A monoclonal antibody specific for immunoglobulin A receptor triggers polymorphonuclear neutrophil superoxide release

Rapid selective priming of FcalphaR on eosinophils by corticosteroids

Preactivation or priming of eosinophils by (proinflammatory) cytokines is important in the pathogenesis of allergic diseases. Several priming-dependent eosinophil responses, such as migration and adhesion, are reduced by treatment with corticosteroids. Many inhibitory effects of corticosteroids are mediated by the glucocorticoid receptor via genomic mechanisms, which are evident only after prolonged interaction (>30 min). However, also faster actions of corticosteroids have been identified, which occur in a rapid, nongenomic manner. In this study, fast effects of corticosteroids were investigated on the function of eosinophil opsonin receptors. Short term corticosteroid treatment of eosinophils for maximal 30 min with dexamethasone (Dex) did not influence eosinophil cell surface CD11b/CD18 expression, adhesion, and/or chemokinesis. In marked contrast, incubation with Dex resulted in a rapid increase in binding of IgA-coated beads to human eosinophils, showing that Dex can up-regulate the activation of FcalphaR (CD89). This priming response by Dex was dose dependent and optimal between 10(-8) and 10(-6) M and was mediated via the glucocorticoid receptor as its selective antagonist RU38486 (10(-6) M) blocked the priming effect. In contrast to FcalphaR, eosinophil FcgammaRII (CD32) was not affected by Dex. Further characterization of the Dex-induced inside-out regulation of FcalphaR revealed p38 MAPK as the central mediator. Dex dose dependently enhanced p38 MAPK phosphorylation and activation in situ as measured by phosphorylation of its downstream target mitogen-activated protein kinase-activated protein kinase 2. The dose responses of the Dex-induced activation of these kinases were similar as seen for the priming of FcalphaR. This work demonstrates that corticosteroids selectively activate the FcalphaR on eosinophils by activation of p38 MAPK.

IgA Fc receptor I signals apoptosis through the FcRγ ITAM and affects tumor growth

The IgA Fc receptor (FcαRI) has dual proinflammatory and anti-inflammatory functions that are transmitted through the immunoreceptor tyrosine-based activation motifs (ITAMs) of the associated FcRγ subunit. Whereas the involvement of FcαRI in inflammation is well documented, little is known of its anti-inflammatory mechanisms. Here we show that monomeric targeting of FcαRI by anti-FcαRI Fab or serum IgA triggers apoptosis in human monocytes, monocytic cell lines, and FcαRI+ transfectants. However, the physiologic ligand IgA induced apoptosis only when cells were cultured in low serum conditions, indicating differences with induction of anti-inflammatory signaling. Apoptosis signaling required the FcRγ ITAM, as cells transfected with FcαRI or with a chimeric FcαRI-FcRγ responded to death-activating signals, whereas cells expressing a mutated FcαRIR209L unable to associate with FcRγ, or an ITAM-mutated chimeric FcαRI-FcRγ, did not respond. FcαRI-mediated apoptosis signals were blocked by treatment with the pan-caspase inhibitor zVAD-fmk, involved proteolysis of procaspase-3, and correlated negatively with SHP-1 concentration. Anti-FcαRI Fab treatment of nude mice injected subcutaneously with FcαRI+ mast-cell transfectants prevented tumor development and halted the growth of established tumors. These findings demonstrate that, on monomeric targeting, FcαRI functions as an FcRγ ITAM-dependent apoptotic module that may be fundamental for controlling inflammation and tumor growth.

Protein kinase Balpha is required for vesicle trafficking and class II presentation of IgA Fc receptor (CD89)-targeted antigen

Ag presentation stimulates Ag-specific adaptive immune responses. FcalphaR (CD89)-mediated capture of IgA-bound exogenous Ag leads to efficient MHC class II Ag presentation by APCs. CD89 signaling is required for trafficking of internalized Ag to specialized multivesicular bodies known as MHC class II compartments (MIIC) and subsequent class II presentation. In the present study, we tested the hypothesis that the vesicle trafficking regulator protein kinase Balpha (PKBalpha) is required for CD89-mediated trafficking to MIIC and Ag presentation. We observed by two independent methods (chemical inhibitors and specific RNA interference) that PKBalpha was required for CD89 trafficking to MIIC and class II Ag presentation. Expression of constitutively active PKBalpha in APCs expressing a mutant CD89 accessory signaling molecule (deficient in CD89/Ag trafficking, processing, and presentation) induced trafficking of CD89 to lamp1-containing late endocytic vesicles, but not class II-containing vesicles (MIIC), or class II Ag presentation. These studies show for the first time that PKBalpha is required for receptor-mediated Ag presentation and suggest the mechanism of action includes regulation of vesicle trafficking.

Immunomodulatory properties of human serum immunoglobulin A: anti-inflammatory and pro-inflammatory activities in human monocytes and peripheral blood mononuclear cells

Our study investigated the immunomodulatory activities of human plasma-derived serum immunoglobulin (Ig)A. Previous findings seem contradictory indicating either pro- or anti-inflammatory activities. We used serum IgA purified from large plasma pools and studied the modulation of the release of cytokines and chemokines from resting and lipopolysaccharide (LPS, endotoxin)-stimulated human adherent monocytes and human peripheral blood mononuclear cells (PBMC). Our results indicate that IgA down-modulates the release of the pro-inflammatory chemokines monocyte chemoattractant protein (MCP) 1, macrophage inflammatory protein (MIP) 1alpha and MIP1beta from LPS-stimulated PBMC and the release of MCP1, MIP1alpha and MIP1beta from LPS-stimulated monocytes. Furthermore, we confirmed previous reports that plasma-derived serum IgA down-modulates the release of the pro-inflammatory cytokines, interleukin (IL)-6 and tumour necrosis factor (TNF)-alpha, from LPS-stimulated monocytes and PBMC, and up-regulates the release of IL-1 receptor antagonist (IL-1RA) from resting and LPS-stimulated monocytes and resting PBMC. This IgA-mediated up-regulation of IL-1RA is independent of the simultaneous up-regulation of IL-1beta release, as shown by blocking the biological activity of IL-1beta with a neutralizing antibody. On the other hand, we also found an IgA-induced pro-inflammatory activity, namely IgA-mediated up-regulation of the release of pro-inflammatory IL-1beta as well as down-regulation of the anti-inflammatory cytokines IL-10 and IL-12p40 from LPS-stimulated monocytes and PBMC and a down-regulation of transforming growth factor (TGF)-beta from resting and LPS-stimulated PBMC. We conclude that human serum IgA has both an anti-inflammatory and a pro-inflammatory capacity and this dual capacity might contribute to the feedback mechanisms maintaining a balance between pro-inflammatory and anti-inflammatory activities.

Critical role for complement receptor 3 (CD11b/CD18), but not for Fc receptors, in killing of Streptococcus pyogenes by neutrophils in human immune serum

During phagocytosis, surface receptors on neutrophils interact with pathogens opsonized with complement factor C3b/iC3b and in some cases with antibodies. In human immune sera antibodies directed against surface-bound M proteins mediated killing of Streptococcus pyogenes by neutrophils. Surprisingly, blocking of the Fc receptors had little effect on the killing. In contrast, inhibition of C3b/iC3b generation, or blocking of the major neutrophil iC3b receptor CD11b/CD18, enabled S. pyogenes to grow efficiently in immune sera. Inhibition of CD11b/CD18, but not of CD32, the major neutrophil signaling Fc receptor, prevented Streptococcus-induced NADPH oxidase-dependent respiratory burst, and blocking of C3b/iC3b formation inhibited Streptococcus-induced activation of Cdc42, a small GTPase critically involved in transmitting pro-inflammatory signals to the cytoskeleton. Consequently, ligation of CD11b/CD18 by bacteria-bound iC3b is necessary for inducing a neutrophil response leading to elimination of S. pyogenes in immune human serum.

IgA Fc receptors

The IgA receptor family comprises a number of surface receptors including the polymeric Ig receptor involved in epithelial transport of IgA/IgM, the myeloid specific IgA Fc receptor (FcalphaRI or CD89), the Fcalpha/muR, and at least two alternative IgA receptors. These are the asialoglycoprotein receptor and the transferrin receptor, which have been implicated in IgA catabolism, and tissue IgA deposition. In this review we focus on the biology of FcalphaRI (CD89). FcalphaRI is expressed on neutrophils, eosinophils, monocytes/macrophages, dendritic cells, and Kupffer cells. This receptor represents a heterogeneously glycosylated transmembrane protein that binds both IgA subclasses with low affinity. A single gene encoding FcalphaRI has been isolated, which is located within the leukocyte receptor cluster on chromosome 19. The FcalphaRI alpha chain lacks canonical signal transduction domains but can associate with the FcR gamma-chain that bears an activation motif (ITAM) in the cytoplasmic domain, allowing activatory functions. FcalphaRI expressed alone mediates endocytosis and recyling of IgA. No FcalphaRI homologue has been defined in the mouse, and progress in defining the in vivo role of FcalphaRI has been made using human FcalphaRI transgenic (Tg) mice. FcalphaRI-Tg mice demonstrated FcalphaRI expression on Kupffer cells and so defined a key role for the receptor in mucosal defense. The receptor functions as a second line of antibacterial defense involving serum IgA rather than secretory IgA. Studies in FcalphaRI-Tg mice, furthermore, defined an essential role for soluble FcalphaRI in the development of IgA nephropathy by formation of circulating IgA-FcalphaRI complexes. Finally, recent work points out a role for human IgA in treatment of infectious and neoplastic diseases.

Characteristics of polymeric lambda-IgA binding to leukocytes in IgA nephropathy

IgA nephropathy (IgAN) is characterized by predominant mesangial polymeric IgA1 (pIgA1) deposits, with increased plasma IgA1 levels. Plasma IgA levels are determined by the rate of IgA production, uptake by leukocytes, and removal by hepatocytes. Fc(alpha) receptor 1 (Fc(alpha)R1) is a candidate molecule for the regulation of IgA levels, but reports of its expression in leukocytes in IgAN are conflicting. Increased binding of endogenous IgA to circulating granulocytes and monocytes in IgAN was demonstrated in this study. Fc(alpha)R1 expression on leukocytes was increased, independently of plasma IgA levels. Fc(alpha)R1 was not saturated in leukocytes, because of internalization of IgA after uptake. Further binding of exogenous IgA isolated from individual subjects was observed with leukocytes from the same subjects. Compared with cells from control subjects, granulocytes but not monocytes from patients with IgAN exhibited a greater binding capacity for exogenous IgA, predominantly pIgA. To circumvent the possibility that endogenous IgA might alter Fc(alpha)R1 expression, granulocytes or monocytes derived from the HL-60 or U937 cell lines were used to explore the nature of IgA binding. A higher affinity for pIgA was demonstrated. Inhibition studies using unlabeled IgA, other serum proteins, or a specific Fc(alpha)R1-blocking antibody suggested binding mechanisms other than Fc(alpha)R1 for pIgA uptake by leukocytes. This study also suggested the migration and/or sequestration of "activated" leukocytes with predominant lambda-IgA in the mononuclear phagocytic system or inflammatory tissues, after the initial binding of lambda-pIgA. These immunologic abnormalities might contribute to the glomerulointerstitial injury in IgAN, in the presence of leukocytic infiltration.

Fc receptor-mediated immunity against Bordetella pertussis

The relevance of specific Abs for the induction of cellular effector functions against Bordetella pertussis was studied. IgG-opsonized B. pertussis was efficiently phagocytosed by human polymorphonuclear leukocytes (PMN). This process was mediated by the PMN IgG receptors, FcgammaRIIa (CD32) and FcgammaRIIIb (CD16), working synergistically. Furthermore, these FcgammaR triggered efficient PMN respiratory burst activity and mediated transfer of B. pertussis to lysosomal compartments, ultimately resulting in reduced bacterial viability. Bacteria opsonized with IgA triggered similar PMN activation via FcalphaR (CD89). Simultaneous engagement of FcalphaRI and FcgammaR by B. pertussis resulted in increased phagocytosis rates, compared with responses induced by either isotype alone. These data provide new insights into host immune mechanisms against B. pertussis and document a crucial role for Ig-FcR interactions in immunity to this human pathogen.

A role for scavenger receptors in phagocytosis of protein-coated particles in rainbow trout head kidney macrophages

In macrophages of higher vertebrates, Fc receptors and receptors for complement and other serum factors, are generally known to enhance the phagocytic process. In lower vertebrates like salmonid fishes, none of these or other phagocytic receptors have been thoroughly characterized. The purpose of this study was to elucidate to what extent these and other receptors are involved in the process of phagocytosis in rainbow trout (Oncorhynchus mykiss) head kidney macrophages. We used tosyl activated, paramagnetic dynabeads (2.8 microm in diameter), specifically coated with 125I labeled Atlantic salmon (Salmo salar) IgM or bovine serum albumin (BSA) as phagocytic probes. The effect of complement opsonization was also investigated by incubating the beads in serum. Our results indicate that neither the Fc- nor the complement-receptor(s) were important for phagocytosis of these beads. Our data support the idea that scavenger receptors are involved in phagocytosis in rainbow trout head kidney macrophages, as the use of a competitive scavenger receptor ligand extensively decreased degradation of the labeled protein coat on the beads.

Human T cell IgD receptors react with O-glycans on both human IgD and IgA1

Previous studies on murine T cell IgD-R have shown that these receptors recognize N-glycans of murine IgD, and not of other Ig isotypes. We have now studied the specificity of IgD-R on human T cells. Human IgD digested with proteinase K to fragments of < 5 kDa inhibit the ability of T cells to form rosettes with IgD-coated ox erythrocytes. The same amount of digested IgG does not. We tested all the human Ig isotypes: IgG1, -2, -3, -4, IgA2, IgE and IgM fail to inhibit significantly at 20 microg/assay. However, IgA1 is as effective as IgD itself, showing approximately 60 % and 80 % inhibition at 5 microg and 10 microg/assay. Human IgA1 and IgD both contain Gal-1 --> 3-GalNac-rich O-linked glycans, and on this basis are both bound to ricin and jacalin. The O-linked glycans may therefore also represent the common moiety binding to IgD-R. Disaccharides Gal-1 --> 3-GalNac, and Gal-1 --> 4-Glc at 10 microg/assay blocked IgD rosetting while Gal-1 --> 6-Glc did not. We conclude that the human IgD-R is a lectin, differing from the murine IgD-R in that it has both IgA1 and IgD as ligands.

Bispecific Molecules Directed to the Fc Receptor for IgA (FcαRI, CD89) and Tumor Antigens Efficiently Promote Cell-Mediated Cytotoxicity of Tumor Targets in Whole Blood

The FcR for IgA (FcαRI, CD89) is primarily expressed on cytotoxic immune effector cells. By chemically cross-linking F(ab′) fragments of the FcR for IgA (FcαRI)-specific mAb (A77) with tumor Ag-specific mAb (anti-HER2/neu and anti-epidermal growth factor receptor), we have developed bispecific molecules (BSM) that simultaneously bind to respective tumor Ags and FcαRI-expressing effector cells in whole blood. These BSM mediated up to 55% of specific lysis of appropriate tumor Ag-expressing target cells (from a variety of tumors) with purified polymorphonuclear leukocytes, monocytes, or whole blood effector cells without preactivation with exogenous cytokines. To our knowledge, this is the first demonstration of Ab-dependent cell-mediated cytotoxic activity via FcαRI in whole blood. Also, monocyte-derived macrophages mediated phagocytosis of HER2/neu-expressing tumor cells (&gt;95% tumor cell loss). These BSM-mediated cytotoxic activities were completely inhibited by F(ab′)2 of A77, demonstrating the specific role of FcαRI as a trigger molecule. Furthermore, the binding of these BSM to monocytes or polymorphonuclear leukocytes in whole blood did not induce modulation of FcαRI in the absence of the target Ag. Therefore, immune effector cells may be “armed” with FcαRI-directed BSM in whole blood. These FcαRI-directed BSM may offer new treatment options for various malignancies and other disease conditions.

Glomerular Fc alphaR expression and disease activity in IgA nephropathy

In this study, we examined the receptors for the Fc portion of immunoglobulin A (IgA) (Fc alphaR) in the glomeruli as well as circulating polymorphonuclear leukocytes and monocytes at the mRNA level by reverse transcription-polymerase chain reaction (RT-PCR) assay and at the protein level by an immunohistochemistry/flow cytometry technique using a specific anti-Fc alphaR monoclonal antibody (My 43). Glomeruli were isolated from biopsy specimens of renal tissues from IgA nephropathy (IgAN; 20 cases) and non-IgA mesangial proliferative glomerulonephritis (PGN; 13 cases) patients, and from normal renal tissue specimens obtained from kidneys removed because of malignancies (five cases) applying the microdissection method. There was a relative increase in Fc alphaR in the circulating phagocytes from IgAN patients compared with those from PGN and healthy controls. Fc alphaR expression was present in approximately 40% of glomeruli samples from IgAN patients at the message levels. Fc alphaR-positive specimens were also strongly positive for expression of tumor necrosis factor-alpha, interleukin-1, and interleukin-6 mRNA. Specimens from PGN patients and healthy controls did not show any detectable Fc alphaR message. Serum IgA levels and severity of hematuria were significantly higher in patients with positive Fc alphaR expression. A message for Fc alphaR was detected in the tissues that were more damaged histologically. Our data suggest that there is some in vivo induction of glomerular Fc alphaR expression, possibly mediated by a synergistic stimulus from IgA and inflammatory cytokines, and the expressed receptor is likely to be involved in the disease process of IgAN.

Size-dependent effect of IgA on the IgA Fc receptor (CD89)

The IgA Fc receptor (FcR; CD89) is expressed on several types of cells of the myeloid cell lineage. We investigated whether different sizes of heat-aggregated IgA (aIgA) bind to CD89 and subsequently induce cellular activation. As a model we used the murine B cell line IIA1.6 transfected with CD89 or IIA1.6 cells transfected with CD89 as well as with the FcR gamma chain to study the binding of IgA to CD89. When these cells expressing CD89 were incubated with monomeric IgA, no significant binding of IgA to the cells was detectable by fluorescence-activated cell sorter analysis; however, incubation of the cells with aggregated IgA resulted in 93 +/- 2% positive cells. Incubation of the cells with different sizes of IgA-containing aggregates revealed optimal binding with aggregates containing five to six molecules of IgA per aggregate. No difference was observed between the binding to CD89 of both IgA1- or IgA2-containing aggregates. Furthermore, the binding of aIgA was found to be CD89-specific, since the binding of IgA was completely inhibited by the CD89-specific monoclonal antibody My43 and no detectable binding occurred to the IIA1.6 parent cell line. Activation studies using interleukin-2 (IL-2) production as a marker, showed that the FcR gamma chain is necessary to induce cellular activation. Only cells transfected with both CD89 and the FcR gamma chain (CD89+/gamma +) enhance the IL-2 production 10-12-fold upon stimulation with aggregates of IgA. Furthermore, triggering of CD89 only results in increase of intracellular calcium concentration ([Ca2+]i) in cells co-expressing FcR gamma chain. Mutation of the tyrosine residues in the FcR gamma chain immunoreceptor tyrosine-based activation motif of the FcR gamma chain abolishes this increase in [Ca2+]i, indicating association and involvement of the FcR gamma chain in CD89-mediated signaling.

IgM monoclonal antibodies recognizing Fc alpha R but not Fc gamma RIII trigger a respiratory burst in neutrophils although both trigger an increase in intracellular calcium levels and degranulation

The role of IgG receptor, Fc gamma RIII, in the triggering of neutrophil functions has been controversial. Here we show that IgM monoclonal antibodies, Leu 11b and 1D3, recognizing Fc gamma RIII, bind to human neutrophils triggering an increase in intracellular calcium concentration and release of myeloperoxidase upon degranulation but do not trigger a respiratory burst detectable as lucigenin-enhanced chemiluminescence. Although many fewer molecules of IgM monoclonal antibody, MY43, recognizing Fc alpha R, bind to the same cells they trigger a much greater increase in intracellular calcium concentration, release of myeloperoxidase and a strong respiratory burst. Since the respiratory bursts triggered by IgG and IgA are equivalent, this demonstrates that Fc gamma RII is responsible for the IgG-mediated response. IgM monoclonal antibody MC2, recognizing the abundant neutrophil cell-surface carbohydrate CD15, also triggers a small rise in intracellular calcium but no respiratory burst.

Fc alpha R expression on polymorphonuclear leukocyte and superoxide generation in IgA nephropathy

Superoxide (O2-) production and Fc alpha R antigen expression of circulating polymorphonuclear leukocytes (PMNL) isolated from patients with IgA nephropathy (IgAN) and non-IgA mesangial proliferative glomerulonephritis (PGN) and healthy volunteers were investigated to establish their biological importance in the immunopathogenesis of mesangial proliferative glomerulonephritis. PMNL from both patient groups showed increased O2- production when stimulated with N-formyl methionyl leucyl phenylalanine (FMLP) and phorbol myristate acetate (PMA). The increased O2- generation demonstrated a positive correlation with the degree of proteinuria. Aggregated IgA caused enhanced O2- production only in patients with IgAN who also showed a significant correlation with proteinuria. Increased expression of Fc alpha R on circulating PMNL was observed in IgAN patients as determined by flow cytometric analysis. The amount of Fc alpha R on PMNL was positively correlated with O2- generation triggered with IgA aggregates. These results suggest that: 1. Circulating PMNL may potentially be participating in the pathogenesis of glomerular injury in mesangial proliferative glomerulonephritis, and 2. IgA aggregates/immune complexes may contribute to the immunopathogenesis of IgAN through augmenting the Fc alpha receptor-mediated generation of superoxide anion.

Stimulation of neutrophil adhesion by antibodies recognizing CD15 (Le(X)) and CD15-expressing carcinoembryonic antigen-related glycoprotein NCA-160

The carbohydrate antigen, CD15 (Le(X)), and its sialylated derivative have recently been shown to be involved in the binding of neutrophils to the endothelial lectins, E-selectin and P-selectin. Neutrophil NCA-160, a carcinoembryonic antigen (CEA)-related glycoprotein, is the major carrier of CD15, which is also expressed on the common beta 2 chain of leucocyte integrins. Rabbit IgG antibodies directed against CEA, which cross-react with neutrophil NCAs, increase the adhesion of neutrophils to plastic. This effect is also observed with F(ab')2 and Fab antibody fragments and a monoclonal antibody (mAb) recognizing the same antigen. Anti-CD15 mAbs inhibit adhesion at higher concentrations, but augment adhesion at lower concentrations. Anti-CEA and anti-CD15 antibodies cause the homotypic adhesion of neutrophils demonstrable by light microscopy and flow cytometry. Anti-(integrin beta 2 chain) mAbs inhibit both adhesion to plastic and homotypic adhesion. These results suggest that binding of ligand to NCA-160 is able to trigger neutrophil adhesion events which have been shown to be integrin mediated. Anti-CD15 mAbs do not, however, induce a respiratory burst from neutrophils.

Receptors for IgA on phagocytic cells

IgA receptors have been detected on monocytes, polymorphonuclear neutrophils (PMNs) and eosinophils, and on phagocytic cells at mucosal sites. These receptors bind both secretory and serum forms of immunoglobulin A (IgA) and require the Ca2 region of the IgA molecule for ligand recognition. Monocytes and PMNs modulate their expression of the IgA receptor upon treatment with cytokines, such as granulocyto-macrophage colony-stimulating factor, and lipopolysaccharide. Purified IgA receptors appear as heavily glycosylated molecules with an average molecular weight of 60 kD, dropping to 32 and 36 kD upon treatment with N-glycanase. The cDNA sequence encoding the IgA receptor has been determined by expression cloning, and predicts that the receptor consists of two Ig-like extracellular domaines, a transmembrane region and a cytoplasmic tail of 41 residues. Ligation of IgA receptors on phagocytic cells by multivalent IgA complexes induces a variety of responses, including superoxide generation, release of inflammatory mediators, phagocytosis, and killing of various pathogenic microorganisms. Thus the apparent role of these receptors is to amplify the protective effects of the IgA antibody, a function of potential importance to mucosal defense.