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

RipE expression correlates with high ATP levels in Ehrlichia, which confers resistance during the extracellular stage to facilitate a new cycle of infection

Ehrlichiosis is a potentially life-threatening disease caused by infection with the obligatory intracellular bacteria Ehrlichia species. Ehrlichia japonica infection of mice provides an animal model of ehrlichiosis as it recapitulates full-spectrum and lethal ehrlichiosis in humans. The E. japonica transposon mutant of EHF0962, which encodes a previously uncharacterized hypothetical protein, is attenuated in both infection and virulence in mice. EHF0962 was hence named here as resistance-inducing protein of Ehrlichia (RipE). Using this ΔripE mutant, we studied how RipE protein contributes to Ehrlichia pathogenesis. Ehrlichia species have an intracellular developmental cycle and a brief extracellular stage to initiate a new cycle of infection. Majority of RipE proteins were expressed on the surface of the smaller infectious dense-core stage of bacteria. Extracellular ΔripE E. japonica contained significantly less adenosine triphosphate (ATP) and lost infectivity more rapidly in culture compared with wild-type (WT) E. japonica. Genetic complementation in the ΔripE mutant or overexpression of ripE in WT E. japonica significantly increased bacterial ATP levels, and RipE-overexpressing E. japonica was more virulent in mice than WT E. japonica. RipE is conserved among Ehrlichia species. Immunization of mice with recombinant RipE induced an in vitro infection-neutralizing antibody, significantly prolonged survival time after a lethal dose of E. japonica challenge, and cross-protected mice from infection by Ehrlichia chaffeensis, the agent of human monocytic ehrlichiosis. Our findings shed light on the extracellular stage of Ehrlichia, highlighting the importance of RipE and ATP levels in Ehrlichia for extracellular resistance and the next cycle of infection. Thus, RipE is a critical Ehrlichia protein for infection as such can be a potential vaccine target for ehrlichiosis.

Ehrlichia effector TRP120 manipulates bacteremia to facilitate tick acquisition

Ehrlichia species are obligatory intracellular bacteria that cause a potentially fatal disease, human ehrlichiosis. The biomolecular mechanisms of tick acquisition of Ehrlichia and transmission between ticks and mammals are poorly understood. Ehrlichia japonica infection of mice recapitulates the full spectrum of human ehrlichiosis. We compared the pathogenicity and host acquisition of wild-type E. japonica with an isogenic transposon mutant of E. japonica that lacks tandem repeat protein 120 (TRP120) (ΔTRP120). Both wild-type and ΔTRP120 E. japonica proliferated similarly in cultures of mammalian and tick cells. Upon inoculation into mice, both wild-type and ΔTRP120 E. japonica multiplied to high levels in various tissues, with similar clinical chemistry and hematologic changes, proinflammatory cytokine induction, and fatal disease. However, the blood levels of ΔTRP120 E. japonica were almost undetectable within 24 h, whereas the levels of the wild type increased exponentially. Greater than 90% of TRP120 was released from infected cells into the culture medium. Mouse blood monocytes exposed to native TRP120 from culture supernatants showed significantly reduced cell surface expression of the transmigration-related markers Ly6C and CD11b. Larval ticks attached to mice infected with either wild-type or ΔTRP120 E. japonica imbibed similar amounts of blood and subsequently molted to nymphs at similar rates. However, unlike wild-type E. japonica, the ΔTRP120 mutant was minimally acquired by larval ticks and subsequent molted nymphs and, thus, failed to transmit to naïve mice. Thus, TRP120 is required for bacteremia but not disease. These findings suggest a novel mechanism whereby an obligatory intracellular bacterium manipulates infected blood monocytes to sustain the tick-mammal transmission cycle.

IMPORTANCE

Effective prevention of tick-borne diseases such as human ehrlichiosis requires an understanding of how disease-causing organisms are acquired. Ehrlichia species are intracellular bacteria that require infection of both mammals and ticks, involving cycles of transmission between them. Mouse models of ehrlichiosis and tick-mouse transmission can advance our fundamental understanding of the pathogenesis and prevention of ehrlichiosis. Herein, a mutant of Ehrlichia japonica was used to investigate the role of a single Ehrlichia factor, named tandem repeat protein 120 (TRP120), in infection of mammalian and tick cells in culture, infection and disease progression in mice, and tick acquisition of E. japonica from infected mice. Our results suggest that TRP120 is necessary only for Ehrlichia proliferation in circulating mouse blood and ongoing bacteremia to permit Ehrlichia acquisition by ticks. This study provides new insights into the importance of bacterial factors in regulating bacteremia, which may facilitate tick acquisition of pathogens.

Zoonotic Bacterial Infections Triggering Cytokine Storm Syndrome

Zoonotic infections can result in life-threatening complications that can manifest with hemophagocytic lymphohistiocytosis (HLH)/cytokine storm syndrome (CSS). Bacteria constitute the largest group of zoonotic infection-related HLH cases. The growing list of zoonotic bacterial infections associated with HLH/CSS include Brucella spp., Rickettsia spp., Ehrlichia, Coxiella burnetii, Mycobacterium spp., and Bartonella spp. Patients most commonly present with fever, cytopenias, hepatosplenomegaly, myalgias, and less frequently with rash, jaundice, and lymphadenopathy.

Effects of Doxycycline Treatment on Hematological Parameters, Viscosity, and Cytokines in Canine Monocytic Ehrlichiosis

This study aimed to analyze the hematological parameters, blood viscosity, and cytokines of dogs infected by Ehrlichia canis untreated and treated with doxycycline. Initially, 47 dogs were examined, and 36 were suspected to have canine monocytic ehrlichiosis, which was confirmed through molecular polymerase chain reaction tests. This study consisted of 25 dogs, with 11 being healthy and 14 testing positive for E. canis. The dogs were divided into experimental groups based on their test results, including a control group of healthy dogs (N = 11), a group of infected dogs without treatment (N = 7), and a group of infected dogs treated with doxycycline (N = 7) at a 10 mg/kg dose every 12 h for 28 days. Blood samples were taken to determine hematological parameters, viscosity, and cytokine levels. It was observed that, regardless of doxycycline treatment, there was a reduction in total leukocytes and lymphocytes in infected dogs with Ehrlichia canis. The eosinophils and platelets decreased in dogs with Ehrlichia canis infections without treatment. Monocytes, eosinophils, and platelets increased when the dogs were treated with doxycycline. Regardless of treatment, infected dogs' blood viscosity was lower than uninfected dogs. Infected dogs showed lower TNF-α and increased IL-1β. There was a correlation between the blood viscosity with the cytokines IL-10 and IL-12 in the infected dogs. The eosinophil count correlated with TNF-α in the group of infected and untreated dogs. In conclusion, treating dogs with monocytic ehrlichiosis using doxycycline can increase platelet and eosinophil levels but may also increase IL-1β and monocyte levels, exacerbating inflammation. Therefore, evaluating viscosity and cytokine levels is important when treating dogs with this condition.

Distinct Role of TNFR1 and TNFR2 in Protective Immunity Against Orientia tsutsugamushi Infection in Mice

Infection with Orientia tsutsugamushi, an obligate intracellular bacterium, can cause mild or severe scrub typhus. Some patients develop acute lung injury, multi-organ failure, and fatal infection; however, little is known regarding key immune mediators that mediate infection control or disease pathogenesis. Using murine models of scrub typhus, we demonstrated in this study the requirement of TNF-TNFR signaling in protective immunity against this infection. Mice lacking both TNF receptors (TNFR1 and TNFR2) were highly susceptible to O. tsutsugamushi infection, displaying significantly increased tissue bacterial burdens and succumbing to infection by day 9, while most wild-type mice survived through day 20. This increased susceptibility correlated with poor activation of cellular immunity in inflamed tissues. Flow cytometry of lung- and spleen-derived cells revealed profound deficiencies in total numbers and activation status of NK cells, neutrophils, and macrophages, as well as CD4 and CD8 T cells. To define the role of individual receptors in O. tsutsugamushi infection, we used mice lacking either TNFR1 or TNFR2. While deficiency in either receptor alone was sufficient to increase host susceptibility to the infection, TNFR1 and TNFR2 played a distinct role in cellular responses. TNF signaling through TNFR1 promoted inflammatory responses and effector T cell expansion, while TNFR2 signaling was associated with anti-inflammatory action and tissue homeostasis. Moreover, TNFRs played an intrinsic role in CD8+ T cell activation, revealing an indispensable role of TNF in protective immunity against O. tsutsugamushi infection.

Effect of GP19 Peptide Hyperimmune Antiserum on Activated Macrophage during Ehrlichia canis Infection in Canine Macrophage-like Cells

In terms of its veterinary importance, vaccine development against Ehrlichia canis is needed. However, the effect of developing vaccines on humoral immune response against E. canis infection is still unknown. Novel GP19_4-43 was synthesized according to E. canis GP19 epitope prediction. To restrict any loss and/or illness in the host animal, rabbits were used in this study to produce GP19_4-43 hyperimmune sera. The effect of GP19_4-43 hyperimmune sera on neutralization was examined in vitro by determining the inhibition of E. canis infection of the macrophage-like cell line (DH82) in the presence of the sera. Four groups of DH82 cells received differing treatments. These included E. canis experimentally infected DH82 cells, E. canis-infected DH82 cells with control rabbit serum (untreated group), E. canis-infected DH82 cells with GP19_4-43 rabbit antiserum (treated group) and uninfected cells (negative control group), respectively. The treated group developed a decrease (p < 0.01) in the percentage of E. canis infected cells after 3 days post-infection at 48.57 ± 1.28. In addition, real-time PCR analyses of cytokine mRNA expression involved with the macrophage, humoral, and cellular immune responses were conducted. The findings revealed an upregulated expression of IFNG in the treated group during the infection. This study demonstrated neutralization in the GP19_4-43 peptide hyperimmune sera of immunized rabbits. Notably, IFN-γ production could be effectively promoted in canine macrophages in relation to the activation of macrophages and adaptive immune responses. The results of this study indicate the potential for the use of this immunogen in further investigations involving immunized and infected dogs as E. canis host species.

Genomic Association between SNP Markers and Diseases in the "Curraleiro Pé-Duro" Cattle

Susceptibility to diseases is inherited and can be transmitted between populations. Single-nucleotide polymorphism (SNPs) in genes related to immune response is associated with diseases in cattle. This study investigated SNPs in the genomic region of cytokines in 702 samples of Curraleiro Pé-Duro cattle and associated them with the occurrence of antibodies in brucellosis, leptospirosis, neosporosis, leukosis, infectious bovine rhinotracheitis (IBR), and bovine viral diarrhea (BVD) tests. DNA samples were evaluated by the kompetitive allele-specific polymerase chain reaction (KASP) method to identify polymorphisms. The gametic phase and SNP haplotypes were determined with the help of PHASE 2.1.1 software. Haplotypes were associated with serological results against Brucella abortus, Leptospira sp., Neospora caninum, leukosis, infectious rhinotracheitis, and BVD using univariate analysis followed by logistic regression. Haplotype 2 of TLR2 was present in 70% of the animals that tested positive for N. caninum infection. Haplotypes of TLR10 and TLR6 and IL10RA were more common in seronegative animals. Haplotypes related to the gene IL10RA were associated with animals negative to all infections. Curraleiro Pé-Duro cattle presented polymorphisms related to resistance to bacterial, viral, and N. caninum infections.

Comparative Analysis of Genome of Ehrlichia sp. HF, a Model Bacterium to Study Fatal Human Ehrlichiosis

BACKGROUND

The genus Ehrlichia consists of tick-borne obligatory intracellular bacteria that can cause deadly diseases of medical and agricultural importance. Ehrlichia sp. HF, isolated from Ixodes ovatus ticks in Japan [also referred to as I. ovatus Ehrlichia (IOE) agent], causes acute fatal infection in laboratory mice that resembles acute fatal human monocytic ehrlichiosis caused by Ehrlichia chaffeensis. As there is no small laboratory animal model to study fatal human ehrlichiosis, Ehrlichia sp. HF provides a needed disease model. However, the inability to culture Ehrlichia sp. HF and the lack of genomic information have been a barrier to advance this animal model. In addition, Ehrlichia sp. HF has several designations in the literature as it lacks a taxonomically recognized name.

RESULTS

We stably cultured Ehrlichia sp. HF in canine histiocytic leukemia DH82 cells from the HF strain-infected mice, and determined its complete genome sequence. Ehrlichia sp. HF has a single double-stranded circular chromosome of 1,148,904 bp, which encodes 866 proteins with a similar metabolic potential as E. chaffeensis. Ehrlichia sp. HF encodes homologs of all virulence factors identified in E. chaffeensis, including 23 paralogs of P28/OMP-1 family outer membrane proteins, type IV secretion system apparatus and effector proteins, two-component systems, ankyrin-repeat proteins, and tandem repeat proteins. Ehrlichia sp. HF is a novel species in the genus Ehrlichia, as demonstrated through whole genome comparisons with six representative Ehrlichia species, subspecies, and strains, using average nucleotide identity, digital DNA-DNA hybridization, and core genome alignment sequence identity.

CONCLUSIONS

The genome of Ehrlichia sp. HF encodes all known virulence factors found in E. chaffeensis, substantiating it as a model Ehrlichia species to study fatal human ehrlichiosis. Comparisons between Ehrlichia sp. HF and E. chaffeensis will enable identification of in vivo virulence factors that are related to host specificity, disease severity, and host inflammatory responses. We propose to name Ehrlichia sp. HF as Ehrlichia japonica sp. nov. (type strain HF), to denote the geographic region where this bacterium was initially isolated.

Isolation and identification of Bengle (Zingiber cassumunar roxb) as a stimulant in phagocytic activity of macrophages

Immunomodulators are pharmacological agents that modify or regulate the immune system through stimulating the functioning of the immune system and, at the same time, inhibiting excessive immune responses. This study was conducted to determine the active compound in Z. cassumunar that is responsible for increasing the immune system based on the parameters of phagocytic activity. The isolation method began with fractionation, which involved extraction with ethanol and successive fractionation with hexane and chloroform. Z. cassumunar extract, hexane fraction, and chloroform fraction were tested on mice macrophage cells for their phagocytic functions. The phagocytic activity of macrophages was measured by active phagocytic cells (averagely 39.194 ±1.597, 27.923 ±2.941, and 62.090 ±6.947) and phagocytic index (in a row, averagely 47.513 ±2.844, 41.129 ±7.195, and 101.527 ±10.555). The results showed that the Z. cassumunar extract,  hexane fraction, and chloroform fraction exhibited more significant phagocytic activities of macrophages (p <0.05) compared with the normal group. Since the chloroform fraction showed the best result, this fraction was further separated by column chromatography. This procedure yielded five sub-fractions, namely F1, F2, F2C, F3, and F4. Based on the phagocytic activity testing, the results were as follows: (1) the active phagocytic cells of F1, F2, F2C, F3 and F4 were 18.860 ±3.191, 27.077 ±4.482, 15.749 ±3.026, 64.333 ±1.780, and 44.943 ±2.944, respectively, and (2) the phagocytic indices were 30.0249 ±3.4231, 44.5969 ±8.3646, 24.5597 ±5.4487, 102.7447 ±1.0806, and 76.5007 ±4.7293. Because F3 produced the best result, this subfraction was then identified using 1H-NMR and 13C-NMR. The identification results showed that F3 was (E)-4-(3,4-dimethoxyphenyl)but-3-en-1-ol as an active compound.

Infectious Complications of Biological and Small Molecule Targeted Immunomodulatory Therapies

The past 2 decades have seen a revolution in our approach to therapeutic immunosuppression. We have moved from relying on broadly active traditional medications, such as prednisolone or methotrexate, toward more specific agents that often target a single receptor, cytokine, or cell type, using monoclonal antibodies, fusion proteins, or targeted small molecules. This change has transformed the treatment of many conditions, including rheumatoid arthritis, cancers, asthma, and inflammatory bowel disease, but along with the benefits have come risks. Contrary to the hope that these more specific agents would have minimal and predictable infectious sequelae, infectious complications have emerged as a major stumbling block for many of these agents. Furthermore, the growing number and complexity of available biologic agents makes it difficult for clinicians to maintain current knowledge, and most review articles focus on a particular target disease or class of agent. In this article, we review the current state of knowledge about infectious complications of biologic and small molecule immunomodulatory agents, aiming to create a single resource relevant to a broad range of clinicians and researchers. For each of 19 classes of agent, we discuss the mechanism of action, the risk and types of infectious complications, and recommendations for prevention of infection.

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.

Type I IFNs drive hematopoietic stem and progenitor cell collapse via impaired proliferation and increased RIPK1-dependent cell death during shock-like ehrlichial infection

Type I interferons (IFNα/β) regulate diverse aspects of host defense, but their impact on hematopoietic stem and progenitor cells (HSC/HSPCs) during infection remains unclear. Hematologic impairment can occur in severe infections, thus we sought to investigate the impact of type I IFNs on hematopoiesis in a tick-borne infection with a virulent ehrlichial pathogen that causes shock-like disease. During infection, IFNα/β induced severe bone marrow (BM) loss, blunted infection-induced emergency myelopoiesis, and reduced phenotypic HSPCs and HSCs. In the absence of type I IFN signaling, BM and splenic hematopoiesis were increased, and HSCs derived from Ifnar1-deficient mice were functionally superior in competitive BM transplants. Type I IFNs impaired hematopoiesis during infection by both limiting HSC/HSPC proliferation and increasing HSPC death. Using mixed BM chimeras we determined that type I IFNs restricted proliferation indirectly, whereas HSPC death occurred via direct IFNαR -mediated signaling. IFNαR-dependent signals resulted in reduced caspase 8 expression and activity, and reduced cleavage of RIPK1 and RIPK3, relative to Ifnar1-deficient mice. RIPK1 antagonism with Necrostatin-1s rescued HSPC and HSC numbers during infection. Early antibiotic treatment is required for mouse survival, however antibiotic-treated survivors had severely reduced HSPCs and HSCs. Combination therapy with antibiotics and Necrostatin-1s improved HSPC and HSC numbers in surviving mice, compared to antibiotic treatment alone. We reveal two mechanisms whereby type I IFNs drive hematopoietic collapse during severe infection: direct sensitization of HSPCs to undergo cell death and enhanced HSC quiescence. Our studies reveal a strategy to ameliorate the type I IFN-dependent loss of HSCs and HSPCs during infection, which may be relevant to other infections wherein type I IFNs cause hematopoietic dysfunction.

The Ehrlichiae are important emerging, tick-borne pathogens that cause immune suppression and cytopenias, though the underlying mechanisms are unclear. In a model of shock-like illness caused by Ixodes ovatus ehrlichia, type I interferons (IFNs) induce hematopoietic dysfunction by reducing hematopoietic stem cell (HSC) proliferation and driving cell death of hematopoietic progenitors (HSPCs). Using mixed bone marrow chimeras, we demonstrate that HSPC loss occurs via intrinsic type I IFN signaling, whereas HSC proliferation is regulated via an extrinsic mechanism. In contrast to sterile inflammation, infection-induced type I IFNs induced RIPK1-dependent loss of hematopoietic progenitors. HSPCs were rescued during infection by inhibiting RIPK1 with Necrostatin-1s. While antibiotic treatment protected against otherwise lethal infection, mice recovering from infection exhibited significantly reduced HSCs and HSPCs. Co-treatment with both antibiotics and Necrostatin-1s significantly increased HSPC frequencies and the number of HSCs compared to antibiotics alone. Blood production is essential for life and necessary for host defense, thus our work reveals a therapeutic strategy to rescue and improve hematopoiesis in patients recovering from serious infectious disease.

Dysfunction of CD19+CD24hiCD27+ B regulatory cells in patients with bullous pemphigoid

Bullous pemphigoid (BP) is an autoimmune blistering skin disease characterized by the production of autoantibodies against the hemidesmosomal protein BP180. B regulatory cells (Bregs) are crucial in maintaining self-tolerance and suppressing autoantibody production. However, it is still unclear whether the dysfunctions of Bregs contributes to the autoantibody production in BP patients. In this study, we found that CD19+CD24hiCD27+ Bregs and IL-10+CD19+ Bregs were significantly increased in the peripheral blood samples of BP patients compared with that in healthy controls. Moreover, compared to Bregs from healthy individuals, we found that Bregs from BP patients fails to suppress the production of specific anti-BP180 autoantibody when co-cultured with patient-derived PBMCs. Additionally, Bregs from BP patients were defective in suppressing the CD4+ T cell proliferation and the cytokines expression (including IFN-γ, TNF-α and IL-4). Notably, we found that patient-derived Bregs produced high level of TNF-α and the TNF inhibitor etanercept could inhibit the autoantibody production in the culture system in vitro. Our results indicate that Bregs from BP patient appear phenotypically pro-inflammatory by their cytokine profile and are defective in immunosuppressive function, which suggest that Bregs play a pro-inflammatory role rather than a regulatory role in the pathogenesis of BP.

MyD88-dependent inflammasome activation and autophagy inhibition contributes to Ehrlichia-induced liver injury and toxic shock

Severe hepatic inflammation is a common cause of acute liver injury following systemic infection with Ehrlichia, obligate Gram-negative intracellular bacteria that lack lipopolysaccharide (LPS). We have previously shown that type I IFN (IFN-I) and inflammasome activation are key host-pathogenic mediators that promote excessive inflammation and liver damage following fatal Ehrlichia infection. However, the underlying signals and mechanisms that regulate protective immunity and immunopathology during Ehrlichia infection are not well understood. To address this issue, we compared susceptibility to lethal Ixodes ovatus Ehrlichia (IOE) infection between wild type (WT) and MyD88-deficient (MyD88^-/-) mice. We show here that MyD88^-/- mice exhibited decreased inflammasome activation, attenuated liver injury, and were more resistant to lethal infection than WT mice, despite suppressed protective immunity and increased bacterial burden in the liver. MyD88-dependent inflammasome activation was also dependent on activation of the metabolic checkpoint kinase mammalian target of rapamycin complex 1 (mTORC1), inhibition of autophagic flux, and defective mitophagy in macrophages. Blocking mTORC1 signaling in infected WT mice and primary macrophages enhanced bacterial replication and attenuated inflammasome activation, suggesting autophagy promotes bacterial replication while inhibiting inflammasome activation. Finally, our data suggest TLR9 and IFN-I are upstream signaling mechanisms triggering MyD88-mediated mTORC1 and inflammasome activation in macrophages following Ehrlichia infection. This study reveals that Ehrlichia-induced liver injury and toxic shock are mediated by MyD88-dependent inflammasome activation and autophagy inhibition.

Human monocytic ehrlichiosis (HME) is the most prevalent emerging infectious disease in the United States. Ehrlichia chaffeensis, etiologic agent of HME, is a Gram negative obligate intracellular bacterium transmitted by infected tick bites and can infect different cell type. Although Ehrlichia lack lipopolysaccharide (LPS), they induce potentially life threatening HME that mimic sepsis or toxic shock associated with multi-organ failure. The clinical diagnosis of HME is difficult, and definitive diagnosis is most often retrospective. Late antibiotic treatment is frequently ineffective in preventing disease progression to fatal multi-organ failure. Liver failure is one of the most serious complications and the most frequent cause of death in patients with HME, however we only have a limited understanding of how this liver failure is caused during fatal Ehrlichia infection. The objective of this study is to determine how LPS-negative Ehrlichia activates inflammatory responses in macrophages during Ehrlichia infection to promote liver damage. We show here that MyD88-signaling causes detrimental derangement of the immune system and subsequent liver damage by regulating two key innate immune events in macrophages: autophagy and inflammasome activation. Targeting host-pathogenic pathways in ehrlichiosis can be incorporated into future design of novel therapeutic approaches for HME.

Tick-Borne Emerging Infections: Ehrlichiosis and Anaplasmosis

Human ehrlichiosis and anaplasmosis are acute febrile tick-borne infectious diseases caused by various members from the genera Ehrlichia and Anaplasma. Ehrlichia chaffeensis is the major etiologic agent of human monocytotropic ehrlichiosis (HME), while Anaplasma phagocytophilum is the major cause of human granulocytic anaplasmosis (HGA). The clinical manifestations of HME and HGA ranges from subclinical to potentially life-threatening diseases associated with multi-organ failure. Macrophages and neutrophils are the major target cells for Ehrlichia and Anaplasma, respectively. The threat to public health is increasing with newly emerging ehrlichial and anaplasma agents, yet vaccines for human ehrlichioses and anaplasmosis are not available, and therapeutic options are limited. This article reviews recent advances in the understanding of HME and HGA.

Fatal Monocytic Ehrlichiosis in Woman, Mexico, 2013

Human monocytic ehrlichiosis is a febrile illness caused by Ehrlichia chaffeensis, an intracellular bacterium transmitted by ticks. In Mexico, a case of E. chaffeensis infection in an immunocompetent 31-year-old woman without recognized tick bite was fatal. This diagnosis should be considered for patients with fever, leukopenia, thrombocytopenia, and elevated liver enzyme levels.

Assay of Genome-Wide Transcriptome and Secreted Proteins on the Same Single Immune Cells by Microfluidics and RNA Sequencing

Given vast heterogeneity of immune cells, searching for gene expression and transcriptional networks belonging to specific cellular functions such as cytokine production has been challenging. To overcome this limitation, we developed a splittable single-cell microchip that integrates a high-density antibody array for cytokine protein detection, while the same single cells with protein profiles can be subsequently sequenced to obtain the genome-wide transcriptome. Combined with bioinformatics algorithms, we discovered a subgroup of highly coexpressed genes correlating with TNFα secretion in mouse macrophage cells. This technology and the data analysis may lead to an unprecedented understanding of regulation mechanisms of the immune system and have the potential to impact disease treatment and drug discovery.

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.

NK Cell-Mediated Regulation of Protective Memory Responses against Intracellular Ehrlichial Pathogens

Ehrlichiae are gram-negative obligate intracellular bacteria that cause potentially fatal human monocytic ehrlichiosis. We previously showed that natural killer (NK) cells play a critical role in host defense against Ehrlichia during primary infection. However, the contribution of NK cells to the memory response against Ehrlichia remains elusive. Primary infection of C57BL/6 mice with Ehrlichia muris provides long-term protection against a second challenge with the highly virulent Ixodes ovatus Ehrlichia (IOE), which ordinarily causes fatal disease in naïve mice. Here, we show that the depletion of NK cells in E. muris-primed mice abrogates the protective memory response against IOE. Approximately, 80% of NK cell-depleted E. muris-primed mice succumbed to lethal IOE infection on days 8-10 after IOE infection, similar to naïve mice infected with the same dose of IOE. The lack of a recall response in NK cell-depleted mice correlated with an increased bacterial burden, extensive liver injury, decreased frequency of Ehrlichia-specific IFN-γ-producing memory CD4+ and CD8+ T-cells, and a low titer of Ehrlichia-specific antibodies. Intraperitoneal infection of mice with E. muris resulted in the production of IL-15, IL-12, and IFN-γ as well as an expansion of activated NKG2D+ NK cells. The adoptive transfer of purified E. muris-primed hepatic and splenic NK cells into Rag2-/-Il2rg-/- recipient mice provided protective immunity against challenge with E. muris. Together, these data suggest that E. muris-induced memory-like NK cells, which contribute to the protective, recall response against Ehrlichia.

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.

Type I interferon contributes to noncanonical inflammasome activation, mediates immunopathology, and impairs protective immunity during fatal infection with lipopolysaccharide-negative ehrlichiae

Ehrlichia species are intracellular bacteria that cause fatal ehrlichiosis, mimicking toxic shock syndrome in humans and mice. Virulent ehrlichiae induce inflammasome activation leading to caspase-1 cleavage and IL-18 secretion, which contribute to development of fatal ehrlichiosis. We show that fatal infection triggers expression of inflammasome components, activates caspase-1 and caspase-11, and induces host-cell death and secretion of IL-1β, IL-1α, and type I interferon (IFN-I). Wild-type and Casp1(-/-) mice were highly susceptible to fatal ehrlichiosis, had overwhelming infection, and developed extensive tissue injury. Nlrp3(-/-) mice effectively cleared ehrlichiae, but displayed acute mortality and developed liver injury similar to wild-type mice. By contrast, Ifnar1(-/-) mice were highly resistant to fatal disease and had lower bacterial burden, attenuated pathology, and prolonged survival. Ifnar1(-/-) mice also had improved protective immune responses mediated by IFN-γ and CD4(+) Th1 and natural killer T cells, with lower IL-10 secretion by T cells. Importantly, heightened resistance of Ifnar1(-/-) mice correlated with improved autophagosome processing, and attenuated noncanonical inflammasome activation indicated by decreased activation of caspase-11 and decreased IL-1β, compared with other groups. Our findings demonstrate that IFN-I signaling promotes host susceptibility to fatal ehrlichiosis, because it mediates ehrlichia-induced immunopathology and supports bacterial replication, perhaps via activation of noncanonical inflammasomes, reduced autophagy, and suppression of protective CD4(+) T cells and natural killer T-cell responses against ehrlichiae.

An animal model of a newly emerging human ehrlichiosis

BACKGROUND

Human ehrlichioses are emerging life-threatening diseases transmitted by ticks. Animal models have been developed to study disease development; however, there is no valid small animal model that uses a human ehrlichial pathogen. The objective of this study was to develop a mouse model for ehrlichiosis with the newly discovered human pathogen, Ehrlichia muris-like agent (EMLA).

METHODS

Three strains of mice were inoculated with different doses of EMLA by the intravenous, intraperitoneal, or intradermal route and evaluated for clinical and pathologic changes during the course of infection.

RESULTS

EMLA infected C57Bl/6, BALB/c, and C3H/HeN mice and induced lethal or persistent infection in a route- and dose-dependent manner. The clinical chemistry and hematologic changes were similar to those of human infection by Ehrlichia chaffeensis or EMLA. Bacterial distribution in tissues differed after intradermal infection, compared with the distribution after intravenous or intraperitoneal injection. Lethal infection did not cause remarkable pathologic changes, but it caused fluid imbalance. EMLA infection of endothelium and mononuclear cells likely plays a role in the severe outcome.

CONCLUSIONS

The EMLA mouse model mimics human infection and can be used to study pathogenesis and immunity and for development of a vector transmission model of ehrlichiosis.

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.

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.

Neutrophils mediate immunopathology and negatively regulate protective immune responses during fatal bacterial infection-induced toxic shock

Ehrlichia chaffeensis is an obligate intracellular bacterium that infects primarily monocytes and macrophages and causes potentially fatal human monocytic ehrlichiosis (HME) that mimics toxic-shock-like syndrome in immunocompetent hosts. Early recruitment of neutrophils to the sites of infection is critical for the control of bacterial infection and inflammatory responses. We recently observed rapid and sustained neutrophil recruitment at a primary site of infection (peritoneum) following lethal murine ehrlichial infection compared to innocuous ehrlichial infection. We examined here the contribution of neutrophils to protective immunity or immunopathology during infection with monocytic Ehrlichia. Unexpectedly, depletion of neutrophils from lethally infected mice enhanced bacterial elimination, decreased immune-mediated pathology, and prolonged survival. Furthermore, compared to lethally infected sham controls, neutrophil depletion in infected mice resulted in amelioration of pathogenic responses, as evidenced by a decreased number of tumor necrosis factor alpha (TNF-α)-producing CD8(+) T cells, which is known to mediate immunopathology and toxic shock in a murine model of fatal ehrlichiosis. Although neutrophil depletion did not influence the number of CD4(+) Th1 cells and NKT cells producing gamma interferon (IFN-γ), it increased the ratio of IFN-γ- to IL-10-producing NKT cells as well as the ratio of IFN-γ to interleukin 10 (IL-10) transcripts in the liver. This may ameliorate the net suppressive effect of IL-10 on IFN-γ-mediated activation of infected macrophages and thus may account for the enhanced bacterial elimination. Finally, transcriptional analysis of gene expression in the liver indicated that neutrophils contribute to overproduction of cytokines and chemokines during fatal ehrlichiosis. In conclusion, these results revealed an unexpected role of neutrophils in supporting bacterial replication indirectly and promoting immunopathology during severe infection with an intracellular bacterium.

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.

The interaction between IL-18 and IL-18 receptor limits the magnitude of protective immunity and enhances pathogenic responses following infection with intracellular bacteria

The binding of IL-18 to IL-18Rα induces both proinflammatory and protective functions during infection, depending on the context in which it occurs. IL-18 is highly expressed in the liver of wild-type (WT) C57BL/6 mice following lethal infection with highly virulent Ixodes ovatus ehrlichia (IOE), an obligate intracellular bacterium that causes acute fatal toxic shock-like syndrome. In this study, we found that IOE infection of IL-18Rα(-/-) mice resulted in significantly less host cell apoptosis, decreased hepatic leukocyte recruitment, enhanced bacterial clearance, and prolonged survival compared with infected WT mice, suggesting a pathogenic role for IL-18/IL-18Rα in Ehrlichia-induced toxic shock. Although lack of IL-18R decreased the magnitude of IFN-γ producing type-1 immune response, enhanced resistance of IL-18Rα(-/-) mice against Ehrlichia correlated with increased proinflammatory cytokines at sites of infection, decreased systemic IL-10 production, increased frequency of protective NKT cells producing TNF-α and IFN-γ, and decreased frequency of pathogenic TNF-α-producing CD8(+) T cells. Adoptive transfer of immune WT CD8(+) T cells increased bacterial burden in IL-18Rα(-/-) mice following IOE infection. Furthermore, rIL-18 treatment of WT mice infected with mildly virulent Ehrlichia muris impaired bacterial clearance and enhanced liver injury. Finally, lack of IL-18R signal reduced dendritic cell maturation and their TNF-α production, suggesting that IL-18 might promote the adaptive pathogenic immune responses against Ehrlichia by influencing T cell priming functions of dendritic cells. Together, these results suggested that the presence or absence of IL-18R signals governs the pathogenic versus protective immunity in a model of Ehrlichia-induced immunopathology.

Molecular and cellular pathobiology of Ehrlichia infection: targets for new therapeutics and immunomodulation strategies

Ehrlichia are small obligately intracellular bacteria in the order Rickettsiales that are transmitted by ticks and associated with emerging life-threatening human zoonoses. Vaccines are not available for human ehrlichiosis, and therapeutic options are limited to a single antibiotic class. New technologies for exploring host-pathogen interactions have yielded recent advances in understanding the molecular interactions between Ehrlichia and the eukaryotic host cell and identified new targets for therapeutic and vaccine development, including those that target pathogen virulence mechanisms or disrupt the processes associated with ehrlichial effector proteins. Animal models have also provided insight into immunopathological mechanisms that contribute significantly to understanding severe disease manifestations, which should lead to the development of immunomodulatory approaches for treating patients nearing or experiencing severe disease states. In this review, we discuss the recent advances in our understanding of molecular and cellular pathobiology and the immunobiology of Ehrlichia infection. We identify new molecular host-pathogen interactions that can be targets of new therapeutics, and discuss prospects for treating the immunological dysregulation during acute infection that leads to life-threatening complications.

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.

IgM production by bone marrow plasmablasts contributes to long-term protection against intracellular bacterial infection

IgM responses are well known to occur early postinfection and tend to be short-lived, which has suggested that this Ig does not significantly contribute to long-term immunity. In this study, we demonstrate that chronic infection with the intracellular bacterium Ehrlichia muris elicits a protective, long-term IgM response. Moreover, we identified a population of CD138(high)IgM(high) B cells responsible for Ag-specific IgM production in the bone marrow. The IgM-secreting cells, which exhibited characteristics of both plasmablasts and plasma cells, contributed to protection against fatal ehrlichial challenge. Mice deficient in activation-induced cytidine deaminase, which produce only IgM, were protected against fatal ehrlichial challenge infection. The IgM-secreting cells that we have identified were maintained in the bone marrow in the absence of chronic infection, as antibiotic-treated mice remained protected against challenge infection. Our studies identify a cell population that is responsible for the IgM production in the bone marrow, and they highlight a novel role for IgM in the maintenance of long-term immunity during intracellular bacterial infection.

Human ehrlichiosis and anaplasmosis

Human ehrlichiosis and anaplasmosis are acute febrile tick-borne diseases caused by various members of the genera Ehrlichia and Anaplasma (Anaplasmataceae). Human monocytotropic ehrlichiosis has become one of the most prevalent life-threatening tick-borne disease in the United States. Ehrlichiosis and anaplasmosis are becoming more frequently diagnosed as the cause of human infections, as animal reservoirs and tick vectors have increased in number and humans have inhabited areas where reservoir and tick populations are high. Ehrlichia chaffeensis, the etiologic agent of human monocytotropic ehrlichiosis (HME), is an emerging zoonosis that causes clinical manifestations ranging from a mild febrile illness to a fulminant disease characterized by multiorgan system failure. Anaplasma phagocytophilum causes human granulocytotropic anaplasmosis (HGA), previously known as human granulocytotropic ehrlichiosis. This article reviews recent advances in the understanding of ehrlichial diseases related to microbiology, epidemiology, diagnosis, pathogenesis, immunity, and treatment of the 2 prevalent tick-borne diseases found in the United States, HME and HGA.

Natural killer cells promote tissue injury and systemic inflammatory responses during fatal Ehrlichia-induced toxic shock-like syndrome

Human monocytotropic ehrlichiosis is caused by Ehrlichia chaffeensis, a Gram-negative bacterium lacking lipopolysaccharide. We have shown that fatal murine ehrlichiosis is associated with CD8(+)T cell-mediated tissue damage, tumor necrosis factor-alpha, and interleukin (IL)-10 overproduction, and CD4(+)Th1 hyporesponsiveness. In this study, we examined the relative contributions of natural killer (NK) and NKT cells in Ehrlichia-induced toxic shock. Lethal ehrlichial infection in wild-type mice induced a decline in NKT cell numbers, and late expansion and migration of activated NK cells to the liver, a main infection site that coincided with development of hepatic injury. The spatial and temporal changes in NK and NKT cells in lethally infected mice correlated with higher NK cell cytotoxic activity, higher expression of cytotoxic molecules such as granzyme B, higher production of interferon-gamma and tumor necrosis factor-alpha, increased hepatic infiltration with CD8alphaCD11c(+) dendritic cells and CD8(+)T cells, decreased splenic CD4(+)T cells, increased serum concentrations of IL-12p40, IL-18, RANTES, and monocyte chemotactic protein-1, and elevated production of IL-18 by liver mononuclear cells compared with nonlethally infected mice. Depletion of NK cells prevented development of severe liver injury, decreased serum levels of interferon-gamma, tumor necrosis factor-alpha, and IL-10, and enhanced bacterial elimination. These data indicate that NK cells promote immunopathology and defective anti-ehrlichial immunity, possibly via decreasing the protective immune response mediated by interferon-gamma producing CD4(+)Th1 and NKT cells.

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.

Ixodes ovatus Ehrlichia exhibits unique ultrastructural characteristics in mammalian endothelial and tick-derived cells

Tick-borne pathogens in the genus Ehrlichia cause emerging zoonoses. Although laboratory mice are susceptible to Ehrlichia infections, many isolates do not cause clinical illness. In contrast, the Ixodes ovatus Ehrlichia-like agent (IOE) causes disease and immune responses in mice comparable to the human illness caused by Ehrlichia chaffeensis. No culture system had been developed for IOE, however, which limited studies of this pathogen. We reasoned that endothelial and tick cell lines could potentially serve as host cells, since the IOE is found in ticks and in endothelial cells in mice. Infected spleen cells from RAG-deficient mice were overlaid onto ISE6 and RF/6A cultures, and colonies typical of Ehrlichia were noted in RF/6A cells within 2 weeks. Infection of ISE6 cells was established after transfer of IOE from RF/6A cells. Electron microscopy revealed densely packed inclusions in infected RF/6A and ISE6 cells; these inclusions contained copious amounts of filamentous structures, apparently originating from Ehrlichial cells. In particular, within RF/6A cells the structures assumed an ordered morphology of finely combed hair. IOE from RF/6A cells, when inoculated into C57BL/6 and RAG-deficient mice, induced fatal disease. These data reveal unique structural features of IOE that may contribute to the pathogen's high virulence.

Mutual modulation between interleukin-10 and interleukin-6 induced by Rhodococcus aurantiacus infection in mice

The interaction between interleukin-10 (IL-10) and interleukin-6 (IL-6) was investigated in the inflammatory response to Rhodococcus aurantiacus (R. aurantiacus) infection, in which both cytokines act as anti-inflammatory cytokines. Compared with wild-type (WT) counterparts, IL-6 gene-deficient (IL-6(-)/(-)) mice mounted a more robust production of IL-10 and tumor necrosis factor-alpha (TNF-alpha) during the initial phase of infection. Administration of anti-IL-10 antibody resulted in all the mice dying within 3 days post-infection as well as a further elevated TNF-alpha release. In vitro challenge of the macrophages from IL-6(-)/(-) and WT mice with heat-killed R. aurantiacus also showed similar results. Addition of exogenous IL-6 depressed IL-10 and TNF-alpha production by either IL-6(-)/(-) mice or IL-6(-)/(-) mouse macrophages. Likewise, WT mouse macrophages pretreated with anti-IL-10 or anti-IL-6 antibody exhibited increased production of TNF-alpha and IL-6 or IL-10 respectively. Moreover, neutralization of both IL-10 and IL-6 induced a further increase in TNF-alpha production by WT mouse cells. Overall, we conclude that IL-10 is a key element in protecting mice against mortality, and that IL-10 and IL-6 production are negatively regulated by each other although they are additive in suppressing TNF-alpha release in R. aurantiacus-infected mouse model.

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.

Regulatory roles of CD1d-restricted NKT cells in the induction of toxic shock-like syndrome in an animal model of fatal ehrlichiosis

CD1d-restricted NKT cells are key players in host defense against various microbial infections. Using a murine model of fatal ehrlichiosis, we investigated the role of CD1d-restricted NKT cells in induction of toxic shock-like syndrome caused by gram-negative, lipopolysaccharide-lacking, monocytotropic Ehrlichia. Our previous studies showed that intraperitoneal infection of wild-type (WT) mice with virulent Ehrlichia (Ixodes ovatus Ehrlichia [IOE]) results in CD8+ T-cell-mediated fatal toxic shock-like syndrome marked by apoptosis of CD4+ T cells, a weak CD4+ Th1 response, overproduction of tumor necrosis factor alpha and interleukin-10, and severe liver injury. Although CD1d-/- mice succumbed to high-dose IOE infection similar to WT mice, they did not develop signs of toxic shock, as shown by elevated bacterial burdens, low serum levels of tumor necrosis factor, normal serum levels of liver enzymes, and the presence of few apoptotic hepatic cells. An absence of NKT cells restored the percentages and absolute numbers of CD4+ and CD8+ T cells and CD11b+ cells in the spleen compared to WT mice and was also associated with decreased expression of Fas on splenic CD4+ lymphocytes and granzyme B in hepatic CD8+ lymphocytes. Furthermore, our data show that NKT cells promote apoptosis of macrophages and up-regulation of the costimulatory molecule CD40 on antigen-presenting cells, including dendritic cells, B cells, and macrophages, which may contribute to the induction of pathogenic T-cell responses. In conclusion, our data suggest that NKT cells mediate Ehrlichia-induced T-cell-mediated toxic shock-like syndrome, most likely via cognate and noncognate interactions with antigen-presenting cells.

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

CD4 T-cell epitopes associated with protective immunity induced following vaccination of mice with an ehrlichial variable outer membrane protein

The ehrlichiae express variable outer membrane proteins (OMPs) that play important roles in both pathogenesis and host defense. Previous studies revealed that OMPs are immunodominant B-cell antigens and that passive transfer of anti-OMP antibodies can protect SCID mice from fatal ehrlichial infection. In this study, we used a model of fatal monocytotropic ehrlichiosis caused by Ehrlichia bacteria from Ixodes ovatus (IOE) to determine whether OMP immunization could generate protective immunity in immunocompetent mice. Immunization of C57BL/6 mice with a purified recombinant OMP expressed by IOE omp19 generated protection from fatal IOE infection and elicited robust humoral and CD4 T-cell responses. To identify CD4 T-cell epitopes within OMPs, we performed enzyme-linked immunospot analyses for gamma interferon (IFN-gamma) production using a panel of overlapping 16-mer peptides from IOE OMP-19. Five immunoreactive peptides comprising residues 30 to 45, 77 to 92, 107 to 122, 197 to 212, and 247 to 264 were identified; the strongest response was generated against OMP-19(107-122). Most of the peptides are conserved between E. muris and E. chaffeensis OMP-19, and they elicited IFN-gamma production in CD4 T cells from E. muris-infected mice, indicating that T-cell epitope cross-reactivity likely contributes to heterologous immunity. Accordingly, CD4 T-cell responses to both OMP-19 and OMP-19(107-122) were of greater magnitude following high-dose IOE challenge of mice that had been immunized by prior infection with E. muris. Our studies cumulatively identify B- and T-cell epitopes that are associated with protective homologous and heterologous immunity during ehrlichial 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.

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.