Demonstration that the leukocyte common antigen CD45 is a protein tyrosine phosphatase

The role of CD45 in T-cell activation--resolving the paradoxes?

CD45 is one of the most abundant transmembrane glycoproteins expressed on the surface of haematopoietic cells. Despite the striking advances of the past few years in the understanding of the actions of CD45 at a molecular level, several paradoxes remain. This article argues that these paradoxes are more apparent than real, and suggests that CD45 plays a key role on the T-cell surface in regulating coupling of the antigen-receptor complex to intracellular signalling pathways.

MAPKAP kinase-2; a novel protein kinase activated by mitogen-activated protein kinase

A novel protein kinase, which was only active when phosphorylated by the mitogen-activated protein kinase (MAP kinase), has been purified 85,000-fold to homogeneity from rabbit skeletal muscle. This MAP kinase activated protein kinase, termed MAPKAP kinase-2, was distinguished from S6 kinase-II (MAPKAP kinase-1) by its response to inhibitors, lack of phosphorylation of S6 peptides and amino acid sequence. MAPKAP kinase-2 phosphorylated glycogen synthase at Ser7 and the equivalent serine (*) in the peptide KKPLNRTLS*VASLPGLamide whose sequence is similar to the N terminus of glycogen synthase. MAPKAP kinase-2 was resolved into two monomeric species of apparent molecular mass 60 and 53 kDa that had similar specific activities and substrate specificities. Peptide sequences of the 60 and 53 kDa species were identical, indicating that they are either closely related isoforms or derived from the same gene. MAP kinase activated the 60 and 53 kDa forms of MAPKAP kinase-2 by phosphorylating the first threonine residue in the sequence VPQTPLHTSR. Furthermore, Mono Q chromatography of extracts from rat phaeochromocytoma and skeletal muscle demonstrated that two MAP kinase isoforms (p42mapk and p44mapk) were the only enzymes in these cells that were capable of reactivating MAPKAP kinase-2. These results indicate that MAP kinase activates at least two distinct protein kinases, suggesting that it represents a point at which the growth factor-stimulated protein kinase cascade bifurcates.

CD22-mediated stimulation of T cells regulates T-cell receptor/CD3-induced signaling

Interaction between B lymphocytes and other cell types is mediated in part by the B-cell adhesion molecule CD22. Recent work has suggested one of the T-cell ligands of B cells to be CD45RO, an isoform of the receptor-linked phosphotyrosine phosphatase CD45. Here we demonstrate direct interaction between CD22 and several isoforms of CD45, including CD45RO, and propose that the interaction may participate in regulation of lymphocyte signaling. Cross-linking of CD3 and CD22 T-cell ligands with anti-CD3 antibody and soluble CD22 is shown to block anti-CD3-induced intracellular calcium increase and to inhibit tyrosine phosphorylation of phospholipase C gamma 1. These effects are consistent with those observed upon coligation of CD3 and CD45 with antibody, providing support to the possibility that ligand-mediated stimulation of CD45 may result in modulation of substrate phosphorylation and lymphocyte activation.

Activation of T cells through a T cell-specific epitope of CD45

The 180- and 190-kDa isoforms of CD45 are preferentially expressed on the helper inducer (memory) subset of CD4 cells. In order to generate monoclonal antibodies against the extracellular domains of these isoforms and determine whether they could regulate the function and activation of these cells, we developed a mAb, anti-4H2D, by immunizing Balb/c mice with an isogenic mouse pre-B cell line expressing the human 190-kDa CD45 isoform. Anti-4H2D reacts with approximately 60% of T cells, 70% of CD4 cells, and 60% of CD8 cells. The CD4 cell population defined by this mAb corresponds functionally and phenotypically to that defined by the CD45RO+CD29+ subset. Western blotting demonstrated that anti-4H2D reacts primarily with the 190-kDa isoform of CD45 and to a minor extent, the 205- and 180-kDa CD45 isoforms. Interestingly, this mAb reacted with only a subpopulation of mature thymocytes and peripheral T cells, despite the fact that the 190-kDa CD45 isoform, as well as CD45RO and CD29, is more widely distributed on cells of hematopoietic origin. The 4H2D epitope was neuraminidase sensitive, indicating that anti-4H2D reacts with a carbohydrate epitope which is present on only a subset of the T cells containing the 190-kDa CD45 isoform epitopes. Functional studies showed that soluble anti-4H2D augmented T cell proliferation induced by the CD2 and CD3 pathways, and treatment of T cells with this mAb up-regulated [Ca2+]i flux induced by both anti-CD2 and anti-CD3 mAbs. These results suggest that the 190-kDa CD45 isoform on human CD4 cells is heterogeneous and that the 190-kDa isoform recognized by anti-4H2D regulates the function and activation of CD4 helper T cells.

cDNA cloning of rat LRP, a receptor like protein tyrosine phosphatase, and evidence for its gene regulation in cultured rat mesangial cells

Protein tyrosine phosphatases (PTPases) are a family of enzymes that play a crucial role in the regulation of signal transduction mediated by reversible protein tyrosine phosphorylation. To understand the significance of PTPases in physiological and pathophysiological processes in the kidney, we isolated three cDNA segments encoding PTPases (LAR, LRP and a novel PTPase) from rat kidney by polymerase chain reaction (PCR). Using PCR product as a probe, we isolated a full-length cDNA of rat LRP. LRP cDNA encoded a single membrane spanning protein consisted of 796 amino acids, with two tandemly located intracellular PTPase domains. By Northern analysis, a ubiquitous pattern of LRP gene expression in rat tissues was demonstrated. In cultured rat mesangial cells, LRP mRNA was detected and the mRNA level was suppressed by either interleukin-1 or interleukin-6 treatment.

Phenylarsine oxide augments tyrosine phosphorylation in hematopoietic cells

Tyrosine phosphorylation and dephosphorylation are implicated in the regulation of cell growth and differentiation. A diverse identification of key regulatory proteins by their content of phosphotyrosine has been hampered by the very low level of tyrosine phosphorylation. This is presumably caused by the relative preponderance of phosphotyrosine phosphatase activity in many cells. We report that treatment of hematopoietic cells with phenylarsine oxide (PAO), a membrane-permeable phosphotyrosine phosphatase inhibitor, induced a dramatic accumulation of phosphotyrosine in a number of cellular proteins. No changes in serine or threonine phosphorylation were detected. The PAO-induced accumulation of phosphotyrosine occurred well before any signs of toxicity or irreversible damage to the cells were seen. Addition of dithiothreitol reversed the effect of PAO. Our data demonstrate that phosphotyrosine phosphatase activity has a major impact on the level of phosphotyrosine in cellular proteins, even in cells with high protein tyrosine kinase activity. Cells with constitutively elevated tyrosine kinase activity are easily detected following treatment with PAO and substrates with an otherwise too low phosphotyrosine content or too rapid phosphate turnover can be studied. This effect of PAO allows determinations of tyrosine phosphorylation-dependent complex formation between proteins.

Evidence for defective transmembrane signaling in B cells from patients with Wiskott-Aldrich syndrome

B lymphocytes from patients expressing the X chromosome-linked immune deficiency disorder, Wiskott-Aldrich syndrome (WAS), fail to produce antibodies in response to stimulation with polysaccharides and other type-2 T cell-independent antigens. To investigate whether this abnormality reflects a defect in the signal transduction cascade normally triggered by ligation of surface immunoglobulin (sIg) on B cells, we have examined early signaling events induced by anti-Ig antibody stimulation of EBV B lymphoblastoid cell lines from WAS patients and healthy controls. Despite the expression of comparable levels of sIg and sIgM on WAS and control EBV B cells, WAS cells failed to manifest the increased proliferation in response to anti-Ig treatment observed in the control cell lines. WAS and control EBV B cells also differed in the magnitude of the change in cytosolic free calcium ([Ca2+]i) induced by sIg ligation; WAS cells showed either markedly diminished or no changes in [Ca2+]i levels whereas control EBV B cells consistently showed increases in [Ca2+]i. Anti-Ig-induced changes in inositol phosphate release were also markedly reduced in WAS compared with control cells. As protein tyrosine phosphorylation is thought to represent a proximal event in the activation of B cells, inducing increases in [Ca2+]i by virtue of tyrosine phosphorylation of phospholipase C (PLC)-gamma, profiles of protein tyrosine phosphorylation and expression of tyrosine-phosphorylated PLC-gamma 1 were compared between WAS and normal EBV B cells before and after sIg cross-linking. These studies revealed that in addition to defective mobilization of Ca2+, the WAS cells manifested little or no increase in tyrosine phosphorylation of PLC-gamma 1 or other intracellular proteins after sIg ligation. Together these results indicate the association of WAS with a defect in the coupling of sIg to signal transduction pathways considered prerequisite for B cell activation, likely at the level of tyrosine phosphorylation. The abnormalities observed in these early transmembrane signaling events in WAS EBV B cells may play a role not only in the nonresponsiveness of WAS patient B cells to certain T independent antigens, but also in the genesis of some of the other cellular deficits exhibited by these patients.

The CD45 tyrosine phosphatase regulates phosphotyrosine homeostasis and its loss reveals a novel pattern of late T cell receptor-induced Ca2+ oscillations

CD45 is a transmembrane tyrosine phosphatase implicated in T cell antigen receptor (TCR)-mediated activation. In T cell variants expressing progressively lower levels of CD45 (from normal to undetectable), CD45 expression was inversely related to spontaneous tyrosine phosphorylation of multiple proteins, including the TCR zeta chain, and was directly correlated with TCR-driven phosphoinositide hydrolysis. The Ca2+ response in these cells was altered in an unexpected fashion. Unlike wild-type cells, stimulated CD45- cell populations did not manifest an early increase in intracellular Ca2+, but did exhibit a delayed and gradual increase in mean intracellular Ca2+. Computer-aided fluorescence imaging of individual cells revealed that CD45- cells experienced late Ca2+ oscillations that were not blocked by removal of extracellular Ca2+. CD45 revertants had the signaling properties of wild-type cells. Thus, CD45 has a profound influence on both TCR-mediated signaling and phosphotyrosine homeostasis, and its loss reveals a novel role for this tyrosine phosphatase in Ca2+ regulation.

The sulfhydryl reagent N-ethylmaleimide induces hyperphosphorylation on tyrosine residues in the Jurkat T-cell line

Tyrosine protein kinases have been shown to be functionally involved in regulation of cellular signalling, proliferation and transformation. The activity of tyrosine protein kinases is counterbalanced by phospho tyrosine phosphatases that maintain constitutively low levels of protein phosphotyrosine in most cells. In this study the effect of N-ethylmaleimide on the protein tyrosine phosphorylation was tested in Jurkat T-cells. Treatment of intact cells for 5-10 mins with 50-100 microM N-ethylmaleimide resulted in a dramatic increase in phosphorylation on tyrosine residues. Phosphoaminoacid analysis revealed an up to ten-fold increase in the content of phosphotyrosine. N-ethylmaleimide blocked the phospho tyrosine phosphatases activity of immunoprecipitated CD45 while in a kinase assay N-ethylmaleimide did not affect the 32P-gamma-ATP phosphorylation of substrates. The N-ethylmaleimide-induced hyperphosphorylation was reversed by treatment with 2 mM dithiotreitol. It is concluded that N-ethylmaleimide offers a novel useful tool for identification of substrates for tyrosine protein kinases and for studies on phosphotyrosine-dependent protein interactions.

Okadaic acid is a potent inducer of AP-1, NF-kappa B, and tumor necrosis factor-alpha in human B lymphocytes

Treatment of human B lymphocytes with an optimal concentration of okadaic acid, an inhibitor of phosphatases 1 and 2A, resulted in the induction of the transcription factor, AP-1 and a marked increase in NF-kappa B levels. In contrast, no effect on the levels of the octamer binding proteins, Oct-1 or Oct-2, were found. Since both AP-1 and NF-kappa B have been reported to be important in the induction of the tumor necrosis factor-alpha (TNF-alpha) gene we examined the effects of okadaic acid on TNF-alpha mRNA levels. Treatment with okadaic acid resulted in a striking increase in TNF-alpha mRNA transcripts within 1 h of stimulation and large amounts of TNF-alpha were released into the culture media. Although okadaic acid provides a potent inductive signal for AP-1 and NF-kappa B it did not induce either B cell proliferation or immunoglobulin secretion.

Variants of the mucosal mast cell line (RBL-2H3) deficient in a functional membrane glycoprotein

We have isolated and characterized subpopulations of the rat mucosal mast cell line, RBL-2H3, carrying either high or low density of a glycoprotein, recently established as mast cell function-associated antigen (MAFA, Ortega et al., 1991), on their surface. These populations were investigated in order to better define the involvement of the MAFA in coupling the immunological stimulation of mast cells to mediator release. The MAFA density on the cell surface of the deficient subpopulation was less than or equal to 10-20% that of the parental population and this phenotype was found to be stably maintained for several months. In contrast, the MAFA-enriched cells had maximally twice the number of copies per cell surface than that of the parental population and this phenotype was less stable. Significantly, low copy number of MAFA on the cell's surface was accompanied by a markedly different secretory response, i.e. (i) a considerable decrease in the secretory response to the Fc epsilon RI-mediated stimulus (ii) a marked enhancement of the ionomycin induced secretion. In order to gain insight into the causes for this decrease in cellular response to the Fc epsilon RI-mediated stimulus, we measured the amplitudes of several biochemical processes which are assigned to the stimulus-secretion coupling cascade. The Fc epsilon RI-mediated uptake of 45Ca2+ by the MAFA-deficient cells was considerably lower than that of the parental and MAFA-enriched cells. Similarly, these cell's Fc epsilon RI-induced rise in [Ca2+]i (both the initial transient as well as the sustained elevation), was markedly lower than that of the parental line and the MAFA-enriched cells. Moreover, the low initial transient rise in [Ca2+]i was found to be correlated with the decrease in Fc epsilon RI-mediated IP3 levels. We therefore examined the cell's content of the phosphatidyl-inositides hydrolyzing enzyme, phospholipase C gamma 1. This was found to be similar in the parental line and in its derived subpopulations. However, PLC gamma 1 activation, as measured by the time course of phosphorylation of its tyrosines, showed a marked difference: while PLC gamma 1 tyrosine phosphorylation, in the parental cells, was only transient (detected already 1 min after antigen addition and declined afterwards to basal levels at ca. 10 min), in the MAFA-deficient cells, tyrosine phosphorylated PLC gamma 1 was also observed 1 min after antigen addition, yet showed no decrease with time in its phosphorylation intensity for up to 30 min.(ABSTRACT TRUNCATED AT 400 WORDS)

Stimulation of a membrane tyrosine phosphatase activity by somatostatin analogues in rat pancreatic acinar cells

A phosphoryl protein tyrosine phosphatase (PTPase) activity has been characterized in rat pancreatic acinar membranes using 32P-labeled poly(Glu,Tyr) as substrate. Acinar membranes exhibited a high affinity for the substrate, with an apparent Km of 0.46 microM and an apparent Vmax of 0.9 nmol.mg protein-1.min-1. Acinar membrane PTPase activity displayed specific characteristics of other PTPases; it was inhibited by the inhibitors Zn2+, orthovanadate and by the divalent cations Mn2+ and Mg2+, and was stimulated by the reducing-agent dithiothreitol. It was also inhibited by soybean trypsin inhibitor and stimulated by trypsin. Gel permeation of pancreatic acinar membranes gave a single peak of enzyme activity with an apparent molecular mass of 70 000 Da. Further purification by HPLC on DEAE revealed two peaks of PTPase activity at 120 mM and 180 mM NaCl. These two peaks reacted in a Western-blot procedure with anti-(peptide) serum directed towards conserved domain of PTPase as a common 67-kDa form associated with lower-molecular-mass proteolytic fragments (31-56 kDa). Incubation of pancreatic acini with somatostatin analogues, SMS 201-995 or BIM 23014, resulted in a stimulation of membrane PTPase activity. The stimulation was rapid and transient, with a maximal level reached within 15 min of addition. The two analogs stimulated PTPase activity in a dose-dependent manner with half-maximal activation occurring at 7 pM and 37 pM and maximal activation at 0.1 nM and 0.1-1 nM for SMS 201-995 and BIM 23014, respectively. The stimulated-membrane PTPase activity also eluted at an apparent molecular mass of 70 kDa in gel-permeation chromatography. The two analogs inhibited the binding of [125I-Tyr3]SMS 201-995 to pancreatic acinar membranes with similar relative potencies to that observed on stimulation of PTPase activity. We conclude that pancreatic acinar membranes possess a low-molecular-mass PTPase which is stimulated by somatostatin analogs at concentrations involving activation of membrane somatostatin receptors.

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

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

CD45 isoform expression during T cell development in the thymus

Various isoforms of leukocyte common antigen, or CD45, are expressed differentially on T cells at different stages of development and activation. We report studies on CD45 isoform expression on various subsets of human T cells using two- and three-color flow cytometry and cell depletion. Bone marrow cells that were depleted of CD3+ and HLA-DR+ cells were CD45RA-RO-. The earliest CD3-CD4-CD8-CD19- thymocytes were CD45RO- with 20%-30% CD45RA+ cells. The most prominent population of CD4+CD8+ double-positive thymocytes were CD45RA-RO+. Even the CD4+CD8+ blasts were greater than 90% CD45RO+. About 80% of single-positive thymocytes (CD4+CD8- or CD4-CD8+) were also CD45RO+. Only 4.3% of CD4+ and 18% of CD8+ single-positive thymocytes were CD45RA+. In contrast, cord blood T cells which represent the stage that immediately follows single-positive thymocytes, contained 90% CD45RA+ cells. Thus, in terms of CD45 isoform expression, single-positive thymocytes are more like double-positive cells than cord blood T cells. These results suggest the following sequence of CD45 isoform switching during T cell development: CD45RA-RO- or RA+RO- (double-negative thymocytes)----RA-RO+ (double-positive and most single-positive thymocytes)----RA+RO- (cord blood T cells), the last switch from CD45RO to CD45RA occurring as a final step of maturation in the thymus.

CD45 isoform expression on human neonatal T cells: expression and turnover of CD45 isoforms on neonatal versus adult T cells after activation

Neonatal T cells are phenotypically similar to "naive" T cells from adult donors in the CD45 isoform expression. Despite the phenotypic similarity, large differences were found between neonatal and adult T cells when T cells were activated. After activation with PHA, adult CD45RA+ T cells began to express CD45RO and no loss of CD45RA expression had yet occurred at Day 3 post-stimulation. Three days after activation, CD45RA+ neonatal T cells also coexpressed CD45RO; however, in contrast to adult T cells, a marked loss of CD45RA was observed. We analyzed the rapid loss of CD45RA found in neonatal T cells. The de novo synthesis of CD45 isoforms in neonatal T cells was essentially the same as that in the adult T cells. Turnover of the CD45RA was very rapid in both resting adult and neonatal T cells. After activation with PHA, the turnover of CD45RA on adult T cells was decreased significantly, while the turnover of CD45RA on neonatal T cells was not changed after activation. Therefore, the regulation of CD45 isoform expression not only involves switches in alternative splicing, but also involves different regulation of turnover of these isoforms from the cell membrane.

Insulin receptor and epidermal growth factor receptor dephosphorylation by three major rat liver protein-tyrosine phosphatases expressed in a recombinant bacterial system

Protein-tyrosine phosphatases (PTPases) play an essential role in the regulation of signal transduction mediated by reversible protein-tyrosine phosphorylation. In order to characterize individual rat hepatic PTPases that might have specificity for autophosphorylated receptor tyrosine kinases, we isolated cDNA segments encoding three PTPases (PTPase 1B, LAR and LRP) that are expressed in insulin-sensitive liver and skeletal muscle tissue, and evaluated their catalytic activity in vitro. The intrinsic PTPase activities of the full-length PTPase 1B protein and the cytoplasmic domains of LAR and LRP were studied by expression of recombinant cDNA constructs in the inducible bacterial vector pKK233-2 using extracts of a host strain of Escherichia coli that lacks endogenous PTPase activity. Each of the cloned cDNAs dephosphorylated a cognate phosphopeptide derived from the regulatory region of the insulin receptor. Despite having only 30-39% sequence identity in their catalytic domains, LAR and PTPase 1B had similar relative activities between the peptide substrate and intact insulin receptors, and also displayed similar initial rates of simultaneous dephosphorylation of insulin and epidermal growth factor (EGF) receptors. In contrast, LRP exhibited a higher rate of dephosphorylation of both intact receptors relative to the peptide substrate, and also dephosphorylated EGF receptors more rapidly than insulin receptors. These studies indicate that three PTPases with markedly divergent structures have the catalytic potential to dephosphorylate both insulin and EGF receptors in intact cells and that redundant PTPase activity may occur in vivo. For these PTPases to have specific physiological actions in intact cells, they must be influenced by steric effects of the additional protein segments of the native transmembrane enzymes, cellular compartmentalization and/or interactions with regulatory proteins.

Autoantibodies to CD45 in systemic lupus erythematosus

Autoantibodies to surface antigens on lymphocytes and other cells of the immune system may contribute to the development of immunoregulatory and other cellular immune abnormalities in patients with active systemic lupus erythematosus. Of special interest in this respect are autoantibodies to CD45 (leukocyte-common antigen, T200), a plasma membrane protein tyrosine phosphatase implicated in the regulation of lymphocyte functional activity, including cytotoxicity, proliferation, and differentiation.

Continuous spectrophotometric assay of protein tyrosine phosphatase using phosphotyrosine

A continuous activity assay for protein tyrosine phosphatases (PTPs), employing phosphotyrosine (P-Tyr) as a substrate, has been developed and applied to measure the activities of two purified enzymes, namely, the full length T-cell protein tyrosine phosphatase (TC PTP) and its truncated form (TC delta C11 PTP). The reaction was followed by changes in ultraviolet absorption and fluorescence resulting from the dephosphorylation of P-Tyr. Both enzymes obey Michaelis-Menten kinetics, with Km = 304 microM, Vmax = 62,000 units/mg for TC PTP and Km = 194 microM, Vmax = 73,000 units/mg for TC delta C11 PTP. The D- and L-forms of P-Tyr are equally effective as substrates. The optimum pH for both enzymes is 4.75. The known effectors of PTPs have the predicted effects on catalytic activity.

Differential expression of a CD45R epitope(6B2) on murine CD5+ B cells: Possible difference in the post-translational modification of CD45 molecules

Although murine peritoneal B cells were homogenously positive for an epitope Lp-2, coded for by the alternative exon 4 of the CD45 gene, they were heterogenous with respect to the expression of another CD45R epitope, 6B2, of unknown exon dependency. While the majority of 6B2-high peritoneal B cells was composed of CD5- B cells, those with low or negative 6B2 were CD5+ B cells. Both 6B2+ and 6B2- peritoneal B cells expressed mainly the same largest CD45R transcripts, with all three alternative exon (4, 5, and 6) sequences. Further, a CD5+ B lymphoma cell line, BCL-1, which was found to be Lp-2+6B2- also had the largest isoform of CD45R molecules with all three alternate structures. Although enzyme digestion studies suggested that the 6B2 epitope resides in protein, not in sugar structures, it is likely that a post-translational modification of CD45R molecules is responsible for the presence or absence of 6B2 epitope expression on peritoneal CD5+ B cells. This event may be related to the differential role of CD45R molecules in regulating lymphocyte function.

Does co-aggregation of the CD45 and CD3 antigens inhibit T cell antigen receptor complex-mediated activation of phospholipase C and protein kinase C?

The binding of agonistic monoclonal antibodies (mAb) to the CD3 antigen in T cells induces a rapid increase in tyrosine phosphorylation, inositive phosphate (IP) production, a rise in intracellular calcium and protein kinase C (PKC) activation. These intracellular signals have been implicated in the control of interleukin-2 and interleukin-2R receptor gene expression, thereby regulating T cell proliferation. Previous studies have shown that co-ligation of the CD45 and CD3 antigens inhibits CD3-induced tyrosine phosphorylation, IP production, calcium signals and T cell proliferation. It has therefore been suggested that the CD45 antigen uncouples the T cell receptor (TcR) from mitogenic signal pathways. In this study co-ligation of the CD3 and CD45 antigens with precisely constructed bispecific mAb did not inhibit CD3-induced T cell proliferation, IP production, calcium signals, diacylglycerol production or PKC activation. Furthermore, co-ligation of CD3 and CD45 antigens already cross-linked with IgM mAb did not lead to inhibition of CD3-induced calcium signals. Inhibitions of CD3-induced intracellular signals were observed following co-ligation of IgG CD45 and CD3 mAb with anti-IgG (F(ab')2 fragments. However, comparable inhibitions were also noted following co-ligation of CD3 with other abundant cell-surface antigens such as CD5 and LFA-1, and inhibitions were only observed when the CD3 mAb used required cross-linking to induce signals. These results suggested that the inhibitory effects of CD45 IgG mAb were not specific and were caused by the prevention of CD3-CD3 cross-linking following CD3 antigen co-ligation with other cell surface molecules. These findings are inconsistent with a specific inhibitory role for the CD45 phosphotyrosine phosphatase in uncoupling the TcR from mitogenic signal pathways.

CD4+ CD45RA+ T cells modulate allergen-induced interleukin 2 responsiveness in human lymphocytes

Peripheral blood lymphocytes from nonallergic individuals acquired responsiveness to interleukin 2 (IL2) after stimulation with ovalbumin (OVA) or Dermatophagoides farinae (Df) antigens when they were pretreated with the CD45RA antibody, which has been shown to define the suppressor inducer subset of CD4+ cells and also to block its suppressor activity. The effect provided by the CD45RA antibody was lost if the lymphocytes had initially been activated with the OVA of Df antigens. The magnitude of the responses was comparable to the allergen-induced responses observed in OVA- or Df-sensitized lymphocytes from allergic patients. The pre-existing IL2 responsiveness in the patients was not increased by the CD45RA antibody pretreatment. However, the CD45RA antibody pretreatment gave rise to Df-induced IL2 responsiveness in the lymphocytes of the patients sensitized with OVA but not with Df; conversely, OVA-induced IL2 responsiveness was enhanced in Df- but not in OVA-sensitized lymphocytes. The CD45RA antibody apparently acts on CD4+ T cells, but not on CD8+ T cells, to induce the IL2 response. A further dissection of normal CD4+ T cells indicated that CD4+45RA- T cells preferentially respond to IL2 after stimulation with OVA or Df antigens. Since normal CD4+45RA+ T cells did not show antigen-induced IL2 responsiveness even after pretreatment with the CD45RA antibody, it is unlikely that the CD45RA antibody stimulates CD4+45RA+ T cells to become responsive to IL2 after antigenic challenge. Alternatively, CD4+45RA+ T cells may modulate the activity of CD4+45RA- T cells, which are potentially responsive to IL2 by antigenic stimulation and thus provide tolerance in nonallergic lymphocytes. Collectively, a defective suppressor activity of CD4+45RA+ T cells may exist in patients with hen-egg allergy and/or bronchial asthma, which may cause lymphocytes to be hyperreactive to OVA or Df antigens.

Interaction of CD4:lck with the T cell receptor/CD3 complex induces early signaling events in the absence of CD45 tyrosine phosphatase

Antibody-mediated ligation of the CD3/T cell antigen receptor (TcR) activates phospholipase C (PLC) via a tyrosine kinase signaling pathway that requires expression of the transmembrane tyrosine phosphatase CD45. In normal T cells, CD3-mediated PLC activation is significantly augmented by co-ligation of CD3 with the CD4 co-receptor; however, unlike CD3-associated tyrosine kinases, antibody-induced activation of the CD4-associated tyrosine kinase p56lck does not require CD45 expression. To explore the role of CD45 in the CD3 and CD4 activation pathways further, we examined the effect of CD3/CD4 cross-linking on tyrosine phosphorylation and activation of phospholipase C in CD45- mutant cells of the T cell leukemia line HPB.ALL. In accord with previous observations, anti-CD3 stimulation of the CD45-deficient cells failed to activate tyrosine kinases, or PLC as measured by mobilization of intracellular calcium. However, we show here that ligation of CD3 with CD4 leads to tyrosine phosphorylation of PLC gamma 1 and elevation in the intracellular free Ca2+ concentration in CD45- cells that is in excess of that seen in CD45+ cells. Since CD4 stimulation alone did not activate PLC, a component of the CD3 signaling pathway must be independent of CD45. Anti-CD4-induced tyrosine phosphorylation and activation of CD4-associated lck was also enhanced in CD45- cells, suggesting that increased lck activation compensates for the defect in CD3/TcR signaling, such that interaction of the CD3 signaling pathway with the CD4-associated pathway activates PLC even in the absence of CD45. The data demonstrate that the requirement for CD45 in coupling CD3/TcR to the PI-PLC activation cascade is not absolute, but rather substantiates a role for CD45 in modifying molecular interactions that control T cell activation.

Protein tyrosine phosphatase containing SH2 domains: characterization, preferential expression in hematopoietic cells, and localization to human chromosome 12p12-p13

Protein tyrosine phosphorylation has been implicated in the growth and functional responses of hematopoietic cells. Recently, approaches have been developed to characterize the protein tyrosine phosphatases that may contribute to regulation of protein tyrosine phosphorylation. One novel protein tyrosine phosphatase was expressed predominantly in hematopoietic cells. Hematopoietic cell phosphatase encodes a 68-kDa protein that contains a single phosphatase conserved domain. Unlike other known protein tyrosine phosphatases, hematopoietic cell phosphatase contains two src homology 2 domains. We also cloned the human homolog, which has 95% amino acid sequence identity. Both the murine and human gene products have tyrosine-specific phosphatase activity, and both are expressed predominantly in hematopoietic cells. Importantly, the human gene maps to chromosome 12 region p12-p13. This region is associated with rearrangements in approximately 10% of cases of acute lymphocytic leukemia in children.

Reversal of CD45R isoform switching in CD8+ T cells

Both CD4+ and CD8+ T cells express either CD45RA or CD45R0 isoform of CD45R in an exclusive way. Recent reports have shown that CD45RA+ T cells lose CD45RA and gain CD45R0 upon activation. This switching has been suggested to be irreversible although more recently, examples of reversal of CD45R isotype switching in CD4+ T cells have been reported. We report here that freshly isolated unprimed CD8+ T cells, when activated with PHA, temporarily lose CD45RA but reexpress an intermediate level of CD45RA 2-3 weeks after activation with PHA. This reversal seems to take place much more slowly in unprimed CD4+ T cells: the majority of CD4+ T cells that had lost CD45RA and gained CD45R0 remained CD45RA-CD45R0+ in 3 weeks after the stimulation. Also, long-term CD8+ CD45RA+ T cell lines stimulated with PHA or OKT3 showed even more rapid recovery of CD45RA while PPD-specific CD4+ T cell clones retained the original CD45R0 phenotype 3 weeks after stimulation with PPD or PHA.

Isoform-specific associations of CD45 with accessory molecules in human T lymphocytes

Association of CD45 with surface molecules was investigated in human T lymphocytes by co-capping. CD45 appeared to be associated with the CD3/T cell receptor complex and with CD4 or CD8 molecules in memory, but not in naive T cells, as previously reported in the mouse. Associations of CD45 isoforms with accessory molecules were then identified with seven anti-CD45R monoclonal antibodies (mAb). An isoform-specific association pattern was observed: CD2 co-capped with CD45 molecules recognized by UCHL1 mAb (CD45R0). LFA-1 with molecules bound by 2H4 mAb (CD45RA), and both CD4 and CD8 with molecules reacting with MCA.347 mAb (whose isoform specificity was not known). Further information on the CD45 isoform(s) associated to CD4 and CD8 was sought by assessing the isoform specificity of MCA.347. Cross-competition experiments showed that it reacts with an epitope clearly different from those recognized by 2H4 and UCHL1, and only partially overlapping the PD7/26 epitope (CD45RB). Moreover, the competition between MCA.347 and PD7/26 was maximal in naive T cells and minimal both in memory T cells and in a subset expressing CD11b, a marker of granular lymphocytes. Immunoprecipitation experiments showed that MCA.347 binds to CD45 molecules with a molecular mass of 220, 205 and 190 kDa, the 190-kDa molecules not being recognized by 2H4, PD7/26 or UCHL1. These data indicate that MCA.347 recognizes amino acid sequences different from those coded by the exon A or B of the gene, and not expressed by CD45R0, suggesting that it binds to sequences coded by the exon C. In conclusion, this work shows that in human T cells different CD45 isoforms are associated to different surface molecules: LFA-1 is associated to CD45RA, CD2 to CD45R0 and CD4 and CD8 presumably to CD45RC. This peculiar behavior of CD45 suggests that it may play a crucial role in lymphocyte activation, probably by modulating the signals delivered to the cell by different receptor systems.

Glycosylation of CD45: carbohydrate processing through Golgi apparatus is required for cell surface expression and protein stability

The importance of glycosylation on cell surface expression, protein stability and function of the CD45 phosphotyrosine phosphatase leukocyte common antigen, has been studied in the K562 erythroleukemic cell line. Cell treatment with the N-glycosylation inhibitor tunicamycin generated unglycosylated CD45 polypeptides of 130 and 140 kDa. Immunofluorescence flow cytometry and Scatchard techniques revealed that CD45 cell surface expression was decreased in a time- and dose-dependent manner upon tunicamycin incubation. Moreover, a remarkable decrease in CD45 phosphatase activity was detected in tunicamycin-treated cells, which correlated with the diminished CD45 cell surface expression. Pulse-chase biosynthetic assays demonstrated that the fate of the unglycosylated CD45 proteins underwent a late non-lysosomal degradation, since it was not prevented either by monensin, an inhibitor of Golgi transport, or by the lysosomal function inhibitor chloroquine. These results demonstrate that intact glycosylation of CD45 molecules through Golgi compartment is required for stability and proper transport towards the plasma membrane of these phosphotyrosine phosphatase proteins.

Protein tyrosine phosphatase containing SH2 domains: characterization, preferential expression in hematopoietic cells, and localization to human chromosome 12p12-p13

Protein tyrosine phosphorylation has been implicated in the growth and functional responses of hematopoietic cells. Recently, approaches have been developed to characterize the protein tyrosine phosphatases that may contribute to regulation of protein tyrosine phosphorylation. One novel protein tyrosine phosphatase was expressed predominantly in hematopoietic cells. Hematopoietic cell phosphatase encodes a 68-kDa protein that contains a single phosphatase conserved domain. Unlike other known protein tyrosine phosphatases, hematopoietic cell phosphatase contains two src homology 2 domains. We also cloned the human homolog, which has 95% amino acid sequence identity. Both the murine and human gene products have tyrosine-specific phosphatase activity, and both are expressed predominantly in hematopoietic cells. Importantly, the human gene maps to chromosome 12 region p12-p13. This region is associated with rearrangements in approximately 10% of cases of acute lymphocytic leukemia in children.

Isoforms of the CD45 common leukocyte antigen family: markers for human T-cell differentiation

The diverse host defense and immunoregulatory functions of human T cells are performed by phenotypically heterogeneous subpopulations. Among the membrane antigens that are differentially expressed by reciprocal human T-cell subsets are the CD45RA and CD45RO isoforms of the common leukocyte antigen family, which have been hypothesized to identify "naive" and "memory" T cells, respectively. The CD45RA antigen is first expressed by T-lineage cells relatively late during their intrathymic maturation and continues to be expressed by most T cells in the immunologically naive neonate. With increasing age and antigenic exposure, however, CD45RA-/RO+ cells become more prevalent in the circulation and comprise the majority of cells in tissues. Analyses of the functional capabilities of CD4+CD45RA+ and CD4+CD45RO+ cells have shown that proliferative responses to "memory" recall antigens or the ability to provide help for antibody production are functions uniquely performed by CD4+CD45RA-/RO+ cells. The major immunoregulatory functions described for CD4+CD45RA+ cells involve suppression of immune responses, either directly or via the induction of suppressor activity by CD8+ cells. Two general models of differentiation have been proposed to describe the lineal relationship of these T-cell subsets. Although these subsets could represent mature, phenotypically and functionally stable progeny arising from separate differentiation pathways, there is considerable experimental support for the hypothesis that CD45RA-/RO+ cells are "memory" cells that derive from "naive" or "virgin" CD45RA+/RO- precursors via an activation-dependent postthymic differentiation pathway. Altered frequencies of CD45RA+ and CD45RO+ T cells have been observed in a variety of different clinical conditions, particularly diseases manifesting altered immune function. These findings have contributed new information concerning the physiological events regulating the in vivo generation of these T-cell subsets. In addition, they may provide clues to the pathogenetic processes associated with certain diseases.

Protein-tyrosine phosphatases and the regulation of insulin action

Protein-tyrosine phosphatases (PTPases) play an important role in the regulation of insulin action by dephosphorylating the active (autophosphorylated) form of the insulin receptor and attenuating its tyrosine kinase activity. PTPases can also modulate post-receptor signalling by catalyzing the dephosphorylation of cellular substrates of the insulin receptor kinase. Dramatic advances have recently been made in our understanding of PTPases as an extensive family of transmembrane and intracellular proteins that are involved in a number of pathways of cellular signal transduction. Identification of the PTPase(s) which act on various components of the insulin action cascade will not only enhance our understanding of insulin signalling but will also clarify the potential involvement of PTPases in the pathophysiology of insulin-resistant disease states. This brief review provides a summary of reversible tyrosine phosphorylation events in insulin action and available data on candidate PTPases in liver and skeletal muscle that may be involved in the regulation of insulin action.

Phenylarsine oxide stimulates a cytosolic tyrosine kinase activity and glucose transport in mouse fibroblasts

In the present report we further approach the mechanism by which insulin and phenylarsine oxide (PAO), a trivalent arsenical compound, regulate glucose transport in mouse fibroblasts (NIH3T3). First, we show that PAO is a powerful stimulatory agent on glucose transport. Second, at least three series of observations indicate that this action of PAO is not mediated through the insulin receptor: (i) the same effect of PAO is observed in NIH3T3 and in transfected cells expressing 6 x 10(6) insulin receptors, while the effect of insulin is markedly increased in the transfected cells; (ii) PAO does not affect the tyrosine phosphorylation of the insulin receptor; (iii) the tyrosine kinase activity of the insulin receptor toward exogenous substrates is not increased by PAO. Since PAO appears to act on glucose transport by a different mechanism than insulin, we have compared the effect of PAO and insulin on tyrosine phosphorylation of cellular proteins. Using Western blot analysis we did not detect common substrates in PAO- and insulin-treated cells. However, we found in cell extracts from both PAO- and insulin-treated cells a 50-kDa protein that is immunoprecipitated by antiphosphotyrosine antibody. In addition, PAO activates a cytosolic tyrosine kinase capable of poly(Glu/Tyr) phosphorylation. As a whole, our data suggest that the 50-kDa protein found in cells incubated with PAO and insulin could be the convergence point of the insulin and PAO signaling pathways.

The coexpression of CD45RA and CD45RO isoforms on T cells during the S/G2/M stages of cell cycle

The tyrosine phosphatase CD45 is alternatively spliced to generate isoforms of different molecular weights (180-220 kDa) which are differentially expressed on hematopoietic cells. Monoclonal antibodies reacting with either the 180-kDa (UCHL-1, CD45RO) or the 200- to 220-kDa (2H4, CD45RA) isoform have been used to subdivide T cell populations based on their expression of one or the other of these two epitopes. CD45RA T cells have "naive" characteristics of unresponsiveness to recall antigens and prominence in cord blood, while CD45RO T cells are considered "memory" T cells because they proliferate to recall antigens and increase following PHA activation of cord blood. However, we have recently demonstrated the expression of the CD45RA isoform on a subpopulation of CD45RO+ T cell clones, suggesting that CD45RA is not a universal marker for naive T cells. Using propidium iodide staining of the DNA to determine cell cycle stage, we now show that CD45RA expression is significantly higher on T cell clones during the S, G2, and M stages of cell cycle when compared to CD45RA expression on cells in Go and G1. Furthermore, CD45RA expression on cells undergoing mitosis is not limited to long-term activated T cell clones, as uncultured peripheral blood T cells in the S/G2/M phase express significantly more CD45RA. The percentage of T cells coexpressing CD45RA and CD45RO also increases following PHA activation, indicating that T cells in the process of division express both isoforms. These results suggest a potential role of the CD45RA isoform during the stages of cell cycle leading to mitosis.

CD3-induced T-cell proliferation and interleukin-2 secretion is modulated by the CD45 antigen

In the present study we have investigated the effect of CD45, CD45RA and CD45RO monoclonal antibodies (MoAbs) on the CD3 receptor-mediated proliferation of human T lymphocytes. It is shown that CD3-induced proliferation of purified resting T cells and quiescent T lymphoblasts (QTL) is promoted via all of the investigated CD45-associated epitopes. It is also shown that the CD45 molecules are required to be cross-linked for costimulation. The MoAbs enhance the interleukin-2 (IL-2) production of CD3-stimulated QTL. The elevation of the IL-2 production correlates with the increase in CD3-induced cell proliferation suggesting that the CD45-driven regulation of T lymphocyte activation is linked to the IL-2 pathway.

Tyrosyl phosphorylation and activation of the myelin basic protein kinase p44mpk during sea star oocyte maturation

The most prominent tyrosyl-phosphorylated protein in maturing sea star oocytes was identified as the 44 kDa myelin basic protein (MBP) kinase p44mpk. Immunoblotting studies with anti-phosphotyrosine PY-20 antibody and phosphoamino acid analysis of in vivo [32P]phosphate-labelled p44mpk showed that the tyrosyl phosphorylation of the kinase correlated with a greater than 10-fold stimulation of its MBP phosphotransferase activity. The activation of p44mpk was reversed almost completely by purified preparations of the protein-tyrosyl phosphatases CD45 and 1B. Purified p44mpk has previously been shown to undergo autophosphorylation in vitro on seryl residues and this was associated with further enhancement of its MBP phosphorylating activity (Sanghera et al. (1991) J. Biol. Chem. 266, 6700-6707). p44mpk also underwent seryl phosphorylation during oocyte maturation, and the protein-seryl/threonyl phosphatase 2A reversed partially the maturation-associated stimulation of its MBP kinase activity. The properties of p44mpk resemble the murine 42 kDa mitogen-activated protein kinase (p42mapk). While p44mpk may feature the phosphorylatable tyrosyl residue that is critical for activation in p42mapk, it lacks the upstream threonyl phosphorylation site that is also required for p42mapk activity (Payne et al. (1991) EMBO J: 10, 885-892). These findings indicate partial differences in the regulatory mechanisms that govern the activities of these isozymes.

CD45 isoforms associated with distinct functions of CD4 cells derived from unusual healthy donors lacking CD45RA- T lymphocytes

We now report two healthy individuals whose T lymphocytes were over 95% positive for CD45RA antigen expression. However, these donors normally expressed both the CD29 high (CD29+) and CD45RO high (CD45RO+) antigens on approximately 40 and 50% of their CD4 cells, respectively. Despite the strong CD45RA expression on the surface of almost all CD4 cells, the CD29 marker allowed T cells from these donors to be divided phenotypically into subsets having distinct in vitro function. CD4+CD29+ cells from these donors responded maximally to recall antigens such as TT and provided strong helper function for B cell Ig synthesis. In contrast, CD4+CD29- cells responded poorly to recall antigens and had poor helper function for B cell Ig synthesis, but had strong suppressor activity. Thus, CD29 antigen expression was still predictive of the in vitro functional activity as previously described for normal donors. Furthermore, biochemical analysis of the distribution of individual CD45 isoforms on the surface of these subsets of CD4 cells revealed distinct differences. The CD4+CD29 high (CD4+CD29+) subset of cells primarily expressed the 180-, 190-, and 205-kDa CD45 isoforms, while the CD4+CD29 low (CD4+CD29-) cells primarily expressed the 190-, 205-, and 220-kDa CD45 isoforms. These results suggest that despite the superficial phenotypic similarity of CD4 cells in these donors, distinctions in the distribution of both CD29 and the 180- and 220-kDa CD45 isoforms exist and might play a role in the different functions of freshly isolated CD4 lymphocytes.