Tyrosine phosphorylation and dephosphorylation in T lymphocyte activation

Age-dependent alterations in the assembly of signal transduction complexes at the site of T cell/APC interaction

TCR interaction with peptide-MHC complexes triggers migration of protein kinases, actin-binding proteins, and other accessory molecules to the T cell/APC synapse. We used confocal immunofluorescence methods to show that the adapter protein LAT (linker for activation of T cells) and the guanine nucleotide exchange factor Vav also move to the APC interface in mouse CD4 T cells conjugated to anti-CD3 hybridoma cells, and in TCR-transgenic CD4 cells conjugated to APC bearing agonist (but not closely related nonagonist) peptides. The proportion of CD4+ T cells able to relocalize LAT or Vav, or to relocate cytoplasmic NT-AT (NF-ATc) from cytoplasm to nucleus, declines about 2-fold in aged mice. The decline in LAT relocalization is accompanied by a similar decline in tyrosine phosphorylation of LAT in CD4 cells stimulated by CD3/CD4 cross-linking. Two-color experiments show that LAT redistribution is strongly associated with relocalization of both NF-ATc and protein kinase C-theta among individual cells. LAT migration to the immunological synapse depends on actin polymerization as well as on activity of Src family kinases, but aging leads to only a small change in the percentage of CD4 cells that redistribute F-actin to the site of APC contact. These results suggest that defects in the ability of T cells from aged donors to move kinase substrates and coupling factors, including LAT and Vav, into the T cell/APC contact region may contribute to the decline with age in NF-ATc-dependent gene expression, and thus to defects in T cell clonal expansion.

Kinetic analysis of the mass transport limited interaction between the tyrosine kinase lck SH2 domain and a phosphorylated peptide studied by a new cuvette-based surface plasmon resonance instrument

We explored the use of a newly developed cuvette-based surface plasmon resonance (SPR) instrument (IBIS) to study peptide-protein interactions. We studied the interaction between the SH2 domain of lck and a phosphotyrosine peptide EPQY*EEIPIYL which was immobilized on a sensor chip. No indications for mass transport limitation (MTL) were observed when standard kinetic approaches were used. However, addition of competing peptide during dissociation revealed a high extent of rebinding. A dissociation rate constant (k(d)) of 0.6+/-0.1 s(-1) was obtained in the presence of large amounts of peptide. A simple bimolecular binding model, applying second-order kinetics for the cuvette system, could not adequately describe the data. Fits were improved upon including a step in the model which describes diffusion of the SH2 domain from the bulk to the sensor, especially for a surface with high binding capacity. From experiments in glycerol-containing buffers, it appeared that the diffusion rate decreased with higher viscosity. It is demonstrated that MTL during association and dissociation can be described by the same diffusion rate. A binding constant (K(D)) of 5.9+/-0.8 nM was obtained from the SPR equilibrium signals by fitting to a Langmuir binding isotherm, with correction for loss of free analyte due to binding. An association rate constant k(a) of 1.1(+/-0.2)x10(8) M(-1) x s(-1) was obtained from k(d)/K(D). The values for k(a) and k(d) obtained in this way were 2-3 orders larger than that from standard kinetic analysis, ignoring MTL. We conclude that in a cuvette the extent of MTL is comparable to that in a flow system.

Peroxynitrite Inhibits T Lymphocyte Activation and Proliferation by Promoting Impairment of Tyrosine Phosphorylation and Peroxynitrite-Driven Apoptotic Death

Peroxynitrite (ONOO−) is a potent oxidizing and nitrating agent produced by the reaction of nitric oxide with superoxide. It readily nitrates phenolic compounds such as tyrosine residues in proteins, and it has been demonstrated that nitration of tyrosine residues in proteins inhibits their phosphorylation. During immune responses, tyrosine phosphorylation of key substrates by protein tyrosine kinases is the earliest of the intracellular signaling pathways following activation through the TCR complex. This work was aimed to evaluate the effects of ONOO− on lymphocyte tyrosine phosphorylation, proliferation, and survival. Additionally, we studied the generation of nitrating species in vivo and in vitro during immune activation. Our results demonstrate that ONOO−, through nitration of tyrosine residues, is able to inhibit activation-induced protein tyrosine phosphorylation in purified lymphocytes and prime them to undergo apoptotic cell death after PHA- or CD3-mediated activation but not upon phorbol ester-mediated stimulation. We also provide evidence indicating that peroxynitrite is produced during in vitro immune activation, mainly by cells of the monocyte/macrophage lineage. Furthermore, immunohistochemical studies demonstrate the in vivo generation of nitrating species in human lymph nodes undergoing mild to strong immune activation. Our results point to a physiological role for ONOO− as a down-modulator of immune responses and also as key mediator in cellular and tissue injury associated with chronic activation of the immune system.

Prostaglandin E2 Modulation of p59fyn Tyrosine Kinase in T Lymphocytes During Sepsis

Prostaglandin E2 (PGE2) has been implicated in the suppression of T cell IL-2 production and proliferation during burn and sepsis. The present study evaluated the potential intracellular mechanism of suppressed T cell responses by assessing the activation of p59fyn kinase in T cells from septic rats as well as the T cells incubated with PGE2. p59fyn is known to regulate T cell functions. Sepsis was induced in rats by implanting fecal pellets containing Escherichia coli (150 CFU) and Bacteroides fragilis (104 CFU) into the abdominal cavity. For the assessment of PGE2 role in sepsis, a group of septic rats were treated with indomethacin, which inhibits endogenous PGE2 synthesis. As assessed by immunoblotting or in vitro kinase assay, a more than 40% inhibition of p59fyn phosphorylation and kinase activity was observed in septic rat T cells compared with the T cells from sterile or control rats. A similar inhibition in p59fyn phosphorylation and kinase activity was observed in PGE2-treated T cells compared with the T cells incubated in the absence of PGE2. The septic-related suppression in p59fyn phosphorylation and kinase activity in T cells was prevented in rats treated with indomethacin. We observed that the inhibition in p59fyn activation in septic or PGE2-treated T cells was due primarily to a suppression in p59fyn phosphorylation and not due to alterations in p59fyn protein expression. These findings suggest that PGE2 released during sepsis could contribute to the sepsis-related suppression in T cell proliferation by attenuating p59fyn phosphorylation and its kinase activity.

Regulation of the serine-base exchange enzyme system by CD4: effects of monoclonal antibodies, jacalin, interleukin 16 and the HIV membrane protein gp120

Phosphatidylserine (PtdSer) is synthesized by an exchange of the polar head group of phospholipids for a serine residue. The enzyme responsible for this reaction, the serine-base exchange enzyme system (serine-BEES) is inhibited during lymphocyte activation. We show here that triggering the CD4 cell surface molecule in several CD4+ T-cell lines regulates the serine-BEES activity, thus resulting in marked changes in PtdSer synthesis. CD4 ligands able to generate an activating signal in T-cells such as the lectin jacalin, down-regulate the synthesis of PtdSer. In contrast, monoclonal antibodies (mAbs) directed against the CD4 molecule, such as IOT4 and IOT4a, which have previously been described as generating an inhibitory signal to T-cells, induced an up-regulation of the serine-BEES and impaired CD3-induced inhibition of PtdSer synthesis. Similarly, the HIV-gp120 envelope glycoprotein, in both soluble and cross-linked forms, induces an increase in PtdSer synthesis. The protein tyrosine kinase p56lck participates in the regulation of serine-BEES activity because the effect of CD4 mAbs was additive to that of amino-hydroxyflavone, an inhibitor of p56lck. Also, CD4 mAbs were inactive in J Cam 1.6 cells or when the CD3 signals were bypassed by using thapsigargin. These results demonstrate that the CD4 surface molecule can transmit both activating and inhibiting intracellular signals depending on the CD4 ligand used. We suggest that PtdSer synthesis would be one of the intracellular signals that could explain the opposite effects of different CD4 ligands on T-cells.

Activation and signal transduction via mitogen-activated protein (MAP) kinases in T lymphocytes

The various mitogen-activated protein (MAP) kinases have central roles in the signalling pathways of T lymphocytes. Their activation is uniquely dependent on dual phosphorylation of a serine/threonine and a tyrosine residue and is regulated by several levels of kinases in parallel cascades. In addition, both the MAP kinases and their upstream, activating kinases are regulated by several phosphatases. Although each of the MAP kinases have many cytoplasmic substrates, their ability to translocate to the nucleus means that they can transmit signals from the cytoplasm directly to transcription factors, which are sometimes nuclear bound. The MAP kinase cascades are activated in T lymphocytes by a variety of different external stimuli. They play an important role in transducing both the signal from T cell receptor and costimulatory molecules, on the T cell surface, and are able to regulate several of the transcription factors controlling the expression of critical genes, including that for IL-2. This review examines how the activation of several MAP kinases is regulated, their role in signal transduction initiated by a variety of stimuli, and how this may lead to different cellular responses.

The role of CD45 and CD45-associated molecules in T cell activation

CD45 (lymphocyte common antigen) is a receptor-linked protein tyrosine phosphatase that is expressed on all leucocytes, and which plays a crucial role in the function of these cells. On T cells the extracellular domain of CD45 is expressed in several different isoforms, and the particular isoform(s) expressed depends on the particular subpopulation of cell, their state of maturation, and whether or not they have previously been exposed to antigen. It has been established that the expression of CD45 is essential for the activation of T cells via the TCR, and that different CD45 isoforms display a different ability to support T cell activation. Although the tyrosine phosphatase activity of the intracellular region of CD45 has been shown to be crucial for supporting signal transduction from the TCR, the nature of the ligands for the different isoforms of CD45 have been elusive. Moreover, the precise mechanism by which potential ligands may regulate CD45 function is unclear. Interestingly, in T cells CD45 has been shown to associate with numerous molecules, both membrane associated and intracellular; these include components of the TCR-CD3 complex and CD4/CD8. In addition, CD45 is reported to associate with several intracellular protein tyrosine kinases including p56lck and p59fyn of the src family, and ZAP-70 of the Syk family, and with numerous proteins of 29-34 kDa. These CD45-associated molecules may play an important role in regulating CD45 tyrosine phosphatase activity and function. However, although the role of some of the CD45-associated molecules (e.g. CD45-AP and LPAP) has become better understood in recent years, the role of others still remains obscure. This review aims to summarize recent findings on the role of CD45 and CD45-associated molecules in T cell activation, and to highlight issues that seem relevant to ongoing research in this area.

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.

Effects of staurosporine on the capacitative regulation of the state of the Ca2+ reserves in activated Jurkat T lymphocytes

Staurosporine (Stp) is an inhibitor of protein kinase C (PKC) that has been used to address the role of this enzyme in a variety of cells. However, Stp can also inhibit protein tyrosine kinases (PTK). We have investigated the effects of Stp on the InsP3-(using mAb C305 directed against the beta chain of the T cell receptor (TcR/CD3 complex) and the thapsigargin (Tg)-dependent release and influx of Ca2+ in human (Jurkat) T cells. The addition of Stp (200 nM) during the sustained phase of the TcR-dependent Ca2+ response resulted in a rapid inhibition of the influx of Ca2+ that was not seen when Ca2+ mobilization was triggered by Tg (1 microM). When the cells were preincubated with Stp (200 nM), there was an inhibition of the mAb C305- but not the Tg-dependent Ca2+ response. The effect of Stp was not the result of the inhibition of PKC as shown by down-regulation of PKC and with the use of the specific PKC inhibitor bis-indolyl maleimide GF 109203X. The effect of Stp on the entry of Ca2+ in activated (mAb C305) Jurkat lymphocytes was dose-related and was not the result of a direct inhibition of plasma membrane Ca2+ channels based on an absence of effect on the Tg-dependent entry of Ca2+ and the use of Ca2+ channel blockers (econazole and Ni2+). These blockers terminated the influx of Ca2+ but the Tg-sensitive Ca2+ reserves were not refilled in marked contrast to the effect of Stp. Quantification of InsP3 revealed that the addition of Stp resulted in an approximate 40% reduction in mAb C305-activated Jurkat cells. The effects of Stp can be explained as follows. Stp decreases the mAb C305-induced production of InsP3 by inhibiting the TcR/CD3-dependent activation of PTK associated with the stimulation of phospholipase C-gamma 1. A decrease in [InsP3] without a return to baseline is sufficient to close the InsP3 Ca2+ channel, endoplasmic Ca2+ ATPases use the incoming Ca2+ to refill the Ca2+ pools and that terminates the capacitative entry of Ca2+. A simple kinetic model reproduced the experimental data.

Translational regulation during activation of porcine peripheral blood lymphocytes: association and phosphorylation of the alpha and gamma subunits of the initiation factor complex eIF-4F

Mature peripheral blood lymphocytes exist in a resting state both in vivo and when maintained in culture, exhibiting low translation rates consistent with their non-proliferative state. Previously we have shown that activation of these quiescent cells with either phorbol ester or concanavalin A leads to a rapid increase in the rate of protein synthesis and phosphate-labelling of initiation factor eIF-4 alpha [Morley, Rau, Kay and Pain (1993) Eur. J. Biochem. 218, 39-48]. We now show that neither the early enhanced translation rate nor the early increased phosphate-labelling of eIF-4 alpha requires the activity of the 70 kDa form of ribosomal protein S6 kinase. In addition, we demonstrate that eIF-4 gamma is phosphorylated in response to cell activation, an event which is correlated with phosphorylation of eIF-4 alpha and enhanced eIF-4F complex formation. In these studies, isoelectric focusing and immunoblot analysis of eIF-4 alpha indicate that phosphate-labelling of eIF-4 alpha following cell activation reflects a modest increase in steady-state phosphorylation, mediated by the enhanced activity of eIF-4 alpha kinase(s) and inhibition of eIF-4 alpha phosphatase activity. In the resting cell, eIF-4 alpha is associated with heat- and acid-stable insulin-responsive protein (PHAS-I; 4E-BP1); following acute stimulation with phorbol ester, there is a 40% decrease in the amount of PHAS-I associated with eIF-4 alpha. Incubation of anti-PHAS-I immunoprecipitates with extracts containing activated or immunprecipitated mitogen-activated protein kinase resulted in a small increase in phosphorylation of recovered PHAS-I and a modest release of eIF-4 alpha from the PHAS-I-eIF-4 alpha complex. These data suggest a possible role for PHAS-I in the regulation of eIF-4F complex formation and the rate of translation in primary cells.

Characterization of lineage restricted forms of a Xenopus CD45 homologue

The leukocyte common antigen, also known as CD45, is a structurally heterogenous molecule ranging in molecular weight from 180 to 220 kDa. CD45 belongs to a family of high molecular weight, cell surface glycoproteins expressed on all hematopoietic lineages with the exception of mature erythrocytes. In higher vertebrates, the highly conserved cytoplasmic domain of CD45 exhibits protein tyrosine phosphatase activity and has been implicated in lymphocyte activation through dephosphorylation of critical tyrosine residues on substrates associated with signal transduction pathways. The monoclonal antibody CL21 recognizes a high molecular weight determinant expressed on the surface of Xenopus leukocytes which was postulated to be a CD45 homologue. In order to determine if lymphocyte subpopulations expressed different molecular weight variants, splenic B cells were identified and isolated on the basis of surface IgM and the CL21 determinant expressed by these cells was compared to the determinant expressed by thymocytes. Immunoprecipitation revealed that IgM + B cells expressed a 220 kDa molecular weight variant whereas thymocytes and IgM-cells expressed a 180 kDa variant. Bone marrow myeloid cells, isolated on the basis of light scatter properties, expressed a determinant which ranged from 150 to 160 kDa. Dephosphorylation experiments utilizing p-nitrophenyl phosphate, 32P-labeled Raytide [tyr(P)], or Kemptide [ser(P)] as substrates demonstrated that immunoprecipitated CL21 antigen exhibited tyrosine specific phosphatase activity which was inhibited by sodium orthovanadate. Thus, data based on the presence of enzymatic activity and lineage restricted molecular weight variants support the hypothesis that the CL21 determinant is the amphibian homologue of mammalian CD45, and suggest that both structural and functional elements of CD45 have been conserved during vertebrate evolution.

CD4 and CD45 regulate qualitatively distinct patterns of calcium mobilization in individual CD4+ T cells

An early consequence of T cell activation is an increase in intracellular calcium concentration. Recent advances in video laser microscopic techniques enable the examination of individual cells over time following stimulation. Such studies have revealed that cells can undergo qualitatively distinct patterns of calcium mobilization, suggesting that different patterns of calcium flux may be associated with different signaling pathways and may differentially affect late events in cell activation. In this report, we identify distinct patterns of calcium mobilization in CD4+ T cells following the antibody-mediated cross-linking of either CD3 or CD4, or following the cross-linking of both CD3 and CD4 simultaneously. These effects can be further modified by the cross-linking of CD45. We find that antibody cross-linking of CD3 alone induces a single spike in the vast majority of cells shortly after the addition of the cross-linking antibody. In contrast, cross-linking CD4 alone induces a delayed pattern of repetitive calcium spikes which are decreased in amplitude compared to CD3 cross-linking. Simultaneous cross-linking of CD3 and CD4 induces a sustained increase in intracellular calcium mobilization which is dependent on the presence of extracellular calcium. This sustained increase in intracellular calcium concentration is also seen following physiologic cross-linking of CD3 and CD4 after T cell interaction with specific antigen and antigen-presenting cells. Finally, the simultaneous cross-linking of CD45, CD3 and CD4 abrogates the sustained increase in calcium seen following CD3 and CD4 cross-linking. These results suggest that the qualitative nature of T cell receptor signaling can be modulated by the molecular association of other signaling molecules, which may be part of the T cell receptor complex or not.

Differential requirement of protein tyrosine kinase and protein kinase C in the generation of IL-2-induced LAK cell and alpha CD3-induced CD3-AK cell responses

This study examined the role of protein tyrosine kinase (PTK) and protein kinase C (PKC) in the signal transduction pathways for lymphocyte activation through IL-2R to generate LAK cells and through TCR-CD3 to generate CD3-AK cells. Two PTK inhibitors [herbimycin A and genistein (PTK-I)] and two PKC inhibitors [calphositin C and staurosporine (PKC-I)] were used in the experiments. It was found that the primary activation pathway through IL-2R was PTK-dependent; that is, generation of both the IL-2-induced proliferative and the cytotoxic responses was completely abrogated by PTK-I and not by PKC-I. Quite different results were obtained with the alpha CD3-induced CD3-AK cell response. First, the alpha CD3-induced proliferation was only partially inhibited by PTK-I or PKC-I alone. Second, generation of CD3-AK cytotoxic response was primarily PKC-dependent; that is, only PKC-I induced significant inhibition. Genistein was found to reduce protein tyrosine phosphorylation in both LAK cells and CD3-AK cells, indicating that CD3-AK cells were also susceptible to PTK-I treatment. Further studies showed that PTK-I and not PKC-I suppressed perforin mRNA expression and N-2-benzyoxycarbonyl-L-lysine thiobeneylester esterase production in LAK cells, and the opposite was true for CD3-AK cells. These results indicate that different pathways were employed in lymphocyte activation through IL-2R and TCR-CD3. The former pathway is primarily PTK-dependent. Activation through TCR-CD3 is a more complex event.(ABSTRACT TRUNCATED AT 250 WORDS)

Isoforms of the transmembrane tyrosine phosphatase CD45 differentially affect T cell recognition

Activation of T cells has been shown to require CD45. CD45 is expressed on T cells as distinct isoforms and these isoforms are expressed differentially on subsets of CD4 T cells. We have generated T cell lines expressing a T cell receptor (TCR) of known specificity, with or without CD4, and examined the effect of different CD45 isoforms on stimulation through the antigen receptor. We find that isoforms differ in their ability to participate in antigen recognition, with the null isoform that is predominantly found on memory CD4 T cells being the most effective. The ability of the CD4 T cells being the most effective. The ability of the CD45 ectodomain to differentially affect sensitivity to specific ligands represents a novel way of regulating the efficacy of signaling through a receptor without altering its specificity. It may play a crucial role both in immunological memory and during intrathymic maturation of T cells.

The murine c-fgr gene product associated with Ly6C and p70 integral membrane protein is expressed in cells of a monocyte/macrophage lineage

The c-fgr gene is a member of the Src family of protooncogene tyrosine kinases. A monoclonal antibody (2H2) that recognizes the specific region of the N-terminal domain of the murine c-fgr gene product (Fgr) has been established. With an immune complex kinase assay in a monocytic leukemia cell line, 2H2 monoclonal antibody was shown to precipitate a 59-kDa protein that corresponds in molecular mass to murine Fgr. Fgr was expressed highly in lymph nodes, slightly in spleen and peripheral blood leukocytes, and barely in the thymus and was not detected in bone marrow. In the presence of a mild detergent, Fgr was coimmunoprecipitated with a 70-kDa protein (p70) or with p70 plus several other molecules that were expressed on the cell-surface membrane of macrophage tumor cell lines PU5-1.8 and J774.1, respectively. By contrast, Fgr was not coimmunoprecipitated with a low-affinity receptor for the Fc portion of IgG that is associated with human Fgr. The molecule was also coimmunoprecipitated with the Ly6C molecule from a macrophage cell line (J774.1) that showed protein-tyrosine kinase activity. Peptide mapping revealed that this kinase activity was derived from Fgr. The similarity of relationship between this intramembrane p70 and/or Ly6C and cytoplasmic Fgr to relationships previously reported between T-cell antigen receptor complex, including CD4 and CD8 coreceptors, and Lck or Fyn in T cells and between surface IgM and Lyn or Blk in B cells, suggests that the Fgr and p70 or Ly6C are, indeed, associated with each other and in the murine system may be responsible for recognition of extracellular substances (either cellular or noncellular) and for signal transduction in cells of monocyte/macrophage lineage.

Molecular associations involving CD16, CD45 and zeta and gamma chains on human natural killer cells

zeta (CD3-zeta) and gamma (Fc epsilon RI gamma) chains associate with CD16, the low affinity receptor for IgG (Fc gamma RIII) on human NK cells and are essential for the cell surface expression of CD16 and for CD16-mediated effector functions. This study has investigated whether, on NK cells, molecules other than CD16 associate with zeta and gamma chains, as a method of identifying other NK cell surface molecules important in NK cell function. Cell surface biotinylated NK cells were lysed in digitonin, and the lysates immunoprecipitated with mAb to CD16, zeta and gamma, and the immunoprecipitates analysed by SDS-PAGE. CD16 mAb co-precipitated zeta and gamma chains (16 and 12kD, respectively) and in addition molecules of 24, 32-35, 100, 150 and 180-200 kD. Also, zeta mAb co-precipitated gamma chain, and molecules of 24-26, 32-35, 48, 50-66, 100, 150 and 180-200 kD; and gamma co-precipitated zeta chain, and molecules of 24-26, 29, 32-35, 37, 45, 49, 50-66 and 100 kD. While significant amounts of zeta and gamma were co-precipitated with CD16, 10 to 12-fold more zeta and gamma were immunoprecipitated with their respective mAb. Furthermore, depletion of CD16 from the lysate resulted in only a partial (10-12%) depletion of zeta and gamma, indicating that only a relatively small proportion (10-12%) of these molecules are associated with CD16. Interestingly, substantial amounts of molecules with electrophoretic mobility similar to CD16 (50-66 kD) were co-precipitated with zeta and gamma chain mAb from lysates depleted of CD16. In contrast to NK cells where zeta associated with a number of different molecules, the majority of zeta in T cells was found to be associated only with the TCR:CD3 complex. NK cells showed a strong association between CD45, CD16 and a 33 kD molecule and often a strong association of zeta with CD16, CD45 and an unidentified molecule of approximately 150 kD. Our results show first, that CD16, zeta and gamma each can be efficiently labelled by cell surface biotinylation, and second, that CD16, zeta and gamma each can form a complex with each other, and with a number of additional molecules including a 33 kD molecule and CD45 potentially important in NK cell function.

Increased phosphorylation of eukaryotic initiation factor 4 alpha during early activation of T lymphocytes correlates with increased initiation factor 4F complex formation

Mature porcine peripheral blood mononuclear cells (PPBMCs) exist in a resting state both in vivo and when maintained in culture, with low translation rates consistent with their non-proliferative state. When cultured in the presence of the appropriate mitogen, there is a 2-4-fold increase in the rate of protein synthesis per ribosome within 4 h of stimulation [Kay, J. E., Ahern, T. and Atkins, M. (1971) Biochim. Biophys. Acta 247, 322-334]. Studies on extracts prepared from unstimulated cells have suggested lesions in initiation factor activity, primarily affecting the binding of mRNA to ribosomes [Ahern, T., Sampson, J. and Kay, J. E. (1974) Nature 248, 519-521]. In these studies, we have demonstrated that activation of quiescent PPBMCs with the phorbol ester phorbol 12-myristate 13-acetate or concanavalin A leads to a rapid 2-4-fold increase in the rate of protein synthesis within 1 h or 4 h, respectively, which is insensitive to the transcriptional inhibitor, 5,6-dichlorobenzimidazole riboside. Relative to control cells, both phorbol ester and concanavalin A induce a 2-4-fold increase in labelling of the eukaryotic initiation factor eIF-4 alpha with phosphate in vivo, which primarily reflects a small net increase in phosphorylation rather than phosphate turnover on eIF-4 alpha. Similarly, with the human leukaemic T cell line JURKAT, stimulation of the T cell receptor with the monoclonal antibody, OKT-3, or treatment with phorbol ester induces a 2-3-fold increase in eIF-4 alpha phosphorylation within 30 min. Analysis of phosphorylation by two-dimensional gel electrophoresis and measurement of kinase activity towards synthetic peptides, indicate that this increased labelling also reflects increased eIF-4 alpha kinase activity rather than phosphate turnover on eIF-4 alpha. Of central importance is the finding that, concomitant with increased rates of protein synthesis following stimulation of PPBMCs with either phorbol ester or concanavalin A, there is a significant increase in the level of eIF-4 alpha recovered in high-molecular-mass complexes. These data suggest that, in quiescent PPBMCs, eIF-4F may be limiting and that the association of eIF-4 alpha and eIF-4 gamma into high-molecular-mass complexes is regulated by phosphorylation and may play a pivotal role in translational control.

Cross-linking of CD59 and of other glycosyl phosphatidylinositol-anchored molecules on neutrophils triggers cell activation via tyrosine kinase

Many membrane proteins are attached via a glycosyl phosphatidylinositol (GPI) anchor. Proteins anchored in this way make no direct contact with the interior of the cell, therefore a role in signaling or activation would seem unlikely. Nevertheless, cross-linking of GPI-anchored proteins on human and murine T lymphocytes has been shown to cause calcium transients and cell activation. Our studies address the non-lethal events caused by the membrane attack complex of complement, which include release of Ca2+ from intracellular stores, and have suggested that the GPI-anchored complement inhibitor CD59 may be involved in signaling these events. We here report that cross-linking of CD59 on human neutrophils using specific monoclonal antibody and second antibody caused a rapid increase in intracellular free Ca2+ concentration (Ca2+ transient) due to release of Ca2+ from stores and also caused neutrophil oxidase activation. All antibodies against CD59 tested were effective and cross-linking of any other GPI-anchored protein expressed on neutrophils also initiated an increase in intracellular free Ca2+ concentration, whereas cross-linking of transmembrane proteins caused little or no response. A tyrosine kinase-dependent activation pathway was indicated by the demonstration of tyrosine phosphorylation on cross-linking and by blocking of the Ca2+ transient with the tyrosine kinase inhibitor herbimycin.