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

Inhibition of CD28/CD3-Mediated Costimulation of Naive and Memory Human T Lymphocytes by Intracellular Incorporation of Polyclonal Antibodies Specific for the Activator Protein-1 Transcriptional Complex

A number of indirect methods have been utilized in demonstrating activator protein-1 transcription factor function in IL-2 promoter activity. However, there has been no direct demonstration that activator protein-1 is involved in CD28-dependent costimulation of IL-2 gene transcription in freshly isolated naive and memory human T lymphocytes. To address this issue, the method of scrape loading was applied to purified peripheral blood T lymphocytes. Since scrape loading relies on adherent cells, peripheral blood human T (PB-T) cells were immobilized on the nonspecific cell attachment factor poly-l-lysine. Cells scraped off poly-l-lysine in the presence of Ig FITC efficiently incorporated Ig, with relatively uniform fluorescence. T cells retained their physical parameters as measured by forward and side light scatter, and functional activity as measured by costimulation of proliferation and IL-2 production after being scraped off this substrate. CD28/CD3-costimulated T cells produced intracellular IL-2 from all subsets measured (CD4+, CD4−, CD45RO+, and CD45RO−). IL-2 production and intracellular accumulation in nonscraped PB-T cells activated with CD28/CD3 coligation were skewed favoring CD45RO+ and CD4+ subsets, as was IL-2 production in scraped PB-T cells. The intracellular incorporation of Abs specific for c-Fos and c-Jun family members by scrape loading inhibited the production and intracellular accumulation of IL-2 within 6 h of costimulation with PMA/ionomycin, or costimulation by CD28 and CD3 ligation. Scrape loading thus provides an efficient mechanism for intracellular incorporation of macromolecules, and the first direct evidence that c-Fos and c-Jun are involved in transcription of the IL-2 gene within its correct chromosomal context, in resting human T lymphocyte subpopulations.

Role of mRNA stability in the different patterns of cytokine production by CD4+ cells from young and old mice

CD4+ cells from young (3 months) and old (19 months) mice were stimulated by plate-bound anti-CD3 monoclonal antibody (mAb) alone or also by soluble anti-CD28 mAb. Supernatants were analysed by enzyme-linked immunosorbent assay (ELISA) to determine cytokine concentrations. Total RNA was extracted from cells, reverse transcribed and the cDNA amplified by polymerase chain reaction (PCR) to evaluate the amount of specific mRNA. The results indicate that anti-CD3 alone is not sufficient to induce interleukin-2 (IL-2) production in CD4+ cells from both young and old mice. However, anti-CD28, together with anti-CD3 mAb, induces a much higher production of IL-2 in CD4+ cells from young as compared with old mice. Conversely, interferon-gamma (IFN-gamma) production is also induced by anti-CD3 alone and is higher in CD4+ cells from old as compared with young mice. Upon addition of anti-CD28 mAb, IFN-gamma production increases in both groups, but it remains much higher in old than in young mice. Also the production of IL-4 and IL-10 is induced by anti-CD3 mAb but it is increased by the addition of anti-CD28 mAb. CD4+ cells from old mice produce more IL-4 and IL-10 as compared with cells from young mice. The amounts of cytokine specific mRNA in CD4+ cells from young and old mice parallel the cytokine levels in culture supernatants. Results on the mRNA turnover indicate that when CD4+ cells are stimulated by anti-CD3 or costimulated also by anti-CD28 mAb, the IFN-gamma, IL-4 and IL-10 specific mRNAs are more stable in old than in young mice, suggesting that mRNA stability has a relevant role in the different patterns of cytokine production.

V7 (CD101) Ligation Inhibits TCR/CD3-Induced IL-2 Production by Blocking Ca2+ Flux and Nuclear Factor of Activated T Cell Nuclear Translocation

Ligation of the V7 (CD101) molecule on T cells with anti-V7 mAb blocks TCR/CD3-induced proliferation by inhibiting IL-2 transcription. To explore the basis for this observation, we analyzed the effects of V7 ligation on CD3/TCR-induced changes in intracellular free Ca2+ and Ca2+-dependent nuclear factor of activated T cells (NF-AT) translocation to the nucleus, which is required for IL-2 transcription. T cells exposed to anti-V7 mAb fluxed Ca2+ transiently, but did not flux Ca2+ in response to subsequent treatment with anti-CD3; however, they recovered the capacity to flux Ca2+ after treatment with pervanadate, indicating that tyrosine dephosphorylation of a critical V7-related substrate is required in the desensitization process. One such substrate, phospholipase C (PLC)-γ1, becomes tyrosine phosphorylated on CD3/TCR activation and mediates inositol triphosphate-dependent Ca2+ flux. Co-cross-linking of T cells with anti-CD3 and anti-V7 resulted in selective inhibition of PLC-γ1 tyrosine phosphorylation, which may explain V7-mediated blockade of anti-CD3-induced Ca2+ flux. Moreover, anti-CD3-induced binding of transcription factors to a consensus NF-AT-binding oligonucleotide, which is dependent on Ca2+, was blocked completely by treatment of the cells with anti-V7, whereas binding to a consensus-activating protein-1 oligonucleotide was unaffected. Western blot analysis of cytoplasmic and nuclear extracts confirmed that anti-V7 prevented nuclear translocation of NF-ATc induced by anti-CD3. We conclude that V7 ligation interferes with T cell activation and IL-2 secretion through a Ca2+ and tyrosine kinase-dependent pathway that inhibits PLC-γ1 phosphorylation and prevents NF-AT translocation to the nucleus.

Overexpression of p65 and c-Jun Substitutes for B7-1 Costimulation by Targeting the CD28RE Within the IL-2 Promoter

The role of Rel and activation protein-1 (AP-1) in IL-2 promoter activity in B7-1- and leukocyte function-associated Ag-3 (LFA-3)-costimulated T cells has been evaluated. We demonstrate that overexpression of c-Jun but not c-Fos increases IL-2 promoter activity in both B7-1- and LFA-3-costimulated Jurkat T cells. Cotransfection of both c-Jun and c-Fos substitutes for B7-1 costimulation in driving an activation protein-1 response element but not for the IL-2 promoter. Overexpression of Rel proteins demonstrated that p65-expressing Jurkat cells transcribed equally well a nuclear factor κβ reporter construct when costimulated with B7-1 or LFA-3, but transcription of IL-2 promoter or CD28 response element (CD28RE)-driven reporters was superior in B7-1-costimulated cells. Combined expression of c-Jun and p65 induced vigorous transcription of IL-2 promoter- and CD28RE-driven reporter constructs in both LFA-3- and B7-1-costimulated Jurkat cells. Mutating the CD28RE but not the upstream nuclear factor κβ-binding site in the IL-2 promoter reduced B7-1-driven transcription >90%. The results implicates a major role of the CD28RE in the integration of p65/c-Jun-mediated transcription within the IL-2 promoter. We suggest that the transition from an autocrine LFA-3-driven immune response to a B7--induced paracrine immune response involves the activation of c-Jun and p65, which target the CD28RE region of the IL-2 promoter.

The regulation and roles of Rel/NF-kappa B transcription factors during lymphocyte activation

The activation of B and T cells by a wide range of stimuli can rapidly induce specific gene expression via a mechanism that promotes the nuclear translocation of different Rel/nuclear factor-kappa B (NF-kappa B) transcription factors which are normally resident in the cytoplasm. Recent findings highlight the crucial roles of specific Rel/NF-kappa B family members in the processes of cell division, apoptosis and differentiation that accompany lymphocyte activation.

Mechanism responsible for T-cell antigen receptor- and CD28- or interleukin 1 (IL-1) receptor-initiated regulation of IL-2 gene expression by NF-kappaB

Initiation of the T-helper lymphocyte activation program is regulated through the T-cell receptor (TCR) and costimulatory receptors. Analysis of TCR and either anti-CD28- or interleukin 1 (IL-1)-mediated activation of the IL-2 promoter shows that costimulatory signals augment promoter activity through NF-kappaB sites. This study comparatively evaluates the mechanisms whereby signals initiated from the TCR and these two costimulatory receptors converge to synergistically increase NF-kappaB transcriptional activity. IL-1 alone stimulates an acute but transient NF-kappaB nuclear localization and a suboptimal NF-kappaB transcriptional response. In contrast, anti-CD3-anti-CD28 or anti-CD3-IL-1 synergistically stimulate prolonged NF-kappaB nuclear localization and NF-kappaB-mediated transcription. Both TCR- and costimulatory receptor-initiated synergistic NF-kappaB responses result from prolonging high rates of cytosolic IkappaB degradation during the second phase of the biphasic NF-kappaB nuclear localization. However, in contrast to previous reports, prolonged nuclear localization of NF-kappaB complexes is not necessarily associated with long-term depletion of IkappaBbeta. In response to either costimulus, c-Rel selectively translocated to the nucleus as a result of induced c-Rel expression and the continued production of c-Rel-IkappaBalpha complexes, which turn over rapidly due to the high rate of IkappaBalpha degradation in the cytosol during the second phase of the response. In contrast, IkappaBbeta is nearly completely degraded during the acute response to either IL-1 or anti-CD3-IL-1 while anti-CD3-anti-CD28 stimulates only a partial reduction (35 to 40%) in cytosolic IkappaBbeta. Cyclosporine (CsA), which inhibits stimulus-induced NF-kappaB transcriptional activity, selectively inhibits the stimulus-induced c-Rel nuclear localization and the rapid formation and degradation of c-Rel-IkappaBalpha complexes in the cytosol. CsA also inhibits both the prolonged, high rate of IkappaBalpha degradation and the lower level of IkappaBbeta turnover during the second phase of the activation response. Together, these results suggest a mechanism by which signals from the T-cell antigen receptor and either CD28 or IL-1 synergistically regulate IL-2 gene transcription by modulating NF-kappaB nuclear translocation.

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.

Transcriptional Repression of the IL-2 Gene in Th Cells by ZEB

Th1- and Th2-type cells mediate distinct effector functions via cytokine secretion in response to immunologic challenge. Precursor Th cells transcribe IFN-γ, IL-2, and IL-4 upon activation. Repeated stimulation of Th precursor cells in the presence of IL-4 leads to terminally differentiated Th2 cells that have lost the ability to transcribe the IL-2 gene. We provide evidence that repression of IL-2 gene expression in Th2 cells and partial repression in Th1 cells are mediated by ZEB, a zinc finger, E box-binding transcription factor. This factor binds to a negative regulatory element, NRE-A, in the IL-2 promoter, thereby acting as a potent repressor of IL-2 transcription.

Role of transcriptional repressor ICER in cyclic AMP-mediated attenuation of cytokine gene expression in human thymocytes

Proliferating human medullary thymocytes can exhibit characteristic T helper cell type 1 cytokine responses exemplified by the immediate early expression of interleukin-2, interferon-gamma, tumor necrosis factor-alpha, and lymphotoxin-beta. Here we report that cAMP-mediated attenuation of the transcription of T helper-1-specific cytokine genes in human medullary thymocytes correlates with the induction of the cAMP-mediated transcriptional repressor ICER (inducible cAMP early repressor). We show that ICER binds specifically to several NFAT/AP-1 (nuclear factor of activated T cells/activating protein-1) composite DNA sites essential for the activation of the interleukin (IL)-2 promoter as well as to a homologous DNA motif present in the proximal segment of the interferon-gamma promoter. In the presence of the minimal NFAT DNA-binding domain, which is sufficient for both DNA binding and AP-1 complex formation, ICER and NFAT form NFAT/ICER ternary complexes on several NFAT/AP-1 DNA composite sites previously identified as essential for the expression of the immunoregulatory cytokines such as IL-2, IL-4, granulocyte-macrophage colony-stimulating factor, and tumor necrosis factor-alpha. In extracts prepared from human medullary thymocytes treated with forskolin and ionomycin, these composite sites bind endogenously expressed ICER either singly or in complexes. Moreover, in Jurkat cells, ectopically expressed ICER represses transcription from NFAT-mediated, phorbol ester/ionophore-activated IL-2, granulocyte-macrophage colony-stimulating factor, and tumor necrosis factor-alpha promoters. We present evidence that ICER interactions with NFAT/AP-1 composite DNA sites correlate with its ability to repress transcription. These findings provide further insight into the mechanisms involved in cAMP-mediated transcriptional attenuation of cytokine expression.

Costimulatory CD80 (B7-1) and CD86 (B7-2) on cerebrospinal fluid cells in multiple sclerosis

The costimulatory CD80 and CD86 molecules were measured by flow cytometry on cerebrospinal fluid (CSF) and blood lymphocytes from patients with possible first attacks of multiple sclerosis (MS, n = 25), clinically definite MS (n = 16), and noninflammatory neurological disease control subjects (n = 30). In patients with demyelinating diseases more CSF B cells expressed CD80 than in control subjects whereas the expression of CD86 by T cells in CSF was low in patients with demyelinating disease and highly variable in the control subjects. In patients with possible first attacks of MS the expression pattern of CD80 and CD86 differed significantly between patients with or without intrathecal synthesis of IgG. Increased expression of the CD80 molecule on CSF B cells may be of importance in the pathogenesis of MS. In contrast, CSF T cell expression of CD86 may be associated with protection from MS.

Functional Consequences of Costimulation by ICAM-1 on IL-2 Gene Expression and T Cell Activation

LFA-1 is a well-recognized adhesion molecule, but its role in providing costimulatory signals to T cells has remained controversial. We have compared the ability of class II-positive transfectants that do and do not coexpress ICAM-1 (ProAd and ProAd-ICAM) to activate Ag-specific Th1 clones and naive CD4-positive T cells isolated from TCR transgenic mice. Ag presentation by ProAd to Th1 clones can induce calcium-dependent signaling events after engagement of the TCR, as evidenced by the nuclear localization of the transcription factors NF-AT and NF-κB. Nevertheless, coexpression of ICAM-1 or B7-1 on ProAd is required to induce detectable levels of IL-2 gene expression in either Th1 clones or naive T cells. In Th1 clones, activation by ProAd-ICAM induces very transient IL-2 mRNA expression that does not result in detectable IL-2 secretion or T cell proliferation. In naive T cells, the duration of IL-2 mRNA expression is longer, allowing for a transient burst of IL-2 protein that is sufficient to drive the cells into the cell cycle. In spite of this initial response, Ag presentation by ProAd-ICAM is a tolerogenic signal to naive T cells, and responding T cells undergo apoptosis 4 to 5 days poststimulation. These data suggest that engagement of LFA-1 can provide sufficient costimulatory signals to induce T cell activation and IL-2 gene expression, but cannot protect against anergy induction or provide for T cell survival.

The Tpl-2 protooncoprotein activates the nuclear factor of activated T cells and induces interleukin 2 expression in T cell lines

Tpl-2 expression is induced within 30-60 min after ConA stimulation of rat splenocytes, suggesting that it may contribute to the induction of IL-2 during T cell activation. Herein we show that wild-type and carboxyl-terminally truncated (activated) Tpl-2 activate the nuclear factor of activated T cells (NFAT) and induce interleukin 2 (IL-2) expression in EL4 cells. In Jurkat cells the truncated Tpl-2 activates NFAT and induces IL-2, whereas wild-type Tpl-2 activates NFAT only when cotransfected with NFAT expression constructs, suggesting that Tpl-2 may induce NFAT activation signals. Experiments in NIH 3T3 cells revealed that the NFATp isoform, but not the NFATc or NFATx isoform, undergoes nuclear translocation when coexpressed with wild-type Tpl-2 and confirmed this hypothesis. Activation of NFAT by anti-CD3 stimulation but not by phorbol 12-myristate 13-acetate and ionomycin in Jurkat cells was inhibited by the kinase-dead Tpl-2K167M, suggesting that Tpl-2 contributes to the transduction of NFAT activation signals originating in the T cell receptor. The Tpl-2-mediated induction of IL-2 was not observed in T cell lymphoma lines other than EL4 and Jurkat, as well as in normal T cells. NFAT activation by Tpl-2, however, was observed in several cell lines including some of nonhematopoietic origin. The activation of NFAT by Tpl-2 in different cell types defines a molecular mechanism that may contribute to its oncogenic potential.

A crucial role of host CD80 and CD86 in rat cardiac xenograft rejection in mice

BACKGROUND

Graft rejection can be initiated by two primary pathways of antigen presentation: (a) direct activation of host T cells by donor-derived antigen presenting cells (APC) and (b) indirect presentation of processed graft antigens by host APC.

METHODS

We investigated the differential roles for direct and indirect antigen presentation by preventing the CD28 costimulatory pathway with monoclonal antibodies to rat or mouse CD80 and CD86 in a rat-to-mouse cardiac transplantation model.

RESULTS

Although the mouse anti-rat monoclonal antibodies to CD80 and CD86 did not significantly prolong the survival of rat cardiac xenografts in mice, the rat anti-mouse monoclonal antibodies to CD80 and CD86 did prolong the survival. Development of the anti-donor antibodies was inhibited, and the deposition of C3, IgM, and IgG on endothelium in the xenografts was mild in the anti-mouse CD80/CD86-treated mice. Infiltration of macrophages, neutrophils, and lymphocytes expressing perforin and interferon-gamma was decreased by the anti-mouse CD80/CD86 treatment.

CONCLUSIONS

These findings suggest that the indirect antigen presentation, which is mediated by CD80 and CD86 pathway on host APC, plays a crucial role in concordant cardiac xenograft rejection.

Interleukin-2, interleukin-15, and their receptors

Both IL-15 and IL-2 are 14-15 kDa members of the four alpha-helical bundle family of cytokines that have T cell growth factor activity. In contrast to the pattern manifested by IL-2, IL-15 mRNA is produced by a wide variety of tissues other than T cells. We have demonstrated that IL-15 expression is posttranscriptionally regulated by multiple elements, including the ten upstream AUGs of the 5' UTR, a 48aa signal peptide and the carboxy-terminus of the mature protein. IL-15 utilizes two distinct receptor signaling pathways. In T cells the IL-15 receptor includes IL-2R beta and gamma c subunits shared with IL-2 as well as an IL-15 specific receptor, IL-15R alpha. However, mast cells respond to IL-15 using a receptor system that does not share elements with the IL-2R system but involves a novel 60-65 kDa IL-15RX subunit. In mast cells, IL-15 signaling involves JAK-2 and STAT-5 activation rather than the JAK-1 and JAK-3 as well as the STAT-3 and STAT-5 used by both IL-2 and IL-15 in activated T cells.

Superinduction of IL-8 in T Cells by HIV-1 Tat Protein Is Mediated Through NF-κB Factors

Elevated levels of circulating IL-8, a potent chemotactic factor for granulocytes and T lymphocytes, are found in HIV-infected individuals. The HIV-1 transactivator protein Tat increased IL-8 secretion in T cell lines following CD3- and CD28-mediated costimulation. Full-length Tat (Tat101) enhanced IL-8 transcription through up-regulated transcription factor binding to the CD28-responsive element (CD28RE) in the IL-8 promoter. Expression of the Tat splice variant Tat72 (72 amino acids) also enhanced IL-8 production following T cell stimulation via a different, most likely post-transcriptional, mechanism. The CD28RE in the IL-8 promoter was characterized as a low-affinity NF-κB binding site recognized by the transcription factors p50 (NF-κB1), p65 (RelA) and c-rel. Transcription factor binding to “classical” NF-κB sites in the HIV-1, the human IL-2, and lymphotoxin promoters, recognized by p50 and p65 following CD3+28-mediated costimulation, was unaffected by Tat101 as was binding to the AP-1 motif in the IL-8 promoter. These experiments identify the CD28RE in the IL-8 promoter as a c-rel recognition site and a Tat101-responsive element. The effect of Tat101 on CD28REs in the IL-8 promoter and the subsequent up-regulation of IL-8 secretion is likely to contribute to the immune dysregulation observed during HIV-1 infection.

Non-responsiveness of antigen-experienced CD4 T cells reflects more stringent co-stimulatory requirements

We recently reported that previously activated T cells, irrespective of the nature of the first stimulus they encountered, are unable to respond to Staphylococcal enterotoxin B (SEB), nor to soluble anti-CD3 monoclonal antibody (mAb) presented by splenic antigen-presenting cells (APC). Such previously activated T cells are, however, fully capable of responding to plate-bound anti-CD3 plus splenic APC. These data suggest differential integration of the T-cell receptor (TCR) and co-stimulatory signalling pathways in naive versus antigen-experienced T cells. Consistent with this hypothesis, anti-CD28 mAb restores the proliferative capacity of resting ex vivo CD45RBlo CD4+ T cells (representing previously activated T cells) to both soluble anti-CD3 mAb and SEB. Interestingly, mAb-mediated engagement of cytotoxic T-lymphocyte antigen-4 (CTLA-4) completely negates the rescue effects mediated by anti-CD28 mAb in CD45RBlo cells. Nevertheless, the non-responsiveness of CD45RBlo CD4+ T cells cannot be reversed by anti-CTLA-4 Fab fragments, indicating that it is not related to negative regulatory effects of CTLA-4 engagement itself. Interestingly, the addition of interleukin-2 (IL-2) restores the proliferative capacity of CD45RBlo CD4+ T cells to SEB and soluble anti-CD3 mAb. Moreover, when rescued by IL-2, the cells are less susceptible to the negative regulatory effects of CTLA-4 engagement. Together, these findings suggest that the non-responsiveness of CD45RBlo CD4+ T cells to certain stimuli may be related to inadequate TCR signalling, primarily affecting IL-2 production.

Simultaneous cross-linking of CD6 and CD28 induces cell proliferation in resting T cells

In the present study, we showed that simultaneous ligation of the monoclonal antibodies (mAb) against CD6 and CD28 induces T-cell proliferation in purified resting T lymphocytes in the absence of T-cell receptor (TCR) occupancy. No cell proliferation was observed when the mAb were cross-linked alone or used simultaneously in the soluble form. T-cell proliferation mediated through CD6/CD28 is accompanied by the up-regulation of interleukin-2 (IL-2) mRNA and expression of IL-2 receptors on the cell surface. In the presence of IL-2-neutralizing mAb the proliferative response of the T cell induced through CD6/CD28 was inhibited dose dependently. Cross-linking mAb to CD6 and CD28 alone or together did not down-regulate the CD3/TCR complex. T-cell proliferation mediated through CD6/CD28 was only partially blocked by the immunosuppressive drug, cyclosporin A (CsA), whereas anti-CD28-induced T-cell proliferation in the presence of the phorbol ester, 12-O-tetradecanoylphorbol-13-acetate (TPA), was unaffected. In sharp contrast T-cell proliferation mediated by anti-CD6 in the presence of TPA was efficiently blocked by CsA. In addition, two protein kinase C (PKC) inhibitors, GF 109203X and H-7 dose-dependently inhibited T-cell proliferation mediated through CD6/CD28, suggesting that PKC activation may be involved. Furthermore, there was a marked differential dose-dependent inhibitory effect of the PKC inhibitors on T-cell proliferation mediated by the co-ligation of anti-CD6 or anti-CD28 in the presence of anti-CD3, with the former being more sensitive to PKC inhibition. Taken collectively, our results suggest that T-cell activation can occur through an antigen-independent pathway by cross-linking the accessory molecules, CD6 and CD28, and that these two cell surface antigens may have distinct signalling pathways.

Analysis of the T-cell micro-environment in Epstein-Barr virus-related post-transplantation B lymphoproliferative disease

Epstein-Barr virus (EBV) post-transplantation B lymphoproliferative disease (BLPD) may undergo regression after immunosuppression withdrawal and restoration of EBV-specific cytotoxic T-cell (CTL) activity in the immunocompromised allografted host. The presence of morphologically normal T cells in the BLPD micro-environment may influence tumour behaviour in vivo. In this immunopathological study, the phenotype and the number of T cells and other immunoregulatory cells have been investigated in seven primary and four recurrent BLPD biopsies from nine solid organ transplant recipients. BLPD with either viral lymphadenopathic or polymorphic lymphoma appearances was found to contain sizeable T-cell populations, mainly of memory/helper (TCR alpha/beta +, CD3+, CD4+, CD45RO+) type. Cytotoxic (TCR alpha/beta +, CD3, CD8+, Tia-1+) T cells were strikingly low in all samples. Low CD28 and CD25 expression suggested that secondary signals for functional and sustained T-cell activation may be deficient in these tumours. No close correlation was found between the degree of T-cell infiltration and clinical outcome, although appreciably higher number of CD8+ T cells were detected in three BLPD tumours showing prolonged clinical remission after treatment. While some level of EBV-specific T-cell function may be present in untreated BLPD, the overall findings of this study suggest that the nature of T-cell infiltrates may reflect a response to immunosuppressive therapy rather than to EBV infection per se. The possibility that a local EBV-specific T-cell response is generated in BLPD undergoing regression after treatment needs to be investigated.

Coordinate transactivation of the interleukin-2 CD28 response element by c-Rel and ATF-1/CREB2

The interleukin-2 CD28 response element (CD28RE) acts as a composite enhancer, in conjunction with a 3'-12-O-tetradecanoylphorbol-13-acetate response element (TRE)-like element, to confer CD28 receptor-dependent inducibility to the interleukin-2 promoter in T-cells. When inserted as a single copy upstream of a basal promoter, this composite enhancer, termed the CD28RE-TRE, is both highly active and CD28-inducible in transactivation assays. A multicomponent nuclear protein complex that binds the CD28RE-TRE was isolated by DNA affinity chromatography from nuclear extracts of mitogen- and CD28 receptor-costimulated human T-cells. Immunological and biochemical analyses of this complex reveal the presence of c-Rel, ATF-1, and CREB2 as major DNA-binding components. Coexpression of c-Rel in combination with ATF-1, CREB2, or ATF-1/CREB2 leads to synergistic transactivation of a CD28RE-TRE reporter plasmid in quiescent Jurkat T-cells. Furthermore, CD28-dependent transactivation of the CD28RE-TRE is specifically inhibited by cAMP response element-binding protein (CREB) dominant-negative expression vectors. Moreover, mutant promoter constructs in which the internal 5'-CD28RE and 3'-TRE-like sequences have been topologically positioned 180 degrees out of phase with one another show loss of mitogen- and CD28-dependent inducibility. Finally, the addition of the CREB-binding transcriptional coactivator p300 leads to a dramatic CREB-dependent increase in both mitogen- and CD28-mediated transactivation of the CD28RE-TRE. These findings demonstrate that full physiological responsiveness to CD28 receptor stimulation in T-cells is dependent on topologically linked sequences within the CD28RE-TRE composite enhancer and provide strong support of a direct role for the CREB family of transcription factors and p300/CREB-binding protein coactivator proteins in cytokine gene induction during T-cell activation.

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.

Hepatocytes induce functional activation of naive CD8+ T lymphocytes but fail to promote survival

Intraperitoneal peptide injection of TCR-transgenic mice or expression of antigen in hepatocytes leads to an accumulation in the liver of specific apoptotic CD8+ T cells expressing activation markers. To determine whether liver cells are capable of directly activating naive CD8+ T cells, we have studied the ability of purified hepatocytes to activate TCR-transgenic CD8+ T cells in vitro. We show that hepatocytes which do not express CD80 and CD86 co-stimulatory molecules are able to induce activation and effective proliferation of specific naive CD8+ T cells in the absence of exogenously added cytokines, a property only shared by professional antigen-presenting cells (APC). Specific T cell proliferation induced by hepatocytes was comparable in magnitude to that seen in response to dendritic cells and was independent of CD4+ T cell help or bystander professional APC co-stimulation. During the first 3 days, the same number of divisions was observed in co-cultures of CD8+ T cells with either hepatocytes or splenocytes. Both APC populations induced expression of early T cell activation markers and specific cytotoxic T lymphocyte (CTL) activity. However, in contrast to T cells activated by splenocytes, T cells activated by hepatocytes lost their cytolytic function after 3 days of co-culture. This correlated with death of activated T cells, suggesting that despite efficient activation, proliferation and transient CTL function, T cells activated by hepatocytes did not survive. Death could be prevented by adding antigen-expressing splenocytes or exogenous IL-2 to the co-culture, indicating that hepatocytes are not involved in direct killing of CD8+ T cells but rather fail to promote survival. Dying cells acquired a CD8(low) TCR(low) B220+ phenotype similar to the one described for apoptotic intrahepatic T cells, suggesting an alternative model to account for the origin of these cells in the liver. The importance of these findings for the understanding of peripheral tolerance and the ability of liver grafts to be accepted is discussed.

The Ca2+/calmodulin-dependent kinase type IV is involved in the CD5-mediated signaling pathway in human T lymphocytes

The CD5 receptor on T lymphocytes is involved in T cell activation and T-B cell interactions. In the present study, we have characterized the signaling pathways induced by anti-CD5 stimulation in human T lymphocytes. In T lymphocytes, anti-CD5 co-stimulation enhances the phytohemagglutinin/anti-CD28-induced interleukin-2 (IL-2) mRNA accumulation 1.6-fold and IL-2 protein secretion 2. 2-fold, whereby the up-regulation is mediated at both the transcriptional and post-transcriptional level. The CD5 signaling pathway up-regulates the IL-2 gene expression by increasing the DNA binding and transactivation activity of activator protein 1 but affects none of the other transcription factors like nuclear factor of activated T cells, nuclear factor kappaB, Oct, and CD28-responsive complex/nuclear factor of mitogen-activated T cells involved in the regulation of the IL-2 promoter activity. The CD5-induced increase of the activator protein 1 activity is mediated through the activation of calcium/calmodulin-dependent (CaM) kinase type IV, and is independent of the activation of mitogen-activated protein kinases Jun N-terminal kinase, extracellular signal-regulated kinase, and p38/Mpk2, and calcium/calmodul-independent kinase type II. The expression of a dominant negative mutant of CaM kinase IV in T lymphocytes transfected with an IL-2 promoter-driven reporter construct completely abrogates the response to CD5 stimulation, indicating that CaM kinase IV is essential to the CD5 signaling pathway. In addition, it is demonstrated that calcium/calmodulin-dependent kinase type IV is also involved in the stabilization of the IL-2 transcripts, which is observed after co-stimulation of phytohemagglutinin/anti-CD28 activated T lymphocytes with anti-CD5.

Expansion of CD4+CD7- T cells, a memory subset with preferential interleukin-4 production, after bone marrow transplantation

BACKGROUND

Inadequate reconstitution of CD4+ lymphocyte and interleukin (IL)-2 production defect are observed after bone marrow or peripheral blood stem cell transplantation (SCT).

METHODS

We studied immune reconstitution after SCT in 33 consecutive patients who received allogeneic SCT (17 patients) or autologous SCT (16 patients). The aims were to assess the regeneration of the CD4+ T-cell subset with regard to helper cell differentiation. CD4+ T-cell subset regeneration and expansion of the CD4+CD7- subset were studied by immunofluorescence analysis. CD4+CD7- cell cytokine secretion was analyzed after cell sorting and costimulation of the CD3 and CD28 pathways, in enzyme-linked immunosorbent assay and reverse transcription-polymerase chain reaction assays.

RESULTS

We report a relative expansion of the CD4+CD7- subset within CD4+ T cells, detected as early as 1 month after bone marrow transplantation and decreasing after day 60. CD4+CD7- T cells preferentially expressed CD45RO and activation markers such as CD57, CD25, and HLA-DR. No relationship was observed between the CD4+CD7- expansion and transplant-related complications. We observed no significant IL-2 production in supernatants from sorted CD4+CD7- T cells, whereas IL-4 levels were comparable to those produced by cells from normal individuals. Autologous CD4+CD7+ cells showed little, if any, IL-4 production, and IL-2 production was lower than that by normal CD4+CD7+ T cells. Reverse transcription-polymerase chain reaction assays showed similar amounts of interferon-gamma transcripts in the two subsets; tumor necrosis factor-alpha, IL-4, and IL-10 transcripts were detected in CD4+CD7- T cells but not in their CD4+CD7+ counterparts.

CONCLUSIONS

These data confirm the IL-2 production defect after bone marrow transplantation and suggest that the CD4+CD7- T-cell subset might be preferentially involved in the enhanced production of IL-4 and low production of IL-2. These data show that the early immune reconstitution in CD4+ T cells after SCT preferentially involves memory T cells with a Th0/Th2 differentiation that might participate in the T-helper cell defect.

Defective integration of activating signals derived from the T cell receptor (TCR) and costimulatory molecules in both CD4+ and CD8+ T lymphocytes of common variable immunodeficiency (CVID) patients

CVID is characterized by hypogammaglobulinaemia and impaired antibody production. Previous studies demonstrated defects at the T cell level. In the present study the response of purified CD4+ and CD8+ T lymphocytes to stimulation with anti-TCR monoclonal antibody (the first signal) in combination with anti-CD4 or anti-CD8, anti-CD2 and anti-CD28 MoAbs (the costimulatory signals) was investigated. Both CD4+ and CD8+ T cells from the patients showed significantly reduced IL-2 release following stimulation via TCR and costimulation via CD4 or CD8 and CD2, respectively. However, normal IL-2 production following TCR plus phorbol myristate acetate (PMA) costimulation and normal expression of an early activation marker, CD69, after TCR+CD28 stimulation indicated that TCR was able to transduce a signal. Furthermore, both IL-2 and IL-4 release were impaired in CD4+ lymphocytes following TCR+CD28 stimulation. In addition, stimulation via TCR+CD28 resulted in significantly decreased expression of CD40 ligand in the patients. These results suggest that the integration of activating signals derived from the TCR and costimulatory molecules is defective in CVID patients; the defect is not confined to costimulation via a single molecule, or restricted to cells producing Th1-type cytokines such as IL-2, and is expressed in both CD4+ and CD8+ T cell subsets.

Simultaneous detection of DNA synthesis and cytokine production in staphylococcal enterotoxin B activated CD4+ T lymphocytes by flow cytometry

Assessment of T cell activation has traditionally been performed by measuring proliferation as a function of 3[H]-thymidine incorporation, or secretion of cytokines from activated peripheral blood mononuclear cells (PBMC) in culture. An alternative method for detection of proliferation at the single cell level utilizes incorporation of bromodeoxyuridine (BrdU), an analog of thymidine, into cellular DNA. After appropriate fixation and permeabilization of the cells, a monoclonal antibody (mAb) against BrdU conjugated with a fluorescent dye is employed to measure by flow cytometry the incorporated BrdU. Here, we report a flow cytometric procedure which can be used for the simultaneous detection of BrdU incorporation, activation markers such as CD69 and CD25, and intracellular cytokines in T cell subsets from activated PBMC. Our observations are consistent with the proposal that cytokine synthesis and cell proliferation occur sequentially in CD4+ T cells stimulated with the superantigen staphylococcal enterotoxin B (SEB). The majority of cells expressing the cytokines IFN-gamma and IL-2 at 48 h appear to have undergone DNA synthesis, however all proliferating cells do not express IFN-gamma or IL-2. The methods presented in this report offer a unique approach for studying simultaneous expression of key cellular activation events in phenotypically resolved lymphocyte populations.

Interleukin-7 Upregulates the Interleukin-2–Gene Expression in Activated Human T Lymphocytes at the Transcriptional Level by Enhancing the DNA Binding Activities of Both Nuclear Factor of Activated T Cells and Activator Protein-1

In the present report, we studied the role of the stromal-derived cytokine interleukin-7 (IL-7) in the IL-2–gene regulation in activated T lymphocytes. Production of IL-2 requires the formation of transcription factors involved in the IL-2 –gene regulation. T-cell receptor (TCR)/CD3 engagement results in the activation of nuclear factor of activated T cells (NFAT), activator protein-1 (AP-1), and nuclear factor κB (NFκB), whereas the CD28 responsive complex (CD28RC) is activated in response to the CD28 signal. Costimulation of phytohemagglutinin/anti-CD28 activated T lymphocytes with IL-7 induces a fivefold enhanced IL-2–mRNA accumulation and a 2.5-fold enhanced protein secretion. The IL-2–gene transcription rate is increased 3.4-fold, indicating that the effect of IL-7 is in part mediated at the transcriptional level. The molecular mechanisms underlying the IL-7 effect involve the upregulation of the DNA binding activity of NFAT (60%) and AP-1 (120%), without affecting the activities of NFκB and CD28RC, which was confirmed by transfection assays. We also show that the IL-7–induced enhancement of the AP-1–DNA binding activity is not cyclosporin A-sensitive. Since AP-1 is part of the NFAT complex, we conclude that the IL-7–signaling pathway is involved in the activation of the fos and jun proteins of which AP-1 consists.

Interleukin-7 Upregulates the Interleukin-2–Gene Expression in Activated Human T Lymphocytes at the Transcriptional Level by Enhancing the DNA Binding Activities of Both Nuclear Factor of Activated T Cells and Activator Protein-1

In the present report, we studied the role of the stromal-derived cytokine interleukin-7 (IL-7) in the IL-2–gene regulation in activated T lymphocytes. Production of IL-2 requires the formation of transcription factors involved in the IL-2 –gene regulation. T-cell receptor (TCR)/CD3 engagement results in the activation of nuclear factor of activated T cells (NFAT), activator protein-1 (AP-1), and nuclear factor κB (NFκB), whereas the CD28 responsive complex (CD28RC) is activated in response to the CD28 signal. Costimulation of phytohemagglutinin/anti-CD28 activated T lymphocytes with IL-7 induces a fivefold enhanced IL-2–mRNA accumulation and a 2.5-fold enhanced protein secretion. The IL-2–gene transcription rate is increased 3.4-fold, indicating that the effect of IL-7 is in part mediated at the transcriptional level. The molecular mechanisms underlying the IL-7 effect involve the upregulation of the DNA binding activity of NFAT (60%) and AP-1 (120%), without affecting the activities of NFκB and CD28RC, which was confirmed by transfection assays. We also show that the IL-7–induced enhancement of the AP-1–DNA binding activity is not cyclosporin A-sensitive. Since AP-1 is part of the NFAT complex, we conclude that the IL-7–signaling pathway is involved in the activation of the fos and jun proteins of which AP-1 consists.

Temporal and subunit-specific modulations of the Rel/NF-kappaB transcription factors through CD28 costimulation

Stimulation of highly purified primary T lymphocytes through CD2 and CD28 adhesion molecules induces a long-term proliferation, dependent on persistent autocrine secretion of interleukin 2 (IL-2), high and prolonged expression of inducible CD25/IL-2 receptor alpha chain (IL-2Ralpha), and secretion of growth factors such as the granulocyte-macrophage colony-stimulating factor (GM-CSF). CD28 costimulation appears to activate cytokine gene expression through conserved kappaB-related CD28 response (CD28RE) or cytokine 1 (CK-1) elements in addition to canonical NF-kappaB-binding sites. In this report, we assess: 1) the evolution of the expression, over an 8-day time period, of the Rel/NF-kappaB family of proteins in costimulated versus TcR/CD3-stimulated primary T cells; 2) the impact of changes on the in vitro occupancy of GM-CSF kappaB and CK-1, as well as IL-2Ralpha kappaB sites; and 3) the differential regulation of newly synthesized p65 and c-Rel by IkappaB proteins. We show that CD2+CD28 stimulation specifically induces, at maximal T cell proliferation phase, sustained nuclear overexpression of NFKB2 p52 and c-Rel subunits which might rely on long-lasting processing of p100 precursor for p52 and increased neosynthesis of c-Rel. This up-regulation correlates with sustained occupancy of GM-CSF kappaB and CK-1 elements by both proteins. Conversely, these subunits do not appear to bind to the IL-2Ralpha kappaB site. Costimulation, but not TcR/CD3 stimulation, appears supported by sustained down-regulation of both IkappaBalpha and -beta regulators. Furthermore, contrary to p65, c-Rel appears to display little affinity for p105, p100 and IkappaBalpha regulators.

p110delta, a novel phosphatidylinositol 3-kinase catalytic subunit that associates with p85 and is expressed predominantly in leukocytes

We have identified a novel p110 isoform of phosphatidylinositol 3-kinase from human leukocytes that we have termed p110delta. In addition, we have independently isolated p110delta from a mouse embryo library on the basis of its ability to interact with Ha-RasV12 in the yeast two-hybrid system. This unique isoform contains all of the conserved structural features characteristic of the p110 family. Recombinant p110delta phosphorylates phosphatidylinositol and coimmunoprecipitates with p85. However, in contrast to previously described p110 subunits, p110delta is expressed in a tissue-restricted fashion; it is expressed at high levels in lymphocytes and lymphoid tissues and may therefore play a role in phosphatidylinositol 3-kinase-mediated signaling in the immune system.

Transcription factor binding sites downstream of the human immunodeficiency virus type 1 transcription start site are important for virus infectivity

When transcriptionally active, the human immunodeficiency virus (HIV) promoter contains a nucleosome-free region encompassing both the promoter/enhancer region and a large region (255 nucleotides [nt]) downstream of the transcription start site. We have previously identified new binding sites for transcription factors downstream of the transcription start site (nt 465 to 720): three AP-1 sites (I, II, and III), an AP3-like motif (AP3-L), a downstream binding factor (DBF) site, and juxtaposed Sp1 sites. Here, we show that the DBF site is an interferon-responsive factor (IRF) binding site and that the AP3-L motif binds the T-cell-specific factor NF-AT. Mutations that abolish the binding of each factor to its cognate site are introduced in an infectious HIV-1 molecular clone to study their effect on HIV-1 transcription and replication. Individual mutation of the DBF or AP3-L site as well as the double mutation AP-1(III)/AP3-L did not affect HIV-1 replication compared to that of the wild-type virus. In contrast, proviruses carrying mutations in the Sp1 sites were totally defective in terms of replication. Virus production occurred with slightly delayed kinetics for viruses containing combined mutations in the AP-1(III), AP3-L, and DBF sites and in the AP3-L and DBF-sites, whereas viruses mutated in the AP-1(I,II,III) and AP3-L sites and in the AP-1(I,II,III), AP3-L, and DBF sites exhibited a severely defective replicative phenotype. No RNA-packaging defect could be measured for any of the mutant viruses as determined by quantification of their HIV genomic RNA. Measurement of the transcriptional activity of the HIV-1 promoter after transient transfection of the HIV-1 provirus DNA or of long terminal repeat-luciferase constructs showed a positive correlation between the transcriptional and the replication defects for most mutants.

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 through B7-2 (CD86) is required for the induction of a lung mucosal T helper cell 2 (TH2) immune response and altered airway responsiveness

The recruitment of eosinophils into the airways after allergen exposure is dependent on interleukin (IL) 5 secreted from antigen-specific CD4+ T cells of the T helper cell (Th) 2 subset. However, while it is established that costimulation through CD28 is required for TCR-mediated activation and IL-2 production, the importance of this mechanism for the induction of a Th2 immune response is less clear. In the present study, we administered the fusion protein CTLA-4 immunoglobulin (Ig) into the lungs before allergen provocation to determine whether CD28/CTLA-4 ligands are required for allergen-induced eosinophil accumulation and the production of Th2 cytokines. Administration of CTLA-4 Ig inhibited the recruitment of eosinophils into the lungs by 75% and suppressed IgE in the bronchoalveolar lavage fluid. CTLA-4 Ig also inhibited the production of IL-4, IL-5, and IL-10 by 70-80% and enhanced interferon-gamma production from CD3-T cell receptor-activated lung Thy1.2+ cells. Allergen exposure upregulated expression of B7-2, but not B7-1, on B cells from the lung within 24 h. Moreover, airway administration of an anti-B7-2 monoclonal antibody (mAb) inhibited eosinophil infiltration, IgE production, and Th2 cytokine secretion comparable in magnitude to that observed with CTLA-4 Ig. Treatment with an anti-B7-1 mAb had a small, but significant effect on eosinophil accumulation, although was less effective in inhibiting Th2 cytokine production. The anti-B7-2, but not anti-B7-1, mAb also inhibited antigen-induced airway hyperresponsiveness in vivo. In all of the parameters assessed, the combination of both the anti-B7-1 and anti-B7-2 mAb was no more effective than anti-B7-2 mAb treatment alone. We propose that strategies aimed at inhibition of CD28 interactions with B7-2 molecules may represent a novel therapeutic target for the treatment of lung mucosal allergic inflammation.

Regulation of the interleukin-2 CD28-responsive element by NF-ATp and various NF-kappaB/Rel transcription factors

The CD28 costimulatory signal enhances antigen-mediated induction of interleukin-2 (IL-2) gene transcription through activation of an enhancer termed the CD28-responsive element (CD28RE). Although various nuclear proteins have been shown to bind to CD28RE, their in vivo functions in the regulation of this enhancer remain elusive. In this report, we show that CD28RE binds distinct transcription factors in cells treated with different mitogenic stimuli. Stimulation of the T-cell receptor (TCR) complex in the absence of a CD28 costimulatory signal induces a member of the nuclear factor of the activated T cells, NF-ATp; however, this treatment fails to activate the CD28RE enhancer activity. Significant activation of CD28RE was detected when the cells were treated with both the TCR stimulators and an anti-CD28 monoclonal antibody (anti-CD28), which induces the NF-kappaB/Rel enhancer binding proteins in addition to NF-ATp. The costimulatory activity of anti-CD28 can be further enhanced by a phorbol ester. Kinetic analyses demonstrate that activation of endogenous IL-2 gene transcription is correlated with the binding of CD28RE by NF-ATp and different NF-kappaB/Rel species. Transient-transfection studies reveal that expression of either NF-ATp or the p50-RelA NF-kappaB heterodimer leads to the potent transactivation of both the CD28RE enhancer and the intact IL-2 promoter in mitogen-stimulated cells. Remarkably, coexpression of these two families of enhancer-binding proteins in Jurkat T cells results in the transactivation of the CD28RE enhancer even in the absence of any cellular stimuli. Together, these results suggest that activation of IL-2 gene transcription by the TCR- and CD28-mediated signals involves the interaction of CD28RE with NF-ATp and various NF-kappaB/Rel transcription factors.

Immune hyperactivation of HIV-1-infected T cells mediated by Tat and the CD28 pathway

Human immunodeficiency virus-type 1 (HIV-1) infection is characterized by a chronic state of immune hyperactivation in patients. Infection of human peripheral blood lymphocytes with HIV-1 in vitro resulted in increased interleukin-2 (IL-2) secretion in response to T cell activation via the CD3 and CD28 receptors. Expression of the HIV-1 transactivator Tat recapitulated this phenotype and was associated with increased IL-2 secretion in response to costimulation with CD3 plus CD28. IL-2 superinduction by Tat occurred at the transcriptional level, was mediated by the CD28-responsive element in the IL-2 promoter, and was exclusively dependent on the 29 amino acids encoded by the second exon of Tat.

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.

CD28-mediated activation of resting human T cells without costimulation of the CD3/TCR complex

Stimulation of resting human T cells by crosslinked CD28 monoclonal antibodies (mAb) induces some early signaling events but does not lead to IL-2 secretion and proliferation. The induction of these functions usually requires the delivery of additional signals such as that provided by costimulation of the T cell receptor (TCR). We analyzed the capacity of a panel of different CD28 mAb to induce cellular functions in purified human T cells. Two patterns of reactivity were observed. "Costimulatory" CD28 mAb like 9.3 required coengagement of the CD3/TCR complex for the induction of IL-2 gene transcription and proliferation. On the other hand, a "stimulatory" pathway could be defined by the use of the CD28 mAb BW 828, which triggered IL-2 synthesis, IL-2R expression, and proliferation without further requirement for additional stimuli. BW 828-induced proliferation was sensitive to inhibition by cyclosporin A and was mainly found in the CD4+CD45R0+ ("memory") T cell subset. These data suggest that T cell stimulation with mAb BW 828 defines a CD28-associated signaling pathway which leads to the induction of effector functions without the need for CD3/TCR coengagement. This pathway might play a role in antigen-independent activation and expansion of T cells.

c-Rel is a target of pentoxifylline-mediated inhibition of T lymphocyte activation

The possible clinical use of the methyl xanthine derivative, pentoxifylline (PF), for the treatment of T cell-dependent diseases is being noted with increasing interest. In this paper, we studied the molecular consequences of PF treatment during lymphocyte activation. We found that in T cells, anti-CD3-induced c-Rel expression was blocked by PF, whereas the induction of other NF-kappaB family members was not significantly affected. However, induction of NF-AT, which has the same signaling requirements as c-Rel induction, was not inhibited by PF. Among genes that respond to these transcription factors, IL-2 mRNA induction was suppressed by PF, whereas IL-2R(alpha) chain mRNA induction was not affected. These observations implicated c-Rel as an IL-2 promoter factor, for which experimental support was obtained from transient transfection experiments. In contrast with the observation in T cells, c-Rel induction was not blocked by PF in B cells. The greater selectivity of PF, compared with FK506, at both the molecular and cellular levels may prove advantageous in manipulating T cell responses in vivo.

Constitutive dephosphorylation and activation of a member of the nuclear factor of activated T cells, NF-AT1, in Tax-expressing and type I human T-cell leukemia virus-infected human T cells

The tax gene product of the type I human T-cell leukemia virus (HTLV-I) transactivates interleukin-2 (IL-2) gene through activation of an enhancer termed CD28 responsive element (CD28RE). Tax activation of the CD28RE is partially mediated by a member of the nuclear factor of activated T cells, NF-AT1. We have previously shown that NF-AT1 is constitutively active in Jurkat T cells stably transfected with the Tax cDNA, although the underlying molecular mechanism and physiological relevance of this finding remain unclear. In this report, we demonstrate that the active form of NF-AT1 is also present in the nuclei of HTLV-I-transformed T cells that express the Tax protein. Interestingly, the constitutive activation of NF-AT1 in these T cells is associated with its dephosphorylation. Furthermore, the dephosphorylated NF-AT1 can be rapidly rephosphorylated when the cells are incubated with cyclosporin A, an immunosuppressant inhibiting the serine/threonine phosphatase calcineurin. These results suggest that activation of NF-AT1 in Tax-expressing and HTLV-I-transformed T cells results from its dephosphorylation, which in turn may be due to deregulation of calcineurin.

Transcription factors of the NFAT family: regulation and function

As targets for the immunosuppressive drugs cyclosporin A and FK506, transcription factors of the NFAT (nuclear factor of activated T cells) family have been the focus of much attention. NFAT proteins, which are expressed in most immune-system cells, play a pivotal role in the transcription of cytokine genes and other genes critical for the immune response. The activity of NFAT proteins is tightly regulated by the calcium/calmodulin-dependent phosphatase calcineurin, a primary target for inhibition by cyclosporin A and FK506. Calcineurin controls the translocation of NFAT proteins from the cytoplasm to the nucleus of activated cells by interacting with an N-terminal regulatory domain conserved in the NFAT family. The DNA-binding domains of NFAT proteins resemble those of Rel-family proteins, and Rel and NFAT proteins show some overlap in their ability to bind to certain regulatory elements in cytokine genes. NFAT is also notable for its ability to bind cooperatively with transcription factors of the AP-1 (Fos/Jun) family to composite NFAT:AP-1 sites, found in the regulatory regions of many genes that are inducibly transcribed by immune-system cells. This review discusses recent data on the diversity of the NFAT family of transcription factors, the regulation of NFAT proteins within cells, and the cooperation of NFAT proteins with other transcription factors to regulate the expression of inducible genes.

Antigen presentation by autoreactive proteolipid protein peptide-specific T cell clones from chronic progressive multiple sclerosis patients: roles of co-stimulatory B7 molecules and IL-12

To assess the role of T cell antigen (Ag) presentation in multiple sclerosis (MS), proteolipid protein (PLP) peptide reactive CD4+ T cell clones (TCCs) from MS patients and normal subjects were studied. TCCs derived from chronic progressive (CP) MS patients were able to proliferate and secret cytokines in response to PLP peptide stimulation in the absence of professional antigen presenting cells (APCs), suggesting that these T cells can simultaneously present and respond to Ags. However, they did not respond to total PLP protein, suggesting that PLP-peptide TCCs were unable to process and present the whole PLP molecule. The ability of the different TCCs to act as APCs in response to Ag stimulation did not correlate with expression of HLA-class II molecules. However, the degree of expression of B7-1 and B7-2 co-stimulatory molecules showed a significant correlation with APC capacity. Furthermore, a combination of anti-B7-1 and anti-B7-2 mAbs effectively inhibited proliferative responses as well as secretion of IL-10, IFN gamma and TGF beta induced by antigen presenting T cells. By contrast, IL-4 secretion was not affected. Finally, IL-12 significantly enhanced the efficiency of T cell Ag presentation by a pathway independent of Ag processing, suggesting that IL-12 might act as an additional co-stimulatory signal for T cell activation during T-T cell interactions. Together, these observations suggest that Ag presentation by T cells might amplify and perpetuate an autoimmune response previously initiated by professional APCs. These properties may account for progression of MS into a CP phase.

CD28 mediates a potent costimulatory signal for rapid degradation of IkappaBbeta which is associated with accelerated activation of various NF-kappaB/Rel heterodimers

Optimal activation of T cells requires at least two signals delivered by the T-cell receptor complex and costimulatory molecules such as CD28. The CD28 signaling participates in the transcription of the interleukin-2 gene through activation of an enhancer termed the CD28-responsive element (CD28RE). Stimulation of CD28 enhances mitogen-mediated induction of CD28RE-binding proteins including members of the NF-kappaB/Rel transcription factor family, although the underlying mechanism remains elusive. In this report, we show that CD28 costimulation leads to biphasic induction of NF-kappaB/Rel heterodimers, including early-phase induction of p50/RelA and c-Rel/RelA and late-phase induction of p50/c-Rel. Interestingly, activation of these NF-kappaB/Rel complexes by the CD28 signal is associated with the rapid degradation of both IkappaBalpha and IkappaBbeta, two major cytoplasmic inhibitors of NF-kappaB/Rel. Although IkappaBalpha degradation can be induced by phorbol ester alone, degradation of IkappaBbeta is largely dependent on the CD28 costimulatory signal. We further demonstrate that CD28-mediated transactivation of the CD28RE enhancer is potently inhibited by an N-terminal truncation mutant of IkappaBbeta that is incapable of responding to the degradation signals. Together, these results suggest that the CD28 costimulatory signal augments activation of NF-kappaB/Rel by promoting degradation of IkappaBbeta as well as enhancing degradation of IkappaBalpha and that induction of NF-kappaB/Rel serves as an essential step in the signal-mediated activation of the CD28RE enhancer.

Regulation of interleukin-2 transcription by inducible stable expression of dominant negative and dominant active mitogen-activated protein kinase kinase kinase in jurkat T cells. Evidence for the importance of Ras in a pathway that is controlled by dual receptor stimulation

Engagement of the T cell receptor induces the activation of several mitogen-activated protein kinase modules, including the extracellular signal-regulated kinase and c-Jun N-terminal kinase (JNK) cascades. Whereas extracellular signal-regulated kinase is activated by T cell receptor/CD3 ligation alone, activation of JNK requires co-stimulation by the CD28 receptor. Activation of MEKK-1, which acts as a mitogen-activated protein kinase kinase kinase in the JNK pathway, was also induced by CD3 plus CD28 (CD3/CD28) ligation in Jurkat cells. To study the significance of the JNK cascade in T lymphocytes, we established stable Jurkat cell lines that inducibly express dominant active (DA) or dominant negative (DN) MEKK-1. Whereas expression of DA-MEKK-1 resulted in the constitutive activation of JNK along with the transcriptional activation of the minimal interleukin-2 (IL-2) promoter, DN-MEKK-1 inhibited JNK responsiveness during CD3/CD28 co-stimulation. In addition to inhibiting CD3/CD28-induced IL-2 mRNA expression, DN-MEKK-1 abrogated the transcriptional activation of the IL-2 promoter and the distal nuclear factor of activated T cells (NFAT)-activating protein 1 (AP-1) response element in that promoter. A c-Jun mutant lacking activation sites for JNK also interfered with the activation of the distal NFAT/AP-1 complex, suggesting that the JNK pathway functions by controlling AP-1 response elements in the IL-2 promoter. Using inducible stable expression of DA- and DN-Ras in Jurkat cells, we found that Ras regulates JNK activation in these cells. Our results suggest that the dual ligation of CD3 and CD28 in T cells triggers a cascade of events that involve Ras, the JNK cascade, and one or more AP-1 response elements in the IL-2 promoter.

High mobility group protein I(Y) is required for function and for c-Rel binding to CD28 response elements within the GM-CSF and IL-2 promoters

CD28 response elements (CD28REs) within cytokine promoters are variant NF-kappaB-binding sites and are essential for transcription in response to CD28 receptor activation in T cells. We show that the CK-1 element (CD28RE) within the GM-CSF promoter binds the RelA and c-Rel transcription factors in response to CD28 activation. We further show that the high mobility group protein HMG I(Y) can bind to the CD28REs of both GM-CSF and IL-2 and that this binding is critical for c-Rel, but not RelA, binding. A second NF-kappaB site in the GM-CSF promoter that binds p50 and RelA, but neither c-Rel nor HMG I(Y), failed to respond to CD28 activation. Expression of HMG I or c-Rel antisense RNA inhibited CD28 activation of the IL-2 and GM-CSF promoters, implying that HMG I(Y) enhancement of c-Rel binding plays an important role in the activity of the CD28REs.

Glucocorticoids modulate CD28 mediated pathways for interleukin 2 production in human T cells: evidence for posttranscriptional regulation

In T cells stimulated through the T cell receptor (TCR), both cyclosporine (CsA) and glucocorticoids (GC) inhibit the transcription of the IL-2 gene. In these cells costimulation via the CD28 cell surface molecule further increases the transcription of IL-2 and stabilizes its mRNA, resulting in a 20-30 fold induction in IL-2 production. This pathway is relatively resistant to the inhibitory effect of CsA. In this study, we asked whether GC interfere with CD28-mediated costimulatory signals for T cell activation. Primary human T cells or Jurkat T cells were stimulated with anti-CD28 and phorbol myristate acetate (PMA) in the presence of dexamethasone (Dex, 10(-10)-10(-5) M). Dex inhibited both the mRNA for IL-2 and IL-2 production in a dose-dependent fashion (minimum effective dose 10(-9) M). In similar experiments employing anti-CD3 mAb and PMA, a 7-20 fold higher concentration of Dex was required to obtain comparable inhibition. To determine if transcriptional modulation is occurring, Jurkat T cells were transfected with a plasmid containing the IL-2 promoter linked to the chloramphenicol acetyl transferase reporter gene. Following stimulation with ionomycin and PMA, high doses (10(-6) M) of Dex inhibited the activity of the IL-2 promoter (approximately 50% inhibition). However, in the presence of anti-CD28 mAb, this promoter became resistant to Dex (< or = 10% inhibition). These results suggest that GC inhibit accessory pathways for IL-2 production via CD28 by predominantly posttranscriptional mechanisms. Inhibition of the CD28 pathway may be an important mechanism for the T cell directed immunosuppressive effects of low-to-moderate doses of GC.