Mechanisms of immunity to Ehrlichia muris: a model of monocytotropic ehrlichiosis

Review: Protective Immunity and Immunopathology of Ehrlichiosis

Human monocytic ehrlichiosis, a tick transmitted infection, ranges in severity from apparently subclinical to a fatal toxic shock-like fatal disease. Models in immunocompetent mice range from an abortive infection to uniformly lethal depending on the infecting Ehrlichia species, dose of inoculum, and route of inoculation. Effective immunity is mediated by CD4+ T lymphocytes and gamma interferon. Lethal infection occurs with early overproduction of proinflammatory cytokines and overproduction of TNF alpha and IL-10 by CD8+ T lymphocytes. Furthermore, fatal ehrlichiosis is associated with signaling via TLR 9/MyD88 with upregulation of several inflammasome complexes and secretion of IL-1 beta, IL-1 alpha, and IL-18 by hepatic mononuclear cells, suggesting activation of canonical and noncanonical inflammasome pathways, a deleterious role for IL-18, and the protective role for caspase 1. Autophagy promotes ehrlichial infection, and MyD88 signaling hinders ehrlichial infection by inhibiting autophagy induction and flux. Activation of caspase 11 during infection of hepatocytes by the lethal ehrlichial species after interferon alpha receptor signaling results in the production of inflammasome-dependent IL-1 beta, extracellular secretion of HMGB1, and pyroptosis. The high level of HMGB1 in lethal ehrlichiosis suggests a role in toxic shock. Studies of primary bone marrow-derived macrophages infected by highly avirulent or mildly avirulent ehrlichiae reveal divergent M1 and M2 macrophage polarization that links with generation of pathogenic CD8 T cells, neutrophils, and excessive inflammation or with strong expansion of protective Th1 and NKT cells, resolution of inflammation and clearance of infection, respectively.

The super repertoire of type IV effectors in the pangenome of Ehrlichia spp. provides insights into host-specificity and pathogenesis

The identification of bacterial effectors is essential to understand how obligatory intracellular bacteria such as Ehrlichia spp. manipulate the host cell for survival and replication. Infection of mammals–including humans–by the intracellular pathogenic bacteria Ehrlichia spp. depends largely on the injection of virulence proteins that hijack host cell processes. Several hypothetical virulence proteins have been identified in Ehrlichia spp., but one so far has been experimentally shown to translocate into host cells via the type IV secretion system. However, the current challenge is to identify most of the type IV effectors (T4Es) to fully understand their role in Ehrlichia spp. virulence and host adaptation. Here, we predict the T4E repertoires of four sequenced Ehrlichia spp. and four other Anaplasmataceae as comparative models (pathogenic Anaplasma spp. and Wolbachia endosymbiont) using previously developed S4TE 2.0 software. This analysis identified 579 predicted T4Es (228 pT4Es for Ehrlichia spp. only). The effector repertoires of Ehrlichia spp. overlapped, thereby defining a conserved core effectome of 92 predicted effectors shared by all strains. In addition, 69 species-specific T4Es were predicted with non-canonical GC% mostly in gene sparse regions of the genomes and we observed a bias in pT4Es according to host-specificity. We also identified new protein domain combinations, suggesting novel effector functions. This work presenting the predicted effector collection of Ehrlichia spp. can serve as a guide for future functional characterisation of effectors and design of alternative control strategies against these bacteria.

A fundamental step for the survival and replication of intravacuolar bacterial pathogens is the establishment of a replicative niche inside host cells by the secretion of bacterial effector proteins in the cytoplasm of the infected cells. These effectors manipulate host signaling pathways, thus allowing to escape the host degradative pathway and uptake nutrients required for intracellular replication of bacteria. In this study, we used S4TE2.0 software for high-throughput computational prediction of bacterial type IV effectors in zoonotic bacteria of the Anaplasmataceae family. The analysis of protein architecture of effectors helped us to identify the cellular pathways targeted during the infection process. The demonstration that effectors are modular components with a broad variety of protein architectures nicely explains their pleotropic mode of action and enlightens their function. We showed that bacterial adaptation to a given host during evolution requires a minimal repertoire of candidate effectors although further experimental determination is needed. T4Es are of increasing interest for basic research, including comprehension of hijacked cellular pathways, manipulated innate immunity, and application for therapeutics. Indeed pathogenomics-driven studies, especially on genetically intractable intracellular bacteria such as Anaplasmataceae, have now a substantial impact for the development of host-targeted antimicrobials, as an alternative to antibiotics.

Immune Response to Tick-Borne Hemoparasites: Host Adaptive Immune Response Mechanisms as Potential Targets for Therapies and Vaccines

Tick-transmitted pathogens cause infectious diseases in both humans and animals. Different types of adaptive immune mechanisms could be induced in hosts by these microorganisms, triggered either directly by pathogen antigens or indirectly through soluble factors, such as cytokines and/or chemokines, secreted by host cells as response. Adaptive immunity effectors, such as antibody secretion and cytotoxic and/or T helper cell responses, are mainly involved in the late and long-lasting protective immune response. Proteins and/or epitopes derived from pathogens and tick vectors have been isolated and characterized for the immune response induced in different hosts. This review was focused on the interactions between tick-borne pathogenic hemoparasites and different host effector mechanisms of T- and/or B cell-mediated adaptive immunity, describing the efforts to define immunodominant proteins or epitopes for vaccine development and/or immunotherapeutic purposes. A better understanding of these mechanisms of host immunity could lead to the assessment of possible new immunotherapies for these pathogens as well as to the prediction of possible new candidate vaccine antigens.

Emerging Roles of Autophagy and Inflammasome in Ehrlichiosis

Human monocytic ehrlichiosis (HME) is a potentially life-threatening tick-borne rickettsial disease (TBRD) caused by the obligate intracellular Gram-negative bacteria, Ehrlichia. Fatal HME presents with acute ailments of sepsis and toxic shock-like symptoms that can evolve to multi-organ failure and death. Early clinical and laboratory diagnosis of HME are problematic due to non-specific flu-like symptoms and limitations in the current diagnostic testing. Several studies in murine models showed that cell-mediated immunity acts as a “double-edged sword” in fatal ehrlichiosis. Protective components are mainly formed by CD4 Th1 and NKT cells, in contrast to deleterious effects originated from neutrophils and TNF-α-producing CD8 T cells. Recent research has highlighted the central role of the inflammasome and autophagy as part of innate immune responses also leading to protective or pathogenic scenarios. Recognition of pathogen-associated molecular patterns (PAMPS) or damage-associated molecular patterns (DAMPS) triggers the assembly of the inflammasome complex that leads to multiple outcomes. Recognition of PAMPs or DAMPs by such complexes can result in activation of caspase-1 and -11, secretion of the pro-inflammatory cytokines IL-1β and IL-18 culminating into dysregulated inflammation, and inflammatory cell death known as pyroptosis. The precise functions of inflammasomes and autophagy remain unexplored in infections with obligate intracellular rickettsial pathogens, such as Ehrlichia. In this review, we discuss the intracellular innate immune surveillance in ehrlichiosis involving the regulation of inflammasome and autophagy, and how this response influences the innate and adaptive immune responses against Ehrlichia. Understanding such mechanisms would pave the way in research for novel diagnostic, preventative and therapeutic approaches against Ehrlichia and other rickettsial diseases.

Antigen-Specific CD4+CD8+ Double-Positive T Cells Are Increased in the Blood and Spleen During Ehrlichia chaffeensis Infection in the Canine Host

Ehrlichia chaffeensis is an obligate intracellular bacterium belonging to the order, Rickettsiales and is a frequent cause of severe and fatal tick-borne infection in people in North America. The reservoir host for E. chaffeensis is the white-tailed deer, while humans and dogs are regarded as common incidental hosts. In dogs, we and others have shown that E. chaffeensis establishes a chronic infection that persists for several weeks to months, while promoting the development of Th1 and Th17 cellular responses and pathogen-specific humoral immunity. We demonstrate here that vaccination with a live, attenuated clone of E. chaffeensis bearing a targeted mutation in the Ech_0230 gene neither promotes the development of long-lived cellular or humoral immunity, nor confers protection against secondary wild-type E. chaffeensis challenge. In dogs, a population of mature CD4+CD8+ double-positive (DP) T cells exists in the periphery that shares similarities with the DP T cell populations that have been described in humans and swine. Little is known about the function of these cells, particularly in the context of infectious diseases. Here, we demonstrate that canine DP T cells expand significantly in response to E. chaffeensis infection. Using in vitro antigen recall assays, we further demonstrate that canine DP T cells undergo clonal expansion, produce IFNγ and IL-17, and upregulate expression of granzyme B and granulysin. Together, our results demonstrate that DP T cells accumulate in the host during E. chaffeensis infection, and suggest that alternative lymphocyte populations may participate in the immune response to tick-borne infections in the incidental host.

Ehrlichioses: An Important One Health Opportunity

Ehrlichioses are caused by obligately intracellular bacteria that are maintained subclinically in a persistently infected vertebrate host and a tick vector. The most severe life-threatening illnesses, such as human monocytotropic ehrlichiosis and heartwater, occur in incidental hosts. Ehrlichia have a developmental cycle involving an infectious, nonreplicating, dense core cell and a noninfectious, replicating reticulate cell. Ehrlichiae secrete proteins that bind to host cytoplasmic proteins and nuclear chromatin, manipulating the host cell environment to their advantage. Severe disease in immunocompetent hosts is mediated in large part by immunologic and inflammatory mechanisms, including overproduction of tumor necrosis factor α (TNF-α), which is produced by CD8 T lymphocytes, and interleukin-10 (IL-10). Immune components that contribute to control of ehrlichial infection include CD4 and CD8 T cells, natural killer (NK) cells, interferon-γ (IFN-γ), IL-12, and antibodies. Some immune components, such as TNF-α, perforin, and CD8 T cells, play both pathogenic and protective roles. In contrast with the immunocompetent host, which may die with few detectable organisms owing to the overly strong immune response, immunodeficient hosts die with overwhelming infection and large quantities of organisms in the tissues. Vaccine development is challenging because of antigenic diversity of E. ruminantium, the necessity of avoiding an immunopathologic response, and incomplete knowledge of the protective antigens.

Vaccination with an Attenuated Mutant of Ehrlichia chaffeensis Induces Pathogen-Specific CD4+ T Cell Immunity and Protection from Tick-Transmitted Wild-Type Challenge in the Canine Host

Ehrlichia chaffeensis is a tick-borne rickettsial pathogen and the causative agent of human monocytic ehrlichiosis. Transmitted by the Amblyomma americanum tick, E. chaffeensis also causes disease in several other vertebrate species including white-tailed deer and dogs. We have recently described the generation of an attenuated mutant strain of E. chaffeensis, with a mutation in the Ech_0660 gene, which is able to confer protection from secondary, intravenous-administered, wild-type E. chaffeensis infection in dogs. Here, we extend our previous results, demonstrating that vaccination with the Ech_0660 mutant protects dogs from physiologic, tick-transmitted, secondary challenge with wild-type E. chaffeensis; and describing, for the first time, the cellular and humoral immune responses induced by Ech_0660 mutant vaccination and wild-type E. chaffeensis infection in the canine host. Both vaccination and infection induced a rise in E. chaffeensis-specific antibody titers and a significant Th1 response in peripheral blood as measured by E. chaffeensis antigen-dependent CD4⁺ T cell proliferation and IFNγ production. Further, we describe for the first time significant IL-17 production by peripheral blood leukocytes from both Ech_0660 mutant vaccinated animals and control animals infected with wild-type E. chaffeensis, suggesting a previously unrecognized role for IL-17 and Th17 cells in the immune response to rickettsial pathogens. Our results are a critical first step towards defining the role of the immune system in vaccine-induced protection from E. chaffeensis infection in an incidental host; and confirm the potential of the attenuated mutant clone, Ech_0660, to be used as a vaccine candidate for protection against tick-transmitted E. chaffeensis infection.

The role of CD8 T lymphocytes in rickettsial infections

Arthropod-borne obligately intracellular bacteria pose a difficult challenge to the immune system. The genera Rickettsia, Orientia, Ehrlichia, and Anaplasma evolved mechanisms of immune evasion, and each interacts differently with the immune system. The roles of CD8 T cells include protective immunity and immunopathology. In Rickettsia infections, CD8 T cells are protective mediated in part by cytotoxicity toward infected cells. In contrast, TNF-α overproduction by CD8 T cells is pathogenic in lethal ehrlichiosis by induction of apoptosis/necrosis in hepatocytes. Yet, CD8 T cells, along with CD4 T cells and antibodies, also contribute to protective immunity in ehrlichial infections. In granulocytic anaplasmosis, CD8 T cells impact pathogen control modestly but could contribute to immunopathology by virtue of their dysfunction. While preliminary evidence indicates that CD8 T cells are important in protection against Orientia tsutsugamushi, mechanistic studies have been neglected. Valid animal models will enable experiments to elucidate protective and pathologic immune mechanisms. The public health need for vaccines against these agents of human disease, most clearly O. tsutsugamushi, and the veterinary diseases, canine monocytotropic ehrlichiosis (Ehrlichia canis), heartwater (Ehrlichia ruminantium), and bovine anaplasmosis (A. marginale), requires detailed immunity and immunopathology investigations, including the roles of CD8 T lymphocytes.

Effects of doxycycline on haematology, blood chemistry and peripheral blood lymphocyte subsets of healthy dogs and dogs naturally infected with Ehrlichia canis

Canine monocytic ehrlichiosis (CME), caused by Ehrlichia canis, is a vector-borne disease with a worldwide distribution. It has been proposed that the pathogenesis, clinical severity and outcome of disease caused by Ehrlichia spp. can be attributed to the immune response rather than to any direct rickettsial effect. Moreover, doxycycline, the antimicrobial of choice for the treatment of CME, has immunomodulatory and anti-inflammatory properties associated with blood leukocyte proliferation function, cytokine synthesis, and matrix metalloproteinase activity. In order to assess the potential effects of doxycycline, dependent and independent of its antimicrobial activity, the present study compared changes in haematology, blood chemistry and circulating lymphocyte subpopulations in 12 healthy dogs and 20 dogs with CME after doxycycline therapy. Some changes were recorded only in the CME affected dogs, probably due to the antimicrobial effect of doxycycline. However, increases in mean corpuscular haemoglobin, mean corpuscular haemoglobin concentration, platelet count and α2-globulins, and decreased plasma creatinine were observed in both healthy and CME affected dogs. The absolute count of B lymphocytes (CD21(+)) increased initially, but then decreased until the end of the study period in both groups. A potential effect of doxycycline unrelated to its antimicrobial activity against E. canis is suggested, taking into account the results observed both in healthy dogs and in dogs with CME.

Early induction of interleukin-10 limits antigen-specific CD4⁺ T cell expansion, function, and secondary recall responses during persistent phagosomal infection

Diverse pathogens have evolved to survive and replicate in the endosomes or phagosomes of the host cells and establish persistent infection. Ehrlichiae are Gram-negative, intracellular bacteria that are transmitted by ticks. Ehrlichiae reside in the endosomes of the host phagocytic or endothelial cells and establish persistent infection in their vertebrate reservoir hosts. CD4(+) T cells play a critical role in protection against phagosomal infections. In the present study, we investigated the expansion, maintenance, and functional status of antigen-specific CD4(+) T cells during persistent Ehrlichia muris infection in wild-type and interleukin-10 (IL-10)-deficient mice. Our study indicated that early induction of IL-10 led to reduced inflammatory responses and impaired bacterial clearance during persistent Ehrlichia infection. Notably, we demonstrated that the functional production of gamma interferon (IFN-γ) by antigen-specific CD4(+) T cells maintained during a persistent phagosomal infection progressively deteriorates. The functional loss of IFN-γ production by antigen-specific CD4(+) T cells was reversed in the absence of IL-10. Furthermore, we demonstrated that transient blockade of IL-10 receptor during the T cell priming phase early in infection was sufficient to enhance the magnitude and the functional capacity of antigen-specific effector and memory CD4(+) T cells, which translated into an enhanced recall response. Our findings provide new insights into the functional status of antigen-specific CD4(+) T cells maintained during persistent phagosomal infection. The study supports the concept that a better understanding of the factors that influence the priming and differentiation of CD4(+) T cells may provide a basis to induce a protective immune response against persistent infections.

Type I interferons promote severe disease in a mouse model of lethal ehrlichiosis

Human monocytic ehrlichiosis (HME) is caused by a tick-borne obligate intracellular pathogen of the order Rickettsiales. HME disease can range from mild to a fatal, toxic shock-like syndrome, yet the mechanisms regulating pathogenesis are not well understood. We define a central role for type I interferons (alpha interferon [IFN-α] and IFN-β) in severe disease in a mouse model of fatal ehrlichiosis caused by Ixodes ovatus Ehrlichia (IOE). IFN-α and IFN-β were induced by IOE infection but not in response to a less virulent strain, Ehrlichia muris. The major sources of type I IFNs during IOE infection were plasmacytoid dendritic cells and monocytes. Mice lacking the receptor for type I IFNs (Ifnar deficient) or neutralization of IFN-α and IFN-β resulted in a reduced bacterial burden. Ifnar-deficient mice exhibited significantly increased survival after IOE infection, relative to that of wild-type (WT) mice, that correlated with increased type II IFN (IFN-γ) production. Pathogen-specific antibody responses were also elevated in Ifnar-deficient mice, and this required IFN-γ. Remarkably, increased IFN-γ and IgM were not essential for protection in the absence of type I IFN signaling. The direct effect of type I IFNs on hematopoietic and nonhematopoietic cells was evaluated in bone marrow chimeric mice. We observed that chimeric mice containing Ifnar-deficient hematopoietic cells succumbed to infection early, whereas Ifnar-deficient mice containing WT hematopoietic cells exhibited increased survival, despite having a higher bacterial burden. These data demonstrate that IFN-α receptor signaling in nonhematopoietic cells is important for pathogenesis. Thus, type I IFNs are induced during a rickettsial infection in vivo and promote severe disease.

Inhibitory effect of interferon gamma on frequency of Ehrlichia canis-infected cells in vitro

Ehrlichia canis is an obligate intracellular bacterium that infects the macrophage-monocyte cells of dogs, causing canine monocytic ehrlichiosis. Interferon-γ (IFN-γ), along with other cytokines, mediates the immune response to such intracellular bacterial invasions. To determine the role of IFN-γ in the immunity of dogs to E. canis infection, peripheral blood mononuclear cells (PBMC) and white blood cells (WBC) were collected from E. canis-infected dogs and added to a culture of E. canis in DH82 cells. The number of E. canis inclusion-positive cells was significantly reduced in cultures containing PBMC and WBC from E. canis-infected dogs compared to uninfected dogs. However, this resistance was inhibited by the addition of an anti-dog IFN-γ antibody. Resistance was also observed when PBMC were added to the Cell Culture Inserts, which prohibited contact of PBMC to DH82 cells, while allowed the diffusion of soluble cell products. The results of this study indicate that resistance was not dependent on cell to cell contact, but was associated with soluble cell products, such as IFN-γ. The addition of recombinant canine IFN-γ to the E. canis culture also reduced the number of infected cells. A commercial recombinant canine IFN-γ, which is sold in Japan, was also effective at reducing E. canis-infected cell number. These results indicate that IFN-γ has an inhibitory effect on the frequency of E. canis-infected cells in vitro and that contact between effector and target cells is not necessary for the resistance.

Recombinant Ehrlichia P29 protein induces a protective immune response in a mouse model of ehrlichiosis

Ehrlichioses are emerging tick-borne bacterial diseases of humans and animals for which no vaccines are available. The diseases are caused by obligately intracellular bacteria belonging to the genus Ehrlichia. Several immunoreactive proteins of ehrlichiae have been identified based on their reactivity with immune sera from human patients and animals. These include the major outer membrane proteins, ankyrin repeat proteins and tandem repeat proteins (TRP). Polyclonal antibodies directed against the tandem repeats (TRs) of Ehrlichia chaffeensis TRP32, TRP47 and TRP120 have been shown to provide protection in mice. In the present study, we evaluated E. muris P29, which is the ortholog of E. chaffeensis TRP47 and E. canis TRP36, as a subunit vaccine in a mouse model of ehrlichiosis. Our study indicated that unlike E. chaffeensis TRP47 and E. canis TRP36, orthologs of E. muris (P29) and E. muris-like agent (EMLA) do not contain tandem repeats. Immunization of mice with recombinant E. muris P29 induced significant protection against a challenge infection. The protection induced by E. muris P29 was associated with induction of strong antibody responses. In contrast to development of P29-specific IgG antibodies following immunization, development of P29-specific IgG antibodies, but not IgM antibodies, was impaired during persistent E. muris infection. Furthermore, our study indicated that CD4+ T cells target P29 during E. muris infection and differentiate into IFN-γ-producing Th1 effector/memory cells. In conclusion, our study indicated that orthologs of E. muris P29 showed considerable variation in the central tandem repeat region among different species, induction of P29-specific IgG antibody response was impaired during persistent E. muris infection, and rP29 induced protective immune responses.

Immune mediators of protective and pathogenic immune responses in patients with mild and fatal human monocytotropic ehrlichiosis

BACKGROUND

Ehrlichia chaffeensis is a bacterial pathogen that causes fatal human monocytic ehrlichiosis (HME) that mimic toxic shock-like syndrome. Murine studies indicate that over activation of cellular immunity followed by immune suppression plays a central role in mediating tissue injury and organ failure during fatal HME. However, there are no human studies that examine the correlates of resistance or susceptibility to severe and fatal HME.

RESULTS

In this study, we compared the immune responses in two patients with mild/non fatal and severe/fatal HME who had marked lymphopenia, thrombocytopenia and elevated liver enzymes. The levels of different immunological factors in the blood of those patients were examined and compared to healthy controls. Our data showed that fatal HME is associated with defective production of Th1 cytokines such as ( IFNγ and IL-2), increased anti-inflammatory (IL-10 and IL-13) and pro-inflammatory (TNF-α, IL-1α, IL-1β, and IL-6) cytokines, increased levels of macrophages, T cells, and NK cells chemokines such as MCP-1, MIP-1α, MIP-1β, but not RANTES and IP-10, increased levels of neutrophils chemokine and growth factor (IL-8 and G-CSF), and elevated expression of tumor necrosis factor receptor (TNFR), and toll like receptors 2 and 4 compared to patients with non fatal HME and healthy controls.

CONCLUSIONS

Fatal Ehrlichia-induced toxic shock is associated with defective Th1 responses, possible immune suppression mediated by IL-10. In addition, marked leukopenia observed in patients with fatal disease could be attributed to enhanced apoptosis of leukocytes and/or elevated chemokine production that could promote migration of immune cells to sites of infection causing tissue injury.

Structure-based vaccines provide protection in a mouse model of ehrlichiosis

Background

Recent advances in bioinformatics have made it possible to predict the B cell and T cell epitopes of antigenic proteins. This has led to design of peptide based vaccines that are more specific, safe, and easy to produce. The obligately intracellular gram negative bacteria Ehrlichia cause ehrlichioses in humans and animals. As yet there are no vaccines to protect against Ehrlichia infection.

Methodology/Principal Findings

We applied the principle of structural vaccinology to design peptides to the epitopes of Ehrlichia muris outer membrane P28-19 (OMP-1/P28) and Ehrlichia Heat shock protein 60 (Hsp60/GroEL) antigenic proteins. Both P28-19 and Ehrlichia Hsp60 peptides reacted with polyclonal antibodies against E. canis and E. chaffeensis and could be used as a diagnostic tool for ehrlichiosis. In addition, we demonstrated that mice vaccinated with Ehrlichia P28-19 and Hsp60 peptides and later challenged with E. muris were protected against the pathogen.

Conclusions/Significance

Our results demonstrate the power of structural vaccines and could be a new strategy in the development of vaccines to provide protection against pathogenic microorganisms.

Immunization with Ehrlichia P28 outer membrane proteins confers protection in a mouse model of ehrlichiosis

The obligately intracellular bacterium Ehrlichia chaffeensis that resides in mononuclear phagocytes is the etiologic agent of human monocytotropic ehrlichiosis (HME). HME is an emerging and often life-threatening, tick-transmitted infectious disease in the United States. Effective primary immune responses against Ehrlichia infection involve generation of Ehrlichia-specific gamma interferon (IFN-γ)-producing CD4(+) T cells and cytotoxic CD8(+) T cells, activation of macrophages by IFN-γ, and production of Ehrlichia-specific antibodies of the Th1 isotype. Currently, there are no vaccines available against HME. We evaluated the ability of 28-kDa outer membrane proteins (P28-OMP-1) of the closely related Ehrlichia muris to stimulate long-term protective memory T and B cell responses and confer protection in mice. The spleens of mice vaccinated with E. muris P28-9, P28-12, P28-19, or a mixture of these three P28 proteins (P28s) using a DNA prime-protein boost regimen and challenged with E. muris had significantly lower bacterial loads than the spleens of mock-vaccinated mice. Mice immunized with P28-9, P28-12, P28-19, or the mixture induced Ehrlichia-specific CD4(+) Th1 cells. Interestingly, mice immunized with P28-14, orthologs of which in E. chaffeensis and E. canis are primarily expressed in tick cells, failed to lower the ehrlichial burden in the spleen. Immunization with the recombinant P28-19 protein alone also significantly decreased the bacterial load in the spleen and liver compared to those of the controls. Our study reports, for the first time, the protective roles of the Ehrlichia P28-9 and P28-12 proteins in addition to confirming previous reports of the protective ability of P28-19. Partial protection induced by immunization with P28-9, P28-12, and P28-19 against Ehrlichia was associated with the generation of Ehrlichia-specific cell-mediated and humoral immune responses.

Ehrlichia canis (Jaboticabal strain) induces the expression of TNF-α in leukocytes and splenocytes of experimentally infected dogs

Canine ehrlichiosis is caused by the bacterium Ehrlichia canis and is characterized by a systemic febrile disease of unknown pathogenesis. This study evaluated the expression of cytokines TNF-α, IL-10, IFN-γ, in splenic cells and blood leukocytes during the acute phase of ehrlichiosis and after treatment with doxycycline hyclate in dogs experimentally infected with the E. canis Jaboticabal strain. The study results showed a significant expression of TNF-α 18 days post-inoculation, reducing by approximately 70% after treatment. There was a unique peak of expression of IL-10 and IFN-γ 18 and 30 days post-inoculation, respectively. This study suggests that TNF-α plays a role in the pathogenesis of the acute phase of canine ehrlichiosis and that treatment with doxycycline hyclate reduces the systemic effects of this cytokine, possibly by reducing or eliminating parasitemia.

Progress and obstacles in vaccine development for the ehrlichioses

Ehrlichia are tick-borne obligately intracellular bacteria that cause significant diseases in veterinary natural hosts, including livestock and companion animals, and are now considered important zoonotic pathogens in humans. Vaccines are needed for these veterinary and zoonotic human pathogens, but many obstacles exist that have impeded their development. These obstacles include understanding genetic and antigenic variability, influence of the host on the pathogen phenotype and immunogenicity, identification of the ehrlichial antigens that stimulate protective immunity and those that elicit immunopathology, development of animal models that faithfully reflect the immune responses of the hosts and understanding molecular host-pathogen interactions involved in immune evasion or that may be blocked by the host immune response. We review the obstacles and progress in addressing barriers associated with vaccine development to protect livestock, companion animals and humans against these host defense-evasive and cell function-manipulative, vector-transmitted pathogens.

MyD88-dependent signaling contributes to host defense against ehrlichial infection

The ehrlichiae are small Gram-negative obligate intracellular bacteria in the family Anaplasmataceae. Ehrlichial infection in an accidental host may result in fatal diseases such as human monocytotropic ehrlichiosis, an emerging, tick-borne disease. Although the role of adaptive immune responses in the protection against ehrlichiosis has been well studied, the mechanism by which the innate immune system is activated is not fully understood. Using Ehrlichia muris as a model organism, we show here that MyD88-dependent signaling pathways play a pivotal role in the host defense against ehrlichial infection. Upon E. muris infection, MyD88-deficient mice had significantly impaired clearance of E. muris, as well as decreased inflammation, characterized by reduced splenomegaly and recruitment of macrophages and neutrophils. Furthermore, MyD88-deficient mice produced markedly lower levels of IL-12, which correlated well with an impaired Th1 immune response. In vitro, dendritic cells, but not macrophages, efficiently produced IL-12 upon E. muris infection through a MyD88-dependent mechanism. Therefore, MyD88-dependent signaling is required for controlling ehrlichial infection by playing an essential role in the immediate activation of the innate immune system and inflammatory cytokine production, as well as in the activation of the adaptive immune system at a later stage by providing for optimal Th1 immune responses.

Persistent infection contributes to heterologous protective immunity against fatal ehrlichiosis

Human monocytotropic ehrlichiosis (HME), an emerging and often life-threatening tick-transmitted disease, is caused by the obligately intracellular bacterium Ehrlichia chaffeensis. HME is modeled in C57BL/6 mice using Ehrlichia muris, which causes persistent infection, and Ixodes ovatus Ehrlichia (IOE), which is either acutely lethal or sublethal depending on the dose and route of inoculation. A persistent primary E. muris infection, but not a sublethal IOE infection, protects mice against an ordinarily lethal secondary IOE challenge. In the present study, we determined the role of persistent infection in maintenance of protective memory immune responses. E. muris-infected mice were treated with doxycycline or left untreated and then challenged with an ordinarily lethal dose of IOE. Compared to E. muris-primed mice treated with doxycycline, untreated mice persistently infected with E. muris had significantly greater numbers of antigen-specific gamma interferon-producing splenic memory T cells, significant expansion of CD4(+) CD25(+) T regulatory cells, and production of transforming growth factor beta1 in the spleen. Importantly, E. muris-primed mice treated with doxycycline showed significantly greater susceptibility to challenge infection with IOE compared to untreated mice persistently infected with E. muris. The study indicated that persistent ehrlichial infection contributes to heterologous protection by stimulating the maintenance of memory T-cell responses.

Major species-specific antibody epitopes of the Ehrlichia chaffeensis p120 and E. canis p140 orthologs in surface-exposed tandem repeat regions

Ehrlichia chaffeensis and E. canis have a small subset of tandem repeat (TR)-containing protein orthologs, including p120/p140, which elicit strong antibody responses. The TR regions of these protein orthologs are immunoreactive, but the molecular characteristics of the p120/p140 epitopes have not been determined. In this study, the immunodeterminants of the E. chaffeensis p120 and E. canis p140 were identified and molecularly defined. Major antibody epitope-containing regions of both p120 and p140 were localized to the TR regions, which reacted strongly by Western immunoblotting with antibodies in sera from E. chaffeensis-infected dogs or patients and E. canis-infected dogs, respectively. Single continuous species-specific major epitopes within the E. chaffeensis p120 and E. canis p140 TRs were mapped to homologous surface-exposed glutamate/aspartate-rich regions (19 to 22 amino acids). In addition, minor cross-reactive epitopes were localized to homologous N- and C-terminal regions of p120 and p140. Furthermore, although the native and recombinant p120 and p140 proteins exhibited higher-than-predicted molecular masses, posttranslational modifications were not present on abnormally migrating p120 and p140 TR recombinant proteins as determined by matrix-assisted laser desorption ionization-time of flight mass spectrometry.

The elimination of Anaplasma phagocytophilum requires CD4+ T cells, but is independent of Th1 cytokines and a wide spectrum of effector mechanisms

Anaplasma phagocytophilum is a Gram-negative, obligate intracellular bacterium that exhibits a striking tropism for neutrophils. When we depleted mice of neutrophils, we found that murine susceptibility to anaplasmal infection was dependent on their presence. While serving as sites of bacterial replication, neutrophils do not seem to act as efficient killer cells in A. phagocytophilum infection, because mice deficient for antimicrobial effectors of neutrophils such as myeloperoxidase, granulocyte elastase, and cathepsin G were fully competent in pathogen elimination. To identify components of the immune system other than neutrophils that control A. phagocytophilum, we studied the course of infection in several gene-deficient mouse strains. IFN-gamma production by NK cells was important for initial defense, but not critical for pathogen elimination. In contrast, bacterial clearance was strictly dependent on CD4(+) T cells, but unexpectedly achieved in the absence of perforin, Fas/FasL and major Th1 cytokines such as IL-12, IFN-gamma, and MCP-1. These findings provide a novel paradigm for the control of an intracellular pathogen, which appears to be strikingly different from the CD4(+) T cell-, IL-12-, and IFN-gamma-dependent immunity to other intracellular bacteria.

Pathologic evidence of ehrlichiosis in calves inoculated with Ehrlichia chaffeensis

An immunocompetent animal disease model based on infection with Ehrlichia chaffeensis would facilitate research toward understanding mechanisms responsible for the broad range of clinical signs associated with human monocytic ehrlichiosis (HME). The adaptability of this model for the experimental feeding of tick species and stages and for testing therapies comparable to those for human diseases are additional advantages of large animal models. Herein, we summarize pathology reports for calves that developed fatal disease after experimental inoculation with E. chaffeensis. Elevated liver enzyme levels and lung pathology among these calves corroborated earlier reports of severe HME. Thus, an experimental disease model based on infection of outbred immunocompetent hosts with E. chaffeensis could be within our grasp for the first time.

Defining the immune response to Ehrlichia species using murine models

Pathogenic bacteria belonging to the family Anaplasmataceae include species of the genera Ehrlichia and Anaplasma. Ehrlichia chaffeensis, first known as the causative agent of human monocytic ehrlichiosis, also infects several vertebrate hosts including white-tailed deer, dogs, coyotes and goats. E. chaffeensis is transmitted from the bite of an infected hard tick, such as Amblyomma americanum. E. chaffeensis and other tick-transmitted pathogens have adapted to both the tick and vertebrate host cell environments. Although E. chaffeensis persists in both vertebrate and tick hosts for long periods of time, little is known about that process. Immunological studies will be valuable in assessing how the pathogen persists in nature in both vertebrate and invertebrate hosts. Understanding the host immune response to the pathogen originating from dual host backgrounds is also important to develop effective methods of diagnosis, control and treatment. In this paper, we provide our perspective of the current understanding of the immune response against E. chaffeensis in relation to other related Anaplasmataceae pathogens.

Protective heterologous immunity against fatal ehrlichiosis and lack of protection following homologous challenge

The roles of antibodies and memory T cells in protection against virulent Ehrlichia have not been completely investigated. In this study, we addressed these issues by using murine models of mild and fatal ehrlichiosis caused by related monocytotropic Ehrlichia strains. Mice were primed with either Ehrlichia muris or closely related virulent ehrlichiae transmitted by Ixodes ovatus (IOE) ticks given intraperitoneally or intradermally. All groups were reinfected intraperitoneally, 30 days later, with a lethal high dose of IOE. Priming with E. muris, but not IOE, induced strong CD4+ and CD8+ memory type 1 T-cell responses, Ehrlichia-specific immunoglobulin G (IgG) antibodies, and persistent infection. Compared to IOE-primed mice, subsequent lethal IOE challenge of E. muris-primed mice, resulted in (i) 100% protection against lethal infection, (ii) strong Ehrlichia-specific secondary gamma interferon (IFN-gamma)-producing effector/effector memory CD4+ and CD8+ T-cell responses, (iii) enhanced secondary anti-ehrlichial antibody response, (iv) accelerated bacterial clearance, and (v) the formation of granulomas in the liver and lung. E. muris-primed mice challenged with IOE had lower levels of serum interleukin-1alpha (IL-1alpha), IL-6, and IL-10 compared to unprimed mice challenged with IOE. Interestingly, the fatal secondary response in IOE-primed mice correlated with (i) decline in the Ehrlichia-specific CD4+ and CD8+ type 1 responses, (ii) marked hepatic apoptosis and necrosis, and (iii) substantial bacterial clearance, suggesting that fatal secondary response is due to immune-mediated tissue damage. In conclusion, protection against fatal ehrlichial infection correlates with strong expansion of IFN-gamma-producing CD4+ and CD8+ effector memory type 1 T cells, which appear to be maintained in the presence of IgG antibodies and persistent infection.

A variable-length PCR target protein of Ehrlichia chaffeensis contains major species-specific antibody epitopes in acidic serine-rich tandem repeats

Ehrlichia chaffeensis and E. canis have a small subset of tandem repeat (TR)-containing proteins that elicit strong host immune responses and are associated with host-pathogen interactions. In a previous study, we molecularly characterized a highly conserved 19-kDa major immunoreactive protein (gp19) of E. canis and identified the corresponding TR-containing ortholog variable-length PCR target (VLPT) protein in E. chaffeensis. In this study, the native 32-kDa VLPT protein was identified and the immunodeterminants defined in order to further understand the molecular basis of the host immune response to E. chaffeensis. Synthetic and/or recombinant polypeptides corresponding to various regions of VLPT were used to localize major antibody epitopes to the TR-containing region. Major antibody epitopes were identified in three nonidentical repeats (R2, R3, and R4), which reacted strongly with antibodies in sera from an E. chaffeensis-infected dog and human monocytotropic ehrlichiosis patients. VLPT-R3 and VLPT-R2 reacted most strongly with antibody, and the epitope was further localized to a nearly identical proximal 17-amino-acid region common between these repeats that was species specific. The epitope in R4 was distinct from that of R2 and R3 and was found to have conformational dependence. VLPT was detected in supernatants from infected cells, indicating that the protein was secreted. VLPT was localized on both reticulate and dense-core cells, and it was found extracellularly in the morula fibrillar matrix and associated with the morula membrane.

Ehrlichia canis gp200 contains dominant species-specific antibody epitopes in terminal acidic domains

Species-specific antibody epitopes within several major immunoreactive protein orthologs of Ehrlichia species have recently been identified and molecularly characterized. In this study, dominant B-cell epitopes within the acidic (pI 5.35) ankyrin repeat-containing 200-kDa major immunoreactive protein (gp200) of Ehrlichia canis were defined. The E. canis gp200 gene (4,263 bp; 1,421 amino acids) was cloned and expressed as four (N-terminal, 1,107 bp; N-internal, 910 bp; C-internal, 1,000 bp; and C-terminal, 1,280 bp) overlapping recombinant proteins. The N-terminal, C-internal, and C-terminal polypeptides (369, 332, and 426 amino acids, respectively) were strongly recognized by antibody, and the major epitope(s) in these polypeptides was mapped to four polypeptide regions (40 to 70 amino acids). Smaller overlapping recombinant polypeptides (14 to 15 amino acids) spanning these regions identified five strongly immunoreactive species-specific epitopes that exhibited conformational dependence. The majority of the epitopes (four) were located in two strongly acidic (pI 4 to 4.9) domains in the distal N- and C-terminal regions of the protein flanking the centralized ankyrin domain-containing region. The amino acid content of the epitope-containing domains included a high proportion of strongly acidic amino acids (glutamate and aspartate), and these domains appear to have important biophysical properties that influence the antibody response to gp200.

T-Cell-independent humoral immunity is sufficient for protection against fatal intracellular ehrlichia infection

Although humoral immunity has been shown to contribute to host defense during intracellular bacterial infections, its role has generally been ancillary. Instead, CD4 T cells are often considered to play the dominant role in protective immunity via their production of type I cytokines. Our studies of highly pathogenic Ehrlichia bacteria isolated from Ixodes ovatus (IOE) reveal, however, that this paradigm is not always correct. Immunity to IOE infection can be induced by infection with a closely related weakly pathogenic ehrlichia, Ehrlichia muris. Type I cytokines (i.e., gamma interferon, tumor necrosis factor alpha, and interleukin-12) were not necessary for E. muris-induced immunity. In contrast, humoral immunity was essential, as shown by the fact that E. muris-infected B-cell-deficient mice were not protected from IOE challenge and because E. muris immunization was effective in CD4-, CD8-, and major histocompatibility complex (MHC) class II-deficient mice. Immunity was unlikely due to nonspecific inflammation, as prior infection with Listeria monocytogenes did not induce immunity to IOE. Antisera from both wild-type and MHC-II-deficient mice provided at least partial resistance to challenge infection, and protection could also be achieved following transfer of total, but not B-cell-depleted, splenocytes obtained from E. muris-immunized mice. The titers of class-switched antibodies in immunized CD4 T-cell- and MHC class II-deficient mice, although lower than those observed in immunized wild-type mice, were significant, indicating that E. muris can induce class switch recombination in the absence of classical T-cell-mediated help. These studies highlight a major protective role for classical T-cell-independent humoral immunity during an intracellular bacterial infection.

Relative importance of T-cell subsets in monocytotropic ehrlichiosis: a novel effector mechanism involved in Ehrlichia-induced immunopathology in murine ehrlichiosis

Infection with gram-negative monocytotropic Ehrlichia strains results in a fatal toxic shock-like syndrome characterized by a decreased number of Ehrlichia-specific CD4(+) Th1 cells, the expansion of tumor necrosis factor alpha (TNF-alpha)-producing CD8(+) T cells, and the systemic overproduction of interleukin-10 (IL-10) and TNF-alpha. Here, we investigated the role of CD4(+) and CD8(+) T cells in immunity to Ehrlichia and the pathogenesis of fatal ehrlichiosis caused by infection with low- and high-dose (10(3) and 10(5) bacterial genomes/mouse, respectively) ehrlichial inocula. The CD4(+) T-cell-deficient mice showed exacerbated susceptibility to a lethal high- or low-dose infection and harbored higher bacterial numbers than did wild-type (WT) mice. Interestingly, the CD8(+) T-cell-deficient mice were resistant to a low dose but succumbed to a high dose of Ehrlichia. The absence of CD8(+) T cells abrogated TNF-alpha and IL-10 production, reduced tissue injury and bacterial burden, restored splenic CD4(+) T-cell numbers, and increased the frequency of Ehrlichia-specific CD4(+) Th1 cells in comparison to infected WT mice. Although fatal disease is perforin independent, our data suggested that perforin played a critical role in controlling bacterial burden and mediating liver injury. Similar to WT mice, mortality of infected perforin-deficient mice was associated with CD4(+) T-cell apoptosis and a high serum concentration of IL-10. Depletion of IL-10 restored the number of CD4(+) and CD8(+) T cells in infected WT mice. Our data demonstrate a novel mechanism of immunopathology in which CD8(+) T cells mediate Ehrlichia-induced toxic shock, which is associated with IL-10 overproduction and CD4(+) T-cell apoptosis.

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.

Role of tumor necrosis factor alpha (TNF-alpha) and interleukin-10 in the pathogenesis of severe murine monocytotropic ehrlichiosis: increased resistance of TNF receptor p55- and p75-deficient mice to fatal ehrlichial infection

Intraperitoneal (i.p.) infection with a high dose of a highly virulent Ehrlichia strain (IOE) results in a toxic shock-like syndrome characterized by severe liver injury and systemic overproduction of tumor necrosis factor alpha (TNF-alpha) by CD8+ T cells. We examined the role of TNF-alpha and TNF receptors in high-dose-IOE-induced shock/liver injury. TNF receptor (TNFR) I/II-/- mice lacking both the p55 and p75 receptors for this cytokine were more resistant to IOE-induced liver injury than their wild-type background controls. TNFR I/II-/- mice survived longer, dying between 15 and 18 days, with evidence of mild liver necrosis/apoptosis. In contrast, wild-type mice were not rescued from the lethal effect of IOE by TNF-alpha neutralization. TNF-alpha-depleted mice developed severe liver injury and succumbed to disease between days 9 and 11 postinfection, similar to sham-treated, infected wild-type mice. Although IFN-gamma production in the spleens of IOE-infected TNFR I/II-/- and TNF-alpha-depleted mice was higher than that detected in wild-type controls, these mice had higher bacterial burdens than infected controls. Following high-dose IOE challenge, TNFR I/II-/- and TNF-alpha-depleted mice have an early increase in IL-10 levels in sera and spleens, which was produced mainly by adherent spleen cells. In contrast, a late burst of interleukin-10 (IL-10) was observed in control mice. Nonadherent spleen cells were the major source of IL-10 in IOE-infected wild-type mice. We conclude that TNFR I/II and TNF-alpha participate in Ehrlichia-induced shock and host defense by regulating liver injury and controlling ehrlichial burden. Our data suggest that fatal ehrlichiosis could be a multistep process, where TNF-alpha is not solely responsible for mortality.

Ehrlichia under our noses and no one notices

Ehrlichia chaffeensis, an obligately intracellular bacterium, resides within a cytoplasmic vacuole in macrophages, establishes persistent infection in natural hosts such as white-tailed deer and canids, and is transmitted transstadially and during feeding by ticks, particularly Amblyomma americanum. Ehrlichial cell walls contain glycoproteins and a family of divergent 28 kDa proteins, but no peptidoglycan or lipopolysaccharide. The dense-cored ultrastructural form preferentially expresses certain glycoproteins, including a multiple repeat unit-containing adhesin. Ehrlichiae attach to L-selectin and E-selectin, inhibit phagolysosomal fusion, apoptosis, and JAK/STAT activation, and downregulate IL-12, IL-15, IL-18, TLR2 and 3, and CD14. Mouse models implicate overproduction of TNF-alpha by antigen-specific CD8 T lymphocytes in pathogenesis and strong type 1 CD4 and CD8 T lymphocyte responses, synergistic activities of IFN-gamma and TNF-alpha, and IgG2a antibodies in immunity. Human monocytotropic ehrlichiosis (HME) manifests as a flu-like illness that progresses in severity to resemble toxic shock-like syndrome, with meningoencephalitis or adult respiratory distress syndrome in some patients, and requires hospitalization in half. In immunocompromised patients, HME acts as an overwhelming opportunistic infection. In one family physician's practice, active surveillance for three years revealed an incidence of 1000 cases per million population. Diagnosis employs serology or polymerase chain reaction, which are not utilized sufficiently to establish the true impact of this emerging virus-like illness.

Essential role for humoral immunity during Ehrlichia infection in immunocompetent mice

Although cellular immunity is essential for host defense during intracellular bacterial infections, humoral immunity can also play a significant role in host defense during infection by some intracellular bacteria, including the ehrlichiae. Antibodies can protect susceptible SCID mice from fatal Ehrlichia chaffeensis infection, an observation that has been hypothesized to involve the opsonization of bacteria released from host cells. To determine whether humoral immunity plays an essential role during ehrlichia infection in immunocompetent mice, we utilized a murine model of fatal monocytotropic ehrlichiosis caused by Ixodes ovatus ehrlichia. Mice lacking either B cells or FcgammaRI were unable to resolve a low-dose (sublethal) I. ovatus ehrlichia infection, which suggested that humoral immunity is essential for resistance. Polyclonal sera generated in I. ovatus ehrlichia-infected mice recognized a conserved ehrlichia outer membrane protein and, when administered to infected mice, caused a significant decrease in bacterial infection. Mice experimentally depleted of complement, or deficient for complement receptors 1 and 2, were also susceptible to sublethal I. ovatus ehrlichia infection, as were mice that lacked the phox91 subunit of NADPH oxidase. The data are consistent with a mechanism whereby bacteria released from infected cells are lysed directly by complement or undergo antibody-mediated FcgammaR-dependent phagocytosis and subsequent exposure to reactive oxygen intermediates. The findings suggest mechanisms whereby antibodies contribute to immunity against intracellular bacteria in immunocompetent mice.

Experimental Ehrlichia canis infection in the dog does not cause immunosuppression

A carrier state develops in some Ehrlichia canis-infected dogs due to ineffective host defenses. The subsequent development of immune-mediated diseases or opportunistic infections in chronic ehrlichiosis suggests dysregulation of immunity; however, the immunobiology of this infection has not been well characterized. In this study, eight dogs were infected with E. canis, and changes in seroreactivity, serum immunoglobulin (Ig) concentrations, peripheral blood T cell subsets, lymphocyte blastogenesis (LBT), and lymphokine-activated killer (LAK) activity were evaluated over 4 months. Infection, which was documented by seroconversion, polymerase chain reaction, and blood culture, caused self-limiting fever and thrombocytopenia. Infected dogs developed an anti-E. canis antibody response but were not immune to re-infection. Serum IgM, IgG, and IgA concentrations were unaffected by E. canis. The percentage of circulating CD4(+) T cells was similar in uninfected and infected dogs at all points. Infected dogs developed a CD8(+) lymphocytosis 6 weeks after inoculation that subsequently subsided, despite organism persistence. Functional defects of cell-mediated immunity, measured as suppression of LAK activity or mitogen-driven LBT, were not observed. These results suggest that immune responses are not grossly impaired in young dogs during the first several months following experimental E. canis infection.

Immunity to the ehrlichiae: new tools and recent developments

PURPOSE OF REVIEW

Discusses recent developments in the study of immunity and host defense against the monocytic ehrlichiae in 2003 and 2004. The review does not address anaplasmoses, as the anaplasmae were recently re-classified into the genus Anaplasma, and are distinct in cell tropism from the ehrlichiae.

RECENT FINDINGS

The features of the immune responses against these emerging Gram-negative obligate intracellular pathogens are only beginning to be understood. Important advances in our ability to study host defense include the development of new experimental mouse models. Recent studies have defined possible mechanisms of innate immune subversion in human monocytes, as well as roles for lymphocyte subsets and type I cytokines during mouse infection. Other studies in the mouse suggest that cytokine production by CD8 T cells may contribute to immunopathology. New data also support a role for humoral immunity during host defense against these intracellular pathogens.

SUMMARY

The use of new animal models will facilitate research of the mechanisms of innate, adaptive, and pathological immune responses, and will enhance our understanding of human immunity to the ehrlichiae as well as to other pathogenic intracellular bacteria.

Evaluation of immunocompetent and immunocompromised mice (Mus musculus) for infection with Ehrlichia chaffeensis and transmission to Amblyomma americanum ticks

Experiments on the natural history of Ehrlichia chaffeensis, the agent of human monocytic ehrlichiosis (HME), would be facilitated by the availability of a laboratory animal model for transmission to vector ticks. Five strains of mice were evaluated for their susceptibility to infection with E. chaffeensis and transmission competence: C57BL/6 mice, inducible nitric oxide synthase (iNOS) deficient mice, MHC I deficient (beta2m /) mice, MHC II deficient mice (Abb /), and B and T cell deficient (Rag1 /) mice. Mice were inoculated with a low passage isolate of E. chaffeensis, and infection and morbidity were monitored for 57 days. Three xenodiagnostic infestations with A. americanum nymphs were performed 1, 8, and 15 days following inoculation. C57BL/6 mice cleared the organism in less than 17 days, with no indication of morbidity, and mounted a rapid, strong antibody response. Transmission to feeding A. americanum nymphs was seen in 1/30 nymphs fed on C57BL/6 mice immediately after inoculation. In MHC I and iNOS deficient mice, pathogen DNA was detected up to 17 or 24 days, respectively, after inoculation. Persistent infection for the duration of the experiment (57 days) was observed in MHC II deficient mice. However, E. chaffeensis was not detected in ticks fed on iNOS, MHC I, or MHC II deficient mice. Susceptibility to infection was greatest in Rag1 knockout mice, with significant morbidity and mortality within 24 days after inoculation. E. chaffeensis DNA was detected in up to 55% of replete nymphs that fed on Rag1 mice. However, E. chaffeensis was not detected in molted adult ticks from the same cohorts.