Antigenic variation in the persistence and transmission of the ehrlichia Anaplasma marginale

Insights into mechanisms of bacterial antigenic variation derived from the complete genome sequence of Anaplasma marginale

Persistence of Anaplasma spp. in the animal reservoir host is required for efficient tick-borne transmission of these pathogens to animals and humans. Using A. marginale infection of its natural reservoir host as a model, persistent infection has been shown to reflect sequential cycles in which antigenic variants emerge, replicate, and are controlled by the immune system. Variation in the immunodominant outer-membrane protein MSP2 is generated by a process of gene conversion, in which unique hypervariable region sequences (HVRs) located in pseudogenes are recombined into a single operon-linked msp2 expression site. Although organisms expressing whole HVRs derived from pseudogenes emerge early in infection, long-term persistent infection is dependent on the generation of complex mosaics in which segments from different HVRs recombine into the expression site. The resulting combinatorial diversity generates the number of variants both predicted and shown to emerge during persistence.

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.

Distinctly different msp2 pseudogene repertoires in Anaplasma marginale strains that are capable of superinfection

Lifelong persistent infection of cattle is a hallmark of the tick transmitted pathogen Anaplasma marginale. Antigenic variation of Major Surface Protein 2 (MSP2) plays an important role in evasion of the host immune response to allow persistence. Antigenic variation of MSP2 is achieved by gene conversion of pseudogenes into the single operon linked expression site and the diversity of variants is defined by the pseudogene repertoire. Once an animal is persistently infected with one strain of A. marginale, infection with a second strain (superinfection) is rare. However, we recently detected animals superinfected with different strains of A. marginale and hypothesized that the msp2 pseudogene repertoire would be distinct in these superinfecting strains, consistent with encoding different sets of surface variants. Five strains of A. marginale were selected in order to identify and compare msp2 pseudogene content; these included strains with similar and different msp1alpha genotypes, and genotypes that were representative of the strains that were found in the superinfected animals. Southern blot analysis of strains associated with superinfection revealed distinctly different msp2 banding patterns, in contrast to a pattern suggesting identical pseudogene content among related strains not associated with superinfection. Indeed, targeted sequence analysis of msp2 pseudogenes showed identical pseudogene repertoires in genotypically closely related strains and varying amounts of dissimilarity in the pseudogene repertoire in strains with distinctly different msp1alpha genotypes, but totally different msp2 pseudogene repertoires between the strains that were found in superinfected animals. This finding supports the hypothesis that the occurrence of superinfection reflects the differences in the msp2 repertoire and corresponding diversity of variants.

Rapid and long-term disappearance of CD4+ T lymphocyte responses specific for Anaplasma marginale major surface protein-2 (MSP2) in MSP2 vaccinates following challenge with live A. marginale

In humans and ruminants infected with Anaplasma, the major surface protein 2 (MSP2) is immunodominant. Numerous CD4(+) T cell epitopes in the hypervariable and conserved regions of MSP2 contribute to this immunodominance. Antigenic variation in MSP2 occurs throughout acute and persistent infection, and sequentially emerging variants are thought to be controlled by variant-specific Ab. This study tested the hypothesis that challenge of cattle with Anaplasma marginale expressing MSP2 variants to which the animals had been immunized, would stimulate variant epitope-specific recall CD4(+) T cell and IgG responses and organism clearance. MSP2-specific T lymphocyte responses, determined by IFN-gamma ELISPOT and proliferation assays, were strong before and for 3 wk postchallenge. Surprisingly, these responses became undetectable by the peak of rickettsemia, composed predominantly of organisms expressing the same MSP2 variants used for immunization. Immune responsiveness remained insignificant during subsequent persistent A. marginale infection up to 1 year. The suppressed response was specific for A. marginale, as responses to Clostridium vaccine Ag were consistently observed. CD4(+)CD25(+) T cells and cytokines IL-10 and TGF-beta1 did not increase after challenge. Furthermore, a suppressive effect of nonresponding cells was not observed. Lymphocyte proliferation and viability were lost in vitro in the presence of physiologically relevant numbers of A. marginale organisms. These results suggest that loss of memory T cell responses following A. marginale infection is due to a mechanism other than induction of T regulatory cells, such as peripheral deletion of MSP2-specific T cells.

Identification of functional promoters in the msp2 expression loci of Anaplasma marginale and Anaplasma phagocytophilum

Organisms in the family Anaplasmataceae are important tick-borne pathogens of livestock worldwide and cause recently emergent infections in humans. Despite their medical importance, very little is known about how these organisms regulate gene expression in the mammalian host, the tick vector, or during transition between the host and vector. However, it is clear that gene regulation, in addition to recombinatorial mechanisms, is essential for these small genome pathogens to adapt to distinctly different environments. In this study, we identify and establish the function of three promoter elements in the locus encoding major outer membrane protein expression sites in both Anaplasma marginale and Anaplasma phagocytophilum. Gene expression from this locus involves both classical and atypical polycistronic transcripts. The identified promoter elements have a structure similar to that defined in Escherichia coli and are functional in driving protein expression in a prokaryotic cell-free transcription and translation system and in recombinant E. coli. The two strongest promoters identified in vitro and with recombinant E. coli were also shown to be functional in A. marginale infected cells, as determined by quantification of downstream transcripts. The promoters in both A. marginale and A. phagocytophilum have similar structure and activity, supporting the conclusion that the two loci are syntenic with conservation of function. In addition, they share structural elements within the promoters that appear to be likely sites for regulation. These data enhance our understanding of how expression of these variable outer membrane proteins may be controlled in the key stages of tick-borne transmission and infection.

Bacterial toxins and the immune system: show me the in vivo targets

Microorganisms that cause persistent infection often exhibit specific adaptations that allow them to avoid the adaptive immune response. Recently, several bacterial toxins have been shown in vitro to disrupt immune cell functions. However, it remains to be established whether these activities are relevant during infection and whether these toxins have specifically evolved to disrupt the adaptive immune system.

Structural basis for segmental gene conversion in generation of Anaplasma marginale outer membrane protein variants

Bacterial pathogens in the genus Anaplasma generate surface coat variants by gene conversion of chromosomal pseudogenes into single-expression sites. These pseudogenes encode unique surface-exposed hypervariable regions flanked by conserved domains, which are identical to the expression site flanking domains. In addition, Anaplasma marginale generates variants by recombination of oligonucleotide segments derived from the pseudogenes into the existing expression site copy, resulting in a combinatorial increase in variant diversity. Using the A. marginale genome sequence to track the origin of sequences recombined into the msp2 expression site, we demonstrated that the complexity of the expressed msp2 increases during infection, reflecting a shift from recombination of the complete hypervariable region of a given pseudogene to complex mosaics with segments derived from hypervariable regions of different pseudogenes. Examination of the complete set of 1183 variants with segmental changes revealed that 99% could be explained by one of the recombination sites occurring in the conserved flanking domains and the other within the hypervariable region. Consequently, we propose an 'anchoring' model for segmental gene conversion whereby the conserved flanking sequences tightly align and anchor the expression site sequence to the pseudogene. Associated with the recombination sites were deletions, insertions and substitutions; however, these are a relatively minor contribution to variant generation as these occurred in less than 2% of the variants. Importantly, the anchoring model, which can account for more variants than a strict segmental sequence identity mechanism, is consistent with the number of msp2 variants predicted and empirically identified during persistent infection.

Ehrlichia chaffeensis expresses macrophage- and tick cell-specific 28-kilodalton outer membrane proteins

Ehrlichia chaffeensis, a tick-transmitted rickettsial agent, causes human monocyte/macrophage-tropic ehrlichiosis. In this study, proteomic approaches were used to demonstrate host cell-specific antigenic expression by E. chaffeensis. The differentially expressed antigens include those from the 28-kDa outer membrane protein (p28-Omp) multigene locus. The proteins expressed in infected macrophages are the products of p28-Omp19 and p28-Omp20 genes, whereas in tick cells, the protein expressed is the p28-Omp14 gene product. The differentially expressed proteins are posttranslationally modified by phosphorylation and glycosylation to generate multiple expressed forms. Host cell-specific protein expression is not influenced by growth temperatures and is reversible. Host cell-specific protein expression coupled with posttranslational modifications may be a hallmark for the pathogen's adaptation to a dual-host life cycle and its persistence.

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.

Simultaneous variation of the immunodominant outer membrane proteins, MSP2 and MSP3, during anaplasma marginale persistence in vivo

Vector-borne bacterial pathogens persist in the mammalian host by varying surface antigens to evade the existing immune response. To test whether the model of surface coat switching and immune evasion can be extended to a vector-borne bacterial pathogen with multiple immunodominant surface proteins, we examined Anaplasma marginale, a rickettsia with two highly immunogenic outer membrane proteins, major surface protein 2 (MSP2) and MSP3. The simultaneous clearance of variants of the two most immunodominant surface proteins of A. marginale followed by emergence of unique variants indicates that the switch rates and immune selection for MSP2 and MSP3 are sufficiently similar to explain the cyclic bacteremia observed during infection in the immunocompetent host.

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.

Expression of multiple outer membrane protein sequence variants from a single genomic locus of Anaplasma phagocytophilum

Anaplasma phagocytophilum is the causative agent of an emerging tick-borne zoonosis in the United States and Europe. The organism causes a febrile illness accompanied by other nonspecific symptoms and can be fatal, especially if treatment is delayed. Persistence of A. phagocytophilum within mammalian reservoir hosts is important for ensuring continued disease transmission. In the related organism Anaplasma marginale, persistence is associated with antigenic variation of the immunoprotective outer membrane protein MSP2. Extensive diversity of MSP2 is achieved by combinatorial gene conversion of a genomic expression site by truncated pseudogenes. The major outer membrane protein of A. phagocytophilum, MSP2(P44), is homologous to MSP2 of A. marginale, has a similar organization of conserved and variable regions, and is also encoded by a multigene family containing some truncated gene copies. This suggests that the two organisms could use similar mechanisms to generate diversity in outer membrane proteins from their small genomes. We define here a genomic expression site for MSP2(P44) in A. phagocytophilum. As in A. marginale, the msp2(p44) gene in this expression site is polymorphic in all populations of organisms we have examined, whether organisms are obtained from in vitro culture in human HL-60 cells, from culture in the tick cell line ISE6, or from infected human blood. Changes in culture conditions were found to favor the growth and predominance of certain msp2(p44) variants. Insertions, deletions, and substitutions in the region of the genomic expression site encoding the central hypervariable region matched sequence polymorphisms in msp2(p44) mRNA. These data suggest that, similarly to A. marginale, A. phagocytophilum uses combinatorial mechanisms to generate a large array of outer membrane protein variants. Such gene polymorphism has profound implications for the design of vaccines, diagnostic tests, and therapy.

The maintenance of sex in parasites

The maintenance of sex is an unresolved paradox in evolutionary biology, given the inherent twofold fitness advantage for asexuals. Parasitic helminths offer a unique opportunity to address this enigma. Parasites that can create novel antigenic strains are able to escape pre-existing host immunity. Viruses produce diversity through mutation with rapid clonal proliferation. The long generation times of helminth parasites prevent them from adopting this strategy. Instead, we argue that sexual reproduction enables parasitic helminths to rapidly generate strain diversity. We use both a stochastic, individual-based model and a simple analytical model to assess the selective value of sexual versus asexual reproduction in helminth parasites. We demonstrate that sexual reproduction can more easily produce and maintain strain diversity than asexual reproduction for long-lived parasites. We also show that sexual parasite populations are resistant to invasion by rare asexual mutants. These results are robust to high levels of cross-immunity between strains. We suggest that the enhancement of strain diversity, despite stochastic extinction of strains, may be critical to the evolutionary success of sex in long-lived parasites.

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

Native major surface protein 1 (MSP1) of Anaplasma marginale, composed of covalently associated MSP1a and MSP1b proteins, stimulates protective immunity in cattle against homologous and heterologous strain challenge. Protective immunity against pathogens in the family Anaplasmataceae involves both CD4(+) T cells and neutralizing immunoglobulin G. Thus, an effective vaccine should contain both CD4(+) T- and B-lymphocyte epitopes that will elicit strong memory responses upon infection with homologous and heterologous strains. Previous studies demonstrated that the predominant CD4(+) T-cell response in MSP1 vaccinates is directed against the MSP1a subunit. The present study was designed to identify conserved CD4(+) T-cell epitopes in MSP1a presented by a broadly represented subset of major histocompatibility complex (MHC) class II molecules that would be suitable for inclusion in a recombinant vaccine. Transmembrane protein prediction analysis of MSP1a from the Virginia strain revealed a large hydrophilic domain (HD), extending from amino acids (aa) 1 to 366, and a hydrophobic region extending from aa 367 to 593. The N terminus (aa 1 to 67) includes one 28-aa form A repeat and one 29-aa form B repeat, which each contain an antibody neutralization-sensitive epitope [Q(E)ASTSS]. In MSP1 vaccinates, recombinant MSP1a HD (aa 1 to 366) stimulated recall proliferative responses that were comparable to those against whole MSP1a excluding the repeat region (aa 68 to 593). Peptide mapping determined a minimum of five conserved epitopes in aa 151 to 359 that stimulated CD4(+) T cells from cattle expressing DR-DQ haplotypes common in Holstein-Friesian breeds. Peptides representing three epitopes (aa 231 to 266, aa 270 to 279, and aa 290 to 319) were stimulatory for CD4(+) T-cell clones and restricted by DR. A DQ-restricted CD4(+) T-cell epitope, present in the N-terminal form B repeat (VSSQSDQASTSSQLG), was also mapped using T-cell clones from one vaccinate. Although form B repeat-specific T cells did not recognize the form A repeat peptide (VSSQS_EASTSSQLG), induction of T-cell anergy by this peptide was ruled out. The presence of multiple CD4(+) T-cell epitopes in the MSP1a HD, in addition to the neutralization-sensitive epitope, supports the testing of this immunogen for induction of protective immunity against A. marginale challenge.

Antigenic characterization of Anaplasma marginale isolates from different regions of Brazil

Antigenic characterization of A. marginale isolates has contributed to identifying the presence of common and restricts epitopes of major surface proteins (MSPs). The data may improve vaccine development to protect against A. marginale isolates from different regions. Brazilian A. marginale isolates were characterized antigenically by Western blot with monoclonal antibodies (MAbs) against MSPs and rabbit anti-MSP-4 from Florida strain. Six A. marginale isolates from MS, MG (AUFV1), SP, PR-L1, PR-HV, RS and Florida strain were tested with ANA22B1 to MSP-1a, AMR36A6 to MSP-1b, ANAF19E2 to MSP-2, AMG75C1 and AMG76B2 to MSP-3 and ANAF16C1 to MSP-5. ANA22B1 recognized MSP-1a epitope in all A. marginale isolates, and reacted with polypeptides of different size ranging 46-105kDa. MSP2 was not detected in MS and SP isolates by ANAF19E2, and only PR-L1 and MG (AUFV1) isolates reacted with MAbs which recognize MSP3 epitope. MSP4 and MSP5 were detected in all A. marginale isolates analyzed. The results revealed conservation of MSP-1a and MSP-5 epitopes among all Brazilian isolates, and showed antigenic variability to MSP-1b, MSP-2 and MSP-3 proteins, agreeing with recent data about the genetic diversity found in the polimorphic multigene family responsible for these proteins.

The highest priority: what microbial genomes are telling us about immunity

Study of microbial genomes has provided new insight into the functions that pathogens require for survival in the animal host. Small genome bacterial pathogens, defined as those < or = 1/3 the size of Escherichia coli, include chlamydiae, rickettsiae and ehrlichiae, mycoplasmas, and spirochetes. The small genome size is believed to result from reductive evolution, a process of initial mutation with loss of function followed by progressive accumulation of mutations and eventual gene deletion. This is most notable in the 1.1 Mb genome of Rickettsia prowazekki in which 24% of the genome is non-coding, as compared to approximately 10% in the 4.4 Mb E. coli. Consequently, these pathogens are thus presumed to retain only the most important functions for survival and propagation. There is consistent evidence from small genomes that the genetic deletion is primarily related to the loss of metabolic function and especially reduction of multiple overlapping pathways and duplicated genes. Thus, these pathogens undergo progressive reduction in their genomes yet maintain the ability to infect, survive within, and cause disease in animals. In the face of this reductive process, what genes and associated functions are maintained? Strikingly, these pathogens devote a high percentage of their genomes to paralogous families of polymorphic surface molecules. This retention suggests that evasion of the immune response is the highest priority of obligate microbial pathogens and provides a strategy for identifying protective antigens for vaccine development to control disease.

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.

Analysis of Borrelia burgdorferi vlsE gene expression and recombination in the tick vector

Expression and recombination of the antigenic variation vlsE gene of the Lyme disease spirochete Borrelia burgdorferi were analyzed in the tick vector. To assess vlsE expression, Ixodes scapularis nymphs infected with the B. burgdorferi strain B31 were fed on mice for 48 or 96 h or to repletion and then crushed and acetone fixed either immediately thereafter (ticks collected at the two earlier time points) or 4 days after repletion. Unfed nymphs also were examined. At all of the time points investigated, spirochetes were able to bind a rabbit antibody raised against the conserved invariable region 6 of VlsE, as assessed by indirect immunofluorescence, but not preimmune serum from the same rabbit. This same antibody also bound to B31 spirochetes cultivated in vitro. Intensity of fluorescence appeared highest in cultured spirochetes, followed by spirochetes present in unfed ticks. Only a dim fluorescent signal was observed on spirochetes at the 48 and 96 h time points and at day 4 postrepletion. Expression of vlsE in vitro was affected by a rise in pH from 7.0 to 8.0 at 34 degrees C. Hence, vlsE expression appears to be sensitive to environmental cues of the type found in the B. burgdorferi natural history. To assess vlsE recombination, nymphs were capillary fed the B. burgdorferi B31 clonal isolate 5A3. Ticks thus infected were either left to rest for 4 weeks (Group I) or fed to repletion on a mouse (Group II). The contents of each tick from both groups were cultured and 10 B. burgdorferi clones from the spirochetal isolate of each tick were obtained. The vlsE cassettes from several of these clones were amplified by PCR and sequenced. Regardless of whether the isolate was derived from Group I or Group II ticks, no changes were observed in the vlsE sequence. In contrast, vlsE cassettes amplified from B. burgdorferi clones derived from a mouse that was infected with B31-5A3 capillary-fed nymphs showed considerable recombination. It follows that vlsE recombination does not occur in the tick vector.

Serodiagnosis of human granulocytic ehrlichiosis by using novel combinations of immunoreactive recombinant proteins

A panel of seven recombinant antigens, derived from Ehrlichia phagocytophila (the agent of human granulocytic ehrlichiosis), was evaluated by class-specific enzyme-linked immunosorbent assays (ELISAs) for utility in the diagnosis of the infection. Fourteen genomic fragments, obtained by serologic expression screening, contained open reading frames (ORFs) encoding 16 immunodominant antigens. Eleven of these antigens were members of the major surface protein (MSP) multigene family. Alignment of their predicted protein sequences revealed a pattern of conserved sequences, which contained short direct repeats, flanking a variable region. In addition, two genomic clones contained two and three MSP ORFs, respectively, indicating that these genes are clustered in tandem copies. The implications for this pattern of both genomic and protein arrangements in antigenic variations of MSPs and in their utilities in a diagnostic assay are discussed. In addition to two MSP recombinant antigens (rHGE-1 and -3) and a fusion protein of these antigens (rErf-1), five further recombinants were evaluated by ELISA. Two of these antigens (rHGE-14 and -15) were novel, while a third (rHGE-2), with no known function, has been described. The final two recombinant antigens (rHGE-9 and -17) represent overlapping segments of the ankyrin gene (ank). The addition of rHGE-9 ELISA data resulted in the detection of 78% (21 of 27) of acute-phase sera. When serologic data for all recombinants are combined, 96.2% (26 of 27) of convalescent-phase patient serum samples and 85.2% (23 of 27) of acute-phase patient serum samples are detected, indicating the potential of these antigens for use in the development of a rapid serologic assay for the detection of E. phagocytophila infection.

C-terminal invariable domain of VlsE is immunodominant but its antigenicity is scarcely conserved among strains of Lyme disease spirochetes

VlsE, the variable surface antigen of Borrelia burgdorferi, contains two invariable domains located at the amino and carboxyl terminal ends, respectively, and a central variable domain. In this study, both immunogenicity and antigenic conservation of the C-terminal invariable domain were assessed. Mouse antiserum to a 51-mer synthetic peptide (Ct) which reproduced the entire sequence of the C-terminal invariable domain of VlsE from B. burgdorferi strain B31 was reacted on immunoblots with whole-cell lysates extracted from spirochetes of 12 strains from the B. burgdorferi sensu lato species complex. The antiserum recognized only VlsE from strain B31, indicating that epitopes of this domain differed among these strains. When Ct was used as enzyme-linked immunosorbent assay (ELISA) antigen, all of the seven monkeys and six mice that were infected with B31 spirochetes produced a strong antibody response to this peptide, indicating that the C-terminal invariable domain is immunodominant. None of 12 monkeys and only 11 of 26 mice that were infected with strains other than B31 produced a detectable anti-Ct response, indicating a limited antigenic conservation of this domain among these strains. Twenty-six of 33 dogs that were experimentally infected by tick inoculation were positive by the Ct ELISA, while only 5 of 18 serum samples from dogs clinically diagnosed with Lyme disease contained detectable anti-Ct antibody. Fifty-seven of 64 serum specimens that were collected from American patients with Lyme disease were positive by the Ct ELISA, while only 12 of 21 European samples contained detectable anti-Ct antibody. In contrast, antibody to the more conserved invariable region IR(6) of VlsE was present in all of these dog and human serum samples.

C-terminal invariable domain of VlsE may not serve as target for protective immune response against Borrelia burgdorferi

VlsE, the variable surface antigen of the Lyme disease spirochete, Borrelia burgdorferi, contains two invariable domains, at the amino and carboxyl termini, respectively, which collectively account for approximately one-half of the entire molecule's length and remain unchanged during antigenic variation. It is not known if these two invariable domains are exposed at the surface of either the antigen or the spirochete. If they are exposed at the spirochete's surface, they may elicit a protective immune response against B. burgdorferi and serve as vaccine candidates. In this study, a 51-mer synthetic peptide that reproduced the entire sequence of the C-terminal invariable domain of VlsE was conjugated to the carrier keyhole limpet hemocyanin and used to immunize mice. Generated mouse antibody was able to immunoprecipitate native VlsE extracted from cultured B. burgdorferi B31 spirochetes, indicating that the C-terminal invariable domain was exposed at the antigen's surface. However, this domain was inaccessible to antibody binding at the surface of cultured intact spirochetes, as demonstrated by both an immunofluorescence experiment and an in vitro killing assay. Mouse antibody to the C-terminal invariable domain was not able to confer protection against B. burgdorferi infection, indicating that this domain was unlikely exposed at the spirochete's surface in vivo. We concluded that the C-terminal invariable domain was exposed at the antigen's surface but not at the surface of either cultured or in vivo spirochetes and thus cannot elicit protection against B. burgdorferi infection.

Highly conserved regions of the immunodominant major surface protein 2 of the genogroup II ehrlichial pathogen Anaplasma marginale are rich in naturally derived CD4+ T lymphocyte epitopes that elicit strong recall responses

Genogroup II ehrlichia, including the agent of human granulocytic ehrlichiosis, Ehrlichia phagocytophila, and the bovine pathogen Anaplasma marginale, express a markedly immunodominant outer membrane protein designated major surface protein 2 (MSP2). MSP2 is encoded by a multigene family, resulting in the expression of variant B cell epitopes. MSP2 variants are sequentially expressed in the repeated cycles of rickettsemia that characterize persistent A. marginale infection and control of each rickettsemic cycle is associated with development of a variant-specific IgG response. Importantly, these persistent rickettsemic cycles are controlled at levels 100-1000 times lower than those responsible for clinical disease during acute infection. Control of rickettsemia during persistence could result from an anamnestic Th lymphocyte response to conserved regions of MSP2 that enhances the primary Ab response against newly emergent variants. Comparison of MSP2 variants reveals conserved N and C termini flanking the central, surface-exposed hypervariable region that represents the variant B lymphocyte epitopes. We demonstrate MSP2-specific CD4(+) T lymphocyte recognition of epitopes common to several strains of A. marginale and the related pathogen A. ovis. Furthermore, T lymphocyte lines from three individuals identified six to nine overlapping peptides representing a minimum of four to seven dominant or subdominant epitopes in these conserved N and C termini. Immunodominant peptides induced high levels of IFN-gamma, a cytokine associated with protection against ehrlichia and needed for rapid generation of variant-specific IgG2. The presented data support the potential importance of a strong Th lymphocyte response to invariant MSP2 epitopes in controlling rickettsemia during persistent infection to subclinical levels.

Antigenic analysis of Anaplasma marginale grown in bovine erythrocytes co-cultured with bovine endothelial cells

Two monoclonal antibodies (mAbs) for A. marginale were used to test the antigenic integrity of A. marginale grown in vitro in bovine erythrocytes co-cultured with endothelial cells. Both the mAbs reacted in the indirect immunofluorescent antibody test with A. marginale grown in vitro and also detected the antigens in Western immunoblots of SDS-PAGE separated antigens made from A. marginale infected erythrocytes from the cultures. Furthermore, active replication was evident as [35S]-methionine is incorporated by A. marginale present in the second passage of a culture maintained for six weeks as shown by immunoprecipitation of labeled antigens by the mAbs. This indicates that A. marginale grown in the in vitro culture system described previously [Waghela et al., Vet. Parasitol. 73 (1997) 43] maintain antigenic character, and with further development the system can be used for preparing immunogens or diagnostic antigens.

Strain diversity in major surface protein 2 expression during tick transmission of Anaplasma marginale

Specific major surface protein 2 (MSP2) variants are expressed by Anaplasma marginale within the tick salivary gland and, following transmission, are expressed during acute rickettsemia. In previous work, we have shown that a restricted pattern of MSP2 variants is expressed in the salivary glands of Dermacentor andersoni ticks infected with the South Idaho strain of A. marginale. Now we demonstrate that the identical restriction does not apply to two other strains of A. marginale, and that different variants are also expressed when the same strain is transmitted by different Dermacentor spp. This indicates that antigenic diversity among strains is maintained in tick transmission and may be a significant constraint to MSP2 vaccine development.