Growth of Cowdria ruminantium, the causative agent of heartwater, in a tick cell line

Possible biased virulence attenuation in the Senegal strain of Ehrlichia ruminantium by ntrX gene conversion from an inverted segmental duplication

Ehrlichia ruminantium is a tick-borne intracellular pathogen of ruminants that causes heartwater, a disease present in Sub-saharan Africa, islands in the Indian Ocean and the Caribbean, inducing significant economic losses. At present, three avirulent strains of E. ruminantium (Gardel, Welgevonden and Senegal isolates) have been produced by a process of serial passaging in mammalian cells in vitro, but unfortunately their use as vaccines do not offer a large range of protection against other strains, possibly due to the genetic diversity present within the species. So far no genetic basis for virulence attenuation has been identified in any E. ruminantium strain that could offer targets to facilitate vaccine production. Virulence attenuated Senegal strains have been produced twice independently, and require many fewer passages to attenuate than the other strains. We compared the genomes of a virulent and attenuated Senegal strain and identified a likely attenuator gene, ntrX, a global transcription regulator and member of a two-component system that is linked to environmental sensing. This gene has an inverted partial duplicate close to the parental gene that shows evidence of gene conversion in different E. ruminantium strains. The pseudogenisation of the gene in the avirulent Senegal strain occurred by gene conversion from the duplicate to the parent, transferring a 4 bp deletion which is unique to the Senegal strain partial duplicate amongst the wild isolates. We confirmed that the ntrX gene is not expressed in the avirulent Senegal strain by RT-PCR. The inverted duplicate structure combined with the 4 bp deletion in the Senegal strain can explain both the attenuation and the faster speed of attenuation in the Senegal strain relative to other strains of E. ruminantium. Our results identify nrtX as a promising target for the generation of attenuated strains of E. ruminantium by random or directed mutagenesis that could be used for vaccine production.

Isolation and Propagation of Laboratory Strains and a Novel Flea-Derived Field Strain of Wolbachia in Tick Cell Lines

Wolbachia are intracellular endosymbionts of several invertebrate taxa, including insects and nematodes. Although Wolbachia DNA has been detected in ticks, its presence is generally associated with parasitism by insects. To determine whether or not Wolbachia can infect and grow in tick cells, cell lines from three tick species, Ixodes scapularis, Ixodes ricinus and Rhipicephalus microplus, were inoculated with Wolbachia strains wStri and wAlbB isolated from mosquito cell lines. Homogenates prepared from fleas collected from cats in Malaysia were inoculated into an I. scapularis cell line. Bacterial growth and identity were monitored by microscopy and PCR amplification and sequencing of fragments of Wolbachia genes. The wStri strain infected Ixodes spp. cells and was maintained through 29 passages. The wAlbB strain successfully infected Ixodes spp. and R. microplus cells and was maintained through 2–5 passages. A novel strain of Wolbachia belonging to the supergroup F, designated wCfeF, was isolated in I. scapularis cells from a pool of Ctenocephalides sp. cat fleas and maintained in vitro through two passages over nine months. This is the first confirmed isolation of a Wolbachia strain from a flea and the first isolation of any Wolbachia strain outside the “pandemic” A and B supergroups. The study demonstrates that tick cells can host multiple Wolbachia strains, and can be added to panels of insect cell lines to improve success rates in isolation of field strains of Wolbachia.

Transcriptomic analysis of Ehrlichia ruminantium during the developmental stages in bovine and tick cell culture

The use of bioinformatics tools to search for possible vaccine candidates has been successful in recent years. In an attempt to search for additional vaccine candidates or improve the current heartwater vaccine design, a genome-wide transcriptional profile of E. ruminantium (Welgevonden strain) replicating in bovine endothelial cells (BA886) and Ixodes scapularis embryonic tick cells (IDE8) was performed. The RNA was collected from the infective extracellular form, the elementary bodies (EBs) and vegetative intracellular form, reticulate bodies (RBs) and was used for transcriptome sequencing. Several genes previously implicated with adhesion, attachment and pathogenicity were exclusively up-regulated in the EBs from bovine and tick cells. Similarly, genes involved in adaptation or survival of E. ruminantium in the host cells were up-regulated in the RBs from bovine cells. Thus, it was concluded that those genes expressed in the EBs might be important for infection of mammalian and tick host cells and these may be targets for both cell and humoral mediated immune responses. Alternatively, those exclusively expressed in the RBs may be important for survival in the host cells. Exported or secreted proteins exclusively expressed at this stage are ideal targets for the stimulation of cytotoxic T-lymphocyte (CTL) immune responses in the host.

Growth of Ehrlichia canis, the causative agent of canine monocytic ehrlichiosis, in vector and non-vector ixodid tick cell lines

Canine monocytic ehrlichiosis is caused by Ehrlichia canis, a small gram-negative coccoid bacterium that infects circulating monocytes. The disease is transmitted by the brown dog tick Rhipicephalus sanguineus s.l. and is acknowledged as an important infectious disease of dogs and other members of the family Canidae worldwide. E. canis is routinely cultured in vitro in the canine monocyte-macrophage cell line DH82 and in non-vector Ixodes scapularis tick cell lines, but not in cells derived from its natural vector. Here we report infection and limited propagation of E. canis in the tick cell line RSE8 derived from the vector R. sanguineus s.l., and successful propagation through six passages in a cell line derived from the experimental vector Dermacentor variabilis. In addition, using bacteria semi-purified from I. scapularis cells we attempted to infect a panel of cell lines derived from non-vector species of the tick genera Amblyomma, Dermacentor, Hyalomma, Ixodes and Rhipicephalus with E. canis and, for comparison, the closely-related Ehrlichia ruminantium, causative agent of heartwater in ruminants. Amblyomma and non-vector Dermacentor spp. cell lines appeared refractory to infection with E. canis but supported growth of E. ruminantium, while some, but not all, cell lines derived from Hyalomma, Ixodes and Rhipicephalus spp. ticks supported growth of both pathogens. We also illustrated and compared the ultrastructural morphology of E. canis in DH82, RSE8 and I. scapularis IDE8 cells. This study confirms that E. canis, like E. ruminantium, is able to grow not only in cell lines derived from natural and experimental tick vectors but also in a wide range of other cell lines derived from tick species not known to transmit this pathogen.

Molecular-genetic and ultrastructural characteristics of 'Candidatus Ehrlichia khabarensis', a new member of the Ehrlichia genus

Recently, a new Ehrlichia genetic variant, Ehrlichia sp. Khabarovsk, was identified in tissue samples of small mammals captured in the Russian Far East. To further characterize Ehrlichia sp. Khabarovsk, tissue homogenate from a naturally infected gray red-backed vole (Myodes rufocanus) was passaged three times in newborn laboratory mice. Using nested PCR Ehrlichia sp. Khabarovsk DNA was detected in tissue samples from infected mice at 1-4 weeks post inoculation. Electron microscopic examination revealed morulae containing gram-negative bacterial cells in monocytes of mouse spleen and liver. The size and ultrastructure of these cells corresponded to those described previously and allowed us to identify the bacteria as Ehrlichia sp. The comparison of ehrlichial 16S rRNA, groEL and gltA genes and putative GroEL and GltA amino acid sequences has demonstrated that Ehrlichia sp. Khabarovsk, like Ehrlichia ruminantium, is more distant from all other Ehrlichia species than these species are between themselves. Phylogenetic analysis has shown that Ehrlichia sp. Khabarovsk belongs to the clade formed by Ehrlichia spp. but clusters separately from other Ehrlichia species and genetic variants. These data indicate that Ehrlichia sp. Khabarovsk can be considered as a new candidate species. We propose to designate it as 'Candidatus Ehrlichia khabarensis' according to the territory where this species was found.

Coinfection of tick cell lines has variable effects on replication of intracellular bacterial and viral pathogens

Ticks transmit various human and animal microbial pathogens and may harbour more than one pathogen simultaneously. Both viruses and bacteria can trigger, and may subsequently suppress, vertebrate host and arthropod vector anti-microbial responses. Microbial coinfection of ticks could lead to an advantage or disadvantage for one or more of the microorganisms. In this preliminary study, cell lines derived from the ticks Ixodes scapularis and Ixodes ricinus were infected sequentially with 2 arthropod-borne pathogens, Borrelia burgdorferi s.s., Ehrlichia ruminantium, or Semliki Forest virus (SFV), and the effect of coinfection on the replication of these pathogens was measured. Prior infection of tick cell cultures with the spirochaete B. burgdorferi enhanced subsequent replication of the rickettsial pathogen E. ruminantium whereas addition of spirochaetes to cells infected with E. ruminantium had no effect on growth of the latter. Both prior and subsequent presence of B. burgdorferi also had a positive effect on SFV replication. Presence of E. ruminantium or SFV had no measurable effect on B. burgdorferi growth. In tick cells infected first with E. ruminantium and then with SFV, virus replication was significantly higher across all time points measured (24, 48, 72h post infection), while presence of the virus had no detectable effect on bacterial growth. When cells were infected first with SFV and then with E. ruminantium, there was no effect on replication of either pathogen. The results of this preliminary study indicate that interplay does occur between different pathogens during infection of tick cells. Further study is needed to determine if this results from direct pathogen-pathogen interaction or from effects on host cell defences, and to determine if these observations also apply in vivo in ticks. If presence of one pathogen in the tick vector results in increased replication of another, this could have implications for disease transmission and incidence.

Ultrastructure of Ehrlichia mineirensis, a new member of the Ehrlichia genus

Recently, we reported the in vitro isolation and the molecular characterization of a new species of Ehrlichia (Ehrlichia mineirensis) from haemolymph of Brazilian Rhipicephalus (Boophilus) microplus ticks. This organism shows an ortholog of Ehrlichia canis major immunogenic protein gp36 with a new structure of tandem repeats. In the present study, we used electron microscopy (high pressure freezing and freeze substitution preparative techniques) to characterize morphologically this new agent growing in IDE8 tick cells. The results showed that E. mineirensis shares ultrastructural features with other members of the genus Ehrlichia (Ehrlichia muris, E. canis and Ehrlichia chaffeensis); typical parasitophorous vacuoles (morulae) contain electron-dense and reticulated Ehrlichiae embedded inside a fibrillar matrix. We observed the characteristic Gram-negative-type cell wall composed of both cytoplasmic and rippled outer membrane. We found organisms undergoing binary fission and rarely altered cells with unusual invagination of the cytoplasmic membrane.

Endogenous tick viruses and modulation of tick-borne pathogen growth

Ticks transmit a wide range of viral, bacterial and protozoan pathogens, many of which can establish persistent infections of lifelong duration in the vector tick and in some cases are transmitted transovarially to the next generation. In addition many ixodid and argasid tick cell lines and, by inference the parent ticks from which they were derived, harbor endogenous viruses (ETV) of which almost nothing is known. In general, low level persistent infections with viral pathogens (arboviruses) are not known to have a deleterious effect on tick survival and fitness, suggesting that they can strike a balance with the tick innate immune response. This tolerance of arbovirus infection may be modulated by the permanent presence of ETV in the host cell. In mosquito cells, temporary or permanent silencing of the genes of an endogenous virus by RNA interference can result in changes in replication rate of a co-infecting arbovirus. We propose that tick cell lines offer a useful model system for in vitro investigation of the modulatory effect of ETV on superinfecting pathogen survival and replication in ticks, using the molecular manipulation techniques applied to insect cells.

In vitro cultivation of Anaplasma marginale and A. phagocytophilum in tick cell lines: a review

Continuous cell lines have been established from several ixodid and argasid tick species, representing an excellent tool suitable for the isolation of pathogens and their subsequent propagation, which in turn allows the production of antigenic material for diagnostic tests, antibody and vaccine production, and also for studies on host-vector-pathogen relationships. This paper reviews the use of tick cells for culture initiation and maintenance of two obligate intracellular bacterial pathogens, Anaplasma marginale and Anaplasma phagocytophilum. These in vitro cultivation systems have been used in a wide range of studies, covering morphological ultrastructural analysis, genetics, proteomics and biological differences between strains, including genome transcriptional and protein expression approaches, enabling comparisons between host and vector cells. Thus, such systems open a new window for a better understanding of interactions between pathogens and tick cells. Last but not least, such systems contribute to the reduction in usage of animals for experimental research, as antigenic material can be produced in reasonably large quantities without the use of in vivo species-specific systems.

Transmission electron microscopy reveals distinct macrophage- and tick cell-specific morphological stages of Ehrlichia chaffeensis

Background

Ehrlichia chaffeensis is an emerging tick-borne rickettsial pathogen responsible for human monocytic ehrlichiosis. Despite the induction of an active host immune response, the pathogen has evolved to persist in its vertebrate and tick hosts. Understanding how the organism progresses in tick and vertebrate host cells is critical in identifying effective strategies to block the pathogen transmission. Our recent molecular and proteomic studies revealed differences in numerous expressed proteins of the organism during its growth in different host environments.

Methodology/Principal Findings

Transmission electron microscopy analysis was performed to assess morphological changes in the bacterium within macrophages and tick cells. The stages of pathogen progression observed included the attachment of the organism to the host cells, its engulfment and replication within a morulae by binary fission and release of the organisms from infected host cells by complete host cell lysis or by exocytosis. E. chaffeensis grown in tick cells was highly pleomorphic and appears to replicate by both binary fission and filamentous type cell divisions. The presence of Ehrlichia-like inclusions was also observed within the nucleus of both macrophages and tick cells. This observation was confirmed by confocal microscopy and immunoblot analysis.

Conclusions/Significance

Morphological differences in the pathogen’s progression, replication, and processing within macrophages and tick cells provide further evidence that E. chaffeensis employs unique host-cell specific strategies in support of adaptation to vertebrate and tick cell environments.

Propagation of a Brazilian isolate of Anaplasma marginale with appendage in a tick cell line (BME26) derived from Rhipicephalus (Boophilus) microplus

Anaplasma marginale is a tick-borne pathogen of cattle responsible for the disease anaplasmosis. Data suggest that Rhipicephalus (Boophilus) microplus and R. annulatus may be the major tick vectors of A. marginale in tropical and subtropical regions of the world. In this work we demonstrated the first infection and propagation of a Brazilian isolate of A. marginale (UFMG1) in the BME26 cell line derived originally from embryos of R. (Boophilus) microplus. The establishment of A. marginale infection in a cell line derived from R. (Boophilus) microplus is relevant for studying the A. marginale/tick interface.

Laboratory maintenance of Ehrlichia chaffeensis and Ehrlichia canis and recovery of organisms for molecular biology and proteomics studies

Tick-borne illnesses are emerging as a major concern for human health in recent years. These include the human monocytic ehrlichiosis caused by the Amblyomma americanum tick-transmitted bacterium, Ehrlichia chaffeensis; human ewingii ehrlichiosis caused by Ehrlichia ewingii (also transmitted by A. americanum ticks); and human granulocytic anaplasmosis caused by the Ixodes scapularis tick-transmitted pathogen, Anaplasma phagocytophilum. Likewise, tick-borne rickettsial pathogens are also a major concern to the health of various vertebrates including dogs, cattle, and several wild animals. In vitro-cultured pathogens grown in a vertebrate host cell and a tick cell culture system will be useful in studies to understand the pathogenic differences as well as to perform experimental infection studies and to generate large quantities of purified antigens. In this unit, methods for culturing E. chaffeensis and Ehrlichia canis (a canine monocytic ehrlichiosis pathogen) in cell lines to represent vertebrate and tick hosts are described. The unit also includes methods useful in purifying bacteria from the host cells and to evaluate proteins by 2-D gel electrophoresis and western blotting.

Tick cell lines: tools for tick and tick-borne disease research

Over 40 cell lines are currently available from 13 ixodid and one argasid tick species. The successful isolation and propagation of several economically important tick-borne pathogens in tick cell lines has created a useful model to study interactions between tick cells and these viral and bacterial disease agents. Tick cell lines have already proved to be a useful tool in helping to define the complex nature of the host-vector-pathogen relationship. With the availability of genomics tools, tick cell lines will become increasingly important as a complement to tick and tick-borne disease research in vivo once genetic transformation and gene silencing using RNA interference become routine.

Applications of a cell culture system for studying the interaction ofAnaplasmamarginale with tick cells

A cell culture system for the tick-borne rickettsiaAnaplasma marginaleoffers new opportunities for research on this economically important pathogen of cattle.A. marginalemultiplies in membrane-bound inclusions in host cells. Whereas erythrocytes appear to be the only site of infection in cattle,A. marginaleundergoes a complex developmental cycle in ticks and transmission occurs via the salivary glands during feeding. We recently developed a cell culture system forA. marginaleusing a cell line derived from embryos ofIxodes scapularis. Here we review the use of this cell culture system for studying the interaction ofA. marginalewith tick cells. Several assays were developed using theA. marginale/tick cell system. An adhesion assay was developed for the identification of proteins required byA. marginalefor adhesion to tick cells. The effect of antibodies against selected major surface proteins in inhibitingA. marginaleinfection was tested in an assay that allowed further confirmation of the role of surface proteins in the infection of tick cells. A drug screening assay forA. marginalewas developed and provides a method of initial drug selection without the use of cattle. The culture system was used to test for enhancing effects of tick saliva and saliva components onA. marginaleinfection. The tick cell culture system has proved to be a good model for studyingA. marginale–tick interactions. Information gained from these studies may be applicable to other closely related tick-borne pathogens that have been propagated in the same tick cell line.

Evolution and function of tandem repeats in the major surface protein 1a of the ehrlichial pathogenAnaplasma marginale

The major surface protein (MSP) 1a of the ehrlichial cattle pathogenAnaplasma marginale, encoded by the single-copy genemsp1α, has been shown to have a neutralization-sensitive epitope and to be an adhesin for bovine erythrocytes and tick cells.msp1αhas been found to be a stable genetic marker for the identification of geographic isolates ofA. marginalethroughout development in acutely and persistently infected cattle and in ticks. The molecular weight of MSP1a varies among geographic isolates ofA. marginalebecause of a varying number of tandemly repeated peptides of 28–29 amino acids. Variation in the sequence of the tandem repeats occurs within and among isolates, and may have resulted from evolutionary pressures exerted by ligand–receptor and host–parasite interactions. These repeated sequences include markers for tick transmissibility that may be important in the identification of ehrlichial pathogens because they may influence control strategies and the design of subunit vaccines.

Unique macrophage and tick cell-specific protein expression from the p28/p30-outer membrane protein multigene locus in Ehrlichia chaffeensis and Ehrlichia canis

Ehrlichia chaffeensis and Ehrlichia canis are tick-transmitted rickettsial pathogens that cause human and canine monocytic ehrlichiosis respectively. We tested the hypothesis that these pathogens express unique proteins in response to their growth in vertebrate and tick host cells and that this differential expression is similar in closely related Ehrlichia species. Evaluation of nine E. chaffeensis isolates and one E. canis isolate demonstrated that protein expression was host cell-dependent. The differentially expressed proteins included those from the p28/30-Omp multigene locus. E. chaffeensis and E. canis proteins expressed in infected macrophages were primarily the products of the p28-Omp 19 and 20 genes or their orthologues. In cultured tick cells, E. canis expressed only the p30-10 protein, an orthologue of the E. chaffeensis p28-Omp 14 protein which is the only protein expressed by E. chaffeensis propagated in cultured tick cells. The expressed Omp proteins were post-translationally modified to generate multiple molecular forms. E. chaffeensis gene expression from the p28/30-Omp locus was similar in tick cell lines derived from both vector (Amblyomma americanum) and non-vector (Ixodes scapularis) ticks. Differential expression of proteins within the p28/p30-Omp locus may therefore be vital for adaptation of Ehrlichia species to their dual host life cycle.

Use of refrigeration as a practical means to preserve viability of in vitro-cultured IDE8 tick cells

In vitro cultivation of the IDE8 cell line, derived from embryonic Ixodes scapularis ticks, constitutes an important system for the study of tick-borne pathogens, as these cells support growth of rickettsial species which are not normally transmitted by this tick. However, since cryopreservation of IDE8 cells is not always successful, there is a need to develop alternative ways to preserve these cells. In the present study, a suspension of IDE8 cells in culture medium was kept under refrigeration at 4 degrees C for up to 60 days. Every 15 days, the suspension was mixed and aliquots were re-cultured in 2-ml tubes, under standardized conditions. In addition, three techniques for cryopreservation, using two different cryoprotectants (DMSO and glycerol), were evaluated. Medium changes were carried out every week and subculturing every 2 weeks. The development of cultures and their respective subcultures, after returning to standard culture temperature, was evaluated by percentage viability and by cellular morphology evaluated in Giemsa-stained cytocentrifuge smears. All cultures and subcultures appeared healthy, showing growth rates comparable to cultures that had not been kept under refrigeration. The results demonstrated that storage under refrigeration at 4 degrees C is an efficient method for preservation of IDE8 cells for up to 60 days and that refrigeration may be preferable to cryopreservation for short-term preservation of IDE8 cells.

Transcription analysis of the major antigenic protein 1 multigene family of three in vitro-cultured Ehrlichia ruminantium isolates

Ehrlichia ruminantium, an obligate intracellular bacterium transmitted by ticks of the genus Amblyomma, causes heartwater disease in ruminants. The gene coding for the major antigenic protein MAP1 is part of a multigene family consisting of a cluster containing 16 paralogs. In the search for differentially regulated genes between E. ruminantium grown in endothelial and tick cell lines that could be used in vaccine development and to determine if differences in the map1 gene cluster exist between different isolates of E. ruminantium, we analyzed the map1 gene cluster of the Senegal and Gardel isolates of E. ruminantium. Both isolates contained the same number of genes, and the same organization as found in the genome sequence of the Welgevonden isolate (H. Van Heerden, N. E. Collins, K. A. Brayton, C. Rademeyer, and B. A. Allsopp, Gene 330:159-168, 2004). However, comparison of two subpopulations of the Gardel isolate maintained in different laboratories demonstrated that recombination between map1-3 and map1-2 had occurred in one subpopulation with deletion of one entire gene. Reverse transcription-PCR on E. ruminantium derived mRNA from infected cells using gene-specific primers revealed that all 16 map1 paralogs were transcribed in endothelial cells. In one vector (Amblyomma variegatum) and several nonvector tick cell lines infected with E. ruminantium, transcripts were found for between 4 and 11 paralogs. In all these cases the transcript for the map1-1 gene was detected and was predominant. Our results indicate that the map1 gene cluster is relatively conserved but can be subject to recombination, and differences in the transcription of map1 multigenes in host and vector cell environments exist.

Characterization of Ehrlichia ruminantium replication and release kinetics in endothelial cell cultures

Ehrlichia ruminantium is the causative agent of Heartwater, a fatal tick-borne disease affecting ruminants in African countries and West Indies and can be used as an inactivated vaccine for wild and domestic animals. In order to improve E. ruminantium production yields we characterize E. ruminantium growth kinetics in terms of duplication time, maximum production yield, and peak of infectivity. After a 24 h period for E. ruminantium attachment/internalization and a lag phase of 12 h, the exponential growth occurred within 36-108 h post-infection (hpi) with a net increase of up to 2.2 orders of magnitude. Maximum E. ruminantium infectivity was observed at 120 hpi and was defined as the best time of harvesting (TOH) for propagation of E. ruminantium cultures. This study showed that considering the quality constraint of the final product (E. ruminantium vaccine), the E. ruminantium suspension should be harvested at 113 hpi. Overall, the characterization of E. ruminantium progression through the average infection cycle, not only can contribute to the maximization of E. ruminantium production yield, with important consequences for the large scale production and utilization of an inactivated Heartwater vaccine, but also to elucidate growth mechanisms of some of the other ehrlichial species, with emerging impact in human and animal health.

Anaplasma marginale(Rickettsiales: Anaplasmataceae): recent advances in defining host–pathogen adaptations of a tick-borne rickettsia

The tick-borne intracellular pathogenAnaplasma marginale(Rickettsiales: Anaplasmataceae) develops persistent infections in cattle and tick hosts. While erythrocytes appear to be the only site of infection in cattle,A. marginaleundergoes a complex developmental cycle in ticks and transmission occurs via salivary glands during feeding. Many geographic isolates occur that vary in genotype, antigenic composition, morphology and infectivity for ticks. In this chapter we review recent research on the host–vector–pathogen interactions ofA. marginale. Major surface proteins (MSPs) play a crucial role in the interaction ofA. marginalewith host cells. The MSP1a protein, which is an adhesin for bovine erythrocytes and tick cells, is differentially regulated and affects infection and transmission ofA. marginalebyDermacentorspp. ticks. MSP2 undergoes antigenic variation and selection in cattle and ticks, and contributes to the maintenance of persistent infections. Phylogenetic studies ofA. marginalegeographic isolates usingmsp4andmsp1α provide information about the biogeography and evolution ofA. marginale:msp1α genotypes evolve under positive selection pressure. Isolates ofA. marginaleare maintained by independent transmission events and a mechanism of infection exclusion in cattle and ticks allows for only the infection of one isolate per animal. Prospects for development of control strategies by use of pathogen and tick-derived antigens are discussed. TheA. marginale/vector/host studies described herein could serve as a model for research on other tick-borne rickettsiae.

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.

Ehrlichia ruminantium grows in cell lines from four ixodid tick genera

Continuous cell lines from the ticks Amblyomma variegatum, Boophilus decoloratus, Boophilus microplus, Hyalomma anatolicum anatolicum, Ixodes scapularis, Ixodes ricinus and Rhipicephalus appendiculatus were tested for ability to support growth of the rickettsial pathogen Ehrlichia (previously Cowdria) ruminantium. Five E.ruminantium isolates, from West Africa, South Africa and the French West Indies, were used. Twelve tick cell lines were inoculated with E.ruminantium derived either from cultures of a bovine endothelial cell strain designated BPC or from other tick cell lines. Successful infection resulted in either continuous growth (in which the pathogen/cell line system could be perpetuated through regular subculture on fresh, uninfected cells for many months or years) or finite growth (in which the pathogen disappeared after one or a few subcultures). Infection with E.ruminantium from BPC was established in I.scapularis, I.ricinus and A.variegatum cell lines; E.ruminantium was transferred from these infected cell lines to B.decoloratus, B.microplus and R. appendiculatus cell lines. H.a.anatolicum cells could not be infected with E.ruminantium by any procedure. All five E.ruminantium isolates grew continuously in at least one tick cell line at temperatures between 28 degrees C and 37 degrees C; three of the isolates were successfully re-established in BPC following prolonged maintenance in tick cells. This study demonstrates that E.ruminantium is not intrinsically restricted to growth in cells from ticks of the natural vector genus Amblyomma.

Isolation of an Anaplasma sp. organism from white-tailed deer by tick cell culture

We used tick cell culture to isolate a bacterium previously referred to as the "white-tailed deer (WTD) agent" from two captive fawns inoculated with blood from wild WTD (Odocoileus virginianus). Buffy coat cells were added to ISE6 tick cell cultures and incubated at 34 degrees C, and 8 days later, Anaplasma-like inclusions were demonstrated in Giemsa-stained culture samples. The microbes became established and could be continuously passaged in tick cells. The identity of a culture isolate designated WTD76 was verified as the WTD agent by using specific PCR primers and by DNA sequencing. Comparison with sequences available in GenBank indicated that the isolate was most closely related first to Anaplasma platys and second to Anaplasma phagocytophilum, supporting its placement in the genus Anaplasma. Transmission electron microscopy of this Anaplasma sp. organism in tick cell cultures revealed large inclusions filled with pleomorphic and rod-shaped bacteria. Tick cells infected with the Anaplasma sp. organism were used to successfully infect a naive deer, thereby proving the infectivity of the isolate for deer.

Babesiosis and heartwater: threats without boundaries

Suppose one of your clients from southern Florida starts talking about cattle egrets while you are vaccinating her cat. It seems she found a nearly dead egret near the cattle pen a few days ago, picked it up, and noticed a number of what looked like small ticks on the legs. Or, suppose you are called out to a small dairy in central Texas to look at some cows that are feverish and anemic. The first animal you examine has a few brown ticks attached just under the tail. Finally, perhaps you are looking at a lame tortoise for a reptile fancier, a new client, and find a large, colorful tick on a hind leg, well up under the shell. Ring any bells? Egrets are great hosts for the immature stages of Amblyomma ticks and have been captured and marked in the eastern Caribbean, then recaptured in the Florida Keys. Those cattle ticks in Texas might be acaricide-resistant Boophilus ticks that originated in Mexico. The Amblyomma tick on the tortoise could well have "hitch-hiked" all the way from South Africa. By now you remember that both Amblyomma and Boophilus ticks are efficient vectors of two tickborne diseases in this hemisphere, heartwater (in the case of Amblyomma) and babesiosis (transmitted by Boophilus ticks). Both of these diseases are exotic to the United States, and because our livestock are considered to be totally susceptible, an introduced infection could result in high initial death losses (approximately 70%); thus, both the ticks and the diseases pose immediate threats to the health and economic security of United States animal industries. Most importantly, you, whether as a small animal or large animal practitioner, are the first line of defense against such exotic diseases and their vectors.

Transcriptional analysis of the major antigenic protein 1 multigene family of Cowdria ruminantium

The major antigenic protein 1 (MAP1) of the tick-borne rickettsial pathogen Cowdria ruminantium is encoded by a multigene family containing conserved and variable genes. The part of a locus containing the map1 multigene family that was characterized contained three homologous, but non-identical map1 genes, designated map1-2, map1-1, and map1. Reverse transcriptase-polymerase chain reaction was used to study the transcriptional activity of these genes in isolates of C. ruminantium grown in bovine endothelial cells, in two different tick cell lines, and in Amblyomma variegatum ticks. The map1 gene was always transcribed, whereas transcription of map1-2 was not detected under any of the tested conditions. The map1-1 gene transcript was detected in A. variegatum ticks, but was not found in virulent C. ruminantium Senegal grown in bovine endothelial cells at 30 or 37 degrees C. Interestingly, transcripts of map1-1 were also found in different passages of the in vitro attenuated Senegal isolate grown in bovine endothelial cells, as well as in the Gardel isolate grown in two tick cell lines. When transcribed, map1-1 was present on a polycistronic messenger together with map1.

Kinetics of experimental infection of sheep with Ehrlichia ruminantium cultivated in tick and mammalian cell lines

Inoculation of sheep with Ehrlichia (previously Cowdria) ruminantium which has been cultivated in mammalian endothelial cell cultures is almost always followed by a severe clinical reaction, whereas inoculation of the agent cultivated in tick cell lines usually does not provoke a clinical response, but may result in seroconversion and/or protection against subsequent challenge with virulent stabilates. A quantitative, real-time PCR assay was developed to determine the kinetics of infection (rickettsaemia) in sheep inoculated with tick cell- and mammalian cell-derived E. ruminantium (Gardel isolate). The method and initial results are described, and the significance of the findings is discussed in relation to the clinical responses of the sheep.

Adaptations of the tick-borne pathogen, Anaplasma marginale, for survival in cattle and ticks

The tick-borne cattle pathogen Anaplasma marginale (Rickettsiales: Anaplasmataceae) multiplies within membrane-bound inclusions in host cell cytoplasm. Many geographic isolates of A. marginale occur that vary in genotype, antigenic composition, morphology and infectivity for ticks. A tick cell culture system for propagation of A. marginale proved to be a good model for study of tick-pathogen interactions. Six major surface proteins (MSPs) identified on A. marginale from bovine erythrocytes were conserved on A. marginale derived from tick cells. MSP1a and MSP1b were adhesins for bovine erythrocytes, while only MSP1a was bound to be an adhesin for tick cells. The tandemly repeated portion of MSP1a was found to be necessary and sufficient for adhesion to both tick cells and bovine erythrocytes. Infectivity of A. marginale isolates for ticks was dependent on the adhesive capacity of the isolate MSP1a, which was found to involve both the adhesive properties and sequence of the repeated peptides. Cattle immunized with A. marginale derived from bovine erythrocytes or tick cells demonstrated a differential antibody response to MSP1a and MSP1b that resulted from the differential expression of these proteins in cattle and ticks cells. MSP2, derived from a multigene family, was found to undergo antigenic variation in cattle and ticks and may contribute to establishment of persistent A. marginale infections. MSP1a has been used as a stable genetic marker for geographic isolates because the molecular weight varies due to differing numbers of the tandem repeats. However, phylogenetic studies of A. marginale isolates from North America using MSP1a and MSP4 demonstrated that MSP4 was a good biogeographic marker, while MSP1a varied greatly among and within geographic areas. Infection and development of A. marginale in cattle and tick cells appears to differ and to be mediated by several surface proteins encoded from the small genome.

Immunogenicity of Ehrlichia ruminantium grown in tick cell lines

Ehrlichia (previously Cowdria) ruminantium, the pathogen which causes heart-water in domestic and wild ruminants, can now be propagated in cell lines from one vector (Amblyomma variegatum) and five non-vector (Ixodes scapularis, I. ricinus, Boophilus decoloratus, B. microplus and Rhipicephalus appendiculatus) tick species. E. ruminantium isolates from West and South Africa and the Caribbean vary in their cell line preference, growth patterns and immunogenic capability. In laboratory trials, certain combinations of tick cell line and E. ruminantium isolate were highly immunogenic in sheep. These trial vaccines were grown under specific in vitro conditions and administered as a single intravenous dose of freshly harvested whole, live culture. Following immunisation and subsequent exposure to virulent E. ruminantium, protected sheep showed no clinical response and a range of serological responses.