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

Long-Term Protective Immunity against Ehrlichia chaffeensis Infection Induced by a Genetically Modified Live Vaccine

Human monocytic ehrlichiosis, an emerging tick-borne disease, is caused by Ehrlichia chaffeensis. Infections with the pathogen are also common in the canine host. Our previous studies demonstrated that functional disruption within the E. chaffeensis phage head-to-tail connector protein gene results in bacterial attenuation, creating a modified live attenuated vaccine (MLAV). The MLAV confers protective immunity against intravenous and tick transmission challenges one month following vaccination. In this study, we evaluated the duration of MLAV protection. Dogs vaccinated with the MLAV were challenged with wild-type E. chaffeensis via intravenous infection at 4-, 8-, and 12-months post-vaccination. Immunized dogs rapidly cleared the wild-type pathogen infection and tested positive for bacteremia less frequently than unvaccinated controls. While immune responses varied among dogs, vaccinees consistently mounted IgG and CD4+ T-cell responses specific to E. chaffeensis throughout the assessment period. Our findings demonstrate that MLAV-mediated immune protection persists for at least one year against wild-type bacterial infection, marking a major advancement in combating this serious tick-borne disease. The data presented here serve as the foundation for further studies, elucidating the molecular mechanisms underlying virulence and vaccine development and aiding in preventing the diseases caused by E. chaffeensis and other tick-borne rickettsial pathogens.

Identification of potential nucleomodulins of Mycoplasma bovis by direct biotinylation and proximity-based biotinylation approaches

Mycoplasma bovis (M. bovis) is a significant bovine pathogen associated with various diseases, including bovine bronchopneumonia and mastitis resulting in substantial economic losses within the livestock industry. However, the development of effective control measures for M. bovis is hindered by a limited understanding of its virulence factors and pathogenesis. Nucleomodulins are newly identified secreted proteins of bacteria that internalize the host nuclei to regulate host cell gene expression and serve as critical virulence factors. Although recent reports have initiated exploration of mycoplasma nucleomodulins, the efficiency of conventional techniques for identification is very limited. Therefore, this study aimed to establish high-throughput methods to identify novel nucleomodulins of M. bovis. Using a direct biotinylation (DB) approach, a total of 289 proteins were identified including 66 high abundant proteins. In parallel, the use of proximity-based biotinylation (PBB), identified 28 proteins. Finally, seven nucleomodulins were verified to be nuclear by transfecting the bovine macrophage cell line BoMac with the plasmids encoding EGFP-fused proteins and observed with Opera Phenix, including the known nucleomodulin MbovP475 and six novel nucleomodulins. The novel nucleomodulins were four ribosomal proteins (MbovP599, MbovP678, MbovP710, and MbovP712), one transposase (MbovP790), and one conserved hypothetical protein (MbovP513). Among them, one unique nucleomodulin MbovP475 was identified with DB, two unique nucleomodulins (MbovP513 and MbovP710) with PBB, and four nucleomodulins by both. Overall, these findings established a foundation for further research on M. bovis nucleomodulin-host interactions for identification of new virulence factors.

Unraveling the Complexity: Ehrlichiosis-Induced Septic Shock and Acute Respiratory Distress Syndrome

Human monocytic ehrlichiosis typically presents with nonspecific cold-like symptoms and a history of recent tick exposure, often responding well to early treatment. Here, we present the case of a 67-year-old immunocompetent male who initially presented with fevers, chills, dysuria, and hematuria, leading to admission to the intensive care unit with septic shock and acute respiratory distress syndrome (ARDS), which was later attributed to Ehrlichia chaffeensis infection. Prompt treatment with doxycycline resulted in a full clinical recovery. This case highlights the rare occurrence of severe ehrlichiosis and provides insights into its effective management based on updated literature.

Prevalence of Anaplasma, Ehrlichia and Rickettsia infections in dogs in Iran: A meta-analysis study

The current study aims to assess the prevalence of different genera of Rickettsiales, intracellular bacteria, in dogs across various regions of Iran. Rickettsiales, as zoonotic pathogens, can have various manifestations in different species, including immunosuppression, anaemia and neuropathy. Due to their close interactions with people and livestock, dogs can serve as reservoirs and transmit these pathogens to humans and other animals. The data from this research can be valuable for managing and controlling related diseases and complications in Iran and possibly neighbouring countries. Pertinent data for this study was gathered without time limitations until 1 March 2022, from different databases. Of all the inspected studies, 26 were eligible based on the inclusion criteria. The Egger test result and asymmetry in the funnel plot revealed significant publication bias; therefore, the meta-analysis model was corrected with the trim-and-fill method. After correction, the prevalence of rickettsial infections among dogs varies by genus - 20.1% for Anaplasma spp. and 10% for Ehrlichia spp. - with an overall prevalence estimated at 18.3% by random-effects analysis. The highest and lowest estimated pooled prevalences were associated with the southwestern (38.5%) and southern (0.3%) provinces of Iran, respectively. The pooled prevalence of rickettsial infection was higher in female dogs (OR = 1.198; p < 0.978; 95% CI, 0.842-1.705) and in dogs ≤2 years (OR = 1.014; p < 0.312; 95% CI, 0.598-1.72), but the difference was not statistically significant. Ultimately, the pooled prevalence of rickettsial infections among dogs is relatively low compared to other countries like Qatar, Iraq, Saudi Arabia and Turkey, which are nearby Iran. Given the significant clinical outcomes of this disease, necessary measures for prevention should be taken.

Antibody reactive immunomes of Ehrlichia chaffeensis and E. canis are diverse and defined by conformational antigenic determinants

For decades, the defined antibody reactive proteins of Ehrlichia chaffeensis and E. canis were limited to a small group with linear antibody epitopes. Recently, our laboratory has utilized an immunomics-based approach to rapidly screen and identify undefined Ehrlichia chaffeensis and E. canis antigenic proteins and antibody epitopes. In this study, we analyzed the remaining portion (~50%) of the E. chaffeensis and E. canis proteomes (n = 444 and n = 405 proteins, respectively), that were not examined in previous studies, to define the complete immunomes of these important pathogens. Almost half of the E. chaffeensis proteins screened (196/444) reacted with antibodies in convalescent HME patient sera, while only 43 E. canis proteins reacted with CME dog sera. New major immunoreactive proteins were identified in E. chaffeensis (n = 7) and E. canis (n = 1), increasing the total number of E. chaffeensis (n = 14) and E. canis proteins (n = 18) that exhibited antibody reactivity comparable to well-defined major antigenic proteins (TRP120 and TRP19). All of the E. chaffeensis but only some E. canis major immunoreactive proteins contained major conformation-dependent antibody epitopes. The E. chaffeensis immunoreactive proteins were generally small (< 250 amino acids; ~27kDa) and the E. canis proteins were slightly larger (> 320 amino acids; ~35 kDa). The majority of these new Ehrlichia major immunoreactive proteins were predicted to be type I secreted effectors, some of which contained transmembrane domains. Characterization of the immunomes of E. chaffeensis and E. canis and understanding the host specific Ehrlichia immune responses will facilitate identification of protective antigens and define the biophysical epitope characteristics vital to effective vaccine development for the ehrlichioses.

Molecular occurrence and genetic diversity of Ehrlichia canis in naturally infected dogs from Thailand

Canine monocytic ehrlichiosis is cause by Ehrlichia canis resulting in hematologic disorders and severe clinical signs. The aim of this study was to scrutinize the molecular detection and genetic diversity of E. canis based on the trp36 gene in dogs from Thailand's northern and central regions. A total of 120 dogs blood samples were amplified for trp36 gene of E. canis using the polymerase chain reaction (PCR). Forty-seven out of 120 dog blood samples (39.16%, 47/120) were positive for E. canis the trp36 DNA with 790 bp of PCR amplicon size. The factor significantly associated with E. canis infection is animal housing status (p < 0.05). Sequence and phylogenetic analysis showed that E. canis trp36 gene of Thailand isolates was clustered into 1st clade with similarity ranging from 95.65 to 100% together with the US genogroup. The 14 haplotypes of the trp36 gene shown in TCS network exhibited that haplotype #1-4 was found in Thailand. The entropy analysis of the trp36 gene illustrated 751 polymorphic sites and 271 entropy peaks of nucleic and amino acid sequences, respectively. Hence, these findings are crucial for better understanding the epidemiology of Ehrlichia infection and could be helpful for implementing control measures in Thailand.

Hepatocyte-specific regulation of autophagy and inflammasome activation via MyD88 during lethal Ehrlichia infection

Hepatocytes play a crucial role in host response to infection. Ehrlichia is an obligate intracellular bacterium that causes potentially life-threatening human monocytic ehrlichiosis (HME) characterized by an initial liver injury followed by sepsis and multi-organ failure. We previously showed that infection with highly virulent Ehrlichia japonica (E. japonica) induces liver damage and fatal ehrlichiosis in mice via deleterious MyD88-dependent activation of CASP11 and inhibition of autophagy in macrophage. While macrophages are major target cells for Ehrlichia, the role of hepatocytes (HCs) in ehrlichiosis remains unclear. We investigated here the role of MyD88 signaling in HCs during infection with E. japonica using primary cells from wild-type (WT) and MyD88-/- mice, along with pharmacologic inhibitors of MyD88 in a murine HC cell line. Similar to macrophages, MyD88 signaling in infected HCs led to deleterious CASP11 activation, cleavage of Gasdermin D, secretion of high mobility group box 1, IL-6 production, and inflammatory cell death, while controlling bacterial replication. Unlike macrophages, MyD88 signaling in Ehrlichia-infected HCs attenuated CASP1 activation but activated CASP3. Mechanistically, active CASP1/canonical inflammasome pathway negatively regulated the activation of CASP3 in infected MyD88-/- HCs. Further, MyD88 promoted autophagy induction in HCs, which was surprisingly associated with the activation of the mammalian target of rapamycin complex 1 (mTORC1), a known negative regulator of autophagy. Pharmacologic blocking mTORC1 activation in E. japonica-infected WT, but not infected MyD88-/- HCs, resulted in significant induction of autophagy, suggesting that MyD88 promotes autophagy during Ehrlichia infection not only in an mTORC1-indpenedent manner, but also abrogates mTORC1-mediated inhibition of autophagy in HCs. In conclusion, this study demonstrates that hepatocyte-specific regulation of autophagy and inflammasome pathway via MyD88 is distinct than MyD88 signaling in macrophages during fatal ehrlichiosis. Understanding hepatocyte-specific signaling is critical for the development of new therapeutics against liver-targeting pathogens such as Ehrlichia.

Type 1 secretion system and effectors in Rickettsiales

Obligate intracellular bacteria in the order Rickettsiales are transmitted by arthropod vectors and cause life-threatening infections in humans and animals. While both type 1 and type 4 secretion systems (T1SS and T4SS) have been identified in this group, the most extensive studies of Rickettsiales T1SS and associated effectors have been performed in Ehrlichia. These studies have uncovered important roles for the T1SS effectors in pathobiology and immunity. To evade innate immune responses and promote intracellular survival, Ehrlichia and other related obligate pathogens secrete multiple T1SS effectors which interact with a diverse network of host targets associated with essential cellular processes. T1SS effectors have multiple functional activities during infection including acting as nucleomodulins and ligand mimetics that activate evolutionarily conserved cellular signaling pathways. In Ehrlichia, an array of newly defined major immunoreactive proteins have been identified that are predicted as T1SS substrates and have conformation-dependent antibody epitopes. These findings highlight the underappreciated and largely uncharacterized roles of T1SS effector proteins in pathobiology and immunity. This review summarizes current knowledge regarding roles of T1SS effectors in Rickettsiales members during infection and explores newly identified immunoreactive proteins as potential T1SS substrates and targets of a protective host immune response.

Ehrlichia canis: Molecular characterization and genetic diversity based on the p28 and trp36 genes

Ehrlichia canis is a common tick-borne intracellular pathogen causing canine monocytic ehrlichiosis (CME) in dogs worldwide. The aims of this study were to investigate the genetic diversity and antigenicity of E. canis based on the p28 and trp36 genes in dogs in Thailand. The E. canis p28 and trp36 genes were amplified by the polymerase chain reaction (PCR) and cloned for sequencing and bioinformatic analyses. 36% (44/120) of dog blood samples were positive for E. canis DNA consisting of p28 (31%, 14/44) and trp36 (69%, 30/44) genes with 792 and 882 bp of PCR products size, respectively. The E. canis TRP36 from all Thailand sequences exhibited encoded nine amino acids (TEDSVSAPA) with 11 copies of tandem repeats along the sequences. The phylogenetic trees of E. canis, using the p28 and trp36 genes, exhibited that the Thailand isolates fell into two clades and one clade with similarity ranging from 55.95 to 100% and 100%, respectively. The results of diversity analysis revealed 10 and 20 haplotypes of the p28 and trp 36 genes, respectively. The entropy analysis of the p28 and trp36 nucleic acid sequences showed 442 and 1321 high entropy peaks respectively, whereas those of the P28 and TRP36 amino acid sequences showed 477 and 388 high entropy peaks, respectively. For B-cell epitopes analysis, the conserved amino acid of P28 and TRP36 sequences has been also demonstrated. Therefore, the results could be utilized to improve the understanding of phylogenetic relationship, genetic diversity and antigenicity of E. canis Thailand isolates.

Proteome analysis of Ehrlichia chaffeensis containing phagosome membranes revealed the presence of numerous bacterial and host proteins

Tick-transmitted Ehrlichia chaffeensis, the causative agent for human monocytic ehrlichiosis, resides and multiplies within a host cell phagosome. Infection progression of E. chaffeensis includes internalization into a host cell by host cell membrane fusion events following engulfment leading to the formation of E. chaffeensis containing vacuole (ECV). Revealing the molecular composition of ECV is important in understanding the host cellular processes, evasion of host defense pathways and in defining host-pathogen interactions. ECVs purified from infected host cells were analyzed to define both host and bacterial proteomes associated with the phagosome membranes. About 160 bacterial proteins and 2,683 host proteins were identified in the ECV membranes. The host proteins included predominantly known phagosome proteins involved in phagocytic trafficking, fusion of vesicles, protein transport, Ras signaling pathway and pathogenic infection. Many highly expressed proteins were similar to the previously documented proteins of phagosome vacuole membranes containing other obligate pathogenic bacteria. The finding of many bacterial membrane proteins is novel; they included multiple outer membrane proteins, such as the p28-Omps, the 120 kDa protein, preprotein translocases, lipoproteins, metal binding proteins, and chaperonins, although the presence of ankyrin repeat proteins, several Type I and IV secretion system proteins is anticipated. This study demonstrates that ECV membrane is extensively modified by the pathogen. This study represents the first and the most comprehensive description of ECV membrane proteome. The identity of many host and Ehrlichia proteins in the ECV membrane will be a valuable to define pathogenic mechanisms critical for the replication of the pathogen within macrophages.

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.

Novel Ehrlichia canis genogroup in dogs with canine ehrlichiosis in Cuba

Background

Canine monocytic ehrlichiosis (CME) is caused by the tick-borne pathogen Ehrlichia canis, an obligate intracellular Gram-negative bacterium of the family Anaplasmataceae with tropism for canine monocytes and macrophages. The trp36 gene, which encodes for the major immunoreactive protein TRP36 in E. canis, has been successfully used to characterize the genetic diversity of this pathogen in different regions of the world. Based on trp36 sequence analysis, four E. canis genogroups, United States (US), Taiwan (TWN), Brazil (BR) and Costa Rica (CR), have been identified. The aim of this study was to characterize the genetic diversity of E. canis in Cuba based on the trp36 gene.

Methods

Whole blood samples (n = 8) were collected from dogs found to be infested with the tick vector Rhipicephalus sanguineus sensu lato (s.l.) and/or presenting clinical signs and symptoms of CME. Total DNA was extracted from the blood samples and trp36 fragments were amplified by PCR. Nucleotide and protein sequences were compared using alignments and phylogenetic analysis.

Results

Four of the trp36 sequences obtained (n = 8) fall within the phylogenetic cluster grouping the US genogroup E. canis strains. The other E. canis trp36 sequences formed a separate and well-supported clade (94% bootstrap value) that is phylogenetically distant from the other major groups and thus represents a new genogroup, herein designated as the ‘Cuba (CUB) genogroup’. Notably, dogs infected with the CUB genogroup presented frequent hemorrhagic lesions.

Conclusions

The results of this study suggest that genetic diversification of E. canis in Cuba is associated with the emergence of E. canis strains with increased virulence.

Immunophenotypical and pathological changes in dogs experimentally infected with Ehrlichia canis

Canine monocytic ehrlichiosis (CME) is one of the most important tick-borne diseases worldwide, with multisystemic presentations. Immune dysregulation has been proposed as the primary mechanism involved in its pathogenesis and in tissue injury in dogs with CME. Experimental infection of German Shepherd dogs in the present study demonstrated that CME caused marked pathological changes in their lymph nodes and spleen, and also gave rise to mononuclear infiltration in organs and tissues. Immunophenotyping of cells in lymph nodes, spleen and injured tissues highlighted differences in lymphocyte subsets, local expression of immunoglobulin subclasses and MHCII molecules between infected and control dogs. These findings suggest that the immunophenotypic and immunopathological changes in dogs with acute experimental CME are related to Th1 bias and compartmentalized immune response.

The "Biological Weapons" of Ehrlichia chaffeensis: Novel Molecules and Mechanisms to Subjugate Host Cells

Ehrlichia chaffeensis is an obligatory intracellular bacterium that causes human monocytic ehrlichiosis, an emerging, potentially fatal tick-borne infectious disease. The bacterium enters human cells via the binding of its unique outer-membrane invasin EtpE to the cognate receptor DNase X on the host-cell plasma membrane; this triggers actin polymerization and filopodia formation at the site of E. chaffeensis binding, and blocks activation of phagocyte NADPH oxidase that catalyzes the generation of microbicidal reactive oxygen species. Subsequently, the bacterium replicates by hijacking/dysregulating host-cell functions using Type IV secretion effectors. For example, the Ehrlichia translocated factor (Etf)-1 enters mitochondria and inhibits mitochondria-mediated apoptosis of host cells. Etf-1 also induces autophagy mediated by the small GTPase RAB5, the result being the liberation of catabolites for proliferation inside host cells. Moreover, Etf-2 competes with the RAB5 GTPase-activating protein, for binding to RAB5-GTP on the surface of E. chaffeensis inclusions, which blocks GTP hydrolysis and consequently prevents the fusion of inclusions with host-cell lysosomes. Etf-3 binds ferritin light chain to induce ferritinophagy to obtain intracellular iron. To enable E. chaffeensis to rapidly adapt to the host environment and proliferate, the bacterium must acquire host membrane cholesterol and glycerophospholipids for the purpose of producing large amounts of its own membrane. Future studies on the arsenal of unique Ehrlichia molecules and their interplay with host-cell components will undoubtedly advance our understanding of the molecular mechanisms of obligatory intracellular infection and may identify hitherto unrecognized signaling pathways of human hosts. Such data could be exploited for development of treatment and control measures for ehrlichiosis as well as other ailments that potentially could involve the same host-cell signaling pathways that are appropriated by E. chaffeensis.

Advances in the Study of the Tick Cattle Microbiota and the Influence on Vectorial Capacity

The information from the tick cattle microbiota suggests that the microbial populations may modulate a successful infection process of the tick-borne pathogens. Therefore, there is a need to know the microbial population and their interactions. In this mini-review, we present several examples of how microbiota regulates the survival of pathogens inside the tick and contributes to fitness, adaptation, and tick immunity, among others. The communication between the tick microbiota and the host microbiota is vital to understanding the pathogen transmission process. As part of the tick microbiota, the pathogen interacts with different microbial populations, including the microorganisms of the host microbiota. These interactions comprise a microsystem that regulates the vectorial capacity involved in tick-borne diseases. The knowledge we have about the vectorial capacity contributes to a better understanding of tick-borne pathogens. Additionally, using approaches based on multi-omics strategies applied to studying the microbiota and its microbiome allows the development of strategies to control ticks. The results derived from those studies reveal the dynamics of the microbiota and potential targets for anti-tick vaccine development. In this context, the anti-microbiota vaccines have emerged as an alternative with a good prognosis. Some strategies developed to control other arthropods vectors, such as paratransgenesis, could control ticks and tick-borne diseases.

Immunoreactive Protein Repertoires of Ehrlichia chaffeensis and E. canis Reveal the Dominance of Hypothetical Proteins and Conformation-dependent Antibody Epitopes

The immunomes of Ehrlichia chaffeensis (E. ch.) and E. canis (E. ca.) have recently be revised to include immunodominant hypothetical proteins with conformational antibody epitopes. In this study, we examined 216 E. ch. and 190 E. ca. highly antigenic proteins according to ANTIGENpro and also performed a genome-wide hypothetical protein analysis (E. ch. n=104; E. ca. n=124) for immunoreactivity. Using cell-free protein expression and immunoanalysis, 118 E. ch. and 39 E. ca. proteins reacted with sera from naturally E. ch.-infected patients or E. ca.-infected dogs. Moreover, 22 E. ch. and 18 E. ca. proteins consistently and strongly reacted with a panel of patient or canine sera. A subset of E. ch. (n=18) and E. ca. (n=9) proteins were identified as immunodominant. Consistent with our previous study, most proteins were classified as hypothetical and the antibody epitopes exhibited complete or partial conformation-dependence. The majority (28/40; 70%) of E. ch. and E. ca. proteins contained transmembrane domains and 19 (48%) were predicted to be secreted effectors. The antigenic repertoires of E. ch. and E. ca. were mostly diverse and suggest that the immunomes of these closely related ehrlichiae are dominated by species-specific conformational antibody epitopes. This study reveals a significant group of previously undefined E. ch. and E. ca. antigens and reaffirms the importance of conformation-dependent epitopes as targets of anti-Ehrlichia immune responses. These findings substantially expand our understanding of host-Ehrlichia immune responses, advance efforts to define the molecular features of protective proteins and improve prospects for effective vaccines for the ehrlichioses.

Ehrlichia TRP effectors: moonlighting, mimicry and infection

Intracellular bacteria have evolved various strategies to evade host defense mechanisms. Remarkably, the obligately intracellular bacterium, Ehrlichia chaffeensis, hijacks host cell processes of the mononuclear phagocyte to evade host defenses through mechanisms executed in part by tandem repeat protein (TRP) effectors secreted by the type 1 secretion system. In the past decade, TRP120 has emerged as a model moonlighting effector, acting as a ligand mimetic, nucleomodulin and ubiquitin ligase. These defined functions illuminate the diverse roles TRP120 plays in exploiting and manipulating host cell processes, including cytoskeletal organization, vesicle trafficking, cell signaling, transcriptional regulation, post-translational modifications, autophagy and apoptosis. This review will focus on TRP effectors and their expanding roles in infection and provide perspective on Ehrlichia chaffeensis as an invaluable model organism for understanding infection strategies of obligately intracellular bacteria.

Anaplasmataceae: Dichotomous Autophagic Interplay for Infection

Autophagy is a vital conserved degradative process that maintains cellular homeostasis by recycling or eliminating dysfunctional cellular organelles and proteins. More recently, autophagy has become a well-recognized host defense mechanism against intracellular pathogens through a process known as xenophagy. On the host-microbe battlefield many intracellular bacterial pathogens have developed the ability to subvert xenophagy to establish infection. Obligately intracellular bacterial pathogens of the Anaplasmataceae family, including Ehrlichia chaffeensis, Anaplasma phaogocytophilium and Orientia tsutsugamushi have developed a dichotomous strategy to exploit the host autophagic pathway to obtain nutrients while escaping lysosomal destruction for intracellular survival within the host cell. In this review, the recent findings regarding how these master manipulators engage and inhibit autophagy for infection are explored. Future investigation to understand mechanisms used by Anaplasmataceae to exploit autophagy may advance novel antimicrobial therapies and provide new insights into how intracellular microbes exploit autophagy to survive.

Genetic variability of Ehrlichia canis TRP36 in ticks, dogs, and red foxes from Eurasia

Ehrlichia canis is among the most prevalent tick-borne pathogens infecting dogs worldwide, being primarily vectored by brown dog ticks, Rhipicephalus sanguineus sensu lato (s.l.). The genetic variability of E. canis has been assessed by analysis of different genes (e.g., disulfide bond formation protein gene, glycoprotein 19, tandem repeat protein 36 - TRP36) in the Americas, Africa, Asia, and in a single dog sample from Europe (i.e., Spain). This study was aimed to assess the variations in the TRP36 gene of E. canis detected in naturally infected canids and R. sanguineus s.l. ticks from different countries in Asia and Europe. DNA samples from dogs (n = 644), foxes (n = 146), and R. sanguineus s.l. ticks (n = 658) from Austria, Italy, Iran, Pakistan, India, Indonesia, Malaysia, the Philippines, Singapore, Thailand, Vietnam, and Taiwan were included in this study. Ehrlichia canis 16S rRNA positive samples (n = 115 from the previous studies; n = 14 from Austria in this study) were selected for molecular examination by analyses of TRP36 gene. Out of 129 E. canis 16S rRNA positive samples from dogs (n = 88), foxes (n = 7), and R. sanguineus s.l. ticks (n = 34), the TRP36 gene was successfully amplified from 52. The phylogenetic analysis of the TRP36 pre-repeat, tandem repeat, and post repeat regions showed that most samples were genetically close to the United States genogroup, whereas two samples from Austria and one from Pakistan clustered within the Taiwan genogroup. TRP36 sequences from all samples presented a high conserved nucleotide sequence in the tandem repeat region (from 6 to 20 copies), encoding for nine amino acids (i.e., TEDSVSAPA). Our results confirm the US genogroup as the most frequent group in dogs and ticks tested herein, whereas the Taiwan genogroup was present in a lower frequency. Besides, this study described for the first time the US genogroup in red foxes, thus revealing that these canids share identical strains with domestic dogs and R. sanguineus s.l. ticks.

Cells within cells: Rickettsiales and the obligate intracellular bacterial lifestyle

The Rickettsiales are a group of obligate intracellular vector-borne Gram-negative bacteria that include many organisms of clinical and agricultural importance, including Anaplasma spp., Ehrlichia chaffeensis, Wolbachia, Rickettsia spp. and Orientia tsutsugamushi. This Review provides an overview of the current state of knowledge of the biology of these bacteria and their interactions with host cells, with a focus on pathogenic species or those that are otherwise important for human health. This includes a description of rickettsial genomics, bacterial cell biology, the intracellular lifestyles of Rickettsiales and the mechanisms by which they induce and evade the innate immune response.

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.

Bacterial nucleomodulins: A coevolutionary adaptation to the eukaryotic command center

Through long-term interactions with their hosts, bacterial pathogens have evolved unique arsenals of effector proteins that interact with specific host targets and reprogram the host cell into a permissive niche for pathogen proliferation. The targeting of effector proteins into the host cell nucleus for modulation of nuclear processes is an emerging theme among bacterial pathogens. These unique pathogen effector proteins have been termed in recent years as "nucleomodulins." The first nucleomodulins were discovered in the phytopathogens Agrobacterium and Xanthomonas, where their nucleomodulins functioned as eukaryotic transcription factors or integrated themselves into host cell DNA to promote tumor induction, respectively. Numerous nucleomodulins were recently identified in mammalian pathogens. Bacterial nucleomodulins are an emerging family of pathogen effector proteins that evolved to target specific components of the host cell command center through various mechanisms. These mechanisms include: chromatin dynamics, histone modification, DNA methylation, RNA splicing, DNA replication, cell cycle, and cell signaling pathways. Nucleomodulins may induce short- or long-term epigenetic modifications of the host cell. In this extensive review, we discuss the current knowledge of nucleomodulins from plant and mammalian pathogens. While many nucleomodulins are already identified, continued research is instrumental in understanding their mechanisms of action and the role they play during the progression of pathogenesis. The continued study of nucleomodulins will enhance our knowledge of their effects on nuclear chromatin dynamics, protein homeostasis, transcriptional landscapes, and the overall host cell epigenome.

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

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

Ehrlichia chaffeensis and E. canis hypothetical protein immunoanalysis reveals small secreted immunodominant proteins and conformation-dependent antibody epitopes

Immunomolecular characterization of Ehrlichia chaffeensis (E. ch.) and E. canis (E. ca.) has defined protein orthologs, including tandem repeat proteins (TRPs) that have immunodominant linear antibody epitopes. In this study, we combined bioinformatic analysis and cell-free protein expression to identify undiscovered immunoreactive E. ch. and E. ca. hypothetical proteins. Antigenicity of the E. ch. and E. ca. ORFeomes (n = 1105 and n = 925, respectively) was analyzed by the sequence-based prediction model ANTIGENpro, and we identified ~250 ORFs in each respective ORFeome as highly antigenic. The hypothetical proteins (E. ch. n = 93 and E. ca. n = 98) present in the top 250 antigenic ORFs were further investigated in this study. By ELISA, 46 E. ch. and 30 E. ca. IVTT-expressed hypothetical proteins reacted with antibodies in sera from naturally E. ch.-infected patients or E. ca.-infected dogs. Moreover, 15 E. ch. and 16 E. ca. proteins consistently reacted with a panel of sera from patients or dogs, including many that revealed the immunoreactivity of “gold standard” TRPs. Antibody epitopes in most (>70%) of these proteins exhibited partial or complete conformation-dependence. The majority (23/31; 74%) of the major immunoreactive proteins identified were small (≤250 aa), and 20/31 (65%) were predicted to be secreted effectors. Unlike the strong linear antibody epitopes previously identified in TRP and OMP orthologs, there were contrasting differences in the E. ch. and E. ca. antigenic repertoires, epitopes and ortholog immunoreactivity. This study reveals numerous previously undefined immunodominant and subdominant antigens, and illustrates the breadth, complexity, and diversity of immunoreactive proteins/epitopes in Ehrlichia.

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.

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.

Sweet and Sour Ehrlichia: Glycoproteomics and Phosphoproteomics Reveal New Players in Ehrlichia ruminantium Physiology and Pathogenesis

Unraveling which proteins and post-translational modifications (PTMs) affect bacterial pathogenesis and physiology in diverse environments is a tough challenge. Herein, we used mass spectrometry-based assays to study protein phosphorylation and glycosylation in Ehrlichia ruminantium Gardel virulent (ERGvir) and attenuated (ERGatt) variants and, how they can modulate Ehrlichia biological processes. The characterization of the S/T/Y phosphoproteome revealed that both strains share the same set of phosphoproteins (n = 58), 36% being overexpressed in ERGvir. The percentage of tyrosine phosphorylation is high (23%) and 66% of the identified peptides are multi-phosphorylated. Glycoproteomics revealed a high percentage of glycoproteins (67% in ERGvir) with a subset of glycoproteins being specific to ERGvir (n = 64/371) and ERGatt (n = 36/343). These glycoproteins are involved in key biological processes such as protein, amino-acid and purine biosynthesis, translation, virulence, DNA repair, and replication. Label-free quantitative analysis revealed over-expression in 31 proteins in ERGvir and 8 in ERGatt. While further PNGase digestion confidently localized 2 and 5 N-glycoproteins in ERGvir and ERGatt, respectively, western blotting suggests that many glycoproteins are O-GlcNAcylated. Twenty-three proteins were detected in both the phospho- and glycoproteome, for the two variants. This work represents the first comprehensive assessment of PTMs on Ehrlichia biology, rising interesting questions regarding ER–host interactions. Phosphoproteome characterization demonstrates an increased versatility of ER phosphoproteins to participate in different mechanisms. The high number of glycoproteins and the lack of glycosyltransferases-coding genes highlight ER dependence on the host and/or vector cellular machinery for its own protein glycosylation. Moreover, these glycoproteins could be crucial to interact and respond to changes in ER environment. PTMs crosstalk between of O-GlcNAcylation and phosphorylation could be used as a major cellular signaling mechanism in ER. As little is known about the Ehrlichia proteins/proteome and its signaling biology, the results presented herein provide a useful resource for further hypothesis-driven exploration of Ehrlichia protein regulation by phosphorylation and glycosylation events. The mass spectrometry proteomics data have been deposited to the ProteomeXchange Consortium with the data set identifier PXD012589.

Molecular characterization of Ehrlichia canis from naturally infected dogs from the state of Rio de Janeiro

The aim of the present study was to evaluate the genetic diversity of Ehrlichia canis in naturally infected dogs from six mesoregions of Rio de Janeiro state. E. canis was diagnosed with a real-time polymerase chain reaction (qPCR) targeting a 93 base pair (bp) fragment of the 16S rDNA gene. To evaluate the genetic diversity of the parasite, we amplified a positive sample from each mesoregion by distinct conventional PCR assays with targets in the gp19 (414 bp), gp36 (814 bp), and p28 (843 bp) genes. A total of 267 samples were collected from dogs in Rio de Janeiro state. Among the samples analyzed, 42.3% (n = 113/267) were 16S rDNA-qPCR positive. When performing PCR for the gp19 and gp36 genes, 100% (n = 113/113) and 5.3% (n = 6/113) of the samples amplified fragments of 414 bp and 814 bp, respectively. The six PCR-positive samples for the gp36 gene also amplified the p28 gene fragment. The characterization based on the gp19 gene demonstrated that it is highly conserved. In protein analysis (TRP36), all samples showed a tandem repeat protein (TRP) that comprised 11 replicates. Seven high-entropy amino acid sites were distributed throughout the gp36 gene. Eleven high-entropy amino acid sites were found throughout the p28 gene. There is a positive selection pressure in both genes (p ≤ 0.05). Comparing and characterizing an organism are useful for providing important information about the host's immune response and identifying new antigenic targets, as well as essential characteristics for the development of vaccines and new diagnostic tools.

Ehrlichia chaffeensis TRP47 enters the nucleus via a MYND-binding domain-dependent mechanism and predominantly binds enhancers of host genes associated with signal transduction, cytoskeletal organization, and immune response

Ehrlichia chaffeensis is an obligately intracellular bacterium that establishes infection in mononuclear phagocytes through largely undefined reprogramming strategies including modulation of host gene transcription. In this study, we demonstrate that the E. chaffeensis effector TRP47 enters the host cell nucleus and binds regulatory regions of host genes relevant to infection. TRP47 was observed in the nucleus of E. chaffeensis-infected host cells, and nuclear localization was dependent on a variant MYND-binding domain. An electrophoretic mobility shift assay (EMSA) demonstrated that TRP47 directly binds host DNA via its tandem repeat domain. Utilizing chromatin immunoprecipitation followed by high-throughput DNA sequencing (ChIP-seq) with E. chaffeensis-infected cells, TRP47 was found to bind at multiple sites in the human genome (n = 2,051 at p < 10-30). Ontology analysis identified genes involved in functions such as immune response, cytoskeletal organization, and signal transduction. TRP47-bound genes included RNA-coding genes, many of these linked to cell proliferation or apoptosis. Comparison of TRP47 binding sites with those of previously-identified E. chaffeensis nucleomodulins identified multiple genes and gene functional categories in common including intracellular transport, cell signaling, and transcriptional regulation. Further, motif analysis followed by EMSA with synthetic oligonucleotides containing discovered motifs revealed a conserved TRP47 DNA-binding motif. This study reveals that TRP47 is a nucleomodulin that enters the nucleus via a MYND-binding domain and appears to play a role in host cell reprogramming by regulation of transcription.

Adenylyl Cyclase-Associated Protein 1: Structure, Regulation, and Participation in Cellular Processes

This review summarizes information available to date about the structural organization, regulation of functional activity of adenylyl cyclase-associated protein 1 (CAP1), and its participation in cellular processes. Numerous data are generalized on the role of CAP1 in the regulation of actin cytoskeleton and its interactions with many actin-binding proteins. Attention is drawn to the similarity of the structure of CAP1 and its contribution to the remodeling of actin filaments in prokaryotes and eukaryotes, as well as to the difference in the interaction of CAP1 with adenylyl cyclase in these cells. In addition, we discuss the participation of CAP1 in various pathological processes.

Ehrlichia chaffeensis TRP120 Effector Targets and Recruits Host Polycomb Group Proteins for Degradation To Promote Intracellular Infection

Ehrlichia chaffeensis has a group of well-characterized type I secreted tandem repeat protein (TRP) effectors that have moonlighting capabilities. TRPs modulate various cellular processes, reprogram host gene transcription as nucleomodulins, function as ubiquitin ligases, and directly activate conserved host cell signaling pathways to promote E. chaffeensis infection. One TRP-interacting host target is polycomb group ring finger protein 5 (PCGF5), a member of the polycomb group (PcG) protein family and a component of the polycomb repressive complex 1 (PRC1). The current study demonstrates that during early infection, PCGF5 strongly colocalizes with TRP120 in the nucleus and later dramatically redistributes to the ehrlichial vacuole along with other PCGF isoforms. Ectopic expression and immunoprecipitation of TRP120 confirmed the interaction of TRP120 with multiple different PCGF isoforms. At 48 h postinfection, a dramatic redistribution of PCGF isoforms from the nucleus to the ehrlichial vacuole was observed, which also temporally coincided with proteasomal degradation of PCGF isoforms and TRP120 expression on the vacuole. A decrease in PRC1-mediated repressive chromatin mark and an altered transcriptional activity in PRC1-associated Hox genes primarily from HOXB and HOXC clusters were observed along with the degradation of PCGF isoforms, suggesting disruption of the PRC1 in E. chaffeensis-infected cells. Notably, small interfering RNA (siRNA)-mediated knockdown of PCGF isoforms resulted in significantly increased E. chaffeensis infection. This study demonstrates a novel strategy in which E. chaffeensis manipulates PRC complexes through interactions between TRP120 and PCGF isoforms to promote infection.

Complete Genome Sequence of Ehrlichia canis Strain YZ-1, Isolated from a Beagle with Fever and Thrombocytopenia

We report the complete genome sequence of Ehrlichia canis strain YZ-1, which was isolated from a beagle with fever, anorexia, depression, lethargy, weight loss, and thrombocytopenia. E. canis is the tick-borne agent of canine and human monocytic ehrlichiosis.

Ehrlichia chaffeensis TRP120 Moonlights as a HECT E3 Ligase Involved in Self- and Host Ubiquitination To Influence Protein Interactions and Stability for Intracellular Survival

Ehrlichia chaffeensis secretes tandem repeat protein (TRP) effectors that are involved in a diverse array of host cell interactions, some of which directly activate cell signaling pathways and reprogram host gene transcription to promote survival in the mononuclear phagocyte. However, the molecular details of these effector-host interactions and roles in pathobiology are incompletely understood. In this study, we determined that the E. chaffeensis effector TRP120 is posttranslationally modified by ubiquitin (Ub) and that ubiquitination occurs through intrinsic and host-mediated HECT ligase activity. A functional HECT E3 ligase domain with a conserved catalytic site was identified in the C-terminal region of TRP120, and TRP120 autoubiquitination occurred in vitro in the presence of host UbcH5b/c E2 enzymes. TRP120 ubiquitination sites were mapped using a high-density microfluidic peptide array and confirmed by ectopic expression of TRP120 lysine mutants in cells. Moreover, we determined that the HECT E3 ubiquitin ligase, Nedd4L, interacts with TRP120 during infection and also mediates TRP120 ubiquitination. Nedd4L knockdown resulted in the reduction of TRP120-Ub, decreased ehrlichial infection, and reduced recruitment of a known TRP120-interacting host protein, PCGF5, to ehrlichial inclusions. TRP120-mediated PCGF5 polyubiquitination was associated with a reduction in PCGF5 levels. Inhibition of ubiquitination with small molecules also significantly decreased ehrlichial infection, indicating that the Ub pathway is critical for ehrlichial intracellular replication and survival. The current study identified a novel E. chaffeensis ubiquitin ligase and revealed an important role for the ubiquitin pathway in effector-host interactions and pathogen-mediated host protein stability in order to promote intracellular survival.

Ehrlichia chaffeensis TRP32 is a Nucleomodulin that Directly Regulates Expression of Host Genes Governing Differentiation and Proliferation

Ehrlichia chaffeensis is an obligately intracellular bacterium that reprograms the mononuclear phagocyte through diverse effector-host interactions to modulate numerous host cell processes, including transcription. In a previous study, we reported that E. chaffeensis TRP32, a type 1 secreted effector, interacts with multiple host nucleus-associated proteins and also auto-activates reporter gene expression in yeast. In this study, we demonstrate that TRP32 is a nucleomodulin that binds host DNA and alters host gene transcription. TRP32 enters the host cell nucleus via a noncanonical translocation mechanism that involves phosphorylation of Y179 located in a C-terminal tri-tyrosine motif. Both genistein and mutation of Y179 inhibited TRP32 nuclear entry. An electromobility shift assay (EMSA) demonstrated TRP32 host DNA binding via its tandem repeat domain. TRP32 DNA binding and motif preference were further confirmed by supershift assays, as well as competition and mutant probe analyses. Using ChIP-Seq, we determined that TRP32 binds a G-rich motif primarily within ±500 bp of the gene transcription start site. An ontology analysis identified genes involved in processes such as immune cell differentiation, chromatin remodeling, and RNA transcription and processing, as primary TRP32 targets. TRP32 bound genes (n=1223) were distributed on all chromosomes and included several global regulators of proliferation and inflammation such as FOS and JUN, AKT3 and NRAS, and non-coding RNA genes, miRNA 21 and miRNA 142. TRP32 target genes were differentially regulated during infection, the majority of which were repressed, and direct repression/activation of these genes by TRP32 was confirmed in vitro with a cellular luciferase reporter assay.

Ehrlichia chaffeensis TRP120 Activates Canonical Notch Signaling To Downregulate TLR2/4 Expression and Promote Intracellular Survival

Ehrlichia chaffeensis preferentially targets mononuclear phagocytes and survives through a strategy of subverting innate immune defenses, but the mechanisms are unknown. We have shown E. chaffeensis type 1 secreted tandem repeat protein (TRP) effectors are involved in diverse molecular pathogen-host interactions, such as the TRP120 interaction with the Notch receptor-cleaving metalloprotease ADAM17. In the present study, we demonstrate E. chaffeensis, via the TRP120 effector, activates the canonical Notch signaling pathway to promote intracellular survival. We found that nuclear translocation of the transcriptionally active Notch intracellular domain (NICD) occurs in response to E. chaffeensis or recombinant TRP120, resulting in upregulation of Notch signaling pathway components and target genes notch1, adam17, hes, and hey Significant differences in canonical Notch signaling gene expression levels (>40%) were observed during early and late stages of infection, indicating activation of the Notch pathway. We linked Notch pathway activation specifically to the TRP120 effector, which directly interacts with the Notch metalloprotease ADAM17. Using pharmacological inhibitors and small interfering RNAs (siRNAs) against γ-secretase enzyme, Notch transcription factor complex, Notch1, and ADAM17, we demonstrated that Notch signaling is required for ehrlichial survival. We studied the downstream effects and found that E. chaffeensis TRP120-mediated activation of the Notch pathway causes inhibition of the extracellular signal-regulated kinase 1/2 (ERK1/2) and p38 mitogen-activated protein kinase (MAPK) pathways required for PU.1 and subsequent Toll-like receptor 2/4 (TLR2/4) expression. This investigation reveals a novel mechanism whereby E. chaffeensis exploits the Notch pathway to evade the host innate immune response for intracellular survival.

IMPORTANCE

E. chaffeensis is an obligately intracellular bacterium and the etiologic agent of human monocytotropic ehrlichiosis (HME), an emerging life-threatening tick-borne zoonosis. Mechanisms by which E. chaffeensis establishes intracellular infection and avoids innate host defenses are not understood, but functionally relevant host-pathogen interactions with type 1 secreted TRP effectors are essential for the ehrlichial cellular reprogramming strategy. This study provides further insight into the molecular strategies used by obligately intracellular pathogens such as E. chaffeensis, which have small genomes and a limited number of effector proteins and exploit evolutionarily conserved host cell programs such as Notch signaling to promote infection and intracellular survival.

Hacker within! Ehrlichia chaffeensis Effector Driven Phagocyte Reprogramming Strategy

Ehrlichia chaffeensis is a small, gram negative, obligately intracellular bacterium that preferentially infects mononuclear phagocytes. It is the etiologic agent of human monocytotropic ehrlichiosis (HME), an emerging life-threatening tick-borne zoonosis. Mechanisms by which E. chaffeensis establishes intracellular infection, and avoids host defenses are not well understood, but involve functionally relevant host-pathogen interactions associated with tandem and ankyrin repeat effector proteins. In this review, we discuss the recent advances in our understanding of the molecular and cellular mechanisms that underlie Ehrlichia host cellular reprogramming strategies that enable intracellular survival.

Ehrlichia chaffeensis Exploits Canonical and Noncanonical Host Wnt Signaling Pathways To Stimulate Phagocytosis and Promote Intracellular Survival

Ehrlichia chaffeensis invades and survives in phagocytes by modulating host cell processes and evading innate defenses, but the mechanisms are not fully defined. Recently we have determined that E. chaffeensis tandem repeat proteins (TRPs) are type 1 secreted effectors involved in functionally diverse interactions with host targets, including components of the evolutionarily conserved Wnt signaling pathways. In this study, we demonstrated that induction of host canonical and noncanonical Wnt pathways by E. chaffeensis TRP effectors stimulates phagocytosis and promotes intracellular survival. After E. chaffeensis infection, canonical and noncanonical Wnt signalings were significantly stimulated during early stages of infection (1 to 3 h) which coincided with dephosphorylation and nuclear translocation of β-catenin, a major canonical Wnt signal transducer, and NFATC1, a noncanonical Wnt transcription factor. In total, the expression of ∼44% of Wnt signaling target genes was altered during infection. Knockdown of TRP120-interacting Wnt pathway components/regulators and other critical components, such as Wnt5a ligand, Frizzled 5 receptor, β-catenin, nuclear factor of activated T cells (NFAT), and major signaling molecules, resulted in significant reductions in the ehrlichial load. Moreover, small-molecule inhibitors specific for components of canonical and noncanonical (Ca(2+) and planar cell polarity [PCP]) Wnt pathways, including IWP-2, which blocks Wnt secretion, significantly decreased ehrlichial infection. TRPs directly activated Wnt signaling, as TRP-coated microspheres triggered phagocytosis which was blocked by Wnt pathway inhibitors, demonstrating a key role of TRP activation of Wnt pathways to induce ehrlichial phagocytosis. These novel findings reveal that E. chaffeensis exploits canonical and noncanonical Wnt pathways through TRP effectors to facilitate host cell entry and promote intracellular survival.

Mutations in Ehrlichia chaffeensis Causing Polar Effects in Gene Expression and Differential Host Specificities

Ehrlichia chaffeensis, a tick-borne rickettsial, is responsible for human monocytic ehrlichiosis. In this study, we assessed E. chaffeensis insertion mutations impacting the transcription of genes near the insertion sites. We presented evidence that the mutations within the E. chaffeensis genome at four genomic locations cause polar effects in altering gene expressions. We also reported mutations causing attenuated growth in deer (the pathogen’s reservoir host) and in dog (an incidental host), but not in its tick vector, Amblyomma americanum. This is the first study documenting insertion mutations in E. chaffeensis that cause polar effects in altering gene expression from the genes located upstream and downstream to insertion sites and the differential requirements of functionally active genes of the pathogen for its persistence in vertebrate and tick hosts. This study is important in furthering our knowledge on E. chaffeensis pathogenesis.

Attenuated Mutants of Ehrlichia chaffeensis Induce Protection against Wild-Type Infection Challenge in the Reservoir Host and in an Incidental Host

Ehrlichia chaffeensis, a tick-borne rickettsial organism, causes the disease human monocytic ehrlichiosis. The pathogen also causes disease in several other vertebrates, including dogs and deer. In this study, we assessed two clonally purified E. chaffeensis mutants with insertions within the genes Ech_0379 and Ech_0660 as vaccine candidates in deer and dogs. Infection with the Ech_0379 mutant and challenge with wild-type E. chaffeensis 1 month following inoculation with the mutant resulted in the reduced presence of the organism in blood compared to the presence of wild-type infection in both deer and dogs. The Ech_0660 mutant infection resulted in its rapid clearance from the bloodstream. The wild-type infection challenge following Ech_0660 mutant inoculation also caused the pathogen's clearance from blood and tissue samples as assessed at the end of the study. The Ech_0379 mutant-infected and -challenged animals also remained positive for the organism in tissue samples in deer but not in dogs. This is the first study that documents that insertion mutations in E. chaffeensis that cause attenuated growth confer protection against wild-type infection challenge. This study is important in developing vaccines to protect animals and people against Ehrlichia species infections.

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.

Canine vector-borne co-infections: Ehrlichia canis and Hepatozoon canis in the same host monocytes

The protozoon Hepatozoon canis and the rickettsia Ehrlichia canis are tick-borne pathogens, transmitted by Rhipicephalus sanguineus, which cause canine hepatozoonosis and canine monocytic ehrlichiosis, respectively. Co-infection of the same host monocytes with H. canis and E. canis confirmed by molecular characterization of the infecting agents and quantitative assessment of co-infected cells is described for the first time in three naturally-infected dogs. Blood smear evaluation indicated that at least 50% of the leukocytes infected with H. canis gamonts contained E. canis morulae. Co-infection of the same host cell demonstrated in this report suggests that infection with one pathogen may permit or enhance invasion or prolonged cellular survival of the other.

Genetic diversity of Ehrlichia canisstrains from naturally infected dogs in Rio de Janeiro, Brazil

The aim of this study was to characterize Ehrlichia canis strains from naturally infected dogs in Rio de Janeiro, Brazil. In addition, all the clinical and hematological findings observed in these dogs were reported. PCR targeting the 16S rRNA gene was used for diagnostic purposes, and the TRP19 and TRP36 genes were sequenced to evaluate the genetic diversity. Fifteen samples were positive for E. canis. The polymerase chain reaction for the TRP19 gene resulted in 11 amplicons (11/15), which were cloned into the pGEM-T easy vector for sequencing. The complete sequence of TRP19 gene was compared to those in the GenBank, revealing high identicalness. Phylogenetic analysis on the TRP36 gene sequences demonstrated two distinct strains from two dogs, named 56C and 70C. The 56C strain was grouped with the strain Cuiaba 16, which is a hybrid strain formed by Brazilian and US genogroups; and the 70C strain was grouped with other strains of the US genogroup, thus suggesting that there are at least two genogroups of E. canis in Rio de Janeiro (US and Brazilian). Those animals, in which the 70C and 56C strains were isolated, showed distinct clinical and hematological manifestations of 1the disease. The appearance of different genotypes may express new phenotypes, thus resulting in different forms of presentation of the disease and making its diagnosis more complex.

Ehrlichia chaffeensis exploits host SUMOylation pathways to mediate effector-host interactions and promote intracellular survival

Ehrlichia chaffeensis is an obligately intracellular Gram-negative bacterium that selectively infects mononuclear phagocytes. We recently reported that E. chaffeensis utilizes a type 1 secretion (T1S) system to export tandem repeat protein (TRP) effectors and demonstrated that these effectors interact with a functionally diverse array of host proteins. By way of these interactions, TRP effectors modulate host cell functions; however, the molecular basis of these interactions and their roles in ehrlichial pathobiology are not well defined. In this study, we describe the first bacterial protein posttranslational modification (PTM) by the small ubiquitin-like modifier (SUMO). The E. chaffeensis T1S effector TRP120 is conjugated to SUMO at a carboxy-terminal canonical consensus SUMO conjugation motif in vitro and in human cells. In human cells, TRP120 was selectively conjugated with SUMO2/3 isoforms. Disruption of TRP120 SUMOylation perturbed interactions with known host proteins, through predicted SUMO interaction motif-dependent and -independent mechanisms. E. chaffeensis infection did not result in dramatic changes in the global host SUMOylated protein profile, but a robust colocalization of predominately SUMO1 with ehrlichial inclusions was observed. Inhibiting the SUMO pathway with a small-molecule inhibitor had a significant impact on E. chaffeensis replication and recruitment of the TRP120-interacting protein polycomb group ring finger protein 5 (PCGF5) to the inclusion, indicating that the SUMO pathway is critical for intracellular survival. This study reveals the novel exploitation of the SUMO pathway by Ehrlichia, which facilitates effector-eukaryote interactions necessary to usurp the host and create a permissive intracellular niche.

Complete Genome Sequence of Ehrlichia muris Strain AS145T, a Model Monocytotropic Ehrlichia Strain

We report here the complete genome sequence of Ehrlichia muris strain AS145(T), which was isolated from a wild mouse in 1983 in Japan. E. muris establishes persistent infections in laboratory mice and is widely used as a surrogate pathogen in a murine model of ehrlichiosis.

The spreading process of Ehrlichia canis in macrophages is dependent on actin cytoskeleton, calcium and iron influx and lysosomal evasion

Ehrlichia canis is an obligate intracellular microorganism and the etiologic agent of canine monocytic ehrlichiosis. The invasion process has already been described for some bacteria in this genus, such as E. muris and E. chaffeensis, and consists of four stages: adhesion, internalisation, intracellular proliferation and intercellular spreading. However, little is known about the spreading process of E. canis. The aim of this study was to analyse the role of the actin cytoskeleton, calcium, iron and lysosomes from the host cell in the spreading of E. canis in dog macrophages in vitro. Different inhibitory drugs were used: cytochalasin D (actin polymerisation inhibitor), verapamil (calcium channel blocker) and deferoxamine (iron chelator). Our results showed a decrease in the number of bacteria in infected cells treated with all drugs when compared to controls. Lysosomes in infected cells were cytochemically labelled with acid phosphatase to allow the visualisation of phagosome-lysosome fusion and were further analysed by transmission electron microscopy. Phagosome-lysosome fusion was rarely observed in vacuoles containing viable E. canis. These data suggest that the spreading process of E. canis in vitro is dependent on cellular components analysed and lysosomal evasion.

Bacterial endosymbiosis in a chordate host: long-term co-evolution and conservation of secondary metabolism

Intracellular symbiosis is known to be widespread in insects, but there are few described examples in other types of host. These symbionts carry out useful activities such as synthesizing nutrients and conferring resistance against adverse events such as parasitism. Such symbionts persist through host speciation events, being passed down through vertical transmission. Due to various evolutionary forces, symbionts go through a process of genome reduction, eventually resulting in tiny genomes where only those genes essential to immediate survival and those beneficial to the host remain. In the marine environment, invertebrates such as tunicates are known to harbor complex microbiomes implicated in the production of natural products that are toxic and probably serve a defensive function. Here, we show that the intracellular symbiont Candidatus Endolissoclinum faulkneri is a long-standing symbiont of the tunicate Lissoclinum patella, that has persisted through cryptic speciation of the host. In contrast to the known examples of insect symbionts, which tend to be either relatively recent or ancient relationships, the genome of Ca. E. faulkneri has a very low coding density but very few recognizable pseudogenes. The almost complete degradation of intergenic regions and stable gene inventory of extant strains of Ca. E. faulkneri show that further degradation and deletion is happening very slowly. This is a novel stage of genome reduction and provides insight into how tiny genomes are formed. The ptz pathway, which produces the defensive patellazoles, is shown to date to before the divergence of Ca. E. faulkneri strains, reinforcing its importance in this symbiotic relationship. Lastly, as in insects we show that stable symbionts can be lost, as we describe an L. patella animal where Ca. E. faulkneri is displaced by a likely intracellular pathogen. Our results suggest that intracellular symbionts may be an important source of ecologically significant natural products in animals.

Ehrlichia moonlighting effectors and interkingdom interactions with the mononuclear phagocyte

Ehrlichia chaffeensis is an obligately intracellular gram negative bacterium with a small genome that thrives in mammalian mononuclear phagocytes by exploiting eukaryotic processes. Herein, we discuss the latest findings on moonlighting tandem repeat protein effectors and their secretion mechanisms, and novel molecular interkingdom interactions that provide insight into the intracellular pathobiology of ehrlichiae.

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.

A white-tailed deer/lone star tick model for studying transmission of Ehrlichia chaffeensis

Animal models for Ehrlichia chaffeensis have been unsuccessful in recapitulating the natural disease cycle. We have developed an animal model for tick feeding and transmission using white-tailed deer (Odocoileus virgianus), the intracellular bacterium (Ehrlichia chaffeensis), and the lone star tick vector (Amblyomma americanum). Here, we report the acquisition and transmission of E. chaffeensis infections by refeeding male ticks in this experimental model. This finding is important because techniques for gene silencing are most successful for unfed adult ticks. Males are able to refeed several days after acquiring a tick-borne pathogen. Using refeeding male lone star ticks and RNA interference technology, we plan to decipher underlying molecular mechanisms involved in transmitting E. chaffeensis to a host via a lone star tick bite.