Biphasic regulation of Fc-receptor mediated phagocytosis of rabbit alveolar macrophages by surfactant phospholipids.
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Dipalmitoyl phosphatidylcholine (DPPC) is a major phospholipid constituent in the pulmonary surfactant, whereas lysophosphatidylcholine (Lyso-PC) is a minor constituent, this membrane phospholipid being produced at inflammatory sites in association with activation of phospholipase A2. To determine the role of these two different forms of phospholipids in the phagocytic function of alveolar macrophages (AM), we examined the effects of DPPC or Lyso-PC on Fc-mediated phagocytosis. We demonstrated a significant decrease of the ingestion activity of AM for anti-sheep erythrocyte immunoglobulin G-coated sheep erythrocytes (EA: IgG) by DPPC. On the other hand, Lyso-PC caused significantly increased ingestion of EA: IgG by AM. These data indicate that increase of Lyso-PC due to the hydrolysis of DPPC through activation of phospholipase A, may up-regulate AM-mediated phagocytic functions in the alveolar milieu associated with infections and inflammation. DPPC may suppress and stabilize the AM-mediated phagocytosis in the normal alveolar environment. (+info)
Characterization and localization of the neonatal Fc receptor in adult human kidney.
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The binding of Fc fragments of Ig on glomerular epithelial cells (GEC) was observed previously, but the receptor could not be identified. In immunofluorescence and immunohistochemical studies using normal adult human kidney sections, the presence of the so-called neonatal Fc receptor (FcRn) was demonstrated on GEC as well as in the brush border of proximal tubular cells. FcRn transcripts were also detected on isolated glomeruli by reverse transcription-PCR. Using an immortalized GEC line, the presence of the FcRn was confirmed by flow cytometry, reverse transcription-PCR, Western blotting, and by the pH dependence of the binding of heat-aggregated IgG. Because it is well established that the FcRn is involved in IgG transcytosis, it is hypothesized that the FcRn in the kidney may play a role in the reabsorption of IgG. Ongoing studies should clarify the role of the FcRn as a potential target for immune complexes on GEC and should assess its relevance in physiology and pathology. (+info)
Identification of a novel Fcalpha receptor expressed by human mesangial cells.
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BACKGROUND: IgA nephropathy (IgAN) is characterized by mesangial deposits of polymeric IgA (pIgA). The pathological consequences of IgA deposition are believed to center on direct interaction between IgA and the glomerular mesangial cell (MC). We have characterized a novel mesangial receptor that recognizes the Fc portion of IgA. METHODS: Five primary MC cultures were evaluated for IgA binding by flow cytometry, and specificity of binding was determined by competitive inhibition. Relative affinities of the receptor for all IgA isoforms were also determined, and binding of pIgA1 was compared to monomer. The identified Fc receptor was then compared with CD89, hitherto the only other Fcalpha receptor reported. CD89 protein and mRNA expression were detected by conventional and intracellular flow cytometry, sequencing of reverse transcription-polymerase chain reaction (RT-PCR) products, and Northern blotting. RESULTS: All MCs constitutively expressed a receptor that bound IgA in an Fcalpha-dependent fashion. The receptor recognized secretory and serum IgA1 and IgA2 equally, but pIgA bound with much greater affinity than monomer. At no time were we able to detect CD89 synthesis, although three novel CD89-related mRNA transcripts were identified by RT-PCR. CONCLUSIONS: We have clearly demonstrated that MCs consistently express an FcalphaR distinct from the myeloid FcalphaR CD89. This novel receptor binds pIgA with high affinity and may therefore mediate the mesangial injury that follows IgA deposition in IgAN. While immunogenically distinct, the mesangial Fcalpha receptor may share some molecular homology with CD89, as mRNA transcripts with partial identity to CD89 were found in all five MC cultures. (+info)
Human intestinal epithelial cells express a novel receptor for IgA.
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Binding and transport of polymeric Igs (pIgA and IgM) across epithelia is mediated by the polymeric Ig receptor (pIgR), which is expressed on the basolateral surface of secretory epithelial cells. Although an Fc receptor for IgA (FcalphaR) has been identified on myeloid cells and some cultured mesangial cells, the expression of an FcalphaR on epithelial cells has not been described. In this study, binding of IgA to a human epithelial line, HT-29/19A, with features of differentiated colonic epithelial cells, was examined. Radiolabeled monomeric IgA (mIgA) showed a dose-dependent, saturable, and cation-independent binding to confluent monolayers of HT-29/19A cells. Excess of unlabeled mIgA, but not IgG or IgM, competed for the mIgA binding, indicating that the binding was IgA isotype-specific and was not mediated by the pIgR. The lack of competition by asialoorosomucoid and the lack of requirement for divalent cations excluded the possibility that IgA binding to HT-29/19A cells was due to the asialoglycoprotein receptor or beta-1, 4-galactosyltransferase, previously described on HT-29 cells. Moreover, the FcalphaR (CD89) protein and message were undetectable in HT-29/19A cells. FACS analysis of IgA binding demonstrated two discrete populations of HT-29/19 cells, which bound different amounts of mIgA. IgA binding to other colon carcinoma cell lines was also demonstrated by FACS analysis, suggesting that an IgA receptor, distinct from the pIgR, asialoglycoprotein receptor, galactosyltransferase, and CD89 is constitutively expressed on cultured human enterocytes. The function of this novel IgA receptor in mucosal immunity remains to be elucidated. (+info)
The IgG Fc contains distinct Fc receptor (FcR) binding sites: the leukocyte receptors Fc gamma RI and Fc gamma RIIa bind to a region in the Fc distinct from that recognized by neonatal FcR and protein A.
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The CH2-CH3 interface of the IgG Fc domain contains the binding sites for a number of Fc receptors including Staphylococcal protein A and the neonatal Fc receptor (FcRn). It has recently been proposed that the CH2-CH3 interface also contains the principal binding site for an isoform of the low affinity IgG Fc receptor II (Fc gamma RIIb). The Fc gamma RI and Fc gamma RII binding sites have previously been mapped to the lower hinge and the adjacent surface of the CH2 domain although contributions of the CH2-CH3 interface to binding have been suggested. This study addresses the question whether the CH2-CH3 interface plays a role in the interaction of IgG with Fc gamma RI and Fc gamma RIIa. We demonstrate that recombinant soluble murine Fc gamma RI and human Fc gamma RIIa did not compete with protein A and FcRn for binding to IgG, and that the CH2-CH3 interface therefore appears not to be involved in Fc gamma RI and Fc gamma RIIa binding. The importance of the lower hinge was confirmed by introducing mutations in the proposed binding site (LL234,235AA) which abrogated binding of recombinant soluble Fc gamma RIIa to human IgG1. We conclude that the lower hinge and the adjacent region of the CH2 domain of IgG Fc is critical for the interaction between Fc gamma RIIa and human IgG, whereas contributions of the CH2-CH3 interface appear to be insignificant. (+info)
Differential effect of cytokine treatment on Fc alpha receptor I- and Fc gamma receptor I-mediated tumor cytotoxicity by monocyte-derived macrophages.
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Macrophages represent an important effector cell for Ab-mediated tumor therapy. Previous studies have documented that cytokines can influence Fc receptor (FcR) expression and function. In this study we examined the tumoricidal activities of the type I receptors for IgG (Fc gamma RI) and the IgA FcR (Fc alpha RI) on monocyte-derived macrophages (MDM) cultured in the presence of IFN-gamma, M-CSF, or GM-CSF. Bispecific Abs were used to target a Her2/neu breast carcinoma cell line, SKBR-3, to Fc alpha RI or Fc gamma RI on MDM. Although Fc alpha RI and Fc gamma RI share a common signaling pathway contingent on association with the gamma-chain (FcR gamma subunit), a marked difference in their efficiency in mediating tumoricidal functions was seen in response to specific cytokines. M-CSF- and GM-CSF-treated MDM mediated efficient phagocytosis of SKBR-3 cells by Fc alpha RI and Fc gamma RI; however, IFN-gamma-treated MDM phagocytosed tumor cells only with the Fc gamma RI-directed bispecific Abs. Similarly, IFN-gamma-cultured MDM lysed tumor cells more efficiently via Fc gamma RI then by Fc alpha RI as measured in Ab-dependent cellular cytotoxicity assays. Conversely, GM-CSF-treated MDM mediated more efficient lysis of tumor cells via Fc alpha RI than Fc gamma RI, while M-CSF-cultured MDM were relatively less efficient in mediating Ab-dependent cellular cytotoxicity through either receptor. With the exception of IFN-gamma-mediated enhancement of Fc gamma RI expression and Fc gamma RI gamma-chain complexes, the regulation of Fc gamma RI- or Fc alpha RI-mediated activity occurred without significant change in either receptor expression or total complexes with gamma subunit. These data suggest that the efficiency of Ab-mediated tumor therapy, which depends on FcR effector cell functions, may be modified by the use of specific cytokines. (+info)
Fcalpha receptor (CD89) mediates the development of immunoglobulin A (IgA) nephropathy (Berger's disease). Evidence for pathogenic soluble receptor-Iga complexes in patients and CD89 transgenic mice.
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The pathogenesis of immunoglobulin A (IgA) nephropathy (IgAN), the most prevalent form of glomerulonephritis worldwide, involves circulating macromolecular IgA1 complexes. However, the molecular mechanism(s) of the disease remain poorly understood. We report here the presence of circulating soluble FcalphaR (CD89)-IgA complexes in patients with IgAN. Soluble CD89 was identified as a glycoprotein with a 24-kD backbone that corresponds to the expected size of CD89 extracellular domains. To demonstrate their pathogenic role, we generated transgenic (Tg) mice expressing human CD89 on macrophage/monocytes, as no CD89 homologue is found in mice. These mice spontaneously developed massive mesangial IgA deposition, glomerular and interstitial macrophage infiltration, mesangial matrix expansion, hematuria, and mild proteinuria. The molecular mechanism was shown to involve soluble CD89 released after interaction with IgA. This release was independent of CD89 association with the FcRgamma chain. The disease was induced in recombination activating gene (RAG)2(-/-) mice by injection of serum from Tg mice, and in severe combined immunodeficiency (SCID)-Tg mice by injection of patients' IgA. Depletion of soluble CD89 from serum abolished this effect. These results reveal the key role of soluble CD89 in the pathogenesis of IgAN and provide an in vivo model that will be useful for developing new treatments. (+info)
The ADP-ribosylating CTA1-DD adjuvant enhances T cell-dependent and independent responses by direct action on B cells involving anti-apoptotic Bcl-2- and germinal center-promoting effects.
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We recently developed a novel immunomodulating gene fusion protein, CTA1-DD, that combines the ADP-ribosylating ability of cholera toxin (CT) with a dimer of an Ig-binding fragment, D, of Staphylococcus aureus protein A. The CTA1-DD adjuvant was found to be nontoxic and greatly augmented T cell-dependent responses to soluble protein Ags after systemic as well as mucosal immunizations. Here we show that CTA1-DD does not appear to form immune complexes or bind to soluble Ig following injections, but, rather, it binds directly to B cells of all isotypes, including naive IgD+ cells. No binding was observed to macrophages or dendritic cells. Immunizations in FcepsilonR (common FcRgamma-chain)- and FcgammaRII-deficient mice demonstrated that CTA1-DD exerted unaltered enhancing effects, indicating that FcgammaR-expressing cells are not required for the adjuvant function. Whereas CT failed to augment Ab responses to high m.w. dextran B512 in athymic mice, CTA1-DD was highly efficient, demonstrating that T cell-independent responses were also enhanced by this adjuvant. In normal mice both CT and CTA1-DD, but not the enzymatically inactive CTA1-R7K-DD mutant, were efficient enhancers of T cell-dependent as well as T cell-independent responses, and both promoted germinal center formation following immunizations. Although CT augmented apoptosis in Ag receptor-activated B cells, CTA1-DD strongly counteracted apoptosis by inducing Bcl-2 in a dose-dependent manner, a mechanism that was independent of the CD19 coreceptor. However, in the presence of CD40 stimulation, apoptosis was low and unaffected by CT, suggesting that the adjuvant effect of CT is dependent on the presence of activated CD40 ligand-expressing T cells. (+info)