The multifaceted functions of neutrophils

Adrenergic modulation of migration, CD11b and CD18 expression, ROS and interleukin-8 production by human polymorphonuclear leukocytes

OBJECTIVES

Adrenergic modulation of immunity has been extensively characterized, however, few information exist regarding polymorphonuclear leukocytes (PMN), despite their key role in immunity and inflammation. We investigated the effect of adrenergic agents on human PMN migration, CD11b and CD18 expression, reactive oxygen species (ROS) and interleukin (IL)-8 production, and on adrenoceptor (AR) expression.

METHODS

Migration was measured by the Boyden chamber assay, CD11b/CD18 expression was assessed by flow cytometry, intracellular ROS were detected by spectrofluorimetry, and IL-8 was quantitated by standard ELISA assay. AR mRNA levels were measured by real-time PCR and PMN morphology was studied by scanning electron microscopy.

RESULTS

Adrenaline(A), noradrenaline and the β-AR agonist isoprenaline reduced N-formyl-Met-Leu-Phe (fMLP)-induced migration, CD11b/CD18 expression, and ROS production, without affecting IL-8. The effect of A on CD11b was antagonized by yohimbine and propranolol, and increased by prazosin. The effect on ROS production was completely abolished by propranolol. PMN expressed α(1A)-, α(1B)-, α(1D)-, α(2A)-, α(2C)-, β(1)-, β(2)-, and β(3)-AR mRNA. A prevented fMLP-induced morphological changes of PMN.

CONCLUSIONS

Adrenergic agents reduced PMN responses mainly through β-AR, although α-AR may contribute at least to CD11b expression. AR-operated pathways in PMN should be investigated in disease conditions and in the response to therapeutic agents.

Molecular and genomic characterization of pathogenic traits of group A Streptococcus pyogenes

Group A streptococcus (GAS) or Streptococcus pyogenes causes various diseases ranging from self-limiting sore throat to deadly invasive diseases. The genome size of GAS is 1.85-1.9 Mb, and genomic rearrangement has been demonstrated. GAS possesses various surface-associated substances such as hyaluronic capsule, M proteins, and fibronectin/laminin/immunoglobulin-binding proteins. These are related to the virulence and play multifaceted and mutually reflected roles in the pathogenesis of GAS infections. Invasion of GAS into epithelial cells and deeper tissues provokes immune and non-immune defense or inflammatory responses including the recruitment of neutrophils, macrophages, and dendritic cells in hosts. GAS frequently evades host defense mechanisms by using its virulence factors. Extracellular products of GAS may perturb cellular and subcellular functions and degrade tissues enzymatically, which leads to the aggravation of local and/or systemic disorders in the host. In this review, we summarize some important cellular and extracellular substances that may affect pathogenic processes during GAS infections, and the host responses to these.

Detection of phosphatidylserine exposure on leukocytes following treatment with human galectins

Cellular turnover represents a fundamental aspect of immunological homeostasis. While many factors appear to regulate leukocyte removal during inflammatory resolution, recent studies suggest that members of the galectin family play a unique role in orchestrating this process. Unlike cellular removal through apoptotic cell death, several members of the galectin family induce surface expression of phosphatidylserine (PS), a phagocytic marker on cells undergoing apoptosis, in the absence of cell death. However, similar to PS on cells undergoing apoptosis, galectin-induced PS exposure sensitizes cells to phagocytic removal. As galectins appear to prepare cells for phagocytic removal without actually inducing apoptotic cell death, this process has recently been coined preaparesis. Given the unique characteristics of galectin-induced PS exposure in the context of preaparesis, we will examine important considerations when evaluating the potential impact of different galectin family members on PS exposure and cell viability.

Neutrophil homeostasis and inflammation: novel paradigms from studying periodontitis

Once viewed as simply antibacterial effector cells packed with antimicrobials, neutrophils are now increasingly appreciated for their regulatory roles in immunity and inflammation. The homeostatic regulation of neutrophils is thus crucial for optimal operation of the immune system. An attractive model to understand mechanistically the role of neutrophils is periodontitis, an oral inflammatory disease that is particularly sensitive to neutrophil alterations in numbers or function. The recruitment and proper activation of neutrophils are largely dependent on leukocyte integrins and complement. This review discusses how these processes are affected by host genetic or microbial factors leading to the development of periodontitis. For instance, both hypo- and hyper-recruitment of neutrophils as a result of deficiencies in the expression of β2 integrins or their negative regulators, respectively, causes unwarranted IL-17-dependent inflammatory bone loss. Moreover, microbial hijacking of C5aR (CD88) signaling in neutrophils impairs their antimicrobial function while promoting destructive inflammatory responses. These studies not only support the concept that neutrophil homeostasis is key to periodontal health but also reveal promising, new therapeutic targets as discussed in the review.

Nonredundant protective properties of FPR2/ALX in polymicrobial murine sepsis

Sepsis is characterized by overlapping phases of excessive inflammation temporally aligned with an immunosuppressed state, defining a complex clinical scenario that explains the lack of successful therapeutic options. Here we tested whether the formyl-peptide receptor 2/3 (Fpr2/3)--ortholog to human FPR2/ALX (receptor for lipoxin A4)--exerted regulatory and organ-protective functions in experimental sepsis. Coecal ligature and puncture was performed to obtain nonlethal polymicrobial sepsis, with animals receiving antibiotics and analgesics. Clinical symptoms, temperature, and heart function were monitored up to 24 h. Peritoneal lavage and plasma samples were analyzed for proinflammatory and proresolving markers of inflammation and organ dysfunction. Compared with wild-type mice, Fpr2/3(-/-) animals exhibited exacerbation of disease severity, including hypothermia and cardiac dysfunction. This scenario was paralleled by higher levels of cytokines [CXCL1 (CXC receptor ligand 1), CCL2 (CC receptor ligand 2), and TNFα] as quantified in cell-free biological fluids. Reduced monocyte recruitment in peritoneal lavages of Fpr2/3(-/-) animals was reflected by a higher granulocyte/monocyte ratio. Monitoring Fpr2/3(-/-) gene promoter activity with a GFP proxy marker revealed an over threefold increase in granulocyte and monocyte signals at 24 h post-coecal ligature and puncture, a response mediated by TNFα. Treatment with a receptor peptido-agonist conferred protection against myocardial dysfunction in wild-type, but not Fpr2/3(-/-), animals. Therefore, coordinated physio-pharmacological analyses indicate nonredundant modulatory functions for Fpr2/3 in experimental sepsis, opening new opportunities to manipulate the host response for therapeutic development.

Generation and characterization of ATP analog-specific protein kinase Cδ

To better study the role of PKCδ in normal function and disease, we developed an ATP analog-specific (AS) PKCδ that is sensitive to specific kinase inhibitors and can be used to identify PKCδ substrates. AS PKCδ showed nearly 200 times higher affinity (Km) and 150 times higher efficiency (kcat/Km) than wild type (WT) PKCδ toward N(6)-(benzyl)-ATP. AS PKCδ was uniquely inhibited by 1-(tert-butyl)-3-(1-naphthyl)-1H-pyrazolo[3,4-d]pyrimidin-4-amine (1NA-PP1) and 1-(tert-butyl)-3-(2-methylbenzyl)-1H-pyrazolo[3,4-d]pyrimidin-4-amine (2MB-PP1) but not by other 4-amino-5-(4-methylphenyl)-7-(t-butyl)pyrazolo[3,4-d]pyrimidine (PP1) analogs tested, whereas WT PKCδ was insensitive to all PP1 analogs. To understand the mechanisms for specificity and affinity of these analogs, we created in silico WT and AS PKCδ homology models based on the crystal structure of PKCι. N(6)-(Benzyl)-ATP and ATP showed similar positioning within the purine binding pocket of AS PKCδ, whereas N(6)-(benzyl)-ATP was displaced from the pocket of WT PKCδ and was unable to interact with the glycine-rich loop that is required for phosphoryl transfer. The adenine rings of 1NA-PP1 and 2MB-PP1 matched the adenine ring of ATP when docked in AS PKCδ, and this interaction prevented the potential interaction of ATP with Lys-378, Glu-428, Leu-430, and Phe-633 residues. 1NA-PP1 failed to effectively dock within WT PKCδ. Other PP1 analogs failed to interact with either AS PKCδ or WT PKCδ. These results provide a structural basis for the ability of AS PKCδ to efficiently and specifically utilize N(6)-(benzyl)-ATP as a phosphate donor and for its selective inhibition by 1NA-PP1 and 2MB-PP1. Such homology modeling could prove useful in designing molecules to target PKCδ and other kinases to understand their function in cell signaling and to identify unique substrates.

The inward rectifier potassium channel Kir2.1 is expressed in mouse neutrophils from bone marrow and liver

Neutrophils are phagocytic cells that play a critical role in innate immunity by destroying bacterial pathogens. Channels belonging to the inward rectifier potassium channel subfamily 2 (Kir2 channels) have been described in other phagocytes (monocytes/macrophages and eosinophils) and in hematopoietic precursors of phagocytes. Their physiological function in these cells remains unclear, but some evidence suggests a role in growth factor-dependent proliferation and development. Expression of functional Kir2 channels has not been definitively demonstrated in mammalian neutrophils. Here, we show by RT-PCR that neutrophils from mouse bone marrow and liver express mRNA for the Kir2 subunit Kir2.1 but not for other subunits (Kir2.2, Kir2.3, and Kir2.4). In electrophysiological experiments, resting (unstimulated) neutrophils from mouse bone marrow and liver exhibit a constitutively active, external K(+)-dependent, strong inwardly rectifying current that constitutes the dominant current. The reversal potential is dependent on the external K(+) concentration in a Nernstian fashion, as expected for a K(+)-selective current. The current is not altered by changes in external or internal pH, and it is blocked by Ba(2+), Cs(+), and the Kir2-selective inhibitor ML133. The single-channel conductance is in agreement with previously reported values for Kir2.1 channels. These properties are characteristic of homomeric Kir2.1 channels. Current density in short-term cultures of bone marrow neutrophils is decreased in the absence of growth factors that are important for neutrophil proliferation [granulocyte colony-stimulating factor (G-CSF) and stem cell factor (SCF)]. These results demonstrate that mouse neutrophils express functional Kir2.1 channels and suggest that these channels may be important for neutrophil function, possibly in a growth factor-dependent manner.

The effect of citrus-derived oil on bovine blood neutrophil function and gene expression in vitro

Research on the use of natural products to treat or prevent microbial invasion as alternatives to antibiotic use is growing. Polymorphonuclear leukocytes (PMNL) play a vital role with regard to the innate immune response that affects severity or duration of mastitis. To our knowledge, effect of cold-pressed terpeneless Valencia orange oil (TCO) on bovine PMNL function has not been elucidated. Therefore, the objective of this study was to investigate the effect of TCO on bovine blood PMNL chemotaxis and phagocytosis capabilities and the expression of genes involved in inflammatory response in vitro. Polymorphonuclear leukocytes were isolated from jugular blood of 12 Holstein cows in mid-lactation and were incubated with 0.0 or 0.01% TCO for 120min at 37°C and 5% CO2, and phagocytosis (2×10(6) PMNL) and chemotaxis (6×10(6) PMNL) assays were then performed in vitro. For gene expression, RNA was extracted from incubated PMNL (6×10(6) PMNL), and gene expression was analyzed using quantitative PCR. The supernatant was stored at -80°C for analysis of tumor necrosis factor-α. Data were analyzed using a general linear mixed model with cow and treatment (i.e., control or TCO) in the model statement. In vitro supplementation of 0.01% of TCO increased the chemotactic ability to IL-8 by 47%; however, migration of PMNL to complement 5a was not altered. Treatment did not affect the production of tumor necrosis factor-α by PMNL. Expression of proinflammatory genes (i.e., SELL, TLR4, IRAK1, TRAF6, and LYZ) coding for proteins was not altered by incubation of PMNL with TCO. However, downregulation of TLR2 [fold change (FC=treatment/control)=-2.14], NFKBIA (FC=1.82), IL1B (FC=-2.16), TNFA (FC=-9.43), and SOD2 (FC=-1.57) was observed for PMNL incubated with TCO when compared with controls. Interestingly, expression of IL10, a well-known antiinflammatory cytokine, was also downregulated (FC=-3.78), whereas expression of IL8 (FC=1.93), a gene coding for the cytokine IL-8 known for its chemotactic function, tended to be upregulated in PMNL incubated with TCO. Incubation of PMNL with TCO enhanced PMNL chemotaxis in vitro. The expression of genes involved in the inflammatory response was primarily downregulated. Results showed that 0.01% TCO did not impair the function of PMNL in vitro. Future studies investigating the use of TCO as an alternative therapy for treatment of mastitis, including dose and duration, for cows during lactation are warranted.

Salmonella evades D-amino acid oxidase to promote infection in neutrophils

Neutrophils engulf and kill bacteria using oxidative and nonoxidative mechanisms. Despite robust antimicrobial activity, neutrophils are impaired in directing Salmonella clearance and harbor viable intracellular bacteria during early stages of infection that can subsequently escape to more-permissive cell types. The mechanisms accounting for this immune impairment are not understood. We report that Salmonella limits exposure to oxidative damage elicited by D-amino acid oxidase (DAO) in neutrophils by expressing an ABC importer specific for D-alanine, a DAO substrate found in peptidoglycan stem peptides. A Salmonella dalS mutant defective for D-alanine import was more susceptible to killing by DAO through exposure to greater oxidative stress during infection. This fitness defect was reversed by selective depletion of neutrophils or by inhibition of DAO in vivo with a small-molecule inhibitor. DalS-mediated subversion of neutrophil DAO is a novel host-pathogen interaction that enhances Salmonella survival during systemic infection.

IMPORTANCE

Neutrophils engulf Salmonella during early stages of infection, but bacterial killing is incomplete. Very little is known about how Salmonella survives in neutrophils to gain access to other cell types during infection. In this study, we show that D-amino acid oxidase (DAO) in neutrophils consumes D-alanine and that importing this substrate protects Salmonella from oxidative killing by neutrophil DAO. Loss of this importer results in increased bacterial killing in vitro, in neutrophils, and in a mouse model of infection, all phenotypes that are lost upon inhibition of DAO. These findings add mechanistic insight into a novel host-pathogen interaction that has consequences on infection outcome.

Regulation of CXCR2 expression and function by a disintegrin and metalloprotease-17 (ADAM17)

The chemokine receptor CXCR2 is expressed at high levels on circulating neutrophils and is critical for directing their migration to sites of inflammation. CXCR2 surface levels are rapidly modulated by 2 mechanisms-cell internalization and recycling upon ligand binding-and by a metalloprotease activity following overt neutrophil activation by nonligand stimuli. The latter process has only been described in human neutrophils, and essentially, nothing is known about its functional relevance and the specific protease involved. We show that targeting ADAM17 in mouse and human neutrophils blocks CXCR2 down-regulation induced by nonligand stimuli but not by chemokine ligands. This was determined by use of a selective ADAM17 inhibitor, an ADAM17 function-blocking antibody, and ADAM17 gene-targeted mice. CXCR2 is known to undergo a marked down-regulation during various inflammatory disorders, and this is associated with impaired neutrophil recruitment. We show that blocking ADAM17 activity reduced CXCR2 down-regulation on circulating neutrophils and enhanced their recruitment during acute inflammation, which was reversed by a CXCR2 inhibitor. Taken together, our findings demonstrate that unlike CXCR2 internalization, ADAM17 induction down-regulates the receptor in an irreversible manner and may serve as a master switch in controlling CXCR2 function, but may also contribute to neutrophil dysfunction during excessive inflammation.

Neutrophil-derived cytokines: facts beyond expression

Polymorphonuclear neutrophils, besides their involvement in primary defense against infections - mainly through phagocytosis, generation of toxic molecules, release of enzymes, and formation of extracellular traps - are also becoming increasingly important for their contribution to the fine regulation in development of inflammatory and immune responses. These latter functions of neutrophils occur, in part, via their de novo production and release of a large variety of cytokines, including chemotactic cytokines (chemokines). Accordingly, the improvement in technologies for molecular and functional cell analysis, along with concomitant advances in cell purification techniques, have allowed the identification of a continuously growing list of neutrophil-derived cytokines, as well as the characterization of their biological implications in vitro and/or in vivo. This short review summarizes crucial concepts regarding the modalities of expression, release, and regulation of neutrophil-derived cytokines. It also highlights examples illustrating the potential implications of neutrophil-derived cytokines according to recent observations made in humans and/or in experimental animal models.

Neutrophil-Mediated Phagocytosis of Staphylococcus aureus

Initial elimination of invading Staphylococcus aureus from the body is mediated by professional phagocytes. The neutrophil is the major phagocyte of the innate immunity and plays a key role in the host defense against staphylococcal infections. Opsonization of the bacteria with immunoglobulins and complement factors enables efficient recognition by the neutrophil that subsequently leads to intracellular compartmentalization and killing. Here, we provide a review of the key processes evolved in neutrophil-mediated phagocytosis of S. aureus and briefly describe killing. As S. aureus is not helpless against the professional phagocytes, we will also highlight its immune evasion arsenal related to phagocytosis.

Innate immune cell-produced IL-17 sustains inflammation in bullous pemphigoid

Bullous pemphigoid (BP) is an autoimmune skin disease characterized by the binding of autoantibodies to components of the hemidesmosome structure, resulting in an inflammatory response and subepidermal blister formation. To investigate the role of immune orientation in the inflammatory processes associated with disease progression, blister fluid, serum, and biopsy specimens were collected from 31 consecutive BP patients. Blister fluids displayed high levels of IL-6, IL-17, IL-22, and IL-23, whereas transforming growth factor-β was increased in BP sera. However, neither immunocytochemistry on a trans-differentiation model of IL-17-producing peripheral blood mononuclear cells nor immunohistochemistry on BP biopsy specimens could demonstrate the presence of T helper type 17 lymphocytes. Instead, innate immune cells, especially neutrophils, produced IL-17 at the skin lesional site. Of note, superpotent topical corticosteroid application quickly and markedly reduced both IL-17 expression and clinical signs of BP. Consistently, IL-17 upregulated matrix-metalloprotease-9 and neutrophil elastase expression, two proteases involved in blister formation, thereof further demonstrating its role in the progress of BP. Finally, IL-17-induced matrix degradation, originated from neutrophil activation, initiated the formation of an amplification loop of the inflammatory response that could represent the underlying phenomenon leading to the maintenance and even disease extent. Thus, our results could open new therapeutic strategies for BP patients.

Social networking of human neutrophils within the immune system

It is now widely recognized that neutrophils are highly versatile and sophisticated cells that display de novo synthetic capacity and may greatly extend their lifespan. In addition, concepts such as "neutrophil heterogeneity" and "neutrophil plasticity" have started to emerge, implying that, under pathological conditions, neutrophils may differentiate into discrete subsets defined by distinct phenotypic and functional profiles. A number of studies have shown that neutrophils act as effectors in both innate and adaptive immunoregulatory networks. In fact, once recruited into inflamed tissues, neutrophils engage into complex bidirectional interactions with macrophages, natural killer, dendritic and mesenchymal stem cells, B and T lymphocytes, or platelets. As a result of this cross-talk, mediated either by contact-dependent mechanisms or cell-derived soluble factors, neutrophils and target cells reciprocally modulate their survival and activation status. Altogether, these novel aspects of neutrophil biology have shed new light not only on the potential complex roles that neutrophils play during inflammation and immune responses, but also in the pathogenesis of several inflammatory disorders including infection, autoimmunity, and cancer.