Localization of culture-derived soluble Babesia bovis antigens in the infected erythrocyte

A recombinant surface protein of Babesia bovis elicits bovine antibodies that react with live merozoites

Ten monoclonal antibodies (MoAbs) were generated against five surface-exposed proteins (16 kDa, 42 kDa, 44 kDa, 60 kDa, 225 kDa) on merozoites of Babesia bovis. A genomic library constructed in the lambda gt11 expression vector was screened with MoAbs in a plaque immunoassay for identification of clones expressing recombinant surface proteins. Two recombinant clones were identified (lambda Bo44-15 and lambda Bo44-16) that encoded a protein recognized by a MoAb specific for an epitope on the native 44-kDa surface protein. Southern blot analysis using radiolabeled Bo44-15 DNA (1.25 kb) against merozoite DNA and bovine leukocyte DNA confirmed the parasite-specificity of the cloned insert and revealed multiple bands of hybridization with merozoite DNA. Western blot analyses of lambda Bo44-15 lysogen preparations demonstrated that recombinant protein production in this clone was IPTG-induced and that the recombinant molecule was a beta-galactosidase fusion protein. Additionally, recombinant 44-kDa protein, purified by immunoaffinity chromatography, reacted with specific MoAb in Western blot assay indicating that the integrity of the epitope was retained during purification. Immune sera from calves immunized with purified recombinant Bo44-15 protein immunoprecipitated metabolically radiolabeled merozoite protein of 44 kDa indicating that antibody induced by recombinant Bo44-15 protein recognized native 44-kDa protein. Also, these sera reacted with the surface of live merozoites as evidenced by indirect immunofluorescence assay. Serum antibody titers determined by this assay had a wide range.

Application of exoantigens of Babesia and Plasmodium in vaccine development

The University of Illinois malaria vaccine programme uses culture-derived soluble exoantigens of Plasmodium falciparum and the squirrel monkey as an experimental model. Exoantigens are soluble polypeptides naturally released into the blood plasma of animals infected with Babesia or Plasmodium species, or into the supernatant medium of in vitro cultures of these organisms. Immunization with soluble B. bovis and B. bigemina exoantigens prepared from culture supernatant fluids protected cattle against homologous and heterologous challenge. Similarly, vaccination of squirrel monkeys with supernatant fluids from P. falciparum cultures containing exoantigen induced protective immunity against acute clinical malaria. Susceptible monkeys have been vaccinated with an aluminium hydroxide-fortified antigenic fraction partially purified from supernatants of P. falciparum strains Indochina I and Genève/SGE-1; this conferred significant clinical protection against needle challenge with the homologous Indochina I strain, and a moderate degree of immunity to the heterologous strain. Following sequential purification by high performance liquid chromatography, the N-terminal amino acid sequences of P. falciparum 100 kDa, 83 kDa and 70 kDa exoantigens were determined. A 29 amino acid peptide constructed from the N-terminal sequence of the P. falciparum (Genève strain) 83 kDa exoantigen has been synthesized. When coupled to a carrier protein, the peptide was immunogenic in rabbits, mice and squirrel monkeys, inducing antibodies which were trophozoite-specific, reactive to native parasite proteins in a two-site enzyme immunoassay (EIA) and in Western blots, and which inhibited P. falciparum growth in vitro. Using this synthetic peptide, EIAs are being developed for the detection of antibodies to P. falciparum blood-phase parasites in individuals living in malaria-endemic areas of Africa, Asia and South America.

Immunoprophylactic control of bovine babesiosis: role of exoantigens of Babesia

A major breakthrough in babesiosis vaccine research was achieved with the development of methods for continuous propagation in vitro of Babesia parasites. The development of a soluble, exoantigen-containing immunogen, free of erythrocytic antigens, has been proposed as a practical and realistic means of immunoprophylactic control of babesiosis. Such immunogens have been tested for immunogenicity of different strains, minimal protective dose, and protective efficacy of a combined B. bovis-B. bigemina immunogen. During the last 7 years, most of these aspects have been studied in a co-operative research programme between the University of Illinois and the Veterinary Research Institute of Venezuela. The Babesia exoantigen vaccine prevents clinical disease, is effective against different parasite strains, induces protective immunity for at least 13 months, does not immunize against host blood groups, is antigenically stable for at least 3 years at 4 degrees C, and is available in large quantities. Continued improvement of currently available culture-derived immunogens will best guarantee the successful production of food-producing animals in the tropics. Since July 1984, 16 field vaccination trials have been conducted in 7 states of Venezuela in areas where large-scale dairy and beef production is crucial for the livestock industry. These ranches encompassed a cattle production of 14,000, of which 3000 have been vaccinated with a combined B. bovis-B. bigemina immunogen in 2 subcutaneous inoculations with a 4-week interval. In the first 5 trials, a 1-year monitoring period following vaccination and natural tick exposure indicated that the culture-derived vaccine has effectively controlled the clinical manifestation of infection. Studies on antigen characterization are shifting the emphasis from crude antigenic preparations to purified polypeptides, isolated and identified using new biochemical techniques. Current research efforts are directed toward isolation of protective antigens and the production of synthetic vaccines. Until optimal molecular vaccines are developed, a polyvalent inactivated B. bovis-B. bigemina immunogen may offer the best combination of potency, efficacy and safety available for effective immunoprophylaxis against bovine babesiosis.

Immunological control of ticks and tick-borne parasitic diseases of livestock

Parasitic diseases inflict major losses on livestock production throughout the world. Currently, control of the diseases relies largely on prophylactic or therapeutic application of anti-parasitic drugs. In many instances, these measures are only partially effective. Moreover, they must be applied frequently, are therefore costly and time-consuming, and lead to the selection of drug resistance within the parasite populations. Thus, it has been recognized for several decades that effective methods of vaccination against parasitic diseases would have a major impact on livestock production. However, despite considerable efforts over the last 30 years, only a few parasite vaccines are currently in use and all of these involve the administration of live organisms.

Induction of protective immunity to Babesia divergens in Mongolian gerbils, Meriones unguiculatus, using culture-derived immunogens

Immunogens derived from microaerophilous stationary phase (MASP) cultures of Babesia divergens grown in bovine erythrocytes were used to inoculate the laboratory host of B. divergens, the Mongolian gerbil, Meriones unguiculatus. Animals inoculated subcutaneously twice with preparations of freeze-thawed merozoites in complete Freund's adjuvant were fully protected against homologous challenge, as were gerbils immunised with a non-viable preparation of parasite-enriched lysed infected bovine erythrocytes. Animals which had been infected with small numbers of parasitised erythrocytes from cultures cooled to 4 degrees C, allowed to recover, then challenged, also survived. All three groups had high antibody titres which dropped immediately after challenge and then rose again. Gerbils given culture supernatants containing soluble merozoite protein coat antigens were partially protected only after receiving a third inoculation. Non-immunised animals all died 4 days after challenge.

Biology of Babesia

Recent advances in the knowledge of the biology of the protozoan family Babesiidae is reviewed with special emphasis on those species which cause economically important diseases in cattle. The life cycle of Babesia of cattle is illustrated taking into account these recent advances. Areas of controversy in the life cycles still exist particularly over the presence or absence of a sexual cycle within the tick vector and of exoerythrocytic schizonts in the mammalian host. In the mammalian host, the major parasite division cycle occurs in erythrocytes and it is these stages which induce the pathogenic effects. The recent progress in the understanding of the biology of Babesia parasites may allow important developments in the control of babesiosis.

Heterologous strain immunity in bovine babesiosis using a culture-derived soluble Babesia bovis immunogen

The cross-protective capacity of culture-derived soluble immunogens against heterologous Babesia bovis strains from different geographical locations of Latin America was examined. Susceptible yearling cattle were either immunized with immunogens derived from Venezuelan or Mexican strains, or were administered a multi-component immunogen containing antigens of the Australian, Mexican and Venezuelan strains. Cattle were challenged with virulent B. bovis organisms of the Argentinian, Colombian, Ecuadorean, Mexican and Venezuelan strains. The major parameters used to evaluate cross-protection were the following: presence, level and duration of parasitemia; maximal PCV reduction; level and duration of fever; determination of fibrinogen and cryofibrinogen; homologous and heterologous antibody levels; and net gains in body weight. Results showed good protection with a Venezuelan B. bovis immunogen after homologous and heterologous challenge exposures. A low degree of cross-immunity was observed when cattle vaccinated with the Mexican immunogen were challenged with each of the heterologous strains.

An update of the isolation and characterization of culture-derived soluble antigens of Babesia bovis

In this paper recent progress concerning the identification of soluble antigens from cultures of Babesia bovis parasites is reviewed. Soluble antigens present in the supernatant of B. bovis cultures have been shown to be efficient immunogens for induction of protective immunity against bovine babesiosis. Immunochemical analysis of culture supernatants has demonstrated that at least 3 parasite antigens are released in vitro. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis showed that at least 2 of these antigens have molecular weights within the range of 37 000-40 000 daltons. Crossed immunoelectrophoretic studies have further revealed an antigenic spectrum consisting of 1 major and 2 minor antigens with mobilities in the albumin and alpha 1 regions. Within the infected erythrocyte, 2 antigens have been localized on or near the erythrocytic membrane, while the third antigen appears to be directly associated with the parasite.