Co-cross-linking of surface immunoglobulin Fc gamma receptors on B lymphocytes uncouples the antigen receptors from their associated G protein

SNPs of Fc-gamma receptor genes and chronic periodontitis

Genetic polymorphisms of host defense genes might increase risk for periodontitis. This case-control study investigated whether 102 single-nucleotide polymorphisms (SNPs) in Fc gamma receptor (FCGR) genes are associated with chronic periodontitis. Cases were 190 Hong Kong Chinese patients with severe chronic periodontitis, and there were 169 periodontitis-free individuals as controls. Genotype CC and allele C of rs445509 (p < 0.0001, OR = 0.30) of FCGR3A were significantly more prevalent among control individuals than among cases, after adjustment for age, sex, and smoking. Smoking was significantly associated with chronic periodontitis (p = 0.003, OR = 3.35). Within the limitations of this study, SNP rs445509 of FCGR3A seem to be negatively associated, while smoking was positively associated, with severe chronic periodontitis among Hong Kong Chinese.

FcgammaRIIb-mediated negative regulation of BCR signalling is associated with the recruitment of the MAPkinase-phosphatase, Pac-1, and the 3'-inositol phosphatase, PTEN

The low-affinity receptor for IgG, FcgammaRIIb, negatively regulates B cell antigen receptor (BCR)-mediated proliferative signalling. FcgammaRIIb has been reported to mediate this inhibition by uncoupling the BCR from the RasMAPkinase pathway. We now show that FcgammaRIIb-mediated negative feedback inhibition also correlates with induction of an Erk-associated phosphatase activity that reflects the rapid association of Erk and the MAPkinase phosphatase, Pac-1, and dephosphorylation and inactivation of ErkMAPkinase. This mechanism of abrogating ongoing ErkMAPkinase signalling therefore provides a rationale for rapid immune-complex-mediated feedback inhibition of active antigen-driven B cell responses. In addition, FcgammaRIIb signalling also induces the recruitment and activation of the 3'-inositol phosphatase, PTEN, which by antagonising PI 3kinase activity and inhibiting BCR-coupling to the anti-apoptotic kinase, Akt, provides an additional mechanism for FcgammaRIIb-mediated negative regulation of BCR-coupling to ErkMAPkinase, cell survival and proliferation.

Positive versus negative signaling by lymphocyte antigen receptors

Antigen receptors on lymphocytes play a central role in immune regulation by transmitting signals that positively or negatively regulate lymphocyte survival, migration, growth, and differentiation. This review focuses on how opposing positive or negative cellular responses are brought about by antigen receptor signaling. Four types of extracellular inputs shape the response to antigen: (a) the concentration of antigen; (b) the avidity with which antigen is bound; (c) the timing and duration of antigen encounter; and (d) the association of antigen with costimuli from pathogens, the innate immune system, or other lymphocytes. Intracellular signaling by antigen receptors is not an all-or-none event, and these external variables alter both the quantity and quality of signaling. Recent findings in B lymphocytes have clearly illustrated that these external inputs affect the magnitude and duration of the intracellular calcium response, which in turn contributes to differential triggering of the transcriptional regulators NF kappa B, JNK, NFAT, and ERK. The regulation of calcium responses involves a network of tyrosine kinases (e.g. lyn, syk), tyrosine or lipid phosphatases (CD45, SHP-1, SHIP), and accessory molecules (CD21/CD19, CD22, FcR gamma 2b). Understanding the biochemistry and logic behind these integrative processes will allow development of more selective and efficient pharmaceuticals that suppress, modify, or augment immune responses in autoimmunity, transplantation, allergy, vaccines, and cancer.

Multiple regions of human Fc gamma RII (CD32) contribute to the binding of IgG

The low affinity receptor for IgG, Fc gamma RII (CD32), has a wide distribution on hematopoietic cells where it is responsible for a diverse range of cellular responses crucial for immune regulation and resistance to infection. Fc gamma RII is a member of the immunoglobulin superfamily, containing an extracellular region of two Ig-like domains. The IgG binding site of human Fc gamma RII has been localized to an 8-amino acid segment of the second extracellular domain, Asn154-Ser161. In this study, evidence is presented to suggest that domain 1 and two additional regions of domain 2 also contribute to the binding of IgG by Fc gamma RII. Chimeric receptors generated by exchanging the extracellular domains and segments of domain 2 between Fc gamma RII and the structurally related Fc epsilon RI alpha chain were used to demonstrate that substitution of domain 1 in its entirety or the domain 2 regions encompassing residues Ser109-Val116 and Ser130-Thr135 resulted in a loss of the ability of these receptors to bind hIgG1 in dimeric form. Site-directed mutagenesis performed on individual residues within and flanking the Ser109-Val116 and Ser130-Thr135 domain 2 segments indicated that substitution of Lys113, Pro114, Leu115, Val116, Phe129, and His131 profoundly decreased the binding of hIgG1, whereas substitution of Asp133 and Pro134 increased binding. These findings suggest that not only is domain 1 contributing to the affinity of IgG binding by Fc gamma RII but, importantly, that the domain 2 regions Ser109-Val116 and Phe129-Thr135 also play key roles in the binding of hIgG1. The location of these binding regions on a molecular model of the entire extracellular region of Fc gamma RII indicates that they comprise loops that are juxtaposed in domain 2 at the interface with domain 1, with the putative crucial binding residues forming a hydrophobic pocket surrounded by a wall of predominantly aromatic and basic residues.

Human immunoglobulin (IgG) induced deletion of IgM rheumatoid factor B cells in transgenic mice

The singular ability of immunoglobulin genes to hypermutate their variable regions, while permitting the generation of high-affinity antibodies against foreign antigens, poses a problem in terms of maintenance of immunological self-tolerance. Immunoglobulin gene hypermutation driven by a foreign antigen has the potential to generate antibodies that cross-react with self-components. Consequently, there must exist a mechanism in the periphery for inactivation of mature autoreactive B cell clones. The classical experimental system used to address this problem is the induction of tolerance to soluble, deaggregated human IgG. We have analyzed the mechanism of induction of tolerance to human IgG using transgenic mice that express a human IgM rheumatoid factor (IgM RF) on a large proportion of their B cells. Injection of deaggregated human IgG caused a specific deletion of those B cells that express an intact IgM RF on their cell surface. The degree of RF B cell deletion was proportional to the reduction in the proliferative response of splenocytes to antigen (aggregated human IgG), or to F(ab')2 fragments of anti-human IgM antibodies. Control experiments showed that IgG administration had little effect on the numbers of mouse Ig-bearing cells or their ability to proliferate to a nonspecific mitogen. Thus, the effects of IgG on the human IgM RF B cell are antigen specific and are not due to nonspecific toxic effects of the human IgG preparation. These experiments demonstrate that peripheral exposure to IgG induces deletion of reactive B cells, without any evidence for anergy, and differ from data obtained by other investigators studying tolerance to soluble protein antigens. The results imply that human Igs have distinct properties as soluble antigens, and that peripheral nonresponsiveness to IgG may be due to lymphocyte deletion.

Interaction of signaling molecules with human Fc gamma RIIb1 and the role of various Fc gamma RIIb isoforms in B-cell regulation

The low-affinity type-IIb IgG Fc-binding receptors (Fc gamma RIIb) are expressed on B cells. When cross-linked with mIgM Fc gamma RIIb are known to down-regulate B-cell activation by interrupting signal transduction upstream from G-protein-activated events. We have studied Fc gamma RII isoforms expressed on resting and activated B cells and the interaction of Fc gamma RIIb1 with molecules transducing the antigen receptor-mediated signals. Expression of Fc gamma RII isoforms was studied by reverse transcription and polymerase chain reaction. Resting B cells express both Fc gamma RIIb2 and Fc gamma RIIb1 isoforms. Activation with anti-IgM or IL-4 induces the splicing of Fc gamma RIIb1 mRNA, while the alternative splicing of Fc gamma RIIb2 mRNA is down-regulated, resulting in the surface expression of Fc gamma RIIb1. Functional differences were found between the two isoforms in inhibiting B-cell activation, suggesting that Fc gamma RIIb2 might influence the threshold of signals necessary for activation of resting B cells, while Fc gamma RIIb1 may regulate in later phases of antibody response. To explore the mechanism by which Fc gamma RII may uncouple antigen receptor-mediated signal transduction, we have investigated the association of signaling molecules with Fc gamma RII. Beside the protein tyrosine kinase (PTK) fyn, protein kinase C (PKC) was found to be co-isolated with Fc gamma RIIb1, suggesting a tight connection between these kinases and Fc gamma RII. We suggest that PKC might be responsible for the activation-induced phosphorylation of Fc gamma RII on serine residues.(ABSTRACT TRUNCATED AT 250 WORDS)

Co-ligation of ICAM-1 (CD54) and membrane IgM negatively affects B cell receptor signaling

A possible role of intercellular adhesion molecule 1 (ICAM-1, CD54) in transmembrane signaling was investigated in B cells from the Burkitt lymphoma cell line MTLM4. Cross-linking of membrane IgM (mIgM) induced an increase in intracellular free Ca2+ as a result of the release from intracellular stores and an influx of extracellular Ca2+. When the B cells were incubated with limiting concentrations of anti-IgM, co-ligation of mIgM and CD54, but not CD19, resulted in an inhibition of the Ca2+ response. Separate cross-linking of mIgM and CD54 under these conditions, using isotype mismatched monoclonal antibodies (mAb), did not affect the mobilization of Ca2+. The CD54-mediated inhibition of the Ca2+ response was also observed in the absence of extracellular Ca2+. All CD54 mAb tested (F10.2, F10.3 and F7.11) interfered with mIgM signaling. The results presented in this report imply that CD54 is linked to intracellular signaling pathways and, via co-ligation with mIgM, interferes in the release of Ca2+ from intracellular stores.

The alternative splicing of human Fc gamma RII mRNA is regulated by activation of B cells with mIgM cross-linking, interleukin-4, or phorbolester

The human type two IgG binding receptors (Fc gamma RII) are encoded by three genes (Fc gamma RIIA, -B and C) resulting in at least six protein isoforms generated by alternative mRNA splicing. Surface expression of Fc gamma RII has been shown to be modulated during B cell activation, although data characterizing the isoform(s) expressed are not available. The extracellular as well as the transmembrane domains of various Fc gamma RII are highly homologous. Only the intracellular domains vary between the different Fc gamma RII isoforms, suggesting differences in signal transduction. Using reverse transcriptase and polymerase chain reaction of mRNA obtained from resting tonsil B cells, we show that the majority of Fc gamma RII mRNA species to be of b2 type, although b1 type and a low level of Fc gamma RIIa type are also present. Culturing the cells for 18 h in the presence of 2.5 U/ml interleukin-4 or 10 micrograms/ml affinity-purified anti-IgM F(ab')2 fragments induced a switch in alternative splicing, resulting in a significant increase of Fc gamma RIIb1 mRNA expression, while the synthesis of Fc gamma RIIb2 mRNA was down-regulated. Stimulation of B cells with 100 ng/ml phorbol 12-myristate 13-acetate induced similar alteration, although only after 48-h treatment. The accumulation of Fc gamma RIIb1 and the reduction of both Fc gamma RIIb2 and Fc gamma RIIa mRNA in activated cells is accompanied by the enhanced expression of Fc gamma RII on the cell surface, representing most probably the Fc gamma RIIb1 isoform. Heat-aggregated IgG inhibited the anti-IgM-induced proliferation of resting but not that of activated B cells, suggesting that aggregation of Fc gamma RIIb2 constitutively expressed on resting B cells might be responsible for the prevention of inadequate activation of resting B cells.

Intravenous immunoglobulins suppress immunoglobulin productions by suppressing Ca(2+)-dependent signal transduction through Fc gamma receptors in B lymphocytes

A high dose intravenous immunoglobulin (IVIG) therapy is used in the treatment of a wide range of autoimmune disorders. However, the mechanisms of the action of IVIGs remain poorly understood. To analyse the mechanisms of effects of IVIGs on immunoglobulin (Ig) production of B cells, the effects of IVIGs on B lymphoblastoid cell lines transformed by Epstein-Barr virus (LCLs) were investigated. The productions of IgG or IgM of LCLs were dose-dependently suppressed by polyethylene glycol (PEG)-treated IVIG or pH 4-treated IVIG though the productions were not or only slightly suppressed by pepsin-treated IVIG. The suppression by IVIGs was blocked by anti-human IgG Fc or anti-Fc gamma RII. C mu gene expression and mu s C terminal gene expression of LCLs were suppressed by PEG-treated IVIG, whereas neither C mu gene expression nor mu s C terminal gene expression of LCLs were suppressed by pepsin-treated IVIG. Although the increase in intracellular calcium concentration in LCLs was not suppressed by pepsin-treated IVIG, the increase was suppressed by PEG-treated IVIG. This suppressing effect of PEG-treated IVIG on intracellular calcium concentration of LCLs was blocked by anti-human IgG Fc or anti- Fc gamma RII. Our results suggest that IVIGs suppressed the Ca(2+)-dependent signal transduction through Fc gamma R on B-cell membrane, consequently, the transcription of C mu mRNA, especially secreted mu mRNA was suppressed in the B cells.

Protein-tyrosine kinase activity tightly associated with human type II Fc gamma receptors

Stimulation of B cells by clustering their surface immunoglobulins (sIg) leads to enhanced phosphorylation of several cellular proteins on Ser and Tyr residues. The type II Fc gamma receptor (Fc gamma RII) is one of those proteins that undergo Ser phosphorylation. Upon affinity isolation of the Fc gamma RII, several molecular entities are coisolated from Triton X-100 lysates of BL41 Burkitt lymphoma line which undergo "in vitro" (cell free) phosphorylation in the immune complex-associated kinase assay. Furthermore, several molecules phosphorylated on Tyr upon sIgM cross-linking in the intact cells are coisolated with Fc gamma RII. The 59-kDa coprecipitated component is identified as the protein-tyrosine kinase (PTK) fyn. Clustering the sIgM molecules enhanced the in vitro phosphorylation of all molecules coprecipitated with Fc gamma RII as well as that of the exogenously added PTK substrate, enolase. Kinase renaturation assays suggest that at least two major renaturable protein kinases (59 kDa and 85-90 kDa) associate with Fc gamma RII. Whereas the 59-kDa component comigrates with the PTK fyn, the 85- to 90-kDa one is an unidentified Ser/Thr kinase. These data suggest that Fc gamma RII exists in the B-cell membrane as part of a multimolecular complex including protein kinases, activities of which are regulated by clustering of the antigen receptors.

Effects of IL-4 and Fc gamma receptor II engagement on Egr-1 expression during stimulation of B lymphocytes by membrane immunoglobulin crosslinking

Egr-1 is an immediate early gene that is rapidly upregulated in response to mitogenic signals induced by antigen receptor crosslinking on murine B lymphocytes. It has been shown that levels of Egr-1 expression are closely correlated with B cell proliferation in several models of B cell activation and tolerance. We compared the expression of Egr-1 during B cell stimulation with Fab'2 and IgG anti-immunoglobulin (anti-Ig), since it is known that Fab'2 anti-Ig is mitogenic while IgG anti-Ig is not, owing to a dominant inhibitory effect of crosslinking the B cell Fc gamma RII to membrane Ig. While mitogenic doses of Fab'2 anti-Ig induce large and rapid increases in Egr-1 expression, IgG anti-Ig results in smaller increases in Egr-1 mRNA, comparable to that seen with submitogenic concentrations of Fab'2 anti-Ig. However, the correlation between Egr-1 expression and B cell proliferation breaks down when IL-4 is added as a co-mitogen to induce B cell proliferation with IgG anti-Ig or submitogenic concentrations of Fab'2 anti-Ig. No corresponding increases in Egr-1 mRNA levels are observed when IL-4 is added. Therefore, IL-4 overcomes Fc receptor-mediated inhibition of B cell proliferation without affecting inhibition of Egr-1 mRNA induction, as demonstrated earlier for c-myc mRNA in this system.

Multiple signaling pathways mediate anti-Ig and IL-4-induced early response gene expression in human tonsillar B cells

We have analyzed the relationship between the signaling pathways coupled to surface immunoglobulin and interleukin (IL)-4 receptors in human B cells from the patterns of expression of a panel of phorbol ester-inducible early response genes (ERG) activated by anti-IgM and IL-4 stimulation in vitro. Anti-IgM stimulation led to the induction of all eleven ERG tested. Two of these, the proto-oncogene, c-fos and an anonymous ERG 1R20 were insensitive to protein kinase C (PKC) inhibition with the drug, staurosporine and retained inducibility after down-regulation of PKC activity by purging with phorbol ester. These observations are consistent with previous data showing anti-IgM signaling through both PKC-dependent and PKC-independent pathways. c-fos and 1R20 were also the only ERG inducible in response to IL-4 stimulation and whilst ionomycin induced only c-fos, dibutyryl cyclic adenosine monophosphate stimulation led to induction of both c-fos and 1R20. These observations lend support to a role for the adenylate cyclase pathway being important for coupling of IL-4-generated signals to B cells responses. None of the anti-IgM-responsive ERG was further induced when B cells were co-stimulated with a combination of anti-IgM and IL-4, suggesting that the signaling cascades from these two agents are integrated downstream of third messenger pathways to synergistically promote B cell proliferation.

The G protein-coupled receptor BLR1 is involved in murine B cell differentiation and is also expressed in neuronal tissues

The BLR1 gene, isolated initially from Burkitt's lymphoma cells (Eur. J. Immunol. 1992. 22: 2795), encodes a G protein-coupled receptor with significant relationship to receptors for chemokines (IL-8, MIP-1 alpha) and neuropeptides. The murine homologue of human BLR1 was cloned and used to investigate its expression in vivo. blr1-specific transcripts are observed in secondary lymphatic organs and to a lesser extent in brain of adult mice but not in other tissues. RNA in situ hybridization localizes blr1 transcription to primary follicles and to the mantle zone of secondary follicles. SCID mice in which mature B cell development is severely impaired exhibit a strongly reduced level of blr1-specific RNA in the spleen. The analysis of murine lymphoid tumor cell lines representing distinct stages of the B cell lineage reveals elevated expression of blr1 in B cell lymphomas but not in pre-B lymphomas or plasmacytomas. Induction of differentiation of resting B cells by cytokines or mitogens down-regulates expression of blr1. RNA in situ hybridization using brain sections of adult mice detects blr1 transcription in the granule and Purkinje cell layer of the cerebellum. Interestingly, the blr1 gene is also expressed during late embryogenesis in fetal liver and brain. In view of the remarkable expression pattern in the B cell lineage we suggest that murine BLR1 may represent a cytokine/neuropeptide receptor exerting regulatory functions on recirculating mature B lymphocytes.

The disulfide bridges of the immunoglobulin-like domains of Fc gamma RIIIB are essential for efficient expression and biological activity

The immunoglobulin G receptor Fc gamma RIIIB belongs to the immunoglobulin superfamily as two extracellular domains show homology to the immunoglobulin domains. Since some residues in these domains, such as the two cysteines, are supposed to form an intrachain disulfide bridge are so commonly conserved, they may be of importance for correct folding. Site-directed mutagenesis and expression in BHK21 confirmed this supposition for the Fc gamma RIIIB. Replacing both cysteines in the first and/or second domain by serines reduced the surface expression level by 50%, whereas the ligand binding capability was 20-30% of that seen in cells expressing the wild-type receptor. Replacing one of the four cysteines resulted in the loss of surface expression. Exchanging the conserved tryptophan in the first domain by phenylalanine only slightly affected the ligand binding (25%), whereas the surface expression remained unchanged.

Phenylarsine oxide (PAO) blocks antigen receptor-induced calcium response and tyrosine phosphorylation of a distinct group of proteins

Antigen receptor (AgR) crosslinking by antigens or AgR-specific antibodies induces a cascade of enzymatic events in lymphocytes which involves activation of several non-receptor tyrosine- and serine/threonine kinases, phosphatases, phospholipases, etc. Here we show data demonstrating that a thiol group-reactive protein tyrosine phosphatase (PTP) inhibitor, phenylarsine oxide (PAO), uncouples a crucial part of the signaling events induced by anti-IgM or anti-Leu-4 (CD3) in human tonsil B lymphocytes, BL41 and Daudi B cell lines and Jurkat T lymphoma cells. PAO treatment (10 microM) resulting in distinct modification of AgR-induced tyrosine phosphorylation pattern inhibited the AgR-mediated calcium response (Ca++ release and influx) of all of these cells completely. Since this treatment did not alter the cell viability and the binding capacity of the AgR crosslinking antibodies, alteration of the tyrosine phosphorylation pattern and blockage of the calcium response indicate prompt inactivation of essential signal transduction element(s).

Cellular signalling mechanisms in B lymphocytes

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Cross-linking of IgG receptors inhibits membrane immunoglobulin-stimulated calcium influx in B lymphocytes

By cross-linking membrane immunoglobulins (mIg), the antigenic stimulation of B lymphocytes induces an increase in intracellular free calcium levels ([Ca2+]i) because of a combination of release from intracellular stores and transmembrane influx. It has been suggested that both events are linked, as in a number of other cases of receptor-induced increase in [Ca2+]i. Conversely, in B lymphocytes, type II receptors for the Fc fragment of IgG (Fc gamma RII) inhibit mIg-mediated signaling. Thus, we have investigated at the level of single cells if these receptors could act on specific phases of mIg Ca2+ signaling. Lipopolysaccharide-activated murine B splenocytes and B lymphoma cells transfected with intact or truncated Fc gamma RII-cDNA were used to determine the domains of Fc gamma RII implicated in the inhibition of the Ca2+ signal. [Ca2+]i was measured in single fura-2-loaded cells by microfluorometry. The phases of release from intracellular stores and of transmembrane influx were discriminated by using manganese, which quenches fura-2, in the external medium as a tracer for bivalent cation entry. The role of membrane potential was studied by recording [Ca2+]i in cells voltage-clamped using the perforated patch-clamp method. Cross-linking of mIgM or mIgG with F(ab')2 fragments of anti-Ig antibodies induced a sustained rise in [Ca2+]i due to an extremely fast and transitory release of Ca2+ from intracellular stores and a long lasting transmembrane Ca2+ influx. The phase of influx, but not that of release, was inhibited by membrane depolarization. The increase in [Ca2+]i occurred after a delay inversely related to the dose of ligand. Co-cross-linking mIgs and Fc gamma RII with intact anti-Ig antibodies only triggered transitory release of Ca2+ from intracellular stores but no Ca2+ influx, even when the cell was voltage-clamped at negative membrane potentials. These transitory Ca2+ rises had similar amplitudes and delays to those induced by cross-linking mIgs alone. Thus, our data show that Fc gamma RII does not mediate an overall inhibition of mIg signaling but specifically affects transmembrane Ca2+ influx without affecting the release of Ca2+ from intracellular stores. Furthermore, this inhibition is not mediated by cell depolarization. Thus, Fc gamma RII represents a tool to dissociate physiologically the phases of release and transmembrane influx of Ca2+ triggered through antigen receptors.

The role of G-proteins versus protein tyrosine kinases in the regulation of lymphocyte activation

The relative roles of G-proteins and protein tyrosine kinases (PTKs) in the regulation of antigen receptor-mediated signalling in B and T cells is controversial. As they, and the biochemical events they control, are potential targets for intervention in various immune dysfunctions, the resolution of the controversy is of great interest. Here, Margaret Harnett and Kevin Rigley provide a timely assessment of current understanding, and propose a model for the interaction of G-proteins and PTKs in antigen receptor-mediated signalling.

Co-crosslinking Fc epsilon RII/CD23 and B cell surface immunoglobulin modulates B cell activation

Previous studies have shown that a highly multivalent from of anti-IgD or anti-IgM, prepared by conjugating the respective antibodies to dextran, causes extensive B cell proliferation with ng/ml concentrations of the anti-immunoglobulin (Ig). A modification of this system has been exploited to investigate the effect of co-crosslinking the Fc epsilon RII and surface Ig by binding DNP to the dextran backbone (DNP-dextran) and employing a DNP-specific monoclonal IgE of either rat or mouse origin. Addition of anti-IgD-(H delta a/1)[DNP-dextran] or anti-IgM-[DNP-dextran] to purified, resting murine B cells resulted in B cell proliferation over a broad dose (0.03-30 micrograms/ml). Addition of DNP-specific rat or mouse IgE dramatically modulated the proliferative response. Proliferation in response to doses greater than 0.3 microgram/ml H delta a/1-[DNP-dextran] was consistently reduced in a dose-dependent manner in the presence of increasing amounts of IgE while proliferation to lower concentrations of H delta a/1-[DNP-dextran] was slightly enhanced or not influenced at all by the IgE anti-DNP. Interleukin-4 (IL-4) significantly increased the IgE effect, in line with its known enhancing effects on Fc epsilon RII levels. Experiments measuring Ig production rather than proliferation demonstrated that in the presence of IgE anti-DNP, B cells produced lower amounts of immunoglobulin (IgG1 or IgM) in response to an anti-Ig signal. Control experiments demonstrated that the IgE effect on proliferation was blocked by monoclonal anti-Fc epsilon RII, but not anti-Fc gamma RII, thus demonstrating the necessity for IgE/Fc epsilon RII interaction. In addition, the necessity for co-crosslinking was shown by the inability of IgE anti-DNP to affect the proliferative response to H delta a/1-dextran even in the presence of various doses of DNP-dextran. These results demonstrate that co-crosslinking of sIg and the Fc epsilon RII results in an altered B cell response to anti-Ig mediated activation. IL-4 does not ablate this inhibition, in contrast to the effect of co-crosslinking Fc gamma RII and surface Ig, suggesting a model whereby IgE can modulate its own production.

Relationships between the degree of cross-linking of surface immunoglobulin and the associated inositol 1,4,5-trisphosphate and Ca2+ signals in human B cells

Cross-linking of surface immunoglobulin (Ig) receptors on human B cells leads to the activation of a tyrosine kinase. The activated tyrosine kinase subsequently phosphorylates a number of substrates, including phospholipase C-gamma. This enzyme breaks down phosphoinositol bisphosphate to form two intracellular messengers, diacylglycerol and inositol 1,4,5-trisphosphate, leading to the activation of protein kinase C and the release of intracellular Ca2+ respectively. We have used h.p.l.c. and flow cytometry to measure accurately the inositol phosphate turnover and Ca2+ release in anti-Ig-stimulated human B cells. In particular, we have examined the effect of dose of the cross-linking antibody on the two responses. The identity of putative messenger inositol phosphates has been verified by structural analysis, and the amounts of both inositol phosphates and Ca2+ present have been quantified. In the Ramos Burkitt lymphoma, which is very sensitive to stimulus through its Ig receptors, both inositol phosphate production and Ca2+ release were found to be related to the dose of anti-Ig antibody applied. This suggests that phospholipase C-mediated signal transduction in human B cells converts the degree of cross-linking of the immunoglobulin receptor quantitatively into intracellular signals.

Regulation of antibody production mediated by Fc gamma receptors, IgG binding factors, and IgG Fc-binding autoantibodies

Fc receptors (FcRs) are immunoglobulin-binding structures that enable antibodies to perform a variety of functions by forming connections between specific recognition and effector cells. Besides eliciting cytotoxicity, inducing secretion of mediators and endocytosis of opsonized particles, FcRs are involved in the regulation of antibody production, both as integral membrane proteins and as soluble molecules released from the cell surface. Most FcRs belong to the same family of proteins as their ligands (immunoglobulin superfamily). This review contains recent data obtained by use of monoclonal antibodies and cloning studies on FcRs and FcR-like molecules. The importance of fine specificity of receptor binding site(s)--that of the conformation of FcRs and their ligands in triggering signaling mechanisms--is analyzed. The regulatory function of membrane-bound and -released FcRs; the correlation between cell cycle, FcR expression, and release; as well as the possible mechanisms of these phenomena are discussed.

A polyclonal model for B-cell tolerance. II. Linkage between signaling of B-cell egress from G0, class II upregulation and unresponsiveness

Overnight exposure of adult splenic B cells to anti-Ig, a surrogate for antigen/tolerogen, can result in a hyporesponsive state in terms of antibody synthesis. Since B cells treated with either intact of F(ab')2 fragments of anti-Ig will exit the G0 phase of the cell cycle and enter G1 or S, respectively, we examined which steps in B-cell activation were required for this form of hyporesponsiveness. We found that B-cell hyporesponsiveness could be induced under conditions leading to either abortive or productive B-cell cycle progression, depending on the immunogenic challenge employed. Thus, PMA + ionomycin, concanavalin A, PMA alone, or ionomycin alone induced hyporesponsiveness. Each of these reagents is able to drive B-cell exit from G0 into G1 and cause class II hyperexpression. We next examined the effect of cyclosporin A (CSA), a reagent that blocks anti-Ig but not by PMA-induced class II hyperexpression. Interestingly, CSA only interfered with the induction of B-cell hyporesponsiveness with anti-Ig. These results suggest that upregulation of MHC class II may be coincident with a CSA-sensitive tolerance pathway in B cells stimulated by anti-Ig. Finally, IL-4 pretreatment was found to ablate hyporesponsiveness induced by either intact anti-Ig or PMA. These results parallel the Fc-dependent induction of hyporesponsiveness reported earlier (G. Warner and D. W. Scott, J. Immunol. 146, 2185, 1991). We propose that crosslinking of surface Ig, leading to cell cycle progression out of G0 as well as class II hyperexpression, in the absence of a cognate T cell signal, leads to B-cell hyporesponsiveness.

Structural requirements of the cytoplasmic domains of the human macrophage Fc gamma receptor IIa and B cell Fc gamma receptor IIb2 for the endocytosis of immune complexes

Two isotypes of the monocyte/macrophage as well as B cell Fc gamma receptor type II (FcRIIa and FcRIIb2, respectively) mainly differ in the length (76 vs. 44 amino acids) and amino acid sequence of their cytoplasmic domains. Only the eight amino acids just behind the putative transmembrane region are identical. Despite this marked difference, both FcRII mediate endocytosis of immune complexes. To determine the functional significance of the cytoplasmic domains, we expressed truncated FcRIIa and FcRIIb2 in FcR- BHK-21 cells. Mutants of both receptors containing only one amino acid (tail-minus) of the cytoplasmic domain failed to mediate immune complex uptake. The significance of the cytoplasmic domain of the receptors could be further demonstrated using a chimeric FcRIII-FcRIIa construct. Therefore we expressed an FcRIII lacking the hydrophobic carboxyl terminus (containing the putative phosphatidyl - inositol - glycan anchor site) fused inframe to the transmembrane and cytoplasmic domain of the FcRIIa in BHK-21 cells. In contrast to the wild type FcRIII, this chimeric receptor mediated immune complex uptake indistinguishable from that mediated by the FcRIIa. Receptor mutants with relatively short cytoplasmic domains (FcRIIb2: 13, and FcRIIa: 16 amino acids) revealed, that these short amino acid stretches are sufficient to allow reduced receptor-mediated endocytosis of bound ligand. Furthermore, using FcRIIa deletion mutants with a cytoplasmic domain consisting of 62, 46, and 28 amino acids, respectively, we found that the capability of these mutants to mediate immune complex uptake decreased gradually with the truncation of the cytoplasmic tails. Thus, only short amino acid sequences of the cytoplasmic domain are sufficient to enable an, albeit reduced, receptor-mediated endocytosis.

Inhibition of B cell proliferation with anti-CD19 monoclonal antibodies: anti-CD19 antibodies do not interfere with early signaling events triggered by anti-IgM or interleukin 4

The 95-kDa antigen recognized by the anti-CD19 panel of monoclonal antibodies is found on the surface of most cells of the B cell lineage. Anti-CD19 antibodies inhibit B cell proliferation in response to anti-Ig plus interleukin 4 (IL4), but enhance the response to mitogenic concentrations of either phorbol 12-myristate 13-acetate (PMA) or Epstein-Barr virus. This dichotomy in the effect of anti-CD19 antibodies suggested that the inhibitory action may be directed at the transmembrane signaling pathways utilized by anti-IgM and IL4. To investigate this hypothesis, an attempt was made to determine the mechanism of signal transduction utilized by the CD19 antigen, and elucidate its effect on transmembrane signaling invoked by anti-immunoglobulin and IL4. Binding of anti-CD19 antibody to B cells did not promote activation of either the phosphoinositide or cAMP signaling pathways. In addition, anti-CD19 antibody did not inhibit phosphatidylinositol bisphosphate (PIP2) hydrolysis induced by anti-IgM or IL4, nor did it interfere with cAMP induction by IL4. We also found that anti-CD19 antibody inhibited PMA plus calcium ionophore-induced B cell proliferation. This evidence indicates that anti-CD19 mAb interrupts the signaling cascade at a point distal to receptor-mediated breakdown of PIP2 and/or activation of adenyl cyclase. This conclusion was fully consistent with experiments in which anti-CD19 antibody was shown to inhibit DNA but not RNA synthesis, and the observation that anti-CD19 antibody must be present between 6 h and 20 h after the initiation of the culture suggesting that anti-CD19 mAb exerts its inhibitory effect in late G0 or G1, after the initial signaling events.

Modulation of type II Fc gamma receptor expression on activated human B lymphocytes

We have monitored Fc gamma RII expression during the activation of human B lymphocytes by simultaneous analysis of monoclonal antibody (mAb) binding and EA rosetting. The expression of Fc gamma RII showed a biphasic time course. Initially, a transient increase of Fc gamma RII with no ligand-binding capacity was observed with mAb staining as early as 10 min after stimulation by the F(ab')2 fragment of anti-human IgM or phorbol 12-myristate 13-acetate and then after 3 to 24 h a decrease in the number of Fc gamma RII+ cells was seen. Trypsin-like serine protease activity also appeared in the lysate of activated B cells at this time. On the 2nd day of activation a significant enhancement of Fc gamma RII expression was observed, mainly on enlarged blast cells as monitored by both mAb and by ligand binding (EA rosette). At the same time, soluble fragments of Fc gamma RII with the ability to bind human Fc were detected in the supernatant of activated B cells, probably as a result of proteolytic cleavage. These findings suggest that activated B cells are identical with the population of mononuclear cells which shed Fc gamma R when incubated at 37 degrees C. The ability of activated but not resting B cells to release Fc gamma RII correlates with the expression of early activation markers and with the appearance of a trypsin-like serine protease activity of the same cells; thus, the release of Fc gamma RII occurs in the early G1 phase of cell cycle as a result of proteolysis. Later the release of Fc gamma RII is accompanied by the enhancement of Fc gamma RII expression before the cells reach the S phase. The fragments of cleaved Fc gamma RII had an apparent molecular mass of 33 and 14-18 kDa under nonreducing conditions, and upon reduction fragments of smaller size were observed.

Elevated levels of intracellular cAMP sensitize resting B lymphocytes to immune complex-induced unresponsiveness

The ability of immune complexes (IC) to regulate B lymphocyte differentiation was investigated. Using an in vitro model, we previously demonstrated that macrophages (M phi) or lymphoid dendritic cells pulsed with IC differentially regulated B cell function, inducing unresponsiveness or stimulation, respectively. The capacity of M phi to induce unresponsiveness was dependent upon two signals, an antigen-specific one supplied by the IC and M phi-secreted prostaglandin (PG)E2. Total inhibition of antibody production was never achieved as a small percentage of B lymphocytes were resistant to IC-induced unresponsiveness. In this study, utilizing an accessory cell-free system, we demonstrate that splenic B cell fractions separated on Percoll density gradients are heterogeneous in their sensitivity to IC-mediated unresponsiveness. Small resting B lymphocytes are exquisitely sensitive to IC-mediated negative signaling and exhibit virtual total ablation of antibody responses. Conversely, large activated B cells are more refractory to this inhibitory pathway. PGE2 and other agents which elevate cAMP potentiate IC-induced unresponsiveness in resting, but not activated B lymphocytes. In addition treatment of resting B cells with PGF2 alpha, which did not elevate cAMP, failed to sensitize these cells to IC-mediated negative signaling. Unresponsiveness induced by IC is selective for specific aspects of B lymphocyte activation, since B cell differentiation but not proliferation is affected. Furthermore, pre-treatment of resting B lymphocytes with interleukin 4 prevents the IC-induced ablation of IgM antibody responses. Overall, our results indicate that the binding of IC by resting B lymphocytes provides a potent mechanism for inhibiting differentiation without affecting proliferation. These observations suggest that in vivo, IC play an important role in regulating the memory B lymphocyte pathway.

The immune complex: possible ways of regulating the antibody response

B cells express antigen, Fc and complement receptors on their surfaces and can thus bind all three components of an immune complex. In addition to the direct effects that they exert on cells, immune complexes may affect localization, presentation and digestion of antigen. In this article, Birgitta Heyman discusses recent developments in antibody-mediated regulation of the humoral immune response, with emphasis on in vivo systems where antigens are injected together with highly purified IgM or IgG antibodies in the absence of adjuvants.

Fc receptor phosphorylation during receptor-mediated control of B-cell activation

It is well known that Fc receptors for IgG (FcRII) on macrophages mediate the endocytosis of antibody-antigen complexes and signal the release of inflammatory and cytotoxic agents. FcRII are also expressed at high levels on B cells where they are less involved in endocytosis than in modulating B-cell activation by membrane immunoglobulins. Although crosslinking of membrane immunoglobulins can result in B-cell differentiation and proliferation through stimulation of phospholipase C, mobilization of intracellular Ca2+, and activation of protein kinase C, crosslinking FcR with membrane immunoglobulins confers a dominant inhibitory signal that prevents or aborts activation. This form of regulation may have a role in the induction of tolerance by IgG and in controlling the B-cell repertoire by anti-idiotypes. The different functions of FcR on B cells and macrophages may reflect the fact that these cell types express closely related but distinct FcR isoforms. We have recently found that the main lymphocyte FcR isoform, FcRII-B1, is unable to mediate endocytosis by way of coated pits and coated vesicles owing to an in-frame insertion of 47 amino acids in its cytoplasmic tail. Here we show that this insert, absent from the FcRII-B2 macrophage isoform, also contains serine phosphorylation sites that may have a role in the ability of FcR to regulate B-cell activation through membrane immunoglobulins.

Signaling through CD19, Fc receptors or transforming growth factor-beta: each inhibits the activation of resting human B cells differently

To understand further the roles that negative regulatory signals may play in B cell immune responses, we compared three inhibitors of B cell proliferation: cross-linking CD19 with monoclonal antibody (mAb), signaling through Fc receptors by intact anti-mu mAb, and transforming growth factor-beta (TGF-beta). Each agent was tested for its ability to block proliferation and specific activation events induced in human tonsilar B cells activated by either cross-linking surface immunoglobulin, signaling through CD20, or direct activation of protein kinase C (PKC) by phorbol 12-myristate 13-acetate. We found that each inhibitor was functionally distinct. Both anti-CD19 mAb and anti-mu mAb inhibited anti-immunoglobulin activated cells and anti-CD20-activated cells, but neither inhibited cells activated by phorbol 12-myristate 13-acetate. TGF-beta, on the other hand, inhibited equally profoundly cells activated by each of the three regimens. These results suggest that TGF-beta blocks B cell activation at a step following the activation of PKC, whereas both signaling through CD19 and Fc receptor block early steps in the PKC activation pathway. Signaling through anti-CD19 mAb was unique in that proliferation of anti-immunoglobulin-activated cells was reduced on day 3 and then augmented subsequently. With all other inhibitory combinations the block was permanent. We conclude that each of these three inhibitors has unique important functions and therefore suggest that the effectiveness of negative signaling in B cell immune regulation will depend on the combinations of specific inhibitors modulating a specific activation program.

Macrophage-secreted prostaglandin E2 potentiates immune complex-induced B cell unresponsiveness

Immune complexes (IC) are potent modulators of immune responses. In this report, we used an in vitro murine model system to investigate how two types of accessory cells pulsed with IC regulated B cell function. We demonstrate that IC-pulsed macrophages (M phi) induce hapten-specific B cell unresponsiveness, whereas IC-pulsed splenic lymphoid dendritic cells (LDC) and and LDC-like tumor line caused an augmentation of the antibody response. The mechanism by which IC-pulsed M phi diminished antibody production required two signals. The first was an antigen-specific signal supplied by the IC, and the second a nonspecific co-factor which was a product of M phi cyclo-oxygenase metabolism. Specifically, it was shown that prostaglandin E2 (PGE2) could function at this co-factor. Interestingly, other prostanoids, such as PGF2 alpha, did not function in this fashion. Purified fluorescein (FL+)-specific B cells pulsed with IC exhibited a similar pattern of hapten-specific unresponsiveness. Treatment of accessory cell-free, FL+B cells with PGE2 rendered them sensitive to subtolerogenic doses of soluble IC. Overall, our results suggest that one mechanism by which unresponsiveness can be induced involves both IC and PGE2, and that elevated levels of PGE2 sensitize B lymphocytes to antigen-specific tolerogenic signals.