Gene expression profiling of human promyelocytic cells in response to infection with Anaplasma phagocytophilum

Lipid droplets in pathogen infection and host immunity

As the hub of cellular lipid metabolism, lipid droplets (LDs) have been linked to a variety of biological processes. During pathogen infection, the biogenesis, composition, and functions of LDs are tightly regulated. The accumulation of LDs has been described as a hallmark of pathogen infection and is thought to be driven by pathogens for their own benefit. Recent studies have revealed that LDs and their subsequent lipid mediators contribute to effective immunological responses to pathogen infection by promoting host stress tolerance and reducing toxicity. In this comprehensive review, we delve into the intricate roles of LDs in governing the replication and assembly of a wide spectrum of pathogens within host cells. We also discuss the regulatory function of LDs in host immunity and highlight the potential for targeting LDs for the diagnosis and treatment of infectious diseases.

Genetic modification, characterization, and co-infection of Franken Sphingomonas and Anaplasma phagocytophilum in tick cells

Paratransgenesis through genetic manipulation of symbiotic or commensal microorganisms has been proposed as an effective and environmentally sound approach for the control of vector-borne diseases, including tick bite-related pathologies, and reducing pathogen transmission. Here, we present a protocol for Sphingomonas transformation with Anaplasma phagocytophilum major surface protein 4 and heat shock protein 70. We describe a step-by-step protocol for in vitro study of interactions between transformed Franken Sphingomonas and Ixodes scapularis ISE6 tick cells during A. phagocytophilum infection. For complete details on the use and execution of this protocol, please refer to Mazuecos et al. (2023).1.

Low NETosis Induced in Anaplasma phagocytophilum-Infected Cells

Anaplasma phagocytophilum are obligatory intracellular bacteria that preferentially replicate inside leukocytes by utilizing biological compounds and processes of these primary host defensive cells. In this study, bioinformatics analysis was conducted to further characterize A. phagocytophilum–host interactions using the neutrophil-like model of human Caucasian promyelocytic leukemia HL60 cells. We detected a hierarchy of molecules involved in A. phagocytophilum-HL60 interactions with overrepresentation in infected human cells of proteins involved in the reactive oxygen species (ROS) pathway and cell surface monocyte markers. As A. phagocytophilum phagocytosis by neutrophils is inhibited, the results suggested a possible explanation for our bioinformatics data: radical oxygen compounds could induce the killing of bacteria activating NETosis, a unique form of defense mechanism resulting in cell death that is characterized by the release of decondensed chromatin and granular contents to the extracellular space, forming neutrophil extracellular traps (NETs) to eliminate invading microorganisms. Thus, we confirmed the existence of a low NETosis induced in A. phagocytophilum-infected cells by immunofluorescence (IF) experiments. These results provide new insights into the complex mechanisms that govern immune response during A. phagocytophilum host interactions.

Anaplasma phagocytophilum Activates NF-κB Signaling via Redundant Pathways

Anaplasma phagocytophilum subverts neutrophil function permitting intracellular survival, propagation and transmission. Sustained pro-inflammatory response, recruitment of new host cells for population expansion, and delayed apoptosis are associated with prolonged nuclear presence of NF-κB. We investigated NF-κB signaling and transcriptional activity with A. phagocytophilum infection using inhibitors of NF-κB signaling pathways, and through silencing of signaling pathway genes. How inhibitors or silencing affected A. phagocytophilum growth, inflammatory response (transcription of the κB-enhanced genes CXCL8 and MMP9), and NF-κB signaling pathway gene expression were tested. Among A. phagocytophilum-infected HL-60 cells, nuclear NF-κB p50, p65, and p52 were detected by immunoblots or iTRAQ proteomics. A. phagocytophilum growth was affected most by the IKKαβ inhibitor wedelolactone (reductions of 96 to 99%) as compared with SC-514 that selectively inhibits IKKβ, illustrating a role for the non-canonical pathway. Wedelolactone inhibited transcription of both CXCL8 (p = 0.001) and MMP9 (p = 0.002) in infected cells. Compared to uninfected THP-1 cells, A. phagocytophilum infection led to >2-fold down regulation of 64 of 92 NF-κB signaling pathway genes, and >2-fold increased expression in only 4. Wedelolactone and SC-514 reversed downregulation in all 64 and 45, respectively, of the genes down-regulated by infection, but decreased expression in 1 gene with SC-514 only. Silencing of 20 NF-κB signal pathway genes increased bacterial growth in 12 (IRAK1, MAP3K1, NFKB1B, MAP3K7, TICAM2, TLR3, TRADD, TRAF3, CHUK, IRAK2, LTBR, and MALT1). Most findings support canonical pathway activation; however, the presence of NFKB2 in infected cell nuclei, selective non-canonical pathway inhibitors that dampen CXCL8 and MMP9 transcription with infection, upregulation of non-canonical pathway target genes CCL13 and CCL19, enhanced bacterial growth with TRAF3 and LTBR silencing provide evidence for non-canonical pathway signaling. Whether this impacts distinct inflammatory processes that underlie disease, and whether and how A. phagocytophilum subverts NF-κB signaling via these pathways, need to be investigated.

Modulation of Host Lipid Pathways by Pathogenic Intracellular Bacteria

Lipids are a broad group of molecules required for cell maintenance and homeostasis. Various intracellular pathogens have developed mechanisms of modulating and sequestering host lipid processes for a large array of functions for both bacterial and host cell survival. Among the host cell lipid functions that intracellular bacteria exploit for infection are the modulation of host plasma membrane microdomains (lipid rafts) required for efficient bacterial entry; the recruitment of specific lipids for membrane integrity of intracellular vacuoles; and the utilization of host lipid droplets for the regulation of immune responses and for energy production through fatty acid β-oxidation and oxidative phosphorylation. The majority of published studies on the utilization of these host lipid pathways during infection have focused on intracellular bacterial pathogens that reside within a vacuole during infection and, thus, have vastly different requirements for host lipid metabolites when compared to those intracellular pathogens that are released into the host cytosol upon infection. Here we summarize the mechanisms by which intracellular bacteria sequester host lipid species and compare the modulation of host lipid pathways and metabolites during host cell infection by intracellular pathogens residing in either a vacuole or within the cytosol of infected mammalian cells. This review will also highlight common and unique host pathways necessary for intracellular bacterial growth that could potentially be targeted for therapeutic intervention.

Allergic Reactions and Immunity in Response to Tick Salivary Biogenic Substances and Red Meat Consumption in the Zebrafish Model

Ticks are arthropod ectoparasite vectors of pathogens and the cause of allergic reactions affecting human health worldwide. In humans, tick bites can induce high levels of immunoglobulin E antibodies against the carbohydrate Galα1-3Galβ1-(3)4GlcNAc-R (α-Gal) present in glycoproteins and glycolipids from tick saliva that mediate anaphylactic reactions known as the alpha-Gal syndrome (AGS) or red meat allergy. In this study, a new animal model was developed using zebrafish for the study of allergic reactions and the immune mechanisms in response to tick salivary biogenic substances and red meat consumption. The results showed allergic hemorrhagic anaphylactic-type reactions and abnormal behavior patterns likely in response to tick salivary toxic and anticoagulant biogenic compounds different from α-Gal. However, the results showed that only zebrafish previously exposed to tick saliva developed allergic reactions to red meat consumption with rapid desensitization and tolerance. These allergic reactions were associated with tissue-specific Toll-like receptor-mediated responses in types 1 and 2 T helper cells (T_H1 and T_H2) with a possible role for basophils in response to tick saliva. These results support previously proposed immune mechanisms triggering the AGS and provided evidence for new mechanisms also potentially involved in the AGS. These results support the use of the zebrafish animal model for the study of the AGS and other tick-borne allergies.

Tick Bites Induce Anti-α-Gal Antibodies in Dogs

Due to the functional inactivation of the gene encoding for the enzyme that is involved in the oligosaccharide galactose-α-1,3-galactose (α-Gal) synthesis, humans and Old-World primates are able to produce a large amount of antibodies against the glycan epitope. Apart from being involved in the hyperacute organ rejection in humans, anti-α-Gal antibodies have shown a protective effect against some pathogenic agents and an implication in the recently recognized tick-induced mammalian meat allergy. Conversely, non-primate mammals, including dogs, have the ability to synthetize α-Gal and, thus, their immune system is not expected to naturally generate the antibodies toward this self-antigen molecule. However, in the current study, we detected specific IgG, IgM, and IgE antibodies to α-Gal in sera of clinically healthy dogs by an indirect enzyme-linked immunosorbent assay (ELISA) for the first time. Furthermore, in a tick infestation experiment, we showed that bites of Ixodes ricinus induce the immune response to α-Gal in dogs and that the resulting antibodies (IgM) might be protective against Anaplasma phagocytophilum. These findings may help lead to a better understanding of the underlying mechanisms involved in mammalian meat allergy and tick-host-pathogen interactions, but they also open up the question about the possibility that dogs could develop an allergy to mammalian meat after tick bites, similar to that in humans.

The redox metabolic pathways function to limit Anaplasma phagocytophilum infection and multiplication while preserving fitness in tick vector cells

Aerobic organisms evolved conserved mechanisms controlling the generation of reactive oxygen species (ROS) to maintain redox homeostasis signaling and modulate signal transduction, gene expression and cellular functional responses under physiological conditions. The production of ROS by mitochondria is essential in the oxidative stress associated with different pathologies and in response to pathogen infection. Anaplasma phagocytophilum is an intracellular pathogen transmitted by Ixodes scapularis ticks and causing human granulocytic anaplasmosis. Bacteria multiply in vertebrate neutrophils and infect first tick midgut cells and subsequently hemocytes and salivary glands from where transmission occurs. Previous results demonstrated that A. phagocytophilum does not induce the production of ROS as part of its survival strategy in human neutrophils. However, little is known about the role of ROS during pathogen infection in ticks. In this study, the role of tick oxidative stress during A. phagocytophilum infection was characterized through the function of different pathways involved in ROS production. The results showed that tick cells increase mitochondrial ROS production to limit A. phagocytophilum infection, while pathogen inhibits alternative ROS production pathways and apoptosis to preserve cell fitness and facilitate infection. The inhibition of NADPH oxidase-mediated ROS production by pathogen infection appears to occur in both neutrophils and tick cells, thus supporting that A. phagocytophilum uses common mechanisms for infection of ticks and vertebrate hosts. However, differences in ROS response to A. phagocytophilum infection between human and tick cells may reflect host-specific cell tropism that evolved during pathogen life cycle.

Anaplasma phagocytophilum modifies tick cell microRNA expression and upregulates isc-mir-79 to facilitate infection by targeting the Roundabout protein 2 pathway

The microRNAs (miRNAs) are a class of small noncoding RNAs that have important regulatory roles in multicellular organisms including innate and adaptive immune pathways to control bacterial, parasite and viral infections, and pathogens could modify host miRNA profile to facilitate infection and multiplication. Therefore, understanding the function of host miRNAs in response to pathogen infection is relevant to characterize host-pathogen molecular interactions and to provide new targets for effective new interventions for the control infectious diseases. The objective of this study was to characterize the dynamics and functional significance of the miRNA response of the tick vector Ixodes scapularis in response to Anaplasma phagocytophilum infection, the causative agent of human and animal granulocytic anaplasmosis. To address this objective, the composition of tick miRNAs, functional annotation, and expression profiling was characterized using high throughout RNA sequencing in uninfected and A. phagocytophilum-infected I. scapularis ISE6 tick cells, a model for tick hemocytes involved in pathogen infection. The results provided new evidences on the role of tick miRNA during pathogen infection, and showed that A. phagocytophilum modifies I. scapularis tick cell miRNA profile and upregulates isc-mir-79 to facilitate infection by targeting the Roundabout protein 2 (Robo2) pathway. Furthermore, these results suggested new targets for interventions to control pathogen infection in ticks.

Modeling Modulation of the Tick Regulome in Response to Anaplasma phagocytophilum for the Identification of New Control Targets

Ticks act as vectors of pathogens affecting human and animal health worldwide, and recent research has focused on the characterization of tick-pathogen interactions using omics technologies to identify new targets for developing novel control interventions. The regulome (transcription factors-target genes interactions) plays a critical role in cell response to pathogen infection. Therefore, the application of regulomics to tick-pathogen interactions would advance our understanding of these molecular interactions and contribute to the identification of novel control targets for the prevention and control of tick infestations and tick-borne diseases. However, limited information is available on the role of tick regulome in response to pathogen infection. In this study, we applied complementary in silico approaches to modeling how Anaplasma phagocytophilum infection modulates tick vector regulome. This proof-of-concept research provided support for the use of network analysis in the study of regulome response to infection, resulting in new information on tick-pathogen interactions and potential targets for developing interventions for the control of tick infestations and pathogen transmission. Deciphering the precise nature of circuits that shape the tick regulome in response to pathogen infection is an area of research that in the future will advance our knowledge of tick-pathogen interactions, and the identification of new antigens for the control of tick infestations and pathogen infection/transmission.

Lipid Droplets: A Significant but Understudied Contributor of Host⁻Bacterial Interactions

Lipid droplets (LDs) are cytosolic lipid storage organelles that are important for cellular lipid metabolism, energy homeostasis, cell signaling, and inflammation. Several bacterial, viral and protozoal pathogens exploit host LDs to promote infection, thus emphasizing the importance of LDs at the host–pathogen interface. In this review, we discuss the thus far reported relation between host LDs and bacterial pathogens including obligate and facultative intracellular bacteria, and extracellular bacteria. Although there is less evidence for a LD–extracellular bacterial interaction compared to interactions with intracellular bacteria, in this review, we attempt to compare the bacterial mechanisms that target LDs, the host signaling pathways involved and the utilization of LDs by these bacteria. Many intracellular bacteria employ unique mechanisms to target host LDs and potentially obtain nutrients and lipids for vacuolar biogenesis and/or immune evasion. However, extracellular bacteria utilize LDs to either promote host tissue damage or induce host death. We also identify several areas that require further investigation. Along with identifying LD interactions with bacteria besides the ones reported, the precise mechanisms of LD targeting and how LDs benefit pathogens should be explored for the bacteria discussed in the review. Elucidating LD–bacterial interactions promises critical insight into a novel host–pathogen interaction.

Tick galactosyltransferases are involved in α-Gal synthesis and play a role during Anaplasma phagocytophilum infection and Ixodes scapularis tick vector development

The carbohydrate Galα1-3Galβ1-(3)4GlcNAc-R (α-Gal) is produced in all mammals except for humans, apes and old world monkeys that lost the ability to synthetize this carbohydrate. Therefore, humans can produce high antibody titers against α-Gal. Anti-α-Gal IgE antibodies have been associated with tick-induced allergy (i.e. α-Gal syndrome) and anti-α-Gal IgG/IgM antibodies may be involved in protection against malaria, leishmaniasis and Chagas disease. The α-Gal on tick salivary proteins plays an important role in the etiology of the α-Gal syndrome. However, whether ticks are able to produce endogenous α-Gal remains currently unknown. In this study, the Ixodes scapularis genome was searched for galactosyltransferases and three genes were identified as potentially involved in the synthesis of α-Gal. Heterologous gene expression in α-Gal-negative cells and gene knockdown in ticks confirmed that these genes were involved in α-Gal synthesis and are essential for tick feeding. Furthermore, these genes were shown to play an important role in tick-pathogen interactions. Results suggested that tick cells increased α-Gal levels in response to Anaplasma phagocytophilum infection to control bacterial infection. These results provided the molecular basis of endogenous α-Gal production in ticks and suggested that tick galactosyltransferases are involved in vector development, tick-pathogen interactions and possibly the etiology of α-Gal syndrome in humans.

Use of Graph Theory to Characterize Human and Arthropod Vector Cell Protein Response to Infection With Anaplasma phagocytophilum

One of the major challenges in modern biology is the use of large omics datasets for the characterization of complex processes such as cell response to infection. These challenges are even bigger when analyses need to be performed for comparison of different species including model and non-model organisms. To address these challenges, the graph theory was applied to characterize the tick vector and human cell protein response to infection with Anaplasma phagocytophilum, the causative agent of human granulocytic anaplasmosis. A network of interacting proteins and cell processes clustered in biological pathways, and ranked with indexes representing the topology of the proteome was prepared. The results demonstrated that networks of functionally interacting proteins represented in both infected and uninfected cells can describe the complete set of host cell processes and metabolic pathways, providing a deeper view of the comparative host cell response to pathogen infection. The results demonstrated that changes in the tick proteome were driven by modifications in protein representation in response to A. phagocytophilum infection. Pathogen infection had a higher impact on tick than human proteome. Since most proteins were linked to several cell processes, the changes in protein representation affected simultaneously different biological pathways. The method allowed discerning cell processes that were affected by pathogen infection from those that remained unaffected. The results supported that human neutrophils but not tick cells limit pathogen infection through differential representation of ras-related proteins. This methodological approach could be applied to other host-pathogen models to identify host derived key proteins in response to infection that may be used to develop novel control strategies for arthropod-borne pathogens.

Alternative Splicing of Differentiated Myeloid Cell Transcripts after Infection by Anaplasma phagocytophilum Impacts a Selective Group of Cellular Programs

Eukaryotic proteome diversity exceeds that encoded within individual genes, and results in part from alternative splicing events of pre-messenger RNA. The diversity of these splicing events can shape the outcome in development and differentiation of normal tissues, and is important in pathogenic circumstances such as cancer and some heritable conditions. A role for alternative splicing of eukaryotic genes in response to viral and intracellular bacterial infections has only recently been recognized, and plays an important role in providing fitness for microbial survival, while potentially enhancing pathogenicity. Anaplasma phagocytophilum survives within mammalian neutrophils by reshaping transcriptional programs that govern cellular functions. We applied next generation RNAseq to ATRA-differentiated HL-60 cells established to possess transcriptional and functional responses similar to A. phagocytophilum-infected human neutrophils. This demonstrated an increase in transcripts with infection and high proportion of alternatively spliced transcript events (ASEs) for which predicted gene ontology processes were in part distinct from those identified by evaluation of single transcripts or gene-level analyses alone. The alternative isoforms are not on average shorter, and no alternative splicing in genes encoding spliceosome components is noted. Although not evident at gene-level analyses, individual spliceosome transcripts that impact nearly all spliceosome components were significantly upregulated. How the distinct GO processes predicted by ASEs are regulated by infection and whether they are relevant to fitness or pathogenicity of A. phagocytophilum should be addressed in more detailed studies.

Anaplasma phagocytophilum infection modulates expression of megakaryocyte cell cycle genes through phosphatidylinositol-3-kinase signaling

Anaplasma phagocytophilum, the agent of human granulocytic anaplasmosis infects neutrophils and other cells from hematopoietic origin. Using human megakaryocytic cell line, MEG-01, we show that expression of cell cycle genes in these cells are altered upon A. phagocytophilum infection. Expression of several cell cycle genes in MEG-01 cells was significantly up regulated at early and then down regulated at later stages of A. phagocytophilum infection. Lactate dehydrogenase (LDH) assays revealed reduced cellular cytotoxicity in MEG-01 cells upon A. phagocytophilum infection. The levels of both PI3KCA (p110 alpha, catalytic subunit) and PI3KR1 (p85, regulatory subunit) of Class I PI3 kinases and phosphorylated protein kinase B (Akt/PKB) and inhibitory kappa B (IκB) were elevated at both early and late stages of A. phagocytophilum infection. Inhibition of PI3 kinases with LY294002 treatment resulted in significant reduction in the expression of tested cell cycle genes, A. phagocytophilum burden and phosphorylated Akt levels in these MEG-01 cells. Collectively, these results suggest a role for PI3K-Akt-NF-κB signaling pathway in the modulation of megakaryocyte cell cycle genes upon A. phagocytophilum infection.

Interferon-γ-dependent control of Anaplasma phagocytophilum by murine neutrophil granulocytes

Background

Anaplasma phagocytophilum is a Gram-negative obligate intracellular bacterium that is transmitted by ticks of the Ixodes ricinus complex. It replicates in neutrophils and elicits febrile disease in humans and animals. Because of its striking tropism for neutrophils, A. phagocytophilum has been used as a model organism to study the immune response against obligate intracellular pathogens. In mice, the control of A. phagocytophilum in the early phase of infection is dependent on natural killer cell-derived interferon-γ (IFN-γ). In contrast, the final elimination strictly requires CD4⁺ T-cells. It is a matter of debate, whether neutrophils serve only as host cells or as killer cells as well.

Results

To study this, we used in vitro generated murine neutrophils with defects in major antimicrobial molecules such as NADPH-oxidase (gp91^phox−/−), myeloperoxidase (MPO^−/−) and inducible nitric oxide synthase (iNOS^−/−). However, bacterial growth in gene-deficient neutrophils was comparable to that in wild-type cells. Whereas gp91^phox and MPO expression remained unchanged, the infection led to an induction of iNOS. In neutrophils stimulated with IFN-γ, bacterial growth was significantly impaired, and iNOS was induced. However, the antibacterial effect of IFN-γ was still seen in iNOS^−/− neutrophils.

Conclusion

Thus, murine in vitro generated neutrophils stimulated with IFN-γ seem to act as killer cells by an iNOS-independent mechanism.

Electronic supplementary material

The online version of this article (doi:10.1186/s13071-017-2274-6) contains supplementary material, which is available to authorized users.

Anaplasma phagocytophilum MSP4 and HSP70 Proteins Are Involved in Interactions with Host Cells during Pathogen Infection

Anaplasma phagocytophilum transmembrane and surface proteins play a role during infection and multiplication in host neutrophils and tick vector cells. Recently, A. phagocytophilum Major surface protein 4 (MSP4) and Heat shock protein 70 (HSP70) were shown to be localized on the bacterial membrane, with a possible role during pathogen infection in ticks. In this study, we hypothesized that A. phagocytophilum MSP4 and HSP70 have similar functions in tick-pathogen and host-pathogen interactions. To address this hypothesis, herein we characterized the role of these bacterial proteins in interaction and infection of vertebrate host cells. The results showed that A. phagocytophilum MSP4 and HSP70 are involved in host-pathogen interactions, with a role for HSP70 during pathogen infection. The analysis of the potential protective capacity of MSP4 and MSP4-HSP70 antigens in immunized sheep showed that MSP4-HSP70 was only partially protective against pathogen infection. This limited protection may be associated with several factors, including the recognition of non-protective epitopes by IgG in immunized lambs. Nevertheless, these antigens may be combined with other candidate protective antigens for the development of vaccines for the control of human and animal granulocytic anaplasmosis. Focusing on the characterization of host protective immune mechanisms and protein-protein interactions at the host-pathogen interface may lead to the discovery and design of new effective protective antigens.

Manipulation of Host Cholesterol by Obligate Intracellular Bacteria

Cholesterol is a multifunctional lipid that plays important metabolic and structural roles in the eukaryotic cell. Despite having diverse lifestyles, the obligate intracellular bacterial pathogens Chlamydia, Coxiella, Anaplasma, Ehrlichia, and Rickettsia all target cholesterol during host cell colonization as a potential source of membrane, as well as a means to manipulate host cell signaling and trafficking. To promote host cell entry, these pathogens utilize cholesterol-rich microdomains known as lipid rafts, which serve as organizational and functional platforms for host signaling pathways involved in phagocytosis. Once a pathogen gains entrance to the intracellular space, it can manipulate host cholesterol trafficking pathways to access nutrient-rich vesicles or acquire membrane components for the bacteria or bacteria-containing vacuole. To acquire cholesterol, these pathogens specifically target host cholesterol metabolism, uptake, efflux, and storage. In this review, we examine the strategies obligate intracellular bacterial pathogens employ to manipulate cholesterol during host cell colonization. Understanding how obligate intracellular pathogens target and use host cholesterol provides critical insight into the host-pathogen relationship.

Genome-Wide Anaplasma phagocytophilum AnkA-DNA Interactions Are Enriched in Intergenic Regions and Gene Promoters and Correlate with Infection-Induced Differential Gene Expression

Anaplasma phagocytophilum, an obligate intracellular prokaryote, infects neutrophils, and alters cardinal functions via reprogrammed transcription. Large contiguous regions of neutrophil chromosomes are differentially expressed during infection. Secreted A. phagocytophilum effector AnkA transits into the neutrophil or granulocyte nucleus to complex with DNA in heterochromatin across all chromosomes. AnkA binds to gene promoters to dampen cis-transcription and also has features of matrix attachment region (MAR)-binding proteins that regulate three-dimensional chromatin architecture and coordinate transcriptional programs encoded in topologically-associated chromatin domains. We hypothesize that identification of additional AnkA binding sites will better delineate how A. phagocytophilum infection results in reprogramming of the neutrophil genome. Using AnkA-binding ChIP-seq, we showed that AnkA binds broadly throughout all chromosomes in a reproducible pattern, especially at: (i) intergenic regions predicted to be MARs; (ii) within predicted lamina-associated domains; and (iii) at promoters ≤ 3000 bp upstream of transcriptional start sites. These findings provide genome-wide support for AnkA as a regulator of cis-gene transcription. Moreover, the dominant mark of AnkA in distal intergenic regions known to be AT-enriched, coupled with frequent enrichment in the nuclear lamina, provides strong support for its role as a MAR-binding protein and genome “re-organizer.” AnkA must be considered a prime candidate to promote neutrophil reprogramming and subsequent functional changes that belie improved microbial fitness and pathogenicity.

Anaplasma phagocytophilum Manipulates Host Cell Apoptosis by Different Mechanisms to Establish Infection

Anaplasma phagocytophilum is an emerging zoonotic pathogen that causes human and animal granulocytic anaplasmosis and tick-borne fever of ruminants. This obligate intracellular bacterium evolved to use common strategies to establish infection in both vertebrate hosts and tick vectors. Herein, we discuss the different strategies used by the pathogen to modulate cell apoptosis and establish infection in host cells. In vertebrate neutrophils and human promyelocytic cells HL-60, both pro-apoptotic and anti-apoptotic factors have been reported. Tissue-specific differences in tick response to infection and differential regulation of apoptosis pathways have been observed in adult female midguts and salivary glands in response to infection with A. phagocytophilum. In tick midguts, pathogen inhibits apoptosis through the Janus kinase/signal transducers and activators of transcription (JAK/STAT) pathway, while in salivary glands, the intrinsic apoptosis pathways is inhibited but tick cells respond with the activation of the extrinsic apoptosis pathway. In Ixodes scapularis ISE6 cells, bacterial infection down-regulates mitochondrial porin and manipulates protein processing in the endoplasmic reticulum and cell glucose metabolism to inhibit apoptosis and facilitate infection, whereas in IRE/CTVM20 tick cells, inhibition of apoptosis appears to be regulated by lower caspase levels. These results suggest that A. phagocytophilum uses different mechanisms to inhibit apoptosis for infection of both vertebrate and invertebrate hosts.

Anaplasma phagocytophilum Uses Common Strategies for Infection of Ticks and Vertebrate Hosts

The tick-borne rickettsial pathogen Anaplasma phagocytophilum develops within membrane-bound inclusions in the host cell cytoplasm. This pathogen has evolved with its tick and vertebrate hosts through dynamic processes involving genetic traits of the pathogen and hosts that collectively mediate pathogen infection, development, persistence, and survival. Herein, we challenge the evidence of tick-host-pathogen coevolution by hypothesizing that A. phagocytophilum utilizes common molecular mechanisms for infection in both vertebrate and tick cells, including remodeling of the cytoskeleton, inhibition of cell apoptosis, and manipulation of the immune response. The discovery of these common mechanisms provides evidence that a control strategy could be developed targeted at both vertebrate and tick hosts for more complete control of A. phagocytophilum and its associated diseases.

Integrated Metabolomics, Transcriptomics and Proteomics Identifies Metabolic Pathways Affected by Anaplasma phagocytophilum Infection in Tick Cells

Anaplasma phagocytophilum is an emerging zoonotic pathogen that causes human granulocytic anaplasmosis. These intracellular bacteria establish infection by affecting cell function in both the vertebrate host and the tick vector, Ixodes scapularis. Previous studies have characterized the tick transcriptome and proteome in response to A. phagocytophilum infection. However, in the postgenomic era, the integration of omics datasets through a systems biology approach allows network-based analyses to describe the complexity and functionality of biological systems such as host-pathogen interactions and the discovery of new targets for prevention and control of infectious diseases. This study reports the first systems biology integration of metabolomics, transcriptomics, and proteomics data to characterize essential metabolic pathways involved in the tick response to A. phagocytophilum infection. The ISE6 tick cells used in this study constitute a model for hemocytes involved in pathogen infection and immune response. The results showed that infection affected protein processing in endoplasmic reticulum and glucose metabolic pathways in tick cells. These results supported tick-Anaplasma co-evolution by providing new evidence of how tick cells limit pathogen infection, while the pathogen benefits from the tick cell response to establish infection. Additionally, ticks benefit from A. phagocytophilum infection by increasing survival while pathogens guarantee transmission. The results suggested that A. phagocytophilum induces protein misfolding to limit the tick cell response and facilitate infection but requires protein degradation to prevent ER stress and cell apoptosis to survive in infected cells. Additionally, A. phagocytophilum may benefit from the tick cell's ability to limit bacterial infection through PEPCK inhibition leading to decreased glucose metabolism, which also results in the inhibition of cell apoptosis that increases infection of tick cells. These results support the use of this experimental approach to systematically identify cell pathways and molecular mechanisms involved in tick-pathogen interactions. Data are available via ProteomeXchange with identifier PXD002181.

Identification and Characterization of Anaplasma phagocytophilum Proteins Involved in Infection of the Tick Vector, Ixodes scapularis

Anaplasma phagocytophilum is an emerging zoonotic pathogen transmitted by Ixodes scapularis that causes human granulocytic anaplasmosis. Here, a high throughput quantitative proteomics approach was used to characterize A. phagocytophilum proteome during rickettsial multiplication and identify proteins involved in infection of the tick vector, I. scapularis. The first step in this research was focused on tick cells infected with A. phagocytophilum and sampled at two time points containing 10–15% and 65–71% infected cells, respectively to identify key bacterial proteins over-represented in high percentage infected cells. The second step was focused on adult female tick guts and salivary glands infected with A. phagocytophilum to compare in vitro results with those occurring during bacterial infection in vivo. The results showed differences in the proteome of A. phagocytophilum in infected ticks with higher impact on protein synthesis and processing than on bacterial replication in tick salivary glands. These results correlated well with the developmental cycle of A. phagocytophilum, in which cells convert from an intracellular reticulated, replicative form to the nondividing infectious dense-core form. The analysis of A. phagocytophilum differentially represented proteins identified stress response (GroEL, HSP70) and surface (MSP4) proteins that were over-represented in high percentage infected tick cells and salivary glands when compared to low percentage infected cells and guts, respectively. The results demonstrated that MSP4, GroEL and HSP70 interact and bind to tick cells, thus playing a role in rickettsia-tick interactions. The most important finding of these studies is the increase in the level of certain bacterial stress response and surface proteins in A. phagocytophilum-infected tick cells and salivary glands with functional implication in tick-pathogen interactions. These results gave a new dimension to the role of these stress response and surface proteins during A. phagocytophilum infection in ticks. Characterization of Anaplasma proteome contributes information on host-pathogen interactions and provides targets for development of novel control strategies for pathogen infection and transmission.

Global DNA methylation changes and differential gene expression in Anaplasma phagocytophilum-infected human neutrophils

Background

Anaplasma phagocytophilum is an obligate intracellular prokaryotic pathogen that both infects and replicates within human neutrophils. The bacterium represses multiple antimicrobial functions while simultaneously increasing proinflammatory functions by reprogramming the neutrophil genome. Previous reports show that many observed phenotypic changes are in part explained by altered gene transcription. We recently identified that large chromosomal regions of the neutrophil genome are differentially expressed during A. phagocytophilum infection. Because of this, we sought to determine whether gene expression programs altered by infection were the result of changes in the host neutrophil DNA methylome.

Results

Within 24 h of infection, marked increases in DNA methylation were observed genome-wide as compared with mock-infected controls and pharmacologic inhibition of DNA methyltransferases resulted in decreased bacterial growth. New regions of DNA methylation were enriched at intron and exon junctions; however, intragenic methylation did not correlate with altered gene expression. In contrast, intergenic DNA methylation was associated with A. phagocytophilum-induced gene expression changes. Within the major histocompatibility complex locus on chromosome 6, a region with marked changes in infection-induced differential gene expression, new regions of methylation were localized to boundaries of active and inactive chromatin.

Conclusions

These data strongly suggest that A. phagocytophilum infection, in addition to altering histone structure, alters DNA methylation and the epigenome of its host cell to promote survival and replication, providing evidence that such bacterial infection can radically alter the epigenome of its host cell.

Electronic supplementary material

The online version of this article (doi:10.1186/s13148-015-0105-1) contains supplementary material, which is available to authorized users.

Nuclease Tudor-SN Is Involved in Tick dsRNA-Mediated RNA Interference and Feeding but Not in Defense against Flaviviral or Anaplasma phagocytophilum Rickettsial Infection

Tudor staphylococcal nuclease (Tudor-SN) and Argonaute (Ago) are conserved components of the basic RNA interference (RNAi) machinery with a variety of functions including immune response and gene regulation. The RNAi machinery has been characterized in tick vectors of human and animal diseases but information is not available on the role of Tudor-SN in tick RNAi and other cellular processes. Our hypothesis is that tick Tudor-SN is part of the RNAi machinery and may be involved in innate immune response and other cellular processes. To address this hypothesis, Ixodes scapularis and I. ricinus ticks and/or cell lines were used to annotate and characterize the role of Tudor-SN in dsRNA-mediated RNAi, immune response to infection with the rickettsia Anaplasma phagocytophilum and the flaviviruses TBEV or LGTV and tick feeding. The results showed that Tudor-SN is conserved in ticks and involved in dsRNA-mediated RNAi and tick feeding but not in defense against infection with the examined viral and rickettsial pathogens. The effect of Tudor-SN gene knockdown on tick feeding could be due to down-regulation of genes that are required for protein processing and blood digestion through a mechanism that may involve selective degradation of dsRNAs enriched in G:U pairs that form as a result of adenosine-to-inosine RNA editing. These results demonstrated that Tudor-SN plays a role in tick RNAi pathway and feeding but no strong evidence for a role in innate immune responses to pathogen infection was found.

Chromatin-bound bacterial effector ankyrin A recruits histone deacetylase 1 and modifies host gene expression

Control of host epigenetics is becoming evident as a mechanism by which symbionts and pathogens survive. Anaplasma phagocytophilum, an obligate intracellular bacterium, down-regulates multiple host defence genes where histone deacetylase 1 (HDAC1) binds and histone 3 is deacetylated at their promoters, including the NADPH oxidase component, CYBB. How HDAC1 is targeted to defence gene promoters is unknown. Ankyrin A (AnkA), an A. phagocytophilum type IV secretion system effector, enters the granulocyte nucleus, binds stretches of AT-rich DNA and alters transcription of antimicrobial defence genes, including down-regulation of CYBB. Here we found AnkA binds to a predicted matrix attachment region in the proximal CYBB promoter. Using the CYBB promoter as a model of cis-gene silencing, we interrogated the mechanism of AnkA-mediated CYBB repression. The N-terminus of AnkA was critical for nuclear localization, the central ANK repeats and C-terminus were important for DNA binding, and most promoter activity localized to the central ANK repeats. Furthermore, a direct interaction between AnkA and HDAC1 was detected at the CYBB promoter, and was critical for AnkA-mediated CYBB repression. This novel microbial manipulation of host chromatin and gene expression provides important evidence of the direct effects that prokaryotic nuclear effectors can exert over host transcription and function.

Effector bottleneck: microbial reprogramming of parasitized host cell transcription by epigenetic remodeling of chromatin structure

Obligate intracellular pathogenic bacteria evolved to manipulate their host cells with a limited range of proteins constrained by their compact genomes. The harsh environment of a phagocytic defense cell is one that challenges the majority of commensal and pathogenic bacteria; yet, these are the obligatory vertebrate homes for important pathogenic species in the Anaplasmataceae family. Survival requires that the parasite fundamentally alter the native functions of the cell to allow its entry, intracellular replication, and transmission to a hematophagous arthropod. The small genomic repertoires encode several eukaryotic-like proteins, including ankyrin A (AnkA) of Anaplasma phagocytophilum and Ank200 and tandem-repeat containing proteins of Ehrlichia chaffeensis that localize to the host cell nucleus and directly bind DNA. As a model, A. phagocytophilum AnkA appears to directly alter host cell gene expression by recruiting chromatin modifying enzymes such as histone deacetylases and methyltransferases or by acting directly on transcription in cis. While cis binding could feasibly alter limited ranges of genes and cellular functions, the complex and dramatic alterations in transcription observed with infection are difficult to explain on the basis of individually targeted genes. We hypothesize that nucleomodulins can act broadly, even genome-wide, to affect entire chromosomal neighborhoods and topologically associating chromatin domains by recruiting chromatin remodeling complexes or by altering the folding patterns of chromatin that bring distant regulatory regions together to coordinate control of transcriptional reprogramming. This review focuses on the A. phagocytophilum nucleomodulin AnkA, how it impacts host cell transcriptional responses, and current investigations that seek to determine how these multifunctional eukaryotic-like proteins facilitate epigenetic alterations and cellular reprogramming at the chromosomal level.

Studies of Anaplasma phagocytophilum in sheep experimentally infected with the human NY-18 isolate: characterization of tick feeding sites

Anaplasma phagocytophilum, transmitted by ticks of the genus Ixodes, was first described in Scotland as the agent of tick-borne fever in sheep and more recently as the cause of human granulocytic anaplasmosis in the U.S. and Europe. We previously reported sheep as an experimental host for the human NY-18 isolate of A. phagocytophilum. While clinical signs were not observed and infected granulocytes were not seen in stained blood smears, these sheep served as a good host for infection of ticks. In this research we characterized tick feeding sites to better understand tick/host/pathogen interactions. Ixodes scapularis adults were allowed to feed for 2 and 4 days on experimentally infected sheep, after which biopsies were taken beneath tick feeding sites for histopathology, PCR and immunohistochemistry (IHC) studies. In addition, the expression of selected immune response genes was studied in blood and feeding site biopsies. While necrosis was too advanced in 4-day biopsies for accurate cell counts, higher numbers of eosinophils and neutrophils were found in 2-day biopsies from infected sheep as compared with the uninfected controls. An unexpected result was the documentation of higher dermal inflammation in infected sheep at sites without ticks. A. phagocytophilum infected granulocytes were localized by immunohistochemistry (IHC) in skin biopsies using rabbit antibodies against the recombinant A. phagocytophilum major surface protein 4 as the primary antibody for indirect peroxidase-anti-peroxidase and fluorescent antibody IHC. These infected cells are likely to be the source of infection for ticks. Sheep therefore served as good hosts for studying host/pathogen/tick interactions of this human strain of A. phagocytophilum, and provided a means of producing infected ticks for future studies on tick/pathogen developmental and transmission cycles.

Comparison and characterization of granulocyte cell models for Anaplasma phagocytophilum infection

Anaplasma phagocytophilum, an obligate intracellular bacterium, modifies functions of its in vivo host, the neutrophil. The challenges of using neutrophils ex vivo necessitate cell line models. However, cell line infections do not currently mimic ex vivo neutrophil infection characteristics. To understand these discrepancies, we compared infection of cell lines to ex vivo human neutrophils and differentiated hematopoietic stem cells with regard to infection capacity, oxidative burst, host defense gene expression, and differentiation. Using established methods, marked ex vivo neutrophil infection heterogeneity was observed at 24-48 h necessitating cell sorting to obtain homogeneously infected cells at levels observed in vivo. Moreover, gene expression of infected cell lines differed markedly from the prior standard of unsorted infected neutrophils. Differentiated HL-60 cells sustained similar infection levels to neutrophils in vivo and closely mimicked functional and transcriptional changes of sorted infected neutrophils. Thus, care must be exercised using ex vivo neutrophils for A. phagocytophilum infection studies because a major determinant of transcriptional and functional changes among all cells was the intracellular bacteria quantity. Furthermore, comparisons of ex vivo neutrophils and the surrogate HL-60 cell model allowed the determination that specific cellular functions and transcriptional programs are targeted by the bacterium without significantly modifying differentiation.

Lessons from Anaplasma phagocytophilum: chromatin remodeling by bacterial effectors

Bacterial pathogens can alter global host gene expression via histone modifications and chromatin remodeling in order to subvert host responses, including those involved with innate immunity, allowing for bacterial survival. Shigella flexneri, Listeria monocytogenes, Chlamydia trachomatis, and Anaplasma phagocytophilum express effector proteins that modify host histones and chromatin structure. A. phagocytophilum modulates granulocyte respiratory burst in part by dampening transcription of several key phagocyte oxidase genes. The A. phagocytophilum protein AnkA localizes to the myeloid cell nucleus where it binds AT-rich regions in the CYBB promoter and decreases its transcription. AT-rich regions of DNA are characteristic of matrix attachment regions (MARs) which are critical for chromatin structure and transcription. MAR-binding proteins, such as SATB1, interact with histone modifying enzymes resulting in altered gene expression. With A. phagocytophilum infection, histone deacetylase 1 (HDAC1) expression is increased and histone H3 acetylation is decreased at the CYBB promoter, suggesting a role for AnkA in altering host epigenetics and modulating gene transcription, at this, and perhaps other loci. This review will focus on how bacterial pathogens alter host epigenetics, by specifically examining A. phagocytophilum AnkA cis-regulation of CYBB transcription and epigenetic changes associated with infection.

Anaplasma phagocytophilum inhibits apoptosis and promotes cytoskeleton rearrangement for infection of tick cells

Anaplasma phagocytophilum causes human granulocytic anaplasmosis. Infection with this zoonotic pathogen affects gene expression in both the vertebrate host and the tick vector, Ixodes scapularis. Here, we identified new genes, including spectrin alpha chain or alpha-fodrin (CG8) and voltage-dependent anion-selective channel or mitochondrial porin (T2), that are involved in A. phagocytophilum infection/multiplication and the tick cell response to infection. The pathogen downregulated the expression of CG8 in tick salivary glands and T2 in both the gut and salivary glands to inhibit apoptosis as a mechanism to subvert host cell defenses and increase infection. In the gut, the tick response to infection through CG8 upregulation was used by the pathogen to increase infection due to the cytoskeleton rearrangement that is required for pathogen infection. These results increase our understanding of the role of tick genes during A. phagocytophilum infection and multiplication and demonstrate that the pathogen uses similar strategies to establish infection in both vertebrate and invertebrate hosts.

Gene expression profile suggests that pigs (Sus scrofa) are susceptible to Anaplasma phagocytophilum but control infection

Background

Anaplasma phagocytophilum infects a wide variety of hosts and causes granulocytic anaplasmosis in humans, horses and dogs and tick-borne fever in ruminants. Infection with A. phagocytophilum results in the modification of host gene expression and immune response. The objective of this research was to characterize gene expression in pigs (Sus scrofa) naturally and experimentally infected with A. phagocytophilum trying to identify mechanisms that help to explain low infection prevalence in this species.

Results

For gene expression analysis in naturally infected pigs, microarray hybridization was used. The expression of differentially expressed immune response genes was analyzed by real-time RT-PCR in naturally and experimentally infected pigs. Results suggested that A. phagocytophilum infection affected cytoskeleton rearrangement and increased both innate and adaptive immune responses by up regulation of interleukin 1 receptor accessory protein-like 1 (IL1RAPL1), T-cell receptor alpha chain (TCR-alpha), thrombospondin 4 (TSP-4) and Gap junction protein alpha 1 (GJA1) genes. Higher serum levels of IL-1 beta, IL-8 and TNF-alpha in infected pigs when compared to controls supported data obtained at the mRNA level.

Conclusions

These results suggested that pigs are susceptible to A. phagocytophilum but control infection, particularly through activation of innate immune responses, phagocytosis and autophagy. This fact may account for the low infection prevalence detected in pigs in some regions and thus their low or no impact as a reservoir host for this pathogen. These results advanced our understanding of the molecular mechanisms at the host-pathogen interface and suggested a role for newly reported genes in the protection of pigs against A. phagocytophilum.

Sheep experimentally infected with a human isolate of Anaplasma phagocytophilum serve as a host for infection of Ixodes scapularis ticks

Anaplasma phagocytophilum, first identified as a pathogen of ruminants in Europe, has more recently been recognized as an emerging tick-borne pathogen of humans in the U.S. and Europe. A. phagocytophilum is transmitted by Ixodes spp., but the tick developmental cycle and pathogen/vector interactions have not been fully described. In this research, we report on the experimental infection of sheep with the human NY-18 isolate of A. phagocytophilum which then served as a host for infection of I. scapularis nymphs and adults. A. phagocytophilum was propagated in the human promyelocytic cell line, HL-60, and the infected cell cultures were then used to infect sheep by intravenous inoculation. Infections in sheep were confirmed by PCR and an Anaplasma-competitive ELISA. Clinical signs were not apparent in any of the infected sheep, and only limited hematologic and mild serum biochemical abnormalities were identified. While A. phagocytophilum morulae were rarely seen in neutrophils, blood film evaluation revealed prominent large granular lymphocytes, occasional plasma cells, and rare macrophages. Upon necropsy, gross lesions were restricted to the lymphoid system. Mild splenomegaly and lymphadenomegaly with microscopic evidence of lymphoid hyperplasia was observed in all infected sheep. Female I. scapularis that were allowed to feed and acquire infection on each of the 3 experimentally infected sheep became infected with A. phagocytophilum as determined by PCR of guts (80-87%) and salivary glands (67-100%). Female I. scapularis that acquired infection as nymphs on an experimentally infected sheep transmitted A. phagocytophilum to a susceptible sheep, thus confirming transstadial transmission. Sheep proved to be a good host for the production of I. scapularis infected with this human isolate of A. phagocytophilum, which can be used as a model for future studies of the tick/pathogen interface.

Expression of heat shock proteins and subolesin affects stress responses, Anaplasma phagocytophilum infection and questing behaviour in the tick, Ixodes scapularis

We characterized the effects of subolesin and heat shock protein (HSP) expression on Ixodes scapularis Say (Acari: Ixodidae) stress responses to heat shock and feeding, questing behaviour and Anaplasma phagocytophilum (Rickettsiales: Anaplasmataceae) infection. Ticks and cultured tick cells were analysed before and after subolesin, hsp20 and hsp70 gene knock-down by RNA interference. The results of these studies confirm that HSPs are involved in the tick cell response to heat stress and that subolesin and HSPs are both involved in the tick response to blood-feeding stress and A. phagocytophilum infection. Subolesin and hsp20 are involved in the tick protective response to A. phagocytophilum infection and hsp70 expression may be manipulated by the pathogen to increase infectivity. Importantly, these results demonstrate that subolesin, hsp20 and hsp70 expression also affect tick questing behaviour. Overall, this research demonstrates a relationship between hsp and subolesin expression and tick stress responses to heat shock and blood feeding, A. phagocytophilum infection and questing behaviour, thereby extending our understanding of the tick-host-pathogen interface.

Mechanisms of obligatory intracellular infection with Anaplasma phagocytophilum

Anaplasma phagocytophilum persists in nature by cycling between mammals and ticks. Human infection by the bite of an infected tick leads to a potentially fatal emerging disease called human granulocytic anaplasmosis. A. phagocytophilum is an obligatory intracellular bacterium that replicates inside mammalian granulocytes and the salivary gland and midgut cells of ticks. A. phagocytophilum evolved the remarkable ability to hijack the regulatory system of host cells. A. phagocytophilum alters vesicular traffic to create an intracellular membrane-bound compartment that allows replication in seclusion from lysosomes. The bacterium downregulates or actively inhibits a number of innate immune responses of mammalian host cells, and it upregulates cellular cholesterol uptake to acquire cholesterol for survival. It also upregulates several genes critical for the infection of ticks, and it prolongs tick survival at freezing temperatures. Several host factors that exacerbate infection have been identified, including interleukin-8 (IL-8) and cholesterol. Host factors that overcome infection include IL-12 and gamma interferon (IFN-γ). Two bacterial type IV secretion effectors and several bacterial proteins that associate with inclusion membranes have been identified. An understanding of the molecular mechanisms underlying A. phagocytophilum infection will foster the development of creative ideas to prevent or treat this emerging tick-borne disease.

Anaplasma phagocytophilum infection induces apoptosis in HL-60 cells

Anaplasma phagocytophilum, the causative agent of human granulocytic anaplasmosis, is an obligate intra-cellular bacterium that survives in neutrophils by delaying apoptosis. The human promyelocytic leukemia cell line HL-60 has been the ultimate choice for culturing Anaplasma in vitro. In this study, we assessed the various events of drug-induced apoptosis in A. phagocytophilum-infected HL-60 cells. Anaplasma infection reduced the cell viability and increased the apoptosis in HL-60 cells and staurosporine or etoposide-induced apoptosis was further exacerbated with Anaplasma infection. Altogether our results suggest that A. phagocytophilum infection is proapoptotic in HL-60 cells unlike in neutrophils where it is antiapoptotic.

Global proteomic analysis of two tick-borne emerging zoonotic agents: anaplasma phagocytophilum and ehrlichia chaffeensis

Anaplasma phagocytophilum and Ehrlichia chaffeensis are obligatory intracellular α-proteobacteria that infect human leukocytes and cause potentially fatal emerging zoonoses. In the present study, we determined global protein expression profiles of these bacteria cultured in the human promyelocytic leukemia cell line, HL-60. Mass spectrometric (MS) analyses identified a total of 1,212 A. phagocytophilum and 1,021 E. chaffeensis proteins, representing 89.3 and 92.3% of the predicted bacterial proteomes, respectively. Nearly all bacterial proteins (≥99%) with known functions were expressed, whereas only approximately 80% of “hypothetical” proteins were detected in infected human cells. Quantitative MS/MS analyses indicated that highly expressed proteins in both bacteria included chaperones, enzymes involved in biosynthesis and metabolism, and outer membrane proteins, such as A. phagocytophilum P44 and E. chaffeensis P28/OMP-1. Among 113 A. phagocytophilum p44 paralogous genes, 110 of them were expressed and 88 of them were encoded by pseudogenes. In addition, bacterial infection of HL-60 cells up-regulated the expression of human proteins involved mostly in cytoskeleton components, vesicular trafficking, cell signaling, and energy metabolism, but down-regulated some pattern recognition receptors involved in innate immunity. Our proteomics data represent a comprehensive analysis of A. phagocytophilum and E. chaffeensis proteomes, and provide a quantitative view of human host protein expression profiles regulated by bacterial infection. The availability of these proteomic data will provide new insights into biology and pathogenesis of these obligatory intracellular pathogens.

Expression of Heat Shock and Other Stress Response Proteins in Ticks and Cultured Tick Cells in Response to Anaplasma spp. Infection and Heat Shock

Ticks are ectoparasites of animals and humans that serve as vectors of Anaplasma and other pathogens that affect humans and animals worldwide. Ticks and the pathogens that they transmit have coevolved molecular interactions involving genetic traits of both the tick and the pathogen that mediate their development and survival. In this paper, the expression of heat shock proteins (HSPs) and other stress response proteins (SRPs) was characterized in ticks and cultured tick cells by proteomics and transcriptomics analyses in response to Anaplasma spp. infection and heat shock. The results of these studies demonstrated that the stress response was activated in ticks and cultured tick cells after Anaplasma spp. infection and heat shock. However, in the natural vector-pathogen relationship, HSPs and other SRPs were not strongly activated, which likely resulted from tick-pathogen coevolution. These results also demonstrated pathogen- and tick-specific differences in the expression of HSPs and other SRPs in ticks and cultured tick cells infected with Anaplasma spp. and suggested the existence of post-transcriptional mechanisms induced by Anaplasma spp. to control tick response to infection. These results illustrated the complexity of the stress response in ticks and suggested a function for the HSPs and other SRPs during Anaplasma spp. infection.

Application of highly sensitive saturation labeling to the analysis of differential protein expression in infected ticks from limited samples

Background

Ticks are vectors of pathogens that affect human and animal health worldwide. Proteomics and genomics studies of infected ticks are required to understand tick-pathogen interactions and identify potential vaccine antigens to control pathogen transmission. One of the limitations for proteomics research in ticks is the amount of protein that can be obtained from these organisms. In the work reported here, individual naturally-infected and uninfected Rhipicephalus spp. ticks were processed using a method that permits simultaneous extraction of DNA, RNA and proteins. This approach allowed using DNA to determine pathogen infection, protein for proteomics studies and RNA to characterize mRNA levels for some of the differentially expressed proteins. Differential protein expression in response to natural infection with different pathogens was characterized by two-dimensional (2-D) differential in gel electrophoresis (DIGE) saturation labeling in combination with mass spectrometry analysis. To our knowledge, this is the first report of the application of DIGE saturation labeling to study tick proteins.

Results

Questing and feeding Rhipicephalus spp. adult ticks were collected in 27 farms located in different Sicilian regions. From 300 collected ticks, only 16 were found to be infected: R. sanguineus with Rickettsia conorii and Ehrlichia canis; R. bursa with Theileria annulata; and R. turanicus with Anaplasma ovis. The proteomic analysis conducted from a limited amount of proteins allowed the identification of host, pathogen and tick proteins differentially expressed as a consequence of infection.

Conclusion

These results showed that DIGE saturation labeling is a powerful technology for proteomics studies in small number of ticks and provided new information about the effect of pathogen infection in ticks.

Molecular events involved in cellular invasion by Ehrlichia chaffeensis and Anaplasma phagocytophilum

Ehrlichia chaffeensis and Anaplasma phagocytophilum are obligatory intracellular bacteria that preferentially replicate inside leukocytes by utilizing biological compounds and processes of these primary host defensive cells. These bacteria incorporate cholesterol from the host for their survival. Upon interaction with host monocytes and granulocytes, respectively, these bacteria usurp the lipid raft domain containing GPI-anchored protein to induce a series of signaling events that result in internalization of the bacteria. Monocytes and neutrophils usually kill invading microorganisms by fusion of the phagosomes containing the bacteria with granules containing both antimicrobial peptides and lysosomal hydrolytic enzymes and/or through sequestering vital nutrients. However, E. chaffeensis and A. phagocytophilum alter vesicular traffic to create a unique intracellular membrane-bound compartment that allows their replication in seclusion from lysosomal killing. These bacteria are quite sensitive to reactive oxygen species (ROS), so in order to survive in host cells that are primary mediators of ROS-induced killing, they inhibit activation of NADPH oxidase and assembly of this enzyme in their inclusion compartments. Moreover, host phagocyte activation and differentiation, apoptosis, and IFN-gamma signaling pathways are inhibited by these bacteria. Through reductive evolution, lipopolysaccharide and peptidoglycan that activate the innate immune response, have been eliminated from these gram-negative bacteria at the genomic level. Upon interaction with new host cells, bacterial genes encoding the Type IV secretion apparatus and the two-component regulatory system are up-regulated to sense and adapt to the host environment. Thus dynamic signal transduction events concurrently proceed both in the host cells and in the invading E. chaffeensis and A. phagocytophilum bacteria for successful establishment of intracellular infection. Several bacterial surface-exposed proteins and porins are recently identified. Further functional studies on Ehrlichia and Anaplasma effector or ligand molecules and cognate host cell receptors will undoubtedly advance our understanding of the complex interplay between obligatory intracellular pathogens and their hosts. Such data can be applied towards treatment, diagnosis, and control of ehrlichiosis and anaplasmosis.

Silencing of genes involved in Anaplasma marginale-tick interactions affects the pathogen developmental cycle in Dermacentor variabilis

BACKGROUND

The cattle pathogen, Anaplasma marginale, undergoes a developmental cycle in ticks that begins in gut cells. Transmission to cattle occurs from salivary glands during a second tick feeding. At each site of development two forms of A. marginale (reticulated and dense) occur within a parasitophorous vacuole in the host cell cytoplasm. However, the role of tick genes in pathogen development is unknown. Four genes, found in previous studies to be differentially expressed in Dermacentor variabilis ticks in response to infection with A. marginale, were silenced by RNA interference (RNAi) to determine the effect of silencing on the A. marginale developmental cycle. These four genes encoded for putative glutathione S-transferase (GST), salivary selenoprotein M (SelM), H+ transporting lysosomal vacuolar proton pump (vATPase) and subolesin.

RESULTS

The impact of gene knockdown on A. marginale tick infections, both after acquiring infection and after a second transmission feeding, was determined and studied by light microscopy. Silencing of these genes had a different impact on A. marginale development in different tick tissues by affecting infection levels, the densities of colonies containing reticulated or dense forms and tissue morphology. Salivary gland infections were not seen in any of the gene-silenced ticks, raising the question of whether these ticks were able to transmit the pathogen.

CONCLUSION

The results of this RNAi and light microscopic analyses of tick tissues infected with A. marginale after the silencing of genes functionally important for pathogen development suggest a role for these molecules during pathogen life cycle in ticks.

Anaplasma phagocytophilum and Anaplasma marginale elicit different gene expression responses in cultured tick cells

The genus Anaplasma (Rickettsiales: Anaplasmataceae) includes obligate tick-transmitted intracellular organisms, Anaplasma phagocytophilum and Anaplasma marginale that multiply in both vertebrate and tick host cells. Recently, we showed that A. marginale affects the expression of tick genes that are involved in tick survival and pathogen infection and multiplication. However, the gene expression profile in A. phagocytophilum-infected tick cells is currently poorly characterized. The objectives of this study were to characterize tick gene expression profile in Ixodes scapularis ticks and cultured ISE6 cells in response to infection with A. phagocypthilum and to compare tick gene expression responses in A. phagocytophilum- and A. marginale-infected tick cells by microarray and real-time RT-PCR analyses. The results of these studies demonstrated modulation of tick gene expression by A. phagocytophilum and provided evidence of different gene expression responses in tick cells infected with A. phagocytophilum and A. marginale. These differences in Anaplasma-tick interactions may reflect differences in pathogen life cycle in the tick cells.

Epigenetic silencing of host cell defense genes enhances intracellular survival of the rickettsial pathogen Anaplasma phagocytophilum

Intracellular bacteria have evolved mechanisms that promote survival within hostile host environments, often resulting in functional dysregulation and disease. Using the Anaplasma phagocytophilum–infected granulocyte model, we establish a link between host chromatin modifications, defense gene transcription and intracellular bacterial infection. Infection of THP-1 cells with A. phagocytophilum led to silencing of host defense gene expression. Histone deacetylase 1 (HDAC1) expression, activity and binding to the defense gene promoters significantly increased during infection, which resulted in decreased histone H3 acetylation in infected cells. HDAC1 overexpression enhanced infection, whereas pharmacologic and siRNA HDAC1 inhibition significantly decreased bacterial load. HDAC2 does not seem to be involved, since HDAC2 silencing by siRNA had no effect on A. phagocytophilum intracellular propagation. These data indicate that HDAC up-regulation and epigenetic silencing of host cell defense genes is required for A. phagocytophilum infection. Bacterial epigenetic regulation of host cell gene transcription could be a general mechanism that enhances intracellular pathogen survival while altering cell function and promoting disease.

Although the main function of defense cells is to eliminate invading infections, some intracellular bacterial pathogens manage to turn defense cells into suitable hosts for bacterial propagation. In doing so, intracellular pathogens dysregulate host cell function and cause disease. With genomic and metabolic resources thousands of times more limited than the host's, intracellular bacteria have evolved very efficient mechanisms to globally subvert the host defense. Here, we define a mechanism by which the intracellular pathogen Anaplasma phagocytophilum globally inhibits host cell defenses by affecting mechanisms of epigenetic control of defense gene expression. Silencing or inhibition of the host protein HDAC1 has a negative effect on intracellular bacterial replication, whereas HDAC1 overexpression leads to defense gene silencing and facilitates intracellular bacterial survival. This study not only provides new insight into a mechanism of host cell subversion, but also identifies a potential target for future development of novel therapeutic intervention strategies.

Silencing of host cell CYBB gene expression by the nuclear effector AnkA of the intracellular pathogen Anaplasma phagocytophilum

Coevolution of intracellular bacterial pathogens and their host cells resulted in the appearance of effector molecules that when translocated into the host cell modulate its function, facilitating bacterial survival within the hostile host environment. Some of these effectors interact with host chromatin and other nuclear components. In this report, we show that the AnkA protein of Anaplasma phagocytophilum, which is translocated into the host cell nucleus, interacts with gene regulatory regions of host chromatin and is involved in downregulating expression of CYBB (gp91(phox)) and other key host defense genes. AnkA effector protein rapidly accumulated in nuclei of infected cells coincident with changes in CYBB transcription. AnkA interacted with transcriptional regulatory regions of the CYBB locus at sites where transcriptional regulators bind. AnkA binding to DNA occurred at regions with high AT contents. Mutation of AT stretches at these sites abrogated AnkA binding. Histone H3 acetylation decreased dramatically at the CYBB locus during A. phagocytophilum infection, particularly around AnkA binding sites. Transcription of CYBB and other defense genes was significantly decreased in AnkA-transfected HL-60 cells. These data suggest a mechanism by which intracellular pathogens directly regulate host cell gene expression mediated by nuclear effectors and changes in host chromatin structure.

Differential expression of the tick protective antigen subolesin in anaplasma marginale- and A. phagocytophilum-infected host cells

Subolesin was recently shown in vaccine and RNA interference (RNAi) studies to protect against tick infestations and to affect tick feeding, reproduction, and development as well as infection of host cells by Anaplasma marginale and A. phagocytophilum. Recent experiments provided evidence that infection of both tick and vertebrate host cells with these two pathogens modified gene expression. We therefore hypothesized that infection of host cells with A. marginale and A. phagocytophilum affects expression of subolesin. Subolesin mRNA levels were determined by real-time reverse transcriptase (RT)-PCR in uninfected and A. marginale-infected Dermacentor variabilis guts and salivary glands and IDE8-cultured tick cells and in uninfected and A. phagocytophilum-infected Ixodes scapularis nymphs, ISE6-cultured tick cells, and the human cell line HL-60. In addition, the effect of subolesin on Anaplasma spp. infection/multiplication was characterized by RNAi in tick tissues and/or cultured tick and human cells. These experiments presented evidence of differential expression of subolesin in A. marginale- and A. phagocytophilum-infected cells. Subolesin was differentially expressed in A. marginale-infected ticks in a tissue-specific manner in which mRNA levels increased in response to A. marginale infection in tick salivary gland cells but not in the gut cells. Subolesin knockdown by RNAi reduced Anaplasma infection/multiplication only in cells in which infection increased subolesin expression, i.e., in A. marginale-infected D. variabilis salivary glands and IDE8 cells. The results reported herein further support the role of subolesin in Anaplasma-host interactions and suggest a putative role of subolesin in vaccines for the control of pathogen infection/multiplication in ticks.

Gene expression profiles of European wild boar naturally infected with Mycobacterium bovis

Global gene expression profiles were analyzed in European wild boar naturally infected with Mycobacterium bovis. Spleen RNA was extracted from 23 M. bovis-infected and 17 uninfected animals and analyzed using a Pigoligoarray representing 20,400 genes. Differentially expressed sequences (N=161) were identified affecting cellular processes such as apoptosis, cell communication and signal transduction, cell growth and/or maintenance, cytoskeleton organization and biogenesis, DNA repair, immune response, metabolism and energy pathways, protein metabolism, regulation of cell proliferation, regulation of gene expression, regulation of nucleic acid metabolism, regulation of physiological processes, and transport. Real-time RT-PCR analysis of mRNA levels was used to corroborate microarray results of selected genes. Immune response genes were among the most represented differentially expressed sequences and were selected for further discussion. Beta-defensin 129, T-cell surface glycoprotein CD8 and B-cell receptor-associated protein 29 were overexpressed in infected animals. Lower expression levels of the immune response genes galectin-1, complement component C1qB and certain HLA class I and class II histocompatibility antigens and immunoglobulin chains were found in infected animals. This study identified new mechanisms by which naturally infected European wild boar respond to M. bovis infection and how the pathogen circumvents host immune responses to establish infection. Gene expression studies in naturally infected wildlife reservoirs of bovine tuberculosis are important for functional genomics and vaccine studies to aid in disease control in wildlife.

Silencing expression of the defensin, varisin, in male Dermacentor variabilis by RNA interference results in reduced Anaplasma marginale infections

Antimicrobial peptides, including defensins, are components of the innate immune system in ticks that have been shown to provide protection against both gram-negative and gram-positive bacteria. Varisin, one of the defensins identified in Dermacentor variabilis, was shown to be produced primarily in hemocytes but transcript levels were also expressed in midguts and other tick cells. In this research, we studied the role of varisin in the immunity of ticks to the gram-negative cattle pathogen, Anaplasma marginale. Expression of the varisin gene was silenced by RNA interference (RNAi) in which male ticks were injected with varisin dsRNA and then allowed to feed and acquire A. marginale infection on an experimentally-infected calf. Silencing expression of varisin in hemocytes, midguts and salivary glands was confirmed by real time RT-PCR. We expected that silencing of varisin would increase A. marginale infections in ticks, but the results demonstrated that bacterial numbers, as determined by an A. marginale msp4 quantitative PCR, were significantly reduced in the varisin-silenced ticks. Furthermore, colonies of A. marginale in ticks used for RNAi were morphologically abnormal from those seen in elution buffer injected control ticks. The colony shape was irregular and in some cases the A. marginale appeared to be free in the cytoplasm of midgut cells. Some ticks were found to be systemically infected with a microbe that may have been related to the silencing of varisin. This appears to be the first report of the silencing of expression of a defensin in ticks by RNAi that resulted in reduced A. marginale infections.