Multiple Ehrlichia chaffeensis Genes Critical for Its Persistent Infection in a Vertebrate Host Are Identified by Random Mutagenesis Coupled with In Vivo Infection Assessment

Ehrlichia chaffeensis, a tick-transmitted obligate intracellular rickettsial agent, causes human monocytic ehrlichiosis. In recent reports, we described substantial advances in developing random and targeted gene disruption methods to investigate the functions of E. chaffeensis genes. We reported earlier that the Himar1 transposon-based random mutagenesis is a valuable tool in defining E. chaffeensis genes critical for its persistent growth in vivo in reservoir and incidental hosts. The method also aided in extending studies focused on vaccine development and immunity. Here, we describe the generation and mapping of 55 new mutations. To define the critical nature of the bacterial genes, infection experiments were carried out in the canine host with pools of mutant organisms. Infection evaluation in the physiologically relevant host by molecular assays and by xenodiagnoses allowed the identification of many proteins critical for the pathogen's persistent in vivo growth. Genes encoding proteins involved in biotin biosynthesis, protein synthesis and fatty acid biosynthesis, DNA repair, electron transfer, and a component of a multidrug resistance (MDR) efflux pump were concluded to be essential for the pathogen's in vivo growth. Three known immunodominant membrane proteins, i.e., two 28-kDa outer membrane proteins (P28/OMP) and a 120-kDa surface protein, were also recognized as necessary for the pathogen's obligate intracellular life cycle. The discovery of many E. chaffeensis proteins crucial for its continuous in vivo growth will serve as a major resource for investigations aimed at defining pathogenesis and developing novel therapeutics for this and related pathogens of the rickettsial family Anaplasmataceae.

Current management of human granulocytic anaplasmosis, human monocytic ehrlichiosis and Ehrlichia ewingii ehrlichiosis

Anaplasma phagocytophilum, Ehrlichia chaffeensis and Ehrlichia ewingii are emerging tick-borne pathogens and are the causative agents of human granulocytic anaplasmosis, human monocytic ehrlichiosis and E. ewingii ehrlichiosis, respectively. Collectively, these are referred to as human ehrlichioses. These obligate intracellular bacterial pathogens of the family Anaplasmataceae are transmitted by Ixodes spp. or Amblyomma americanum ticks and infect peripherally circulating leukocytes to cause infections that range in clinical spectra from asymptomatic seroconversion to mild, severe or, in rare instances, fatal disease. This review describes: the ecology of each pathogen; the epidemiology, clinical signs and symptoms of the human diseases that each causes; the choice methods for diagnosing and treating human ehrlichioses; recommendations for patient management; and is concluded with suggestions for potential future research.

Tick-borne ehrlichiosis infection in human beings

Human monocytic ehrlichiosis is a tick-borne infectious disease transmitted by several tick species, especially Amblyomma spp caused by Ehrlichia chaffeensis. E. chaffeensis is an obligatory intracellular, tick-transmitted bacterium that is maintained in nature in a cycle involving at least one and perhaps several vertebrate reservoir hosts. Two additional Ehrlichia spp, Anaplasma (formerly Ehrlichia) phagocytophila (the agent of human granulocytic ehrlichiosis [HGE]) and E. ewingii (a cause of granulocytic ehrlichiosis in dogs) act as human pathogens. Human E. chaffeensis infections have generally been reported in North America, Asia and Europe, but recently human cases have been reported in Brazil only. Human monocytic ehrlichiosis is diagnosed by demonstration of a four-fold or greater change in antibody titer to E. chaffeensis antigen by IFA in paired serum samples, or a positive PCR assay and confirmation of E. chaffeensis DNA, or identification of morulae in leukocytes and a positive IFA titer to E. chaffeensis antigen, or immunostaining of E. chaffeensis antigen in a biopsy or autopsy sample, or culture of E. chaffeensis from a clinical specimen.

Enhanced spatial models for predicting the geographic distributions of tick-borne pathogens

BACKGROUND

Disease maps are used increasingly in the health sciences, with applications ranging from the diagnosis of individual cases to regional and global assessments of public health. However, data on the distributions of emerging infectious diseases are often available from only a limited number of samples. We compared several spatial modelling approaches for predicting the geographic distributions of two tick-borne pathogens: Ehrlichia chaffeensis, the causative agent of human monocytotropic ehrlichiosis, and Anaplasma phagocytophilum, the causative agent of human granulocytotropic anaplasmosis. These approaches extended environmental modelling based on logistic regression by incorporating both spatial autocorrelation (the tendency for pathogen distributions to be clustered in space) and spatial heterogeneity (the potential for environmental relationships to vary spatially).

RESULTS

Incorporating either spatial autocorrelation or spatial heterogeneity resulted in substantial improvements over the standard logistic regression model. For E. chaffeensis, which was common within the boundaries of its geographic range and had a highly clustered distribution, the model based only on spatial autocorrelation was most accurate. For A. phagocytophilum, which has a more complex zoonotic cycle and a comparatively weak spatial pattern, the model that incorporated both spatial autocorrelation and spatially heterogeneous relationships with environmental variables was most accurate.

CONCLUSION

Spatial autocorrelation can improve the accuracy of predictive disease risk models by incorporating spatial patterns as a proxy for unmeasured environmental variables and spatial processes. Spatial heterogeneity can also improve prediction accuracy by accounting for unique ecological conditions in different regions that affect the relative importance of environmental drivers on disease risk.

Global stability of equilibria in a tick-borne disease model

In this short note we establish global stability results for a four- dimensional nonlinear system that was developed in modeling a tick-borne disease by H.D. Gaff and L.J. Gross (Bull. Math. Biol., 69 (2007), 265-288) where local stability results were obtained. These results provide the parameter ranges for controlling long-term population and disease dynamics.

TRANSMISSION OF EHRLICHIA CHAFFEENSIS FROM LONE STAR TICKS (AMBLYOMMA AMERICANUM ) TO WHITE-TAILED DEER (ODOCOILEUS VIRGINIANUS)

Amblyomma americanum is an aggressive ixodid tick that has been implicated as a vector for several bacterial agents. Among these is Ehrlichia chaffeensis, which causes human monocytic (or monocytotropic) ehrlichiosis. In this study, experimental tick transmission of E. chaffeensis from infected lone star ticks to deer was revisited, and the question of whether it would be possible to re-isolate the organism from deer was asked, because this had not been done previously. Here, we were able to transmit a wild strain of E. chaffeensis from acquisition-fed lone star ticks to white-tailed deer. Ehrlichia chaffeensis was re-isolated from one white-tailed deer on multiple days during the infection and from another deer on one day during the infection. Peak rickettsemias for E. chaffeensis-infected deer were 17 DPI with acquisition-fed ticks and 14 DPI with needle-inoculated deer. This study supports the role of the lone star tick and white-tailed deer as vector and reservoir host for E. chaffeensis, demonstrating culture re-isolation of E. chaffeensis in deer infected by experimental tick transmission for the first time.

Differential clearance and immune responses to tick cell-derived versus macrophage culture-derived Ehrlichia chaffeensis in mice

Human monocytic ehrlichiosis is caused by a tick-transmitted rickettsia, Ehrlichia chaffeensis. We recently reported that E. chaffeensis grown in tick cells expresses different proteins than bacteria grown in macrophages. Therefore, we tested the hypothesis that immune responses against E. chaffeensis would be different if the mice are challenged with bacteria grown in macrophages or tick cells. We assessed the E. chaffeensis clearance from the peritoneum, spleen, and liver by C57BL/6J mice using a TaqMan-based real-time reverse transcription-PCR assay. Macrophage-grown E. chaffeensis was cleared in 2 weeks from the peritoneum, whereas the pathogen from tick cells persisted for nine additional days and included three relapses of increasing bacterial load separated by three-day intervals. Tick cell-grown bacteria also persisted in the livers and spleens with higher bacterial loads compared to macrophage-grown bacteria and fluctuated over a period of 35 days. Three-day periodic cycles were detected in T-cell CD62L/CD44 ratios in the spleen and bone marrow in response to infections with both tick cell- and macrophage-grown bacteria and were accompanied by similar periodic cycles of spleen cell cytokine secretions and nitric oxide and interleukin-6 by peritoneal macrophages. The E. chaffeensis-specific immunoglobulin G response was considerably higher and steadily increased in mice infected with the tick cell-derived E. chaffeensis compared to DH82-grown bacteria. In addition, antigens detected by the immunoglobulins were significantly different between mice infected with the E. chaffeensis originating from tick cells or macrophages. The differences in the immune response to tick cell-grown bacteria compared to macrophage-grown bacteria reflected a delay in the shift of gene expression from the tick cell-specific Omp 14 gene to the macrophage-specific Omp 19 gene. These data suggest that the host response to E. chaffeensis depends on the source of the bacteria and that this experimental model requires the most natural inoculum possible to allow for a realistic understanding of host resistance.

Degradation of p22phox and inhibition of superoxide generation by Ehrlichia chaffeensis in human monocytes

Ehrlichia chaffeensis is an obligate intracellular bacterium which replicates in monocytes or macrophages, the primary producers of reactive oxygen species (ROS). However, effects of ROS on E. chaffeensis infection and whether E. chaffeensis modulates ROS generation in host monocytes are unknown. Here, E. chaffeensis was shown to lose infectivity upon exposure to O(2)(-) or hydrogen peroxide. Upon incubation with human monocytes, E. chaffeensis neither induced O(2)(-) generation by human monocytes, nor colocalized with nicotinamide adenine dinucleotide phosphate (NADPH) oxidase components. Instead, it actively blocked O(2)(-) generation by monocytes stimulated with phorbol myristate acetate and caused the rapid degradation of p22(phox), a component of NADPH oxidase. These effects were not seen in neutrophil, which is another potent ROS generator, but a cell type that E. chaffeensis does not infect. Trypsin pretreatment of monocytes prevented the inhibition of O(2)(-) generation by E. chaffeensis. The degradation of p22(phox) by E. chaffeensis was specific to subsets of monocytes with bound and/or intracellular bacteria, and the degradation could be reduced by heat treatment of the bacterium, lipopolysaccharide pretreatment of monocytes, or the incubation with haemin. The degradation of p22(phox) by E. chaffeensis and its prevention by haemin or protease inhibitors also occurred in isolated monocyte membrane fractions, indicating that host cytoplasmic signalling is not required for these processes. The amount of gp91(phox) was stable under all conditions examined in this study. These findings point to a unique survival mechanism of ROS-sensitive obligate intraleucocytic bacteria that involves the destabilization of p22(phox) following the binding of bacteria to host cell surface proteins.

Modeling Tick-Borne Disease: A Metapopulation Model

Recent increases in reported outbreaks of tick-borne diseases have led to increased interest in understanding and controlling epidemics involving these transmission vectors. Mathematical disease models typically assume constant population size and spatial homogeneity. For tick-borne diseases, these assumptions are not always valid. The disease model presented here incorporates non-constant population sizes and spatial heterogeneity utilizing a system of differential equations that may be applied to a variety of spatial patches. We present analytical results for the one patch version and find parameter restrictions under which the populations and infected densities reach equilibrium. We then numerically explore disease dynamics when parameters are allowed to vary spatially and temporally and consider the effectiveness of various tick-control strategies.

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.

The developmental cycle of Ehrlichia chaffeensis in vertebrate cells

Ehrlichia chaffeensis, an obligatory intracellular bacterium, has two forms in mammalian cells: small dense-cored cells (DC) with dense nucleoid and larger reticulate cells (RC) with uniformly dispersed nucleoid. We have determined by electron microscopy that DC but not RC attaches to and enters into the host cells and RC but not DC multiples inside the host cells. Analysis of outer membrane protein expression by confocal microscopy showed that RC expressed the 28 kDa outer membrane protein (p28), the intermediate form, which were transforming from RC to DC, expressed both gp120 and p28, and the mature DC expressed gp120 only. The TCID50 of DC is 6 log10 higher than RC. We conclude that E. chaffeensis has a developmental cycle, in which the DC attaches to and enters into the host cells, and transforms into RC and the RC multiplies by binary fission for 48 h and then matures into DC at 72 h.

Lymph node hemophagocytosis in rickettsial diseases: a pathogenetic role for CD8 T lymphocytes in human monocytic ehrlichiosis (HME)?

BACKGROUND

Human monocytic ehrlichiosis (HME) and Rocky Mountain spotted fever (RMSF) are caused by Ehrlichia chaffeensis and Rickettsia rickettsii, respectively. The pathogenesis of RMSF relates to rickettsia-mediated vascular injury, but it is unclear in HME.

METHODS

To study histopathologic responses in the lymphatic system for correlates of immune injury, lymph nodes from patients with HME (n = 6) and RMSF (n = 5) were examined. H&E-stained lymph node tissues were examined for five histopathologic features, including hemophagocytosis, cellularity, necrosis, and vascular congestion and edema. The relative proportions of CD68 macrophages, CD8 and CD4 T lymphocytes, and CD20 B lymphocytes were evaluated by immunohistochemical staining.

RESULTS

Hemophagocytosis was similar in HME and RMSF, and was greater than in control cases (p = .015). Cellularity in HME was not different from controls, whereas RMSF lymph nodes were markedly less cellular (p < 0.002). E. chaffeensis-infected mononuclear phagocytes were infrequent compared to R. rickettsii-infected endothelial cells. More CD8 cells in lymph nodes were observed with HME (p < .001), but no quantitative differences in CD4 lymphocytes, macrophages, or B lymphocytes were identified.

CONCLUSION

Hemophagocytosis, CD8 T cell expansion, and the paucity of infected cells in HME, suggest that E. chaffeensis infection leads to macrophage activation and immune-mediated injury.

Identification and prevalence of Ehrlichia chaffeensis infection in Haemaphysalis longicornis ticks from Korea by PCR, sequencing and phylogenetic analysis based on 16S rRNA gene

Genomic DNAs extracted from 1,288 Haemaphysalis longicornis ticks collected from grass vegetation and various animals from nine provinces of Korea were subjected to screening by genus-specific (Ehrlichia spp. or Anaplasma spp.) real-time TaqMan PCR and speciesspecific (E. chaffeensis) nested-PCR based on amplification of 16S rRNA gene fragments. In all, 611 (47.4%) ticks tested positive for genus-specific amplification of 116 bp fragment of 16S rRNA of Ehrlichia spp. or Anaplasma spp. Subsequently, 396 bp E. chaffeensis-specific fragment of 16S rRNA was amplified from 4.2% (26/611) tick samples. The comparison of the nucleotide sequence of 16S rRNA gene from one tick (EC-PGHL, GeneBank accession number AY35042) with the sequences of 20 E. chaffeensis strains available in the database showed that EC-PGHL was 100% identical or similar to the Arkansas (AF416764), the Sapulpa (U60476) and the 91HE17 (U23503) strains. The phylogenetic analysis also revealed that the E. chaffeensis EC-PGHL formed a single cluster with the above strains. This is the first study to report molecular detection and phylogenetic analysis of E. chaffeensis from H. longicornis ticks in Korea. The implicit significance of E. chaffeensis infection in H. longicornis ticks in Korea is discussed.

Evaluation of antibiotic susceptibilities of Ehrlichia canis, Ehrlichia chaffeensis, and Anaplasma phagocytophilum by real-time PCR

We determined MICs of antibiotics against Anaplasma phagocytophilum, Ehrlichia chaffeensis, and Ehrlichia canis by real-time quantitative PCR. The doubling times of the organisms were established: 19 h for E. chaffeensis, 26 h for A. phagocytophilum, and 28 h for E. canis. In comparison to the reference method for determining sensitivities, which uses Diff-Quick staining, our PCR assay was very sensitive and specific. We confirmed that doxycycline and rifampin are highly active against these bacteria and found variable susceptibilities to fluoroquinolones; A. phagocytophilum was susceptible, but E. canis and E. chaffeensis were only partly susceptible. Beta-lactam compounds, cotrimoxazole, macrolide compounds, and telithromycin showed no activity against any of the three organisms. Thiamphenicol was found to be more active than chloramphenicol. For the first time, we showed that these three species have numerous point mutations in their 23S RNA genes, with those at positions 754, 2057, 2058, 2059, and 2611 (Escherichia coli numbering) known to confer resistance to macrolide compounds in other bacteria. The role of each of these mutations in resistance to these drugs should be investigated in the future. Our study confirms previous reports that quantitative PCR is a reliable method for determining antibiotic susceptibility; therefore, it might be useful for screening new drugs.

Survival strategy of obligately intracellular Ehrlichia chaffeensis: novel modulation of immune response and host cell cycles

Ehrlichia chaffeensis is an obligatory intracellular bacterium which resides in an early endosome in monocytes. E. chaffeensis infection in a human monocyte cell line (THP1) significantly altered the transcriptional levels of 4.5% of host genes, including those coding for apoptosis inhibitors, proteins regulating cell differentiation, signal transduction, proinflammatory cytokines, biosynthetic and metabolic proteins, and membrane trafficking proteins. The transcriptional profile of the host cell revealed key themes in the pathogenesis of Ehrlichia. First, E. chaffeensis avoided stimulation of or repressed the transcription of cytokines involved in the early innate immune response and cell-mediated immune response to intracellular microbes, such as the interleukin-12 (IL-12), IL-15, and IL-18 genes, which might make Ehrlichia a stealth organism for the macrophage. Second, E. chaffeensis up-regulated NF-kappaB and apoptosis inhibitors and differentially regulated cell cyclins and CDK expression, which may enhance host cell survival. Third, E. chaffeensis also inhibited the gene transcription of RAB5A, SNAP23, and STX16, which are involved in membrane trafficking. By comparing the transcriptional response of macrophages infected with other bacteria and that of macrophages infected with E. chaffeensis, we have identified few genes that are commonly induced and no commonly repressed genes. These results illustrate the stereotyped macrophage response to other pathogens, in contrast with the novel host response to obligate intracellular Ehrlichia, whose survival depends entirely on a long evolutionary process of outmaneuvering macrophages.

Survival, replication, and antibody susceptibility of Ehrlichia chaffeensis outside of host cells

Ehrlichia chaffeensis, an obligate intracellular, tick-transmitted bacterium, is susceptible to antibody-mediated host defense, but the mechanism by which this occurs is not understood. One possible explanation is that antibodies directly access the bacteria in the extracellular environment of the host, perhaps during bacterial intercellular transfer. Accordingly, we investigated whether bacteria could be found outside of host cells during infection. Host cell-free plasma obtained from infected mice was found to contain ehrlichiae, and the host cell-free ehrlichiae readily transferred disease to susceptible SCID recipients. The host cell-free ehrlichiae were found during infection of both immunocompetent BALB/c and immunocompromised BALB/c-scid mice and reached levels as high as 10(8)/ml in plasma during persistent infection in SCID mice. Approximately 10% of the blood-borne bacteria were found outside of host cells. Although it is generally accepted that replication of ehrlichiae occurs only within host cells, the cell-free bacteria were shown to undergo DNA replication and cell division in vitro for 3 to 5 days when incubated at 37 degrees C in plasma. Paradoxically, both infectivity and virulence were lost after 24 h of ex vivo culture. The data indicate that E. chaffeensis is exposed to the extracellular milieu during infection, presumably during intercellular transfer, and reveal that these intracellular bacteria do not require the environment of the host cell for replication. Our findings reveal a possible mechanism by which antibodies can access the intracellular bacteria upon their release into the extracellular milieu and mediate host defense and also have implications for understanding the replication and transmission of this vector-borne pathogen.

Rapid activation of protein tyrosine kinase and phospholipase C-gamma2 and increase in cytosolic free calcium are required by Ehrlichia chaffeensis for internalization and growth in THP-1 cells

Ehrlichia chaffeensis, a bacterium that cannot survive outside the eukaryotic cell, proliferates exclusively in human monocytes and macrophages. In this study, signaling events required for ehrlichial infection of human monocytic cell line THP-1 were characterized. Entry and proliferation of E. chaffeensis in THP-1 cells were significantly blocked by various inhibitors that can regulate calcium signaling, including 8-(diethylamino)octyl-3,4,5-trimethoxybenzoate and 2-aminoethoxydiphenyl borate (intracellular calcium mobilization inhibitors), verapamil and 1-[beta-[3-(4-methoxyphenyl)propyl]-4-methoxyphenethyl]-1H-imidazole (SKF-96365) (calcium channel inhibitors), neomycin and 1-(6-[[17beta-3-methoxyestra-1,3,5(10)-trien-17-yl]amino]hexyl)-1H-pyrrole-2,5-dione (U-73122) (phospholipase C [PLC] inhibitors), monodansylcadaverine (a transglutaminase [TGase] inhibitor), and genistein (a protein tyrosine kinase [PTK] inhibitor). Addition of E. chaffeensis resulted in rapid increases in the level of inositol 1,4,5-trisphosphate (IP(3)) and the level of cytosolic free calcium ([Ca(2+)](i)) in THP-1 cells, which were prevented by pretreatment of THP-1 cells with inhibitors of TGase, PTK, and PLC. E. chaffeensis induced rapid tyrosine phosphorylation of PLC-gamma2, and the presence of a PLC-gamma2 antisense oligonucleotide in THP-1 cells significantly blocked ehrlichial infection. Furthermore, tyrosine-phosphorylated proteins and PLC-gamma2 were colocalized with ehrlichial inclusions, as determined by double-immunofluorescence labeling. The heat-sensitive component of viable E. chaffeensis cells was essential for these signaling events. E. chaffeensis, therefore, can recruit interacting signal-transducing molecules and induce the following signaling events required for the establishment of infection in host cells: protein cross-linking by TGase, tyrosine phosphorylation, PLC-gamma2 activation, IP(3) production, and an increase in [Ca(2+)](i).

Human granulocytic ehrlichiosis presenting as facial diplegia in a 42-year-old woman

Neurologic manifestations of human ehrlichiosis are unusual and have been described almost exclusively in human monocytic ehrlichiosis associated with Ehrlichia chaffeensis. We report here a case of a previously healthy 42-year-old woman who developed bilateral facial nerve palsies in association with infection by the agent of human granulocytic ehrlichiosis (aoHGE). The diagnosis was made by specific polymerase chain reaction amplification of aoHGE sequences from samples of the patient's blood and cerebrospinal fluid (CSF), as well as propagation of aoHGE in culture of HL60 cells inoculated with the patient's CSF. To our knowledge, this is the first report directly demonstrating the presence of aoHGE in CSF, and it underscores the importance of considering HGE in patients presenting with a nonspecific febrile illness and unexplained neurologic manifestations. HGE should also be considered in the differential diagnosis of bilateral facial palsy-a rare occurrence.

Infection of the laboratory mouse with the intracellular pathogen Ehrlichia chaffeensis

To determine the basis of susceptibility and resistance to human monocytic ehrlichiosis (HME), immunocompetent and immunocompromised mice were infected with Ehrlichia chaffeensis and bacterial loads were measured by PCR and by immunohistochemistry. Immunocompetent (C. B-17 and C57BL/6) mice cleared the bacteria within 10 days, but immunocompromised SCID and SCID/BEIGE mice developed persistent infection in the spleen, liver, peritoneal cavity, brain, lung, and bone marrow and became moribund within 24 days. Both immunocompromised strains lack T and B lymphocytes, but the SCID/BEIGE strain is also deficient in natural killer (NK) cell function. During advanced stages of disease, the infections were associated with wasting, splenomegaly, lymphadenopathy, liver granulomas and necroses, intravascular coagulation, and granulomatous inflammation. Histochemical and immunohistochemical localization studies confirmed the presence of bacteria in tissues, and viable bacteria were cultured from infected animals. The data reveal that T and/or B cells play an essential role during resistance of immunocompetent mice to infection with E. chaffeensis and demonstrate the utility of immunocompromised mice as an experimental model for the study of HME.

A single tissue culture system for the propagation of the agents of the human ehrlichioses

Two newly emergent human diseases found in the United States, human monocytotropic ehrlichiosis (HME) and human granulocytotropic ehrlichiosis (HGE), are caused by pathogens of the genus Ehrlichia. The causative agent of HGE can be propagated in HL-60 human promyelocytic leukemia cells. Herein, we report the development of a method to propagate E. chaffeensis, the causative agent of HME, in HL-60 cells, thus providing a common system for the study of both species. The continuous propagation of E. chaffeensis requires the induction of HL-60 differentiation along the monocytic pathway toward phenotypically mature macrophages by the addition of 25-OH vitamin D3 to the growth medium.

Cultivation of Ehrlichia chaffeensis in mouse embryo, Vero, BGM, and L929 cells and study of Ehrlichia-induced cytopathic effect and plaque formation

We successfully propagated Ehrlichia chaffeensis in mouse embryo, Vero, BGM, and L929 cells inoculated with host cell-free ehrlichiae, indicating that E. chaffeensis is capable of entry, survival, and growth in a relatively wide range of cell types derived from different species. We demonstrated rapid adaptation of E. chaffeensis in these cell lines, so that typical morulae could be detected as early as 5 days after inoculation. E. chaffeensis-induced cytopathic effect with different morphological characteristics in mouse embryo, Vero, and L929 cells. The earliest cytopathic effect appeared in untreated and irradiated mouse embryo cells at 4 days postinoculation. As the infected foci gradually expanded, the center of the foci showed necrotic cells with pyknotic nuclei and degraded morulae. E. chaffeensis caused cell lysis in untreated and irradiated L929 cells, with formation of distinct, round macroscopic plaques at 18 days postinoculation. In untreated and irradiated Vero cells, E. chaffeensis produced infected foci composed of loosely interwoven necrotic cells, spaces of detached cells, cells filled with morulae, and uninfected cells, resulting in characteristic reticular foci. Irradiated cells generally contained many large morulae and presented larger cytopathic foci. DH82 and BGM cells did not develop obvious cytopathic foci under the conditions employed. The findings reported herein offer the opportunity to study the pathogenic mechanism of cell injury by E. chaffeensis, the basis for quantification of infectious E. chaffeensis, improved approaches for recovery of ehrlichiae from human patients and tick hosts, and additional methods for cultivation of E. chaffeensis for molecular analysis.

Abrogation of gamma interferon-induced inhibition of Ehrlichia chaffeensis infection in human monocytes with iron-transferrin

Ehrlichia chaffeensis is an obligate intracellular bacterium which infects cells of the macrophage/monocyte lineage. To test whether gamma interferon (IFN-gamma) inhibits infection of monocytes with E. chaffeensis, human peripheral blood monocytes were incubated with recombinant human IFN-gamma for 3 h and then exposed to E. chaffeensis. With 2,000 U of IFN-gamma per ml, maximal inhibition of infection by E. chaffeensis was observed. THP-1 cells, a human monocyte cell line, pretreated with phorbol myristic acetate or not pretreated, were incubated with various concentrations of IFN-gamma. Maximum inhibition was obtained at 1,000 U of IFN-gamma per ml with phorbol myristic acetate-treated THP-1 cells. However, nontreated cells did not achieve a similar level of anti-ehrlichial activity even with 10,000 U of IFN-gamma per ml. IFN-gamma given within 6 h postinfection was effective in inhibiting E. chaffeensis. Nitric oxide production was not demonstrated in the monocyte medium incubated with IFN-gamma and E. chaffeensis. None of the reactive oxygen intermediate scavengers tested blocked the IFN-gamma-induced anti-ehrlichial activity. Deferoxamine, an intracellular iron chelator, at 15 microM completely inhibited the survival of E. chaffeensis. Iron-saturated transferrin at 1.67 mg/ml completely reversed the IFN-gamma-induced ehrlichial killing. These results indicate that (i) E. chaffeensis is sensitive to intracytoplasmic iron depletion, (ii) E. chaffeensis is sensitive to IFN-gamma-induced killing, and (iii) the anti-ehrlichial activity induced in human monocytes by IFN-gamma is mediated by limitation of available cytoplasmic iron and is not due to the generation of reactive oxygen intermediates or nitric oxide.