Human granulocytic ehrlichiosis agent inhibits superoxide anion generation by human neutrophils

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.

PERK-mediated antioxidant response is key for pathogen persistence in ticks

A crucial phase in the life cycle of tick-borne pathogens is the time spent colonizing and persisting within the arthropod. Tick immunity is emerging as a key force shaping how transmissible pathogens interact with the vector. How pathogens remain in the tick despite immunological pressure remains unknown. In persistently infected Ixodes scapularis, we found that Borrelia burgdorferi (causative agent of Lyme disease) and Anaplasma phagocytophilum (causative agent of granulocytic anaplasmosis) activate a cellular stress pathway mediated by the endoplasmic reticulum receptor PKR-like ER kinase (PERK) and the central regulatory molecule eIF2α. Disabling the PERK pathway through pharmacological inhibition and RNA interference (RNAi) significantly decreased microbial numbers. In vivo RNAi of the PERK pathway not only reduced the number of A. phagocytophilum and B. burgdorferi colonizing larvae after a bloodmeal but also significantly reduced the number of bacteria that survive the molt. An investigation into PERK pathway-regulated targets revealed that A. phagocytophilum and B. burgdorferi induce activity of the antioxidant response regulator, nuclear factor erythroid 2-related factor 2 (Nrf2). Tick cells deficient for nrf2 expression or PERK signaling showed accumulation of reactive oxygen and nitrogen species in addition to reduced microbial survival. Supplementation with antioxidants rescued the microbicidal phenotype caused by blocking the PERK pathway. Altogether, our study demonstrates that the Ixodes PERK pathway is activated by transmissible microbes and facilitates persistence in the arthropod by potentiating an Nrf2-regulated antioxidant environment. IMPORTANCE Recent advances demonstrate that the tick immune system recognizes and limits the pathogens they transmit. Innate immune mediators such as antimicrobial peptides and reactive oxygen/nitrogen species are produced and restrict microbial survival. It is currently unclear how pathogens remain in the tick, despite this immune assault. We found that an antioxidant response controlled by the PERK branch of the unfolded protein response is activated in ticks that are persistently infected with Borrelia burgdorferi (Lyme disease) or Anaplasma phagocytophilum (granulocytic anaplasmosis). The PERK pathway induces the antioxidant response transcription factor, Nrf2, which coordinates a gene network that ultimately neutralizes reactive oxygen and nitrogen species. Interfering with this signaling cascade in ticks causes a significant decline in pathogen numbers. Given that innate immune products can cause collateral damage to host tissues, we speculate that this is an arthropod-driven response aimed at minimizing damage to "self" that also inadvertently benefits the pathogen. Collectively, our findings shed light on the mechanistic push and pull between tick immunity and pathogen persistence within the arthropod vector.

PERK-mediated antioxidant response is key for pathogen persistence in ticks

A crucial phase in the lifecycle of tick-borne pathogens is the time spent colonizing and persisting within the arthropod. Tick immunity is emerging as a key force shaping how transmissible pathogens interact with the vector. How pathogens remain in the tick despite immunological pressure remains unknown. In persistently infected Ixodes scapularis , we found that Borrelia burgdorferi (Lyme disease) and Anaplasma phagocytophilum (granulocytic anaplasmosis) activate a cellular stress pathway mediated by the endoplasmic reticulum receptor PERK and the central regulatory molecule, eIF2α. Disabling the PERK pathway through pharmacological inhibition and RNAi significantly decreased microbial numbers. In vivo RNA interference of the PERK pathway not only reduced the number of A. phagocytophilum and B. burgdorferi colonizing larvae after a bloodmeal, but also significantly reduced the number of bacteria that survive the molt. An investigation into PERK pathway-regulated targets revealed that A. phagocytophilum and B. burgdorferi induce activity of the antioxidant response regulator, Nrf2. Tick cells deficient for nrf2 expression or PERK signaling showed accumulation of reactive oxygen and nitrogen species in addition to reduced microbial survival. Supplementation with antioxidants rescued the microbicidal phenotype caused by blocking the PERK pathway. Altogether, our study demonstrates that the Ixodes PERK pathway is activated by transmissible microbes and facilitates persistence in the arthropod by potentiating an Nrf2-regulated antioxidant environment.

Microbes and the fate of neutrophils

Neutrophils or polymorphonuclear neutrophils (PMNs) are an important component of innate host defense. These phagocytic leukocytes are recruited to infected tissues and kill invading microbes. There are several general characteristics of neutrophils that make them highly effective as antimicrobial cells. First, there is tremendous daily production and turnover of granulocytes in healthy adults-typically 1011 per day. The vast majority (~95%) of these cells are neutrophils. In addition, neutrophils are mobilized rapidly in response to chemotactic factors and are among the first leukocytes recruited to infected tissues. Most notably, neutrophils contain and/or produce an abundance of antimicrobial molecules. Many of these antimicrobial molecules are toxic to host cells and can destroy host tissues. Thus, neutrophil activation and turnover are highly regulated processes. To that end, aged neutrophils undergo apoptosis constitutively, a process that contains antimicrobial function and proinflammatory capacity. Importantly, apoptosis facilitates nonphlogistic turnover of neutrophils and removal by macrophages. This homeostatic process is altered by interaction with microbes and their products, as well as host proinflammatory molecules. Microbial pathogens can delay neutrophil apoptosis, accelerate apoptosis following phagocytosis, or cause neutrophil cytolysis. Here, we review these processes and provide perspective on recent studies that have potential to impact this paradigm.

Ehrlichia chaffeensis Uses an Invasin To Suppress Reactive Oxygen Species Generation by Macrophages via CD147-Dependent Inhibition of Vav1 To Block Rac1 Activation

The obligatory intracellular pathogen Ehrlichia chaffeensis lacks most factors that could respond to oxidative stress (a host cell defense mechanism). We previously found that the C terminus of Ehrlichia surface invasin, entry-triggering protein of Ehrlichia (EtpE; EtpE-C) directly binds mammalian DNase X, a glycosylphosphatidylinositol-anchored cell surface receptor and that binding is required to induce bacterial entry and simultaneously to block the generation of reactive oxygen species (ROS) by host monocytes and macrophages. However, how the EtpE-C-DNase X complex mediates the ROS blockade was unknown. A mammalian transmembrane glycoprotein CD147 (basigin) binds to the EtpE-DNase X complex and is required for Ehrlichia entry and infection of host cells. Here, we found that bone marrow-derived macrophages (BMDM) from myeloid cell lineage-selective CD147-null mice had significantly reduced Ehrlichia-induced or EtpE-C-induced blockade of ROS generation in response to phorbol myristate acetate. In BMDM from CD147-null mice, nucleofection with CD147 partially restored the Ehrlichia-mediated inhibition of ROS generation. Indeed, CD147-null mice as well as their BMDM were resistant to Ehrlichia infection. Moreover, in human monocytes, anti-CD147 partially abrogated EtpE-C-induced blockade of ROS generation. Both Ehrlichia and EtpE-C could block activation of the small GTPase Rac1 (which in turn activates phagocyte NADPH oxidase) and suppress activation of Vav1, a hematopoietic-specific Rho/Rac guanine nucleotide exchange factor by phorbol myristate acetate. Vav1 suppression by Ehrlichia was CD147 dependent. E. chaffeensis is the first example of pathogens that block Rac1 activation to colonize macrophages. Furthermore, Ehrlichia uses EtpE to hijack the unique host DNase X-CD147-Vav1 signaling to block Rac1 activation.IMPORTANCEEhrlichia chaffeensis is an obligatory intracellular bacterium with the capability of causing an emerging infectious disease called human monocytic ehrlichiosis. E. chaffeensis preferentially infects monocytes and macrophages, professional phagocytes, equipped with an arsenal of antimicrobial mechanisms, including rapid reactive oxygen species (ROS) generation upon encountering bacteria. As Ehrlichia isolated from host cells are readily killed upon exposure to ROS, Ehrlichia must have evolved a unique mechanism to safely enter phagocytes. We discovered that binding of the Ehrlichia surface invasin to the host cell surface receptor not only triggers Ehrlichia entry but also blocks ROS generation by the host cells by mobilizing a novel intracellular signaling pathway. Knowledge of the mechanisms by which ROS production is inhibited may lead to the development of therapeutics for ehrlichiosis as well as other ROS-related pathologies.

Neutrophils in innate immunity and systems biology-level approaches

The innate immune system is the first line of host defense against invading microorganisms. Polymorphonuclear leukocytes (PMNs or neutrophils) are the most abundant leukocyte in humans and essential to the innate immune response against invading pathogens. Compared to the acquired immune response, which requires time to develop and is dependent on previous interaction with specific microbes, the ability of neutrophils to kill microorganisms is immediate, nonspecific, and not dependent on previous exposure to microorganisms. Historically, studies of PMN-pathogen interaction focused on the events leading to killing of microorganisms, such as recruitment/chemotaxis, transmigration, phagocytosis, and activation, whereas postphagocytosis sequelae were infrequently considered. In addition, it was widely accepted that human neutrophils possessed limited capacity for new gene transcription and thus, relatively little biosynthetic capacity. This notion has changed dramatically within the past 20 years. Further, there is now more effort directed to understand the events occurring in PMNs after killing of microbes. Herein, we give an updated review of the systems biology-level approaches that have been used to gain an enhanced view of the role of neutrophils during host-pathogen interaction and neutrophil-mediated diseases. We anticipate that these and future systems-level studies will continue to provide information important for understanding, treatment, and control of diseases caused by pathogenic microorganisms. This article is categorized under: Physiology > Organismal Responses to Environment Physiology > Mammalian Physiology in Health and Disease Biological Mechanisms > Cell Fates.

Role and Function of the Type IV Secretion System in Anaplasma and Ehrlichia Species

The obligatory intracellular pathogens Anaplasma phagocytophilum and Ehrlichia chaffeensis proliferate within membrane-bound vacuoles of human leukocytes and cause potentially fatal emerging infectious diseases. Despite the reductive genome evolution in this group of bacteria, genes encoding the type IV secretion system (T4SS), which is homologous to the VirB/VirD4 system of the plant pathogen Agrobacterium tumefaciens, have been expanded and are highly expressed in A. phagocytophilum and E. chaffeensis in human cells. Of six T4SS effector proteins identified in them, roles and functions have been described so far only for ankyrin repeat domain-containing protein A (AnkA), Anaplasma translocated substrate 1 (Ats-1), and Ehrlichia translocated factor 1 (Etf-1, ECH0825). These effectors are abundantly produced and secreted into the host cytoplasm during infection, but not toxic to host cells. They contain eukaryotic protein motifs or organelle localization signals and have distinct subcellular localization, target to specific host cell molecules to promote infection. Ats-1 and Etf-1 are orthologous proteins, subvert two important innate immune mechanisms against intracellular infection, cellular apoptosis and autophagy, and manipulate autophagy to gain nutrients from host cells. Although Ats-1 and Etf-1 have similar functions and roles in obligatory intracellular infection, they are specifically adapted to the distinct membrane-bound intracellular niche of A. phagocytophilum and E. chaffeensis, respectively. Ectopic expression of these effectors enhances respective bacterial infection, whereas intracellular delivery of antibodies against these effectors or targeted knockdown of the effector with antisense peptide nucleic acid significantly impairs bacterial infection. Thus, both T4SSs have evolved as important survival and nutritional virulence mechanism in these obligatory intracellular bacteria. Future studies on the functions of Anaplasma and Ehrlichia T4SS effector molecules and signaling pathways will undoubtedly advance our understanding of the complex interplay between obligatory intracellular pathogens and their hosts. Such data can be applied toward the treatment and control of anaplasmosis and ehrlichiosis.

Development of TEM-1 β-lactamase based protein translocation assay for identification of Anaplasma phagocytophilum type IV secretion system effector proteins

Anaplasma phagocytophilum, the aetiologic agent of human granulocytic anaplasmosis (HGA) is an obligate intracellular Gram-negative bacterium with the genome size of 1.47 megabases. The intracellular life style and small size of genome suggest that A. phagocytophilum has to modulate a multitude of host cell physiological processes to facilitate its replication. One strategy employed by A. phagocytophilum is through its type IV secretion system (T4SS), which translocates bacterial effectors into target cells to disrupt normal cellular activities. In this study we developed a TEM-1 β-lactamase based protein translocation assay and applied this assay for identification of A. phagocytophilum T4SS effectors. An A. phagocytophilum hypothetical protein, APH0215 is identified as a T4SS effector protein and found interacting with trans-Golgi network in transfected cells. Hereby, this protein translocation assay developed in this study will facilitate the identification of A. phagocytophilum T4SS effectors and elucidation of HGA pathogenesis.

Ehrlichia chaffeensis and Its Invasin EtpE Block Reactive Oxygen Species Generation by Macrophages in a DNase X-Dependent Manner

The obligatory intracellular pathogen Ehrlichia chaffeensis lacks most genes that confer resistance to oxidative stress but can block reactive oxygen species (ROS) generation by host monocytes-macrophages. Bacterial and host molecules responsible for this inhibition have not been identified. To infect host cells, Ehrlichia uses the C terminus of its surface invasin, entry-triggering protein of Ehrlichia (EtpE; EtpE-C), which directly binds the mammalian cell surface receptor glycosylphosphatidylinositol-anchored protein DNase X. We investigated whether EtpE-C binding to DNase X blocks ROS production by mouse bone marrow-derived macrophages (BMDMs). On the basis of a luminol-dependent chemiluminescence assay, E. chaffeensis inhibited phorbol myristate acetate (PMA)-induced ROS generation by BMDMs from wild-type, but not DNase X^−/−, mice. EtpE-C is critical for inhibition, as recombinant EtpE-C (rEtpE-C)-coated latex beads, but not recombinant N-terminal EtpE-coated or uncoated beads, inhibited PMA-induced ROS generation by BMDMs from wild-type mice. DNase X is required for this inhibition, as none of these beads inhibited PMA-induced ROS generation by BMDMs from DNase X^−/− mice. Previous studies showed that E. chaffeensis does not block ROS generation in neutrophils, a cell type that is a potent ROS generator but is not infected by E. chaffeensis. Human and mouse peripheral blood neutrophils did not express DNase X. Our findings point to a unique survival mechanism of ROS-sensitive obligate intramonocytic bacteria that involves invasin EtpE binding to DNase X on the host cell surface. This is the first report of bacterial invasin having such a subversive activity on ROS generation.

Ehrlichia chaffeensis preferentially infects monocytes-macrophages and causes a life-threatening emerging tick-transmitted infectious disease called human monocytic ehrlichiosis. Ehrlichial infection, and hence the disease, depends on the ability of this bacterium to avoid or overcome powerful microbicidal mechanisms of host monocytes-macrophages, one of which is the generation of ROS. Our findings reveal that an ehrlichial surface invasin, EtpE, not only triggers bacterial entry but also blocks ROS generation by host macrophages through its host cell receptor, DNase X. As ROS sensitivity is an Achilles’ heel of this group of pathogens, understanding the mechanism by which E. chaffeensis rapidly blocks ROS generation suggests a new approach for developing effective anti-infective measures. The discovery of a ROS-blocking pathway is also important, as modulation of ROS generation is important in a variety of ailments and biological processes.

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-Occupied Vacuole Interactions with the Host Cell Cytoskeleton

Anaplasma phagocytophilum is an obligate intracellular bacterial pathogen of humans and animals. The A. phagocytophium-occupied vacuole (ApV) is a critical host-pathogen interface. Here, we report that the intermediate filaments, keratin and vimentin, assemble on the ApV early and remain associated with the ApV throughout infection. Microtubules localize to the ApV to a lesser extent. Vimentin, keratin-8, and keratin-18 but not tubulin expression is upregulated in A. phagocytophilum infected cells. SUMO-2/3 but not SUMO-1 colocalizes with vimentin filaments that surround ApVs. PolySUMOylation of vimentin by SUMO-2/3 but not SUMO-1 decreases vimentin solubility. Consistent with this, more vimentin exists in an insoluble state in A. phagocytophilum infected cells than in uninfected cells. Knocking down the SUMO-conjugating enzyme, Ubc9, abrogates vimentin assembly at the ApV but has no effect on the bacterial load. Bacterial protein synthesis is dispensable for maintaining vimentin and SUMO-2/3 at the ApV. Withaferin A, which inhibits soluble vimentin, reduces vimentin recruitment to the ApV, optimal ApV formation, and the bacterial load when administered prior to infection but is ineffective once vimentin has assembled on the ApV. Thus, A. phagocytophilum modulates cytoskeletal component expression and co-opts polySUMOylated vimentin to aid construction of its vacuolar niche and promote optimal survival.

Persistent Infections and Immunity in Ruminants to Arthropod-Borne Bacteria in the Family Anaplasmataceae

Tick-transmitted gram-negative bacteria in the family Anaplasmataceae in the order Rickettsiales cause persistent infection and morbidity and mortality in ruminants. Whereas Anaplasma marginale infection is restricted to ruminants, Anaplasma phagocytophilum is promiscuous and, in addition to causing disease in sheep and cattle, notably causes disease in humans, horses, and dogs. Although the two pathogens invade and replicate in distinct blood cells (erythrocytes and neutrophils, respectively), they have evolved similar mechanisms of antigenic variation in immunodominant major surface protein 2 (MSP2) and MSP2(P44) that result in immune evasion and persistent infection. Furthermore, these bacteria have evolved distinct strategies to cause immune dysfunction, characterized as an antigen-specific CD4 T-cell exhaustion for A. marginale and a generalized immune suppression for A. phagocytophilum, that also facilitate persistence. This indicates highly adapted strategies of Anaplasma spp. to both suppress protective immune responses and evade those that do develop. However, conserved subdominant antigens are potential targets for immunization.

Anaplasma phagocytophilum up-regulates some anti-apoptotic genes in neutrophils and pro-inflammatory genes in mononuclear cells of sheep

Anaplasma phagocytophilum, the causative agent of tick-borne fever (TBF) in sheep and cattle and human granulocytic anaplasmosis, has the unique ability to selectively infect and multiply within the hostile environment of the neutrophil. Previous studies have shown that sheep with TBF are more susceptible to other infections and that infected neutrophils have reduced phagocytic ability and delayed apoptosis. This suggests that survival of A. phagocytophilum in these short-lived cells involves the ability to subvert or resist their bacterial killing, but also to modify the host cells such that the host cells survive long after infection. The present study shows that infection of sheep by A. phagocytophilum is characterized by up-regulation of some anti-apoptotic genes (BCL2, BIRC3 and CFLAR) in neutrophils and up-regulation of genes encoding the pro-inflammatory cytokines interferon-γ, interleukin (IL)-1β and IL-6 in mononuclear cells during the period of bacteraemia. Infection with A. phagocytophilum was also characterized by significant up-regulation of CYBB, which is associated with the respiratory burst of neutrophils.

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: 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.

Innate immunity against Granulibacter bethesdensis, an emerging gram-negative bacterial pathogen

Acetic acid bacteria were previously considered nonpathogenic in humans. However, over the past decade, five genera of Acetobacteraceae have been isolated from patients with inborn or iatrogenic immunodeficiencies. Here, we describe the first studies of the interactions of the human innate immune system with a member of this bacterial family, Granulibacter bethesdensis, an emerging pathogen in patients with chronic granulomatous disease (CGD). Efficient phagocytosis of G. bethesdensis by normal and CGD polymorphonuclear leukocytes (CGD PMN) required heat-labile serum components (e.g., C3), and binding of C3 and C9 to G. bethesdensis was detected by immunoblotting. However, this organism survived in human serum concentrations of ≥90%, indicating a high degree of serum resistance. Consistent with the clinical host tropism of G. bethesdensis, CGD PMN were unable to kill this organism, while normal PMN, in the presence of serum, reduced the number of CFU by about 50% after a 24-h coculture. This finding, together with the observations that G. bethesdensis was sensitive to H(2)O(2) but resistant to LL-37, a human cationic antimicrobial peptide, suggests an inherent resistance to O(2)-independent killing. Interestingly, 10 to 100 times greater numbers of G. bethesdensis were required to achieve the same level of reactive oxygen species (ROS) production induced by Escherichia coli in normal PMN. In addition to the relative inability of the organism to elicit production of PMN ROS, G. bethesdensis inhibited both constitutive and FAS-induced PMN apoptosis. These properties of reduced PMN activation and resistance to nonoxidative killing mechanisms likely play an important role in G. bethesdensis pathogenesis.

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.

Coxiella burnetii acid phosphatase inhibits the release of reactive oxygen intermediates in polymorphonuclear leukocytes

Coxiella burnetii, the etiological agent of Q fever, is a small, Gram-negative, obligate intracellular bacterium. Replication of C. burnetii during infection has been shown to be increased by decreasing oxidative stress using p47(phox -/-) and iNOS(-/-) mice in vivo and by pharmacologic inhibitors in vitro. Building upon this model, we investigated the role polymorphonuclear leukocytes (PMN) play in the control of infection, since NADPH oxidase-mediated release of reactive oxygen intermediates (ROI) is a primary bactericidal mechanism for these cells that is critical for early innate clearance. Earlier studies suggested that C. burnetii actively inhibited release of ROI from PMN through expression of an unidentified acid phosphatase (ACP). Recent genomic annotations identified one open reading frame (CBU0335) which may encode a Sec- and type II-dependent secreted ACP. To test this model, viable C. burnetii propagated in tissue culture host cells or axenic media, C. burnetii extracts, or purified recombinant ACP (rACP) was combined with human PMN induced with 4-phorbol 12-myristate 13-acetate (PMA). The release of ROI was inhibited when PMN were challenged with viable C. burnetii, C. burnetii extracts, or rACP but not when PMN were challenged with electron beam-inactivated C. burnetii. C. burnetii extracts and rACP were also able to inhibit PMA-induced formation of NADPH oxidase complex on PMN membranes, suggesting a molecular mechanism responsible for this inhibition. These data support a model in which C. burnetii eludes the primary ROI killing mechanism of activated PMN by secreting at least one acid phosphatase.

Type IV secretion in the obligatory intracellular bacterium Anaplasma phagocytophilum

Anaplasma phagocytophilum is an obligatory intracellular bacterium that infects neutrophils, the primary host defence cells. Consequent effects of infection on host cells result in a potentially fatal systemic disease called human granulocytic anaplasmosis. Despite ongoing reductive genome evolution and deletion of most genes for intermediary metabolism and amino acid biosynthesis, Anaplasma has also experienced expansion of genes encoding several components of the type IV secretion (T4S) apparatus. Two A. phagocytophilum T4S effector molecules are currently known; Anaplasma translocated substrate 1 (Ats-1) and ankyrin repeat domain-containing protein A (AnkA) have C-terminal positively charged amino acid residues that are recognized by the T4S coupling protein, VirD4. AnkA and Ats-1 contain eukaryotic protein motifs and are uniquely evolved in the family Anaplasmataceae; Ats-1 contains a mitochondria-targeting signal. They are abundantly produced and secreted into the host cytoplasm, are not toxic to host cells, and manipulate host cell processes to aid in the infection process. At the cellular level, the two effectors have distinct subcellular localization and signalling in host cells. Thus in this obligatory intracellular pathogen, the T4S system has evolved as a host-subversive survival factor.

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.

Role of neutrophils in innate immunity: a systems biology-level approach

The innate immune system is the first line of host defense against invading microorganisms. Polymorphonuclear leukocytes (PMNs or neutrophils) are the most abundant leukocyte in humans and essential to the innate immune response against invading pathogens. Compared with the acquired immune response, which requires time to develop and is dependent on previous interaction with specific microbes, the ability of neutrophils to kill microorganisms is immediate, non-specific, and not dependent on previous exposure to microorganisms. Historically, studies on PMN-pathogen interaction focused on the events leading to killing of microorganisms, such as recruitment/chemotaxis, transmigration, phagocytosis, and activation, whereas post-phagocytosis sequelae were infrequently considered. In addition, it was widely accepted that human neutrophils possessed limited capacity for new gene transcription and thus, relatively little biosynthetic capacity. This notion has changed dramatically within the past decade. Further, there is now more effort directed to understand the events occurring in PMNs after killing of microbes. Herein we review the systems biology-level approaches that have been used to gain an enhanced view of the role of neutrophils during host-pathogen interaction. We anticipate that these and future systems-level studies will ultimately provide information critical to our understanding, treatment, and control of diseases caused by pathogenic microorganisms.

Impairment of gamma interferon signaling in human neutrophils infected with Anaplasma phagocytophilum

Anaplasma phagocytophilum, the causative agent of tick-borne human granulocytic anaplasmosis (HGA), is an intracellular bacterium which survives and multiplies inside polymorphonuclear neutrophil granulocytes (PMN). Increased bacterial burden in gamma interferon (IFN-gamma)-deficient mice suggested a major role of IFN-gamma in the control of A. phagocytophilum. Here we investigated whether infection of human PMN with A. phagocytophilum impairs IFN-gamma signaling thus facilitating intracellular survival of the bacterium. The secretion of the IFN-gamma-inducible chemokines IP-10/CXCL10 and MIG/CXCL9 was markedly inhibited in infected neutrophils. Molecular analyses revealed that, compared to uninfected PMN, A. phagocytophilum decreased the expression of the IFN-gamma receptor alpha-chain CD119, diminished the IFN-gamma-induced phosphorylation of STAT1, and enhanced the expression of SOCS1 and SOCS3 in PMN. Since IFN-gamma activates various antibacterial effector mechanisms of PMN, the impaired IFN-gamma signaling in infected cells likely contributes to the survival of A. phagocytophilum inside PMN and to HGA disease development.

The natural history of Anaplasma phagocytophilum

Anaplasma phagocytophilum is the recently designated name replacing three species of granulocytic bacteria, Ehrlichia phagocytophila, Ehrlichia equi and the agent of human granulocytic ehrlichiosis, after the recent reorganization of the families Rickettsiaceae and Anaplasmataceae in the order Rickettsiales. Tick-borne fever (TBF), which is caused by the prototype of A. phagocytophilum, was first described in 1932 in Scotland. A similar disease caused by a related granulocytic agent was first described in horses in the USA in 1969; this was followed by the description of two distinct granulocytic agents causing similar diseases in dogs in the USA in 1971 and 1982. Until the discovery of human granulocytic anaplasmosis (HGA) in the USA in 1994, these organisms were thought to be distinct species of bacteria infecting specific domestic animals and free-living reservoirs. It is now widely accepted that the agents affecting different animal hosts are variants of the same Gram-negative obligatory intracellular bacterium, which is transmitted by hard ticks belonging to the Ixodes persulcatus complex. One of its fascinating features is that it infects and actively grows in neutrophils by employing an array of mechanisms to subvert their bactericidal activity. It is also able to survive within an apparently immune host by employing a complex mechanism of antigenic variation. Ruminants with TBF and humans with HGA develop severe febrile reaction, bacteraemia and leukopenia due to neutropenia, lymphocytopenia and thrombocytopenia within a week of exposure to a tick bite. Because of the severe haematological disorders lasting for several days and other adverse effects on the host's immune functions, infected animals and humans are more susceptible to other infections.

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.

Neisseria gonorrhoeae suppresses the oxidative burst of human polymorphonuclear leukocytes

Symptomatic infection with Neisseria gonorrhoeae (Gc) results in a potent polymorphonuclear leukocyte (PMN)-driven inflammatory response, but the mechanisms by which Gc withstands PMN attack are poorly defined. Here we report that Gc can suppress the PMN oxidative burst, a central component of the PMN antimicrobial arsenal. Primary human PMNs remained viable after exposure to liquid-grown, exponential-phase, opacity-associated protein (Opa)-negative Gc of strains FA1090 and MS11 but did not generate reactive oxygen species (ROS), even after bacterial opsonization. Liquid-grown FA1090 Gc expressing OpaB, an Opa protein previously correlated with PMN ROS production, elicited a minor PMN oxidative burst. PMN ROS production in response to Opa(-) and OpaB+ Gc was markedly enhanced if bacteria were agar-grown or if liquid-grown bacteria were heat-killed. Liquid-grown Opa(-) Gc inhibited the PMN oxidative burst elicited by isogenic dead bacteria, formylated peptides or Staphylococcus aureus but did not inhibit PMN ROS production by OpaB+ Gc or phorbol esters. Suppression of the oxidative burst required Gc-PMN contact and bacterial protein synthesis but not phagocytosis. These results suggest that viable Gc directly inhibits PMN signalling pathways required for induction of the oxidative burst, which may contribute to gonococcal pathogenesis during inflammatory stages of gonorrhoeal disease.

Crystal structures of the staphylococcal toxin SSL5 in complex with sialyl Lewis X reveal a conserved binding site that shares common features with viral and bacterial sialic acid binding proteins

Staphylococcus aureus is a significant human pathogen. Among its large repertoire of secreted toxins is a group of staphylococcal superantigen-like proteins (SSLs). These are homologous to superantigens but do not have the same activity. SSL5 is shown here to bind to human granulocytes and to the cell surface receptors for human IgA (Fc alphaRI) and P-selectin [P-selectin glycoprotein ligand-1 (PSGL-1)] in a sialic acid (Sia)-dependent manner. Co-crystallization of SSL5 with the tetrasaccharide sialyl Lewis X (sLe(X)), a key determinant of PSGL-1 binding to P-selectin, led to crystal structures of the SSL5-sLe(X) complex at resolutions of 1.65 and 2.75 A for crystals at two pH values. In both structures, sLe(X) bound to a specific site on the surface of the C-terminal domain of SSL5 in a conformation identical with that bound by P-selectin. Conservation of the key carbohydrate binding residues indicates that this ability to bind human glycans is shared by a substantial subgroup of the SSLs, including SSL2, SSL3, SSL4, SSL5, SSL6, and SSL11. This indicates that the ability to target human glycans is an important property of this group of toxins. Structural comparisons also showed that the Sia binding site in SSL5 contains a substructure that is shared by other Sia binding proteins from bacteria as well as viruses and represents a common binding motif.

Identification of novel surface proteins of Anaplasma phagocytophilum by affinity purification and proteomics

Anaplasma phagocytophilum is the etiologic agent of human granulocytic anaplasmosis (HGA), one of the major tick-borne zoonoses in the United States. The surface of A. phagocytophilum plays a crucial role in subverting the hostile host cell environment. However, except for the P44/Msp2 outer membrane protein family, the surface components of A. phagocytophilum are largely unknown. To identify the major surface proteins of A. phagocytophilum, a membrane-impermeable, cleavable biotin reagent, sulfosuccinimidyl-2-[biotinamido]ethyl-1,3-dithiopropionate (Sulfo-NHS-SS-Biotin), was used to label intact bacteria. The biotinylated bacterial surface proteins were isolated by streptavidin agarose affinity purification and then separated by electrophoresis, followed by capillary liquid chromatography-nanospray tandem mass spectrometry analysis. Among the major proteins captured by affinity purification were five A. phagocytophilum proteins, Omp85, hypothetical proteins APH_0404 (designated Asp62) and APH_0405 (designated Asp55), P44 family proteins, and Omp-1A. The surface exposure of Asp62 and Asp55 was verified by immunofluorescence microscopy. Recombinant Asp62 and Asp55 proteins were recognized by an HGA patient serum. Anti-Asp62 and anti-Asp55 peptide sera partially neutralized A. phagocytophilum infection of HL-60 cells in vitro. We found that the Asp62 and Asp55 genes were cotranscribed and conserved among members of the family Anaplasmataceae. With the exception of P44-18, all of the proteins were newly revealed major surface-exposed proteins whose study should facilitate understanding the interaction between A. phagocytophilum and the host. These proteins may serve as targets for development of chemotherapy, diagnostics, and vaccines.

Differential interaction of dendritic cells with Rickettsia conorii: impact on host susceptibility to murine spotted fever rickettsiosis

Spotted fever group rickettsioses are emerging and reemerging infectious diseases, some of which are life-threatening. In order to understand how dendritic cells (DCs) contribute to the host resistance or susceptibility to rickettsial diseases, we first characterized the in vitro interaction of rickettsiae with bone marrow-derived DCs (BMDCs) from resistant C57BL/6 (B6) and susceptible C3H/HeN (C3H) mice. In contrast to the exclusively cytosolic localization within endothelial cells, rickettsiae efficiently entered and localized in both phagosomes and cytosol of BMDCs from both mouse strains. Rickettsia conorii-infected BMDCs from resistant mice harbored higher bacterial loads compared to C3H mice. R. conorii infection induced maturation of BMDCs from both mouse strains as judged by upregulated expression of classical major histocompatibility complex (MHC) and costimulatory molecules. Compared to C3H counterparts, B6 BMDCs exhibited higher expression levels of MHC class II and higher interleukin-12 (IL-12) p40 production upon rickettsial infection and were more potent in priming naïve CD4(+) T cells to produce gamma interferon. In vitro DC infection and T-cell priming studies suggested a delayed CD4(+) T-cell activation and suppressed Th1/Th2 cell development in C3H mice. The suppressive CD4(+) T-cell responses seen in C3H mice were associated with a high frequency of Foxp3(+) T regulatory cells promoted by syngeneic R. conorii-infected BMDCs in the presence of IL-2. These data suggest that rickettsiae can target DCs to stimulate a protective type 1 response in resistant hosts but suppressive adaptive immunity in susceptible hosts.

Immune evasion and immunosuppression by Anaplasma phagocytophilum, the causative agent of tick-borne fever of ruminants and human granulocytic anaplasmosis

Anaplasma phagocytophilum, the causative agent of tick-borne fever (TBF) in sheep and cattle and human granulocytic anaplasmosis, has the unique ability to infect and multiply within neutrophils, eosinophils and monocytes, cells at the frontline of the immune system. Infection with A. phagocytophilum is also characterized by severe leukopenia due to lymphocytopenia, neutropenia and thrombocytopenia lasting for several days. By itself TBF does not cause high mortality rates but infected animals are more susceptible to other secondary infections, pregnant animals may abort and there is a severe reduction in milk yield in dairy cattle. The susceptibility to secondary infections can be attributed to the leukopenia that accompanies the disease and the organism's adverse effects on lymphocyte and neutrophil functions. One of its fascinating features is that it infects and actively grows in neutrophils by employing an array of mechanisms to subvert their bactericidal activity. These include its ability to inhibit phagosome-lysosome fusion, to suppress respiratory burst and to delay the apoptotic death of neutrophils. It is also able to survive within an apparently immune host by employing a complex mechanism of antigenic variation.

Degradation of p22phox and inhibition of superoxide generation by Ehrlichia chaffeensis in human monocytes

Ehrlichia chaffeensis is an obligate intracellular bacterium which replicates in monocytes or macrophages, the primary producers of reactive oxygen species (ROS). However, effects of ROS on E. chaffeensis infection and whether E. chaffeensis modulates ROS generation in host monocytes are unknown. Here, E. chaffeensis was shown to lose infectivity upon exposure to O(2)(-) or hydrogen peroxide. Upon incubation with human monocytes, E. chaffeensis neither induced O(2)(-) generation by human monocytes, nor colocalized with nicotinamide adenine dinucleotide phosphate (NADPH) oxidase components. Instead, it actively blocked O(2)(-) generation by monocytes stimulated with phorbol myristate acetate and caused the rapid degradation of p22(phox), a component of NADPH oxidase. These effects were not seen in neutrophil, which is another potent ROS generator, but a cell type that E. chaffeensis does not infect. Trypsin pretreatment of monocytes prevented the inhibition of O(2)(-) generation by E. chaffeensis. The degradation of p22(phox) by E. chaffeensis was specific to subsets of monocytes with bound and/or intracellular bacteria, and the degradation could be reduced by heat treatment of the bacterium, lipopolysaccharide pretreatment of monocytes, or the incubation with haemin. The degradation of p22(phox) by E. chaffeensis and its prevention by haemin or protease inhibitors also occurred in isolated monocyte membrane fractions, indicating that host cytoplasmic signalling is not required for these processes. The amount of gp91(phox) was stable under all conditions examined in this study. These findings point to a unique survival mechanism of ROS-sensitive obligate intraleucocytic bacteria that involves the destabilization of p22(phox) following the binding of bacteria to host cell surface proteins.

Differential expression of VirB9 and VirB6 during the life cycle of Anaplasma phagocytophilum in human leucocytes is associated with differential binding and avoidance of lysosome pathway

Anaplasma phagocytophilum, an obligate intracellular bacterium, is the aetiologic agent of human granulocytic anaplasmosis (HGA). A. phagocytophilum virB/D operons encoding type IV secretion system are expressed in cell culture and in the blood of HGA patients. In the present study, their expression across the A. phagocytophilum intracellular developmental cycle was investigated. We found that mRNA levels of both virB9 and virB6 were upregulated during infection of human neutrophils in vitro. The antibody against the recombinant VirB9 protein was prepared and immunogold and immunofluorescence labelling were used to determine the VirB9 protein expression by individual organisms. Majority of A. phagocytophilum spontaneously released from the infected host cells poorly expressed VirB9. At 1 h post infection, VirB9 was not detectable on most bacteria associated with neutrophils. However, VirB9 was strongly expressed by A. phagocytophilum during proliferation in neutrophils. In contrast, with HL-60 cells, approximately 80% of A. phagocytophilum organisms associated at 1 h post infection expressed VirB9 protein and were colocalized with lysosome-associated membrane protein-1 (LAMP-1), whereas, VirB9-undetectable bacteria were not colocalized with LAMP-1. These results indicate developmental regulation of expression of components of type IV secretion system during A. phagocytophilum intracellular life cycle and suggest that bacterial developmental stages influence the nature of binding to the hosts and early avoidance of late endosome-lysosome pathway.

Mechanisms of evasion of neutrophil killing by Anaplasma phagocytophilum

PURPOSE OF REVIEW

This review summarizes recent knowledge regarding the strategies employed by Anaplasma phagocytophilum to evade or subvert neutrophil killing mechanisms and modify other neutrophil pathways to promote its survival.

RECENT FINDINGS

A. phagocytophilum evades neutrophil oxidative killing by preventing fusion of cytochrome b558-carrying specific granules and secretory vesicles with the membrane of its cytoplasmic compartment. It also directly detoxifies superoxide anion. Additionally, the bacterium alters the interaction of transcription factors with the CYYB promoter, which results in greatly reduced gp91phox levels and a consequent decline in respiratory burst capability. A. phagocytophilum not only fails to activate the normal neutrophil apoptosis differentiation program stimulated by bacterial uptake, but also delays spontaneous apoptosis by manipulating the expression of pro and antiapoptotic genes. Maintenance of the proapoptotic factor Bfl-1 contributes, at least in part, to the preservation of mitochondrial membrane integrity and inhibition of caspase 3 activation.

SUMMARY

A. phagocytophilum combats neutrophil oxidative killing by scavenging O2, inhibiting NADPH oxidase assembly on its vacuolar membrane, and modifying promoter activity for a key NADPH oxidase component, gp91phox. Uptake of this unique pathogen fails to induce neutrophil apoptosis. Furthermore, A. phagocytophilum extends the life of its otherwise short-lived host cell by dysregulating neutrophil gene expression and molecular machinery to potentially maximize its survival and dissemination within its mammalian host.

Ultrastructural study of the gametocytes and merogonic stages of Fallisia audaciosa (Haemosporina: Garniidae) that infect neutrophils of the lizard Plica umbra (Reptilia: Iguanidae)

Little is known regarding the ultrastructure of the genus Fallisia (Apicomplexa: Haemosporina: Garniidae). This report describes the fine structure of some developmental stages of Fallisia audaciosa that infect neutrophils in the peripheral blood of the Amazonian lizard Plica umbra (Reptilia: Iguanidae). The parasites lie within a parasitophorous vacuole and exhibit the basic structures of members of the Apicomplexa, such as the pellicle and the cytostome. Invaginations of the inner membrane complex were seen in the gametocytes and may be concerned with nutrition. The meronts were irregularly shaped before division, a feature unusual among members of the Apicomplexa. The unusual presence of a parasitic protozoan within neutrophils, in some way interfering with or modulating the microbicidal activity of such cells, is discussed.

Early transcriptional response of human neutrophils to Anaplasma phagocytophilum infection

Anaplasma phagocytophilum, an unusual obligate intracellular pathogen that persists within neutrophils, causes human anaplasmosis (previously known as human granulocytic ehrlichiosis). To study the effects of this pathogen on the transcriptional profile of its host cell, we performed a comprehensive DNA microarray analysis of the early (4-h) transcriptional response of human neutrophils to A. phagocytophilum infection. A. phagocytophilum infection resulted in the up- and down-regulation of 177 and 67 neutrophil genes, respectively. These data were verified by quantitative reverse transcription-PCR of selected genes. Notably, the up-regulation of many antiapoptotic genes, including the BCL2A1, BIRC3, and CFLAR genes, and the down-regulation of the proapoptotic TNFSF10 gene were observed. Genes involved in inflammation, innate immunity, cytoskeletal remodeling, and vesicular transport also exhibited differential expression. Vascular endothelial growth factor was also induced. These data suggest that A. phagocytophilum may alter selected host pathways in order to facilitate its survival within human neutrophils. To gain further insight into the bacterium's influence on host cell gene expression, this report presents a detailed comparative analysis of our data and other gene expression profiling studies of A. phagocytophilum-infected neutrophils and promyelocytic cell lines.

Leishmania pifanoi amastigotes avoid macrophage production of superoxide by inducing heme degradation

Whereas infections of macrophages by promastigote forms of Leishmania mexicana pifanoi induce the production of superoxide, infections by amastigotes barely induce superoxide production. Several approaches were employed to gain insight into the mechanism by which amastigotes avoid eliciting superoxide production. First, in experiments with nitroblue tetrazolium, we found that 25% of parasitophorous vacuoles (PVs) that harbor promastigotes are positive for the NADPH oxidase complex, in contrast to only 2% of PVs that harbor amastigotes. Second, confocal microscope analyses of infected cells labeled with antibodies to gp91phox revealed that this enzyme subunit is found in PVs that harbor amastigotes. Third, in immunoblots of subcellular fractions enriched with PVs from amastigote-infected cells and probed with antibodies to gp91phox, only the 65-kDa premature form of gp91phox was found. In contrast, subcellular fractions from macrophages that ingested zymosan particles contained both the 91- and 65-kDa forms of gp91phox. This suggested that only the immature form of gp91phox is recruited to PVs that harbor amastigotes. Given that gp91phox maturation is dependent on the availability of heme, we found that infections by Leishmania parasites induce an increase in heme oxygenase 1 (HO-1), the rate-limiting enzyme in heme degradation. Infections by amastigotes performed in the presence of metalloporphyrins, which are inhibitors of HO-1, resulted in superoxide production by infected macrophages. Taken together, we propose that Leishmania amastigotes avoid superoxide production by inducing an increase in heme degradation, which results in blockage of the maturation of gp91phox, which prevents assembly of the NADPH oxidase enzyme complex.

Ehrlichia subversion of host innate responses

Anaplasma (formerly Ehrlichia) phagocytophilum and Ehrlichia chaffeensis, upon infection of humans, replicate in host leukocyte granulocytes and monocytes/macrophages, respectively. These unusual Gram-negative bacteria lack genes for biosynthesis of the lipopolysaccharide and peptidoglycan that activate host leukocytes. Caveolae-mediated endocytosis directs A. phagocytophilum and E. chaffeensis to an intracellular compartment secluded from oxygen-dependent and -independent killing. Furthermore, these bacteria orchestrate a remarkable series of events that culminate in suppression of NADPH oxidase, phagocyte activation and differentiation pathways, apoptosis, and interferon-gamma signaling in host leukocytes. They offer a fascinating example of how pathogens employ intricate strategies to usurp and subvert host cell function.

Insights into pathogen immune evasion mechanisms: Anaplasma phagocytophilum fails to induce an apoptosis differentiation program in human neutrophils

Polymorphonuclear leukocytes (PMNs or neutrophils) are essential to human innate host defense. However, some bacterial pathogens circumvent destruction by PMNs and thereby cause disease. Anaplasma phagocytophilum, the agent of human granulocytic anaplasmosis, survives within PMNs in part by altering normal host cell processes, such as production of reactive oxygen species (ROS) and apoptosis. To investigate the molecular basis of A. phagocytophilum survival within neutrophils, we used Affymetrix microarrays to measure global changes in human PMN gene expression following infection with A. phagocytophilum. Notably, A. phagocytophilum uptake induced fewer perturbations in host cell gene regulation compared with phagocytosis of Staphylococcus aureus. Although ingestion of A. phagocytophilum did not elicit significant PMN ROS, proinflammatory genes were gradually up-regulated, indicating delayed PMN activation rather than loss of proinflammatory capacity normally observed during phagocytosis-induced apoptosis. Importantly, ingestion of A. phagocytophilum failed to trigger the neutrophil apoptosis differentiation program that typically follows phagocytosis and ROS production. Heat-killed A. phagocytophilum caused some similar initial alterations in neutrophil gene expression and function, which included delaying normal PMN apoptosis and blocking Fas-induced programmed cell death. However, at 24 h, down-regulation of PMN gene transcription may be more reliant on active infection. Taken together, these findings suggest two separate antiapoptotic processes may work concomitantly to promote bacterial survival: 1) uptake of A. phagocytophilum fails to trigger the apoptosis differentiation program usually induced by bacteria, and 2) a protein or molecule on the pathogen surface can mediate an early delay in spontaneous neutrophil apoptosis.

Interferon-gamma deficiency reveals that 129Sv mice are inherently more susceptible to Anaplasma phagocytophilum than C57BL/6 mice

Immunocompetent mice 129Sv (129) and C57BL/6 (B6) mice are similarly susceptible to Anaplasma phagocytophilum. We now show that 129 mice lacking interferon-gamma (IFN-gamma) develop more severe infection with A. phagocytophilum than IFN-gamma deficient B6 mice. These data demonstrate that there is an inherent increased susceptibility of 129 mice, compared with B6 mice, to A. phagocytophilum that can only be discerned in the absence of IFN-gamma.

Anaplasma phagocytophilum modulates gp91phox gene expression through altered interferon regulatory factor 1 and PU.1 levels and binding of CCAAT displacement protein

Infection of neutrophil precursors with Anaplasma phagocytophilum, the causative agent of human granulocytic ehrlichiosis, results in downregulation of the gp91(phox) gene, a key component of NADPH oxidase. We now show that repression of gp91(phox) gene transcription is associated with reduced expression of interferon regulatory factor 1 (IRF-1) and PU.1 in nuclear extracts of A. phagocytophilum-infected cells. Loss of PU.1 and IRF-1 correlated with increased binding of the repressor, CCAAT displacement protein (CDP), to the promoter of the gp91(phox) gene. Reduced protein expression of IRF-1 was observed with or without gamma interferon (IFN-gamma) stimulation, and the defect in IFN-gamma signaling was associated with diminished binding of phosphorylated Stat1 to the Stat1 binding element of the IRF-1 promoter. The diminished levels of activator proteins and enhanced binding of CDP account for the transcriptional inhibition of the gp91(phox) gene during A. phagocytophilum infection, providing evidence of the first molecular mechanism that a pathogen uses to alter the regulation of genes that contribute to an effective respiratory burst.

Neutrophil NADPH oxidase is reduced at the Anaplasma phagocytophilum phagosome

The intracellular organism Anaplasma phagocytophilum causes human granulocytic ehrlichiosis and specifically infects and multiplies in neutrophilic granulocytes. Previous reports have suggested that, for its survival, this bacterium suppresses the neutrophil respiratory burst. To investigate the mechanism of survival, we first assessed the kinetics of A. phagocytophilum entry into neutrophils by using double-labeling confocal microscopy. At 30, 60, 120, and 240 min of incubation, 25, 50, 55, and 70% of neutrophils contained bacteria, respectively. The neutrophil respiratory burst in the presence of A. phagocytophilum was assessed by a kinetic cytochrome c assay and by measurement of oxygen consumption. Neutrophils in the presence of A. phagocytophilum did not produce a significant respiratory burst, but A. phagocytophilum did not inhibit the neutrophil respiratory burst when phorbol myristate acetate was added. Immunoelectron microscopy of neutrophils infected with A. phagocytophilum or Escherichia coli revealed that NADPH oxidase subunits gp91(phox) and p22(phox) were significantly reduced at the A. phagocytophilum phagosome after 1 and 4 h of incubation. In neutrophils incubated simultaneously with A. phagocytophilum and E. coli for 30, 60, and 90 min, gp91(phox) was present on 20, 14, and 10% of the A. phagocytophilum phagosomes, whereas p22(phox) was present in 11, 5, and 4% of the phagosomes, respectively. Similarly, on E. coli phagosomes, gp91(phox) was present in 62, 64, and 65%, whereas p22(phox) was detected in 54, 48, and 48%. We conclude that A. phagocytophilum does not suppress a global respiratory burst and that, under identical conditions in the same cells, A. phagocytophilum, but not E. coli, significantly reduces gp91(phox) and p22(phox) from its phagosome membrane.

Anaplasma phagocytophilum utilizes multiple host evasion mechanisms to thwart NADPH oxidase-mediated killing during neutrophil infection

Anaplasma phagocytophilum, the etiologic agent of human anaplasmosis, is a bacterial pathogen that specifically colonizes neutrophils. Neutrophils utilize the NADPH oxidase complex to generate superoxide (O(2)(-)) and initiate oxidative killing of microorganisms. A. phagocytophilum's unique tropism for neutrophils, however, indicates that it subverts and/or avoids oxidative killing. We therefore examined the effects of A. phagocytophilum infection on neutrophil NADPH oxidase assembly and reactive oxygen species (ROS) production. Following neutrophil binding, Anaplasma invasion requires at least 240 min. During its prolonged association with the neutrophil plasma membrane, A. phagocytophilum stimulates NADPH oxidase assembly, as indicated by increased cytochrome b(558) mobilization to the membrane, as well as colocalization of Rac and p22(phox). This initial stimulation taxes the host neutrophil's finite oxidase reserves, as demonstrated by time- and bacterial-dose-dependent decreases in secondary activation by N-formyl-methionyl-leucyl-phenylalanine (FMLP) or phorbol myristate acetate (PMA). This stimulation is modest, however, and does not diminish oxidase stores to nearly the extent that Escherichia coli, serum-opsonized zymosan, FMLP, or PMA do. Despite the apparent activation of NADPH oxidase, no change in ROS-dependent chemiluminescence is observed upon the addition of A. phagocytophilum to neutrophils, indicating that the bacterium may scavenge exogenous O(2)(-). Indeed, A. phagocytophilum rapidly detoxifies O(2)(-) in a cell-free system. Once internalized, the bacterium resides within a protective vacuole that excludes p22(phox) and gp91(phox). Thus, A. phagocytophilum employs at least two strategies to protect itself from neutrophil NADPH oxidase-mediated killing.

Obligatory intracellular parasitism by Ehrlichia chaffeensis and Anaplasma phagocytophilum involves caveolae and glycosylphosphatidylinositol-anchored proteins

Obligatory intracellular, human ehrlichiosis agents Ehrlichia chaffeensis and Anaplasma phagocytophilum create unique replicative compartments devoid of lysosomal markers in monocytes/macrophages and granulocytes respectively. The entry of these bacteria requires host phospholipase C (PLC)-gamma2 and protein tyrosine kinases, but their entry route is still unclear. Here, using specific inhibitors, double immunofluorescence labelling and the fractionation of lipid rafts, we demonstrate that bacterial entry and intracellular infection involve cholesterol-rich lipid rafts or caveolae and glycosylphosphatidylinositol (GPI)-anchored proteins. By fluorescence microscopy, caveolar marker protein caveolin-1 was co-localized with both early and replicative bacterial inclusions. Additionally, tyrosine-phosphorylated proteins and PLC-gamma2 were found in bacterial early inclusions. In contrast, clathrin was not found in any inclusions from either bacterium. An early endosomal marker, transferrin receptor, was not present in the early inclusions of E. chaffeensis, but was found in replicative inclusions of E. chaffeensis. Furthermore, several bacterial proteins from E. chaffeensis and A. phagocytophilum were co-fractionated with Triton X-100-insoluble raft fractions. The formation of bacteria-encapsulating caveolae, which assemble and retain signalling molecules essential for bacterial entry and interact with the recycling endosome pathway, may ensure the survival of these obligatory intracellular bacteria in primary host defensive cells.

Bacterial responses to neutrophil phagocytosis

PURPOSE OF REVIEW

This review focuses on adaptive bacterial interactions with neutrophils, emphasizing information communicated within the past year about bacterial factors that respond to contact with or phagocytosis by PMN.

RECENT FINDINGS

Since the discovery of type III and IV secretion, progress in the analysis of bacterial interactions with host phagocytes has been extensive but largely focused on the macrophage. The remarkable growth of information about bacterial subversion of macrophage metabolism has been summarized in several excellent reviews. The scope of progress on neutrophil-bacteria interactions is more limited and dominated by recent studies of the granulocyte pathogen, Anaplasma phagocytophilum, the agent of granulocytic ehrlichiosis.

SUMMARY

For many pathogens, contact with or ingestion by phagocytes elicits a vigorous but varied microbial response. The response repertoire includes activation of type III and type IV secretion systems that inject effector molecules into the host cell. Effectors modify host cell signaling and metabolic pathways to favor survival of the microbe. Whereas microbial secretory structures are few in kind and relatively conserved, effector molecules are numerous and variable. Effectors may promote phagocytosis by nonphagocytic cells or suppress phagocytosis by macrophages and neutrophils. They may suppress assembly or misdirect localization of the phagocyte NADPH oxidase that is responsible for generating toxic oxidants, and they may suppress phagosome-lysosome fusion. Phagocytosed bacteria may also up-regulate the expression of defensive proteins that attenuate the effects of phagocyte-derived antimicrobial toxins. These pathogenic stratagems probably have their origins in the competition among single-celled organisms, eukaryotes versus prokaryotes, that arose early in evolution.

Murine neutrophils require alpha1,3-fucosylation but not PSGL-1 for productive infection with Anaplasma phagocytophilum

Anaplasma phagocytophilum causes human granulocytic ehrlichiosis, the second most common tick-borne disease in the United States. Mice are natural reservoirs for this bacterium and man is an inadvertent host. A phagocytophilum's tropism for human neutrophils is linked to neutrophil expression of P-selectin glycoprotein ligand-1 (PSGL-1), as well as sialylated and alpha1,3-fucosylated glycans. To determine whether A phagocytophilum uses similar molecular features to infect murine neutrophils, we assessed in vitro bacterial binding to neutrophils from and infection burden in wild-type mice; mice lacking alpha 1,3-fucosyltransferases Fuc-TIV and Fuc-TVII; or mice lacking PSGL-1. Binding to Fuc-TIV-/-/Fuc-TVII-/- neutrophils and infection of Fuc-TIV-/-/Fuc-TVII-/- mice were significantly reduced relative to wild-type mice. A phagocytophilum binding to PSGL-1-/- neutrophils was modestly reduced, whereas sialidase treatment significantly decreased binding to both wild-type and PSGL-1-/- neutrophils. A phagocytophilum similarly infected PSGL-1-/- and wild-type mice in vivo. A phagocytophilum induced comparable levels of chemokines from wild-type and PSGL-1-/- neutrophils in vitro, while those induced from Fuc-TIV-/-/Fuc-TVII-/- neutrophils were appreciably reduced. Therefore, A phagocytophilum infection in mice, as in humans, requires sialylation and alpha1,3-fucosylation of neutrophils. However, murine infection does not require neutrophil PSGL-1 expression, which has important implications for understanding how A phagocytophilum binds and infects neutrophils.

Invasion and survival strategies ofAnaplasma phagocytophilum

Anaplasma phagocytophilum is an aetiological agent of human granulocytic ehrlichiosis, an emerging tick-borne zoonosis in the United States and Europe. This obligate intracellular bacterium is unique in that it colonizes polymorphonuclear leucocytes (neutrophils). Neutrophils are key players in innate immunity. These short-lived phagocytes ingest invading microorganisms and destroy them by various means, which include fusing the bacteria-containing phagosome with acidic lysosomes as well as directing toxic oxidative and proteolytic compounds into the phagosomal lumen. Its tropism for neutrophils indicates that A. phagocytophilum uses strategies for evading and/or neutralizing these microbicidal activities. This review focuses on some of the mechanisms that A. phagocytophilum uses for neutrophil adhesion, surviving within the hostile intracellular environment of its host neutrophil and for effectively disseminating to naïve host cells.

Mechanisms of pathogenesis: evasion of killing by polymorphonuclear leukocytes

Few microorganisms evade killing by neutrophils. Summarized here are the mechanisms used by Yersinia, group A streptococci, Helicobacter, Ehrlichia and Francisella to block phagocytosis, disrupt phagosome maturation or perturb the respiratory burst. Also discussed are mechanisms used by neutrophils to control organisms that replicate inside macrophages.

Mechanisms to create a safe haven by members of the family Anaplasmataceae

Members of the family Anaplasmataceae are obligatory intracellular bacteria with unique host cell specificities. Depending on each bacterial species, granulocytes, platelets, endothelial cells, monocytes, macrophages, red blood cells, and cells of invertebrates are specifically infected. This unique host cell specificity has been the major hurdle to overcome in order to cultivate this group of bacteria. Because these bacteria cannot survive outside host cells, once released from a host cell, they need to rapidly induce signals for their own internalization into another host cell unique to each species. How these bacteria enter and continue to survive and replicate within the host milieu, then exit the host cell is largely unknown. Recently, however, unique strategies employed by some of these bacteria for successful parasitism of mammalian leukocytes have begun to be uncovered. When these bacteria interact with host cells, signals are transduced both inside the host cells and inside the bacteria. These signals disable the alarm system, as well as microbicidal mechanisms, of the leukocytes and condition the host cells to accept these intruders to share space and nutrient resources. Signals transduced inside the bacteria allow them to finely tune their metabolism and physiology in the new host cell environment and to disguise themselves as "insiders" so that their sojourn does not upset the host cell physiology until they have sufficiently multiplied. This paper discusses our recent findings on these topics.

Early induction and late abrogation of respiratory burst in A. phagocytophilum-infected neutrophils

Human granulocytic ehrlichiosis is an emerging tick-borne disease caused by the obligate intracellular pathogen, Anaplasma phagocytophilum. This organism is unique because it survives and propagates in neutrophil vacuoles. During phagocytosis professional phagocytes increase oxygen consumption (respiratory burst) through the activity of NADPH-oxidase, which generates superoxide anion (O2(-)) and hydrogen peroxide (H(2)O(2)). We assayed the ability of A. phagocytophilum-infected neutrophils to generate O2(-) in response to early infection (0, 3, and 5 hours) by using 2',7'-dichlorodihydrofluorescein diacetate (DCFH-DA), a fluorogenic probe commonly used to detect cellular production of reactive oxygen species (ROS). Our flow cytometric analyses showed that cell-free A. phagocytophilum induced respiratory burst 6.9-fold greater than that of LPS, an effect still evident to a lesser degree after 3 h, but absent by 5h. A. phagocytophilum initially induces and then represses respiratory burst in neutrophils. The long-term reduction in respiratory burst activity may be important for survival of A. phagocytophilum and other infectious agents in neutrophils. The effect of this may be to generate a limited functional equivalent of chronic granulomatous disease that predisposes to infections by opportunistic pathogens.

Neutrophil granulocytes--Trojan horses for Leishmania major and other intracellular microbes?

Polymorphonuclear neutrophil granulocytes (PMNs) possess numerous effector mechanisms to kill ingested pathogens as the first line of defence. However, several microorganisms evade intracellular killing in neutrophils, survive and retain infectivity. There is increasing evidence that several pathogens even multiply within neutrophils. Taking Leishmania major as a prototypic intracellular pathogen, we suggest an evasion strategy that includes the manipulation of PMNs in such a way that the pathogens are able to use the granulocytes as host cells. The ability to survive and maintain infectivity in PMNs subsequently enables these organisms to establish productive infection. These organisms can use granulocytes as Trojan horses before they enter their definitive host cells, the macrophages.