The B cell antigen receptor complex: association of Ig-alpha and Ig-beta with distinct cytoplasmic effectors

A 13-amino-acid motif in the cytoplasmic domain of Fc gamma RIIB modulates B-cell receptor signalling

The Fc receptor on B lymphocytes, Fc gamma RIIB (beta 1 isoform), helps to modulate B-cell activation triggered by the surface immunoglobulin complex. Crosslinking of membrane immunoglobulin by antigen or anti-Ig F(ab')2 antibody induces a transient increase in cytosolic free Ca2+, a rise in inositol-3-phosphate, activation of protein kinase C, and enhanced protein tyrosine phosphorylation. Crosslinking Fc gamma RIIB with the surface immunoglobulin complex confers a dominant signal that prevents or aborts lymphocyte activation triggered through the ARH-1 motifs of the signal transduction subunits Ig-alpha and Ig-beta. Here we show that Fc gamma RIIB modulates membrane immunoglobulin-induced Ca2+ mobilization by inhibiting Ca2+ influx, without changing the pattern of tyrosine phosphorylation. A 13-amino-acid motif in the cytoplasmic domain of Fc gamma RIIB is both necessary and sufficient for this effect. Tyrosine at residue 309 in this motif is phosphorylated upon co-crosslinking with surface immunoglobulin; mutation of this residue aborts the inhibitory effect of Fc gamma RIIB. This inhibition is directly coupled to signalling mediated through Ig-alpha and Ig-beta as evidenced by chimaeric IgM/alpha and IgM/beta molecules. The 13-residue motif in Fc gamma RIIB controls lymphocyte activation by inhibiting a Ca2+ signalling pathway triggered through ARH-1 motifs as a result of recruitment of novel SH2-containing proteins that interact with this Fc gamma RIIB cytoplasmic motif.

Ig alpha and Ig beta are functionally homologous to the signaling proteins of the T-cell receptor

Signal transduction by antigen receptors and some Fc receptors requires the activation of a family of receptor-associated transmembrane accessory proteins. One common feature of the cytoplasmic domains of these accessory molecules is the presence is at least two YXXA repeats that are potential sites for interaction with Src homology 2 domain-containing proteins. However, the degree of similarity between the different receptor-associated proteins varies from that of T-cell receptor (TCR) zeta and Fc receptor RIIIA gamma chains, which are homologous, to the distantly related Ig alpha and Ig beta proteins of the B-cell antigen receptor. To determine whether T- and B-cell antigen receptors are in fact functionally homologous, we have studied signal transduction by chimeric immunoglobulins bearing the Ig alpha or Ig beta cytoplasmic domain. We found that Ig alpha and Ig beta cytoplasmic domains were able to activate Ca2+ flux, interleukin-2 secretion, and phosphorylation of the same group of cellular substrates as the TCR in transfected T cells. Chimeric proteins were then used to examine the minimal requirements for activation of the Fyn, Lck, and ZAP kinases in T cells. Both Ig alpha and Ig beta were able to trigger Fyn, Lck, and ZAP directly without involvement of TCR components. Cytoplasmic tyrosine residues in Ig beta were required for recruitment and activation of ZAP-70, but these amino acids were not essential for the activation of Fyn and Lck. We conclude that Fyn and Lck are able to recognize a clustered nonphosphorylated immune recognition receptor, but activation of these kinases is not sufficient to induce cellular responses such as Ca2+ flux and interleukin-2 secretion. In addition, the molecular structures involved in antigen receptor signaling pathways are conserved between T and B cells.

B cell antigen receptors

Antigen receptors have a dual role on the surface of B lymphocytes, namely the binding and internalization of antigens and the activation of the antigen-recognizing B cell. The structural requirements for internalization are still a matter of controversy; but as far as signalling is concerned it is becoming increasingly apparent that the antigen receptor is functionally divided into the ligand-binding immunoglobulin (Ig) molecule and the signal-transducing Ig-alpha/Ig-beta heterodimer. This knowledge has been exploited to design variant antigen receptors which combine both functions in a single molecule.

The hetero-oligomeric antigen receptor complex and its coupling to cytoplasmic effectors

T-cell and B-cell antigen receptors are representative of a family of multisubunit receptors that utilize Src-family kinases as proximal cytoplasmic effectors in signal transduction. Recent studies have shown that distinct receptor subunits mediate ligand and effector interactions and demonstrate that physical interaction with effectors, and their activation, is a function of a 26 amino acid motif found in multiple receptor subunits. Further, receptor ligation induces tyrosine phosphorylation of this motif, and this initiates SH2-mediated association and activation of Src-family kinases and, apparently, ZAP70 kinases. Finally, this association triggers SH3-mediated binding of Lyn and Fyn to PI3-K, resulting in PI3-K activation. An integrated model of signal transduction is presented.

Ig alpha and Ig beta are functionally homologous to the signaling proteins of the T-cell receptor

Signal transduction by antigen receptors and some Fc receptors requires the activation of a family of receptor-associated transmembrane accessory proteins. One common feature of the cytoplasmic domains of these accessory molecules is the presence is at least two YXXA repeats that are potential sites for interaction with Src homology 2 domain-containing proteins. However, the degree of similarity between the different receptor-associated proteins varies from that of T-cell receptor (TCR) zeta and Fc receptor RIIIA gamma chains, which are homologous, to the distantly related Ig alpha and Ig beta proteins of the B-cell antigen receptor. To determine whether T- and B-cell antigen receptors are in fact functionally homologous, we have studied signal transduction by chimeric immunoglobulins bearing the Ig alpha or Ig beta cytoplasmic domain. We found that Ig alpha and Ig beta cytoplasmic domains were able to activate Ca2+ flux, interleukin-2 secretion, and phosphorylation of the same group of cellular substrates as the TCR in transfected T cells. Chimeric proteins were then used to examine the minimal requirements for activation of the Fyn, Lck, and ZAP kinases in T cells. Both Ig alpha and Ig beta were able to trigger Fyn, Lck, and ZAP directly without involvement of TCR components. Cytoplasmic tyrosine residues in Ig beta were required for recruitment and activation of ZAP-70, but these amino acids were not essential for the activation of Fyn and Lck. We conclude that Fyn and Lck are able to recognize a clustered nonphosphorylated immune recognition receptor, but activation of these kinases is not sufficient to induce cellular responses such as Ca2+ flux and interleukin-2 secretion. In addition, the molecular structures involved in antigen receptor signaling pathways are conserved between T and B cells.

Signal transduction by lymphocyte antigen receptors

Despite the differences in the antigens that they recognize and in the effector functions they carry out, B and T lymphocytes utilize remarkably similar signal transduction components to initiate responses. They both use oligomeric receptors that contain distinct recognition and signal transduction subunits. Antigen receptors on both cells interact with at least two distinct families of PTKs via common sequence motifs, ARAMs, in the cytoplasmic tails of their invariant chains, which have likely evolved from a common evolutionary precursor. Coreceptors appear to serve to increase the sensitivity of both of these receptor systems through events that influence ligand binding and signal transduction. The critical role of tyrosine phosphorylation of downstream signaling components, such as phospholipase C, is the net result of changes in the balance of the action of antigen receptor-regulated PTKs and PTPases. The identification of downstream effectors, including calcineurin and Ras, that regulate cellular responses, such as lymphokine gene expression, promises the future possibility of connecting the complex pathway from the plasma membrane to the nucleus in lymphocytes. Insight gained from studies of the signaling pathways downstream of TCR and BCR stimulation is likely to contribute significantly to future understanding of mechanisms responsible for lymphocyte differentiation and for the discrimination of self from nonself in developing and mature cells.

The cDNAs encoding two forms of the LYN protein tyrosine kinase are expressed in rat mast cells and human myeloid cells

Two isoforms of lck/yes-related novel (LYN) protein tyrosine kinase (PTK) appear to play a role in B-cell-IgM and FcERI receptor signaling. The cDNAs lynA and lynB encoding these two forms were isolated and sequenced; they were derived from rat mucosal mast cell and human myeloid cell lines. The nucleotide (nt) and deduced amino acid (aa) sequences share 94 and 97% identity between rat and mouse lyn, respectively, and 88 and 96% identity between rat and human lyn. In all three species, a region of 20 aa is uniformly inserted at an identical site and its sequence is highly conserved. This suggests an important regulatory role for this region mediated by this PTK.

The alpha/beta sheath and its cytoplasmic tyrosines are required for signaling by the B-cell antigen receptor but not for capping or for serine/threonine-kinase recruitment

The B-cell antigen receptor is composed of membrane immunoglobulin sheathed by an alpha/beta heterodimer. The complex is noncovalently associated with protein kinase activity, and crosslinking of the receptor leads to capping and transmembrane signaling. Here we show that the sheath is not necessary either for this capping or for the association of membrane immunoglobulin with the detergent-insoluble cytoskeletal fraction that occurs following crosslinking. It is also not required for association of membrane immunoglobulin with a casein-kinase-like serine/threonine kinase. The sheath is essential, however, for transmembrane signaling. Provision of just the cytoplasmic domain of the beta sheath polypeptide to a mutant, unsheathed IgM molecule was sufficient to restore full signaling capability as judged by the phosphorylation of a variety of cellular proteins, including the B-cell-specific transmembrane protein CD22. This signaling was destroyed by mutating one of the tyrosines in the beta cytoplasmic domain. These results not only suggest that receptor signaling is mediated through phosphorylation of the tyrosines in the sheath's cytoplasmic domains but, together with previous work, indicate that different motifs within the sheath mediate presentation and signaling.

B-cell activation by wild type and mutant Ig-beta cytoplasmic domains

In B lymphocytes, the cytoplasmic domains of the membrane immunoglobulin-associated heterodimeric Ig-alpha and Ig-beta proteins link membrane immunoglobulin to intracellular signalling molecules. We constructed chimeric genes encoding the extracellular and transmembrane domain of human CD8 alpha and the cytoplasmic domain of Ig-alpha or Ig-beta and examined the ability of the chimeric proteins to induce signalling in the murine B-cell lymphoma A20. Crosslinking of CD8/Ig-alpha or CD8/Ig-beta induced both calcium mobilization and protein tyrosine phosphorylation, although induction by CD8/Ig-alpha was somewhat stronger. We also carried out mutagenesis of residues within the "Reth" motif of the CD8/Ig-beta cytoplasmic domain and determined the effects of these mutations on signalling in the murine B-cell hybridoma LK 35.2. Mutants in which alanine was substituted for glutamine 202, threonine 205, and isoleucine 209 retained the ability to induce protein tyrosine phosphorylation and calcium mobilization. In contrast, substitution of alanine for leucine 198 abrogated these responses, suggesting a critical role for this residue in interaction with cytoplasmic signalling proteins.

Heavy-chain directed B-cell maturation: continuous clonal selection beginning at the pre-B cell stage

A number of laboratories have demonstrated a biased representation of certain V-region segments in the primary B-cell repertoire. This may reflect clonal selection at the pre-B cell stage of differentiation. Here, Robert Schwartz and David Stollar suggest that pre-B cells undergo positive selection directed by the presence of surface heavy chain with low affinity to autoantigen. This mechanism would account for the anti-self property of the pre-immune B-cell repertoire.

Fc receptor-mediated signal transduction

Receptors for the Fc domain of IgG (Fc gamma Rs) on leukocytes mediate a pleiotropic response following cross-linking by immune complexes. Signaling events following cross-linking of B and T cell antigen receptors, Fc epsilon RI, and Fc gamma Rs share common elements. In each, signaling is initiated by receptor cross-linking by antigen or immune complexes and results in the activation of src family kinases and ZAP-70-related tyrosine kinases, which associate with members of the receptor complex. Subsequent events include phosphorylation on tyrosine of multiple cellular substrates including phospholipase C gamma 1 and PI3-kinase. The [Ca2+]i flux is an event secondary to phospholipase C gamma 1 activation. Protein tyrosine kinase inhibitors block both early events such as [Ca2+]i flux and the later effects of cytokine release and cellular proliferation.

The gamma chain of the high-affinity receptor for IgE is a major functional subunit of the T-cell antigen receptor complex in gamma delta T lymphocytes

T-cell activation is a consequence of the clonotypic T-cell antigen receptor (TCR) binding to an antigen followed by signal transduction via the invariant subunits of the TCR/CD3 complex. gamma delta TCR cells are a small subset of T cells that populate both the epithelial and lymphoid tissues and have unique antigen specificity and function. However, the composition of invariant chains within the gamma delta TCR/CD3 complex has not been well characterized. Here we report that, unlike the majority of alpha beta T cell, gamma delta T cells isolated from spleen and intestinal epithelial tissue express high levels of the gamma chain of the high-affinity receptor for IgE (Fc epsilon RI gamma) as one invariant subunit of their TCR/CD3 complex. Fc epsilon RI gamma exists as both a homodimer and a heterodimer associated with the TCR zeta chain. Moreover, stimulation of the gamma delta TCR results in rapid tyrosine phosphorylation of Fc epsilon RI gamma. Our results suggest that utilization of distinct receptor signaling components may enable the coupling of antigen stimulation to the activation of different signal transduction pathways in alpha beta and gamma delta T cells.

The human IgE network

IgE and its receptors are believed to have evolved as a mechanism to protect mammals against parasites. But other and intrinsically innocuous antigens can subvert this system to provoke an allergic response. For human populations in industrialized countries, allergy and asthma now represent a far greater threat than parasitic infection, and the main impetus for current studies of the IgE system is the hope of understanding and intervening in the aetiology of allergic diseases.

Conserved cytoplasmic tyrosine residues of the gamma subunit are required for a phagocytic signal mediated by Fc gamma RIIIA

Fc receptors for immunoglobulins are found on many cells and are important in host defense. We transfected Fc gamma RIIIA, present on macrophages and natural killer (NK) cells, into COS-1 cells to study its role in phagocytosis and calcium mobilization in the absence of other Fc gamma receptors. Human Fc gamma RIIIA-alpha (CD16) was cotransfected with its associated chains, either Fc gamma RIIIA gamma or zeta. Both gamma and zeta were observed to induce a phagocytic signal, but gamma was at least sixfold more effective than zeta. Conservative substitution by phenylalanine of either one of the two cytoplasmic tyrosine residues in the gamma chain resulted in markedly diminished phagocytosis and calcium mobilization. Tyrphostin 23, an inhibitor of tyrosine kinases, reversibly inhibited phagocytosis. Further, in vitro kinase assays with the wild type and mutant gamma chains demonstrated that the wild type gamma chain, but not the mutant gamma chains, is phosphorylated. These results suggest that the cytoplasmic tyrosine residues and tyrosine phosphorylation are required for Fc gamma RIIIA to mediate two signal transduction events: phagocytosis and calcium mobilization.

B-cell antigen receptor motifs have redundant signalling capabilities and bind the tyrosine kinases PTK72, Lyn and Fyn

BACKGROUND

The 13 cell antigen receptor (BCR) is a multimeric protein complex consisting of an antigen recognition structure (membrane immunoglobulin) and two associated proteins, lg-alpha and Ig-beta It has been proposed that signalling through the BCR involves Ig-alpha and Ig-beta. Both of these proteins contain within their cytoplasmic domains an amino-acid motif that is present in a number of immune recognition receptors, including the BCR, T-cell antigen receptor and Fc receptor complexes. This motif, termed the antigen-receptor homology motif (ARH1), appears to have signal transduction ability.

RESULTS

We now show that the presence of cytoplasmic regions containing the ARM motif from either Ig-alpha or Ig-beta is sufficient to confer signalling capability on an otherwise non-functional fusion protein. Both Ig-alpha- and Ig-beta-containing chimeras induced, in an apparently redundant fashion, signalling events seen upon membrane immunoglobulin crosslinking, including tyrosine phosphorylation of particular proteins, phosphoinositicle breakdown and calcium mobilization. Furthermore, crosslinking of the chimeras resulted in tyrosine phosphorylation of the Ig-alpha and Tg-beta tails and their association with the tyrosine kinases PTK72, p53/56(lyn) and p59(fyn).

CONCLUSIONS

These observations indicate that Ig-alpha and Ig-beta are responsible for coupling membrane immunoglobulin to intracellular signalling components. Moreover, they demonstrate that a number of tyrosine kinases associate directly with the cytoplasmic domains of both Ig-alpha and Ig-beta. Stimulation of the chimeras, which results in tyrosine phosphorylation of the ig-alpha and Ig-beta tails, is a prerequisite for some of these associations. The implications of these findings for the mechanism by which the BCR initiates the signalling reactions are discussed.

Mapping of sites on the Src family protein tyrosine kinases p55blk, p59fyn, and p56lyn which interact with the effector molecules phospholipase C-gamma 2, microtubule-associated protein kinase, GTPase-activating protein, and phosphatidylinositol 3-kinase

Engagement of the B-cell antigen receptor complex induces immediate activation of receptor-associated Src family tyrosine kinases including p55blk, p59fyn, p53/56lyn, and perhaps p56lck, and this response is accompanied by tyrosine phosphorylation of distinct cellular substrates. These kinases act directly or indirectly to phosphorylate and/or activate effector proteins including p42 (microtubule-associated protein kinase) (MAPK), phospholipases C-gamma 1 (PLC gamma 1) and C-gamma 2 (PLC gamma 2), phosphatidylinositol 3-kinase (PI 3-K), and p21ras-GTPase-activating protein (GAP). Although coimmunoprecipitation results indicate that the Src family protein tyrosine kinases interact physically with some of these effector molecules, the molecular basis of this interaction has not been established. Here, we show that three distinct sites mediate the interaction of these kinases with effectors. The amino-terminal 27 residues of the unique domain of p56lyn mediate association with PLC gamma 2, MAPK, and GAP. Binding to PI 3-K is mediated through the Src homology 3 (SH3) domains of the Src family kinases. Relatively small proportions of cellular PI 3-K, PLC gamma 2, MAPK, and GAP, presumably those which are tyrosine phosphorylated, bind to the SH2 domains of these kinases. Comparative analysis of binding activities of Blk, Lyn, and Fyn shows that these kinases differ in their abilities to associate with MAPK and PI 3-K, suggesting that they may preferentially bind and subsequently phosphorylate distinct sets of downstream effector molecules in vivo. Fast protein liquid chromatography Mono Q column-fractionated MAPK maintains the ability to bind bacterially expressed Lyn, suggesting that the two kinases may interact directly.

Mapping of sites on the Src family protein tyrosine kinases p55blk, p59fyn, and p56lyn which interact with the effector molecules phospholipase C-gamma 2, microtubule-associated protein kinase, GTPase-activating protein, and phosphatidylinositol 3-kinase

Engagement of the B-cell antigen receptor complex induces immediate activation of receptor-associated Src family tyrosine kinases including p55blk, p59fyn, p53/56lyn, and perhaps p56lck, and this response is accompanied by tyrosine phosphorylation of distinct cellular substrates. These kinases act directly or indirectly to phosphorylate and/or activate effector proteins including p42 (microtubule-associated protein kinase) (MAPK), phospholipases C-gamma 1 (PLC gamma 1) and C-gamma 2 (PLC gamma 2), phosphatidylinositol 3-kinase (PI 3-K), and p21ras-GTPase-activating protein (GAP). Although coimmunoprecipitation results indicate that the Src family protein tyrosine kinases interact physically with some of these effector molecules, the molecular basis of this interaction has not been established. Here, we show that three distinct sites mediate the interaction of these kinases with effectors. The amino-terminal 27 residues of the unique domain of p56lyn mediate association with PLC gamma 2, MAPK, and GAP. Binding to PI 3-K is mediated through the Src homology 3 (SH3) domains of the Src family kinases. Relatively small proportions of cellular PI 3-K, PLC gamma 2, MAPK, and GAP, presumably those which are tyrosine phosphorylated, bind to the SH2 domains of these kinases. Comparative analysis of binding activities of Blk, Lyn, and Fyn shows that these kinases differ in their abilities to associate with MAPK and PI 3-K, suggesting that they may preferentially bind and subsequently phosphorylate distinct sets of downstream effector molecules in vivo. Fast protein liquid chromatography Mono Q column-fractionated MAPK maintains the ability to bind bacterially expressed Lyn, suggesting that the two kinases may interact directly.

Signal transduction by immunoglobulin is mediated through Ig alpha and Ig beta

Immunoglobulin (Ig) antigen receptors are composed of a noncovalently-associated complex of Ig and two other proteins, Ig alpha and Ig beta. The cytoplasmic domain of both of these Ig associated proteins contains a consensus sequence that is shared with the signaling proteins of the T cell and Fc receptor. To test the idea that Ig alpha-Ig beta heterodimers are the signaling components of the Ig receptor, we have studied Ig mutations that interfere with signal transduction. We find that specific mutations in the transmembrane domain of Ig that inactivate Ca2+ and phosphorylation responses also uncouple IgM from Ig alpha-Ig beta. These results define amino acid residues that are essential for the assembly of the Ig receptor. Further, receptor activity can be fully reconstituted in Ca2+ flux and phosphorylation assays by fusing the cytoplasmic domain of Ig alpha with the mutant Igs. In contrast, fusion of the cytoplasmic domain of Ig beta to the inactive Ig reconstitutes only Ca2+ responses. Thus, Ig alpha and Ig beta are both necessary and sufficient to mediate signal transduction by the Ig receptor in B cells. In addition, our results suggest that Ig alpha and Ig beta can activate different signaling pathways.

T cell activation by clustered tyrosine kinases

Many cellular recognition events in the immune system are initiated by aggregation of cell surface receptors that lack intrinsic protein-tyrosine kinase activity. Receptor-associated kinases related to the src protooncogene product have been found to be essential for cellular activation and may interact with the cytoplasmic domains of the antigen receptor chains. We show here that anti-CD16 antibody-mediated clustering of chimeric transmembrane proteins bearing a CD16 extracellular domain and a Src family kinase intracellular domain is not sufficient to initiate a cellular activation signal in T cells, whereas clustering of similar chimeras bearing Syk or ZAP-70 kinase sequences triggers calcium mobilization. Aggregation of the Syk chimera alone, or coaggregation of chimeras bearing Fyn and ZAP-70 kinases, suffices to initiate cytolytic effector function. The pattern of tyrosine phosphorylation induced by clustering of the Syk chimera is similar to the pattern induced by aggregation of T cell receptor.

Association of immunoglobulin G Fc receptor II with Src-like protein-tyrosine kinase Fgr in neutrophils

The interaction of Fc receptors with antibody-antigen complexes activates multiple biological functions in hematopoietic cells. Recently, protein-tyrosine phosphorylation has been suggested to be involved in Fc receptor-mediated cell signaling. Here we show that the Src-like protein-tyrosine kinase Fgr, which is specifically expressed in mature myelomonocytic cells, coimmunoprecipitates with IgG Fc receptor II (Fc gamma RII), but not with Fc gamma RIII from detergent lysates of human peripheral neutrophils. Crosslinking of Fc gamma RII induced a rapid increase in the tyrosine kinase activity and comodulation of Fgr. These results suggest that Fgr is physically and functionally associated with Fc gamma RII and involved in Fc gamma RII-mediated signal transduction pathways.

Transmembrane signalling through the T-cell-receptor-CD3 complex

Recent data support the existence of activation motifs within different subunits of the T-cell-receptor-CD3 complex. This architecture generates a receptor composed of discrete modules, each capable of being coupled to an effector pathway. Although new T-cell specific protein tyrosine kinases have recently been identified, the nature of the proximal non-receptor protein tyrosine kinase linking the T-cell receptor complex to essential signalling effectors remains unknown. Developmentally regulated differences in T-cell-receptor-CD3 assembly or stability may lead to the expression of isoforms displaying different sets of activation motifs. Whether this may be the basis of differential signalling during T-cell development is still a matter of speculation.

The CD19-CR2-TAPA-1 complex, CD45 and signaling by the antigen receptor of B lymphocytes

A paradigm describing the response of T lymphocytes to antigen holds that signals from antigen receptors must be modulated by non-antigen-specific, accessory membrane proteins for an appropriate cellular response to occur, such as differentiation, activation and tolerance. Recent studies suggest that this paradigm applies also to B lymphocytes. Signaling through membrane IgM in these cells requires CD45, a phosphotyrosine phosphatase, and is amplified by a complex containing CD19, complement receptor 2 (CD21), and TAPA-1, which recruits the intracellular enzyme, phosphatidylinositol 3-kinase.

CD19 of B cells as a surrogate kinase insert region to bind phosphatidylinositol 3-kinase

Antigen receptors on B and T lymphocytes transduce signals by activating nonreceptor protein tyrosine kinases (PTKs). A family of receptor PTKs contains kinase insert regions with the sequence tyrosine-X-X-methionine (where X is any amino acid) that when phosphorylated mediate the binding and activation of phosphatidylinositol 3-kinase (PI 3-kinase). The CD19 membrane protein of B cells enhances activation through membrane immunoglobulin M (mIgM) and was found to contain a functional analog of the kinase insert region. Ligation of mIgM induced phosphorylation of CD19 and association with PI 3-kinase. Thus, CD19 serves as a surrogate kinase insert region for mIgM by providing the means for PI 3-kinase activation by nonreceptor PTKs.

Physical and functional association of p56lck with Fc gamma RIIIA (CD16) in natural killer cells

The transmembrane receptor for immunoglobulin G immune complexes on natural killer (NK) cells and macrophages, Fc gamma RIIIA (CD16), mediates cellular activation through a tyrosine kinase-dependent pathway. We show that Fc gamma RIII crosslinking results in activation of the src-related kinase p56lck in NK cells and demonstrate a physical association of p56lck with Fc gamma RIIIA in immunoprecipitates from NK cells obtained using anti-Fc gamma RIII antibodies or immune complexes. Our studies show that the zeta chain, the signal transducing subunit of Fc gamma RIIIA and of T cell receptor, associates with p56lck and, in NK cells, is a substrate for this kinase. Such direct association of p56lck with the zeta subunit as confirmed by demonstrating the interaction in heterologous cells transfected with cDNA expressing p56lck and zeta. Our findings demonstrate both functional and physical association of p56lck with Fc gamma RIIIA, through direct interaction of the kinase with the zeta and/or the gamma signal transducer subunits of the receptor. These data suggest a possible mechanism by which activation via Fc gamma RIIIA occurs.

The structure of the mu/pseudo light chain complex on human pre-B cells is consistent with a function in signal transduction

Prior to immunoglobulin (Ig) light (L) chain rearrangement, pre-B cells can express mu heavy (H) chains at the cell surface in association with pseudo (psi) L chains. This complex may be essential for B cell development. We have investigated the composition of the mu/psi L chain complex of a human pre-B cell line, in view of its potential role in transmembrane signal transduction. The mu/lambda receptor of a mature B cell line was analyzed in comparison. The mu/psi L chain complex is associated with disulfide-linked molecules that are homologous or identical to the mb-1 and B29 proteins, known to be integral components of membrane Ig receptors on mature B cells. Both receptors contain tyrosine (Tyr) kinase activity. In the mu/lambda receptor, the lyn and lck Tyr kinases could clearly be identified. The mb-1 and B29 proteins in both mu/lambda and mu/psi L chain receptors are substrates for in vitro phosphorylation on Tyr, but also on serine (Ser) and threonine (Thr) residues. The undefined mu-associated Ser/Thr kinase also phosphorylates the src-related kinases in the mu/lambda receptor and a 43-kDa mu-associated protein that is present in both complexes. The 43-kDa protein may be an integral part of both receptor types, or a transiently associated molecule instrumental in the signaling process. We conclude that the mu/psi L receptor on human pre-B cells fulfills the presently known criteria to function as a signal transduction unit.

CD22 associates with the human surface IgM-B-cell antigen receptor complex

The B-cell surface molecule CD22, when cross-linked, modulates signaling through the surface IgM (sIgM)-B-cell receptor (BCR) complex. Here we analyzed the basis of this interaction between CD22 and the human sIgM complex. After lysis of B cells or B-cell lines in digitonin, CD22 coimmunoprecipitated a kinase activity that in vitro-phosphorylated two polypeptides of 150 and 130 kDa on tyrosine residues. By immunoblot analysis with a rabbit anti-serum specific for a synthetic peptide of CD22, we found these proteins to be CD22 itself. Furthermore, the phosphorylated 150-kDa CD22 was found in the sIgM-BCR complex maintained by digitonin, along with Ig alpha/mb-1, Ig beta/B29, and a 75-kDa polypeptide precipitated by an antiserum specific to protein-tyrosine kinase PTK72. CD22 is likely to be an important signaling partner in the sIgM-BCR complex since it is very rapidly and strikingly phosphorylated after sIgM is cross-linked and since it contains the antigen recognition homology I (ARHI) motif, present in other antigen receptor molecules.

Differential signaling through the Ig-alpha and Ig-beta components of the B cell antigen receptor

The B cell antigen receptor is a complex containing the antigen-binding immunoglobulin molecules and the Ig-alpha/Ig-beta heterodimer which presumably connects the B cell antigen receptor to intracellular signaling components. To analyze the functional properties of the cytoplasmic parts of the B cell antigen receptor, we used the K46 B lymphoma line (IgG2a, kappa) to express chimeric molecules composed of the extracellular and transmembrane part of the CD8 alpha molecule and the cytoplasmic sequence of either the Ig-alpha (CD8 alpha/Ig-alpha), the Ig-beta (CD8 alpha/Ig-beta) protein or the membrane-bound gamma 2a heavy chain (CD8 alpha/gamma 2a). From these three types of chimeric molecules only (CD8 alpha/Ig-alpha and CD8 alpha/Ig-beta, but not CD8 alpha/gamma 2a, could transduce signals, thus providing the first evidence that the cytoplasmic tail of Ig-alpha and Ig-beta have a signaling capacity. After cross-linking with anti-CD8 alpha antibodies, both molecules induced a similar increase in intracellular free calcium ion and in MAP kinase phosphorylation. Protein tyrosine kinases, however, were strongly activated via the CD8 alpha/Ig-alpha and only marginally via the CD8 alpha/Ig-beta molecule. This suggests that the Ig-alpha and Ig-beta proteins have distinct roles during signal transduction through the B cell antigen receptor.

Biochemistry of B lymphocyte activation

The activation of B lymphocytes from resting cells proceeds from the events of early activation to clonal proliferation to final differentiation into either an antibody-secreting plasma cell or a memory B cell. This is a complex activation process marked by several alternative pathways, depending on the nature of the initial antigenic stimulus. Over the past 5-10 years, there has been an explosion of studies examining the biochemical nature of various steps in these pathways. Some of that progress is reviewed here. In particular, we have described in detail what is known about the structure and function of the AgR, as this molecule plays a pivotal role in B cell responses of various types. We have also reviewed recent progress in understanding the mechanism of action of contact-dependent T cell help and of the cytokine receptors, particularly the receptors for IL-2, IL-4, and IL-6. Clearly, all of these areas represent active areas of investigation and great progress can be anticipated in the next few years.