Identification of a Theileria annulata antigen expressed in multiple stages of the parasite life cycle

Humoral and cellular immunity in response to an in silico-designed multi-epitope recombinant protein of Theileria annulata

Tropical theileriosis is a lymphoproliferative disease caused by Theileria annulata and is transmitted by Ixodid ticks of the genus Hyalomma. It causes significant losses in livestock, especially in exotic cattle. The existing methods for controlling it, chemotherapeutic agents and a vaccine based on an attenuated schizont stage parasite, have several limitations. A promising solution to control this disease is the use of molecular vaccines based on potential immunogenic proteins of T. annulata. For this purpose, we selected five antigenic sequences of T. annulata, i.e. SPAG-1, Tams, TaSP, spm2, and Ta9. These were subjected to epitope prediction for cytotoxic T lymphocytes, B-cells, and helper T lymphocytes. CTL and B-cell epitopes with a higher score whereas those of HTL with a lower score, were selected for the construct. A single protein was constructed using specific linkers and evaluated for high antigenicity and low allergenicity. The construct was acidic, hydrophobic, and thermostable in nature. Secondary and tertiary structures of this construct were drawn using the PSIPRED and RaptorX servers, respectively. A Ramachandran plot showed a high percentage of residues in this construct in favorable, allowed, and general regions. Molecular docking studies suggested that the complex was stable and our construct could potentially be a good candidate for immunization trials. Furthermore, we successfully cloned it into the pET-28a plasmid and transformed it into the BL21 strain. A restriction analysis was performed to confirm the transformation of our plasmid. After expression and purification, recombinant protein of 49 kDa was confirmed by western blotting. An ELISA detected increased specific antibody levels in the sera of the immunized animals compared with the control group, and flow cytometric analysis showed a stronger cell-mediated immune response. We believe our multi-epitope recombinant protein has the potential for the large-scale application for disease prevention globally in the bovine population. This study will act as a model for similar parasitic challenges.

Evaluation of sporozoite and macroschizont antigen (Spm2) of Theileria annulata for its diagnostic potential

Theileria are tick-borne apicomplexan parasites, which mainly infect ruminants in tropical and subtropical regions of the world. The present study was directed to investigate the serological methods for the diagnosis of theileriosis in crossbred cattle. Blood samples (n = 176) were collected from the regional cattle populations of Bihar state situated at the Gangetic plains of India. Microscopic examination of blood smears from the cattle revealed the presence of tick-borne infectious organisms (Theileria and Anaplasma) in the region. PCR-based detection of Tams1 (Theileria annulata merozoite surface antigen) gene and the sequencing of 18S rRNA amplicon from the blood samples confirmed T. annulata as the primary causative agent of theileriosis in cattle of the Bihar region. Similarly, the amplification of the msp5 gene confirmed Anaplasma marginale infection. For the large-scale epidemiological investigation, sporozoite and macroschizont (spm2) partial gene from T. annulata was cloned in pET-28a (+) vector and overexpressed in E. coli BL21 cells. Overexpressed recombinant-Spm2 (43 kDa) was purified by Ni-NTA affinity chromatography and was used for immunodetection of theileriosis in cattle serum samples. Sequence analysis of the cloned partial spm2 gene in this study showed multiple SNPs (single nucleotide polymorphisms) in T. annulata. Recombinant-Spm2 antigen was explicitly recognised by the immunoglobulins (IgG) of the cattle naturally infected with Theileria. Further, an indirect enzyme-linked immunosorbent assay (ELISA) was developed using partial r-Spm2 antigen that exhibited high sensitivity (100 %) and specificity (90.9 %). Thus, this study suggests that partial r-Spm2 can be used as a diagnostic antigen for seroepidemiological studies of T. annulata infection in crossbred cattle.

Development of an indirect ELISA based on the recombinant Spm2 protein for detection of tropical theileriosis

Tropical theileriosis, caused by Theileria annulata, is distributed worldwide and causes great economic losses in dairy. The reliable diagnostic method is critical for prevention and control of the disease. In this study, a sporozoite and macroschizont gene 2 (spm2) protein from T. annulata was used to develop an indirect ELISA for tropical theileriosis. Specificity test showed that there were no cross-reactions with antibodies raised against other bovine piroplasm species using ELISA or western blotting. The specificity and sensitivity were 98.4% and 98.7%, respectively, with a threshold of 35.5% of the specific mean antibody rate (AbR). Furthermore, a total of 196 field sera samples collected from Xinjiang and Gansu provinces were detected by the spm2 ELISA and IFA. The results obtained with the spm2 ELISA and IFA in this study had the moderate agreement. The average positive rates of T. annulata sera samples detected in the present study were close to the prevalence of previous reports in these endemic areas. This indicated that the Spm2 ELISA could be used as a reliable diagnostic tool for serological survey of T. annulata infection in areas where Theileria parva is not present.

High-resolution genotyping and mapping of recombination and gene conversion in the protozoan Theileria parva using whole genome sequencing

BACKGROUND

Theileria parva is a tick-borne protozoan parasite, which causes East Coast Fever, a disease of cattle in sub-Saharan Africa. Like Plasmodium falciparum, the parasite undergoes a transient diploid life-cycle stage in the gut of the arthropod vector, which involves an obligate sexual cycle. As assessed using low-resolution VNTR markers, the crossover (CO) rate in T. parva is relatively high and has been reported to vary across different regions of the genome; non-crossovers (NCOs) and CO-associated gene conversions have not yet been characterised due to the lack of informative markers. To examine all recombination events at high marker resolution, we sequenced the haploid genomes of two parental strains, and two recombinant clones derived from ticks fed on cattle that had been simultaneously co-infected with two different parasite isolates.

RESULTS

By comparing the genome sequences, we were able to genotype over 64 thousand SNP markers with an average spacing of 127 bp in the two progeny clones. Previously unrecognized COs in sub-telomeric regions were detected. About 50% of CO breakpoints were accompanied by gene conversion events. Such a high fraction of COs accompanied by gene conversions demonstrated the contributions of meiotic recombination to the diversity and evolutionary success of T. parva, as the process not only redistributed existing genetic variations, but also altered allelic frequencies. Compared to COs, NCOs were more frequently observed and more uniformly distributed across the genome. In both progeny clones, genomic regions with more SNP markers had a reduced frequency of COs or NCOs, suggesting that the sequence divergence between the parental strains was high enough to adversely affect recombination frequencies. Intra-species polymorphism analysis identified 81 loci as likely to be under selection in the sequenced genomes.

CONCLUSIONS

Using whole genome sequencing of two recombinant clones and their parents, we generated maps of COs, NCOs, and CO-associated gene conversion events for T. parva. The data comprises one of the highest-resolution genome-wide analyses of the multiple outcomes of meiotic recombination for this pathogen. The study also demonstrates the usefulness of high throughput sequencing typing for detailed analysis of recombination in organisms in which conventional genetic analysis is technically difficult.

Immunity and vaccine development in the bovine theilerioses

There are three economically important bovine Theileria species: Theileria annulata, which causes tropical theileriosis and occurs across north Africa and most of central Asia; Theileria parva, which causes East Coast fever and is found in East and Central Africa; and Theileria sergenti, which is predominantly a problem in Japan and Korea. Theileria annulata preferentially infects macrophages in vivo. It is controlled largely by means of live, attenuated vaccines, which are produced by prolonged tissue culture of the schizont-infected cells. The immunity induced in animals, which have either recovered from an infection or have been vaccinated (with an attenuated vaccine), is broad, solid and cell mediated. It is considered that the main effector cells are cytostatic macrophages that produce nitric oxide. Subsidiary roles for bovine leucocyte antigen (BoLA)-restricted, transiently appearing, cytotoxic T cells, and possibly also natural killer (NK) cells, have been identified. Cytokines such as tumour necrosis factor alpha (TNF-alpha) may have important roles, particularly in the induction of pathology. Matrix metalloproteinases have been implicated in the metastatic behaviour of schizont-infected cells. The nature of the protective schizont target antigens remains unknown. Attempts to develop a subunit vaccine have focused upon a sporozoite antigen (SPAG-1) and a merozoite antigen (Tams1). Both SPAG-1 and Tams1 have given partial protection using different delivery systems and adjuvants, but further vaccine development will probably require identification of a range of other antigens, especially from the schizont stage. Theileria parva has a tropism for T cells. Vaccination is currently by the 'infection and treatment' method, which involves challenging with a controlled dose of sporozoite stabilate and the simultaneous administration of long-acting tetracyclines. The immunity thus induced is mediated by BoLA-restricted cytotoxic T cells, which recognize polymorphic schizont antigens. These antigens have not been characterized at the molecular level. However, the polymorphic nature of the target antigens underlies the fact that the immunity is very strain specific--a situation that distinguishes T. parva from T. annulata. Interestingly, it is not possible to produce an attenuated vaccine to T. parva, as T. parva requires up to two orders of magnitude more schizonts in order to achieve transfer to the new host. A suggested reason for this is that the macrophage targets of T. annulata are phagocytes and thus the schizont has a natural, efficient route of entry whilst the preferred host of T. parva is the non-phagocytic T cell. Analysis of the cytotoxic T-cell response has revealed evidence of BoLA haplotype dominance plus competition between parasite epitopes. Subunit vaccination using a recombinant sporozoite antigen (p67) has proved very promising, with levels of protection of the order of 70% being achieved. A proportion of the protected calves exhibits complete sterile immunity. Interestingly, the basis for this immunity is not clear, since there is no correlation between the titre of antibodies that inhibit sporozoite penetration of lymphocytes and protection. Similarly, there is no significant T-cell response that distinguishes the protected and susceptible animals. These data are very encouraging, but other components, particularly those derived from the schizont, need to be identified and characterized. The mild Theileria species of Japan and Korea (termed T. sergenti in the literature) cause fever and severe chronic anaemia. The schizont stage of the life cycle is very rare and the host cell type is not known. The pathology is associated with chronic piroplasm infection. Immunity can be induced by immunizing with crude piroplasm extracts. Serological analysis of immune sera reveals that the immunodominant antigen is a polypeptide of 30-33 kDa, which corresponds to the protective T. annulata polypeptide Tams1. (ABSTRACT T