Differential response of bovine T-cell lines to membrane and soluble antigens of Theileria parva schizont-infected cells

An Ad/MVA vectored Theileria parva antigen induces schizont-specific CD8+ central memory T cells and confers partial protection against a lethal challenge

The parasite Theileria parva is the causative agent of East Coast fever (ECF), one of the most serious cattle diseases in sub-Saharan Africa, and directly impacts smallholder farmers’ livelihoods. There is an efficient live-parasite vaccine, but issues with transmission of vaccine strains, need of a cold chain, and antibiotics limit its utilization. This has fostered research towards subunit vaccination. Cytotoxic T lymphocytes (CTL) are crucial in combating the infection by lysing T. parva-infected cells. Tp1 is an immunodominant CTL antigen, which induces Tp1-specific responses in 70–80% of cattle of the A18 or A18v haplotype during vaccination with the live vaccine. In this study, human adenovirus serotype 5 (HAd5) and modified vaccinia Ankara (MVA) were assessed for their ability to induce Tp1-specific immunity. Both viral vectors expressing the Tp1 antigen were inoculated in cattle by a heterologous prime-boost vaccination regimen. All 15 animals responded to Tp1 as determined by ELISpot. Of these, 14 reacted to the known Tp1 epitope, assayed by ELISpot and tetramer analyses, with CTL peaking 1-week post-MVA boost. Eleven animals developed CTL with specific cytotoxic activity towards peripheral blood mononuclear cells (PBMC) pulsed with the Tp1 epitope. Moreover, 36% of the animals with a Tp1 epitope-specific response survived a lethal challenge with T. parva 5 weeks post-MVA boost. Reduction of the parasitemia correlated with increased percentages of central memory lymphocytes in the Tp1 epitope-specific CD8⁺ populations. These results indicate that Tp1 is a promising antigen to include in a subunit vaccine and central memory cells are crucial for clearing the parasite.

A vaccine expressing parasitic proteins offers more convenient East Coast fever prophylaxis. Current vaccination for the cattle disease, caused by the parasite Theileria parva and a detriment to sub-Saharan African farmers, involves inconvenient injection with live parasites before antibiotic treatment (ITM). A collaboration led by Nicholas Svitek, of the Kenyan International Livestock Research Institute, designed a candidate to provoke cellular immune responses against the parasitic antigen Tp1—an ITM vaccine candidate. In tests on cattle, 93% created Tp1-targeting T cells, and 33% survived a lethal dose of T. parva. The East Coast fever reduction seen in animals in this research outperformed a recent study and was able to generate the same immune memory cells that ITM inspires to provide long-lasting protection. Future research might integrate more antigens with this Tp1 vaccine to provide more comprehensive protection.

A highly sensitive, non-radioactive assay for T cell activation in cattle: applications in screening for antigens recognised by CD4(+) and CD8(+) T cells

We describe a highly sensitive, non-radioactive assay for T cell activation, based on the rapid induction of class II MHC expression by constitutively negative bovine endothelial cells, when cultured in the presence of supernatants derived from activated bovine T cells. We demonstrate the effectiveness of this assay in detecting rBoIFNgamma and activation of immune CD4(+) and CD8(+) T cell lines and clones in response to specific antigen and transfected COS-7 cells, respectively. We also demonstrate its utility in identifying purified pathogen fractions that activate immune CD4(+) T cell clones.

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

Prospects for subunit vaccines against tick-borne diseases

Tick-borne parasites are a serious impediment to the improvement of live-stock production in the developing world. The major parasites affecting cattle include Theileria parva, T. annulata, Babesia bigemina, B. bovis, Anaplasma marginale and Cowdria ruminantium. The control of these infections is dependent on the use of acaricides to decrease transmission by the tick vectors, and immunization of susceptible animals with live vaccines. The use of acaricide is hampered by the development of resistance, and live vaccines require cold chain facilities, which are generally unreliable in developing countries. There is therefore a need for improved vaccines that can circumvent these problems. There is a subunit vaccine being developed for T. parva based on the major surface antigen of the sporozoite (p67). A similar antigen, SPAG 1, has been identified as a candidate for T. annulata. Although several candidate antigens have been identified for Babesia spp., progress towards development of a subunit vaccine based on these antigens has been hampered by polymorphism among isolates and between species, and lack of knowledge of the immune effector mechanisms responsible for protection. The search for protective antigens of A. marginale has focused on outer membrane proteins; immunization with a variety of these antigens alone or in combination, has yielded promising results. As with Babesia, further definition of immune effector mechanisms is needed to optimize immunization strategies. The work on identifying the protective antigens of C. ruminantium is in its embryonic stages; however, two antigens have been identified and are currently being evaluated. There is high expectancy for subunit vaccines for all these diseases; however there is need for further work to elucidate the immune mechanisms in order to select appropriate antigen delivery systems.

Identification of candidate vaccine antigens of bovine hemoparasites Theileria parva and Babesia bovis by use of helper T cell clones

Current vaccines for bovine hemoparasites utilize live attenuated organisms or virulent organisms administered concurrently with antiparasitic drugs. Although such vaccines can be effective, for most hemoparasites the mechanisms of acquired resistance to challenge infection with heterologous parasite isolates have not been clearly defined. Selection of potentially protective antigens has traditionally made use of antibodies to identify immunodominant proteins. However, numerous studies have indicated that induction of high antibody titers neither predicts the ability of an antigen to confer protective immunity nor correlates with protection. Because successful parasites have evolved antibody evasion tactics, alternative strategies to identify protective immunogens should be used. Through the elaboration of cytokines, T helper 1-(Th1)-like T cells and macrophages mediate protective immunity against many intracellular parasites, and therefore most likely play an important role in protective immunity against bovine hemoparasites. CD4+ T cell clones specific for soluble or membrane antigens of either Theileria parva schizonts or Babesia bovis merozoites were therefore employed to identify parasite antigens that elicit strong Th cell responses in vitro. Soluble cytosolic parasite antigen was fractionated by gel filtration, anion exchange chromatography or hydroxylapatite chromatography, or a combination thereof, and fractions were tested for the ability to induce proliferation of Th cell clones. This procedure enabled the identification of stimulatory fractions containing T. parva proteins of approximately 10 and 24 kDa. Antisera raised against the purified 24 kDa band reacted with a native schizont protein of approximately 30 kDa. Babesia bovis-specific Th cell clones tested against fractionated soluble Babesia bovis merozoite antigen revealed the presence of at least five distinct antigenic epitopes. Proteins separated by gel filtration revealed four patterns of reactivity, and proteins separated by anion exchange revealed two patterns of reactivity when selected T cell clones were assayed for stimulation by antigenic fractions. Studies using a continuous-flow electrophoresis apparatus have indicated the feasibility of identifying T cell-stimulatory proteins from parasite membranes as well as from the cytosolic fraction of B. bovis merozoites. The Th cell clones reactive with these different hemoparasites expressed either unrestricted or Th1 cytokine profiles, and were generally characterized by the production of high levels of IFN-gamma. A comprehensive study of T cell and macrophage responses to defined parasite antigens will help elucidate the reasons for vaccine failure or success, and provide clues to the mechanisms of acquired immunity that are needed for vaccine development.

Immune CD4+ T cells specific for Theileria parva-infected lymphocytes recognize a 24-kilodalton protein

Theileria parva is a protozoan parasite that infects and transforms bovine lymphocytes. Here we report the partial purification of a T. parva-specific protein from infected lymphocytes that is recognized by CD4+ parasite-specific T-cell clones derived from immune cattle. T. parva-infected lymphocytes were homogenized in Dulbecco's phosphate-buffered saline in the presence of protease inhibitors. The antigen was purified from a postmicrosomal supernatant by using a combination of DEAE-cellulose chromatography and hydroxylapatite column chromatography. After labelling with 125I, the antigen preparation was subjected to sodium dodecyl sulfate-polyacrylamide gel electrophoresis and found to contain 8 to 10 proteins. This preparation was subjected to chromatography in phosphate-buffered saline on HPLC TSK-250/125 columns coupled in tandem. A radiolabelled protein of M(r) 24,000 correlated with antigenic activity.

Induction of feline immunodeficiency virus-specific cytolytic T-cell responses from experimentally infected cats

We have examined the in vitro induction and activity of feline immunodeficiency virus (FIV)-specific cytolytic T cells obtained from cats experimentally infected for 7 to 17 weeks or 20 to 22 months with the Petaluma isolate of FIV. Normal or FIV-infected autologous and allogeneic T lymphoblastoid cells were used as target cells in chromium-51 or indium-111 release assays. When effector cells consisted of either fresh peripheral blood mononuclear cells or concanavalin A- and interleukin-2-stimulated cells, only low levels of cytotoxicity were observed. However, the levels of FIV-specific cytotoxicity were consistently higher in both groups of cats following in vitro stimulation of the effector cells with irradiated, FIV-infected autologous T lymphoblastoid cells and interleukin-2. The effector cells lysed autologous but not allogeneic FIV-infected target cells and were composed predominantly of CD8+ T cells, indicating that the FIV-specific cytotoxicity measured in this system is mediated by CD8+, major histocompatibility complex class I-restricted T cells. These studies show that FIV-specific cytolytic T cells can be detected as early as 7 to 9 weeks postinfection, and they define a system to identify virus-encoded epitopes important in the induction of protective immunity against lentiviruses.

Theileria parva: CD4+ helper and cytotoxic T-cell clones react with a schizont-derived antigen associated with the surface of Theileria parva-infected lymphocytes

Theileria parva is a protozoan parasite which infects and transforms bovine lymphocytes, resulting in a fatal lymphoproliferative disease. There is evidence that immunity to the intralymphocytic schizont stage is mediated by T cells. We have previously reported derivation of CD4+ T-cell clones which recognize parasite-derived antigens presented on the surface of infected cells in conjunction with MHC molecules and partial characterization of the antigens. The present study further evaluated one of these antigens, demonstrating that it could be derived from cells infected with different parasite stocks as well as from purified theilerial schizonts and that it was recognized by primed, but not unprimed, bovine lymphocytes including cytolytic CD4+ T cells. Using a cloned CD4+ cytolytic cell line, lysis of schizont-infected cells was shown to be MHC-restricted but not parasite-strain restricted. In addition we demonstrated that T cells which respond to the HSS antigen preparation were generated in cattle immunized with parasites from any of the three subspecies of T. parva. The antigenic material was fractionated by sequential subjection to anion-exchange chromatography, hydroxylapatite chromatography, and gel filtration using HPLC, which resulted in recovery of approximately 20% of the antigenic material with more than 10(6)-fold purification in selected fractions. To assess the molecular size of the proteins in the highly purified antigenic fractions, the T. parva-infected lymphocytes were metabolically labeled before fractionation with 3H-amino acids and the material was analyzed by SDS-polyacrylamide gel electrophoresis and liquid scintillation counting of gel slices. The major protein in these fractions had a molecular mass of 9-10 kDa.

Babesia bovis: Bovine helper T cell lines reactive with soluble and membrane antigens of merozoites

Babesia bovis-specific T cell lines were established from cattle infected with either tick-derived or cultured parasites by stimulation of peripheral blood mononuclear cells with a crude parasite membrane fraction. Induction and enrichment of CD4+ T cells occurred over time. All cell lines responded vigorously and in a dose-dependent, MHC-restricted manner to intact merozoites, and to soluble and membrane fractions derived from merozoites by homogenization and high-speed centrifugation. Solubilization of the membrane fraction with nondenaturing zwitterionic or nonionic detergents yielded antigenic extracts which also stimulated the T cells. However, a differential response was observed, in that cell lines from one animal proliferated vigorously to the detergent extracts of the membrane fraction, whereas cell lines from a second animal proliferated only weakly to these extracts. SDS-PAGE analysis revealed common protein bands of 90 and 22 kDa in the various immunogenic fractions. Cell lines from the animal infected with cultured parasites also responded to parasite culture supernatant "exoantigens" and to the related parasite, Babesia bigemina. We conclude that antigens present in merozoite membranes and soluble parasite extracts preferentially stimulate CD4+ T cells from cattle immune to Babesia bovis. The differential pattern of response of T cell lines from different cattle suggests that more than one protein or epitope is immunodominant for T cells.

Induction of proliferative responses of T cells from Babesia bovis-immune cattle with a recombinant 77-kilodalton merozoite protein (Bb-1)

A major portion of a Babesia bovis-specific gene encoding a 77-kDa merozoite protein (Bb-1) produced during natural infection in cattle and in microaerophilous culture was subcloned into the pGEX1N expression vector. Recombinant Bb-1 protein fused to glutathione S-transferase (Bb-1-GST) was used to examine cellular immune responses in B. bovis-immune cattle. Sera from rabbits immunized with Bb-1-GST reacted with fusion protein and with the native antigen present in crude B. bovis but not with B. bigemina merozoites. Bb-1-GST but not GST induced strong proliferation of T lymphocytes from these immune cattle, and Bb-1-reactive T-cell lines which consisted of a mixed population of either CD4+ and CD8+ cells or CD4+, CD8+, and "null" (gamma delta T) cells were established by in vitro stimulation of peripheral blood mononuclear cells with the recombinant fusion protein. Three CD4+ CD8- and three CD4- CD8+ Bb-1-specific T-cell clones were identified after limiting-dilution cloning of the cell lines. The studies described here demonstrate that the 77-kDa protein of B. bovis contains T-cell epitopes capable of eliciting proliferation of two types of T cells in immune cattle, an important consideration for the design of a recombinant subunit vaccine.

Cell-mediated immune responses to Babesia bovis merozoite antigens in cattle following infection with tick-derived or cultured parasites

Peripheral blood mononuclear cells from cattle experimentally infected with Babesia bovis were examined for parasite-specific cell-mediated immune responses. Unfractionated merozoites and soluble and membrane fractions derived from merozoites were all antigenic for immune cattle, although the membrane fraction was the most stimulatory. Cattle responded to different antigenic fractions in a differential manner, and only that animal immunized with autologous cultured parasites responded to parasitized erythrocyte culture supernatants. Plastic-adherent cells (presumably monocytes/macrophages) were required for a proliferative response to babesial antigens but not to the T-cell mitogen concanavalin A, suggesting that babesial proteins are not simply mitogenic for T cells. Lymphocyte responses directed against a different hemoparasite from Mexico, Babesia bigemina, indicate that this parasite shares cross-reactive T-cell epitopes with B. bovis. These studies define a system whereby T lymphocytes from babesia-immune cattle can be used in proliferation assays to identify babesial merozoite antigens which are immunogenic for T cells. Because identification of helper T-cell epitopes is important for the design of a babesial subunit vaccine which will evoke anamnestic responses, the studies described here provide a basis for such experiments.

Theileria parva: bovine helper T cell clones specific for both infected lymphocytes and schizont membrane antigens

Two Theileria parva-specific bovine helper T cell clones were used to identify T. parva-derived antigens expressed on the surface of schizont infected lymphoblastoid cells. Although the clones proliferated in response to both the immunizing (Muguga) and heterologous stocks of T. parva, the patterns of the responses differed, showing that the two clones recognized different antigenic epitopes. Both clones were stimulated by autologous infected cells, without an additional source of antigen-presenting cells, as well as by purified schizonts and by a subcellular membrane fraction prepared from infected lymphoblastoid cells, when antigen-presenting cells were present. The membrane fraction was shown to be enriched for schizont membranes as indicated by the presence of a schizont surface antigen detected by immunoblotting using a schizont-specific monoclonal antibody. Elimination of schizonts with the anti-theilerial drug, parvaquone, resulted in reduced antigenicity of the membrane fraction as detected by both the T cell clones and the schizont-specific monoclonal antibody. We conclude that the T. parva-infected cell surface antigens recognized by the T cell clones are of schizont membrane origin. Although the antigens have not yet been characterized biochemically, the monoclonal antibody-specific epitope appears to be distinguishable from the T cell epitopes.