesicle trafficking. This strongly suggests that Apicomplexa have developed in parallel molecular machines similar in structure and function to the ones used for endo- and exocytosis in eukaryotic cells. (C) 2007 Elsevier Ltd. All rights reserved.

Bovine babesiosis, caused by Babesia bovis, B. bigemina, and B. divergens, is a significant impediment to livestock production in countries with tropical/subtropical and temperate climates. Previous studies conducted on the immunoprophylaxis against the disease and diagnosis of these parasites has demonstrated the presence of similar antigens. The objective of this article was to identify and partially characterize antigens conserved among these three species. Immunochemical analysis using sera from cattle immunized individually with antigens from these three Babesia species revealed a number of antigens recognized by heterologous antisera. Cross-reactions were more evident in sera from cattle immunized with B. bovis/B. bigemina which recognized several antigens (15 kDa to >200 kDa) in B. divergens. Immunoscreening of a B. divergens cDNA library with bovine serum to B. bigemina allowed the isolation of five clones and DNA sequencing of plasmid BdJF5 showed a 680 bp cDNA insert. Basic Local Alignment Search Tool (BLAST) analysis of the predicted amino acid sequence revealed 47% identity with a protein identified as alphaNAC. Serum from mice immunized with a recombinant Glutathione S-Transferase-BdJF5 fusion protein immunoprecipitated a 20 kDa B. bovis antigen. However, 30 kDa and 18 kDa antigens were immunoprecipitated from B. divergens and immunoblotting analysis revealed the recognition of a 35 kDa B. bigemina antigen. An indirect fluorescence antibody assay on merozoites showed strong reaction with B. divergens and weak recognition of B. bovis and B. bigemina. Despite the existent antigenic polymorphism among the Babesia spp., these results demonstrated that common antigens occur between European B. divergens and Mexican B. bovis/B. bigemina.

A leading bisthiazolium drug, T16, designed to mimic choline, was shown to exert potent antibabesial activity, with 50% inhibitory concentrations of 28 and 7 nM against Babesia divergens and B. canis, respectively. T16 accumulated inside Babesia-infected erythrocytes (cellular accumulation ratio, >60) by a saturable process with an apparent K(m) of 0.65 microM. Subcellular fractionation of Babesia parasites revealed the accumulation of T16 into a low-density fraction, while in malaria-infected erythrocytes a significant fraction of the drug was associated with heme malaria pigment. T16 exerts an early and specific inhibition of the de novo biosynthesis of phosphatidylcholine both in B. divergens- and Plasmodium falciparum-infected erythrocytes. Choline accumulation into isolated Babesia parasites was highly sensitive to inhibition by T16. These data are consistent with the hypothesis that bisthiazolium drugs target the de novo phosphatidylcholine biosynthesis of intraerythrocytic hematozoan parasites. In malaria parasites, which generate ferriprotoporphyrin IX during hemoglobin digestion, T16 binding to heme may enhance the accumulation and activity of the drug. The selectivity of accumulation and potent activity of this class of drug into parasite-infected erythrocytes offers unique advantages over more traditional antihematozoan drugs.

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.

Abstract: Bovine babesiosis, caused by Babesia bovis , B. bigemina , and B. divergens , is a significant impediment to livestock production in countries with tropical/subtropical and temperate climates. Previous studies conducted on the immunoprophylaxis against the disease and diagnosis of these parasites has demonstrated the presence of similar antigens. The objective of this article was to identify and partially characterize antigens conserved among these three species. Immunochemical analysis using sera from cattle immunized individually with antigens from these three Babesia species revealed a number of antigens recognized by heterologous antisera. Cross-reactions were more evident in sera from cattle immunized with B. bovis / B. bigemina which recognized several antigens (15 kDa to >200 kDa) in B. divergens . Immunoscreening of a B. divergens cDNA library with bovine serum to B. bigemina allowed the isolation of five clones and DNA sequencing of plasmid BdJF5 showed a 680 bp cDNA insert. Basic Local Alignment Search Tool (BLAST) analysis of the predicted amino acid sequence revealed 47% identity with a protein identified as αNAC. Serum from mice immunized with a recombinant Glutathione S-Transferase-BdJF5 fusion protein immunoprecipitated a 20 kDa B. bovis antigen. However, 30 kDa and 18 kDa antigens were immunoprecipitated from B. divergens and immunoblotting analysis revealed the recognition of a 35 kDa B. bigemina antigen. An indirect fluorescence antibody assay on merozoites showed strong reaction with B. divergens and weak recognition of B. bovis and B. bigemina . Despite the existent antigenic polymorphism among the Babesia spp., these results demonstrated that common antigens occur between European B. divergens and Mexican B. bovis/B. bigemina .

The Bd37gene encoding for a glycosyl-phosphatidyl-inositol anchored protein of Babesia divergens displays genetic polymorphisms among isolates. Five major polymorphic groups (clades) were shown by PCR-RFLP among different B. divergens isolates. Each group has been characterized according to a reference Bd37 gene (Rouen87, W8843, Y5, 6303E and 1705B). Recombinant (GST fusion) protein (Bd37r) expressed from the Bd37 gene, was used as antigen in a saponin-based formulation and was able to protect gerbils, after 2 injections at low dose vaccine concentration (1 μg per dose), against a virulent challenge with the B. divergens Rouen87 isolate. In spite of polymorphism of Bd37 gene, Bd37r induced complete immunoprotection against challenges with each of the 5 reference isolate groups defined by PCR-RFLP.

Throughout Europe, bovine babesiosis is mainly caused by Babesia divergens, an Apicomplexan parasite transmitted by tick bites. The intra-erythrocytic development of B. divergens merozoites leads to haemolytic anaemia, and bovine babesiosis is responsible for economic losses in the agro-business industry. A totally efficient recombinant vaccine based on the merozoite surface protein Bd37 and saponin QuilA was recently described. In the present study we determined that protective immunity elicited by the Bd37 recombinant protein was related to the presence of hydrophobic residues in the protein. Using polymeric fusion of Bd37 as well as cell-free in vitro protein expression, we successfully expressed recombinant proteins containing hydrophobic sequences without the need of GST fusion. We used different hydrophobic sequences and different recombinant Bd37 proteins to demonstrate that antigen hydrophobicity affects the immune system, turning an inefficient protein into a 100% protective vaccine. Some hypotheses about the hydrophobic effect and its potential application to other parasitic protozoa vaccine are also discussed.

Abstract: 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.

In Europe, Babesia divergens is the major agent responsible for babesiosis in cattle and can occasionally infect splenectomised humans. Recently, we reported the characterisation of a 37 kDa exoantigen (Bd37) anchored in the merozoite membrane of B. divergens by a glycosylphosphatidyl-inositol. After phospholipase hydrolyse of the glycosylphosphatidyl-inositol anchor, the Bd37 antigen could be isolated in the plasma of the infected host and from the in vitro culture supernatants. Immunisation of mice with a gel-filtration protective fraction of B. divergens exoantigens, produced a monoclonal antibody (MAb), called F4.2F8-INT, directed against Bd37. In the present study, we report data on passive protection using MAb F4.2F8-INT. This MAb was able to completely protect against virulent challenges with B. divergens isolates Rouen 1987 (Rouen87) and Weybridge 8843 (W8843) but had no protective effect against another French isolate from Massif Central (6303E). Physical characterisation of the epitope recognised by F4.2F8-INT allowed us to explain the differences observed between these isolates by western blotting and passive protection. These results suggest that the antigen carrying this epitope could be used as a target in the development of a recombinant vaccine against B. divergens babesiosis.

A bstract : 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 [ 35 S]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.

Babesia divergens is an Apicomplexa transmitted to bovines by its acarian vector, the tick I. ricinus. Babesia divergens merozoites have an intraerythrocytic development in the blood of infected mammals. The nucleocytoplasmic transport system in this parasite is not yet characterized and no protein involvement in such transport has been described. In this report, we describe the cloning of a protein that shares important homologies with Ran binding protein 1. This protein in Eukaryote belongs to the nucleocytoplasmic transport system.

Pulsed-field gel electrophoresis of intact chromosomes from 2 isolates of each of the 2 most pathogenic species of large Babesia parasites that infect dogs, i.e. Babesia canis (European species) and B. rossi (South African species), revealed 5 chromosomes in their haploid genome. The size of chromosomes 1-5 was found to be different in the 2 species, ranging from 0.8 to 6.0 Mbp. The genome size was estimated to be approximately 14.5 Mbp for B. canis and 16 Mbp for B. rossi, respectively. Within each species, the size of chromosomes 1-3 of B. canis and 1-2 of B. rossi was conserved between the 2 isolates, whereas the size of chromosomes 4-5 of B. canis and 3-5 of B. rossi was variable. Chromosomes 1-5 hybridized with a 28-mer telomeric oligonucleotide probe derived from Plasmodium berghei. When NotI-digested chromosomes of the 4 isolates were hybridized with the telomeric probe a maximum of 10 fragments was revealed. Moreover, hybridization of this telomeric probe to a Southern blot of genomic DNA from the 4 isolates, digested with a series of restriction enzymes, revealed a species-specific restriction map. Hybridization of intact or NotI-digested chromosomes of both species with 2 sets of 3 cDNA-antigen probes derived from each species, revealed no cross-hybridization between these B. canis and B. rossi genes.

The immunoprotective potential of Babesia divergens antigens released in supernatants of in vitro cultures of the parasite is generally known. Among a number of parasite molecules, a 37 kDa protein has been found in the supernatants of Babesia divergens cultures. In this report the cloning and biochemical characterization of this protein, called Bd37, are described. In addition, the processing of the protein was studied in vitro. Results suggest that Bd37 is encoded by a single copy gene. Bd37 appears to be a merozoite-associated molecule attached to the surface by a glycosylphosphatidylinositol moiety containing a palmitate residue attached to the inositol ring. In addition, it is demonstrated that both extremities of the protein are linked by a disulphide bond. Results further indicate that a soluble, hydrophilic form of Bd37 can be released from the merozoite surface by GPI-specific phospholipase D. The potential role the Bd37 protein and the GPI anchor are discussed.

Anaplasma marginale is the etiological agent of anaplasmosis, a tick-transmitted disease with an important economic impact that affects cattle throughout the world. Although, North American isolates of A. marginale and their antigens have been extensively studied, relatively little information is available on the antigenic composition of South American isolates. The characterization of diverse geographical isolates of A. marginale will result in a thorough antigenic profile and may lead to the identification of additional diagnostic and immunoprophylactic tools. Short-term cultures of a Venezuelan isolate (Ta) of A. marginale were maintained for up to 13 days in vitro. During that period, the A. marginale remained viable and were propagated in the bovine erythrocyte culture system. During the initial days of culture, cell division and reinvasion were evidenced by a significant rise in parasitemia up to a 50%. A. marginale antigens were identified by metabolic labeling with (35S) methionine, followed by fractionation and immunoprecipitation with homologous and heterologous bovine sera. This yielded a complete antigenic set for the Ta isolate of A. marginale, including soluble, secreted and corpuscular polypeptide antigens. Fifteen immunodominant polypeptides were recognized by the bovine sera in the soluble and corpuscular fractions with relative molecular weights of 200, 150, 100–110, 86, 60, 50, 47, 40, 37, 33, 31, 25, 23, 19 and 16 kDa. Seven polypeptides were present in the exoantigen fraction. The 31 and 19 kDa antigens were recognized by the ANAR76A1 and ANAF16C1 monoclonal antibodies, respectively which are specific for MSP-4 and MSP-5 from North American isolates of A. marginale. Metabolic labeling with (14C) glucosamine prior to immunoprecipitation with bovine sera allowed the identification of glycoprotein antigens of 200, 100–150, 60, 55, 50, 45–43, 37, 33, 31, 22, 19 and 16 kDa in the soluble fraction.

To study the antigens secreted by promastigote and amastigote forms of Leishmania infantum which are able to induce a humoral response in human patients and dogs, we have carried out immunoprecipitation assays with different supernatants of in vitro cultured parasites, metabolically labelled with [35S]methionine, using serum samples from human patients and dogs. In addition, some metabolic labelling experiments were performed daily during the in vitro culture parasite's life cycle to follow the time course excretion-secretion of parasitic antigens. The results demonstrated that the two different hosts developed an antibody response against secreted antigens of both stages of Leishmania infantum. Nevertheless, the humoral response directed against the excreted-secreted antigens of the promastigote forms was qualitatively and quantitatively different when we compare the human and the dog immune responses. On the other hand, when the excreted-secreted antigens of the amastigote forms are immunoprecipitated with either human or canine immune serum, the humoral response is similar. In addition, the time course study showed that excretion-secretion of antigens was qualitatively and quantitatively modulated during the parasitic in vitro life cycle.

In the search for immunoprotective antigens of the intraerythrocytic Babesia canis rossi parasite, a new cDNA was cloned and sequenced. Protein sequence database searches suggested that the 41-kDa protein belongs to the phosphofructokinase B type family (PFK-B). However, because of the low level sequence identity (< 20%) of the protein both with adenosine and sugar kinases from this family, its structural and functional features were further investigated using molecular modelling and enzymatic assays. The sequence/structure comparison of the protein with the crystal structure of a member of the PFK-B family, Escherichia coli ribokinase (EcRK), suggested that it might also form a stable and active dimer and revealed conservation of the ATP-binding site. However, residues specifically involved in the ribose-binding sites in the EcRK sequence (S and N) were substituted in its sequence (by H and M, respectively), and were suspected of binding adenosine compounds rather than sugar ones. Enzymatic assays using a purified glutathione S-transferase fusion protein revealed that this protein exhibits rapid catalysis of the phosphorylation of adenosine with an apparent K-m value of 70 nM, whereas it was inactive on ribose or other carbohydrates. As enzymatic assays confirmed the results of the structure/function analysis indicating a preferential specificity towards adenosine compounds, this new protein of the PFK-B family corresponds to an adenosine kinase from B. canis rossi. It was named BcrAK.