Molecular regulation of the IL-2 gene: rheostatic control of the immune system

Innate Control of Adaptive Immunity: Beyond the Three-Signal Paradigm

Activation of cells in the adaptive immune system is a highly orchestrated process dictated by multiples cues from the innate immune system. Although the fundamental principles of innate control of adaptive immunity are well established, it is not fully understood how innate cells integrate qualitative pathogenic information to generate tailored protective adaptive immune responses. In this review, we discuss complexities involved in the innate control of adaptive immunity that extend beyond TCR engagement, costimulation, and priming cytokine production but are critical for the generation of protective T cell immunity.

IL-15 functions as a danger signal to regulate tissue-resident T cells and tissue destruction

In this Opinion article, we discuss the function of tissues as a crucial checkpoint for the regulation of effector T cell responses, and the notion that interleukin-15 (IL-15) functions as a danger molecule that communicates to the immune system that the tissue is under attack and poises it to mediate tissue destruction. More specifically, we propose that expression of IL-15 in tissues promotes T helper 1 cell-mediated immunity and provides co-stimulatory signals to effector cytotoxic T cells to exert their effector functions and drive tissue destruction. Therefore, we think that IL-15 contributes to tissue protection by promoting the elimination of infected cells but that when its expression is chronically dysregulated, it can promote the development of complex T cell-mediated disorders associated with tissue destruction, such as coeliac disease and type 1 diabetes.

Transmembrane TNF-TNFR2 Impairs Th17 Differentiation by Promoting Il2 Expression

The double-edged sword nature by which IL-2 regulates autoimmunity and the unpredictable outcomes of anti-TNF therapy in autoimmunity highlight the importance for understanding how TNF regulates IL-2. Transmembrane TNF (tmTNF) preferentially binds TNFR2, whereas soluble TNF (sTNF) binds TNFR1. We previously showed reduced IL-2 production in TNFR1(-/-) TNFR2(-/-) CD4(+) T cells. In this study, we generated TNFR1(-/-), TNFR2(-/-), or TNFR1(-/-) TNFR2(-/-) 5C.C7 TCR Il2-GFP mice and report that CD4(+) T cell-intrinsic tmTNF/TNFR2 stimulates Il2 promoter activity and Il2 mRNA stability. We further used tmTNF Foxp3 gfp reporter mice and pharmacological TNF blockade in wild-type mice to report a tmTNF/TNFR2 interaction for Il2 expression. IL-17 is critical for host defense, but its overabundance promotes autoimmunity. IL-2 represses Th17 differentiation, but the role for TNFR2 in this process is not well understood. We report elevated expression of TNFR2 under Th17-polarization conditions. Genetic loss-of-function experimental models, as well as selective TNF blockade by etanercept and XPro1595 in wild-type mice, demonstrate that impaired tmTNF/TNFR2, but not sTNF/TNFR1, promotes Th17 differentiation in vivo and in vitro. Under Th17-polarizing conditions, elevated IL-17 production by TNFR2-knockout CD4(+) T cells was associated with increased STAT3 activity and decreased STAT5 activity. Increased IL-17 production in TNFR2-knockout T cells was prevented by adding exogenous IL-2. We conclude that CD4(+) T cell-intrinsic tmTNF/TNFR2 promotes IL-2 production that inhibits the generation of Th17 cells in a Foxp3-independent manner. Moreover, under Th17-polarizing conditions, selective blockade of CD4(+) T cell-intrinsic TNFR2 appears to be sufficient to promote Th17 differentiation.

Biphasic regulation of Il2 transcription in CD4+ T cells: roles for TNF-alpha receptor signaling and chromatin structure

We describe a novel biphasic regulation of Il2 transcription in naive CD4(+) T cells. Few ( approximately 5%) CD4(+) T cells transcribe Il2 within 6 h of anti-TCR-beta plus anti-CD28 stimulation (early phase). Most naive CD4(+) T cells do not initiate Il2 transcription until after an additional approximately 12 h of T cell stimulation (late phase). In comparison, essentially all previously activated (Pre-Ac) CD4(+) T cells that transcribe Il2 do so with an early-phase response. Late-phase Il2 expression mostly requires c-Rel, CD28, and TNFR signaling. In contrast, early-phase transcription is only partly c-Rel and CD28 dependent and TNFR independent. There was also increased stable DNA accessibility at the Il2 locus and elevated c-Rel expression in resting Pre-Ac CD4(+) cells. Upon T cell activation, a faster and greater increase in DNA accessibility as well as c-Rel nuclear expression were observed in Pre-Ac CD4(+) cells relative to naive CD4(+) T cells. In addition, both acetylated histone H3 and total H3 decreased at the Il2 locus upon rechallenge of Pre-Ac CD4(+) T cells, whereas increased acetylated histone H3 with no change in total H3 was observed following activation of naive CD4(+) T cells. We propose a model in which nucleosome disassembly facilitates rapid initiation of Il2 transcription in CD4(+) T cells, and suggest that a threshold level of c-Rel must be reached for Il2 promoter activity in both naive and Pre-Ac CD4(+) T cells. This is provided, at least partially, by TNFR signaling during priming, but not during recall.

B7-1 mediated costimulation regulates pancreatic autoimmunity

Costimulation by B7-1 and B7-2 molecules results in divergent biological effects. This is particularly striking in the NOD mouse, since the lack of B7-2 leads to complete protection from diabetes whereas the B7-1 deficiency causes exacerbation of disease. We tested the hypothesis that B7-1 costimulation suppresses pancreatic autoimmunity. We describe that the lack of B7-1 not only causes aberrant thymocyte maturation but also significantly enhances expansion, survival, and effector function of islet specific T cells in periphery. We also observed a significant reduction in the proportion of T-regulatory (T-regs) cells. Immunophenotypic analysis of T and APCs revealed a significantly lower frequency of T cells expressing the negative costimulatory receptor PD-1 in B7-1KO mice whereas the proportion of B7-H1 positive APCs was found to be significantly higher. Blocking studies in B7-1KO mice suggest that B7-H1 provides negative signals for anti islet CD4 and CD8 T-cell expansion but is differentially required for their priming. Our data demonstrate that deficiency of B7-1 mediated costimulation causes multitude of immunological defects, which involve reduction in T-regs and a concomitant enhancement of expansion, survival and effector potential of auto reactive T cells.

Negative regulation of interleukin-2 and p38 mitogen-activated protein kinase during T-cell activation by the adaptor ALX

Activation of naïve T cells requires synergistic signals produced by the T-cell receptor (TCR) and by CD28. We previously identified the novel adaptor ALX, which, upon overexpression in Jurkat T cells, inhibited activation of the interleukin-2 (IL-2) promoter by TCR/CD28, suggesting that it is a negative regulator of T-cell activation. To further understand the physiological role of ALX, ALX-deficient mice were generated. Purified T cells from ALX-deficient mice demonstrated increased IL-2 production, CD25 expression, and proliferation in response to TCR/CD28 stimulation. Enhanced IL-2 production and proliferation were also observed when ALX-deficient mice were primed in vivo with ovalbumin-complete Freund's adjuvant and then restimulated ex vivo. Consistent with our initial overexpression studies, these data demonstrate that ALX is a negative regulator of T-cell activation. While TCR/CD28-mediated activations of phosphotyrosine induction, extracellular signal-regulated kinase 1/2, Jun N-terminal protein kinase, IkappaB kinase alpha/beta, and Akt were unaltered, constitutive activation of p38 mitogen-activated protein kinase and its upstream regulators MKK3/6 were observed for ALX-deficient splenocytes. The phenotype of ALX-deficient mice resembled the phenotype of those deficient in the transmembrane adaptor LAX, and an association between ALX and LAX proteins was demonstrated. These results suggest that ALX, in association with LAX, negatively regulates T-cell activation through inhibition of p38.

Cutting Edge: Cbl-b: One of the Key Molecules Tuning CD28- and CTLA-4-Mediated T Cell Costimulation

Cbl-b negatively regulates CD28-dependent T cell activation. In this report, we tested the hypothesis that CD28 and CTLA-4 have opposite roles in tuning T cell activation threshold by controlling the levels of Cbl-b protein expression. We demonstrate that CD28 costimulation potentiates TCR-induced Cbl-b degradation, whereas CTLA-4-B7 interaction is required for Cbl-b re-expression. In support of this finding, Cbl-b expression in CTLA-4 knockout (KO) T cells is significantly reduced, and treating CTLA-4KO mice with human CTLA-4Ig to block CD28-B7 interaction restores Cbl-b expression on T cells. Furthermore, CD28 and CTLA-4 costimulatory effects are compromised in Cbl-bKO T cells. These observations indicate that CD28 and CTLA-4 tightly regulate Cbl-b expression which is critical for establishing the threshold for T cell activation.

The ALX Src homology 2 domain is both necessary and sufficient to inhibit T cell receptor/CD28-mediated up-regulation of RE/AP

Activation of naive T cells occurs when two signals are received. The first signal is received through the T cell antigen receptor (TCR), and a second costimulatory signal is primarily provided by CD28. We have recently identified a novel adaptor molecule, ALX, which is expressed exclusively in hematopoietic cells. ALX contains several sites for potential protein-protein interaction, including an Src homology 2 (SH2) domain, four PXXP polyproline sequences, and two likely sites of tyrosine phosphorylation. Overexpression of ALX inhibits the transcriptional activation of the interleukin 2 promoter during T cell activation, specifically affecting CD28-mediated activation of the RE/AP element of the interleukin 2 promoter. To understand how ALX functions downstream of CD28, we generated a panel of site-directed mutants as well as truncations in which potential protein-binding sites were mutated or absent. We found that the ALX SH2 domain is both necessary and sufficient to mediate inhibition of RE/AP activation. Mutation of the SH2 domain did not affect ALX expression, relative localization in the cytoplasm and nucleus, phosphorylation, or a mobility shift in response to TCR signaling alone. However, an activation-induced mobility shift triggered by CD28 was reduced in the ALX SH2 domain mutant. In addition, the isolated ALX SH2 domain was found to associate with a phosphoprotein from Jurkat T cells on TCR/CD28 stimulation. Therefore, the ALX SH2 domain plays a critical role in ALX function downstream of CD28.

Cloning and characterization of ALX, an adaptor downstream of CD28

T cell activation requires two signals: specific recognition of antigen through the T cell receptor (TCR) and a costimulatory signal provided primarily by CD28 in naïve T cells. We cloned a novel gene with considerable homology to RIBP/TSAd/Lad, an adaptor involved in T cell activation and interleukin-2 (IL-2) promoter activation. Expression of this gene is limited to the spleen and thymus. We have named this gene ALX, adaptor in lymphocytes of unknown function X. Because the related adaptor RIBP is involved in IL-2 regulation, we investigated whether ALX had a similar function. ALX overexpression in Jurkat T cells results in inhibition of IL-2 promoter activation after stimulation with superantigen. The IL-2 promoter contains several binding sites for transcription factors including the composite element RE/AP, which is the primary site of CD28 transcriptional activation. ALX overexpression had the greatest effect on the activation of a RE/AP reporter as opposed to an AP-1 reporter. Interestingly, ALX overexpression strongly inhibited RE/AP activation in response to anti-CD28/phorbol 12-myristate 13-acetate (PMA) stimulation but had minimal effect when anti-TCR/PMA was used. Therefore, it appears that ALX may function downstream of CD28 costimulation during T cell activation. In addition, the mobility of ALX shifts upon TCR/CD28 costimulation to a greater extent than what is observed with either stimulus alone demonstrating that ALX is a target of both TCR and CD28 costimulatory signaling pathways.

Kinetic perspectives of T cell antigen receptor signaling. A two-tier model for T cell full activation

T-cell activation consists of multiple layers of signaling events. Interleukin-2 production is of interest for many, since its expression determines a critical difference between partial and full T-cell activation. To achieve full activation of T cells, it is necessary for the T-cell antigen receptor (TCR) to be engaged for an extended period of time. However, why extended stimulation is required for full T-cell activation is not understood at the molecular level. In this review, orchestrated events of TCR signal transduction will be analyzed in a kinetic manner and connected toward the understanding of the mechanism of T-cell activation. Based on recent results, a model of the mechanism that dictates the threshold between partial and full T-cell activation is proposed.

Cutting edge: regulation of T cell activation threshold by CD28 costimulation through targeting Cbl-b for ubiquitination

Optimal T cell activation requires signaling through the TCR and CD28 costimulatory receptor. CD28 costimulation is believed to set the threshold for T cell activation. Recently, Cbl-b, a ubiquitin ligase, has been shown to negatively regulate CD28-dependent T cell activation. In this report, we show that CD28 costimulation selectively induces greater ubiquitination and degradation of Cbl-b in wild-type T cells than CD3 stimulation alone, and TCR-induced Cbl-b ubiquitination and degradation are significantly reduced in CD28-deficient T cells. Stimulation of CD28-deficient T cells with higher doses of anti-CD3 results in increased ubiquitination of Cbl-b, which correlates with enhanced T cell responses. Our results demonstrate that CD28 costimulation regulates the threshold for T cell activation, at least in part, by promoting Cbl-b ubiquitination and degradation.

Cutting edge: A-kinase anchor proteins are involved in maintaining resting T cells in an inactive state

A-kinase anchor proteins (AKAPs) target protein kinase A (PKA) to different subcellular locations and are thought to play important roles in the cAMP signaling pathway. The aims of this study were to determine whether T cells express AKAPs and, if so, to establish their physiological significance. CD4(+) T cells were found to express eight AKAPs. Disruption of the AKAP-PKA interaction caused high levels of IL-2, IL-4, IL-5, and IFN-gamma production in the absence of stimulation via CD3epsilon and CD28 molecules. Disruption of the AKAP-PKA interaction acted synergistically with suboptimal doses of Ag in boosting proliferative responses of T cells. Finally, disruption of the AKAP-PKA interaction rendered T cells insensitive to cAMP-elevating agents. It was concluded that AKAPs, through their association with PKA, are involved in maintaining T cell homeostasis and in regulating the sensitivity of T cells to incoming cAMP signals.

Opposing roles of serine/threonine kinases MEKK1 and LOK in regulating the CD28 responsive element in T-cells

T-cell activation requires signals from both the T-cell receptor (TcR) and other co-stimulatory molecules such as CD28. TcR- and CD28-mediated signals are integrated during T-cell activation resulting in the expression of cytokine genes such as interleukin-2 (IL-2). An enhancer element (CD28RE) of the IL-2 gene specifically responsive to CD28 signals has been previously identified and characterized. This response element and an adjacent Activated Protein-1 (nuclear factor-interleukin-2B) site together (RE/AP1) were shown to complex with c-rel, AP-1 and other factors. However, details of the signal transduction pathways leading from CD28 to the composite response element remain poorly understood. We present data showing that overexpression of the serine threonine kinase, mitogen-activated protein kinase/extracellular-signal-regulated kinase kinase kinase-1 (MEKK1), but not nuclear factor-kappa B inducing kinase, or MAP kinase/ERK kinase-1 (MEK1), can significantly increase the level of CD28RE/AP1-driven luciferase (Luc) reporter gene expression in Jurkat E6-1 cells. A MEKK1 dominant negative mutant blocked such activation induced by stimulation with Raji B cells and the superantigen staphylococcus enterotoxin E (SEE), as well as via CD3/CD28. Mutations in either site of the RE/AP1 element abolished MEKK1-induced Luc expression. Calcineurin inhibitors, CsA and FK520, or inhibitors of p38 kinase (SB 203580), or MEK1 (PD 098059), did not affect MEKK1-induced reporter activation. These results directly implicate MEKK1 in the CD28 signalling pathway that activates the CD28 response element. Co-expression of the lymphocyte-oriented kinase (LOK) kinase attenuated Raji/SEE-induced IL-2 production in Jurkat cells, as well as MEKK1 and Raji/SEE-induced reporter gene activation. These data suggest that MEKK1 and LOK may have opposing roles in regulating the CD28RE/AP1 element.

Inhibitor of apoptosis protein from Orgyia pseudotsugata nuclear polyhedrosis virus provides a costimulatory signal required for optimal proliferation of developing thymocytes

The inhibitors of apoptosis proteins (IAPs) constitute a family of endogenous inhibitors that control apoptosis in the cell by inhibiting caspase processing and activity. IAPs are also implicated in cell division, cell cycle regulation, and cancer. To address the role of IAPs in thymus development and homeostasis, we generated transgenic mice expressing IAP generated from the baculovirus Orgyia pseudotsugata nuclear polyhedrosis virus (OpIAP). Developing thymocytes expressing OpIAP show increased nuclear levels of NF-kappaB and reduced cytoplasmic levels of its inhibitor, IkappaBalpha. In mature thymocytes, OpIAP induces optimal activation and proliferation after TCR triggering in the absence of a costimulatory signal. OpIAP expression in immature thymocytes blocks TCR-induced apoptosis. Taken together, our data illustrate the pleiotropism of OpIAP in vivo.

Endogenous CD28 expressed on myeloma cells up-regulates interleukin-8 production: implications for multiple myeloma progression

CD28 is the major costimulatory molecule on T cells. CD28 activation, in conjunction with T-cell receptor engagement, up-regulates transcription of several cytokines, including interleukin-2 (IL-2), through transcriptional activation of the RE/AP composite element. Although CD28 is not normally expressed on B cells or plasma cells, more than 90% of extramedullary myelomas (a late stage B-cell neoplasm) express CD28. The functional significance of this is unknown. The results of this study demonstrate that CD28 stimulates transcriptional activation of RE/AP-based reporters in B cells and myeloma cells. However, CD28 stimulation does not up-regulate IL-2 production in myeloma cell lines, demonstrating that the IL-2 promoter may not be a relevant RE/AP-containing target of CD28 in myelomas. Instead, an RE/AP composite element has been identified within the promoter of the IL-8 gene, a chemokine that promotes angiogenesis. Furthermore, stimulation of endogenous CD28 expressed by 3 myeloma cell lines increased IL-8 production. Therefore, the study demonstrates that CD28 is functional in myelomas to up-regulate transcription of endogenous genes, including IL-8. The proposal is made that aberrant expression of CD28 may play a role in the progression of multiple myeloma.

Decreased Ras-mitogen-activated protein kinase signaling may cause DNA hypomethylation in T lymphocytes from lupus patients

OBJECTIVE

Previous studies have shown that inhibiting T cell DNA methylation causes a lupus-like disease by modifying gene expression. T cells from patients with lupus exhibit diminished levels of DNA methyltransferase (MTase) enzyme activity, hypomethylated DNA, and changes in gene expression similar to those exhibited by T cells treated with methylation inhibitors, suggesting that DNA hypomethylation may contribute to human lupus. Since it is known that DNA MTase levels are regulated by the ras-mitogen-activated protein kinase (MAPK) pathway, this study sought to determine whether decreased ras-MAPK signaling could account for the DNA hypomethylation in lupus T cells.

METHODS

DNA MTase messenger RNA (mRNA) from lupus patients and from healthy controls was quantitated by Northern analysis, and ras-MAPK signaling was determined by immunoblotting with antibodies to the activated forms of extracellular receptor-associated kinase (ERK). Results were compared with those in T cells in which ras-MAPK signaling was inhibited with a soluble inhibitor of MAPK ERK I (MEK1).

RESULTS

T cells from patients with active lupus had diminished DNA MTase mRNA levels and decreased signaling through the ras-MAPK pathway. Inhibiting signaling through the ras-MAPK pathway with the MEK1 inhibitor decreased DNA MTase mRNA and enzyme activity to the levels seen in lupus T cells, and resulted in DNA hypomethylation resembling that seen in lupus T cells.

CONCLUSION

These results suggest that a decrease in signaling through the ras-MAPK pathway may be responsible for the decreased MTase activity and DNA hypomethylation in patients with lupus.

Immunobiology of allogeneic peripheral blood mononuclear cells mobilized with granulocyte-colony stimulating factor

The use of mobilized peripheral blood (PB) stem cells for autologous transplantation initially generated much enthusiasm because of enhanced engraftment in comparison to marrow stem cells and avoidance of general anesthesia for the donor. Its application to the allogeneic setting seemed inevitable. For obvious ethical reasons, allogeneic donors are mobilized with cytokines only, mainly granulocyte colony-stimulating factor (G-CSF). Results from preliminary studies suggest that in comparison to standard bone marrow transplants, outcomes such as engraftment, host-versus-graft reaction, graft-versus-host disease, graft-versus-leukemia and immunological reconstitution may be different. Surprisingly, G-CSF, previously recognized as a late acting lineage-specific factor for neutrophil production, not only disrupts homeostasis between stem cells and their microenvironment, but also induces significant quantitative and qualitative changes in the accessory cell compartment, affecting lymphocytes, monocytes, natural killer, dendritic, and stromal cells. Furthermore, mobilization of huge numbers of non-professional antigen presenting cells (CD34+ stem cells) amplifies the tolerizing potential of PB stem cell grafts. Thus, G-CSF mobilization provides PB transplants with different immunobiologic properties in comparison to standard bone marrow grafts. Whether these immunobiologic differences will lead to better transplant outcomes remains to be shown through much awaited results of large randomized clinical trials.

Ligation of CD28 In Vivo Induces CD40 Ligand Expression and Promotes B Cell Survival

Functional activation of T cells requires ligation of Ag receptors with specific peptides presented by MHC molecules on APCs concurrent with appropriate contacts of cell surface accessory molecules. Among these accessory molecules, interactions between CD28/CTLA-4 with B7 family members (CD80 and CD86) and CD40 with CD40 ligand (CD40L) play a decisive role in regulating the progression of balanced immune responses. However, most information regarding the role of accessory molecules in immune responses has been derived in the context of signals from the TCRs. Little understanding has been achieved regarding the consequence of ligation of costimulation molecules in absence of signals from the TCR. By employing an in vivo murine system, we show, herein, that ligation of CD28 alone with anti-CD28 Abs leads to a dramatic enlargement of the peripheral lymphoid organs characterized primarily by the expansion of B cells. B cells from anti-CD28-treated mice are resistant to spontaneous and anti-IgM-induced apoptosis. These cells are also unsusceptible to FasL-mediated apoptosis. Interestingly, this in vivo effect of CD28 on B cells is largely mediated by inducing the expression of CD40L, since coadministration of a blocking Ab against CD40L inhibited CD28-mediated B cell survival and expansion. Therefore, CD28-mediated expression of CD40L may play an important role in the regulation of lymphocyte homeostasis.

Cutting Edge: Nuclear Factor of Activated T Cells and AP-1 Are Insufficient for IL-2 Promoter Activation: Requirement for CD28 Up-Regulation of RE/AP

IL-2 gene transcription in T cells requires both TCR and costimulatory signals. IL-2 promoter activation in Jurkat T cells stimulated with superantigen presented by Raji B cells requires CD28 activation. The addition of rCTLA4Ig, which blocks CD28 binding to its ligand, to the cultures decreased IL-2 promoter activation by >80%. Interestingly, CTLA4Ig did not significantly inhibit the activation of either NF of activated T cells (NFAT) or AP-1 reporters. Therefore, activation of NFAT and AP-1 is insufficient for IL-2 promoter activation. In contrast, an RE/AP reporter was blocked by CTLA4Ig by >90%. Thus, the requirement for CD28 in IL-2 promoter activation appears to be due to RE/AP and not the NFAT or AP-1 sites. In addition, these data suggest that transcriptional activation of RE/AP is not mediated by NFAT, because activation of a NFAT reporter is not affected by the addition of CTLA4Ig.

Molecular regulation of interleukin-2 expression by CD28 co-stimulation and anergy

The consequences of T-cell receptor engagement (signal 1) are profoundly affected by the presence or absence of co-stimulation (signal 2). T-cell receptor (TCR) stimulation in the absence of CD28-mediated co-stimulation not only results in little interleukin (IL)-2 production, but induces a long lasting hyporesponsive state known as T-cell clonal anergy. The addition of CD28 ligation to signal 1, on the other hand, results in the production of copious amounts of IL-2. Our laboratory has utilized CD4+ Th 1 clones in an effort to understand the molecular events resulting in enhanced IL-2 production by co-stimulation and the inhibition of IL-2 production in anergy. Our current studies have focused on defining the post-transcriptional effects of CD28-enhanced IL-2 production. The data suggest that a major component of CD28's ability to regulate IL-2 production occurs at the level of message stability and involves the 3'-untranslated region of the message. In terms of anergy, our recent studies support the notion that it is not the result of TCR engagement in the absence of co-stimulation, but rather signal 1 in the absence of IL-2 receptor signaling and proliferation. Furthermore, T-cell anergy appears to be an active negative state in which IL-2 production is inhibited both at the level of signal transduction and by cis-dominant repression at the level of the IL-2 promoter.

Interleukin-2 and systemic lupus erythematosus--fifteen years later

Systemic lupus erythematosus (SLE) is a highly heterogeneous disorder in which multiple immunologic abnormalities have been described. In this review, we thoroughly analyse the impaired T cell production of, and response to, interleukin-2 (IL-2) characteristic of patients with SLE. Since it was first reported, several articles have provided us with enlightening, but somewhat confusing, data that reveal the complexity of the subject. The IL-2 production by T cells is part of a complex network in which a discrete alteration is capable of disrupting the whole system. On the other hand, regulatory mechanisms exist that, in an attempt to compensate the primary alteration, provoke secondary defects. Evidence indicates that this defect is not intrinsic, but rather, results from multiple microenvironmental influences that act on the T cell and modify its activation state and its cytokine production. Abnormalities in co-stimulatory mechanisms and in cytokines that may be related to the IL-2 production deficiency, have been described in patients with SLE. We also consider the information derived from murine SLE models, IL-2 knockout models and reports concerning the immune dysregulation present in patients with SLE.

Self-veto mechanism of CD8+ cytotoxic effector T cells. Peptide-induced paralysis affects the peptide-MHC-recognizing cytotoxic T lymphocytes and is independent of Fas/Fas ligand interactions

The lytic activity of CD8+ cytotoxic T lymphocyte (CTL) cell lines or clones can be inhibited by addition of the peptide recognized by these cells. The mechanisms underlying this phenomenon are not fully understood. Here we have analyzed peptide-induced CTL paralysis using in vivo generated ovalbumin (OVA)-specific CTL. Lytic activity of OVA-specific CTL was inhibited by addition of the immunodominant OVA-peptide SIINFEKL in a dose-dependent manner. Paralysis was induced rapidly and binding of the peptide to MHC class I molecules was required. Using mixing experiments with CTL populations of different peptide specificities restricted to the same MHC class I molecule we identified a veto-like mechanism: the cytotoxic activity of the peptide-recognizing CTL was inhibited while the lytic activity of the peptide-presenting CTL was unaltered. Only CD8+ CTL but not CD4+ T cells or B+ cells induced paralysis. After removal of the peptide-presenting CTL by magnetic cell sorting, paralysis was maintained and paralyzed CTL showed no signs of apoptosis. Loss of cytotoxicity could be induced in CTL populations from Fas-deficient (lpr+/lpr+) or Fas ligand-deficient (gld+/gld+) mice and mixtures thereof, implying that Fas/Fas ligand interactions are not involved during induction of paralysis. Hence, peptide-induced paralysis of CTL is due to a self-veto mechanism rather than to mutual killing of CTL. These findings may have implications for in vivo immunization with peptides, viral escape and peripheral tolerance mechanisms.

Low CD3+CD28-induced interleukin-2 production correlates with decreased reactive oxygen intermediate formation in neonatal T cells

The capacity of neonatal T cells to secrete interleukin-2 (IL-2) has been reported to be variable. We analysed IL-2 production in purified neonatal and adult T cells using polyclonal activator phorbol ester + calcium ionophore (PDBu + iono) or receptor-mediated anti-CD3/anti-CD3+ anti-CD28 stimulation. PDBu + iono induced equally high IL-2 levels in both groups and, when stimulated with plate-bound anti-CD3 monoclonal antibody (mAb), the IL-2 secretion by neonatal cells was undetectable and adult cells produced low amounts of IL-2 (mean 331 +/- 86 pg/ml). The addition of anti-CD28 mAb to anti-CD3-stimulated cells markedly increased IL-2 production in both cell types, but levels of IL-2 in neonatal T cells remained clearly lower than those of adult T cells (respective mean values: 385 +/- 109 pg/ml and 4494 +/- 1199 pg/ml). As NF-kappa B is a critical transcription factor in the control of IL-2 expression, we next analysed its nuclear translocation in neonatal and adult T cells using the electrophoretic mobility shift assay and, because induction of reactive oxygen intermediates (ROI) is required for the activation of NF-kappa B, we also analysed levels of intracellular ROI in these cells using the ROI-reactive fluorochrome DCFH-DA and flow cytometry. In neonatal T cells NF-kappa B activation and ROI formation after anti-CD3 stimulation were low compared with adult T cells and, although addition of anti-CD28 mAb increased induction of NF-kappa B and ROI formation, levels similar to those of adults were not achieved. After PDBu + iono stimulation, the cells showed similar ROI formation and IL-2 secretion. Our results suggest that reduced IL-2 production by neonatal T cells is specific for anti-CD3 and anti-CD3+ anti-CD28-mediated stimulation and that these activators cannot effectively activate the ROI-NF-kappa B signalling pathway in neonatal T cells.

Inhibition of Mitogen-Activated Protein Kinase Kinase Blocks T Cell Proliferation But Does Not Induce or Prevent Anergy

Three mitogen-activated protein kinase pathways are up-regulated during the activation of T lymphocytes, the extracellular signal-regulated kinase (ERK), Jun NH2-terminal kinase, and p38 mitogen-activated protein kinase pathways. To examine the effects of blocking the ERK pathway on T cell activation, we used the inhibitor U0126, which has been shown to specifically block mitogen-activated protein kinase/ERK kinase (MEK), the kinase upstream of ERK. This compound inhibited T cell proliferation in response to antigenic stimulation or cross-linked anti-CD3 plus anti-CD28 Abs, but had no effect on IL-2-induced proliferation. The block in T cell proliferation was mediated by down-regulating IL-2 mRNA levels. Blocking Ag-induced proliferation by inhibiting MEK did not induce anergy, unlike treatments that block entry into the cell cycle following antigenic stimulation. Surprisingly, induction of anergy in T cells exposed to TCR cross-linking in the absence of costimulation was also not affected by blocking MEK, unlike cyclosporin A treatment that blocks anergy induction. These results suggest that inhibition of MEK prevents T cell proliferation in the short term, but does not cause any long-term effects on either T cell activation or induction of anergy. These findings may help determine the viability of using mitogen-activated protein kinase inhibitors as immune suppressants.

Impaired Induction of the CD28-Responsive Complex in Granulocyte Colony-Stimulating Factor Mobilized CD4 T Cells

Use of the CD28/B7 costimulatory signal for T-cell activation was analyzed in granulocyte colony-stimulating factor (G-CSF) mobilized peripheral blood mononuclear cells (G-PBMCs) and in peripheral blood mononuclear cells obtained before administration of G-CSF (preG-PBMCs). CTLA4Ig inhibition of OKT3-stimulated proliferation was significantly lower in G-PBMCs compared with preG-PBMCs (39.9% ± 5.6% and 72.2% ± 5.4%, respectively; P < .001). Furthermore, as shown in electrophoretic mobility-shift assays, the inducible level of the T-cell transcription factor CD28 responsive complex (CD28RC) was suppressed in CD4 cells derived from G-PBMC. However, depletion of CD14 cells from G-PBMCs restored CD28RC induction to normal levels. Taken together, these findings suggest that the large number of CD14 monocytes in G-PBMCs may limit T-cell responsiveness by suppressing the induction of the CD28RC.

Impaired Induction of the CD28-Responsive Complex in Granulocyte Colony-Stimulating Factor Mobilized CD4 T Cells

Use of the CD28/B7 costimulatory signal for T-cell activation was analyzed in granulocyte colony-stimulating factor (G-CSF) mobilized peripheral blood mononuclear cells (G-PBMCs) and in peripheral blood mononuclear cells obtained before administration of G-CSF (preG-PBMCs). CTLA4Ig inhibition of OKT3-stimulated proliferation was significantly lower in G-PBMCs compared with preG-PBMCs (39.9% ± 5.6% and 72.2% ± 5.4%, respectively; P < .001). Furthermore, as shown in electrophoretic mobility-shift assays, the inducible level of the T-cell transcription factor CD28 responsive complex (CD28RC) was suppressed in CD4 cells derived from G-PBMC. However, depletion of CD14 cells from G-PBMCs restored CD28RC induction to normal levels. Taken together, these findings suggest that the large number of CD14 monocytes in G-PBMCs may limit T-cell responsiveness by suppressing the induction of the CD28RC.

The p38 mitogen-activated protein kinase pathway in activated and anergic Th1 cells

Stimulation of T cells through the TCR leads to activation of the mitogen-activated protein kinase (MAPK) family members ERK (extracellular signal-regulated kinase) and JNK (jun NH2-terminal kinase). These kinases act in synergy to increase the activity of the transcription factor AP-1 which is involved in the transcriptional upregulation of IL-2. Recently a third MAPK member, p38, has been identified. The effects of T cell activation on this pathway have not yet been elucidated. Using two murine Th1 clones, we demonstrate that the p38 pathway is induced upon anti-CD3 plus anti-CD28 crosslinking or PMA plus ionomycin stimulation. p38 activity was induced fully by anti-CD3 or PMA alone and is not enhanced by costimulation even at low levels of TCR signaling. p38 activity peaked at 20 min and was significantly decreased by 2 hr. Anergic (tolerant) Th1 cells showed decreased p38 activity as well as decreased ERK and JNK activities even though levels of these proteins remained unchanged.

Antigen-unspecific B cells and lymphoid dendritic cells both show extensive surface expression of processed antigen-major histocompatibility complex class II complexes after soluble protein exposure in vivo or in vitro

Intravenous (i.v.) injection of high amounts of soluble proteins often results in the induction of antigen-specific tolerance or deviation to helper rather than inflammatory T cell immunity. It has been proposed that this outcome may be due to antigen presentation to T cells by a large cohort of poorly costimulatory or IL-12-deficient resting B cells lacking specific immunoglobulin receptors for the protein. However, previous studies using T cell activation in vitro to assess antigen display have failed to support this idea, showing evidence of specific peptide-major histocompatibility complex (MHC) class II ligand only on purified dendritic cells (DC) or antigen-specific B cells isolated from protein injected mice. Here we reexamine this question using a recently derived monoclonal antibody specific for the T cell receptor (TCR) ligand formed by the association of the 46-61 determinant of hen egg lysozyme (HEL) and the mouse MHC class II molecule I-Ak. In striking contrast to conclusions drawn from indirect T cell activation studies, this direct method of TCR ligand analysis shows that i.v. administration of HEL protein results in nearly all B cells in lymphoid tissues having substantial levels of HEL 46-61-Ak complexes on their surface. DC readily isolated from spleen also display this TCR ligand on their surface. Although the absolute number of displayed ligands is greater on such DC, the relative specific ligand expression compared to total MHC class II levels is similar or greater on B cells. These results demonstrate that in the absence of activating stimuli, both lymphoid DC and antigen-unspecific B cells present to a similar extent class II-associated peptides derived from soluble proteins in extracellular fluid. The numerical advantage of the TCR ligand-bearing B cells may permit them to interact first or more often with naive antigen-specific T cells, contributing to the induction of high-dose T cell tolerance or immune deviation.

CD28 mediates transcriptional upregulation of the interleukin-2 (IL-2) promoter through a composite element containing the CD28RE and NF-IL-2B AP-1 sites

Mutagenesis studies have demonstrated the requirement for the CD28-responsive element (CD28RE) within the interleukin-2 (IL-2) promoter for transcriptional upregulation by CD28. Here, we demonstrate that CD28 responsiveness is conferred by a composite element containing both the CD28RE and the NF-IL-2B AP-1 sites (RE/AP). Mutations at either site within the RE/AP composite element abolish activity. The RE/AP composite element is a site for signal integration within the IL-2 promoter, since its activation is dependent on at least two separate signalling pathways being activated, through the T-cell receptor, CD28, and/or phorbol myristate acetate. Activation is maximal when all three signals occur simultaneously. By using a panel of CD28 cytoplasmic domain mutants, it was found that the transcriptional activation of the RE/AP composite element correlates exactly with the pattern of IL-2 secretion induced by these mutants upon stimulation. Similar to the upregulation of IL-2 secretion, the transcriptional upregulation of the RE/AP composite element by CD28 is FK506 insensitive. The pattern of activation of the RE/AP composite element is different from that observed for either an NFAT or consensus AP-1 site, implying that RE/AP represents a unique element. Using gel shift analysis, we demonstrate that stimulation by CD28 induces the association of the NF-kappaB family member c-Rel to the CD28RE within the RE/AP composite element. The transcriptional upregulation of IL-2 by CD28 appears, therefore, to be mediated through the RE/AP composite element, involving the association of c-Rel with the CD28RE.

Costimulation by B7-1 and LFA-3 targets distinct nuclear factors that bind to the interleukin-2 promoter: B7-1 negatively regulates LFA-3-induced NF-AT DNA binding

We have characterized the regulation of nuclear factors involved in transcriptional control of the interleukin-2 (IL-2) promoter-enhancer activity in Jurkat T cells stimulated with superantigen presented on HLA-DR transfectants combined with the ligands LFA-3 (CD58) and B7-1 (CD80). Gel shift analyses showed that NF-AT was strongly induced in LFA-3-costimulated Jurkat T cells, suggesting that NF-AT is a key target nuclear factor for the CD2-LFA-3 pathway. Studies using HLA-DR-B7-1-LFA-3 triple transfectants showed that the LFA-3-induced NF-AT DNA binding activity was negatively regulated by B7-1 costimulation. In contrast, induction of a CD28 response complex containing only c-Rel proteins was seen after B7-1 costimulation. Both LFA-3 costimulation and B7-1 costimulation induced the AP-1 and NF-kappaB nuclear factors. Distinct compositions of the NF-AT complexes were seen in B7-1- and LFA-3-costimulated cells. LFA-3 induced primarily Jun-D, Fra-1, and Fra-2, while B7-1 induced June-D-Fos complexes. In contrast, AP-1 and NF-kappaB complexes induced in B7-1- and LFA-3-costimulated T cells showed similar contents. Transient transfection of Jurkat T cells with a construct encoding the IL-2 enhancer-promoter region (position -500 to +60) linked to a luciferase reporter gene revealed that B7-1 costimulation was required to induce strong transcriptional activity. Combined B7-1-LFA-3 costimulation resulted in a synergistic increase in IL-2 transcriptional activity. Multimers of the AP-1, NF-AT, NF-kappaB, and CD28 response elements showed distinct kinetics and activity after LFA-3 and B7-1 costimulation and revealed that B7-1 and LFA-3 converge to superinduce transcriptional activity of the AP-1, NF-AT, and CD28 response elements. Transcriptional studies with an IL-2 enhancer-promoter carrying a mutation in the CD28 response element site revealed that the activity was reduced by 80% after B7-1 and B7-1-LFA-3 costimulation whereas the transcriptional activity induced by LFA-3 was unaffected. Our data strongly suggest a selectivity in induction of nuclear factors by the CD2-LFA-3 and CD28-B7-1 pathways. This selectivity may contribute to regulation of the levels of IL-2 induced by LFA-3 and B7-1 costimulation and favor autocrine and paracrine T-cell responses, respectively.

c-rel regulation of IL-2 gene expression may be mediated through activation of AP-1

T cell activation by antigen/MHC induces the expression of several genes critical to the immune response, including interleukin-2. T cells from mice deficient for the NF-kappa B family member c-rel cannot activate IL-2 gene expression. However, mutating the NF-kappa B site in the IL-2 promoter has only moderate effects. To investigate additional ways c-Rel could regulate IL-2 gene expression, we determined whether c-rel overexpression could increase the activity of other transcription factors involved in IL-2 promoter regulation: NF-AT, Oct/OAP (ARRE-1), and AP-1. In Jurkat TAg cells, overexpression of c-Rel increased AP-1 activation approximately 17-fold. Moreover, AP-1 activity stimulated by anti-TCR Abs or PMA/ionomycin was further increased by c-Rel overexpression. c-Rel overexpression did not affect NF-AT or ARRE-1 activity. Additionally, overexpression of c-Rel activated the nonconsensus AP-1 site from the IL-2 promoter (NF-IL-2B), although to a lesser extent, approximately sixfold. AP-1 activation required both the DNA binding and transactivation domains of c-Rel. Our results may provide an explanation for the effect on IL-2 gene activation in c-rel-deficient mice.

Costimulation by CD28 sFv expressed on the tumor cell surface or as a soluble bispecific molecule targeted to the L6 carcinoma antigen

Interaction of the CD80 (B7-1) and CD86 (B7-2) molecules on antigen presenting cells with the receptors CD28 and CTLA-4 on T cells generates signals important in the regulation of immune responses. Because this receptor system involves multiple receptor-ligand interactions, determining the function for individual receptors has been difficult. One approach is the use of antibodies and their derivatives with singular specificity as substitute ligands to explore the activities of these molecules. We have constructed recombinant mono- and bi-specific sFv molecules specific for the CD28 receptor that are capable of binding and generating costimulatory signals to activate T cells. We demonstrate that these soluble molecules are capable of higher levels of costimulation than soluble CD80Ig at equivalent concentrations. We also constructed artificial adhesion receptors on the cell surface using two different CD28-specific sFvIgs fused to the CD80 cytoplasmic and transmembrane domains. In this report, we compared costimulation by a soluble bispecific (alpha CD28-alpha L6) single chain sFvIg fusion protein to that generated by L6 antigen positive (L6+) H3347 tumor cells transduced with cell surface expressed forms of alpha CD28 sFv's. We show that the bispecific protein can target potent CD28 costimulatory activity to L6+ tumor cells in vitro. We also show that transfection of the cell surface forms of the two different CD28 sFvIgs into H3347 tumor cells allows them to generate significant costimulatory signals to activated T cells. Finally, we demonstrate that tumor cell presentation of either the soluble bispecific or transduced cell surface sFv generate similar costimulatory effects resulting in T cell activation.

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.

Neonatal tolerance revisited: turning on newborn T cells with dendritic cells

For some time it has been thought that antigenic challenge in neonatal life is a tolerogenic rather than immunogenic event. Reexamination of the classic neonatal tolerance experiments of Billingham, Brent, and Medawar showed that tolerance is not an intrinsic property of the newborn immune system, but that the nature of the antigen-presenting cell determines whether the outcome is neonatal tolerance or immunization.

Graft-versus-host disease and the Th1/Th2 paradigm

Graft-versus-host disease (GVHD) is the major complication after allogeneic bone marrow transplantation (BMT) and is initiated by alloreactive donor T cells recognizing foreign histocompatibility antigens of the host. There is now substantial experimental and clinical evidence to implicate a dysregulation of cytokine networks as a primary cause for the induction and maintenance of GVHD. In this article, current knowledge of the involvement of cytokines in GVHD is reviewed. The balance between type 1 cytokines (interleukin-2, interferon-gamma) and type 2 cytokines (interleukin-4, interleukin-10) is hypothesized to govern the extent to which a cell-mediated immune response and a systemic inflammatory response develop after allogeneic BMT. Because type 2 cytokines can inhibit the production of the proinflammatory cytokines interleukin-1 and tumor necrosis factor-alpha, a type 1 to type 2 shift in the initial response of donor T cells to host alloantigens may interrupt the cytokine cascade after allogeneic BMT and may offer a new approach to the prevention and treatment of acute GVHD. Interventions to specifically eliminate or modify the response of donor T cells to alloantigens in order to reduce GVHD may obviate the need for T cell depletion in clinical BMT and thus avoid the increased risk of relapse of malignancy and impairment of donor cell engraftment.

In vivo anergized CD4+ T cells express perturbed AP-1 and NF-kappa B transcription factors

Anergy is a major mechanism to ensure antigen-specific tolerance in T lymphocytes in the adult. In vivo, anergy has mainly been studied at the cellular level. In this study, we used the T-cell-activating superantigen staphylococcal enterotoxin A (SEA) to investigate molecular mechanisms of T-lymphocyte anergy in vivo. Injection of SEA to adult mice activates CD4+ T cells expressing certain T-cell receptor (TCR) variable region beta-chain families and induces strong and rapid production of interleukin 2 (IL-2). In contrast, repeated injections of SEA cause CD4+ T-cell deletion and anergy in the remaining CD4+ T cells, characterized by reduced expression of IL-2 at mRNA and protein levels. We analyzed expression of AP-1, NF-kappa B, NF-AT, and octamer binding transcription factors, which are known to be involved in the regulation of IL-2 gene promoter activity. Large amounts of AP-1 and NF-kappa B and significant quantities of NF-AT were induced in SEA-activated CD4+ spleen T cells, whereas Oct-1 and Oct-2 DNA binding activity was similar in both resting and activated T cells. In contrast, anergic CD4+ T cells contained severely reduced levels of AP-1 and Fos/Jun-containing NF-AT complexes but expressed significant amounts of NF-kappa B and Oct binding proteins after SEA stimulation. Resolution of the NF-kappa B complex demonstrated predominant expression of p50-p65 heterodimers in activated CD4+ T cells, while anergic cells mainly expressed the transcriptionally inactive p50 homodimer. These alterations of transcription factors are likely to be responsible for repression of IL-2 in anergic T cells.

Detection of intracytoplasmic cytokine using flow cytometry and directly conjugated anti-cytokine antibodies

Recently, there have been several reports demonstrating improvements in the flow cytometric detection of intracellular cytokines. These advances, although significant, have not yielded techniques that have easily been translated into broad use. To address this issue, we have coupled a fixation and permeabilization method with the use of directly labelled monoclonal anti-cytokine antibodies, providing both improved signal and simpler staining. The kinetics of in situ cytokine production in both CD4 and CD8 cells are shown for IL-2, IL-4, IL-5 and IFN-gamma. Based on these data, 6 h was chosen for optimal detection of this combination of cytokines. We show the specificity of this technique by blocking cytokine staining using a molar excess of recombinant cytokine. Additionally, unlabelled anti-cytokine antibodies are demonstrated to block specific staining of labelled antibody, providing an objective means to place statistical markers. Using such controls, we routinely detected as few as 0.1% false positive cells, allowing the flow cytometric detection of IL-5, which is below the threshold of detection of published methods. To further prove the specificity of staining, we stained using two anti-IL-5 mAbs known to recognize different epitopes and demonstrate that the same cells stain with both antibodies. Without permeabilization we could detect a fraction of cells with low intensity staining for cytokine. This staining was further examined using differential two color staining for intracellular and extracellular cytokine, clearly demonstrating no cells staining exclusively for extracellular cytokine, confirming a lack of passive transfer of cytokine to nearby cells. We show that cytokine flow cytometry is useful in examining the increased IL-5 production characteristic of eosinophilic states and that IL-5 production is limited to the CD27 negative subpopulation. These data illustrate the unique capability of cytokine flow cytometry to correlate cytokine expression with cell surface phenotype without cell separation. In summary, using directly conjugated anti-cytokine antibodies, cytokine flow cytometry becomes a specific and versatile technique for the assessment of complex cytokine production phenotypes in fresh ex vivo T cell subpopulations.

Accumulation of sequence-specific RNA-binding proteins in the cytosol of activated T cells undergoing RNA degradation and apoptosis

Engagement of the T cell antigen receptor (TCR) causes transformed T cell hybridomas to produce lymphokines and then die by an apoptotic mechanism. Here we show that these functional effects of TCR-mediated signaling are associated with the accumulation of several cytosolic mRNA-binding proteins including the previously described AU-A, -B, and -C proteins as well as a novel 70-kDa protein. The results indicate that the 70-kDa protein derived from a 90-kDa precursor present in unactivated T cells and that both proteins bound to two independent sites at the 3'-end of the interleukin-2 mRNA, one in the coding region and the other in an AU-rich segment of the 3'-untranslated region. Glucocorticoids, TCR engagement by monoclonal antibodies, pharmacologic mimics of TCR signaling, or high concentrations of protein or RNA synthesis inhibitors all induced apoptosis, the cytosolic appearance of the RNA-binding proteins, and an increased rate of RNA turnover. Moreover, drugs that interfere with TCR-mediated signals, such as cyclosporin A and staurosporin, prevented both apoptosis and the appearance of the RNA-binding factors. The fact that the accumulation of these factors occurred in the presence of inhibitors of transcription and translation suggested that these proteins are present in an inactive form in unstimulated T cells and are activated when apoptosis is induced.

Pertussis toxin-sensitive GTP-binding proteins regulate activation-induced apoptotic cell death of human natural killer cells

Apoptosis of natural killer (NK) cells can be induced by non-specific physical damage (UV irradiation, heat shock) or by simultaneous ligation of the CD16 and the interleukin-2 receptor (IL-2R) molecules, but not with either anti-CD16 or IL-2 alone. Whereas blockade of GTP-binding protein (G protein)-mediated signal transduction using ADP-ribosylating bacterial toxins or the GTPase-resistant GTP analog guanosine 5'-0-(3-thiotriphosphate (GTP gamma S) does not affect non-specific induction of NK cell apoptosis, such interventions do inhibit induction of apoptosis by anti-CD16/IL-2. The G proteins involved in the regulation of activation-induced NK apoptosis are sensitive to pertussis toxin (PTX) and to the non-specific GTP analog GTP gamma S but not to cholera toxin, Pseudomonas exotoxin A or diphtheria toxin. A pertussis toxin mutant that lacks ADP-ribosylating activity, but conserves the membrane translocating and T cell-mitogenic effects of the native molecule, fails to inhibit NK apoptosis. To exert their apoptosis-inhibitory effect, PTX and GTP gamma S must be employed before cells are activated. Later addition has no effect, suggesting the implication of G proteins in the transmission of apoptosis-inducing signals, but not in the effector stage of apoptosis. Pre-incubation with PTX or GTP gamma S does not affect the activation of NK cells by CD16 cross-linking, IL-2 stimulation- or both, as assessed by the induction of CD69 expression, protein tyrosine phosphorylation and calcium mobilization. Moreover, neither PTX nor GTP gamma S compromise the effector function of NK cells or the susceptibility of target cells to NK-mediated lysis. These data suggest apoptosis as a novel mechanism by which NK responses may be controlled in vivo, as well as an experimental and therapeutical strategy to counteract endogenous down-regulation of NK responses.

Costimulation requirement for AP-1 and NF-kappa B transcription factor activation in T cells

The transcriptional activity of the IL-2 promoter requires T-cell costimulation delivered by the TCR and the auxiliary receptor CD28. Several transcription factors participate in IL-2 promoter activation, among which are AP-1-like factors and NF-kappa B. Protein phosphorylation has an important role in the regulation of these two factors: (1) it induces the transactivating capacity of the AP-1 protein c-Jun; and (2) it is involved in the release of the cytoplasmic inhibitor, I kappa B, from NF-kappa B, allowing translocation of the latter into the nucleus. We have recently shown that both phosphorylation processes require T-cell costimulation. Furthermore, in activated T cells, the kinetics of the two phosphorylation events are essentially similar. According to our results, however, the kinases responsible for the two processes are distinct entities. Whereas TPCK inhibits phosphorylation of I kappa B and, consequently, activation of NF-kappa B, it markedly enhances the activity of JNK, the MAP kinase-related kinase that phosphorylates the transactivation domain of c-Jun. We, therefore, propose the activation scheme presented in FIGURE 3 for T-cell costimulation. Costimulation results in the activation of a signaling pathway that leads to the simultaneous induction of the two transcription factors, AP-1 and NF-kappa B. Integration of the signals generated by TCR and CD28 engagement occurs along this pathway, which then bifurcates to induce I kappa B phosphorylation and NF-kappa B activation on the one hand, and JNK activation and c-Jun phosphorylation on the other. We are currently engaged in defining where the two signals integrate along the AP-1/NF-kappa B 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.

Negative transcriptional regulation in anergic T cells

Anergy is a mechanism of T-lymphocyte tolerance induced by antigen-receptor stimulation in the absence of costimulation, whereby T cells exhibit a defect in antigen-induced transcription of the interleukin 2 (IL-2) gene. Here we present evidence for a mechanism of negative IL-2 gene regulation in anergic T cells. High amounts of binding activity to the negative regulatory element A (NRE-A) of the IL-2 promotor were detected in nuclear extracts from human T cells shortly after induction of anergy. Rapid induction of this nuclear complex is blocked by cyclosporin A and is found to be independent of protein synthesis. Plasmid DNAs, containing either the human phorbol 12-myristate 13-acetate-responsive element (PRE) or both NRE-A and PRE, were used as template for in vitro transcription assays in the presence of T-cell nuclear extracts. Under these conditions nuclear extracts from both anergic and rested T-cell clones, after crosslinking of CD3 and CD28, induced transcription of plasmids containing only PRE. However, when plasmids containing NRE-A and PRE were used, transcription was only induced by nuclear extracts from rested but not anergic T cells. These findings suggest the functional relevance of transcriptional repression of the IL-2 gene in anergic T cells.

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.

In vivo stimulation of I kappa B phosphorylation is not sufficient to activate NF-kappa B

NF-kappa B is a major inducible transcription factor in many immune and inflammatory reactions. Its activation involves the dissociation of the inhibitory subunit I kappa B from cytoplasmic NF-kappa B/Rel complexes, following which the Rel proteins are translocated to the nucleus, where they bind to DNA and activate transcription. Phosphorylation of I kappa B in cell-free experiments results in its inactivation and release from the Rel complex, but in vivo NF-kappa B activation is associated with I kappa B degradation. In vivo phosphorylation of I kappa B alpha was demonstrated in several recent studies, but its role is unknown. Our study shows that the T-cell activation results in rapid phosphorylation of I kappa B alpha and that this event is a physiological one, dependent on appropriate lymphocyte costimulation. Inducible I kappa B alpha phosphorylation was abolished by several distinct NF-kappa B blocking reagents, suggesting that it plays an essential role in the activation process. However, the in vivo induction of I kappa B alpha phosphorylation did not cause the inhibitory subunit to dissociate from the Rel complex. We identified several protease inhibitors which allow phosphorylation of I kappa B alpha but prevent its degradation upon cell stimulation, presumably through inhibition of the cytoplasmic proteasome. In the presence of these inhibitors, phosphorylated I kappa B alpha remained bound to the Rel complex in the cytoplasm for an extended period of time, whereas NF-kappa B activation was abolished. It appears that activation of NF-kappa B requires degradation of I kappa B alpha while it is a part of the Rel cytoplasmic complex, with inducible phosphorylation of the inhibitory subunit influencing the rate of degradation.

Ca2+ mobilization in physiologically stimulated single T cells gradually increases with peptide concentration (analog signaling)

We have investigated Ca2+ mobilization in single T cells stimulated with their physiological ligand, i.e. antigenic peptide bound to major histocompatibility complex (MHC) molecules on antigen-presenting cells (APC). Fibroblasts expressing I-Ed class II molecules were pulsed with a peptide derived from the lambda 2(315) immunoglobulin light chain. Onto such antigen-pulsed fibroblasts were sedimented cloned Th1 cells loaded with Fura-2. Changes in cytosolic Ca2+ concentration in single T cells were continually monitored by use of an imaging system based on fluorometry. Ca2+ mobilization was both peptide-specific and MHC-restricted. Within seconds of the initial APC-T cell contact, a Ca2+ spike could be observed. The Ca2+ response gradually declined over a 25-min period, during which oscillations were noted. Various parameters characterizing the magnitude of the Ca2+ response (latency, increase rate, max and mean Ca2+ increase, frequency and period of oscillations) all correlated with the amount of peptide used for pulsing the fibroblasts. Thus, Ca2+ mobilization in single T cells appears not to be an all or none phenomenon. Rather, activation is incremental (analog signaling), the degree of Ca2+ mobilization probably being related to the number of stimulatory peptide-MHC complexes on the surface of the APC. The extent of calcium mobilization and lymphokine production (interleukin (IL)-2, IL-3, interferon-gamma) correlated, at least at the population level.