Optimization of the cellular import of functionally active SH2-domain-interacting phosphopeptides

Phosphopeptides interacting with src homology 2 (SH2) domains can activate essential signaling enzymes in vitro. When delivered to cells, they may disrupt protein-protein interactions, thereby influencing intracellular signaling. We showed earlier that phosphopeptides corresponding to the inhibitory motif of Fcgamma receptor IIb and a motif of the Grb2-associated binder 1 adaptor protein activate SH2-containing tyrosine phosphatase 2 in vitro. To study the ex vivo effects of these peptides, we have now compared different methods for peptide delivery: (i) permeabilization of the target cells and (ii) the use of cell-permeable vectors, which are potentially able to transport biologically active compounds into B cells. We found octanoyl-Arg(8) to be an optimal carrier for the delivery of phosphopeptides to the cells. With this strategy, the function of cell-permeable SHP-2-binding phosphopeptides was analyzed. These peptides modulated the protein phosphorylation in B cells in a dose- and time-dependent manner.

Signaling pathways leading to apoptosis or survival in immature and mature B cells

In the absence of phosphorylated Gab2, immature B cells are unable to maintain a prolonged signal transduction, which is necessary for the transcriptional regulation and activation of the cells. Thus, the lack of Gab2 phosphorylation may drive immature B cells to apoptosis.

Co-clustering of Fcgamma and B cell receptors induces dephosphorylation of the Grb2-associated binder 1 docking protein

The immunoreceptor tyrosine-based inhibitory motif (ITIM) of human type IIb Fcgamma receptor (FcgammaRIIb) is phosphorylated on its tyrosine upon co-clustering with the B cell receptor (BCR). The phosphorylated ITIM (p-ITIM) binds to the SH2 domains of polyphosphoinositol 5-phosphatase (SHIP) and the tyrosine phosphatase, SHP-2. We investigated the involvement of the molecular complex composed of the phosphorylated SHIP and FcgammaRIIb in the activation of SHP-2. As a model compound, we synthesized a bisphosphopeptide, combining the sequences of p-ITIM and the N-terminal tyrosine phosphorylated motif of SHIP with a flexible spacer. This compound bound to the recombinant SH2 domains of SHP-2 with high affinity and activated the phosphatase in an in vitro assay. These data suggest that the phosphorylated FcgammaRII-SHIP complexes formed in the intact cells may also activate SHP-2. Grb2-associated binder 1 (Gab1) is a multisite docking protein, which becomes tyrosine-phosphorylated in response to various types of signaling, including BCR. In turn it binds to the SH2 domains of SHP-2, SHIP and the p85 subunit of phosphatidyl inositol 3-kinase (PtdIns3-K) and may regulate their activity. Gab1 is a potential substrate of SHP-2, thus its binding to FcgammaRIIb may modify the Gab1-bound signaling complex. We show here that Gab1 is part of the multiprotein complex assembled by FcgammaRIIb upon its co-clustering with BCR. Gab1 may recruit SH2 domain-containing molecules to the phosphorylated FcgammaRIIb. SHP-2, activated upon the binding to FcgammaRIIb-SHIP complex, partially dephosphorylates Gab1, resulting in the release of PtdIns3-K and ultimately in the inhibition of downstream activation pathways in BCR/FcgammaRIIb co-aggregated cells.

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.

Fcgamma receptor-mediated inhibition of human B cell activation: the role of SHP-2 phosphatase

Co-clustering of the type II receptors binding the Fc part of IgG (FcgammaRIIb) and B cell receptors results in the translocation of cytosolic, negative regulatory molecules to the phosphorylated immunoreceptor tyrosine-based inhibitory motif (P-ITIM) of the FcgammaRIIb. SH2 domain-containing protein tyrosine phosphatases (SHP-1 and SHP-2), and the polyphosphoinositol 5'-phosphatase (SHIP) have been reported earlier to bind to murine FcgammaRIIb P-ITIM. However, neither the functional substrates of these enzymes, nor the mechanism of the inhibition are fully resolved. We show here that the human FcgammaRIIb binds SHP-2 when co-clustered with the B cell receptors, whereas its synthetic P-ITIM peptide bindes SHP-2 and SHIP in lysates of the Burkitt's lymphoma cell line BL41. The P-ITIM peptide binding enhances SHP-2 activity, resulting in dephosphorylation and release of P-ITIM-bound SHIP and Shc. Moreover, P-ITIM-bound SHP-2 dephosphorylates synthetic peptides corresponding to the sites of tyrosine phosphorylation on SHIP and Shc, indicating that these proteins are its potential substrates. Thus SHP-2-induced dephosphorylation may modulate the intracellular localization and/or activity of SHIP and Shc, thereby inhibiting further activation pathways which they mediate.

Cooperation between SHP-2, phosphatidyl inositol 3-kinase and phosphoinositol 5-phosphatase in the Fc gamma RIIb mediated B cell regulation

Co-clustering B cell receptors (BCR) and type II receptors binding the Fc part of IgG (Fc gamma RIIb) inhibits B cell activation and antibody production. Tyrosine phosphorylation of an intracellular motif of Fc gamma RIIb has been shown to be a prerequisite of the inhibition. After being phosphorylated by BCR-activated tyrosine kinases, the immunoreceptor tyrosine-based inhibitory motif (P-ITIM) of Fc gamma RIIb recruits SH2 domain containing protein tyrosine phosphatase(s) (PTPs) and polyphosphoinositol 5-phosphatase (SHIP) to the vicinity of BCR, which in turn dephosphorylate their specific substrates. This leads to the interruption of signal transduction, consequently to the anergy and/or apoptosis of the cell. The downstream signaling pathways affected by Fc gamma RIIb-BCR co-clustering are not clarified yet, neither the substrates of PTPs are known. We have studied the Fc gamma RIIb mediated B cell inhibition on human Burkitt lymphoma cell line (BL41). From the lysates of BL41 cells SHP-2 and phosphatidylinositol 3-kinase (PI3-K), as well as the protein tyrosine kinase (PTK) Lyn bind both to the BCR-co-clustered Fc gamma RIIb and to its P-ITIM peptide. Lyn hyperphosphorylates the P-ITIM associated molecules, including SHIP in the in vitro protein tyrosine kinase activity assay. The P-ITIM-compelled multi-phosphoprotein complex binds to and activates SHP-2, which in turn dephosphorylates SHIP and Shc and probably other substrates. Subcellular localisation of these signaling molecules is regulated by the phosphotyrosine-SH2 domain interactions, thus dephosphorylation may result in the re-direction of Shc and SHIP within the cell, consequently, in the modulation of their activity. Finally, co-clustering Fc gamma RIIb and BCR or Fc gamma RIIb and CD19 on the intact cells inhibited PI3-K activity as detected in the anti-phosphotyrosine (anti-PY) precipitates. The results indicate that SHP-2 bound to and activated by the BCR co-clustered Fc gamma RIIb, may down-regulate PI3-K activity by dephosphorylating a yet unidentified regulatory molecule, which recruits PI3-K to the cell membrane.

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.

Fc gammaRIIb inhibits both B cell receptor- and CD19-induced Ca2+ mobilization in Fc gammaR-transfected human B cells

Fc gammaRIIb (CD32) controls antibody production by down-regulating cell activation, when co-clustered with B cell antigen receptors (BCR) in vivo, via immune complexes consisting of secreted IgG and antigen. Fc gammaRIIb-BCR co-ligation in vitro was shown to inhibit the Ca2+ influx from the extracellular space, the mechanism of which is not fully understood. Human B cells express Fc gammaRIIb1 and Fc gammaRIIb2, differing only in a 19 amino acid long insert in the cytoplasmic tail of the former. To elucidate whether Fc gammaRIIb1 and Fc gammaRIIb2 isoforms show any difference in the down-regulation of B cells, we have studied the effect of co-clustering of BCR and Fc gammaRIIb1 or Fc gammaRIIb2 on the Ca2+ signaling in a Burkitt's lymphoma cell line, ST486, transfected with the two isoforms respectively. We have shown here, for the first time, that co-aggregation of BCR and Fc gammaRIIb may also inhibit Ca2+ release from the endoplasmic reticulum pool of human B cells. Both isoforms mediated this inhibition and the inhibitory effect depended on the ratio of BCR to Fc gammaRIIb cross-linking. In contrast to Fc gammaRIIb, the CD21/CD19 complex was shown to up-regulate B cell response by lowering the activation threshold. We have shown here that co-clustering of Fc gammaRIIb with CD19 inhibited the CD19-induced Ca2+ influx. Furthermore, the three party co-aggregation of Fc gammaRIIb with BCR and CD19 resulted in a decreased Ca2+ response, as compared to the BCR- plus CD19-induced one, indicating that Fc gammaRIIb may inhibit CD19-induced enhancement of B cell activation. On the basis of these data we suggest that IgG-containing and C3d-fixing immune complexes may down-regulate the B cell response by interfering with both BCR- and CD19-mediated Ca2+ mobilization.

Integration of activatory and inhibitory signals in human B-cells

Fc gamma receptors type IIb1 (Fc gammaRIIb1) inhibit B-cell activation when co-ligated with B-cell antigen receptors (BCR) by immune complexes. In murine B-cells the inhibition is mediated by the interaction of the phosphorylated immunoreceptor tyrosine-based inhibitory motif (P-ITIM) of Fc gammaRIIb1 with the SH2 domain containing protein tyrosine phosphatase. SHP1. To clarify the mechanism of Fc gammaRIIb mediated inhibition of human B-cells we have studied the association of signaling molecules with human Fc gammaRIIb1 after co-ligating with BCR. Fc gammaRIIb1 were affinity purified from the Burkitt lymphoma cell line, BL41. Several tyrosine phosphorylated proteins were co-isolated with Fc gammaRIIb1 at 145, 110, and 50 60 kDa, which were not present in Fc gammaRIIb1 free immune complexes. Among these molecules we have identified the p52 Shc adaptor protein. Furthermore, we have shown that the insolubilised synthetic peptide corresponding P-ITIM bound Shc, Lyn and the p75 and p 10 unidentified tyrosine phosphorylated proteins. Here we describe that the cell membrane associated Shc is partially dephosphorylated in BCR-Fc gammaRIIb1 co-ligated samples, suggesting that its function in regulating p21ras monomeric G protein is impaired. Indeed, we have detected a lower p21ras activity in BCR-Fc gammaRIIb1 co-crosslinked samples. These data indicate that co-ligation of BCR and Fc gammaRIIb1 interrupts signal transduction between protein tyrosine kinase activation and p21ras mediated activation pathway. Since in contrast to the mouse B-cells both Fc gammaRIIb1 and Fc gammaRIIb2 are expressed in human B-cells, we have investigated the inhibitory function of the two receptors in Fc gammaRIIb negative Burkitt lymphoma cell line ST486 transfected with Fc gammaRIIb1 and Fc gammaRIIb2, respectively. Both Fc gammaRIIb1 and Fc gammaRIIb2 inhibited the rise of intracellular Ca2+ induced by the crosslinking of BCR. The rate of the inhibition depended on the ratio of the co-crosslinked receptors (BCR-Fc gammaRIIb1) to the crosslinked BCR (BCR-BCR). Co-crosslinking of the two receptors inhibited not only the capacitive Ca2+ entry but rather the total Ca2+ response in both Fc gammaRIIb1 and Fc gammaRIIb2 transfected human B-cells. CD19 represents the signal transduction unit of complement receptor, CR2 (CD21), and is responsible for the complement activating IgM-immune complex induced enhancement of B-cell activation. Co-crosslinking of CD19 and BCR was shown to enhance B-cell activation due to the recruitment of further signaling molecules to the activator complex by the phosphorylated tyrosine residues of CD19. Here we show a novel finding that co-ligation of CD19 with Fc gammaRIIb1 inhibits the CD19-induced upregulation of Ca2+ response. The results indicate that IgG plus complement containing immune complexes may inhibit B-cell activation in vivo, due to the Fc gammaRIIb1-mediated interruption of signal transduction via both BCR and CD19.

Human type II Fcgamma receptors inhibit B cell activation by interacting with the p21(ras)-dependent pathway

Co-ligation of antigen receptors and type II Fcgamma receptors (FcgammaRIIb) on B cells interrupts signal transduction and ultimately inhibits antibody production. We have identified p52 Shc in the FcgammaRIIb1-specific immunoprecipitates isolated from the membrane fraction of BL41 Burkitt lymphoma cells following B cell receptor-FcgammaRIIb1 co-ligation. The insolubilized synthetic peptide representing the phosphorylated form of the tyrosine-based inhibitory motif of FcgammaRIIb also binds Shc from the lysates of activated but not from resting BL41 cells. This suggests that the binding does not depend on the interaction of FcgammaRIIb1-phosphotyrosine with the SH2 domain of Shc. Tyr phosphorylation of FcgammaRIIb1-associated Shc is low, indicating an impaired function. Shc is implicated in regulating p21(ras) activation; thus, we have compared p21(ras) activities in BL41 cells treated in different ways. p21(ras) activity is reduced when B cell receptor and FcgammaRIIb1 are co-ligated. p21(ras) couples protein-tyrosine kinase-dependent events to the Ser/Thr kinase-mediated signaling pathway leading to the activation of mitogen-activated protein kinases (MAPK). Our results show that B cell receptor-FcgammaRIIb1 co-cross-linking partially inhibits mitogen-activated protein kinase activity. We conclude that FcgammaRIIb1-dependent inhibition of human B cell activation may be based on interrupting signal transduction between protein-tyrosine kinases and the p21(ras)/mitogen-activated protein kinase-dependent activation pathway.

Fc gamma RII-mediated regulation of human B cells

Recent studies have provided considerable insight into the mechanism of BCR-mediated B-cell activation, but the inhibitory signals transferred by Fc gamma Rs leading to down-regulation of BCR-activated B lymphocytes are not clarified yet. In the present paper the authors give an overview on new findings regarding BCR structure and signal transduction mechanisms induced by the B-cell antigen receptor complex and outline, partly based on their own observations, the possible mechanisms resulting in Fc gamma R-mediated inhibition of B cells.

The polymorphic human TLX-B/CD46/MCP system and its implications in transplantation and reproduction

TLX antigens have been found on most peripheral blood cells, trophoblasts, seminal vesicle cells and sperms. These antigens seem to be associated with the membrane cofactor protein (MCP) and the CD46 antigen. Alloantibodies to TLX antigens with Fc tau RII-blocking features were obtained by transfusion of leucocytes or platelets. Preliminary population studies revealed that alloantibodies to TLX/CD46/MCP recognize four overlapping specificities. The terminology TLX-B was introduced with specificities TLX-B1, B2, B3, B4 and frequencies obtained in the population were: 38%, 46%, 42% and 26%, respectively. Family studies showed an independent segregation of the TLX and HLA alleles. At the cellular protein on trophoblast, the alloantibody detected a glycoprotein of 66-67 kDa molecular mass, which may correspond to the alpha chain of the TLX/CD46/MCP isotypes. A direct association of the alloantibody with Fc tau RII could be excluded thus its FcR blocking feature is probably based on an indirect functional effect. After transfusion and in pregnancy the induction of TLX alloantibody production depended on the mismatching in the TLX/CD46/MCP phenotypes. Probable associations were revealed in the case of recurrent habitual abortion between the lack of Fc tau R blocking antibody production and the matched TLX specificities of the couples. After transfusion, TLX alloantibody production with Fc tau R and MLR blocking function was induced only when the recipient was lacking the TLX specificities expressed on the donor cells. Suppression of MLR was found only when TLX specificity in sera corresponded to the TLX specificity of the effector cell. The immunopathological importance of these findings in transplantation and reproductive medicine has yet to be clarified.

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.

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.

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.

Effect of human IgG1 peptides on the antigen-specific antibody response of mice in vivo

The effect of synthetic peptides--corresponding to the amino acid sequences 289-301 (Y48) and 293-301 (Y91) within the CH-2 domain in the human IgG1 was studied on the oxazolone-specific primary and secondary antibody response isotype distribution and on the sheep erythrocyte (SRBC)-specific primary IgM response. High responder (Balb/c) and low responder (C57Bl/6) mice to oxazolone hapten were treated intraperitoneally with various doses of peptides simultaneously with the first and second contact sensitization. The relative levels of oxazolone-specific IgM, IgG3, IgG1, IgG2a and IgG2b antibodies were determined by a solid phase radioimmunoassay. Y48 and Y91 peptides in a dose range of 10(-5) - 10(-8) M/animal enhanced the oxazolone-specific antibody response. This effect was more striking under suboptimal conditions: using smaller antigen dose for sensitization, cyclophosphamide pretreatment or using genetically low responder mice. SRBC-specific primary IgM response was enhanced by Y91 peptide, Y48 was ineffective.

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.

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.

Modulatory effect of synthetic human IgG Fc peptides on the in vitro immune response of murine spleen cells

Synthetic peptides representative of defined surface-exposed sequences within the CH-2 and CH-3 domains of human IgG1 induce IgM production by murine spleen cells, even in cultures depleted of T-lymphocytes. This stimulation was not altered by simultaneous administration of dextran sulphate in suboptimal concentration, its effect being additive to that of the peptides. Cell proliferation was augmented only at 10(-4) M doses of peptides. IL-1 production by adherent cells was also increased as a result of peptide treatment; whilst administration of exogenous IL-1, 4 h later, seemed to abrogate the effect of peptide treatment on the augmentation of IgM production. Peptide treatment failed to induce IL-2 and/or IL-4 production. The effect of IgG peptides seems to be exerted directly on B-cells at an early step of activation and to be mediated at least in part by IL-1.

Fine specificity of a rabbit antibody interacting with human IgG Fc receptor-like molecules

A polyclonal rabbit antibody raised against an Fc receptor (FcR)-like membrane glycoprotein fraction of chronic leukaemic lymphocytes has previously been prepared and partially characterized. This antibody, called AbA, was found to precipitate a 70-kDa and a 45-kDa fraction of the detergent lysate of U937 cells and to inhibit ligand binding to Fc gamma R on the P388D1 murine macrophage cell line. In the present work we have characterised this antibody further. All Fc gamma RII-positive B lymphoblastoid cell lines, as well as resting human B lymphocytes, were positively stained with the AbA antibody. U937 cells were found to be negative, but after stimulation with phorbol ester (PMA), 50% of the cells became positive. AbA antibody did not react with human T cell lines or with the T + 0 cell subset of peripheral blood. Monocytes were also negative. On the other hand, AbA antibody exhibited a dose-dependent inhibition of antibody-mediated cytotoxic reaction (ADCC) of monocytes, while not affecting K cell-mediated ADCC. It had an inhibitory effect of EA rosette formation of B cells and stimulated U937 cells. Furthermore, it interacted with the soluble form of Fc gamma RII released by activated B lymphocytes, and--similarly to IgG--precipitated a 33 kDa fraction from the supernatant of B cells.

Synthesis and functional studies of tuftsin analogs containing isopeptide bond

In the present paper a new approach is reported, to increase the resistance of tuftsin toward enzymatic cleavage by the introduction of an isopeptide bond into the molecule. The tetrapeptides H-Lys(Thr)-Pro-Arg-OH and H-Lys(Ala)-Pro-Arg-OH, the pentapeptides H-Thr-Lys(Ala)-Pro-Arg-OH, H-Thr-Lys(Thr)-Pro-Arg-OH and H-Ala-Lys(Ala)-Pro-Arg-OH and their For- and Boc-protected derivatives were built up by stepwise elongation of the chain, using conventional solution-phase methods. Preliminary experiments confirmed that from the Lys residue in position 2 of tuftsin the alpha-peptide bond between the Thr and Lys is cleaved with a significantly higher rate by leucine aminopeptidase than the epsilon-peptide bond. Several of the isopeptide derivatives increased to a higher extent the interleukin (IL-1) secretion by monocytes than tuftsin or [Ala1]-tuftsin.

Distinctive role of IgG1 and IgG3 isotypes in Fc gamma R-mediated functions

Polyclonal and monoclonal anti-Rh (D) antibodies of IgG1 and IgG3 subclass were evaluated for their capacity to sensitize erythrocytes and (i) to trigger monocyte and K-cell mediated antibody-dependent cellular cytotoxicity (ADCC); (ii) to mediate binding to monocyte and lymphocyte Fc gamma R; (iii) to stimulate phagocytosis by monocytes. All antibodies were equally effective in mediating monocyte or activated U937 cell ADCC but IgG1 was more active than IgG3 in K-cell mediated ADCC. IgG3-sensitized erythrocytes inhibited IgG1-induced lysis, suggesting that each subclass engages the same Fc gamma R receptor but that lysis requires a further 'signal' that the IgG3 molecule can not deliver. Two monoclonal IgG3 anti-D antibodies were shown to have higher binding (two times) and phagocytic (three times) indices than IgG1 antibody for monocytes; similar differences were observed for polyclonal IgG1 and IgG3 antibodies. The same pattern was observed in an EA rosette assay when a total lymphocyte population was used; however, this difference was not seen with a B-cell depleted (T+ null cell) lymphocyte population.

Lymphocytes stimulated by allogeneic B cell lines cleave the third component of complement and fix C3 fragments. Their nonspecific lytic capacity is elevated against complement receptor type 2-carrying targets

Human blood lymphocytes stimulated in mixed cultures by allogeneic B cell lines were shown to cleave C3 molecules. The B cell lines were derived from Burkitt lymphoma patients: 1) established from their EBV-negative lymphoma, 2) the EBV-positive sublines converted in vitro, and 3) lymphoblastoid cell lines (LCL) i.e., B lymphocytes transformed in vitro by EBV. These cell lines differed considerably in their capacity to stimulate allogeneic lymphocytes. The split products of C3 were detected in the supernatants and on the surface of the activated lymphocytes at levels which correlated with the strength of stimulation. Lymphocytes cultured with LCL had the highest levels of thymidine incorporation blast transformation, C3 cleavage, and C3 fragment fixation. Lymphocytes exposed to the EBV-negative Burkitt lymphomas were stimulated weakly and their C3-activating capacity was low. Irrespective of the efficiency of lymphocyte stimulation induced in the cultures, 60 to 70% of the blasts were found to fix C3 fragments. The majority of the lymphocytes which fixed C3 fragments were T blasts that carried the CD3 marker and expressed IL-2R (CD25). CD4 and CD8 cells were represented with equal frequency in the C3-fragment fixing and C3-fragment negative populations. Pre-exposure of the MLC-activated lymphocytes to human serum increased their cytotoxic capacity toward CR type 2-carrying targets. The enhanced lysis was abrogated by F(ab)2 rabbit anti-human C3d or rabbit anti-CR type 2 antibodies. The results suggest that the C3 fragments fixed on the lymphocytes bind to CR on the targets and elevate the avidity of binding between the two interacting cells. This was also indicated by an increase in the frequency of lymphocyte-target conjugates.

Lymphocytes stimulated by allogeneic B cell lines cleave the third component of complement and fix C3 fragments. Their nonspecific lytic capacity is elevated against complement receptor type 2-carrying targets

Human blood lymphocytes stimulated in mixed cultures by allogeneic B cell lines were shown to cleave C3 molecules. The B cell lines were derived from Burkitt lymphoma patients: 1) established from their EBV-negative lymphoma, 2) the EBV-positive sublines converted in vitro, and 3) lymphoblastoid cell lines (LCL) i.e., B lymphocytes transformed in vitro by EBV. These cell lines differed considerably in their capacity to stimulate allogeneic lymphocytes. The split products of C3 were detected in the supernatants and on the surface of the activated lymphocytes at levels which correlated with the strength of stimulation. Lymphocytes cultured with LCL had the highest levels of thymidine incorporation blast transformation, C3 cleavage, and C3 fragment fixation. Lymphocytes exposed to the EBV-negative Burkitt lymphomas were stimulated weakly and their C3-activating capacity was low. Irrespective of the efficiency of lymphocyte stimulation induced in the cultures, 60 to 70% of the blasts were found to fix C3 fragments. The majority of the lymphocytes which fixed C3 fragments were T blasts that carried the CD3 marker and expressed IL-2R (CD25). CD4 and CD8 cells were represented with equal frequency in the C3-fragment fixing and C3-fragment negative populations. Pre-exposure of the MLC-activated lymphocytes to human serum increased their cytotoxic capacity toward CR type 2-carrying targets. The enhanced lysis was abrogated by F(ab)2 rabbit anti-human C3d or rabbit anti-CR type 2 antibodies. The results suggest that the C3 fragments fixed on the lymphocytes bind to CR on the targets and elevate the avidity of binding between the two interacting cells. This was also indicated by an increase in the frequency of lymphocyte-target conjugates.

Elevated NK sensitivity of Raji cells carrying acceptor-bound C3 fragments

The majority of cell lines derived from Burkitt lymphomas carry CR2 on their plasma membrane cell lines of haematopoietic origin can activate C3 present in human serum through the alternative pathway. However, only the lines that carry CR2 were shown to bind C3 fragments. This bond can be either fixation to acceptor sites or attachment to the CR. Our studies with Raji cells showed that when the possibility for the covalent acceptor bond was eliminated by using methylamine (MA)- or zymosan-treated serum, considerably lower amounts of C3 were bound. In the zymosan-treated serum C3 fragments are present that can bind to receptors but their capacity for acceptor bond is absent. These results indicate that when Raji cell are incubated in human serum some of the generated C3 fragments are bound to acceptors and a lower proportion through the specific interaction with complement receptors. Pretreatment of the CR2 carrying cell lines with human serum elevated their sensitivity to the lytic effect of human blood lymphocytes. We showed in this work that MA-treated serum did not induce this elevation. Zymosan-treated serum under conditions that excluded activation of the residual native C3 molecules, i.e., in the presence of EDTA, did not have the enhancing effect either. These results suggest that the increased lytic efficiency imposed by human serum was due to cleavage of C3 molecules by Raji and fixation of the C3 fragments by acceptor sites. Natural killer cells carry CR3; therefore it is likely that the attached C3 fragments bind also to the effector cells. The C3 molecules could elevate thereby the avidity between the target and the lytic lymphocytes. The observation that C3 fragments are not bound to the surface of CR2 negative lines in spite of their capacity to activate C3 suggests that the receptor molecule is either involved in the activation and/or serves also as an acceptor.

The effect of synthetic peptides corresponding to Fc sequences in human IgG1 on various steps in the B cell activation pathway

The influence of synthetic peptides comprising sequences in the exposed positions of the Fc region of human IgG 1 was tested on B lymphocyte activation. CH 2 domain peptides having an inhibitory effect on antibody-dependent cellular cytotoxicity, as well as the whole Fc fragment, induced the appearance of the early signs of activation on resting B lymphocytes such as increase in cell volume and HLA-DR antigen expression or leukocyte migration inhibitory factor production. The peptides did not induce proliferation of resting B cells even when B cell growth factor (BCGF)-containing supernatants were added. Exposure to Fc fragment, however, induced a weak proliferation which was significantly enhanced by BCGF. On the other hand, both the Fc fragment and the CH 2 or CH 3 domain peptides enhanced the IgM synthesis of human blood mononuclear cells when a suboptimal dose of pokeweed mitogen was present. This effect was lost when Fc fragment or the peptides were added on the third day of culture. These results suggest that the early steps of B cell activation can be induced by Fc fragment and by small molecular weight Fc peptides, which are potential ligands of Fc receptors. The Fc fragment activates B cells to the state where they respond to BCGF, but the peptides do not possess this activity. Furthermore, both Fc fragment and Fc peptides are able to enhance the IgM synthesis, when accessory cells and the appropriate differentiating factors are present.

Alternative pathway of complement activation by stimulated T lymphocytes. II. Elevation of cytotoxic potential against complement receptor-carrying cell lines

Exposure of lectin-stimulated (concanavalin A, phytohemagglutinin and pokeweed mitogen) blood lymphocytes to human serum or to purified C3 increased their cytotoxic capacity towards complement receptor positive targets such as Raji and Daudi cells. The lysis of complement receptor-negative lymphoblastoid cell lines was not influenced. The lytic capacity of lymphocytes exposed to 12-O-tetradecanoylphorbol 13-acetate was not elevated by human serum. Lectin-stimulated lymphocytes were previously shown to activate and bind C3. The results using lymphocytes activated in different ways and targets with or without complement receptor expression suggest that the C3b deposited on lymphocytes binds to the complement receptor on the targets. This contact elevates the avidity between the two cells as indicated also by the increased frequency of the lymphocyte-target conjugates. On the basis of immune adherence the C3 fragment bound on the lymphocytes was identified as C3b. The increase of the conjugate formation and cytotoxicity was abrogated when the target cells, Raji, were pre-exposed to purified C3d which occupy the CR2 receptor. The majority of lymphocytes responsible for the cytotoxicity were CD8+.

Alternative pathway of complement activation by stimulated T lymphocytes. I. Binding of C3 fragments

Human blood lymphocytes cultured for 3 days with concanavalin A (Con A), phytohemagglutinin or pokeweed mitogen, in mixed lymphocyte culture with added interleukin 2 and stimulated by a lymphoblastoid cell line were found to activate and bind C3 molecules when exposed to human serum. The split products of C3 were detected in the supernatants and on the surface of the activated cells. The surface-attached C3 fragment on the Con A blast was identified as C3b by immune adherence i.e. binding of CR1 carrying human erythrocytes. In the Con A-stimulated population the majority of cells that activated and bound C3 were CD3 and Fc gamma receptor (CD16)-positive but complement receptor-negative blasts. In this cell subset both CD4 and CD8-positive cells were detected but their frequency suggested that a proportion of them carried both markers.

CH2 and CH3 domain deleted IgG1 paraproteins inhibit differently Fc receptor mediated binding and cytolysis

Domain deleted paraproteins are suitable tools to study the interaction between IgG domains and Fc receptor (FcR) binding sites. The effect of the C gamma 2 or C gamma 3 domain deleted paraproteins was compared on antibody dependent cellular cytotoxicity (ADCC) and on FcR mediated rosette formation. The C gamma 2 domain deleted paraprotein (TIM) had no significant effect on lymphocyte or monocyte mediated ADCC, while the C gamma 3 domain deleted paraprotein (SIZ) inhibited both types of cytotoxicity. FcR dependent rosette formation was also inhibited by SIZ but TIM was ineffective. The data further confirm our previous results suggesting a significant role of C gamma 2 domain in the transfer of killing signal in ADCC and that of C gamma 3 domain in the high affinity binding to lymphocyte FcR.

Mapping the functional topography of Fc gamma with monoclonal antibodies: localization of epitopes interacting with the binding sites of Fc receptor on human K cells

A panel of monoclonal antibodies (mAb) specific for the C gamma 2, C gamma 3 or inter C gamma 2/C gamma 3 domain epitopes was tested for inhibition of antibody-dependent cellular cytotoxicity (ADCC) specific for anti-D IgG-coated erythrocytes. Significant inhibition of ADCC was demonstrable for some antibodies having specificity for C gamma 2 or C gamma 3 domain epitopes, while others gave no inhibition. Fab fragments of a representative C gamma 2-specific antibody (A55) and C gamma 3-specific antibody (x3a8) retained their inhibitory capacity in lymphocyte-mediated ADCC, but only A55 Fab inhibited monocyte-mediated lysis. Furthermore, the Fab portion of A55 completely abolished the complement-dependent enhancement of ADCC mediated by concanavalin A-stimulated cells, while x3a8 Fab had no effect in this system. On the other hand, x3a8 Fab inhibited the binding of anti-D IgG-sensitized erythrocytes to lymphocytes while A55 Fab did not influence this latter interaction. The results suggest that C gamma 2 domain-FcR interaction is essential for the triggering of lytic process both in lymphocyte and in monocyte-mediated ADCC, while C gamma 3 domain has no role in the latter but is responsible for the appropriate contact between effector lymphocytes and target cells. A site in the region of Lys274 appears to be critical for triggering of both lymphocyte and monocyte-mediated ADCC.

Complement-dependent cellular cytotoxicity: lymphoblastoid lines that activate complement component 3 (C3) and express C3 receptors have increased sensitivity to lymphocyte-mediated lysis in the presence of fresh human serum

Lymphocyte-mediated lysis of cells of the Raji, Daudi, Jijoye, and Bjab lines was elevated when fresh human serum was added to the assay. A higher proportion of effector-target conjugates was observed in the presence of human serum. In similar experiments lysis of 1301, Rael, and P3HR-1 cells was unaltered. All cell lines activated the alternative pathway of complement but they varied in the expression of receptors for complement component 3 (C3) and in the ability to fix the C3 cleavage products on their membrane. The enhancement of lysis in the presence of human serum occurred only with those cells that bound C3. This characteristic was correlated to the expression of C3 receptors. Analysis of the nature of the deposited C3 was performed with Raji cells. Raji cells exposed to human serum bound C3b as indicated by the immunoadherence test. The C3b was further processed to C3bi, because the immunoadherence declined with time and conjugate formation increased with Daudi cells, which carry the C3 receptors CR2 and CR3. This suggests that in the lytic assay lymphocytes with C3bi receptors are recruited in the presence of human serum. We assume that the bridge of C3 molecules between targets and effectors increases the avidity of their interaction.

Ligand inhibition studies on the role of Fc receptors in antibody-dependent cell-mediated cytotoxicity

Subjection of human peripheral blood lymphocytes to a temp shift from 4 to 37 degrees C resulted in a shedding of Fc receptors (termed FcRI) from 40-50% of FcR-positive cells followed by their re-expression within 4 hr; a phenomenon which had no effect on the cells' antibody-dependent killing capacity. Removal of lymphocytes having an immobile form of the Fc receptor resistant to the effects of the temp shift (termed FcRII), or removal of lymphocytes bearing both FcRI and FcRII, resulted in a similar amount of reduction in ADCC activity. This was attributed, therefore, to the loss of FcRII-positive cells. The influence of isolated (shedded) FcRI and Clq on ADCC activity was investigated. Soluble FcRI was shown to inhibit ADCC mediated through the immobile Fc receptors (FcRII), despite its lack of an ability to block EA rosette formation through these receptors. Clq also had a dose-dependent inhibitory effect on ADCC. These observations are consistent with earlier findings that FcRII possesses two active binding sites; and suggest that a prerequisite for killing in ADCC is the interaction of these with the C gamma 2 and C gamma 3 domains. The ability of synthetic peptides representative of human gamma 1-chain sequences to inhibit ADCC was determined, in an attempt to locate those sites within the IgG antibody Fc region involved in interaction with two FcR binding sites. Preliminary evidence was obtained to suggest that one of these is situated within the C gamma 2 domain, in the region of residues 274 (Lys)-294 (Glu).

Activation of lymphocytes alters Fc receptor-beta 2-microglobulin interrelationship on the lymphocyte surface

The effect of anti-beta 2-microglobulin (anti-B2Mi) on the expression of Fc receptors (FcR) of human lymphocytes was compared on resting and activated cells. Previously we reported that anti-B2Mi induces a "co-shedding" of FcR with the beta 2-microglobulin (B2Mi)-anti-B2Mi complexes when used under the conditions where the redistribution of membrane molecules is allowed (Sármay et al., Cell. Immunol. 56, 452, 1980; Sármay et al. Immunology 36, 339, 1979). Furthermore our group also described two types of FcR-bearing cells, one which shed their FcR during a temperature shift from 4 to 37 degrees C (FcRI+ cells) and the other which has an immobile type FcR under the same circumstances (FcRI+ cells) (Sándor et al., Immunology 38, 553, 1979; Sármay et al., Immunology 34, 315, 1978). In this work we have characterized the FcR released from the membrane as a consequence of anti-B2Mi treatment. We have found that they are the mobile, FcRI type. It was proved that the shedding of this FcRI is a consequence of the anti-B2MI-induced transformation of FcRII into the FcRI form on the membrane of the antibody-treated lymphocytes. On the activated T cells, however, anti-B2Mi is incapable of inducing the same phenomenon in the early phase of activation. In contrast, FcR expression is blocked by anti-B2Mi treatment similarly to that on resting lymphocytes, on the surface of activated B cells, or on activated T cells in the later phases of activation.

Recognition of autologous and allogeneic lymphocytes and tumor cells by human natural killer cells

The shedding of the mobile Fc receptor (FcR1) and the depletion of the immobile Fc receptor (FcR11) bearing human lymphocytes revealed that human natural killer cells belong to the FcR11-bearing population. Anti-beta-2-microglobulin treatment of the effector cells decreased natural cytotoxicity against some target cells and the detectability of HLA antigens, indicating that histocompatibility antigens or related structures may be involved in natural cytotoxicity. Using a panel of 29 autologous and allogenic PHA-stimulated target cells and peripheral lymphocytes from the same donors as the effector cells, distinct cytotoxic responses against allogeneic and autologous target cells were observed. A computer analysis of selective natural cytotoxicity distinguished seven different groups of target cells that may represent common structures for NK recognition.

Shedding and reappearance of Fc, C3 and SRBC receptors on peripheral lymphocytes from normal donors and chronic lymphatic leukaemia (CLL) patients

The shedding and resynthesis of Fc, C3 receptors, and receptors binding SRBC has been studied in normal peripheral lymphocytes, in lymphocytes from CLL patients and in a Raji cell line during 4 h incubation at 37 degrees. During the first part of the incubation, an increased release of receptors was observed. The shed, functionally active, receptors were detectable in the incubating medium on the basis of their ability to inhibit rosette formation specifically. After 4 h incubation the original ratios of the different rosette forming cells were observed, showing that the cells were capable of resynthesizing their shed receptors. This later phenomenon may be the result of an active metabolic process. The shedding was not detectable on Raji cells. Different patterns of receptor shedding and resynthesis were demonstrated by the lymphocytes of CLL patients. The correlations between the reappearance of the receptors on the cells, and the metabolism of the lymphocytes, is discussed.