Discovery of a recombinant Babesia canis supernatant antigen that protects dogs against virulent challenge infection

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. canis 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 39kDa 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 39-40kDa doublet was released into the supernatant of B. canis cultures starting 15min after the chase. This doublet was specifically precipitated by VC-serum, thus corroborating that the 39-40kDa doublet in SPA preparations was of parasite origin. Partial amino acid sequencing allowed the discovery of the gene that encoded the 39-40kDa 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 39kDa 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.

Classification of Babesia canis strains in Europe based on polymorphism of the Bc28.1-gene from the Babesia canis Bc28 multigene family

The vast majority of clinical babesiosis cases in dogs in Europe is caused by Babesia canis. Although dogs can be vaccinated, the level of protection is highly variable, which might be due to genetic diversity of B. canis strains. One of the major merozoite surface antigens of B. canis is a protein with a Mr of 28 kDa that belongs to the Bc28 multigene family, that comprises at least two genes, Bc28.1 and a homologous Bc28.2 gene. The two genes are relatively conserved but they are very distinct in their 3' ends, enabling the design of specific primers. Sequencing of the Bc28.1 genes from 4 genetically distinct B. canis laboratory strains (A8, B, 34.01 and G) revealed 20 mutations at conserved positions of which three allowed the classification of B. canis strains into three main groups (A, B and 34.01/G) by RFLP. This assay was subsequently used to analyze blood samples of 394 dogs suspected of clinical babesiosis from nine countries in Europe. All blood samples were first analyzed with a previously described assay that allowed detection of the different Babesia species that infect dogs. Sixty one percent of the samples contained detectable levels of Babesia DNA. Of these, 98.3% were positive for B. canis, the remaining cases were positive for B. vogeli. Analysis of the Bc28.1 gene, performed on 178 of the B. canis samples, revealed an overall dominance of genotype B (62.4%), followed by genotypes A (37.1%) and 34 (11.8%). Interestingly, a great variation in the geographical distribution and prevalence of the three B. canis genotypes was observed; in the North-East genotype A predominated (72.1% A against 27.9% B), in contrast to the South-West where genotype B predominated (10.3% A against 89.7% B). In the central part of Europe intermediate levels were found (26.0-42.9% A against 74.0-57.1% B, from West to East). Genotype 34 was only identified in France (26.9% among 78 samples) and mostly as co-infection with genotypes A or B (61.9%). A comparative analysis of the classification of 35 B. canis strains in genotypes A and B using a previously described 18SrDNA-derived PCR-RFLP test revealed a partial but no direct correlation with the classification based on polymorphism of the Bc28.1-gene described here.

Soluble parasite antigens from Babesia canis do not directly activate the kallikrein system in dogs infected with Babesia canis

Soluble parasite antigens (SPA) from Babesia canis have been shown to induce protective immunity when used as vaccine. In order to explain the immune mechanisms of vaccination, the precise role of SPA in the pathogenesis of canine babesiosis is under investigation. Earlier studies suggested that the plasma kallikrein system is central in the pathogenesis of babesiosis, malaria and trypanosomosis, and significant plasma kallikrein activation during acute B. bovis and P. knowlesi infections has been described. In the studies presented here dogs were experimentally infected with B. canis to investigate whether the plasma kallikrein system is activated during babesiosis infection. Results showed that prekallikrein levels decreased during episodes of peak parasitaemia. No effect was found on the kallikrein levels. In order to determine whether B. canis SPA could activate plasma kallikrein, dogs were infused with variable amounts of B. canis SPA and plasma samples were taken for (pre-) kallikrein determination. The results indicated that B. canis SPA did not affect plasma (pre-) kallikrein levels. In addition, the effect of B. canis SPA on (pre-) kallikrein levels in normal dog plasma was determined in vitro. Again, no effect on (pre-) kallikrein levels was found. The results suggest that, although the kallikrein pathway may be involved in B. canis-associated pathology, the system is not directly activated by B. canis SPA. Furthermore, infusion of B. canis SPA as well as stroma of normal dog erythrocytes triggered the production of the acute phase reactant, C-reactive protein. This suggests that the inflammatory response that is triggered during B. canis infection could be in part due to the release and exposure of self molecules. The implications of these findings are discussed.

Vaccination against large Babesia species from dogs

The original observation of Sibinovic that soluble parasite antigens (SPA) of B. canis could be used to protect dogs against challenge infection formed the starting point for the development of an effective vaccine. With the advent of in vitro cultivation techniques for haemoprotozoan parasites an important tool became available for the commercial production of the vaccine antigens. A first generation vaccine was developed for dogs, but it appeared that the level of protection induced was not complete. In contrast to what was found with the SPA from serum/plasma of infected animals, protection induced with SPA from a single Babesia canis strain protected against a homologous challenge infection only. Further research led to the discovery that a combination of SPA of B. canis and SPA of B. rossi induced a broad spectrum of immunity. This improved vaccine, Nobivac Piro, not only induces protection against heterologous B. canis infection, but also against heterologous B. rossi infection.

Recombinant protein Bd37 protected gerbils against heterologous challenges with isolates ofBabesia divergenspolymorphic for thebd37gene

The Bd37gene encoding for a glycosyl-phosphatidyl-inositol anchored protein ofBabesia divergensdisplays genetic polymorphisms among isolates. Five major polymorphic groups (clades) were shown by PCR-RFLP among differentB. divergensisolates. 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 theB. divergensRouen87 isolate. In spite of polymorphism ofBd37gene, Bd37r induced complete immunoprotection against challenges with each of the 5 reference isolate groups defined by PCR-RFLP.

Association between sequence polymorphism in an epitope of Babesia divergens Bd37 exoantigen and protection induced by passive transfer

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.

Antibodies raised against Bcvir15, an extrachromosomal double-stranded RNA-encoded protein from Babesia canis, inhibit the in vitro growth of the parasite

As part of a search for homologous members of the Plasmodium falciparum Pf60 multigene family in the intraerythrocytic protozoan parasite Babesia canis, we report here the characterization of a cDNA of 1,115 bp, which was designated Bcvir for its potential viral origin. The Bcvir cDNA contained two overlapping open reading frames (ORFs) (ORF1 from nucleotide [nt] 61 to 486 and ORF2 from nt 417 to 919), where Bcvir15, the deduced ORF1 peptide (M(1) to I(141)), is the main expressed product. The Bcvir cDNA was derived from an extrachromosomal dsRNA element of 1.2 kbp that was always found associated with a double-stranded RNA (dsRNA) of 2.8 kbp by hybridization, and no copy of this cDNA sequence was found in B. canis genomic DNA. Biochemical characterization of Bcvir15, by using polyclonal rabbit sera directed against recombinant proteins, indicated that it is a soluble protein which remained associated with the cytoplasm of the B. canis merozoite. Interestingly, purified immunoglobulins from the anti-glutathione S-transferase-Bcvir15 (at a concentration of 160 micro g/ml) induced 50% inhibition of the in vitro growth of B. canis, and the inhibitory effect was associated with morphological damage of the parasite. Our data suggest that the extrachromosomal dsRNA-encoded Bcvir15 protein might interfere with the intracellular growth of the parasite rather than with the process of invasion of the host cell by the merozoite. Epitope mapping of Bcvir15 identified three epitopes that might be essential for the function of the protein.

Babesia divergens: cloning and biochemical characterization of Bd37

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.

Chromosome number, genome size and polymorphism of European and South African isolates of large Babesia parasites that infect dogs

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.

Characterization and molecular cloning of an adenosine kinase from Babesia canis rossi

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 Km 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.

Secreted antigens of the amastigote and promastigote forms of Leishmania infantum inducing a humoral response in humans and dogs

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.

Babesia canis canis, Babesia canis vogeli, Babesia canis rossi: differentiation of the three subspecies by a restriction fragment length polymorphism analysis on amplified small subunit ribosomal RNA genes

The parasites Babesia canis and Babesia gibsoni (phylum Apicomplexa) are responsible for canine babesiosis throughout the world. Babesia canis was previously described as a group of three biologically different subspecies, namely B. canis canis, B. canis vogeli, and B. canis rossi. We report partial sequences of small subunit ribosomal RNA gene (ssu-rDNA) of each subspecies amplified in vitro with primers derived from a semi-conserved region of the ssu-rDNA genes in other Babesia species. The polymerase chain reaction combined with a restriction fragment length polymorphism analysis, using HinfI and TaqI restriction enzymes, confirmed the separation of B. canis into three subspecies. These sequences were compared with previously published sequences of other Babesia species. A phylogenetic approach showed that the three subspecies of B. canis belong to the clade of Babesia species sensu stricto where B. canis canis clusters with B. canis rossi whereas B. canis vogeli might form a monophyletic group with the cluster B. divergens and B. odocoilei. Our results show that the three subspecies of B. canis can readily be differentiated at the molecular level and suggest that they might be considered as true species.

Human babesiosis

The first demonstrated case of human babesiosis in the world was reported in Europe, in 1957. Since then, a further 28 babesial infections in man have been reported in Europe. Most (83%) of the infections were in asplenic individuals and most (76%) were with Babesia divergens, a cattle parasite. Parasitaemias varied from 1%-80% of red blood cells. The usual clinical manifestations of severe B. divergens infection were severe intravascular haemolysis with haemoglobinuria. The most efficient treatment consisted of a massive blood-exchange transfusion, followed immediately by chemotherapy with clindamycin. Hundreds of cases of human infection with Babesia spp. have been reported in the U.S.A. Most cases were infected by ticks carrying the rodent parasite B. microti, but other emerging. Babesia spp. (currently known as WA1, CA1, and MO1) are increasingly involved. Several cases were the result of blood transfusion. In terms of clinical manifestations, human infections with B. microti varied widely, from asymptomatic infection to a severe, rapidly fatal disease. Parasitaemia ranged between <1% and 85%. The splenectomized, the elderly, the immunocompromised and HIV-infected patients were predisposed to severe infection. Infection with B. microti often remained subclinical or asymptomatic and were only detected through serological surveys. The currently recommended treatment of symptomatic cases is quinine plus clindamycin. A few other cases of human babesial infection have been described in China, Egypt, Mexico, South Africa and Taiwan.

Continuous in vitro culture of Babesia divergens in a serum-free medium

Babesia divergens was cultivated in RPMI 1640 (25 mM HEPES) supplemented with 10% human serum (RPMI-10% HS) with a high percentage of parasitized erythrocytes (PPE) (> or = 40%). Standardization of in vitro tests, purification of exoantigens, biochemical studies and the safety of the culture handler motivated the development of a serum-free defined medium. Removal of serum greatly reduced the PPE but, after a period of adaptation, the culture was continuous and the parasite was able to develop a 3% routine PPE. Addition of vitamins or reduced glutathione in basal medium (RPMI) did not improve the PPE. The supplementation of basal medium with lipidic carrier (Albumax I or bovine serum albumin-Cohn's fraction V) promoted the growth of B. divergens with high PPE (> 30%) close to those obtained in RPMI-10% HS. Neither protein nor lipid fractions alone were able to restore the growth of B. divergens. Nevertheless, the whole lipid fraction from serum or Albumax I added to delipidated albumin partially restored the growth (7% PPE), indicating that the presentation of specific lipids by a carrier is crucial for the parasite. All the data indicate that Albumax I can replace human serum offering the advantages of safety, standardization for chemosensitivity tests, and exoantigen purification.

A modification in restriction pattern of the Plasmodium falciparum Pf60 multigene family associated with a specific antigenic variation switch in the Palo Alto line

The wide occurrence of molecular rearrangements associated with expression of specific members within multigene families led us to investigate whether this also happens during antigenic variation of malaria parasites. We have investigated here the Pf60 multigene family restriction patterns of four distinct variants of the Plasmodium falciparum Palo Alto line propagated in Saimiri monkeys. The O and its cloned Oc variant both express the O serotype, while the R variant (derived from O parasites) and the Vc variant (derived from Oc parasites) express distinct serotypes. We show that a specific modification of the restriction pattern could be associated with antigenic switching in this line. The DNA of the variants which expressed the O serotype (O and Oc) had a specific 5.5 kb Hind III/Eco RI restriction fragment which was absent from the R or Vc parasite DNA, whereas both R and Vc DNA presented a 3.5 kb Hind III/Eco RI restriction fragment, which was absent from the O and Oc parasites. These results indicate that both expression and silencing of the O serotype were associated with specific restriction patterns, suggesting that some molecular rearrangement or some modification of the DNA might control expression of the variant surface antigen in malaria parasites.

Plasmodium falciparum: typing of malaria parasites based on polymorphism of a novel multigene family

Pf60.1, a marker recently isolated from the human malaria parasite Plasmodium falciparum, defines a large multigene family encoding antigens of 60 kDa, expressed by the blood stages (Carcy et al., Molecular and Biochemical Parasitology, 1994, 68, 221-233). Southern blotting showed that DNA from all strains and field isolates analyzed contained a large number of Pf60.1 copies. Considerable RFLP was observed. This diversity could be likewise visualized by analyzing PCR fragments amplified using primers derived from the Pf60.1 insert. Specific, multiple-band patterns were generated from laboratory strains, cloned lines, or wild isolates. This was further outlined after RsaI digestion of the PCR products. The sensitivity of this amplification was such that products could be visualized using a DNA amount representing less than one genome equivalent. Moreover, amplification was observed in some strains using a single primer, suggesting that some members of the Pf60.1 family are adjacent in an inverted orientation. This analysis confirmed the genetic similarity of a subset of laboratory strains. The results described here show that the extended diversity of this P. falciparum gene family provides a useful and sensitive PCR approach for strain typing.

A 37-kilodalton glycoprotein of Babesia divergens is a major component of a protective fraction containing low-molecular-mass culture-derived exoantigens

The supernatants of in vitro cultures of Babesia divergens Rouen 1987 in human erythrocytes, obtained by using a semidefined medium based on human high-density lipoproteins, were fractionated by gel filtration chromatography into four fractions, F1 to F4. The crude supernatant as well as each fraction adjuvanted with Quil-A protected gerbils from mortality due to a homologous infectious challenge. Analysis of the humoral response of the 10 protected gerbils with fraction F4, containing major proteins with molecular masses lower than 50 kDa, showed that a few antigens (from 50 to 17 kDa) could be important candidates for an improved vaccine against B. divergens babesiosis. As an immunodominant response was directed against the 37-kDa antigen (Bd37) in two different B. divergens strains tested, a polyclonal antibody directed against Bd37 was produced in a rabbit. In an immunofluorescence assay, the anti-Bd37 antiserum strongly labelled small internal vesicles of the merozoites and the cell surface was diffusely labelled after fixation, whereas on live merozoites, this labelling was not observed. [3H]glucosamine-radiolabelling experiments demonstrate that Bd37 is a glycoprotein. The Bd37 protein can also be labelled with [14C]palmitate but not with [3H]myristic acid. In Triton X-114 temperature phase partitioning of B. divergens-infected erythrocyte extracts, Bd37 was exclusively found into the detergent phase, indicating that the palmitoylated Bd37 protein was in the membrane fraction. In the in vitro supernatant, the glycoprotein Bd37 was found in a nonpalmitoylated form, indicating excretion and/or release of the glycoprotein from the merozoite.

A large multigene family expressed during the erythrocytic schizogony of Plasmodium falciparum

We report the identification of a large multigene family of Plasmodium falciparum using a clone isolated with a polyclonal antiserum raised to a Babesia divergens merozoite protein. The recombinant antigen reacted with human sera collected from individuals exposed to malaria. The deduced protein sequence contains a motif homologous to the consensus sequence of merozoite rhoptry proteins encoded by multigene families in several Babesia species. Antibodies raised to the recombinant protein reacted with a 60-kDa merozoite protein both on B. divergens and on P. falciparum immunoblots. The insert hybridized to a large number of fragments on P. falciparum Southern blots and to most chromosomes of the parasite. Specifically, approx. 3-kb RNAs were detected in 4-16-nucleus schizonts. Ten distinct cDNAs were isolated that differed in the size, position and number of restriction sites in the region homologous to the original genomic clone. With about 140 copies per haploid genome, this is the first large multigene family described in malaria parasites. The existence of a multigene family encoding proteins present in the invasive stage of malaria parasites suggests an important role in invasion and denotes a significant potential for generating diversity.

Immunogenicity and antigenicity of a Plasmodium falciparum protein fraction (90-110 kDa) able to protect squirrel monkeys against asexual blood stages

A monkey vaccination trial using a Plasmodium falciparum protein fraction containing antigens of 90-110 kDa is reported. The fraction was obtained by electroelution from preparative polyacrylamide gels. Three monkeys out of five resisted a heavy challenge dose of highly virulent parasites. Using specific antisera, several components of the fraction were identified, namely heat shock protein 90 (hsp90), Ag44/RhopH3, ABRA, 96tR/GBP130 and Pf96 protease. The fraction did not contain KAHRP, nor the SERP antigen. The antibody response of the monkeys was studied on these individual antigens purified by preparative immunoprecipitation. Surprisingly, hsp90 was found in the immunoprecipitates obtained with SERP antisera. Interestingly, the response to hsp90 correlated with protection, high antibody titres being found only in the protected monkeys. In contrast, no correlation with protection could be found for the response to the other antigens.

Plasmodium falciparum proteinases: cloning of the putative gene coding for the merozoite proteinase for erythrocyte invasion (MPEI) and determination of hydrolysis sites of spectrin by Pf37 proteinase

Numerous proteinase activities have been shown to be essential for the survival of Plasmodium falciparum. One approach to antimalarial chemotherapy, would be to block specifically one or several of these activities, by using compounds structurally analogous to the substrates of these proteinases. Such a strategy requires a detailed knowledge of the active site of the proteinase, in order to identify the best substrate for the proteinase. Aiming at developing such a strategy, two proteinases previously identified in our laboratory, were chosen for further characterization of their molecular structure and properties: the merozoite proteinase for erythrocytic invasion (MPEI), involved in the erythrocyte invasion by the merozoites, and the Pf37 proteinase, which hydrolyses human spectrin in vitro.

Characterization of a new 60 kDa apical protein of Plasmodium falciparum merozoite expressed in late schizogony

Immunological cross-reactivity studies between the Apicomplexa Babesia divergens and Plasmodium falciparum allowed us to identify a P falciparum 60 kDa protein (Pf60) using an antiserum directed against a B divergens 37 kDa culture-derived exoantigen. In immunofluorescence assays (IFA), Pf60 appears as a doublet of fluorescent spots associated to the apical pole of merozoites. The doublet co-locates with two rhoptry components: the protein RAP-1 and the 140/130/110 (105) kDa rhoptry protein complex suggesting the rhoptry location of Pf60. The biosynthesis of Pf60, established by labeling experiments with [35S]methionine on synchronized cultures, and by immunofluorescence detection, occurred during late schizogony. The physico-chemical properties of Pf60, the absence of identified precursor forms and the absence of co-precipitation with other proteins indicated a new class of rhoptry protein. Pf60 was detected in all the different geographic P falciparum strains so far tested, with a slight variability in molecular mass ranging from 58 to 60 kDa. During the invasion process of erythrocytes by merozoites, the IFA showed the presence of the Pf60 in the apex of free merozoites, but not in invading merozoite, as well as in new ring-infected erythrocytes. Furthermore, immunoprecipitation assays indicated the presence of Pf60 in the culture medium, and its absence in new ring-infected erythrocytes. All together these results suggest a possible involvement of the Pf60 protein in the invasion process.

Babesia divergens: characterization of a 17-kDa merozoite membrane protein

Large amounts of viable merozoites were purified from in vitro cultures of Babesia divergens by a two-step sieving procedure. A monoclonal antibody produced against B. divergens merozoites, mAb DG7, stained the merozoite plasma membrane and an intra-parasitic linear organelle in indirect immunofluorescence. Immunogold labeling in electron microscopy demonstrated that the antigen recognized by mAb DG7 was localized just beneath the merozoite plasma membrane. Immunoprecipitations of metabolically labeled ([35S]methionine) B. divergens antigens showed that the epitope recognized by mAb DG7 was present on a 17-kDa polypeptide (Bd17) and was shared in all B. divergens geographical isolates tested so far. Bd17 was always present in the in vitro culture supernatants of all these isolates. Furthermore, Triton X-114 phase separation of babesial antigens demonstrated the hydrophilic character of Bd17 which suggests that it is an extrinsic protein present on the cytosol side of the parasite membrane. When added to the culture medium, mAb DG7, purified from ascite fluids, drastically altered the growth of the parasite with concentrations inhibiting 50% of development (IC50) ranging between 16.6 and 26.1 micrograms/ml).

Cellular and humoral immune responses induced in cattle by vaccination with Babesia divergens culture-derived exoantigens correlate with protection

Previous results with the Babesia divergens gerbil vaccination model were extended in studies with cattle. Two calves were vaccinated with culture-derived B. divergens exoantigens, and two others were treated with control supernatant; both preparations were adjuvanted with Quil-A saponin. A parasite-specific humoral response was observed after the first vaccine injection and was boosted by two succeeding vaccine injections. Sera from the two vaccinated calves immunoprecipitated eight major parasitic proteins (with molecular masses ranging between 17 and 110 kDa) whose patterns were close to those observed in gerbil vaccine assays. The cellular immune response, monitored by lymphoproliferation assays, was slightly delayed in comparison with the humoral response; a significant proliferation occurred only after the second vaccine injection. Mononuclear cell proliferation was dose dependent in the presence of (i) lysates of B. divergens-parasitized erythrocytes, (ii) exoantigens of the whole supernatant, or (iii) protective exoantigens of two low-molecular-mass fractions obtained after supernatant gel filtration chromatography. An infectious challenge was administered 3 weeks after the third vaccine injection, with 3.6 x 10(10) B. divergens-parasitized erythrocytes. Erythrocyte count, rectal temperature, and parasitemia of the animals were monitored daily until they returned to initial values. All parameters indicated that the exoantigens induced protection from B. divergens infection for the two vaccinated calves.

Babesia divergens vaccine

A vaccine strategy against Babesia divergens bovine babesiosis was successfully developed after perfecting of an efficient in vitro culture. Crude supernatants and purified fractions were able to induce a vaccine protection in gerbils against B. divergens infection. More, supernatants induced an effective vaccine protection in cattle. The role of B. divergens exoantigens of 17, 37, 46, 70 and 90 kDa in the development of the immune response was clearly demonstrated in gerbils, cattle, and man.

Analysis of immune responses of different hosts to Babesia divergens isolates from different geographic areas and capacity of culture-derived exoantigens to induce efficient cross-protection

The immunoprecipitation of [35S]methionine-radiolabelled antigens from different Babesia divergens isolates by using bovine, gerbil, and human immune sera has shown that many B. divergens proteins contain epitopes shared between isolates. The cross-protective capacity of culture-derived soluble immunogens from the B. divergens Rouen 1987 isolate was tested against different B. divergens isolates. Results showed complete protection against the 7107b French isolate and substantial protection against the Weybridge 8843 English isolate (80% protection) and the Munich 87 German isolate (60% protection). In order to explain these vaccination results and to assess both the common and variable antigenicity of B. divergens, the antigenic patterns of the challenge isolates (Rouen 1987, 7107b, Weybridge 8843, and Munich 87) were compared by immunoprecipitation, using gerbil antisera raised against the Rouen 1987 vaccine isolate. Differences in the antigenic patterns and in the cross-protection of gerbils in these heterologous challenges were examined by studying the virulence and the antigenic status of each isolate.

Lipid trafficking between high density lipoproteins and Babesia divergens-infected human erythrocytes

A two-fold increase in the amount of phospholipids was observed in Babesia divergens infected human red blood cells. In vitro incubation with [32P]-phosphorus and [3H]-glycerol demonstrated that B divergens has the ability to synthesize the phospholipid backbone. On the other hand, the low incorporation of [14C]acetate indicated the absence of a de novo fatty acid synthesis and suggested the necessity of an exogenous lipid source for the parasite. Several intra-erythrocytic growth cycles of B divergens could be achieved in vitro, using a serum-free medium supplemented only with fractions of human high density lipoproteins (HDL). At an HDL concentration of 0.5 mg/ml (protein concentration) and with a 1% starting parasitaemia, parasite growth was similar to that observed under standard culture conditions with 10% human serum, at least for the first 24 h, a time equivalent to three parasite erythrocytic life-cycles. Lipid transfer from HDL to the intra-erythrocytic parasites was demonstrated by uptake and exchange of fluorescent NBD-phosphatidylcholine (NBD-PC) loaded HDL at different temperatures. Kinetic experiments with [3H]-oleyl-PC-loaded HDL demonstrated a unidirectional transfer of lipids from radiolabelled HDL to the parasite; partial conversion of PC to phosphatidylethanolamine (PE) was also observed. In the semi-defined medium, the HDL fraction appeared to be the major source of lipids for the growth of B divergens in human erythrocytes.

Heat shock response of Babesia divergens and identification of the hsp70 as an immunodominant early antigen during ox, gerbil and human babesiosis

Using antisera (alpha-R and alpha-C7Ag) directed against the conserved Gly-Gly-Met-Pro-epitope of the hsp70 family, a single antigen was identified in the human Babesia divergens Rouen 1987 isolate by Western immunoblotting and immunoprecipitation experiments. This B divergens hsp70 is highly conserved as shown by the analysis of five other geographical B divergens isolates from different hosts (human and bovine). Indirect immunofluorescence assay performed on the asexual intraerythrocytic stages showed that the hsp70 is mainly cytoplasmic and stage-independent. Heat-shock experiments, with 20 min incubation at 40 degrees C followed by a 10 to 50 min shift to 37 degrees C in the presence of [35S]-methionine, led to an increase of two hsp of 85 and 70 kDa while protein synthesis in general decreased within 10 min. Immunoprecipitations of [35S]-methionine radiolabelled proteins with human, ox and gerbil antisera raised against various B divergens isolates, showed the presence of a B divergens 70 kDa protein which was demonstrated to be a hsp70 by coupling immunoblotting assays with alpha-C7Ag serum on the same immunoprecipitated material. During human babesiosis, the B divergens hsp70 appears as an early antigen during the acute phase. These results are in agreement with the use of the B divergens hsp70 as an essential valence antigen in an anti-babesiosis vaccine.