Tyrosine Phosphorylation Of CD28 Research Articles

Modulation of B Lymphocyte Antigen Receptor Signal Transduction by a CD19/CD22 Regulatory Loop

CD19 and CD22 are B lymphocyte cell-surface molecules that positively and negatively regulate antigen receptor signal transduction, respectively. Biochemical studies with B cells from CD19-deficient and CD22-deficient mice indicated that these two regulatory molecules influenced each other's functions: CD22 expression negatively regulated CD19 tyrosine phosphorylation, while optimal CD22 function was dependent on CD19 expression. Functional CD19 and CD22 interactions were also assessed in vivo by generating CD19/CD22 double-deficient mice. Remarkably, the CD19 mutation was dominant to the CD22 mutation in most instances. B lymphocytes from CD19/CD22-deficient and CD19-deficient mice were functionally equivalent despite the negative influence normally provided by CD22 expression. These data collectively suggest that CD19 activates the CD22/SHP1 inhibitory pathway that then acts primarily on CD19.

Identification of Tyrosine Phosphorylation Sites in the CD28 Cytoplasmic Domain and Their Role in the Costimulation of Jurkat T Cells

The cytoplasmic domain of CD28 contains four tyrosine residues. Because signal transduction by CD28 appears to involve its tyrosine phosphorylation, we determined sites of CD28 tyrosine phosphorylation using mutants of mouse CD28 that retained tyrosine at one position, with the remaining three positions mutated to phenylalanine. When expressed in Jurkat cells and stimulated by mAb, only the mutants with tyrosine at position 170 or 188 were tyrosine phosphorylated. Phosphorylation of Tyr170 recruits phosphatidylinositol 3-kinase to CD28. Tyr188 has not been associated with any specific signaling event, but we found that ligation of CD28 by the natural ligand B7.2 also induced phosphorylation of Tyr188, suggesting that this event is of physiological importance. Consistent with that possibility, mutation of Tyr188 to phenylalanine severely impaired the ability of mouse CD28 to deliver a costimulus for the expression of CD69 and the production of IL-2. The functional consequences of the mutation of Tyr188 were unique; mutation of the other three tyrosines, individually or in combination, did not impair costimulation. Therefore, of the four CD28 tyrosine residues only Tyr188 is required for signaling in Jurkat cells, suggesting that its phosphorylation is a key event in the costimulation of T cells.

CD80 and CD86 Are Not Equivalent in Their Ability to Induce the Tyrosine Phosphorylation of CD28

Ligation of either CD80 (B7-1) or CD86 (B7-2), two principal ligands for CD28, is thought to skew the immune response toward Th1 or Th2 differentiation. We have examined early signal transduction pathways recruited following T cell stimulation with either CD80 or CD86. Purified human peripheral T cells or Jurkat T cells were stimulated with Chinese hamster ovary (CHO) cells expressing either human CD80 (CHO-CD80) or human CD86 (CHO-CD86) or with anti-CD28 monoclonal antibody (mAb). In the presence of phorbol 12-myristate 13-acetate, both CHO-CD80 and CHO-CD86, like anti-CD28 mAb, were capable of stimulating cytokine production from both human peripheral T cells and Jurkat T cells. Both CHO-CD80 and CHO-CD86, in the presence of anti-CD3 mAb, costimulated NFAT-dependent transcriptional activation. Several intracellular signaling proteins, such as CBL and VAV, were phosphorylated on tyrosine in response to CD80, CD86, and anti-CD28 mAb. Surprisingly, although stimulation of Jurkat T cells with either CHO-CD80 or anti-CD28 mAb resulted in robust tyrosine phosphorylation of CD28 itself, ligation with CHO-CD86 was unable to induce detectable CD28 tyrosyl phosphorylation over a range of stimulation conditions. In addition, the association of phosphoinositide 3-kinase with CD28 and enhanced tyrosine phosphorylation of phospholipase Cgamma were seen after anti-CD28 mAb and CHO-CD80 stimulation but to a much lesser extent after CHO-CD86 stimulation. Thus, ligation of CD28 with either CD80 or CD86 leads to shared early signal transduction events such as the tyrosine phosphorylation of CBL and VAV, to NFAT-mediated transcriptional activation, and to the costimulation of interleukin-2 and granulocyte-macrophage colony-stimulating factor production. However, CD80 and CD86 also induce distinct signal transduction pathways including the tyrosine phosphorylation of CD28 and phospholipase Cgamma1 and the SH2-dependent association of phosphoinositide 3-kinase with CD28. These quantitative, if not qualitative, differences between signaling initiated by these two ligands for CD28 may contribute to functional differences (e.g. Th1 or Th2 differentiation) in T cell responses.

CD72-mediated B cell activation involves recruitment of CD19 and activation of phosphatidylinositol 3-kinase.

Occupancy of the B cell glycoprotein, CD72 results in syk-independent activation of phospholipase-C gamma and calcium mobilization. The cytoplasmic tail of CD72 does not contain an immunoreceptor tyrosine-based activation motif to directly transduce signals into the B lymphocyte. Hence, we investigated whether other coreceptors such as CD19 and its associated phosphatidylinositol 3-kinase (PI 3-K) were involved in CD72 signaling. Two specific inhibitors of PI 3-K inhibited CD72-stimulated B cell proliferation in a dose-dependent manner. Activation of B lymphocytes via CD72 resulted in recruitment and activation of PI 3-K, which was mediated by CD19. Accordingly, CD72 ligation induced CD19 tyrosine phosphorylation. Thus, lipid products generated as a result of PI 3-K activation may have an important function in CD72-mediated B lymphocyte activation. The kinetics of CD19 tyrosine phosphorylation induced by CD72 ligation were strikingly different from those seen following B cell antigen receptor (BCR) stimulation. A transient increase in the tyrosine phosphorylation of the complement receptors, CD21 and CD35 was observed in BCR- but not CD72-stimulated cells. Co-cross-linking of CD72 and CD19 failed to induce syk tyrosine phosphorylation suggesting that even under these conditions, CD72 signaling was independent of syk activation. A transient and stimulation-dependent physical association between CD19 and CD72 was observed in CD72-ligated cells. These observations suggest a mechanism by which CD72 can recruit CD19 and influence activation of CD19-associated PI 3-K, which appears to be critical for CD72-mediated B cell activation.

CD72-mediated B cell activation involves recruitment of CD19 and activation of phosphatidylinositol 3-kinase

Occupancy of the B cell glycoprotein, CD72 results in syk-independent activation of phospholipase-C gamma and calcium mobilization. The cytoplasmic tail of CD72 does not contain an immunoreceptor tyrosine-based activation motif to directly transduce signals into the B lymphocyte. Hence, we investigated whether other coreceptors such as CD19 and its associated phosphatidylinositol 3-kinase (PI 3-K) were involved in CD72 signaling. Two specific inhibitors of PI 3-K inhibited CD72-stimulated B cell proliferation in a dose-dependent manner. Activation of B lymphocytes via CD72 resulted in recruitment and activation of PI 3-K, which was mediated by CD19. Accordingly, CD72 ligation induced CD19 tyrosine phosphorylation. Thus, lipid products generated as a result of PI 3-K activation may have an important function in CD72-mediated B lymphocyte activation. The kinetics of CD19 tyrosine phosphorylation induced by CD72 ligation were strikingly different from those seen following B cell antigen receptor (BCR) stimulation. A transient increase in the tyrosine phosphorylation of the complement receptors, CD21 and CD35 was observed in BCR- but not CD72-stimulated cells. Co-cross-linking of CD72 and CD19 failed to induce syk tyrosine phosphorylation suggesting that even under these conditions, CD72 signaling was independent of syk activation. A transient and stimulation-dependent physical association between CD19 and CD72 was observed in CD72-ligated cells. These observations suggest a mechanism by which CD72 can recruit CD19 and influence activation of CD19-associated PI 3-K, which appears to be critical for CD72-mediated B cell activation.

CD19 as a Membrane-Anchored Adaptor Protein of B Lymphocytes: Costimulation of Lipid and Protein Kinases by Recruitment of Vav

CD19 is a coreceptor that amplifies signaling by membrane immunoglobulin (mIg) to promote responses of the B lymphocyte to T-dependent antigens. Vav is a guanine nucleotide exchange factor for the Rho, Rac, Cdc42 family of small GTPases. We found that coligating mIg and CD19 causes a synergistic increase in the tyrosine phosphorylation of CD19. Phosphorylated tyrosine-391 of CD19 binds Vav to mediate a sustained increase in intracellular Ca 2+ concentration. This response correlates with activation by the CD19–Vav complex of phosphatidylinositol 4-phosphate 5-kinase for the synthesis of phosphatidylinositol 4,5-bisphosphate. Interaction of CD19 with Vav also mediates the synergistic activation of the mitogen-activated protein kinase JNK. Therefore, CD19 is a membrane adaptor protein that recruits Vav for the activation of lipid and protein kinases.

Differential induction of DNA-binding activities following CD19 cross-linking in human B lineage cells.

The B cell-specific cell surface molecule CD19 is expressed at all stages of B cell development, including normal plasma cells, and mediates signal transduction via interaction with cytoplasmic effector proteins. Cross-linking CD19 on early human B lineage cells induces the formation of a CD19/Vav/phosphatidylinositol-3 kinase complex, tyrosine phosphorylation of CD19 and Vav, and activation of the Ras pathway. To further explore the ramifications of CD19 signaling, the current study examined whether phosphorylation of Elk-1, activation of activator protein-1 (AP-1), or activation of nuclear factor-kappaB (NF-kappaB) transcription factors occurred following CD19 cross-linking. The cells used were the BLIN-1 pre-B cell line expressing low levels of cell surface mu heavy chain associated with surrogate light chain and the 1E8 immature B cell line expressing cell surface mu/kappa. Lysates from CD19 cross-linked 1E8 cells induced robust phosphorylation of an Elk-1 fusion protein in vitro, whereas no phosphorylation of Elk-1 fusion protein occurred using lysates from CD19 cross-linked BLIN-1 cells. An electrophoretic mobility shift assay employing AP-1 and NF-kappaB consensus oligonucleotides was used to demonstrate that AP-1 -binding activity increased, while constitutive NF-kappaB-binding activity was not enhanced, following 2 h of CD19 cross-linking in 1E8 cells. Supershift experiments revealed that JunD and c-Fos proteins mediated anti-CD19 induced AP-1-binding activity in 1E8 cells. In contrast, CD19 cross-linking in BLIN-1 cells resulted in the induction of NF-kappaB, but had no apparent effect on AP-1-binding activity. These data suggest that CD19-mediated signal transduction activates different transcription factors at juxtaposed stages of B cell development that may culminate in the activation or suppression of distinct sets of genes.

The SH3 domain of Itk/Emt binds to proline-rich sequences in the cytoplasmic domain of the T cell costimulatory receptor CD28.

Engagement of the transmembrane receptor CD28 potentiates T cell survival, proliferation, and activation. The biochemical basis by which CD28 controls these outcomes is unclear, although early events following cross-linking of the receptor are characterized by tyrosine phosphorylation of CD28 and other cellular substrates. We demonstrate that following CD28 ligation, a CD28-associated tyrosine kinase activity is increased in parallel to activation of the T cell-specific tyrosine kinase Itk (Itk/Emt), while Lck and Fyn kinase activities are not increased. We show that Itk forms an inducible complex with CD28, mediated by the SH3 domain of Itk and the diproline motifs of CD28. Site-directed mutagenesis of the N-terminal diproline motif of CD28 abrogates the association of CD28 with the SH3 domain of Itk, while mutations within the C-terminal diproline motif have little effect. Peptides corresponding to the N-terminal diproline motif were more efficient at abrogating the interaction between CD28 and the SH3 domain of Itk, than peptides corresponding to the C-terminal diproline motif. In addition, peptides corresponding to the N-terminal diproline motif of CD28 activated the tyrosine kinase activity of Itk to levels similar to those observed following Ab-mediated cross-linking of CD28. Together, our data show that the SH3 domain of Itk binds to a proline-rich motif within the cytoplasmic tail of CD28, and define a mechanism by which CD28 couples to and activates a downstream tyrosine kinase.

Activation of syk in an immature B cell line does not require lyn activity

BKS-2 is an immature B cell lymphoma that undergoes apoptotic cell death when signaled via its surface IgM receptor. To study the signaling components of surface IgM mediated apoptosis in B lymphoma cells, we generated mutants of BKS-2 that were resistant to anti-IgM induced apoptosis. One mutant cell line, 1.B5, did not undergo apoptotic cell death upon treatment with anti-IgM antibodies and also did not exhibit elevation of intracellular Ca 2+ in response to cross-linking of surface IgM. This appeared to be due to a defect in protein tyrosine kinase (PTK) activity since fewer proteins were tyrosine phosphorylated in the mutant cells stimulated with anti-IgM when compared to wild type BKS-2. Subsequently, we showed that protein tyrosine kinases lyn and blk were inducibly tyrosine phosphorylated in the wild type BKS-2 but not in 1.B5 mutant cells in response to anti-IgM. Also the kinase activity of lyn was elevated in the wild type but not in mutant cells upon triggering through surface IgM. Furthermore, tyrosine phosphorylation of CD19, a known substrate of lyn, was inducible in anti-IgM stimulated BKS-2 cells but severely reduced in 1.B5 cells. In contrast, kinase activity of another src kinase, blk, was increased on anti-IgM stimulation in both wild type and mutant cells. Surprisingly, syk, a non-src protein tyrosine kinase important for surface IgM mediated signaling, was tyrosine phosphorylated in the lyn deficient mutant cells as well as in the wild type BKS-2 cells. Furthermore, anti-IgM induced increase in kinase activity of syk was similar in the mutant and wild type cells. Thus, in contrast to other studies that propose syk to be a downstream target of src family kinases, syk may act upstream of lyn in immature B cells. Consistent with a functional syk, its target, phospholipase γ2 (PLC-γ2) was normally tyrosine phosphorylated in mutant cells.

CD38 ligation in human B cell progenitors triggers tyrosine phosphorylation of CD19 and association of CD19 with lyn and phosphatidylinositol 3-kinase.

CD38 is a 45-kDa transmembrane glycoprotein highly expressed in lymphoid progenitors. Ligation of CD38 with specific Abs inhibits growth and induces apoptosis in human immature B cells. CD38 ligation also triggers tyrosine phosphorylation of syk, c-cbl, and phospholipase C-gamma and activates phosphatidylinositol 3-kinase (PI3-K). In the present study, we investigated whether the cell surface membrane molecules used in B cell receptor-mediated signaling, such as Ig alpha, Ig beta, and CD19, could be involved in the CD38-mediated signaling cascade. In the B cell receptor-negative immature B cell lines RS4;11, 380, and REH, Ig alpha and Ig beta were expressed exclusively in the cytoplasm and were not tyrosine phosphorylated after CD38 ligation. By contrast, CD19 was markedly tyrosine phosphorylated and was associated with lyn and PI3-K. PI3-K activation appears to be directly linked to the growth-arresting effects of CD38 ligation, which are reduced by PI3-K inhibitors. Ligation of either CD38 or CD19 resulted in a similar pattern of protein tyrosine phosphorylation; both signaling pathways caused tyrosine phosphorylation of c-cbl. Levels of CD38 surface expression were not affected by prolonged incubation with anti-CD19 Ab, while CD19 expression markedly decreased. These results indicate that CD19 is a major component of the CD38 signaling cascade in B cell precursors, serving as a cell surface membrane docking site for cytoplasmic kinases. CD38 and CD19 are not physically linked, but activate an overlapping set of kinases in human immature B cells.

Role of CD19 tyrosine 391 in synergistic activation of B lymphocytes by coligation of CD19 and membrane Ig.

CD19 and CD21, which form a complex on B lymphocytes, are required for normal Ab responses to T-dependent Ags. Coligation of the CD21/CD19 complex with membrane IgM (mIgM) powerfully enhances B cell activation in vitro and in vivo. To determine how CD19-mIgM synergy is produced, we examined immediate and downstream signaling events after ligation of either complex alone or after CD19-mIgM coligation in normal and lymphoblastoid B cells. Ligation of mIgM alone is known to result in tyrosine phosphorylation of CD19 and association of CD19 with phosphatidylinositol 3-kinase and Vav, and these events are not enhanced by coligation. In contrast, tyrosine phosphorylation of phosphatidylinositol 3-kinase and Vav is markedly enhanced after CD19-mIgM coligation. Coligation also results in synergistic, prolonged enhancement of mitogen-activated protein kinase activity, relative to ligation of either receptor complex alone. Mutation of CD19 Y391, the site at which Vav binds, but not mutation of CD19 Y482 and Y513, the sites of association with phosphatidylinositol 3-kinase, blocks the enhancement of mitogen-activated protein kinase activation. These findings suggest that the synergistic enhancement of B cell activation following CD19-mIgM coligation results from greater tyrosine phosphorylation of Vav and Vav-dependent enhanced activation of mitogen-activated protein kinases.

Membrane IgM-induced tyrosine phosphorylation of CD19 requires a CD19 domain that mediates association with components of the B cell antigen receptor complex.

CD19 enhances membrane IgM (mIgM) signaling and is required for B lymphocyte responses to T-dependent Ags. CD19 is tyrosine phosphorylated when mIgM is ligated and binds SH2 domain-containing signaling proteins. We suggest that the basis for phosphorylation is the association of CD19 with Syk and other components of the mIgM complex. IgM, CD22, Ig-alpha, Ig-beta, and Syk were coimmunoprecipitated with CD19 from detergent lysates of B lymphocytes. The association was maintained with a chimeric form of CD19 containing only the transmembrane domain and the membrane proximal 17 amino acids of the cytoplasmic domain encoded by exon 6. This sequence is sufficient to mediate the association, as a synthetic peptide of the exon 6-encoded region adsorbs IgM and Syk. Deletion of the juxtamembrane 17 amino acids of the cytoplasmic domain encoded by CD19 exon 6 abolishes association of CD19 with the mIgM complex. Deletion of these amino acids, which contain no tyrosines, also reduces mIgM-induced tyrosine phosphorylation of the remainder of the CD19 cytoplasmic domain. Coligating this mutant CD19 to mIgM restores phosphorylation. Thus, a discrete region of the cytoplasmic domain regulates the tyrosine phosphorylation of CD19 in the activation of B cells by mIgM.

Co-ligation of the Antigen and Fc Receptors Gives Rise to the Selective Modulation of Intracellular Signaling in B Cells: REGULATION OF THE ASSOCIATION OF PHOSPHATIDYLINOSITOL 3-KINASE AND INOSITOL 5′-PHOSPHATASE WITH THE ANTIGEN RECEPTOR COMPLEX

Cross-linking of the Fc receptor (FcR) to surface immunoglobulin (sIg) on B cells inhibits the influx of extracellular calcium and abrogates the proliferative signal. The mechanism by which this occurs is not well understood. In this report we show that co-cross-linking the FcR to the antigen receptor gives rise to very selective modulation of signal transduction in B cells. Co-cross-linking sIg and the FcR enhanced the phosphorylation of the FcR, the adapter protein, Shc, and the inositol 5'-phosphatase Ship. Furthermore, phosphorylation of the FcR induced its association with Ship. Cross-linking of the FcR and sIg decreased the tyrosine phosphorylation of CD19, which led to a reduction in the association of phosphatidylinositol 3-kinase. In addition, the phosphorylation of several other proteins of 73, 39, and 34 kDa was reduced. Activation of the cells with either F(ab')2 or intact anti-IgG induced very similar changes in levels of tyrosine phosphorylation of most other proteins, and no differences in the activation of several protein kinases were observed. These results indicate that the inhibitory signal that is transmitted through the FcR is not mediated by a global shutdown of tyrosine phosphorylation but is, rather, a selective mechanism involving localized changes in the interactions of adapter proteins and the enzymes Ship and phosphatidylinositol 3-kinase with the antigen receptor complex.

Differential Phosphorylation of the T Lymphocyte Costimulatory Receptor CD28: ACTIVATION-DEPENDENT CHANGES AND REGULATION BY PROTEIN KINASE C

Treatment of T lymphocytes with phorbol ester and anti-CD28 monoclonal antibody (mAb) can induce proliferation and interleukin 2 production by triggering still undefined intracellular signaling pathways. We have developed a deglycosylation procedure that allows the precise identification of a distinct CD28 protein band, facilitating the analysis of activation-dependent changes in the phosphorylation state of CD28. Phorbol 12-myristate 13-acetate (PMA) treatment induced the in vitro phosphorylation of CD28 on threonine as detected in immune complex kinase assays. This effect of PMA was (i) rapid, preceding a PMA-induced increase in CD28 surface expression; (ii) occurred using kinase buffer containing either manganese or magnesium; and (iii) was found in human peripheral T cells, Jurkat T cells, and in a Jurkat subclone, J.Cam1, that is deficient in Lck tyrosine kinase activity. In contrast, anti-CD28 monoclonal antibody stimulation led to in vitro phosphorylation of CD28 on tyrosine that was manganese-dependent and required Lck tyrosine kinase activity, as it was undetectable in J.Cam1 cells. Importantly, CD28 was phosphorylated on tyrosine in vivo as detected with anti-phosphotyrosine antibodies after stimulation with anti-CD28 monoclonal antibody. The in vivo tyrosine phosphorylation of CD28 was inhibited by PMA treatment and was absent in J.Cam1 cells. Thus, the CD28 coreceptor can trigger different intracellular signaling pathways, depending upon the nature of the initial costimulatory signal.

Specific binding of Fyn and phosphatidylinositol 3-kinase to the B cell surface glycoprotein CD19 through their src homology 2 domains.

CD19 is a B cell surface protein capable of forming non-covalent molecular complexes with a number of other B cell surface proteins including the CD21/CD81/Leu-13 complex as well as with surface immunoglobulin. CD19 tyrosine phosphorylation increases after B cell activation, and is proposed to play a role in signal transduction through its cytoplasmic domain, which contains nine tyrosine residues. Several second messenger proteins have been shown to immunoprecipitate with CD19, including p59 Fyn (Fyn), p59 Lyn (Lyn) and phosphatidylinositol-3 kinase (PI-3 kinase). These associations are predicted to occur via the src-homology 2 (SH2) domains of the second messenger proteins. Two of the cytoplasmic tyrosines in the CD19 cytoplasmic region contain the consensus binding sequence for the PI-3 kinase SH2 domain (YPO4-X-X-M). However, the reported consensus binding sequence for the Fyn and Lyn SH2 domains (YPO4-X-X-I/L) is not found in CD19. We investigated the capacity of CD19 cytoplasmic tyrosines to bind both Fyn and PI-3 kinase SH2-domain fusion proteins. In activated B cells, both Fyn and PI-3 kinase SH2-domain fusion proteins precipitate CD19. Using synthetic tyrosine-phosphorylated peptides comprising each of the CD19 cytoplasmic tyrosines and surrounding amino acids, we investigated the ability of the Fyn SH2 and PI-3 kinase SH2 fusion proteins to bind to the different CD19 cytoplasmic phosphotyrosine peptides. ELISA revealed that the two CD19 cytoplasmic tyrosine residues contained within the Y-X-X-M sequences (Y484 and Y515) bound preferentially to the PI-3 kinase SH2-domain fusion proteins. Two different tyrosines (Y405 and Y445) bound preferentially to the Fyn SH2-domain fusion protein via a novel sequence, Y-E-N-D/E, different from that previously reported for the Fyn SH2 domain. In precipitation studies, peptide Y484 was able to compete with tyrosine phosphorylated CD19 specifically for binding to the PI-3 kinase SH2 domain fusion proteins, while peptides Y405 and Y445 were able to compete specifically for binding to the Fyn SH2 domain fusion proteins. These results indicate that CD19 may be capable of binding both Fyn and PI-3 kinase concurrently, suggesting a mechanism for CD19 signal transduction, in which binding of PI-3 kinase to the Fyn SH3 domain results in activation of PI-3 kinase.

Host cell requirements for efficient infection of quiescent primary B lymphocytes by Epstein-Barr virus

Quiescent primary B lymphocytes are efficiently immortalized by Epstein-Barr virus (EBV). This process requires both the delivery and expression of the viral genome and results in activation of the cell division cycle. Infection of B lymphocytes depends on a direct interaction between the viral glycoprotein gp340/220 and CD21, the C3dg complement receptor. This interaction is required for the adsorption of EBV. In addition, several lines of evidence suggest that the interaction of EBV with CD21 modulates the phenotype of cells. CD21 forms part of a multimeric signal transduction complex with CD19, TAPA-1, and Leu-13. In normal B lymphocytes, CD19 becomes tyrosine phosphorylated following stimulation of the antigen receptor and recruits the signal-transducing enzyme phosphatidylinositol 3-kinase kinase. Here, we investigated the involvement of signal transduction pathways in efficient infection. Protein synthesis is not required for events leading to the transcription of the viral genome, suggesting that the early stages of infection do not depend on the expression of novel cell genes and consistent with the Wp promoter being the first viral promoter used upon infection. Since the stimulation of cells with gp340/220 leads to an increase in the level of CD19 tyrosine phosphorylation, we investigated the potential contribution of both tyrosine and phosphatidylinositol 3-kinase kinases to efficient infection. Both kinases contribute to the posttranscriptional control of viral gene expression following infection, but neither is required for the entry or initial transcription of the virus. Thus, it appears that EBV exploits a host signal transduction pathway to efficiently infect primary cells.

CD28 signal transduction: tyrosine phosphorylation and receptor association of phosphoinositide-3 kinase correlate with Ca(2+)-independent costimulatory activity.

The interaction of CD28 with its counter-receptor, B7, induces a cosignal in T cells required to prevent clonal anergy and to promote antigen-dependent interleukin-2 production. The molecular basis of the CD28 cosignal is not well understood but involves the activation of protein tyrosine kinase(s) (PTK). In this report we demonstrate that CD28 cross-linking on Jurkat T leukemic cells causes the activation of at least two PTK pathways. A CD28-induced, p56lck kinase-independent pathway causes tyrosine-phosphorylation of a 110-kDa substrate while recruitment of p56lck kinase activity is apparently required for CD28-induced tyrosine-phosphorylation of 97- and 68-kDa substrates as well as CD28-induced increases in intracellular calcium. The tyrosine phosphorylation of p110, but not p97 or p68, correlated with CD28 calcium-independent costimulatory activity. The pp110 molecule was tentatively identified as the catalytic subunit of phosphoinositide (PI)-3 kinase based upon its coimmunoprecipitation with the p85 regulatory subunit of PI-3 kinase. PI-3 kinase protein and catalytic activity were found complexed with the CD28 receptor if the receptor was "activated" by cross-linking on the surface of intact cells prior to detergent solubilization. The kinetics of association of PI-3 kinase with the "activated" CD28 receptor was rapid, occurring within 30 s of receptor cross-linking and was stable for at least 30 min. Analysis of the CD28 cytoplasmic peptide sequence revealed a putative PI-3 kinase src homology 2 binding motif and CD28 tyrosine phosphorylation site, DYMNM. Tyrosine phosphorylation of CD28 was detected in pervanadate-treated Jurkat B2.7 cells, but not untreated cells. Pervanadate-induced tyrosine phosphorylation of CD28 correlated with receptor association of PI-3 kinase in the absence of CD28 cross-linking, suggesting that CD28 association with PI-3 kinase uses a tyrosine phosphorylation-dependent mechanism. These data provide a model for CD28 signal transduction and support a role for PI-3 kinase in mediating the CD28 calcium-independent, cyclosporin A-insensitive costimulatory signal.

Signal transduction through the CD19 receptor during discrete developmental stages of human B-cell ontogeny.

We present evidence that the CD19 receptor is functionally operative and transmits pleiotropic signals throughout the pro-B, pre-pre-B, pre-B, early B, and mature B cell stages of human B-cell ontogeny. The signaling ability of CD19 does not depend on the existence of a functional B-cell antigen receptor complex (ARC). In B-cell precursors (BCP) lacking a functional ARC, CD19 is physically and functionally associated with Src family protein tyrosine kinases (PTK). The engagement of the CD19 receptor on BCP with a high affinity anti-CD19 monoclonal antibody (mAb) or its homoconjugate rapidly activates the associated PTK and results in tyrosine phosphorylation of CD19. Moreover, this proximal PTK activation step triggers downstream stimulation of several different intracellular messenger systems. Remarkably, CD19 becomes rapidly phosphorylated on tyrosine residues upon engagement of several other surface receptors as well, suggesting that it may function as a common response element linked via tyrosine phosphorylation to multiple BCP/B-cell receptors and signaling pathways. Furthermore, in all B-lineage lymphoid cell populations, co-approximation of the receptors CD19 and CD72 (ligand for the CD5 T-cell receptor) generates a stronger signal than the engagement of either individual receptor. These convergent observations constitute a strong argument for an important regulatory function of CD19 in human BCP and prompt the hypothesis that the CD19 receptor may play an important role in cognate interactions between B- and T-lineage lymphoid compartments as well as the coordinate production of BCP at multiple stages of human B-cell ontogeny.

Tyrosine phosphorylation of CD19 in pre-B and mature B cells.

Cross-linking of B cell surface immunoglobulins (sIg) results in activation of mature B cells and stimulates a molecular signaling mechanism for antigen-specific B cell expansion and differentiation. This signaling pathway is dependent on tyrosine (Tyr) phosphorylation and results in the activation of sIg-associated src family kinases and p72SYK. Rapid Tyr phosphorylation occurs on multiple protein substrates. Here we show that activation of B cells by cross-linking sIg results in an increase in Tyr phosphorylation of the lineage-restricted B cell surface antigen CD19, and show that it is a major substrate of activated Tyr kinase following sIg stimulation. Lower levels of constitutive CD19 Tyr phosphorylation occurred in most sIg+ mature B cell lines examined and in normal dense tonsillar B cells. We also find that when CD19 is Tyr-phosphorylated it becomes competent to interact with SH2 domains suggesting a mechanism whereby, following B cell activation, CD19 could be linked to intracellular signaling pathways. In sIg- pre-B cell lines, CD19 was expressed but was not constitutively phosphorylated on tyrosine. Upon CD19 cross-linking, Tyr phosphorylation of CD19 was induced in sIg- pre-B cell lines. CD19 cross-linking also directly induced Tyr phosphorylation of CD19 and other substrates in mature B cells. The ability of CD19 to signal in the absence of sIg expression may provide important stimulation in pre-B cell development.

CD19 Is a Substrate of the Antigen Receptor-Associated Protein Tyrosine Kinase in Human B Cells

The B cell antigen receptor, mIg, is part of a multimolecular complex including Igα, Igβ, and CD19. We provide evidence here that upon ligation of mIg CD19 becomes phosphorylated on tyrosine residues. Further, protein tyrosine kinase lyn, which is activated by the antigen receptor, can be readily co-immunoprecipitated with CD19 suggesting this is the enzyme responsible for the antigen receptor mediated tyrosine phosphorylation of CD19.