Infection of the laboratory mouse with the intracellular pathogen Ehrlichia chaffeensis

Efficacy and Immune Correlates of OMP-1B and VirB2-4 Vaccines for Protection of Dogs from Tick Transmission of Ehrlichia chaffeensis

Ehrlichia chaffeensis, an obligatory intracellular bacterium, causes human monocytic ehrlichiosis, an emerging disease transmitted by the Lone Star tick, Amblyomma americanum. Here, we investigated the vaccine potential of OMP-1B and VirB2-4. Among the highly expressed and immunodominant E. chaffeensis porin P28s/OMP-1s, OMP-1B is predominantly expressed by E. chaffeensis in A. americanum ticks, whereas VirB2-4 is a pilus protein of the type IV secretion system essential for E. chaffeensis infection of host cells. Immunization with recombinant OMP-1B (rOMP-1B) or recombinant VirB2-4 (rVirB2-4) protected mice from E. chaffeensis infection as effectively as Entry-triggering protein of Ehrlichia immunization. Dogs vaccinated with a nanoparticle vaccine composed of rOMP-1B or rVirB2-4 and an immunostimulating complex developed high antibody titers against the respective antigen. Upon challenge with E. chaffeensis-infected A. americanum ticks, E. chaffeensis was undetectable in the blood of rOMP-1B or rVirB2-4 immunized dogs on day 3 or 6 post-tick attachment and for the duration of the experiment, whereas dogs sham-vaccinated with the complex alone were persistently infected for the duration of the experiment. E. chaffeensis exponentially replicates in blood-feeding ticks to facilitate transmission. Previously infected ticks removed from OMP-1B-immunized dogs showed significantly lower bacterial load relative to ticks removed from sham-immunized dogs, suggesting in-tick neutralization. Peripheral blood leukocytes from rVirB2-4-vaccinated dogs secreted significantly elevated amounts of interferon-γ soon after tick attachment by ELISpot assay and reverse transcription-quantitative PCR, suggesting interferon-γ-mediated Ehrlichia inhibition. Thus, Ehrlichia surface-exposed proteins OMP-1B and VirB2-4 represent new potential vaccine candidates for blocking tick-borne ehrlichial transmission. IMPORTANCE Ehrlichia are tick-borne pathogens that cause a potentially fatal illness-ehrlichiosis-in animals and humans worldwide. Currently, no vaccine is available for ehrlichiosis, and treatment options are limited. Ticks are biological vectors of Ehrlichia, i.e., Ehrlichia exponentially replicates in blood-sucking ticks before infecting animals. Ticks also inoculate immunomodulatory substances into animals. Thus, it is important to study effects of candidate vaccines on Ehrlichia infection in both animals and ticks and the immune responses of animals shortly after infected tick challenge. Here, we investigated the efficacy of vaccination with functionality-defined two surface-exposed outer membrane proteins of Ehrlichia chaffeensis, OMP-1B and VirB2-4, in a mouse infection model and then in a dog-tick transmission model. Our results begin to fill gaps in our understanding of Ehrlichia-derived protective antigens against tick-transmission and immune correlates and mechanisms that could help future development of vaccines for immunization of humans and animals to counter tick-transmitted ehrlichiosis.

An intracellular nanobody targeting T4SS effector inhibits Ehrlichia infection

Infection with obligatory intracellular bacteria is difficult to treat, as intracellular targets and delivery methods of therapeutics are not well known. Ehrlichia translocated factor-1 (Etf-1), a type IV secretion system (T4SS) effector, is a primary virulence factor for an obligatory intracellular bacterium, Ehrlichia chaffeensis In this study, we developed Etf-1-specific nanobodies (Nbs) by immunizing a llama to determine if intracellular Nbs block Etf-1 functions and Ehrlichia infection. Of 24 distinct anti-Etf-1 Nbs, NbD7 blocked mitochondrial localization of Etf-1-GFP in cotransfected cells. NbD7 and control Nb (NbD3) bound to different regions of Etf-1. Size-exclusion chromatography showed that the NbD7 and Etf-1 complex was more stable than the NbD3 and Etf-1 complex. Intracellular expression of NbD7 inhibited three activities of Etf-1 and E. chaffeensis: up-regulation of mitochondrial manganese superoxide dismutase, reduction of intracellular reactive oxygen species, and inhibition of cellular apoptosis. Consequently, intracellular NbD7 inhibited Ehrlichia infection, whereas NbD3 did not. To safely and effectively deliver Nbs into the host cell cytoplasm, NbD7 was conjugated to cyclized cell-permeable peptide 12 (CPP12-NbD7). CPP12-NbD7 effectively entered mammalian cells and abrogated the blockade of cellular apoptosis caused by E. chaffeensis and inhibited infection by E. chaffeensis in cell culture and in a severe combined-immunodeficiency mouse model. Our results demonstrate the development of an Nb that interferes with T4SS effector functions and intracellular pathogen infection, along with an intracellular delivery method for this Nb. This strategy should overcome current barriers to advance mechanistic research and develop therapies complementary or alternative to the current broad-spectrum antibiotic.

Ehrlichia chaffeensis Outer Membrane Protein 1-Specific Human Antibody-Mediated Immunity Is Defined by Intracellular TRIM21-Dependent Innate Immune Activation and Extracellular Neutralization

Antibodies are essential for immunity against Ehrlichia chaffeensis, and protective mechanisms involve blocking of ehrlichial attachment or complement and Fcγ-receptor-dependent destruction. In this study, we determined that major outer membrane protein 1 (OMP-19) hypervariable region 1 (HVR1)-specific human monoclonal antibodies (huMAbs) are protective through conventional extracellular neutralization and, more significantly, through a novel intracellular TRIM21-mediated mechanism.

Antibodies are essential for immunity against Ehrlichia chaffeensis, and protective mechanisms involve blocking of ehrlichial attachment or complement and Fcγ-receptor-dependent destruction. In this study, we determined that major outer membrane protein 1 (OMP-19) hypervariable region 1 (HVR1)-specific human monoclonal antibodies (huMAbs) are protective through conventional extracellular neutralization and, more significantly, through a novel intracellular TRIM21-mediated mechanism. Addition of OMP-1-specific huMAb EHRL-15 (IgG1) prevented infection by blocking attachment/entry, a mechanism previously reported; conversely, OMP-1-specific huMAb EHRL-4 (IgG3) engaged intracellular TRIM21 and initiated an immediate innate immune response and rapid intracellular degradation of ehrlichiae. EHRL-4-TRIM21-mediated inhibition was significantly impaired in TRIM21 knockout THP-1 cells. EHRL-4 interacted with cytosolic Fc receptor TRIM21, observed by confocal microscopy and confirmed by co-immunoprecipitation. E. chaffeensis-EHRL-4-TRIM21 complexes caused significant upregulation of proinflammatory cytokine/chemokine transcripts and resulted in rapid (<30 min) nuclear accumulation of NF-κB and TRIM21 and ehrlichial destruction. We investigated the role of TRIM21 in the autophagic clearance of ehrlichiae in the presence of EHRL-4. Colocalization between EHRL-4-opsonized ehrlichiae, polyubiquitinated TRIM21, autophagy regulators (ULK1 and beclin 1) and effectors (LC3 and p62), and lysosome-associated membrane protein 2 (LAMP2) was observed. Moreover, autophagic flux defined by conversion of LC3I to LC3II and accumulation and degradation of p62 was detected, and EHRL-4-mediated degradation of E. chaffeensis was abrogated by the autophagy inhibitor 3-methyladenine. Our results demonstrate that huMAbs are capable of inhibiting E. chaffeensis infection by distinct effector mechanisms: extracellularly by neutralization and intracellularly by engaging TRIM21, which mediates a rapid innate immune response that mobilizes the core autophagy components, triggering localized selective autophagic degradation of ehrlichiae.

Chloroquine inhibits endosomal viral RNA release and autophagy-dependent viral replication and effectively prevents maternal to fetal transmission of Zika virus

Zika virus (ZIKV) infection can cause neonatal microcephaly and neurological disorders. Currently, there is no designated drug for treating ZIKV infection and preventing neonatal microcephaly. In this study, we evaluated the effect of chloroquine, an anti-malaria drug, in ZIKV infected cells and mouse models. Chloroquine significantly inhibited ZIKV infection in multiple mammalian cell lines. Chloroquine treatment significantly improved the survival of ZIKV-infected 1-day old suckling SCID Beige mice and reduced viremia in adult SCID Beige mice. Importantly, chloroquine protected the fetus from maternal infection by reducing placenta to fetus viral transmission. We found that chloroquine exerts at least two mechanisms in protecting against ZIKV infection: 1) inhibiting endosomal disassembly of the internalized virus and thus reducing the release of viral RNA to the cytoplasm for replication; 2) inhibiting ZIKV RNA replication through blocking ZIKV induced autophagy. Our study suggests that chloroquine treatment warrants to be considered as a mitigation strategy for treating ZIKV infection and preventing ZIKV-associated microcephaly in pregnant women.

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.

Molecular Pathogenesis of Ehrlichia chaffeensis Infection

Ehrlichia chaffeensis is an obligatory intracellular and cholesterol-dependent bacterium that has evolved special proteins and functions to proliferate inside leukocytes and cause disease. E. chaffeensis has a multigene family of major outer membrane proteins with porin activity and induces infectious entry using its entry-triggering protein to bind the human cell surface protein DNase X. During intracellular replication, three functional pairs of two-component systems are sequentially expressed to regulate metabolism, aggregation, and the development of stress-resistance traits for transmission. A type IV secretion effector of E. chaffeensis blocks mitochondrion-mediated host cell apoptosis. Several type I secretion proteins are secreted at the Ehrlichia-host interface. E. chaffeensis strains induce strikingly variable inflammation in mice. The central role of MyD88, but not Toll-like receptors, suggests that Ehrlichia species have unique inflammatory molecules. A recent report about transient targeted mutagenesis and random transposon mutagenesis suggests that stable targeted knockouts may become feasible in Ehrlichia.

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 omentum is a site of protective IgM production during intracellular bacterial infection

Infection of mice with the bacterium Ehrlichia muris elicits a protective T cell-independent (TI) IgM response mediated primarily by a population of CD11c-expressing plasmablasts in the spleen. Although splenic marginal zone (MZ) B cells are considered to be important for TI responses to blood-borne pathogens, MZ B cells were not responsible for generating plasmablasts in response to Ehrlichia muris. Moreover, antigen-specific serum IgM was decreased only modestly in splenectomized mice and in mice that lacked spleen, lymph nodes, and Peyer's patches (SLP mice). Both splenectomized and SLP mice were protected against lethal ehrlichial challenge infection. Moreover, we found a high frequency of Ehrlichia-specific plasmablasts in the omentum of both conventional and SLP mice. Omental plasmablasts elicited during Ehrlichia infection lacked expression of CD138 but expressed CD11c in a manner similar to that of their splenic counterparts. Selective ablation of CD11c-expressing B cells nearly eliminated the omental Ehrlichia-specific plasmablasts and reduced antigen-specific serum IgM, identifying the omental B cells as a source of IgM production in the SLP mice. Generation of the omental plasmablasts was route dependent, as they were detected following peritoneal infection but not following intravenous infection. Our data identify the omentum as an important auxiliary site of IgM production during intracellular bacterial infection.

Ehrlichia chaffeensis infection in the reservoir host (white-tailed deer) and in an incidental host (dog) is impacted by its prior growth in macrophage and tick cell environments

Ehrlichia chaffeensis, transmitted from Amblyomma americanum ticks, causes human monocytic ehrlichiosis. It also infects white-tailed deer, dogs and several other vertebrates. Deer are its reservoir hosts, while humans and dogs are incidental hosts. E. chaffeensis protein expression is influenced by its growth in macrophages and tick cells. We report here infection progression in deer or dogs infected intravenously with macrophage- or tick cell-grown E. chaffeensis or by tick transmission in deer. Deer and dogs developed mild fever and persistent rickettsemia; the infection was detected more frequently in the blood of infected animals with macrophage inoculum compared to tick cell inoculum or tick transmission. Tick cell inoculum and tick transmission caused a drop in tick infection acquisition rates compared to infection rates in ticks fed on deer receiving macrophage inoculum. Independent of deer or dogs, IgG antibody response was higher in animals receiving macrophage inoculum against macrophage-derived Ehrlichia antigens, while it was significantly lower in the same animals against tick cell-derived Ehrlichia antigens. Deer infected with tick cell inoculum and tick transmission caused a higher antibody response to tick cell cultured bacterial antigens compared to the antibody response for macrophage cultured antigens for the same animals. The data demonstrate that the host cell-specific E. chaffeensis protein expression influences rickettsemia in a host and its acquisition by ticks. The data also reveal that tick cell-derived inoculum is similar to tick transmission with reduced rickettsemia, IgG response and tick acquisition of E. chaffeensis.

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.

TLR2 and Nod2 mediate resistance or susceptibility to fatal intracellular Ehrlichia infection in murine models of ehrlichiosis

Our murine models of human monocytic ehrlichiosis (HME) have shown that severe and fatal ehrlichiosis is due to generation of pathogenic T cell responses causing immunopathology and multi-organ failure. However, the early events in the liver, the main site of infection, are not well understood. In this study, we examined the liver transcriptome during the course of lethal and nonlethal infections caused by Ixodes ovatus Ehrlichia and Ehrlichia muris, respectively. On day 3 post-infection (p.i.), although most host genes were down regulated in the two groups of infected mice compared to naïve counterparts, lethal infection induced significantly higher expression of caspase 1, caspase 4, nucleotide binding oligomerization domain-containing proteins (Nod1), tumor necrosis factor-alpha, interleukin 10, and CCL7 compared to nonlethal infection. On day 7 p.i., lethal infection induced highly significant upregulation of type-1 interferon, several inflammatory cytokines and chemokines, which was associated with increased expression levels of Toll-like receptor-2 (TLR2), Nod2, MyD88, nuclear factor-kappa B (NF-kB), Caspase 4, NLRP1, NLRP12, Pycard, and IL-1β, suggesting enhanced TLR signals and inflammasomes activation. We next evaluated the participation of TLR2 and Nod2 in the host response during lethal Ehrlichia infection. Although lack of TLR2 impaired bacterial elimination and increased tissue necrosis, Nod2 deficiency attenuated pathology and enhanced bacterial clearance, which correlated with increased interferon-γ and interleukin-10 levels and a decreased frequency of pathogenic CD8(+) T cells in response to lethal infection. Thus, these data indicate that Nod2, but not TLR2, contributes to susceptibility to severe Ehrlichia-induced shock. Together, our studies provide, for the first time, insight into the diversity of host factors and novel molecular pathogenic mechanisms that may contribute to severe HME.

Molecular basis of antibody mediated immunity against Ehrlichia chaffeensis involves species-specific linear epitopes in tandem repeat proteins

Humoral immune mechanisms are an important component of protective immunity to Ehrlichia species. However, the molecular basis of antibody mediated immunity is not completely defined, and the role of most molecularly characterized major immunoreactive proteins is unknown. In previous studies, we mapped major species-specific continuous epitopes in three surface exposed and secreted tandem repeat proteins (TRP32, TRP47 and TRP120). In this study, we report that protection is provided by antibodies against these molecularly defined TRP epitopes using in vitro and in vivo models. Protection was demonstrated in vitro after prophylactic and therapeutic administration of epitope-specific anti-TRP antibodies, suggesting that the protective mechanisms involve extracellular and intracellular antibody-mediated effects. In vivo passive transfer of individual epitope-specific TRP sera significantly reduced the ehrlichial load and splenomegaly, and protected mice against lethal infection. Moreover, the combination of antibodies to all three TRPs provided enhanced reduction in ehrlichial load similar to that of Ehrlichia chaffeensis immune sera. IgG1 was the predominant antibody isotype in the epitope-specific TRP mouse sera. These results demonstrate that antibodies against linear epitopes in TRP32, TRP47 and TRP120 are protective during E. chaffeensis infection and involves extracellular and intracellular antibody-mediated mechanisms.

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.

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.

Natural history of Ehrlichia chaffeensis: vertebrate hosts and tick vectors from the United States and evidence for endemic transmission in other countries

Ehrlichia chaffeensis, an intracellular gram-negative zoonotic bacterium, is the causative agent of human monocytotropic ehrlichiosis (HME). In humans, the disease can range from a mild, non-specific illness with few to no clinical signs to a moderately severe to fatal disease, especially those with compromised immune systems. E. chaffeensis is maintained in a complex cycle involving white-tailed deer (WTD; Odocoileus virginianus) as a primary reservoir and the lone star tick (LST; Amblyomma americanum) as a primary vector. Numerous other species are naturally exposed to E. chaffeensis and disease has been documented in some domestic animals and wildlife including domestic dogs and ring-tailed lemurs. The organism has been found throughout the natural range of the LST and as the tick continues to expand its range, the geographic range of risk for E. chaffeensis infections will likely continue to expand. Recent data have indicated that E. chaffeensis, or a closely related organism, has been found in many species of ticks and vertebrate hosts in numerous countries.

Diminished hematopoietic activity associated with alterations in innate and adaptive immunity in a mouse model of human monocytic ehrlichiosis

Human monocytic ehrlichiosis (HME) is a tick-borne disease caused by Ehrlichia chaffeensis. Patients exhibit diagnostically important hematological changes, including anemia and thrombocytopenia, although the basis of the abnormalities is unknown. To begin to understand these changes, we used a mouse model of ehrlichiosis to determine whether the observed hematological changes induced by infection are associated with altered hematopoietic activity. Infection with Ehrlichia muris, a pathogen closely related to E. chaffeensis, resulted in anemia, thrombocytopenia, and a marked reduction in bone marrow cellularity. CFU assays, conducted on days 10 and 15 postinfection, revealed a striking decrease in multipotential myeloid and erythroid progenitors. These changes were accompanied by an increase in the frequency of immature granulocytes in the bone marrow and a decrease in the frequency of B lymphocytes. Equally striking changes were observed in spleen cellularity and architecture, and infected mice exhibited extensive extramedullary hematopoiesis. Splenomegaly, a characteristic feature of E. muris infection, was associated with an expanded and disorganized marginal zone and a nearly 66-fold increase in the level of Ter119(+) erythroid cells, indicative of splenic erythropoiesis. We hypothesize that inflammation associated with ehrlichia infection suppresses bone marrow function, induces the emigration of B cells, and establishes hematopoietic activity in the spleen. We propose that these changes, which may be essential for providing the innate and acquired immune cells to fight infection, are also responsible in part for blood cytopenias and other clinical features of HME.

Presence of a novel Ehrlichia sp. in Ixodes granulatus found in Okinawa, Japan

Ehrlichia-specific DNA fragments of Ehrlichia omp-1 and groEL genes were found in two I. granulatus ticks which had been collected from wild small mammals in a subtropical zone in Japan. The DNA sequences of groEL and 16SrDNA of the suspected Ehrlichia were clustered into a group of E. chaffeensis, E. muris, and Ehrlichia sp. HF565 found in I. ovatus, but were distinctly different. Therefore the Ehrlichia strain was designated as a novel Ehrlichia sp. 360. The Ehrlichia sp. 360 was detected in I. granulatus but not in any other ticks. This suggests that I. granulatus is a probable vector of Ehrlichia sp. 360 in Japan.

Liver transcriptome profiles associated with strain-specific Ehrlichia chaffeensis-induced hepatitis in SCID mice

Infection of humans with Ehrlichia chaffeensis, the etiologic agent of human monocytic ehrlichiosis, can cause hepatitis of various levels of severity. When the three human isolates of E. chaffeensis, each belonging to a different genogroup, are inoculated into severe combined immunodeficiency mice, the order of severity of clinical signs and bacterial burden detected in the liver is as follows (from greatest to least severity and highest to lowest burden): strain Wakulla, followed by strain Liberty, followed by strain Arkansas. In this article, we used microarray analysis to define transcriptional profiles characteristic of the histopathological features in the mouse liver. Cytokine and chemokine profiles and their receptor profiles were strikingly different among the three strains of E. chaffeensis: gamma interferon, CCL5, CXCL1, CXCL2, CXCL7, CXCL9, interleukin 2 receptor gamma (IL2Rgamma), IL21R, CCR2, and CXCR6 were highly upregulated with strain Arkansas; and tumor necrosis factor (TNF), CCL2, CCL3, CCL5, CCL6, CCL12, CCL20, CXCL2, CXCL7, CXCL9, CXCL13, TNF receptor superfamily 9 (TNFRSF9), TNFRSF13beta, IL1R2, IL2Rgamma, IL20Rbeta, IL21R, CCR1, CCR2, and CXCR4 were highly upregulated with strain Wakulla. With strain Liberty, only CXCL13 was highly upregulated, and IL13Ralpha2 was downregulated. In livers infected with the Arkansas strain, monocytes/macrophages and NK cells were enriched in the granulomas and an increase in NK cell marker mRNAs was detected. Livers infected with the Wakulla strain displayed infiltration of significantly more neutrophils and an increase in neutrophil marker mRNAs. Genes commonly upregulated in liver tissue infected with the three strains are other host innate immune and inflammatory response genes, including those encoding several acute-phase proteins. Genes downregulated commonly are related to host physiologic functions. The results suggest that marked modulation of host cytokine and chemokine profiles by E. chaffeensis strains underlies the distinct host liver disease.

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.

Isolation and molecular detection of Ehrlichia from vertebrate animals

Human monocytic ehrlichiosis (HME), caused by Ehrlichia chaffeensis, was first recognized in 1986. Infection with this pathogen can be fatal in immune compromised and elderly humans. E. chaffeensis can also infect dogs and several wild animals. The clinical symptoms of HME include fever, headache, malaise, myalgia, confusion, rash, lymphadenopathy, and nausea. White-tailed deer serve as the major reservoir host for the natural maintenance of E. chaffeensis. E. canis is primarily responsible for the canine monocytic ehrlichiosis and is endemic throughout the world. It has a significant impact on the health of dogs. The isolation and growth of Ehrlichia species from vertebrate host samples is difficult and time consuming. In this unit, methods to recover E. chaffeensis and E. canis from infected blood samples collected from dogs, deer, and human patients are described. PCR and RT-PCR methods for sensitive detection of Ehrlichia infection are also discussed.

Proteomic analysis of and immune responses to Ehrlichia chaffeensis lipoproteins

Ehrlichia chaffeensis is an obligately intracellular gram-negative bacterium and is the etiologic agent of human monocytic ehrlichiosis (HME). Although E. chaffeensis induces the generation of several cytokines and chemokines by leukocytes, E. chaffeensis lacks lipopolysaccharide and peptidoglycan. Bioinfomatic analysis of the E. chaffeensis genome, however, predicted genes encoding 15 lipoproteins and 3 posttranslational lipoprotein-processing enzymes. The present study showed that by use of multidimensional liquid chromatography followed by tandem mass spectrometry, all predicted lipoproteins as well as lipoprotein-processing enzymes were expressed by E. chaffeensis cultured in the human promyelocytic leukemia cell line HL-60. Consistent with this observation, a signal peptidase II inhibitor, globomycin, was found to inhibit E. chaffeensis infection and lipoprotein processing in HL-60 cell culture. To study in vivo E. chaffeensis lipoprotein expression and host immune responses to E. chaffeensis lipoproteins, 13 E. chaffeensis lipoprotein genes were cloned into a mammalian expression vector. When the DNA constructs were inoculated into naïve dogs, or when dogs were infected with E. chaffeensis, the animals developed delayed-type hypersensitivity reactions at cutaneous sites of the DNA construct deposition and serum antibodies to these lipoproteins. This is the first demonstration of lipoprotein expression and elicitation of immune responses by a member of the order Rickettsiales. Multiple lipoproteins expressed by E. chaffeensis in vitro and in vivo may play key roles in pathogenesis and immune responses in HME.

Molecular Pathology of Rickettsial Lung Infections

Rickettsial infections of humans comprise a diverse group of infections caused by pathogens that are obligate intracellular bacteria with a genetic relationship, including the genera Rickettsia, Orientia, Ehrlichia, and Anaplasma. The host cells of these pathogens largely belie the systemic clinical manifestations, because Rickettsia and Orientia infect endothelial cells, and Ehrlichia and Anaplasma infect circulating leukocytes (monocytes and neutrophils, respectively). Thus, the predominant manifestations (fever, headache, myalgia, with or without rash) do not usually focus attention on the respiratory system; however, the underlying pathogenesis of these infections involves degrees of vascular compromise either by direct injury and inflammation or by the action of vasoactive proinflammatory molecules such as cytokines, chemokines, and prostaglandins. Given that the lung possesses the largest vascular bed in the human body, it is not surprising that pulmonary involvement is periodically identified and, when severely affected, is considered a potentially life-threatening complication.1,2

Increased pathogenesis and inflammation of airways from respiratory syncytial virus infection in T cell deficient nude mice

Respiratory syncytial virus (RSV) infection is ubiquitous and leads to various outcomes between immunocompetent and immunocompromised individuals. This study aimed to compare RSV infection and inflammatory responses between immunocompetent BALB/c mice and immunodeficient nude mice. RSV titers in both infected BALB/c mice and nude mice peaked on the third day post-inoculation, but the nude mice had longer lasting and higher levels of viral replication. RSV infection induced a more severe grade of pulmonary histopathology and larger numbers of leukocytes in airways of nude mice than that of BALB/c mice. RSV infection increased pulmonary macrophages and natural killer (NK) cells in both strains of mice. Furthermore, infected nude mice had larger numbers of pulmonary macrophages and NK cells than infected BALB/c mice. Whereas the RSV infected BALB/c mice secreted more tumor necrosis factor -alpha (TNF-alpha), interleukin-12 (IL-12), interferon-gamma (IFN-gamma) and IL-10 than control BALB/c mice, the infected nude mice had higher levels of TNF-alpha, IL-12 and IL-10 than the infected BALB/c mice. The inflammation induced by RSV infection did not correspond with the immune response of T cells. Macrophages and NK cells were potent immunocytes and inflammatory cells in RSV infection especially when T lymphocytes were deficient. Therefore, nude mice may be a good model for severe and persistent RSV infection in immunocompromised hosts.

Virulence potential of Ehrlichia chaffeensis strains of distinct genome sequences

Human monocytic ehrlichiosis, one of the most frequent life-threatening tick-borne zoonoses, is caused by Ehrlichia chaffeensis that lacks endotoxin and peptidoglycan. While sequence polymorphisms in several genes in E. chaffeensis strains have been reported, global genomic divergence and biological differences among strains are unknown. The objectives of the present study were to compare the genome sequences of strains of E. chaffeensis and to examine the virulence potentials of the strains with defined genome sequences. Genomic DNA was extracted from purified E. chaffeensis strains Wakulla and Liberty, and comparative genome hybridization was performed using a densely tiled microarray of 147,027 chromosome positions of the E. chaffeensis strain Arkansas genome. The results revealed that 4,663 and 5,325 positions in the chromosomes of strains Wakulla and Liberty, respectively, were different from those in the chromosome of strain Arkansas, including three common major polymorphic chromosomal regions. Of various functional categories, the differences were most concentrated in genes predicted to encode cell envelope proteins. Of all the open reading frames (ORFs), 21 omp-1 (p28 gene) paralogs, nine genes encoding hypothetical proteins, two genes encoding ankyrin repeat proteins, and hemE contained the most differences. Several highly polymorphic ORFs were confirmed by sequencing. When the E. chaffeensis strains were inoculated into severe combined immunodeficiency mice, the order of the severity of clinical signs and the bacterial burden detected in mice was Wakulla > Liberty > Arkansas. Severe diffuse inflammation and granulomatous inflammation were evident in the livers of mice infected with strains Wakulla and Arkansas, respectively, but not in the livers of mice infected with strain Liberty. These results revealed distinct virulence phenotypes of E. chaffeensis strains with defined genome sequences.

Fatal recall responses mediated by CD8 T cells during intracellular bacterial challenge infection

The roles(s) of CD8 T cells during infections by intracellular bacteria that reside in host cell endocytic compartments are not well understood. Our previous studies in a mouse model of human monocytotropic ehrlichiosis indicated that CD8 T cells are not essential for immunity. However, we have observed an unexpected role for these cells during challenge infection. Although immunocompetent mice cleared a primary low-dose (nonfatal) Ixodes ovatus ehrlichia infection, a secondary low-dose challenge infection resulted in fatal disease and loss of control of infection. The outcome was CD8-dependent, because CD8-deficient mice survived secondary low-dose challenge infection. Moreover, effector and/or memory phenotype CD8 T cells were responsible, because adoptive transfer of purified CD44(high) CD8 T cells to naive mice induced fatal responses following a primary low-dose infection. The fatal responses were perforin- and Fas ligand-independent, and were associated with high serum concentrations of TNF-alpha and CCL2, and low levels of IL-10. Accordingly, blockade of either TNF-alpha or CCL2 ameliorated fatal recall responses, and in vitro coculture of memory CD8 T cells and Ixodes ovatus ehrlichia-infected peritoneal exudate cells resulted in substantial increases in TNF-alpha and CCL2. Thus, during monocytotropic ehrlichiosis, inflammatory cytokine production, by CD8 T cells and/or other host cells, can trigger chemokine-dependent disease. These findings highlight a novel role for CD8 T cells, and reveal that live vaccines for intracellular bacteria can, under some conditions, induce undesirable consequences.

An intradermal environment promotes a protective type-1 response against lethal systemic monocytotropic ehrlichial infection

Immune responses against monocytotropic ehrlichiosis during infection with a strain of Ehrlichia from Ixodes ovatus (IOE) were evaluated using a model that closely reproduces the pathology and immunity associated with tick-transmitted human monocytotropic ehrlichiosis. C57BL/6 mice were inoculated intradermally or intraperitoneally with high-dose highly virulent IOE or intraperitoneally with mildly virulent Ehrlichia muris. Intradermal (i.d.) infection with IOE established mild, self-limited disease associated with minimal hepatic apoptosis, and all mice survived past 30 days. Intraperitoneal (i.p.) infection with IOE resulted in acute, severe toxic shock-like syndrome and severe multifocal hepatic apoptosis and necrosis, and all mice succumbed to disease. Compared to i.p. infection with IOE, intradermally infected mice had a 100- to 1,000-fold lower bacterial load in the spleen with limited dissemination. Compared to mice infected intraperitoneally with IOE, i.d. infection stimulated a stronger protective type-1 cell-mediated response on day 7 of infection, characterized by increased percentages of both CD4+ and CD8+ splenic T cells, generation of a greater number of IOE-specific, gamma interferon-producing CD4+ Th1 cells, and higher levels of tumor necrosis factor (TNF-alpha) in the spleen but lower concentrations of serum TNF-alpha and interleukin-10. These data suggest that under the conditions of natural route of challenge (i.e., i.d. inoculation), the immune response has the capacity to confer complete protection against monocytotropic ehrlichiosis, which is associated with a strong cell-mediated type-1 response and decreased systemic production of pro- and anti-inflammatory cytokines.

Balancing protective immunity and immunopathology: a unifying model of monocytotropic ehrlichiosis

Interactions among pathogens, antigen-presenting cells (APCs) and lymphocytes are critical in maintaining balance in the daily challenges to the immune system. Monocytotropic ehrlichiosis, caused by Ehrlichia chaffeensis, is a multisystem inflammatory ailment. A complex interaction between Ehrlichia and the immune systems of a number of mammalian hosts, in human disease and animal models, has been described. The presence of an overwhelming ehrlichial infection in immunocompromised individuals suggests that severe tissue damage is most likely due to direct bacterial effect. However, clinical and experimental observations indicate that this is an oversimplified concept. First, immunocompetent patients with severe ehrlichiosis have a low bacterial burden. Second, severe and fatal murine ehrlichiosis in immunocompetent animals, which mimics human disease, is associated with a low bacterial burden in different organs and late systemic and local overproduction of TNF-alpha by T cells. In order to counterbalance overshooting immune responses, T cells and APCs secrete anti-inflammatory cytokines that are key for maintaining a healthy balance between protection and immunopathology. CD4+ T cell-mediated immunity and antibody responses of a Th1 phenotype play critical roles in protection against Ehrlichia. Of particular importance for the generation of protective immunity is the induction of activation programs in APCs directly by pathogens or by T cell-derived factors. In this study, we consider the roles of innate and adaptive immune responses in terms of protection from severe ehrlichiosis and their potential roles in immunopathology.

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.

Anaplasma and Ehrlichia Infection

Human and animal infections by Anaplasmataceae are increasingly recognized as important and potentially fatal arthropod-transmitted diseases. Since the first recognition and implementation of diagnostic methods for human infection by Ehrlichia chaffeensis and Anaplasma phagocytophilum, the incidence of infections has linearly increased. Moreover, diagnostic and epidemiological testing indicates that "ehrlichia" infections are globally distributed and suggests that additional agents of human and veterinary "ehrlichiosis" will be identified. With increasing disease identification has come recognition that the infections can be severe, with approximately one-half of patients requiring hospitalization for complications including respiratory distress, myocarditis, neurological complications, hepatitis, a septic or toxic shock-like disease, opportunistic infections, and death in 0.5 to 3.0%. An understanding of the diseases, pathophysiology, pathogenesis, control, and management will best be developed through fundamental investigations. Advances in comprehension as to the separate contributions of bacteria and host are crucial since most data now suggest that alterations in host neutrophil function and protection from innate and adaptive immunity also contribute to disease manifestations. It is reasonable to operate from the hypothesis that these host cell functional changes ultimately benefit bacterial survival while inadvertently eliciting clinical disease in poorly adapted hosts. A firmer basis for the scientific understanding of Anaplasmataceae biology will allow logical and rational approaches toward infection control, prevention, and treatment.

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.

Susceptibility of mice (Mus musculus) to repeated infestation with Amblyomma americanum (Acari: Ixodidae) ticks

Laboratory mice, Mus musculus (L.), BALB/c strain, were assessed for their ability to develop resistance to repeated infestation by Amblyomma americanum (L.) ticks. Mice were infested five consecutive times with A. americanum nymphs. No decrease in tick viability was seen after five infestations, suggesting that BALB/c mice do not develop immune-mediated resistance to A. americanum. In contrast, tick viability was significantly reduced in the second infestation of a New Zealand White rabbit, a laboratory animal known to develop resistance to A. americanum.

Histologic, serologic, and molecular analysis of persistent ehrlichiosis in a murine model

Human monocytotropic ehrlichiosis caused by Ehrlichia chaffeensis was reported in 1987. An animal model to study acute fatal ehrlichiosis in mice that has been developed closely resembles the fatal form of human monocytotropic ehrlichiosis. However, animal models for persistent infection in the genus Ehrlichia in immunocompetent mice have not been characterized. We report the histopathological progression of Ehrlichia muris infection in immunocompetent mice (AKR and C57BL/6 strains) correlated with their antibody response determined by indirect immunofluorescence and Western immunoblotting, and the distribution and quantity of the ehrlichial load by immunohistochemistry, polymerase chain reaction (PCR), and real-time PCR in lungs, liver, and spleen. Mild to moderate correlation was observed between histopathological grading in these organs and relative ehrlichial loads. The highest ehrlichial loads were present between days 4 and 14 after infection. E. muris was detected in tissues examined up to 150 days after infection by real-time PCR. Analysis of the serological response revealed several immunodominant antigens, including 200-, 180-, 100-, 73/75-, 45-, and 28-kd proteins. In conclusion, we have provided for the first time a complete histopathological, serological, immunohistochemical, and quantitative analysis of an animal model for the study of persistent ehrlichial infection.

Production of IFN-gamma by CD4 T cells is essential for resolving ehrlichia infection

To address the role of cellular immunity during ehrlichia infection, we have used a newly described model of monocytic ehrlichiosis that results from infection of mice by an ehrlichia that was isolated from an Ixodes ovatus tick (Ixodes ovatus ehrlichia, IOE). Immunocompetent C57BL/6 and BALB/c mice exhibited a dose-dependent susceptibility to IOE infection. Mice infected with a high dose inoculum ( approximately 1000 organisms) exhibited pronounced thrombocytopenia, lymphopenia, anemia, and morbidity within 12 days postinfection. Infection was associated with bacterial colonization of a number of tissues. In contrast, mice infected with a low dose inoculum ( approximately 100 organisms) exhibited only transient disease and were able to resolve the infection. SCID mice were highly susceptible to low-dose infection, indicating that adaptive immunity was required. Resistance to sublethal challenge in both C57BL/6 and BALB/c mice was CD4-, but not CD8-, dependent and required IL-12p40-dependent cytokines, IFN-gamma, and TNF-alpha, but not IL-4. CD4 T cells purified from infected mice proliferated in vitro in response to IOE Ags. T cell proliferation was associated with production of IFN-gamma, and the production of this cytokine by CD4 T cells rescued IFN-gamma-deficient mice from fatal infection. Exogenous IFN-gamma was capable of inducing microbiocidal activity in infected macrophages. The data suggest that classical immune mechanisms involving CD4 cells and type 1 cytokines are responsible for macrophage activation and for elimination of this intracellular bacterial pathogen.

Invasion of the central nervous system by intracellular bacteria

Infection of the central nervous system (CNS) is a severe and frequently fatal event during the course of many diseases caused by microbes with predominantly intracellular life cycles. Examples of these include the facultative intracellular bacteria Listeria monocytogenes, Mycobacterium tuberculosis, and Brucella and Salmonella spp. and obligate intracellular microbes of the Rickettsiaceae family and Tropheryma whipplei. Unfortunately, the mechanisms used by intracellular bacterial pathogens to enter the CNS are less well known than those used by bacterial pathogens with an extracellular life cycle. The goal of this review is to elaborate on the means by which intracellular bacterial pathogens establish infection within the CNS. This review encompasses the clinical and pathological findings that pertain to the CNS infection in humans and includes experimental data from animal models that illuminate how these microbes enter the CNS. Recent experimental data showing that L. monocytogenes can invade the CNS by more than one mechanism make it a useful model for discussing the various routes for neuroinvasion used by intracellular bacterial pathogens.

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

Ehrlichia species can cause life-threatening infections or chronic persistent infections. Mechanisms of protective immunity were examined in an Ehrlichia muris mouse model of monocytotropic ehrlichiosis. C57BL/6 mice possessed strong genetic resistance to E. muris of an undetermined mechanism. CD8 T lymphocytes were particularly important, as revealed by 81% fatalities for E. muris-infected, major histocompatibility complex class I gene knockout mice compared with no deaths for wild-type C3H mice. Moreover, 80% of C3H mice depleted of CD8 and CD4 cells died of E. muris infection compared with only 44% of CD4 cell-depleted mice. CD8 T lymphocytes were demonstrated for the first time in an Ehrlichia infection to exhibit cytotoxic T-lymphocyte activity against Ehrlichia-infected target cells. Both gamma interferon and tumor necrosis factor were shown to play synergistic roles in protective immunity in vivo for the first time, as demonstrated by 75% fatalities when both cytokines were neutralized compared with minimal mortality when they were depleted separately. Passive transfer of antibodies, but not Fab fragments, to E. muris protected C3H/SCID mice against lethal infection. The mechanism of increased susceptibility (22% lethality) of C57BL/6 major histocompatibility complex class II gene knockout mice and CD4 cell-depleted C3H mice (i.e., through a gamma interferon or antibody mechanism), as well as the more important role of CD8 T lymphocytes (in the form of cytotoxic T-lymphocyte activity and/or gamma interferon production), remains to be elucidated. Protective immunity against monocytotropic E. muris is mediated by a combination of CD8 and CD4 T lymphocytes, gamma interferon, tumor necrosis factor alpha, and antibodies.

Delayed clearance of Ehrlichia chaffeensis infection in CD4+ T-cell knockout mice

Human monocytic ehrlichiosis is an emerging tick-borne disease caused by the rickettsia Ehrlichia chaffeensis. To examine the role of helper T cells in host resistance to this macrophage-tropic bacterium, we assessed E. chaffeensis infections in three mouse strains with differing functional levels of helper T cells. Wild-type, C57BL/6J mice resolved infections in approximately 2 weeks. Major histocompatibility complex class II (MHCII) knockout, B6.129-Abb(tm1) mice lacking helper T cells developed persistent infections that were not resolved even after several months. CD4+ T-cell-deficient, B6.129S6-Cd4(tm1Knw) mice cleared the infection, but the clearance took 2 weeks longer than it did for wild-type mice. C57BL/6J mice resolved infection more rapidly following a second experimental challenge, but B6.129S6-Cd4(tm1Knw) mice did not. The B6.129S6-Cd4(tm1Knw) mice also developed active E. chaffeensis-specific immunoglobulin G responses that were slightly lower in concentration and slower to develop than that observed in C57BL/6J mice. E. chaffeensis-specific cytotoxic T cells were not detected following a single bacterial challenge in any mouse strain, including wild-type C57BL/6J mice. However, the cytotoxic T-cell activity developed in all three mouse strains, including the MHCII and CD4+ T-cell knockouts, when challenged with a second E. chaffeensis infection. The data reported here suggest that the cell-mediated immunity, orchestrated by CD4+ T cells is critical for conferring rapid clearance of E. chaffeensis.

Overproduction of TNF-α by CD8+Type 1 Cells and Down-Regulation of IFN-γ Production by CD4+Th1 Cells Contribute to Toxic Shock-Like Syndrome in an Animal Model of Fatal Monocytotropic Ehrlichiosis

Human monocytotropic ehrlichiosis (HME) is an emerging, life-threatening, infectious disease caused by Ehrlichia chaffeensis, an obligate intracellular bacterium that lacks cell wall LPS. We have previously developed an animal model of severe HME using a strain of Ehrlichia isolated from Ixodes ovatus ticks (IOE). To understand the basis of susceptibility to severe monocytotropic ehrlichiosis, we compared low and high doses of the highly virulent IOE strain and the less virulent Ehrlichia muris strain that are closely related to E. chaffeensis in C57BL/6 mice. Lethal infections caused by high or low doses of IOE were accompanied by extensive liver damage, extremely elevated levels of TNF-alpha in the serum, high frequency of Ehrlichia-specific, TNF-alpha-producing CD8(+) T cells in the spleen, decreased Ehrlicha-specific CD4(+) T cell proliferation, low IL-12 levels in the spleen, and a 40-fold decrease in the number of IFN-gamma-producing CD4(+) Th1 cells. All groups contained negligible numbers of IL-4-producing cells in the spleen. Transfer of Ehrlichia-specific polyclonal Abs and IFN-gamma-producing Ehrlichia-specific CD4(+) and CD8(+) type 1 cells protected naive mice against lethal IOE challenge. Interestingly, infection with high dose E. muris provided protection against rechallenge with a lethal dose of IOE. Cross-protection was associated with substantial expansion of IFN-gamma-producing CD4(+) and CD8(+) cells, but not TNF-alpha-producing CD8(+) T cells, a high titer of IgG2a, and a low serum level of TNF-alpha. In conclusion, uncontrolled TNF-alpha production by CD8(+) T cells together with a weak CD4(+) Th1 cell response are associated with immunopathology and failure to clear IOE in the fatal model of HME.

Ehrlichia chaffeensis: a prevalent, life-threatening, emerging pathogen

Ehrlichia chaffeensis are small, obligately intracellular, endosomal bacteria with tropism for macrophages. Persistent infection in reservoir white-tailed deer is transmitted by lone star ticks. Flu-like illness can progress to severe multisystem disease with toxic shock-like syndrome, meningitis, or ARDS. The case-fatality rate is 2.7%. Leukopenia and thrombocytopenia are diagnostically useful. Granulomas are associated with control of the infection. Ehrlichial proteins and glycoproteins have been sequenced and expressed for diagnostic serology and vaccine development. Mouse models (mild disease and persistent infection with E. muris and fatal monocytotropic ehrlichiosis with a Japanese tick isolate) revealed that CD4 and CD8 T type 1 lymphocyte responses, IFN-gamma, TNF-alpha, and antibodies play roles in protective immunity, while a weak CD4 T-helper response, overproduction of TNF-alpha, and very high IL-10 are associated with toxic shock-like mortality. Protection against fatal ehrlichiosis was achieved by prior infection with low virulence E. muris. Acute clinical diagnosis is difficult except by PCR. Response to doxycycline is dramatic.

Evaluation of transovarial transmission and transmissibility of Ehrlichia chaffeensis (Rickettsiales: Anaplasmataceae) in Amblyomma americanum (Acari: Ixodidae)

It has long been assumed that Ehrlichia chaffeensis (Anderson, Dawson & Wilson), is not transmitted transovarially in the lone star tick vector Amblyomma americanum (L.). To test this hypothesis, three beagle dogs, Canis familiaris (L.) (Carnivora: Canidae), were subcutaneously infected with E. chaffeensis (Arkansas strain). Uninfected nymphal lone star ticks were placed on the infected dogs and allowed to feed to repletion. These nymphal ticks were allowed to molt, and five of five adult female ticks sampled were confirmed to be infected by polymerase chain reaction (PCR). Forty infected adult ticks, the majority of which were female with several males included to stimulate feeding, were then placed on two uninfected dogs. Fourteen females were removed early and the rest were allowed to feed to repletion. After feeding to repletion, the six remaining females detached and two of these females were tested preoviposition, whereas the remaining four were allowed to lay eggs. All six of these fully engorged females tested negative by PCR. The egg clutches laid by four engorged adult females, and the larvae that hatched from these eggs along with larvae from a previously untested egg clutch, were shown to be uninfected by PCR. This seems to support the long-held assumption of lack of transovarial transmission. Also of interest, we found that these PCR-confirmed infected adult ticks reared in the laboratory did not transmit E. chaffeensis to uninfected dogs.

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.

Mechanisms of humoral immunity during Ehrlichia chaffeensis infection

Our laboratory has been investigating the mechanisms of host defense during Ehrlichia chaffeensis infection in the mouse. Although major roles are clearly played by T cells, we found that antibodies could also control infection in both normal and immunocompromised SCID mice, and could protect the latter from lethal infection. Antibodies are not generally effective during such intracellular infections, so we would like to understand exactly how antibodies can mediate immunity in this model. We have found that much of the humoral immune response is directed at the bacterial outer membrane proteins (OMPs), and that highly effective OMP antibodies (mostly IgG2a) exhibited picomolar affinities and very long binding half-lives. These antibodies, which could mediate bacterial clearance from tissues as early as 24 hours after administration, require host Fc receptors for their function(s). In contrast, we have failed to find any role for complement or reactive nitrogen intermediates, or for neutrophils, or for NK cells. One possible mechanism is that antibodies or immune complexes trigger microbiocidal activities in infected macrophages that lead to the elimination of bacteria residing inside host macrophages. Alternatively, it is proposed that antibodies opsonize bacteria exposed during intercellular transfer. This notion is supported by studies that have demonstrated the presence of bacteria in the extracellular milieu during infection, and suggests that our understanding of the behavior of the bacterium in the host may be key to our understanding of its susceptibility to antibody-mediated host defenses.

Hepatic pathology in human monocytic ehrlichiosis. Ehrlichia chaffeensis infection

Ehrlichia chaffeensis causes human monocytic ehrlichiosis (HME) that usually includes fever, myalgias, and pancytopenia and, in 80% to 90% of patients, elevations in serum transaminase levels. Thus, the pathology of liver injury was studied in liver tissues from 7 patients with laboratory-confirmed HME. H&E and immunohistochemical stains for E chaffeensis and leukocyte markers were examined. Scattered lobular lymphohistiocytic foci and diffuse lymphohistiocytic infiltration and Kupffer cell hyperplasia with increased phagocytosis frequently were present. Various degrees of liver cell injury and death were observed. Cholestasis was evident in 6 cases, sometimes with bile duct epithelial injury. Rare to abundant E chaffeensis-infected mononuclear cells infiltrating lobules or portal regions or in Kupffer cells were observed in 5 patients. The inflammation was out of proportion to the infection in 6 cases. In the absence of infected hepatocytes or biliary epithelial cells, these findings suggest that host inflammatory or immune responses contribute to the liver injury seen in HME.

Distribution of ehrlichiae in tissues as determined by in-situ hybridization

Specific identification of ehrlichiae in the tissues and determination of their distribution is difficult. In this study, an in-situ hybridization method was developed to detect ehrlichial 16S rRNA in tissue specimens from mice experimentally infected with the HF strain. This strain is closely related to Ehrlichia chaffeensis, the agent of human monocytic ehrlichiosis. HF strain-specific 16S rRNA was detected in endothelial cells and monocyte-macrophages in the liver, lungs, bone marrow, spleen, lymph nodes, and large and small intestinal tissues. The results suggest that the in-situ hybridization method with a digoxigenin-labelled RNA probe specific to ehrlichial 16S rRNA will be useful for post-mortem diagnosis and for the histopathological investigation of ehrlichial infection.

Ehrlichia chaffeensis: a prototypical emerging pathogen

Ehrlichia chaffeensis is an obligately intracellular, tick-transmitted bacterium that is maintained in nature in a cycle involving at least one and perhaps several vertebrate reservoir hosts. The moderate to severe disease caused by E. chaffeensis in humans, first identified in 1986 and reported for more than 1,000 patients through 2000, represents a prototypical "emerging infection." Knowledge of the biology and natural history of E. chaffeensis, and of the epidemiology, clinical features, and laboratory diagnosis of the zoonotic disease it causes (commonly referred to as human monocytic ehrlichiosis [HME]) has expanded considerably in the period since its discovery. In this review, we summarize briefly the current understanding of the microbiology, pathogenesis, and clinical manifestations associated with this pathogen but focus primarily on discussing various ecological factors responsible for the recent recognition of this important and potentially life-threatening tick-borne disease. Perhaps the most pivotal element in the emergence of HME has been the staggering increases in white-tailed deer populations in the eastern United States during the 20th century. This animal serves as a keystone host for all life stages of the principal tick vector (Amblyomma americanum) and is perhaps the most important vertebrate reservoir host for E. chaffeensis. The contributions of other components, including expansion of susceptible human populations, growth and broadening geographical distributions of other potential reservoir species and A. americanum, and improvements in confirmatory diagnostic methods, are also explored.

Antibodies highly effective in SCID mice during infection by the intracellular bacterium Ehrlichia chaffeensis are of picomolar affinity and exhibit preferential epitope and isotype utilization

Although often considered to be ineffective against intracellular bacteria, Abs, in the absence of lymphocytes, have been shown previously to protect SCID mice from lethal infection by the obligate intracellular bacterium Ehrlichia chaffeensis, even when administered well after infection has been established. To identify characteristics of Abs that are critical for host defense during this intracellular infection, a panel of Ehrlichia-specific mAbs was generated and analyzed. Among 100 Abs recovered, 39 recognized an amino-terminal hypervariable region of an outer membrane protein (OMP), demonstrating that the OMPs are both antigenically variable and immunodominant. A subset of 16 representative OMP-specific Abs was further examined to identify characteristics that were essential for in vivo efficacy. The highly effective Abs recognized a linear epitope within the first hypervariable region of OMP-1g. Only IgG were found to be effective, and among the effective IgG, the following hierarchy was observed: IgG2a > IgG3 = IgG2b. The most striking characteristics of the highly effective Abs were their picomolar binding affinities and long binding t(1/2). Thus, although epitope recognition and isotype use may contribute to efficacy, high affinity may be a critical characteristic of Abs that can act effectively during this intracellular bacterial infection.

Severe Ehrlichia chaffeensis infection in a lung transplant recipient: a review of ehrlichiosis in the immunocompromised patient

We describe a case of human ehrlichiosis in a lung transplant recipient and review published reports on ehrlichiosis in immunocompromised patients. Despite early therapy with doxycycline, our patient had unusually severe illness with features of thrombotic thrombocytopenic purpura. Of 23 reported cases of ehrlichiosis in immunocompromised patients, organ failure occurred in all patients and 6 (25%) died.

DNA vaccination with map1 gene followed by protein boost augments protection against challenge with Cowdria ruminantium, the agent of heartwater

A DNA vaccine encoding the immunodominant MAP1 protein of Cowdria ruminantium (Crystal Springs (CS) strain) was shown to partially protect DBA/2 mice against homologous lethal challenge. To enhance the protective capacity of this DNA vaccine, the effects of length of interval between vaccinations and of prime-boost regimes were investigated. Increasing the interval between vaccinations from 2 to 12 weeks did not result in better protection (P=0.900). However, boosting DNA vaccine-primed mice with recombinant MAP1 protein significantly augmented protection on homologous challenge in various trials from 13-27 to 53-67% (P<0.050). The augmented protection by the prime-boost regimen correlated with augmented T(H1) type immune responses that were induced by the DNA vaccine. These responses were characterized by production of IFN-gamma, IL-2 and anti-MAP1 antibodies of predominantly IgG2a isotype, and were critical for protection against C. ruminantium infection. Cytokine analyses were done at 48h after in vitro stimulation of splenocytes with C. ruminantium or control antigens. In contrast, splenocytes of DNA vector control mice produced no cytokines and these mice were fully susceptible to challenge. In addition, DBA/2 mice immunized with the recombinant MAP1 protein without DNA vaccine priming produced non-protective T(H2) type immune responses which were characterized by production of IL-4, IL-5, IL-10 and IgG1 anti-MAP1 antibodies. A second DNA vaccine containing map1 gene from the Mbizi strain of C. ruminantium also delivered by a prime-boost regime, conferred less protection against heterologous challenge. Hence, in developing DNA vaccines against heartwater that contain map1 gene, a prime-boost regimen should be adopted and gene sequence heterogeneity of field isolates should also be considered.