Structure of the expression site reveals global diversity in MSP2 (P44) variants in Anaplasma phagocytophilum

Genomic analyses indicate the North American Ap-ha variant of the tick-vectored bacterium Anaplasma phagocytophilum was introduced from Europe

BACKGROUND

Anaplasma phagocytophilum is a tick-vectored, obligately intracellular bacterium that infects a diversity of vertebrate hosts. In North America, the Ap-ha variant of A. phagocytophilum can cause dangerous infections in humans, whereas symptomatic human infections in Europe are rare. Conversely, the European host-generalist ecotype of A. phagocytophilum frequently causes illness in domestic ruminants while no comparable infections have been recorded from North America. Despite these differences in pathogenicity, the Ap-ha variant is closely aligned phylogenetically with the European host-generalist ecotype. Furthermore, North American populations of A. phagocytophilum are less genetically diverse than those in Europe. Taken together, these observations suggest that the North American Ap-ha variant may represent an introduced population of this bacterium.

METHODS

Data from publicly available whole genomes of A. phagocytophilum were used to compare phylogeographic patterns and the extent of genetic divergence between the North American Ap-ha variant and the European host-generalist ecotype.

RESULTS

The results confirm that North American Ap-ha samples are phylogenetically nested within the diversity of the European host-generalist ecotype, and that Ap-ha likely radiated within the last 100 years. As expected, the Ap-ha variant also exhibited relatively low genetic diversity levels compared to the European host-generalist ecotype. Finally, North American Ap-ha harbored significantly more derived alleles than the European host-generalist A. phagocytophilum population.

CONCLUSIONS

Collectively, these results support the hypothesis that the Ap-ha variant was recently introduced to North America from Europe and underwent a strong genetic bottleneck during this process (i.e. a 'founder event'). Adaptation to novel vectors may have also played a role in shaping genetic diversity and divergence patterns in these pathogenic bacteria. These findings have implications for future studies aimed at understanding evolutionary patterns and pathogenicity variation within A. phagocytophilum.

Comparative Whole Genome Analysis of an Anaplasma phagocytophilum Strain Isolated from Norwegian Sheep

Anaplasma phagocytophilum is a Gram-negative obligate intracellular tick-borne alphaproteobacteria (family Anaplasmatacea, order Rickettsiales) with a worldwide distribution. In Norway, tick borne fever (TBF), caused by A. phagocytophilum, presents a major challenge in sheep farming. Despite the abundance of its tick vector, Ixodes ricinus, and A. phagocytophilum infections in wild and domestic animals, reports of infections in humans are low compared with cases in the U.S. Although A. phagocytophilum is genetically diverse and complex infections (co-infection and superinfection) in ruminants and other animals are common, the underlying genetic basis of intra-species interactions and host-specificity remains unexplored. Here, we performed whole genome comparative analysis of a newly cultured Norwegian A. phagocytophilum isolate from sheep (ApSheep_NorV1) with 27 other A. phagocytophilum genome sequences derived from human and animal infections worldwide. Although the compared strains are syntenic, there is remarkable genetic diversity between different genomic loci including the pfam01617 superfamily that encodes the major, neutralization-sensitive, surface antigen Msp2/p44. Blast comparisons between the msp2/p44 pseudogene repertoires from all the strains showed high divergence between U. S. and European strains and even between two Norwegian strains. Based on these comparisons, we concluded that in ruminants, complex infections can be attributed to infection with strains that differ in their msp2/p44 repertoires, which has important implications for pathogen evolution and vaccine development. We also present evidence for integration of rickettsial DNA into the genome of ISE6 tick cells.

Biostatistical prediction of genes essential for growth of Anaplasma phagocytophilum in a human promyelocytic cell line using a random transposon mutant library

Anaplasma phagocytophilum (Ap), agent of human anaplasmosis, is an intracellular bacterium that causes the second most common tick-borne illness in North America. To address the lack of a genetic system for these pathogens, we used random Himar1 transposon mutagenesis to generate a library of Ap mutants capable of replicating in human promyelocytes (HL-60 cells). Illumina sequencing identified 1195 non-randomly distributed insertions. As the density of mutants was non-saturating, genes without insertions were either essential for Ap, or spared randomly. To resolve this question, we applied a biostatistical method for prediction of essential genes. Since the chances that a transposon was inserted into genomic TA dinucleotide sites should be the same for all loci, we used a Markov chain Monte Carlo model to estimate the probability that a non-mutated gene was essential for Ap. Predicted essential genes included those coding for structural ribosomal proteins, enzymes involved in metabolism, components of the type IV secretion system, antioxidant defense molecules and hypothetical proteins. We have used an in silico post-genomic approach to predict genes with high probability of being essential for replication of Ap in HL-60 cells. These results will help target genes to investigate their role in the pathogenesis of human anaplasmosis.

Clinical and immunological responses in sheep after inoculation with Himar1-transformed Anaplasma phagocytophilum and subsequent challenge with a virulent strain of the bacterium

In Norway, the tick-transmitted bacterium Anaplasma phagocytophilum is estimated to cause tick-borne fever (TBF) in 300 000 lambs on pastures each year, resulting in economic and animal welfare consequences. Today, prophylactic measures mainly involve the use of acaricides, but a vaccine has been requested by farmers and veterinarians for decades. Several attempts have been made to produce a vaccine against A. phagocytophilum including antigenic surface proteins, inactivated whole cell vaccines and challenge followed by treatment. In the current study, a virulent wild type strain of A. phagocytophilum named Ap.Norvar1 (16S rRNA sequence partial identical to sequence in GenBank acc.no M73220) was subject to genetic transformation with a Himar1-transposon, which resulted in three bacterial mutants, capable of propagation in a tick cell line (ISE6). In order to test the immunogenicity and pathogenicity of the live, mutated bacteria, these were clinically tested in an inoculation- and challenge study in sheep. One group was inoculated with the Ap.Norvar1 as an infection control. After inoculation, the sheep inoculated with mutated bacteria and the Ap.Norvar1 developed typical clinical signs of infection and humoral immune response. After challenge with Ap.Norvar1, 28 days later all groups inoculated with mutated bacteria showed clinical signs of tick-borne fever and bacteremia while the group initially inoculated with the Ap.Norvar1, showed protection against clinical disease. The current study shows a weak, but partial protection against infection in animals inoculated with mutated bacteria, while animals that received Ap.Norvar1 both for inoculation and challenge, responded with homologues protection.

A lifelong study of a pack Rhodesian ridgeback dogs reveals subclinical and clinical tick-borne Anaplasma phagocytophilum infections with possible reinfection or persistence

Background

Various tick-borne infections often occur without specific clinical signs and are therefore notoriously hard to diagnose separately in veterinary practice. Longitudinal studies over multiple tick seasons performing clinical, serological and molecular investigations in parallel, may elucidate the relationship between infection and disease. In this regard, six related Rhodesian Ridgeback dogs living as a pack became subject of lifetime studies due to ongoing tick infestations and recurring clinical problems. Blood samples for diagnostic tests were obtained throughout the years 2000 to 2009.

Methods

Data collected from clinical observations, hemograms, serology and detection of Anaplasma phagocytophilum, either by microscopy or by DNA amplification and typing, were placed in a time line. This dataset essentially presents as a prospective study enabling the association of the Anaplasma infections with occurring disease.

Results

All six dogs were infected, and two of them developed particular clinical symptoms that could be associated with Anaplasma infections over time. More specifically, episodes of general malaise with fever and purpura with thrombocytopenia and bacterial inclusions in granulocytes, were found concurrently with Anaplasma DNA and specific antibodies in peripheral blood samples. DNA from A. phagocytophilum variant 4 (of 16S rRNA) was found in multiple and sequential samples. DNA-sequences from variant 1 and the human granulocytic ehrlichiosis (HGE) agent were also detected.

Conclusions

In this study two lifelong cases of canine anaplasmosis (CGA) are presented. The data show that dogs can be naturally infected concurrently with A. phagocytophilum variant 1, variant 4 and the HGE agent. The ongoing presence of specific antibodies and Anaplasma DNA in one dog indicates one year of persisting infection. Treatment with doxycycline during recurring clinical episodes in the other dog resulted in transient clinical improvement and subsequent disappearance of specific antibodies and DNA suggesting that re-infection occurred.

Electronic supplementary material

The online version of this article (10.1186/s13071-018-2806-8) contains supplementary material, which is available to authorized users.

Persistent Infections and Immunity in Ruminants to Arthropod-Borne Bacteria in the Family Anaplasmataceae

Tick-transmitted gram-negative bacteria in the family Anaplasmataceae in the order Rickettsiales cause persistent infection and morbidity and mortality in ruminants. Whereas Anaplasma marginale infection is restricted to ruminants, Anaplasma phagocytophilum is promiscuous and, in addition to causing disease in sheep and cattle, notably causes disease in humans, horses, and dogs. Although the two pathogens invade and replicate in distinct blood cells (erythrocytes and neutrophils, respectively), they have evolved similar mechanisms of antigenic variation in immunodominant major surface protein 2 (MSP2) and MSP2(P44) that result in immune evasion and persistent infection. Furthermore, these bacteria have evolved distinct strategies to cause immune dysfunction, characterized as an antigen-specific CD4 T-cell exhaustion for A. marginale and a generalized immune suppression for A. phagocytophilum, that also facilitate persistence. This indicates highly adapted strategies of Anaplasma spp. to both suppress protective immune responses and evade those that do develop. However, conserved subdominant antigens are potential targets for immunization.

Mini-review: Strategies for Variation and Evolution of Bacterial Antigens

Across the eubacteria, antigenic variation has emerged as a strategy to evade host immunity. However, phenotypic variation in some of these antigens also allows the bacteria to exploit variable host niches as well. The specific mechanisms are not shared-derived characters although there is considerable convergent evolution and numerous commonalities reflecting considerations of natural selection and biochemical restraints. Unlike in viruses, mechanisms of antigenic variation in most bacteria involve larger DNA movement such as gene conversion or DNA rearrangement, although some antigens vary due to point mutations or modified transcriptional regulation. The convergent evolution that promotes antigenic variation integrates various evolutionary forces: these include mutations underlying variant production; drift which could remove alleles especially early in infection or during life history phases in arthropod vectors (when the bacterial population size goes through a bottleneck); selection not only for any particular variant but also for the mechanism for the production of variants (i.e., selection for mutability); and overcoming negative selection against variant production. This review highlights the complexities of drivers of antigenic variation, in particular extending evaluation beyond the commonly cited theory of immune evasion. A deeper understanding of the diversity of purpose and mechanisms of antigenic variation in bacteria will contribute to greater insight into bacterial pathogenesis, ecology and coevolution with hosts.

Lambs immunized with an inactivated variant of Anaplasma phagocytophilum

Background

Anaplasma phagocytophilum (formerly Ehrlichia phagocytophila) is an obligate intracellular bacterium causing the disease tick-borne fever (TBF) in domestic ruminants. An effective vaccine against the infection has been demanded for livestock by sheep farmers and veterinary practitioners for years.

Findings

In the present study, we immunized lambs with an inactivated suspension of 1 × 10⁸ killed A. phagocytophilum organisms mixed with adjuvant (Montanide ISA 61VG; Seppic). Twelve 9-months-old lambs of the Norwegian White Sheep breed were used. A full two-dose series of immunization was given subcutaneously to six lambs with a 4 week interval between injections. One month after the last immunization, all lambs were challenged with the homologous viable variant of A. phagocytophilum. After challenge, all lambs showed clinical responses for several days, although the immunized lambs reacted with an anamnestic response, i.e. significant reduction in infection rate and a significantly higher antibody titer.

Conclusion

Immunization with inactivated A. phagocytophilum did not protect lambs TBF.

Comparative genomics of first available bovine Anaplasma phagocytophilum genome obtained with targeted sequence capture

BACKGROUND

Anaplasma phagocytophilum is a zoonotic and obligate intracellular bacterium transmitted by ticks. In domestic ruminants, it is the causative agent of tick-borne fever, which causes significant economic losses in Europe. As A. phagocytophilum is difficult to isolate and cultivate, only nine genome sequences have been published to date, none of which originate from a bovine strain.Our goals were to; 1/ develop a sequencing methodology which efficiently circumvents the difficulties associated with A. phagocytophilum isolation and culture; 2/ describe the first genome of a bovine strain; and 3/ compare it with available genomes, in order to both explore key genomic features at the species level, and to identify candidate genes that could be specific to bovine strains.

RESULTS

DNA was extracted from a bovine blood sample infected by A. phagocytophilum. Following a whole genome capture approach, A. phagocytophilum DNA was enriched 197-fold in the sample and then sequenced using Illumina technology. In total, 58.9% of obtained reads corresponded to the A. phagocytophilum genome, covering 85.3% of the HZ genome. Then by performing comparisons with nine previously-sequenced A. phagocytophilum genomes, we determined the core genome of these ten strains. Following analysis, 1281 coding DNA sequences, including 1001 complete sequences, were detected in the A. phagocytophilum bovine genome, of which four appeared to be unique to the bovine isolate. These four coding DNA sequences coded for "hypothetical proteins of unknown function" and require further analysis. We also identified nine proteins common to both European domestic ruminants tested.

CONCLUSION

Using a whole genome capture approach, we have sequenced the first A. phagocytophilum genome isolated from a cow. To the best of our knowledge, this is the first time that this method has been used to selectively enrich pathogenic bacterial DNA from samples also containing host DNA. The four proteins unique to the A. phagocytophilum bovine genome could be involved in host tropism, therefore their functions need to be explored.

Prevalence of human-active and variant 1 strains of the tick-borne pathogen Anaplasma phagocytophilum in hosts and forests of eastern North America

Anaplasmosis is an emerging infectious disease caused by infection with the bacterium Anaplasma phagocytophilum. In the eastern United States, A. phagocytophilum is transmitted to hosts through the bite of the blacklegged tick, Ixodes scapularis. We determined the realized reservoir competence of 14 species of common vertebrate hosts for ticks by establishing the probability that each species transmits two important strains of A. phagocytophilum (A. phagocytophilum human-active, which causes human cases, and A. phagocytophilum variant 1, which does not) to feeding larval ticks. We also sampled questing nymphal ticks from ∼ 150 sites in a single county over 2 years and sampled over 6 years at one location. White-footed mice (Peromyscus leucopus) and Eastern chipmunks (Tamias striatus) were the most competent reservoirs for infection with the A. phagocytophilum human-active strain. Across the county, prevalence in ticks for both strains together was 8.3%; ticks were more than two times as likely to be infected with A. phagocytophilum human-active as A. phagocytophilum variant 1.

Molecular detection of co-infections with Anaplasma phagocytophilum and/or Babesia canis canis in Dirofilaria-positive dogs from Slovakia

Recently, several arthropod-borne infections have been introduced into previously non-endemic regions in Europe as the result of various global changes. At the same time, endemic regions are expanding and the risk of co-infections is rising, due to climate change that allows vectors to move and spread infectious diseases into new areas. The aim of the current study was to confirm simultaneous infections with Anaplasma phagocytophilum and/or Babesia canis canis in Dirofilaria-infected dogs from Slovakia, central Europe. Genomic DNA was isolated from 366 blood samples of microfilaraemic dogs without clinical signs of infection. Samples were further screened for the presence of canine tick-borne pathogens using PCR and sequencing. This survey revealed co-infection with four arthropod-borne pathogens, in particular, Dirofilaria repens, Dirofilaria immitis, A. phagocytophilum, and B. canis canis. While D. repens, responsible for canine subcutaneous dirofilariosis, is scattered through the whole territory of the country, D. immitis occurs only in endemic areas of southeastern and southwestern Slovakia in mixed infection with D. repens. Co-infection with A. phagocytophilum was reported in 3.27% of the dogs positive for D. repens; mixed infection with D. repens and B. canis canis was detected in 3.55% of the tested blood samples. Eastern Slovak Lowland represents a natural focus of B. canis canis and is a highly endemic area for canine dirofilariosis. The presence of triple infection with D. repens, A. phagocytophilum, and B. canis canis was detected in one dog originating from the eastern lowland region of Slovakia. This study highlights the importance of co-infected, clinically healthy dogs in the spreading of several different arthropod-borne pathogens and the necessity for detailed epidemiological surveys, especially in newly infested areas.

An emerging tick-borne disease of humans is caused by a subset of strains with conserved genome structure

The prevalence of tick-borne diseases is increasing worldwide. One such emerging disease is human anaplasmosis. The causative organism, Anaplasma phagocytophilum, is known to infect multiple animal species and cause human fatalities in the U.S., Europe and Asia. Although long known to infect ruminants, it is unclear why there are increasing numbers of human infections. We analyzed the genome sequences of strains infecting humans, animals and ticks from diverse geographic locations. Despite extensive variability amongst these strains, those infecting humans had conserved genome structure including the pfam01617 superfamily that encodes the major, neutralization-sensitive, surface antigen. These data provide potential targets to identify human-infective strains and have significance for understanding the selective pressures that lead to emergence of disease in new species.

Anaplasma phagocytophilum: deceptively simple or simply deceptive?

Anaplasma phagocytophilum is an obligate intracellular rickettsial pathogen transmitted by ixodid ticks. This bacterium colonizes myeloid and nonmyeloid cells and causes human granulocytic anaplasmosis--an important immunopathological vector-borne disease in the USA, Europe and Asia. Recent studies uncovered novel insights into the mechanisms of A. phagocytophilum pathogenesis and immunity. Here, we provide an overview of the underlying events by which the immune system responds to A. phagocytophilum infection, how this pathogen counteracts host immunity and the contribution of the tick vector for microbial transmission. We also discuss current scientific gaps in the knowledge of A. phagocytophilum biology for the purpose of exchanging research perspectives.

Antigen variability in Anaplasma phagocytophilum during chronic infection of a reservoir host

Anaplasma phagocytophilum is an obligately intracellular, tick-transmitted, bacterial pathogen of humans and other animals. In order to evade host immunity during the course of infection, A. phagocytophilum utilizes gene conversion to shuffle approximately 100 functional pseudogenes into a single expression cassette of the msp2(p44) gene, which encodes the major surface antigen, major surface protein 2 (Msp2). The role and extent of msp2(p44) recombination in a reservoir host for A. phagocytophilum have not been evaluated. In the current study, we explored patterns of recombination and expression site variability of the msp2(p44) gene in three chronically infected woodrats, a reservoir for the disease in the Western USA. All three woodrats developed persistent infection of at least 6 months duration; two of them maintained active infection for at least 8 months. In total, we detected the emergence of 60 unique msp2(p44) expression site variants with no common temporal patterns of expression site recombination among the three A. phagocytophilum populations. Both the strength of infection (i.e. pathogen load) and the genetic diversity of pseudogenes detected at the msp2(p44) expression site fluctuated periodically during the course of infection. An analysis of the genomic pseudogene exhaustion rate showed that the repertoire of pseudogenes available to the A. phagocytophilum population could in theory become depleted within a year. However, the apparent emergence of variant pseudogenes suggests that the pathogen could potentially evade host immunity indefinitely. Our findings suggest a tightly co-evolved relationship between A. phagocytophilum and woodrats in which the pathogen perpetually evades host immunity yet causes no detectable disease.

Typical and atypical manifestations of Anaplasma phagocytophilum infection in dogs

Eighteen clinically ill dogs, naturally infected with Anaplasma phagocytophilum, were examined at a veterinary practice in Baxter, Minnesota. A clinical examination, complete blood cell count, enzyme- linked immunosorbent assay (ELISA) for A phagocytophilum, Borrelia burgdorferi, and Ehrlichia canis antibodies and Dirofilaria immitis antigen, and a polymerase chain reaction test for A phagocytophilum DNA were obtained for all dogs. Physical examination findings included fever, arthropathy, lymphadenopathy, epistaxis, acute gastritis, cervical hyperpathia, and central nervous system dysfunction. Complete blood cell count abnormalities included thrombocytopenia, morulae in neutrophils, anemia, leukopenia, eosinopenia, lymphopenia, and monocytosis. Seroreactivity to A phagocytophilum was found in 61%, B burgdorferi antibodies in 17%, and D immitis antigen in 5% of the dogs. Fever, arthropathy, neurologic dysfunction, and epistaxis are clinical syndromes that can be associated with A phagocytophilum infection. Treatment with doxycycline resulted in rapid resolution of clinical signs in all dogs.

Evolution of antigen variation in the tick-borne pathogen Anaplasma phagocytophilum

Anaplasma phagocytophilum is an obligately intracellular tick-transmitted bacterial pathogen of humans and other animals. During the course of infection, A. phagocytophilum utilizes gene conversion to shuffle ∼100 functional pseudogenes into a single expression cassette of the msp2(p44) gene, which codes for the major surface antigen and major surface protein 2 (MSP2). The role and extent of msp2(p44) recombination, particularly in hosts that only experience acute infections, is not clear. In the present study, we explored patterns of recombination and expression of the msp2(p44) gene of A. phagocytophilum in a serially infected mouse model. Even though the bacterium was passed rapidly among mice, minimizing the opportunities for the host to develop adaptive immunity, we detected the emergence of 34 unique msp2(p44) expression cassette variants. The expression of msp2(p44) pseudogenes did not follow a consistent pattern among different groups of mice, although some pseudogenes were expressed more frequently than others. In addition, among 263 expressed pseudogenes, 3 mosaic sequences each consisting of 2 different pseudogenes were identified. Population genetic analysis showed that genetic diversity and subpopulation differentiation tended to increase over time until stationarity was reached but that the variance that was observed in allele (expressed pseudogene) frequency could occur by drift alone only if a high variance in bacterial reproduction could be assumed. These findings suggest that evolutionary forces influencing antigen variation in A. phagocytophilum may comprise random genetic drift as well as some innate but apparently nonpurifying selection prior to the strong frequency-dependent selection that occurs cyclically after hosts develop strong adaptive immunity.

Mechanisms of obligatory intracellular infection with Anaplasma phagocytophilum

Anaplasma phagocytophilum persists in nature by cycling between mammals and ticks. Human infection by the bite of an infected tick leads to a potentially fatal emerging disease called human granulocytic anaplasmosis. A. phagocytophilum is an obligatory intracellular bacterium that replicates inside mammalian granulocytes and the salivary gland and midgut cells of ticks. A. phagocytophilum evolved the remarkable ability to hijack the regulatory system of host cells. A. phagocytophilum alters vesicular traffic to create an intracellular membrane-bound compartment that allows replication in seclusion from lysosomes. The bacterium downregulates or actively inhibits a number of innate immune responses of mammalian host cells, and it upregulates cellular cholesterol uptake to acquire cholesterol for survival. It also upregulates several genes critical for the infection of ticks, and it prolongs tick survival at freezing temperatures. Several host factors that exacerbate infection have been identified, including interleukin-8 (IL-8) and cholesterol. Host factors that overcome infection include IL-12 and gamma interferon (IFN-γ). Two bacterial type IV secretion effectors and several bacterial proteins that associate with inclusion membranes have been identified. An understanding of the molecular mechanisms underlying A. phagocytophilum infection will foster the development of creative ideas to prevent or treat this emerging tick-borne disease.

Cloning of the major outer membrane protein expression locus in Anaplasma platys and seroreactivity of a species-specific antigen

Anaplasma platys infects peripheral blood platelets and causes infectious cyclic thrombocytopenia in canines. The genes, proteins, and antigens of A. platys are largely unknown, and an antigen for serodiagnosis of A. platys has not yet been identified. In this study, we cloned the A. platys major outer membrane protein cluster, including the P44/Msp2 expression locus (p44ES/msp2ES) and outer membrane protein (OMP), using DNA isolated from the blood of four naturally infected dogs from Venezuela and Taiwan, Republic of China. A. platys p44ES is located within a 4-kb genomic region downstream from a putative transcriptional regulator, tr1, and a homolog of the Anaplasma phagocytophilum, identified here as A. platys omp-1X. The predicted molecular masses of the four mature A. platys P44ES proteins ranged from 43.3 to 43.5 kDa. Comparative analyses of the deduced amino acid sequences of Tr1, OMP-1X, and P44/Msp2 proteins from A. platys with those from A. phagocytophilum showed sequence identities of 86.4% for Tr1, 45.9% to 46.3% for OMP-1X, and 55.0% to 56.9% for P44/Msp2. Comparison between A. platys and Anaplasma marginale proteins showed sequence identities of 73.1% for Tr1/Tr, 39.8% for OMP-1X/OMP1, and 41.5% to 42.1% for P44/Msp2. A synthetic OMP-1X peptide was shown to react with A. platys-positive sera but not with A. platys-negative sera or A. phagocytophilum-positive sera. Together, determination of the genomic locus of A. platys outer membrane proteins not only contributes to the fundamental understanding of this enigmatic pathogen but also helps in developing A. platys-specific PCR and serodiagnosis.

Potential infectivity of Anaplasma phagocytophilum strains in Ixodes ricinus ticks from Serbia

Anaplasma phagocytophilum strains from different geographical regions are characterised by diverse potential infectivity for humans and domesticated animals. We investigated the potential pathogenicity of A. phagocytophilum strains in Ixodes ricinus ticks from 11 geographically different localities in Serbia. Sequences obtained in this study showed a high variability of p44 paralogues. Some of them, however, formed groups with similarities greater than 86% ('similarity groups'). Previous studies showed that 'similarity groups' were nearly always country specific. Our results correlated with this observation, and we also observed significant clustering of paralogues according to vector and reservoir origin of A. phagocytophilum strains. According to the high genetic similarity of sequences isolated from ticks collected in four localities, namely Avala, Batrovci, Hajducka cesma and Ljubovija, with paralogues with proven pathogenicity isolated from human granulocytic anaplasmosis (HGA) patients and A. phagocytophilum infected sheep, we could assume that strains with potential infectivity for humans and domestic animals were present in Serbia.

Reservoir competence of the redwood chipmunk (Tamias ochrogenys) for Anaplasma phagocytophilum

Granulocytic anaplasmosis (GA) is an emerging tick-transmitted disease that persists in rodent- Ixodes ricinus-complex tick cycles across the Holarctic. Although the putative reservoir for anaplasmosis in the western United States is the dusky-footed woodrat (Neotoma fuscipes), this rodent was not shown reservoir-competent because of failure of infection from woodrats to other animals via ticks. Redwood chipmunks are common in habitats where Anaplasma phagocytophilum is common, have high PCR- and seroprevalence, and are infested with a diversity of Ixodes spp. ticks. Experimental infection of seven wild-caught A. phagocytophilum-negative redwood chipmunks induced persistent periods of recurrent rickettsemia during the persistent phase of infection. Of three animals for which xenodiagnosis was attempted, all successfully infected pools of I. pacificus larvae during the primary rickettsemia. We show that chipmunks are reservoir-competent for GA and may be important for maintaining infection in nature.

Variant -and individual dependent nature of persistent Anaplasma phagocytophilum infection

BACKGROUND

Anaplasma phagocytophilum is the causative agent of tick-borne fever in ruminants and human granulocytotropic anaplasmosis (HGA). The bacterium is able to survive for several months in immune-competent sheep by modifying important cellular and humoral defence mechanisms. Little is known about how different strains of A. phagocytophilum propagate in their natural hosts during persistent infection.

METHODS

Two groups of five lambs were infected with each of two 16S rRNA gene variants of A. phagocytophilum, i.e. 16S variant 1 which is identical to GenBank no M73220 and 16S variant 2 which is identical to GenBank no AF336220, respectively. The lambs were infected intravenously and followed by blood sampling for six months. A. phagocytophilum infection in the peripheral blood was detected by absolute quantitative real-time PCR.

RESULTS

Both 16S rRNA gene variants of A. phagocytophilum established persistent infection for at least six months and showed cyclic bacteraemias, but variant 1 introduced more frequent periods of bacteraemia and higher number of organisms than 16S rRNA gene variant 2 in the peripheral blood.

CONCLUSION

Organisms were available from blood more or less constantly during the persistent infection and there were individual differences in cyclic activity of A. phagocytophilum in the infected animals. Two 16S rRNA gene variants of A. phagocytophilum show differences in cyclic activity during persistent infection in lambs.

Anaplasma phagocytophilum and Ehrlichia chaffeensis: subversive manipulators of host cells

Anaplasma spp. and Ehrlichia spp. cause several emerging human infectious diseases. Anaplasma phagocytophilum and Ehrlichia chaffeensis are transmitted between mammals by blood-sucking ticks and replicate inside mammalian white blood cells and tick salivary-gland and midgut cells. Adaptation to a life in eukaryotic cells and transmission between hosts has been assisted by the deletion of many genes that are present in the genomes of free-living bacteria (including genes required for the biosynthesis of lipopolysaccharide and peptidoglycan), by the acquisition of a cholesterol uptake pathway and by the expansion of the repertoire of genes encoding the outer-membrane porins and type IV secretion system. Here, I review the specialized properties and other adaptations of these intracellular bacteria.

Antigen diversity in the parasitic bacterium Anaplasma phagocytophilum arises from selectively-represented, spatially clustered functional pseudogenes

Anaplasma phagocytophilum is a tick-transmitted bacterial pathogen of humans and other animals, and is an obligate intracellular parasite. Throughout the course of infection, hosts acquire temporary resistance to granulocytic anaplasmosis as they develop immunity specific for the major antigen, major surface protein 2 (Msp2). However, the bacterium then utilizes a novel recombination mechanism shuffling functional pseudogenes sequentially into an expression cassette with conserved 5' and 3' ends, bypassing host immunity. Approximately 100 pseudogenes are present in the only fully sequenced human-origin HZ genome, representing the possibility for almost unlimited antigenic diversity. In the present study, we identified a select group of 20% of the A. phagocytophilum HZ msp2 pseudogenes that have matched preferentially to human, canine, and equine expression cassettes. Pseudogenes cluster predominantly in one spatial run limited to a single genomic island in less than 50% of the genome but phylogenetically related pseudogenes are neither necessarily located in close proximity on the genome nor share similar percent identity with expression cassettes. Pseudogenes near the expression cassette (and the origin) are more likely to be expressed than those farther away. Taken together, these findings suggest that there may be natural selection pressure to retain pseudogenes in one cluster near the putative origin of replication, even though global recombination shuffles pseudogenes around the genome, separating pseudogenes that share genetic origins as well as those with similar identities.

Diversity of Anaplasma phagocytophilum strains, USA

We analyzed the structure of the expression site encoding the immunoprotective protein MSP2/P44 from multiple Anaplasma phagocytophilum strains in the United States. The sequence of p44ESup1 had diverged in Ap-variant 1 strains infecting ruminants. In contrast, no differences were detected between A. phagocytophilum strains infecting humans and domestic dogs.

Structural analysis of a p44/msp2 expression site of Anaplasma phagocytophilum in naturally infected ticks in Japan

Anaplasma phagocytophilum, an agent of human granulocytic anaplasmosis, infects neutrophils and causes an emerging tickborne febrile disease. The genome of this bacterium contains a large number of p44/msp2-related genes encoding 44 kDa major outer-membrane proteins, and it is known that a specific p44/msp2 gene is predominantly transcribed from a single expression locus. This study successfully characterized the genomic expression site for p44/msp2 (3.8 kb) in uncultured A. phagocytophilum from Ixodes persulcatus ticks inhabiting a northern part of Japan. Comparative analysis of the sequences revealed that the structures of the expression sites in Japanese A. phagocytophilum were similar to those of US strains from human patients and European strains from a dog and sheep, but omp-1N (upstream from p44/msp2) and a truncated recA (downstream from p44/msp2) in the p44/msp2 expression site seemed to share similarities with those of US and European strains. The central hypervariable region sequences of Japanese p44/msp2 were found to be quite diverse (24.4-100 % amino acid similarities) and distinct from their closest relatives from US human patients or animal host origins (56.3-97.6 % amino acid similarities) with some exceptions. Thus, this study provides significant information about the molecular characteristics of A. phagocytophilum in East Asia, as well as the global diversity of p44/msp2.

Variant-specific and diminishing immune responses towards the highly variable MSP2(P44) outer membrane protein of Anaplasma phagocytophilum during persistent infection in lambs

Anaplasma phagocytophilum is the causative agent of tick-borne fever in small ruminants and has been identified as the zoonotic agent of human granulocytic anaplasmosis. The Norwegian strains of the rickettsia are naturally persistent in lambs and represent a suitable experimental system for analyzing the mechanisms of persistence. Variation of the outer membrane protein MSP2(P44) by recombination of variable pseudogene segments into an expression site is believed to play a key role in persistence of the organism. The goal of the present study was to analyze the dynamics of the immune response towards A. phagocytophilum and MSP2(P44) during persistent infection of lambs. Responses to the hypervariable region of MSP2(P44) were detected shortly after appearance of the respective variants in cyclic rickettsemic peaks, consistent with a process of antigenic variation. In addition, there was a diminishing antibody response to MSP2(P44) and to other A. phagocytophilum antigens overall with time of infection, that was not associated with clearance of the infection.

An entropy-optimized multilocus approach for characterizing the strains of Anaplasma phagocytophilum infecting horses in the Czech Republic

Anaplasma phagocytophilum is a tick-borne rickettsial pathogen that has measurable genetic heterogeneity across its geographical range and reservoir spectrum. In the present study, publicly available sequences of the genes that have prevailingly been used for typing A. phagocytophilum were analysed to identify the segments giving the highest resolution with respect to the predictability of host and geographical provenances of the strains. Selected partial sequences of 16S rRNA, groL, msp4 and ank genes were then employed in a tentative multilocus typing scheme used to characterize the strains causing equine granulocytic anaplasmosis (EGA). We were able to both identify alleles characteristic for equine strains of A. phagocytophilum and distinguish two unique genetic variants infecting horses in the Czech Republic. This resolution far exceeded the discriminatory potential of any of the four sequenced genes when used singly. The two novel A. phagocytophilum variants appeared to be phylogenetically closer to the strains reported as causing human disease in Slovenia than to strains thus far isolated from other European EGA cases. A decline in the quality of recently deposited A. phagocytophilum sequences was also demonstrated.

Differential expression and glycosylation of anaplasma phagocytophilum major surface protein 2 paralogs during cultivation in sialyl Lewis x-deficient host cells

Many microbial pathogens alter expression and/or posttranslational modifications of their surface proteins in response to dynamics within their host microenvironments to retain optimal interactions with their host cells and/or to evade the humoral immune response. Anaplasma phagocytophilum is an intragranulocytic bacterium that utilizes sialyl Lewis x (sLe(x))-modified P-selectin glycoprotein ligand 1 as a receptor for infecting myeloid cells. Bacterial populations that do not rely on this receptor can be obtained through cultivation in sLe(x)-defective cell lines. A. phagocytophilum major surface protein 2 [Msp2(P44)] is encoded by members of a paralogous gene family and is speculated to play roles in host adaptation. We assessed the complement of Msp2(P44) paralogs expressed by A. phagocytophilum during infection of sLe(x)-competent HL-60 cells and two HL-60 cell lines defective for sLe(x) expression. Multiple Msp2(P44) and N-terminally truncated 25- to 27-kDa isoforms having various isoelectric points and electrophoretic mobilities were expressed in each cell line. The complement of expressed msp2(p44) paralogs and the glycosyl residues modifying Msp2(P44) varied considerably among bacterial populations recovered from sLe(x)-competent and -deficient host cells. Thus, loss of host cell sLe(x) expression coincided with both differential expression and glycosylation of A. phagocytophilum Msp2(P44). This reinforces the hypothesis that this bacterium is able to generate a large variety of surface-exposed molecules that could provide great antigenic diversity and result in multiple binding properties.

Possible differential host tropism in Anaplasma phagocytophilum strains in the Western United States

Despite organization into a single species, Anaplasma phagocytophilum, of bacteria from diverse hosts, there is evidence that different hosts vary in susceptibility to local strains of this bacterial pathogen. In particular, there is evidence that clinical hosts (humans, horses, and dogs) in the western part of the United States may be infected with strains of bacteria that differ from those in the reported wildlife reservoir, the dusky-footed woodrat. There is a lack of spatial overlap between clinical cases with woodrat exposure, genetic studies show highly distinctive strains in woodrats compared with those from clinical hosts, and experimental studies failed to transfer infection from woodrats into horses. These data suggest that multiple distinct A. phagocytophilum strains could circulate in western U.S. ecosystems. Host barriers to different bacterial strains would suggest the need for careful discrimination of A. phagocytophilum strains and further research to understand the host-pathogen interactions that result in differential outcomes in infection.

Co-phylogenetic analysis of Anaplasma phagocytophilum and its vectors, Ixodes spp. ticks

The coevolutionary history of Ixodes spp. ticks, the obligately tick-transmitted bacterial pathogen Anaplasma phagocytophilum, and its various rodent reservoir hosts world-wide is not known. According to coevolution theory, the most recently evolved of tick-bacterial complexes could have difficulty maintaining A. phagocytophilum in nature, because transmissibility has not been efficiently maximized. This study was intended to examine the phylogeographic history of I. ricinus-subgroup ticks and A. phagocytophilum, provide an estimate for the date of the divergence of A. marginale and A. phagocytophilum, and evaluate whether there is correspondence between tick and Anaplasma spp. trees. Analysis of Ixodes spp. ticks showed a New World clade consisting of I. scapularis and I. pacificus, European I. ricinus as a sister group to this clade, and Asian I. persulcatus as basal. Of the three A. phagocytophilum genes evaluated, the most resolution was provided by the ankA gene. ankA sequences formed an Old World clade with eastern North America strains as a sister clade. California strains were highly diverse and did not form a clade. Base substitution rates were very comparable along both A. marginale and A. phagocytophilum lineages. Based on 16S rDNA analysis, maximum and minimum divergence times of A. phagocytophilum and A. marginale were calculated to be 78,296,703 and 43,415,708 years, respectively. If A. phagocytophilum did closely coevolve with specific I. ricinus-subgroup tick species, then A. phagocytophilum strains could have specialized on local tick species and optimized local infectivity in the Old World and eastern US. However, lack of absolute resolution of tick trees and conflicting prevalence data (with low prevalence in Asia and western North America) preclude us from inferring a tight coevolutionary relationship of tick species from this phylogeographic analysis.

In situ detection of Anaplasma spp. by DNA target-primed rolling-circle amplification of a padlock probe and intracellular colocalization with immunofluorescently labeled host cell von Willebrand factor

Endothelial cell culture and preliminary immunofluorescent staining of Anaplasma-infected tissues suggest that endothelial cells may be an in vivo nidus of mammalian infection. To investigate endothelial cells and other potentially cryptic sites of Anaplasma sp. infection in mammalian tissues, a sensitive and specific isothermal in situ technique to detect localized Anaplasma gene sequences by using rolling-circle amplification of circularizable, linear, oligonucleotide probes (padlock probes) was developed. Cytospin preparations of uninfected or Anaplasma-infected cell cultures were examined using this technique. Via fluorescence microscopy, the technique described here, and a combination of differential interference contrast microscopy and von Willebrand factor immunofluorescence, Anaplasma phagocytophilum and Anaplasma marginale were successfully localized in situ within intact cultured mammalian cells. This work represents the first application of this in situ method for the detection of a microorganism and forms the foundation for future applications of this technique to detect, localize, and analyze Anaplasma nucleotide sequences in the tissues of infected mammalian and arthropod hosts and in cell cultures.

Anaplasma phagocytophilum MSP2(P44)-18 predominates and is modified into multiple isoforms in human myeloid cells

Anaplasma phagocytophilum is the etiologic agent of human granulocytic anaplasmosis. MSP2(P44), the bacterium's major surface protein, is encoded by a paralogous gene family and has been implicated in a variety of pathobiological processes, including antigenic variation, host adaptation, adhesion, porin activity, and structural integrity. The consensus among several studies performed at the DNA and RNA levels is that a heterogeneous mix of a limited number of msp2(p44) transcripts is expressed by A. phagocytophilum during in vitro cultivation. Such analyses have yet to be extended to the protein level. In this study, we used proteomic and molecular approaches to determine that MSP2(P44)-18 is the predominant if not the only paralog expressed and is modified into multiple 42- to 44-kDa isoforms by A. phagocytophilum strain HGE1 during infection of HL-60 cells. The msp2(p44) expression profile was homogeneous for msp2(p44)-18. Thus, MSP2(P44)-18 may have a fitness advantage in HL-60 cell culture in the absence of selective immune pressure. Several novel 22- to 27-kDa MSP2 isoforms lacking most of the N-terminal conserved region were also identified. A. phagocytophilum MSP2(P44) orthologs expressed by other pathogens in the family Anaplasmataceae are glycosylated. Gas chromatography revealed that recombinant MSP2(P44)-18 is modified by glucose, galactose, xylose, mannose, and trace amounts of other glycosyl residues. These data are the first to confirm differential modification of any A. phagocytophilum MSP2(P44) paralog and the first to provide evidence for expression of truncated versions of such proteins.

Sequential analysis of Anaplasma phagocytophilum msp2 transcription in murine and equine models of human granulocytic anaplasmosis

Anaplasma phagocytophilum causes human granulocytic anaplasmosis by inducing immunopathologic responses. Its immunodominant Msp2 protein is encoded by a family of >100 paralogs. Msp2 (msp2) expression modulates in the absence of immune pressure, and prolonged in vitro passage modulates in vivo virulence. Because programmed MSP2 expression occurs in Anaplasma marginale, we hypothesized a similar event in A. phagocytophilum in vivo, with specific Msp2 expression triggering immunopathologic injury or clinical manifestations of disease. We examined msp2 transcripts in 11 B6 mice and 6 horses inoculated with low- or high-passage A. phagocytophilum Webster strain. Blood was sequentially obtained through 3 weeks postinfection for msp2 reverse transcription-PCR. Horses were additionally assessed for clinical manifestations, seroconversion, complete blood count, blood chemistry, and cytokine gene transcription. In both species, there was no consistent emergence of msp2 transcripts, and all 22 msp2 variants were detected in both passage groups. Clinical severity was much higher for high-passage-infected than for low-passage-infected horses, preceded by higher levels of blood gamma interferon transcription on day 7. Antibody was first detected on day 7, and all horses seroconverted by day 22, with a trend toward lower antibody titers in low-passage-infected animals. Leukocyte and platelet counts were similar between experimental groups except on day 13, when low-passage-infected animals had more profound thrombocytopenia. These findings corroborate studies with mice, where msp2 diversity did not explain differences in hepatic histopathology, but differ from the paradigm of low-passage A. phagocytophilum causing more significant clinical illness. Alteration in transcription of msp2 has no bearing on clinical disease in horses, suggesting the existence of a separate proinflammatory component differentially expressed with changing in vitro passage.

Outer membrane protein sequence variation in lambs experimentally infected with Anaplasma phagocytophilum

Anaplasma phagocytophilum has long been known to cause tick-borne fever in ruminants and has been identified more recently as the causative agent of the emerging disease human granulocytic anaplasmosis. The related organism Anaplasma marginale uses gene conversion of the expression site for two major outer membrane proteins (OMPs) to generate extensive sequence and antigenic variation in these OMPs. This is thought to present a continuously varying repertoire of epitopes to the mammalian host and allow disease persistence. Recent genomic and structural data on human strains of A. phagocytophilum, together with animal studies in model systems, have implicated an orthologous OMP of A. phagocytophilum in a similar mechanism of variation. However, to date there has been little investigation of the mechanisms of antigenic variation or disease persistence in hosts naturally infected with field strains of A. phagocytophilum. Approximately 300,000 lambs in Norway suffer severe disease caused by A. phagocytophilum annually. We show here the persistent and cyclic nature of infection in these animals that is accompanied by loosely programmed sequence variation of the major OMP expression site in each rickettsemic peak. These data will allow analysis of interactions between A. phagocytophilum and the host immune system in naturally occurring persistent infections and provide an important comparison with enduring infections of cattle caused by A. marginale.

Anaplasma phagocytophilum p44 mRNA expression is differentially regulated in mammalian and tick host cells: involvement of the DNA binding protein ApxR

The natural life cycle of Anaplasma phagocytophilum, an obligatory intracellular bacterium that causes human granulocytic anaplasmosis, consists of alternate infection of two distinct hosts, ticks and mammals, in which bacterial surface proteins are expected to have a critical role. The present study investigated regulation of A. phagocytophilum p44 genes, which encode the P44 major surface proteins. Quantitative real-time reverse transcription-PCR analysis revealed that the amount of p44 mRNA obtained from spleens of A. phagocytophilum-infected SCID mice was approximately 10-fold greater than the amount obtained from salivary glands of A. phagocytophilum-infected Ixodes scapularis nymphs. Similarly, the amount of p44 mRNA obtained from A. phagocytophilum-infected HL-60 cells per bacterium was significantly greater than the amount obtained from infected ISE6 tick cells. The relative amount of p44 mRNA was approximately threefold higher in A. phagocytophilum-infected HL-60 cells cultured at 37 degrees C than in A. phagocytophilum-infected HL-60 cells cultured at 28 degrees C. Although there are more than 100 p44 paralogs, we observed expression mainly from the p44 expression locus (p44E) in various host environments. Interestingly, transcription of the A. phagocytophilum gene encoding the DNA binding protein ApxR was also significantly greater in A. phagocytophilum-infected HL-60 cells than in infected ISE6 tick cells. Gel mobility shift and DNase I protection assays revealed recombinant ApxR binding to the promoter regions of p44E and apxR. ApxR also transactivated the p44E and apxR promoter regions in a lacZ reporter assay. These results indicate that p44 genes and apxR are specifically up-regulated in the mammalian host environment and suggest that ApxR not only is positively autoregulated but also acts as a transcriptional regulator of p44E.

Immune evasion and immunosuppression by Anaplasma phagocytophilum, the causative agent of tick-borne fever of ruminants and human granulocytic anaplasmosis

Anaplasma phagocytophilum, the causative agent of tick-borne fever (TBF) in sheep and cattle and human granulocytic anaplasmosis, has the unique ability to infect and multiply within neutrophils, eosinophils and monocytes, cells at the frontline of the immune system. Infection with A. phagocytophilum is also characterized by severe leukopenia due to lymphocytopenia, neutropenia and thrombocytopenia lasting for several days. By itself TBF does not cause high mortality rates but infected animals are more susceptible to other secondary infections, pregnant animals may abort and there is a severe reduction in milk yield in dairy cattle. The susceptibility to secondary infections can be attributed to the leukopenia that accompanies the disease and the organism's adverse effects on lymphocyte and neutrophil functions. One of its fascinating features is that it infects and actively grows in neutrophils by employing an array of mechanisms to subvert their bactericidal activity. These include its ability to inhibit phagosome-lysosome fusion, to suppress respiratory burst and to delay the apoptotic death of neutrophils. It is also able to survive within an apparently immune host by employing a complex mechanism of antigenic variation.