Differential contribution of Lck and Fyn protein tyrosine kinases to intraepithelial lymphocyte development

The developmental stages and the role of protein tyrosine kinases (PTK) in the maturation of CD3⁺CD8αα⁺ intraepithelial lymphocytes (IEL) have not been extensively characterized. However, comparisons of thymic and extrathymic T cell development indicate that these processes involve some distinct signaling and selection events. We used mice deficient in Lck, Fyn, or both Lck and Fyn to analyze the role that these src-family PTK play in IEL development. In contrast to thymocyte development, we found that all IEL subsets develop in mice deficient for either kinase alone. However, lck^-/- animals exhibited reduced numbers of TcRαβ⁺CD8αα⁺ IEL, indicating that Lck is important in the development of these cells. Mice which lack both Lck and Fyn fail to generate TcRαβ⁺ IEL, suggesting that signaling through the preTcR, mediated by Lck and, to a lesser extent Fyn, is required for maturation of all TcRαβ⁺ IEL lineages. Interestingly, a small population of TcRγδ⁺CD8αα⁺ cells are apparent in lck^-/-fyn^-/- animals, demonstrating that TcRαβ⁺CD8αα⁺ and TcRγδ⁺CD8αα⁺ IEL have distinct PTK requirements for their development or expansion. CD3^?-CD8α^?CD44⁺ and CD3^?CD8αα⁺CD16/32⁺B220⁺ cells comprise the majority of IEL in both lck^-/-fyn^-/- and rag^-/- mice, while they are poorly represented in wild-type controls. Comparison of the cell surface phenotype of these putative precursor IEL in lck^-/-fyn^-/- and rag^-/- animals suggests that IEL maturation in these animals is arrested at an equivalent developmental stage. Overall, the data presented demonstrate that signals mediated by Lck or Fyn direct TcRαβ⁺CD8αα⁺ IEL maturation but are dispensable for the development of TcRγδ⁺CD8αα⁺ IEL.     

Signal transduction properties of the T cell activation monoclonal antibody panel: ubiquitous increase in cellular substrate tyrosine phosphorylation in the absence of detectable calcium mobilization

A panel of coded monoclonal antibodies (mAbs) submitted to the T cell section of the Vth International Workshop and Conference on Human Leukocyte Differentiation Antigens was examined for the ability to induce cellular activation. Intracellular calcium levels were examined by loading resting T cells with the Ca++-specific dye Indo-1, incubating the cells with Ab, cross-linking with goat anti-mouse Ab and analyzing by flow cytometry. Only 11 out of 68 Abs induced a detectable rise in Ca++; two of these Abs induced a substantial rise in Ca++. In resting T cells, 63 of 68 Abs induced increased tyrosine phosphorylation of substrates compared to negative control. Approximately half of the Abs that induced tyrosine phosphorylation induced substrates different from those seen following treatment of cells with anti-CD3 Ab. All mAbs that induced a rise in Ca++ also induced increased tyrosine phosphorylation. These experiments show a distinct difference in the ability of cross-linked Ab to induce changes in tyrosine phosphorylation and intracellular free Ca+. Furthermore, these results indicate that transient increases in cellular substrate phosphorylation may have questionable biologic significance in T cells.     

The role of tyrosine phosphorylation in the interaction of cellular tyrosine kinases with the T cell receptor ζ chain tyrosine-based activation motif

Immunoglobulin receptor family tyrosine-based activation motifs (ITAM) define a conserved signaling sequence, EX₂ YX₂L/IX₇YX₂L/I, that mediates coupling of the T cell antigen receptor (TCR) to protein tyrosine kinases (PTK). In the present study, we explored the role of phosphorylation of the two ITAM tyrosine residues in the interactions of the motif with the PTK ZAP-70 and p59fyn. The data show that the phosphorylation of a single tyrosine within the motif enables binding of p59fyn, whereas phosphorylation of both tyrosines within the motif is required for maximal binding of the PTK ZAP-70. Quantitative binding experiments show that nanomolar concentrations of the doubly phosphorylated ζ1-ITAM are sufficient for ZAP-70 recruitment, whereas micromolar levels of singly phosphorylated ITAM are necessary for p59fyn binding. ZAP-70 binds with low efficiency to a singly phosphorylated ITAM, but shows preferential binding to the C-terminal phosphotyrosine in the ITAM, whereas p59fyn binds selectively to the N-terminal phosphotyrosine. The present data thus show that there is the potential for a singly phosphorylated ITAM to couple to cellular PTK. Moreover, the data suggest a mechanism for heterogeneity in signal transduction responses by the TCR, since ITAM could differentially couple the TCR to downstream signaling events depending on their phosphorylation state.     

Roles of Lck,Syk and ZAP-70 tyrosine kinases in TCR-mediated phosphorylation of the adapter protein Shc

The adapter protein Shc has been implicated in mitogenic signaling via growth factor receptors, antigen receptors and cytokine receptors. Recent studies have suggested that tyrosine phosphorylation of Shc may play a key role in T lymphocyte proliferation via interaction of phosphorylated Shc with downstream molecules involved in activation of Ras and Myc proteins. However, the sites on Shc that are tyrosine phosphorylated in response to TCR engagement and the ability of different T cell tyrosine kinases to phosphorylate Shc have not been defined. In this report, we show that during TCR signaling, the tyrosines Y239, Y240 and Y317 of Shc are the primary sites of tyrosine phosphorylation. Mutation of all three tyrosines completely abolished tyrosine phosphorylation of Shc following TCR stimulation. Our data also suggest that multiple T cell tyrosine kinases contribute to tyrosine phosphorylation on Shc. In T cells, CD4/Lck-dependent tyrosine phosphorylation on Shc was markedly diminished when Y317 was mutated, suggesting a preference of Lck for the Y317 site. The syk-family kinases (Syk and ZAP-70) were able to phosphorylate the Y239 and Y240 sites, and less efficiently the Y317 site. Moreover, co-expression of Syk or ZAP-70 with Lck resulted in enhanced phosphorylation of Shc on all three sites, suggesting a synergy between the syk -family and scr -family kinases. Of the two potential Grb2 binding sites (Y239 and Y317), Y239 appears to play a greater role in recruiting Sos through Grb2. These studies have implications for Ras activation and mitogenic signaling during T cell activation.     

Reduced Cbl phosphorylation and degradation of the zeta-chain of the T-cell receptor/CD3 complex in T cells with low Lck levels

T cells with short interfering RNA-mediated Lck-knockdown (kd) display paradoxical hyper-responsiveness upon TCR ligation. We have previously reported a possible mechanism for T-cell activation in cells with low levels of Lck depending on Grb2-SOS1 recruitment to the zeta-chain of TCR/CD3 (Methi et al., Eur. J. Immunol. 2007, 37: 2539-2548). Here, we show that short interfering RNA-mediated targeting of Lck caused a dramatic reduction in c-Cbl phosphorylation and a general reduction in protein ubiquitination after TCR stimulation. Specifically, this resulted in reduced ubiquitination of the zeta-chain, yet internalization of TCR/CD3 appeared to be normal after receptor engagement. However, zeta-chain levels were elevated in Lck-kd cells, and confocal microscopy revealed reduced colocalization of CD3-containing vesicles with endosomal and lysosomal compartments. We hypothesize that prolonged stability of internalized T-cell receptor complex may result in extended signaling in T cells with low Lck levels.     

Fibronectin induces phosphorylation of a 120-kDa protein and synergizes with the T cell receptor to activate cytotoxic T cell clones

Fibronectin (FN) has been shown to act as a costimulator in both CD4⁺ and CD8⁺ T cell activation through the T cell receptor (TcR). Consistent with previous studies, we found that FN is able to both enhance the maximal amount of TcR-triggered degranulation and lower the threshold for activation. The density of immobilized anti-CD3 or anti-TcR required to induce degranulation and tyrosine phosphorylation of cellular proteins by several cytotoxic T lymphocyte clones is quantitatively about tenfold lower in the presence of FN. We further demonstrate that FN alone stimulates transient tyrosine phosphorylation of a 120-kDa protein (pp120) in CD8⁺ T cells and when FN is coimmobilized with substimulatory amounts of anti-CD3 or anti-TcR there is a synergistic response, resulting in prolonged and enhanced phosphorylation of pp120. To determine if FN acts as a costimulator in CD8⁺ cells solely through mediating adhesion events or if it also transduces signals in T cells we conducted remote stimulation experiments. Degranulation was induced when FN and sub-stimulatory anti-CD3 were presented on separate surfaces, indicating that FN induces independent transmembrane signals capable of augmenting TcR-induced signals resulting in a functional response. Both FN plus TcR-induced tyrosine phosphorylation of pp120 and degranulation are inhibited by RGD-containing peptides, implying that an RGD-dependent FN receptor is mediating phosphorylation of pp120 and enhancing TcR-mediated degranulation.     

The CD45 protein tyrosine phosphatase is required for the completion of the activation program leading to lymphokine production in the Jurkat human T cell line.

Stimulation of the T cell antigen receptor, TCR-CD3, induces tyrosine phosphorylation of specific cellular proteins through activation of a tyrosine kinase. The possible regulatory role of the CD45 protein tyrosine phosphatase in this process was explored by studying the functional properties of cellular variants of the Jurkat T cell line which have been selected to have normal levels of the TCR-CD3 complex, but low or negative expression of CD45. These variants had less than 20% of the normal membrane tyrosine phosphatase activity. Triggering the TCR-CD3 receptor on the CD45 variants with anti-CD3 mAb induced the activation of a tyrosine kinase. Tyrosine phosphorylation of cellular substrates as well as of the CD3 zeta chain was qualitatively comparable to normal cells although the extent of stimulation was lower. No differences were observed between the variants and the normal cells in the duration of the tyrosine phosphorylation signal. The increase in intracellular calcium concentration following receptor stimulation was also less efficient, suggesting that CD45 is necessary for optimal generation of the second messengers of the activation. The CD45 deficient cells secreted highly reduced levels of lymphokines (IL-2, IL-3 or GM-CSF) after activation by anti-CD3 mAb combined with the phorbol ester TPA. This impaired lymphokines production is related to the absence of CD45 since a CD45+ revertant subclone, isolated from one CD45- clone, produced normal levels of cytokines upon activation via CD3, while CD45- subclones were unable to secrete cytokines following activation via CD3. However, upon activation with Ca2+ ionophore and PMA, all CD45- (sub)clones secreted cytokines at levels comparable to those produced by CD45+ cells. These results show that CD45 is required for cytokine production after activation via the TCR-CD3 complex.     

CD154-CD40-independent up-regulation of B7-2 on splenic antigen-presenting cells and efficient T cell priming by staphylococcal enterotoxin A

It has been demonstrated that in vivo T cell priming requires CD154-CD40 interaction, which is suggested to be critical in the induction of co-stimulatory activities on antigen-presenting cells (APC). In the current study, we demonstrate that in vivo administration of a high dose of a superantigen, staphylococcal enterotoxin A (SEA), could up-regulate B7-2 on most splenic APC independently of the CD154-CD40 interaction, followed by efficient expansion of SEA-reactive V(beta)3(+) T cells in CD154- or CD40-deficient mice. However, the CD154-CD40 interaction may be involved in SEA-mediated T cell activation, since a contribution of the CD154-CD40 interaction was observed when a lower dose of SEA was injected. CD154-independent T cell priming by SEA appeared also independent of the TRANCE-RANK pathway, which was shown to be capable of mediating CD154-independent activation of naive T cells during the infection of some viruses. These results indicate that SEA, which provokes rapid and efficient T cell responses without adjuvant, could utilize multiple CD154/TRANCE-independent pathways, to prime T cells.     

T‐cell development is regulated by the coordinated function of proximal and distal Lck promoters active at different developmental stages

Expression of Lck, a T‐cell lineage‐specific tyrosine kinase critical for T‐cell development and activation, can be mediated by either proximal or distal lck promoter. We generated BAC transgenic mice in which BAC lck promoter was deleted and bred these transgenes to an Lck knockout background. Lck‐PROX mice, in which only the proximal promoter is functional, have maximal Lck protein and normal thymic development through CD4^?CD8^? double negative (DN) and CD4⁺CD8⁺ double positive (DP) stages, but undetectable Lck later in development and reduced mature single positive thymocytes. In contrast, Lck‐DIST mice, in which only distal promoter was functional, are deficient in Lck protein in DN and DP thymocytes and severely defective in early T‐cell development, with a block at the DN3‐DN4 beta checkpoint equivalent to complete Lck knockouts. The ability of the proximal lck promoter to support thymic development is independent of Fyn; while, in contrast, the distal lck promoter alone is completely unable to support development in the absence of Fyn. Notably, normal thymocyte development is restored by presence of both proximal and distal promoters, even when independently expressed on different lck genes. These results define distinct and complementary requirements for proximal and distal lck promoters during T‐cell development.     

Signaling through human killer cell activating receptors triggers tyrosine phosphorylation of an associated protein complex

Our understanding of the biology of human natural killer (NK) cells has significantly advanced in recent years upon identification of a family of NK cell-expressed genes that encode killer cell inhibitory receptors (KIR). Individual KIR can selectively bind various HLA class I allotypes and consequently transduce inhibitory signals that block NK cell lysis of ligand-bearing target cells. A distinct subset of related and linked genes express truncated versions of KIR that are otherwise highly homologous in amino acid sequence. Interestingly, these receptors appear to transmit stimulatory signals into NK cells and have been termed killer cell activating receptors (KAR). In this report, we demonstrate that recognition of HLA-Cw3 by the p50 KAR, NKAT8, can potentiate the cytotoxic response of appropriate NK cell clones. Specific cross-linking of this KAR with a monoclonal antibody resulted in intracellular calcium mobilization, protein tyrosine phosphorylation, and phosphorylation of the MAP kinases, ERK1 and ERK2. In addition, we identified a KAR-associated disulfide-linked dimer of a 13-kDa protein that was absent in the Jurkat T cell line and is predicted to participate in these activation signaling events. Upon treatment of NK cells with pervanadate, the disulfide-linked p13 and additional proteins of 25, 30, 37 and 50 – 95 kDa were identified as KAR-associated tyrosine phosphoproteins. Importantly, p13 was inducibly tyrosine phosphorylated upon cross-linking of NKAT8, which strongly suggests that the associated p13 provides KAR with appropriate cytoplasmic structure to couple with tyrosine kinase-mediated signaling effectors.     

3H11, a unique cell surface molecule involved in the function of the CD45RA+subset of CD4+ cells

We have developed a mAb anti-3H11 by immunizing mice with aT cell line derived from the Callithrix Jacchus (common marmoset).anti-3H11 is reactive with 48% of unfractlonated T cells, 62%of CD4⁺ cells and 39% of CD8⁺ cells. Among CD4 cells, anti-3H11preferentially reacts with the CD45RA⁺ T cell subset. The majorityof helper activity for pokeweed mitogen (PWM)-driven B cellIgG synthesis and T cell response to recall antigen such astetanus toxold was found within the 3H11-CD4 cell population,whereas anti-3H11⁺CD4⁺ cells provided poor helper function forPWM-driven B cell IgG synthesis and were more responsive toconcanavalln A and autologous mixed lymphocyte reaction. Biochemicalcharacterization showed that anti-3H11 precipitated a singleprotein band with a relative molecular weight of 32,000 from¹²⁵l-surface labeled cell lysate. Biochemical, phenotyplc andfunctional studies revealed that the 3H11 molecule appearedto be different from previously established molecules on theT cell surface. Interestingly, addition of anti-3H11 to thecombination of CD4 and B cells in the presence of CDS cellsbut not to the combination of CD4 and B cells resulted in enhancementof the suppression of PWM-driven B cell IgG synthesis. Moreover,anti-3H11 had a co-mitogenic effect on T cells via the CD2 andCD3 pathways, and this co-mitogenic activity is restricted tothe CD45RA⁺ T cells. Taken together, our results show that the3H11 molecule is a novel antigen which may play an importantrole in the activation and function of the CD45RA⁺ subset ofT cells.     

Analysis of the tyrosine phosphorylation and calcium fluxing of human CD6 isoforms with different cytoplasmatic domains

CD6 is a cell surface glycoprotein that functions both as a co-stimulatory and adhesion receptor on T cells. Recently we have described CD6 isoforms (CD6a, b, c, d, e) that arise via alternative splicing of exons encoding the cytoplasmic region of the molecule. CD6 becomes phosphorylated on tyrosine (Tyr) residues following stimulation through the T cell receptor (TCR) complex. Since the phosphorylation of Tyr residues renders some cell surface receptors competent for interactions with proteins of intracellular signaling pathways, we wanted to determine which region(s) and residues in the cytoplasmic domain of CD6 were important for phosphorylation on Tyr residues. We engineered and stably expressed chimeric receptors that consisted of the extracellular region of mouse CD6 and the cytoplasmic regions of either naturally occurring isoforms of human CD6, truncated proteins, or point mutants. We were able to demonstrate that of the nine Tyr residues in the cytoplasmic domain of the largest isoform CD6a, the two C-terminal Tyr residues (Tyr 629/662) are critical for the phosphorylation of CD6 following TCR cross-linking. Isoform CD6e, which is missing a region that contains two proline-rich motifs, is not phosphorylated. We further analyzed the ability of the different CD6 isoforms and truncated receptors to mobilize intracellular calcium after CD6/TCR co-ligation. All CD6 isoforms, including CD6e, as well as the truncation mutant Δ 555, which is missing approximately the C-terminal half of the cytoplasmic domain, are able to increase Ca²⁺ influx. Taken together, these results suggest that the region of CD6 which is critical for Ca²⁺ mobilization is located N-terminal from amino acid 555 and is therefore different from the region located at the C terminus of CD6, which is necessary for tyrosine phosphorylation.     

病毒性肝炎患者血清IL-6和T细胞亚群的关系

采用ＭＴＴ比色法和抗体致敏的红细胞花环试验（间接法）　检测了９１例病毒性肝炎患者血清ＩＬ－６和Ｔ细胞亚群。　结果显示，病毒性肝炎患者血清ＩＬ－６活性均明显高于正常对照组（Ｐ＜０．０１），以重型肝炎为最高；且与肝细胞损害程度呈正比（Ｐ＜０．０１）；ＣＤ４~＋细胞、ＣＤ４＋／ＣＤ８~＋均明显降低，与血清ＩＬ－６呈负相关；ＣＯ８~＋细胞明显增高，与血清ＩＬ－６呈正相关（Ｐ＜０．０１）。表明，血清ＩＬ－６与机体细胞免疫功能密切相关，二者互为因果，共同参与病毒性肝炎的免疫病理损害，可作为判断病情和预后及免疫调节治疗的监测指标。     

The characterization of tyrosine phosphorylation of 78 and 92 kDa proteins in rat basophilic RBL-2H3 cells

Previously, we have demonstrated that tyrosine phosphorylation of 78 and 92 kDa proteins in rat basophilic leukemia cells (RBL-2H3) is involved in a signal transduction system for high-affinity IgE receptor (FcRI)-mediated histamine secretion. However, it is not clarified whether the tyrosine phosphorylation of 78 and 92 kDa proteins in RBL-2H3 cells is regulated by activation of protein kinase C (PKC) or phosphatidylinositol 3-kinase (PI3-kinase). In this study, therefore, the effect of depletion of PKC in RBL-2H3 cells, or the influence of PKC, PI3-kinase and tyrosine kinase inhibitors on histamine release from RBL-2H3 cells was examined. The elimination of PKC in RBL-2H3 cells induced significant suppression of histamine release, although the tyrosine phosphorylation of 78 and 92 kDa proteins was not inhibited. The inhibition of histamine release was also observed by the treatment with a PKC inhibitor such as H-7, calphostin C, a PI3-kinase inhibitor such as wortmannin or a tyrosine kinase inhibitor such as ST638, genistein, hervimycin A, although the tyrosine phosphorylation of both proteins was inhibited by only ST638. These results suggest that the 78 kDa protein in RBL-2H3 cells is not identical to the protein-tyrosine kinase PTK72 and the tyrosine phosphorylation of 78 and 92 kDa proteins in RBL-2H3 cells occurs upstream of PKC and PI3-kinase activation or is regulated independently of the PKC- and PI3-kinase-dependent signaling pathway.accepted by M. Katori     

T cell receptor-induced Fas ligand expression in cytotoxic T lymphocyte clones is blocked by protein tyrosine kinase inhibitors and cyclosporin A

Fas/APO-1 is a member of the tumor necrosis factor receptor family of proteins, that induces apoptosis when cross-linked with monoclonal antibody (mAb) or with its physiological ligand. Recently, both a perforin-based and a Fas-based mechanism have been proposed to account for T cell-mediated cytotoxicity. In the present study we used a murine CD8⁺ cytotoxic T lymphocyte (CTL) clone (KB5.C20) specific for H-2K^b and a T cell receptor (TcR)-negative variant of the same clone (2005^?D4) to test (i) whether the same cell can exert both cytotoxic effector mechanisms and (ii) the role of TcR engagement in the induction of Fas-based cytotoxicity. We demonstrate that both the TcR⁺ and TcR^? clones were able to express the Fas ligand after stimulation with phorbol 12-myristate 13-acetate (PMA)/ionomycin, and that TcR engagement of the KB5.C20 clone by means of antigen-bearing cells or of its anticlonotypic mAb (Désiré-1), which leads to Ca²⁺-dependent, presumably perforin-based, cytotoxicity, was also able to induce Fas-based cytotoxicity. In addition, using inhibitors we investigated the signal transduction pathway(s) involved in the induction of Fas-based cytotoxicity and expression of the Fas ligand mRNA in the CTL clones. The involvement of src-like protein tyrosine kinases (PTK) in Fas ligand induction through TcR engagement, was strongly suggested by inhibition with the src-like PTK inhibitor herbimycin A. Inhibition of Fas ligand induction by genistein, a more general TPK inhibitor, even upon stimulation by PMA plus ionomycin, suggested the possible involvement of PTK activities downstream of protein kinase C (PKC) in Fas ligand induction in CTL. Finally, the implication of the Ca²⁺/calmodulin-dependent protein phosphatase calcineurin in Fas ligand induction was demonstrated by the partial inhibition of Fas ligand induction with cyclosporin A. Thus, in CTL clones, Fas ligand expression is inducible by TcR engagement through a pathway similar to that involved in expression of some lymphokine genes.     

Determination of the tyrosine phosphorylation sites in the T cell transmembrane glycoprotein CD5

Studies of CD5-deficient mice indicate that the transmembrane glycoprotein CD5 negatively regulates antigen receptor-mediated signals in thymocytes, lymph node T cells and B1a cells. CD5 contains four tyrosine residues in its cytoplasmic domain and is phosphorylated on tyrosine residues following antigen receptor ligation. Recently it has been proposed that CD5 function is dependent on the recruitment of the tyrosine phosphatase SHP-1 to tyrosine-phosphorylated CD5 and subsequent dephosphorylation of signaling molecules. In this study we investigated the requirements for, and sites of, CD5 tyrosine phosphorylation. Using a T cell line deficient in the tyrosine kinase p56(lck) and the same cell line reconstituted with this kinase, we show that p56(lck) expression is required for efficient CD5 tyrosine phosphorylation. Using tyrosine-phosphorylated peptides corresponding to CD5 cytoplasmic sequences we also show that the Src homology 2 (SH2) domain of p56(lck) binds prominently to pY429SQP, with 30-fold less affinity to pY463DLQ and not to pY441PAL. A number of murine CD5 Y --> F and deletion mutants were expressed in Jurkat T cells. The Y441F mutant was tyrosine phosphorylated at levels comparable to wild-type, but the Y429F and Y463F mutants were phosphorylated at lower levels. Two deletion mutants, which contain only one tyrosine residue (Y378) located at the interface of the transmembrane and cytoplasmic domains, were not tyrosine phosphorylated, suggesting that Y378 is not readily available for phosphorylation. Taken together these results suggest that both Y429 and Y463 can recruit p56(lck), and that these residues are the only prominent sites for CD5 tyrosine phosphorylation.     

B cell proliferation following CD40 stimulation results in the expression and activation of Src protein tyrosine kinase

Resting normal human B cells express negligible c-src mRNA or Src protein tyrosine kinase; however, upon induction of proliferation, these cells express high levels of both mRNA and protein and show a concomitant increase in tyrosine kinase activity of immunoprecipitated Src. Src expression was most pronounced upon stimulation with CD154, and to a lesser extent CD70, Staphylococcus aureus, Cowan strain I and phorbol ester, and correlated with the activation of the cells. Transfection of cDNA for human wild-type or kinase-dead Src into Raji B cells resulted in an increase and decrease, respectively, of the cell numbers in culture, showing a direct correlation of proliferation to the expression of Src that was corroborated using anti-sense oligodeoxynucleotides and chemical inhibitors. Furthermore, the human B cell lines, Namalwa, Daudi and Raji express low levels of Src but express very high levels of Src after stimulation with CD154 that showed a correlation with increased activation. This is the first report of Src detectable in normal B cells. The finding that Src expression is inducible and correlates with stimulation by CD154 and the proliferation of the B cells suggests that Src may play a specific role in normal and transformed B cell activation/proliferation pathways mediated primarily through CD40 stimulation.     

Defective expression and tyrosine phosphorylation of the T cell receptor zeta chain in peripheral blood T cells from systemic lupus erythematosus patients

We have reported that tyrosine phosphorylation and expression of the T cell receptor zeta chain (TCR ζ) was decreased in two systemic lupus erythematosus (SLE) patients with an abnormal TCR ζ lacking exon‐7. To examine further the TCR ζ defect and any possible relationship with specific clinical features, we studied the expression of TCR ζ in peripheral blood T cells from 44 patients with SLE, 53 with other rheumatic diseases (30 rheumatoid arthritis (RA), 11 systemic sclerosis (SSc) and 12 primary Sjögren's syndrome(SjS)) and 39 healthy individuals. Flow cytometric analysis demonstrated a significant decrease in the expression of TCR ζ in SLE (P < 0·001), but not in the other rheumatic diseases. Immunoprecipitation experiments confirmed that the expression of TCR ζ in SLE T cells was decreased dramatically (normal: 111·4 ± 22·6%, SLE: 51·6 ± 37·4%, P < 0·0001). The decrease in TCR ζ did not correlate with disease activity, or with the dose of prednisolone (PSL). There were, however, three SLE patients in whom the level of TCR ζ expression normalized after treatment, suggesting that mechanisms responsible for the TCR ζ defect appear to be heterogeneous. These results confirm the defective expression and altered tyrosine phosphorylation of TCR ζ in a large proportion of SLE patients, suggesting that it may play an important role in T cell dysfunction in SLE.     

Regulation of the p70zap tyrosine protein kinase in T cells by the CD45 phosphotyrosine phosphatase

Two classes of protein tyrosine kinases (PTK) are utilized by the T cell antigen receptor (TcR)/CD3 complex for initiation of the signaling cascade, the Src-family PTK p56^lck and p59^fyn, and the Syk-family PTK p70^zap and p72^syk. In addition, the CD45 phosphotyrosine phosphatase (PTPase) is required for the induction of tyrosine phosphorylation by the TcR/CD3, presumably by positively regulating Src-family PTK. Here we report that CD45 also regulates the Syk-family PTK p70^zap (or ZAP-70). In CD45-negative T cells, p70^zap was constitutively phosphorylated on tyrosine and co-immunoprecipitated with the TcR-ζ chain. In resting wild-type CD45-positive cells, p70^zap was mainly unphosphorylated, but it was rapidly phosphorylated on tyrosine upon treatment of the cells with anti-CD3 or PTPase inhibitors. Finally, p70^zap co-distributed with CD45 in intact T cells, and tyrosine phosphorylated p70^zap was dephosphorylated by CD45 in vitro. These findings suggest that CD45 plays an important role, direct or indirect, in the regulation of p70^zap and its function in TcR/CD3 signaling.     

Protein phosphatase-2A modulates the serine and tyrosine phosphorylation of paxillin in Lewis lung carcinoma tumor variants

Cellular adhesion and motility, processes regulated by focal adhesion assembly and disassembly, can influence a tumor cell's ability to metastasize. Focal adhesion dynamics are, in turn, influenced by the serine and tyrosine phosphorylation state of paxillin. Using Lewis lung carcinoma (LLC) tumor variants, this study shows the importance of the serine/threonine protein phosphatase-2A (PP-2A) in maintaining adherence and restricting tumor cell motility, and modulating the serine and tyrosine phosphorylation of paxillin. Treating non-metastatic LLC-C8 tumor variants with okadaic acid to inhibit PP-2A activity resulted in cell rounding and increased motility. These effects on motility and adherence were accompanied by increased serine and decreased tyrosine phosphorylation of paxillin. These results suggest PP-2A regulation of paxillin phosphorylation may have a role in controlling tumor cell adherence and motility. This revised version was published online in July 2006 with corrections to the Cover Date.