Regulation of in vitro and in vivo immune functions by the cytosolic adaptor protein SKAP-HOM

SKAP-HOM is a cytosolic adaptor protein representing a specific substrate for the Src family protein tyrosine kinase Fyn. Previously, several groups have provided experimental evidence that SKAP-HOM (most likely in cooperation with the cytosolic adaptor protein ADAP) is involved in regulating leukocyte adhesion. To further assess the physiological role of SKAP-HOM, we investigated the immune system of SKAP-HOM-deficient mice. Our data show that T-cell responses towards a variety of stimuli are unaffected in the absence of SKAP-HOM. Similarly, B-cell receptor (BCR)-mediated total tyrosine phosphorylation and phosphorylation of Erk, p38, and JNK, as well as immunoreceptor-mediated Ca(2+) responses, are normal in SKAP-HOM(-/-) animals. However, despite apparently normal membrane-proximal signaling events, BCR-mediated proliferation is strongly attenuated in the absence of SKAP-HOM(-/-). In addition, adhesion of activated B cells to fibronectin (a ligand for beta1 integrins) as well as to ICAM-1 (a ligand for beta2 integrins) is strongly reduced. In vivo, the loss of SKAP-HOM results in a less severe clinical course of experimental autoimmune encephalomyelitis following immunization of mice with the encephalitogenic peptide of MOG (myelin oligodendrocyte glycoprotein). This is accompanied by strongly reduced serum levels of MOG-specific antibodies and lower MOG-specific T-cell responses. In summary, our data suggest that SKAP-HOM is required for proper activation of the immune system, likely by regulating the cross-talk between immunoreceptors and integrins.

The CD2v protein of African swine fever virus interacts with the actin-binding adaptor protein SH3P7

The predicted extracellular domain of the CD2v protein of African swine fever virus (ASFV) shares significant similarity to that of the CD2 protein in T cells but has a unique cytoplasmic domain of unknown function. Here we have shown that CD2v is expressed as a glycoprotein of approximately 105 kDa in ASFV-infected cells. In the absence of an extracellular ligand, the majority of CD2v appears to localize to perinuclear membrane compartments. Furthermore, we have shown using the yeast two-hybrid system and by direct binding studies that the cytoplasmic tail of CD2v binds to the cytoplasmic adaptor protein SH3P7 (mAbp1, HIP55), which has been reported to be involved in diverse cellular functions such as vesicle transport and signal transduction. A cDNA clone encoding a variant form of SH3P7 could also be identified and was found to be expressed in a wide range of porcine tissues. Deletion mutagenesis identified proline-rich repeats of sequence PPPKPC in the ASFV CD2v protein to be necessary and sufficient for binding to the SH3 domain of SH3P7. In ASFV-infected cells, CD2v and SH3P7 co-localized in areas surrounding the perinuclear virus factories. These areas also stained with an antibody that recognizes a Golgi network protein, indicating that they contained membranes derived from the Golgi network. Our data provide a first molecular basis for the understanding of the immunomodulatory functions of CD2v in ASFV-infected animals.

B cell signaling. Introduction

The B cell antigen receptor (BCR) is composed of the membrane form of the immunoglobulin (Ig) and the Ig-alpha/Ig-beta heterodimer, which function as the antigen recognition component and the signaling component, respectively. A signal transmitted by BCR modulates gene expression, adhesion or survival, thereby determining the fate of antigen-encountered B cells. BCR proximal signaling occurs within cholesterol- and sphingolipid-rich plasma membrane microdomains termed lipid rafts, and involves tyrosine kinases such as Lyn, Syk and Btk and the adapter molecule SLP65/BLNK. Although the distal signaling cascades via BCR are not yet fully elucidated, various components are already identified, such as lipid kinases and small G-proteins. BCR signaling is regulated by various membrane molecules termed co-receptors such as CD19 and CD22. The BCR co-receptors appear to be required for normal immune functions. Viral proteins such as LMP2 also regulate BCR signaling to maintain viral latency. Various aspects of BCR signaling and its regulatory mechanisms are discussed in this issue.

Multitasking of Ig-alpha and Ig-beta to regulate B cell antigen receptor function

Since their discovery as signaling subunits of the B cell antigen receptor (BCR), Ig-alpha and Ig-beta are discussed to serve either a redundant or distinct function for B cell development, maintenance, and activation. Dependent upon the experimental system that has been used to address this issue, evidence could be provided to support both possibilities. Only recently has it become clear that Ig-alpha and Ig-beta possess a unique signaling identity but that both together are required to orchestrate proper B cell function in vivo. Here we discuss some of the underlying mechanisms that may involve direct coupling to discrete subsets of BCR effector proteins, such as protein tyrosine kinases or the intracellular adaptor SLP-65/BLNK.

SHP-1 Requires Inhibitory Co-receptors to Down-modulate B Cell Antigen Receptor-mediated Phosphorylation of Cellular Substrates

Signaling through the B cell antigen receptor (BCR) is negatively regulated by the SH2 domain-containing protein-tyrosine phosphatase SHP-1, which requires association with tyrosine-phosphorylated proteins for activation. Upon BCR ligation, SHP-1 has been shown to associate with the BCR, the cytoplasmic protein-tyrosine kinases Lyn and Syk, and the inhibitory co-receptors CD22 and CD72. How SHP-1 is activated by BCR ligation and regulates BCR signaling is, however, not fully understood. Here we demonstrate that, in the BCR-expressing myeloma line J558L mu 3, CD72 expression reduces the BCR ligation-induced phosphorylation of the BCR component Ig alpha/Ig beta and its cytoplasmic effectors Syk and SLP-65. Substrate phosphorylation was restored by expression of dominant negative mutants of SHP-1, whereas the SHP-1 mutants failed to enhance phosphorylation of the cellular substrates in the absence of CD72. This indicates that SHP-1 is efficiently activated by CD72 but not by other pathways in J558L mu m3 cells and that inhibition of SHP-1 specifically activated by CD72 reverses CD72-induced dephosphorylation of cellular substrates in these cells. Taken together, BCR-induced SHP-1 activation is likely to require inhibitory co-receptors such as CD72, and SHP-1 appears to mediate the negative regulatory effect of CD72 on BCR signaling by dephosphorylating Ig alpha/Ig beta and its downstream signaling molecules Syk and SLP-65.

Association of SLP-65/BLNK with the B cell antigen receptor through a non-ITAM tyrosine of Ig-alpha

The cytoplasmic adaptor protein SLP-65 (BLNK or BASH) is a critical downstream effector of the B cell antigen receptor (BCR). Tyrosine-phosphorylated SLP-65 assembles intracellular signaling complexes such as the Ca(2 +) initiation complex encompassing phospholipase C-gamma2 and Bruton's tyrosine kinase. It is, however, unclear how the SLP-65 signaling module can be recruited to the plasma membrane. Here we show that following B cell stimulation, SLP-65 associates directly with the BCR signaling subunit, the Ig-alpha / Ig-beta heterodimer. The interaction is mediated by the Src homology 2 domain of SLP-65 and the phosphorylated Ig-alpha tyrosine 204, which is located outside of the immunoreceptor tyrosine-based activation motif. Our data identify an unexpected BCR phosphorylation pattern and indicate that Ig-alpha has the capability to serve as transmembrane adaptor in BCR signaling.

An unsolved problem of the clonal selection theory and the model of an oligomeric B-cell antigen receptor

The B cell antigen receptor (BCR) plays a central role in the development, survival and activation of B lymphocytes. As the pre-BCR, it controls allelic exclusion of heavy chains and the expansion of pre-B cells. As the BCR, it controls the positive and negative selection of immature B cells as well as the survival and activation of mature B cells. Recent studies of receptors have shown that it is the ligand that brings them into the conformation necessary for signaling. How the multiple and structurally diverse antigens could fulfill this task for the BCR is unknown, and we regard this as an unsolved problem of Burnet's clonal selection theory This question and our recent biochemical studies lead us to propose a new model for the BCR, according to which the BCR exists as a precise oligomeric complex on the B cell surface. In this form, it can signal positive selection and survival of B cells. Binding to self- or foreign antigen results in a distortion of the oligomeric complex that gives the signal for negative selection of immature and activation of mature B cells.

Signal transduction elements of the B cell antigen receptor and their role in immunodeficiencies

The primary function of B lymphocytes is to contribute to the elimination of foreign antigens by producing large amounts of soluble antibodies. The activation of B cells through their antigen receptor triggers a dynamic network of intracellular signaling proteins. The recent identification of the cytoplasmic adaptor protein SLP-65 (also called BLNK or BASH) provided insight in how the antigen receptor-regulated protein tyrosine kinases couple to downstream signaling cascades, including the mobilization of Ca2+ ions, activation of mitogen-activated kinases and reorganization of the cytoskeleton architecture. While these events have been mostly studied in mature B cells, it is now clear that the components of the antigen receptor and its downstream effector elements play also a central role during early and late B cell development, and in the apoptotic elimination of B cells with reactivity to self-antigens. Thus, genetic defects affecting the expression of antigen receptor subunits or its intracellular signaling proteins can interfere with B cell development and activation, and can cause severe antibody deficiencies in mouse and man. In this article I summarize our current picture of the B cell antigen receptor, how the extracellular signal is transported into the cell interior, and how dysregulation of these processes contribute to immune defects.

The serine and threonine residues in the Ig-alpha cytoplasmic tail negatively regulate immunoreceptor tyrosine-based activation motif-mediated signal transduction

The B cell antigen receptor (BCR) is a multiprotein complex consisting of the membrane-bound Ig molecule and the Ig-alpha/Ig-beta heterodimer. On BCR engagement, Ig-alpha and Ig-beta become phosphorylated not only on tyrosine residues of the immunoreceptor tyrosine-based activation motif but also on serine and threonine residues. We have mutated all serine and threonine residues in the Ig-alpha tail to alanine and valine, respectively. The mutated Ig-alpha sequence was expressed either as a single-chain Fv/Ig-alpha molecule or in the context of the complete BCR. In both cases, the mutated Ig-alpha showed a stronger tyrosine phosphorylation than the wild-type Ig-alpha and initiated increased signaling on stimulation. These findings suggest that serine/threonine kinases can negatively regulate signal transduction from the BCR.

Abnormal development and function of B lymphocytes in mice deficient for the signaling adaptor protein SLP-65

During signal transduction through the B cell antigen receptor (BCR), several signaling elements are brought together by the adaptor protein SLP-65. We have investigated the role of SLP-65 in B cell maturation and function in mice deficient for SLP-65. While the mice are viable, B cell development is affected at several stages. SLP-65-deficient mice show increased proportions of pre-B cells in the bone marrow and immature B cells in peripheral lymphoid organs. B1 B cells are lacking. The mice show lower IgM and IgG3 serum titers and poor IgM but normal IgG immune responses. Mutant B cells show reduced Ca2+ mobilization and reduced proliferative responses to B cell mitogens. We conclude that while playing an important role, SLP-65 is not always required for signaling from the BCR.

Interaction of SLP adaptors with the SH2 domain of Tec family kinases

Activation of lymphocytes through their antigen receptors leads to mobilization of intracellular Ca(2+) ions. This process requires expression of SLP adaptors and involves phosphorylation of phospholipase C-gamma isoforms by the Tec-related protein tyrosine kinase Btk in B cells and Itk in T cells. The SH2 domain of Btk and Itk is essential for phospholipase C-gamma phosphorylation and mutations in this domain lead to the X-linked agammaglobulinemia immuno deficiency in humans. Here we show that, in contrast to SH2 domains from other signaling proteins, the Btk and Itk SH2 domains exhibit a restricted binding specificity. They bind selectively to tyrosine-phosphorylated SLP-65 and SLP-76 in activated B and T cells, respectively. Our findings suggest that Btk/Itk and phospholipase C-gamma both bind via their SH2 domain to phosphorylated SLP adaptors, and that this association is required for the activation of phospholipase C-gamma.

Analysis of tyrosine phosphorylation-dependent interactions between stimulatory effector proteins and the B cell co-receptor CD22

The B cell-restricted transmembrane glycoprotein CD22 is rapidly phosphorylated on tyrosine in response to cross-linking of the B cell antigen receptor, thereby generating phosphotyrosine motifs in the cytoplasmic domain which recruit intracellular effector proteins that contain Src homology 2 domains. By virtue of its interaction with these effector proteins CD22 modulates signal transduction through the B cell antigen receptor. To define further the molecular mechanism by which CD22 mediates its co-receptor function, phosphopeptide mapping experiments were conducted to determine which of the six tyrosine residues in the cytoplasmic domain are involved in recruitment of the stimulatory effector proteins phospholipase Cgamma (PLCgamma), phosphoinositide 3-kinase (PI3K), Grb2, and Syk. The results obtained indicate that the protein tyrosine kinase Syk interacts with multiple CD22-derived phosphopeptides in both immunoprecipitation and reverse Far Western assays. In contrast, the Grb2.Sos complex was observed to bind exclusively to the fourth phosphotyrosine motif (Y828ENV) from CD22 and does so via a direct interaction based on Far Western and reverse Far Western blotting. Although both PLCgamma and PI3K were observed to bind to multiple phosphopeptides in precipitation experiments, subsequent studies using reverse Far Western blot analysis demonstrated that only the carboxyl-terminal phosphopeptide of CD22 (Y863VTL) binds directly to either one. This finding suggests that PLCgamma and PI3K may be recruited to CD22 either through a direct interaction with Tyr863 or indirectly through an association with one or more intermediate proteins.

Characterization of the B cell-specific adaptor SLP-65 and other protein tyrosine kinase substrates by two-dimensional gel electrophoresis

The identification of substrates for protein tyrosine kinases in B cells is a critical step to a better understanding of the molecular mechanism(s) of lymphocyte activation through the antigen receptor. The substrate proteins were immunopurified from stimulated B cells and separated by two-dimensional gel electrophoresis techniques using either the isoelectric focussing (IEF)/SDS-PAGE or the non-equilibrium PH gradient electrophoresis (NEPHGE)/SDS-PAGE method. The biochemical characteristics of the proteins (isoelectric point and relative molecular mass) obtained and the subsequent use of antibodies that are specific for different cellular proteins confirmed the participation of HS1, Vav, Ig-alpha, Lyn and Btk in antigen receptor-mediated signal transduction. The heat shock cognate protein HSC70 was identified as a novel substrate protein in activated B cells. An important signaling function has previously been suggested for a 65-kDa protein (p65), whose phosphorylation can be detected before that of other substrate proteins. The analysis identified p65 as a so far unknown protein. Based on p65 peptide sequences, the full length cDNA was isolated and found to encode a B cell-specific adaptor protein, called SLP-65.

SH3P7 is a cytoskeleton adapter protein and is coupled to signal transduction from lymphocyte antigen receptors

Lymphocytes respond to antigen receptor engagement with tyrosine phosphorylation of many cellular proteins, some of which have been identified and functionally characterized. Here we describe SH3P7, a novel substrate protein for Src and Syk family kinases. SH3P7 migrates in sodium dodecyl sulfate-polyacrylamide gel electrophoresis as a 55-kDa protein that is preferentially expressed in brain, thymus, and spleen. It contains multiple amino acid sequence motifs, including two consensus tyrosine phosphorylation sites of the YXXP type and one SH3 domain. A region of sequence similarity, which we named SCAD, was found in SH3P7 and three actin-binding proteins. The SCAD region may represent a new type of protein-protein interaction domain that mediates binding to actin. Consistent with this possibility, SH3P7 colocalizes with actin filaments of the cytoskeleton. Altogether, our data implicate SH3P7 as an adapter protein which links antigen receptor signaling to components of the cytoskeleton.

Induction of the antigen receptor expression on B lymphocytes results in rapid competence for signaling of SLP-65 and Syk

The binding of antigen to the B cell antigen receptor (BCR) results in the activation of protein tyrosine kinases (PTKs) such as Lyn and Syk, and the phosphorylation of several substrate proteins including HS1 and SLP-65. How these signaling elements are connected to the BCR is not well understood. Using an expression vector for a tamoxifen-regulated Cre recombinase, we have developed a method that allows the inducible expression of the BCR. Disruption of the VH leader reading frame of the immunoglobulin heavy chain by two loxP sites is overcome by Cre-mediated DNA recombination and results in the cell surface expression of the BCR starting 4 h after exposure of transfected B cells to tamoxifen. This method can, in principle, be employed for the inducible expression of any secreted or type I transmembrane protein. By monitoring the activation of signaling elements in pervanadate-stimulated B cells expressing different levels of the BCR, we show here that phosphorylation of SLP-65 and Syk, but not of Lyn, is strictly dependent on the expression of the BCR on the cell surface. These data suggest that the BCR reorganizes its signaling molecules as soon as it appears on the cell surface.

SLP-65: a new signaling component in B lymphocytes which requires expression of the antigen receptor for phosphorylation

The B cell antigen receptor (BCR) consists of the membrane-bound immunoglobulin (Ig) molecule as antigen-binding subunit and the Ig-alpha/Ig-beta heterodimer as signaling subunit. BCR signal transduction involves activation of protein tyrosine kinases (PTKs) and phosphorylation of several proteins, only some of which have been identified. The phosphorylation of these proteins can be induced by exposure of B cells either to antigen or to the tyrosine phosphatase inhibitor pervanadate/H2O2. One of the earliest substrates in B cells is a 65-kD protein, which we identify here as a B cell adaptor protein. This protein, named SLP-65, is part of a signaling complex involving Grb-2 and Vav and shows homology to SLP-76, a signaling element of the T cell receptor. In pervanadate/H2O2-stimulated cells, SLP-65 becomes phosphorylated only upon expression of the BCR. These data suggest that SLP-65 is part of a BCR transducer complex.

Initiation and processing of signals from the B cell antigen receptor

Current models of signal transduction from the antigen receptors on B and T cells still resemble equations with several unknown elements. Data from recent knockout experiments in cell lines and mice contradict the assumption that Src-family kinase and tyrosine kinases of the Syk/Zap-70 family are the transducer elements that set signaling from these receptors in motion. Using a functional definition of signaling elements, we discuss the current knowledge of signaling events from the BCR and suggest the existence of an as-yet-unknown BCR transducer complex.

Evidence for a preformed transducer complex organized by the B cell antigen receptor

The B cell antigen receptor (BCR) consists of the membrane-bound immunoglobulin (mIg) molecule and the Ig-alpha/Ig-beta heterodimer, which functions as signaling subunit of the receptor. Stimulation of the BCR activates protein tyrosine kinases (PTKs) that phosphorylate a number of substrate proteins, including the Ig-alpha/Ig-beta heterodimer of the BCR itself. How the PTKs become activated after BCR engagement is not known at present. Here, we show that BCR-negative J558L cells treated with the protein tyrosine phosphatase inhibitor pervanadate/H2O2 display only a weak substrate phosphorylation. However, in BCR-positive transfectants of J558L, treatment with pervanadate/H2O2 induces a strong phosphorylation of several substrate proteins. Treatment with pervanadate/H2O2 does not result in receptor crosslinking, yet the pattern of protein phosphorylation is similar to that observed after BCR stimulation by antigen. The response requires cellular integrity because tyrosine phosphorylation of most substrates is not visible in cell lysates. Cells that express a BCR containing an Ig-alpha subunit with a mutated immunoreceptor tyrosine-based activation motif display a delayed response. The data suggest that, once expressed on the surface, the BCR organizes protein tyrosine phosphatases, PTKs, and their substrates into a transducer complex that can be activated by pervanadate/H202 in the absence of BCR crosslinking. Assembly of this preformed complex seems to be a prerequisite for BCR-mediated signal transduction.

Tyrosine-phosphorylated forms of Ig beta, CD22, TCR zeta and HOSS are major ligands for tandem SH2 domains of Syk

The protein tyrosine kinase Syk plays an important role in signal transduction from the B cell antigen receptor and possibly also from the TCR. We have examined the binding specificity of Syk-derived SH2 domains in vitro and found that the tandem SH2 domains have two major ligands in activated Ramos B cells as well as in activated Jurkat T cells. The SH2-binding proteins in Ramos B cells were identified as the tyrosine-phosphorylated forms of the Ig alpha beta heterodimer and of CD22. Binding to the Ig alpha beta heterodimer seems to occur predominantly via Ig beta, indicating that the two receptor components might couple to distinct signaling pathways. In Jurkat T cells one of the SH2-binding proteins represents the tyrosine-phosphorylated TCR zeta chain. The identity of the second SH2 ligand, called HOSS, is not known. HOSS is discussed as a putative member of the receptor family characterized by the immunoreceptor tyrosine-based activation motif.

The beta D-sheet residues of the Lck-derived SH2 domain determine specificity of the interaction with tyrosine-phosphorylated ligands in Ramos B cells

Src homology 2 (SH2) domains are noncatalytic regions that are conserved among a group of cellular signaling proteins. SH2 domains share the common property of binding phosphotyrosine-containing peptides. Previously, we showed that SH2 domains expressed as recombinant glutathione S-transferase-fusion proteins (GST-SH2) from GTPase-activating protein, Shc, zeta-chain-associated protein tyrosine kinase Zap-70, and Src-like tyrosine kinases precipitated distinct sets of phospho-proteins from activated B cells. To determine the intrinsic structural motifs responsible for the binding specificity within the different SH2 domains, we created chimeric SH2 domains especially focusing on crystal structure-defined contact residues. Recombinant SH2 domains of Lck, Zap-70, and Shc were tested in Ramos B cell lysates for phosphotyrosine-dependent protein binding. Biomolecular interaction analysis (BIAcore) was used to characterize the interaction between the various recombinant SH2 molecules and defined phosphorylated peptides. In agreement with the crystal structure data from the Src and the Lck SH2 domains, our results show that most of the "specificity information" of the Lck SH2 domain is provided by the beta D-sheet, located downstream of the SH2 conserved consensus motif GTFLVRES. In addition, the overall affinity is critically influenced by residues located at the N terminus of the SH2 domain.

In vitro characterization of major ligands for Src homology 2 domains derived from protein tyrosine kinases, from the adaptor protein SHC and from GTPase-activating protein in Ramos B cells

Antigen receptors of B lymphocytes transmit their activation signal to the cell interior by associating with and activation of specific non-receptor tyrosine kinases. Most of these kinases as well as other cytoplasmic effectors contain at least one Src homology 2 (SH2) domain, known to bind tyrosine-phosphorylated proteins. We examined the binding specificity of SH2 domains from different signaling molecules in B cells and found that each of the SH2 domains tested bound distinct subsets of stimulation-dependent phosphoproteins in vitro. SH2 domains from Src-like tyrosine kinases bound predominantly to the HS1 phosphoprotein. The tandem SH2 domains of the ZAP-70 tyrosine kinase bound to phosphorylated Ig-beta but only weakly to Ig-alpha. Also the SHC-derived SH2 domain formed complexes with the tyrosine-phosphorylated Ig-alpha/beta heterodimer, while the C- and N-terminal SH2 domains of GTPase-activating protein displayed completely different binding preferences. These results suggest that cytoplasmic effector molecules can be recruited to the activated B cell receptor in an SH2-phosphotyrosine-mediated manner. The data also provide a possible explanation for the notion that Ig-alpha and Ig-beta might couple to different biochemical pathways.

Glycosyl-phosphatidylinositol linkage as a mechanism for cell-surface expression of immunoglobulin D

The B-cell antigen receptor of the IgM and IgD class is a multimeric complex consisting of the membrane-bound form of the immunoglobulin molecule and two other proteins, Ig-alpha and Ig-beta. The Ig-alpha and Ig-beta proteins form a disulphide-linked alpha/beta heterodimer and are encoded by the mb-1 (ref 9, 10) and B29 genes, respectively. Surface expression of the membrane-bound IgM molecule requires assembly with the alpha/beta heterodimer. The IgD molecule, however, can be expressed on the cell surface in an alpha/beta-dependent and -independent form. We show here that in the alpha/beta-independent form the IgD molecule is anchored in the plasma membrane through a glycosyl-phosphatidylinositol linker. In the presence of the alpha/beta heterodimer, most of the otherwise glycosyl-phosphatidylinositol-linked IgD molecule is expressed on the cell surface as transmembrane proteins.

The B cell antigen receptor of class IgD can be expressed on the cell surface in two different forms

Membrane-bound immunoglobulins of the IgM and IgD class are expressed on the B cell surface in association with a disulfide-linked heterodimer consisting of alpha and beta subunits. While the alpha component of the IgM antigen receptor (IgM-alpha, 34 kDa) is encoded by the B cell-specific gene mb-1, the gene coding for IgD-alpha (35 kDa) has not yet been identified. We show here that the alpha component of the IgD antigen receptor is also encoded by the mb-1 gene. The difference in molecular weight between IgM-alpha and IgD-alpha thus seems to be due to post-translational modifications of the mb-1 gene product. We also demonstrate that the previously described myeloma variant J558L delta m2.6 expresses an alternative form of the IgD antigen receptor, which does not contain an alpha/beta heterodimer.

Membrane IgM and IgD molecules fail to transduce Ca2+ mobilizing signals when expressed on differentiated B lineage cells

We have measured Ca2+ mobilization in a panel of B lineage cell lines after stimulation with anti-Ig to assess whether membrane Ig transduces a functional signal in cells that are representative of immature, mature, or terminally differentiated stages. For these studies, three transfected cell lines which express the same IgM molecule (300-19 microns lambda 36/8, K46-17 microns lambda, and J558L microns lambda 3) as well as two lines expressing an identical IgD molecule (K46 delta m2.6 and J558L delta m8.8) were used. Cross-linking of membrane Ig on IgM+ or IgD+ lymphomas (K46-17 microns lambda or K46 delta m2.6) resulted in a Ca2+ mobilization response that is similar to that seen in mature, resting B cells. Both intracellular release and extracellular influx of Ca2+ were observed. In contrast, ligation of membrane Ig on an IgM+ pre-B cell line (300 - 19 microns lambda 36/8) induced extracellular influx of Ca2+ but no detectable intracellular release. Finally, cross-linking of membrane Ig on IgM+ or IgD+ plasmacytomas (J558L microns 3 or J558L delta m8.8) or an IgD+ B cell hybridoma (B1.8.delta 1) expressing an endogenous Ig gene, did not result in a detectable Ca2+ mobilization response. Importantly, stimulation of cells with the GTP-binding protein activator, aluminum fluoride, resulted in a comparable Ca2+ mobilization response in all cell lines. In view of the fact that aluminum fluoride induced a Ca2+ response in the terminally differentiated B cell lines, J558L microns 3, J558L delta m8.8, and B1.8.delta 1, it is likely that there is an alteration in the signal transduction cascade at some point proximal to GTP binding protein activation. This finding suggests that differentiation of the B cell is accompanied by the loss or alteration of one or more components that couple membrane Ig to subsequent signal transduction elements. Finally, it has previously been demonstrated that the IgM+ cell lines described above, express the recently described membrane Ig-associated protein, B34. Thus, it is apparent based on the fact that the J558L microns 3 cell line does not mobilize Ca2+ after stimulation with anti-Ig, that coexpression of B34 in association with membrane Ig does not constitute a functional receptor complex capable of activating GTP-binding proteins that in turn regulate Ca2+ mobilization.

Membrane IgM and IgD molecules fail to transduce Ca2+ mobilizing signals when expressed on differentiated B lineage cells

We have measured Ca2+ mobilization in a panel of B lineage cell lines after stimulation with anti-Ig to assess whether membrane Ig transduces a functional signal in cells that are representative of immature, mature, or terminally differentiated stages. For these studies, three transfected cell lines which express the same IgM molecule (300-19 microns lambda 36/8, K46-17 microns lambda, and J558L microns lambda 3) as well as two lines expressing an identical IgD molecule (K46 delta m2.6 and J558L delta m8.8) were used. Cross-linking of membrane Ig on IgM+ or IgD+ lymphomas (K46-17 microns lambda or K46 delta m2.6) resulted in a Ca2+ mobilization response that is similar to that seen in mature, resting B cells. Both intracellular release and extracellular influx of Ca2+ were observed. In contrast, ligation of membrane Ig on an IgM+ pre-B cell line (300 - 19 microns lambda 36/8) induced extracellular influx of Ca2+ but no detectable intracellular release. Finally, cross-linking of membrane Ig on IgM+ or IgD+ plasmacytomas (J558L microns 3 or J558L delta m8.8) or an IgD+ B cell hybridoma (B1.8.delta 1) expressing an endogenous Ig gene, did not result in a detectable Ca2+ mobilization response. Importantly, stimulation of cells with the GTP-binding protein activator, aluminum fluoride, resulted in a comparable Ca2+ mobilization response in all cell lines. In view of the fact that aluminum fluoride induced a Ca2+ response in the terminally differentiated B cell lines, J558L microns 3, J558L delta m8.8, and B1.8.delta 1, it is likely that there is an alteration in the signal transduction cascade at some point proximal to GTP binding protein activation. This finding suggests that differentiation of the B cell is accompanied by the loss or alteration of one or more components that couple membrane Ig to subsequent signal transduction elements. Finally, it has previously been demonstrated that the IgM+ cell lines described above, express the recently described membrane Ig-associated protein, B34. Thus, it is apparent based on the fact that the J558L microns 3 cell line does not mobilize Ca2+ after stimulation with anti-Ig, that coexpression of B34 in association with membrane Ig does not constitute a functional receptor complex capable of activating GTP-binding proteins that in turn regulate Ca2+ mobilization.

Molecular components of the B cell antigen receptor complex of class IgD differ partly from those of IgM

Two classes of immunoglobulin, IgM and IgD, are present as antigen receptors on the surface of mature B lymphocytes. We show here that IgD molecules are noncovalently associated in the B cell membrane with a heterodimer consisting of two proteins of 35 kd (IgD-alpha) and 39 kd (Ig-beta), respectively. The two novel proteins are not found in the IgD-expressing myeloma J558L delta m, which fails to bring IgD antigen receptor onto the cell surface. In a surface IgD positive variant line of this myeloma, however, membrane-bound IgD molecules are associated with the heterodimer, suggesting that the formation of an antigen receptor complex is required for surface IgD expression. We further demonstrate that the IgD-associated heterodimer differs partly from that of the IgM antigen receptor and that its binding to the heavy chain only requires the presence of the last constant domain and the transmembrane part of the delta m chain.