Comparative transcriptome analysis of geographically distinct virulent and attenuated Babesia bovis strains reveals similar gene expression changes through attenuation

Chromosome-level genome assembly of Babesia caballi reveals diversity of multigene families among Babesia species

BACKGROUND

Babesia caballi is an intraerythrocytic parasite from the phylum Apicomplexa, capable of infecting equids and causing equine piroplasmosis. However, since there is limited genome information available on B. caballi, molecular mechanisms involved in host specificity and pathogenicity of this species have not been fully elucidated yet.

RESULTS

Genomic DNA from a B. caballi subclone was purified and sequenced using both Illumina and Nanopore technologies. The resulting assembled sequence consisted of nine contigs with a size of 12.9 Mbp, rendering a total of 5,910 protein-coding genes. The phylogenetic tree of Apicomplexan species was reconstructed using 263 orthologous genes. We identified 481 ves1-like genes and named "ves1c". In contrast, expansion of the major facilitator superfamily (mfs) observed in closely related B. bigemina and B. ovata species was not found in B. caballi. A set of repetitive units containing an open reading frame with a size of 297 bp was also identified.

CONCLUSIONS

We present a chromosome-level genome assembly of B. caballi. Our genomic data may contribute to estimating gene expansion events involving multigene families and exploring the evolution of species from this genus.

Whole genome sequence and diversity in multigene families of Babesia ovis

Ovine babesiosis, caused by Babesia ovis, is an acute, lethal, and endemic disease worldwide and causes a huge economic loss to animal industry. Pathogen genome sequences can be utilized for selecting diagnostic markers, drug targets, and antigens for vaccine development; however, those for B. ovis have not been available so far. In this study, we obtained a draft genome sequence for B. ovis isolated from an infected sheep in Turkey. The genome size was 7.81 Mbp with 3,419 protein-coding genes. It consisted of 41 contigs, and the N50 was 526 Kbp. There were 259 orthologs identified among eight Babesia spp., Plasmodium falciparum, and Toxoplasma gondii. A phylogeny was estimated on the basis of the orthologs, which showed B. ovis to be closest to B. bovis. There were 43 ves genes identified using hmm model as well. They formed a discriminating cluster to other ves multigene family of Babesia spp. but showed certain similarities to those of B. bovis, B. caballi, and Babesia sp. Xinjiang, which is consistent with the phylogeny. Comparative genomics among B. ovis and B. bovis elucidated uniquely evolved genes in these species, which may account for the adaptation.

Babesia divergens egress from host cells is orchestrated by essential and druggable kinases and proteases

Apicomplexan egress from host cells is fundamental to the spread of infection and is poorly characterized in Babesia spp., parasites of veterinary importance and emerging zoonoses. Through the use of video microscopy, transcriptomics and chemical genetics, we have implicated signaling, proteases and gliding motility as key drivers of egress by Babesia divergens. We developed reverse genetics to perform a knockdown screen of putative mediators of egress, identifying kinases and proteases involved in distinct steps of egress (ASP3, PKG and CDPK4) and invasion (ASP2, ASP3 and PKG). Inhibition of egress leads to continued intracellular replication, indicating exit from the replication cycle is uncoupled from egress. Chemical genetics validated PKG, ASP2 and ASP3 as druggable targets in Babesia spp. All taken together, egress in B. divergens more closely resembles T. gondii than the more evolutionarily-related Plasmodium spp. We have established a molecular framework for biological and translational studies of B. divergens egress.

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

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

A novel promising diagnostic candidate selected by screening the transcriptome of Babesia gibsoni (Wuhan isolate) asexual stages in infected beagles

BACKGROUND

Babesia gibsoni is one of the causative agents of canine babesiosis worldwide. Some dogs infected with B. gibsoni show severe clinical signs with progressive anemia, hemoglobinuria and splenomegaly. However, most infected dogs present a state of chronic infection and thereby may be a persistent pathogen carrier, increasing the risk of pathogen spreading. To date, little is known about this pathogen, with genomic and transcriptomic data in particular generally unavailable. This lack of knowledge extensively limits the development of effective diagnostic strategies and vaccines.

METHODS

High-throughput RNA sequencing of total RNA of B. gibsoni asexual stages collected from infected beagles was performed. The unigenes were annotated in seven databases. The genes were sorted according to their fragments per kilobase per million (FPKM) value, which was used as an indicator for expression level. The gene with the highest FPKM value was cloned from the genome of B. gibsoni and further tested for immunogenicity, cellular localization and efficacy as a potential diagnostic candidate for detecting B. gibsoni in sera collected from beagles.

RESULTS

A total of 62,580,653 clean reads were screened from the 64,336,475 raw reads, and the corresponding 70,134 transcripts and 36,587 unigenes were obtained. The gene with the highest FPKM value was screened from the unigenes; its full length was 1276 bp, and it was named BgP30. The BgP30 gene comprised three exons and two introns, with a 786-bp open reading frame, and encoded 261 amino acids with a predicted molecular weight of 30 kDa. The cellular localization assay confirmed the existence of P30 protein in B. gibsoni parasites. Moreover, P30 was detected in the serum of experimentally B. gibsoni-infected beagles, from 15 days up to 422 days post-infection, suggesting its usefulness as a diagnostic candidate for both acute and chronic infections.

CONCLUSIONS

We sequenced the transcriptome of B. gibsoni asexual stages for the first time. The BgP30 gene was highly expressed in the transcriptome screening experiments, with further studies demonstrating that it could induce immune response in B. gibsoni-infected dogs. These results lead us to suggest that bgP30 may be a good diagnostic candidate marker to detect both acute and chronic B. gibsoni infections.

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.

Comparative genomic analysis of Babesia duncani responsible for human babesiosis

Background

Human babesiosis, caused by parasites of the genus Babesia, is an emerging and re-emerging tick-borne disease that is mainly transmitted by tick bites and infected blood transfusion. Babesia duncani has caused majority of human babesiosis in Canada; however, limited data are available to correlate its genomic information and biological features.

Results

We generated a B. duncani reference genome using Oxford Nanopore Technology (ONT) and Illumina sequencing technology and uncovered its biological features and phylogenetic relationship with other Apicomplexa parasites. Phylogenetic analyses revealed that B. duncani form a clade distinct from B. microti, Babesia spp. infective to bovine and ovine species, and Theileria spp. infective to bovines. We identified the largest species-specific gene family that could be applied as diagnostic markers for this pathogen. In addition, two gene families show signals of significant expansion and several genes that present signatures of positive selection in B. duncani, suggesting their possible roles in the capability of this parasite to infect humans or tick vectors.

Conclusions

Using ONT sequencing and Illumina sequencing technologies, we provide the first B. duncani reference genome and confirm that B. duncani forms a phylogenetically distinct clade from other Piroplasm parasites. Comparative genomic analyses show that two gene families are significantly expanded in B. duncani and may play important roles in host cell invasion and virulence of B. duncani. Our study provides basic information for further exploring B. duncani features, such as host-parasite and tick-parasite interactions.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12915-022-01361-9.

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.

Thrombospondin Related Anonymous Protein Superfamily in Vector-Borne Apicomplexans: The Parasite’s Toolkit for Cell Invasion

Apicomplexan parasites transmitted by vectors, including Babesia spp. and Plasmodium spp., cause severe disease in both humans and animals. These parasites have a complex life cycle during which they migrate, invade, and replicate in contrasting hosts such as the mammal and the invertebrate vector. The interaction of parasites with the host cell is mediated by adhesive proteins which play a key role in the different cellular processes regarding successful progression of the life cycle. Thrombospondin related anonymous protein (TRAP) is a superfamily of adhesins that are involved in motility, invasion and egress of the parasite. These proteins are stored and released from apical organelles and have either one or two types of adhesive domains, namely thrombospondin type 1 repeat and von Willebrand factor type A, that upon secretion are located in the extracellular portion of the molecule. Proteins from the TRAP superfamily have been intensively studied in Plasmodium species and to a lesser extent in Babesia spp., where they have proven to be functionally relevant throughout the entire parasite’s journey both in the arthropod vector and in the mammalian host. In recent years new findings provided answers to the role of TRAP proteins and in some cases the function of these adhesins during the parasite’s life cycle was redefined. In this review we will discuss the current knowledge of the diverse roles of the TRAP superfamily in vector-borne parasites from Class Aconoidasida. We will focus on the varied approaches that allowed the understanding of protein function and the relevance of TRAP- superfamily throughout the entire parasite’s cell cycle.

A Culture-Adapted Strain of Babesia bovis Has Reduced Subpopulation Complexity and Is Unable to Complete Its Natural Life Cycle in Ticks

Babesia bovis natural field strains are composed of several geno-phenotypically distinct subpopulations. This feature, together with possible epigenetic modifications, may facilitate adaptation to variable environmental conditions. In this study we compare geno-phenotypical features among long-term (more than 12 years) (LTCP) and short-term cultured B. bovis parasites (STCP) derived from the B. bovis S74-T3Bo strain. LTCPs intraerythrocytic forms are smaller in size than STCPs and have faster in vitro growth rate. In contrast to its parental strain, the LTCP lack expression of the sexual stage specific 6cysA and 6cysB proteins and are unable to develop sexual forms upon in vitro sexual stage induction. Consistently, in contrast to its parental strain, LTCPs have reduced virulence and are not transmissible to cattle by vector competent Rhipicephalus microplus (R. microplus). Similar to previous comparisons among attenuated and virulent B. bovis strains, the LTCP line has decreased genomic diversity compared to the STCP line. Thus, LTCP may contribute to our understanding of adaptive mechanisms used by the parasites in response to environmental changes, protective immunity, virulence, and transmission by ticks. In addition, LTCPs may be considered as candidates for a non-tick transmissible vaccine against bovine babesiosis.

Complete genomes of the eukaryotic poultry parasite Histomonas meleagridis: linking sequence analysis with virulence / attenuation

Background

Histomonas meleagridis is a protozoan parasite and the causative agent of histomonosis, an important poultry disease whose significance is underlined by the absence of any treatment and prophylaxis. The recent successful in vitro attenuation of the parasite urges questions about the underlying mechanisms.

Results

Whole genome sequence data from a virulent and an attenuated strain originating from the same parental lineage of H. meleagridis were recruited using Oxford Nanopore Technology (ONT) and Illumina platforms, which were combined to generate megabase-sized contigs with high base-level accuracy. Inspecting the genomes for differences identified two substantial deletions within a coding sequence of the attenuated strain. Additionally, one single nucleotide polymorphism (SNP) and indel targeting coding sequences caused the formation of premature stop codons, which resulted in the truncation of two genes in the attenuated strain. Furthermore, the genome of H. meleagridis was used for characterizing protein classes of clinical relevance for parasitic protists. The comparative analysis with the genomes of Trichomonas vaginalis, Tritrichomonas foetus and Entamoeba histolytica identified ~ 2700 lineage-specific gene losses and 9 gene family expansions in the H. meleagridis lineage.

Conclusions

Taken as a whole, the obtained data provide the first hints to understand the molecular basis of attenuation in H. meleagridis and constitute a genomics platform for future research on this important poultry pathogen.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-021-08059-2.

Congenital Transmission of Apicomplexan Parasites: A Review

Apicomplexans are a group of pathogenic protists that cause various diseases in humans and animals that cause economic losses worldwide. These unicellular eukaryotes are characterized by having a complex life cycle and the ability to evade the immune system of their host organism. Infections caused by some of these parasites affect millions of pregnant women worldwide, leading to various adverse maternal and fetal/placental effects. Unfortunately, the exact pathogenesis of congenital apicomplexan diseases is far from being understood, including the mechanisms of how they cross the placental barrier. In this review, we highlight important aspects of the diseases caused by species of Plasmodium, Babesia, Toxoplasma, and Neospora, their infection during pregnancy, emphasizing the possible role played by the placenta in the host-pathogen interaction.

Recent Advances in Molecular Genetic Tools for Babesia

Development of in vitro culture and completion of genome sequencing of several Babesia parasites promoted the efforts to establish transfection systems for these parasites to dissect the gene functions. It has been more than a decade since the establishment of first transfection for Babesia bovis, the causative agent of bovine babesiosis. However, the number of genes that were targeted by genetic tools in Babesia parasites is limited. This is partially due to the low efficiencies of these methods. The recent adaptation of CRISPR/Cas9 for genome editing of Babesia bovis can accelerate the efforts for dissecting this parasite’s genome and extend the knowledge on biological aspects of erythrocytic and tick stages of Babesia. Additionally, glmS ribozyme as a conditional knockdown system is available that could be used for the characterization of essential genes. The development of high throughput genetic tools is needed to dissect the function of multigene families, targeting several genes in a specific pathway, and finally genome-wide identification of essential genes to find novel drug targets. In this review, we summarized the current tools that are available for Babesia and the genes that are being targeted by these tools. This may draw a perspective for the future development of genetic tools and pave the way for the identification of novel drugs or vaccine targets.

Epidemiology, haematology and molecular characterization of haemoprotozoon and rickettsial organisms causing infections in cattle of Jammu region, North India

BACKGROUND

The present study was aimed at establishing the prevalence, epidemiology and molecular characterization of major haemoprotozoons (Babesia and Theileria) and rickettsia (Anaplasma) of cattle in Jammu region (North India) using microscopy and Polymerase Chain Reaction (PCR). Hematology, microscopy and PCR based prevalence studies were undertaken with 278 whole blood samples from cattle. Molecular prevalence studies were followed by genetic characterization of the isolates of Babesia, Anaplasma and Theileria spp. based on 18S rRNA, 16S rRNA and Tams1 gene, respectively. The data related to metrology and epidemiological variables like temperature, rainfall, season, age and type of livestock rearing was analyzed and correlated with occurrence of disease by statistical methods.

RESULTS

The prevalence based on microscopy was 12.9% (36/278) whereas PCR recorded 30.22% (84/278) animals positive for haemoparasitic infections. All the samples found positive by microscopy were also recorded positive by PCR. Thus the study revealed prevalence of Babesia bigemina, Anaplasma marginale and Theileria annulata to be 9.7, 16.5 and 0.7% respectively. The metrological and epidemiological variables made inroads for the propagation of vector ticks and occurrence of infection. Haematological alterations predominantly related to decrease in haemoglobin, red blood cell count and packed cell volume were evident in diseased animals and collaterally affected the productivity. Further the genetic characterization of Babesia bigemina. (MN566925.1, MN567603, MN566924.1), Anaplasma marginale. (MH733242.1, MN567602.1) and Theileria annulata (MT113479) provided a representative data of the isolates circulating in the region and their proximity with available sequences across the world.

CONCLUSIONS

Despite holding much significance to the animal sector, comprehensive disease mapping has yet not been undertaken in several parts of India. The present study provides a blue print of disease mapping, epidemiological correlations and genomic diversity of Babesia bigemina, Anaplasma marginale and Theileria annulata circulating in the region.

The key to egress? Babesia bovis perforin-like protein 1 (PLP1) with hemolytic capacity is required for blood stage replication and is involved in the exit of the parasite from the host cell

Bovine babesiosis is a tick-borne disease caused by apicomplexan parasites of the Babesia genus that represents a major constraint to livestock production worldwide. Currently available vaccines are based on live parasites which have archetypal limitations. Our goal is to identify candidate antigens so that new and effective vaccines against Babesia may be developed. The perforin-like protein (PLP) family has been identified as a key player in cell traversal and egress in related apicomplexans and it was also identified in Babesia, but its function in this parasite remains unknown. The aim of this work was to define the PLP family in Babesia and functionally characterize PLP1, a representative member of the family in Babesia bovis. Bioinformatic analyses demonstrate a variable number of plp genes (four to eight) in the genomes of six different Babesia spp. and conservation of the family members at the secondary and tertiary structure levels. We demonstrate here that Babesia PLPs contain the critical domains present in other apicomplexan PLPs to display the lytic capacity. We then focused on the functional characterization of PLP1 of B. bovis, both in vitro and in vivo. PLP1 is expressed and exposed to the host immune system during infection and has high hemolytic capacity under a wide range of conditions in vitro. A B. bovis plp1 knockout line displayed a decreased growth rate in vitro compared with the wild type strain and a peculiar phenotype consisting of multiple parasites within a single red blood cell, although at low frequency. This phenotype suggests that the lack of PLP1 has a negative impact on the mechanism of egression of the parasite and, therefore, on its capacity to proliferate. It is possible that PLP1 is associated with other proteins in the processes of invasion and egress, which were found to have redundant mechanisms in related apicomplexans. Future work will be focused on unravelling the network of proteins involved in these essential parasite functions.

A Comparative Genomic Study of Attenuated and Virulent Strains of Babesia bigemina

Cattle babesiosis is a socio-economically important tick-borne disease caused by Apicomplexa protozoa of the genus Babesia that are obligate intraerythrocytic parasites. The pathogenicity of Babesia parasites for cattle is determined by the interaction with the host immune system and the presence of the parasite’s virulence genes. A Babesia bigemina strain that has been maintained under a microaerophilic stationary phase in in vitro culture conditions for several years in the laboratory lost virulence for the bovine host and the capacity for being transmitted by the tick vector. In this study, we compared the virulome of the in vitro culture attenuated Babesia bigemina strain (S) and the virulent tick transmitted parental Mexican B. bigemina strain (M). Preliminary results obtained by using the Basic Local Alignment Search Tool (BLAST) showed that out of 27 virulence genes described and analyzed in the B. bigemina virulent tick transmitted strain, only five were fully identified in the attenuated laboratory strain. In all cases, the identity and coverture of the identified genes of the wildtype strain were higher than those of the laboratory strain. This finding is putatively associated with the continuous partial loss of virulence genes in the laboratory strain after several passages of the parasite population under optimal in vitro growth conditions. The loss of virulence factors might be reflected in the absence of symptoms of the disease in cattle inoculated with the attenuated strain despite the presence of infection in the bovine host cells.

The repertoire of serine rhomboid proteases of piroplasmids of importance to animal and human health

Babesia, Theileria and Cytauxzoon are tick-borne apicomplexan protozoans of the order Piroplasmida, notorious for the diseases they cause in livestock, pets and humans. Host cell invasion is their Achilles heel, allowing for the development of drug or vaccine-based therapies. In other apicomplexans, cleavage of the transmembrane domain of adhesins by the serine rhomboid proteinase ROM4 is required for successful completion of invasion. In this study, we record and classify the rhomboid repertoire encoded in the genomes of 10 piroplasmid species pertaining to the lineages Babesia sensu stricto (s.s., Clade VI), Theileria sensu stricto (Clade IV), Theileria equi (Clade IV), Cytauxzoon felis (Clade IIIb) and Babesia microti (Clade I), as defined by Schnittger et al. (2012). Fifty-six piroplasmid rhomboid-like proteins were assigned by phylogenetic analysis and bidirectional best hit to the ROM4, ROM6, ROM7 or ROM8 groups, and their crucial motifs for conformation and function were identified. Forty-four of these rhomboids had either been incorrectly classified or misannotated. Babesia s.s. encode five or three ROM4 proteinase paralogs, whereas the remaining piroplasmids encode two ROM4 paralogs. All piroplasmids encode a single ROM6, ROM7 and ROM8. Thus, an increased paralog number of ROM4 is the only feature distinguishing Babesia s.s. from other piroplasmid lineages. Piroplasmid ROM6 is related to the mammalian mitochondrial rhomboid and, accordingly, N-terminal mitochondrial targeting signal sequences was found in some cases. ROM6 is the only rhomboid encoded by piroplasmids that is ubiquitous in other organisms. ROM8 represents a pseudoproteinase that is highly conserved between studied piroplasmids, suggesting that it is important in regulatory functions. ROM4, ROM6, ROM7 and ROM8 are exclusively present in Aconoidasida, which comprises piroplasmids and Plasmodium, suggesting a relevant functional role in erythrocyte invasion. The correct classification and designation of piroplasmid rhomboids presented in this study facilitates an informed choice for future in-depth study of their functions.

A framework for signaling throughout the life cycle of Babesia species

Babesia species are tick-borne intracellular parasites that infect the red blood cells of their mammalian host, leading to severe or fatal disease. Babesia spp. infect a wide range of mammalian species and cause a significant economic burden globally, predominantly through disease in cattle. Several Babesia spp. are increasingly being recognized as zoonotic pathogens of humans. Babesia spp. have complex life cycles involving multiple stages in the tick and the mammalian host. The parasite utilizes complex signaling pathways during replication, egress, and invasion in each of these stages. They must also rapidly respond to their environment when switching between the mammalian and tick stages. This review will focus on the signaling pathways and environmental stimuli that Babesia spp. utilize in the bloodstream and for transmission to the tick, with an emphasis on the role of phosphorylation- and calcium-based signaling during egress and invasion. The expanding availability of in vitro and in vivo culture systems, genomes, transcriptomes, and transgenic systems available for a range of Babesia spp. should encourage further biological and translational studies of these ubiquitous parasites.

Bovine Babesiosis in Turkey: Impact, Current Gaps, and Opportunities for Intervention

Bovine babesiosis is a global tick-borne disease that causes important cattle losses and has potential zoonotic implications. The impact of bovine babesiosis in Turkey remains poorly characterized, but several Babesia spp., including B. bovis, B. bigemina, and B. divergens, among others and competent tick vectors, except Rhipicephalus microplus, have been recently identified in the country. Bovine babesiosis has been reported in all provinces but is more prevalent in central and highly humid areas in low and medium altitude regions of the country housing approximately 70% of the cattle population. Current control measures include acaricides and babesicidal drugs, but not live vaccines. Despite the perceived relevant impact of bovine babesiosis in Turkey, basic research programs focused on developing in vitro cultures of parasites, point-of-care diagnostic methods, vaccine development, “omics” analysis, and gene manipulation techniques of local Babesia strains are scarce. Additionally, no effective and coordinated control efforts managed by a central animal health authority have been established to date. Development of state-of-the-art research programs in bovine babesiosis to address current gaps in knowledge and implementation of long-term plans to control the disease will surely result in important economic, nutritional, and public health benefits for the country and the region.

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

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

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.

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.

Use of molecular markers can help to understand the genetic diversity of Babesia bovis

Cattle babesiosis is a tick-borne disease responsible for significant losses for the livestock industries in tropical areas of the world. These piroplasms are under constant control of the host immune system, which lead to a strong selective pressure for arising more virulent or attenuated phenotypes. Aiming to better understand the most critical genetic modifications in Babesia bovis genome, related to virulence, an in silico analysis was performed using DNA sequences from GenBank. Fourteen genes (sbp-2, sbp-4, trap, msa-1, msa-2b, msa-2c, Bv80 (or Bb-1), 18S rRNA, acs-1, ama-1, β-tub, cp-2, p0, rap-1a) related to parasite infection and immunogenicity and ITS region were selected for alignment and comparison of several isolates of Babesia bovis from different geographic regions around the world. Among the 15 genes selected for the study of diversity, only 7 genes (sbp-2, sbp-4, trap, msa-1, msa-2b, msa-2c, Bv80) and the ITS region presented sufficient genetic variation for the studies of phylogeny. Despite this genetic diversity observed into groups, there was not sufficient information available to associate molecular markers with virulence of isolates. However, some genetic groups no were correlated with geographic region what could indicate some typical evolutionary characteristics in the relation between parasite-host. Further studies using these genes in herds presenting diverse clinical conditions are required. The better understanding of evolutionary mechanisms of the parasite may contribute to improve prophylactic and therapeutic measures. In this way, we suggest that genes used in our study are potential markers of virulence and attenuation and have to be analyzed with the use of sequences from animals that present clinical signs of babesiosis and asymptomatic carriers.

Investigation into limiting dilution and tick transmissibility phenotypes associated with attenuation of the S24 vaccine strain

Background

Babesia bovis is the causal agent of Asiatic redwater, transmitted by the pandemic tick Rhipicephalus (Boophilus) microplus. Disease control may target the tick vector using acaricides or anti-tick vaccines, or the parasite using chemoprophylaxis or anti-parasite vaccines. Current anti-parasite vaccines comprise live blood vaccines using attenuated B. bovis strains. Attenuation is attained by rapid passage that may result in different phenotypes such as reduced virulence, non-transmissibility by the tick vector, inability to sequester in the host (lack of limiting dilution) and limited genetic diversity. Attenuation and phenotypes may be linked to selection of subpopulations during rapid passage. The South African B. bovis S24 vaccine strain comprise a subpopulation that present low virulence, non-transmissibility, lack of limiting dilution phenotype and the presence of a single A558 Bv80 allele. The S24 strain could be co-transmitted with a field strain (05-100) suggesting sexual recombination. The present study investigated the change in phenotype for the S24 vaccine strain during rapid passage and co-transmission.

Methods

Vaccine phenotype change during passage as well as co-transmissibility was monitored using Bv80 allele specific PCR, limiting dilution and Illumina-based genome sequencing.

Results

The S24 population could not be rescued from the S16 passage as previously attained suggesting that selection of the S24 vaccine strain was a serendipitous and stochastic event. Passage from S16 to S24 also resulted in loss of the limiting dilution phenotype. Genome sequencing indicated sexual recombination during co-transmission with the 05-100 field strain. Analysis of the recombinant strain indicate that VESA1, smORF and SBP2 family members are present and may be responsible for the limiting dilution phenotypes, while various regions may also be responsible for the tick transmission phenotype.

Conclusions

The molecular basis for tick transmission and limiting dilution phenotypes may be defined in future using selection based on these traits in combination with sexual recombination.

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.

Unravelling the cellular and molecular pathogenesis of bovine babesiosis: is the sky the limit?

The global impact of bovine babesiosis caused by the tick-borne apicomplexan parasites Babesia bovis, Babesia bigemina and Babesia divergens is vastly underappreciated. These parasites invade and multiply asexually in bovine red blood cells (RBCs), undergo sexual reproduction in their tick vectors (Rhipicephalus spp. for B. bovis and B. bigemina, and Ixodes ricinus for B. divergens) and have a trans-ovarial mode of transmission. Babesia parasites can cause acute and persistent infections to adult naïve cattle that can occur without evident clinical signs, but infections caused by B. bovis are associated with more severe disease and increased mortality, and are considered to be the most virulent agent of bovine babesiosis. In addition, babesiosis caused by B. divergens has an important zoonotic potential. The disease caused by B. bovis and B. bigemina can be controlled, at least in part, using therapeutic agents or vaccines comprising live-attenuated parasites, but these methods are limited in terms of their safety, ease of deployability and long-term efficacy, and improved control measures are urgently needed. In addition, expansion of tick habitats due to climate change and other rapidly changing environmental factors complicate efficient control of these parasites. While the ability to cause persistent infections facilitates transmission and persistence of the parasite in endemic regions, it also highlights their capacity to evade the host immune responses. Currently, the mechanisms of immune responses used by infected bovines to survive acute and chronic infections remain poorly understood, warranting further research. Similarly, molecular details on the processes leading to sexual reproduction and the development of tick-stage parasites are lacking, and such tick-specific molecules can be targets for control using alternative transmission blocking vaccines. In this review, we identify and examine key phases in the life-cycle of Babesia parasites, including dependence on a tick vector for transmission, sexual reproduction of the parasite in the midgut of the tick, parasite-dependent invasion and egression of bovine RBCs, the role of the spleen in the clearance of infected RBCs (IRBCs), and age-related disease resistance in cattle, as opportunities for developing improved control measures. The availability of integrated novel research approaches including "omics" (such as genomics, transcriptomics, and proteomics), gene modification, cytoadhesion assays, RBC invasion assays and methods for in vitro induction of sexual-stage parasites will accelerate our understanding of parasite vulnerabilities. Further, producing new knowledge on these vulnerabilities, as well as taking full advantage of existing knowledge, by filling important research gaps should result in the development of next-generation vaccines to control acute disease and parasite transmission. Creative and effective use of current and future technical and computational resources are needed, in the face of the numerous challenges imposed by these highly evolved parasites, for improving the control of this disease. Overall, bovine babesiosis is recognised as a global disease that imposes a serious burden on livestock production and human livelihood, but it largely remains a poorly controlled disease in many areas of the world. Recently, important progress has been made in our understanding of the basic biology and host-parasite interactions of Babesia parasites, yet a good deal of basic and translational research is still needed to achieve effective control of this important disease and to improve animal and human health.

Babesia bovis RON2 contains conserved B-cell epitopes that induce an invasion-blocking humoral immune response in immunized cattle

Background

Babesia bovis belongs to the phylum Apicomplexa and is the major causal agent of bovine babesiosis, the most important veterinary disease transmitted by arthropods. In apicomplexan parasites, the interaction between AMA1 and RON2 is necessary for the invasion process, and it is a target for vaccine development. In B. bovis, the existence of AMA1 has already been reported; however, the presence of a homolog of RON2 is unknown. The aim of this study was to characterize RON2 in B. bovis.

Results

The B. bovis ron2 gene has a similar synteny with the orthologous gene in the B. bigemina genome. The entire ron2 gene was sequenced from different B. bovis strains showing > 99% similarity at the amino acid and nucleotide level among all the sequences obtained, including the characteristic CLAG domain for cytoadherence in the amino acid sequence, as is described in other Apicomplexa. The in silico transcription analysis showed similar levels of transcription between attenuated and virulent B. bovis strains, and expression of RON2 was confirmed by western blot in the B. bovis T3Bo virulent strain. Four conserved peptides, containing predicted B-cell epitopes in hydrophilic regions of the protein, were designed and chemically synthesized. The humoral immune response generated by the synthetic peptides was characterized in bovines, showing that anti-RON2 antibodies against peptides recognized intraerythrocytic merozoites of B. bovis. Only peptides P2 and P3 generated partially neutralizing antibodies that had an inhibitory effect of 28.10% and 21.42%, respectively, on the invasion process of B. bovis in bovine erythrocytes. Consistently, this effect is additive since inhibition increased to 42.09% when the antibodies were evaluated together. Finally, P2 and P3 peptides were also recognized by 83.33% and 87.77%, respectively, of naturally infected cattle from endemic areas.

Conclusions

The data support RON2 as a novel B. bovis vaccine candidate antigen that contains conserved B-cell epitopes that elicit partially neutralizing antibodies.

Electronic supplementary material

The online version of this article (10.1186/s13071-018-3164-2) contains supplementary material, which is available to authorized users.

Up-regulated expression of spherical body protein 2 truncated copy 11 in Babesia bovis is associated with reduced cytoadhesion to vascular endothelial cells

The factors involved in gain or loss of virulence in Babesia bovis are unknown. Spherical body protein 2 truncated copy 11 (sbp2t11) transcripts in B. bovis were recently reported to be a marker of attenuation for B. bovis strains. Increased cytoadhesion of B. bovis-infected red blood cells (iRBC) to vascular endothelial cells is associated with severe disease outcomes and an indicator of parasite virulence. Here, we created a stable B. bovis transfected line over-expressing sbp2t11 to determine whether up-regulation of sbp2t11 is associated with changes in cytoadhesion. This line was designated sbp2t11up and five B. bovis clonal lines were derived from the sbp2t11up line by limiting dilution for characterisation. We compared the ability of iRBCs from the sbp2t11up line and its five derivative clonal lines to adhere to bovine brain endothelial cells, using an in vitro cytoadhesion assay. The same lines were selected for in vitro cytoadhesion and the levels of sbp2t11 transcripts in each selected line were quantified. Our results demonstrate that up-regulation of sbp2t11 is accompanied by a statistically significant reduction in cytoadhesion. Confirmed up-regulation of sbp2t11 in B. bovis concomitant with the reduction of iRBC in vitro cytoadhesion to bovine brain endothelial cell is consistent with our previous finding that up-regulation of sbp2t11 is an attenuation marker in B. bovis and suggests the involvement of sbp2t11 transcription in B. bovis virulence.

Characterization of new microsatellite markers based on the transcriptome sequencing of Clematis finetiana

Background

Clematis is the biggest genus in the family Ranunculaceae with about 300 species. Clematis is also a globally important commercial group of flowers, especially in the United States and European countries. Their petals with different colors and shapes make the genus the “Queen of the Vines”. However, the genomic information and phylogeny of Clematis based on existing molecular studies are limited. In this paper, new microsatellites (SSR) markers were identified from the transcriptome data of C. finetiana obtained using the Illumina paired-end sequencing technology.

Results

Sequences on a total of 71,900 high-quality unigenes with the mean length of 865 bp were produced in this study. There were 6192unigenes annotated and classified into 49 functional sub-groups in three main ontology categories in GO (Gen Ontology) database,14,022 unigenes mapped to COGs (Clusters of Orthologous Groups) database and classified into 25 functional categories, and 21,494 unigenes obtained and divided into 128 pathways of KEGG (Kyoto Encyclopedia of Genes) Database. A total of 7532 SSRs were discovered from 6337 unigenes. We randomly tested 210 primer pairs, of which 52 primer pairs were able to generate specific products, and 19 possessed polymorphism in the 13 wild populations of six species from Clematis, which were used as a test material.

Conclusions

The dataset of C. finetiana transcriptome and the identified new SSR markers will promote genetic research and breeding effort in Clematis.

Electronic supplementary material

The online version of this article (10.1186/s41065-018-0060-x) contains supplementary material, which is available to authorized users.

Mitigated clinical disease in water buffaloes experimentally infected with Babesia bovis

Water buffaloes (Bubalus bubalis) are raised in tropical and subtropical regions of the world, and act as hosts of Babesia bovis parasites and the tick vector Rhipicephalus microplus. As no clinical cases of B. bovis-infection have been reported, we hypothesized that, unlike bovines, water buffaloes respond asymptomatically to an acute infection. To test this hypothesis, we inoculated two groups of 24-month-old Mediterranean breed water buffaloes with 10⁸ erythrocytes infected with two Argentine B. bovis isolates: BboM2P (n = 5) or BboS2P (n = 5). These strains displayed mild (BboM2P) or high (BboS2P) pathogenicity in Bos taurus calves of the same age (n = 5 and n = 1, respectively), when tested in parallel. In water buffaloes, no changes in body temperature were observed with both strains, and no hematocrit changes were detected in BboM2P-inoculated animals. In contrast, in the BboS2P-inoculated water buffalo group significant but relatively minor reductions in haematocrit values were noted compared to the infected bovine. The parasitemia attained in water buffaloes was considerably lower than in bovines and could only be detected by nested PCR, or indirectly via serology, whereas in most bovines, it could also be detected in Giemsa-stained smears under the light microscope. Our results show that water buffaloes present no or significantly mitigated clinical symptoms to B. bovis infections and suggest that they are able to substantially reduce and/or eliminate B. bovis parasites from circulation by an efficient innate immune mechanism.

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.

The Babesia bovis hap2 gene is not required for blood stage replication, but expressed upon in vitro sexual stage induction

Babesia bovis, is a tick borne apicomplexan parasite responsible for important cattle losses globally. Babesia parasites have a complex life cycle including asexual replication in the mammalian host and sexual reproduction in the tick vector. Novel control strategies aimed at limiting transmission of the parasite are needed, but transmission blocking vaccine candidates remain undefined. Expression of HAP2 has been recognized as critical for the fertilization of parasites in the Babesia-related Plasmodium, and is a leading candidate for a transmission blocking vaccine against malaria. Hereby we identified the B. bovis hap2 gene and demonstrated that it is widely conserved and differentially transcribed during development within the tick midgut, but not by blood stage parasites. The hap2 gene was disrupted by transfecting B. bovis with a plasmid containing the flanking regions of the hap2 gene and the GPF-BSD gene under the control of the ef-1α-B promoter. Comparison of in vitro growth between a hap2-KO B. bovis clonal line and its parental wild type strain showed that HAP2 is not required for the development of B. bovis in erythrocytes. However, xanthurenic acid-in vitro induction experiments of sexual stages of parasites recovered after tick transmission resulted in surface expression of HAP2 exclusively in sexual stage induced parasites. In addition, hap2-KO parasites were not able to develop such sexual stages as defined both by morphology and by expression of the B. bovis sexual marker genes 6-Cys A and B. Together, the data strongly suggests that tick midgut stage differential expression of hap2 is associated with the development of B. bovis sexual forms. Overall these studies are consistent with a role of HAP2 in tick stages of the parasite and suggest that HAP2 is a potential candidate for a transmission blocking vaccine against bovine babesiosis.

Babesia bovis, is a tick borne apicomplexan parasite responsible for important cattle losses globally. Babesia parasites have a complex life cycle including asexual replication in the mammalian host and sexual reproduction in the tick vector. Novel control strategies aimed at limiting transmission of the parasite are needed, but transmission blocking vaccine candidates remain undefined. In this study we analyze the conservation and role of the hap2 gene in the erythrocyte stage of the life cycle of the parasite and found that expression of the gene is not required for the development of the parasite in erythrocytic stages, using a hap2 mutated parasite line. In addition, we developed an in vitro system for the induction of sexual forms of B. bovis and found expression of the hap2 gene and surface localization of the protein. However, hap2-KO parasites are unable to develop sexual stages. We concluded that HAP2 is a leading candidate for a transmission blocking vaccine against bovine babesiosis due of the high level of conservation, surface exposure, and specific expression in tick stage and in in vitro induced sexual stages parasites.

New approaches and omics tools for mining of vaccine candidates against vector-borne diseases

Vector-borne diseases (VBDs) present a major threat to human and animal health, as well as place a substantial burden on livestock production. As a way of sustainable VBD control, focus is set on vaccine development. Advances in genomics and other "omics" over the past two decades have given rise to a "third generation" of vaccines based on technologies such as reverse vaccinology, functional genomics, immunomics, structural vaccinology and the systems biology approach. The application of omics approaches is shortening the time required to develop the vaccines and increasing the probability of discovery of potential vaccine candidates. Herein, we review the development of new generation vaccines for VBDs, and discuss technological advancement and overall challenges in the vaccine development pipeline. Special emphasis is placed on the development of anti-tick vaccines that can quell both vectors and pathogens.

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.

Geno- and phenotypic characteristics of a transfected Babesia bovis 6-Cys-E knockout clonal line

BACKGROUND

Babesia bovis is an intra-erythrocytic tick-transmitted apicomplexan protozoan parasite. It has a complex lifestyle including asexual replication in the mammalian host and sexual replication occurring in the midgut of host tick vector, typically, Rhipicephalus microplus. Previous evidence showed that certain B. bovis genes, including members of 6-Cys gene family, are differentially expressed during tick and mammalian stages of the parasite's life cycle. Moreover, the 6-Cys E gene is differentially expressed in the T3Bo strain of B. bovis tick stages, and anti 6-Cys E antibodies were shown to be able to inhibit in vitro growth of the phenotypically distinct B. bovis Mo7clonal line.

METHODS

In this study, the 6-Cys E gene of B. bovis T3Bo strain was disrupted by transfection using a plasmid containing 6-Cys gene E 5' and 3' regions to guide homologous recombination, and the egfp-bsd fusion gene under control of a ef-1α promoter, yielding a B. bovis clonal line designated 6-Cys EKO-cln. Full genome sequencing of 6-Cys EKO-cln parasites was performed and in vitro inhibition assays using anti 6-Cys E antibodies.

RESULTS

Full genome sequencing of 6-Cys EKO-cln B. bovis demonstrated single insertion of egfp-bsd gene that disrupts the integrity of 6-Cys gene E. Undistinguishable growth rate of 6-Cys EKO-cln line compared to wild-type 6-Cys E intact T3Bo B. bovis strain in in vitro cultures indicates that expression of gene 6-Cys E is not essential for blood stage replication in this strain. In vitro inhibition assays confirmed the ability of anti-6 Cys E antibodies to inhibit the growth of the wild-type Mo7 and T3Bo B. bovis parasites, but no significant inhibition was found for 6-Cys EKO-cln line parasites.

CONCLUSIONS

Overall, the data suggest that the anti-6 Cys E antibody neutralising effect on the wild type strains is likely due to mechanical hindrance, or cross-reactivity, rather than due to functional requirements of 6-Cys gene E product for survival and development of the erythrocyte stages. Further investigation is underway to determine if the 6-Cys E protein is required for replication and sexual stage development of B. bovis during tick stages.

Expression of 6-Cys Gene Superfamily Defines Babesia bovis Sexual Stage Development within Rhipicephalus microplus

Babesia bovis, an intra-erythrocytic tick-borne apicomplexan protozoan, is one of the causative agents of bovine babesiosis. Its life cycle includes sexual reproduction within cattle fever ticks, Rhipicephalus spp. Six B. bovis 6-Cys gene superfamily members were previously identified (A, B, C, D, E, F) where their orthologues in Plasmodium parasite have been shown to encode for proteins required for the development of sexual stages. The current study identified four additional 6-Cys genes (G, H, I, J) in the B. bovis genome. These four genes are described in the context of the complete ten 6-Cys gene superfamily. The proteins expressed by this gene family are predicted to be secreted or surface membrane directed. Genetic analysis comparing the 6-Cys superfamily among five distinct B. bovis strains shows limited sequence variation. Additionally, A, B, E, H, I and J genes were transcribed in B. bovis infected tick midgut while genes A, B and E were also transcribed in the subsequent B. bovis kinete stage. Transcription of gene C was found exclusively in the kinete. In contrast, transcription of genes D, F and G in either B. bovis infected midguts or kinetes was not detected. None of the 6-Cys transcripts were detected in B. bovis blood stages. Subsequent protein analysis of 6-Cys A and B is concordant with their transcript profile. The collective data indicate as in Plasmodium parasite, certain B. bovis 6-Cys family members are uniquely expressed during sexual stages and therefore, they are likely required for parasite reproduction. Within B. bovis specifically, proteins encoded by 6-Cys genes A and B are markers for sexual stages and candidate antigens for developing novel vaccines able to interfere with the development of B. bovis within the tick vector.

Transcriptome Sequencing and Development of Genic SSR Markers of an Endangered Chinese Endemic Genus Dipteronia Oliver (Aceraceae)

Dipteronia Oliver (Aceraceae) is an endangered Chinese endemic genus consisting of two living species, Dipteronia sinensis and Dipteronia dyeriana. However, studies on the population genetics and evolutionary analyses of Dipteronia have been hindered by limited genomic resources and genetic markers. Here, the generation, de novo assembly and annotation of transcriptome datasets, and a large set of microsatellite or simple sequence repeat (SSR) markers derived from Dipteronia have been described. After Illumina pair-end sequencing, approximately 93.2 million reads were generated and assembled to yield a total of 99,358 unigenes. A majority of these unigenes (53%, 52,789) had at least one blast hit against the public protein databases. Further, 12,377 SSR loci were detected and 4179 primer pairs were designed for experimental validation. Of these 4179 primer pairs, 435 primer pairs were randomly selected to test polymorphism. Our results show that products from 132 primer pairs were polymorphic, in which 97 polymorphic SSR markers were further selected to analyze the genetic diversity of 10 natural populations of Dipteronia. The identification of SSR markers during our research will provide the much valuable data for population genetic analyses and evolutionary studies in Dipteronia.

Bumped kinase inhibitor prohibits egression in Babesia bovis

Babesiosis is a global zoonotic disease acquired by the bite of a Babesia-infected Ixodes tick or through blood transfusion with clinical relevance affecting humans and animals. In this study, we evaluated a series of small molecule compounds that have previously been shown to target specific apicomplexan enzymes in Plasmodium, Toxoplasma and Cryptosporidium. The compounds, bumped kinase inhibitors (BKIs), have strong therapeutic potential targeting apicomplexa-specific calcium dependent protein kinases (CDPKs). We investigated if BKIs also show inhibitory activities against piroplasms such as Babesia. Using a subset of BKIs that have promising inhibitory activities to Plasmodium and Toxoplasma, we determined that their actions ranged from 100% and no inhibition against Babesia bovis blood stages. One specific BKI, RM-1-152, showed complete inhibition against B. bovis within 48h and was the only BKI that showed noticeable phenotypic changes to the parasites. Focusing our study on this BKI, we further demonstrated that RM-1-152 has Babesia-static activity and involves the prohibition of merozoite egress while replication and re-invasion of host cells are unaffected. The distinct, abnormal phenotype induced by RM-1-152 suggests that this BKI can be used to investigate less studied cellular processes such as egression in piroplasm.

Co-transmission of the non-transmissible South African Babesia bovis S24 vaccine strain during mixed infection with a field isolate

The South African Babesia bovis live blood vaccine, originating from a field isolate attenuated by 23 serial syringe passages in splenectomized calves, has lost the ability to infect the natural vector Rhipicephalus (Boophilus) microplus. In this study, infection with mixed parasites from the vaccine strain and a field isolate, resulted in transmission of both genotype populations. Comparing the field isolate and transmitted combination indicated no significant difference in their virulence, while challenge of vaccinated cattle with these isolates showed the ability of the vaccine to protect against both. Limiting dilution of the transmitted combination, followed by infection of splenectomized cattle (n=34) yielded no single infections for the vaccine strain genotype, seven clonal lines of the field isolate and one mixture of vaccine strain and field isolate. Only one of two field isolate clonal lines selected for vector transmission study was transmitted. Showing that B. bovis isolates can contain both tick transmissible and non-transmissible subpopulations. The findings of this study also indicate the probability of vaccine co-infection transmission occurring in the field, which may result in new genotype populations of B. bovis. However, the impact of this recombination with field isolates is considered negligible since a genotypically diverse population of B. bovis is already present in South Africa.

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

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