The zeta chain is associated with a tyrosine kinase and upon T-cell antigen receptor stimulation associates with ZAP-70, a 70-kDa tyrosine phosphoprotein

Human peripheral blood mononuclear cells stimulated by Schistosoma mansoni antigens: association between protein tyrosine kinases, mitogen-activated protein kinases and cytokine production

Concerning schistosomiasis, little is known about the intracellular signaling response of human peripheral blood mononuclear cells (PBMC) to Schistosoma mansoni antigens. To understand the critical role of protein tyrosine kinases (PTKs) in PBMC activation by S. mansoni antigens, we investigated how inhibition of PTKs by genistein, a tyrosine kinase inhibitor, affects proliferation, cytokine production and activation of mitogen-activated protein kinases (MAPKs). Our studies showed that PTKs have an important role in proliferation of PBMC from chronic schistosomiasis patients as cells stimulated with S. mansoni soluble antigens in the presence of genistein had an impaired proliferation. In contrast, PTK inhibition failed to cause any effect on MAPKs activity. We also evaluated the cytokine production for interleukin (IL)-2, interferon gamma (IFN-gamma), and IL-10 in culture supernatants of PBMC treated with or without PTKs inhibitor. Our results show that PBMC from chronic patients produced a high amount of IL-10 when stimulated with soluble egg antigen preparation (SEA), however, the amount produced of IL-2 and IFN-gamma was not significant. In the presence of PTKs inhibitor only the production of IL-10 was decreased. The findings suggest that PTKs are involved on signal transduction pathway for PBMC activation, but may not be an absolute requirement for all signaling responses to S. mansoni antigens.

Immune defects observed in patients with primary malignant brain tumors

Malignant glioblastomas (gliomas) account for approximately one third of all diagnosed brain tumors. Yet, a decade of research has made little progress in advancing the treatment of these tumors. In part this lack of progress is linked to the challenge of discovering how glial tumors are capable of both modulating host immune function and neutralizing immune-based therapies. Patients with gliomas exhibit a broad suppression of cell-mediated immunity. The impaired cell-mediated immunity observed in patients with gliomas appears to result from immunosuppressive factor(s) secreted by the tumor. This article reviews what has been elucidated about the immune defects of patients harboring glioma and the glioma-derived factors which mediate this immunosuppression. A model involving systemic cytokine dysregulation is presented to suggest how the immune defects arise in these individuals.

Role of the Src homology 2 domains and interdomain regions in ZAP-70 phosphorylation and enzymatic activity

The protein tyrosine kinase ZAP-70, which mediates T-cell antigen receptor (TCR) signalling, contains three distinct functional modules, two tandemly arranged SH2 domains, a kinase domain and a linker region (interdomain B) that connects them. ZAP-70 enzymatic activation is strictly dependent on the binding, via its SH2 domains, to the triggered TCR and on tyrosine phosphorylation. Here we utilized recombinant ZAP-70 and carried out a mutational analysis to understand the structural requirements for its activation. We show that deletion of both SH2 domains corresponding to the first 254 residues moderately increases ZAP-70 enzymatic activity on an exogenous substrate in vitro, results in increased tyrosine phosphorylation and produces subtle conformational changes, as judged by altered SDS/PAGE migration. Mutation of Tyr292, 315 and 319 to Phe in the interdomain B region, which constitute the major phosphorylation sites both in vitro and in vivo, did not affect ZAP-70 enzymatic activity. Moreover, deletion analysis of the interdomain B region established residues 320-619 as a minimal region endowed with full kinase activity. We propose that binding of ZAP-70 to the TCR promotes, through conformational changes, its extensive phosphorylation on tyrosine. However, Tyr292, 315 and 319 do not affect ZAP-70 enzymatic activity and may influence ZAP-70 signalling only indirectly by mediating its association with intracellular transducers.

Characterization of TCR-induced receptor-proximal signaling events negatively regulated by the protein tyrosine phosphatase PEP

The proline-, glutamic acid-, serine- and threonine-enriched protein tyrosine phosphatase PEP, which is expressed primarily in hematopoietic cells, was recently discovered to be physically associated with the 50-kDa cytosolic protein tyrosine kinase (PTK) Csk, an important suppressor of Src family PTK, including Lck and Fyn in T cells. We report that this phosphatase has an inhibitory effect on TCR-induced transcriptional activation of the c-fos proto-oncogene and elements from the IL-2 gene promoter. Catalytically inactive mutants of PEP had no effects in these assays. Expression of PEP also reduced activation of the N-terminal c-Jun kinase Jnk2 in response to receptor ligation, but not in response to UV light. In agreement with a more receptor-proximal site of action, we found that PEP reduced the TCR-induced increase in tyrosine phosphorylation of an Lck mutant, Lck-Y505F, which is only phosphorylated on tyrosine 394, the positive regulatory site. Finally, we observed that PEP reduced c-fos activation in a synergistic manner with Csk, supporting the notion that these two enzymes form a functional team acting on Src family kinases involved in TCR signaling.

Live cell fluorescence imaging of T cell MEKK2: redistribution and activation in response to antigen stimulation of the T cell receptor

T cell activation requires engagement of the T cell receptor (TCR) at the interface of conjugates formed with antigen-presenting cells. TCR engagement is accompanied by a redistribution of specific signaling molecules to the cytoplasmic region of the TCR complex. In this study, immunocytochemistry and live cell fluorescence imaging demonstrate that T cell MEK kinase 2 (MEKK2) is translocated to the T cell/antigen-presenting cell interface in response to antigen activation. MEKK2 translocation occurs more rapidly as the antigen concentration is increased. Biochemical activation of MEKK2 follows TCR stimulation, and expression of a dominant-negative MEKK2 inhibits TCR-mediated conjugate stabilization and ERK and p38 MAP kinase phosphorylation. Live cell fluorescence imaging thus enables characterization of signal transducers that are dynamically translocated following TCR engagement.

SLAP, a dimeric adapter protein, plays a functional role in T cell receptor signaling

Engagement of the T cell antigen receptor (TCR) leads to rapid activation of protein tyrosine kinases, which in turn phosphorylate downstream enzymes and adapter proteins. Some adapter proteins, such as SLP-76, Vav, and LAT, positively regulate TCR-mediated signal transduction, whereas others, such as Cbl, play an inhibitory role. SLAP (Src-like adapter protein), an adapter protein containing a Src homology 3 and a Src homology 2 domain, was isolated from a yeast interacting screen by using N-terminal Cbl as bait. N-terminal Cbl interacts with SLAP in vivo and in vitro in a tyrosine phosphorylation-independent manner. We observed that SLAP is expressed in T cells, and upon TCR activation, SLAP interacts with ZAP-70, Syk, LAT, and TCRzeta chain in Jurkat T cells. In transiently transfected COS-7 cells, SLAP forms separate complexes with ZAP-70, Syk, and LAT through its Src homology 2 domain. Overexpression of a C-terminal-truncated SLAP mutant down-regulates nuclear factor of activated T cells-AP1 activity. We have evidence that SLAP forms homodimers through its C-terminal region. Serial truncations and mutations in the C terminus of SLAP demonstrate that there is a correlation between the loss of dimerization and the inhibition of nuclear factor of activated T cells-AP1 activity. The in vivo association of SLAP with key signaling molecules and its inhibition of T cell activation suggests that SLAP plays an important role in TCR-mediated signal transduction.

T cell activation stimulates the association of enzymatically active tyrosine-phosphorylated ZAP-70 with the Crk adapter proteins

Engagement of the T cell antigen receptor initiates signal transduction involving tyrosine phosphorylation of multiple effector molecules and the formation of multimolecular complexes at the receptor site. Adapter proteins that possess SH2 and SH3 protein-protein interaction domains are implicated in the assembly of cell activation-induced signaling complexes. We found that Crk adapter proteins undergo activation-induced interaction with the zeta-chain associated protein (ZAP-70) tyrosine kinase in the human T cell line, Jurkat. Incubation of various glutathione S-transferase fusion proteins with a lysate of activated Jurkat cells resulted in selective association of ZAP-70 with Crk, but not Grb2 or Nck, adapter proteins. In addition, tyrosine-phosphorylated ZAP-70 co-immunoprecipitated with Crk from a lysate of activated Jurkat cells, and ZAP-70 association with GST-Crk was observed in a lysate of activated human peripheral blood T cells. Association between the two molecules was mediated by direct physical interaction and involved the Crk-SH2 domain and phosphotyrosyl-containing sequences on ZAP-70. The association required intact Lck, considered to be an upstream regulator of ZAP-70, because it could not take place in activated JCaM1 cells, which express normal levels of ZAP-70 but are devoid of Lck. Finally, glutathione S-transferase-Crk fusion proteins were found to interact predominantly with membrane-residing tyrosine-phosphorylated ZAP-70 that exhibited autophosphorylation activity as well as phosphorylation of an exogenous substrate, CFB3. These findings suggest that Crk adapter proteins play a role in the early activation events of T lymphocytes, apparently, by direct interaction with, and regulation of, the membrane-residing ZAP-70 protein tyrosine kinase.

Structural analysis of the lymphocyte-specific kinase Lck in complex with non-selective and Src family selective kinase inhibitors

BACKGROUND

The lymphocyte-specific kinase Lck is a member of the Src family of non-receptor tyrosine kinases. Lck catalyzes the initial phosphorylation of T-cell receptor components that is necessary for signal transduction and T-cell activation. On the basis of both biochemical and genetic studies, Lck is considered an attractive cell-specific target for the design of novel T-cell immunosuppressants. To date, the lack of detailed structural information on the mode of inhibitor binding to Lck has limited the discovery of novel Lck inhibitors.

RESULTS

We report here the high-resolution crystal structures of an activated Lck kinase domain in complex with three structurally distinct ATP-competitive inhibitors: AMP-PNP (a non-selective, non-hydrolyzable ATP analog); staurosporine (a potent but non-selective protein kinase inhibitor); and PP2 (a potent Src family selective protein tyrosine kinase inhibitor). Comparison of these structures reveals subtle but important structural changes at the ATP-binding site. Furthermore, PP2 is found to access a deep, hydrophobic pocket near the ATP-binding cleft of the enzyme; this binding pocket is not occupied by either AMP-PNP or staurosporine.

CONCLUSIONS

The potency of staurosporine against Lck derives in part from an induced movement of the glycine-rich loop of the enzyme upon binding of this ligand, which maximizes the van der Waals interactions present in the complex. In contrast, PP2 binds tightly and selectively to Lck and other Src family kinases by making additional contacts in a deep, hydrophobic pocket adjacent to the ATP-binding site; the amino acid composition of this pocket is unique to Src family kinases. The structures of these Lck complexes offer useful structural insights as they demonstrate that kinase selectivity can be achieved with small-molecule inhibitors that exploit subtle topological differences among protein kinases.

TCRzeta is transported to and retained in the Golgi apparatus independently of other TCR chains: implications for TCR assembly

It is generally assumed that TCR assembly occurs in the endoplasmic reticulum (ER), and ER retention/degradation signals have been identified in several of the TCR chains. These signals are probably responsible for retention of incompletely assembled TCR complexes and free TCR chains in the ER. This study focused on the intracellular localization and transport of partially assembled TCR complexes as determined by confocal microscopy analyses. We found that none of the TCR chains except for TCRzeta were allowed to exit the ER in T cell variants in which the hexameric CD3gammaepsilonTi alphabetaCD3 deltaepsilon complex was not formed. Interestingly, TCRzeta was exported from the ER independently of other TCR chains and was predominantly located in a compartment identified as the Golgi apparatus. Furthermore, in the TCRzeta-negative cell line MA5.8, the hexameric CD3gammaepsilonTi alphabetaCD3 deltaepsilon complex was allowed to exit the ER and was also predominantly located in the Golgi apparatus. However, neither hexameric TCR complexes nor TCRzeta chains were efficiently expressed at the cell surface without the other. The observations that TCRzeta and hexameric TCR complexes are transported from the ER to the Golgi apparatus independently of each other and that these partial TCR complexes are unable to be efficiently expressed at the cell surface suggest that final TCR assembly occurs in the Golgi apparatus.

Age-related impairment of p56lck and ZAP-70 activities in human T lymphocytes activated through the TcR/CD3 complex

Cellular immune responses decrease with aging. Lymphocytes of aged individuals do not perform as well as cells from young subjects in a number of in vitro assays including cell proliferation, cytokine production, and protection against apoptosis. Here, we have tested the hypothesis that a decrease in T cell responses in tymphocytes from elderly subjects could parallel a decrease in the activity of protein tyrosine kinases (PTK) associated with signal transduction in T lymphocytes. We report that anti-CD3-triggered T lymphocyte proliferation was significantly decreased in T lymphocytes from elderly subjects, but the decrease was not due to an alteration of the percentage or mean fluorescence intensities of CD3, CD4, and CD45. Of significance, the activities of p56lck and ZAP-70 in vitro were significantly decreased in T lymphocytes from elderly subjects compared to young individuals. However, the level of expression of the two kinases did not change with aging. The activity of p59fyn did not show changes with aging, suggesting that p59fyn did not compensate for the decreased activity of p56lck. We also found that the extent of tyrosine phosphorylation of the adaptor protein p95vav was similar in activated T lymphocytes from elderly and young subjects. Our results suggest that the altered cellular immune responses observed in T lymphocytes with aging may be the result, at least in part, of an alteration in early events associated with signal transduction through the TcR/CD3 complex that translates into decreased activities of p56lck and ZAP-70. Impairment in the activities of these twokey components of T cell signaling may contribute to reduced immune functions associated with aging.

The hematopoietic-specific adaptor protein gads functions in T-cell signaling via interactions with the SLP-76 and LAT adaptors

BACKGROUND

The adaptor protein Gads is a Grb2-related protein originally identified on the basis of its interaction with the tyrosine-phosphorylated form of the docking protein Shc. Gads protein expression is restricted to hematopoietic tissues and cell lines. Gads contains a Src homology 2 (SH2) domain, which has previously been shown to have a similar binding specificity to that of Grb2. Gads also possesses two SH3 domains, but these have a distinct binding specificity to those of Grb2, as Gads does not bind to known Grb2 SH3 domain targets. Here, we investigated whether Gads is involved in T-cell signaling.

RESULTS

We found that Gads is highly expressed in T cells and that the SLP-76 adaptor protein is a major Gads-associated protein in vivo. The constitutive interaction between Gads and SLP-76 was mediated by the carboxy-terminal SH3 domain of Gads and a 20 amino-acid proline-rich region in SLP-76. Gads also coimmunoprecipitated the tyrosine-phosphorylated form of the linker for activated T cells (LAT) adaptor protein following cross-linking of the T-cell receptor; this interaction was mediated by the Gads SH2 domain. Overexpression of Gads and SLP-76 resulted in a synergistic augmentation of T-cell signaling, as measured by activation of nuclear factor of activated T cells (NFAT), and this cooperation required a functional Gads SH2 domain.

CONCLUSIONS

These results demonstrate that Gads plays an important role in T-cell signaling via its association with SLP-76 and LAT. Gads may promote cross-talk between the LAT and SLP-76 signaling complexes, thereby coupling membrane-proximal events to downstream signaling pathways.

Interaction between CD45-AP and protein-tyrosine kinases involved in T cell receptor signaling

CD45-AP associates specifically with CD45, a protein-tyrosine phosphatase essential for antigen receptor-mediated signal transduction. CD45 modulates the activity of Src family protein-tyrosine kinases involved at the onset of antigen receptor-mediated signaling by dephosphorylating their regulatory tyrosyl residues. We have shown that lymphocyte responses to antigen receptor stimulation are impaired in CD45-AP-null mice. To examine the possibility that CD45-AP coordinates the interaction between CD45 and its substrates, we investigated the associations of CD45-AP with several protein-tyrosine kinases. Endogenous CD45-AP coimmunoprecipitated with Lck and ZAP-70 in both CD45-positive T cells and their CD45-negative variants after stimulation by antigen receptor ligation. Concomitantly, CD45 coimmunoprecipitated with Lck and ZAP-70 after T cell receptor-mediated stimulation of CD45-positive cells. Recombinant CD45-AP exhibited specific binding to Lck and ZAP-70 protein-tyrosine kinases, but not to Fyn or Csk, in lysates of both CD45-positive and -negative T cells. Specific interactions were demonstrated between the respective recombinant proteins as well. These results demonstrate that CD45-AP associates directly and selectively with Lck and ZAP-70 in response to T cell receptor-mediated stimulation. The associations of CD45-AP with Lck and ZAP-70 may mediate the functional interactions of these kinases with CD45 during antigen receptor stimulation.

TCR Activation Inhibits Chemotaxis Toward Stromal Cell-Derived Factor-1: Evidence for Reciprocal Regulation Between CXCR4 and the TCR

Stromal cell-derived factor-1 (SDF-1), a C-X-C family chemokine, is a potent T lymphocyte chemoattractant. We investigated the effects of T cell activation on the chemotactic response to SDF-1. Anti-CD3 Ab stimulation of either Jurkat T cells or murine peripheral CD4+ T lymphocytes produced a dramatic inhibition of SDF-1-induced chemotaxis. In contrast, the SDF-1 responses of Jurkat clones with deficiencies in key TCR signaling components (Lck, CD45, and TCR-β), were only marginally reduced by anti-CD3 stimulation. Similar to PMA treatment, which abolished both CXCR4 receptor expression and the chemotactic response of Jurkat cells to SDF-1, anti-CD3 Ab treatment reduced cell surface expression of CXCR4 to 65% of the control value, an effect that was blocked by protein kinase C inhibitors. Our data suggest that initial T cell activation events inhibit the response of Jurkat T cells to CXCR4 stimulation. In contrast, SDF-1 treatment resulted in a reduction of tyrosine phosphorylation of the TCR downstream effectors, ZAP-70, SLP-76, and LAT (linker for activation of T cells), suggesting that this chemokine potentially regulates the threshold for T cell activation.

Interdomain B in ZAP-70 regulates but is not required for ZAP-70 signaling function in lymphocytes

The protein tyrosine kinase ZAP-70 plays an important role in T-cell activation and development. After T-cell receptor stimulation, ZAP-70 associates with the receptor and is phosphorylated on many tyrosines, including Y292, Y315, and Y319 within interdomain B. Previously, we demonstrated that Y292 negatively regulates ZAP-70 function and that Y315 positively regulates ZAP-70 function by interacting with Vav. Recent studies have suggested that Y319 also positively regulate ZAP-70 function. Paradoxically, removal of interdomain B (to create the construct designated Delta), containing the Y292, Y315, and Y319 sites, did not eliminate the ability of ZAP-70 to induce multiple gene reporters in Syk-deficient DT-40 B cells and ZAP-70/Syk-deficient Jurkat cells. Here we show that Delta still utilizes the same pathways as wild-type ZAP-70 to mediate NF-AT induction. This is manifested by the ability of Delta to restore induction of calcium fluxes and mitogen-activated protein kinase activation and by the ability of dominant negative Ras and FK506 to block the induction of NF-AT activity mediated by Delta. Biochemically we show that the stimulated tyrosine phosphorylation of Vav, Shc, and ZAP-70 itself is diminished, whereas that of Slp-76 is increased in cells reconstituted with Delta. Deletion of interdomain B did not affect the ability of ZAP-70 to bind to the receptor. The in vitro kinase activity of ZAP-70 lacking interdomain B was markedly reduced, but the kinase activity was still required for the protein's in vivo activity. Based on these data, we concluded that interdomain B regulates but is not required for ZAP-70 signaling function leading to cellular responses.

Prolactin receptor signaling: shared components with the T-cell antigen receptor in Nb2 lymphoma cells

Previously, we reported that activation of the human prolactin receptor (PRLR) produced a protein phosphorylation pattern strikingly similar to that provoked by Concanavalin A (Con A), an activator of the T-cell antigen receptor (TCR). These results suggested that certain signaling components of the TCR may be shared by the activated PRLR. Additional studies here assessed the levels of TCR expression following PRLR stimulation and the effect of TCR activation on PRL-stimulated proliferation in lactogen-dependent pre-T Nb2-11 lymphoma cells. The results indicated that the TCR was expressed on the surface of approx 4% of exponentially proliferating and prolactin- (PRL) treated cells. In contrast, approx 45% of quiescent cells, cultured in the absence of PRL for 24 h, expressed the TCR at the cell surface, suggesting that lactogen withdrawal may up-regulate TCR cell-surface expression. Moreover, TCR activation with anti-CD3 antibodies attenuated PRL-stimulated Nb2-11 cell proliferation in a concentration-dependent manner. In other experiments, immunoprecipitation and immunoblotting of Nb2-11 lysates revealed that activation of the PRLR resulted in rapid tyrosyl phosphorylation of ZAP-70, a critical TCR-associated tyrosine kinase. In addition, ZAP-70 was found to associate transiently with the putative guanine nucleotide exchange factor and substrate, Vav, in PRL-treated cells. ZAP-70 was also found to associate constitutively with the PRLR; PRL stimulation provoked the transient recruitment of Vav to the complex. These observations suggest that PRL signaling reflects the transient formation of a PRLR-ZAP-70-Vav complex and its immunomodulatory actions involve diverse interactions that affect TCR expression and signaling mechanisms.

ZAP-70 association with T cell receptor zeta (TCRzeta): fluorescence imaging of dynamic changes upon cellular stimulation

The nonreceptor protein tyrosine kinase ZAP-70 is a critical enzyme required for successful T lymphocyte activation. After antigenic stimulation, ZAP-70 rapidly associates with T cell receptor (TCR) subunits. The kinetics of its translocation to the cell surface, the properties of its specific interaction with the TCRzeta chain expressed as a chimeric protein (TTzeta and Tzetazeta), and its mobility in different intracellular compartments were studied in individual live HeLa cells, using ZAP-70 and Tzetazeta fused to green fluorescent protein (ZAP-70 GFP and Tzetazeta-GFP, respectively). Time-lapse imaging using confocal microscopy indicated that the activation-induced redistribution of ZAP-70 to the plasma membrane, after a delayed onset, is of long duration. The presence of the TCRzeta chain is critical for the redistribution, which is enhanced when an active form of the protein tyrosine kinase Lck is coexpressed. Binding specificity to TTzeta was indicated using mutant ZAP-70 GFPs and a truncated zeta chimera. Photobleaching techniques revealed that ZAP-70 GFP has decreased mobility at the plasma membrane, in contrast to its rapid mobility in the cytosol and nucleus. Tzetazeta- GFP is relatively immobile, while peripherally located ZAP-70 in stimulated cells is less mobile than cytosolic ZAP-70 in unstimulated cells, a phenotype confirmed by determining the respective diffusion constants. Examination of the specific molecular association of signaling proteins using these approaches has provided new insights into the TCRzeta-ZAP-70 interaction and will be a powerful tool for continuing studies of lymphocyte activation.

Molecular interaction between the Fyn-associated protein SKAP55 and the SLP-76-associated phosphoprotein SLAP-130

It has previously been reported that in resting T-lymphocytes the protein tyrosine kinase p59 constitutively co-precipitates with four phosphoproteins of 43, 55, 85, and 120 kDa, respectively. We have recently cloned the 55-kDa protein that was termed Src kinase-associated phosphoprotein of 55 kDa (SKAP55). Here we demonstrate that the recently characterized SH2-domain-containing leukocyte protein 76-associated phosphoprotein of 130 kDa (SLAP-130) is one of the components of the Fyn complex and that it also co-precipitates with SKAP55 in human T-cells. We establish that SKAP55 and SLAP-130 associate with each other when both molecules are co-expressed in COS cells. By co-transfection of truncated mutants of SKAP55 and SLAP-130 as well as by using the two-hybrid selection system, we further demonstrate that the association between SLAP-130 and SKAP55 is direct and involves the Src homology 3 domain of SKAP55 and the proline-rich sequence of SLAP-130.

Host response to EBV infection in X-linked lymphoproliferative disease results from mutations in an SH2-domain encoding gene

X-linked lymphoproliferative syndrome (XLP or Duncan disease) is characterized by extreme sensitivity to Epstein-Barr virus (EBV), resulting in a complex phenotype manifested by severe or fatal infectious mononucleosis, acquired hypogammaglobulinemia and malignant lymphoma. We have identified a gene, SH2D1A, that is mutated in XLP patients and encodes a novel protein composed of a single SH2 domain. SH2D1A is expressed in many tissues involved in the immune system. The identification of SH2D1A will allow the determination of its mechanism of action as a possible regulator of the EBV-induced immune response.

Phosphorylation and coupling of zeta-chains to activated T-cell receptor (TCR)/CD3 complexes from peripheral blood T-cells of elderly humans

Aging is often accompanied by altered T-cell signaling and functions. Signals mediated through the T-cell receptor (TCR)/CD3 complex are associated with tyrosine phosphorylations of zeta-chains by the regulated activities of protein tyrosine kinases p56(lck) and p59(fyn) as well as protein tyrosine phosphatases. In the present investigation, the coupling and phosphorylation of zeta-chains to TCR/CD3 immunocomplexes were examined in peripheral blood T-cells from 13 elderly and young humans stimulated by ligation of the TCR/CD3 with cross-linked anti-CD3epsilon monoclonal antibody OKT3. Western blots analyzing the non-covalent coupling of zeta-chains to TCR/CD3 immunocomplexes from Brij-96 detergent lysates of anti-CD3 ligated T-cells showed that the levels of zeta-chains within TCR/CD3 immunocomplexes from T-cells of elderly and young subjects did not significantly differ. By contrast, the levels of phosphorylated zeta-chains generated during in vitro phosphorylations of TCR/CD3 immunocomplexes from elderly subjects were significantly reduced and averaged 44% of those observed for anti-CD3epsilon ligated T-cells from young subjects. Analyses of the levels of zeta-chain coupling and phosphorylations in T-cells from each of the 13 elderly individuals also showed that the reductions in zeta-chain phosphorylations were heterogeneous and unrelated to modest reductions in coupling. Furthermore, the age-related decreases in zeta-chain phosphorylations were not due to diminished frequencies of CD3epsilon+ cells or densities of CD3epsilon surface receptors and could be observed without reductions in epsilon-chain phosphorylations. These results suggest that aberrancies of zeta-chain phosphorylations can occur in T-cells of elderly humans independent from any uncoupling of zeta-chains to activated TCR/CD3 complexes.

Regulation of Tyrosine Phosphorylation in Isolated T Cell Membrane by Inhibition of Protein Tyrosine Phosphatases

Jurkat T cells activated by the phosphotyrosine phosphatase inhibitors H2O2 or vanadate were found to have a similar pattern of tyrosine phosphorylation when compared with T cells stimulated by anti-CD3 Ab cross-linking, suggesting that protein tyrosine phosphatase (PTP) inhibitors affect the early steps of TCR signaling. To study the role of PTPs in the most proximal membrane events of tyrosine phosphorylation, subcellular fractions of T cells were treated with the PTP inhibitors in the presence of ATP. In the membrane fraction, tyrosine phosphorylation of Lck, Fyn, and CD3ζ can be induced by PTP inhibitors, but not by anti-CD3. Detailed characterization of this cell-free system showed that the pattern and the order of induced tyrosine phosphorylation is similar to that induced in intact cells. Upon removal of the PTP inhibitor, the tyrosine-phosphorylated proteins, including Lck, Fyn, Syk, Zap70, and CD3ζ, are rapidly dephosphorylated. Preliminary characterizations indicate that a PTP distinct from CD45, SHP1, and SHP2 is present in T cell membranes and the inhibition of this yet unidentified PTP is most likely responsible for the Lck-dependent tyrosine phosphorylation triggered by PTP inhibitors.

Identification of the Fc epsilonRI-activated tyrosine kinases Lyn, Syk, and Zap-70 in human basophils

BACKGROUND

In human blood basophils, cross-linking the high-affinity IgE receptor Fc epsilonRI with multivalent antigen activates a signaling pathway leading to Ca2+ mobilization, actin polymerization, shape changes, secretion, and cytokine production.

METHODS AND RESULTS

The role of tyrosine kinases in human Fc epsilonRI signaling was explored by using human basophils isolated by Percoll gradient centrifugation followed by negative and/or positive selection with antibody-coated magnetic beads. Fc epsilonRI cross-linking of more than 95% pure basophil preparations activates the protein-tyrosine kinases Lyn and Syk, previously linked to Fc epsilonRI-coupled rodent mast cell activation, as well as Zap-70, previously implicated in T-cell receptor signaling, and causes the tyrosine phosphorylation of multiple proteins. The presence of Lyn, Syk, and Zap-70 in basophils was confirmed by Western blotting in lysates of highly purified basophils and independently by confocal fluorescence microscopy in cells labeled simultaneously with kinase-specific antibodies and with the basophil-specific antibody 2D7. Comparable amounts of Lyn and Syk were found in basophils and B cells, whereas T cells appear to have greater amounts of Zap-70 than basophils. The tyrosine kinase inhibitor piceatannol spares IgE-mediated Lyn activation but inhibits IgE-induced Syk and Zap-70 activation as well as overall protein tyrosine phosphorylation and secretion. Overall protein-tyrosine phosphorylation increases steadily over a range of anti-IgE concentrations that are low to optimal for secretion. However, tyrosine phosphorylation continues to increase at high anti-IgE concentrations that elicit very little secretion (the characteristic high-dose inhibition of secretion).

CONCLUSIONS

Our data demonstrate the association of anti-IgE-stimulated, protein-tyrosine phosphorylation by a cascade of tyrosine kinases, including Zap-70 as well as Lyn and Syk, with the initiation of Fc epsilonRI-mediated signaling in human basophils.

De Novo-Developed T Cells Have Compromised Response to Existing Alloantigens: Using Ld-Specific Transgenic 2C T Cells as Tracers in a Mouse Heart Transplantation Model

In this study, the phenotype, TCR signaling events, and function of T cells developed de novo during adulthood in the presence of extrathymic alloantigen were investigated. C57BL/6 mice(H-2b) were first transplanted heterotopically with BALB/c hearts (H-2d) and treated with rapamycin for 2 wk to create a tolerant status. Three weeks postoperation, the mice were whole body irradiated and transplanted with bone marrow cells from 2C mice, which are transgenic for TCR, and most of their T cells are Ld-specific CD8 cells. The 2C T cells developed de novo in the C57BL/6 mice were not able to reject the heart allograft. No clonal deletion, TCR down-regulation, or CD8 down-regulation was found in the tolerized 2C T cells. There was no characteristic phenotype of these cells in terms of CD25, ICAM-1, CD44, and MEL-14 expression. Early TCR signaling events such as intracellular calcium concentration flux, tyrosine phosphorylation, Lck and Fyn kinase activities, and Lck and Fyn protein levels in the tolerized 2C T cells were comparable to their normal counterparts, but the tolerized T cells were defective in IL-2 production and proliferation upon H-2d alloantigen stimulation in vitro. Exogenous IL-2 could not reverse the compromised proliferation. The results of this study indicate that during adulthood, the de novo-developed T cells become tolerant to extrathymic Ag without clonal deletion. These newly minted T cells are functionally defective although they are indistinguishable from normal T cells in phenotypes and in some early signaling events.

Defects in actin-cap formation in Vav-deficient mice implicate an actin requirement for lymphocyte signal transduction

BACKGROUND

Antigen-receptor interactions on lymphocytes result in local clustering of actin, receptors and signaling molecules into an asymmetric membrane structure termed a cap. Although actin polymerization is known to be required, the mechanisms underlying cap formation are unclear. We have studied the events underlying cap formation using mice bearing a null mutation in vav (vav-/-), a gene that encodes a guanine-nucleotide exchange factor for the GTPase Rac.

RESULTS

Lymphocytes from vav-/- mice failed to form T-cell receptor caps following activation and had a defective actin cytoskeleton. The vav-/- T cells were deficient in interleukin-2 (IL-2) production and proliferation, and the peak of Ca2+ mobilization was reduced although of normal duration. Activation of Jun N-terminal kinase or stress-activated kinase (JNK or SAPK) and mitogen-activated protein kinase (MAPK) and the induction of the transcription factor NF-ATc1 and egr-1 genes was normal. Despite the reduced Ca2+ mobilization, translocation of cytoplasmic NF-ATc to the nucleus was normal, reflecting that the lower levels of Ca2+ in vav-/- cells were still sufficient to activate calcineurin. Treatment of lymphocytes with cytochalasin D, which blocks actin polymerization, inhibited cap formation and produced defects in signaling and IL-2 transcriptional induction in response to antigen-receptor signaling that were nearly identical to those seen in vav-/- cells. In transfection studies, either constitutively active Vav or Rac could complement constitutively active calcineurin to activate NF-AT-dependent transcription.

CONCLUSIONS

These results indicate that Vav is required for cap formation in lymphocytes. Furthermore, the correlation between cap formation, IL-2 production and proliferation supports the hypothesis that an actin-dependent pathway is a source of specialized growth regulatory signals.

Activation of p21ras/MAPK signal transduction molecules decreases with age in mitogen-stimulated T cells from rats

Signal transduction is ubiquitously involved in the initiation of physiological signals that lead to growth and proliferation of cells. The signaling cascade mediated by the mitogen-activated protein kinase (MAPK) is considered essential for T cell growth and function. Therefore, it was of interest to determine the influence of age on the induction of MAPK in mitogen-activated T cells. T cells from young (4-6 months) and old (24-26 months) rats responded to concanavalin A (Con A) stimulation by increasing MAPK, c-jun amino terminal kinase (JNK), and p21ras activities. The time course of induction of MAPK/JNK and p21ras activities was similar in T cells isolated from young and old rats. The induction of JNK activity did not change significantly with age; however, the induction of MAPK and p21ras activities was significantly less (50 to 65%) in T cells from old rats than in T cells from young rats. Although the relative protein levels of p42 and p44 MAPK did not change with age, the proportion of the phosphorylated p44 MAPK decreased with age. In addition, it was found that the in vitro kinase activities of the T cell receptor-associated protein tyrosine kinase Lck (p56Lck) and ZAP-70 but not Fyn (p59Fyn) were lower in T cells from old rats than in T cells from young rats. The decline in activities of these signaling molecules with age was not associated with changes in their corresponding protein levels. Thus, our results demonstrate that aging alters the activation of the signal transduction cascade that leads to T cell activation.

Tyrosine kinase chimeras for antigen-selective T-body therapy

Protein tyrosine kinases (PTKs) transmit activation signals in almost every cell type, including immune effector cells. The aberrant or constitutive activation of PTKs can often cause neoplastic transformation. The use of chimeric receptors based on PTKs may enable us to elucidate the signaling pathways of normal immune cells and other cell types, and the abnormal events that can lead to malignant transformation. In this review, we focus on antigen specific chimeric PTKs in which antibody-derived scFv are joined to the Syk family of PTKs. These chimeric receptors yielded reagents that can selectively redirect immune effector cells and specifically activate them to produce cytokines or lyse their target. The advantages of using such PTK-based chimeras to redirect lymphocytes to tumor targets and their potential as an immunotherapeutic approach to malignant disease is discussed.

Characterization of Signal Transduction Through the TCR-ζ Chain Following T Cell Stimulation with Analogue Peptides of Type II Collagen 260–267

The immunodominant T cell determinant of type II collagen (CII) recognized by DBA/1 mice (I-Aq) is CII 260–267. The aims of this study were to determine the role of the amino acid residues within CII 245–270 in T cell signal transduction. To that end, we utilized I-Aq-restricted, CII-specific T cell hybridomas and examined tyrosine phosphorylation of TCR-ζ following stimulation with either wild-type CII 245–270 or a panel of analogue peptides. A variety of patterns occurred, ranging from increased phosphorylation of TCR-ζ to either partial or a complete abrogation of phosphorylation. Critical substitutions also completely abrogated the phosphorylation of ZAP70, a downstream molecule in TCR-ζ signaling. Evaluation of the supernatants of the T cell hybridomas for cytokine production in response to the peptides revealed a close correlation between the induction of phosphorylation of TCR-ζ and the amount of cytokine induced. Selected analogue peptides were tested as tolerogens in neonatal mice. Analogues that did not induce the phosphorylation of ζ chain, such as B3 (CII 251–270s263F→N), were completely unable to induce tolerance, while analogues that caused a partial phosphorylation, such as B6 (CII 251–270s267Q→T) and A3 (CII 245–270s269P→A), induced partial tolerance judged by intermediate degrees of suppression of arthritis. We conclude that discrete alterations in specific amino acid residues of antigenic peptides had profound effects on T cell signaling and that the signaling correlated with T cell cytokine secretion and T cell function in the induction of tolerance and suppression of arthritis.

Harnessing Syk Family Tyrosine Kinases as Signaling Domains for Chimeric Single Chain of the Variable Domain Receptors: Optimal Design for T Cell Activation

T cells of tumor bearers often show defective TCR-mediated signaling events and, therefore, exhibit impaired immune responses. As such, patients with heavy tumor burden are often not amenable to adoptive T cell therapy. To overcome this limitation, we have developed a chimeric receptor that joins an extracellular single chain Fv (scFv) of a specific Ab for Ag recognition to an intracellular protein tyrosine kinase (PTK) for signal propagation. Stimulation through the scFv-PTK receptor should bypass defective TCR-proximal events and directly access the T cell’s effector mechanisms. In this study we describe the optimization of a scFv-PTK configuration, leading to complete T cell activation. The cytosolic PTK Syk is superior to its family member, Zap-70, for intracellular signaling. As a transmembrane (TM) domain, CD4 performs better than CD8 when plastic-immobilized Ag serves as a stimulator. However, when APC are used to trigger chimeric receptors, the need for a flexible spacer between the scFv and TM domains becomes apparent. The CD8α-derived hinge successfully performs this task in chimeric scFv-Syk receptors regardless of its cysteine content. A cytotoxic T cell hybridoma expressing chimeric receptor genes composed of scFv-CD8hinge-CD8TM-Syk or scFv-CD8hinge-CD4TM-Syk is efficiently stimulated to produce IL-2 upon interaction with APC and specifically lyses appropriate target cells in a non-MHC-restricted manner.

Early activation defects in T lymphocytes from aged mice

Aging affects both calcium signals and protein kinase cascades in mouse T lymphocytes. The decline in calcium signal development largely represents differences between naive and memory T cells; the latter are resistant to increases in calcium concentration, and are more common in aged mice. Aging leads to declines in phosphorylation of a wide range of substrates in T cells stimulated by either anti-CD3 antibodies or by substances, such as phorbol myristate acetate (PMA) or ionomycin, that act at intracellular sites, but some phosphoproteins respond only in old T cells, and others respond regardless of age. Tyrosine phosphorylation of the CD3 zeta chain declines with age, both in resting T cells and after activation, but the proportion of Zap-70 that is bound to CD3 zeta increases in T cells from old mice. Zap-70 function and phosphorylation of CD3 zeta-associated Zap-70 change only slightly after stimulation of T cells by anti-CD3 and anti-CD4, and are at similar levels in activated old and young T cells. Nonetheless, induction of Raf-1, MEK, and ERK kinase activity declines with age in CD4 T cells. The effect of aging on T-cell activation is not simply an overall decline in signal intensity, but a set of qualitative changes that differ among subsets and depend at least partly on the nature of the stimulus.

Signaling efficiency of the T cell receptor controlled by a single amino acid in the beta chain constant region

A single amino acid residue, Gln136, located within the connecting peptide domain of Cbeta controls the ability of the alpha/beta TCR to transmit a full signal. TCRs in which this Cbeta residue is mutated to Phe, the residue found in TCR-gamma, are unresponsive to antigenic ligands. Interestingly, this Cbeta residue is either polar or charged in every species studied thus far, including the trout and the skate. In contrast, the analogous residue in Cgamma is always hydrophobic. In spite of their compromised antigen responsiveness, the mutant TCR complex contains the CD3-gamma, -delta, -epsilon, and -zeta chains, and undergoes zeta chain phosphorylation and ZAP-70 recruitment. However, the biological response of the mutant TCR could be rescued with a calcium ionophore, implying that mutant TCRs are defective in generating a calcium-mediated signal. The implications of the differences between Cbeta and Cgamma are considered.

Human airway epithelial cells stimulate T-lymphocyte lck and fyn tyrosine kinase

Previous studies have shown that human airway epithelial cells (AEC) can stimulate allogeneic peripheral blood T-lymphocyte (PBT) proliferation. We now sought to determine which AEC surface molecule/T-cell coreceptors are involved in this process. AEC-induced PBT proliferation was inhibited by 25 microM genestein or herbamycin A (0.9 and 1.8 microM), both tyrosine kinase inhibitors. Anti-phosphotyrosine immunoblots performed on PBT lysates after coculture with AEC demonstrated phosphorylation of 56kD and 60kD bands. To determine whether CD3 associated p59fyn, or CD4 and CD8 associated p56lck phosphotyrosine kinases (PTK) were involved, we assayed kinase activity in lymphocyte lysates immunoprecipitated with anti-p56lck and p59fyn mAbs. PBT cells or murine T-cell line transfectants expressing human CD4 (3G4) or human CD8alpha (3G8) were cocultured with AEC or A549, an alveolar-like cell line lacking class II Ag expression. After A549 or AEC coculture, p56lck activity in PB T-cells peaked at 2 min whereas p59fyn kinase activity continued to rise at 8 min. AEC (expressing class II Ags) stimulate PTK activity in both 3G8 and 3G4 cells. A549 stimulated p56lck in 3G8, but not in 3G4 cells. This activation of p56lck was not blocked by preincubation of A549 with anti-class I or anti-CD1d mAbs. An antibody generated in our laboratory, which recognizes an epithelial specific surface molecule (mAb L12) and which blocks AEC driven PBT proliferation, was shown to block PTK activity of peripheral blood T-cell lysates, though not of 3G8 lysates. These studies suggest that AEC are capable of stimulating CD4 and CD8 associated lck and CD3 associated fyn kinases through class II dependent and independent pathways.

Proximity and orientation underlie signaling by the non-receptor tyrosine kinase ZAP70

Signaling by the antigen receptor of T lymphocytes initiates different developmental transitions, each of which require the tyrosine kinase ZAP70. Previous studies with agonist and antagonist peptides have indicated that ZAP70 might respond differently to different structures of the TCR-CD3 complex induced by bound peptides. The roles of membrane proximity and orientation in activation of ZAP70 signaling were explored using synthetic ligands and their binding proteins designed to produce different architectures of membrane-bound complexes composed of ZAP70 fusion proteins. Transient membrane recruitment of physiological levels of ZAP70 with the membrane-permeable synthetic ligand FK1012A leads to rapid phosphorylation of ZAP70 and activation of the ras/MAPK and Ca2+/calcineurin signaling pathways. ZAP70 SH2 domains are not required for signaling when the kinase is artifically recruited to the membrane, indicating that the SH2 domains function solely in recruitment and not in kinase activation. Using additional synthetic ligands and their binding proteins that recruit ZAP70 equally well but orient it at the cell membrane in different ways, we define a requirement for a specific presentation of ZAP70 to its downstream targets. These results provide a mechanism by which ZAP70, bound to the phosphorylated receptor, could discriminate between conformational changes induced by the binding of different MHC-peptide complexes to the antigen receptor and introduce an approach to exploring the role of spatial orientation of signaling complexes in living cells.

Functional analysis of immunoreceptor tyrosine-based activation motif (ITAM)-mediated signal transduction: the two YxxL segments within a single CD3zeta-ITAM are functionally distinct

Functional analysis of the immunoreceptor tyrosine-based activation motif (ITAM) derived from the membrane-proximal ITAM of CD3zeta demonstrates that mutations at either the tyrosine or leucine residues in the N-terminal YxxL segment of the ITAM abolish all signal transduction functions of this ITAM. In contrast, mutations at the tyrosine or leucine residues in the C-terminal YxxL segment abrogate signals for interleukin (IL)-2 production but do not prevent tyrosine phosphorylation of the N-terminal tyrosine of the ITAM, lck association with the ITAM, activation of phospholipase C-gamma1 or calcium mobilization. Cross-linking of chimeric receptors containing a C-terminal YxxL leucine mutation induces tyrosine phosphorylation of ZAP70 but without stable binding to the phosphorylated ITAM. These results indicate that the two YxxL segments in an ITAM are functionally distinct and that both are essential for ZAP70 binding and IL-2 production. Furthermore, tyrosine phosphorylation of ZAP70 per se is not sufficient to trigger the downstream events leading to IL-2 production. Substitution of an alanine for the bulky side chain at the Y+1 position of the N-terminal YxxL segment reduces the receptor cross-linking requirement necessary to achieve cellular activation and the absolute dependence on lck in this process. Our results reveal that both the number of ITAM as well as the specific amino acid residues within a single ITAM determine the extent of chimeric receptor cross-linking required to trigger tyrosine phosphorylation-dependent signaling events.

Phenotypic features of selective T cell deficiency characterized by absence of CD8+ T lymphocytes and undetectable mRNA for ZAP-70 kinase

Selective T cell deficiency is a rare immune deficiency characterized by the absence of CD8+ T lymphocytes and depressed/absent T cell function. This syndrome has been associated with mutations in the gene for ZAP-70, a tyrosine kinase that has profound effects on signaling via the T cell receptor. In this paper we describe a patient with selective T cell deficiency and certain phenotypic features that are unique among the small number of patients described. The patient had virtually absent T cell function, hypogammaglobulinemia, and no response to vaccination. The T lymphocytes failed to respond to mitogenic stimuli, even in the presence of exogenous interleukin 2. Similar to other patients with this disorder, the T cells were capable of proliferating when stimulated by pharmacologic agents such as phorbol ester and ionomycin. While peripheral blood T cells had limited capability to increase cytosolic Ca2+ levels in response to mitogenic stimulation, thymocytes responded to a large panel of antibodies and mitogens. This report broadens the spectrum of clinical presentations associated with selective T cell deficiency and, for the first time, compares the responses of both peripheral T cells and thymocytes. The data support the concept that the defect in signal transduction resulting from the absence of ZAP-70 is primarily manifested following export of T lymphocytes from the thymus and that selection of CDS-positive T cells is dependent on the presence of ZAP-70.

Differential T cell receptor-mediated signaling in naive and memory CD4 T cells

Naive and memory CD4 T cells differ in cell surface phenotype, function, activation requirements, and modes of regulation. To investigate the molecular bases for the dichotomies between naive and memory CD4 T cells and to understand how the T cell receptor (TCR) directs diverse functional outcomes, we investigated proximal signaling events triggered through the TCR/CD3 complex in naive and memory CD4 T cell subsets isolated on the basis of CD45 isoform expression. Naive CD4 T cells signal through TCR/CD3 similar to unseparated CD4 T cells, producing multiple tyrosine-phosphorylated protein species overall and phosphorylating the T cell-specific ZAP-70 tyrosine kinase which is recruited to the CD3zeta subunit of the TCR. Memory CD4 T cells, however, exhibit a unique pattern of signaling through TCR/CD3. Following stimulation through TCR/CD3, memory CD4 T cells produce fewer species of tyrosine-phosphorylated substrates and fail to phosphorylate ZAP-70, yet unphosphorylated ZAP-70 can associate with the TCR/CD3 complex. Moreover, a 26/28-kDa phosphorylated doublet is associated with CD3zeta in resting and activated memory but not in naive CD4 T cells. Despite these differences in the phosphorylation of ZAP-70 and CD3-associated proteins, the ZAP-70-related kinase, p72syk, exhibits similar phosphorylation in naive and memory T cell subsets, suggesting that this kinase could function in place of ZAP-70 in memory CD4 T cells. These results indicate that proximal signals are differentially coupled to the TCR in naive versus memory CD4 T cells, potentially leading to distinct downstream signaling events and ultimately to the diverse functions elicited by these two CD4 T cell subsets.

Lck-independent triggering of T-cell antigen receptor signal transduction by staphylococcal enterotoxins

Superantigens (SAgs) activate T-cells in a manner specific to the Vbeta region of the T-cell antigen receptor. Stimulations by SAgs provoke drastic T-cell activation that leads to programmed cell death or the anergic state of responding cells. To characterize the signal transduction pathway initiated by SAgs, mutant lines derived from the human leukemic T-cell line Jurkat were tested for their reactivities against prototypic SAgs, staphylococcal enterotoxins. The J.CaM1.6 cell line, which lacks Lck expression and lost reactivity against T-cell antigen receptor-mediated stimulation, was activated by staphylococcal enterotoxins in a manner indistinguishable from the Jurkat cell line. In contrast, the J.45. 01 cell line, which lacks expression of functional CD45, showed severely impaired reactivity. The role of Lck appears to be replaced by another Src family protein-tyrosine kinase, Fyn. In J.CaM1.6 cells, Fyn was rapidly phosphorylated and activated after staphylococcal enterotoxin treatment. The kinase-inactive mutant of Fyn significantly suppressed the reactivity against staphylococcal enterotoxin E in J.CaM1.6 cells, and the expression of the active form of Fyn reconstituted reactivity against staphylococcal enterotoxin E in J.45.01 cells. These results demonstrate that SAgs activate T-cells in an Lck-independent pathway and that Fyn plays a critical role in the process.

Interaction between the SH2 domains of ZAP-70 and the tyrosine-based activation motif 1 sequence of the zeta subunit of the T-cell receptor

One of the key steps involved in T-cell activation is binding of the tyrosine kinase ZAP-70 via its two SH2 domains to peptide segments termed tyrosine-based activation motifs (ITAM) which are present in three of the T-cell receptor (TCR) subunits. The crystal structure of the ZAP-70 SH2 domains complexed to phosphopeptide revealed that the amino-terminal phosphotyrosine-binding pocket is formed at the interface between the two SH2 domains. This study was designed to further characterize the binding between TCR zeta ITAM1 and the ZAP-70 SH2 domains as well as to assess the change in conformation of SH2 domain structure upon zeta ITAM1 binding. BIAcore analysis of wild type and nonfunctional single-point mutants of ZAP-70 SH2 domains demonstrated that the amino-terminal SH2 domain can bind phosphopeptide in the absence of a functional carboxyl-terminal SH2 domain. In addition, the amino-terminal SH2 domain prefers the RREEpYDVLDK sequence of zeta chain ITAM1 over the GQNQLpYNELNL sequence. To assess changes in protein conformation upon ITAM binding to ZAP-70 SH2 domains, fluorescence spectroscopy and analytical ultracentrifugation experiments were performed. A significant blue shift in the tryptophan emission spectrum of the SH2 domains was observed in the presence of saturating amounts of phosphopeptide, indicating a loss in solvent exposure for the tryptophan residues in the protein-phosphopeptide complex. This was accompanied by changes in the frictional coefficient consistent with a compacting of the protein structure. Finally, thermal denaturation experiments showed an increase in stability and cooperativity in unfolding for the protein-phosphopeptide complex relative to the protein alone.

A spontaneously arising mutation in the DLAARN motif of murine ZAP-70 abrogates kinase activity and arrests thymocyte development

Development of immature CD4+ CD8+ thymocytes into functionally mature CD4+ and CD8+ T cells is driven by selection events that require signals transduced through the T cell antigen receptor (TCR). Transduction of TCR signals in the thymus involves tyrosine phosphorylation of the protein tyrosine kinase ZAP-70 by p56(lck) and results in induction of ZAP-70 enzymatic activity. We have identified a novel, spontaneously arising point mutation within a highly conserved motif (DLAARN) in the kinase domain of murine ZAP-70 that uncouples tyrosine phosphorylation of ZAP-70 from induction of ZAP-70 kinase activity. Mice homozygous for this mutation are devoid of mature T cells because thymocyte development is arrested at the CD4+ CD8+ stage of differentiation. The developmental arrest is due to the inability of CD4+ CD8+ thymocytes to propagate TCR signals in the absence of ZAP-70 kinase activity despite tyrosine phosphorylation of TCR-associated ZAP-70 molecules.

Differential expression of protein tyrosine kinases and their phosphorylation in murine Th1 cells anergized with class II MHC-peptide complexes

In resting T cell clones, antigen presentation with immobilized anti-CD3 or anti-T cell receptor (TCR) is known to result in a state of anergy as characterized by unresponsiveness to normal antigenic restimulation. Similarly, T cell unresponsiveness could be induced by immobilized (plate-coated) complexes of purified class II MHC and antigenic peptide. It is not clearly defined whether the engagement of TCR by immobilized anti-TCR or immobilized class II MHC-peptide complexes generates similar or differential signals during the induction of T cell unresponsiveness. In order to address the initial signalling events induced by TCR occupancy with anti-TCR and class II MHC-peptide molecules, the expression of three critical protein tyrosine kinases (PTK) and their phosphorylation were investigated in the present study using a murine T cell clone (HS17) restricted for IAS and myelin basic protein (MBP (91-103)) peptide. The anergic T cells induced by immobilized IAS-MBP (91-103) complex or anti-TCR (H57) showed differential expression of lck (56 kDa) and Zap-70 (70 kDa) proteins. In both systems, however, the induction of T cell unresponsiveness was accompanied by increased level of fyn (59 kDa) expression. When analysed for the total tyrosine phosphorylation of PTK, anergic HS17 T cells induced by both molecules showed increased phosphorylation associated with only the fyn protein. These results suggest that the signal transduction events induced by immobilized class II MHC-peptide complexes and anti-TCR are distinct, although both can initiate signals that lead to increased fyn expression and phosphorylation. In addition, the present study supports the evidence for the important functional association of fyn protein with direct TCR engagement in T cell signalling.

Reconstitution of T cell antigen receptor-induced Erk2 kinase activation in Lck-negative JCaM1 cells by Syk

The two related protein-tyrosine kinases Syk and Zap are rapidly phosphorylated on tyrosine residues and enzymatically activated upon crosslinking of the T cell antigen receptor. We have previously reported that the activation of Syk is less dependent on the Src family kinase Lck than the activation of Zap. Here we report that overexpression of Syk in the Lck-negative JCaM1 cells enabled the T cell antigen receptor/CD3 complex to induce a normal activation of the mitogen-activated protein kinase (MAPK) pathway and expression of a nuclear factor of activated T cells reporter construct. In contrast, Zap and other protein-tyrosine kinases were unable to reconstitute these signaling pathways when expressed at the same levels. In parallel, Syk was phosphorylated on tyrosine, while Zap was not. The Syk-mediated T cell antigen receptor-induced MAPK activation was detectable within 1 min of receptor stimulation and peaked at 3-5 min. The capacity of Syk to reconstitute the MAPK response required the catalytic activity of Syk, an intact autophosphorylation site (Y518 and Y519), both Src homology 2 domains and it was blocked by the inhibitory N17-mutated dominant-negative Ras construct. A Y341-->F mutant of Syk, which is deficient in its interaction with phospholipase Cy1 and Vav, was less efficient than wild-type Syk. Our results suggest that Syk, in contrast to Zap, can transduce signals from the T cell antigen receptor independently of Lck.

Production and characterization of a novel monoclonal antibody against phosphorylated T cell receptor zeta chain

One of the earliest events following T cell receptor (TCR) triggering is the activation of the protein kinase Lck and induction of tyrosine phosphorylation of zeta, the major signal transduction subunit of the T cell receptor complex. Here we report the generation and characterization of a monoclonal antibody specific for human phosphozeta. The antibody was produced by immunizing mice with a truncated recombinant form of human zeta together with the Lck enzyme. The C415.9A antibody recognizes recombinant as well as cellular phosphozeta but is unreactive with unphosphorylated zeta or other tyrosine phosphorylated proteins. Using this antibody, we have demonstrated aberrant TCR-zeta tyrosine phosphorylation in Jurkat T cell transduction mutants. Therefore, this antibody can be used to elucidate T cell signal transduction mechanisms by analyzing and monitoring tyrosine phosphorylation of zeta in vitro and in vivo directly. Furthermore, this antibody could find application in the analysis of abnormal T cell signaling in autoimmune disease, cancer, and immunodeficiency disorders.

Human immunodeficiency virus infection abolishes CD4-dependent activation of ZAP-70 by inhibition of p56lck

The effect of early human immunodeficiency virus-1 infection in vitro on proximal signal transduction events in primary peripheral blood lymphocytes was investigated with respect to CD4-mediated costimulation of CD3/T cell-receptor signalling. Tyrosine phosphorylation profiles induced by CD4 and CD3 + CD4 ligation were profoundly abrogated in virally infected cells, although CD4 receptor expression remained intact during early infection. Furthermore, the association of the tyrosine kinase p56lck with the CD4 receptor was reduced in virally infected cells. The downmodulation of CD4-mediated CD3 signalling coincided with the subsequent inhibition of the activity and tyrosine phosphorylation of the downstream kinase ZAP-70 in virally infected cells. The observed virally mediated cosignalling defects during early infection may account for the inhibition of distal signal events and thus contribute to HIV pathogenesis, such as reduced immune response to antigenic exposure, anergy, and apoptosis.

Binding of HIV-1 to its receptor induces tyrosine phosphorylation of several CD4-associated proteins, including the phosphatidylinositol 3-kinase

Cell surface CD4 molecules are known to be important in several physiological responses of T lymphocytes. The use of human immunodeficiency virus (HIV) particles or purified gp120 molecules as CD4 cross-linking agents has been shown to result in a cascade of intracellular biochemical events. In addition, we and other have provided evidence suggesting that virus-mediated CD4 multimerization can lead to modulation of HIV-1 long terminal repeat-dependent activity and virus production. We were thus interested in measuring the effect of HIV-1 particles on intracellular tyrosine-phosphorylation levels, mostly of CD4-associated proteins. Using the T cell line CEM-T4, we observed that HIV-1 induces an increase in tyrosine phosphorylation of four major proteins physically complexed to the CD4 molecule. Immunoblot analysis permitted the identification of two of these proteins, p56lck and phosphatidylinositol 3-kinase (PI 3-kinase) p85 alpha. No concomitant variation in the level of these two CD4-associated proteins was observed after HIV-1-induced CD4 cross-linking. To our knowledge, this is the first report linking HIV-1-mediated CD4 multimerization to an increase in tyrosine phosphorylation of the PI 3-kinase complex. The four CD4-associated molecules described in this report are most likely implicated in virus-induced CD4-linked signaling events.

Effect of dexamethasone on T-cell receptor/CD3 expression

Glucocorticoid hormones (GCH) are anti-inflammatory and immunosuppressive agents that inhibit T-cell growth and activation. Since the T-cell receptor (TCR)/CD3 complex mediates T-lymphocyte activation, we studied the effect of in vitro dexamethasone (DEX), a synthetic GCH, on TCR/CD3 expression. DEX-treatment of a hybridoma T-cell line and normal un-transformed T-cell clones induced a decrease of the TCR/ CD3 membrane expression after 4 days. After 4 weeks, TCR/CD3 was undetectable. However, the amount of mRNAs coding TCR/CD3 chains, including TCR alpha, TCR beta, CD3 gamma, CD3 theta and CD3 epsilon, as well as the amount of CD3 epsilon protein, a major component of the complex, were unaltered. By contrast, a decrease of the mRNAs deriving from the TCR zeta gene locus, as well as of the TCR zeta protein which is responsible for the membrane expression of the TCR/CD3 complex, was induced. These data suggest that the down-modulation of TCR expression is due to the diminution of TCR zeta gene products in DEX-treated cells.

ITIMs and ITAMs. The Yin and Yang of antigen and Fc receptor-linked signaling machinery

The initial stages of an immune response are regulated at the level of the cell-surface antigen and Fc receptors. The extracellular portions of these receptors provide immune specificity and determine the nature of the responding effector cells, whereas the intracellular portion transduces signals into the cell and determines the intensity and duration of the immune response. Recent studies led to the identification of two types of modules within the cytoplasmic region of receptor subunits that are critical for the activation and termination of signal transduction pathways. Phosphorylation of the conserved tyrosine residues within the two modules, the immunoreceptor tyrosine-based activation motif (ITAM) and the immunoreceptor tyrosine-based inhibition motif (ITIM), is followed by the recruitment of different sets of SH2-containing molecules to the receptor site. These proteins regulate the receptor-linked signal transduction pathways in a positive or a negative fashion, which is a reminiscent of the ancestral Yin-Yang principle.

Enhancement of lymphocyte responsiveness by a gain-of-function mutation of ZAP-70

The protein tyrosine kinase ZAP-70 plays an essential role in T-cell activation and development. After T-cell receptor stimulation, ZAP-70 is associated with the receptor and is phosphorylated on many tyrosine residues, including tyrosine 292 (Y-292), in the region between the C-terminal SH2 domain and the kinase domain (interdomain B). Here we show that a mutation of Y-292 (292F) or deletion of interdomain B enhanced the ability of ZAP-70 to reconstitute B-cell receptor stimulation-dependent NF-AT induction in a B-cell line deficient in Syk. In contrast, in a T-cell line, expression of 292F led to basal NF-AT induction independent of T-cell receptor stimulation. These results demonstrate that the role of Y-292 is to negatively regulate the function of ZAP-70 in lymphocytes. This appears to be a dominant function of interdomain B because deletion of most of interdomain B also resulted in a mutant of ZAP-70 with enhanced ability to reconstitute Syk-deficient DT-40 B cells. Since our biochemical studies did not reveal an effect of the 292F mutation on either the kinase activity of ZAP-70 or on the ability of ZAP-70 to bind to the receptor, we propose a model in which Y-292 interacts with an inhibitory protein to negatively regulate ZAP-70 function.

Binding of human immunodeficiency virus type 1 to CD4 induces association of Lck and Raf-1 and activates Raf-1 by a Ras-independent pathway

We have analyzed CD4-mediated signaling during the early stages of human immunodeficiency virus type 1 (HIV-1) infection. Binding of purified HIV-1 virions or recombinant HIV-1 glycoprotein gp120 to CD4 receptors resulted in association and tyrosine phosphorylation and activation of tyrosine kinase Lck and serine/threonine kinase Raf-1. The association between Lck and Raf-1 was mediated by stimulation of the CD4 receptors, since it was abolished by preincubation of the virus with soluble CD4 and was not detected in CD4-negative A201 T cells. However, the Lck-Raf-1 association was restored in A201 cells permanently transfected with human CD4 cDNA and stimulated with anti-CD4 antibodies. In addition, a catalytically active Lck was required for the association of Lck and Raf-1. Surprisingly, the CD4-mediated signaling, induced by the HIV-1 binding, did not result in stimulation of the Ras GTP-binding activity or its association with Raf-1, indicating that the signaling pathway generated by the HIV-1 binding is not identical to the classical Ras/Raf-1 pathway. Furthermore, overexpression of activated Raf-1 in Jurkat T cells stimulated the HIV long terminal repeat promoter activity and significantly enhanced HIV-1 replication. This suggests that the Lck-Raf-1 pathway, rapidly stimulated by the binding of HIV-1 or gp120 to CD4 receptors, may play an essential role in the transcriptional activation of the integrated HIV-1 provirus as well as in its pathogenicity.