Titration in cattle of infectivity and immunogenicity of autologous cell lines infected with Theileria parva

Comparative transcriptional analysis identifies genes associated with the attenuation of Theileria parva infected cells after long-term in vitro culture

Autologous administration of attenuated Theileria parva-infected cells induces immunity to T. parva in cattle. The mechanism of attenuation, however, is largely unknown. Here, we used RNA sequencing of pathogenic and attenuated T. parva-infected T-cells to elucidate the transcriptional changes underpinning attenuation. We observed differential expression of several host genes, including TRAIL, PD-1, TGF-β and granzymes that are known to regulate inflammation and proliferation of infected cells. Importantly, many genes linked with the attenuation of the related T. annulata-infected cells were not dysregulated in this study. Furthermore, known T. parva antigens were not dysregulated in attenuated relative to pathogenic cells, indicating that attenuation is not due to enhanced immunogenicity. Overall this study suggests that attenuation is driven by a decrease in proliferation and restoration of the inflammatory profile of T. parva-infected cells. Additionally, it provides a foundation for future mechanistic studies of the attenuation phenotype in Theileria-infected cells.

Inherited Tolerance in Cattle to the Apicomplexan Protozoan Theileria parva is Associated with Decreased Proliferation of Parasite-Infected Lymphocytes

Theileria parva is the causative agent of East Coast fever and Corridor disease, which are fatal, economically important diseases of cattle in eastern, central and southern Africa. Improved methods of control of the diseases are urgently required. The parasite transforms host lymphocytes, resulting in a rapid, clonal expansion of infected cells. Resistance to the disease has long been reported in cattle from T. parva-endemic areas. We reveal here that first- and second-generation descendants of a single Bos indicus bull survived severe challenge with T. parva, (overall survival rate 57.3% compared to 8.7% for unrelated animals) in a series of five field studies. Tolerant cattle displayed a delayed and less severe parasitosis and febrile response than unrelated animals. The in vitro proliferation of cells from surviving cattle was much reduced compared to those from animals that succumbed to infection. Additionally, some pro-inflammatory cytokines such as IL1β, IL6, TNFα or TGFβ which are usually strongly expressed in susceptible animals and are known to regulate cell growth or motility, remain low in tolerant animals. This correlates with the reduced proliferation and less severe clinical reactions observed in tolerant cattle. The results show for the first time that the inherited tolerance to T. parva is associated with decreased proliferation of infected lymphocytes. The results are discussed in terms of whether the reduced proliferation is the result of a perturbation of the transformation mechanism induced in infected cells or is due to an innate immune response present in the tolerant cattle.

Influence of host and parasite genotypes on immunological control ofTheileriaparasites

Infections withTheileria parvain the African buffalo are invariably asymptomatic, whereas infections in cattle usually result in clinical disease, the severity of which varies in different populations of cattle. The parasite exhibits antigenic heterogeneity, which in cattle manifests as differences between parasite strains in their cross-protective properties. A series of studies on T cell responses toT. parvain cattle have demonstrated that class I MHC-restricted cytotoxic T lymphocytes (CTL), specific for parasitized lymphoblasts, are important mediators of immunity. Cytotoxic T cell responses frequently display parasite strain-restricted specificities which appear to correlate with the capacity of strains to cross-protect. The strain specificity of CTL responses varies in animals immunized with the same parasite strain and is influenced by both host and parasite genotype. Recent studies have provided evidence that there is competition between epitopes for induction of CTL responses, which can result in a bias to strain-specific epitopes. These properties of the CTL response have important implications for vaccination. Thus, in designing a vaccine, it may be possible, by selecting parasite proteins containing appropriate CTL epitopes, to generate CTL responses that protect against a wide range of parasite strains. Although there are no comparable data on CTL responses in the buffalo, it is considered that the features of the immune response described for cattle would be advantageous for survival of parasite populations in the buffalo. Specifically, a bias in the immune responses to strain-specific determinants should favour establishment of infection in buffalo already carrying the parasite and allow fluctuation in the levels of different parasite strains during the course of persistent infection.

A rapid and sensitive intracellular flow cytometric assay to identify Theileria parva infection within target cells

Theileria parva is an intracellular protozoan parasite transmitted by ticks that causes a fatal lymphoproliferative disease of cattle known as East Coast Fever. Vaccination against the disease currently relies on inoculation of the infective sporozoite stage of the parasite and simultaneous treatment with long-acting formulations of oxytetracycline. Sporozoites are maintained as frozen stabilates of triturated infected ticks and the method requires accurate titration of stabilates to determine appropriate dose rates. Titration has traditionally been undertaken in cattle and requires large numbers of animals because of individual variation in susceptibility to infection. An alternative tissue culture-based method is laborious and time consuming. We have developed a flow cytometric method for quantifying the infectivity of sporozoite stabilates in vitro based on the detection of intracellular parasite antigen. The method allows clear identification of parasitized cells with a high degree of sensitivity and specificity. Analysis of infected cells between 48 and 72 h post-infection clearly defines the potential transforming capability of different stabilates.

Current status of vaccine development against Theileria parasites

The tick-borne protozoan parasites Theileria parva and Theileria annulata cause economically important diseases of cattle in tropical and sub-tropical regions. Because of shortcomings in disease control measures based on therapy and tick control, there is a demand for effective vaccines against these diseases. Vaccines using live parasites have been available for over two decades, but despite their undoubted efficacy they have not been used on a large scale. Lack of infrastructure for vaccine production and distribution, as well as concerns about the introduction of vaccine parasite strains into local tick populations have curtailed the use of these vaccines. More recently, research has focused on the development of subunit vaccines. Studies of immune responses to different stages of the parasites have yielded immunological probes that have been used to identify candidate vaccine antigens. Immunisation of cattle with antigens expressed in the sporozoite, schizont or merozoite stages has resulted in varying degrees of protection against challenge. Although the levels of protection achieved have not been sufficient to allow exploitation for vaccination, there are clearly further lines of investigation, relating to both the choice of antigens and the antigen delivery systems employed, that need to be pursued to fully explore the potential of the candidate vaccines. Improved knowledge of the molecular biology and immunology of the parasites gained during the course of these studies has also opened up opportunities to refine and improve the quality of live vaccines.

Culture-derived parasites in vaccination of cattle against tick-borne diseases

The major economically important tick-borne diseases of cattle are theileriosis, babesiosis, anaplasmosis, and cowdriosis. Culture-derived attenuated schizonts of Theileria annulata have proved to be safe for all types of cattle and they protect against tick-borne theileriosis. T. parva was also successfully grown in vitro; however, inoculation of cattle with allogeneic schizont-infected cells resulted in rejection and destruction of the parasites together with the host cells. The number of schizont-infected cells needed for immunization is greater than for T. annulata theileriosis. Culture-propagated Babesia bovis and B. bigemina were used for large scale vaccination in the field. An avirulent population of Babesia spp. was obtained by in vitro cloning; inoculation of cattle did not induce clinical babesiosis, but produced specific antibodies. Culture-derived exoantigens of Babesia spp. proved to be completely safe for cattle, however, they conferred less protection than live parasites. Cell-cultured Cowdria ruminantium was highly infective for susceptible animals but, attenuated in vitro, could offer a potential source for vaccination. Anaplasma marginale, successfully grown in tick cell culture, may be developed for vaccines. Factors that should be considered in the developing of vaccines against tick-borne diseases include: the protective immune response to the pathogenic parasite developmental stages, virulence, immunological strain differences, and antigenic variations in cattle and in culture.

Pathogenicity of Theileria parva is influenced by the host cell type infected by the parasite

Theileria parva has been shown to infect and transform B cells and T cells at similar frequencies in vitro. However, the majority of parasitized cells in the tissues of infected cattle are alpha/beta T cells. The aim of this study was to determine whether the cell type infected with T. parva influenced the pathogenicity of the parasite. The initial approach, which involved inoculation of cattle with autologous cloned cell lines of different phenotypes, failed to resolve the issue, because of prolonged period of culture required to clone and characterize the cell lines resulted in attenuation of the cells. As an alternative approach, cattle were inoculated with purified populations of autologous cells that had been incubated in vitro with T. parva sporozoites for 48 h. As few as 3 x 10(4) peripheral blood mononuclear cells (PBMC) treated in this way were found to produce severe clinical reactions with high levels of parasitosis. Infections of similar severity were produced with purified populations of CD2+, CD4+, and CD8+ T cells. By contrast, infected B cells gave rise to mild self-limiting infections even when administered at a 10-fold-higher dose. In animals that received infected CD4+ or CD8+ T cells, the parasitized cells in the lymph nodes on day 11 of infection were all within the CD4+ and CD8+ populations, respectively, indicating that there had been minimal transfer of the parasite between cell types. Phenotypic analyses of cultures of PBMC infected in vitro with saturating concentrations of sporozoites revealed that parasitized B cells were abundant in the cultures after 1 week but were subsequently overgrown by T cells. The results of these experiments indicate that the cell type infected by T. parva influences the pathogenicity of the parasite.

Generation and characterization of cloned Theileria parva parasites

A 3-step procedure for cloning Theileria parva parasites was developed. The first step involved the in vitro infection of a fixed number of bovine lymphocytes with titrated sporozoites. The cell lines obtained from infections initiated using sporozoite/lymphocyte ratios below 1:100 were then selected for cloning as these contained schizont-infected cells, each of which was derived from infection with a single sporozoite. In the second step, these cell lines were cloned by limiting dilution. As sporozoites infect lymphocytes and transform to induce clonal multiplication, this step produced infected cell lines containing both cloned parasites and cloned lymphocytes. In the third step, the cloned cell lines were used to infect cattle and isolation of the parasite in ticks was made during piroplasm parasitaemia. Finally, sporozoites were harvested from infected ticks and used for further characterization. Sporozoites derived from cloned cell lines of T. parva Muguga, Marikebuni, Boleni, Uganda and buffalo-derived 7014 were characterized using monoclonal antibody profiles, DNA restriction fragment length polymorphism detected using repetitive and telomeric probes, in vivo infectivity and, in one case, cross-immunity studies. Additionally, several distinct schizont-infected lymphocyte clones were isolated from the Muguga, Mariakani and buffalo-derived 7014 stocks. The combined results of the characterization revealed that the cloning procedure selected clones of T. parva from the parental stocks which were known to contain a mixture of genetically different parasite populations. The cloning method and the clones generated will be of value in studies of the biology of the parasite and in elucidating the strain specificity of immune responses in cattle.

Expression of Tac antigen component of bovine interleukin-2 receptor in different leukocyte populations infected with Theileria parva or Theileria annulata

The Tac antigen component of the bovine interleukin-2 receptor was expressed as a Cro-beta-galactosidase fusion protein in Escherichia coli and used to raise antibodies in rabbits. These antibodies were used for flow cytofluorimetric analysis to investigate the expression of Tac antigen in a variety of Theileria parva-infected cell lines and also in three Theileria annulata-infected cell lines. Cells expressing Tac antigen on their surface were found in all T. parva-infected cell lines tested whether these were of T- or B-cell origin. T cells expressing Tac antigen could be CD4- CD8-, CD4+ CD8-, CD4- CD8+, or CD4+ CD8+. Tac antigen expression was observed both in cultures which had been maintained in the laboratory for several years and in transformed cell lines which had recently been established by infection of lymphocytes in vitro with T. parva. Northern (RNA) blot analysis demonstrated Tac antigen transcripts in RNA isolated from all T. parva-infected cell lines. Three T. annulata-infected cell lines which were not of T-cell origin were also tested. Two of them expressed Tac antigen on their surface. Abundant Tac antigen mRNA was detected in these T. annulata-infected cell lines, but only trace amounts were demonstrated in the third cell line, which contained very few Tac antigen-expressing cells. In all cell lines tested, whether cloned or uncloned, a proportion of the cells did not express detectable levels of Tac antigen on their surface. This was also the case for a number of other leukocyte surface markers. In addition, we showed that the interleukin-2 receptors were biologically functional, because addition of recombinant interleukin-2 to cultures stimulated cell proliferation. Recombinant interleukin-2 treatment also resulted in increased amounts of steady-state Tac antigen mRNA. The relevance of interleukin-2 receptor expression on Theileria-infected cells is discussed.

Characterization of species and strains of Theileria

A variety of methods is now available for characterizing species and strains of Theileria. For many practical purposes involving field control of theileriosis, characterization on a broad basis may be sufficient, but in other areas much more precise characterization is required. Such precision can be usefully exploited only when cloned parasite populations are involved, and methods to improve parasite characterization and parasite cloning should be developed concurrently. The current methods of immunization against theileriosis involve the use of live parasite populations which are generally poorly defined and, in addition, have the capacity to undergo biological change (by selection, mutation or genetic recombination) within hosts and vectors. Such changes may be difficult to define and identify, but could have profound effects on immunization strategies. Improved methods of parasite characterization and selection, which are now becoming available, will enable parasite stocks for immunization to be identified and selected more precisely, and any biological changes that occur can be monitored. Improved methods of parasite characterization will also open the way to a better understanding of Theileria genetics and the mechanisms of heritability, which appear to differ in some fundamental ways from patterns of Mendelian inheritance. Controlled matings between selected and defined populations of parasites can be envisaged, with the aim of producing hybrid parasites for immunization. In addition, the prospects of modifying the theilerial genome by genetic manipulation become very real: transfection vectors tailored by restriction enzymes could be used to insert or modify gene sequences to develop parasites with appropriate sets of characters. It may also be possible to identify parasite genes which trigger the cytotoxic response which is so important in immunity (Eugui and Emery, 1981; Emery et al., 1981; Preston et al., 1983). Such genes might then be transfected into bovine host lymphocytes to generate immunity against the whole parasite (Iams, 1985). The gene products which are responsible for stimulating immune responses could also be synthesized artificially and used for vaccination. Methods of characterizing Theileria range from Giemsa's staining to DNA hybridization; all have a role to play, and by judicious selection of appropriate methods for particular circumstances, it is becoming possible to characterize theilerial parasites very precisely. Improved methods of characterization can, in turn, lead to a better understanding of parasite biology and to the development of improved methods of immunization and control.

Comparative analysis of infection and transformation of lymphocytes from African buffalo and Boran cattle with Theileria parva subsp. parva and T. parva subsp. lawrencei

This study compared infection and transformation of peripheral blood mononuclear cells (PBM) of Boran cattle and African buffalo in vitro to determine whether differences occurred which could account for the greater susceptibility of Boran cattle to infection with Theileria parva subsp. parva and T. parva subsp. lawrencei. PBM from buffalo and cattle had a similar percentage of cells which bound T. parva subsp. parva sporozoites (24 to 34%) and in which schizonts developed during the first week after infection (18 to 23%). Using a limiting dilution culture system, it was established, however, that a significantly higher proportion of cattle PBM transformed into continuously replicating cell lines after infection with T. parva subsp. parva than did buffalo PBM. The evidence suggests that the low capacity of T. parva subsp. parva to establish infections in buffalo compared with cattle is related to the lower frequency of buffalo cells which undergo transformation. With T. parva subsp. lawrencei, however, the frequency of transformation of buffalo PBM was higher than that for cattle PBM. The frequency of cells transformed by T. parva subsp. lawrencei, therefore, cannot account for the greater resistance of buffalo to infections with T. parva subsp. lawrencei. Buffalo must have other mechanisms, either innate or acquired, which control infection with T. parva subsp. lawrencei more efficiently than in cattle.

Induction of immunity against infection with Theileria parva (East Coast fever) in cattle using plasma membranes from parasitized lymphoblasts

Protection against challenge with Theileria parva was conferred on three of four calves given three or four inocula of plasma membranes prepared from 6 to 12 X 10(8) autologous parasitized lymphoblasts from cultured cell lines. In contrast, calves remained susceptible to infection following immunization with membranes prepared from allogeneic parasitized lymphoblasts. Similarly, calves vaccinated with either gamma-irradiated autologous or allogeneic infected cells also died of East Coast fever after challenge. The results raise the possibility of vaccination against T. parva using subcellular material from infected lymphoblasts.

Immunity in theileriosis

The theilerioses can be separated on the basis of their principal pathogenic features, into a lymphoproliferative group caused by Theileria parva and T. annulata in cattle, and T. hirci in goats and sheep, and a haemoproliferative group caused by T. sergenti and T. mutans both in cattle. In the former group, proliferation of parasites within lymphoid cells followed by lymphodestruction are the main pathogenic features; whereas in the latter group, invasion and destruction of erythrocytes, causing anaemia, are more important. In addition, a number of other theilerial parasites which cause mild or inapparent infections, are found in domestic livestock. This review focuses on T. parva, the causative agent of East Coast fever (ECF) in cattle in East and Central Africa, because it is the most pathogenic species and the immunology of ECF has been more intensively studied than that of the other theilerioses.