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

Dependence of surface monoclonal antibody binding on dynamic changes in FcgammaRIIb expression

Receptors for the Fc region of immunoglobulin G (FcgammaRs) are expressed on a broad range of haematopoietic cell types and are responsible for regulating antibody production and linking the humoral and effector responses. In response to a number of stimuli, such as cytokine signals or inflammation, FcgammaR expression at the cell surface is dynamically regulated. On B cells, we observed what appeared to be a correlation between CD22 expression and FcgammaRIIb expression when the latter was varied in a number of models. Further investigation revealed that this was specific to a particular anti-CD22 monoclonal antibody, which appeared to require stabilization by interaction with FcgammaRIIb for optimal binding to CD22. Since alterations in the regulation of FcgammaR expression are important in controlling immune responses and have been associated with a number of immune-mediated disease states, we suggest that it might be prudent to confirm the expression of cell surface markers by two independent methods. Furthermore, because the efficacy of therapeutic antibodies may depend upon their interaction with FcgammaRs, our results are relevant to their design and assessment.

Characterization of a trypsin-like serine protease of activated B cells mediating the cleavage of surface proteins

Activated B cells may cleave their surface receptors due to the proteolytic activity on the cell membrane or in its vicinity. We attempted to isolate and characterize the protease(s) responsible for this cleavage. Zymograms prepared from the supernatant and the plasma membrane fraction of activated human B cells and BL41/95 cell line exhibited a 85-90 kDa doublet band with protease activity, while that of resting B cells did not. Soybean trypsin inhibitor (STI), Nalpha-p-tosyl-L-lysine chloromethyl ketone (TLCK) and EDTA treatment abolished the activity of this protease. The excess of Zn(2+) ions in EDTA did not restore the enzymatic activity, while it was completely recovered in the presence of Ca(2+). We affinity-purified a 85-90 kDa protease from the supernatant of BL41/95 cells using STI coupled to Sepharose 4B beads, and measured its kinetic parameters. For the arginyl substrate K(M) was 358+/-59 microM and for the lysyl substrate 582+/-103 microM. TLCK and benzamidine inhibited the protease at micromolar, while STI at nanomolar concentrations. Both the inhibition profile and the substrate specificity suggest that it is a trypsin-like serine protease. We assume that the 85-90 kDa serine protease expressed on and secreted by activated B cells and BL41/95 cell line is responsible for the cleavage of various membrane proteins, including Fcgamma receptors; thus it may play a crucial role in regulating B cell's function.

Bacterially expressed human Fc gamma RIIb is soluble and functionally active after in vitro refolding

A recombinant soluble form of the human Fc gamma receptor was produced by engineering a cDNA construct containing the extracellular part of the mature protein. After expression in bacteria as inclusion body, the polypeptide was highly purified and was refolded in vitro with a method that was developed for the renaturation of immunoglobulin fragments. With this method oxidation of the disulfide bridges within the domains of the protein is done in the presence of an artificial 'chaperone' which protects the polypeptide molecules from unwanted protein protein interactions thereby inhibiting the incorrect oxidation of the SH-groups. and misfolding of the protein. The refolded recombinant soluble Fc gamma RIIb showed several characteristics of the native receptor: (i) it was recognized by a series of monoclonal antibodies specific for, and in most cases produced against the native cell-surface receptor: (ii) it is bound to its ligand (the Fc-region of different immunoglobulins) under very diverse conditions: and (iii) it is competed strongly and specifically with the native cell surface receptor for both ligand and antibody binding in experiments with distinct read-outs; (iv) monoclonal antibodies produced against the recombinant protein specifically recognized Fc gamma RIIb on different cells. From these data it was concluded that the recombinant soluble Fc-receptor was in a native, functionally active form, and its function was not affected by the lack of glycosylation.

Immunoreceptor tyrosine-based inhibition motif-bearing receptors regulate the immunoreceptor tyrosine-based activation motif-induced activation of immune competent cells

ITIM-bearing receptors, a family which only recently has been recognized, play a key role in the regulation of the ITAM-induced activation of immune competent cells. The mechanism of ITM-mediated regulation in various cells was recently clarified. The present review focuses on ITIM bearing membrane proteins that negatively regulate the activation of cells when co-crosslinked with ITAM containing receptors, illustrates the inhibitory processes by the negative regulation of B-, NK-, T-cells and mast cells and summarizes current views on the mechanism of ITIM-mediated inhibition.

Soluble Fc gamma receptors: interaction with ligands and biological consequences

Soluble Fc gamma receptors are produced by cleavage of the membrane receptors or by alternative splicing. They are found in biologic fluids. After a brief description of the structure and mode of production of soluble Fc gamma R, we address the question of ligands and function of the soluble Fc gamma R by using recombinant molecules and transgenic animals. We show that soluble Fc gamma R are not only IgG-binding factors which interfere with, and block, Fc-dependent immune reactions but also molecules that interact, in vitro, with non-Ig-ligands such as CR3 and CR4 and are trigger or regulate immune functions via these receptors.

Membrane protein secretases

A diverse range of membrane proteins of Type 1 or Type II topology also occur as a circulating, soluble form. These soluble forms are often derived from the membrane form by proteolysis by a group of enzymes referred to collectively as 'secretases' or 'sheddases'. The cleavage generally occurs close to the extracellular face of the membrane, releasing physiologically active protein. This secretion process also provides a mechanism for down-regulating the protein at the cell surface. Examples of such post-translational proteolysis are seen in the Alzheimer's amyloid precursor protein, the vasoregulatory enzyme angiotensin converting enzyme, transforming growth factor-alpha, the tumour necrosis factor ligand and receptor superfamilies, certain cytokine receptors, and others. Since the proteins concerned are involved in pathophysiological processes such as neurodegeneration, apoptosis, oncogenesis and inflammation, the secretases could provide novel therapeutic targets. Recent characterization of these individual secretases has revealed common features, particularly sensitivity to certain metalloprotease inhibitors and upregulation of activity by phorbol esters. It is therefore likely that a closely related family of metallosecretases controls the surface expression of multiple integral membrane proteins. Current knowledge of the various secretases are compared in this Review, and strategies for cell-free assays of such proteases are outlined as a prelude to their ultimate purification and cloning.

Physiological enzymatic cleavage of leukocyte membrane molecules

Certain membrane molecules are enzymatically cleaved from the cell surface and then released into the extracellular medium in the form of soluble fragments. This process, commonly initiated by cell stimulation, may regulate the surface expression of such molecules, and may also be responsible for the production of their soluble forms in vivo. Here, Vladimír Bazil provides an overview of the molecules that are cleaved from cells, focusing particularly on leukocyte receptors. In addition, he discusses the mechanisms and putative enzymes involved in this process, as well as the potential physiological significance of such events.

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)

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.

Regulation of ICAM-3 (CD50) membrane expression on human neutrophils through a proteolytic shedding mechanism

The regulation of the cell surface expression of ICAM-3 (CD50) was investigated in human neutrophils. Immunofluorescence flow cytometry analysis revealed a remarkable and very rapid down-regulation of the ICAM-3 cell surface expression upon neutrophil activation with stimulating agents such as phorbol myristate acetate (PMA) or calcium ionophore. A similar low expression of ICAM-3 was observed on neutrophils from patients undergoing hemodialysis with cell-activating cellulosic membranes. Internalization assays with 125I-labeled anti-ICAM-3 monoclonal antibody (mAb) suggested that ICAM-3-down-regulation was due to antigen release from the cell surface towards the outer milieu, rather than to antigen internalization. Immunoprecipitation studies confirmed this down-regulatory effect, and revealed the presence of ICAM-3 in cell-free supernatants from activated neutrophils. Furthermore, the presence of a soluble form of ICAM-3 with a range of concentrations of 0-296 ng/ml in the plasma from healthy human volunteers was detected by using a two-site mAb radioimmunoassay. A proteolytic mechanism likely accounts for this process since protease inhibitors virtually abrogated the PMA-induced down-regulation of ICAM-3. Functional studies showed that anti-ICAM-3 mAb were able to trigger homotypic neutrophil aggregation both before and after ICAM-3 down-regulation, indicating that the fraction of ICAM-3 molecules remaining on the neutrophil surface upon activation are still capable of sustaining cell adhesion. In contrast, the loss of L-selectin (CD62L) on activated neutrophils was almost complete, thus leading to an impairment of L-selectin-mediated neutrophil-endothelial cell adhesion. These results indicate that ICAM-3 is released to the medium upon neutrophil stimulation and that both ICAM-3 and L-selectin have a role in the neutrophil adhesive phenomena.

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.

Production and characterization of two murine monoclonal antibodies directed against epitopes exclusive to soluble fragments of Fc epsilon RII/CD23

Two murine monoclonal antibodies (mAb) designated as SU1 and SU3 directed against soluble Fc epsilon RII/CD23 have been generated by fusing X.63.AG.8653 (a mouse myeloma cell line) with spleen cells from mice immunized with an Epstein Barr virus (EBV)-transformed B cell line (RPMI-8866). The antibodies have been shown to be capable of detecting affinity purified soluble Fc epsilon RII/CD23 in an enzyme-linked immunosorbent assay. Indirect immunofluorescence has shown that the SU1 and SU3 mAb do not stain RPMI-8866, a Fc epsilon RII/CD23+ B cell line. By studying the migration profiles of affinity purified SU1- and SU3-reactive molecules on sodium dodecylsulfate-polyacrylamide gel electrophoresis it has been shown that SU1 mAb immunoprecipitates 33- and 12-kDa components, while the SU3 mAb recognized 25- and 45-kDa proteins from culture supernatants of RPMI-8866 cells. Moreover, affinity purified SU1- and SU3-reactive proteins have been shown to be recognized by human IgE but not by the human IgG molecule. These results provide evidence that SU1 and SU3 mAb may recognize some putative post-cleavage epitopes on the N-terminal end of the low affinity receptor which appear, perhaps, following the process of fragmentation. In addition, the effect of these antibodies on continuous growth of a panel of lymphoblastoid cell lines indicates that SU1 mAb was found incapable of influencing the spontaneous proliferation of EBV-immortalized B cell lines; whereas SU3 mAb completely blocked the spontaneous growth and proliferation of all B cell lines tested. The results are discussed in relation to the appearance of a functional post-cleavage epitope on soluble Fc epsilon RII/CD23.

Proteolysis increases the Fc-mediated binding of murine IgG2b to human EBV-transformed B cells, but decreases the expression of Fc gamma RII and Fc epsilon RII

We have previously described a polymorphic human Fc receptor for murine IgG2b (mIgG2b). This receptor was defined by the binding of (complexed) mIgG2b to monocytes and Epstein-Barr virus (EBV)-transformed B cells. Three per cent of normal individuals were high responders with respect to mIgG2b (mIgG2b-HR), whereas the other individuals were low responders for mIgG2b (mIgG2b-LR). In the present study we investigated the effect of proteolytic enzymes on the Fc-mediated binding of mIgG2b to EBV-B cells. Pronase, human leucocyte elastase and cathepsin G caused an increased binding (in an EA-rosetting assay) of mIgG2b to EBV-B cells from mIgG2b-HR, but not from mIgG2b-LR. Simultaneous immunofluorescence studies demonstrated that these proteolytic enzymes strongly reduced the expression of Fc gamma RII and Fc epsilon RII on these cells, whereas HLA class I or HLA class II molecules were not affected. These findings strongly suggest that binding of mIgG2b is not mediated by Fc gamma RII or Fc epsilon RII. We also studied the effect of proteolysis on mIgG2b-HR EBV-B cells from an HLA class II-negative individual. In this case EA-mIgG2b rosetting was decreased after proteolysis, suggesting that HLA class II molecules may have a role in protecting the binding site for mIgG2b against proteolytic destruction.

CD43, the major sialoglycoprotein of human leukocytes, is proteolytically cleaved from the surface of stimulated lymphocytes and granulocytes

CD43, the major sialoglycoprotein of human leukocytes, whose expression is defective in patients with the Wiskott-Aldrich syndrome, was down-regulated by phorbol 12-myristate 13-acetate (PMA) on granulocytes but not on lymphocytes. However, CD43 expressed on both of these leukocyte subpopulations was down-regulated after crosslinking by anti-CD43 monoclonal antibodies, a stimulation that may simulate the effect of a natural CD43 ligand. Soluble, labeled CD43 molecules were isolated from culture supernatants of both surface-iodinated granulocytes activated by PMA and lymphocytes stimulated with anti-CD43 antibodies. Thus, in this case down-regulation represents release from the cell surface into the culture medium, rather than internalization. The apparent molecular masses of the released molecules and of soluble CD43 isolated from human serum were identical. Importantly, PMA-induced down-regulation of CD43 on granulocytes was markedly blocked both by the metalloprotease inhibitor 1,10-phenanthroline and by the serine protease inhibitors N alpha-(p-tosyl)-L-lysine chloromethyl ketone and Pefabloc SC, which inhibit two different classes of proteases, thus indicating that the release is proteolytic.

CD44 and its interaction with extracellular matrix

It is now generally accepted that CD44 is a cell adhesion receptor and that hyaluronan is one of its ligands. Like many cell adhesion receptors, CD44 is broadly distributed, and its ligand, hyaluronan, is a common component of extracellular matrices and extracellular fluids. Yet a great variety of responses has been reported to result from CD44 ligation. These include cell adhesion, cell migration, induction (or at least support) of hematopoietic differentiation, effects on other cell adhesion mechanisms, and interaction with cell activation signals. This diversity of responses indicates that downstream events following ligand binding by CD44 may vary depending on the cell type expressing CD44 and on the environment of that cell. CD44 is expressed on cells in the early stages of hematopoiesis and has been shown to participate in at least some aspects of the hematopoietic process. In mature lymphocytes, CD44 is upregulated in response to antigenic stimuli and may participate in the effector stage of immunological responses. Along with other adhesion receptors that show alterations in expression after activation, CD44 probably contributes to differences in the recirculation patterns of different lymphocyte subpopulations. CD44 ligand-binding function on lymphocytes is strictly regulated, such that most CD44-expressing cells do not constitutively bind ligand. Ligand-binding function may be activated as a result of differentiation, inside-out signaling, and/or extracellular stimuli. This regulation, which in some situations can be rapid and transient, potentially provides exquisite specificity to what would otherwise be a common interaction. CD44 is not a single molecule, but a diverse family of molecules generated by alternate splicing of multiple exons of a single gene and by different posttranslational modifications in different cell types. It is not yet clear how these modifications influence ligand-binding function. The significance of the multiple isoforms of CD44 is not understood, but association of some isoforms with malignancies has been observed. And in at least some experimental systems, a contribution of CD44 isoforms to metastatic behavior has been demonstrated.

A trypsin-like serine protease activity on activated human B cells and various B cell lines

We have studied the trypsin-like serine protease activity of human tonsillar B lymphocytes. The lysate of the low-density, in vivo activated B cells as well as the lysate of cells stimulated with anti-human IgM F(ab')2 show elevated trypsin-like serine protease activity compared to the resting subset as monitored by the cleavage of Tos-Gly-Pro-Arg-pNA. The cleavage is sensitive to N-tosyl-L-lysyl-chloromethyl ketone and benzamidine but not to iodoacetamide. Experiments with intact cells give similar results. The finding that the intact cells hydrolyze the substrate, while their supernatant does not, suggests that the protease activity is cell membrane associated. It is possible that C3 is a substrate of the enzyme since the activated B cells cleave C3, whereas the resting B cells do not, and also C3 inhibits the enzyme-substrate reaction. In addition to the ex vivo B cells, we studied the serine protease activity of certain well-characterized B cell lines. The results show a correlation between the phenotype and the enzyme expression of the cell lines. BL41, an Epstein-Barr virus (EBV)-negative Burkitt lymphoma line, with a resting phenotype, has low activity, while its EBV genome-carrying convertants E95-A-BL41, E95-C-BL41, EHR-A-BL41 and BL41/95 that have the phenotype of activated B cells, have high proteolytic activity. The lymphoblastoid cell line WW-1-LCL which has the phenotype of an immunoblast, has the highest serine protease activity. On the basis of the above data, we suggest that a rather tight correlation exists between the degree of activation and the appearance of serine protease(s) on the surface of human B cells.

Functional studies on Fc receptors

Fc receptors (FcRs) are cell surface molecules that recognize and bind to the constant domains of immunoglobulins. In doing so, they enable antibodies to perform several biological functions, by forming a link between specific antigen recognition and FcR-bearing cells. Here, Gabriella Sármay provides an overview of recent studies on FcRs in Hungarian laboratories, concentrating on their role in selected biological systems.

Prognostic significance of the heterogenous expression of IgG Fc receptors in B-cell chronic lymphocytic leukemia

Receptors for the Fc part of IgG (Fc gamma R) are expressed in three forms on peripheral blood lymphocytes. The presence of the releasable form (Fc gamma R(REL.)) as well as of the two nonreleasable forms with lower (Fc gamma R(LOW)) and higher (Fc gamma R(HIGH)) cellular avidity was correlated with survival in 63 patients with B-cell chronic lymphocytic leukemia (B-CLL). High percentage of cells with Fc gamma R(LOW) as well as high "absolute" number of cells carrying the two nonreleasable forms of Fc gamma R were connected to unfavorable prognosis. Combining these three parameters, an Fc gamma R constellation was defined which pointed to a favorable prognosis (in 24 patients) when all three parameters were low, but detected short survivors when all three data were high (in 14 patients). The Fc gamma R constellation was capable of identifying patients with better or worse prognosis within groups that were homogeneous regarding some other known prognostic factors. Fc gamma R constellation as a prognostic factor was shown to be independent of age, sex, and Rai and Binet stages, but it was found to be connected with the total tumor mass score (TTM). The three forms of Fc gamma R on B cells might reflect stages of B-cell activation. Differences in Fc gamma R constellations between patients with B-CLL would thus correspond to differently activated B-cell clones with variable prognosis.

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.