Effects of Anaplasma phagocytophila on NADPH oxidase components in human neutrophils and HL-60 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.

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.

Diminished adhesion of Anaplasma phagocytophilum-infected neutrophils to endothelial cells is associated with reduced expression of leukocyte surface selectin

Anaplasma phagocytophilum propagates within neutrophils and causes a disease marked by inflammatory tissue injury or complicated by opportunistic infections. We hypothesized that infection with A. phagocytophilum modifies the binding of neutrophils to endothelial cells and the expression of neutrophil adhesion molecules and studied these changes in vitro. Infected dimethyl sulfoxide-differentiated HL-60 cells and neutrophils showed reduced binding to cultured brain and systemic endothelial cells and lost expression of P-selectin glycoprotein ligand 1 (PSGL-1, CD162) and L-selectin (CD62L) (to 33 and 5% of control values, respectively), at a time when the levels of beta(2) integrin and immunoglobulin superfamily adhesion molecules and activation markers Mac-1 and intercellular adhesion molecule 1 increased (5 to 10 times that of the control). The loss of CD162 and CD62L expression was inhibited by EDTA, which suggests that neutrophil activation and sheddase cleavage occurred. The loss of selectin expression and the retained viability of the neutrophils persisted for at least 18 h with A. phagocytophilum infection, whereas Escherichia coli and Staphylococcus aureus rapidly killed neutrophils. The adhesion defect might increase the numbers of infected cells and their persistence in the blood prior to tick bites. However, decreased CD162 expression and poor endothelial cell binding may partly explain impaired host defenses, while simultaneous neutrophil activation may aggravate inflammation. These observations may help us to understand the modified biological responses, host inflammation, and immune response that occur with A. phagocytophilum infections.

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.

Repression of rac2 mRNA expression by Anaplasma phagocytophila is essential to the inhibition of superoxide production and bacterial proliferation

Anaplasma phagocytophila, the etiologic agent of human granulocytic ehrlichiosis, is an emerging bacterial pathogen that invades neutrophils and can be cultivated in HL-60 cells. Infected neutrophils and HL-60 cells fail to produce superoxide anion (O(2)(-)), which is partially attributable to the fact that A. phagocytophila inhibits transcription of gp91(phox), an integral component of NADPH oxidase. cDNA microarray and RT-PCR analyses demonstrated that transcription of the gene encoding Rac2, a key component in NADPH oxidase activation, was down-regulated in infected HL-60 cells. Quantitative RT-PCR demonstrated that rac2 mRNA expression was reduced 7-fold in retinoic acid-differentiated HL-60 cells and 50-fold in neutrophils following A. phagocytophila infection. Rac2 protein expression was absent in infected HL-60 cells. Rac1 and Rac2 are interchangeable in their abilities to activate NADPH oxidase. HL-60 cells transfected to express myc-tagged rac1 and gp91(phox) from the CMV immediate early promoter maintained the ability to generate O(2)(-) 120 h postinfection. A. phagocytophila proliferation was severely inhibited in these cells. These results directly attribute the inhibition of rac2 and gp91(phox) transcription to the loss of NADPH oxidase activity in A. phagocytophila-infected cells and demonstrate its importance to bacterial intracellular survival.

Roles of p38 mitogen-activated protein kinase, NF-kappaB, and protein kinase C in proinflammatory cytokine mRNA expression by human peripheral blood leukocytes, monocytes, and neutrophils in response to Anaplasma phagocytophila

Anaplasma phagocytophila, an obligately intracellular bacterium of granulocytes, causes human granulocytic ehrlichiosis. Within 2 h after addition of A. phagocytophila, interleukin-1beta (IL-1beta), tumor necrosis factor alpha (TNF-alpha), and IL-6 mRNAs are induced in human peripheral blood leukocytes (PBLs) or monocytes in vitro. However, neutrophils generate only IL-1beta mRNA. In the present study, signaling pathways for induction of these three cytokines were examined. TNF-alpha and IL-6 mRNA expression by PBLs was inhibited with SB 203580 (a p38 mitogen-activated protein kinase [MAPK] inhibitor), MG-132 (a proteasome inhibitor), and SN-50 (an NF-kappaB inhibitor). Activation of p38 MAPK and NF-kappaB mRNAs in monocytes was detectable within 15 to 30 min after addition of A. phagocytophila. Expression of these two cytokine mRNAs in PBLs and monocytes was also dependent on protein kinase C (PKC), protein kinase A (PKA), and protein tyrosine kinase (PTK). IL-1beta mRNA expression by neutrophils was not dependent on p38 MAPK, and p38 MAPK was not activated in neutrophils incubated with A. phagocytophila. IL-1beta mRNA induction by PBLs, monocytes, and neutrophils was dependent on PKC and PKA. Neutrophil expression of IL-1beta mRNA was dependent on transglutaminase, phospholipase C, and PTK, all of which are also required for internalization of A. phagocytophila. However, monocyte expression of IL-1beta mRNA was less dependent on these enzymes. These results suggest that A. phagocytophila transduces different signals between its host neutrophils and monocytes for proinflammatory cytokine generation.