Review: Theileria schizonts induce fundamental alterations in their host cells

The sporozoites of Theileria annulata and T. parva invade bovine leukocytes, where they differentiate into schizonts. The latter can immortalize and induce fundamental changes in their host cells. T. annulata infects mainly major histocompatibility complex class II cells, whereas T. parva preferentially transforms T-lymphocytes, which proliferate continuously without the need for exogenously added growth factors. Most of the cell lines appear to be independent of a growth factor but may express several cytokines that influence the outcome of the disease. The mechanisms underlying this transformation are not well understood. The infected cells show increased activity of casein kinase II and Jun NH2-terminal kinase (JNK), whereas extracellular signal-related kinase 1 and 2 and P38 are not activated. In addition, several transcriptional factors such as NFkB and AP-1 are activated. It has been postulated that parasite proteins either expressed on the surface of the schizonts or secreted into the host cell cytoplasm may interfere with the signal-transduction pathway of the host cells. A possible candidate may the casein kinase II homologue that was identified in schizonts of both T. annulata and T. parva.

Review: the cellular basis of the immunity to and immunopathogenesis of tropical theileriosis

The intracellular protozoan parasite Theileria annulata causes a severe and often fatal disease of pure and crossbred cattle in tropical and subtropical countries. Animals that recover from the infection are immune against challenge with homologous parasite strains. In the present review we refer to the role of immunocompetent cells and their products in containing the infection or in facilitating the progress of the disease. Parasite-infected host cells produce cytokines, which, depending on their concentration and timing of production, may enhance the establishment of the infection. Thus, cell lines producing high levels of proinflammatory cytokines cause severe postvaccinal reactions when inoculated into cattle. This may be supported by an aberrant non-specific activation of naive T-cells, leading to the production of high levels of gamma-interferon (IFN-y). Under these circumstances development of the specific immune response may be inhibited. At this stage, innate immune reactions are operating to contain the infection. Natural killer cells and macrophages may represent the most important part of this immunity. Antibodies and specific T-lymphocytes, CD4+ T-cells and cytotoxic T-lymphocytes (CTLs), play the most important role in a challenge infection. In this context, CD4+ T-cells produce cytokines required for the clonal expansion of CTLs that kill their target cells in a major histocompatibility complex (MHC) class I-restricted manner. In addition, CD4+ T-cells produce macrophage-activating cytokines such as IFN-gamma. Such activated macrophages produce mediators such as NO, which destroy the intracellular schizonts. Attempts have been directed toward the identification of parasite antigens involved in the induction of immunity. To date, only a limited number of sporozoite and merozoite antigens have been identified and examined for their immunogenicity, and the protection achieved is partial. An effective vaccine must include schizont proteins, notably, those proteins that are secreted into the host cell cytoplasm because these may have access to the MHC class I and II compartments to be presented to CTLs and CD4+ T-cells, respectively. Several schizont proteins have been identified and these are now under investigation.

Innate and adaptive immune responses co-operate to protect cattle against Theileria annulata

For many years it was assumed that Theileria annulata resembled T. parva, parasitizing lymphocytes and causing lymphoproliferative disease, with the two species being controlled by similar protective immune responses. Patricia Preston et al. here review the evidence that has led to a different view of T. annulata. It is now thought that the schizonts of T. annulata inhabit macrophages and B cells, and that tropical theileriosis is not a lymphoproliferative disease. Both innate and adaptive responses contribute to recovery from infection and resistance to challenge and cytokines produced by infected and uninfected cells influence the outcome of infection. Partial protection has been stimulated recently by defined recombinant antigens; efficacy depended upon the delivery system.

Is interleukin 2 necessary for the autocrine proliferation of Theileria-infected bovine cells?

Theileria-infected bovine lymphoblastoid cells are induced to proliferate permanently such that the division of the parasites and that of their host cells occur synchronously. The mechanism by which the parasites induce the transformation of their host cells is unknown. We investigated the growth-factor dependency of a number of Theileria-infected cell lines. Supernatants of the majority of the cell lines tested in our study showed no growth-enhancing activity. However, supernatants of a limited number of cell lines occasionally enhanced the growth of parasitized cells that were used as responder cells. Thus, they contained a growth factor whose biological effect was not eliminated by an anti-interleukin-2 (anti-IL-2) antibody. Moreover, neither the proliferation of T. parva-infected cells nor that of T. annulata-infected cells was impaired by this antibody. In contrast, the anti-IL-2 antibody substantially prevented bovine peripheral blood mononuclear cells from undergoing a proliferative response upon stimulation with concanavalin A (Con A). In line with these results we observed that unlike Con A-stimulated lymphocytes, the infected cell lines did not express IL-2 mRNA. Taken together, our results suggest that Theileria-infected cells do not secrete IL-2 and that IL-2 does not play an important role in the autocrine proliferation of the parasitized host cells.

Evidence for strain specificity in cytotoxic T-lymphocyte-mediated, major histocompatibility complex class I-dependent killing of Theileria annulata-infected cells

Cattle immunised against Theileria annulata with one parasite strain have been found to be immune to re-challenge with different strains of the parasite. However, recent evidence of apparent strain specificity has been documented in cattle immunised with attenuated parasite-infected cells. In this study the strain specificity of major histocompatibility complex class I-restricted cytotoxic T-lymphocytes (CTL), a major anti-parasite effector mechanism, was examined. CTL generated following challenge with the Hissar (Indian) strain effectively lysed autologous cells infected with this strain of the parasite. However, CTL were less effective against cells infected with the Gharb (Moroccan) strain and showed virtually no reactivity against the Ankara (Turkish) strain, providing the first direct evidence for strain specificity in immune responses against T. annulata.

Detection and differentiation of Theileria annulata and Theileria parva using macroschizont-derived DNA probes

A lamda gt11 expression library based on T.annulata-infected cells was screened with an antiserum raised in rabbits against partially purified schizonts of T.annulata. Two clones were detected, sequenced and designated as SA288 and SB288 (Shayan et al., submitted for publication). From the sequences of these two genes oligonucleotide primers were designed for specific amplification of parasite DNA by polymerase chain reaction (PCR). We could show that these genes are of parasitic origin and do occur in all T.annulata stocks tested in the present study. In addition, a target sequence for SA288 could also be identified in T.parva-schizonts. None of them reacted with genomic DNA of different Babesia spp. A third primer pair was designed from the DNA-sequence of a gene encoding for the T.parva-specific casein kinase II-alpha subunit. Using this primer pair, a target sequence could only be detected in T.parva. Taken together, the primers described here can be used as molecular tools in PCR for the detection of Theileria parasites and to distinguish T.annulata from T.parva.

Macroschizonts of Theileria annulata as vaccine and diagnostic tools

T. annulata-infected cells present infection-associated peptides. These peptides represent target molecules of the cytotoxic acting cells. Their preparation and characterization may help to develop a sub-unit vaccine. Our studies show that macroschizont-infected bovine cells can be used as parasite antigen in serology for the detection of parasite-specific antibodies in serum of infected animals. Primers derived from the macroschizont of T. annulata can be used as molecular tools for the detection of parasite DNA in blood samples of carrier cattle.

Theileria-mediated constitutive expression of the casein kinase II-alpha subunit in bovine lymphoblastoid cells

Theileria-infected cells are induced to undergo a transformation that is reversible, since their proliferation is inhibited after elimination of the schizonts by the theilericidal drug buparvaquone. The molecular mechanisms of the transformation remain unknown. The experiments described in the present report deal with the role of casein kinase (CK) II, a serine/threonine protein kinase, in the permanent proliferation of the parasitized cells and show that the CK II-alpha subunit is expressed in both T. annulata- and T. parva-infected cells and that its expression is closely related to the presence of the parasites in the host-cell cytoplasm. Thus, elimination of the schizonts by buparvaquone leads to the inhibition of CK II-alpha subunit mRNA expression without affecting the expression of actin. Cells treated with 5,6-dichloro-1-beta-D-ribofuranosylbenzimidazole (DRB) are inhibited in a dose-dependent manner from under-going DNA synthesis as measured by [3H]-thymidine incorporation and from expressing CK II. Furthermore, a host-cell-specific CK II-alpha antisense inhibits DNA synthesis in a dose-dependent manner. In the present study, 6 microM antisense reduced [3H]-thymidine incorporation by Theileria-infected bovine cells to about 50%. Using a primer derived from T. parva CK II, we detected a parasite-specific CK II mRNA in T. parva-infected cell lines. Interestingly. DRB also inhibited the expression of the parasite-specific CK II. However, to date we have not detected a target sequence for this primer in T. annulata schizonts.

Production of interferon by Theileria annulata- and T. parva-infected bovine lymphoid cell lines

Theileria annulata and T. parva-infected lymphoblastoid cells were examined for their capacity to produce interferon (IFN). Supernatants of such cells were tested in biological assay for their antiviral activity. Only T. parva-infected cells of T-cell origin were capable of producing IFN-gamma. Supernatants of some but not all T. annulata-infected cells showed also antiviral activity, which was greatly reduced after exposure to a pH of 2. Northern-blot analysis of the cells using an IFN-gamma cDNA probe confirmed the results obtained for T. parva-infected cells in a biological assay. No IFN-gamma mRNA was detected in T. annulata-infected cells. The importance of IFN for the pathogenesis of theileriosis is discussed.

Effect of buparvaquone on the expression of interleukin 2 receptors in Theileria annulata-infected cells

Theileria annulata-infected cells were cultured in the presence or absence of human recombinant interleukin 2 (hrIL-2). This growth factor proved to be capable of enhancing the growth of the infected cells: its effect was marked, particularly when the cells were seeded at low densities, and it varied from cell line to cell line. The infected cells produced a factor that possessed the biological activities of IL-2, since their supernatants could enhance the proliferation of concanvalin A-stimulated (Con A) blasts. The reactivity of the parasitized cells to hrIL-2 was abolished following their treatment with the antitheilerial drug buparvaquone. In addition, the drug inhibited the binding of 125I-IL-2 to T. annulata-infected cells but failed to suppress its binding to Con A blasts. Northern blot analysis revealed that the drug had no effect on the expression of the alpha chain of the IL-2 receptor (IL-2R). Therefore, it is possible that buparvaquone interferes with the expression of the beta chain of the IL-2R. The role of IL-2 and the IL2R in the permanent proliferation of T. annulata-infected cells is discussed.

Effect of cyclosporin A on the proliferation of bovine lymphocytes to concanavalin A and on the growth of Theileria annulata-infected bovine cells

After being inoculated by Hyalomma ticks, the sporozoites of Theileria annulata invade bovine lymphocytes, where they subsequently differentiate to schizonts. The infected cells are induced to a continuous proliferation which can be enhanced by human recombinant interleukin 2 (hrIL-2). In the present study, we examined the influence of cyclosporin A (CsA) on the growth of schizont-containing cells and compared with its effect on bovine peripheral blood lymphocytes (PBL) responding to Concanavalin A (ConA). In both cell types, the proliferation was inhibited in a dose dependent manner, which was not restorable in T. annulata-infected cells even after addition of hrIL-2. In contrast, ConA-blasts were able to undergo a proliferative response provided they were treated with high doses of CsA. Both, T. annulata-infected cells and bovine ConA-blasts express IL-2 receptors (IL-2R). The binding of radiolabelled hrIL-2 to ConA-blasts and T. annulata-infected cells was only partially inhibited after treatment with CsA. CsA was not toxic for the parasites, since the treated cells still contained schizonts which did not show any morphological abnormality.

Application of luminol-dependent chemiluminescence to assay opsonizing antibodies to procyclic forms of Trypanosoma congolense in the sera of dogs experimentally infected with heterologous stocks

Luminol-dependent chemiluminescence (LCL) responses of dog granulocytes were used to assay opsonizing antibodies to procyclic culture forms of T. congolense. A high degree of sensitivity was demonstrated and LCL levels were high, when the phagocytic cells were incubated with the sera of infected dogs even at dilutions as high as 1:400 as compared to pre-infection or negative sera. The levels of opsonizing antibodies were elevated in all the dogs throughout the time of observation. The technique was sensitive, could be automated and, therefore, allows a rapid evaluation of large numbers of serum samples for Trypanosoma specific antibodies.

Buparvaquone but not cyclosporin A prevents Theileria annulata-infected bovine lymphoblastoid cells from stimulating uninfected lymphocytes

The influence of Buparvaquone on the morphology, proliferation, and stimulation with T and B cell mitogens of Theileria annulata-infected cells was studied. In addition, the stimulatory capacity of the infected cells before and after treatment with Buparvaquone or cyclosporin A (CsA) was also examined and compared to that of ConA-stimulated bovine peripheral blood cells (PBL). After incubation of the cells for 4 days with Buparvaquone only few schizonts were detectable in the cells. Prolongation of the incubation time to 8, 12, or 14 days eliminated completely the parasites. Despite the elimination of the parasites, the cells were still unable to undergo a proliferative response to Con A or PWM. However, the drug did not interfere with the response of normal PBL to these mitogens. Furthermore, Buparvaquone but not CsA inhibits the generation of mixed lymphocyte reaction (MLR). None of the drugs could prevent ConA-blasts from stimulating autologous PBL. These results suggest that the antigen expressed by the infected cells and recognised by the responder PBL was induced by the schizonts.

In vitro proliferative and cytotoxic responses of PBL from Theileria annulata-immune cattle

Peripheral blood lymphocytes were isolated from healthy calves and were subsequently infected with sporozoites of Theileria annulata in vitro. The infected cells were passaged for 50 times and thereafter inoculated into animals from which they were previously isolated. Within 4-5 days, schizont-containing cells were demonstrable in the lymph nodes of all animals. Few days later, merozoites were detected in erythrocytes. A slight decrease in the counts of lymphocytes and leucocytes was also found. After 2 months these animals and a group of uninfected calves were heavily infected by tick-infestation and showed severe symptoms of theileriosis with 60% schizont-containing cells in the lymph nodes and a parasitaemia of about 35%. Because of the severity of the infection, all control calves were treated with Halofuginone. In contrast, the initially immunized cattle (by inoculation of culture cells), survived the infection without chemotherapy. Less than 10% of their lymph node cells contained schizonts, whereas less than 1% of their erythrocytes were found to be infected with merozoites. In all immunized animals, specific cytotoxic PBL, with the capacity to lyse autologous but not allogeneic infected cells, were demonstrated. In addition, a population of PBL were found to be able to inhibit the growth of T.annulata-infected culture cells in vitro. However, in comparison to PBL of immune animals, PBL of acute infected calves were superior in their capacity to inhibit the proliferation of schizont-containing cells. In mixed lymphocyte reactions, T. annulata-infected cells could induce a more pronounced proliferative response in PBL from immune than in PBL of uninfected animals.

The intermediate-affinity interleukin (IL)2 receptor expressed on Theileria annulata-infected cells comprises a single IL 2-binding protein. Partial characterization of bovine IL2 receptors

Bovine high-, intermediate- and low-affinity interleukin 2 receptors (IL2R) were studied by ligand binding and affinity labeling using 125I-labeled IL2 and homobifunctional chemical cross-linking reagents. High- (Kd = 17 pM) and low-affinity (Kd greater than 6 nM) IL 2R were detected on concanavalin A-activated peripheral blood lymphocytes (PBL). Theileria annulata (TA)-infected autonomously growing PBL (TA-PBL) express predominantly intermediate-affinity IL2R (Kd = 1 nM). Affinity-labeling studies revealed that the high-affinity IL2R comprises a 55-kDa (L chain) and an additional 90-kDa IL 2-binding protein (H chain). TA-PBL express predominantly the H chain. In contrast to the human chain, the bovine form was not separable by sodium dodecyl sulfate-polyacrylamide gel electrophoresis/urea into the two distinct bands H1 and H2 and ran in parallel to the human H2 band. These results suggest (a) that the bovine intermediate-affinity IL2R comprises a single H chain and (b) that the single H chain and the L chain are sufficient to form the functional high-affinity bovine IL2R.

Enhancement of neutrophil adherence to Toxocara canis larvae by the C3 component of complement and IgG antibodies

In the present study, the binding human neutrophils to Toxocara canis larvae was examined in vitro. It was found that IgG and/or the C3 component of complement were able to enhance the attachment of the cells to the parasites. This was associated with the generation of strong chemiluminescence reactions. Despite the binding of the cells, the larvae were able to escape and to maintain their infectivity. This was apparent, since the parasites could survive and migrate into different organs including the brain after having been inoculated into mice.

The role of antibodies in immunity against Theileria annulata infection in cattle

The role of antibodies in protecting cattle against Theileria annulata infection was investigated. It was found that serum samples from immune cattle had a neutralizing effect on the sporozoites preventing in this way the invasion of lymphocytes. However, the same antiserum neither caused lysis of macroschizont- infected lymphoblastoid cells nor inhibited their proliferation in vitro. Free merozoites but not the infected erythrocytes were opsonized by immune serum. In addition, complement alone, without antibody participation, was able to lyse free merozoites. The importance of humoral factors in immunity to T. annulata infection is discussed.

The role of interleukin 2 (IL 2) in the proliferation of Theileria annulata-infected bovine lymphocytes

The role of interleukin 2 (IL 2) in the proliferation of Theileria annulata-infected lymphoblastoid cells was studied. It was found that these cells neither require nor produce IL 2. This was based on the following findings: (a) The growth of T. annulata-infected cells was not enhanced by addition of IL 2 to the culture medium. (b) IL 2-dependent Con A-stimulated bovine peripheral blood lymphocytes (Con A-blasts) no longer required IL 2 after being infected with sporozoites of T. annulata. (c) The supernatants of T. annulata-infected cells did not support growth of IL 2-dependent Con A-blasts.

Application of chemi- and bioluminescence in studies of immunological reactions against protozoa

The opsonization and lysis of different protozoa by antibodies and/or complement was followed using luminol-dependent chemiluminescence and bioluminescence. The addition of immune serum to variable antigen type populations of Trypanosoma evansi led to the specific opsonization of trypanosomes resulting in an intense metabolic activation and chemiluminescence response of phagocytic cells. In comparison to those of uninfected control mice, the phagocytosis of coccidia merozoites by spleen cells from mice infected with Eimeria falciformis was enhanced during the acute stage of a primary infection. Opsonizing activity was demonstrated in phosphate-buffered saline extracts of gut contents of mice infected for 10 days. The incubation of E. falciformis merozoites together with guinea-pig complement resulted in slow lysis of the cells. The addition of mouse serum collected greater than 6 days after an infection led to an accelerated lysis of the merozoites, indicating the appearance of complement-fixing antibodies in the serum. Heat-inactivated immune serum alone had no lysing activity on merozoites. In the presence of complement, bovine lymphoblastoid cells infected with Theileria annulata were lysed by anti-lymphoblastoid cell serum raised in mice but not by serum from cattle which had developed immunity to Theileria annulata.

Variant specific opsonization of Trypanosoma evansi measured by luminol-dependent chemiluminescence

Using luminol-dependent chemiluminescence (LCL), the specificity of antibodies to variable antigen type (VAT)-populations of Trypanosoma evansi was studied in four infected ponies. Trypanosomes of each wave of parasitemia were isolated and multiplied in irradiated mice. Their opsonization by serum collected during the infection was investigated with LCL and results for isolated VAT-populations are shown in the paper. Antibodies specific to each VAT-population were first found three days after the maximum of a parasitemic wave. There was no cross reactivity between different VAT-populations. LCL proved to be a rapid and automatic method for the demonstration of antibodies with specificity to variable antigen types of trypanosomes.

Antigenic variation of Trypanosoma evansi in rabbits

In order to study the occurrence and sequential appearance of antigenic variation during the course of T. evansi-infection, rabbits were infected with a clone prepared from T. evansi, Java/55/PTV/1 (stock 1, clone 5). During the course of infection, trypanosome-populations were isolated from the cerebrospinal fluid and from each parasitaemia peak. Specific antisera against the clone, 6 parasitaemia-populations and two different liquor-populations (all isolated from 1 rabbit), were produced in rabbits. These antisera were used for the characterization and comparison of trypanosome populations with each other by means of immunolysis as well as neutralisation-infectivity-(NIF)-test. It was found that in all infected rabbits, the first parasitaemia contained trypanosomes, which were almost identical to those present in the clone. However, the clone contained already minor variant antigenic types ("minor-VATs"). All trypanosome-populations isolated from rabbits were heterogeneous. Heterogeneity was more pronounced in the later phases of the infection. After each fluctuating parasitaemia new dominant variants ("major VAT") were expressed. The sequential appearance of antigenic variation was similar among all infected animals. The effectivity of immunolysis was checked by the NIF-test. As it was expected, complete lysis of all trypanosomes of a population was not achieved, because some "minor VATs" could escape the influence of the antiserum. By this way, they formed the basis of new "major VATs" which dominated in the next fluctuating parasitaemia.

Immediate type hypersensitivity in murine coccidiosis

Subcutaneous injection of soluble merozoite antigen into the footpad of mice previously infected with Eimeria falciformis caused an immediate type hypersensitivity as measured by footpad swelling (FPS). This reaction was first demonstrable ten days after the infection. Maximal values were achieved on day 13, and later on the intensity of FPS decreased. Increase in the dose of oocysts or in the number of infections did not enhance the degree of FPS. The reaction was transferable to normal recipients with immune serum.