Regulation of interleukin-2 gene enhancer activity by the T cell accessory molecule CD28

Phosphorylation of each of the distal three tyrosines of the CD28 cytoplasmic tail is required for CD28-induced T cell IL-2 secretion

Signaling by the CD28 T cell costimulatory receptor is known to involve recruitment and activation of phosphatidylinositol 3-kinase (PI3-kinase) which is dependent upon phosphorylation of tyrosine 173 of the CD28 cytoplasmic tail, present in a YMNM motif. However, whether this phosphorylation is required for CD28 costimulation and whether or not phosphorylation of any of the other three tyrosines of the CD28 cytoplasmic tail (tyrosines 188, 191 and 200) is also important for CD28 induced responses is unclear. To address this we examined the ability of chimeric receptors, consisting of the extracellular plus transmembrane membrane domain of human CD8 alpha linked to different mutated human CD28 cytoplasmic tails, to induce IL-2 secretion in Jurkat T leukemia cells in the presence of PMA and ionomycin. A receptor in which tyrosine 173 of the CD28 tail was mutated to phenylalanine was able to induce IL-2. By contrast, receptors which contained single tyrosine 188, 191 or 200 to phenylalanine substitutions were unable to induce IL-2. These results imply that in this system phosphorylation of tyrosine 173 and hence activation of PI3-kinase is not required for CD28 induced IL-2 secretion. Further, they imply that phosphorylation of each of tyrosines 188, 191 and 200 is necessary for this response. Despite an apparent requirement for phosphorylation of all three of these tyrosines, however, receptors which contain tyrosine only at positions 191 or 200 and a truncated receptor which does not contain tyrosine 200 induce normal IL-2. These last findings, therefore, illustrate the complexity of CD28 mediated activation signals.

Costimulation regulates the kinetics of interleukin-2 response to bacterial superantigens

The aim of this study was to investigate the mechanisms by which B7-related costimulatory molecules (CD80, CD86) increase T-cell responsiveness to extracellular ligands. As a model study, the in vitro response of purified splenic CD4+ T cells to a bacterial superantigen, SEB, was characterized. Previous analysis of this experimental model led us to conclude that expression of B7-related molecules is strictly required in order to activate CD4+ T cells in the presence of bacterial superantigens. In the present report, we demonstrate that antigen-presenting cell-derived costimulatory signals regulate the kinetics of interleukin-2 (IL-2) production by SEB-activated splenic CD4+ T cells. Indeed, experiments performed with purified subpopulations of antigen-presenting cells and using B7-transfected cell lines indicated that increased levels of CD80 and/or CD86 cell surface expression is associated with a faster kinetics of IL-2 production in response to SEB. Accordingly, blocking of CD80 or CD86-derived signals by specific monoclonal antibodies led to a slower kinetics of IL-2 production in response to SEB. Thus these data demonstrate that similar strength of signal through the T-cell receptor can lead to immune responses displaying distinct kinetics depending on the level of costimulatory ligands on APC.

Minimal residual disease: detection, clinical relevance, and treatment strategies

Improvement of established treatment strategies for cancer has resulted in increased survival times for patients with malignancies. However, success of surgery, chemotherapy, and radiotherapy is limited, as even combined and repeated therapy regimens and high-dose chemotherapy can only reduce tumor burden by several logarithmic steps and are not able to completely eradicate all neoplastic cells. If clinically complete remission is achieved--that is, if no sign of the tumor is detectable by standard diagnostic procedures, remaining minimal residual disease (MRD) can eventually give rise to clinically manifest relapse. More sensitive methods are, therefore, necessary to detect single tumor cells for exact staging, to assess the metastatic potential of an individual tumor, to evaluate the sensitivity to prior therapy, and to detect MRD-positive patients with remaining malignant cells who are at higher risk for relapse. Novel treatment approaches must be created to eradicate minimal residual disease after conventional therapy.

A dynamic assembly of diverse transcription factors integrates activation and cell-type information for interleukin 2 gene regulation

The interleukin 2 (IL-2) gene is subject to two types of regulation: its expression is T-lymphocyte-specific and it is acutely dependent on specific activation signals. The IL-2 transcriptional apparatus integrates multiple types of biochemical information in determining whether or not the gene will be expressed, using multiple diverse transcription factors that are each optimally activated or inhibited by different signaling pathways. When activation of one or two of these factors is blocked IL-2 expression is completely inhibited. The inability of the other, unaffected factors to work is explained by the striking finding that none of the factors interacts stably with its target site in the IL-2 enhancer unless all the factors are present. Coordinate occupancy of all the sites in the minimal enhancer is apparently maintained by continuous assembly and disassembly cycles that respond to the instantaneous levels of each factor in the nuclear compartment. In addition, the minimal enhancer undergoes specific increases in DNase I accessibility, consistent with dramatic changes in chromatin structure upon activation. Still to be resolved is what interaction(s) conveys T-lineage specificity. In the absence of activating signals, the minimal IL-2 enhancer region in mature T cells is apparently unoccupied, exactly as in non-T lineage cells. However, in a conserved but poorly studied upstream region, we have now mapped several novel sites of DNase I hypersensitivity in vivo that constitutively distinguish IL-2 producer type T cells from cell types that cannot express IL-2. Thus a distinct domain of the IL-2 regulatory sequence may contain sites for competence- or lineage-marking protein contacts.

Severe combined immunodeficiency due to defective binding of the nuclear factor of activated T cells in T lymphocytes of two male siblings

Peripheral blood lymphocytes (PBL) and alloreactive T cell lines of two male infants born to consanguinous parents and presenting with severe combined immunodeficiency (SCID) showed a pronounced deficiency in T cell activation. Although phenotypically normal, the proliferative response of the childrens' T cells was strongly reduced but could be improved by the addition of interleukin-2 (IL-2). Furthermore both childrens' T cells were unable to produce the cytokines IL-2, interferon-gamma (IFN-gamma), IL-4 and tumor necrosis factor-alpha (TNF-alpha). This multiple cytokine production deficiency could not be restored by IL-2 or co-stimulatory signals provided by antigen-presenting cells (APC). Moreover, mRNA for IL-2 and IFN-gamma could not be detected. In contrast, expression of the activation-dependent cell surface markers CD25 and CD69 was within normal limits. To determine whether the functional defect of the patients' T cells was due to the absence or abnormal binding of transcription factors involved in cytokine gene expression, electrophoretic mobility shift assays were used to examine the DNA binding of AP-1, Oct, CREB, SP1, NF-kappa B and the nuclear factor of activated T cells (NF-AT) to their respective response elements in the promoter of the IL-2 gene. Whereas AP-1, NF-kappa B, Oct, CREB and SP1 displayed normal binding activities in nuclear extracts, the binding of NF-AT to its IL-2 promoter response element was barely detectable both before and after T cell stimulation. Our results strongly suggest that this NF-AT/DNA binding defect is responsible for the multiple cytokine deficiency and the SCID phenotype observed in the two infant brothers.

CD28: a signalling perspective

CD28 and the related molecule cytotoxic T lymphocyte-associated molecule-4 (CTLA-4), together with their natural ligands B7.1 and B7.2, have been implicated in the differential regulation of several immune responses. CD28 provides signals during T cell activation which are required for the production of interleukin 2 and other cytokines and chemokines, and it has also been implicated in the regulation of T cell anergy and programmed T cell death. The biochemical signals provided by CD28 are cyclosporin A-resistant and complement those provided by the T cell antigen receptor to allow full activation of T cells. Multiple signalling cascades which may be independent of, or dependent on, protein tyrosine kinase activation have been demonstrated to be activated by CD28, including activation of phospholipase C, p21ran, phosphoinositide 3-kinase, sphingomyelinase/ceramide and 5-lipoxygenase. The relative contributions of these cascades to overall CD28 signalling are still unknown, but probably depend on the state of activation of the T cell and the level of CD28 activation. The importance of these signalling cascades (in particular the phosphoinositide 3-kinase-mediated cascade) to functional indications of CD28 activation, such as interleukin 2 gene regulation, has been investigated using pharmacological and genetic manipulations. These approaches have demonstrated that CD28-activated signalling cascades regulate several transcription factors involved in interleukin 2 transcriptional activation. This review describes in detail the structure and expression of the CD28 and B7 families, the functional outcomes of CD28 ligation and the signalling events that are thought to mediate these functions.

Costimulation of cytokine gene expression in T cells by the human T leukemia/lymphotropic virus type 1 trans activator Tax

Many cell signals such as CD28 and CD4 binding can costimulate cytokine gene expression in activated T cells. We have found that the human T leukemia/lymphotropic virus type 1 viral protein Tax can also strongly costimulate expression of interleukin-2 (IL-2), IL-3, and granulocyte-macrophage colony-stimulating factor (GM-CSF) mRNA in T cells activated with the phorbol ester phorbol myristate acetate (PMA) and calcium ionophore, which can mimic activation through the antigen specific T-cell receptor. Reporter constructs also showed strong synergy between both stimuli and showed that Tax and the PMA-Ca2+ ionophore act through different regions of the IL-2 and GM-CSF genes. Furthermore, the Tax-responsive regions (TxRR) from both GM-CSF and IL-2 respond to costimulation through the CD28 surface receptor. The GM-CSF and IL-2 TxRRs showed significantly higher levels of NF-kappaB/rel binding, following induction by Tax, compared with that of the PMA-Ca2+ ionophore with only Tax capable of inducing c-Rel binding to a Consensus kappaB element within the GM-CSF TxRR. Tax protein mutants, however, showed that a pathway(s) other than NF-kappaB/rel induction could also cooperate with the PMA-Ca2+ ionophore to activate the GM-CSF and IL-2 genes. This high-level costimulation by Tax, through multiple pathways, may be important in the early stages of leukemia and in the nervous system disorder tropical spastic paraparesis.

CTLA-4 ligation blocks CD28-dependent T cell activation

CTLA-4 is a CD28 homologue believed to be a negative regulator of T cell function. However, the mechanism of this downregulatory activity is not well understood. The present study was designed to examine the effect of CTLA-4 ligation on cytokine production, cell survival, and cell cycle progression. The results demonstrate that the primary effect of CTLA-4 ligation is not the induction of apoptosis. Instead, CTLA-4 signaling blocks IL-2 production, IL-2 receptor expression, and cell cycle progression of activated T cells. Moreover, the effect of CTLA-4 signaling was manifested after initial T cell activation. Inhibition of IL-2 receptor expression and cell cycle progression was more pronounced at late (72 h) time points after initial activation. The effects of anti-CTLA-4 mAbs were most apparent in the presence of optimal CD28-mediated costimulation consistent with the finding that CTLA-4 upregulation was CD28-dependent. Finally, the addition of exogenous IL-2 to the cultures restored IL-2 receptor expression and T cell proliferation. These results suggest that CTLA-4 signaling does not regulate cell survival or responsiveness to IL-2, but does inhibit CD28-dependent IL-2 production.

Anergic T cells are defective in both jun NH2-terminal kinase and mitogen-activated protein kinase signaling pathways

T helper type 1 cells (Th1) become anergic when stimulated through the antigen receptor in the absence of costimulation. They do not produce IL-2 or proliferate in response to subsequent stimulation. Previous studies have indicated that anergic T cells are defective in the trnsactivational activity of the transcription factor, AP-1, which is required for optimal IL-2 transcription. Using two murine Th1 cell clones, we demonstrate that anergic Th1 cells have defects in both jun NH2-terminal kinase (JNK) and extracellular signal-regulated kinase (ERK) activities. These kinases have been shown to be important for the upregulation of AP-1 activity. Furthermore, our data show that ERK and JNK activities are restored when anergy is induced in the presence of the protein synthesis inhibitor cycloheximide, or when anergic T cells are allowed to proliferate in response to exogenous IL-2. These treatments have previously been shown to prevent or reverse the anergic state. Our results suggest that defects in both JNK and ERK may result in the decreased AP-1 activity and the reduced IL-2 transcription observed in anergic T cells.

Rel-deficient T cells exhibit defects in production of interleukin 3 and granulocyte-macrophage colony-stimulating factor

The c-rel protooncogene encodes a subunit of the NF-kappa B-like family of transcription factors. Mice lacking Rel are defective in mitogenic activation of B and T lymphocytes and display impaired humoral immunity. In an attempt to identify changes in gene expression that accompany the T-cell stimulation defects associated with the loss of Rel, we have examined the expression of cell surface activation markers and cytokine production in mitogen-stimulated Rel-/- T cells. The expression of cell surface markers including the interleukin 2 receptor alpha (IL-2R alpha) chain (CD25), CD69 and L-selectin (CD62) is normal in mitogen-activated Rel-/- T cells, but cytokine production is impaired. In Rel-/- splenic T cell cultures stimulated with phorbol 12-myristate 13-acetate and ionomycin, the levels of IL-3, IL-5, granulocyte- macrophage colony-stimulating factor (GM-CSF), tumor necrosis factor alpha (TNF-alpha), and gamma interferon (IFN-gamma) were only 2- to 3-fold lower compared with normal T cells. In contrast, anti-CD3 and anti-CD28 stimulated Rel-/- T cells, which fail to proliferate, make little or no detectable cytokines. Exogenous IL-2, which restitutes the proliferative response of the anti-CD3- and anti-CD28-treated Rel-/- T cells, restores production of IL-5, TNF-alpha, and IFN-gamma, but not IL-3 and GM-CSF expression to approximately normal levels. In contrast to mitogen-activated Rel-/- T cells, lipopolysaccharide-stimulated Rel-/- macrophages produce higher than normal levels of GM-CSF. These findings establish that Rel can function as an activator or repressor of gene expression and is required by T lymphocytes for production of IL-3 and GM-CSF.

Nuclear appearance of a factor that binds the CD28 response element within the interleukin-2 enhancer correlates with interleukin-2 production

Activation of T lymphocytes requires the combined signaling of the T cell receptor and costimulatory molecules such as CD28. The ability of T cells to produce interleukin-2 (IL-2) is a critical control point in T lymphocyte activation. The IL-2 enhancer contains a functional motif named CD28 response element (CD28RE) that serves a role as a target for mitogenic T cell activation signals. The CD28RE sequence reveals similarity to the consensus kappaB binding motif. Here we demonstrate that CD28RE binds an inducible protein with a molecular mass of approximately 35 kDa called nuclear factor of mitogenic-activated T cells (NF-MATp35) that is clearly different from the known NF- kappaB/Rel family members. Induction of NF-MATp35 was shown to depend on de novo protein synthesis and was restricted to T cells that received a mitogenic combination of T cell stimuli, not necessarily including CD28 signaling. Nonmitogenic T cell stimulation did not result in appearance of NF-MATp35. These results indicate that mitogenic combinations of T cell activation signals are integrated at the level of NF-MATp35 induction. Similar to its effect on IL-2 production, cyclosporin A inhibited the induction of NF-MATp35. Taken together, these data demonstrate that the nuclear appearance of NF-MATp35 shows excellent correlation with IL-2 production, which is a unique characteristic among nuclear factors implicated in the control of IL-2 gene expression.

Transcriptional regulation of the interleukin-2 gene in normal human peripheral blood T cells. Convergence of costimulatory signals and differences from transformed T cells

To study transcriptional regulation in normal human T cells, we have optimized conditions for transient transfection. Interleukin-2 (IL-2) promoter-reporter gene behavior closely parallels the endogenous gene in response to T cell receptor and costimulatory signals. As assessed with mutagenized promoters, the most important IL-2 cis-regulatory elements in normal T cells are the proximal AP-1 site and the NF- kappaB site. Both primary activation, with phytohemagglutinin or antibodies to CD3, and costimulation, provided by pairs of CD2 antibodies or B7-positive (B cells) or B7-negative (endothelial) accessory cells, are mediated through the same cis-elements. Interestingly, the nuclear factor of activated T cell sites are much less important in normal T cells than in Jurkat T cells. We conclude that IL-2 transcriptional regulation differs in tumor cell lines compared with normal T cells and that different costimulatory signals converge on the same cis-elements in the IL-2 promoter.

The relative contribution of the CD28 and gp39 costimulatory pathways in the clonal expansion and pathogenic acquisition of self-reactive T cells

The zona pellucida (ZP), an ovarian extracellular structure, contains three major glycoproteins: ZP1, ZP2, and ZP3. A ZP3 peptide contains both an autoimmune oophoritis-inducing T cell epitope and a B cell epitope that induces autoantibody to ZP. This study investigates two major T cell costimulation pathways in this disease model. Herein we show that blockage of glycoprotein (gp)39 and CD40 interaction with gp39 monoclonal antibody (mAb) results in the failure to induce both autoimmune oophoritis and autoantibody production. Inhibition of ligand binding to the CD28 receptor with the fusion protein, murine CTLA4-immunoglobulin (Ig), also results in failure to generate antibody to ZP and significantly reduces disease severity and prevalence. Surprisingly, the frequencies of antigen-specific T cells in anti-gp39 mAb-treated mice, CTLA4-Ig treated mice, and in mice given control hamster IgG or control fusion protein L6, were equivalent as determined by limiting dilution analysis (approximately equals 1:5,000). These T cells, which produced comparable amounts of interleukin 4 and interferon gamma in vitro, were able to transfer oophoritis to normal recipients. When anti-gp39 mAb and CTLA4-Ig were given together, the effect was additive, leading to inhibition of T cell activation as determined by in vitro proliferation and limiting dilution analysis (approximately equals 1:190,000); disease and antibody responses were absent in these mice. By studying these two costimulatory pathways in parallel, we have shown that autoimmune disease and autoantibody production are inhibitable by blocking either the gp39 or the CD28 pathway, whereas inhibition of clonal expansion of the effector T cell population occurs only when both pathways are blocked.

Bone marrow-derived dendritic cell progenitors (NLDC 145+, MHC class II+, B7-1dim, B7-2-) induce alloantigen-specific hyporesponsiveness in murine T lymphocytes

The functional maturation of dendritic cells (DC) and other antigen-presenting cells is believed to reflect the upregulation of cell surface major histocompatibility complex (MHC) class II and other T cell co-stimulatory molecules, especially the CD28 ligands B7-1 (CD80) and B7-2 (CD86). In this study, we propagated cells exhibiting characteristics of DC precursors from the bone marrow (BM) of B10 mice (H-2b; I-A+) in response to granulocyte-macrophage colony stimulating factor (GM-CSF). The methods used were similar to those employed previously to propagate DC progenitors from normal mouse liver. Cells expressing DC lineage markers (NLDC 145+, 33D1+, N418+) harvested from 8-10-day GM-CSF stimulated BM cell cultures were CD45+, heat-stable antigen+, CD54+, CD44+, MHC class II+, B7-1dim but B7-2- (costimulatory molecule-deficient). Supplementation of cultures with interleukin-4 (IL-4) in addition to GM-CSF however, resulted in marked upregulation of MHC class II and B7-2 expression. These latter cells exhibited potent allostimulatory activity in primary mixed leukocyte cultures. In contrast, the cells stimulated with GM-CSF alone were relatively weak stimulators and induced alloantigen-specific hyporesponsiveness in allogeneic T cells (C3H; H-2k; I-E+) detected upon restimulation in secondary MLR. This was associated with blockade of IL-2 production. Reactivity to third-party stimulators was intact. The hyporesponsiveness induced by the GM-CSF stimulated, costimulatory molecule-deficient cells was prevented by incorporation of anti-CD28 monoclonal antibody in the primary MLR and was reversed by addition of IL-2 to restimulated T cells. The findings show that MHC class II+ B7-2- cells with a DC precursor phenotype can induce alloantigen-specific hyporesponsiveness in vitro. Under the appropriate conditions, such costimulatory molecule-deficient cells could contribute to the induction of donor-specific unresponsiveness in vivo.

CD28-mediated costimulation in the absence of phosphatidylinositol 3-kinase association and activation

T-cell activation involves two distinct signal transduction pathways. Antigen-specific signaling events are initiated by T-cell receptor recognition of cognate peptide presented by major histocompatibility complex molecules. Costimulatory signals, which are required for optimal T-cell activation and for overcoming the induction of anergy, can be provided by the homodimeric T-cell glycoprotein CD28 through its interaction with the counterreceptors B7-1 and B7-2 on antigen-presenting cells. Ligation of CD28 results in its phosphorylation on tyrosines and the subsequent recruitment and activation of phosphatidylinositol 3-kinase (PI 3-kinase). It has been suggested that the induced association of CD28 and PI 3-kinase is required for costimulation. We report here that ligation of CD19, a heterologous B-cell receptor that also associates with and activates PI 3-kinase upon ligation, failed to costimulate interleukin-2 production. Moreover, pharmacological inhibition of PI 3-kinase activity failed to block costimulation mediated by CD28. By mutational analysis, we demonstrate that disruption of PI 3-kinase association with CD28 also did not abrogate costimulation. These results argue that PI 3-kinase association with CD28 is neither necessary nor sufficient for costimulation of interleukin-2 production. Finally, we identify specific amino acid residues required for CD28-mediated costimulatory activity.

Simultaneous quantitation of multiple cytokine mRNAs by RT-PCR utilizing plate based EIA methodology

Cytokines are small protein hormones produced during an immune response that are responsible for mediation and regulation of many aspects of immunity. Measurement of cytokines by several different methods has led to a broader understanding of the immune response. This paper describes a sensitive, reproducible, and quantitative RT-PCR assay for the simultaneous measurement of multiple cytokines. The main features of the methodology are: RNA competitors which control for all aspects of the process from RNA extraction, through reverse transcription (RT) and PCR amplification; a general cloning vector, pQPCR1, for building RNA competitors that does not require prior analyte cDNA cloning; and analysis by plate based EIA. This RT-PCR-EIA system is shown to be more sensitive than agarose gel electrophoresis followed by EtBr staining, measuring PCR product in the sub-nanogram range. It also extends the linear dynamic range of detection to a four log fold range of analyte concentration. The assay is reproducible, with coefficients of variation (CVs) in the 10-20% range. Moreover, the cloning vector is designed to accommodate multiple primer templates, thus allowing simultaneous quantitation of many different analytes from a single RT reaction. The described system is versatile and adapts to numerous analytes.

Novel NFAT sites that mediate activation of the interleukin-2 promoter in response to T-cell receptor stimulation

The transcription factors NFAT and AP-1 have been shown to be essential for inducible interleukin-2 (IL-2) expression in activated T cells. NFAT has been previously reported to bind to two sites in the IL-2 promoter: in association with AP-1 at the distal antigen response element at -280 and at -135. On the basis of DNase I footprinting with recombinant NFAT and AP-1 proteins, gel shift assays, and transfection experiments, we have identified three additional NFAT sites in the IL-2 promoter. Strikingly, all five NFAT sites are essential for the full induction of promoter activity in response to T-cell receptor stimulation. Four of the five NFAT sites are part of composite elements able to bind AP-1 in association with NFAT. These sites display a diverse range of cooperativity and interdependency on NFAT and AP-1 proteins for binding. One of the NFAT sites directly overlaps the CD28-responsive element. We present evidence that CD28 inducibility is conferred by the AP-1 component in NFAT-AP-1 composite elements. These findings provide further insight into the mechanisms involved in the regulation of the IL-2 promoter.

Selective CD28pYMNM mutations implicate phosphatidylinositol 3-kinase in CD86-CD28-mediated costimulation

CD28 costimulatory signals are required for lymphokine production and T cell proliferation. CD28 signaling recruits the intracellular proteins PI 3-kinase, ITK, and GRB-2/SOS. PI 3-kinase and GRB-2/SOS bind the CD28 cytoplasmic pYMNM motif via SH2 domains. We generated CD28 pYMNM mutants and found that Y191 mutation (Y191CD28F) disrupted both PI 3-kinase and GRB-2 binding, while M194 mutation (M194CD28C) disrupted only PI 3-kinase binding. Both mutants still bound ITK. We have assessed the ability of these selective mutants to support IL-2 production upon TCR zeta/CD3 ligation in the presence of CHO-CD86 (B7-2) cells. Both Y191CD28F and M194CD28C mutants failed to generate IL-2. These data directly implicate PI 3-kinase in CD28-mediated costimulation leading to IL-2 secretion. Wortmannin, an inhibitor of PI 3-kinase, induced cell apoptosis and as such was unsuitable for use in this study.

Signalling via CD28 of human naive neonatal T lymphocytes

Accessory molecules play a crucial role in the development of the T cell response to antigenic challenge. We have examined the role of CD28 in modulating the 'naive' neonatal T cell response to anti-CD2-mediated activation. To compare the role of CD28, neonatal and adult T cells were stimulated with a pair of mitogenic anti-CD2 antibodies in the presence or absence of anti-CD28 MoAb. With anti-CD2 alone, neonatal T cells proliferated slightly but produced no detectable IL-2, whereas adult T cells proliferated vigorously, with significant IL-2 production. Costimulation with anti-CD28 MoAb greatly enhanced the proliferative response of neonatal T cells to levels equivalent to those of adult T cells, whereas adult T cells showed only slight increases. Although IL-2 secretion was increased in the presence of anti-CD28 MoAb, neonatal T cell IL-2 production remained lower than in adults. In contrast, enhancement of IL-2 mRNA expression in neonates was similar to adult levels. Anti-CD28 MoAb costimulation increased NF kappa B levels in neonates, albeit to levels lower than that of adults. The cellular mechanism governing the diminished proliferative response of neonatal T lymphocytes to anti-CD2 may therefore be due to decreased NF kappa B induction, reduced IL-2 mRNA expression and deficient IL-2 production. Although anti-CD28 MoAb costimulation enhances all of the above signals, NF kappa B and IL-2 levels remain lower than in adults, suggesting the need for further activation requirements in the neonate.

p56Lck and p59Fyn regulate CD28 binding to phosphatidylinositol 3-kinase, growth factor receptor-bound protein GRB-2, and T cell-specific protein-tyrosine kinase ITK: implications for T-cell costimulation

T-cell activation requires cooperative signals generated by the T-cell antigen receptor zeta-chain complex (TCR zeta-CD3) and the costimulatory antigen CD28. CD28 interacts with three intracellular proteins-phosphatidylinositol 3-kinase (PI 3-kinase), T cell-specific protein-tyrosine kinase ITK (formerly TSK or EMT), and the complex between growth factor receptor-bound protein 2 and son of sevenless guanine nucleotide exchange protein (GRB-2-SOS). PI 3-kinase and GRB-2 bind to the CD28 phosphotyrosine-based Tyr-Met-Asn-Met motif by means of intrinsic Src-homology 2 (SH2) domains. The requirement for tyrosine phosphorylation of the Tyr-Met-Asn-Met motif for SH2 domain binding implicates an intervening protein-tyrosine kinase in the recruitment of PI 3-kinase and GRB-2 by CD28. Candidate kinases include p56Lck, p59Fyn, zeta-chain-associated 70-kDa protein (ZAP-70), and ITK. In this study, we demonstrate in coexpression studies that p56Lck and p59Fyn phosphorylate CD28 primarily at Tyr-191 of the Tyr-Met-Asn-Met motif, inducing a 3- to 8-fold increase in p85 (subunit of PI 3-kinase) and GRB-2 SH2 binding to CD28. Phosphatase digestion of CD28 eliminated binding. In contrast to Src kinases, ZAP-70 and ITK failed to induce these events. Further, ITK binding to CD28 was dependent on the presence of p56Lck and is thus likely to act downstream of p56Lck/p59Fyn in a signaling cascade. p56Lck is therefore likely to be a central switch in T-cell activation, with the dual function of regulating CD28-mediated costimulation as well as TCR-CD3-CD4 signaling.

Role of B7 signaling in the differentiation of naive CD4+ T cells to effector interleukin-4-producing T helper cells

Signaling through the T cell receptor must be accompanied by costimulatory signals for the differentiation of naive T cells to cytokine-producing effector T helper cells. The costimulatory signal through CD28 is required for T cell activation resulting in increased interleukin (IL)-2 production in vitro, but its role in the production of IL-4 and in the in vivo response is still unclear. We have examined the effects of blocking CTLA-4 (the CD28 homologue) ligand interactions on the in vivo development of IL-4-producing T helper effector cells during a primary mucosal immune response to the nematode parasite Heligmosomoides polygyrus and during a primary systemic immune response to immunogenic anti-IgD antibodies. Our results demonstrate that CD28 and/or CTLA-4 signaling is required for T cell priming leading to IL-4 cytokine production, B cell activation, and IgE secretion during both immune responses, suggesting that other signaling molecules do not substitute for these molecules in either of these two different immune responses. Furthermore, the CD28 ligands, B7-1 and B7-2, can substitute for each other in providing the required T cell costimulatory ligand interactions during the primary immune response to H. polygyrus. In contrast, memory T cells during the challenge immune response do not require CD28/CTLA-4 ligand interactions for IL-4 production and T helper effector function.

Interleukin-10 differentially regulates B7-1 (CD80) and B7-2 (CD86) expression on human peripheral blood dendritic cells

Most of the immunosuppressive effects of interleukin-10 (IL-10) are related to functional inhibition of antigen-presenting cells (APC). Herein, we investigate the influence of recombinant (r)IL-10 on human dendritic cells (DC) purified from peripheral blood of healthy volunteers. First, we found that rIL-10 inhibited in a dose-dependent manner the proliferative responses as well as the production of IL-2 and interferon-gamma (IFN-gamma) in mixed lymphocyte reaction (MLR) between purified T cells and DC. This rIL-10 effect could be attributed to a direct effect on DC, as DC preincubated with rIL-10 were found to be deficient in the induction of alloreactive T cells even when anti-IL-10 neutralizing mAb was added at the time of MLR. Flow cytometric analysis indicated that rIL-10 did not modify the expression of ICAM-1 (CD54) and B7-1 (CD80), but decreased HLA-DR and B7-2 (CD86) expression at the DC surface. We conclude that the inhibitory effect of rIL-10 on primary alloreactive T cell responses involves down-regulation of class II MHC and B7-2 expression at the DC surface.

RelA is a potent transcriptional activator of the CD28 response element within the interleukin 2 promoter

T-cell activation requires two different signals. The T-cell receptor's recognition of a specific antigen on antigen-presenting cells provides one, and the second signal comes from costimulatory molecules such as CD28. In contrast, T cells that are stimulated with antigen in the absence of the CD28 costimulatory signal can become anergic (nonresponsive). The CD28 response element (CD28RE) has been identified as the DNA element mediating interleukin 2 (IL-2) gene activation by CD28 costimulation. Our previous work demonstrates that the Rel/NF-kappa B family proteins c-Rel, RelA (p65), and NFKB1 (p50) are involved in the complex that binds to the CD28RE. We also showed that c-Rel, but not NFKB1 (p50), can bind to the CD28RE and activate CD28RE-driven transcription in cotransfection assays. However, the role of RelA (p65) in CD28 signaling has not yet been addressed. We provide evidence that RelA (p65) itself bound directly to the CD28RE of the IL-2 promoter and other lymphokine promoters. In addition, RelA (p65) was a potent transcriptional activator of the CD28RE in vivo. We show that a RelA (p65)-c-Rel heterodimer bound to the CD28RE and synergistically activated the CD28RE enhancer activity. We also demonstrate that activated Raf-1 kinase synergized with RelA (p65) in activating the CD28RE enhancer activity. Interestingly, a soluble anti-CD28 monoclonal antibody alone, in the absence of other stimuli, also synergized with RelA (p65) in activating the CD28RE. Furthermore, we show that RelA (p65) activated expression of the wild-type IL-2 promoter but not the CD28RE-mutated IL-2 promoter. A combination of RelA (p65) and NFKB1 (p50) also activated the IL-2 promoter through the CD28RE site. These results demonstrate the functional regulation of the CD28RE, within the IL-2 promoter, by Rel/NF-kappa B transcription factors.

Regulation of granulocyte-macrophage colony-stimulating factor and interleukin 3 expression

Granulocyte-macrophage colony stimulating factor (GM-CSF) and interleukin 3 (IL-3) are multilineage acting hematopoietic growth factors which have overlapping but distinct biological properties. Cellular sources of IL-3 are confined to activated T cells, natural killer (NK) cells, mast cells and possibly megakaryocytes, while these cells and activated macrophages, fibroblasts and endothelial cells are important sources of GM-CSF. In vitro studies have implicated both cytokines in the autocrine growth of human myeloid or murine mast cell leukemias. The human GM-CSF and IL-3 genes map to the long arm of chromosome 5, show similar genomic structures, and share several conserved elements in their 5' and 3' flanking regions. The promoters of these genes contain a variety of positive and negative regulatory regions, and the level of expression of these genes is controlled by both transcriptional and post-transcriptional mechanisms.

CD28 costimulation can promote T cell survival by enhancing the expression of Bcl-XL

T cell activation through the TCR can result in either cell proliferation or cell death. The role of costimulatory receptors in regulating T cell survival has not been defined. Here, we present data demonstrating that CD28 costimulation enhances the in vitro survival of activated T cells. One mechanism for this enhancement is the ability of CD28 costimulation to augment the production of IL-2, which acts as an extrinsic survival factor for T cells. In addition, CD28 costimulation augments the intrinsic ability of T cells to resist apoptosis. Although CD28 signal transduction had no effect on Bcl-2 expression, CD28 costimulation was found to augment the expression of Bcl-XL substantially. Transfection experiments demonstrated that this level of Bcl-XL could prevent T cell death in response to TCR cross-linking, Fas cross-linking, or IL-2 withdrawal. These data suggest that an important role of CD28 costimulation is to augment T cell survival during antigen activation.

Sphingomyelin-ceramide turnover in CD28 costimulatory signaling

When the CD28 membrane glycoprotein of T cells binds to its ligand, a signal is transmitted that is required for T cell receptor-induced proliferation and cytokine secretion: T cells are not stimulated by the CD28 signal alone. Ligation of CD28 initiated sphingomyelin hydrolysis and generated ceramide. Treatment of T cells with either exogenous sphingomyelinase or a cell-permeable ceramide analogue. C6-ceramide, mimicked the CD28 signal by inducing T cell proliferation and interleukin-2 gene transcription. Stabilization of interleukin-2 mRNA was also observed in C6-ceramide-treated cells. Thus, the sphingomyelin-ceramide pathway is a candidate for mediating the costimulatory signal.

The inositol 1,4,5-trisphosphate receptor is essential for T-cell receptor signaling

Antigen-specific activation of T lymphocytes, via stimulation of the T-cell antigen receptor (TCR) complex, is marked by a rapid and sustained increase in the concentration of cytoplasmic free Ca2+ ([Ca2+]i). It has been suggested that the second messenger inositol 1,4,5-trisphosphate (IP3) produced after TCR stimulation binds to the IP3 receptor (IP3R), an intracellular Ca(2+)-release channel, and triggers the increase in [Ca2+]i that activates transcription of the gene for T-cell growth factor interleukin 2 (IL-2). However, the role of the IP3R in T-cell signaling and possibly in plasma membrane Ca2+ influx in T cells remains unproven. Stable transfection of T cells (Jurkat) with antisense type 1 IP3R cDNA prevented type 1 IP3R expression, providing a tool for dissecting the role of IP3 signaling during T-cell activation. T cells lacking type 1 IP3R failed to increase [Ca2+]i or produce IL-2 after TCR stimulation. Moreover, depletion of intracellular Ca2+ stores without TCR activation stimulated Ca2+ influx in cells lacking the type 1 IP3R. These results establish that the type 1 IP3R is required for intracellular Ca2+ release that triggers antigen-specific T-cell proliferation but not for plasma membrane Ca2+ influx.

CD28 signals through acidic sphingomyelinase

T cell receptor recognition of antigen can lead either to T lymphocyte differentiation and proliferation or to a state of unresponsiveness, which is dependent on whether appropriate costimulatory signals are provided to the mature T cell. We have investigated a novel intracellular signaling pathway provided by the costimulatory molecule CD28. CD28 engagement triggers the activation of an acidic sphingomyelinase (A-SMase), which results in the generation of ceramide, an important lipid messenger intermediate. A-SMase activation by CD28 occurred in resting as well as in activated primary T cells or leukemic Jurkat cells. In contrast, ligation of either CD3 or CD2 did not result in A-SMase activation. Overexpression of recombinant A-SMase in Jurkat T cells substituted for CD28 with regard to nuclear factor-kB activation. These data suggest that CD28 provides an important costimulatory signal by activation of an acidic sphingomyelinase pathway.

Regulation of interleukin 2 gene expression by CD28 costimulation in mouse T-cell clones: both nuclear and cytoplasmic RNAs are regulated with complex kinetics

T-cell receptor (TCR) signalling is required to induce expression of the interleukin 2 (IL-2) gene in mouse T cells. Additional costimulation through CD28 augments IL-2 production by 30- to 100-fold. Using IL-2 RNA accumulation and transcription reporter assays, we have addressed potential mechanisms of CD28 regulation at various time points of stimulation. The kinetic regulation of IL-2 mRNA by TCR and CD28 signals is complex: (i) at the earliest detectable time point, CD28 signalling causes a 20-fold increase compared with TCR signalling alone; (ii) both groups rapidly accumulate mRNA for the first 4 h; (iii) IL-2 mRNA then disappears from cells stimulated through the TCR alone but plateaus or increases slightly in cells costimulated through CD28; and (iv) after 8 h, the mRNA disappears in cultures with the anti-CD28 antibody. Transcription reporter assays did not show a specific effect of CD28 signalling on IL-2 enhancer driven transcription. This was true for either a 353- or a 1.9-kb enhancer, over a broad range of kinetics and TCR occupancy, and with several TCR signal mimics. The early component of CD28 costimulation is nuclear, however, since the initial enhancement of mRNA is also found in unspliced IL-2 RNA. Between 2 and 6 h, there is a marked difference in the rates of decay of IL-2 mRNA in the presence and absence of the CD28 signalling. Rapid decay of IL-2 mRNA commences after 8 h even in the presence of CD28 signals, although the decay occurs at a rate slower than that seen after 4 h of anti-TCR stimulation alone. This complexity suggests the existence of two interesting molecular mechanisms by which CD28 costimulates lymphokine gene expression.

Co-expression of B7-1 and ICAM-1 on tumors is required for rejection and the establishment of a memory response

Although the transfection of B7-1 cDNA into a few mouse tumor cell lines can induce anti-tumor T cell immunity, its expression alone is ineffective in many other tumor cell lines tested. We were interested to study what factors limit B7-1 co-stimulatory activity, and decided to investigate whether B7-1 requires the cooperation of ICAM-1 to provide the minimal co-stimulatory signal for establishing an efficient anti-tumor immunity. We show that the transfection of B7-1 cDNA into three ICAM-1+ (plasmocytoma J558L, T lymphomas EL-4 and RMA), but not into two ICAM-1- tumors cell lines (adenocarcinoma TS/A and melanoma B16.F1), is sufficient to induce their complete rejection in syngeneic mice. The expression of ICAM-1 is necessary for the rejection of the B7 expressing tumors, since the primary response elicited by B7-1+ EL-4 and RMA clones expressing reduced levels of ICAM-1 is severely reduced. Furthermore, super-transfection of ICAM-1 cDNA into B7-1+ adenocarcinoma and melanoma clones optimizes their primary rejection. Histologic examination of transfected tumors reveals that B7-1 and ICAM-1 exert a potent pro-inflammatory activity. The intra-tumor infiltration is composed of both eosinophils and lymphomonocytes, and is already massive 5 days after the tumor challenge. The primary rejection of the B7-1+ ICAM-1+ tumors depends critically on CD8+ T cells, natural killer cells and granulocytes, but is independent of CD4+ T cells. Remarkably, in addition to its effects on the early phases of the immune response, the co-expression of ICAM-1 and B7-1 on tumors is also necessary for the efficient induction of a memory response. In fact, only the primary challenge with B7-1+, ICAM-1+ tumor cells protects the majority of the mice from a second injection of parental tumor cells. Collectively, our findings indicate that B7-1 and ICAM-1 are fundamental components for triggering the primary rejection of tumors and establishing a protective memory response. These findings may help to define new strategies for the rational application of co-stimulation in tumor immunotherapy.

T cell antigen CD28 binds to the GRB-2/SOS complex, regulators of p21ras

The T cell molecule CD28 provides a co-stimulatory signal that is required for T cell proliferation, and has been implicated in the control of T cell anergy. An important clue to the signaling mechanism of CD28 is the finding that CD28 can bind to phosphatidylinositol 3-kinase (PI 3-kinase) by means of a cytoplasmic phospho-YMNM (pYMNM) motif. A remaining issue concerns whether CD28 can recruit other intracellular signaling molecules. In this study, we show that CD28 uses the same pYMNM motif to recruit a second intracellular protein, GRB-2. CD28-associated GRB-2, as detected by anti-GRB-2 immunoblotting, was found in human peripheral T cells, HPB-ALL and Jurkat cells. As in the case of PI3-kinase, antibody-induced cross-linking of CD28 induces a time-dependent recruitment of GRB-2. Likewise, mutation of the pY-191 residue within the pYMNM motif reduces GRB-2 binding. Peptide binding studies show that the SH2 domain of GRB-2 binds to the pYMNM motif with an affinity comparable to GRB-2/SHC, but some 10- to 100-fold lower than the CD28/PI 3-kinase. Despite this, CD28/GRB-2 and CD28/PI 3-kinase complexes are found to co-exist in peripheral T cells. Finally, immunoblotting shows that CD28 also associates with the gene product of the human homolog of the Drosophila Son of sevenless gene (SOS), a GRB-2-complexed guanine nucleotide exchange factor responsible for converting p21ras to a GTP-bound active state. CD28-associated GRB2/SOS is likely to serve an important link in the regulation of p21ras and lymphokine expression mediated by CD28.

Co-stimulation of T cells via CD28 inhibits human IgE production; reversal by pertussis toxin

In lymphocyte cultures, IgE production was achieved by stimulating T cells with anti-CD2 and IL-2. Here we show that anti-CD28, in the presence or absence of IL-2, reduces this IgE production approximately 10-fold. This inhibition of IgE production was almost completely reversed by Pertussis toxin (PT). PT had no effect on IgE production when the cells were stimulated in the absence of anti-CD28. No major effects of PT were found on IgM production. PT had no effect on purified B cells, stimulated with IL-4 and anti-CD40. In the presence of saturating amounts of rIL-4 similar results were obtained, albeit the absolute amounts of IgE produced were higher in all situations. Furthermore, PT-induced IgE production was still dependent on IL-4, as was evident from experiments in which anti-IL-4 was added to the culture. The IgE enhancing effect was dependent on the adenosine diphosphate (ADP)-ribosyltransferase activity of PT, because a mutant molecule lacking this activity was not able to restore anti-CD28-induced inhibition of IgE synthesis. Thus, we show that co-stimulation with anti-CD28 causes an inhibition of T cell-dependent IgE production by B cells, which inhibition can be specifically overcome by PT. An analysis of the molecular pathways underlying this phenomenon may contribute to our understanding of the regulation of IgE synthesis in (patho)physiological conditions.

Analysis of proteins that interact with the IL-2 regulatory region in patients with rheumatic diseases

In order to investigate transcriptional regulation of lymphokine genes in rheumatic diseases, peripheral blood mononuclear cells from patients with systemic lupus erythematosus (SLE), rheumatoid arthritis (RA), and systemic sclerosis (SSc) were analysed for expression of DNA-binding proteins. Nuclear extracts prepared from unstimulated and mitogen-activated cells were studied for their ability to bind to 32P-labelled oligonucleotides containing the AP-1, NF-AT, NF-B and CD28RC sites of the IL-2 promoter. Using gel mobility-shift assay, detection of protein binding to the AP-1 site was reduced in SLE compared with controls. NF-AT binding activity was enhanced in all groups of patients, and was associated with measures of disease activity in RA. In addition, SSc patients showed increased NF-kappa B binding activity. Altered patterns of DNA-binding proteins suggest disturbed intracellular signalling which may contribute to abnormal lymphokine production in rheumatic diseases.

The antitumor effect of tumor-draining lymph node cells activated by both anti-CD3 monoclonal antibody and activated B cells as costimulatory-signal-providing cells

To establish an efficient cell-culture system for adoptive immunotherapy, we attempted to use lipopolysaccharide(LPS)-activated B cells (LPS blasts) as costimulatory-signal-providing cells in the in vitro induction of antitumor effector cells. Both normal and tumor-draining lymph node cells were efficiently activated by both anti-CD3 monoclonal antibody (mAb) and LPS blasts, and subsequently expanded by a low dose of interleukin-2 (IL-2; anti-CD3 mAb and LPS blasts/IL-2). The expanded cells were predominantly CD8+ T cells and showed a low level of tumor-specific cytotoxic T lymphocyte (CTL) activity. The adoptive transfer of B16-melanoma-draining lymph node cells expanded by anti-CD3 mAb and LPS blasts/IL-2 showed significant antitumor effect against the established metastases of B16 in combination with intraperitoneal injections of IL-2. This treatment cured all B16-bearing mice. In addition, these mice also showed tumor-specific protective immunity against B16 at the rechallenge. Considering that activated B cells express several kinds of costimulatory molecules, these findings thus indicate an efficacy of costimulation that is derived from activated B cells for the in vitro induction of tumor-specific CTL, in co-operation with anti-CD3 mAb. The culture system presented here may thus be therapeutically useful, providing potent effectors for adoptive immunotherapy against various types of cancer.

Calcium/calmodulin-dependent protein kinase II downregulates both calcineurin and protein kinase C-mediated pathways for cytokine gene transcription in human T cells

Engagement of the T cell receptor for antigen activates phospholipase C resulting in an increase in intracellular free calcium concentration ([Ca2+]i) and activation of protein kinase C (PKC). Increased [Ca2+]i activates Ca2+/calmodulin-dependent kinases including the multifunctional Ca2+/calmodulin-dependent protein kinase II (CaM-K II), as well as calcineurin, a type 2B protein phosphatase. Recent studies have identified calcineurin as a key enzyme for interleukin (IL)-2 and IL-4 promoter activation. However, the role of CaM-K II remains unknown. We have used mutants of these kinases and phosphatases (gamma B*CaM-K and delta CaM-AI, respectively) to explore their relative role in cytokine gene transcription and their interactions with PKC-dependent signaling systems. gamma B*CaM-K and delta CaM-AI, known to exhibit constitutive Ca(2+)-independent activity, were cotransfected (alone or in combination) in Jurkat T cells with a plasmid containing the intact IL-2 promoter driving the expression of the chloramphenicol acetyltransferase reporter gene. Cotransfection of gamma B*CaM-K with the IL-2 promoter construct downregulated its transcription in response to stimulation with ionomycin and phorbol myristate acetate (PMA). The inhibitory effect of CaM-K II on IL-2 promoter was associated with decreased transcription of its AP-1 and NF-AT transactivating pathways. Under the same conditions, delta CaM-AI superinduced IL-2 promoter activity (approximately twofold increase). When both mutants were used in combination, gamma B*CaM-K inhibited the induction of the IL-2 promoter by delta CaM-AI. Similar results were obtained when a construct containing the IL-4 promoter also was used. gamma B*CaM-K also downregulated the activation of AP-1 in response to transfection with a constitutively active mutant of PKC or stimulation with PMA. These results suggest that CaM-K II may exert negative influences on cytokine gene transcription in human T cells, and provide preliminary evidence for negative cross-talk with the calcineurin- and PKC-dependent signaling systems.

Human alveolar macrophages present antigen ineffectively due to defective expression of B7 costimulatory cell surface molecules

Alveolar macrophages, resident phagocytic cells in the lung that derive from peripheral blood monocytes, are paradoxically ineffective in presenting antigen to T cells. We found that antigen presentation by alveolar macrophages could be restored by the addition of anti-CD28 mAb to cultures of T cells and macrophages, indicating that costimulation by alveolar macrophages via the CD28 pathway was defective. In addition, we found that alveolar macrophages activated with IFN-gamma failed to express B7-1 or B7-2 antigens, which normally ligate CD28 on T cells and provide a costimulatory signal required for the activation of T cells. These observations are the first to demonstrate the inability of a "professional" antigen-presenting cell type to effectively express the costimulatory molecules B7-1 and B7-2. Inasmuch as immune reactions within the lung are inevitably associated with inflammatory injury to pulmonary tissue, these observations suggest that reduced expression of B7-1 and B7-2 by alveolar macrophages may be advantageous, as a critical mechanism involved in the induction of peripheral tolerance to the abundance of antigens to which mucosal tissues are continuously exposed.