CD45 tyrosine phosphatase-activated p59fyn couples the T cell antigen receptor to pathways of diacylglycerol production, protein kinase C activation and calcium influx.

The role of the CD45 phosphotyrosine phosphatase in coupling the T cell antigen receptor complex (TCR) to intracellular signals was investigated. CD45- HPB-ALL T cells were transfected with cDNA encoding the CD45RA+B+C- isoform. The tyrosine kinase activity of p59fyn was found to be 65% less in CD45- cells than in CD45+ cells, whereas p56lck kinase activity was comparable in both sub-clones. In CD45- cells the TCR was uncoupled from protein tyrosine phosphorylation, phospholipase C gamma 1 regulation, inositol phosphate production, calcium signals, diacylglycerol production and protein kinase C activation. Restoration of TCR coupling to all these pathways correlated with the increased p59fyn activity observed in CD45-transfected cells. Co-aggregation of CD4- or CD8-p56lck kinase with the TCR in CD45- cells restored TCR-induced protein tyrosine phosphorylation, phospholipase C gamma 1 regulation and calcium signals. Receptor-mediated calcium signals were largely due (60-90%) to Ca2+ influx, and only a minor component (10-40%) was caused by Ca2+ release from intracellular stores. Maximal CD3-mediated Ca2+ influx occurred at CD3 mAb concentrations at which inositol phosphate production was non-detectable. These results indicate that CD45-regulated p59fyn plays a critical role in coupling the TCR to specific intracellular signalling pathways and that CD4- or CD8-p56lck can only restore signal transduction coupling in CD45- cells when brought into close association with the TCR.     

High levels of CD45 are coordinately expressed with CD4 and CD8 on avian thymocytes.

In this paper we describe the avian homolog of mammalian CD45. We show that this Ag is expressed on all leukocytes but not on erythroid cells or their immediate precursors. Immunoprecipitations demonstrated that B lineage cells from the bursa of Fabricius expressed a higher molecular mass variant (215 kDa) than did T lineage cells from the thymus (190 kDa), and crucially, these high molecular mass molecules had intrinsic phosphotyrosine phosphatase activity characteristic of mammalian CD45. We show that levels of CD45 expression as detected by mAb LT40 in the avian thymus are heterogeneous and further that mAb LT40 can deplete all phosphotyrosine phosphatase activity from thymocyte membrane preparations. Therefore total levels of CD45 are heterogeneous among avian thymocytes. Specifically, 87 to 89% of thymocytes expressed fourfold higher levels of surface CD45 (CD45hi) than the remaining 11 to 13% (CD45lo). The CD45lo population contained exclusively thymocytes with the phenotype CD3-4-8lo, characteristic of the immediate precursors to the CD3-4+8+ thymic population which are CD45hi. The shift from low to high levels of surface CD45 expression therefore occurred at the same stage as the transition from CD4-8lo to CD4+8+ and before the expression of CD3. The protein tyrosine kinase activity associated with CD4 and CD8 (p56lck) and the phosphatase activity of CD45 have been implicated elsewhere in jointly regulating peripheral T cell signal transduction and subsequent cellular responses. The coordinated expression of high levels of CD45 with both CD4 and CD8 in the avian thymus supports the possibility that these molecules may function together in regulating thymocyte growth and/or differentiation.     

Differential activation of phosphotyrosine protein phosphatase activity in a murine T cell hybridoma by monoclonal antibodies to CD45.

The effect of two anti-CD45 (T200, LCA, Ly5) antibodies on the activation of the murine T-cell hybridoma 13.13 has been evaluated. These studies have been carried out in a system that did not require cross-linking or coclustering of antibodies. Activation of 13.13 cells with the anti-CD3 monoclonal antibody, 145.2C11, gave rise to rapid increases in intracellular calcium and interleukin-2 production. Additionally, within 1 min, phosphorylation on tyrosine of four major proteins of about 130,000, 110,000, 80,000, and 37,000 daltons could be seen. Pretreatment of the cells with the anti-CD45 mAb M1/89.18.7.HK markedly inhibited all three biological responses, while an alternate anti-CD45 antibody, M1/9.3.4.HL.2, had little effect. The two antibodies bound to CD45 with similar affinities, and no differences in the lateral mobility of antibody-CD45 complexes in the cell membrane were observed. The inhibition of activation of the cells by M1/89.18.7.HK was abrogated significantly both by the phosphotyrosine protein phosphatase inhibitor orthovanadate and by excess M1/9.3.4.HL.2. If M1/89.18.7.HK was added to the 13.13 cells after they had already been activated with anti-CD3, it very effectively stimulated dephosphorylation of substrates that had been phosphorylated on tyrosines prior to adding the anti-CD45 antibody. These results indicate that the phosphotyrosine protein phosphatase activity of CD45 is critical to its biological function and that bivalent (i.e. uncross-linked) anti-CD45 antibodies can give rise to markedly different responses. One of the antibodies, M1/89.18.7.HK, appears to behave much like a receptor ligand and is able to activate the enzymatic activity associated with the CD45 transmembrane protein.     

Physical associations between CD45 and CD4 or CD8 occur as late activation events in antigen receptor-stimulated human T cells.

Activation of human PBL T cells with solid phase anti-CD3 mAb or during the course of an MLR response gives rise to the association of CD4 or CD8 molecules with the protein tyrosine phosphatase, CD45, on the cell surface. This paired association of cell-surface molecules occurs late in the activation cycle and appears to be dependent upon Ti-CD3-mediated signaling because mitogen-driven activation does not induce formation of the complex. Maximal association occurred 72 to 96 h after exposure to anti-CD3 mAb on both CD4+ and CD8+ T cells. In contrast, association between CD8 and CD45 during an MLR response did not occur until day 6 of a MLR whereas CD4-CD45 association was detected by 72 h of culture. The kinetics of association between CD4 or CD8 and CD45 was measured by fluorescence resonance energy transfer and confirmed by immunoprecipitation of dithiobis succinimidylpropionate or disuccinimidyl suberate cross-linked 125I-labeled resting or activated T cells. The molecules that co-precipitated with either CD4 or CD8 and had an apparent kDa of 180 to 205 could be immunodepleted with anti-CD45 mAb. Furthermore, CD4 or CD8 immunoprecipitates from 96-h activated T cells contained significant levels of protein tyrosine phosphatase activity whereas corresponding immunoprecipitates from resting or recently activated T cells showed little protein tyrosine phosphatase activity. This association may allow CD45 to engage and dephosphorylate lck or another CD4- or CD8-associated substrate in order to reset the receptor complex to receive a new set of stimuli. Our observations suggest that synergistic signaling provided as a consequence of CD4 or CD8 association with the TCR after antigenic stimulation may develop on a different temporal scale than that observed after soluble anti-CD4+ anti-CD3 heteroconjugate antibody cross-linking.     

Specific CD45 isoforms differentially regulate T cell receptor signaling.

Multiple isoforms of T cell CD45 tyrosine phosphatase are expressed as a result of alternative RNA splicing among extracellular exons. To discern the presence and identity of distinct functions among CD45 isoforms, we compared thymic T cell activation responses by elevating expression of two CD45 isoforms normally found on quiescent T cells. We report that CD45RABC significantly increased CD4+ thymic T cell proliferation in both a mixed lymphocyte reaction and following anti-T cell receptor (TCR) antibody stimulation. Additionally, CD45RABC enhanced Ca2+ mobilization and phosphotyrosine accumulation, and suppressed the inhibitory effect of anti-CD4 antibodies. By contrast, CD45R0 did not enhance TCR signaling or phosphotyrosine levels in CD4+ thymic T cells and required a TCR co-stimulus to augment cellular proliferation. These studies provide genetic evidence that alternative CD45 isoforms are functionally distinct and disclose a unique mechanism by which T cell immunologic responsiveness can be modified.     

Activation of tyrosine phosphorylation in human T cells via the CD2 pathway. Regulation by the CD45 tyrosine phosphatase

In this study we compare the effect of CD3 and CD2 ligation on tyrosine kinase activation in human peripheral blood T cells. Using antiphosphotyrosine antibody to detect tyrosine phosphorylation of cellular substrates, we demonstrate that mAb stimulation of either CD3 or CD2 results in tyrosine phosphorylation of the TCR-zeta chain and 135- and 100-kDa proteins. However, differences are observed between CD3 and CD2 ligation; only the former results in rapid tyrosine phosphorylation of 72-, 65-, and 40-kDa substrates. Co-aggregation of CD2 and CD45, a tyrosine phosphatase, results in inhibition of intracellular calcium elevation and T cell proliferation. We demonstrate in this study that this manipulation also inhibits polyphosphoinositide hydrolysis and tyrosine phosphorylation of the 100-kDa substrate. The failure of tyrosine phosphorylation of the 100-kDa substrate is specific in that phosphorylation of the 135-kDa protein is not inhibited. Similar results are observed when CD2 and CD45 are independently cross-linked rather than co-aggregated. The observation that CD45 cross-linking alters tyrosine phosphorylation of T cell substrates and effects polyphosphoinositide hydrolysis is further evidence that tyrosine phosphorylation regulates early events in T cell activation including, perhaps, phospholipase C activity.     

The induction of T cell unresponsiveness by rapidly modulating CD3

The immunomodulatory effects of an IgM anti-CD3 mAb (38.1) were investigated. 38.1 was distinct from other anti-CD3 mAb, in that it was rapidly modulated from the cell surface in the absence of a secondary antibody. Although 38.1 induced an immediate increase in intracellular free calcium [Ca2+]i by highly purified T cells, it did not induce entry of the cells into the cell cycle in the absence of accessory cells (AC) or a protein kinase C-activating phorbol ester. Clearing of 38.1 from the surface of AC-depleted T cells, documented both by immunofluorescence and by functional activity, was rapid, with markedly reduced levels of initially bound mAb observed after a 1 to 2 h incubation at 37 degrees C and complete modulation noted after a 5-h incubation. Despite rapid modulation of 38.1, the T cells continued to express substantial amounts of surface CD3, suggesting there is a rapid rate of turnover of CD3 molecules on resting T cells. After modulation of 38.1 bound CD3, T cells were markedly inhibited in their capacity to respond to PHA. Inhibition could be overcome by culturing the cells with supplemental AC or IL-2. The inhibitory effects of 38.1 could be mimicked by briefly pulsing cells with the calcium ionophore, ionomycin, that had no effect on surface expression of CD3. 38.1- or ionomycin-pulsed cells were inhibited in their subsequent response to PHA even when exposures were carried out in the presence of EGTA to prevent increases in [Ca2+]i from extracellular sources. Inhibition could not be accounted for by an inability of the ionomycin-treated or 38.1-modulated T cells to increase [Ca2+]i in response to PHA. These studies demonstrate that a state of T cell nonresponsiveness can be induced by modulating CD3 with an anti-CD3 mAb in the absence of co-stimulatory signals. A brief increase in [Ca2+]i resulting from mobilization of internal calcium stores appears to be sufficient to induce this state of T cell nonresponsiveness.     

CD5 negatively regulates the T-cell antigen receptor signal transduction pathway: involvement of SH2-containing phosphotyrosine phosphatase SHP-1

The negative regulation of T- or B-cell antigen receptor signaling by CD5 was proposed based on studies of thymocytes and peritoneal B-1a cells from CD5-deficient mice. Here, we show that CD5 is constitutively associated with phosphotyrosine phosphatase activity in Jurkat T cells. CD5 was found associated with the Src homology 2 (SH2) domain containing hematopoietic phosphotyrosine phosphatase SHP-1 in both Jurkat cells and normal phytohemagglutinin-expanded T lymphoblasts. This interaction was increased upon T-cell receptor (TCR)-CD3 cell stimulation. CD5 co-cross-linking with the TCR-CD3 complex down-regulated the TCR-CD3-increased Ca2+ mobilization in Jurkat cells. In addition, stimulation of Jurkat cells or normal phytohemagglutinin-expanded T lymphoblasts through TCR-CD3 induced rapid tyrosine phosphorylation of several protein substrates, which was substantially diminished after CD5 cross-linking. The CD5-regulated substrates included CD3zeta, ZAP-70, Syk, and phospholipase Cgammal but not the Src family tyrosine kinase p56(lck). By mutation of all four CD5 intracellular tyrosine residues to phenylalanine, we found the membrane-proximal tyrosine at position 378, which is located in an immunoreceptor tyrosine-based inhibitory (ITIM)-like motif, crucial for SHP-1 association. The F378 point mutation ablated both SHP-1 binding and the down-regulating activity of CD5 during TCR-CD3 stimulation. These results suggest a critical role of the CD5 ITIM-like motif, which by binding to SHP-1 mediates the down-regulatory activity of this receptor.     

Novel redistribution of an intracellular pool of CD45 accompanies T cell activation

The major tyrosine phosphatase activity against angiotensin detected in membranes of the antigen-specific T cell hybridoma 2B4 is contained in the cytoplasmic tail of the CD45 molecule. When these cells are stimulated with either an antibody directed against the T cell antigen receptor or an activating anti-Thy-1 antibody, there is a rapid redistribution of CD45 in the cells. The redistribution can be observed in two ways: morphology and subcellular fractionation. Morphologic examination of resting cells reveals intense CD45 staining of the Golgi as well as surface staining. Upon activation the Golgi is rapidly cleared of CD45. This redistribution is specific for CD45 and is not observed for an intrinsic Golgi protein, mannosidase II, or a protein traversing the secretory pathway, the T cell receptor. In activated cells, in contrast to resting cells, approximately 30% of the total cellular CD45 is precipitated either at 280 x g or at 200,000 x g through a 2.2 M sucrose cushion after cell homogenization. This fraction is not accessible to cell surface labeling. CD45 redistribution does not require hydrolysis of phosphatidylinositides and cannot be reproduced by the addition of phorbol ester and calcium ionophore. It does require the presence of an intact functional T cell receptor on the cell surface. These studies suggest that the residence time of CD45 within an intracellular organelle can be acutely regulated by a signal mediated via the T cell receptor. This regulation may control access of this phosphatase to critical substrates.     

Induction of human T cell proliferation by a monoclonal antibody to CD5.

A mouse mAb, TS 43, which recognized the human CD5 molecule, was found to induce the proliferation of human peripheral blood T cells. TS 43 mAb precipitated from 125I-radiolabeled T cells a 67-kDa band, which comigrated with the 67-kDa band precipitated by the anti-CD5 mAb OKT1. Preclearing of cell lysates with OKT1 mAb abolished the capacity of TS 43 mAb to precipitate radiolabeled material from T cell lysates. Furthermore, a mouse T cell hybridoma transfected with human CD5 was stained by TS 43 mAb. T cell proliferation mediated by TS 43 mAb was monocyte dependent, and was accompanied by IL-2R expression and by IL-2 synthesis. T cell activation by TS 43 mAb involved a rise in intracellular calcium level (CA2+)i and was dependent on the expression of the TCR/CD3 complex because no rise in (Ca2+)i was observed in a TCR-beta-deficient Jurkat T cell mutant. This study indicates that CD5 should be added to the list of surface molecules that can signal T cells to proliferate.     

CD27, a member of the nerve growth factor receptor family, is preferentially expressed on CD45RA+ CD4 T cell clones and involved in distinct immunoregulatory functions.

CD27 is a disulfide-linked 120-kDa homodimer expressed on the majority of peripheral T cells at variable density that belongs to the recently defined nerve growth factor receptor family. mAb reactive with CD27 can either enhance or inhibit T cell activation, suggesting a crucial role in the process of T cell activation. We now show that CD27 is preferentially expressed on the CD45RA+CD45RO-CD29low subset of CD4 cells. CD27 expression on this subset is maintained for a prolonged period in culture after PHA activation. In contrast, CD45RA-CD45RO(+)-CD29high subset of CD4 cells express very low level of CD27, and its expression is lost within 2 wk after PHA activation. To further analyze the differential expression of CD27 on these reciprocal subsets of CD4 cells, we developed T cell clones by stimulating isolated CD4+CD45RA+ and CD4+CD45RO+ populations with PHA. T cell clones derived from cells originally CD45RA+ retained both CD45RA and CD27 expression, whereas T cell clones derived from cells originally CD45RO+ were CD45RA- and CD27-. In functional assays, IL-4 production could only be induced in CD45RA-CD27- CD4 clones by stimulation with PMA and ionomycin. Four of six CD45RA+ CD4 clones had suppressor activity in PWM-driven IgG synthesis, whereas five of six CD45RA- CD4 clones had helper activity. Of interest, the suppressor activity of CD45RA+CD27+ clones was partially blocked by pretreatment with anti-CD27 mAb (1A4). Anti-1A4 pretreatment of these T cell clones resulted in elevation of intracellular cAMP levels. Thus, CD27 appears to play a role in the function of CD45RA+CD27+ CD4 cells, and may be involved in suppressor activity of these cells at least in part via its effects on cAMP production.     

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.     

Cell surface comodulation of CD4 and T cell receptor by anti-CD4 monoclonal antibody

In our study we have used anti-CD4 mAb to investigate the cell surface association between CD4 and the Ag-specific TCR complex on mature peripheral T cells. Anti-CD4 mAb was administered in vivo and in vitro and its effects on CD4 and CD3 cell surface expression were determined. In vivo, anti-CD4 mAb reduced cell surface expression of its ligand, CD4, and secondarily also reduced cell surface expression of CD3/TCR on CD4+ splenic T cells. In vitro, multivalent cross-linking of CD4 by anti-CD4 mAb and either FcR+ cells or anti-Ig mAb also resulted in decreased surface expression of CD4 and specific comodulation of CD3/TCR. The secondary reduction in cell surface CD3/TCR expression induced by CD4 cross-linking could be pharmacologically disrupted by high doses of PMA, indicating that the comodulation of CD3 with CD4 was dependent upon intracellular mediators, possibly including protein kinase C. These results demonstrate that, in the presence of anti-CD4 mAb, CD4 is functionally associated with the CD3/TCR complex, and that this association is dependent upon the activity of intracellular mediators. Such intracellular mediators might induce the coordinate down-modulation of physically unassociated CD4 and CD3/TCR molecules, or, alternatively, might promote a physical interaction between CD4 and CD3/TCR molecules.     

CD2/LFA-3 ligation induces phospholipase-C gamma 1 tyrosine phosphorylation and regulates CD3 signaling.

Activation of T cells through the TCR/CD3 receptor complex with either specific Ag or antibody results in tyrosine phosphorylation of intracellular protein substrates and phosphatidylinositol-phospholipase C (PLC) signaling, leading to the generation of PI breakdown products and the mobilization of intracellular calcium. Stimulation of the T cell surface receptor CD2 similarly propagates early signals through phosphatidylinositol-PLC activation. Previous reports have shown that CD3 activation leads to tyrosine phosphorylation of the PLC isozyme PLC gamma 1. In this report, we investigated the potential similarity between CD3-induced signaling through PLC gamma 1 and that induced by CD2. We show that stimulation of CD2 receptors on T cells caused tyrosine phosphorylation of PLC gamma 1. Cross-linking of CD2 with CD3 receptors augmented the phosphorylation of PLC gamma 1 on tyrosine, whereas ligation of the CD45 tyrosine phosphatase with CD2 receptors prevented PLC gamma 1 tyrosine phosphorylation. T cells stimulated by ligation of CD2 with its counter-receptor in the form of a soluble LFA-3/Ig fusion protein cross-linked on the cell surface, resulted in a low, but detectable level of PLC gamma 1 phosphorylation with prolonged kinetics, whereas that induced by cross-linking with anti-CD2 was stronger but transient. Co-ligation of LFA-3/Ig with suboptimal concentrations of anti-CD3 resulted in profound augmentation of PLC gamma 1 tyrosine phosphorylation, mobilization of intracellular calcium and T cell proliferation. To explore the relationship between CD3- and CD2-stimulated signaling, T cells were desensitized through 1 h incubation with anti-CD3. CD3 receptor modulation potently down-regulated CD2-induced PLC gamma 1 tyrosine phosphorylation and calcium mobilization. In contrast, PMA or ionomycin treatment did not alter CD2-stimulated tyrosine phosphorylation of PLC gamma 1, suggesting that tyrosine kinase inhibition by CD3 receptor modulation was not caused by signaling events downstream of PLC gamma 1. Taken together, these results support the hypothesis that CD2 provides a potent co-stimulatory signal for CD3-induced T cell activation that is associated with tyrosine kinase(s) and PLC gamma 1.     

CD45 cross-linking regulates phospholipase C activation and tyrosine phosphorylation of specific substrates in CD3/Ti-stimulated T cells.

In lymphocytes, CD45 regulates the increase in cytoplasmic calcium concentration that occurs after receptor cross-linking. Here we show that T cell receptor complex (CD3/Ti)-mediated inositol phosphate production was inhibited by CD45 ligation in Jurkat cells. CD3/Ti signaling in normal T cells was also inhibited by CD45 ligation, but coupling of CD4 with CD3/Ti gave augmented calcium signals that were entirely resistant to the inhibitory effect of CD45. In contrast, CD3-induced T cell proliferation was suppressed by immobilized CD45 mAb even in the presence of CD4 mAb. The effect of CD45 and CD4 ligation on tyrosine phosphorylation during T cell activation was directly examined by immunoblotting with anti-phosphotyrosine. Using immobilized mAb, CD45 ligation suppressed the tyrosine phosphorylation of specific substrates induced by CD3/Ti stimulation, including almost complete suppression of 150-, 36-, and 35-kDa proteins and partial suppression of 76- and 80-kDa proteins. Other tyrosine-phosphorylated proteins induced by CD3/Ti stimulation, including 135- and 21-kDa proteins, were not suppressed by simultaneous ligation of CD3/Ti and CD45. Simultaneous ligation of CD3 and CD4 enhanced tyrosine phosphorylation of all substrates, but did not overcome the CD45-mediated suppression of tyrosine phosphorylation of the 35- and 36-kDa proteins. The CD45-mediated suppression of phospholipase C activation is therefore modulated by association with CD4 without altering the specific inhibition of tyrosine phosphorylation and T cell proliferation after co-ligation of CD45 and CD3/Ti.     

Mechanism of peripheral T cell activation by coengagement of CD44 and CD2.

A number of CD44 antibodies are known to augment peripheral T cell proliferation stimulated with suboptimal concentrations of activating pairs of CD2 mAb. These findings have implicated the CD44 adhesion receptor in the activation of peripheral T cells via CD2. We have investigated early events after CD44 and CD2 coengagement on peripheral T cells. CD44 and CD2 coengagement resulted in enhanced [Ca2+]i mobilization. However, the increase in [Ca2+]i mobilization did not occur until at least 3 min after CD2 and CD44 coengagement, suggesting that other events precede the elevation in [Ca2+]i. Using a T cell/fibroblast adhesion assay, we could demonstrate a dramatic increase in T cell adhesiveness after about 1 min after CD44 and CD2 coengagement. The increase in T cell adhesiveness was comparable to that induced by PMA. In the absence of antibodies or treatment with mAb directed to other T cell surface Ag, there was little if any adhesion between unstimulated peripheral T cells and fibroblasts. Enhancement of T cell adhesiveness through CD44 engagement was not mediated by a direct effect on lymphocyte-function associated Ag-3, the known ligand of CD2. However, cross-linking of CD44 resulted in epitopic modulation of CD2 as demonstrated by the increased expression of the T11(3) activation epitope. Furthermore, anti-CD44 could substitute for anti-T11(2) in the activation of peripheral T cells via CD2. These results suggest that CD44 ligation has profound effects on CD2-mediated events by inducing epitopic modulation of CD2.     

Impaired calcium mobilization in CD4+ and CD8+ T cells in a retrovirus-induced immunodeficiency syndrome, murine AIDS.

After infection with LP-BM5 murine leukemia viruses, susceptible strains of mice develop a severe and progressive immunodeficiency disease, termed murine AIDS (MAIDS), features of which include markedly impaired T cell response to mitogens or specific Ag stimulation and decreased production of IL-2. Since an elevation of intracellular calcium concentration resulting from binding of Ag to the TCR is associated with IL-2 production, T cells from mice either uninfected or infected with LP-BM5 murine leukemia viruses were examined by a calcium mobilization assay. Both CD4+ and CD8+ T cells from infected mice manifested impaired calcium mobilization responses upon in vitro stimulation with anti-CD3 mAb or Con A. The abnormalities appeared early after virus inoculation and showed no difference in time course between subsets of T cells. Frequencies of prestimulation calcium-positive cells among both CD4+ and CD8+ cells in mice with MAIDS were significantly higher than those for uninfected mice. These abnormalities were associated with presence of the MAIDS-inducing defective virus genome, but were not induced by infection of mice genetically resistant to development of MAIDS or with nonpathogenic helper murine leukemia virus, a virus component that induces high spontaneous proliferation of T cells, even in MAIDS-resistant mice.     

Reduced proliferation in T lymphocytes in aged humans is predominantly in the CD8+ subset, and is unrelated to defects in transmembrane signaling which are predominantly in the CD4+ subset

Peripheral blood lymphocytes (PBL) from elderly donors have a reduced proliferative response to phytohemagglutinin (PHA) and anti-CD3 monoclonal antibodies (mAb) compared to those from young donors. To examine whether this is due to intrinsic deficiencies in proliferative potential of T-cell subsets, we compared the growth of unsorted PBL vs sorted CD4+ or CD8+ CD11- cells after anti-CD3 mAb or PHA stimulation. Unsorted PBL of elderly donors (greater than 65 years) showed a significant decrease in proliferation compared to young donors (20-30 years) when stimulated with anti-CD3 mAb or PHA. Sorted CD4+ and CD8+ cells were grown in culture in the absence of accessory cells under optimized growth conditions (CD28 mAb, interleukin 2 and beta-mercaptoethanol present). CD4+ cells from elderly donors showed no reduced growth after anti-CD3 mAb stimulation and only slightly decreased growth after stimulation with PHA. CD8+ CD11- cells from elderly donors, however, showed a 20-30% reduction in the proportion of cells proliferating in response to the mitogens and up to 40% reduction in the rate of cell-cycle progression of the responding cells. We examined whether this reduced proliferation is related to decreased efficiency of signal transduction by comparing this to the mobilization of intracellular free calcium ([Ca2+]i) and calcium channel activity after stimulation with anti-CD3 mAb or PHA. [Ca2+]i was measured in CD4 and CD8 subsets of young and elderly donors using a flow cytometric assay with the dye indo-1. Compared to cells from young donors, CD4+ cells from elderly donors showed a [Ca2+]i response which was up to 26% lower after stimulation with CD3 and 10% lower after stimulation with PHA. This appeared to be related to decreased calcium channel activity in elderly donors, rather than mobilization of intracellular Ca2+ stores. CD8+ cells from elderly donors, however, had a slightly, but significantly, greater [Ca2+]i response to CD3 mAb and PHA than did cells from young donors. Since the age-dependent defect in proliferation is mainly in CD8+ cells, but the [Ca2+]i decline is predominantly in the CD4+ subset, these results suggest that the reduced proliferation of T cells from older donors is not related to decreased efficiency of transmembrane signal transduction.     

Mechanisms of immune memory. T cell activation and CD3 phosphorylation correlates with Ta1 (CDw26) expression

The Ta1 (CDw26) Ag distinguishes a subset of circulating T lymphocytes that is the major population proliferating to recall Ag challenge. Unlike receptors for growth factors such as IL-2 and transferrin, the Ta1 Ag is present on T cell lines and clones irrespective of cell cycle. The appearance of Ta1 on T cells that respond to recall Ag allowed us to investigate activation requirements that may be associated with T cell immune memory. Ta1+ peripheral blood T cells were induced to proliferate by mAb recognizing either the invariant chains of the TCR, or by pairs of mitogenic antibodies directed to the CD2 molecule. In contrast, Ta1- cells were not stimulated by these antibodies. In addition, Ta1-cells did not proliferate maximally after addition of the phorbol ester PMA in combination with the calcium ionophore Ionomycin, suggesting that the intracellular targets of these agents may not be fully active. Anti-CD3-induced elevation of intracellular calcium levels was equivalent in the two subpopulations, suggesting that calcium mobilization mechanisms were intact. In contrast, PMA-induced phosphorylation of TCR CD3 chains was significantly greater in Ta1+ cells as compared to Ta1- T cells. Taken together, our results indicate that Ta1 expression, which is associated with T cell activation and memory, may be causally related to TCR and CD2-mediated activation mechanisms. The PMA inducible TCR phosphorylation in Ta1+ memory cells associated with their increased ability to proliferate after CD3/TCR or CD2 stimulation suggests that intracellular phosphorylation events may be causally associated with T cell immune memory.     

The role of protein kinase C in transmembrane signaling by the T cell antigen receptor complex. Effects of stimulation with soluble or immobilized CD3 antibodies

Phorbol esters, such as phorbol myristate acetate (PMA), are known to be potent co-stimulants with calcium ionophores for activation of T lymphocytes. The most extensively studied intracellular effect of PMA is its ability to activate the cytoplasmic enzyme protein kinase C (pkC). Herein, we examined the role of pkC activation during T cell activation. During physiologic activation, this enzyme is activated by diacylglycerol which is generated through the hydrolysis of polyphosphoinositides. Therefore, we studied the activation of T lymphocytes induced by a synthetic diacylglycerol, dioctanoylglycerol. In contrast to PMA, this compound can be metabolized in T cells and presumably more closely mimics physiologic activation of pkC. Dioctanoylglycerol together with reagents that induce increases in intracellular free Ca2+ concentration, Ca2+ ionophores, or anti-cluster designation (CD)3 monoclonal antibodies (mAb) were able to induce interleukin 2 receptor expression and proliferation of T lymphocytes. Previous studies have demonstrated that the stimulation of T cells via the CD3/T cell antigen receptor complex by mAb against CD3 leads to an increase in cytoplasmic free Ca2+ and to an activation of pkC. Paradoxically, however, soluble CD3 antibodies do not cause proliferation of resting purified T cells. Inasmuch as immobilization of CD3 mAb has been shown to influence the agonist properties of such antibodies, we compared the ability of soluble and immobilized CD3 mAb to activate pkC. We demonstrated herein that soluble CD3 mAb cause only a very transient activation of pkC in the T cell leukemic line Jurkat. This pkC activation is markedly prolonged when Jurkat cells are stimulated with immobilized rather than soluble CD3 antibodies. These studies suggest that activation of pkC plays a major role in T cell activation and that the activation of pkC is influenced by the form in which CD3 mAb is presented to T cells.