Antigen receptor-mediated regulation of sustained polyphosphoinositide turnover in a human T cell line. Evidence for a receptor-regulated pathway for production of phosphatidylinositol 4,5-bisphosphate

Stimulation of the human T cell line, Jurkat, by the addition of monoclonal antibodies reactive with the T cell antigen receptor complex (CD3/Ti) leads to sustained increases in levels of inositol 1,4,5-trisphosphate. To investigate the possibility that the production of polyphosphoinositides is regulated during CD3/Ti stimulation, we studied Jurkat cells whose inositol phospholipids had been labeled to steady state with [3H]inositol, as well as Jurkat cells during nonequilibrium labeling with [32P]orthophosphate. The addition of CD3 monoclonal antibodies led to a 4-5-fold increase in [3H]inositol trisphosphate that was sustained for greater than 20 min. Within 60 s of CD3/Ti stimulation, [3H] phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P2) and [3H]phosphatidylinositol 4-phosphate (PtdIns4P) decreased by 65 and 35%, respectively. This change in [3H]PtdIns(4,5)P2 persisted for greater than 20 min. The decrease in [3H]PtdIns4P, however, was transient, and, after 5 min, the levels of [3H]PtdIns4P were comparable in stimulated and unstimulated cells. To examine the rate of flux through inositol phospholipids, we measured the CD3/Ti-stimulated changes in the ratio, 32P cpm/3H cpm, in each inositol phospholipid. CD3/Ti stimulation led to accelerated fluxes through PtdIns(4,5)P2 and phosphatidylinositol (PtdIns) that were maintained for greater than 20 min. After the initial 30 s, however, there was no detectable effect of anti-CD3 on flux through Ptsins4p. This observation suggested that, during CD3/Ti stimulation, production of PtdIns(4,5)P2 from PtdIns might occur via a small pool of PtdIns4P with a very high turnover. The existence of such a pool was established by determining that, in stimulated cells, the 32P-specific activity of the 1-position phosphate of PtdIns(4,5)P2 was 8-10-fold that of PtdIns4P. We conclude that, during the initial 60 s of CD3/Ti stimulation, there is a substantial depletion of cellular PtdIns(4,5)P2 and PtdIns4P. Thereafter, a CD3/Ti-regulated pathway generates PtdIns(4,5)P2 from PtdIns through a small, but highly labile, pool of PtdIns4P.     

HTLV-III large envelope protein (gp120) suppresses PHA-induced lymphocyte blastogenesis

The addition of inactivated preparations of purified human T cell lymphotropic virus (HTLV-III) was found to inhibit normal human lymphocyte phytohemagglutinin (PHA)-induced blastogenesis but had no effect on concanavalin A (Con A), pokeweek mitogen, or allogeneic stimulation. The inhibition was concentration-dependent and also dependent on adding the virus preparation before or at the same time as PHA. The CD4 molecule is the receptor for HTLV-III binding. Immunopurified large envelope protein (gp120) from HTLV-III was found to bind to the CD4 molecule and also inhibited PHA-induced lymphocyte blastogenesis. These results suggest that the gp120 viral protein may alter immune function by binding to the CD4 molecule, which in turn serves as an "off" signal to lymphocyte response to PHA stimulation. Alternatively, by binding to the CD4 molecule, gp120 may interfere with the interaction of this molecule with class II histocompatibility antigens on accessory cells, thus selectively suppressing PHA response.     

Sustained diacylglycerol accumulation resulting from prolonged G protein-coupled receptor agonist-induced phosphoinositide breakdown in hepatocytes

Studies in various cells have led to the idea that agonist-stimulated diacylglycerol (DAG) generation results from an early, transient phospholipase C (PLC)-catalyzed phosphoinositide breakdown, while a more sustained elevation of DAG originates from phosphatidylcholine (PC). We have examined this issue further, using cultured rat hepatocytes, and report here that various G protein-coupled receptor (GPCR) agonists, including vasopressin (VP), angiotensin II (Ang.II), prostaglandin F2alpha, and norepinephrine (NE), may give rise to a prolonged phosphoinositide hydrolysis. Preincubation of hepatocytes with 1-butanol to prevent conversion of phosphatidic acid (PA) did not affect the agonist-induced DAG accumulation, suggesting that phospholipase D-mediated breakdown of PC was not involved. In contrast, the GPCR agonists induced phosphoinositide turnover, assessed by accumulation of inositol phosphates, that was sustained for up to 18 h, even under conditions where PLC was partially desensitized. Pretreatment of hepatocytes with wortmannin, to inhibit synthesis of phosphatidylinositol 4-phosphate and phosphatidylinositol 4,5-bisphosphate (PIP2), prevented agonist-induced inositol phosphate and DAG accumulation. Upon VP stimulation the level of PIP) declined, but only transiently, while increases in inositol 1,4,5-trisphosphate (InsP3) and DAG mass were sustained, suggesting that efficient resynthesis of PIP2 allowed sustained PLC activity. This was confirmed when cells were pretreated with wortmannin to prevent resynthesis of PIP2. Furthermore, metabolism of InsP3 was rapid, compared to that of DAG, with a more than 20-fold difference in half-life. Thus, rapid metabolism of InsP3 and efficient resynthesis of PIP2 may account for the larger amount of DAG generated and the more sustained time course, compared to InsP3. The results suggest that DAG accumulation that is sustained for many hours in response to VP, Ang.II, NE, and prostaglandin F2alpha in hepatocytes is mainly due to phosphoinositide breakdown.     

Inositol lipids and phosphates in the regulation of the growth and differentiation of haemopoietic and other cells

Stimulation of phosphatidylinositol 4,5-bisphosphate hydrolysis is an important signalling reaction involved in the responses of cells to some, but not all, stimuli that promote cell proliferation. Active agents in this regard include antigens activating T and B lymphocytes, angiotensin (employing a receptor encoded by the mas oncogene), bombesin and platelet-derived growth factor PDGF). However, accumulating evidence suggests that inositol lipids and phosphates also have other roles in the regulation of cell growth and differentiation. Growth factor receptors that encode tyrosine kinases (such as that for PDGF) activate a kinase that synthesises phosphatidylinositol 3-phosphate, a novel lipid, and loss of this kinase-activating function abolishes growth-promoting activity. Human interleukin-4, a lymphokine that activates B lymphocytes, appears to employ phosphatidylinositol 4,5-bisphosphate hydrolysis as a brief initial signal that is followed by a sustained rise in cyclic adenosine monophosphate (cAMP): both signals are needed for the successful induction of the surface antigen CD23. Moreover, the same inositol lipid signalling pathway as is employed by antigen-stimulated mature T lymphocytes to provoke proliferation may be redeployed in immature T cells to trigger their elimination when they encounter self-antigens. Finally, studies of HL60 promyelocytic cells have shown that these cells contain high concentrations of inositol 3,4,5,6-tetrakisphosphate, 1,3,4,5,6-pentakisphosphate and hexakisphosphate, three inositol polyphosphates that are probably formed independently of inositol lipid metabolism. When these cells are induced to differentiate either towards neutrophils (in the presence of dimethylsulphoxide) or macrophages (in phorbol myristate acetate), cessation of growth and acquisition of differentiated characteristics are accompanied by large and different changes in the concentrations of these inositol phosphates that may be characteristic of these two pathways of differentiation.     

Stimulation of AIDS lymphocytes with calcium ionophore (A23187) and phorbol ester (PMA): studies of cytoplasmic free Ca, IL-2 receptor expression, IL-2 production, and proliferation

We have studied whether the decreased lymphocyte proliferative responses of AIDS lymphocytes to stimulation by mitogens and antigens may be overcome when challenged with a combination of calcium ionophore A23187 and phorbol ester PMA. Comparison of the proliferative response of lymphocytes from nine patients with AIDS with the response of lymphocytes from nine control subjects showed that the response of AIDS lymphocytes was severely decreased when stimulated with PHA and no further response could be achieved by stimulation with A23187/PMA. On the other hand, no significant difference between the PHA-induced rise of cytoplasmic free calcium concentration ([Ca2+]1) in normal and AIDS lymphocytes was observed. The percentage of cells expressing IL-2 receptors (CD25) was also normal both after addition of PHA and after addition of A23187/PMA and the expression was normal on both CD4 and CD8 cells. The production of IL-2 in normal lymphocytes stimulated with A23187/PMA was 33 times higher than that after stimulation with PHA. In AIDS lymphocytes the production of IL-2 induced by all activators was severely decreased compared to control subjects, although the production of IL-2 after stimulation with A23187/PMA was higher than that in control lymphocytes after stimulation with PHA. The present study shows that a direct activation of protein kinase C combined with mobilization of cytoplasmic calcium does not overcome the lymphocyte proliferative deficiency of AIDS lymphocytes.     

HIV-induced immunodeficiency. Relatively preserved phytohemagglutinin as opposed to decreased pokeweed mitogen responses may be due to possibly preserved responses via CD2/phytohemagglutinin pathway

We studied the proliferative response of PBL to the mitogens PHA and PWM and Candida albicans Ag in 301 HIV seropositive homosexual men, of whom 55 had AIDS. The responses to PHA were reduced only in the clinically ill HIV seropositive subjects. In contrast, the responses to PWM were profoundly reduced in most HIV seropositive subjects including the asymptomatic group. Further analysis of 16 HIV seropositive subjects showed that the proliferative responses were reduced in both CD4 and CD8 T cell subsets. A total of 15 HIV seropositive individuals with low responses to PWM, of whom seven had AIDS and eight controls were chosen for the following studies. Expression of T3, Ti, delta receptors, and CD2 was investigated and showed an increased percentage of CD2 receptors positive cells in HIV seropositive subjects without AIDS. The proliferative responses of PBL to stimulation with PHA, PWM, antibodies to CD3, or antibodies to CD2 were investigated and showed significant correlation in controls, whereas in contrast, only the responses to PHA and CD2ab correlated in patients with AIDS. The proliferative responses to CD2ab and CD3ab in controls were larger than the responses to both PHA and PWM. In patients, these responses were less suppressed than the responses to PWM indicating that stimulation with mitogens is more complex than a simple stimulation of Ti/T3 and CD2 receptors. Further investigations were done on resting T cells, i.e., lymphocytes depleted of macrophages and pre-activated cells. Addition of PHA to these cells resulted in preactivation with expression of IL-2R (CD25) but not in proliferation. In contrast, addition of PHA plus SRBC, which bind to the CD2 receptors caused IL-2R expression, IL-2 production, and proliferation. Addition of PWM + SRBC did not result in proliferation. A comparison of the responses to PHA + SRBC of resting T cells from 26 HIV seropositive individuals, of whom seven had AIDS and 12 seronegative controls, showed that these responses were normal or only slightly decreased in the 19 seropositive men without AIDS whereas it was decreased in AIDS patients. Nevertheless, all AIDS patients showed clear-cut responses in this assay. Thus, the discrepancy between responses to PHA and PWM may be explained by an at least partially preserved function of the PHA/CD2-dependent pathway. We suggest that the defect induced by the HIV infection primarily concerns T3/Ti-induced responses.     

Unsaturated fatty acids suppress interleukin-2 production and transferrin receptor expression by concanavalin A-stimulated rat Iymphocytes

The proliferation of T-lymphocytes is dependent upon their ability to synthesize and secrete the cytokine, interleukin-2, and to express cell surface receptors for interleukin-2 and transferrin. We have previously reported that certain fatty acids inhibit mitogen-stimulated T-lymphocyte proliferation. We now report that unsaturated fatty acids decrease the concentration of interleukin-2 in the culture medium of such cells by up to 45%. This suggests that unsaturated fatty acids inhibit lymphocyte proliferation by suppressing interleukin-2 production. However, lymphocyte proliferation was only partially restored by addition of exogenous interleukin-2 to cell culture medium in the presence of unsaturated fatty acids, indicating that these fatty acids also affect other processes involved in the control of proliferation. Saturated fatty acids, which also inhibit lymphocyte proliferation, did not affect the interleukin-2 concentration in the culture medium suggesting a different mechanism for their action. Neither saturated nor unsaturated fatty acids affected the expression of the interleukin-2 receptor by mitogenstimulated lymphocytes. In contrast, unsaturated fatty acids decreased expression of the transferrin receptor by up to 50%. These observations suggest that the mechanism by which unsaturated fatty acids inhibit lymphocyte proliferation involves suppression of interleukin-2 production and of transferrin receptor expression. The mechanism for the inhibitory action of saturated fatty acids is not clear.     

Human leptin enhances activation and proliferation of human circulating T lymphocytes

Leptin is an adipocyte-secreted hormone that centrally regulates weight control. However, leptin receptor is expressed not only in the central nervous system, but also in other systems such as reproductive and hematopoietic tissues. Human leptin has previously been shown to enhance cytokine production by murine peritoneal macrophages and human circulating monocytes. In this paper we have assessed the presence of leptin receptors in peripheral human T lymphocytes and we have studied their functional role. Both CD4(+) and CD8(+) T lymphocytes express leptin receptors. Moreover, we show that human leptin dose-dependently enhances proliferation and activation of human circulating T lymphocytes when they are costimulated by PHA or Con A. Leptin alone was not able to activate T lymphocytes. To confirm a direct effect of leptin on T lymphocytes, monocytes were extracted by adhesion to culture flasks. The early activation surface marker CD69 was then induced in both CD4(+) and CD8(+) T lymphocytes after 8 h stimulation with PHA or Con A. Leptin dose-dependently enhanced stimulated CD69 expression. Moreover, leptin dose-dependently enhanced the expression of the late activation markers CD25 and CD71 in both CD4(+) and CD8(+) T lymphocytes after 48 h stimulation with PHA or Con A. Finally, we have found that leptin modulates CD4(+) T lymphocyte activation toward Th1 phenotype by stimulating the synthesis of IL-2 and IFN-gamma. These results demonstrate the presence of the leptin receptor in human circulating CD4(+) and CD8(+) T lymphocytes and a functional role of leptin as a modulator (enhancer) of lymphocyte stimulation with a shift toward Th1 cytokine-production profile. This function of leptin may have some relevance in the pathophysiology of immunologic alterations related to obesity.     

T lymphocytes and their CD4 subset are direct targets for the inhibitory effect of calcitriol

We studied the direct effects of the hormone calcitriol on the activation and proliferation of pure T lymphocytes and their subsets. Calcitriol inhibited the proliferation of T lymphocytes stimulated in the absence of monocytes with phytohemagglutinin (PHA) and either a monocytic culture supernatant or a combination of monocyte-derived interleukin 1 and interleukin 6. This inhibition was not influenced by the concentration of the stimulating agents. The minimal effective concentration of calcitriol was 10(-10) M. In contrast, the interleukin 2 (10 U/ml)-driven growth of PHA-stimulated T lymphocytes was not significantly altered by calcitriol at 10(-8) M. The hormone had also no influence on the T lymphocyte proliferation induced by a combination of PHA and the anti-CD28 monoclonal antibody 9.3. Pure T lymphocytes, after incubation for 5 days with PHA and monocytic factors, expressed a high level of transferrin receptors. This phenomenon was strongly suppressed on both CD4 and CD8 subsets when 10(-8) M calcitriol had been present during the culture. Moreover, the proliferation of pure CD4 cells was directly inhibited by calcitriol in similar conditions as for unseparated T lymphocytes. We conclude that T lymphocytes and their CD4 subset are direct targets for the inhibitory effect of calcitriol.     

The control of transferrin receptor synthesis in mitogen-stimulated human lymphocytes

Human lymphocytes cultured in the presence of the plant mitogenic lectin phytohemagglutin in (PHA) become activated and leave the G0 phase of the cell cycle. In the presence of PHA and lymphokines produced in situ the cells will enter S phase and undergo cell division. We have determined the time course of appearance for the receptor for transferrin as an initial attempt to understand the molecular mechanisms regulating the onset of lymphocyte differentiation and proliferation in the 48 h following PHA addition. Using three different assay methods we have shown that the increase in the number of surface receptor molecules is due to the accumulation of newly synthesized receptor and not to the redistribution of a previously existing pool of receptor molecules. The total amount of transferrin receptor increased at least four-fold. In vitro translation of RNA from activated lymphocytes indicates that the new receptor synthesis is due, at least in part, to increased availability of mRNA encoding the transferrin receptor.     

Characterization of an actin-myosin head interface in the 40-113 region of actin using specific antibodies as probes.

A method of membrane permeabilization of T lymphocytes with the bacterial cytotoxin streptolysin O has allowed the effect of guanine nucleotide analogues on phosphatidylinositol metabolism and protein kinase C (PKC) activation to be investigated. The data demonstrate that, in permeabilized cells, phosphorylation of the gamma subunit of the CD3 antigen can be induced in response to the PKC activator phorbol 12,13-dibutyrate, the polyclonal mitogen phytohaemagglutinin (PHA) and the stimulatory guanine nucleotide analogue guanosine 5'-[gamma-thio]triphosphate (GTP[S]). Application of a pseudo-substrate inhibitor of PKC indicated that CD3gamma-chain phosphorylation induced in response to all three agonists was mediated by PKC. PHA and GTP[S] also stimulated inositol phospholipid turnover and inositol phosphate accumulation. The kinetics and concentration-dependence of PHA-induced inositol phospholipid hydrolysis correlated with PHA-induced CD3gamma phosphorylation, suggesting that PHA may regulate CD3gamma phosphorylation via diacylglycerol produced as a consequence of inositol phospholipid hydrolysis. However, there was an inconsistency in that PHA induced greater (greater than 200%) levels of inositol phospholipid turnover than did GTP[S], but much weaker (less than 50%) levels of CD3-antigen phosphorylation. There was also a discrepancy between GTP[S] effects on phosphatidylinositol turnover and PKC activation, in that the half-maximal GTP[S] concentration for inositol phosphate production and CD3gamma-chain phosphorylation was 0.75 microM and 75 microM respectively. Moreover, 10 microM-GTP[S] induced maximal inositol phosphate production, but only 10% of maximal CD3gamma-chain phosphorylation. The data are consistent with the idea that other signal-transduction pathways, in addition to those involving inositol phosphate production, exist for the regulation of PKC in T lymphocytes.     

Suppression of T lymphocyte function by measles virus is due to cell cycle arrest in G1

Measles virus suppresses T lymphocyte functions in vitro. When measles virus-infected T lymphocytes are stimulated with PHA or 12-O-tetradecanoylphorbol-13-acetate, plus calcium ionophore, the cells secrete IL-2 and express the IL-2R or Tac Ag to a similar extent as uninfected cells, yet proliferation is reduced by 50 to 90%. Stimulated infected T cells also express the cell surface activation Ag 4F2, transferrin R, and HLA-DR. The secretion of IFN-gamma by infected T cells in response to PHA is not suppressed at 24 to 72 h after stimulation. Total RNA synthesis at 48 and 72 h after stimulation is reduced in infected T lymphocytes. Infectious measles virus progeny are produced during this interval. Thus infected T lymphocytes can become activated in response to mitogenic stimuli and the cells support efficient viral replication before the block in cell proliferation.     

Mathematical model of phosphatidylinositol-4,5-bisphosphate hydrolysis mediated by epidermal growth factor receptor generating diacylglycerol

Phosphatidylinositol-4,5-bisphosphate (PIP2) is hydrolyzed in response to the tyrosine phosphorylation of the epidermal growth factor receptor (EGFR) and plays an important role in regulating cell proliferation and differentiation through the generation of second messengers diacylglycerol (DAG) and trisphosphate inositol (IP3) which lead to the activation of protein kinase C (PKC) and increased levels of intracellular calcium, respectively. In the paper, a mathematical model was established to simulate the accumulation of DAG due to PIP2 hydrolysis mediated by EGFR. Molecular mechanisms between DAG, PIP2, EGFR and phosphatidylinositol transfer protein (PITP) were explained successfully, and positive cooperativity which existed between phospholipase C-gamma1 (PLC-gamma1) and PIP2 was also explained. In the model the effects of parameters on simulation of PIP2 hydrolysis were analyzed and the efficacies of some molecular intervention strategies were predicted. To test the coherence between the model and the biological response to epidermal growth factor (EGF) in cells, the levels of DAG and the tyrosine phosphorylation-EGFRs in NIH3T3 mouse embryonic fibroblast (MEF) were determined by biochemical experiments which showed that the accumulation of DAG was a sigmoidal function of phosphorylation-EGFR concentration, and the consistency between the mathematical model and experimental results was confirmed. In brief, this mathematical model provided a new idea for the further study of the dynamic change of biological characteristics in inositol phospholipid hydrolysis, predicting the efficacy of molecular intervention and the relationship between the metabolisms of inositol phospholipid and other signal transduction pathways.     

Inhibitors of arachidonic acid lipoxygenase impair the stimulation of inositol phospholipid hydrolysis by the T lymphocyte mitogen phytohaemagglutinin

Piriprost and nordihydroguiaretic acid (NDGA), specific inhibitors of arachidonate lipoxygenase, inhibited phytohaemagglutinin (PHA)-stimulated breakdown of inositol lipids in human T lymphocytes. The dual inhibitors eicosatetraynoic acid (ETYA) and BW 755C, which inhibit both lipoxygenase and cyclooxygenase, also had similar actions, whereas indomethacin and acetylsalicyclic acid, which inhibit cyclooxygenase alone, did not. The effects of lipoxygenase inhibitors and dual inhibitors were reversible. These agents did not inhibit phosphatidylinositol-4,5-bisphosphate-specific phospholipase C (PIP2-PLC) in vitro. Bromophenacyl bromide, and irreversible inhibitor of phospholipase A2, also abolished PHA-stimulated inositol lipid breakdown without affecting PIP2-PLC in vitro. The results are consistent with a role for the PHA-stimulated generation of arachidonic acid and its conversion to lipoxygenase metabolites (e.g. leukotrienes and/or hydroxyeicosatetraenoic acids) as intermediate steps in the signal transduction pathway between cell-surface mitogen receptors and the stimulation of PIP2-PLC in lymphocytes.     

Functional properties of the 50 kd protein associated with the E-receptor on human T lymphocytes: suppression of IL 2 production by anti-p50 monoclonal antibodies

Monoclonal antibody 9.6 is specific for a 50 kd T cell surface protein (p50) associated with the sheep erythrocyte (E)-receptor on human T lymphocytes. This antibody interferes with many T cell functions. We have examined the effect of antibody 9.6 on lymphocyte proliferation and interleukin 2 (IL 2) production triggered by mitogens, soluble antigens, and alloantigens to elucidate the mechanism(s) of its immunosuppressive action. At concentrations as low as 50 ng/ml, 9.6 suppressed lymphocyte proliferation and the elaboration of IL 2 by T cells stimulated by PHA, alloantigens, or low concentrations of the phorbol ester TPA (less than or equal to ng/ml). Furthermore, in cultures stimulated by a combination of PHA plus TPA, 9.6 did not inhibit the acquisition of IL 2 receptors but inhibited proliferation and IL 2 production. Immunoaffinity-purified IL 2 completely restored lymphocyte proliferation in cultures inhibited by 9.6. Studies of kinetics of inhibition by 9.6 showed that this antibody inhibited lymphocyte proliferation induced by PHA, alloantigen, and PPD even when added at 24, 48, and 72 hr, respectively, after the initiation of these cultures, suggesting that 9.6 does not block lectin binding or antigen recognition by T cells and that it can inhibit lymphocyte proliferation even after cells have undergone one or more rounds of cell division. A dose-response analysis of lymphocyte proliferation induced by PHA or by TPA demonstrated that the degree of inhibition by 9.6 decreased with increasing concentrations of these mitogens. Antibody 9.6 did not inhibit lymphocyte response induced by optimal concentrations of PHA (50 to 100 micrograms/ml; PHA-M) but inhibited proliferation of maximally induced lymphocytes by using a synergistic combination of low concentrations of PHA (5 micrograms/ml, PHA-M) plus TPA (1 ng/ml). Taken together, these findings indicate that 1) 9.6 inhibits lymphocyte proliferation by affecting IL 2 production, 2) 9.6 does not inhibit the acquisition of 9.6 receptors induced by a synergistic combination of PHA plus TPA, and 3) p50 molecules may be involved in multiple pathways of T cell activation.     

Costimulation of T cell receptor/CD3-mediated activation of resting human CD4+ T cells by leukocyte function-associated antigen-1 ligand intercellular cell adhesion molecule-1 involves prolonged inositol phospholipid hydrolysis and sustained increase of intracellular Ca2+ levels.

Activation of resting human CD4+ T cells mediated by mAb ligation of the TCR/CD3 complex requires costimulatory signals to result in proliferation; these can be provided by intercellular cell adhesion molecule-1 (ICAM-1, CD54) a natural ligand of leukocyte function-associated Ag-1 (LFA-1, CD11a/CD18). We analyzed early signaling events involved in T cell activation to determine the contribution by the LFA-1/ICAM-1 interaction. We studied in detail the hydrolysis of phosphatidylinositol(4,5)bisphosphate and intracellular levels of free Ca2+ during stimulation with beads coated with the CD3 mAb OKT3 alone or in combination with purified ICAM-1 protein. Our investigations show no response to LFA-1/ICAM-1 alone, but that costimulation by LFA-1/CAM-1 interaction induces prolonged inositol phospholipid hydrolysis (up to 4 h), resulting in generation of both inositol(1,4,5)phosphate3 and inositol(1,3,4,5)phosphate4 and their derivatives. Based on studies with cycloheximide, this costimulatory effect of prolonged inositol phospholipid hydrolysis appears dependent in part on de novo protein synthesis. A sustained increase in intracellular levels of free Ca2+ level is also observed after LFA-1/ICAM-1 costimulation, which is at least partly dependent on extracellular sources of Ca2+. Kinetic studies indicate that costimulation requires a minimal period of 4 h of LFA-1/ICAM-1 interaction to provide maximal costimulation for OKT3-mediated T cell proliferation. Thus, the necessary costimulation required for OKT3-mediated proliferation in this model system may be provided by an extended LFA-1/ICAM-1 interaction that in combination with OKT3 mAb leads to signal-transducing events, resulting in prolonged phospholipase C activation and phosphatidylinositol(4,5)bisphosphate hydrolysis, and a sustained increase in intracellular levels of free Ca2+.     

Stimulation of human platelets with low concentrations of thrombin: evidence for equimolar accumulation of inositol trisphosphates and phosphatidic acid

1. Gel-filtered human platelets prelabeled with [32P]Pi or [3H]glycerol were exposed to 0-0.3 U/ml of thrombin and analyzed for radioactivities and masses in the phosphoinositides, inositol trisphosphates (IP3), phosphatidic acid (PA) and diacylglycerol (DAG) at 15 and 180 sec of stimulation. 2. At thrombin concentrations below 0.1 U/ml, PA and IP3 accumulated in equimolar amounts. 3. The production and disappearance of the metabolites of the polyphosphoinositide cycle was balanced during 180 sec of stimulation with 0.03-0.1 U/ml of thrombin. 4. Under these conditions no increase in [3H]DAG or [3H]monoacylglycerol could be detected. 5. The data indicate that all DAG is converted to PA and support our conclusion that phosphatidylinositol 4,5-bisphosphate represents the major source for production of DAG upon stimulation of human platelets with low concentrations of thrombin.     

Human T cell activation by OKT3 is inhibited by a monoclonal antibody to CD44.

The CD44 molecule, also known as Hermes lymphocyte homing receptor, human Pgp-1, and extracellular matrix receptor III, has been shown to play a role in T cell adhesion and activation. Specifically, anti-CD44 mAb block binding of lymphocytes to high endothelial venules, inhibit T cell-E rosetting, and augment T cell proliferation induced by the CD2 or CD3-TCR pathways. We have characterized an anti-CD44 mAb (212.3) which immunoprecipitates a 90-kDa protein and is specific for CD44 as shown by peptide mapping and antibody competition studies. Interestingly, our studies with 212.3 demonstrate that this CD44-specific mAb completely inhibits T cell proliferation stimulated by the anti-CD3 mAb, OKT3. Inhibition is not a result of reduced cell viability, but is associated with 1) inhibition of IL-2 production, 2) inhibition of IL-2R expression, and 3) inhibition of OKT3-mediated increases in intracellular Ca2+ levels. In addition, 212.3 does not inhibit proliferation by the T cell mitogens PHA or PWM nor does it inhibit proliferation in a mixed lymphocyte reaction. Similar to other anti-CD44 mAb, 212.3 also augments T cell proliferation induced by mAb directed against the T11(2) and T11(3) epitopes of CD2. Thus, these studies describe a novel CD44-specific mAb (212.3) that inhibits T cell activation by OKT3 by blocking early signal transduction. Furthermore, these studies suggest that "receptor cross-talk" between the CD3-TCR complex and CD44 may regulate T cell activation.     

Cholera toxin partially inhibits the T-cell response to phytohaemagglutinin through the ADP-ribosylation of a 45 kDa membrane protein.

This study examines the influence of cholera toxin (CT) on T lymphocyte activation by the mitogenic lectin phytohaemagglutinin (PHA). CT suppressed lectin-induced [3H]thymidine uptake in a dose-dependent fashion and acted synergistically with PHA in the generation of intracellular cyclic AMP. The toxin was assumed to act on Gs, because it also stimulated ADP-ribosylation of a 45 kDa membrane protein in vitro; no additional substrates were seen. The inhibitory effect of the adenylate cyclase/cyclic AMP pathway was shown to be directed at a concomitant stimulatory pathway, namely inositol phospholipid turnover. Lectin-stimulated 32P incorporation into both phosphatidylinositol as well as its 4,5-biphosphate derivative was depressed in the presence of CT or exogenous dibutyryl cyclic AMP. This, in turn, was associated with reduced activation of C-kinase as determined by decreased lectin-induced translocation from the cytosol to the surface membrane. These results indicate that Gs probably acts as a transducer between the PHA receptor and adenylate cyclase and may give rise to an exaggerated adenylate cyclase response in the presence of CT. It would seem as if reduction in inositol phospholipid turnover is related to the elevation of cyclic AMP rather than a CT effect on a putative transducer which acts directly on phospholipase C. Our study does not exclude the existence of non-CT-sensitive transducers in this capacity.     

Breakdown of phosphatidylinositol 4,5-bisphosphate in a T-cell leukaemia line stimulated by phytohaemagglutinin is not dependent on Ca2+ mobilization.

Addition of phytohaemagglutinin (PHA) to the [32P]Pi-prelabelled JURKAT cells, a human T-cell leukaemia line, resulted in a decrease of [32P]phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P2] to about 35% of the control value. The decrease was almost complete within 30s after the PHA addition. This decrease was followed by an increase in the 32P-labelling of phosphatidic acid (maximally 2.8-fold at 2 min). The stimulation of myo-[2-3H]inositol-prelabelled JURKAT cells by PHA induced an accumulation of [2-3H]inositol trisphosphate in the presence of 5 mM-LiCl. The result indicates hydrolysis of PtdIns (4,5)P2 by a phospholipase C. The PHA stimulation of JURKAT cells induced about 6-fold increase in the cytosolic free Ca2+ concentration, [Ca2+]i, which was reported by Quin-2, a fluorescent Ca2+ indicator. Studies with partially Ca2+-depleted JURKAT cells, with the Ca2+ ionophore A23187, and with 8-(diethylamino)-octyl-3,4,5-trimethoxybenzoate indicate that the breakdown of PtdIns(4,5)P2 is not mediated through changes of [Ca2+]i. These results therefore indicate that the PHA-induced breakdown of PtdIns(4,5)P2 in JURKAT cells is not dependent on the Ca2+ mobilization.