Identification of immunodominant polypeptides common between Anaplasma centrale and Anaplasma marginale

Evidence confirming the phylogenetic position of Anaplasma centrale (ex Theiler 1911) Ristic and Kreier 1984

In 1911, Sir Arnold Theiler isolated and described a parasite that was very similar to Anaplasma marginale but which was more centrally located within the erythrocytes of the host cells, and was much less pathogenic than A. marginale. He named the parasite A. marginale variety centrale. The name Anaplasma centrale, referring to the same organism, was published in Validation List No. 15 in 1984, but the publication was based on an erroneous assumption that Theiler had indicated that it was a separate species. Many authors have subsequently accepted this organism as a separate species, but evidence to indicate that it is a distinct species has never been presented. The near full-length 16S rRNA gene sequence, and the deduced amino acid sequences for groEL and msp4 from several isolates of A. marginale and A. centrale from around South Africa were compared with those of the A. marginale type strain, St Maries, and the A. centrale Israel strain and other reference sequences. Phylogenetic analyses of these sequences demonstrated that A. centrale consistently forms a separate clade from A. marginale, supported by high bootstrap values (≥90 %), revealing that there is divergence between these two organisms. In addition, we discuss distinctive characteristics which have been published recently, such as differences in Msp1a/Msp1aS gene structure, as well as genome architecture that provide further evidence to suggest that A. centrale is, in fact, a separate species. Our results, therefore, provide evidence to support the existing nomenclature, and confirm that A. centrale (ex Theiler 1911) Ristic and Kreier 1984 is, indeed, a distinct species.

The natural history of Anaplasma marginale

The intracellular pathogen Anaplasma marginale (Rickettsiales: Anaplasmataceae), described by Sir Arnold Theiler in 1910, is endemic worldwide in tropical and subtropical areas. Infection of cattle with A. marginale causes bovine anaplasmosis, a mild to severe hemolytic disease that results in considerable economic loss to both dairy and beef industries. Transmission of A. marginale to cattle occurs biologically by ticks and mechanically by biting flies and by blood-contaminated fomites. Both male ticks and cattle hosts become persistently infected with A. marginale and serve as reservoirs of infection. While erythrocytes are the major site of infection in cattle, A. marginale undergoes a complex developmental cycle in ticks that begins by infection of gut cells, and transmission to susceptible hosts occurs from salivary glands during feeding. Major surface proteins (MSPs) play a crucial role in the interaction of A. marginale with host cells, and include adhesion proteins and MSPs from multigene families that undergo antigenic change and selection in cattle, thus contributing to maintenance of persistent infections. Many geographic strains of A. marginale have been identified worldwide, which vary in genotype, antigenic composition, morphology and infectivity for ticks. Isolates of A. marginale may be maintained by independent transmission events and a mechanism of infection/exclusion in cattle and ticks. The increasing numbers of A. marginale genotypes identified in some geographic regions most likely resulted from intensive cattle movement. However, concurrent A. marginale strain infections in cattle was reported, but these strains were more distantly related. Phylogenetic studies of selected geographic isolates of A. marginale, using msp4 and msp1alpha, provided information about the biogeography and evolution of A. marginale, and msp1alpha genotypes appear to have evolved under positive selection pressure. Live and killed vaccines have been used for control of anaplasmosis and both types of vaccines have advantages and disadvantages. Vaccines have effectively prevented clinical anaplasmosis in cattle but have failed to block A. marginale infection. Vaccines are needed that can prevent clinical disease and, simultaneously, prevent infection in cattle and ticks, thus eliminating these hosts as reservoirs of infection. Advances in genomics, proteomics, immunology and biochemical and molecular technologies during the last decade have been applied to research on A. marginale and related organisms, and the recent development of a cell culture system for A. marginale has provided a format for studying the pathogen/tick interface. Recent advancements and new research methodologies should provide additional opportunities for development of new strategies for control and prevention of bovine anaplasmosis.

Genetic diversity of anaplasma species major surface proteins and implications for anaplasmosis serodiagnosis and vaccine development

The genus Anaplasma (Rickettsiales: Anaplasmataceae) includes several pathogens of veterinary and human medical importance. An understanding of the diversity of Anaplasma major surface proteins (MSPs), including those MSPs that modulate infection, development of persistent infections, and transmission of pathogens by ticks, is derived in part, by characterization and phylogenetic analyses of geographic strains. Information concerning the genetic diversity of Anaplasma spp. MSPs will likely influence the development of serodiagnostic assays and vaccine strategies for the control of anaplasmosis.

Molecular conservation of MSP4 and MSP5 in Anaplasma marginale and A. centrale vaccine strain

Anaplasma centrale msp4 and msp5 genes were cloned and sequenced, and the recombinant proteins were expressed. The identity between Anaplasma marginale and A. centrale MSP4 was 83% in the nucleotide sequences and 91.7% in the encoded protein sequences. A. centrale msp5 nucleotide sequences shared 86.8% identity with A. marginale msp5, and there was 92.9% homology between A. centrale and A. marginale encoded amino acids of the MSP5 protein. Southern blots hybridized with probes derived from the msp4 and msp5 central regions indicate that msp4 and msp5 of A. centrale are encoded by single copy genes. Recombinant MSP4 and MSP5 fusion proteins reacted with anti-A. marginale monoclonal antibodies ANAR76A1 and ANAF16C, respectively, demonstrating the conservation of conformation-sensitive B-cell epitopes between A. centrale and A. marginale. These data demonstrate the structural and antigenic conservation of MSP4 and MSP5 in A. centrale and A. marginale. This conservation is consistent with the cross-protective immunity between A. marginale and A. centrale and supports the development of improved vaccines based upon common outer membrane proteins.

Assessment of a low virulence Australian isolate of Anaplasma marginale for pathogenicity, immunogenicity and transmissibility by Boophilus microplus

A 14-year-old cow (Dawn) born and kept in a Boophilus microplus-free region gave birth to a calf, which showed the presence of an Anaplasma marginale infection after splenectomy. The calf's grand dam was from a B. microplus infected area and we assume the infection originated via the transplacental route over two generations. An isolate, prepared from the calf, had similar or lower pathogenicity as Anaplasma centrale, and previously exposed steers were resistant to challenge by four A. marginale field isolates. Two attempts to transmit the isolate using B. microplus were unsuccessful. Our results indicate that Dawn A. marginale may be a useful vaccine in Australia and warrants larger scale validation of its safety and potency locally as well as of the protection it affords against African and New World isolates.

Antigens and alternatives for control of Anaplasma marginale infection in cattle

Anaplasmosis, a tick-borne cattle disease caused by the rickettsia Anaplasma marginale, is endemic in tropical and subtropical areas of the world. The disease causes considerable economic loss to both the dairy and beef industries worldwide. Analyses of 16S rRNA, groESL, and surface proteins have resulted in the recent reclassification of the order Rickettsiales. The genus Anaplasma, of which A. marginale is the type species, now also includes A. bovis, A. platys, and A. phagocytophilum, which were previously known as Ehrlichia bovis, E. platys, and the E. phagocytophila group (which causes human granulocytic ehrlichiosis), respectively. Live and killed vaccines have been used for control of anaplasmosis, and both types of vaccines have advantages and disadvantages. These vaccines have been effective in preventing clinical anaplasmosis in cattle but have not blocked A. marginale infection. Thus, persistently infected cattle serve as a reservoir of infective blood for both mechanical transmission and infection of ticks. Advances in biochemical, immunologic, and molecular technologies during the last decade have been applied to research of A. marginale and related organisms. The recent development of a cell culture system for A. marginale provides a potential source of antigen for the development of improved killed and live vaccines, and the availability of cell culture-derived antigen would eliminate the use of cattle in vaccine production. Increased knowledge of A. marginale antigen repertoires and an improved understanding of bovine cellular and humoral immune responses to A. marginale, combined with the new technologies, should contribute to the development of more effective vaccines for control and prevention of anaplasmosis.

Immunological relationship between Neospora caninum and Besnoitia besnoiti

Neospora caninum is a coccidian parasite identified as a major cause of abortion in cattle. A combined infection of N. caninum with another taxonomically related parasite of cattle, Besnoitia besnoiti can occur in geographical areas endemic for both species. Both infections are routinely diagnosed serologically, and incorrect diagnosis could occur if immunological cross-reactivity exists between the two parasites. To investigate the possible degree of cross-reactivity, we compared results obtained with two serological techniques, immunofluorescent antibody test (IFA), and Western blot analysis on known positive and negative sera. The test sera were derived from naturally infected cattle and from experimentally infected Mongolian gerbils. In IFA of bovine sera, no cross-reactvity was detected at the commonly used serum dilution cutoffs of 1:200 for N. caninum and 1:256 for B. besnoiti. However, at 1:64 dilution of both cattle and gerbil sera, anti-N. caninum sera reacted with B. besnoiti antigen in some individual samples. Anti-B. besnoiti serum did not react with N. caninum antigen at any dilution. This low level one directional cross-reactivity was confirmed by Western blot analysis. B. besnoiti antigen showed two immunoreactive bands when probed with anti-N. caninum serum, while no bands appeared when N. caninum antigen was probed with B. besnoiti antiserum. Immunization and challenge experiments in the highly susceptible Mongolian gerbil (Meriones unguiculatus) showed essentially no cross-protection between N. caninum and B. besnoiti.

Expression of major surface protein 2 variants with conserved T-cell epitopes in Anaplasma centrale vaccinates

Major surface protein 2 (MSP-2), identified as a protection-inducing immunogen against Anaplasma marginale challenge, is an immunodominant outer membrane protein with orthologues in all examined Anaplasma species. Although immunization with live Anaplasma centrale has long been used to induce protection against acute disease upon challenge with virulent A. marginale, its MSP-2 structure and whether MSP-2 variants are generated during persistence of the vaccine strain was unknown. In this study, we showed that the A. centrale vaccine strain persisted for a minimum of 4 years postvaccination and generated sequential MSP-2 variants. Comparison of amino acid sequences encoded by A. centrale msp-2 transcripts from the initial postimmunization period and from sequential time points during persistence of the vaccine strain revealed a central hypervariable domain flanked by conserved amino and carboxy-terminal regions. This structure corresponded to that shown in A. marginale MSP-2, where the central hypervariable region encodes variant B-cell epitopes in the extracellular domain and the flanking transmembrane domains are rich in CD4(+)-T-cell epitopes. Importantly, at least four CD4(+)-T-cell epitopes are conserved between the two species, a finding consistent with A. marginale challenge triggering a recall response of CD4(+) T cells induced by A. centrale vaccination. The genomic arrangement is conserved between A. centrale and A. marginale with multiple msp-2 pseudogenes and a single operon-linked expression site for the full-length msp-2. This conservation of both genomic structure for generating MSP-2 variants and the CD4(+)-T-cell epitopes between these two genetically distinct Anaplasma species indicates that they present a similar repertoire of MSP-2 epitopes to the immune system and that this similarity may be responsible for all or part of the A. centrale vaccine efficacy.

Immunity following use of Australian tick fever vaccine: a review of the evidence

OBJECTIVE

To review the evidence available on the degree and duration of immunity provided by Australian tick fever vaccines against Babesia bovis, B. bigemina and Anaplasma marginale infections in Australia and overseas.

BACKGROUND

Vaccines containing attenuated strains of B bovis and B bigemina as well as A. centrale grown in splenectomised calves have been used in Australia since 1964 to immunise cattle against tick fever. About 800,000 doses of vaccine are supplied annually and much of the evidence for protection is field evidence rather than conventional immunological measures or pen trials.

CONCLUSIONS

Immunity to Babesia bovis and B. bigemina--A single inoculation generally provides sound, long-lasting protection both in Australia and overseas. No evidence was found of a loss of immunity with time. Vaccine failures to B. bovis do occur, but are uncommon and evidently caused by a number of factors, including immune responsiveness of the vaccinated animals, and immunogenicity of the vaccine strain. Immunity to Anaplasma marginale--The vaccine containing A. centrale provides partial, variable protection against A. marginale. Protection against challenge in Australia is adequate in most cases to prevent disease and use of the vaccine in this country appears to be justified. Protection against antigenically diverse, highly virulent stocks of A. marginale in other countries is, at times, clearly inadequate and better vaccines are required in situations where the challenge is severe.

Analysis of the 16S rRNA gene sequence of Anaplasma centrale and its phylogenetic relatedness to other ehrlichiae

The nucleotide sequence of the Anaplasma centrale 16S rRNA gene was determined and compared with the sequences of ehrlichial bacteria. The sequence of A. centrale was closely related to Anaplasma marginale by both level-of-similarity (98.08% identical) and distance analysis. A species-specific PCR was developed based upon the alignment data. The PCR can detect A. centrale DNA extracted from 10 infected bovine red blood cells in a reaction mixture. A. centrale DNA was amplified in the reaction, but not other related ehrlichial species.

Identification of antigenic differences that discriminate between cattle vaccinated with Anaplasma centrale and cattle naturally infected with Anaplasma marginale

Monoclonal antibodies were raised against the vaccine strain of Anaplasma centrale used in Australia. A monoclonal antibody that reacted with an 80 kDa antigen was used to develop an A. centrale-specific fluorescent antibody test that will be useful for confirming species identity in patent infections. Another monoclonal antibody that reacted with a 116 kDa antigen was used to develop an A. centrale-specific competitive inhibition enzyme-linked immunosorbent assay (ELISA) for the serological identification of vaccinated cattle. The sensitivity of the ELISA was 100% in cattle experimentally infected with A. centrale, 97.1% in a vaccinated beef herd and 98.3% in a vaccinated dairy herd. The specificity of the ELISA was 98.6% in non-vaccinated cattle outside the Anaplasma marginale-endemic area, 97.9% in non-vaccinated cattle within the A. marginale-endemic area and 100% in cattle experimentally infected with A. marginale. The ELISA detected antibodies to A. centrale in cattle up to 9 years after vaccination with no apparent decrease in sensitivity. The assay has proved extremely valuable in Australia for investigating reported failures of multivalent live vaccines used to protect cattle against anaplasmosis and babesiosis, and should be similarly useful elsewhere in the world where these types of vaccines are used, e.g. Israel and South America.

Characterization of Anaplasma isolates from eland (Taurotragus oryx). Pathogenicity in cattle and sheep and DNA profiles analysis

Two eland Anaplasma isolates, AnapE1, from Kenya, and AnapE2, from South Africa were characterised. Their characterization was based on their pathogenicity to intact and splenectomized cattle and sheep and also their DNA profiles. Their DNA profiles were analysed and compared to Anaplasma marginale, A. ovis and A. centrale after endonuclease restrictions and probing with Anaplasma DNA probes, AC5-12 and AC-1. The results of the pathogenicity trials showed AnapE1 to be similar to A. ovis and AnapE2 an isolate of A. marginale. On DNA profiles, AnapE1 was close to A. ovis, with differences that occur even in same Anaplasma species isolates from different locations. On the other hand, AnapE2, resembled one of the A. marginale isolates known to occur in South Africa. The DNA profiles correlated well with the pathogenicity results. It is concluded that elands are carriers of both A. marginale and A. ovis parasites and are therefore important reservoirs that need attention in epidemiology of anaplasmosis.

Evaluation of Anaplasma marginale major surface protein 3 (MSP3) as a diagnostic test antigen

An immunodominant surface protein, major surface protein 3 (MSP3), has been proposed as an antigen suitable for use in the diagnosis of bovine anaplasmosis. We further characterized MSP3 to examine its potential as a test antigen for the serological diagnosis of carrier cattle. The specificity of this antigen in detecting infected cattle as well as the conservation of MSP3 between strains of Anaplasma marginale was evaluated by using immunoblots of A. marginale proteins separated by one- and two-dimensional polyacrylamide gel electrophoreses. Immune sera from animals infected with Anaplasma ovis, Ehrlichia risticii, and Ehrlichia ewingii reacted with the MSP3 antigen of A. marginale. One-dimensional gel electrophoresis of A. marginale proteins demonstrated size polymorphism of MSP3 between different geographic isolates. Two-dimensional gel electrophoresis revealed at least three different antigens migrating at the 86-kDa molecular size, and sera from animals infected with different strains of A. marginale reacted with different 86-kDa antigens. These results indicate that MSP3 may share cross-reactive epitopes with antigens found in A. ovis and some Ehrlichia spp. In addition, MSP3 is not conserved between different isolates of A. marginale, and at least in the isolate from Florida, what was previously identified as MSP3 is actually a group of three or more 86-kDa antigens with different isoelectric points. The cross-reactivity of MSP3 with some Ehrlichia spp., the variability of MSP3 between isolates, and the multiple 86-kDa antigens recognized by various sera suggest that MSP3 is not a suitable candidate for use as a recombinant test antigen.

Molecular basis for vaccine development against anaplasmosis and babesiosis

Immunization of livestock against the erythroparasitic pathogens Anaplasma marginale, Babesia bigemina, and Babesia bovis with safe and effective killed vaccines is not yet feasible on a practical basis. However, the immune protection afforded by recovery from natural infection and premunition indicates that microbial epitopes capable of inducing immunity exist and that the bovine immune system can be primed appropriately. Induction of protection by immunization with killed parasite fractions, enriched for polypeptides with surface exposed epitopes, supports a focus on surface epitopes, including apical complex organellar epitopes in Babesia, for vaccine development. Cloning, sequencing, and expression of genes encoding these key surface polypeptides has allowed examination of polypeptide function and detailed analysis of epitope conservation in light of genetic polymorphism. In this paper, the characterization of these polypeptides at the epitope level and their roles in inducing protective immunity are reviewed. Definition of these epitopes, in combination with improved understanding of immune mechanisms, provides the basis for development of effective recombinant vaccines against anaplasmosis and babesiosis.

The immunoprotective Anaplasma marginale major surface protein 2 is encoded by a polymorphic multigene family

An Anaplasma marginale Florida msp-2 gene was cloned and expressed in Escherichia coli. Pulsed-field gel electrophoresis and Southern blot analysis revealed the presence of multiple msp-2 gene copies that were widely distributed throughout the chromosomes of all three strains examined. Genomic polymorphism among copies was greatest in the 5' end of msp-2 but also occurred in 3' regions. The presence of gene-copy-specific epitopes was indicated by the reactivity of the cloned msp-2 copy with some, but not all, monoclonal antibodies that bound native MSP-2. Multiple antigenically distinct MSP-2 molecules were expressed within strains and were coexpressed by individual A. marginale organisms. These results suggest that expression of polymorphic msp-2 gene copies is responsible for the significant percentages of A. marginale organisms within strains that do not react with individual anti-MSP-2 monoclonal antibodies. Sequence analysis revealed highly significant MSP-2 homology with two rickettsial surface proteins, A. marginale MSP-4 and Cowdria ruminantium MAP-1. Immunization with MSP-4 has been shown to induce protective immunity in a manner similar to that of immunization with MSP-2. These findings support the hypothesis that A. marginale surface proteins are targets of protective immune responses but are antigenically polymorphic.

Heterologous strain challenge of cattle immunized with Anaplasma marginale outer membranes

In this study, we tested the hypothesis that outer membrane immunization would induce protection against an antigenically variant strain. Outer membranes were isolated from the Virginia strain of Anaplasma marginale using density gradient centrifugation, combined with saponin adjuvant, and used to immunize Friesian cattle in Zimbabwe. Immunized cattle developed high antibody titers (80,000-160,000) against outer membrane polypeptides including MSP-2 and MSP-5 in both the homologous Virginia and heterologous Zimbabwe strains. Outer membrane immunized cattle were protected significantly following challenge with 10(4) Zimbabwe strain parasitized erythrocytes, demonstrated by significant differences in prepatent period and peak rickettsemia compared with adjuvant immunized control cattle. One outer membrane immunized animal was completely protected against infection. However, there were no overall significant differences in severity of anemia between cattle immunized with outer membrane and the control group, indicating that a significant reduction in rickettsemia does not necessarily result in less severe anemia.

Anaplasma marginale: failure of sera from immune cattle to confer protection in passive-transfer experiments

High levels of immunity to Anaplasma marginale were induced in cattle either by vaccination using sonically disrupted A. marginale-infected erythrocytes or by repeated infection with different strains of the rickettsia. In both instances, high levels of anti-A. marginale antibody were detected in the sera of the immune cattle by immunoblotting. Serum from one animal that had been made immune by repeated infection was transferred intravenously to A. marginale-susceptible calves (three non-splenectomised and two splenectomised) undergoing initial A. marginale infection at serum doses of 2-10 ml/kg. Neither the course nor the outcome of infection as indicated by the parasite levels attained or the level of anaemia induced was altered in the calves that received the immune serum relative to the course or outcome of infection in control calves (two non-splenectomised and two splenectomised) that received serum from an two splenectomised) that received serum from an A. marginale-naive donor animal. In a similar experiment, a pool of sera from four steers that had been vaccinated with sonically disrupted A. marginale initial bodies was transfused into two intact A. marginale-susceptible calves during the early stage of A. marginale infection at a dose of 10 ml/kg. No difference was observed in the course or outcome of infection in these calves relative to the course or outcome of infection in the two non-splenectomised calves that were transfused with non-immune serum.(ABSTRACT TRUNCATED AT 250 WORDS)