Characterization of a variant erythrocyte surface antigen (VESA1) expressed by Babesia bovis during antigenic variation

Advances in understanding red blood cell modifications by Babesia

Babesia are tick-borne protozoan parasites that can infect livestock, pets, wildlife animals, and humans. In the mammalian host, they invade and multiply within red blood cells (RBCs). To support their development as obligate intracellular parasites, Babesia export numerous proteins to modify the RBC during invasion and development. Such exported proteins are likely important for parasite survival and pathogenicity and thus represent candidate drug or vaccine targets. The availability of complete genome sequences and the establishment of transfection systems for several Babesia species have aided the identification and functional characterization of exported proteins. Here, we review exported Babesia proteins; discuss their functions in the context of immune evasion, cytoadhesion, and nutrient uptake; and highlight possible future topics for research and application in this field.

Integration of DNA Repair, Antigenic Variation, Cytoadhesion, and Chance in Babesia Survival: A Perspective

Apicomplexan parasites live in hostile environments in which they are challenged chemically and their hosts attempt in many ways to kill them. In response, the parasites have evolved multiple mechanisms that take advantage of these challenges to enhance their survival. Perhaps the most impressive example is the evolutionary co-option of DNA repair mechanisms by the parasites as a means to rapidly manipulate the structure, antigenicity, and expression of the products of specific multigene families. The purpose of variant proteins that mediate cytoadhesion has long been thought to be primarily the avoidance of splenic clearance. Based upon known biology, I present an alternative perspective in which it is survival of the oxidative environment within which Babesia spp. parasites live that has driven integration of DNA repair, antigenic variation, and cytoadhesion, and speculate on how genome organization affects that integration. This perspective has ramifications for the development of parasite control strategies.

Babesiosis and the human immune system

Immunological phenomena have been described in infections such as infective endocarditis. However, none has been reported in the context of Babesiosis.

Babesiosis is a tick-borne illness caused by the protozoa of the genus Babesia and causes infections that range from asymptomatic to severe and sometimes are fatal. This report presents the first case of ANCA/ANA positive severe babesiosis in an asplenic patient treated with repeated red blood cell exchange transfusion.

Identification of a novel variant erythrocyte surface antigen-1 (VESA1) in Babesia orientalis

Babesia orientalis, belonging to the phylum Apicomplexa, is mainly accountable for water buffalo babesiosis, which adversely affected the livestock industry in China. Variant erythrocyte surface antigen-1 (VESA1), an antigen that helps infected erythrocytes to escape from host immune responses, was first reported in Babesia bovis. Various VESA1 proteins have also been characterized in other Babesia species. Nevertheless, there is no research on the identification and characterization of VESA1 proteins in Babesia orientalis. In this study, the BoVESA1 gene was amplified from both gDNA and cDNA. The results revealed that it is an intronless gene with a full length of 753 bp, encoding a protein of 250 amino acids with a predicted molecular weight of 28 kDa. The coding sequence (CDS) was cloned into the pGEX-6p-1 vector using a homologous recombination kit and expressed as a glutathione-S-transferase (GST)-fusion protein with a molecular weight of 53 kDa. The tertiary structure of BoVESA1 was predicted using the I-TASSER software. The recombinant protein was subjected to western blotting; the immunogenicity of recombinant BoVESA1 (rBoVESA1) was identified by incubating it with B. orientalis-positive serum. The native BoVESA1 was identified using the lysates of B. orientalis-infected water buffalo erythrocytes incubated with the anti-rBoVESA1 mouse serum. The results showed a band of ~ 28 kDa, which is similar to the predicted size. Immunofluorescence assay (IFA) using anti-rBoVESA1 serum probed indicated a strong signal in the infected RBCs, while the negative control showed no signal. In conclusion, the VESA1 protein was first identified in B. orientalis. This study facilitated further investigation of B. orientalis, and the results indicated that BoVESA1 may serve as a potential candidate antigen for diagnosis and detection of B. orientalis infection.

Comparative analysis of gene expression between Babesia bovis blood stages and kinetes allowed by improved genome annotation

Throughout their life cycle, Babesia parasites alternate between a mammalian host, where they cause babesiosis, and the tick vector. Transition between hosts results in distinct environmental signals that influence patterns of gene expression, consistent with the morphological and functional changes operating in the parasites during their life stages. In addition, comparing differential patterns of gene expression among mammalian and tick parasite stages can provide clues for developing improved methods of control. Hereby, we upgraded the genome assembly of Babesia bovis, a bovine hemoparasite, closing a 139 kbp gap, and used RNA-Seq datasets derived from mammalian blood and tick kinete stages to update the genome annotation. Of the originally annotated genes, 1,254 required structural changes, and 326 new genes were identified, leading to a different predicted proteome compared to the original annotation. Next, the RNA-Seq data was used to identify B. bovis genes that were differentially expressed in the vertebrate and arthropod hosts. In blood stages, 28% of the genes were upregulated up to 300 fold, whereas 26% of the genes in kinetes, a tick stage, were upregulated up to >19,000 fold. We thus discovered differentially expressed genes that may play key biological roles and serve as suitable targets for the development of vaccines to control bovine babesiosis.

Novel Babesia bovis exported proteins that modify properties of infected red blood cells

Babesia bovis causes a pathogenic form of babesiosis in cattle. Following invasion of red blood cells (RBCs) the parasite extensively modifies host cell structural and mechanical properties via the export of numerous proteins. Despite their crucial role in virulence and pathogenesis, such proteins have not been comprehensively characterized in B. bovis. Here we describe the surface biotinylation of infected RBCs (iRBCs), followed by proteomic analysis. We describe a multigene family (mtm) that encodes predicted multi-transmembrane integral membrane proteins which are exported and expressed on the surface of iRBCs. One mtm gene was downregulated in blasticidin-S (BS) resistant parasites, suggesting an association with BS uptake. Induced knockdown of a novel exported protein encoded by BBOV_III004280, named VESA export-associated protein (BbVEAP), resulted in a decreased growth rate, reduced RBC surface ridge numbers, mis-localized VESA1, and abrogated cytoadhesion to endothelial cells, suggesting that BbVEAP is a novel virulence factor for B. bovis.

Babesia bovis Rad51 ortholog influences switching of ves genes but is not essential for segmental gene conversion in antigenic variation

The tick-borne apicomplexan parasite, Babesia bovis, a highly persistent bovine pathogen, expresses VESA1 proteins on the infected erythrocyte surface to mediate cytoadhesion. The cytoadhesion ligand, VESA1, which protects the parasite from splenic passage, is itself protected from a host immune response by rapid antigenic variation. B. bovis relies upon segmental gene conversion (SGC) as a major mechanism to vary VESA1 structure. Gene conversion has been considered a form of homologous recombination (HR), a process for which Rad51 proteins are considered pivotal components. This could make BbRad51 a choice target for development of inhibitors that both interfere with parasite genome integrity and disrupt HR-dependent antigenic variation. Previously, we knocked out the Bbrad51 gene from the B. bovis haploid genome, resulting in a phenotype of sensitivity to methylmethane sulfonate (MMS) and apparent loss of HR-dependent integration of exogenous DNA. In a further characterization of BbRad51, we demonstrate here that ΔBbrad51 parasites are not more sensitive than wild-type to DNA damage induced by γ-irradiation, and repair their genome with similar kinetics. To assess the need for BbRad51 in SGC, RT-PCR was used to observe alterations to a highly variant region of ves1α transcripts over time. Mapping of these amplicons to the genome revealed a significant reduction of in situ transcriptional switching (isTS) among ves loci, but not cessation. By combining existing pipelines for analysis of the amplicons, we demonstrate that SGC continues unabated in ΔBbrad51 parasites, albeit at an overall reduced rate, and a reduction in SGC tract lengths was observed. By contrast, no differences were observed in the lengths of homologous sequences at which recombination occurred. These results indicate that, whereas BbRad51 is not essential to babesial antigenic variation, it influences epigenetic control of ves loci, and its absence significantly reduces successful variation. These results necessitate a reconsideration of the likely enzymatic mechanism(s) underlying SGC and suggest the existence of additional targets for development of small molecule inhibitors.

B. bovis establishes highly persistent infections in cattle, in part by using cytoadhesion to avoid passage through the spleen. While protective, a host antibody response targeting the cytoadhesion ligand is quickly rendered ineffective by antigenic variation. In B. bovis, antigenic variation relies heavily upon segmental gene conversion (SGC), presumed to be a form of homologous recombination (HR), to generate variants. As Rad51 is generally considered essential to HR, we investigated its contribution to SGC. While diminishing the parasite’s capacity for HR-dependent integration of exogenous DNA, the loss of BbRad51 did not affect the parasite’s sensitivity to ionizing radiation, overall genome stability, or competence for SGC. Instead, loss of BbRad51 diminished the extent of in situ transcriptional switching (isTS) among ves gene loci, the accumulation of SGC recombinants, and the mean lengths of SGC sequence tracts. Given the overall reductions in VESA1 variability, compromise of the parasite’s capacity for in vivo persistence is predicted.

Knockout of Babesia bovis rad51 ortholog and its complementation by expression from the BbACc3 artificial chromosome platform

Babesia bovis establishes persistent infections of long duration in cattle, despite the development of effective anti-disease immunity. One mechanism used by the parasite to achieve persistence is rapid antigenic variation of the VESA1 cytoadhesion ligand through segmental gene conversion (SGC), a phenomenon thought to be a form of homologous recombination (HR). To begin investigation of the enzymatic basis for SGC we initially identified and knocked out the Bbrad51 gene encoding the B. bovis Rad51 ortholog. BbRad51 was found to be non-essential for in vitro growth of asexual-stage parasites. However, its loss resulted in hypersensitivity to methylmethane sulfonate (MMS) and an apparent defect in HR. This defect rendered attempts to complement the knockout phenotype by reinsertion of the Bbrad51 gene into the genome unsuccessful. To circumvent this difficulty, we constructed an artificial chromosome, BbACc3, into which the complete Bbrad51 locus was inserted, for expression of BbRad51 under regulation by autologous elements. Maintenance of BbACc3 makes use of centromeric sequences from chromosome 3 and telomeric ends from chromosome 1 of the B. bovis C9.1 line. A selection cassette employing human dihydrofolate reductase enables recovery of transformants by selection with pyrimethamine. We demonstrate that the BbACc3 platform is stably maintained once established, assembles nucleosomes to form native chromatin, and expands in telomere length over time. Significantly, the MMS-sensitivity phenotype observed in the absence of Bbrad51 was successfully complemented at essentially normal levels. We provide cautionary evidence, however, that in HR-competent parasites BbACc3 can recombine with native chromosomes, potentially resulting in crossover. We propose that, under certain circumstances this platform can provide a useful alternative for the genetic manipulation of this group of parasites, particularly when regulated gene expression under the control of autologous elements may be important.

Interplay between Attenuation- and Virulence-Factors of Babesia bovis and Their Contribution to the Establishment of Persistent Infections in Cattle

Bovine babesiosis is an acute and persistent tick-borne global disease caused mainly by the intraerythrocytic apicomplexan parasites Babesia bovis and B. bigemina. B. bovis infected erythrocytes sequester in blood capillaries of the host (cytoadhesion), causing malaria-like neurological signs. Cytoadhesion and antigenic variation in B. bovis are linked to the expression of members of the Variant Erythrocyte Surface Antigen (VESA) gene family. Animals that survive acute B. bovis infection and those vaccinated with attenuated strains remain persistently infected, suggesting that B. bovis parasites use immune escape mechanisms. However, attenuated B. bovis parasites do not cause neurological signs in vaccinated animals, indicating that virulence or attenuation factors play roles in modulating parasite virulence phenotypes. Artificial overexpression of the SBP2t11 protein, a defined attenuation factor, was associated with reduced cytoadhesion, suggesting a role for this protein as a key modulator of virulence in the parasite. Hereby, we propose a model that might be functional in the modulation of B. bovis virulence and persistence that relies on the interplay among SBP2t, VESA proteins, cytoadhesion, and the immune responses of the host. Elucidation of mechanisms used by the parasite to establish persistent infection will likely contribute to the design of new methods for the control of bovine babesiosis.

Variable and Variant Protein Multigene Families in Babesia bovis Persistence

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

Whole-genome assembly of Babesia ovata and comparative genomics between closely related pathogens

Background

Babesia ovata, belonging to the phylum Apicomplexa, is an infectious parasite of bovids. It is not associated with the manifestation of severe symptoms, in contrast to other types of bovine babesiosis caused by B. bovis and B. bigemina; however, upon co-infection with Theileria orientalis, it occasionally induces exacerbated symptoms. Asymptomatic chronic infection in bovines is usually observed only for B. ovata. Comparative genomic analysis could potentially reveal factors involved in these distinguishing characteristics; however, the genomic and molecular basis of these phenotypes remains elusive, especially in B. ovata. From a technical perspective, the current development of a very long read sequencer, MinION, will facilitate the obtainment of highly integrated genome sequences. Therefore, we applied next-generation sequencing to acquire a high-quality genome of the parasite, which provides fundamental information for understanding apicomplexans.

Results

The genome was assembled into 14,453,397 bp in size with 5031 protein-coding sequences (91 contigs and N50 = 2,090,503 bp). Gene family analysis revealed that ves1 alpha and beta, which belong to multigene families in B. bovis, were absent from B. ovata, the same as in B. bigemina. Instead, ves1a and ves1b, which were originally specified in B. bigemina, were present. The B. ovata and B. bigemina ves1a configure one cluster together even though they divided into two sub-clusters according to the spp. In contrast, the ves1b cluster was more dispersed and the overlap among B. ovata and B. bigemina was limited. The observed redundancy and rapid evolution in sequence might reflect the adaptive history of these parasites. Moreover, same candidate genes which potentially involved in the distinct phenotypes were specified by functional analysis. An anamorsin homolog is one of them. The human anamorsin is involved in hematopoiesis and the homolog was present in B. ovata but absent in B. bigemina which causes severe anemia.

Conclusions

Taking these findings together, the differences demonstrated by comparative genomics potentially explain the evolutionary history of these parasites and the differences in their phenotypes. Besides, the draft genome provides fundamental information for further characterization and understanding of these parasites.

Electronic supplementary material

The online version of this article (10.1186/s12864-017-4230-4) contains supplementary material, which is available to authorized users.

Genome-wide analysis of gene expression and protein secretion of Babesia canis during virulent infection identifies potential pathogenicity factors

Infections of dogs with virulent strains of Babesia canis are characterized by rapid onset and high mortality, comparable to complicated human malaria. As in other apicomplexan parasites, most Babesia virulence factors responsible for survival and pathogenicity are secreted to the host cell surface and beyond where they remodel and biochemically modify the infected cell interacting with host proteins in a very specific manner. Here, we investigated factors secreted by B. canis during acute infections in dogs and report on in silico predictions and experimental analysis of the parasite’s exportome. As a backdrop, we generated a fully annotated B. canis genome sequence of a virulent Hungarian field isolate (strain BcH-CHIPZ) underpinned by extensive genome-wide RNA-seq analysis. We find evidence for conserved factors in apicomplexan hemoparasites involved in immune-evasion (e.g. VESA-protein family), proteins secreted across the iRBC membrane into the host bloodstream (e.g. SA- and Bc28 protein families), potential moonlighting proteins (e.g. profilin and histones), and uncharacterized antigens present during acute crisis in dogs. The combined data provides a first predicted and partially validated set of potential virulence factors exported during fatal infections, which can be exploited for urgently needed innovative intervention strategies aimed at facilitating diagnosis and management of canine babesiosis.

Diversity of extracellular proteins during the transition from the ‘proto-apicomplexan’ alveolates to the apicomplexan obligate parasites

The recent completion of high-coverage draft genome sequences for several alveolate protozoans – namely, the chromerids, Chromera velia and Vitrella brassicaformis; the perkinsid Perkinsus marinus; the apicomplexan, Gregarina niphandrodes, as well as high coverage transcriptome sequence information for several colpodellids, allows for new genome-scale comparisons across a rich landscape of apicomplexans and other alveolates. Genome annotations can now be used to help interpret fine ultrastructure and cell biology, and guide new studies to describe a variety of alveolate life strategies, such as symbiosis or free living, predation, and obligate intracellular parasitism, as well to provide foundations to dissect the evolutionary transitions between these niches. This review focuses on the attempt to identify extracellular proteins which might mediate the physical interface of cell–cell interactions within the above life strategies, aided by annotation of the repertoires of predicted surface and secreted proteins encoded within alveolate genomes. In particular, we discuss what descriptions of the predicted extracellular proteomes reveal regarding a hypothetical last common ancestor of a pre-apicomplexan alveolate – guided by ultrastructure, life strategies and phylogenetic relationships – in an attempt to understand the evolution of obligate parasitism in apicomplexans.

Large, rapidly evolving gene families are at the forefront of host-parasite interactions in Apicomplexa

The Apicomplexa is a phylum of parasitic protozoa, which includes the malaria parasite Plasmodium, amongst other species that can devastate human and animal health. The past decade has seen the release of genome sequences for many of the most important apicomplexan species, providing an excellent basis for improving our understanding of their biology. One of the key features of each genome is a unique set of large, variant gene families. Although closely related species share the same families, even different types of malaria parasite have distinct families. In some species they tend to be found at the ends of chromosomes, which may facilitate aspects of gene expression regulation and generation of sequence diversity. In others they are scattered apparently randomly across chromosomes. For some families there is evidence they are involved in antigenic variation, immune regulation and immune evasion. For others there are no known functions. Even where function is unknown these families are most often predicted to be exposed to the host, contain much sequence diversity and evolve rapidly. Based on these properties it is clear that they are at the forefront of host–parasite interactions. In this review I compare and contrast the genomic context, gene structure, gene expression, protein localization and function of these families across different species.

Bioinformatic prediction of the exportome of Babesia bovis and identification of novel proteins in parasite-infected red blood cells

Babesia bovis is a pathogen of considerable economic significance to the livestock industry worldwide but the precise mechanisms by which this parasite causes disease in susceptible cattle remain poorly understood. It is clear, however, that alterations to the structure and function of red blood cells in which the parasites reside and replicate play an important role in pathogenesis and that these are secondary to the export of numerous, currently unknown and uncharacterised parasite-encoded proteins. Using a rational bioinformatic approach, we have identified a set of 362 proteins (117 of which are hypothetical) that we predict encompasses the B. bovis exportome. These exported proteins are likely to be trafficked to various cellular locations, with a subset destined for the red blood cell cytosol or the red blood cell cytoskeleton. These proteins are likely to play important roles in mediating the pathogenesis of babesiosis. We have selected three novel proteins and confirmed their predicted export and localisation within the host red blood cell by immunofluorescence using specific antibodies raised against these proteins. Complete characterisation of these novel exported parasite proteins will help elucidate their function within the host red blood cell and assist in identification of new therapeutic targets for babesiosis.

Unusual chromatin structure associated with monoparalogous transcription of the Babesia bovis ves multigene family

Rapid antigenic variation in Babesia bovis involves the variant erythrocyte surface antigen-1 (VESA1), a heterodimeric protein with subunits encoded by two branches of the ves multigene family. The ves1α and ves1β gene pair encoding VESA1a and 1b, respectively, are transcribed in a monoparalogous manner from a single locus of active ves transcription (LAT), just one of many quasi-palindromic ves loci. To determine whether this organization plays a role in transcriptional regulation, chromatin structure was first assessed. Limited treatment of isolated nuclei with micrococcal nuclease to assay nucleosomal patterning revealed a periodicity of 156-159 bp in both bulk chromatin and specific gene coding regions. This pattern also was maintained in the intergenic regions (IGr) of non-transcribed ves genes. In contrast, the LAT IGr adopts a unique pattern, yielding an apparent cluster of five closely-spaced hypersensitive sites flanked by regions of reduced nucleosomal occupancy. ves loci fall into three patterns of overall sensitivity to micrococcal nuclease or DNase I digestion, with only the LAT being consistently very sensitive. Non-transcribed ves genes are inconsistent in their sensitivity to the two enzymatic probes. Non-linear DNA structure in chromatin was investigated to determine whether unique structure arising as a result of the quasi-palindromic nature of the LAT may effect transcriptional control. The in vitro capacity of ves IGr sequences to adopt stable higher-order DNA structure is demonstrated here, but the presence of such structure in vivo was not supported. Based upon these results a working model is proposed for the chromatin structural remodeling responsible for the sequential expression of ves multigene family members from divergently-organized loci.

Sirtuins of parasitic protozoa: in search of function(s)

The SIR2 family of NAD⁺-dependent protein deacetylases, collectively called sirtuins, has been of central interest due to their proposed roles in life-span regulation and ageing. Sirtuins are one group of environment sensors of a cell interpreting external information and orchestrating internal responses at the sub-cellular level, through participation in gene regulation mechanisms. Remarkably conserved across all kingdoms of life SIR2 proteins in several protozoan parasites appear to have both conserved and intriguing unique functions. This review summarises our current knowledge of the members of the sirtuin families in Apicomplexa, including Plasmodium, and other protozoan parasites such as Trypanosoma and Leishmania. The wide diversity of processes regulated by SIR2 proteins makes them targets worthy of exploitation in anti-parasitic therapies.

Characterization of the unusual bidirectional ves promoters driving VESA1 expression and associated with antigenic variation in Babesia bovis

Rapid clonal antigenic variation in Babesia bovis involves the variant erythrocyte surface antigen-1 (VESA1) protein expressed on the infected-erythrocyte surface. Because of the significance of this heterodimeric protein for demonstrated mechanisms of parasite survival and virulence, there is a need to understand how expression of the ves multigene family encoding this protein is controlled. As an initial step toward this goal, we present here initial characterization of the ves promoter driving transcription of VESA1a and -1b subunits. A series of transfection constructs containing various sequence elements from the in vivo locus of active ves transcription (LAT) were used to drive expression of the firefly luciferase gene in a dual luciferase-normalized assay. The results of this approach reveal the presence of two bidirectional promoter activities within the 434-bp intergenic region (IGr), influenced by putative regulatory sequences embedded within the flanking ves1α and ves1β genes. Repressor-like effects on the apposing gene were observed for intron 1 of both ves1α and ves1β. This effect is apparently not dependent upon intronic promoter activity and acts only in cis. The expression of genes within the ves family is likely modulated by local elements embedded within ves coding sequences outside the intergenic promoter region in concert with chromatin modifications. These results provide a framework to help us begin to understand gene regulation during antigenic variation in B. bovis.

Comparison of protective immune responses to apicomplexan parasites

Members of the phylum Apicomplexa, which includes the species Plasmodium, Eimeria, Toxoplasma, and Babesia amongst others, are the most successful intracellular pathogens known to humankind. The widespread acquisition of antimicrobial resistance to most drugs used to date has sparked a great deal of research and commercial interest in the development of vaccines as alternative control strategies. A few antigens from the asexual and sexual stages of apicomplexan development have been identified and their genes characterised; however, the fine cellular and molecular details of the effector mechanisms crucial for parasite inhibition and stimulation of protective immunity are still not entirely understood. This paper provides an overview of what is currently known about the protective immune response against the various types of apicomplexan parasites and focuses mainly on the similarities of these pathogens and their host interaction. Finally, the evolutionary relationships of these parasites and their hosts, as well as the modulation of immune functions that are critical in determining the outcome of the infection by these pathogenic organisms, are discussed.

Recent insights into alteration of red blood cells by Babesia bovis: moovin' forward

Over the past decade or so, our understanding of the biology of apicomplexan parasites has increased dramatically, particularly in the case of malaria. Notable achievements are the availability of complete genome sequences, transcriptome and proteome profiles and the establishment of in vitro transfection techniques for asexual-stage malaria parasites. Interestingly, despite their major economic importance and striking similarities with malaria, Babesia parasites have been relatively ignored, but change is on the horizon. Here, we bring together recent work on Babesia bovis parasites which are beginning to unravel the molecular mechanisms that underlie the pathogenesis of babesiosis and highlight some opportunities and challenges that lie ahead.

The Babesia bovis VESA1 virulence factor subunit 1b is encoded by the 1beta branch of the ves multigene family

Babesia bovis, an intraerythrocytic parasite of cattle, establishes persistent infections of extreme duration. This is accomplished, at least in part, through rapid antigenic variation of a heterodimeric virulence factor, the variant erythrocyte surface antigen-1 (VESA1) protein. Previously, the VESA1a subunit was demonstrated to be encoded by a 1alpha member of the ves multigene family. Since its discovery the 1beta branch of this multigene family has been hypothesized to encode the VESA1b polypeptide, but formal evidence for this connection has been lacking. Here, we provide evidence that products of ves1beta genes are rapidly variant in antigenicity and size-polymorphic, matching known VESA1b polypeptides. Importantly, the ves1beta-encoded antigens are co-precipitated with VESA1a during immunoprecipitation with anti-VESA1a monoclonal antibodies, and antisera to ves1beta polypeptide co-precipitate VESA1a. Further, the ves1beta-encoded antigens significantly co-localize with VESA1a on the infected-erythrocyte membrane surface of live cells. These characteristics all match known properties of VESA1b, allowing us to conclude that the ves1beta gene divergently apposing the ves1beta gene within the locus of active ves transcription (LAT) encodes the 1b subunit of the VESA1 cytoadhesion ligand. However, the extent and stoichiometry of VESA1a and 1b co-localization on the surface of individual cells is quite variable, implicating competing effects on transcription, translation, or trafficking of the two subunits. These results provide essential information facilitating further investigation into this parasite virulence factor.

Universal primers suitable to assess population dynamics reveal apparent mutually exclusive transcription of the Babesia bovis ves1alpha gene

Babesia bovis is an intraerythrocytic hemoparasite of widespread distribution, which adversely affects livestock production in many regions of the world. This parasite establishes persistent infections of long duration, at least in part through rapid antigenic variation of the VESA1 protein on the infected-erythrocyte surface. To understand the dynamics of in vivo antigenic variation among the parasite population it is necessary to have sensitive and broadly applicable tools enabling monitoring of variation events in parasite antigen genes. To address this need for B. bovis, "universal" primers for the polymerase chain reaction have been designed for the ves1alpha gene, spanning from exon 2 to near the 3' end of cysteine-lysine-rich domain (CKRD) sequences in exon 3. These primers robustly amplified this segment, with minimal bias, from essentially the entire repertoire of full-length ves1alpha sequences in the B. bovis Mexico isolate genome, and are equivalently present in other isolates. On purified genomic DNA, this primer set can achieve a sensitivity of 10 genome equivalents or less. When applied to the amplification of cDNA derived from the B. bovis C9.1 clonal line evidence consistent with mutually exclusive transcription of the ves1alpha gene was obtained, concomitant with detection of numerous mutational events among members of the parasite population. These characteristics of the primers will facilitate the application of polymerase chain reaction-based methodologies to the study of B. bovis population and antigenic switching dynamics.

An overview of the Babesia, Plasmodium and Theileria genomes: a comparative perspective

Babesia, Plasmodium and Theileria form a triad of apicomplexan hemoparasites and are accountable for significant mortality and morbidity to humans and animals globally. Understanding the pathobiology of these three genera is crucial as multiple drug resistant strains continue to arise in endemic areas along with pesticide and acaricide resistant vector hosts. Vastly improved sequencing technology has produced whole genome sequences of several apicomplexan species and subsequent comparative analyses of these genomes have identified unique as well as common features among the different species, information that will help in the pursuit of alternative therapies, management and perhaps elimination of the disease. This review, therefore, summarizes comparisons of genome structure, protein families, metabolic pathways and organelle biology in these three apicomplexans and how such knowledge has and will continue to enhance the field.

Human babesiosis

Human babesiosis is an emerging intraerythrocytic infection caused by protozoal parasites transmitted by ixodid ticks. Babesiosis is endemic in the northeastern and upper midwestern regions of the United States and is found sporadically in other parts of the United States, Europe, Asia, Africa, and South America. Babesial infections range from asymptomatic to severe and occasionally are fatal. Specific laboratory diagnosis of babesial infection is made by morphologic examination of Giemsa-stained blood smears, serology, and amplification of babesial DNA using polymerase chain reaction. The combination of atovaquone and azithromycin is the treatment of choice for mild-to-moderate illness, whereas clindamycin and quinine and exchange transfusion are indicated for severe disease.

Genome sequence of Babesia bovis and comparative analysis of apicomplexan hemoprotozoa

Babesia bovis is an apicomplexan tick-transmitted pathogen of cattle imposing a global risk and severe constraints to livestock health and economic development. The complete genome sequence was undertaken to facilitate vaccine antigen discovery, and to allow for comparative analysis with the related apicomplexan hemoprotozoa Theileria parva and Plasmodium falciparum. At 8.2 Mbp, the B. bovis genome is similar in size to that of Theileria spp. Structural features of the B. bovis and T. parva genomes are remarkably similar, and extensive synteny is present despite several chromosomal rearrangements. In contrast, B. bovis and P. falciparum, which have similar clinical and pathological features, have major differences in genome size, chromosome number, and gene complement. Chromosomal synteny with P. falciparum is limited to microregions. The B. bovis genome sequence has allowed wide scale analyses of the polymorphic variant erythrocyte surface antigen protein (ves1 gene) family that, similar to the P. falciparum var genes, is postulated to play a role in cytoadhesion, sequestration, and immune evasion. The ∼150 ves1 genes are found in clusters that are distributed throughout each chromosome, with an increased concentration adjacent to a physical gap on chromosome 1 that contains multiple ves1-like sequences. ves1 clusters are frequently linked to a novel family of variant genes termed smorfs that may themselves contribute to immune evasion, may play a role in variant erythrocyte surface antigen protein biology, or both. Initial expression analysis of ves1 and smorf genes indicates coincident transcription of multiple variants. B. bovis displays a limited metabolic potential, with numerous missing pathways, including two pathways previously described for the P. falciparum apicoplast. This reduced metabolic potential is reflected in the B. bovis apicoplast, which appears to have fewer nuclear genes targeted to it than other apicoplast containing organisms. Finally, comparative analyses have identified several novel vaccine candidates including a positional homolog of p67 and SPAG-1, Theileria sporozoite antigens targeted for vaccine development. The genome sequence provides a greater understanding of B. bovis metabolism and potential avenues for drug therapies and vaccine development.

Vector-transmitted blood parasites cause some of the most widely distributed, serious, and poorly controlled diseases globally, including the most severe form of human malaria caused by Plasmodium falciparum. In livestock, tick-transmitted blood parasites include the protozoa Theileria parva, the cause of East Coast fever and Babesia bovis, the cause of tick fever, to which well over half of the world's cattle population are at risk. There is a critical need to better understand the mechanisms by which these parasites are transmitted, persist, and cause disease in order to optimize methods for control, including development of vaccines. This manuscript presents the genome sequence of B. bovis, and provides a whole genome comparative analysis with P. falciparum and T. parva. Genome-wide characterization of the B. bovis antigenically variable ves1 family reveals interesting differences in organization and expression from the related P. falciparum var genes. The second largest gene family (smorf) in B. bovis was newly discovered and may itself be involved in persistence, highlighting the utility of this approach in gene discovery. Organization and structure of the B. bovis genome is most similar to that of Theileria, and despite common features in clinical outcome is limited to microregional similarity with P. falciparum. Comparative gene analysis identifies several previously unknown proteins as homologs of vaccine candidates in one or more of these parasites, and candidate genes whose expression might account for unique properties such as the ability of Theileria to reversibly transform leukocytes.

New insights into the altered adhesive and mechanical properties of red blood cells parasitized by Babesia bovis

Sequestration of parasite-infected red blood cells (RBCs) in the microvasculature is an important pathological feature of both bovine babesiosis caused by Babesia bovis and human malaria caused by Plasmodium falciparum. Surprisingly, when compared with malaria, the cellular and molecular mechanisms that underlie this abnormal circulatory behaviour for RBCs infected with B. bovis have been relatively ignored. Here, we present some novel insights into the adhesive and mechanical changes that occur in B. bovis-infected bovine RBCs and compare them with the alterations that occur in human RBCs infected with P. falciparum. After infection with B. bovis, bovine RBCs become rigid and adhere to vascular endothelial cells under conditions of physiologically relevant flow. These alterations are accompanied by the appearance of ridge-like structures on the RBC surface that are analogous, but morphologically and biochemically different, to the knob-like structures on the surface of human RBCs infected with P. falciparum. Importantly, albeit for a limited number of parasite lines examined here, the extent of these cellular and rheological changes appear to be related to parasite virulence. Future investigations to identify the precise molecular composition of ridges and the proteins that mediate adhesion will provide important insight into the pathogenesis of both babesiosis and malaria.

Antigenic variation by protozoan parasites: insights from Babesia bovis

Antigenic variation of surface membrane proteins by protozoan parasites enables these pathogenic organisms to avoid host immune responses and thus perpetuate long-term infections. Babesia bovis, the causative agent of severe babesiosis in cattle, was previously shown to undergo antigenic variation through modifications to its primary surface antigen, a protein called VESA1. In this issue, Al-Khedery and Allred provide a detailed description of the genes that encode VESA1 and present convincing evidence for progressive, segmental gene conversion in the generation of variant forms of this surface antigen.

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.

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

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

Antigenic variation as an exploitable weakness of babesial parasites

Babesia bovis and its bovine host interact in many ways, resulting in a range of disease and infection phenotypes. Host responses to the parasite elicit or select for a variety of responses on the part of the parasite, the full range of which is not yet known. One well-established phenomenon, thought to aid parasite survival by evasion of host adaptive immune responses, is the sequential expansion of antigenically variant populations during an infection, a phenomenon referred to as "antigenic variation". Antigenic variation in B. bovis, like that in the human malarial parasite, Plasmodium falciparum, is intimately linked to a second survival mechanism, cytoadhesion. In cytoadhesion, mature parasite-containing erythrocytes bind to the capillary and post-capillary venous endothelium through parasite-derived ligands. The reliance of these parasites on both functions, and on their linkage, may provide opportunities to develop anti-babesial and, perhaps, anti-malarial protection strategies. The development of inhibitors of DNA metabolism in B. bovis may be used to abrogate the process of antigenic variation, whereas small molecular mimics may provide the means to vaccinate against a wide range of variants or to prevent the surface export of variant antigen ligands. In this article, aspects of antigenic variation and cytoadhesion in bovine babesiosis are explored, with a discussion of opportunities for prophylactic or therapeutic intervention in these intertwined processes.

Antigenic variation inBabesia bovisoccurs through segmental gene conversion of thevesmultigene family, within a bidirectional locus of active transcription

Antigenic variation in Babesia bovis is one aspect of a multifunctional virulence/survival mechanism mediated by the heterodimeric variant erythrocyte surface antigen 1 (VESA1) protein that also involves endothelial cytoadhesion with sequestration of mature parasitized erythrocytes. The ves1alpha gene encoding the VESA1a subunit was previously identified. In this study, we present the unique organization of the genomic locus from which ves1alpha is transcribed, and identify a novel branch of the ves multigene family, ves1beta. These genes are found together, closely juxtaposed and divergently oriented, at the locus of active transcription. We provide compelling evidence that variation of both transcriptionally active genes occurs through a mechanism of segmental gene conversion involving sequence donor genes of similar organization. These results also suggest the possibility of epigenetic regulation through in situ switching among gene loci, further expanding the potential repertoire of variant proteins.

Cellular adhesive phenomena in apicomplexan parasites of red blood cells

The apicomplexan parasites Babesia and Plasmodium are related, yet phylogenetically distinct haemoprotozoa that infect red blood cells and cause severe diseases of major human and veterinary importance. A variety of cellular and molecular interactions are pivotal in many aspects of the pathogenicity of these two parasites. Comparison of the cellular and molecular mechanisms that culminate in accumulation of parasitised red blood cells in the microvasculature of cattle infected with Babesia bovis (babesiosis) and humans infected with Plasmodium falciparum (falciparum malaria) is particularly instructive given the striking similarities in the pathophysiology of these two important medical and veterinary diseases. While such adhesive phenomena have been studied extensively in malaria, they have received relatively little attention in babesiosis. In this review, we summarise the findings of more than 25 years of research into cellular adhesive phenomena in malaria and speculate on how this body of work can now be applied to Babesia parasites. Such information is fundamental if we are to learn more about the biology of Babesia parasites, the cellular and molecular mechanisms by which they cause infection and disease and how to develop novel therapeutic strategies or vaccines for both Babesia and malaria infections.

Ability of the vector tick Boophilus microplus to acquire and transmit Babesia equi following feeding on chronically infected horses with low-level parasitemia

The protozoan parasite Babesia equi replicates within erythrocytes. During the acute phase of infection, B. equi can reach high levels of parasitemia, resulting in a hemolytic crisis. Horses that recover from the acute phase of the disease remain chronically infected. Subsequent transmission is dependent upon the ability of vector ticks to acquire B. equi and, following development and replication, establishment of B. equi in the salivary glands. Although restriction of the movement of chronically infected horses with B. equi is based on the presumption that ticks can acquire and transmit the parasite at low levels of long-term infection, parasitemia levels during the chronic phase of infection have never been quantified, nor has transmission been demonstrated. To address these epidemiologically significant questions, we established long-term B. equi infections (>1 year), measured parasitemia levels over time, and tested whether nymphal Boophilus microplus ticks could acquire and, after molting to the adult stage, transmit B. equi to naive horses. B. equi levels during the chronic phase of infection ranged from 10(3.3) to 10(6.0)/ml of blood, with fluctuation over time within individual horses. B. microplus ticks fed on chronically infected horses with mean parasite levels of 10(5.5) +/- 10(0.48)/ml of blood acquired B. equi, with detection of B. equi in the salivary glands of 7 to 50% of fed ticks, a range encompassing the percentage of positive ticks that had been identically fed on a horse in the acute phase of infection with high parasitemia levels. Ticks that acquired B. equi from chronically infected horses, as well as those fed during the acute phase of infection, successfully transmitted the parasite to naive horses. The results unequivocally demonstrated that chronically infected horses with low-level parasitemia are competent mammalian reservoirs for tick transmission of B. equi.

Antigenic variation and cytoadhesion in Babesia bovis and Plasmodium falciparum: different logics achieve the same goal

Babesia bovis is a protozoal hemoparasite of cattle which behaves in certain crucial respects like Plasmodium falciparum, despite being phylogenetically distant and having many differences in its life cycle. The shared behavioral attributes of rapid antigenic variation and cytoadhesion/sequestration are thought to contribute significantly to immune evasion, establishment of persistent infections, and disease pathology. Although differing in their genetic and biochemical strategies for achieving these behaviors, information from studies of each parasite may further our understanding of the overall host-parasite interaction. In this review we contrast the molecular basis and 'genetic logic' for these critical behaviors in the two parasites, with emphasis on the biology of B. bovis.

Babesiosis: persistence in the face of adversity

Many babesial parasites establish infections of long duration in immune hosts. Among different species, at least four mechanisms are known that could facilitate evasion of the host immune response, although no one species is (yet) known to use them all. This update strives to illustrate the ramifications of these mechanisms and the interplay between them.

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.

Molecular technology and antigenic variation among intraerythrocytic hemoparasites: do we see reality?

Antigenic variation is one mechanism of immune evasion utilized by many microorganisms--encompassing such broad evolutionary groups as viruses, bacteria, and protozoa--to survive the onslaught of a specifically activated host immune system. Because of its importance to the survival of many infectious agents there is considerable interest in understanding this phenomenon. With knowledge of the molecular mechanisms by which these microbes deliberately manipulate their genomes, it may be possible to disrupt the molecular machinery of the responsible genetic mechanisms. Among intraerythrocytic parasites, genetic mechanisms that have been observed or postulated to control antigenic variation include segmental gene conversion, epigenetically controlled in situ transcriptional switching, alterations of chromosomal structure associated with transcriptional control, and recombination during sexual reproduction. Likely, more than one type of mechanism is used by all organisms that undergo antigenic variation. In this paper, both the observed mechanisms and some of the molecular technology used to detect these mechanisms are discussed. While often seemingly straightforward from a technical standpoint, sometimes subtle differences in the methods used to study this process may affect what is observed. Some examples of this phenomenon are discussed in the context of a small selection of intraerythrocytic parasites.

Antigenic variation in babesiosis:is there more than one ˈwhyˈ?

Many intraerythrocytic hemoparasites survive the host immune system through rapid antigenic variation. Among babesial parasites antigenic variation has been demonstrated convincingly only for Babesia bovis and Babesia rodhaini. The molecular basis for antigenic variation in babesial parasites and its possible connection with cytoadherence and sequestration are discussed.

The malaria-infected red blood cell: structural and functional changes

The asexual stage of malaria parasites of the genus Plasmodium invade red blood cells of various species including humans. After parasite invasion, red blood cells progressively acquire a new set of properties and are converted into more typical, although still simpler, eukaryotic cells by the appearance of new structures in the red blood cell cytoplasm, and new proteins at the red blood cell membrane skeleton. The red blood cell undergoes striking morphological alterations and its rheological properties are considerably altered, manifesting as red blood cells with increased membrane rigidity, reduced deformability and increased adhesiveness for a number of other cells including the vascular endothelium. Elucidation of the structural changes in the red blood cell induced by parasite invasion and maturation and an understanding of the accompanying functional alterations have the ability to considerably extend our knowledge of structure-function relationships in the normal red blood cell. Furthermore, interference with these interactions may lead to previously unsuspected means of reducing parasite virulence and may lead to the development of novel antimalarial therapeutics.

Selection of Babesia bovis-infected erythrocytes for adhesion to endothelial cells coselects for altered variant erythrocyte surface antigen isoforms

Sequestration of Babesia bovis-infected erythrocytes (IRBCs) in the host microvasculature is thought to constitute an important mechanism of immune evasion. Since Ig is considered to be important for protection from disease, an in vitro assay of B. bovis sequestration was used to explore the ability of anti-B. bovis Ig to interfere with IRBC cytoadhesion, and to identify IRBC surface Ags acting as endothelial cell receptors. Bovine infection sera reactive with the IRBC surface inhibited and even reversed the binding of IRBCs to bovine brain capillary endothelial cells (BBECs). This activity is at least partially attributable to serum IgG. IgG isolated from inhibitory serum captured the variant erythrocyte surface ag 1 (VESA1) in surface-specific immunoprecipitations of B. bovis-IRBCs. Selection for the cytoadhesive phenotype concurrently selected for antigenic and structural changes in the VESA1 Ag. In addition, the anti-VESA1 mAb, 4D9.1G1, proved capable of effectively inhibiting and reversing binding of adhesive, mAb-reactive parasites to BBECs, and by immunoelectron microscopy localized VESA1 to the external tips of the IRBC membrane knobs. These data are consistent with a link between antigenic variation and cytoadherence in B. bovis and suggest that the VESA1 Ag acts as an endothelial cell ligand on the B. bovis-IRBC.

The ves multigene family of B. bovis encodes components of rapid antigenic variation at the infected erythrocyte surface

B. bovis, an intraerythrocytic protozoal parasite, establishes chronic infections in cattle in part through rapid variation of the polymorphic, heterodimeric VESA1 protein on the infected erythrocyte surface and sequestration of mature parasites. We describe the characterization of the ves1 alpha gene encoding the VESA1a subunit, thus providing a description of a gene whose product is involved in rapid antigenic variation in a babesial parasite. This three-exon gene, a member of a multigene family (ves), encodes a polypeptide with no cleavable signal sequence, a single predicted transmembrane segment, and a cysteine/lysine-rich domain. Variation appears to involve creation and modification or loss of a novel, transcribed copy of the gene.

Antigenic variation in vector-borne pathogens

Several pathogens of humans and domestic animals depend on hematophagous arthropods to transmit them from one vertebrate reservoir host to another and maintain them in an environment. These pathogens use antigenic variation to prolong their circulation in the blood and thus increase the likelihood of transmission. By convergent evolution, bacterial and protozoal vector-borne pathogens have acquired similar genetic mechanisms for successful antigenic variation. Borrelia spp. and Anaplasma marginale (among bacteria) and African trypanosomes, Plasmodium falciparum, and Babesia bovis (among parasites) are examples of pathogens using these mechanisms. Antigenic variation poses a challenge in the development of vaccines against vector-borne pathogens.

A polymorphic multigene family encoding an immunodominant protein from Babesia microti

Human babesiosis in the United States is caused predominantly by Babesia microti, a tick-transmitted blood parasite. Improved testing methods for the detection of infection with this parasite are needed, since asymptomatic B. microti infection represents a potential threat to the blood supply in areas where B. microti is endemic. We performed immunoscreening of an expression library of genomic DNA from a human isolate of B. microti (strain MN1). Among 17 unique immunoreactive clones, we identified 9 which represent a related family of genes with little sequence homology to other known sequences but with an architecture resembling that of several surface proteins of Plasmodium. Within this family, a tandem array of a degenerate six-amino-acid repeat (SEAGGP, SEAGWP, SGTGWP, SGTVGP) was found in various lengths between relatively well conserved segments at the N and C termini. In order to examine within-clone variation, we developed a PCR protocol for direct recovery of a specific bmn1-6 homologue directly from 30 human blood isolates, 4 corresponding hamster isolates, and 5 geographically corresponding Peromyscus leucopus (white-footed mouse) isolates. Isolates from the hamsters had the same sequences as those found in the corresponding human blood, suggesting that genetic variation of bmn1-6 does not occur during passage. However, clones from different patients were often substantially different from each other with regard to the number and location of the degenerate repeats within the bmn1-6 homologue. Moreover, we found that strains that were closely related geographically were also closely related at the sequence level; nine patients, all from Nantucket Island, Mass., harbored clones that were indistinguishable from each other but that were distinct from those found in other northeastern or upper midwestern strains. We conclude that considerable genetic and antigenic diversity exists among isolates of B. microti from the United States and that geographic clustering of subtypes may exist. The nature of the bmn1-6 gene family suggests a mechanism of antigenic variation in B. microti that may occur by recombination, differential expression, or a combination of both mechanisms.

Cytoadherence of Babesia bovis-infected erythrocytes to bovine brain capillary endothelial cells provides an in vitro model for sequestration

Babesia bovis, an intraerythrocytic parasite of cattle, is sequestered in the host microvasculature, a behavior associated with cerebral and vascular complications of this disease. Despite the importance of this behavior to disease etiology, the underlying mechanisms have not yet been investigated. To study the components involved in sequestration, B. bovis parasites that induce adhesion of the infected erythrocytes (IRBCs) to bovine brain capillary endothelial cells (BBEC) in vitro were isolated. Two clonal lines, CD7(A+I+) and CE11(A+I-), were derived from a cytoadherent, monoclonal antibody 4D9.1G1-reactive parasite population. This antibody recognizes a variant, surface-exposed epitope of the variant erythrocyte surface antigen 1 (VESA1) of B. bovis IRBCs. Both clonal lines were cytoadhesive to BBEC and two other bovine endothelial cell lines but not to COS7 cells, FBK-4 cells, C32 melanoma cells, or bovine brain pericytes. By transmission electron microscopy, IRBCs were observed to bind to BBEC via the knobby protrusions on the IRBC surface, indicating involvement of components associated with these structures. Inhibition of protein export in intact, trypsinized IRBCs ablated both erythrocyte surface reexpression of parasite protein and cytoadhesion. IRBCs allowed to recover surface antigen expression regained the ability to bind endothelial cells, demonstrating that parasite protein export is required for cytoadhesion. We propose the use of this assay as an in vitro model to study the components involved in B. bovis cytoadherence and sequestration.

A model for the sequential dominance of antigenic variants in African trypanosome infections

Trypanosoma brucei infects various domestic and wild mammals in equatorial Africa. The parasite's genome contains several hundred alternative and highly diverged surface antigens, of which only a single one is expressed in any cell. Individual cells occasionally change expression of their surface antigen, allowing them to escape immune surveillance. These switches appear to occur in a partly random way, creating a diverse set of antigenic variants. In spite of this diversity, the parasitaemia develops as a series of outbreaks, each outbreak dominated by relatively few antigenic types. Host-specific immunity eventually clears the dominant antigenic types and a new outbreak follows from antigenic types that have apparently been present all along at low frequency. This pattern of sequential dominance by different antigenic types remains unexplained. I use a mathematical model of parasitaemia and host immunity to show that small variations in the rate at which each type switches to other types can explain the observations. My model shows that randomly chosen switch rates do not provide sufficiently ordered parasitaemias to match the observations. Instead, minor modifications of switch rates by natural selection are required to develop a sequence of ordered parasitaemias.

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.