Transient transfection of purified Babesia bovis merozoites

Establishment of a stable transfection and gene targeting system in Babesia divergens

Babesia divergens is an emerging tick-borne pathogen considered as the principal causative agent of bovine babesiosis in Europe with a notable zoonotic risk to human health. Despite its increasing impact, considerable gaps persist in our understanding of the molecular interactions between this parasite and its hosts. In this study, we address the current limitation of functional genomic tools in B. divergens and introduce a stable transfection system specific to this parasite. We define the parameters for a drug selection system hdhfr-WR99210 and evaluate different transfection protocols for highly efficient generation of transgenic parasites expressing GFP. We proved that plasmid delivery into bovine erythrocytes prior to their infection is the most optimal transfection approach for B. divergens, providing novel evidence of Babesia parasites' ability to spontaneously uptake external DNA from erythrocytes cytoplasm. Furthermore, we validated the bidirectional and symmetrical activity of ef-tgtp promoter, enabling simultaneous expression of external genes. Lastly, we generated a B. divergens knockout line by targeting a 6-cys-e gene locus. The observed dispensability of this gene in intraerythrocytic parasite development makes it a suitable recipient locus for further transgenic application. The platform for genetic manipulations presented herein serves as the initial step towards developing advanced functional genomic tools enabling the discovery of B. divergens molecules involved in host-vector-pathogen interactions.

Advances in Babesia Vaccine Development: An Overview

Babesiosis is a tick-borne zoonotic disease, which is caused by various species of intracellular Babesia parasite. It is a problem not only for the livestock industry but also for global health. Significant global economic losses, in particular in cattle production, have been observed. Since the current preventive measures against babesiosis are insufficient, there is increasing pressure to develop a vaccine. In this review, we survey the achievements and recent advances in the creation of antibabesiosis vaccine. The scope of this review includes the development of a vaccine against B. microti, B. bovis, B. bigemina, B. orientalis and B. divergens. Here, we present different strategies in their progress and evaluation. Scientists worldwide are still trying to find new targets for a vaccine that would not only reduce symptoms among animals but also prevent the further spread of the disease. Molecular candidates for the production of a vaccine against various Babesia spp. are presented. Our study also describes the current prospects of vaccine evolution for successful Babesia parasites elimination.

Identification of novel immune correlates of protection against acute bovine babesiosis by superinfecting cattle with in vitro culture attenuated and virulent Babesia bovis strains

The apicomplexan tickborne parasites Babesia bovis and B. bigemina are the major causative agents of bovine babesiosis, a disease that negatively affects the cattle industry and food safety around the world. The absence of correlates of protection represents one major impediment for the development of effective and sustainable vaccines against bovine babesiosis. Herein we superinfected cattle with attenuated and virulent strains of B. bovis to investigate immune correlates of protection against acute bovine babesiosis. Three 6-month-old Holstein calves were infected intravenously (IV) with the in vitro culture attenuated Att-S74-T3Bo B. bovis strain (106 infected bovine red blood cells (iRBC)/calf) while three age-matched Holstein calves were inoculated IV with normal RBC as controls (106 RBC/calf). All Att-S74-T3Bo-infected calves showed a significant increase in temperature early after inoculation but recovered without treatment. Att-S74-T3Bo-infected calves also developed: (a) monocytosis, neutropenia, and CD4+ lymphopenia in peripheral blood on days 3 to 7 post-inoculation; (b) significant levels of TNFα, CXCL10, IFNγ, IL-4, and IL-10 in sera at day 6 after infection; and (c) IgM and IgG against B. bovis antigens, starting at days 10 and 30 post-inoculation, respectively. At 46 days post-Att-S74-T3Bo inoculation, all experimental calves were infected IV with the homologous virulent B. bovis strain Vir-S74-T3Bo (107 iRBC/calf). All Att-S74-T3Bo-infected calves survived superinfection with Vir-S74-T3Bo without displaying signs of acute babesiosis. In contrast, control animals showed signs of acute disease, starting at day 10 post-Vir-S74-T3Bo infection, and two of them were humanely euthanized at days 13 and 14 after inoculation due to the severity of their symptoms. Also, control calves showed higher (P<0.05) parasite load in peripheral blood compared to animals previously exposed to Att-S74-T3Bo. No significant alterations in the profile of leukocytes and cytokines were observed in Att-S74-T3Bo-inoculated after Vir-S74-T3Bo infection. In conclusion, data demonstrate novel changes in the profile of blood immune cells and cytokine expression in peripheral blood that are associated with protection against acute bovine babesiosis. These identified immune correlates of protection may be useful for designing effective and sustainable vaccines against babesiosis in cattle.

Establishment of a Transient and Stable Transfection System for Babesia duncani Using a Homologous Recombination Strategy

Genetic modification provides an invaluable molecular tool to dissect the biology and pathogenesis of pathogens. However, no report is available about the genetic modification of Babesia duncani, a pathogen responsible for human babesiosis that is widespread in North America, suggesting the necessity to develop a genetic manipulation method to improve the strategies for studying and understanding the biology of protozoan pathogens. The establishment of a genetic modification method requires promoters, selectable markers, and reporter genes. Here, the double-copy gene elongation factor-1α (ef-1α) and its promoters were amplified by conventional PCR and confirmed by sequencing. We established a transient transfection system by using the ef-1αB promoter and the reporter gene mCherry and achieved stable transfection through homologous recombination to integrate the selection marker hDHFR-eGFP into the parasite genome. The potential of this genetic modification method was tested by knocking out the thioredoxin peroxidase-1 (TPX-1) gene, and under the drug pressure of 5 nM WR99210, 96.3% of the parasites were observed to express green fluorescence protein (eGFP) by flow cytometry at day 7 post-transfection. Additionally, the clone line of the TPX-1 knockout parasite was successfully obtained by the limiting dilution method. This study provided a transfection method for B. duncani, which may facilitate gene function research and vaccine development of B. duncani.

Harnessing Mycobacterium bovis BCG Trained Immunity to Control Human and Bovine Babesiosis

Babesiosis is a disease caused by tickborne hemoprotozoan apicomplexan parasites of the genus Babesia that negatively impacts public health and food security worldwide. Development of effective and sustainable vaccines against babesiosis is currently hindered in part by the absence of definitive host correlates of protection. Despite that, studies in Babesia microti and Babesia bovis, major causative agents of human and bovine babesiosis, respectively, suggest that early activation of innate immune responses is crucial for vertebrates to survive acute infection. Trained immunity (TI) is defined as the development of memory in vertebrate innate immune cells, allowing more efficient responses to subsequent specific and non-specific challenges. Considering that Mycobacterium bovis bacillus Calmette-Guerin (BCG), a widely used anti-tuberculosis attenuated vaccine, induces strong TI pro-inflammatory responses, we hypothesize that BCG TI may protect vertebrates against acute babesiosis. This premise is supported by early investigations demonstrating that BCG inoculation protects mice against experimental B. microti infection and recent observations that BCG vaccination decreases the severity of malaria in children infected with Plasmodium falciparum, a Babesia-related parasite. We also discuss the potential use of TI in conjunction with recombinant BCG vaccines expressing Babesia immunogens. In conclusion, by concentrating on human and bovine babesiosis, herein we intend to raise awareness of BCG TI as a strategy to efficiently control Babesia infection.

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.

Stable transfection system for Babesia sp. Xinjiang

BACKGROUND

Stable transfection systems have been described in many protozoan parasites, including Plasmodium falciparum, Cryptosporidium parvum, Babesia bovis, Babesia ovata, and Babesia gibsoni. For Babesia sp. Xinjiang (Bxj), which is the causative pathogen of ovine babesiosis and mainly prevails across China, the platform of those techniques remains absent. Genetic manipulation techniques are powerful tools to enhance our knowledge on parasite biology, which may provide potential drug targets and diagnostic markers.

METHODS

We evaluated the inhibition efficiency of blasticidin (BSD) and WR99210 to Bxj. Then, a plasmid was constructed bearing selectable marker BSD, green fluorescent protein (GFP) gene, and rhoptry-associated protein-1 3' terminator region (rap 3' TR). The plasmid was integrated into the elongation factor-1 alpha (ef-1α) site of Bxj genome by cross-over homologous recombination technique. Twenty μg of plasmid was transfected into Bxj merozoites. Subsequently, drug selection was performed 24 h after transfection to generate transfected parasites.

RESULTS

Transfected parasite lines, Bxj-c1, Bxj-c2, and Bxj-c3, were successfully obtained after transfection, drug selection, and colonization. Exogenous genes were integrated into the Bxj genome, which were confirmed by PCR amplification and sequencing. In addition, results of western blot (WB) and indirect immunofluorescence assay (IFA) revealed that GFP-BSD had expressed for 11 months.

CONCLUSIONS

In our present study, stable transfection system for Bxj was successfully developed. We anticipate that this platform will greatly facilitate basic research of Bxj.

3D Printed Microneedle Array for Electroporation

In-vitro transfection of cells by electroporation is a widely used approach in cell biology and medicine. The transfection method is highly dependent on the cell culture's electrical resistance, which is strongly determined by differences in the membranes, but also on the morphology of the electrodes. Microneedle (MN)-based electrodes have been used to concentrate the electrical field during electroporation, and therefore maximize its effect on cell membrane permeability. So far, the methods used for the fabrication of MN electrodes have been relatively limited with respect to the needle design. In this work, we provide a method to fabricate MNs using 3D printing, which is a technology that provides a high degree of flexibility with respect to geometry and dimensions. Pyramidal-shaped MN designs were fabricated and tested on HCT116 cancer cells. Customization of the tips of the pyramids permits tailoring of the electrical field in the vicinity of the cell membranes. The fabricated device enables low-voltage (2 V) electroporation, eliminating the need for the use of specialized chemical buffers. The results show the potential of this method, which can be exploited and optimized for many different applications, and offer a very accessible approach for in-vitro electroporation and cell studies. The MNs can be customized to create complex structures, for example, for a multi-culture cell environment.

Establishment of a stable transfection method in Babesia microti and identification of a novel bidirectional promoter of Babesia microti

Babesia microti, an emerging human pathogen, is primarily transmitted through a bite of an infected tick and blood transfusions in human. Stable transfection technique has been reported in many protozoan parasites over the past few years. However, in vivo transient and stable transfection method has not been established for Babesia microti. Here, for the first time, we present a method of transient as well as stable transfection of the Babesia microti (B. microti) in the in vivo conditions. We have identified a novel promoter of B. microti. We also demonstrated that Plasmodium berghei DHFR promoter is recognized and functional in B. microti. We show that BM-CTQ41297 promoter control the expression of two genes, which are present on either side and thus represents a bi-functional promoter in B. microti. The predicted promoter activity values using Promoter 2.0 program is higher for BM- CTQ41297 promoter than strong promoters such as β-actin, ef-1β, and many other promoters. Furthermore, we discovered a non-essential locus for the genetic manipulation of the parasite, allowing us to stably integrate foreign genes; GFP, mCherry, into the B. microti. The transfection using an electroporation method and genetic manipulation of B. microti is now achievable and it is possible to obtain transfected viable parasites under in vivo growing conditions. The growth curve analysis of transfected and WT B. microti are similar indicating no defects in the transgenic parasites. This study will enable other researchers in understanding the B. microti biology, host modulation and diverse parasite developmental stages using reverse genetics and holds great potential to identify novel drug targets and vaccine development.

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

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

Transient transfection of Babesia ovis using heterologous promoters

Babesia species, etiological agents of babesiosis, a recognized emerging tick-borne disease, are a significant animal and human health concern with a worldwide socio-economic impact. The development of genetic manipulation techniques, such as transfection technology, is pivotal to improve knowledge regarding the biology of these poorly studied parasites towards better disease control strategies. For Babesia ovis, responsible for ovine babesiosis, a tick-borne disease of small ruminants, these tools are not yet available. The present study was based on the existence of interchangeable cross-species functional promoters between Babesia species. Herein, we describe for the first time B. ovis transient transfection using two heterologous promoters, the ef-1α-B intergenic regions from B. bovis and B. ovata. Their ability to drive expression of a reporter luciferase in B. ovis supports their cross-species functionality. Also, the ef-1α-B promoter region from B. ovata resulted in statistically significantly higher luminescence values in comparison to the control, thus a possibly suitable promoter for stable gene expression. Evaluation of transfection efficiency using qPCR demonstrated that higher luminescence levels were due to promoter strength rather than a higher transfection efficiency. These findings represent a step forward in the development of methods for B. ovis genetic manipulation, an undoubtedly necessary tool to study this parasite basic biology, including its life cycle, the parasite interactions with host cells and virulence factors.

Transient transfection of Cryptosporidium baileyi

Here we demonstrate the transient transfection of C. baileyi. We describe the transfection of this apicomplexan parasite and the cultivation in ovo. The functionality of heterologous sequences in C. baileyi was demonstrated by the expression and detection of the mCherry protein in ovo. The mCherry protein was expressed in parasitic stages up to the oocyst stage under the control of the heterologous promoter region of the C. parvum actin gene.

Advances in the application of genetic manipulation methods to apicomplexan parasites

Apicomplexan parasites such as Babesia, Theileria, Eimeria, Cryptosporidium and Toxoplasma greatly impact animal health globally, and improved, cost-effective measures to control them are urgently required. These parasites have complex multi-stage life cycles including obligate intracellular stages. Major gaps in our understanding of the biology of these relatively poorly characterised parasites and the diseases they cause severely limit options for designing novel control methods. Here we review potentially important shared aspects of the biology of these parasites, such as cell invasion, host cell modification, and asexual and sexual reproduction, and explore the potential of the application of relatively well-established or newly emerging genetic manipulation methods, such as classical transfection or gene editing, respectively, for closing important gaps in our knowledge of the function of specific genes and proteins, and the biology of these parasites. In addition, genetic manipulation methods impact the development of novel methods of control of the diseases caused by these economically important parasites. Transient and stable transfection methods, in conjunction with whole and deep genome sequencing, were initially instrumental in improving our understanding of the molecular biology of apicomplexan parasites and paved the way for the application of the more recently developed gene editing methods. The increasingly efficient and more recently developed gene editing methods, in particular those based on the CRISPR/Cas9 system and previous conceptually similar techniques, are already contributing to additional gene function discovery using reverse genetics and related approaches. However, gene editing methods are only possible due to the increasing availability of in vitro culture, transfection, and genome sequencing and analysis techniques. We envisage that rapid progress in the development of novel gene editing techniques applied to apicomplexan parasites of veterinary interest will ultimately lead to the development of novel and more efficient methods for disease control.

Transient transfection of intraerythrocytic Babesia gibsoni using elongation factor-1 alpha promoter

The development of gene manipulation techniques has been reported in many protozoan parasites over the past few years. However, these techniques have not yet been established for Babesia gibsoni. Here, we report for the first time, the successful transient transfection of B. gibsoni. The plasmid containing the firefly luciferase reporter gene (pBS-ELA) was transfected into B. gibsoni by an AMAXA 4D Nucleofector™ device. Transfection using program FA113 and Lonza buffer SF showed the highest luciferase expression. Twenty micrograms of plasmid produced the highest relative transfection efficiency. The fluorescent protein-expressing parasites were determined by GFP-containing plasmid (pBS-EGA) at 48 and 72h post transfection. This finding is the first step towards a stable transfection method for B. gibsoni, which may contribute to a better understanding of the biology of the parasite.

Transfected Babesia bovis Expressing a Tick GST as a Live Vector Vaccine

The Rhipicephalus microplus tick is a notorious blood-feeding ectoparasite of livestock, especially cattle, responsible for massive losses in animal production. It is the main vector for transmission of pathogenic bacteria and parasites, including Babesia bovis, an intraerythrocytic apicomplexan protozoan parasite responsible for bovine Babesiosis. This study describes the development and testing of a live B. bovis vaccine expressing the protective tick antigen glutathione-S-transferase from Haemaphysalis longicornis (HlGST). The B. bovis S74-T3B parasites were electroporated with a plasmid containing the bidirectional Ef-1α (elongation factor 1 alpha) promoter of B. bovis controlling expression of two independent genes, the selectable marker GFP-BSD (green fluorescent protein–blasticidin deaminase), and HlGST fused to the MSA-1 (merozoite surface antigen 1) signal peptide from B. bovis. Electroporation followed by blasticidin selection resulted in the emergence of a mixed B. bovis transfected line (termed HlGST) in in vitro cultures, containing parasites with distinct patterns of insertion of both exogenous genes, either in or outside the Ef-1α locus. A B. bovis clonal line termed HlGST-Cln expressing intracellular GFP and HlGST in the surface of merozoites was then derived from the mixed parasite line HlGST using a fluorescent activated cell sorter. Two independent calf immunization trials were performed via intravenous inoculation of the HlGST-Cln and a previously described control consisting of an irrelevant transfected clonal line of B. bovis designated GFP-Cln. The control GFP-Cln line contains a copy of the GFP-BSD gene inserted into the Ef-1α locus of B. bovis in an identical fashion as the HIGST-Cln parasites. All animals inoculated with the HlGST-Cln and GFP-Cln transfected parasites developed mild babesiosis. Tick egg fertility and fully engorged female tick weight was reduced significantly in R. microplus feeding on HlGST-Cln-immunized calves. Collectively, these data show the efficacy of a transfected HlGST-Cln B. bovis parasite to induce detectable anti-glutathione-S-transferase antibodies and a reduction in tick size and fecundity of R. microplus feeding in experimentally inoculated animals.

The cattle tick Rhipicephalus microplus is a hematophagous ectoparasite, responsible for the transmission of lethal parasites such as Babesia sp, limiting cattle production in tropical and subtropical regions of the world. There is an urgent emerging need for improved methods of control for these currently neglected tick and tick borne diseases. It is hypothesized that a dual attenuated-live vector vaccine containing a stably transfected tick antigen elicits protective immune responses against the parasite and the tick vector in vaccinated cattle. Live Babesia vaccines based on attenuated parasites are the only effective method available for preventing acute babesiosis. On the other hand, glutathione-S-transferase from Haemaphysalis longicornis (HlGST) is a known effective antigen against Rhipicephalus microplus, the most common vector for B. bovis. This study describes the development and testing of a transfected, B. bovis vaccine expressing HlGST against the tick R. microplus. A B. bovis clonal line designated HlGST-Cln expressing HlGST and GFP/BSD, and separately a control transfected B. bovis clonal line expressing only GFP/BSD was used to vaccinate calves in two independent experiments. All immunized calves developed mild babesiosis, and only calves immunized with the HlGST-Cln parasite line generated anti-HlGST antibodies. Tick egg fertility and fully engorged female tick weight were reduced significantly in R. microplus feeding on HlGST-Cln-vaccinated calves. Taken together, these data demonstrates the ability of transfected B. bovis to elicit antibodies against a heterologous tick antigen in cattle and to induce partial protection in the vaccinated animals against the cattle tick for the first time, representing a step toward the goal to produce a live vector anti-tick vaccine.

Identification of interchangeable cross-species function of elongation factor-1 alpha promoters in Babesia bigemina and Babesia bovis

Background

Tick-borne Babesia bigemina is responsible for acute and potentially lethal hemolytic disease in cattle. The development of genetic manipulation tools necessary to the better understanding of parasite biology is currently limited by the lack of a complete parasite genome and experimental tools such as transfection. Effective promoters, required to regulate expression of transgenes, such as the elongation factor-1 alpha (ef-1α), have been identified in other apicomplexans such as Babesia bovis and Plasmodium falciparum.

Methods

The B. bigemina ef-1a locus was defined by searching a partial genome library of B. bigemina (Sanger Institute). Presence of an intron in the 5’ untranslated region was determined by 5’ Rapid Amplification of cDNA Ends (RACE) analysis. Promoter activity was determined by measurement of luciferase expression at several time points after electroporation, efficiency of transfections and normalization of data was determined by quantitative PCR and by the percentage of parasitized erythrocytes.

Results

The ef-1α locus contains two identical head to head ef-1α genes separated by a 1.425 kb intergenic (IG) region. Significant sequence divergence in the regions upstream of the inverted repeats on each side of the B. bigemina IG region suggest independent regulation mechanisms for controlling expression of each of the two ef-1α genes. Plasmid constructs containing the 5’ and 3’ halves of the IG regions controlling the expression of the luciferase gene containing a 3’ region of a B. bigemina rap-1a gene, were generated for the testing of luciferase activity in transiently transfected parasites. Both halves of the ef-1α IG region tested showed the ability to promote high level production of luciferase. Moreover, both B. bigemina ef-1α promoters are also active in transiently transfected B. bovis and conversely, a B. bovis ef-1α promoter is active in transiently transfected B. bigemina.

Conclusions

Collectively these data demonstrate the existence of two distinct promoters with homologous and heterologous promoter function in B. bigemina and B. bovis which is described for the first time in Babesia species. This study is of significance for development of interspecies stable transfection systems for B. bigemina and for B. bovis.

Electronic supplementary material

The online version of this article (doi:10.1186/s13071-016-1859-9) contains supplementary material, which is available to authorized users.

Genomic resources for a unique, low-virulence Babesia taxon from China

BACKGROUND

Babesiosis is a socioeconomically important tick-borne disease of animals (including humans) caused by haemoprotozoan parasites. The severity of babesiosis relates to host and parasite factors, particularly virulence/pathogenicity. Although Babesia bovis is a particularly pathogenic species of cattle, there are species of Babesia of ruminants that have limited pathogenicity. For instance, the operational taxonomic unit Babesia sp. Xinjiang (abbreviated here as Bx) of sheep from China is substantially less virulent/pathogenic than B. bovis is in cattle. Although the reason for this distinctiveness is presently unknown, it is possible that Bx has a reduced ability to adhere to cells or evade/suppress immune responses, which might relate to particular proteins, such as the variant erythrocyte surface antigens (VESAs).

RESULTS

We sequenced and annotated the 8.4 Mb nuclear draft genome of Bx and compared it with those of B. bovis and B. bigemina by synteny analysis; we also investigated the genetic relationship of Bx with selected Babesia species and related apicomplexans for which genomic datasets are available, and explored the VESA complement in Bx.

CONCLUSIONS

The availability of the Bx genome now provides unique opportunities to elucidate aspects of the molecular biology, biochemistry and physiology of Bx, and to explore the reason(s) for its limited virulence and/or apparent ability to evade immune attack by the host animal. Moreover, the present genomic resource and an in vitro culture system for Bx raises the prospect of establishing a functional genomic platform to explore essential genes as new intervention targets against babesiosis.

RBC invasion and invasion-inhibition assays using free merozoites isolated after cold treatment of Babesia bovis in vitro culture

Babesia bovis is an apicomplexan hemoprotozoan that can invade bovine red blood cells (RBCs), where it multiplies asexually. RBC invasion assays using free viable merozoites are now routinely used to understand the invasion mechanism of B. bovis, and to evaluate the efficacy of chemicals and antibodies that potentially inhibit RBC invasion by the parasite. The application of high-voltage pulses (high-voltage electroporation), a commonly used method to isolate free merozoites from infected RBCs, reduces the viability of the merozoites. Recently, a cold treatment of B. bovis in vitro culture was found to induce an effective release of merozoites from the infected RBCs. In the present study, we incubated in vitro cultures of B. bovis in an ice bath to liberate merozoites from infected RBCs and then evaluated the isolated merozoites in RBC invasion and invasion-inhibitions assays. The viability of the purified merozoites (72.4%) was significantly higher than that of merozoites isolated with high-voltage electroporation (48.5%). The viable merozoites prepared with the cold treatment also invaded uninfected bovine RBCs at a higher rate (0.572%) than did merozoites prepared with high-voltage electroporation (0.251%). The invasion-blocking capacities of heparin, a polyclonal rabbit antibody directed against recombinant B. bovis rhoptry associated protein 1, and B. bovis-infected bovine serum were successfully demonstrated in an RBC invasion assay with the live merozoites prepared with the cold treatment, suggesting that the targets of these inhibitors were intact in the merozoites. These findings indicate that the cold treatment technique is a useful tool for the isolation of free, viable, invasion-competent B. bovis merozoites, which can be effectively used for RBC invasion and invasion-inhibition assays in Babesia research.

Transfection of Babesia bovis by Double Selection with WR99210 and Blasticidin-S and Its Application for Functional Analysis of Thioredoxin Peroxidase-1

Genetic manipulation is an essential technique to analyze gene function; however, limited methods are available for Babesia bovis, a causative pathogen of the globally important cattle disease, bovine babesiosis. To date, two stable transfection systems have been developed for B. bovis, using selectable markers blasticidin-S deaminase (bsd) or human dihydrofolate reductase (hdhfr). In this work, we combine these two selectable markers in a sequential transfection system. Specifically, a parent transgenic B. bovis line which episomally expresses green fluorescent protein (GFP) and human dihydrofolate reductase (hDHFR), was transfected with a plasmid encoding a fusion protein consisting of red fluorescent protein (RFP) and blasticidin-S deaminase (BSD). Selection with WR99210 and blasticidin-S resulted in the emergence of parasites double positive for GFP and RFP. We then applied this method to complement gene function in a parasite line in which thioredoxin peroxidase-1 (Bbtpx-1) gene was knocked out using hDHFR as a selectable marker. A plasmid was constructed harboring both RFP-BSD and Bbtpx-1 expression cassettes, and transfected into a Bbtpx-1 knockout (KO) parasite. Transfectants were independently obtained by two transfection methods, episomal transfection and genome integration. Complementation of Bbtpx-1 resulted in full recovery of resistance to nitrosative stress, via the nitric oxide donor sodium nitroprusside, which was impaired in the Bbtpx-1 KO parasites. In conclusion, we developed a sequential transfection method in B. bovis and subsequently applied this technique in a gene complementation study. This method will enable broader genetic manipulation of Babesia toward enhancing our understanding of the biology of this parasite.

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.

Bovine babesiosis in the 21st century: advances in biology and functional genomics

Bovine babesiosis caused by the protozoan parasite, Babesia bovis, remains a significant cause of avoidable economic losses to the livestock industry in many countries throughout the world. The molecular mechanisms underlying the pathophysiology of severe disease in susceptible cattle are not well understood and the tools available to study the biology of the parasite, including technologies for genetic manipulation, have only recently been developed. Recent availability of multiple parasite genomes and bioinformatic tools, in combination with the development of new biological reagents, will facilitate our better understanding of the parasite. This will ultimately assist in the identification of novel targets for the development of new therapeutics and vaccines. Here we describe some recent advances in Babesia research and highlight some important challenges for the future.

Babesia bovis: a bipartite signal directs the glutamyl-tRNA synthetase to the apicoplast

Babesia bovis contains a prokaryotic derived organelle known as the apicoplast. Many participants of the metabolic pathways within the apicoplast are encoded in the nuclear genome and post-translationally imported with the help of a bipartite signal. Recently, an all encompassing algorithm was derived to predict apicoplast targeted proteins for many non-Plasmodium apicomplexans in which it reported the presence of 260 apicoplast targeted proteins in Babesia. One of these proteins is glutamyl tRNA synthetase (GltX). This study investigates if the putative bipartite signal of GltX alone is sufficient to direct proteins into the apicoplast. Using a transient transfection system consisting of a green fluorescent protein as the reporter, we tested the signal and transit portions of the bipartite signal in apicoplastic transport. We first identified the transcript of gltX to be expressed during the asexual blood stages and subsequently confirmed that the complete bipartite signal is responsible for directing the reporter protein into a compartment distinct from the nucleus and the mitochondrion. As GltX bipartite signal successfully guided the reporter protein into the apicoplast, our finding implies that it also directs native GltX into the same organelle.

Tick-borne diseases in cattle: applications of proteomics to develop new generation vaccines

Tick-borne diseases (TBDs) affect 80% of the world's cattle population, hampering livestock production throughout the world. Livestock industry is important to rural populations not only as food supply, but also as a source of income. Tick control is usually achieved by using acaricides which are expensive, deleterious to the environment and can induce chemical resistance of vectors; the development of more effective and sustainable control methods is therefore required. Theileriosis, babesiosis, anaplasmosis and heartwater are the most important TBDs in cattle. Immunization strategies are currently available but with variable efficacy. To develop a new generation of vaccines which are more efficient, cheaper and safer, it is first necessary to better understand the mechanisms by which these parasites are transmitted, multiply and cause disease; this becomes especially difficult due to their complex life cycles, in vitro culture conditions and the lack of genetic tools to manipulate them. Proteomics and other complementary post-genomic tools such as transcriptomics and metabolomics in a systems biology context are becoming key tools to increase knowledge on the biology of infectious diseases. Herein, we present an overview of the so called "Omics" studies currently available on these tick-borne pathogens, giving emphasis to proteomics and how it may help to discover new vaccine candidates to control TBDs.

Characterization of acyl carrier protein and LytB in Babesia bovis apicoplast

The apicoplast is a highly specialized organelle that mediates required functions in the growth and replication of apicomplexan parasites. Despite structural conservation of the apicoplast among different parasite genera and species, there are also critical differences in the metabolic requirements of different parasites and at different stages of the life cycle. To specifically compare apicoplast pathways between parasites that have both common and unique stages, we characterized the apicoplast in Babesia bovis, which has only intraerythrocytic asexual stages in the mammalian host, and compared it to that of Plasmodium falciparum, which has both asexual intraerythrocytic and hepatic stages. Specifically focusing on the type II fatty acid (FASII) and isoprenoid (MEP) biosynthesis pathways, we searched for pathway components and retention of active sites within the genome, localized key components [acyl carrier protein (ACP) and 4-hydroxy-3-methylbut-2-enyl diphosphate reductase (LytB)] to the apicoplast, and demonstrated that the N-terminal bipartite signals of both proteins are required and sufficient for trafficking to the apicoplast lumen. Using specific pharmacologic inhibition, we demonstrated that MEP biosynthesis may be disrupted and its presence is required for intraerythrocytic growth of B. bovis asexual stages, consistent with the genomic pathway analysis and with its requirement in the asexual erythrocytic stages of P. falciparum. In contrast, FASII biosynthesis may or may not be present and specific drug targets did not have any inhibitory effect to B. bovis intraerythrocytic growth, which is consistent with the lack of requirement for P. falciparum intraerythrocytic growth. However, genomic analysis revealed the loss of FASII pathway components in B. bovis whereas the pathway is intact for P. falciparum but regulated to be expressed when needed (hepatic stages) and silent when not (intraerythrocytic stages). The results indicate specialized molding of apicoplast biosynthetic pathways to meet the requirements of individual apicomplexan parasites and their unique intracellular niches.