Differential roles of Fc gamma RII and Fc gamma RIII in immune complex stimulation of human neutrophils

Heterogeneity in neutrophil responses to immune complexes

Immune complexes (ICs) can trigger inflammation and thrombosis, in part, by activating neutrophils. Much attention has focused on the serologic characteristics of ICs and Fc receptors associated with cellular activation, but few studies have examined host susceptibility to neutrophil activation by ICs. Here, we use a novel whole blood system to investigate the ability of ICs to cause neutrophil activation and degranulation. Using monoclonal anti-platelet factor 4/heparin (PF4/heparin), anti-protamine/heparin antibodies, patient-derived anti-PF4/heparin antibodies, and heat-aggregated immunoglobulin G as model ICs, we demonstrate that heparin-containing ICs cause robust, heparin-dependent neutrophil activation and degranulation which is mediated by both FcγRIIa and complement. Longitudinal testing over a 1-year period shows that an individual's neutrophil response to ICs represents a fixed phenotype resulting in high, intermediate, or low reactivity. Examination of individuals at the extremes of reactivity (high vs low) shows that phenotypic variation resides in the cellular compartment and is correlated with host white blood cell count and absolute neutrophil count, but not age, sex, race, polymorphisms in neutrophil Fcγ receptors, or CR1, CR3, and Fcγ receptor expression on neutrophils. Together, these studies demonstrate that susceptibility to neutrophil activation by ICs is intrinsic to the host and is likely genetic in origin. These findings may be relevant to the heterogeneous clinical outcomes seen in patients with heparin-induced thrombocytopenia and other IC-mediated disorders and could potentially identify patients at high risk for thrombotic and inflammatory complications.

Influence of FcγRIIIb polymorphism on its ability to cooperate with FcγRIIa and CR3 in mediating the oxidative burst of human neutrophils

Considering that human neutrophil FcγRIIa and FcγRIIIb receptors interact synergistically with CR3 in triggering neutrophil functional responses, allelic polymorphisms in these receptors might influence such interactions. We assessed whether FcγRIIIb polymorphisms affect FcγR/CR cooperation in mediating the neutrophil oxidative burst (OB), in particular the FcγRIIIb/CR3 cooperation that occurs via lectin-saccharide-like interactions. The OB of human neutrophil antigen (HNA)-1a-, HNA-1b-, and HNA-1a/-1b-neutrophils stimulated with immune complexes, opsonized or not with serum complement, was measured by the luminol-enhanced chemiluminescence assay. Compared with HNA-1a-neutrophils, HNA-1b-neutrophils exhibited reduced FcγR-stimulated OB, but increased FcγR/CR-stimulated OB. It suggests that (i) FcγR and CR cooperate more effectively in HNA-1b-neutrophils, and (ii) the HNA-1b allotype influences the FcγRIIIb cooperation with FcγRIIa, but not with CR3. HNA-1a- and HNA-1b-neutrophils exhibited similar OB responses elicited via CR3 alone or via FcγR/CR-independent pathways. In addition, the level of FcγRIIIb, FcγRIIa, and CR3 expression did not differ significantly among the neutrophil groups studied. Together, these results demonstrate that the HNA-1b allotype influences the functional cooperation between FcγRIIIb and FcγRIIa, and suggest that the difference in the glycosylation pattern between HNA-1a and HNA-1b does not affect the FcγRIIIb cooperation with CR3.

Initial cytoplasmic and phagosomal consequences of human neutrophil exposure to Staphylococcus epidermidis

Microorganisms are recognized by specific phagocyte surface receptors. Liganded receptors then signal a series of events leading to phagocytosis and destruction of the organism by oxidative, lytic, and associated processes. Some organisms, such as Mycobacterium tuberculosis (Mtb), Cryptococcus neoformans (Cf), and others, evade such destruction, surviving and sometimes multiplying within the phagosome to later cause disease. To study such evasion, we developed protocols which permit simultaneous kinetic measurement of early cytoplasmic signaling and of phagosomal pH (pH(p)) and oxidative burst, on a cell-by-cell basis, of polymorphonuclear (PMN) leukocytes exposed to fluorescently labeled, nonpathogenic Staphylococcus epidermidis (Se). The availability of a new, highly sensitive pH probe, pHrodo, permits observation of increasing pH(p). Simultaneous labeling of the organism, applicable to any phagocyte target, with a probe insensitive to pH and oxidative species, such as AlexaFluor350, permits distinction between binding and functional responses to it by ratioing fluorescences. Addition of an extracellular-specific quencher (Trypan blue) permits distinction between bound and phagosome-enclosed targets, so that conditions within the closed phagosome can be studied. We found that opsonization is required for functional activation of PMN by Se, that the organism causes early alkalinization of the phagosome (in contrast to Cf which hyperacidifies it), and that extracellular Ca(2+) is not required for cytoplasmic Ca(2+) signaling but contributes markedly to binding of Se to PMN and to ensuant bactericidal functions. These findings lead to a new approach to the study of select organisms, like Cf and Mtb, which evade killing by manipulating the phagosomal environment.

Copy number, linkage disequilibrium and disease association in the FCGR locus

The response of a leukocyte to immune complexes (ICs) is modulated by receptors for the Fc region of IgG (FcγRs), and alterations in their affinity or function have been associated with risk of autoimmune diseases, including systemic lupus erythematosus (SLE). The low-affinity FcγR genomic locus is complex, containing regions of copy number variation (CNV) which can alter receptor expression and leukocyte responses to IgG. Combined paralogue ratio tests (PRTs) were used to distinguish three intervals within the FCGR locus which undergo CNV, and to determine FCGR gene copy number (CN). There were significant differences in FCGR3B and FCGR3A CNV profiles between Caucasian, East Asian and Kenyan populations. A previously noted association of low FCGR3B CN with SLE in Caucasians was supported [OR = 1.57 (1.08–2.27), P = 0.018], and replicated in Chinese [OR = 1.65 (1.25–2.18), P = 4 × 10⁻⁴]. There was no association of FCGR3B CNV with vasculitis, nor with malarial or bacterial infection. Linkage disequilibrium (LD) between multi-allelic FCGR3B CNV and SLE-associated SNPs in the FCGR locus was defined for the first time. Despite LD between FCGR3B CNV and a variant in FcγRIIB (I232T) which abolishes inhibitory function, both reduced CN of FCGR3B and homozygosity of the FcγRIIB-232T allele were individually strongly associated with SLE risk. Thus CN of FCGR3B, which controls IC responses and uptake by neutrophils, and variations in FCGR2B, which controls factors such as antibody production and macrophage activation, are important in SLE pathogenesis. Further interpretations of contributions to pathogenesis by FcγRs must be made in the context of LD involving CNV regions.

Measurement of phagocytosis and of the phagosomal environment in polymorphonuclear phagocytes by flow cytometry

Phagocytes are the most important early components of the immune response, programmed to recognize, engulf, and destroy immune complexes (formed when antibodies recognize their specific antigens), foreign particles, bacteria, mycobacteria, apoptotic cells, etc. Neutrophils, monocytes, macrophages, and dendritic cells all participate in this process. Flow cytometry permits observation of phagocytes that have responded and, with the appropriate fluorescent probes, of the environment in the phagosome that has enclosed the foreign matter. This unit gives the background and the protocols for performing such studies.

The association of Fcgamma receptor IIIb genetic polymorphism and susceptibility to periodontitis in Taiwanese individuals

AIM

The allelic polymorphism of FcgammaRIIIb, the neutrophil-specific receptor involved in the phagocytosis of immunoglobulin G-opsonized bacteria, has functionally distinct capacities that are important in host defence mediated by neutrophils. The aim of this study was to identify whether the polymorphism of FcgammaRIIIb is associated with periodontitis in Taiwanese individuals.

MATERIALS AND METHODS

This case-control study included of 93 aggressive periodontitis (AgP) patients, 372 chronic periodontitis (CP) patients and 158 healthy controls (HC). The FcgammaRIIIb genotypes were determined by PCR using allele-specific primers. The risk for periodontitis associated with genotypes was calculated as the odds ratio (OR).

RESULTS

A significant difference was observed in the distribution of the FcgammaRIIIb genotype between either AgP and HC, or AgP and CP, but not between CP and HC. The OR for carriage of the NA2 allele (NA1NA2+NA2NA2 versus NA1NA1) in AgP was 3.27 [95% confidence interval (CI)=1.57-7.51, p=0.0027] and 2.94 (95% CI=1.49-6.48, p=0.0037), as compared with HC and CP. After adjusting for possible confounding factors, the association was still significant.

CONCLUSIONS

The results of the present study suggest that subjects carrying at least one copy of the FcgammaRIIIb-NA2 allele might be associated with susceptibility to AgP. However, the clinical implications of the FcgammaRIIIb allelic polymorphism should be determined by further studies.

Critical but overlapping role of FcgammaRIII and FcgammaRIV in activation of murine neutrophils by immobilized immune complexes

Immune complex-induced activation of neutrophils through cell surface FcRs plays a central role in the pathogenesis of autoimmune inflammatory diseases. These diseases are often modeled using genetically modified mice. However, in contrast to the number of studies on human cells, the identity of FcRs involved in immune complex activation of murine neutrophils is at present unknown. Furthermore, little is known about the cellular functions mediated by the recently identified murine FcgammaRIV. In this study, we tested the identity of FcRs involved in the activation of neutrophils by plate-bound immune complexes, using various knockout mouse strains, function-blocking mAbs, or the combination of both approaches. Activation of murine neutrophils by immobilized IgG immune complexes was abrogated in FcR gamma-chain-deficient cells, but not by the single or combined deficiency of the gamma-chain-associated FcgammaRI and FcgammaRIII, or by blocking Abs against either FcgammaRIII or FcgammaRIV alone. However, treatment of FcgammaRIII-deficient neutrophils with FcgammaRIV-blocking Abs or simultaneous blocking of FcgammaRIII and FcgammaRIV in wild-type cells completely inhibited the immune complex-induced cellular responses. In parallel studies, activation of human neutrophils by immobilized immune complexes was abrogated by blocking Abs against either FcgammaRIIA or FcgammaRIIIB alone. Taken together, neutrophil activation by immobilized immune complexes requires the murine FcgammaRIII/FcgammaRIV or the human FcgammaRIIA/FcgammaRIIIB molecules. Although both of the two human receptors are required for this response, the two murine receptors play overlapping, redundant roles. These results promote our understanding of autoimmune diseases and identify an IgG-dependent cellular function of FcgammaRIV.

Sequential chemotactic and phagocytic activation of human polymorphonuclear neutrophils

Human polymorphonuclear neutrophils (PMN) chemotax to a foreign entity. When the chemoattractants' origins are reached, specific receptors bind to the invader's surface, initiating phagocytosis, phagosome formation, and fusion with granule membranes, generating the bactericidal oxidative burst, and releasing lytic enzymes, specific peptides, and proteins. We explored the initial signaling involved in these functions by observing naïve, unprimed PMN in suspension using fluorescent indicators of cytoplasmic signals (Delta[Ca(2+)](i) and DeltapH(i)) and of bactericidal entities (oxidative species and elastase) exposed to N-formyl-methionyl-leucyl-phenylalanine (fMLP) and/or multivalent immune complexes (IC). fMLP and IC each initiate a rapid transient rise in [Ca(2+)](i), mostly from intracellular stores, simultaneously with a drop in pH(i); these are followed by a drop in [Ca(2+)](i) and a rise in pH(i), with the latter being due to a Na(+)/H(+) antiport. The impact of a second stimulation depends on the order in which stimuli are applied, on their dose, and on their nature. Provided that [Ca(2+)](i) is restored, 10(-7) M fMLP, previously shown to elicit maximal Delta[Ca(2+)](i) but no bactericidal functions, did not prevent the cells' responses with Delta[Ca(2+)](i) to a subsequent high dose of fMLP or IC; conversely, cells first exposed to 120 mug/ml IC, previously shown to elicit maximal Delta[Ca(2+)](i) and bactericidal functions, exhibited no subsequent Delta[Ca(2+)](i) or DeltapH(i) to either stimulus. While exposure to 10(-7) M fMLP, which saturates the PMN high-affinity receptor, did not elicit bactericidal release from these naïve unprimed PMN in suspension, 10(-5) M fMLP did, presumably via the low-affinity receptor, using a different Ca(2+) source.

CD16b associates with high-density, detergent-resistant membranes in human neutrophils

CD16b is unique in that it is the only Fc receptor linked to the plasma membrane by a GPI (glycosylphosphatidylinositol) anchor. GPI-anchored proteins often preferentially localize to DRMs (detergent-resistant membranes) that are rich in sphingolipids and cholesterol and play an important role in signal transduction. Even though the responses to CD16b engagement have been intensively investigated, the importance of DRM integrity for CD16b signalling has not been characterized in human neutrophils. We provide direct evidence that CD16b constitutively partitions with both low- and high-density DRMs. Moreover, upon CD16b engagement, a significant increase in the amount of the receptor is observed in high-density DRMs. Similarly to CD16b, CD11b also resides in low- and high-density DRMs. In contrast with CD16b, the partitioning of CD11b in DRMs does not change in response to CD16b engagement. We also provide evidence for the implication of Syk in CD16b signalling and its partitioning to DRMs in resting and activated PMNs (polymorphonuclear neutrophils). Additionally, DRM-disrupting agents, such as nystatin and methyl-beta-cyclodextrin, alter cellular responses to CD16b receptor ligation. Notably, a significant increase in the mobilization of intracellular Ca2+ and in tyrosine phosphorylation of intracellular substrates after CD16b engagement is observed. Altogether, the results of this study provide evidence that high-density DRMs play a role in CD16b signalling in human neutrophils.

Fcgamma and complement receptors: expression, role and co-operation in mediating the oxidative burst and degranulation of neutrophils of Brazilian systemic lupus erythematosus patients

We have investigated the individual role of FcgammaR and CR, as well as their cooperation, in mediating the oxidative burst and degranulation of neutrophils of Brazilian systemic lupus erythematosus (SLE) patients. Neutrophils were stimulated with the immune complexes (IC)-IgG or -F(ab')2, opsonized or not with normal or SLE human serum. The oxidative burst was decreased in neutrophils of active SLE patients compared to healthy controls when this response was mediated by FcgammaR and/or CR, while the degranulation was unaffected. The SLE hypocomplementemia did not affect the oxidative burst mediated only by CR. FcgammaRII and CR1 expression on neutrophils of active SLE patients was reduced, while the expression of FcgammaRIII and CR3 was unaffected. These results suggest that the different FcgammaR and CR may be involved or cooperate in different ways in the mediation of the oxidative burst and the degranulation. Moreover, the decreased oxidative burst of neutrophils of active SLE patients may not depend only on SLE hypocomplementemia for IC opsonization. These observations are directed at the understanding of how each of these immune system components (FcgammaR, CR and complement) influences the precise biological neutrophil responses both in physiological and pathological conditions. Since the Brazilian population comprises many races, these results are important because they are directed at a specific population of SLE patients.

Both the epitope specificity and isotype are important in the antitumor effect of monoclonal antibodies against Her-2/neu antigen

The Her-2/neu oncogene, which encodes a growth factor receptor, was implicated in the malignancy of human adenocarcinomas. Antibodies directed to this molecule have been previously shown to have an antitumor effect in vivo. In an attempt to understand the mechanisms of the antitumor activity, we generated 2 monoclonal antibodies (mAbs), HRO G1 and HRT G1, that recognize different epitopes on Her-2/neu. Both of the mAbs bound HER2/neu on the tumor surface, resulting in phosphorylation of HER2/neu. We also generated IgG2a and IgG2b mAbs from these 2 mAbs, respectively. The results of in vitro studies showed that these anti-Her-2/neu mAbs could not inhibit the growth of the tumor cells that express Her-2/neu molecules by themselves. However, in an antibody-dependent cellular cytotoxicity study using mouse splenocytes as effector cells, HRT mAbs had antitumor activities superior to those of HRO mAbs, indicating that the epitope specificity may also partake in antibody-dependent cellular cytotoxicity with antibody isotype. In a complement-dependent cytotoxicity study, the IgG2a and IgG2b mAbs showed stronger effects than IgG1 isotype mAbs irrespective of the epitope specificities. The results of in vivo studies also showed that HRT mAbs had superior antitumor activity to those of HRO mAbs. The antitumor activity was most prominent in the HRT G2b isotype among HRT mAbs. HRT G1 also showed a moderate antitumor effect, while HRT G2a showed only slight inhibition effect. These data indicate that both the epitope specificity and the differences in Fc region of mAbs could play important roles in the antitumor activities.

Differential role of neutrophil Fcgamma receptor IIIB (CD16) in phagocytosis, bacterial killing, and responses to immune complexes

OBJECTIVE

To determine the roles played by the neutrophil Fcgamma receptor type II (FcgammaRII) (CD32) and FcgammaRIIIb (CD16) in phagocytosis, bacterial killing, and activation by immune complexes (ICs) and to test the hypothesis that inhibition of pathologic effector neutrophil function is possible without compromising host defense.

METHODS

Receptor function was probed by enzymic removal of FcgammaRIIIb from the cell surface and by use of Fab/F(ab')(2) fragments of monoclonal antibodies to block receptor-ligand binding. Cells were challenged with (a) serum-opsonized Staphylococcus aureus, (b) serum- and IgG-opsonized latex particles, and (c) synthetic soluble and insoluble ICs to mimic bacterial and inflammatory stimuli.

RESULTS

Phosphatidylinositol-phospholipase C treatment removed >97% of surface FcgammaRIIIb from neutrophils previously treated with tumor necrosis factor alpha to mobilize intracellular stores of receptor. This treatment profoundly inhibited activation of primed neutrophils by soluble ICs of the type found in diseased rheumatoid joints, but had no effect on phagocytosis and killing of serum-opsonized S aureus.

CONCLUSION

FcgammaRIIIb plays a major role in the secretion of toxic products in response to ICs, but little or no role in the phagocytosis and killing of serum-opsonized bacteria. The selective suppression of effector neutrophil function is therefore possible. FcgammaRIIIb, or its intracellular signaling pathway, is a potential therapeutic target in inflammatory diseases such as rheumatoid arthritis, because disruption of its function should decrease inflammatory tissue damage, but not jeopardize host protection against infection.

Soluble Fcgamma receptor IIIb alters the function of polymorphonuclear neutrophils but extends their survival

We have established that polymorphonuclear neutrophil (PMN)-binding anti-Fcgamma receptor IIIb (FcgammaRIIIb) autoantibodies (autoAb) inhibit the function of these cells but extend their survival. Here, we show that recombinant FcgammaRIIIb (rFcgammaRIIIb), as well as purified FcgammaRIIIb (pFcgammaRIIIb), deteriorated the PMN adherence and respiratory burst in a dose-dependent manner. Furthermore, rFcgammaRIIIb and pFcgammaRIIIb reduced the level of annexin V-binding PMN from 23.6 +/- 1.6 % to 6.3 +/- 1.0 and 11.0 +/- 1.0 %, respectively, while human serum albumin exerted no effects. Incubation of rFcgammaRIIIb with those autoAb binding to soluble FcgammaRIIIb resulted in the attachment of such immune complexes (IC) to the cells, thereby also delaying apoptosis (44.9 +/- 5.9 versus 18.0 +/- 2.0 % annexin V-binding PMN after 16 hours). Soluble FcgammaRIIIb, in concert with FcgammaRIIIb / anti-FcgammaRIIIb IC, produced similar effects in that the percentage of annexin V-binding PMN declined to 16.0 +/-1.9 %. It was thus suggested that FcgammaRIIIb / anti-FcgammaRIIIb IC inserted the Fc region of their IgG into the membrane FcgammaRIIIb. Such an interpretation is consistent with our finding that, whereas aggregated IgG and anti-FcgammaRIIIb monoclonal Ab prevented membrane FcgammaRIIIb / IC interaction, neither soluble FcgammaRIIIb, nor anti-cgammaRII did so. We conclude that the function and the life span of PMN are influenced synergistically by soluble FcgammaRIIIb and anti-FcgammaRIIIb autoAb.

Neutrophil chemotaxis and superoxide production are induced by cross-linking FcgammaRII receptors

Neutrophils express two types of receptor for the Fc region of IgG, FcgammaRII and FcgammaRIIIB. Via these receptors, neutrophils bind IgG complexes that contain more than one IgG molecule. This binding activates functional processes, such as the respiratory burst and chemotaxis. Neutrophils were treated with biotinylated anti-Fc receptor monoclonal antibodies and chemotaxis toward streptavidin, a cross-linking agent, was determined. Cross-linking FcgammaRII and not FcgammaRIIIB induced neutrophil chemotaxis. Superoxide production in response to immobilized anti-Fc receptor antibodies was also examined. Anti-FcgammaRII Fab bound to ELISA plates induced superoxide production, while anti-FcgammaRIIIB Fab did not. Pretreatment of neutrophils with anti-FcgammaRII Fab reduced superoxide generated by immobilized anti-FcgammaRII antibody. The data demonstrate that FcgammaRII and not FcgammaRIIIB are responsible for neutrophil chemotaxis and superoxide production upon Fc receptor activation.

Salmonella typhimurium induces epithelial IL-8 expression via Ca(2+)-mediated activation of the NF-kappaB pathway

Interactions between the enteric pathogen Salmonella typhimurium and the luminal surface of the intestine provoke an acute inflammatory response, mediated in part by epithelial cell secretion of the chemokine IL-8 and other proinflammatory molecules. This study investigated the mechanism by which this pathogen induces IL-8 secretion in physiologically polarized model intestinal epithelia. IL-8 secretion induced by both the prototypical proinflammatory cytokine TNF-alpha and S. typhimurium was NF-kappaB dependent. However, NF-kappaB activation and IL-8 secretion induced by S. typhimurium, but not by TNF-alpha, was preceded by and required an increase in intracellular [Ca(2+)]. Additionally, agonists that increased intracellular [Ca(2+)] by receptor-dependent (carbachol) or independent (thapsigargin, ionomycin) means also induced IL-8 secretion. Furthermore, the ability of S. typhimurium mutants to induce IkappaB-alpha degradation, NF-kappaB translocation, and IL-8 transcription and secretion correlated precisely with their ability to induce an intracellular [Ca(2+)] increase in model intestinal epithelia, but not with their ability to invade these cells. Finally, S. typhimurium, but not TNF-alpha, induced a Ca(2+)-dependent phosphorylation of IkappaB-alpha. These results indicate that S. typhimurium-induced activation of NF-kappaB-dependent epithelial inflammatory responses proceeds by a Ca(2+)-mediated activation of an IkappaB-alpha kinase. These observations raise the possibility that pharmacologic intervention of the acute inflammatory response can be selectively matched to the specific class of initiating event.

LXA4, aspirin-triggered 15-epi-LXA4, and their analogs selectively downregulate PMN azurophilic degranulation

The eicosanoid lipoxin A4 (LXA4) is biosynthesized in vivo by cells present at inflammatory sites and appears to be an endogenous anti-inflammatory mediator. Further, in the presence of aspirin, the 15-epimer of LXA4 (15-epi-LXA4) is biosynthesized and may mediate some of aspirin's desirable bioactions. LXA4, 15-epi-LXA4, and their stable analogs inhibit inflammation in established animal models, indicating that these compounds may be useful for treating inflammatory disease states. To investigate the cellular mechanisms by which these lipid mediators downregulate inflammation, we investigated whether these eicosanoids could influence receptor-mediated degranulation of human neutrophils, an event thought to play a major causative role in several inflammatory disease states. LXA4, 15-epi-LXA4, and their stable analogs potently (IC50 < 1 nM) and selectively downregulated neutrophil release of azurophilic granule contents but did not affect other neutrophil secretory functions. Thus the cellular basis of action of these natural off-switches to inflammation appears to involve downregulation of neutrophil azurophilic granule release.

A microscopic study of Fc gamma RIII-mediated respiratory burst in neutrophils

We investigated the relationship between the respiratory burst in neutrophils and the membrane distribution of the IgG receptor, Fc gamma RIII. Fc gamma RIII receptors were labeled with a fluoresceinated antibody that does not block binding of immune complexes. The respiratory burst was detected using covalently bound rosamine stain previously described for flow cytometric applications. This method allows visualization of intracellular oxidant production in fixed cells using attenuated illumination with a laser. Strong cytosolic oxidation of rosamine was observed only in those cells that displayed prominent receptor endocytosis upon interaction with insoluble immune complexes. Soluble immune complexes or insoluble complexes in the presence of cytochalasin B did not stimulate endocytosis of Fc gamma RIII and induced no rosamine oxidation. Extracellular superoxide production measured by the cytochrome c test did not correlate with intracellular rosamine oxidation: it was maximal in cytochalasin-treated cells and did not require any visible receptor rearrangement. Our results demonstrate the utility of the rosamine stain as an intracellular marker of the oxidative burst, support the role of Fc gamma RIII in neutrophil activation and emphasize the compartmental regulation of the oxidative burst.

Activation of human neutrophils by soluble immune complexes: role of Fc gamma RII and Fc gamma RIIIb in stimulation of the respiratory burst and elevation of intracellular Ca2+

Activation of control, unprimed neutrophils with soluble immune complexes fails to generate a respiratory burst. However, if the cells are primed with either tumor necrosis factor-alpha or granulocyte-macrophage colony-stimulating factor prior to addition of soluble immune complexes, then a rapid and transient burst of reactive oxidant secretion is observed. In unprimed neutrophils the soluble immune complexes stimulate an intracellular Ca2+ transient that arises from the mobilization of intracellular Ca2+. However, in primed cells, an "extra" intracellular Ca2+ signal is observed that arises from Ca2+ influx. After removal of Fc gamma RIIIb by treatment with pronase or PI-PLC, the soluble immune complexes fail to activate a respiratory burst in unprimed neutrophils and the "extra" Ca2+ signal is not observed. These results indicate that during priming Fc gamma RIIIb becomes functionally activated and thence its ligation leads to stimulated Ca2+ influx and the generation of intracellular signals that lead to NADPH oxidase activation. Experiments using Fab/F(ab')2 fragments to specifically crosslink either Fc gamma RII or Fc gamma RIIIb and experiments with neutrophils from an individual with Fc gamma RIIIb gene deficiency confirm this important function for Fc gamma RIIIb in neutrophil activation.

Circulating promyelocytes and low levels of CD16 expression on polymorphonuclear leukocytes accompany early-onset periodontitis

Early-onset periodontitis (EOP) is characterized by rapidly progressive alveolar bone loss, chemotactic defects of neutrophils, and significant familial aggregation. We found immature myeloid lineage cells, defined as promyelocytes, in the peripheral blood in patients with EOP. A hematological examination of peripheral blood cells showed normal reference values regarding cell proportions. Flow cytometry revealed significantly lower expression of CD16, a glycosylphosphatidylinositol (GPI)-anchored protein, on peripheral neutrophils in patients compared with those in age- and sex-matched healthy controls, whereas the levels of CD11a and CD11b expression were similar. The chemotactic response of neutrophils was lower toward not only formyl-methionyl-leucyl-phenylalanine but also complement fragment C5a than that of healthy controls. The expression of another GPI-anchored protein, CD14, was equally expressed by controls and patients. Therefore, the low level of CD16 expression was not due to the incomplete synthesis of the GPI anchor. GPI anchors of CD16 on neutrophils from controls and patients were both partially resistant to phosphatidylinositol-specific phospholipase C. The presence of promyelocytes in peripheral blood, low expression of CD16, and low chemotactic response of neutrophils suggest that patients with EOP have an abnormal maturation system in myeloid lineage cells in the bone marrow, which may be associated with the onset and course of EOP.

Stimulation of intracellular Ca2+ levels in human neutrophils by soluble immune complexes. Functional activation of FcgammaRIIIb during priming

Soluble immune complexes bind to unprimed neutrophils and generate intracellular Ca2+ transients but fail to activate the NADPH oxidase. Following priming of the neutrophils with either tumor necrosis factor alpha or granulocyte-macrophage colony-stimulating factor, stimulation of the cells with the soluble immune complexes leads to an enhanced Ca2+ signal and significant secretion of reactive oxidants. The enhanced Ca2+ signal observed in primed neutrophils results from the influx of Ca2+ from the external environment and is partly sensitive to tyrosine kinase inhibitors. This is in contrast to the Ca2+ signal observed in unprimed neutrophils, which arises from the mobilization of intracellular stores. When the surface expression of FcgammaRIIIb on primed neutrophils was decreased either through incubation with Pronase or phosphoinositide-specific phospholipase C, the extra enhanced Ca2+ mobilization seen in primed cells was significantly lowered, while the initial rise in intracellular Ca2+ was unaffected. Depletion of FcgammaRIIIb had no significant effect on the Ca2+ transients in unprimed neutrophils. Cross-linking FcgammaRII, but not FcgammaRIIIb, induced increases in intracellular Ca2+ in unprimed neutrophils, while cross-linking either of these receptors increased Ca2+ levels in primed neutrophils. The FcgammaRII-dependent intracellular Ca2+ rise in primed cells was unaffected by incubation in Ca2+-free medium, whereas the FcgammaRIIIb-dependent transient was significantly decreased when Ca2+ influx was prevented in Ca2+-free medium supplemented with EGTA. Cross-linking either FcgammaRII or FcgammaRIIIb in primed or unprimed cells failed to stimulate substantial levels of inositol 1,4,5-trisphosphate production. These results indicate that following stimulation of primed neutrophils with soluble immune complexes the enhanced Ca2+ mobilization observed is the result of a functional activation of the glycosylphosphatidylinositol-linked FcgammaRIIIb.

Effect of iC3b binding to immune complexes upon the phagocytic response of human neutrophils: synergistic functions between Fc gamma R and CR3

We compared the phagocytosis of immune complexes (IC) and iC3b-opsonized derivatives (iC3b-IC) by human neutrophils. The phagocytosis of iC3b-IC via Fc gamma R and CR3 was much greater than that of IC via Fc gamma R alone. Adding ethanol to the cells decreased iC3b-IC phagocytosis to that of IC, which was not affected by these reagents, suggesting that the enhanced phagocytosis is attributable to CR3-mediated phospholipase D activation. The IC phagocytosis was inhibited more effectively by anti-Fc gamma IIIB, whereas the iC3b-IC phagocytosis was partly inhibited only by anti-Fc gamma RII. The main Fc gamma R might differ in IC and iC3b-IC phagocytosis.

Heterotypic Fc gamma R clusters evoke a synergistic Ca2+ response in human neutrophils

Both Fc gamma receptors on human neutrophils (Fc gamma RIIa and Fc gamma RIIIb) are capable of initiating signal transduction after multivalent cross-linking. However, immune complexes most likely activate neutrophils by a combined homotypic and heterotypic cross-linking of Fc gamma Rs. We have investigated the effect of homotypic and heterotypic Fc gamma R cluster formation on changes in the intracellular free Ca2+ concentration. Combined heterotypic and homotypic cluster formation resulted in a Ca2+ response that was strongly enhanced as compared to the sum of both individual Fc gamma R responses. This synergistic response was caused by the formation of heterotypic clusters of Fc gamma Rs and not by the simultaneous formation of homotypic clusters. This conclusion was supported by experiments with a bispecific antibody binding to both Fc gamma RIIa and Fc gamma RIIIb. The heterotypic Fc gamma R cross-linking results in efficient activation of Ca2+ influx, probably caused by a more pronounced depletion of intracellular Ca2+ stores. Stimulation with immune complexes also induced Ca2+ influx in normal neutrophils, but not in Fc gamma RIIIb-deficient neutrophils. The synergism between both Fc gamma Rs was also apparent in other responses of neutrophils, such as the activation of the respiratory burst. This study shows that the two different Fc gamma Rs on neutrophils complement each other in mediating an important cellular response.

Oxidative modification of low density lipoproteins by human polymorphonuclear leukocytes

Oxidatively modified low density lipoproteins are thought to play an important role in the generation of macrophage-derived foam cells in early atherosclerotic lesions. Cultured endothelial cells, monocytes, macrophages and smooth muscle cells can modify low density lipoproteins, either by a free radical mechanism or by the action of lipoxygenases. Previous studies demonstrated that activated human polymorphonuclear leukocytes can oxidize low density lipoprotein lipids. Stimulation of the cells with phorbol 12-myristate 13-acetate resulted in an increase both in superoxide anion production and in low density lipoprotein oxidation. The present results show that the oxidative modification of low density lipoproteins by human polymorphonuclear leukocytes can be inhibited by superoxide dismutase but not by the lipoxygenase inhibitor, (5,8,11,14)-eicosatetraynoic acid. The low density lipoproteins oxidized by polymorphonuclear leukocytes were recognized by the scavenger receptor of macrophages (P 388 cell line). It is proposed that the superoxide anion is an important factor in the oxidative modification of low density lipoproteins induced by polymorphonuclear leukocytes, and that under conditions of increased oxidative metabolism in vivo, polymorphonuclear leukocytes can contribute to foam cell formation by a scavenger receptor-dependent process at lesion sites.

Changes in complement and immunoglobulin G receptor expression on neutrophils associated with Porphyromonas gingivalis-induced inhibition of phagocytosis

We examined the phagocytic capacity and receptor expression on neutrophils stimulated with Porphyromonas gingivalis soluble products. Stimulated neutrophils had decreased phagocytic capacities and altered expression of CR1, CR3, Fc gamma RII, and Fc gamma RIII. For cases in which TLCK (N-alpha-p-tosyl-L-lysine chloromethyl ketone) neutralized the effects of the stimuli, the P. gingivalis-derived factors causing the phenomena seem to be trypsin-like proteases.

Human IgG Fc receptor heterogeneity: molecular aspects and clinical implications

Receptors for the Fc domain of IgG (Fc gamma R) provide a critical link between specific humoral responses and the cellular branch of the immune system. When hFc gamma R interact with immunoglobulin, a variety of biological responses are triggered. These include phagocytosis, endocytosis, antibody-dependent cellular cytotoxicity (ADCC), release of inflammatory mediators, and enhancement of antigen presentation. In the last few years our understanding of the Fc gamma receptor structure has increased dramatically, due to the availability of monoclonal antibodies (mAb) and cDNA probes. Fc gamma R are members of the immunoglobulin superfamily and three main classes, hFc gamma RI, hFc gamma RII, and hFc gamma RIII are recognized in man generating at least 12 different isoforms. A further level of complexity is introduced by various genetic polymorphisms and, importantly, recent evidence points at the relevance of this Fc gamma R heterogeneity.