The human T3 gamma chain is phosphorylated at serine 126 in response to T lymphocyte activation

S101, an Inhibitor of Proliferating T Cells, Rescues Mice From Superantigen-Induced Shock

Superantigens (SAgs) are extremely potent bacterial toxins, which evoke a virulent immune response, inducing nonspecific T-cell proliferation, rapid cytokine release, and lethal toxic shock, for which there is no effective treatment. We previously developed a small molecule, S101, which potently inhibits proliferating T cells. In a severe mouse model of toxic shock, a single injection of S101 given together with superantigen challenge rescued 100% of the mice. Even when given 2 hours after challenge, S101 rescued 40% of the mice. S101 targets the T-cell receptor, inflammatory response, and actin cytoskeleton pathways. S101 inhibits the aryl hydrocarbon receptor, a ligand-activated transcription factor that is involved in the differentiation of T-helper cells, especially Th17, and regulatory T cells. Our results provide the rationale for developing S101 to treat superantigen-induced toxic shock and other pathologies characterized by T-cell activation and proliferation.

Ligand-induced TCR down-regulation is not dependent on constitutive TCR cycling

TCR internalization takes place both in resting T cells as part of constitutive TCR cycling, after PKC activation, and during TCR triggering. It is still a matter of debate whether these pathways represent distinct pathways. Thus, some studies have indicated that ligand-induced TCR internalization is regulated by mechanisms distinct from those involved in constitutive internalization, whereas other studies have suggested that the ligand-induced TCR internalization pathway is identical with the constitutive pathway. To resolve this question, we first identified requirements for constitutive TCR cycling. We found that in contrast to PKC-induced TCR internalization where both CD3gamma-S(126) and the CD3gamma leucine-based internalization motif are required, constitutive TCR cycling required neither PKC nor CD3gamma-S(126) but only the CD3gamma leucine-based motif. Having identified these requirements, we next studied ligand-induced internalization in cells with abolished constitutive TCR cycling. We found that ligand-induced TCR internalization was not dependent on constitutive TCR internalization. Likewise, constitutive internalization and recycling of the TCR were independent of an intact ligand-induced internalization of the TCR. In conclusion, ligand-induced TCR internalization and constitutive cycling of the TCR represents two independent pathways regulated by different mechanisms.

The CD3 gamma leucine-based receptor-sorting motif is required for efficient ligand-mediated TCR down-regulation

TCR down-regulation plays an important role in modulating T cell responses both during T cell development and in mature T cells. At least two distinct pathways exist for down-regulation of the TCR. One pathway is activated following TCR ligation and is dependent on tyrosine phosphorylation. The other pathway is dependent on protein kinase C (PKC)-mediated activation of the CD3 gamma di-leucine-based receptor-sorting motif. Previous studies have failed to demonstrate a connection between ligand- and PKC-induced TCR down-regulation. Thus, although an apparent paradox, the dogma has been that ligand- and PKC-induced TCR down-regulations are not interrelated. By analyses of a newly developed CD3 gamma-negative T cell variant, freshly isolated and PHA-activated PBMC, and a mouse T cell line, we challenged this dogma and demonstrate in this work that PKC activation and the CD3 gamma di-leucine-based motif are indeed required for efficient ligand-induced TCR down-regulation.

Agonist-induced T cell receptor down-regulation: molecular requirements and dissociation from T cell activation

T cell receptor (TCR) down-regulation is a consequence of specific receptor engagement and plays an important role in modulating the T cell response. We have investigated the role of protein kinase C (PKC) and protein tyrosine kinases (PTK) in the induction of TCR down-regulation. We report that the mutation of S126 in the CD3-gamma chain that is known to inhibit phorbol-12-myristate 13-acetate-induced TCR down-regulation does not affect down-regulation induced by a specific agonist. In addition, agonist-induced TCR down-regulation is not affected by blockade or depletion of PKC, neither by blockade or lack of PTK, while the same treatments efficiently interfere with T cell activation. These results demonstrate that TCR down-regulation is induced by early events which follow specific engagement by an agonist and can be dissociated from those required for full T cell activation.

T cell receptor alpha-chain tail is required for protein kinase C-mediated down-regulation, but not for signaling

Antigen stimulation through the T cell receptor (TCR) induces phosphorylation of the associated CD3 gamma delta epsilon- and zeta-chain cytoplasmic tails. These events lead to the induction of the intracellular signaling pathways with concomitant receptor down-regulation. The TCR is down-regulated from the cell surface by the activation of protein kinase, C (PKC) and subsequent serine phosphorylation of the CD3 gamma-chain. We report here that the TCR alpha-chain cytoplasmic tail is also necessary for PKC-mediated internalization of the TCR complex. The requirement for the TCR alpha-chain cytoplasmic tail is specific for internalization of the TCR complex, since down-regulation of CD4 is still intact in hybridoma cells expressing a tailless TCR alpha-chain. The absence of TCR internalization directly correlates with defective PKC-mediated phosphorylation of the CD3 gamma-chain. Despite deficient PKC-mediated TCR down-regulation, the tailless alpha beta TCR still transduces antigenic signals resulting in the production of interleukin-2. Although the TCR tails are not obviously required for signal transduction, the TCR alpha-tail may serve as a targeting domain for PKC-mediated down-regulation of the TCR complex.

Differential cytosolic tail dependence and intracellular fate of T-cell receptors internalized upon activation with superantigen or phorbol ester

Activation of T lymphocytes by T-cell receptor (TCR) ligands such as peptide/MHC complexes, superantigens or anti-TCR mAbs, or by pharmacological activators of protein kinase C such as phorbol esters, results in the internalization and cell surface downregulation of TCRs. We investigated the role of internalization motifs located in the cytosolic region of CD3 gamma in the internalization of TCR complexes induced by enterotoxin superantigens, anti-TCR mAbs or phorbol esters. To this end, a series of CD3 gamma mutants were expressed in a CD3 gamma-deficient variant of the human T-cell line Jurkat. We found that serine126 and the di-leucine motif (Leu131-Leu132) are required for phorbol-ester-induced TCR downregulation, but they are not necessary for enterotoxin superantigen or antibody-induced TCR downregulation. Moreover, the tyrosine-based motifs (residues 138 to 141 and 149 to 152) are not required either for phorbol aster or for superantigen or antibody-induced TCR downregulation. Confocal microscopy analysis reveals that TCR complexes accumulate in an early endocytic/recycling compartment upon activation of cells with phorbol esters, whereas TCRs internalized upon activation with superantigen or anti-TCR mAbs are routed to lysosomes. Consistent with this intracellular localization, TCRs internalized in response to phorbol ester are not degraded and can be reexpressed on the cell surface. In contrast, TCRs internalized upon superantigen activation are degraded.

Molecular characterization of the di-leucine-based internalization motif of the T cell receptor

Several cell surface receptors including the T cell receptor (TCR) are phosphorylated and down-regulated following activation of protein kinases. We have recently shown that both phosphorylation of Ser-126 and the presence of the di-leucine sequence Leu-131 and Leu-132 in CD3 gamma are required for protein kinase C (PKC)-mediated TCR down-regulation. To identify additional residues required for PKC-mediated phosphorylation of CD3 gamma and for TCR down-regulation, an alanine scanning of CD3 gamma was done. Mutations of Arg-124, Ser-126, Lys-128, and Gln-129 inhibited both phosphorylation and TCR down-regulation, whereas mutation of Asp-127 only inhibited down-regulation. Further analyses demonstrated a discrepancy between the ability to be phosphorylated on CD3 gamma and to down-regulate the TCR in several transfectants. Phosphorylation was not as strictly dependent on the nature and position of the phosphoacceptor group and basic residues as were the subsequent steps involved in TCR down-regulation. Our results suggest that PKC-mediated TCR down-regulation may be regarded as a two-step process. 1) Recognition and phosphorylation of CD3 gamma by PKC. In this process Arg-124, Ser-126, Lys-128, and Gln-129 are important. 2) Recognition of phosphorylated CD3 gamma by molecules involved in receptor internalization. In this process Ser(P)-126, Asp-127, Leu-131, and Leu-132 are important.

T cell antigen receptor ubiquitination is a consequence of receptor-mediated tyrosine kinase activation

Engagement of the T cell antigen receptor results in both its phosphorylation and its ubiquitination. T cell antigen receptor ubiquitination was evaluated in Jurkat, a well characterized human T leukemia cell line. Treatment of cells with the tyrosine kinase inhibitor herbimycin A resulted in an inhibition of receptor ubiquitination. Consistent with this, pervanadate, which increases cellular tyrosine phosphorylation, enhanced receptor ubiquitination. A requirement for receptor-mediated tyrosine kinase activity for ubiquitination was confirmed in cells lacking the tyrosine kinase p56lck and also in cells that are defective in expression of CD45, a tyrosine phosphatase that regulates the activity of p56lck. The need for tyrosine kinase activation for ubiquitination was not bypassed by directly activating protein kinase C and stimulating endocytosis of receptors. These observations establish ubiquitination of the T cell antigen receptor as a tyrosine kinase-dependent manifestation of transmembrane signaling and suggest a role for tyrosine phosphorylation in the ligand-dependent ubiquitination of mammalian transmembrane receptors.

Okadaic acid enhances human T cell activation and phosphorylation of an internal substrate induced by phorbol myristate acetate

Okadaic acid is a potent tumor promoter and an inhibitor of serine/threonine-specific protein phosphatases. We studied the effect of okadaic acid in human T cell activation and phosphorylation of internal substrates. Okadaic acid at up to 4 nM enhanced phorbol myristate acetate (PMA)-induced proliferation and CD25 (IL-2 receptor, p55) expression, although it showed no activation by itself. Okadaic acid induced hyperphosphorylation of a 60 kDa protein in T cells as well as non-T cells, as reported in fibroblasts and keratinocytes. Preincubation with 4 nM okadaic acid enhanced PMA induced phosphorylation of the 80 kDa protein, an internal substrate of protein kinase C in T cells. These results suggest that okadaic acid inhibited dephosphorylation of protein kinase C specific substrates, and as a result, enhanced T cell activation mediated by protein kinase C pathway.

CD3.TCR1, A Human CD3 epitope expressed on viable γδ lymphocytes exclusively

T lymphocytes express either the alpha/beta or the gamma/delta receptor (TCR) in a mutually exclusive fashion. Both structures are associated on the cell membrane with the CD3 proteins which are thought to transduce signals resulting from antigen recognition. The CD3 complex is present in both alpha/beta and gamma/delta cells and includes at least five proteins (designated gamma, delta, epsilon, zeta and eta). We have developed here a novel mAb, anti-CD3.TCR1, which immunoprecipitates the CD3 molecules from both alpha/beta and gamma/delta cells lysates following solubilization with Triton X-100. While the SDS-PAGE migration profile of the material recognized by either anti-CD3.TCR1 or anti-OKT3 are superimposable in both cell types, this mAb recognizes viable untreated gamma/delta T lymphocytes exclusively. These findings further support the view that molecular interactions within the TCR/CD3 protein complex are distinct in the two T lymphocyte populations.

Inhibitory effect of human syncytiotrophoblast plasma membrane vesicles on Jurkat cells activated by phorbol ester and calcium ionophore

The effects of syncytiotrophoblast plasma membrane vesicles (STPM) on stimulated Jurkat leukemic T cells have been investigated. STPM inhibited IL-2 production and the expression of protein P55 of the IL-2 receptor (IL-2R P55), when Jurkat cells were stimulated by a combination of calcium ionophore A23187 (CaI) + phorbol 12-myristate 13-acetate (PMA). STPM also inhibited IL-2R P55 when cells were stimulated by PMA alone, a situation in which IL-2 production is negligible. On the other hand, STPM had no effect on the sustained mobilization of intracellular Ca2+ induced by CaI nor on the PKC-dependent CD3 down regulation induced by PMA. Finally STPM had no effect on intracellular cAMP levels. These results show that (i) the inhibitory effect of STPM on IL-2R P55 expression is independent of the inhibition of IL-2 production, and (ii) the inhibitory effects of STPM are at least partially independent of phosphatidylinositol 4,5-bisphosphate hydrolysis. They suggest that STPM affect a signaling pathway activated by PMA but possibly PKC independent.

The T cell receptor/CD3 complex and CD2 stimulate the tyrosine phosphorylation of indistinguishable patterns of polypeptides in the human T leukemic cell line Jurkat

Stimulation of the T cell receptor (TcR)/CD3 complex of Jurkat T cells with a monoclonal antibody to the CD3 epsilon chain induced the tyrosine phosphorylation of multiple polypeptides, ranging in size from 21 to 155 kDa. The protein tyrosine phosphorylation was characterized by its rapidity and its transient nature, returning to baseline levels by 60 min. Protein tyrosine kinase activity was also induced when the Jurkat T cells were stimulated with a mitogenic pair of antibodies directed against CD2. Comparison of the polypeptides which were phosphorylated on tyrosine in response to stimulation of the two receptors, by either one- or two-dimensional analysis, failed to reveal any differences. These data suggest that the TcR/CD3 complex and CD2 activated the same tyrosine kinase or kinases. A model is proposed in which CD2 functions as a signal amplifier in physiological responses to antigen/major histocompatibility complex without changing the qualitative nature of the signal generated via the TcR/CD3 complex.

T cell receptor/CD3 complex internalization following activation of a cytolytic T cell clone: evidence for a protein kinase C-independent staurosporine-sensitive step

The fate of the T cell receptor (TcR)/CD3 complex was examined on a cytotoxic T lymphocyte (CTL) clone (KB5.C20) activated either via binding of an anti-TcR monoclonal antibody (mAb) or by a Ca2+ ionophore and phorbol 12-myristate 13-acetate (PMA). After binding of the anti-TcR mAb, electron microscopy revealed internalization through coated vesicles followed by slow degradation of the antibody as shown by use of radiolabeled mAb. The influence of activation on TcR/CD3 internalization was analyzed. The Ca2+ ionophore alone had no effect on internalization, whereas PMA induced an accelerated internalization of anti-TcR mAb. PMA-induced internalization was dependent on protein kinase C (PKC) as shown by its absence in PKC-depleted cells or in the presence of the PKC inhibitor staurosporine. Anti-TcR mAb-induced internalization was maintained in PKC-depleted cells, but unexpectedly remained sensitive to inhibition by staurosporine. The monovalent anti-TcR mAb Fab fragment is non-stimulatory for the CTL. It was poorly internalized but its internalization was induced by PMA. Surprisingly, on PKC-depleted cells, the Fab was internalized more readily than in untreated cells and this internalization was sensitive to inhibition by staurosporine. Inhibition of PMA-induced phosphorylation of gamma and epsilon subunits of CD3 was demonstrated after depletion of PKC or in the presence of staurosporine, confirming that PKC function was inhibited in those conditions. Cross-linking of the TcR via plastic-coated anti-TcR mAb led to phosphorylation of CD3 gamma and epsilon and also of zeta, known to be phosphorylated on tyrosines. All of these phosphorylation events were inhibited by treatment with staurosporine. Our results indicate that staurosporine inhibits the receptor internalization induced by anti-TcR mAb by means other than inhibition of PKC, suggesting that other kinases may control a step of this internalization process.

Modulation of cellular processes by H7, a non-selective inhibitor of protein kinases

H7 has been described as a potent inhibitor of protein kinase C (PKC) and has been widely used to investigate the regulatory role of this enzyme in intact cell systems. In this comparative study between H7 and the microbial alkaloid, staurosporine, we found that the former inhibited rat brain PKC and cAMP dependent protein kinase with IC50 values of 18 and 16 microM respectively whereas the latter was a much more potent inhibitor of both kinases with IC50 values of 9.5 nM and 42 nM respectively. H7, at concentrations up to 100 microM, failed to block cellular events induced by phorbol esters, agents which specifically stimulate PKC, yet was a potent inhibitor of IL-2 induced T cell proliferation with an IC50 value of 19 microM. In contrast, staurosporine was a potent inhibitor of both phorbol ester induced p47 phosphorylation in platelet (I50 value = 540 nM) and also CD3 and CD4 down-regulation in T cells (I50 values 200 nM and 50 nM respectively). Staurosporine was also a potent inhibitor of IL-2 induced T cell proliferation I50 value = 9 nM). These results provide a strong argument against the use of H7 to probe for PKC involvement in cellular processes.

Identification of two distinct phosphoproteins as components of the human B cell antigen receptor complex

In human B cells, the molecules that, upon receptor occupancy, couple membrane immunoglobulin to intracellular signal transduction pathways have never been identified. We here describe two phosphoproteins as integral parts of the B cell antigen receptor complex. Membrane IgM is non-covalently associated with a disulfide-linked heterodimer of glycoproteins. These molecules can be demonstrated on B cell lines and freshly isolated polyclonal B cell populations and are subject to phosphorylation at serine residues. Identification of these constituents of the B cell receptor complex opens up the opportunity to study coupling of the B cell antigen receptor to the intracellular signal transduction machinery at the molecular level.

Activation of the immune system in the testis

The present concept for the mechanism of prolonged transplant survival in the testis suggests that lymphocyte activation is inhibited locally in this site by testicular products. As the testis produces several immunoregulatory factors, the behaviour of lymphocytes in the testis may depend on the net effect of all these factors on intralymphocytic events. In the present article, the extra- and intracellular events associated with lymphocyte activation are reviewed and the steps of lymphocyte activation which might be subject to interference by testicular factors are identified.

Characterization of the CD3 gamma and delta invariant subunits of the sheep T cell antigen receptor

The CD3 gamma and delta chains of the sheep T cell antigen receptor have been characterized by cDNA cloning. In comparison to human and mouse protein sequences, short oligopeptide stretches have been deleted from both the gamma and delta chains. However, apart from further variability in the number and position of potential N-linked glycosylation sites, the general structure of the CD3 gamma and delta proteins is markedly conserved. Particularly remarkable are two motifs consisting of 8 and 32 amino acids, respectively, which locate in the C-terminal half of each protein and which have been precisely conserved in both the CD3 gamma and delta chains of all three species. These motifs probably contribute structural properties essential for the function of the CD3/T cell receptor complex. mRNA of the sheep CD3 gamma (1.5 and 1.2 kb) and CD3 delta (1.5 kb) chains are transcribed from closely linked, single-copy genes, suggesting that common mechanisms for the regulation of CD3 genes are also preserved between species.

The phosphorylation of the CD3 gamma chain of T lymphocytes is modulated by beta-endorphin

The neuropeptide beta-endorphin can modulate the response of T and B cells to mitogenic or antigenic stimulation. In the present report we describe a novel mechanism by which beta-endorphin can interfere with T cell activation. It is shown here that beta-endorphin can modulate the phorbol ester-induced phosphorylation of the gamma chain of the CD3 complex. The effect of beta-endorphin is dose dependent and appears to be mediated via interaction of beta-endorphin with an opiate receptor on lymphocytes. Evidence is presented that the modulatory effect of beta-endorphin is specific for the phosphorylation of the CD3 gamma chain. beta-Endorphin does not affect the phosphorylation of total cell protein, nor does it have any effect on the phosphorylation of the CD4 determinant on T cells. The possible consequence of a change in CD3 gamma chain phosphorylation is discussed.

Heterogeneity of protein kinase C expression and regulation in T lymphocytes

The purpose of the present study was to examine protein kinase C (PKC) isotype expression in T lymphoblasts derived from peripheral blood and the T leukaemic cell Jurkat. Using antisera reactive with PKC alpha, beta 1, and beta 2 and gamma, it was observed that T cells expressed two PKC isotypes, PKC alpha and beta 1. No PKC gamma was detected in T lymphocytes. In lymphoblasts, high levels of PKC beta compared to PKC alpha were found whereas Jurkat cells expressed high levels of alpha compared to PKC beta. Differences in the calcium sensitivity of phorbol ester-induced phosphorylation were observed in Jurkat and T lymphoblasts which correlated with the relative levels of PKC alpha and beta isotypes expressed by the cells.

Ionomycin-regulated phosphorylation of the myeloid calcium-binding protein p14

Two associated calcium-binding proteins (CaBPs) have recently been identified specifically in cells of myeloid origin. These proteins have relative molecular masses (Mr) of 8,000 and 14,000 and are variously referred to as the cystic fibrosis antigen, the L1 light chain, MRP-8 or p8, and the L1 heavy chain, MRP14 or p14, respectively. The expression of p8 and p14 seems to be confined to a specific stage of myeloid cell differentiation, because both proteins are expressed in circulating neutrophils and monocytes but not in normal tissue macrophages. In chronic inflammatory conditions, however, such as rheumatoid arthritis, macrophages in affected tissues express both p8 and p14. These proteins are members of a family of CaBPs of low Mr, which include S-100 alpha and beta proteins, calcyclin (2A9), intestinal CaBP and p11. All the proteins have an Mr of approximately 10,000 with the exception of p14 which has a longer C-terminal sequence after the second calcium-binding domain. Little is known about their function, although by analogy with calmodulin they could be molecules involved in intracellular signalling that are activated by an increase in the intracellular Ca2+ concentration ([Ca2+]). Here we report that p14 is phosphorylated in both monocytes and neutrophils. The level of p14 phosphorylation can be increased by elevating the [Ca2+]i using the ionophore ionomycin, but is not affected by activation of protein kinase C using phorbol 12,13-dibutyrate. The phosphorylated residue is threonine at position 113, which is the penultimate amino acid in p14 and contained in the longer 'tail' sequence. Part of this sequence is identical to the neutrophil immobilizing factors NIF-1 and NIF-2, indicating that the phosphorylation event could have a role in the generation of NIF activity in the p14 protein.

DNA fragmentation and cell death mediated by T cell antigen receptor/CD3 complex on a leukemia T cell line

An anti-T cell receptor (TcR) monoclonal antibody (mAb), LC4, directed against a human leukemic T cell line, SUP-T13, caused DNA fragmentation ("apoptosis") and cell death upon binding to this cell line. Cross-linking of receptor molecules was necessary for this effect since F(ab')2, but not Fab', fragments of LC4 could induce cell death. Five anti-CD3 mAb tested also caused apoptosis, but only when they were presented on a solid phase. Interestingly, soluble anti-CD3 mAb induced calcium flux and had an additive effect on the calcium flux and interleukin 2 receptor expression induced by LC4, but these anti-CD3 mAb reversed the growth inhibition and apoptosis caused by LC4. The calcium ionophore A23187, but not the protein kinase C activator phorbol 12-myristate 13-acetate (PMA), also induced apoptosis, suggesting that protein kinase C activation alone does not cause apoptosis, although PMA is growth inhibitory. These results suggest that two distinct biological phenomena can accompany stimulation of the TcR/CD3 complex. In both cases, calcium flux and interleukin 2 receptor expression is induced, but only in one case is apoptosis and cell death seen. The signal initiating apoptosis can be selectively prevented by binding CD3 portion of the receptor in this cell line. This difference in signals mediated by the TcR/CD3 complex may be important in explaining the process of thymic selection, as well as in choosing anti-TcR mAb for therapeutic use.

Molecular interactions, T-cell subsets and a role of the CD4/CD8:p56lck complex in human T-cell activation

Several T-cell structures are capable of generating intracellular signals linked to T-cell proliferation. Crosslinking of CD2, CD4 and CD45 with Ti/CD3 to several of these antigens can augment the minimal signal induced by antigen binding to the Ti/CD3 complex. Importantly, some of these regulatory structures (CD4, CD8 and CD45) are also expressed on subsets of T cells with distinct activation requirements and functional programs (helper, suppressor, suppressor-inducer and cytotoxic function). The CD4+ CD45RA+ (2H4+) subset responds well to self-Ia, poorly to soluble antigen and possesses suppressor-inducer function. A reciprocal subset CD4+ CD45RA- (4B4+) is preferentially activated by soluble recall antigens and possesses helper function. Each of these subsets can be distinguished by virtue of the differential expression of CD45 antigens. Importantly, the anti-2H4 antibody which reacts with a specific region near the N-terminus of two CD45 isoforms can effectively block its function. Crosslinking of CD4 with the Ti/CD3 complex preferentially activated the CD4+ CD45+ RA- subset, while soluble antibodies to CD2 preferentially affected the CD45 CD45RA+ subset. Thus, CD3 and CD4 more effectively synergize in the activation process on the CD4+ CD45RA- subset, a result consistent with the ability of this subpopulation to respond to recall antigens. The regulatory role of the CD4, CD8 and CD45 antigens may be mediated by an interactive network of protein-tyrosine phosphorylation and dephosphorylation. We have shown the CD4 and CD8 antigens to be associated with the T cell-specific protein-tyrosine kinase (p56lck). p56lck is a member of a family of protein-tyrosine kinases with an established ability to activate and transform mammalian cells. The CD4/CD8:p56lck complex is catalytically active as shown by its ability to phosphorylate various members of the Ti/CD3 complex. By contrast, the CD45 antigens possess protein-tyrosine phosphatase activity within their intracellular domains and are postulated to function by virtue of a regulatory interaction with CD4/CD8:p56lck and its potential substrates. Thus, the differences in the response of the CD4+ CD45RA+/- subsets to various stimuli and the expansion of T-cell subsets with distinct immunoregulatory programs may be governed by a pathway of tyrosine-mediated events.

Kinases and phosphatases in T-cell activation

Protein phosphorylation-dephosphorylation plays an important role in signal transduction in T lymphocytes. In this review, Denis Alexander and Doreen Cantrell focus on the identification, regulation and functions of the kinases and phosphatases that control phosphorylation events in T cells.

Dephosphorylation of the human T lymphocyte CD3 antigen

Previous studies demonstrated that activation of T lymphocytes by phorbol ester or mitogenic lectin leads to phosphorylation of Ser 126 of the CD3 antigen gamma chain, whereas treatment with ionomycin results in phosphorylation of both Ser 123 and 126 [Davies, A. A. et al. (1987) J. Biol. Chem. 262, 10918-10921]. In the present study, the dephosphorylation of Ser 123 and Ser 126 of the gamma chain was investigated. Phorbol-ester-induced phosphorylation of the gamma-chain Ser 126 in vivo was reversed following removal of phorbol ester. Dephosphorylation of both Ser 123 and 126 was also observed in vitro using the microsome fraction of T lymphocytes. In order to identify the phosphatases acting at these two sites, the immunoprecipitated gamma chain was used as substrate either following treatment with protein kinase C in vitro, in which case phosphorylation occurs mainly at Ser 123, or following in vivo phosphorylation of Ser 126. Purified oligomeric forms of the polycation-stimulated phosphatases were more effective in dephosphorylating both phosphorylated forms of the gamma chain compared with equivalent amounts of ATP,Mg2+-dependent phosphatases or calcineurin. By using phosphopeptide analogues of the CD3 gamma chain containing Ser 123 or Ser 126 as substrates (A3 and A6), it was shown that polycation-stimulated phosphatases selectively dephosphorylated Ser 123 compared to Ser 126. In order to determine which phosphatases dephosphorylate the gamma chain in microsomes, A3 and A6 were used as substrates for characterising phosphatases in microsomes from human T leukaemia Jurkat 6 cells. Three phosphopeptide phosphatases (250-400 kDa) co-eluted through five purification steps with three forms of polycation-stimulated phosphorylase phosphatase. The partially purified A3/A6 phosphopeptide phosphatases were insensitive to Ca2+, calmodulin and inhibitor-1, and dephosphorylated A3 preferentially compared with A6. A latent form of microsomal ATP,Mg2+-dependent phosphorylase phosphatase was stimulated 10-fold by trypsinisation, but did not dephosphorylate phosphopeptides A3 and A6. The results show that high-Mr forms of polycation-stimulated phosphatases are the only enzymes in human T leukaemia cell microsomes which dephosphorylate gamma chain phosphopeptides. The data point to an important role for polycation-stimulated phosphatases in regulating the phosphorylation state, and so function(s), of the CD3 antigen.

Evidences for protein kinase C. Activation in T lymphocytes by stimulation of either the CD2 or CD3 antigens

Phosphorylation of protein kinase C (PKC) substrates in T lymphocytes was analyzed after stimulation with specific pairs of anti-CD2 monoclonal antibodies (mAb) or an anti-CD3 mAb. The results show that the appropriate stimulation of both CD2 or CD3 antigens results in phosphorylation of a 80-kDa putative PKC substrate and that this phosphorylation event is sensitive to a PKC inhibitor, sphinganine. CD2- and CD3-dependent phosphorylation was found to be strongly dependent on an extensive cross-linking of surface antigens. The biological importance of cross-linking of CD2 and CD3 was also evident for other biological responses such as interleukin 2 production and induction of an autocrine growth response. Finally, we also present evidence for interaction between the CD2 and CD3 signal transducing pathways.

Evidence that a GTP binding protein regulates phosphorylation of the CD3 antigen in human T lymphocytes

The role of guanine nucleotide binding regulatory proteins (G proteins) in the regulation of phosphorylation of the gamma subunit of the CD3 antigen has been examined. CD3 gamma chain phosphorylation in isolated T cell microsomes was stimulated by the G protein activator guanosine 5'-0 thiotriphosphate (GTP gamma S), but cyclic adenosine monophosphate and guanosine 5'-diphosphate were ineffective at inducing gamma chain phosphorylation. The effect of GTP gamma S was rapid and transient; a half maximal effect was observed with 50 microM of the nucleotide. gamma polypeptide phosphorylated in vitro in GTP gamma S stimulated microsomes incorporated phosphate on Serines 123 and 126. These data are consistent with the involvement of a G protein in the signalling mechanisms that regulate the phosphorylation of the CD3 gamma chain.

Protein kinase C is required for responses to T cell receptor ligands but not to interleukin-2 in T cells

We have tested the role of protein kinase C in mRNA expression and T cell proliferation mediated through the T cell receptor and through the interleukin-2 (IL-2) receptor. Chronic treatment of a mouse T cell clone with phorbol esters caused a complete loss of protein kinase C activity and a concomitant loss of proliferation to T cell receptor ligands (antigen, lectins, antireceptor antibodies). In contrast, kinase C-depleted T cells retained the ability to proliferate to IL-2. Loss of the T cell receptor response was not due to decreased cell surface expression of receptor or impairment of early receptor function (phosphatidylinositol turnover, calcium mobilization). Kinase C-depleted T cells showed no induction of mRNAs for activation-associated genes on exposure to the T cell receptor ligand Concanavalin A; expression of a subset of the same mRNAs in response to IL-2 was unaffected. We conclude that kinase C is required for mRNA expression and subsequent proliferation mediated through the T cell receptor pathway but is not involved in mRNA expression and proliferation in response to IL-2.