Major histocompatibility complex class II DR-restricted memory CD4(+) T lymphocytes recognize conserved immunodominant epitopes of Anaplasma marginale major surface protein 1a

Are CD45RO+ and CD45RA- genuine markers for bovine memory T cells?

Effective vaccination induces memory T cells, which protect the host against pathogen re-infections. Therefore, detection of memory T cells is essential for evaluating vaccine efficacy, which was originally dependent on cytokine induction assays. Currently, two isoforms of CD45 tyrosine phosphatase, CD45RO expression and CD45RA exclusion (CD45RO+/ CD45RA-) are used extensively for detecting memory T cells in cattle. The CD45RO+/CD45RA- markers were first established in humans around three decades ago, and were adopted in cattle soon after. However, in the last two decades, some published data in humans have challenged the initial paradigm, and required multiple markers for identifying memory T cells. On the contrary, memory T cell detection in cattle still mostly relies on CD45RO+/CD45RA- despite some controversial evidence. In this review, we summarized the current literature to examine if CD45RO+/CD45RA- are valid markers for detecting memory T cells in cattle. It seems CD45RA and CD45RO (CD45RA/RO) as markers for identifying bovine memory T cells are questionable.

Targeted mutagenesis in Anaplasma marginale to define virulence and vaccine development against bovine anaplasmosis

Tick-borne Anaplasma species are obligate, intracellular, bacterial pathogens that cause important diseases globally in people, agricultural animals, and dogs. Targeted mutagenesis methods are yet to be developed to define genes essential for these pathogens. In addition, vaccines conferring protection against diseases caused by Anaplasma species are not available. Here, we describe a targeted mutagenesis method for deletion of the phage head-to-tail connector protein (phtcp) gene in Anaplasma marginale. The mutant did not cause disease and exhibited attenuated growth in its natural host (cattle). We then assessed its ability to confer protection against wild-type A. marginale infection challenge. Additionally, we compared vaccine protection with the mutant to that of whole cell A. marginale inactivated antigens as a vaccine (WCAV) candidate. Upon infection challenge, non-vaccinated control cattle developed severe disease, with an average 57% drop in packed cell volume (PCV) between days 26-31 post infection, an 11% peak in erythrocytic infection, and apparent anisocytosis. Conversely, following challenge, all animals receiving the live mutant did not develop clinical signs or anemia, or erythrocyte infection. In contrast, the WCAV vaccinees developed similar disease as the non-vaccinees following A. marginale infection, though the peak erythrocyte infection reduced to 6% and the PCV dropped 43%. This is the first study describing targeted mutagenesis and its application in determining in vivo virulence and vaccine development for an Anaplasma species pathogen. This study will pave the way for similar research in related Anaplasma pathogens impacting multiple hosts.

Molecular characterization of Anaplasma marginale based on the msp1a and msp1b genes

Anaplasma marginale is an intracellular rickettsial bacterium causing anaplasmosis in ruminants. A. marginale is transmitted biologically by ticks and mechanically by blood-sucking vectors. Anaplasmosis occurs in tropical and subtropical areas of the world. This disease causes huge economic losses due to decreasing meat yield and milk production. The aims of this study were to determine the genetic diversity and antigenicity of A. marginale based on the msp1a and msp1b genes in cattle in Thailand. The A. marginale msp1a and msp1b genes were amplified by the polymerase chain reaction (PCR). There have been four copies of MSP1a tandem repeats among A. marginale Thailand strain, and thirteen different MSP1a tandem repeats were found including repeats B, 25, 27, M, 3, S, C, H, β, 80, 4, TH1 and TH2. Notably, this study showed two copies of the novel conserved tandem sequences namely Thailand Type 1 (TH1) and Type 2 (TH2). The phylogenetic analysis revealed that A. marginale msp1a and msp1b genes were genetically diverse and showed 9 and 5 clades with similarity ranging from 98 to 100% and 79.5 to 100%, respectively, when compared within the isolates of this study. The results of diversity analysis showed 18 and 16 haplotypes of the msp1a and msp1b genes, respectively. The entropy analyses of msp1a and msp1b nucleic acid sequences showed 39 and 900 high entropy peaks with values ranging from 0.35 to 0.85 and from 0.41 to 1.48, respectively, while those of MSP1a and MSP1b amino acid sequences exhibited 75 and 72 high entropy peaks with values ranging from 0.35 to 1.06 and from 0.41 to 1.55, respectively. In addition, B-cell and T-cell epitopes have also been investigated in this study. Hence, our results could be employed to improve the insight input of molecular phylogenetics, genetic diversity and antigenicity of A. marginale Thailand strain.

The detection of long-lasting memory foot-and-mouth disease (FMD) virus serotype O-specific CD4+ T cells from FMD-vaccinated cattle by bovine major histocompatibility complex class II tetramer

Foot‐and‐mouth disease (FMD) is a highly contagious, economically devastating disease of cloven‐hooved animals. The development of long‐lasting effective FMD vaccines would greatly benefit the global FMD control programme. Deep analysis of adaptive immunity in cattle vaccinated against FMD is technically challenging due to the lack of species‐specific tools. In this study, we aimed to identify CD4⁺ T‐cell epitopes in the FMD virus (FMDV) capsid and to phenotype the CD4⁺ T cells that recognize them using bovine major histocompatibility complex (BoLA) class II tetramer. A BoLA class II tetramer based on the DRA/DRB3*020:02 allele and FMDV antigen‐stimulated PBMCs from bovine vaccinates were used to successfully identify four epitopes in the FMDV capsid, three of which have not been previously reported; two epitopes were identified in the structural protein VP1, one in VP3 and one in VP4. Specificity of the three novel epitopes was confirmed by proliferation assay. All epitope‐expanded T‐cell populations produced IFN‐γ in vitro, indicating a long‐lasting Th1 cell phenotype after FMD vaccination. VP3‐specific CD4⁺ T cells exhibited the highest frequency amongst the identified epitopes, comprising >0·004% of the CD4⁺ T‐cell population. CD45RO⁺CCR7⁺ defined central memory CD4⁺ T‐cell subpopulations were present in higher frequency in FMDV‐specific CD4⁺ T‐cell populations from FMD‐vaccinated cattle ex vivo. This indicates an important role in maintaining cell adaptive immunity after FMD vaccination. Notably, FMDV epitope‐loaded tetramers detected the presence of FMDV‐specific CD4⁺ T cells in bovine PBMC more than four years after vaccination. This work contributes to our understanding of vaccine efficacy.

Detection and Characterisation of Anaplasma marginale and A. centrale in South Africa

Bovine anaplasmosis is endemic in South Africa and it has a negative economic impact on cattle farming. An improved understanding of Anaplasma marginale and Anaplasma marginale variety centrale (A. centrale) transmission, together with improved tools for pathogen detection and characterisation, are required to inform best management practices. Direct detection methods currently in use for A. marginale and A. centrale in South Africa are light microscopic examination of tissue and organ smears, conventional, nested, and quantitative real-time polymerase chain reaction (qPCR) assays, and a reverse line blot hybridisation assay. Of these, qPCR is the most sensitive for detection of A. marginale and A. centrale in South Africa. Serological assays also feature in routine diagnostics, but cross-reactions prevent accurate species identification. Recently, genetic characterisation has confirmed that A. marginale and A. centrale are separate species. Diversity studies targeting Msp1a repeats for A. marginale and Msp1aS repeats for A. centrale have revealed high genetic variation and point to correspondingly high levels of variation in A. marginale outer membrane proteins (OMPs), which have been shown to be potential vaccine candidates in North American studies. Information on these OMPs is lacking for South African A. marginale strains and should be considered in future recombinant vaccine development studies, ultimately informing the development of regional or global vaccines.

Co-infections with multiple genotypes of Anaplasma marginale in cattle indicate pathogen diversity

BACKGROUND

Only a few studies have examined the presence of Anaplasma marginale and Anaplasma centrale in South Africa, and no studies have comprehensively examined these species across the whole country. To undertake this country-wide study we adapted a duplex quantitative real-time PCR (qPCR) assay for use in South Africa but found that one of the genes on which the assay was based was variable. Therefore, we sequenced a variety of field samples and tested the assay on the variants detected. We used the assay to screen 517 cattle samples sourced from all nine provinces of South Africa, and subsequently examined A. marginale positive samples for msp1α genotype to gauge strain diversity.

RESULTS

Although the A. marginale msp1β gene is variable, the qPCR functions at an acceptable efficiency. The A. centrale groEL gene was not variable within the qPCR assay region. Of the cattle samples screened using the assay, 57% and 17% were found to be positive for A. marginale and A. centrale, respectively. Approximately 15% of the cattle were co-infected. Msp1α genotyping revealed 36 novel repeat sequences. Together with data from previous studies, we analysed the Msp1a repeats from South Africa where a total of 99 repeats have been described that can be attributed to 190 msp1α genotypes. While 22% of these repeats are also found in other countries, only two South African genotypes are also found in other countries; otherwise, the genotypes are unique to South Africa.

CONCLUSIONS

Anaplasma marginale was prevalent in the Western Cape, KwaZulu-Natal and Mpumalanga and absent in the Northern Cape. Anaplasma centrale was prevalent in the Western Cape and KwaZulu-Natal and absent in the Northern Cape and Eastern Cape. None of the cattle in the study were known to be vaccinated with A. centrale, so finding positive cattle indicates that this organism appears to be naturally circulating in cattle. A diverse population of A. marginale strains are found in South Africa, with some msp1α genotypes widely distributed across the country, and others appearing only once in one province. This diversity should be taken into account in future vaccine development studies.

Genetic diversity and molecular epidemiology of Anaplasma

Anaplasma are obligate intracellular bacteria of cells of haematopoietic origin and are aetiological agents of tick-borne diseases of both veterinary and medical interest common in both tropical and temperate regions. The recent disclosure of their zoonotic potential has greatly increased interest in the study of these bacteria, leading to the recent reorganisation of Rickettsia taxonomy and to the possible discovery of new species belonging to the genus Anaplasma. This review is particularly focused on the common and unique characteristics of Anaplasma marginale and Anaplasma phagocytophilum, with an emphasis on genetic diversity and evolution, and the main distinguishing features of the diseases caused by the different Anaplasma spp. are described as well.

Loss of Immunization-Induced Epitope-Specific CD4 T-Cell Response following Anaplasma marginale Infection Requires Presence of the T-Cell Epitope on the Pathogen and Is Not Associated with an Increase in Lymphocytes Expressing Known Regulatory Cell Phenotypes

We have shown that in cattle previously immunized with outer membrane proteins, infection with Anaplasma marginale induces a functionally exhausted CD4 T-cell response to the A. marginale immunogen. Furthermore, T-cell responses following infection in nonimmunized cattle had a delayed onset and were sporadic and transient during persistent infection. The induction of an exhausted T-cell response following infection presumably facilitates pathogen persistence. In the current study, we hypothesized that the loss of epitope-specific T-cell responses requires the presence of the immunizing epitope on the pathogen, and T-cell dysfunction correlates with the appearance of regulatory T cells. In limited studies in cattle, regulatory T cells have been shown to belong to γδ T-cell subsets rather than be CD4 T cells expressing forkhead box protein P3 (FoxP3). Cattle expressing the DRB3*1101 haplotype were immunized with a truncated A. marginale major surface protein (MSP) 1a that contains a DRB3*1101-restricted CD4 T-cell epitope, F2-5B. Cattle either remained unchallenged or were challenged with A. marginale bacteria that express the epitope or with A. marginale subsp. centrale that do not. Peripheral blood and spleen mononuclear cells were monitored for MSP1a epitope F2-5B-specfic T-cell proliferative responses and were stained for γδ T-cell subsets or CD4(+) CD25(+) FoxP3(+) T cells before and during infection. As hypothesized, the induction of T-cell exhaustion occurred only following infection with A. marginale, which did not correlate with an increase in either CD4(+) CD25(+) FoxP3(+) T cells or any γδ T-cell subset examined.

A genome-wide association study of immune response traits in Canadian Holstein cattle

Background

Breeding for enhanced immune response (IR) has been suggested as a tool to improve inherent animal health. Dairy cows with superior antibody-mediated (AMIR) and cell-mediated immune responses (CMIR) have been demonstrated to have a lower occurrence of many diseases including mastitis. Adaptive immune response traits are heritable, and it is, therefore, possible to breed for improved IR, decreasing the occurrence of disease. The objective of this study was to perform genome-wide association studies to determine differences in genetic profiles among Holstein cows classified as High or Low for AMIR and CMIR. From a total of 680 cows with immune response phenotypes, 163 cows for AMIR (81 High and 82 Low) and 140 for CMIR (75 High and 65 Low) were selectively genotyped using the Illumina Bovine SNP50 BeadChip. Results were validated using an unrelated population of 164 Holstein bulls IR phenotyped for AMIR and 146 for CMIR.

Results

A generalized quasi likelihood score method was used to determine single nucleotide polymorphisms (SNP) and chromosomal regions associated with immune response. After applying a 5% chromosomal false discovery rate, 186 SNPs were significantly associated with AMIR. The majority (93%) of significant markers were on chromosome 23, with a similar peak found in the bull population. For CMIR, 21 SNP markers remained significant. Candidate genes within 250,000 base pairs of significant SNPs were identified to determine biological pathways associated with AMIR and CMIR. Various pathways were identified, including the antigen processing and presentation pathway, important in host defense. Candidate genes included those within the bovine Major Histocompatability Complex such as BoLA-DQ, BoLA-DR and the non-classical BoLA-NC1 for AMIR and BoLA-DQ for CMIR, the complement system including C2 and C4 for AMIR and C1q for CMIR, and cytokines including IL-17A, IL17F for AMIR and IL-17RA for CMIR and tumor necrosis factor for both AMIR and CMIR. Additional genes associated with CMIR included galectins 1, 2 and 3, BCL2 and β-defensin.

Conclusions

The significant genetic variation associated with AMIR and CMIR in this study may imply feasibility to include immune response in genomic breeding indices as an approach to improve inherent animal health.

Functional and immunological relevance of Anaplasma marginale major surface protein 1a sequence and structural analysis

Bovine anaplasmosis is caused by cattle infection with the tick-borne bacterium, Anaplasma marginale. The major surface protein 1a (MSP1a) has been used as a genetic marker for identifying A. marginale strains based on N-terminal tandem repeats and a 5′-UTR microsatellite located in the msp1a gene. The MSP1a tandem repeats contain immune relevant elements and functional domains that bind to bovine erythrocytes and tick cells, thus providing information about the evolution of host-pathogen and vector-pathogen interactions. Here we propose one nomenclature for A. marginale strain classification based on MSP1a. All tandem repeats among A. marginale strains were classified and the amino acid variability/frequency in each position was determined. The sequence variation at immunodominant B cell epitopes was determined and the secondary (2D) structure of the tandem repeats was modeled. A total of 224 different strains of A. marginale were classified, showing 11 genotypes based on the 5′-UTR microsatellite and 193 different tandem repeats with high amino acid variability per position. Our results showed phylogenetic correlation between MSP1a sequence, secondary structure, B-cell epitope composition and tick transmissibility of A. marginale strains. The analysis of MSP1a sequences provides relevant information about the biology of A. marginale to design vaccines with a cross-protective capacity based on MSP1a B-cell epitopes.

Breadth of the CD4+ T cell response to Anaplasma marginale VirB9-1, VirB9-2 and VirB10 and MHC class II DR and DQ restriction elements

MHC class II molecules influence antigen-specific CD4+ T lymphocyte responses primed by immunization and infection. CD4+ T cell responses are important for controlling infection by many bacterial pathogens including Anaplasma marginale and are observed in cattle immunized with the protective A. marginale outer membrane (OM) vaccine. Immunogenic proteins that comprise the protective OM vaccine include type IV secretion system (T4SS) proteins VirB9-1, VirB9-2 and VirB10, candidates for inclusion in a multiepitope vaccine. Our goal was to determine the breadth of the VirB9-1, VirB9-2 and VirB10 T cell response and MHC class II restriction elements in six cattle with different MHC class II haplotypes defined by DRB3, DQA and DQB allele combinations for each animal. Overlapping peptides spanning each T4SS protein were tested in T cell proliferation assays with autologous antigen-presenting cells (APC) and artificial APC expressing combinations of bovine DR and DQ molecules. Twenty immunostimulatory peptides were identified; three representing two or more epitopes in VirB9-1, ten representing eight or more epitopes in VirB9-2 and seven representing seven or more epitopes in VirB10. Of the eight DRA/DRB3 molecules, four presented 15 peptides, which was biased as DRA/DRB3*1201 presented ten and DRA/DRB3*1101 presented four peptides. Four DQA/DQB molecules composed of two intrahaplotype and two interhaplotype pairs presented seven peptides, of which five were uniquely presented by DQ molecules. In addition, three functional mixed isotype (DQA/DRB3) restriction elements were identified. The immunogenicity and broad MHC class II presentation of multiple VirB9-1, VirB9-2 and VirB10 peptide epitopes justify their testing as a multiepitope vaccine against A. marginale.

Adaptive immunity to Anaplasma pathogens and immune dysregulation: implications for bacterial persistence

Anaplasma marginale is an obligate intraerythrocytic bacterium that infects ruminants, and notably causes severe economic losses in cattle worldwide. Anaplasma phagocytophilum infects neutrophils and causes disease in many mammals, including ruminants, dogs, cats, horses, and humans. Both bacteria cause persistent infection - infected cattle never clear A. marginale and A. phagocytophilum can also cause persistent infection in ruminants and other animals for several years. This review describes correlates of the protective immune response to these two pathogens as well as subversion and dysregulation of the immune response following infection that likely contribute to long-term persistence. I also compare the immune dysfunction observed with intraerythrocytic A. marginale to that observed in other models of chronic infection resulting in high antigen loads, including malaria, a disease caused by another intraerythrocytic pathogen.

Identification of major histocompatibility complex restriction and anchor residues of foot-and-mouth disease virus-derived bovine T-cell epitopes

Despite intensive research on the identification of T-cell epitopes in cattle after foot-and-mouth disease virus (FMDV) infection during the last 20 years, knowledge of major histocompatibility complex (MHC) restriction and anchor residues of such epitopes is still sparse. Therefore, as a first step, we tested lymphocytes from two experimentally FMDV serotype A24-vaccinated and -challenged cattle for recognition of FMDV-derived pentadecapeptides in proliferation assays. Two epitopes were identified: amino acid residues 66 to 80 within the structural protein 1D and amino acid residues 22 to 36 within the structural protein 1A. The latter epitope was recognized by lymphocytes from both cattle. Peptide-specific proliferation was caused by a response of CD4(+) T helper cells as identified by carboxyfluorescein diacetate succinimidyl ester proliferation assays. Having identified one epitope that was recognized by two cattle, we hypothesized that these animals should have common MHC class II alleles. Cloning and sequencing of DRB3, DQA, and DQB alleles revealed that both animals possessed DQA allele 22021 and DQB allele 1301 but had no common DRB3 allele. A parallel analysis of amino acid residues involved in MHC presentation by peptides with alanine substitutions showed that the amino acid residues in positions 5 and 9 within the pentadecapeptide representing the 1A epitope were important for MHC binding in both cattle. These data indicate that the epitope located on FMDV protein 1A can be presented by MHC class II DQ molecules encoded by DQA allele 22021 and DQB allele 1301 and present the first evidence of the binding motif of this particular DQ molecule.

Molecular characterization of Ehrlichia interactions with tick cells and macrophages

Several tick-transmitted Anaplasmataceae family rickettsiales of the genera Ehrlichia and Anaplasma have been discovered in recent years. Some species are classified as pathogens causing emerging diseases with growing health concern for people. They include human monocytic ehrlichiosis, human granulocytic ewingii ehrlichiosis and human granulocytic anaplasmosis which are caused by Ehrlichia chaffeensis, E. ewingii and Anaplasma phagocytophilum, respectively. Despite the complex cellular environments and defense systems of arthropod and vertebrate hosts, rickettsials have evolved strategies to evade host clearance and persist in both vertebrate and tick host environments. For example, E. chaffeensis growing in vertebrate macrophages has distinct patterns of global host cell-specific protein expression and differs considerably in morphology compared with its growth in tick cells. Immunological studies suggest that host cell-specific differences in Ehrlichia gene expression aid the pathogen, extending its survival. Bacteria from tick cells persist longer when injected into mice compared with mammalian macrophage-grown bacteria, and the host response is also significantly different. This review presents the current understanding of tick-Ehrlichia interactions and implications for future.

Rapid deletion of antigen-specific CD4+ T cells following infection represents a strategy of immune evasion and persistence for Anaplasma marginale

Acquired T cell immunity is central for protection against infection. However, the immunological consequences of exposing memory T cells to high Ag loads during acute and persistent infection with systemic pathogens are poorly understood. We investigated this by using infection with Anaplasma marginale, a ruminant pathogen that replicates to levels of 10(9) bacteria per ml of blood during acute infection and maintains mean bacteremia levels of 10(6) per ml during long-term persistent infection. We established that immunization-induced Ag-specific peripheral blood CD4(+) T cell responses were rapidly and permanently lost following infection. To determine whether these T cells were anergic, sequestered in the spleen, or physically deleted from peripheral blood, CD4(+) T lymphocytes from the peripheral blood specific for the major surface protein (MSP) 1a T cell epitope were enumerated by DRB3*1101 tetramer staining and FACS analysis throughout the course of immunization and challenge. Immunization induced significant epitope-specific T lymphocyte responses that rapidly declined near peak bacteremia to background levels. Concomitantly, the mean frequency of tetramer(+)CD4(+) cells decreased rapidly from 0.025% before challenge to a preimmunization level of 0.0003% of CD4(+) T cells. Low frequencies of tetramer(+)CD4(+) T cells in spleen, liver, and inguinal lymph nodes sampled 9-12 wk postchallenge were consistent with undetectable or unsustainable Ag-specific responses and the lack of T cell sequestration. Thus, infection of cattle with A. marginale leads to the rapid loss of Ag-specific T cells and immunologic memory, which may be a strategy for this pathogen to modulate the immune response and persist.

Total, membrane, and immunogenic proteomes of macrophage- and tick cell-derived Ehrlichia chaffeensis evaluated by liquid chromatography-tandem mass spectrometry and MALDI-TOF methods

Ehrlichia chaffeensis, a tick-transmitted rickettsial, is the causative agent of human monocytic ehrlichiosis. To examine protein expression patterns, we analyzed total, membrane, and immunogenic proteomes of E. chaffeensis originating from macrophage and tick cell cultures. Total proteins resolved by one-dimensional gel electrophoresis and subjected to liquid chromatography-electrospray ionization ion trap mass spectrometry allowed identification of 134 and 116 proteins from macrophage- and tick cell-derived E. chaffeensis, respectively. Because a majority of immunogenic proteins remained in the membrane fraction, individually picked total and immunogenic membrane proteins were also surveyed by liquid chromatography-tandem mass spectrometry and matrix-assisted laser desorption ionization-time of flight methods. The analysis aided the identification of 48 additional proteins. In all, 278 genes of the E. chaffeensis genome were verified as functional genes. They included genes for DNA and protein metabolism, energy metabolism and transport, membrane proteins, hypothetical proteins, and many novel proteins of unknown function. The data reported in this study suggest that the membrane of E. chaffeensis is very complex, having many expressed proteins. This study represents the first and the most comprehensive analysis of E. chaffeensis-expressed proteins. This also is the first study confirming the expression of nearly one-fourth of all predicted genes of the E. chaffeensis genome, validating that they are functionally active genes, and demonstrating that classic shotgun proteomic approaches are feasible for tick-transmitted intraphagosomal bacteria. The identity of novel expressed proteins reported in this study, including the large selection of membrane and immunogenic proteins, will be valuable in elucidating pathogenic mechanisms and developing effective prevention and control methods.

Physical linkage of naturally complexed bacterial outer membrane proteins enhances immunogenicity

The outer membrane proteins (OMPs) of bacterial pathogens are essential for their growth and survival and especially for attachment and invasion of host cells. Since the outer membrane is the interface between the bacterium and the host cell, outer membranes and individual OMPs are targeted for development of vaccines against many bacterial diseases. Whole outer membrane fractions often protect against disease, and this protection cannot be fully reproduced by using individual OMPs. Exactly how the interactions among individual OMPs influence immunity is not well understood. We hypothesized that one OMP rich in T-cell epitopes can act as a carrier for an associated OMP which is poor in T-cell epitopes to generate T-dependent antibody responses, similar to the hapten-carrier effect. Major surface protein 1a (MSP1a) and MSP1b1 occur as naturally complexed OMPs in the Anaplasma marginale outer membrane. Previous studies demonstrated that immunization with the native MSP1 heteromer induced strong immunoglobulin G (IgG) responses to both proteins, but only MSP1a stimulated strong CD4+ T-cell responses. Therefore, to test our hypothesis, constructs of CD4+ T-cell epitopes from MSP1a linked to MSP1b1 were compared with individually administered MSP1a and MSP1b1 for induction of MSP1b-specific IgG. By linking the T-cell epitopes from MSP1a to MSP1b1, significantly higher IgG titers against MSP1b1 were induced. Understanding how the naturally occurring intermolecular interactions between OMPs influence the immune response may lead to more effective vaccine design.

Anaplasmosis: focusing on host-vector-pathogen interactions for vaccine development

Anaplasma marginale and A. phagocytophylum are intracellular rickettsiae that cause bovine anaplasmosis and human granulocytic anaplasmosis, respectively. The ultimate vaccine for the control of anaplasmosis would be one that reduces infection and transmission of the pathogen by ticks. Effective vaccines for control of anaplasmosis are not available despite attempts using different approaches, such as attenuated strains, infected erythrocyte and tick cell-derived purified antigens, and recombinant pathogen and tick-derived proteins. Three lines of functional analyses were conducted by our laboratory to characterize host-tick-Anaplasma interactions to discover potential vaccine candidate antigens to control tick infestations and the infection and transmission of Anaplasma spp.: (1) characterization of A. marginale adhesins involved in infection and transmission of the pathogen, (2) global expression analysis of genes differentially expressed in HL-60 human promyelocytic cells in response to infection with A. phagocytophilum, and (3) identification and characterization of tick-protective antigens by expression library immunization (ELI) and analysis of expressed sequence tags (EST) in a mouse model of tick infestations and by RNA interference in ticks. These experiments have resulted in the characterization of the A. marginale MSP1a as an adhesin for bovine erythrocytes and tick cells, providing support for its use as candidate vaccine antigen for the control of bovine . Microarray analysis of genes differentially expressed in human cells infected with A. phagocytophilum identified key molecules involved in pathogen infection and multiplication. The screening for tick-protective antigens resulted in vaccine candidates reducing tick infestation, molting, and oviposition and affecting Anaplasma infection levels in ticks.

Phylogenetic analysis of Anaplasma marginale strains from Paraná State, Brazil, using the msp1alpha and msp4 genes

Anaplasma marginale is an obligate intraerythrocytic rickettsial pathogen (order, Rickettsiales: family, Anaplasmataceae) that causes bovine anaplasmosis. This disease is widely distributed in tropical and sub-tropical regions of the world and causes important economic losses to cattle production. Major surface protein (MSP)1a (msp1alpha gene) is one of the six MSPs identified on A. marginale from cattle, whose sequence and size vary according to the number of tandem 28- to 29-amino acid repeats. This study characterized the msp1alpha and msp4 genes obtained from three distinct Brazilian herds from the State of Paraná. Three strains of the msp1alpha and one strain of the msp4 gene were sequenced. The strains evaluated revealed PCR products of different size, representing three, five and six internal repeats. Sequence analyses confirmed the number of tandem sequence copies and revealed a high degree of sequence identity with strains from other Brazilian States, as well as strains from the USA, Europe and Israel. The msp1alpha DNA and amino acid sequences from A. marginale and DNA sequences of msp4 strains did not reveal distinct phylogeographical segregation. However, the amino acid sequences of msp4 demonstrated definite phylogeographical relationship. These results suggest that the amino acid sequences of msp4 should be used for phylogenetic identification of A. marginale strains and may be an important tool for the epidemiology and control of anaplasmosis. Additionally, the close similarity of the Paraná strains of A. marginale with strains from USA, Europe and Asia may reflect the introduction of these genes during the development of the Brazilian bovine herd.

DNA vaccine construct incorporating intercellular trafficking and intracellular targeting motifs effectively primes and induces memory B- and T-cell responses in outbred animals

We developed a vaccine construct in which a BVP22 domain and an invariant-chain major histocompatibility complex class II-targeting motif capable of enhancing dendritic cell antigen uptake and presentation were fused to a sequence encoding a B- and T-cell antigen from the Anaplasma marginale major surface protein 1a and tested whether this construct would prime and expand immune responses in outbred calves. A single inoculation with this construct effectively primed the immune responses, as demonstrated by a significant enhancement of CD4(+) T-cell proliferation compared to that in calves identically inoculated but inoculated with a DNA construct lacking the targeting domains and compared to that in calves inoculated with an empty vector. These proliferative responses were mirrored by priming and expansion of gamma interferon-positive CD4(+) T cells and immunoglobulin G responses against the linked B-cell epitope. Priming by the single immunization induced memory that underwent rapid recall following reexposure to the antigen. These results demonstrate that DNA vaccines targeting key intercellular and intracellular events significantly enhance priming and expansion and support the feasibility of single-dose DNA immunization in outbred populations.

Anaplasma marginale field challenge: Protection by an inactivated immunogen that shares partial sequence of msp1α variable region with the challenge strain

Twenty four Hereford heifers free of anaplasmosis were allotted into three groups of eight animals each and inoculated three times with adjuvant in Puck saline as control or 50 microg and 100 microg of total protein of Anaplasma marginale initial bodies from three Mexican strains which share the same variable region of msp1alpha and msp4. Inoculation with the adjuvant or the immunogen at either of the two protein doses did not induce any undesirable changes attributable to inoculation in vaccinates or controls. On day 78 post vaccination animals were released in a ranch where bovine Anaplasmosis is endemic. The A. marginale strain prevalent in this ranch shares some of the msp1alpha tandem repeats with and the strains used in the vaccine. After release, all animals became infested with Boophilus microplus ticks and flies. During the challenge period, between days 279 and 300, loss of PCV due to clinical anaplasmosis in control animals was statistically higher from vaccinated animals. Likewise, controls mean peak rickettsemia was also significantly higher (p< or =0.01) than vaccinates' rickettsemias. The antibody responses of all vaccinates after the third vaccination reached OD values above 2.0 on day 49 and were different from controls (p<0.01). IgG(2) responses from both groups of vaccinates were different from controls (p<0.01). Vaccinates which required treatment, also showed the lowest IgG(2) and substantial IgG(1) responses. After contact with the rickettsia, controls developed clinical disease and 7 out of 8 required treatment, while vaccinates in general showed no substantial changes in hematocrit or rickettsemia and only one animal in each group required treatment. Our present results show that vaccination with either 50 microg or 100 microg of protein from purified IB derived from three strains induced protection to resist the challenge with the a field strain that shares some of the tandem repeats of MSP1a.

Quantitation of Anaplasma marginale major surface protein (MSP)1a and MSP2 epitope-specific CD4+ T lymphocytes using bovine DRB3*1101 and DRB3*1201 tetramers

Antigen-specific CD4+ T cells play a critical role in protective immunity to many infectious pathogens. Although the antigen-specific CD4+ T cells can be measured by functional assays such as proliferation or cytokine enzyme-linked immunospot, such assays are limited to a specific function and cannot quantify anergic or suppressed T cells. In contrast, major histocompatiblity complex (MHC) class II tetramers can enumerate epitope-specific CD4+ T cells independent of function. In this paper, we report the construction of bovine leukocyte antigen MHC class II tetramers using a novel mammalian cell system to express soluble class II DRA/DRB3 molecules and defined immunodominant peptide epitopes of Anaplasma marginale major surface proteins (MSPs). Phycoerythrin-labeled tetramers were either loaded with exogenous peptide or constructed with the peptide epitope linked to the N terminus of the DRB3 chain. A DRB3*1101 tetramer loaded with MSP1a peptide F2-5B (ARSVLETLAGHVDALG) and DRB3*1201 tetramers loaded with MSP1a peptide F2-1-1b (GEGYATYLAQAFA) or MSP2 peptide P16-7 (NFAYFGGELGVRFAF) specifically stained antigen-specific CD4+ T cell lines and clones. Tetramers constructed with the T-cell epitope linked to the DRB3 chain were slightly better at labeling CD4+ T cells. In one cell line, the number of tetramer-positive T cells increased to approximately 94% of the CD4+ T cells after culture for 21 weeks with specific antigen. This novel technology should be useful to track the fate of antigen-specific CD4+ T-cell responses in cattle after immunization or infection with persistent pathogens, such as A. marginale, that modulate the host immune response.

Immunogenicity of Mycobacterium bovis BCG expressing Anaplasma marginale MSP1a antigen

Humoral and cellular immune responses of mice inoculated with recombinant Mycobacterium bovis BCG expressing the MSP1a antigen of Anaplasma marginale were evaluated. The msp1a gene was amplified by PCR and cloned into the mycobacterial expression vectors pUS2000 and pMIP12. Immunization of isogenic BALB/c mice with the rBCG/pUS2000-msp1a construct induced significant seroconversion to MSP1a (p<0.001), which was 26 times above pre-immunization levels at day 63 post-initial immunization and which remained stable for the duration of the experiment (6 months). In contrast, rBCG/pMIP12-msp1a induced seroconversion at a level of 6 times above pre-immunization values, which peaked at day 63. Western blot analysis showed that sera derived from mice vaccinated with either rBCG construct recognized both native and recombinant forms of A. marginale MSP1a. In contrast to the humoral response data, immunization with rBCG/pMIP12-msp1a was found to induce a markedly stronger cellular response than that recorded for BCG/pUS2000-msp1a. These observations clearly demonstrated the immunogenicity of recombinant BCG expressing the MSP1a antigen and suggested that the immune responses were influenced by the level of antigen expression. The results of this research warrant studies of recombinant M. bovis BCG expressing MSP1a in cattle to test for protective antibody production for control of bovine anaplasmosis.

Immune control of Babesia bovis infection

Babesia bovis causes an acute and often fatal infection in adult cattle, which if resolved, leads to a state of persistent infection in otherwise clinically healthy cattle. Persistently infected cattle are generally resistant to reinfection with related parasite strains, and this resistance in the face of infection is termed concomitant immunity. Young animals are generally more resistant than adults to B. bovis infection, which is dependent on the spleen. Despite the discovery of B. bovis over a century ago, there are still no safe and effective vaccines that protect cattle against this most virulent of babesial pathogens. Immunodominant antigens identified by serological reactivity and dominant T-cell antigens have failed to protect cattle against challenge. This review describes the innate and acquired immune mechanisms that define resistance in young calves and correlate with the development of concomitant immunity in older cattle following recovery from clinical disease. The first sections will discuss the innate immune responses by peripheral blood- and spleen-derived macrophages in cattle induced by B. bovis merozoites and their products that limit parasite replication, and comparison of natural killer cell responses in the spleens of young (resistant) and adult (susceptible) cattle. Later sections will describe a proteomic approach to discover novel antigens, especially those recognized by immune CD4+ T lymphocytes. Because immunodominant antigens have failed to stimulate protective immunity, identification of subdominant antigens may prove to be important for effective vaccines. Identification of CD4+ T-cell immunogenic proteins and their epitopes, together with the MHC class II restricting elements, now makes possible the development of MHC class II tetramers and application of this technology to both quantify antigen-specific lymphocytes during infection and discover novel antigenic epitopes. Finally, with the imminent completion of the B. bovis genome-sequencing project, strategies using combined genomic and proteomic approaches to identify novel vaccine candidates will be reviewed. The availability of an annotated B. bovis genome will, for the first time, enable identification of non-immunodominant proteins that may stimulate protective immunity.

Intrahaplotype and interhaplotype pairing of bovine leukocyte antigen DQA and DQB molecules generate functional DQ molecules important for priming CD4+ T-lymphocyte responses

Antigen-specific CD4(+) T-lymphocyte responses are restricted by major histocompatibility complex class II molecules, which influence T-cell priming during infection. Human leukocyte antigen (HLA) and bovine leukocyte antigen (BoLA) DRB3 and DQ genes are polymorphic, but unlike HLA, many BoLA haplotypes have duplicated DQ genes, and antibody-blocking studies indicated that BoLA-DQ molecules present various pathogen epitopes. Limited experimentation also suggested that BoLA-DQ molecules formed by interhaplotype pairing of A and B chains are functional. To compare antigen presentation by DR and DQ molecules and to definitively demonstrate functional BoLA-DQ molecules derived from interhaplotype pairing, different combinations of DR or DQ A and B proteins were expressed with CD80 in 293-F cells for use as antigen-presenting cells (APCs). This approach identified 11 unique restriction elements including five DR and six DQ pairs for antigen-specific CD4(+) T-cell responses against tick-transmitted bovine hemoparasites Anaplasma marginale or Babesia bovis. Interhaplotype pairing of DQ A and B molecules was demonstrated. Testing of six expressed DQA/B pairs from an animal with duplicated DQ haplotypes (DH16A/DH22H) demonstrated that an interhaplotype pair, DQA*2206/DQB*1301, presented A. marginale peptide B. In DH22H and DH16A homozygous animals, DQA*2206 was tightly linked with DQB*1402, and DQA*22021 was linked with DQB*1301. APCs from these donors could not present peptide B, confirming that DQA*2206/DQB*1301 encoded a functional interhaplotype pair. Functional BoLA-DQ molecules are generated by both intrahaplotype and interhaplotype pairing of A and B chains and play a similar role to BoLA-DR in priming helper T-cell responses to important pathogens.

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.

Enhancement of antigen acquisition by dendritic cells and MHC class II-restricted epitope presentation to CD4+ T cells using VP22 DNA vaccine vectors that promote intercellular spreading following initial transfection

Induction of immune responses against microbial antigens using DNA is an attractive strategy to mimic the immunity induced by live vaccines. Although DNA vaccines are efficacious in murine models, the requirement for multiple immunizations using high doses in outbred animals and humans has hindered deployment. This requirement is, in part, a result of poor vaccine spreading and suboptimal DC transfection efficiency. Incorporation of a signal that directs intercellular spreading of a DNA-encoded antigen is proposed to mimic live vaccine spreading and increase dendritic cell (DC) presentation. Bovine herpes virus 1 tegument protein, BVP22, is capable of trafficking to surrounding cells. To test the hypothesis that BVP22 enhances spreading and antigen presentation to CD4+ T cells, a DNA construct containing BVP22, fused in-frame to a sequence encoding a T cell epitope of Anaplasma marginale, was generated. A construct with reversed BVP22 sequence served as a negative control. Immunocytometric analysis of transfected primary keratinocytes, human embryonic kidney 293, COS-7, and Chinese hamster ovary cells showed that BVP22 enhanced intercellular spreading by > or = 150-fold. Flow cytometric analysis of antigen-presenting cells (APCs) positively selected from cocultures of transfected cells and APCs showed that 5% of test APCs were antigen-positive, compared with 0.6% of control APCs. Antigen-specific CD4+ T cell proliferation demonstrated that BVP22 enhanced DC antigen presentation by > or = 20-fold. This first report of the ability of BVP22 to increase DNA-encoded antigen acquisition by DCs and macrophages, with subsequent enhancement of major histocompatibility complex class II-restricted CD4+ T cell responses, supports incorporating a spreading motif in a DNA vaccine to target CD4+ T cell-dependent immunity in outbred animals.

Mapping of B-cell epitopes in the N-terminal repeated peptides of Anaplasma marginale major surface protein 1a and characterization of the humoral immune response of cattle immunized with recombinant and whole organism antigens

Major surface protein (MSP) 1a of the genus type species Anaplasma marginale (Rickettsiales: Anaplasmataceae) together with MSP1b forms the MSP1 complex. MSP1a has been shown to be involved in adhesion, infection and tick transmission of A. marginale, as well as to contribute to protective immunity in cattle. A differential antibody response to MSP1a and MSP1b was observed in cattle immunized with A. marginale derived from bovine erythrocytes (anti-MSP1a response) or cultured tick cells (anti-MSP1b response). In this study, we further characterized the MSP1a antibody response of cattle using several immunogens, including recombinant MSP1a (rMSP1a) protein, erythrocyte- or tick cell culture-derived A. marginale, or a combination of tick cell culture-derived A. marginale and rMSP1a. The MSP1a antibody response to all these immunogens was directed primarily against the N-terminal region of MSP1a that contains tandemly repeated peptides, whereas low antibody levels were detected against the C-terminal portion. Linear B-cell epitopes of MSP1a were mapped using synthetic peptides representing the entire sequence of the protein that were prepared by SPOT synthesis technology. Only two peptides in the N-terminal repeats were recognized by sera from immunized cattle. These peptides shared the sequence SSAGGQQQESS, which is likely to contain the linear B-cell epitope that was recognized by the pools of bovine sera. The average differential of antibody titers against MSP1a minus those against MSP1b correlated with lower percent reductions in PCV. A preferential antibody response to MSP1a was observed in cattle immunized with erythrocyte-derived, cell culture-derived plus rMSP1a or rMSP1a alone, and the percent reduction PCV was significantly lower in these cattle as compared with the other immunization groups. These results provide insight into the bovine antibody response against A. marginale and the role of MSP1a in protection of cattle against A. marginale infection.

Conservation of Babesia bovis small heat shock protein (Hsp20) among strains and definition of T helper cell epitopes recognized by cattle with diverse major histocompatibility complex class II haplotypes

Babesia bovis small heat shock protein (Hsp20) is recognized by CD4+ T lymphocytes from cattle that have recovered from infection and are immune to challenge. This candidate vaccine antigen is related to a protective antigen of Toxoplasma gondii, Hsp30/bag1, and both are members of the alpha-crystallin family of proteins that can serve as molecular chaperones. In the present study, immunofluorescence microscopy determined that Hsp20 is expressed intracellularly in all merozoites. Importantly, Hsp20 is also expressed by tick larval stages, including sporozoites, so that natural tick-transmitted infection could boost a vaccine-induced response. The predicted amino acid sequence of Hsp20 from merozoites is completely conserved among different B. bovis strains. To define the location of CD4+ T-cell epitopes for inclusion in a multiepitope peptide or minigene vaccine construct, truncated recombinant Hsp20 proteins and overlapping peptides were tested for their ability to stimulate T cells from immune cattle. Both amino-terminal (amino acids [aa] 1 to 105) and carboxy-terminal (aa 48 to 177) regions were immunogenic for the majority of cattle in the study, stimulating strong proliferation and IFN-gamma production. T-cell lines from all individuals with distinct DRB3 haplotypes responded to aa 11 to 62 of Hsp20, which contained one or more immunodominant epitopes for each animal. One epitope, DEQTGLPIKS (aa 17 to 26), was identified by T-cell clones. The presence of strain-conserved T helper cell epitopes in aa 11 to 62 of the ubiquitously expressed Hsp20 that are presented by major histocompatibility complex class II molecules represented broadly in the Holstein breed supports the inclusion of this region in vaccine constructs to be tested in cattle.

Delayed clearance of Ehrlichia chaffeensis infection in CD4+ T-cell knockout mice

Human monocytic ehrlichiosis is an emerging tick-borne disease caused by the rickettsia Ehrlichia chaffeensis. To examine the role of helper T cells in host resistance to this macrophage-tropic bacterium, we assessed E. chaffeensis infections in three mouse strains with differing functional levels of helper T cells. Wild-type, C57BL/6J mice resolved infections in approximately 2 weeks. Major histocompatibility complex class II (MHCII) knockout, B6.129-Abb(tm1) mice lacking helper T cells developed persistent infections that were not resolved even after several months. CD4+ T-cell-deficient, B6.129S6-Cd4(tm1Knw) mice cleared the infection, but the clearance took 2 weeks longer than it did for wild-type mice. C57BL/6J mice resolved infection more rapidly following a second experimental challenge, but B6.129S6-Cd4(tm1Knw) mice did not. The B6.129S6-Cd4(tm1Knw) mice also developed active E. chaffeensis-specific immunoglobulin G responses that were slightly lower in concentration and slower to develop than that observed in C57BL/6J mice. E. chaffeensis-specific cytotoxic T cells were not detected following a single bacterial challenge in any mouse strain, including wild-type C57BL/6J mice. However, the cytotoxic T-cell activity developed in all three mouse strains, including the MHCII and CD4+ T-cell knockouts, when challenged with a second E. chaffeensis infection. The data reported here suggest that the cell-mediated immunity, orchestrated by CD4+ T cells is critical for conferring rapid clearance of E. chaffeensis.

The hypervariable region of Anaplasma marginale major surface protein 2 (MSP2) contains multiple immunodominant CD4+ T lymphocyte epitopes that elicit variant-specific proliferative and IFN-gamma responses in MSP2 vaccinates

Major surface protein 2 (MSP2) is an immunodominant outer membrane protein of Anaplasma marginale and Anaplasma phagocytophilum pathogens that cause bovine anaplasmosis and human granulocytic ehrlichiosis, respectively. MSP2 has a central hypervariable region (HVR) flanked by highly conserved amino and carboxyl termini. During A. marginale infection, dynamic and extensive amino acid sequence variation in MSP2 occurs through recombination of msp2 pseudogenes into the msp2 expression site, followed by sequential segmental gene conversions to generate additional variants. We hypothesized that MSP2 variation leads to significant changes in Th cell recognition of epitopes in the HVR. T cell epitopes were mapped using T cells from native MSP2-immunized cattle and overlapping peptides spanning the most abundant of five different MSP2 HVRs in the immunogen. Several epitopes elicited potent effector/memory Th cell proliferative and IFN-gamma responses, including those in three discreet blocks of sequence that undergo segmental gene conversion. Th cell clones specific for an epitope in the block 1 region of the predominant MSP2 variant type failed to respond to naturally occurring variants. However, some of these variants were recognized by oligoclonal T cell lines from MSP2 vaccinates, indicating that the variant sequences contain immunogenic CD4(+) T cell epitopes. In competition/antagonism assays, the nonstimulatory variants were not inhibitory for CD4(+) T cells specific for the agonist peptide. Dynamic amino acid sequence variation in MSP2 results in escape from recognition by some effector/memory MSP2-specific Th cells. Antigenic variation in MSP2 Th cell and B cell epitopes may contribute to immune evasion that allows long-term persistence of A. marginale in the mammalian reservoir.

DNA-encoded fetal liver tyrosine kinase 3 ligand and granulocyte macrophage-colony-stimulating factor increase dendritic cell recruitment to the inoculation site and enhance antigen-specific CD4+ T cell responses induced by DNA vaccination of outbred animals

DNA-based immunization is a contemporary strategy for developing vaccines to prevent infectious diseases in animals and humans. Translating the efficacy of DNA immunization demonstrated in murine models to the animal species that represent the actual populations to be protected remains a significant challenge. We tested two hypotheses directed at enhancing DNA vaccine efficacy in outbred animals. The first hypothesis, that DNA-encoding fetal liver tyrosine kinase 3 ligand (Flt3L) and GM-CSF increases dendritic cell (DC) recruitment to the immunization site, was tested by intradermal inoculation of calves with plasmid DNA encoding Flt3L and GM-CSF followed by quantitation of CD1(+) DC. Peak DC recruitment was detected at 10-15 days postinoculation and was significantly greater (p < 0.05) in calves in the treatment group as compared with control calves inoculated identically, but without Flt3L and GM-CSF. The second hypothesis, that DNA encoding Flt3L and GM-CSF enhances immunity to a DNA vector-expressed Ag, was tested by analyzing the CD4(+) T lymphocyte response to Anaplasma marginale major surface protein 1a (MSP1a). Calves immunized with DNA-expressing MSP1a developed strong CD4(+) T cell responses against A. marginale, MSP1a, and specific MHC class II DR-restricted MSP1a epitopes. Administration of DNA-encoding Flt3L and GM-CSF before MSP1a DNA vaccination significantly increased the population of Ag-specific effector/memory cells in PBMC and significantly enhanced MSP1a-specific CD4(+) T cell proliferation and IFN-gamma secretion as compared with MHC class II DR-matched calves vaccinated identically but without Flt3L and GM-CSF. These results support use of these growth factors in DNA vaccination and specifically indicate their applicability for vaccine testing in outbred animals.