Characterization of a sialic acid- and P-selectin glycoprotein ligand-1-independent adhesin activity in the granulocytotropic bacterium Anaplasma phagocytophilum

Temporal patterns of gene expression in response to inoculation with a virulent Anaplasma phagocytophilum strain in sheep

The aim of this study was to characterize the gene expression of host immune- and cellular responses to a Norwegian virulent strain of Anaplasma phagocytophilum, the cause of tick-borne fever in sheep. Ten sheep were intravenously inoculated with a live virulent strain of A. phagocytophilum. Clinical-, observational-, hematological data as well as bacterial load, flow cytometric cell count data from peripheral blood mononuclear cells and host's gene expression post infection was analysed. The transcriptomic data were assessed for pre-set time points over the course of 22 days following the inoculation. Briefly, all inoculated sheep responded with clinical signs of infection 3 days post inoculation and onwards with maximum bacterial load observed on day 6, consistent with tick-borne fever. On days, 3-8, the innate immune responses and effector processes such as IFN1 signaling pathways and cytokine mediated signaling pathways were observed. Several pathways associated with the adaptive immune responses, namely T-cell activation, humoral immune responses, B-cell activation, and T- and B-cell differentiation dominated on the days of 8, 10 and 14. Flow-cytometric analysis of the PBMCs showed a reduction in CD4+CD25+ cells on day 10 and 14 post-inoculation and a skewed CD4:CD8 ratio indicating a reduced activation and proliferation of CD4-T-cells. The genes of important co-stimulatory molecules such as CD28 and CD40LG, important in T- and B-cell activation and proliferation, did not significantly change or experienced downregulation throughout the study. The absence of upregulation of several co-stimulatory molecules might be one possible explanation for the low activation and proliferation of CD4-T-cells during A. phagocytophilum infection, indicating a suboptimal CD4-T-cell response. The upregulation of T-BET, EOMES and IFN-γ on days 8-14 post inoculation, indicates a favoured CD4 Th1- and CD8-response. The dynamics and interaction between CD4+CD25+ and co-stimulatory molecules such as CD28, CD80, CD40 and CD40LG during infection with A. phagocytophilum in sheep needs further investigation in the future.

Essential domains of Anaplasma phagocytophilum invasins utilized to infect mammalian host cells

Anaplasma phagocytophilum causes granulocytic anaplasmosis, an emerging disease of humans and domestic animals. The obligate intracellular bacterium uses its invasins OmpA, Asp14, and AipA to infect myeloid and non-phagocytic cells. Identifying the domains of these proteins that mediate binding and entry, and determining the molecular basis of their interactions with host cell receptors would significantly advance understanding of A. phagocytophilum infection. Here, we identified the OmpA binding domain as residues 59 to 74. Polyclonal antibody generated against a peptide spanning OmpA residues 59 to 74 inhibited A. phagocytophilum infection of host cells and binding to its receptor, sialyl Lewis x (sLe^x-capped P-selectin glycoprotein ligand 1. Molecular docking analyses predicted that OmpA residues G61 and K64 interact with the two sLe^x sugars that are important for infection, α2,3-sialic acid and α1,3-fucose. Amino acid substitution analyses demonstrated that K64 was necessary, and G61 was contributory, for recombinant OmpA to bind to host cells and competitively inhibit A. phagocytophilum infection. Adherence of OmpA to RF/6A endothelial cells, which express little to no sLe^x but express the structurally similar glycan, 6-sulfo-sLe^x, required α2,3-sialic acid and α1,3-fucose and was antagonized by 6-sulfo-sLe^x antibody. Binding and uptake of OmpA-coated latex beads by myeloid cells was sensitive to sialidase, fucosidase, and sLe^x antibody. The Asp14 binding domain was also defined, as antibody specific for residues 113 to 124 inhibited infection. Because OmpA, Asp14, and AipA each contribute to the infection process, it was rationalized that the most effective blocking approach would target all three. An antibody cocktail targeting the OmpA, Asp14, and AipA binding domains neutralized A. phagocytophilum binding and infection of host cells. This study dissects OmpA-receptor interactions and demonstrates the effectiveness of binding domain-specific antibodies for blocking A. phagocytophilum infection.

Anaplasma phagocytophilum causes the potentially deadly bacterial disease granulocytic anaplasmosis. The pathogen replicates inside white blood cells and, like all other obligate intracellular organisms, must enter host cells to survive. Multiple A. phagocytophilum surface proteins called invasins cooperatively orchestrate the entry process. Identifying these proteins’ domains that are required for function, and determining the molecular basis of their interaction with host cell receptors would significantly advance understanding of A. phagocytophilum pathogenesis. In this study, the binding domains of two A. phagocytophilum surface proteins, OmpA and Asp14, were identified. The specific OmpA residues that interact with its host cell receptor were also defined. An antibody cocktail generated against the binding domains of OmpA, Asp14, and a third invasin, AipA, blocked the ability of A. phagocytophilum to infect host cells. The data presented within suggest that binding domains of OmpA, Asp14, and AipA could be exploited to develop a vaccine for granulocytic anaplasmosis.

Anaplasma phagocytophilum surface protein AipA mediates invasion of mammalian host cells

Anaplasma phagocytophilum, which causes granulocytic anaplasmosis in humans and animals, is a tick-transmitted obligate intracellular bacterium that mediates its own uptake into neutrophils and non-phagocytic cells. Invasins of obligate intracellular pathogens are attractive targets for protecting against or curing infection because blocking the internalization step prevents survival of these organisms. The complement of A. phagocytophilum invasins is incompletely defined. Here, we report the significance of a novel A. phagocytophilum invasion protein, AipA. A. phagocytophilum induced aipA expression during transmission feeding of infected ticks on mice. The bacterium upregulated aipA transcription when it transitioned from its non-infectious reticulate cell morphotype to its infectious dense-cored morphotype during infection of HL-60 cells. AipA localized to the bacterial surface and was expressed during in vivo infection. Of the AipA regions predicted to be surface-exposed, only residues 1 to 87 (AipA1-87 ) were found to be essential for host cell invasion. Recombinant AipA1-87 protein bound to and competitively inhibited A. phagocytophilum infection of mammalian cells. Antiserum specific for AipA1-87 , but not other AipA regions, antagonized infection. Additional blocking experiments using peptide-specific antisera narrowed down the AipA invasion domain to residues 9 to 21. An antisera combination targeting AipA1-87 together with two other A. phagocytophilum invasins, OmpA and Asp14, nearly abolished infection of host cells. This study identifies AipA as an A. phagocytophilum surface protein that is critical for infection, demarcates its invasion domain, and establishes a rationale for targeting multiple invasins to protect against granulocytic anaplasmosis.

Breaking in and grabbing a meal: Anaplasma phagocytophilum cellular invasion, nutrient acquisition, and promising tools for their study

Anaplasma phagocytophilum invades neutrophils to cause the emerging infection, human granulocytic anaplasmosis. Here, we provide a focused review of the A. phagocytophilum invasin-host cell receptor interactions that promote bacterial entry and the degradative and membrane traffic pathways that the organism exploits to route nutrients to the organelle in which it resides. Because its obligatory intracellular nature hinders knock out-complementation approaches, we also discuss the current methods used to study A. phagocytophilum gene function and the potential benefit of applying novel tools that have advanced studies of other obligate intracellular bacterial pathogens.

Anaplasma phagocytophilum--a widespread multi-host pathogen with highly adaptive strategies

The bacterium Anaplasma phagocytophilum has for decades been known to cause the disease tick-borne fever (TBF) in domestic ruminants in Ixodes ricinus-infested areas in northern Europe. In recent years, the bacterium has been found associated with Ixodes-tick species more or less worldwide on the northern hemisphere. A. phagocytophilum has a broad host range and may cause severe disease in several mammalian species, including humans. However, the clinical symptoms vary from subclinical to fatal conditions, and considerable underreporting of clinical incidents is suspected in both human and veterinary medicine. Several variants of A. phagocytophilum have been genetically characterized. Identification and stratification into phylogenetic subfamilies has been based on cell culturing, experimental infections, PCR, and sequencing techniques. However, few genome sequences have been completed so far, thus observations on biological, ecological, and pathological differences between genotypes of the bacterium, have yet to be elucidated by molecular and experimental infection studies. The natural transmission cycles of various A. phagocytophilum variants, the involvement of their respective hosts and vectors involved, in particular the zoonotic potential, have to be unraveled. A. phagocytophilum is able to persist between seasons of tick activity in several mammalian species and movement of hosts and infected ticks on migrating animals or birds may spread the bacterium. In the present review, we focus on the ecology and epidemiology of A. phagocytophilum, especially the role of wildlife in contribution to the spread and sustainability of the infection in domestic livestock and humans.

Anaplasma phagocytophilum outer membrane protein A interacts with sialylated glycoproteins to promote infection of mammalian host cells

Anaplasma phagocytophilum is the tick-transmitted obligate intracellular bacterium that causes human granulocytic anaplasmosis (HGA). A. phagocytophilum binding to sialyl Lewis x (sLe(x)) and other sialylated glycans that decorate P selectin glycoprotein 1 (PSGL-1) and other glycoproteins is critical for infection of mammalian host cells. Here, we demonstrate the importance of A. phagocytophilum outer membrane protein A (OmpA) APH_0338 in infection of mammalian host cells. OmpA is transcriptionally induced during transmission feeding of A. phagocytophilum-infected ticks on mice and is upregulated during invasion of HL-60 cells. OmpA is presented on the pathogen's surface. Sera from HGA patients and experimentally infected mice recognize recombinant OmpA. Pretreatment of A. phagocytophilum organisms with OmpA antiserum reduces their abilities to infect HL-60 cells. The OmpA N-terminal region is predicted to contain the protein's extracellular domain. Glutathione S-transferase (GST)-tagged versions of OmpA and OmpA amino acids 19 to 74 (OmpA(19-74)) but not OmpA(75-205) bind to, and competitively inhibit A. phagocytophilum infection of, host cells. Pretreatment of host cells with sialidase or trypsin reduces or nearly eliminates, respectively, GST-OmpA adhesion. Therefore, OmpA interacts with sialylated glycoproteins. This study identifies the first A. phagocytophilum adhesin-receptor pair and delineates the region of OmpA that is critical for infection.

Anaplasma phagocytophilum Asp14 is an invasin that interacts with mammalian host cells via its C terminus to facilitate infection

Anaplasma phagocytophilum, a member of the family Anaplasmataceae, is the tick-transmitted obligate intracellular bacterium that causes human granulocytic anaplasmosis. The life cycle of A. phagocytophilum is biphasic, transitioning between the noninfectious reticulate cell (RC) and infectious dense-cored (DC) forms. We analyzed the bacterium's DC surface proteome by selective biotinylation of surface proteins, NeutrAvidin affinity purification, and mass spectrometry. Transcriptional profiling of selected outer membrane protein candidates over the course of infection revealed that aph_0248 (designated asp14 [14-kDa A. phagocytophilum surface protein]) expression was upregulated the most during A. phagocytophilum cellular invasion. asp14 transcription was induced during transmission feeding of A. phagocytophilum-infected ticks on mice and was upregulated when the bacterium engaged its receptor, P-selectin glycoprotein ligand 1. Asp14 localized to the A. phagocytophilum surface and was expressed during in vivo infection. Treating DC organisms with Asp14 antiserum or preincubating mammalian host cells with glutathione S-transferase (GST)-Asp14 significantly inhibited infection of host cells. Moreover, preincubating host cells with GST-tagged forms of both Asp14 and outer membrane protein A, another A. phagocytophilum invasin, pronouncedly reduced infection relative to treatment with either protein alone. The Asp14 domain that is sufficient for cellular adherence and invasion lies within the C-terminal 12 to 24 amino acids and is conserved among other Anaplasma and Ehrlichia species. These results identify Asp14 as an A. phagocytophilum surface protein that is critical for infection, delineate its invasion domain, and demonstrate the potential of targeting Asp14 in concert with OmpA for protecting against infection by A. phagocytophilum and other Anaplasmataceae pathogens.

Anaplasma phagocytophilum: deceptively simple or simply deceptive?

Anaplasma phagocytophilum is an obligate intracellular rickettsial pathogen transmitted by ixodid ticks. This bacterium colonizes myeloid and nonmyeloid cells and causes human granulocytic anaplasmosis--an important immunopathological vector-borne disease in the USA, Europe and Asia. Recent studies uncovered novel insights into the mechanisms of A. phagocytophilum pathogenesis and immunity. Here, we provide an overview of the underlying events by which the immune system responds to A. phagocytophilum infection, how this pathogen counteracts host immunity and the contribution of the tick vector for microbial transmission. We also discuss current scientific gaps in the knowledge of A. phagocytophilum biology for the purpose of exchanging research perspectives.

Anaplasma phagocytophilum dihydrolipoamide dehydrogenase 1 affects host-derived immunopathology during microbial colonization

Anaplasma phagocytophilum is a tick-borne rickettsial pathogen that provokes an acute inflammatory response during mammalian infection. The illness caused by A. phagocytophilum, human granulocytic anaplasmosis, occurs irrespective of pathogen load and results instead from host-derived immunopathology. Thus, characterizing A. phagocytophilum genes that affect the inflammatory process is critical for understanding disease etiology. By using an A. phagocytophilum Himar1 transposon mutant library, we showed that a single transposon insertion into the A. phagocytophilum dihydrolipoamide dehydrogenase 1 gene (lpda1 [APH_0065]) affects inflammation during infection. A. phagocytophilum lacking lpda1 revealed enlargement of the spleen, increased splenic extramedullary hematopoiesis, and altered clinicopathological abnormalities during mammalian colonization. Furthermore, LPDA1-derived immunopathology was independent of neutrophil infection and correlated with enhanced reactive oxygen species from NADPH oxidase and nuclear factor (NF)-κB signaling in macrophages. Taken together, these findings suggest the presence of different signaling pathways in neutrophils and macrophages during A. phagocytophilum invasion and highlight the importance of LPDA1 as an immunopathological molecule.

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 infects mast cells via alpha1,3-fucosylated but not sialylated glycans and inhibits IgE-mediated cytokine production and histamine release

Mast cells are sentinels for infection. Upon exposure to pathogens, they release their stores of proinflammatory cytokines, chemokines, and histamine. Mast cells are also important for the control of certain tick-borne infections. Anaplasma phagocytophilum is an obligate intracellular tick-transmitted bacterium that infects neutrophils to cause the emerging disease granulocytic anaplasmosis. A. phagocytophilum adhesion to and infection of neutrophils depend on sialylated and α1,3-fucosylated glycans. We investigated the hypotheses that A. phagocytophilum invades mast cells and inhibits mast cell activation. We demonstrate that A. phagocytophilum binds and/or infects murine bone marrow-derived mast cells (BMMCs), murine peritoneal mast cells, and human skin-derived mast cells. A. phagocytophilum infection of BMMCs depends on α1,3-fucosylated, but not sialylated, glycans. A. phagocytophilum binding to and invasion of BMMCs do not elicit proinflammatory cytokine secretion. Moreover, A. phagocytophilum-infected cells are inhibited in the release of tumor necrosis factor alpha (TNF-α), interleukin 6 (IL-6), IL-13, and histamine following stimulation with IgE or antigen. Thus, A. phagocytophilum mitigates mast cell activation. These findings potentially represent a novel means by which A. phagocytophilum usurps host defense mechanisms and shed light on the interplay between mast cells and vector-borne bacterial pathogens.

Neutrophils exposed to A. phagocytophilum under shear stress fail to fully activate, polarize, and transmigrate across inflamed endothelium

Anaplasma phagocytophilum is an obligate intracellular bacterium that has evolved mechanisms to hijack polymorphonuclear neutrophil (PMN) receptors and signaling pathways to bind, infect, and multiply within the host cell. E-selectin is upregulated during inflammation and is a requisite endothelial receptor that supports PMN capture, rolling, and activation of integrin-mediated arrest. Ligands expressed by PMN that mediate binding to endothelium via E-selectin include sialyl Lewis x (sLe(x))-expressing ligands such as P-selectin glycoprotein ligand-1 (PSGL-1) and other glycolipids and glycoproteins. As A. phagocytophilum is capable of binding to sLe(x)-expressing ligands expressed on PMN, we hypothesized that acute bacterial adhesion to PMN would subsequently attenuate PMN recruitment during inflammation. We assessed the dynamics of PMN recruitment and migration under shear flow in the presence of a wild-type strain of A. phagocytophilum and compared it with a strain of bacteria that binds to PMN independent of PSGL-1. Acute bacterial engagement with PMN resulted in transient PMN arrest and minimal PMN polarization. Although the wild-type pathogen also signaled activation of beta2 integrins and elicited a mild intracellular calcium flux, downstream signals including PMN transmigration and phosphorylation of p38 mitogen-activated protein kinase (MAPK) were inhibited. The mutant strain bound less well to PMN and failed to activate beta2 integrins and induce a calcium flux but did result in decreased PMN arrest and polarization that may have been partially mediated by a suppression of p38 MAPK activation. This model suggests that A. phagocytophilum binding to PMN under shear flow during recruitment to inflamed endothelium interferes with normal tethering via E-selectin and navigational signaling of transendothelial migration.

Anaplasma phagocytophilum and Ehrlichia chaffeensis: subversive manipulators of host cells

Anaplasma spp. and Ehrlichia spp. cause several emerging human infectious diseases. Anaplasma phagocytophilum and Ehrlichia chaffeensis are transmitted between mammals by blood-sucking ticks and replicate inside mammalian white blood cells and tick salivary-gland and midgut cells. Adaptation to a life in eukaryotic cells and transmission between hosts has been assisted by the deletion of many genes that are present in the genomes of free-living bacteria (including genes required for the biosynthesis of lipopolysaccharide and peptidoglycan), by the acquisition of a cholesterol uptake pathway and by the expansion of the repertoire of genes encoding the outer-membrane porins and type IV secretion system. Here, I review the specialized properties and other adaptations of these intracellular bacteria.

Fucosylation enhances colonization of ticks by Anaplasma phagocytophilum

Fucosylated structures participate in a wide range of pathological processes in eukaryotes and prokaryotes. The impact of fucose on microbial pathogenesis, however, has been less appreciated in arthropods of medical relevance. Thus, we used the tick-borne bacterium Anaplasma phagocytophilum- the agent of human granulocytic anaplasmosis to understand these processes. Here we show that A. phagocytophilum uses alpha1,3-fucose to colonize ticks. We demonstrate that A. phagocytophilum modulates the expression of alpha1,3-fucosyltransferases and gene silencing significantly reduces colonization of tick cells. Acquisition but not transmission of A. phagocytophilum was affected when alpha1,3-fucosyltransferases were silenced during tick feeding. Our results uncover a novel mechanism of pathogen colonization in arthropods. Decoding mechanisms of pathogen invasion in ticks might expedite the development of new strategies to interfere with the life cycle of A. phagocytophilum.

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.

Anaplasma phagocytophilum dense-cored organisms mediate cellular adherence through recognition of human P-selectin glycoprotein ligand 1

Anaplasma phagocytophilum is an obligate intracellular bacterium that infects granulocytes to cause human granulocytic anaplasmosis. The susceptibilities of human neutrophils and promyelocytic HL-60 cells to A. phagocytophilum are linked to bacterial usage of P-selectin glycoprotein ligand 1 (PSGL-1) as a receptor for adhesion and entry. A. phagocytophilum undergoes a biphasic developmental cycle, transitioning between a smaller electron dense-cored cell (DC), which has a dense nucleoid, and a larger, pleomorphic electron lucent reticulate cell (RC), which has a dispersed nucleoid. The pathobiological roles of each form have not been elucidated. To ascertain the role of each form, we used electron microscopy to monitor bacterial binding, entry, and intracellular development within HL-60 cells. Only DCs were observed binding to and inducing uptake by HL-60 cells. By 12 h, internalized DCs had transitioned to RCs, which had initiated replication. By 24 h, large RC numbers were observed within individual inclusions. Reinfection had occurred by 36 h, as individual, vacuole-enclosed DCs and RCs were again observed. The abilities of DC- and RC-enriched A. phagocytophilum populations to bind and/or infect HL-60 cells or Chinese hamster ovary cells transfected to express PSGL-1 (PSGL-1 CHO) were compared. Only DCs bound PSGL-1 CHO cells and did so in a PSGL-1-blocking antibody-inhibitable manner. These results demonstrate that the respective roles of A. phagocytophilum DCs and RCs are consistent with analogous forms of other obligate intracellular pathogens that undergo biphasic development and hint that the PSGL-1-targeting adhesin(s) may be upregulated or optimally posttranslationally modified on DCs.

Neutrophils phagocytose activated platelets in vivo: a phosphatidylserine, P-selectin, and β2 integrin–dependent cell clearance program

Activated platelets express ligands, which are recognized by counterreceptors on neutrophils. Here, we show that the ensuing cell-to-cell interaction programs neutrophil phagocytic function, resulting in activated platelet clearance. Neutrophils that have internalized platelets circulate in the blood of patients with acute myocardial infarction, and the extent of platelet clearance correlates with expression of platelet activation, including P-selectin. Activated platelets injected intravenously in experimental animals are detectable in circulating neutrophils 60 minutes after, and within 3 hours, more than 70% circulating neutrophils have internalized platelets. Platelet clearance comprises 2 events: adhesion to neutrophils, which requires divalent cations and depends on P-selectin, on the P-selectin glycoprotein ligand-1 (PSGL-1), and on the CD11b/CD18 β2 integrin; and internalization, which is abrogated by the phosphatidylserine-binding protein annexin A5. Adhesion to platelets causes neutrophil degranulation and is blocked by antibodies specific for P-selectin and PSGL-1, either in a synthetic medium in vitro or in the whole blood, therefore in the presence of a physiologic array of plasma cofactors and opsonins. The data suggest that the interaction between circulating platelets and neutrophils influences innate immune functions, possibly contributing to regulate vascular inflammation.

Differential expression and glycosylation of anaplasma phagocytophilum major surface protein 2 paralogs during cultivation in sialyl Lewis x-deficient host cells

Many microbial pathogens alter expression and/or posttranslational modifications of their surface proteins in response to dynamics within their host microenvironments to retain optimal interactions with their host cells and/or to evade the humoral immune response. Anaplasma phagocytophilum is an intragranulocytic bacterium that utilizes sialyl Lewis x (sLe(x))-modified P-selectin glycoprotein ligand 1 as a receptor for infecting myeloid cells. Bacterial populations that do not rely on this receptor can be obtained through cultivation in sLe(x)-defective cell lines. A. phagocytophilum major surface protein 2 [Msp2(P44)] is encoded by members of a paralogous gene family and is speculated to play roles in host adaptation. We assessed the complement of Msp2(P44) paralogs expressed by A. phagocytophilum during infection of sLe(x)-competent HL-60 cells and two HL-60 cell lines defective for sLe(x) expression. Multiple Msp2(P44) and N-terminally truncated 25- to 27-kDa isoforms having various isoelectric points and electrophoretic mobilities were expressed in each cell line. The complement of expressed msp2(p44) paralogs and the glycosyl residues modifying Msp2(P44) varied considerably among bacterial populations recovered from sLe(x)-competent and -deficient host cells. Thus, loss of host cell sLe(x) expression coincided with both differential expression and glycosylation of A. phagocytophilum Msp2(P44). This reinforces the hypothesis that this bacterium is able to generate a large variety of surface-exposed molecules that could provide great antigenic diversity and result in multiple binding properties.

Anaplasma phagocytophilum infects cells of the megakaryocytic lineage through sialylated ligands but fails to alter platelet production

Anaplasma phagocytophilum is an obligate intracellular bacterial pathogen that principally inhabits neutrophils. However, infection with A. phagocytophilum results in a moderate to marked thrombocytopenia. In host neutrophils, A. phagocytophilum uses sialylated ligands, primarily P-selectin glycoprotein ligand-1 (PSGL-1), to enter its host cell. PSGL-1 is expressed on a wide array of haematopoietic cells, including megakaryocytes. In this study, it was hypothesized that (i) cells of the megakaryocytic lineage (MEG-01 cells) would be susceptible to A. phagocytophilum infection and (ii) infection may induce alterations in platelet production contributing to infection-induced thrombocytopenia. It was found that MEG-01 cells are susceptible to infection. MEG-01 cells expressing abundant sialylated ligands were the most susceptible to infection, and the absence of sialylation, or blocking of PSGL-1, limited infection susceptibility. However, infected MEG-01 cells produced proplatelets and platelet-like particles comparable to uninfected cells. These results highlight a novel target of pathogen infection and suggest that the pathogen may utilize similar strategies to gain access to megakaryocytes. Direct pathogen modification of platelet production may not play a role in infection-induced thrombocytopenia.

Anaplasma phagocytophilum PSGL-1-independent infection does not require Syk and leads to less efficient AnkA delivery

Anaplasma phagocytophilum is an obligate intracellular bacterium that infects neutrophils to cause granulocytic anaplasmosis in humans and mammals. P-selectin glycoprotein ligand-1 (PSGL-1) and the tetrasaccharide sialyl Lewis x (sLe(x)), which caps the PSGL-1 N-terminus, are confirmed A. phagocytophilum receptors. A. phagocytophilum is capable of sLe(x)-modified PSGL-1-dependent and -independent infection. PSGL-1 N-terminus-mediated entry is dependent on spleen tyrosine kinase (Syk). Here, we determined that PSGL-1-independent entry does not alter bacterial replication and investigated whether it involves Syk using NCH-1A2, an enriched subpopulation of A. phagocytophilum NCH-1 obtained through cultivation in a sLe(x)-deficient HL-60 cell line, HL-60 A2. Pharmacological inhibition of Syk nearly abolishes NCH-1 infection, but does not alter NCH-1A2 invasion and only marginally reduces NCH-1A2 propagation. This phenomenon was confirmed by a competitive infection assay using PSGL-1-dependent and -independent A. phagocytophilum organisms transformed to express mCherry or green fluorescent protein respectively. We also assayed for delivery and tyrosine phosphorylation of the A. phagocytophilum effector, AnkA, following NCH-1or NCH-1A2 incubation with HL-60 or HL-60 A2 cells in the presence of PSGL-1 blocking antibody. PSGL-1 N-terminus recognition promotes optimal AnkA delivery while binding to sLe(x) or the unknown receptor is comparably less important for this process.

Sialyl-Lewis x-independent infection of human myeloid cells by Anaplasma phagocytophilum strains HZ and HGE1

Anaplasma phagocytophilum, the causative agent of human granulocytic anaplasmosis, is an obligate intracellular bacterium that infects neutrophils and neutrophil precursors. Bacterial recognition of P-selectin glycoprotein ligand-1 (PSGL-1) and the alpha2,3-sialylated- and alpha1,3-fucosylated-moiety sialyl-Lewis x (sLe(x)), which modifies the PSGL-1 N terminus, is important for adhesion to and invasion of myeloid cells. We have previously demonstrated that A. phagocytophilum organisms of the NCH-1 strain that utilize an sLe(x)-modified PSGL-1-independent means of entry can be enriched for by cultivation in undersialylated HL-60 cells that are unable to construct sLe(x). Because it was unknown whether other A. phagocytophilum isolates share this ability, we extended our studies to the geographically diverse strains HZ and HGE1. HL-60 A2 is a clonal cell line that is defective for sialylation and alpha1,3-fucosyltransferase. HL-60 A2 cell surfaces, therefore, not only lack sLe(x) but also are virtually devoid of any other sialic acid- and/or alpha1,3-fucose-modified glycan. By cultivating HZ and HGE1 in HL-60 A2 cells, we enriched for bacterial subpopulations (termed HZA2 and HGE1A2) that bind and/or infect myeloid cells in the absence of sialic acid and alpha1,3-fucose and in the presence of antibody that blocks the N terminus of PSGL-1. Thus, multiple A. phagocytophilum isolates share the ability to use sLe(x)-modified PSGL-1-dependent and -independent routes of entry into myeloid cells. HZA2 and HGE1A2 represent enriched bacterial populations that will aid dissection of the complexities of the interactions between A. phagocytophilum and host myeloid cells.

The skin as interface in the transmission of arthropod-borne pathogens

Animal skin separates the inner world of the body from the largely hostile outside world and is actively involved in the defence against microbes. However, the skin is no perfect defence barrier and many microorganisms have managed to live on or within the skin as harmless passengers or as disease-causing pathogens. Microbes have evolved numerous strategies that allow them to gain access to the layers underneath the epidermis where they either multiply within the dermis or move to distant destinations within the body for replication. A number of viruses, bacteria and parasites use arthropod vectors, like ticks or mosquitoes, to deliver them into the dermis while taking their blood meal. Within the dermis, successful pathogens subvert the function of a variety of skin resident cells or cells of the innate immune system that rush to the site of infection. In this review several interactions with cells of the skin by medically relevant vector-borne pathogens are discussed to highlight the different ways in which these pathogens have come to survive within the skin and to usurp the defence mechanisms of the host for their own ends.

IL-12/23p40-dependent clearance of Anaplasma phagocytophilum in the murine model of human anaplasmosis

Human anaplasmosis is an emerging infectious disease transmitted by ticks that can be potentially fatal in the immunocompromised and the elderly. The mechanisms of defense against the causative agent, Anaplasma phagocytophilum, are not completely understood; however, interferon (IFN)-gamma plays an important role in pathogen clearance. Here, we show that IFN-gamma is regulated through an early IL-12/23p40-dependent mechanism. Interleukin (IL)-12/23p40 is regulated in macrophages and dendritic cells after activation by microbial agonists and cytokines and constitutes a subunit of IL-12 and IL-23. IL-12/23p40-deficient mice displayed an increased A. phagocytophilum burden, accelerated thrombocytopenia and increased neutrophil numbers in the spleen at day 6 postinfection. Infection of MyD88- and mitogen-activated kinase kinase 3 (MKK3)-deficient mice suggested that the early susceptibility due to IL-12/23p40 deficiency was not dependent on signaling through MyD88 or MKK3. The lack of IL-12/23p40 reduced IFN-gamma production in both CD4(+) and CD8(+) T cells although the effect was more pronounced in CD4(+) T cells. Our data suggest that the immune response against A. phagocytophilum is a multifactorial and cooperative process. The IL-12/23p40 subunit drives the CD4(+) Th1 immune response in the early phase of infection and IL-12/23p40-independent mechanisms ultimately contribute to pathogen elimination from the host.