Early transcriptional response of human neutrophils to Anaplasma phagocytophilum infection

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.

Multiple Myeloma with Skin and Renal Involvement and Anaplasma Phagocytophilum Co-Infection in a Dog

An 11-year-old male Cocker Spaniel was presented with acute onset of apathy, polyuria and polydipsia. Blood examination showed hypoalbuminemia and hyperglobulinemia, while electrophoresis revealed monoclonal gammopathy. Subsequent tests showed glycosuria, proteinuria, a positive serology test for Anaplasma spp. and ultrasonographical changes of the liver and spleen. Urine electrophoresis confirmed the presence of Bence-Jones protein. The dog’s condition deteriorated, and the dog had developed skin lesions on the neck and body. The owner decided to euthanize the dog and agreed to bone marrow and skin biopsy. Bone marrow cytology revealed an increased number of plasma cells and several cellular atypia. Histopathology of the skin lesions showed a round cell tumor of lymphoid or plasmacytoid origin. Subsequent immunohistology supported the diagnosis of multiple myeloma cutaneous involvement.

This case report describes unusual features observed in a dog with multiple myeloma.

Current Understanding of the Neutrophil Transcriptome in Health and Disease

Neutrophils are key cells of the innate immune system. It is now understood that this leukocyte population is diverse in both the basal composition and functional plasticity. Underlying this plasticity is a post-translational framework for rapidly achieving early activation states, but also a transcriptional capacity that is becoming increasingly recognized by immunologists. Growing interest in the contribution of neutrophils to health and disease has resulted in more efforts to describe their transcriptional activity. Whilst initial efforts focused predominantly on understanding the existing biology, investigations with advanced methods such as single cell RNA sequencing to understand interactions of the entire immune system are revealing higher flexibility in neutrophil transcription than previously thought possible and multiple transition states. It is now apparent that neutrophils utilise many forms of RNA in the regulation of their function. This review collates current knowledge on the nuclei structure and gene expression activity of human neutrophils across homeostasis and disease, before highlighting knowledge gaps that are research priority areas.

Increasing Incidence of Anaplasmosis in the United States, 2012 Through 2016

Anaplasmosis is a tick-borne disease that is primarily caused by the rickettsial bacterium Anaplasma phagocytophilum. Anaplasmosis is a febrile disease with common symptoms, including headaches, fever, and lethargy, but it can cause serious organ failure and even death if left untreated. Human infections are reported annually to the Centers for Disease Control and Prevention (CDC) through the National Notifiable Diseases Surveillance System (NNDSS). This report analyzed the cases of anaplasmosis reported by the NNDSS from 2012 to 2016. In total, there were 15,778 reported A. phagocytophilum infections, and the incidence rate was 7.27 cases per million persons per year, with the number of reported cases increasing each year. The demographic group most affected was middle-aged and elderly white males. Infections were most abundant in the coastal northeast and northern midwest regions. Our study provides useful results for a basic understanding of incidence, distribution, and severity of A. phagocytophilum infections.

Intracellular proliferation of Anaplasma phagocytophilum is promoted via modulation of endoplasmic reticulum stress signaling in host cells

Anaplasma phagocytophilum, an obligate intracellular bacterium that propagates within host granulocytes, is considered to modify the host intracellular environment for pathogenesis. However, the mechanism(s) underlying such host modifications remain unclear. Here, we aimed to investigate the relation between A. phagocytophilum and endoplasmic reticulum (ER) stress in THP-1 cells. A. phagocytophilum activated the three ER stress sensors: inositol-requiring enzyme-1 (IRE1), protein kinase RNA-like endoplasmic reticulum kinase (PERK), and activating transcription factor-6 (ATF6). IRE1 activation occurred immediately after host cell invasion by A. phagocytophilum; however, the activated IRE1-induced splicing of X-box-binding protein 1 was not promoted during A. phagocytophilum infection. This suppression was sustained even after the doxycycline-mediated elimination of intracellular A. phagocytophilum. IRE1 knockdown accelerated A. phagocytophilum-induced apoptosis and decreased intracellular A. phagocytophilum. These data suggest that A. phagocytophilum utilizes IRE1 activation to promote its own intracellular proliferation. Moreover, PERK and ATF6 partially mediated A. phagocytophilum-induced apoptosis by promoting the expression of CCAAT/enhancer-binding protein homologous protein, which induces the transcription of several proapoptotic genes. Thus, A. phagocytophilum possibly manipulates the host ER stress signals to facilitate intracellular proliferation and infection of surrounding cells before/after host cell apoptosis.

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.

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.

Could ehrlichial infection cause some of the changes associated with leukemia, myelodysplastic diseases and autoimmune disorders, and offer antibiotic treatment options?

We hypothesize that a large group of medical conditions of unknown etiology including leukemia, multiple myeloma, myelodysplastic and autoimmune disorders, may be associated with or caused by an obscure group of intracellular obligate parasitic bacteria named Ehrlichia/Anaplasma (EA). Ensconced in the stem cells of the bone marrow, EA may disrupt the normal development and function of many of the cells of immunity, manifesting itself as different syndromes. Recent studies of the activity of EA suggest direct effects on the immune system consistent with the manifestations of leukemia. We reference here three leukemia patients with direct or indirect evidence of EA infection. Moreover, EA have been shown to be most sensitive to rifamycins. Two moribund leukemia patients with levels of platelets and white cells incompatible with life were treated with therapeutic doses of Rifampin. Though they did not survive, their condition improved dramatically for a time, suggesting Rifampin provided some therapeutic benefit. We assert that these results warrant more extensive study.

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.

Comparison and characterization of granulocyte cell models for Anaplasma phagocytophilum infection

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

Global H3K4me3 genome mapping reveals alterations of innate immunity signaling and overexpression of JMJD3 in human myelodysplastic syndrome CD34+ cells

The molecular bases of myelodysplastic syndromes (MDS) are not fully understood. Trimethylated histone 3 lysine 4 (H3K4me3) is present in promoters of actively transcribed genes and has been shown to be involved in hematopoietic differentiation. We performed a genome-wide H3K4me3 CHIP-Seq (chromatin immunoprecipitation coupled with whole genome sequencing) analysis of primary MDS bone marrow (BM) CD34+ cells. This resulted in the identification of 36 genes marked by distinct higher levels of promoter H3K4me3 in MDS. A majority of these genes are involved in nuclear factor (NF)-κB activation and innate immunity signaling. We then analyzed expression of histone demethylases and observed significant overexpression of the JmjC-domain histone demethylase JMJD3 (KDM6b) in MDS CD34+ cells. Furthermore, we demonstrate that JMJD3 has a positive effect on transcription of multiple CHIP-Seq identified genes involved in NF-κB activation. Inhibition of JMJD3 using shRNA in primary BM MDS CD34+ cells resulted in an increased number of erythroid colonies in samples isolated from patients with lower-risk MDS. Taken together, these data indicate the deregulation of H3K4me3 and associated abnormal activation of innate immunity signals have a role in the pathogenesis of MDS and that targeting these signals may have potential therapeutic value in MDS.

Differential neutrophil gene expression in early bovine pregnancy

BACKGROUND

In food production animals, especially cattle, the diagnosis of gestation is important because the timing of gestation directly affects the running of farms. Various methods have been used to detect gestation, but none of them are ideal because of problems with the timing of detection or the accuracy, simplicity, or cost of the method. A new method for detecting gestation, which involves assessing interferon-tau (IFNT)-stimulated gene expression in peripheral blood leukocytes (PBL), was recently proposed. PBL fractionation methods were used to examine whether the expression profiles of various PBL populations could be used as reliable diagnostic markers of bovine gestation.

METHODS

PBL were collected on days 0 (just before artificial insemination), 7, 14, 17, 21, and 28 of gestation. The gene expression levels of the PBL were assessed with microarray analysis and/or quantitative real-time reverse transcription (q) PCR. PBL fractions were collected by flow cytometry or density gradient cell separation using Histopaque 1083 or Ficoll-Conray solutions. The expression levels of four IFNT-stimulated genes, interferon-stimulated protein 15 kDa (ISG15), myxovirus-resistance (MX) 1 and 2, and 2'-5'-oligoadenylate synthetase (OAS1), were then analyzed in each fraction through day 28 of gestation using qPCR.

RESULTS

Microarray analysis detected 72 and 28 genes in whole PBL that were significantly higher on days 14 and 21 of gestation, respectively, than on day 0. The upregulated genes included IFNT-stimulated genes. The expression levels of these genes increased with the progression of gestation until day 21. In flow cytometry experiments, on day 14 the expression levels of all of the genes were significantly higher in the granulocyte fraction than in the other fractions. Their expression gradually decreased through day 28 of gestation. Strong correlations were observed between the expression levels of the four genes in the granulocyte fractions obtained with flow cytometry and with density gradient separation.

CONCLUSIONS

The expression profiles of ISG15, MX1, MX2, and OAS1 could be a useful diagnostic biomarker of bovine gestation. Assessing the expression levels of these genes in a granulocyte fraction obtained with density gradient separation is a practical way of detecting gestation in cows within three weeks of insemination.

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.

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

Background

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

Results

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

Conclusions

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

Receptor interacting protein-2 contributes to host defense against Anaplasma phagocytophilum infection

The Gram-negative obligate intracellular bacterium Anaplasma phagocytophilum is the causative agent of human granulocytic anaplasmosis (HGA), an emerging tick-borne infectious disease occurring worldwide. HGA is generally self-limiting; however, the underlying mechanisms, particularly the innate immune pathways that mediate the immune clearance of A. phagocytophilum, are less understood. We herein report an unexpected role for Receptor interacting protein-2 (Rip2), the adaptor protein for the Nod-like receptors (NLRs), Nod1/Nod2, in the host immune response against A. phagocytophilum infection. Although A. phagocytophilum genome is reported to lack the genes encoding the known ligands of Nod1 and Nod2, its infection upregulated the transcription of Rip2 in human primary neutrophils. Our results revealed that Rip2-deficient mice had significantly higher bacterial load than wild-type controls throughout the infection period. In addition, the Rip2-deficient mice took strikingly longer duration to clear A. phagocytophilum infection. Detailed analysis identified that interferon gamma (IFNγ) and interleukin (IL)-18 but not IL-12, macrophage inflammatory protein-2, and KC response were diminished in A. phagocytophilum-challenged Rip2-deficient mice. Together, these results revealed that Rip2 plays important roles in the immune control of A. phagocytophilum and may contribute to our understanding of the host response to Rickettsiales.

Infection with Anaplasma phagocytophilum activates the phosphatidylinositol 3-Kinase/Akt and NF-κB survival pathways in neutrophil granulocytes

Anaplasma phagocytophilum, a Gram-negative, obligate intracellular bacterium infects primarily neutrophil granulocytes. Infection with A. phagocytophilum leads to inhibition of neutrophil apoptosis and consequently contributes to the longevity of the host cells. Previous studies demonstrated that the infection inhibits the executionary apoptotic machinery in neutrophils. However, little attempt has been made to explore which survival signals are modulated by the pathogen. The aim of the present study was to clarify whether the phosphatidylinositol 3-kinase (PI3K)/Akt and NF-κB signaling pathways, which are considered as important survival pathways in neutrophils, are involved in A. phagocytophilum-induced apoptosis delay. Our data show that infection of neutrophils with A. phagocytophilum activates the PI3K/Akt pathway and suggest that this pathway, which in turn maintains the expression of the antiapoptotic protein Mcl-1, contributes to the infection-induced apoptosis delay. In addition, the PI3K/Akt pathway is involved in the activation of NF-κB in A. phagocytophilum-infected neutrophils. Activation of NF-κB leads to the release of interleukin-8 (IL-8) from infected neutrophils, which, in an autocrine manner, delays neutrophil apoptosis. In addition, enhanced expression of the antiapoptotic protein cIAP2 was observed in A. phagocytophilum-infected neutrophils. Taken together, the data indicate that upstream of the apoptotic cascade, signaling via the PI3K/Akt pathway plays a major role for apoptosis delay in A. phagocytophilum-infected neutrophils.

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.

Ehrlichia and bone marrow cells: could Ehrlichial infection explain the unsuspected etiology of some diseases of the immune system?

A large group of diseases of unknown etiology, including leukemia, systemic lupus erythematosus, myelodysplastic disease, multiple sclerosis, amyotrophic lateral sclerosis, and rheumatoid arthritis, all present with some elements of immune system disturbance. The Ehrlichia/anaplasma (EA) are an obscure group of obligate parasitic intracellular pathogens that excrete intracellularly a substance called host transcriptional protein, which can alter transcription in cell division. Infection with EA may lead to changes in transcription in proliferating cells, such as those in the marrow, and alter the biology of the products such as T and B cells. Normally 60% of B cells produced in the marrow may be self reactive, but are eliminated before release from the marrow. Changes in transcription could allow self reactive cells to escape into the peripheral circulation and injure normal tissue, creating the dysfunctions which characterize the different immune system diseases and give them their separate identities. A number of studies previously published, and new information presented here, suggest that EA infections may be an underlying, undiagnosed cause for these and other immune system diseases. This hypothesis, long overlooked, has never been subjected to adequate, rigorous study sufficient to prove or disprove its truth. If so, patients may be treated with antibiotics, and marrow transplant manipulations already used in treatment of diseases such as lupus and leukemia may become more effective.

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

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

Mature human neutrophils constitutively express the transcription factor EGR-1

The immediate early response gene, Early Growth Response 1 (EGR-1) has emerged as a central regulator of early inflammatory and immune processes by rapidly regulating the transcription of a wide array of downstream effector genes. Neutrophils, which are among the first circulating leukocytes to respond to inflammatory signals, exhibit a broad set of transcriptional changes immediately upon exposure to inflammatory and pathogenic stimuli. Such transcriptional changes are likely to be controlled by early gene transcription factors such as EGR-1. We therefore examined the regulation and role of EGR-1 in mature human neutrophils exposed to the inflammatory stimuli fMLP and IL-8. We report that human neutrophils rapidly and transiently up-regulate EGR-1 mRNA upon stimulation with fMLP or IL-8. However in contrast to that seen in other cells, EGR-1 mRNA expression profiles were not predictive of protein expression. Instead, we show that human neutrophils constitutively express EGR-1 protein. The cellular content of EGR-1 did not change over time or upon neutrophil activation. Confocal microscopy revealed that EGR-1 was present in both the cytoplasm and nuclei of un-stimulated neutrophils and that activation did not change this subcellular localization or promote nuclear translocation. Using chromatin immunoprecipitation, we demonstrate that EGR-1 is associated with the promoter regions of the immune regulatory genes IL-1 beta, TGFbeta-1 and MIF in both resting and activated neutrophils with increased promoter association observed upon cell activation. This novel pattern of EGR-1 protein expression may underlie the ability of the neutrophil to respond rapidly to inflammatory stimuli.

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.

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.

The elimination of Anaplasma phagocytophilum requires CD4+ T cells, but is independent of Th1 cytokines and a wide spectrum of effector mechanisms

Anaplasma phagocytophilum is a Gram-negative, obligate intracellular bacterium that exhibits a striking tropism for neutrophils. When we depleted mice of neutrophils, we found that murine susceptibility to anaplasmal infection was dependent on their presence. While serving as sites of bacterial replication, neutrophils do not seem to act as efficient killer cells in A. phagocytophilum infection, because mice deficient for antimicrobial effectors of neutrophils such as myeloperoxidase, granulocyte elastase, and cathepsin G were fully competent in pathogen elimination. To identify components of the immune system other than neutrophils that control A. phagocytophilum, we studied the course of infection in several gene-deficient mouse strains. IFN-gamma production by NK cells was important for initial defense, but not critical for pathogen elimination. In contrast, bacterial clearance was strictly dependent on CD4(+) T cells, but unexpectedly achieved in the absence of perforin, Fas/FasL and major Th1 cytokines such as IL-12, IFN-gamma, and MCP-1. These findings provide a novel paradigm for the control of an intracellular pathogen, which appears to be strikingly different from the CD4(+) T cell-, IL-12-, and IFN-gamma-dependent immunity to other intracellular bacteria.

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

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

Whole genome transcription profiling of Anaplasma phagocytophilum in human and tick host cells by tiling array analysis

Background

Anaplasma phagocytophilum (Ap) is an obligate intracellular bacterium and the agent of human granulocytic anaplasmosis, an emerging tick-borne disease. Ap alternately infects ticks and mammals and a variety of cell types within each. Understanding the biology behind such versatile cellular parasitism may be derived through the use of tiling microarrays to establish high resolution, genome-wide transcription profiles of the organism as it infects cell lines representative of its life cycle (tick; ISE6) and pathogenesis (human; HL-60 and HMEC-1).

Results

Detailed, host cell specific transcriptional behavior was revealed. There was extensive differential Ap gene transcription between the tick (ISE6) and the human (HL-60 and HMEC-1) cell lines, with far fewer differentially transcribed genes between the human cell lines, and all disproportionately represented by membrane or surface proteins. There were Ap genes exclusively transcribed in each cell line, apparent human- and tick-specific operons and paralogs, and anti-sense transcripts that suggest novel expression regulation processes. Seven virB2 paralogs (of the bacterial type IV secretion system) showed human or tick cell dependent transcription. Previously unrecognized genes and coding sequences were identified, as were the expressed p44/msp2 (major surface proteins) paralogs (of 114 total), through elevated signal produced to the unique hypervariable region of each – 2/114 in HL-60, 3/114 in HMEC-1, and none in ISE6.

Conclusion

Using these methods, whole genome transcription profiles can likely be generated for Ap, as well as other obligate intracellular organisms, in any host cells and for all stages of the cell infection process. Visual representation of comprehensive transcription data alongside an annotated map of the genome renders complex transcription into discernable patterns.

Anaplasma phagocytophilum-induced gene expression in both human neutrophils and HL-60 cells

Anaplasma phagocytophilum (Ap), the etiologic agent of the tick-borne disease human granulocytic anaplasmosis, is an obligate intracellular pathogen unique in its ability to target and replicate within neutrophils. We define and compare the spectra of host gene expression in response to Ap infection of human neutrophils and of HL-60 cells using long (70-mer)-oligonucleotide array technology. In addition to apoptosis-related genes, genes involved in signaling pathways, transcriptional regulation, immune response, host defense, cell adhesion, and cytoskeleton were modulated in neutrophils infected with Ap. Ap infection affected the same pathways in HL-60 cells but transcriptional changes occurred more slowly and in a reduced spectrum of genes. Gene expression changes detected by microarray were confirmed for randomly selected genes by QRT-PCR and Western blot studies. These studies demonstrate for the first time that the ERK pathway is activated in Ap-infected neutrophils and also define multiple pathways that are activated during intracellular Ap infection, which together serve to prolong the cell survival that is needed to allow bacterial replication and survival in neutrophils, which otherwise would rapidly apoptose.

Differential expression of inflammatory and immune response genes in sheep infected with Anaplasma phagocytophilum

Anaplasma phagocytophilum infects a wide variety of host species and causes the diseases tick-borne fever (TBF) in ruminants and granulocytic anaplasmosis in humans, horses and dogs. TBF in sheep has become one of the more prevalent tick-borne diseases in some regions of Europe. A. phagocytophilum infection modifies host gene expression and immune response. The objective of this research was to characterize differential gene expression in sheep experimentally and naturally infected with A. phagocytophilum by microarray hybridization and real-time RT-PCR. The results of these studies demonstrated in sheep the activation of inflammatory and innate immune pathways and the impairment of adaptive immunity during A. phagocytophilum infection. The characterization of the genes and their expression profiles in sheep in response to A. phagocytophilum infection advances our understanding of the molecular mechanisms of pathogen infection and the pathogenesis of TBF. Collectively, these results expand current information on the mammalian host response to A. phagocytophilum infection.

Expression and regulation of CCL18 in synovial fluid neutrophils of patients with rheumatoid arthritis

Rheumatoid arthritis (RA) is characterized by the recruitment of leukocytes and the accumulation of inflammatory mediators within the synovial compartment. Release of the chemokine CCL18 has been widely attributed to antigen-presenting cells, including macrophages and dendritic cells. This study investigates the production of CCL18 in polymorphonuclear neutrophils (PMN), the predominant cell type recruited into synovial fluid (SF). Microarray analysis, semiquantitative and quantitative reverse transcriptase polymerase chain reaction identified SF PMN from patients with RA as a novel source for CCL18 in diseased joints. Highly upregulated expression of other chemokine genes was observed for CCL3, CXCL8 and CXCL10, whereas CCL21 was downregulated. The chemokine receptor genes were differentially expressed, with upregulation of CXCR4, CCRL2 and CCR5 and downregulation of CXCR1 and CXCR2. In cell culture experiments, expression of CCL18 mRNA in blood PMN was induced by tumor necrosis factor alpha, whereas synthesis of CCL18 protein required additional stimulation with a combination of IL-10 and vitamin D3. In comparison, recruited SF PMN from patients with RA were sensitized for CCL18 production, because IL-10 alone was sufficient to induce CCL18 release. These results suggest a release of the T cell-attracting CCL18 by PMN when recruited to diseased joints. However, its production is tightly regulated at the levels of mRNA expression and protein synthesis.

Expression of perilipin in human promyelocytic cells in response to Anaplasma phagocytophilum infection results in modified lipid metabolism

The obligate intracellular pathogen Anaplasma phagocytophilum is transmitted by ticks and causes human granulocytic anaplasmosis, tick-borne fever of ruminants, and equine and canine granulocytic anaplasmosis. In a previous study, the perilipin (PLIN) gene was identified as one of the genes differentially expressed in human promyelocytic HL-60 cells in response to infection with A. phagocytophilum. PLIN is a major adipocyte lipid droplet-associated phosphoprotein that plays a central role in lipolysis and cholesterol synthesis. Host cholesterol and other lipids are required by A. phagocytophilum for infection and multiplication in human cells. In this study, it was hypothesized that PLIN may be involved in infection of human HL-60 cells by A. phagocytophilum. To test this hypothesis, a combination of real-time RT-PCR, immunofluorescence and RNA interference was used to study the expression of PLIN. The results of these studies demonstrated that A. phagocytophilum modulates lipid metabolism by increasing PLIN mRNA levels and facilitates infection of HL-60 cells. The results of these studies expand our knowledge of the role of lipid metabolism in A. phagocytophilum infection and multiplication in HL-60 cells and suggest a mechanism by which A. phagocytophilum modulates lipid metabolism.

ASC/PYCARD and caspase-1 regulate the IL-18/IFN-gamma axis during Anaplasma phagocytophilum infection

Anaplasma phagocytophilum is an obligate intracellular pathogen that resides within neutrophils and can cause fever, pancytopenia, or death. IFN-gamma plays a critical role in the control of A. phagocytophilum; however, the mechanisms that regulate IFN-gamma production remain unclear. In this study, we demonstrate that apoptotic specklike protein with a caspase-activating recruiting domain (ASC)/PYCARD, a central adaptor molecule in the Nod-like receptor (NLR) pathway, regulates the IL-18/IFN-gamma axis during A. phagocytophilum infection through its effect on caspase-1. Caspase-1- and asc-null mice were more susceptible than control animals to A. phagocytophilum infection due to the absence of IL-18 secretion and reduced IFN-gamma levels in the peripheral blood. Moreover, caspase-1 and ASC deficiency reduced CD4+ T cell-mediated IFN-gamma after in vitro restimulation with A. phagocytophilum. The NLR family member IPAF/NLRC4, but not NALP3/NLRP3, was partially required for IFN-gamma production in response to A. phagocytophilum. Taken together, our data demonstrate that ASC and caspase-1 are critical for IFN-gamma-mediated control of A. phagocytophilum infection.

Using genomic approaches to unravel livestock (host)-tick-pathogen interactions

Ticks and tick-borne diseases are a major constraint on livestock farming in many developing countries, which has a huge impact on their economies. Genomic information is becoming more abundant for many of the species involved, which if exploited successfully could be used to develop new control strategies. Here, we review the genomic resources that are now available and discuss how this information is currently being harnessed or can be used in the future to explore the complex interplay that occurs between livestock hosts, tick vectors and tick-borne pathogens.

Coxiella burnetii inhibits apoptosis in human THP-1 cells and monkey primary alveolar macrophages

Coxiella burnetii, the cause of human Q fever, is an aerosol-borne, obligate intracellular bacterium that targets host alveolar mononuclear phagocytic cells during infection. In all cell types examined, C. burnetii establishes a replicative niche in a lysosome-like parasitophorous vacuole where it carries out a lengthy infectious cycle with minimal cytopathic effects. The persistent and mild nature of C. burnetii infection in vitro suggests that the pathogen modulates apoptosis to sustain the host cell. In the current study, we examined the ability of C. burnetii to inhibit apoptotic cell death during infection of human THP-1 monocyte-derived macrophages and primary monkey alveolar macrophages. C. burnetii-infected cells demonstrated significant protection from death relative to uninfected cells following treatment with staurosporine, a potent inducer of intrinsic apoptosis. This protection correlated with reduced cleavage of caspase-9, caspase-3, and poly(ADP-ribose) polymerase (PARP), all proteolytic events that occur during apoptosis. Reduced PARP cleavage was also observed in cells treated with tumor necrosis factor alpha to induce extrinsic apoptosis. Apoptosis inhibition was a C. burnetii-driven process as infected cells treated with rifampin or chloramphenicol, inhibitors of bacterial RNA and protein synthesis, respectively, showed significantly reduced protection against staurosporine-induced apoptosis. C. burnetii infection affected the expression of multiple apoptosis-related genes and resulted in increased synthesis of the antiapoptotic proteins A1/Bfl-1 and c-IAP2. Collectively, these data suggest that C. burnetii modulates apoptotic pathways to inhibit host cell death, thus providing a stable, intracellular niche for the course of the pathogen's infectious cycle.

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.

Anaplasma phagocytophilum delays spontaneous human neutrophil apoptosis by modulation of multiple apoptotic pathways

Anaplasma phagocytophilum infects human neutrophils and inhibits the intrinsic pathway of spontaneous neutrophil apoptosis by protecting mitochondrial membrane integrity. In the present study, we investigated the molecular signalling of the extrinsic pathway and the interaction between the intrinsic and extrinsic pathways in the inhibition of spontaneous human neutrophil apoptosis by A. phagocytophilum. Cell surface Fas clustering during spontaneous neutrophil apoptosis was significantly blocked by A. phagocytophilum infection. The cleavage of pro-caspase 8, caspase 8 activation and the cleavage of Bid, which links the intrinsic and extrinsic pathways, in the extrinsic pathway of spontaneous neutrophil apoptosis were inhibited by A. phagocytophilum infection. Inhibition of this pathway was active as the cleavage of pro-caspase 8 and Bid in anti-Fas-induced neutrophil apoptosis was also inhibited by A. phagocytophilum infection. Likewise, A. phagocytophilum infection inhibited the pro-apoptotic Bax translocation to mitochondria, activation of caspase 9, the initiator caspase in the intrinsic pathway, and the degradation of a potent caspase inhibitor, X-chromosome-linked inhibitor of apoptosis protein (XIAP), during spontaneous neutrophil apoptosis. These data point to a novel mechanism induced by A. phagocytophilum involving both extrinsic and intrinsic pathways to ensure to delay the apoptosis of host neutrophils.

Host gene expression profiling in pathogen–host interactions

Microarray technology is a powerful high-throughput tool for the analysis of host-pathogen interactions that permits simultaneous interrogation of the transcriptional status of thousands of genes. Emerging topics from microarray-based studies employing diverse pathogens and cell types suggest an initial common host response largely characterised by features of the innate immune response. However, specific host gene expression patterns that reflect differences between bacteria of related genera, different species of a particular genus, as well as strains within a single species can also be discerned. These differences are indicative of virulence determinant functions and suggest adaptive survival strategies. These studies have led to a more comprehensive understanding of the host response and identified new avenues of research for potential control strategies against pathogens.

Anaplasma phagocytophilum causes global induction of antiapoptosis in human neutrophils

Anaplasma phagocytophilum (Ap), the agent of the tick-borne disease human granulocytic anaplasmosis, is an obligate intracellular pathogen unique in its ability to target and replicate within neutrophils. It profoundly inhibits neutrophil apoptosis, prolonging neutrophil survival from hours to days. To determine the basis of antiapoptosis, we compared gene expression in Ap-infected vs mock-infected human neutrophils. Antiapoptosis genes were consistently and significantly up-regulated (2- to 15-fold) within 1-3 h. These genes synergistically inhibit apoptosis through several interconnected pathways, including p38MAPK (MAP2K3), ERK (IER3), PI3K (PRKCD), and NF-kappaB (BCL2A1, NFKB1, NFKBIA, GADD45B). Both extrinsic death receptor (TNFAIP3, CFLAR, SOD2) and intrinsic mitochondrial (BCL2A1, PIM2, BIRC3) pathways were affected as confirmed by reductions in both caspase 3 and caspase 8 activities. Several important antiapoptotic genes noted to be up-regulated in Ap-infected neutrophils were not up-regulated during Ap infection of HL-60 cells (which is not antiapoptotic). In conclusion, just as apoptosis may be triggered through multiple molecular pathways, effective antiapoptosis of neutrophils is achieved rapidly and redundantly by this intracellular pathogen dependent on cell survival.