Signal-Specific Activation and Regulation of Human Neutrophil Fcγ Receptors

Human neutrophil Fc gamma receptors: different buttons for different responses

Neutrophils are fundamental cells in host defense. These leukocytes are quickly recruited from the blood to sites of infection or tissue damage. At these sites, neutrophils initiate several innate immune responses, including phagocytosis, production of reactive oxygen species, degranulation to release proteases and other antimicrobial compounds, production of inflammatory mediators, and formation of neutrophil extracellular traps. In addition to their role in innate immunity, neutrophils are now recognized as cells that also regulate adaptive immunity, via interaction with dendritic cells and lymphocytes. Neutrophils also respond to adaptive immunity by interacting with antibody molecules. Indeed, antibody molecules allow neutrophils to have antigen-specific responses. Neutrophils express different receptors for antibodies. The receptors for immunoglobulin G molecules are known as Fcγ receptors. Upon Fcγ receptor aggregation on the cell membrane, these receptors trigger distinct signal transduction cascades that activate particular cellular responses. In this review, we describe the major Fcγ receptors expressed on human neutrophils and discuss how each Fcγ receptor activates a choice of signaling pathways to stimulate particular neutrophil responses.

Role of CD11b/CD18 in priming of human leukocytes by endotoxin glycoforms from Escherichia coli

The primary objective of this study was to determine the role of β2 integrin α-subunit (CD11b) in the mechanism of human polymorphonuclear leukocyte (PML) priming by S or Re endotoxin glycoforms from Escherichia coli for fMLP-induced respiratory burst. Similar priming activity of S and Re endotoxin glycoforms for fMLP-induced reactive oxygen species (ROS) generation from primed PML was found. Anti-CD11b antibodies (clone ICRF 44) as well as isotype-matched immunoglobulin G1 (clone MOPC-21) do not influence the fMLP-induced ROS generation from unprimed PML. Antibodies against CD11b do not change fMLP-induced ROS generation from endotoxin-primed PML as well. The involvement of different isoforms of Fcγ receptors in fMLP-induced ROS generation from activated PML is proposed.

Fc gamma receptors in respiratory syncytial virus infections: implications for innate immunity

RSV infections are a major burden in infants less than 3 months of age. Newborns and infants express a distinct immune system that is largely dependent on innate immunity and passive immunity from maternal antibodies. Antibodies can regulate immune responses against viruses through interaction with Fc gamma receptors leading to enhancement or neutralization of viral infections. The mechanisms underlying the immunomodulatory effect of Fc gamma receptors on viral infections have yet to be elucidated in infants. Herein, we will discuss current knowledge of the effects of antibodies and Fc gamma receptors on infant innate immunity to RSV. A better understanding of the pathogenesis of RSV infections in young infants may provide insight into novel therapeutic strategies such as vaccination.

Pentraxins: structure, function, and role in inflammation

The pentraxins are an ancient family of proteins with a unique architecture found as far back in evolution as the Horseshoe crab. In humans the two members of this family are C-reactive protein and serum amyloid P. Pentraxins are defined by their sequence homology, their pentameric structure and their calcium-dependent binding to their ligands. Pentraxins function as soluble pattern recognition molecules and one of the earliest and most important roles for these proteins is host defense primarily against pathogenic bacteria. They function as opsonins for pathogens through activation of the complement pathway and through binding to Fc gamma receptors. Pentraxins also recognize membrane phospholipids and nuclear components exposed on or released by damaged cells. CRP has a specific interaction with small nuclear ribonucleoproteins whereas SAP is a major recognition molecule for DNA, two nuclear autoantigens. Studies in autoimmune and inflammatory disease models suggest that pentraxins interact with macrophage Fc receptors to regulate the inflammatory response. Because CRP is a strong acute phase reactant it is widely used as a marker of inflammation and infection.

Soy protein inhibits inflammation-induced VCAM-1 and inflammatory cytokine induction by inhibiting the NF-κB and AKT signaling pathway in apolipoprotein E-deficient mice

PURPOSE

Inflammation is a hallmark of many diseases, such as atherosclerosis, autoimmune diseases, obesity, and cancer. Isoflavone-free soy protein diet (SPI(-)) has been shown to reduce atherosclerotic lesions in a hyperlipidemic mouse model compared to casein (CAS)-fed mice, despite unchanged serum lipid levels. However, possible mechanisms contributing to the athero-protective effect of soy protein remain unknown. Therefore, we investigated whether and how SPI(-) diet inhibits inflammatory responses associated with atherosclerosis.

METHODS

Apolipoprotein E knockout (apoE-/-) mice (5-week) were fed CAS or SPI(-) diet for 1 or 5 week to determine LPS- and hyperlipidemia-induced acute and chronic inflammatory responses, respectively. Expression of NF-κB-dependent inflammation mediators such as VCAM-1, TNF-α, and MCP-1 were determined in aorta and liver. NF-κB, MAP kinase, and AKT activation was determined to address mechanisms contributing to the anti-inflammatory properties of soy protein/peptides.

RESULTS

Isoflavone-free soy protein diet significantly reduced LPS-induced VCAM-1 mRNA and protein expression in aorta compared to CAS-fed mice. Reduced VCAM-1 expression in SPI(-)-fed mice also paralleled attenuated monocyte adhesion to vascular endothelium, a critical and primary processes during inflammation. Notably, VCAM-1 mRNA and protein expression in lesion-prone aortic arch was significantly reduced in apoE-/- mice fed SPI(-) for 5 weeks compared with CAS-fed mice. Moreover, dietary SPI(-) potently inhibited LPS-induced NF-κB activation and the subsequent upregulation of pro-inflammatory cytokines, including TNF-α, IL-6, IL-1β, and MCP-1. Interestingly, SPI(-) inhibited NF-κB-dependent inflammatory responses by targeting I-κB phosphorylation and AKT activation with no effect on MAP kinase pathway. Of the five putative soy peptides, four of the soy peptides inhibited LPS-induced VCAM-1, IL-6, IL-8, and MCP-1 protein expression in human vascular endothelial cells in vitro.

CONCLUSIONS

Collectively, our findings suggest that anti-inflammatory properties of component(s) of soy protein/peptides may be a possible mechanism for the prevention of chronic inflammatory diseases such as atherosclerosis.

Fc gammaRIIIb triggers raft-dependent calcium influx in IgG-mediated responses in human neutrophils

Human neutrophils constitutively express a unique combination of FcγRs, namely FcγRIIa and FcγRIIIb. Numerous lines of evidence support the concept that these FcγRs generate only partially characterized intracellular signals. However, despite the fact that both receptors are likely to be engaged simultaneously in a physiological setting, no recent publications have investigated the distinct, although partially convergent, results of their joint activation in IgG-dependent responses. To examine the significance of the co-expression of FcγRIIa and FcγRIIIb on human neutrophils, we analyzed the neutrophil responses to stimuli that engage these FcγRs, namely the phagocytosis of human IgG-opsonized zymosan and the responses to heat-aggregated IgGs. Blocking antibodies to either FcγR significantly decreased the phagocytic index and the stimulated production of superoxide anions. Both receptors are required for optimal IgG-dependent responses by human neutrophils. On the other hand, only blocking antibodies to FcγRIIIb, but not to FcγRIIa, inhibited the mobilization of calcium in response to heat-aggregated IgGs. Furthermore, phagocytosis of IgG-opsonized zymosan by human neutrophils required an extracellular influx of calcium that was blocked only by antibodies against FcγRIIIb. We also observed that this calcium influx as well as the IgG-dependent phagocytosis were dependent on the integrity of the plasma membrane detergent-resistant microdomains to which both isoforms were recruited following stimulation by heat-aggregated IgGs. These data clarify the mechanisms that regulate the FcγRs constitutively expressed on human neutrophils, describe a specific contribution of FcγRIIIb at the level of the mobilization of calcium, and provide evidence for a crucial role of detergent-resistant microdomains in this process.

Neutrophil phenotype and function in partial hepatectomy in man

PURPOSE

Hepatic resections are still associated with considerable morbidity mainly because of postoperative infection. Adequate function of neutrophils is a crucial element in host defense. The aim of the study was to characterize neutrophils during partial hepatectomy.

METHODS

Fourteen patients undergoing partial liver resection were enrolled. Twenty-four hours pre-, intra- (after induction of anesthesia, after preparation of the liver, and 15 min after release of the Pringle maneuver), as well as postoperatively (3 h after Pringle; 24, 48, and 120 h after surgery), blood samples were obtained. In addition, healthy volunteers (n = 5) were investigated. Adhesion molecules (CD 62, CD 18), Fcy receptors (CD 16, CD 32), and phagocytosis by neutrophils were characterized by fluorescence-activated cell sorter analysis. Spontaneous and stimulated (formyl-methionyl-leucyl-phenylalanine) oxygen radical generation was measured by lucigenin-enhanced chemiluminescence.

RESULTS

Numeric upregulation of CD 62 and CD 18 on neutrophils was seen before the use of Pringle maneuver and persisted thereafter (p < 0.05). Spontaneous numeric expression of Fcy receptors (CD16 and CD 32) was unchanged during liver dissection but downregulated after Pringle maneuver was opened (p < 0.05). Although numeric Fcy receptors were downregulated, phagocytosis of heterologous opsonized Escherichia coli bacteria by neutrophils was unaffected. Spontaneous oxygen radical production peaked sharply 15 min after release of the Pringle maneuver (p < 0.05), contrary to stimulated oxygen radical production, which was depressed 3 h after the release of the Pringle maneuver (ns).

CONCLUSIONS

Uneventful partial hepatectomy in man resulted already in a significant change in the phenotype but in less significant changes in the functions of neutrophils.

FcgammaRIIA and FcgammaRIIIB mediate nuclear factor activation through separate signaling pathways in human neutrophils

Receptors for IgG Abs (Fcgamma receptors) are capable of triggering diverse cell responses in leukocytes. In neutrophils, two Fcgamma receptors, namely FcgammaRIIA and FcgammaRIIIB, are constitutively expressed. The signaling pathways that regulate FcgammaRIIA-mediated phagocytosis have been relatively well described. However, the different signaling pathways that lead to NF activation after engagement of each Fcgamma receptor have only been partially described. To address this problem, neutrophils were stimulated by cross-linking selectively each type of Fcgamma receptor with specific mAbs, and NF activation was then analyzed. FcgammaRIIIB, but not FcgammaRIIA, promoted a robust increase in phosphorylated ERK in the nucleus, and also efficient phosphorylation of the NF Elk-1. Complete mAb 3G8 (anti-FcgammaRIIIB) induced a higher response than did F(ab')(2) fragments of mAb 3G8, suggesting a possible synergistic effect of both FcgammaR receptors. However, mAb IV.3 (anti-FcgammaRIIA) alone did not cause an increase of phosphorylated ERK in the nucleus. FcgammaRIIIB-induced nuclear phosphorylation of ERK, and of Elk-1, was not affected by Syk, PI3K, or MEK inhibitors. In contrast, FcgammaRIIA- or FcgammaRIIIB-mediated phosphorylation of cytoplasmic ERK depended on Syk, PI3K, and MEK. Also, ERK, but not MEK, was constitutively present in the nucleus, and FcgammaRIIIB cross-linking did not increase the levels of nuclear ERK or MEK. These data clearly show that different neutrophil Fcgamma receptors possess different signaling capabilities. FcgammaRIIIB, but not FcgammaRIIA, activates a unique signaling pathway leading to the nuclear-restricted phosphorylation of ERK and Elk-1, independently of Syk, PI3K, or MEK.

Activation of human plasmacytoid dendritic cells by TLR9 impairs Fc gammaRII-mediated uptake of immune complexes and presentation by MHC class II

Human plasmacytoid dendritic cells (pDCs)(2) exploit Ag uptake receptors like CD32a for internalization of exogenous Ags. Activation of pDC by TLR9 ligand CpG-C induces strong maturation. Surprisingly, we observed that CpG-C-stimulated pDCs showed impaired Ag-specific T cell proliferation whereas the induction of allogeneic T cell proliferation was not affected. We demonstrated that signals from TLR9 caused a rapid down-regulation of the capacity of pDC to take-up Ab-Ag complexes without altering their CD32a expression, thus explaining the reduced Ag presentation. The recent contrasting biological responses that were observed upon TLR9 ligation in pDCs prompted us to study the effect of several TLR9 ligands. We observed that type I IFN-inducer CpG-A, localizing in the early endosomal compartment, did not affect CD32a function, whereas CpGs localizing in the late endosomes and inducing pDC maturation clearly inhibited CD32a-mediated Ag uptake and presentation. We conclude that TLR9 ligands not only determine the type of response, i.e., type I IFN production (innate immunity) or maturation (adaptive immunity), but also directly affect Ag presentation capacity of pDCs. We hypothesize that pDC, once activated via TLR9-ligands reaching the late endosomes, can only present initially sampled Ags and thus are protected from uptake and processing of additional potential self-Ags.

Antiendothelial cell antibodies mediate enhanced leukocyte adhesion to cytokine-activated endothelial cells through a novel mechanism requiring cooperation between Fc{gamma}RIIa and CXCR1/2

Antiendothelial cell antibodies (AECAs) are commonly detectable in diseases associated with vascular injury, including systemic lupus erythematosus (SLE), systemic sclerosis, Takayasu arteritis, Wegener granulomatosis, Behçet syndrome, and transplant arteriosclerosis. Here, we explore the hypothesis that these antibodies might augment polymorphonuclear leukocyte (PMN) adhesion to endothelium in inflammation. Initially, we established that a mouse IgG mAb bound to endothelial cells (ECs) significantly increased PMN adhesion to cytokine-stimulated endothelium in an FcgammaRIIa-dependent manner. Neutralizing antibodies, and adenoviral transduction of resting ECs, demonstrated that the combination of E-selectin, CXCR1/2, and beta(2) integrins is both necessary and sufficient for this process. We observed an identical mechanism using AECA IgG isolated directly from patients with SLE. Assembled immune complexes also enhanced PMN adhesion to endothelium, but, in contrast to adhesion because of AECAs, this process did not require CXCR1/2, was not inhibited by pertussis toxin, and was FcgammaRIIIb rather than FcgammaRIIa dependent. These data are the first to demonstrate separate nonredundant FcgammaRIIa and FcgammaRIIIb-mediated mechanisms by which EC-bound monomeric IgG and assembled immune complexes amplify leukocyte adhesion under dynamic conditions. Furthermore, the observation that FcgammaRIIa and CXCR1/2 cooperate to enhance PMN recruitment in the presence of AECAs suggests a mechanism whereby AECAs may augment tissue injury during inflammatory responses.

Anti-OxLDL IgG blocks OxLDL interaction with CD36, but promotes FcgammaR, CD32A-dependent inflammatory cell adhesion

Generation of antibodies against oxidized-low-density lipoproteins (oxLDL) during atherosclerosis could result in the formation and deposition of oxLDL immune complexes (oxLDL-IC) on the vascular endothelial cells. Inflammatory cells express scavenger receptor (SR such as CD36) and Fcgamma receptor (FcgammaR: CD32A and CD64) that can bind to oxLDL and oxLDL-IC, respectively. Hence, depending on anti-oxLDL IgG titer, circulating monocytes could adhere to endothelium to oxLDL-IC-coated vascular bed via either FcgammaR and/or CD36. In this study, we determined the relative contribution of SR and FcgammaR in mediating monocyte interaction with oxLDL-IC deposited on vascular bed. At saturating levels of anti-oxLDL IgG concentration, monocytic cells adhered to oxLDL-IC and this adhesion is completely blocked by anti-CD32A mAb. Using CHOK1-CD32A-CD36 cells expressing equal levels of CD32A and CD36, it was observed that at lower concentrations of anti-oxLDL IgG, CD32A and CD36 contribute about 75% and 25% of cell adhesion, respectively, while at higher concentrations of anti-oxLDL IgG the adhesion is completely CD32A-dependent. CD32A-dependent adhesion was further confirmed with peripheral blood monocytes and platelets that express 2- to 5-fold higher levels of CD36 compared to CD32A. Further, PBMC adhesion to oxLDL-IC-deposited endothelial cells induced secretion of pro-inflammatory chemokines, MCP-1 and IL-8. Our results demonstrate that anti-oxLDL IgG blocks oxLDL interaction with SR such as CD36, whereas oxLDL-IC formation promotes monocyte adhesion and subsequent chemokine release through FcgammaR. These findings suggest a role for FcgammaR-mediated inflammatory cell activation in the progression of atherosclerosis.