Tyrosine phosphatase CD45 is essential for coupling T-cell antigen receptor to the phosphatidyl inositol pathway

Regulation of the calcium/NF-AT T cell activation pathway by the D2 domain of CD45

CD45 contains tandem repeated protein tyrosine phosphatase (PTP) domains and is essential for the initiation of the earliest activation events resulting from Ag ligation of the TCR. The second PTP domain (D2) contains four CK2 phosphorylation sites in a unique 19-aa insert, which are targets of CK2 phosphorylation. This study was designed to evaluate the roles of these Ser residues in T cell activation. Transient transfection of the CD45- T cell line, J45.01, with CD45 cDNA incorporating four Ser to Ala (S/A) mutations in the 19-aa insert did not affect the magnitude of NF-AT activation resulting from TCR ligation. However, the basal level of NF-AT activity in unstimulated cells expressing the CD45 S/A mutation was elevated 9- to 10-fold. Increased basal NF-AT was dependent on extracellular Ca2+ stores as judged by EGTA treatment. In additional experiments, isolation of stable clones derived from transfection of the CD45 S/A mutant into CD45- H45.01 cells showed sustained calcium flux after TCR engagement. The sustained calcium flux returned to baseline levels after addition of EGTA, suggesting that the expression of the CD45 S/A mutant may have prevented deactivation of plasma membrane calcium channels. Consideration of both transient and stable transfection systems suggests that in addition to being essential for initial events in T cell triggering, the intact CD45 D2, 19-aa insert is necessary for regulation of TCR-mediated calcium signaling pathways.

Partial T cell activation with an altered superantigenic ligand

T cells have the capacity to respond to ligands as full, weak, partial or null agonists, or indeed as antagonists. In the present paper, it is reported that staphylococcal enterotoxin B (SEB) mutated in a T cell receptor (TCR) contact site (SEBDelta61Y) behaves as an altered ligand for a T cell clone (AC20) that expresses the Vbeta17 TCR. The T cells were partially activated by SEBDelta61Y, as shown by TCR down-modulation and up-regulation of the IL-2 receptor. However, these cells did not secrete IL-2, IL-3, IL-4 or IFN-gamma, nor did they proliferate. Analysis of intracellular protein tyrosine phosphorylation after cellular activation provided further evidence that SEBDelta61Y could transduce a signal via the Vbeta17 TCR. The events following receptor ligation were clearly different when the T cells were stimulated with SEB or SEBDelta61Y, manifested as both quantitatively and qualitatively different patterns of phosphorylation of intracellular substrates. In contrast, only quantitative differences were apparent when a transfectant expressing the same alpha/beta TCR was stimulated with the different superantigens. Together, these results provide the first demonstration that altered TCR ligands are not restricted to peptides substituted at secondary TCR contact residues. Rather, an altered superantigenic ligand mutated in the TCR binding site can behave as a partial agonist.

The role of lipid rafts in T cell antigen receptor (TCR) signalling

Plasma membranes of many cell types contain domains enriched in specific lipids and cholesterol, called lipid rafts. In T lymphocytes, key T cell antigen receptor (TCR) signalling molecules associate with rafts, and disrupting raft-association of certain of these abrogates TCR signalling. The TCR itself associates with lipid rafts, and TCR cross-linking causes aggregation of raft-associated proteins. Furthermore, raft aggregation promotes tyrosine phosphorylation and recruitment of signalling proteins, but excludes the tyrosine phosphatase CD45. Together the data suggest that lipid rafts are important in controlling appropriate protein interactions in resting and activated T cells, and that aggregation of rafts following receptor ligation may be a general mechanism for promoting immune cell signalling.

A model system for activation-induced alternative splicing of CD45 pre-mRNA in T cells implicates protein kinase C and Ras

Multiple isoforms of the protein tyrosine phosphatase CD45 are expressed on the surface of human T cells. Interestingly, the expression of these isoforms has been shown to vary significantly upon T-cell activation. In this report, we describe a novel cell line-based model system in which we can mimic the activation-induced alternative splicing of CD45 observed in primary T cells. Of the many proximal signaling events induced by T-cell stimulation, we show that activation of protein kinase C and activation of Ras are important for the switch toward the exclusion of CD45 variable exons, whereas events related to Ca(2+) flux are not. In addition, the ability of cycloheximide to block the activation-induced alternative splicing of CD45 suggests a requirement for de novo protein synthesis. We further demonstrate that sequences which have previously been implicated in the tissue-specific regulation of CD45 variable exons are likewise necessary and sufficient for activation-induced splicing. These results provide an initial understanding of the requirements for CD45 alternative splicing upon T-cell activation, and they confirm the importance of this novel cell line in facilitating a more detailed analysis of the activation-induced regulation of CD45 than has been previously possible.

Regulation of phosphatidylserine synthesis in Jurkat T cell clones: caffeine bypasses CD3/TCR-induced protein tyrosine kinases and calcium signals

Phosphatidylserine synthesis as measured by the incorporation of [(3)H]serine into phosphatidylserine (PtdSer) through the serine-base exchange enzyme system (serine-BEES) is markedly inhibited in Jurkat cells treated with caffeine. The caffeine-induced inhibition was compared to that observed in cells treated with either CD3 mAb or thapsigargin. While CD3- and thapsigargin-induced inhibition was related to the release of Ca(2+) from the endoplasmic reticulum (ER), a process that deprives the serine-BEES of its major cofactor, caffeine modified PtdSer synthesis in the absence of decreased Ca(2+) content of ER. Using Jurkat clones differing by the expression of cell surface markers or protein tyrosine kinases implicated in the CD3/TCR signal transmission pathway, we have shown that CD3 mAb-induced inhibition of PtdSer synthesis necessitates the expression of both the CD3/TCR and the protein tyrosine phosphatase CD45 at the cell surface as well as the presence of p56(lck) and ZAP-70 protein tyrosine kinases. By contrast, thapsigargin, a blocker of the Ca(2+)-ATPase of the ER, known for its Ca(2+) releasing properties, inhibited PtdSer synthesis in all the Jurkat clones tested, indicating that this compound bypasses the CD3/TCR-induced signals. Despite its lack of effect on Ca(2+) release from ER and on protein tyrosine phosphorylations, caffeine inhibited PtdSer synthesis in all the Jurkat clones. The use of several cAMP-inducing drugs and of others xanthine derivatives indicated that caffeine modify PtdSer synthesis either by a direct action on the serine-BEES or by a modification of the structure of the phospholipids used as substrate by the enzyme.

Role of protein phosphatases in the regulation of human mast cell and basophil function

Many extracellular stimuli mediate physiological change in target cells by altering the phosphorylation state of proteins. These alterations result from the dynamic interplay of protein kinases, which mediate phosphorylations, and protein phosphatases, which catalyse dephosphorylations. The antigen-mediated aggregation of high-affinity receptors for IgE on mast cells and basophils triggers rapid changes in the phosphorylation of many proteins and culminates in the generation of inflammatory mediators involved in allergic inflammatory diseases such as asthma. Although protein kinases have an established role in this process, less is known about the involvement of protein phosphatases. This imbalance has been redressed in recent years by the availability of phosphatase inhibitors, such as okadaic acid, that facilitate investigations of the role of protein phosphatases in intact cells. Here we review a number of studies in which inhibitors of protein phosphatases have been used to shed light on the potential importance of these enzymes in the regulation of human mast cell and human basophil function.

Biochemical and functional analysis of mice deficient in expression of the CD45-associated phosphoprotein LPAP

The role of the CD45-associated phosphoprotein (LPAP / CD45-AP) during an immune response remains unclear. To understand better the function of LPAP we generated LPAP-deficient mice by disrupting exon 2 of the LPAP gene. LPAP-null mice were healthy and did not show gross abnormalities compared to their wild-type littermates. However, immunofluorescence analysis of T and B lymphocytes revealed a reduced expression of CD45, which did not affect a particular subpopulation. In contrast to a recent report (Matsuda et al., J. Exp. Med. 1998. 187: 1863 - 1870) we neither observed significant alterations of the assembly of the CD45 / lck-complex nor of polyclonal T-cell responses. However, lymphnodes from LPAP-null mice showed increased cellularity, which could indicate that expression of LPAP might be required to prevent expansion of lymphocytes in particular lymphatic organs rather than potentiating immune responses.

Aggregation of lipid rafts accompanies signaling via the T cell antigen receptor

The role of lipid rafts in T cell antigen receptor (TCR) signaling was investigated using fluorescence microscopy. Lipid rafts labeled with cholera toxin B subunit (CT-B) and cross-linked into patches displayed characteristics of rafts isolated biochemically, including detergent resistance and colocalization with raft-associated proteins. LCK, LAT, and the TCR all colocalized with lipid patches, although TCR association was sensitive to nonionic detergent. Aggregation of the TCR by anti-CD3 mAb cross-linking also caused coaggregation of raft-associated proteins. However, the protein tyrosine phosphatase CD45 did not colocalize to either CT-B or CD3 patches. Cross-linking of either CD3 or CT-B strongly induced tyrosine phosphorylation and recruitment of a ZAP-70(SH2)(2)-green fluorescent protein (GFP) fusion protein to the lipid patches. Also, CT-B patching induced signaling events analagous to TCR stimulation, with the same dependence on expression of key TCR signaling molecules. Targeting of LCK to rafts was necessary for these events, as a nonraft- associated transmembrane LCK chimera, which did not colocalize with TCR patches, could not reconstitute CT-B-induced signaling. Thus, our results indicate a mechanism whereby TCR engagement promotes aggregation of lipid rafts, which facilitates colocalization of LCK, LAT, and the TCR whilst excluding CD45, thereby triggering protein tyrosine phosphorylation.

Domain organization of the extracellular region of CD45

CD45 is a large, heavily glycosylated, transmembrane protein phosphotyrosine phosphatase found on all nucleated cells of haematopoietic origin. In lymphocytes, the cytoplasmic phosphatase is necessary for efficient signalling through the antigen receptor but in contrast little is known about the interactions of the extracellular region of the molecule. This consists of a mucin-like region, a novel cysteine-containing region and a region containing three putative fibronectin type III domains. To confirm this organization and to identify parts potentially important for function, we have expressed fragments of the extracellular domain of rat CD45 as recombinant soluble proteins. Proteins corresponding to two, three and four domains of CD45 were expressed in secreted forms. Single domains and constructs for proteins with truncations of the predicted domains were not expressed. This is consistent with the proposed structural organization. Determination of the positions of the disulphide bonds in the N-terminal cysteine-containing region and the first fibronectin type III domain identified novel disulphide bonds within the fibronectin type III domain and an unusual inter-domain disulphide linkage. Circular dichroism spectroscopy indicated that this region of rat CD45 has mainly beta-strand secondary structure and no alpha-helical content. These studies support the proposed domain organization of CD45.

CD45 Is Essential for FcεRI Signaling by ZAP70, But Not Syk, in Syk-Negative Mast Cells

The ZAP70/Syk family of protein tyrosine kinases plays an important role in Ag receptor signaling. Structural similarity of Syk and ZAP70 suggests their functional overlap. Previously, it was observed that expression of either ZAP70 or Syk reconstitutes Ag receptor signaling in Syk-negative B cells. However, in CD45-deficient T cells, Syk, but not ZAP70, restores T cell receptor-signaling pathway. To study the function of Syk, ZAP70, and CD45 in mast cells, a Syk/CD45 double-deficient variant of RBL-2H3 cells was characterized. After transfection, stable cell lines were isolated that expressed ZAP70, Syk, CD45, ZAP70 plus CD45, and Syk plus CD45. IgE stimulation did not induce degranulation in parental double-deficient cells, nor in the cells expressing only CD45. ZAP70 expression did not restore FcεRI signaling unless CD45 was coexpressed in the cells. However, Syk alone restored the IgE signal transduction pathway. The coexpression of CD45 with Syk had no significant effects on the responses to FcεRI-aggregation. There was much better binding of Syk than ZAP70 to the phosphorylated FcεRIγ-ITAM. Furthermore, unlike Syk, ZAP70 required CD45 to display receptor-induced increase in kinase activity. Therefore, in mast cells, ZAP70, but not Syk, requires CD45 for Ag receptor-induced signaling.

Greatly reduced efficiency of both positive and negative selection of thymocytes in CD45 tyrosine phosphatase-deficient mice

The T cell repertoire is shaped by positive and negative selection of thymocytes. TCR-mediated signals that determine these selection processes are only partly understood. The CD45 tyrosine phosphatase has been shown to be important for signal transduction through the TCR, but there has been disagreement about whether CD45 is a positive or negative regulator of TCR signaling. Using CD45-deficient mice expressing transgenic TCR, we show that in the absence of CD45 there is a large increase in the thresholds of TCR stimulation required for both positive and negative selection. Our results conclusively demonstrate that in double-positive thymocytes CD45 is a positive regulator of the TCR signals that drive thymic selection events.

Dephosphorylation of ZAP-70 and inhibition of T cell activation by activated SHP1

Studies with motheaten mice, which lack the SHP1 protein tyrosine phosphatase, indicate that this enzyme plays an important negative role in T cell antigen receptor (TCR) signaling. The physiological substrates for SHP1 in T lymphocytes, however, have remained unclear or controversial. To define these targets for SHP1 we have compared the effects of constitutively active and inactive mutants of SHP1 on TCR signaling. Expression of wild-type SHP1 had a very small effect on the TCR-induced tyrosine phosphorylation of ZAP-70 and Syk, even when SHP1 was overexpressed 20 - 100-fold over endogenous SHP1. Inactive SHP1-D421A and wild-type SHP2 were without effects. Constitutively active SHP1-DeltaSH2 had a more pronounced effect on ZAP-70 and Syk, even when expressed at near physiological levels. SHP1-DeltaSH2 also inhibited events downstream of ZAP-70 and Syk, such as activation of the mitogen-activated protein kinase Erk2 and the transcriptional activation of the interleukin-2 gene. In contrast, a constitutively active SHP2-DeltaSH2 had no statistically significant effect (although it caused a slight augmentation in some individual experiments). None of the constructs influenced the anti-CD3-induced tyrosine phosphorylation of the TCR zeta-chain or phospholipase Cgamma1, indicating that Src family kinase function was intact. Taken together, our findings support the notion that ZAP-70 and Syk can be direct substrates for SHP1 in intact cells. However, the two SH2 domains of SHP1 did not facilitate its recognition of ZAP-70 and Syk as substrates in intact cells. Therefore, we suggest that SHP1 is not actively recruited to inhibit TCR signaling induced by ligation of this receptor alone. Instead, we propose that ligation of a distinct inhibitory receptor leads to the recruitment of SHP1 via its SH2 domains, activation of SHP1 and subsequently inhibition of TCR signals if the inhibitory receptor is juxtaposed to the TCR.

CD45 Negatively Regulates Lyn Activity by Dephosphorylating Both Positive and Negative Regulatory Tyrosine Residues in Immature B Cells

Using CD45-deficient clones from the immature B cell line, WEHI-231, we previously demonstrated that CD45 selectively dephosphorylates the Src-family protein tyrosine kinase Lyn and inhibits its kinase activity. To further define the mechanisms of CD45 action on Lyn, we metabolically labeled Lyn from CD45-positive and -negative WEHI-231 cells and analyzed cyanogen bromide fragments by SDS-PAGE analysis. Phosphoamino acid analysis confirmed that Lyn is tyrosine phosphorylated with little serine or threonine phosphorylation. In CD45-negative cells, two bands at 8.2 and 4.1 kDa were phosphorylated in the absence of B cell Ag receptor (BCR) ligation. The 8.2-kDa band corresponded to a fragment containing the positive regulatory site (Tyr397), as assessed by its size and its phosphorylation in an in vitro kinase assay. The 4.1-kDa band was phosphorylated by COOH-terminal Src kinase, suggesting that it contains the COOH-terminal negative regulatory site (Tyr508). CD45 was also shown to dephosphorylate autophosphorylated Lyn in vitro. Thus, CD45 dephosphorylates not only the negative but also the positive regulatory tyrosine residues of Lyn. Furthermore, coimmunoprecipitations using anti-Igα Ab demonstrated that Lyn associated with the resting BCR was constitutively phosphorylated and activated in CD45-negative cells. In the parental cells, both regulatory sites were phosphorylated on BCR ligation. Taken collectively, these results suggest that CD45 keeps both BCR-associated and total cytoplasmic pools of Lyn in an inactive state, and a mechanism by which Lyn is activated by relative reduction of CD45 effect may be operative on BCR ligation.

A cell surface ADP-ribosyltransferase modulates T cell receptor association and signaling

ART-1, a cell surface ADP-ribosyltransferase, is imbedded in the membrane by a glycosylphosphatidylinositol anchor. Function of this enzyme in mouse T lymphocytes is to transfer ADP-ribose groups from NAD to arginine residues, exposed on the extracellular domain of cell surface molecules. As a consequence, T cell responses are modulated. To explore the precise action of the enzyme, the T cell lymphoma EL-4 was transfected with the ART-1 gene, and its effects were examined. It is shown that ART-1 ADP-ribosylates distinct cell surface molecules, causing inhibition of T cell receptor signaling, concomitant to suppression of p56(lck) kinase activation. These effects are explained by failure of T cell receptors and co-receptors to associate into a contiguous and functional receptor cluster.

Regulation of Integrin-Mediated T Cell Adhesion by the Transmembrane Protein Tyrosine Phosphatase CD45

The transmembrane protein tyrosine phosphatase CD45 is required for Ag receptor signal transduction in lymphocytes. Recently, a role for CD45 in the regulation of macrophage adhesion has been demonstrated as well. To investigate further the role of CD45 in the regulation of adhesion, we examined integrin-mediated adhesion to fibronectin of two T cell lines and their CD45-deficient variants. The absence of CD45 correlated with enhanced adhesion to fibronectin via integrin α5β1 (VLA-5), but not α4β1 (VLA-4) in both cell lines. Adhesion returned to normal levels upon transfection of wild-type CD45 into the CD45-deficient lines. Transfection of chimeric or mutant molecules expressing some, but not all, CD45 domains and activities demonstrated that both the transmembrane domain and the tyrosine phosphatase activity of CD45 were required for regulation of integrin-dependent adhesion, but the highly glycosylated extracellular domain was dispensable. In contrast, only a catalytically active CD45 cytoplasmic domain was required for TCR signaling. Transfectants that restored normal levels of adhesion to fibronectin coimmunoprecipitated with the transmembrane protein known as CD45-associated protein. These studies demonstrate a novel role for CD45 in adhesion regulation and suggest a possible function for its association with CD45-associated protein.

Expression of the p56(Lck) Y505F mutation in CD45-deficient mice rescues thymocyte development

Mice deficient in the transmembrane protein tyrosine phosphatase CD45 exhibit a block in thymocyte development. To determine whether the block in thymocyte development was due to the inability to dephosphorylate the inhibitory phosphorylation site (Y505) in p56(lck) (Lck), we generated CD45-deficient mice that express transgenes for the Lck Y505F mutation and the DO11.10 T-cell antigen receptor (TCR). CD4 single-positive T cells developed and accumulated in the periphery. Treatment with antigen resulted in thymocyte apoptosis and the loss of transgenic-TCR-bearing cells. Peripheral CD45-deficient T cells from the mice expressing both transgenes responded to antigen by increasing CD69 expression, interleukin-2 production, and proliferation. These results indicate that thymocyte development requires the dephosphorylation of the inhibitory site in Lck by CD45.

The regulation of antigen-receptor signaling by protein tyrosine phosphatases: a hole in the story

Antigen-receptor signaling requires Src-family kinases to initiate tyrosine phosphorylation. CD45 dephosphorylates the inhibitory site in Src-family kinases before antigen-receptor engagement and thus serves to 'prime' the kinases. It has been unclear why CD45 does not also dephosphorylate 'activated' kinases or motifs within the cytoplasmic domains of antigen-receptors and thus prevent signal transduction. Recent reports raise the possibility that CD45 is excluded from engaged antigen-receptors by mechanisms that may include the formation of lipid microdomains.

Interactions of CD45-associated protein with the antigen receptor signaling machinery in T-lymphocytes

CD45 is a transmembrane protein tyrosine phosphatase playing an essential role during T-cell activation. This function relates to the ability of CD45 to regulate p56(lck), a cytoplasmic protein tyrosine kinase necessary for T-cell antigen receptor (TCR) signaling. Previous studies have demonstrated that CD45 is constitutively associated in T-lymphocytes with a transmembrane molecule termed CD45-AP (or lymphocyte phosphatase-associated phosphoprotein). Even though the exact role of this polypeptide is unclear, recent analyses of mice lacking CD45-AP have indicated that its expression is also required for optimal T-cell activation. Herein, we wished to understand better the function of CD45-AP. The results of our studies showed that in T-cells, CD45-AP is part of a multimolecular complex that includes not only CD45, but also TCR, the CD4 and CD8 coreceptors, and p56(lck). The association of CD45-AP with TCR, CD4, and CD8 seemed to occur via the shared ability of these molecules to bind CD45. However, binding of CD45-AP to p56(lck) could take place in the absence of other lymphoid-specific components, suggesting that it can be direct. Structure-function analyses demonstrated that such an interaction was mediated by an acidic segment in the cytoplasmic region of CD45-AP and by the kinase domain of p56(lck). Interestingly, the ability of CD45-AP to interact with Lck in the absence of other lymphoid-specific molecules was proportional to the degree of catalytic activation of p56(lck). Together, these findings suggest that CD45-AP is an adaptor molecule involved in orchestrating interactions among components of the antigen receptor signaling machinery. Moreover, they raise the possibility that one of the functions of CD45-AP is to recognize activated Lck molecules and bring them into the vicinity of CD45.

SHP2-interacting transmembrane adaptor protein (SIT), a novel disulfide-linked dimer regulating human T cell activation

T lymphocytes express several low molecular weight transmembrane adaptor proteins that recruit src homology (SH)2 domain-containing intracellular molecules to the cell membrane via tyrosine-based signaling motifs. We describe here a novel molecule of this group termed SIT (SHP2 interacting transmembrane adaptor protein). SIT is a disulfide-linked homodimeric glycoprotein that is expressed in lymphocytes. After tyrosine phosphorylation by src and possibly syk protein tyrosine kinases SIT recruits the SH2 domain-containing tyrosine phosphatase SHP2 via an immunoreceptor tyrosine-based inhibition motif. Overexpression of SIT in Jurkat cells downmodulates T cell receptor- and phytohemagglutinin-mediated activation of the nuclear factor of activated T cells (NF-AT) by interfering with signaling processes that are probably located upstream of activation of phospholipase C. However, binding of SHP2 to SIT is not required for inhibition of NF-AT induction, suggesting that SIT not only regulates NF-AT activity but also controls NF-AT unrelated pathways of T cell activation involving SHP2.

Galectin-1, a natural ligand for the receptor-type protein tyrosine phosphatase CD45

Galectin-1 binds preferentially to N-acetyllactosamine residues on oligosaccharides associated with several cell surface glycoconjugates. In the present work, placental galectin-1 has been identified to be a natural ligand for the receptor-type protein tyrosine phosphatase CD45. The binding of galectin-1 to CD45 was detected by affinity chromatography of NP 40 solubilized Jurkat T cell membranes on galectin-1 agarose followed by immunoblotting of the galectin-1 agarose bound fraction applying monoclonal antibodies to CD45 isoforms. The PTPase activity of the galectin-1 agarose binding membrane fraction could be inhibited by sodium orthovanadate. Preincubation of Jurkat T cell membrane preparations with galectin-1 decreased the membrane-associated PTPase activity in a concentration-dependent manner. Incubation of Jurkat cells with galectin-1 suppressed the immunoprecipitated PTPase activity of CD45. Galectin-1 stimulates the cell surface expression of phosphatidylserine an early indicator of apoptosis. In CD45+ Jurkat T cells, galectin-1 induces higher levels of phosphatidylserine when compared with CD45- Jurkat cells. These observations indicate that galectin-1-mediated ligation of CD45 is involved in the induction of apoptosis in Jurkat T cells.

Phosphorylation of CD45 by casein kinase 2. Modulation of activity and mutational analysis

CD45 is a receptor-type protein-tyrosine phosphatase (PTP) that is required for antigen-specific stimulation and proliferation in lymphocytes. This study was designed to determine the nature of specific kinases in lymphocytes that phosphorylate CD45 and to determine the effect of phosphorylation on CD45 PTP activity. A major cytoplasmic lymphocyte kinase that phosphorylated CD45 was identified as casein kinase 2 (CK2) by use of an in-gel kinase assay in combination with immunoprecipitation, immunodepletion, and specific inhibition. Mutational analysis of CK2 consensus sites showed that the target for CK2 was in an acidic insert of 19 amino acids in the D2 domain, and Ser to Ala mutations at amino acids 965, 968, 969, and 973 abrogated CK2 phosphorylation of CD45. CK2 phosphorylation increased CD45 activity 3-fold toward phosphorylated myelin basic protein, and this increase was reversible by PP2A treatment. Mutation of Ser to Glu at the CK2 sites had the same effect as phosphorylation and also tripled the Vmax of CD45. CD45 isolated in vivo was highly phosphorylated and could not be phosphorylated by CK2 without prior dephosphorylation with phosphatase PP2A. We conclude that CK2 is a major lymphocyte kinase that is responsible for in vivo phosphorylation of CD45, and phosphorylation at specific CK2 sites regulates CD45 PTP activity.

Cutting Edge: The CD45 Tyrosine Phosphatase Is an Inhibitor of Lck Activity in Thymocytes

A widely accepted model for regulation of the Lck tyrosine kinase is that it is activated by CD45-mediated dephosphorylation of its COOH-terminal negative regulatory tyrosine (Tyr505). Previous work from our laboratory, however, found that despite hyperphosphorylation of Tyr505, the activity of Lck from CD45− T cell lines was actually increased due to hyperphosphorylation of the positive regulatory tyrosine, residue 394. To avoid potential complications introduced by transformed cells, in this study we have characterized the effect of CD45 on Lck activity in normal cells. Lck in thymocytes from CD45−/− mice was hyperphosphorylated on tyrosine residues. Importantly, and in disagreement with the model that CD45 only activates Lck in vivo, the kinase activity of Lck from cells lacking CD45 was substantially increased. These results support a model in which CD45 dephosphorylates both Tyr505 and Tyr394, the net effect in normal thymocytes being a decrease in enzymatic activity.

The second domain of the CD45 protein tyrosine phosphatase is critical for interleukin-2 secretion and substrate recruitment of TCR-zeta in vivo

The CD45 protein tyrosine phosphatase (PTPase) has been shown to regulate the activity of Lck and Fyn protein tyrosine kinases in T cells. However, it is not clear that these constitute the only CD45 substrates. Moreover, the manner by which PTPase activity and substrate recruitment are regulated, is poorly understood. Previous in vitro studies suggest that the first cytoplasmic PTPase domain (D1) of CD45 is the active PTPase, which may be regulated by an enzymatically inactive second PTPase domain (D2). However, the function of CD45 D2 in vivo is unknown. In this study, reconstitution of CD45(-) T cells with specific CD45 PTPase mutants allowed demonstration of a critical role for D2 in TCR-mediated interleukin (IL)-2 production. Specifically, replacement of CD45 D2 with that of the LAR PTPase to form a CD45/LAR:D2 chimera, abrogates CD45-dependent IL-2 production. This effect cannot be accounted for by loss of PTPase activity per se. The expression of D1 substrate-trapping mutants reveals an in vivo interaction between CD45 and TCR-zeta that is dependent on CD45 D2. Thus, cells expressing CD45 lacking D2 exhibit abnormal TCR-mediated signaling characterized by hyperphosphorylation of zeta and deficient ZAP-70 phosphorylation. These data suggest an essential role for CD45 D2 in TCR-regulated IL-2 production through substrate recruitment of the zeta chain.

CD148: A Receptor-Type Protein Tyrosine Phosphatase Involved in the Regulation of Human T Cell Activation

Following ligation of the TCR and costimulatory molecules such as CD28, T cells proliferate and secrete cytokines. Several other cell surface molecules have been identified that are capable of augmenting activation mediated via the TCR. These include CD2, CD27, CD40 ligand, and signaling lymphocytic activation molecule. Here, we have characterized the expression and function of CD148, a recently identified receptor-type protein tyrosine phosphatase. CD148 is expressed at low levels on resting T cells, but is up-regulated following in vitro activation. Cross-linking CD148 with immobilized anti-CD148 mAb induced vigorous proliferation of anti-CD3 mAb-activated, highly purified peripheral blood T cells in an IL-2-dependent, cyclosporin A-sensitive manner. This effect was greatest after 8 days of in vitro culture, suggesting that this molecule is involved in the latter stages of a T cell response. CD148-induced proliferation was significantly greater for CD8+ T cells than for CD4+ T cells. Thus, CD148 is a receptor-type protein tyrosine phosphatase involved in the activation of T lymphocytes.

CD45 partially mediates heterotypic adhesion between murine leukemia/lymphoma cell line L5178Y and marrow stromal cells

We raised mAbs to whole L5178Y leukemia/lymphoma (LL) cells to identify adhesion proteins involved in adherence between LL cells and marrow stromal cells. One mAb, 4C, and its subclones 4C.1 and 4C.2 inhibited adherence of L5178Y LL cells to MLT. a nontransformed murine marrow stromal cell line. These MoAbs are directed against CD45RA. Control anti-CD45 mAbs and isotype mAbs were non-inhibitory. Other anti-CD45 mAbs, M1/9.3, RA3-3A1/6.1 and RA3-2C2/1 do not compete with mAb 4C.1 for binding to the L5178Y cell surface, but mAb 4C.1 competes for binding of mAb RA3-2C2/1. Effects of mAb 4C on tyrosine-phosphatase activity of CD45 in L5178Y cells are minimal, suggesting direct involvement of CD45 as an adhesion protein.

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

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

T cell activation deficiency associated with an aberrant pattern of protein tyrosine phosphorylation after CD3 perturbation in Down's syndrome

Children affected by Down's syndrome (DS) have an increased susceptibility to viral or bacterial infections and leukemia, associated with several abnormalities of the immune system. We investigated whether the T cell defect was qualitative in nature and associated with abnormalities of the early events occurring during cell activation. The proliferative response of lymphocytes from DS individuals after CD3 cross-linking was clearly depressed, as already reported. In contrast, phorbol ester and ionomycin were able to induce cell cycle progression in DS, suggesting a defect in the early stages of the signal transduction through a T cell receptor/CD3 (TCR/CD3) complex upstream of protein kinase C activation. The functional impairment in DS was not related either to a decrease of circulating mature-type CD3+ cells, which express high levels of surface of CD3 molecules, or to a decrease of the CD4+ subpopulation. The analysis of phosphotyrosine-containing proteins after the cross-linking of CD3 molecules in DS lymphocytes revealed a partial signaling, characterized by increased phosphorylation of proteins of 42-44 kD, comparable to that observed in control subjects, but not of proteins of 70 and 21 kD. Moreover, although the "anti-anergic" gamma element of IL-2, IL-4, IL-7, and IL-15 receptors was normally tyrosine-phosphorylated during cell activation, the CD3 zeta-associated protein kinase (ZAP-70) was not. Our results indicate that in DS there is a T cell activation defect, characterized by partial signal transduction through a TCR/CD3 complex, and associated with a selective failure of ZAP-70 tyrosine phosphorylation.

Epitope-specific signaling through CD45 on T lymphocytes leads to cAMP synthesis in monocytes after ICAM-1-dependent cellular interaction

We recently demonstrated that different CD45 monoclonal antibodies (mAb) are able to induce cellular aggregation in human peripheral blood mononuclear cells (PBMC) through LFA-1/ICAM-1 interactions. Such interactions could be down-modulated by protein kinase (PK) A/G inhibitors, but were unaffected by inhibitors of PKC, suggesting the involvement of PKA or PKG in CD45 mAb-induced adhesion. In this study we show that after incubation of PBMC with several (but not all) mAb to CD45, CD45RO and CD45RA, intracellular cAMP, but not cGMP concentrations readily increase, reaching a maximum 30 min after start of activation. As evidenced by several lines of investigation cAMP accumulation was independent of Fc receptor-associated signaling as well as tyrosine phosphatase activity of CD45. In highly pure T lymphocytes, CD45 mAb were unable to induce cAMP synthesis, but readily did so after addition of autologous monocytes. After paraformaldehyde fixation of both quiescent or IFN-gamma/TNF-alpha-preactivated monocytes, cAMP production was no longer detectable, suggesting monocytes as the cell of origin for the increased cAMP synthesis. Further, cAMP accumulation in monocytes occurred after reconstitution to T lymphocytes preincubated with CD45 mAb and extensively washed. Importantly, pretreatment of T lymphocyte/monocyte mixtures with LFA-1 mAb and/or ICAM-1 mAb down-regulated CD45 mAb-induced cAMP synthesis. Finally, we demonstrate that CD45 mAb are not only capable of inducing cAMP production, but also of directly stimulating PKA enzyme activity. Based on the data presented, we propose that CD45 signaling in T lymphocytes subsequently activates cAMP accumulation and PKA activation in monocytes via LFA-1/ICAM-1-dependent cellular interactions.

Negative regulation of T cell antigen receptor signal transduction by hematopoietic tyrosine phosphatase (HePTP)

The hematopoietic tyrosine phosphatase (HePTP) is predominantly expressed in thymocytes and T lymphocytes and at lower levels in other hematopoietic cells. Expression of the gene is enhanced by the T cell growth factor interleukin-2, suggesting a role for HePTP in T cell proliferation or differentiation. We report that HePTP blocks T cell antigen receptor (TCR)-induced transcriptional activation of a reporter gene driven by a nuclear factor of activated T cells(NFAT)/AP-1 element taken from the interleukin-2 gene promoter. This effect was specific to HePTP and was abolished by a mutation (C270S) that impaired its phosphatase activity. Co-expression of HePTP also reduced TCR-induced activation of the mitogen-activated protein kinase Erk2 and the TCR-induced appearance of phosphorylated Erk. In contrast, HePTP did not affect the activation of the N-terminal c-Jun kinase, Jnk. Together these findings suggest that HePTP plays an active negative role in TCR signaling by dephosphorylating one or several signaling molecules between the receptor and the mitogen-activated protein kinase pathway.

Expression of ADP-Ribosyltransferase on Normal T Lymphocytes and Effects of Nicotinamide Adenine Dinucleotide on Their Function

ADP-ribosyltransferase (ADPRT) is a glycosylphosphatidylinositol-anchored cell surface enzyme on CTL. Expression of this enzyme correlates with suppression of CTL functions in the presence of its substrate β-nicotinamide adenine dinucleotide (NAD). To investigate the immunoregulatory importance of ADPRT on normal lymphocytes in vivo, NAD was injected into mice and the effects on cell-mediated and humoral immunity were assessed. Induction of both delayed-type hypersensitivity and CTL, but not Ab responses, are shown to be suppressed by NAD. Consistent with this, mature T cells, but not B cells or macrophages, express ADPRT and are able to ADP-ribosylate cell surface proteins. ADP-ribosylated molecules were identified as LFA-1, CD8, CD27, CD43, CD44, and CD45. Concomitant to ADP-ribosylation of these molecules, T cell trafficking to secondary lymphoid organs is suppressed by NAD. To examine whether this is due to effects of NAD on cell activation, Ag-stimulated responses were assayed in vitro. NAD is shown to inhibit induction of cell proliferation, cytotoxicity, and cytokine secretion. It is suggested that ADPRT regulates T cells on the level of transmembrane signaling via ADP-ribosylation of cell surface molecules. This effect is reported to be indirect, as it involves transmission of signals through TCRs, which are not ADP-ribosylated.

Physical and functional interactions between receptor-like protein-tyrosine phosphatase alpha and p59fyn

We have examined the in vivo activity of receptor-like protein-tyrosine phosphatase alpha (PTPalpha) toward p59(fyn), a widely expressed Src family kinase. In a coexpression system, PTPalpha effected a dose-dependent tyrosine dephosphorylation and activation of p59(fyn), where maximal dephosphorylation correlated with a 5-fold increase in kinase activity. PTPalpha expression resulted in increased accessibility of the p59(fyn) SH2 domain, consistent with a PTPalpha-mediated dephosphorylation of the regulatory C-terminal tyrosine residue of p59(fyn). No p59(fyn) dephosphorylation was observed with an enzymatically inactive mutant form of PTPalpha or with another receptor-like PTP, CD45. Many enzyme-linked receptors are complexed with their substrates, and we examined whether PTPalpha and p59(fyn) underwent association. Reciprocal immunoprecipitations and assays detected p59(fyn) and an appropriate kinase activity in PTPalpha immunoprecipitates and PTPalpha and PTP activity in p59(fyn) immunoprecipitates. No association between CD45 and p59(fyn) was detected in similar experiments. The PTPalpha-mediated activation of p59(fyn) is not prerequisite for association since wild-type and inactive mutant PTPalpha bound equally well to p59(fyn). Endogenous PTPalpha and p59(fyn) were also found in association in mouse brain. Together, these results demonstrate a physical and functional interaction of PTPalpha and p59(fyn) that may be of importance in PTPalpha-initiated signaling events.

Analysis of antigen receptor signalling using mouse gene targeting

Gene targeting in mice has enabled the study of antigen receptor signalling in primary lymphocytes. Furthermore, it has provided the tools to directly assess the function of individual signalling proteins by mutation of the genes that code for them. Some of the results that gene targeting has produced have confirmed previous views of the function of particular proteins. Others have given surprising results and overturned accepted viewpoints.

Inhibition of HIV-1 replication by a monoclonal antibody directed toward the complementarity determining region 3-like domain of CD4 in CD45 expressing and CD45-deficient cells

Monoclonal antibodies directed toward the complementarity determining region (CDR)3-like loop of the aminoterminal domain of CD4 have been shown to inhibit the replication of human immunodeficiency virus (HIV) in CD4 positive T cells. The mechanism of action of these antibodies is not yet elucidated, although several observations suggest that they inhibit viral transcription by signal transduction through the CD4 molecule, potentially implicating the activation of a protein tyrosine kinase (PTK) cascade. Since CD45 is the major protein tyrosine phosphatase associated to the plasma membrane in T cells, and has been shown to regulate the activity of several PTK, we postulated that CD45 may be necessary for the inhibitory action of the CDR3-like specific anti-CD4 antibodies. Therefore we tested the effect of one such anti-CD4 monoclonal antibody, 13B8.2, in repressing HIV replication in CD45 positive cell lines and CD45 deficient variants. Our data show that cells respond to 13B8.2 postinfection treatment regardless of CD45 expression, indicating that neither CD45 nor PTK regulated by CD45 are implicated in the mechanism of action of this antibody.

CD45 and Src-related protein tyrosine kinases regulate the T cell response to phorbol esters

Protein kinase C (PKC)-dependent activation of the Ras signal transduction cascade is essential for induction of the IL-2 promoter during stimulation of T lymphocytes via the T cell receptor (TCR). In this study, the effects of PKC-activating phorbol myristate acetate (PMA) on Ras-dependent activation of transcription from the ets/AP-1 Ras-responsive promoter element were examined in human T cells. Pretreatment of Jurkat cells with the Src-family PTK inhibitor herbimycin A resulted in a 50% inhibition of transactivation of the reporter following incubation with PMA. Evidence was also obtained to suggest the participation of the leukocyte-specific protein tyrosine phosphatase CD45, a regulator of Src-like PTKs, in the PMA-induced activation of the Ras/Raf pathway. First, PMA-induced transactivation of ets/AP-1 is diminished 75% in CD45-negative variants, compared with CD45-positive cells. Second, engagement of CD45 by monoclonal antibodies suppresses the PMA response from the reporter construct. Taken together, these data suggest that Src-related proteins mediate PKC-dependent activation of the Ras/Raf pathway and implicate CD45 in the TCR-independent activation of T lymphocytes induced by agents such as PMA.

The structure and function of CD26 in the T-cell immune response

CD26 is a widely distributed 110 kD cell-surface glycoprotein with known dipeptidyl-peptidase IV (DPP-IV) activity in its extracellular domain. This ecto-enzyme is capable of cleaving amino terminal dipeptides from polypeptides with either L-proline or L-alanine in the penultimate position. On human T cells, CD26 expression appears late in thymic differentiation and is preferentially restricted to the CD4+ helper/memory population, and CD26 can deliver a potent co-stimulatory T-cell activation signal. The cDNA sequence of CD26 predicts a type II membrane protein with only 6 amino acids in its cytoplasmic region, suggesting that, in addition to DPP-IV enzyme activity, other signal-inducing molecules may be associated with CD26. Considerable evidence exists that CD26 interacts, presumably in its extracellular domain, with both CD45, a protein tyrosine phosphatase, and adenosine deaminase (ADA), each of which is capable of functioning in a signal transduction pathway. In addition, CD26 is the receptor for ADA, and ADA on the cell surface is involved in an important immunoregulatory mechanism by which released ADA binds to the cell-surface ADA. This multifunctional molecule may be involved in cell migration and the HIV-1-associated loss of CD4+ cells through the process of programmed cell death. Thus, CD26 appears to play a key role in a number of aspects of lymphocyte function.

Cellular functions regulated by Src family kinases

Src family protein tyrosine kinases are activated following engagement of many different classes of cellular receptors and participate in signaling pathways that control a diverse spectrum of receptor-induced biological activities. While several of these kinases have evolved to play distinct roles in specific receptor pathways, there is considerable redundancy in the functions of these kinases, both with respect to the receptor pathways that activate these kinases and the downstream effectors that mediate their biological activities. This chapter reviews the evidence implicating Src family kinases in specific receptor pathways and describes the mechanisms leading to their activation, the targets that interact with these kinases, and the biological events that they regulate.

Dimerization-induced inhibition of receptor protein tyrosine phosphatase function through an inhibitory wedge

The function and regulation of the receptorlike transmembrane protein tyrosine phosphatases (RPTPs) are not well understood. Ligand-induced dimerization inhibited the function of the epidermal growth factor receptor (EGFR)-RPTP CD45 chimera (EGFR-CD45) in T cell signal transduction. Properties of mutated EGFR-CD45 chimeras supported a general model for the regulation of RPTPs, derived from the crystal structure of the RPTPalpha membrane-proximal phosphatase domain. The phosphatase domain apparently forms a symmetrical dimer in which the catalytic site of one molecule is blocked by specific contacts with a wedge from the other.

A point mutation within CD45 exon A is the cause of variant CD45RA splicing in humans

The leukocyte common antigen (CD45) is alternatively spliced, generating various isoforms expressed on hemopoietic cells. The splicing pattern of CD45 in T cells is altered in some individuals who show abnormal expression of high molecular weight isoforms containing exon A. The variant splicing pattern was shown to be associated with heterozygosity for a silent point mutation within CD45 exon A. This C to G transition is located 77 nucleotides downstream of the splice acceptor junction of exon A (198 bp total length). Here we report that this mutation is the cause of abnormal splicing. To isolate the mutant gene, somatic cell hybrids of lymphocytes with a CD45 splicing defect and a mouse lymphoid line were produced and clones expressing different isoforms of CD45 were isolated. Expression of the high molecular weight isoform containing exon A was associated with the mutation within exon A. All hybrids expressing the low molecular weight isoforms lacking exon A contained the normal allele of CD45 only. In addition, minigenes including this mutation were constructed and transfected into various cell lines (COS-7, HeLa, CHO). Semi-quantitative reverse transcription polymerase chain reaction showed an increase of more than tenfold in splicing to CD45RA (concomitant with a decrease in splicing to CD45RO) when compared with the normal minigene. Taken together, these results demonstrate a causal relationship between the mutation in CD45 exon A and the variant splicing pattern observed. The involvement of trans-acting splicing factors that interact with this region of CD45 pre-mRNA is currently under investigation.

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.

Antiproliferative action of interferon-alpha requires components of T-cell-receptor signalling

Signal transduction through both cytokine and lymphocyte antigen receptors shares some common pathways by which they initiate cellular responses, such as activation of mitogen-activated protein kinase(s). However, other signalling components appear to be uniquely coupled to each receptor. For example, the interferon receptors transduce regulatory signals through the JAK/STAT pathway, resulting in an inhibition of growth and of antiviral effects, whereas this pathway apparently plays no role in T-cell-receptor (TCR)-dependent gene expression. Conversely, signal transduction through the TCR requires the tyrosine kinases Lck and ZAP-70 and the tyrosine phosphatase CD45. Here we show that, unexpectedly, transmission of growth-inhibitory signals by interferon-alpha (IFN-alpha) in T cells requires the expression and association of CD45, Lck and ZAP-70 with the IFN-alpha-receptor signalling complex.

Murine mast cells and monocytes express distinctive sets of CD45 isoforms

CD45 isoform expression patterns were examined in various mast and monocyte cell populations. The reverse transcription-polymerase chain reaction (RT/PCR) and Southern analysis showed these myeloid cells express characteristic sets of CD45 isoforms. Mast cells produce mRNA for two splice variants, one containing exons 5, 7 and 8 of the alternatively expressed exons (therefore lacking exons 4 and 6) and another containing variable exons 7 and 8. Monocytes express three prominent CD45 mRNA species, one which includes exons 5, 7 and 8, another with exons 7 and 8 and the third containing only exon 8 of the variable exons. These results show that there are clear differences within the myeloid lineage sub-populations with respect to CD45 exon usage which appear to delineate mast cell and monocyte specific patterns.

Normal macrophage functions, but impaired induction of gamma delta T cells, at the site of bacterial infection in CD45 exon 6-deficient mice

We investigated the protective functions of macrophages and gamma delta T cells in adult CD45 exon 6-deficient (CD45 -/-) mice against an intraperitoneal (i.p.) infection with Listeria monocytogenes. gamma delta T cells are preferentially localized in the spleen, liver, and intraperitoneal cavity of the adult CD45-/- mice. Increased numbers of gamma delta T cells were observed after i.p. infection with L. monocytogenes in the peritoneal cavity of C57BL/6 (CD45 +/+) mice but not in CD45 -/- mice. The gamma delta T cells showed predominant usage of V delta 5 and V delta 6 rearranged to J delta 1 in the infected CD45 -/- mice which are the same as those used by resident gamma delta T cells of noninfected CD45 +/+ and CD45 -/- mice. Furthermore, we analyzed the protective abilities of the CD45 -/-, CD45 +/+, and gamma delta T cell-depleted mice at the early stage of the listerial infection. The numbers of bacteria in the spleens and livers of the CD45 -/- mice 5 days after the listerial infection were almost ten times larger than those in the CD45 -/- and gamma delta T cell-depleted CD45 +/+ mice. Macrophages showed normal antigen presentation, nitric oxide production and bactericidal activity for L. monocytogenes despite their lacking CD45 surface expression, suggesting that CD45-negative macrophages have a minimal influence on the increased bacterial multiplication in the CD45-/- mice. These results suggest that the gamma delta T cells are induced by the bacterial infection in a CD45-dependent manner, and that unresponsiveness of the gamma delta T cells results in only weak protection against L. monocytogenes in CD45 -/- mice.

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.

Functional Interactions Between the Thrombin Receptor and the T-Cell Antigen Receptor in Human T-Cell Lines

The proteolytically activated thrombin receptor (TR) is expressed by T lymphocytes, which suggests that thrombin may modulate T-cell activation at sites of hemostatic stress. We examined the relationship between TR function and T-cell activation in the Jurkat human T-cell line and in T-cell lines with defined defects in T-cell antigen receptor (TCR) function. Stimulation with thrombin or the synthetic TR peptide SFLLRN produced intracellular Ca2+ transients in Jurkat cells. As the concentration of TR agonist was increased, peak Ca2+ mobilization increased, but influx of extracellular Ca2+ decreased. TR signaling was enhanced in a TCR-negative Jurkat line and in T-cell lines deficient in the tyrosine kinase lck or the tyrosine phosphatase CD45, both of which are essential for normal TCR function. TCR cross-linking with anti-CD3 IgM desensitized TR signaling in Jurkat cells, but not in CD45-deficient cells. A proteinase-activated receptor (PAR-2)–specific agonist peptide, SLIGKV, produced small Ca2+ transients in both MEG-01 human megakaryocytic cells and Jurkat cells, but was less potent than the TR-specific agonist TFRIFD in both cell types. Like TR signaling, PAR-2 signaling was enhanced in TCR-negative or lck-deficient Jurkat clones. These findings provide evidence for functional cross-talk between proteolytically activated receptors and the TCR.

Restoration of thymocyte development and function in zap-70-/- mice by the Syk protein tyrosine kinase

The Syk family of protein tyrosine kinases, consisting of ZAP-70 and Syk, associate with the pre- and alphabeta T cell antigen receptors (TCRs) and undergo tyrosine phosphorylation and activation following receptor engagement. Thymocyte development in zap-70-/- mice is blocked at the CD4+CD8+ TCR(lo) stage. The presence of Syk in the thymus has raised the possibility that Syk may be able to mediate TCR function. To determine if Syk can play a role in thymocyte development, we generated zap-70-/- mice expressing a human syk cDNA. Syk expression restored both thymocyte development and function. In addition, Syk function required the CD45 transmembrane protein tyrosine phosphatase. Hence, ZAP-70 and Syk can play overlapping functions and exhibit similar regulatory mechanisms in mediating alphabeta T cell development.