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

The human T-cell lymphoma/leukemia viruses

The primate T-cell lymphoma/leukemia viruses belong to an oncogenic genus of complex retroviruses. Members of this genus have been shown to be pathogenic in man. The human T-cell lymphoma/leukemia virus (HTLV) type I has been linked in the etiology of T-cell malignancies and "autoimmune-like" neurologic and rheumatic disorders; a related virus, HTLV-II, is becoming increasingly associated with similar disorders. Cell transformation is thought to be caused predominantly by the effects of the viral regulatory protein, Tax. An additional induced host cell molecule, adult T-cell lymphoma-derived factor, may contribute to cell immortalization. Like the DNA tumor viruses, HTLV activates transcription of cellular proto-oncogenes and inhibits cellular mechanisms of tumor suppression, cell cycle arrest, and apoptosis. However, individuals who are able to mount a strong cell-mediated immune response and limit viral entry into uninfected cells do not develop associated malignancies. Unfortunately, HTLV-induced malignancies are difficult to treat with conventional chemotherapy, and disease progression is often rapid with a median survival of less than 2 years. There are, however, some novel approaches that have yet to be fully tested that may have greater efficacy in the treatment of HTLV-induced diseases. In the future, better screening and detection methods, along with new vaccines and therapies, may contribute to the increased prevention and control of HTLV infection and its associated diseases.

Changes in chromatin accessibility across the GM-CSF promoter upon T cell activation are dependent on nuclear factor kappaB proteins

Granulocyte/macrophage colony-stimulating factor (GM-CSF) is a key cytokine in myelopoiesis and aberrant expression is associated with chronic inflammatory disease and myeloid leukemias. This aberrant expression is often associated with constitutive nuclear factor (NF)-kappaB activation. To investigate the relationship between NF-kappaB and GM-CSF transcription in a chromatin context, we analyzed the chromatin structure of the GM-CSF gene in T cells and the role of NF-kappaB proteins in chromatin remodeling. We show here that chromatin remodeling occurs across a region of the GM-CSF gene between -174 and +24 upon T cell activation, suggesting that remodeling is limited to a single nucleosome encompassing the proximal promoter. Nuclear NF-kappaB levels appear to play a critical role in this process. In addition, using an immobilized template assay we found that the ATPase component of the SWI/SNF chromatin remodeling complex, brg1, is recruited to the GM-CSF proximal promoter in an NF-kappaB-dependent manner in vitro. These results suggest that chromatin remodeling across the GM-CSF promoter in T cells is a result of recruitment of SWI/SNF type remodeling complexes by NF-kappaB proteins binding to the CD28 response region of the promoter.

Targeting of p300 to the interleukin-2 promoter via CREB-Rel cross-talk during mitogen and oncogenic molecular signaling in activated T-cells

In this report, we explore the mechanisms of targeting of p300 to the interleukin-2 (IL-2) promoter in response to mitogenic and oncogenic molecular signals. Recruitment of p300 by cAMP-responsive element-binding protein-Rel cross-talk at the composite CD28 response element (CD28RE)-TRE element of the IL-2 promoter is essential for promoter inducibility during T-cell activation, and CD28RE-TRE is the exclusive target of the human T-cell lymphotropic virus type I oncoprotein Tax. The intrinsic histone acetyltransferase activity of p300 is dispensable for activation of the IL-2 promoter, and the N-terminal 743 residues contain the minimal structural requirements for synergistic transactivation of the CD28RE-TRE, the IL-2 promoter, and endogenous IL-2 gene expression. Mutational analysis of p300 reveals differential structural requirements for the N-terminal p300 module by individual cis-elements within the IL-2 promoter. These findings provide evidence that p300 assembles at the IL-2 promoter to form an enhanceosome-like signal transduction target that is centrally integrated at the CD28RE-TRE element of the IL-2 promoter through specific protein module-targeted associations in activated T-cells.

A NF-kappa B/Sp1 region is essential for chromatin remodeling and correct transcription of a human granulocyte-macrophage colony-stimulating factor transgene

The GM-CSF gene is expressed following activation of T cells. The proximal promoter and an upstream enhancer have previously been characterized using transfection and reporter assays in T cell lines in culture. A 10.5-kb transgene containing the entire human GM-CSF gene has also been shown to display inducible, position-independent, copy number-dependent transcription in mouse splenocytes. To determine the role of individual promoter elements in transgene function, mutations were introduced into the proximal promoter and activity assessed following the generation of transgenic mice. Of four mutations introduced into the transgene promoter, only one, in an NF-kappaB/Sp1 region, led to decreased induction of the transgene in splenocytes or bone marrow-derived macrophages. This mutation also affected the activity of reporter gene constructs stably transfected into T cell lines in culture, but not when transiently transfected into the same cell lines. The mutation alters the NF-kappaB family members that bind to the NF-kappaB site as well as reducing the binding of Sp1 to an adjacent element. A DNase I hypersensitive site that is normally generated at the promoter following T cell activation on the wild-type transgene does not appear in the mutant transgene. These results suggest that the NF-kappaB/Sp1 region plays a critical role in chromatin remodeling and transcription on the GM-CSF promoter in primary T cells.

Molecular pathways in virus-induced cytokine production

Virus infections induce a proinflammatory response including expression of cytokines and chemokines. The subsequent leukocyte recruitment and antiviral effector functions contribute to the first line of defense against viruses. The molecular virus-cell interactions initiating these events have been studied intensively, and it appears that viral surface glycoproteins, double-stranded RNA, and intracellular viral proteins all have the capacity to activate signal transduction pathways leading to the expression of cytokines and chemokines. The signaling pathways activated by viral infections include the major proinflammatory pathways, with the transcription factor NF-kappaB having received special attention. These transcription factors in turn promote the expression of specific inducible host proteins and participate in the expression of some viral genes. Here we review the current knowledge of virus-induced signal transduction by seven human pathogenic viruses and the most widely used experimental models for viral infections. The molecular mechanisms of virus-induced expression of cytokines and chemokines is also analyzed.

Reciprocal interactions between human T-lymphotropic virus type 1 and prostaglandins: implications for viral transmission

Human T-lymphotropic virus type 1 (HTLV-1), the etiologic agent of adult T-cell leukemia/lymphoma, is transmitted through breast milk and seminal fluid, which are rich in prostaglandins (PGs). We demonstrate that PGE(2) upregulates the HTLV-1 long terminal repeat promoter through the protein kinase A pathway, induces replication of HTLV-1 in peripheral blood mononuclear cells (PBMC) derived from asymptomatic carriers, and enhances transmission of HTLV-1 to cord blood mononuclear cells (CBMC). Furthermore, HTLV-1 Tax transactivates a promoter for cyclooxygenase 2, a PG synthetase, and induces PGE(2) expression in PBMC or CBMC. Thus, HTLV-1 interacts with and benefits from PGs, constituents of its own vehicle for transmission.

Functional interaction between the HIV transactivator Tat and the transcriptional coactivator PC4 in T cells

The human immunodeficiency virus (HIV) transactivator Tat is a potent activator of transcription from the HIV long terminal repeat and is essential for efficient viral gene expression and replication. Tat has been shown to interact with components of the basal transcription machinery and transcriptional activators. Here we identify the cellular coactivator PC4 as a Tat-interacting protein using the yeast two-hybrid system and confirmed this interaction both in vitro and in vivo by coimmunoprecipitation. We found that this interaction has a functional outcome in that PC4 overexpression enhanced activation of the HIV long terminal repeat in transient transfection studies in a Tat-dependent manner. The domains of PC4 and Tat required for the interaction were mapped. In vitro binding studies showed that the basic transactivation-responsive binding domain of Tat is required for the interaction with PC4. The minimum region of PC4 required for Tat binding was amino acids 22-91, whereas mutation of the lysine-rich domain between amino acids 22 and 43 prevented interaction with Tat. Tat-PC4 interactions may be controlled by phosphorylation, because phosphorylation of PC4 by casein kinase II inhibited interactions with Tat both in vivo and in vitro. We propose that PC4 may be involved in linking Tat to the basal transcription machinery.

An ordered array of cold shock domain repressor elements across tumor necrosis factor-responsive elements of the granulocyte-macrophage colony-stimulating factor promoter

The tumor necrosis factor-alpha-responsive region of the human granulocyte-macrophage colony-stimulating factor (GM-CSF) promoter (-114 to -31) encompasses binding sites for NF-kappaB, CBF, AP-1, ETS, and NFAT families of transcription factors. We show both here and previously that mutation of any one of these binding sites greatly reduces tumor necrosis factor-alpha induction of the GM-CSF promoter. Interspersed between these elements are sequences that when mutated lead to an increase in GM-CSF promoter activity. We have previously shown that two of these repressor elements bind proteins known as cold shock domain (CSD) factors and that overexpression of CSD proteins leads to repression of GM-CSF promoter activity in fibroblasts. CSD proteins are single strand DNA- and RNA-binding proteins that contact 5'-CCTG-3' sequences in the GM-CSF repressor elements. We show here that two newly identified repressor sequences in the proximal promoter can also bind CSD proteins. We have characterized the CSD-containing protein complexes that bind to the GM-CSF promoter and identified a novel protein related to mitochondrial single strand binding protein that forms part of one of these complexes. The four CSD-binding sites on the promoter occur in pairs on opposite strands of the DNA and appear to form an ordered array of binding elements. A similar ordered array of CSD sites are present in the promoters of the granulocyte colony-stimulating factor and interleukin-3 genes, implying a common mechanism for negative regulation of these myeloid growth factors.

Nuclear factor of activated T cells contributes to the function of the CD28 response region of the granulocyte macrophage-colony stimulating factor promoter

The granulocyte macrophage colony stimulating factor (GM-CSF) promoter contains a 10 bp element known as CK-1 or CD28RE that specifically responds to the co-stimulatory signal delivered to T cells via the CD28 surface receptor. This element is a variant NFkappaB site that does not function alone but requires an adjacent promoter region that includes a classical NFkappaB element, an Sp-1 site and a putative activator protein-1 (AP-1)-like binding site. The entire region is referred to as the CD28 response region (CD28RR). The GM-CSF CK-1 element has been shown to bind NFkappaB proteins, in particular c-Rel, whose binding and function is dependent on the architectural transcription factor HMGI(Y). It has been previously suggested that the nuclear factor of activated T cells (NFAT) family of proteins also plays a role in the activity of this region. We show here that recombinant NFATp but not AP-1 can bind to the GM-CSF CD28RR. NFATp present in activated Jurkat T cell extracts can also interact with the CD28RR. The binding of NFATp and Rel proteins requires the same core CK-1 sequences, and appears to be mutually exclusive. We investigated the functional significance of NFATp binding to CK-1 by overexpressing the protein in Jurkat T cells and found that NFATp cannot activate the CD28RR alone but can cooperate with signals generated by phorbol 12-myristate 13-acetate/calcium ionophore. The CD28RR is therefore a complex region that can bind and respond to a combination of transcription factors and signals.

HTLV type I Tax activation of the CXCR4 promoter by association with nuclear respiratory factor 1

Human T lymphotropic virus type I trans-activator Tax protein regulates expression of several cellular genes that are involved in cellular activation, proliferation, and transformation. Tax mediates its regulatory activity through interaction with cellular transcription factors such as members of the cAMP-responsive element-binding factors/ATF family or the NF-kappaB/Rel family. In this study we have demonstrated that Tax trans-activates the promoter for CXCR4, a coreceptor for T cell-tropic HIV-1 through its association with nuclear respiratory factor 1 (NRF1). The promoter region for CXCR4 contains an NRF1-binding site, which is crucial for basal and Tax-induced activity. Glutathione S-transferase (GST) pull-down experiments showed association of GST-Tax fusion protein with NRF1 in vitro. Expression of Tax, in addition to stimulation with phorbol myristate acetate and ionomycin, increased formation of the NRF1 complex in a gel-mobility shift assay, indicating that Tax association with NRF1 in vivo facilitates its DNA binding. HTLV-I Tax activation of CXCR4 may contribute to the rapid progression of HIV disease observed in certain coinfected individuals.

Involvement of NF-AT in type I human T-cell leukemia virus Tax-mediated Fas ligand promoter transactivation

Human T-cell leukemia virus type I (HTLV-I)-infected T-cells constitutively express surface Fas ligand (FasL), which may serve as a mechanism of viral pathogenesis. HTLV-I induces transcription of FasL gene through the viral transactivator Tax, although the underlying molecular mechanism remains unclear. In the present study, we have analyzed both the cis-activating element and transactivating factors involved in Tax activation of the FasL promoter. We show that the 486-base pair upstream region of the human FasL gene is sufficient for Tax-mediated transactivation in Jurkat T-cells. Interestingly, a palindromic DNA sequence (GGAAACTTCC) covering the consensus NF-ATp binding site (GGAAA), is required for Tax activation of FasL promoter. The involvement of NF-AT in this transactivation event is suggested by the finding that Tax fails to activate the FasL promoter in a Jurkat T-cell line defective in capacitative calcium entry, a signaling mechanism involved in NF-AT activation. Furthermore, activation of FasL promoter by Tax is largely attenuated in the nonlymphoid F9 embryonal and COS kidney cells deficient in NF-ATp expression. DNA-protein interaction assays reveal that the NF-AT-like motif in the FasL promoter is bound by both NF-ATp and NF-AT4 in Jurkat T-cells expressing Tax. The binding of NF-ATp, although not NF-AT4, to this enhancer also occurs along with HTLV-I-mediated infection of human peripheral blood T-cells. Furthermore, exogenously transfected NF-ATp binds to the NF-AT-like enhancer and participates in Tax-mediated FasL promoter transactivation. These results suggest an important role for proteins of the NF-AT family in HTLV-I Tax-mediated FasL gene transactivation.

Factors secreted by human T lymphotropic virus type I (HTLV-I)-infected cells can enhance or inhibit replication of HIV-1 in HTLV-I-uninfected cells: implications for in vivo coinfection with HTLV-I and HIV-1

It remains controversial whether human T lymphotropic virus type I (HTLV-I) coinfection leads to more rapid progression of human immunodeficiency virus (HIV) disease in dually infected individuals. To investigate whether HTLV-I infection of certain cells can modulate HIV-1 infection of surrounding cells, primary CD4(+) T cells were treated with cell-free supernatants from HTLV-I-infected MT-2 cell cultures. The primary CD4+ T cells became resistant to macrophage (M)-tropic HIV-1 but highly susceptible to T cell (T)-tropic HIV-1. The CC chemokines RANTES (regulated on activation, normal T cell expressed and secreted), macrophage inflammatory protein (MIP)-1alpha, and MIP-1beta in the MT-2 cell supernatants were identified as the major suppressive factors for M-tropic HIV-1 as well as the enhancers of T-tropic HIV-1 infection, whereas soluble Tax protein increased susceptibility to both M- and T-tropic HIV-1. The effect of Tax or CC chemokines on T-tropic HIV-1 was mediated, at least in part, by increasing HIV Env-mediated fusogenicity. Our data suggest that the net effect of HTLV-I coinfection in HIV-infected individuals favors the transition from M- to T-tropic HIV phenotype, which is generally indicative of progressive HIV disease.

HTLV-I and HTLV-II virus expression increase with HIV-1 coinfection

Coinfections with HIV-1 and HTLV-I or HTLV-II have been associated with unique immunophenotypes and an increased risk for development of neurodegenerative conditions. These findings may result from an increased HTLV-I or II viral burden in dually infected individuals. To investigate this possibility, HTLV-I/II tax/rex messenger RNA and viral antigen expression in peripheral blood mononuclear cells (PBMCs) were measured in 37 HTLV-I- or HTLV-II-infected subjects with or without HIV-1 coinfection. Tax/rex messenger RNA was detected in 14 of 24 PBMC samples from dually infected subjects, compared with only 1 of 13 PBMC samples from singly infected subjects (58% versus 7%; p < .003). The reverse transcription-polymerase chain reaction (RT-PCR) assay correlated with HTLV-I/II viral antigen detection in PBMC cultures but not with HIV-1 viral load levels in plasma. These findings may provide clues regarding the pathophysiologic consequences of HIV/HTLV-I and HIV/HTLV-II coinfections.

Constitutive expression of the HTLV-I pX and env regions in Jurkat T-cells induces differential activation of SRE, CRE and NF kappa B pathways

Human T-cell leukemia virus type I (HTLV-I) causes adult T-cell leukemia/lymphoma (ATLL). HTLV Tax, the viral transcriptional activator, can activate a variety of cellular genes. HTLV-mediated T-cell transformation, however, may involve additional viral proteins expressed from singly- as well as doubly-spliced viral mRNA. To determine the combined effect of these viral proteins on cellular gene expression in Jurkat T-cells, we derived stable transfectants that constitutively express the HTLV-I pX and env regions (J3.9). J3.9 cells show substantially increased mRNA levels of egr-1 and c-jun but no induction of either CD25 or GM-CSF by Northern blotting. This pattern corresponded to the activation of an egr-1 but not a GM-CSF promoter-driven reporter construct in transient gene expression assays. In DNA electrophoretic mobility shift assays (EMSA), nuclear extract from J3.9 cells has significantly increased binding to CRE and SRE but not nuclear factor kappa B (NF kappa B) DNA oligos, as compared to J-Neo cell extract. These results suggest that low level expression of pX and env region gene products in Jurkat T-cells stimulates persistent activation of CRE- and SRE- but not NF kappa B-induced cellular genes.

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.

Deoxyribonucleic acid triplex formation inhibits granulocyte macrophage colony-stimulating factor gene expression and suppresses growth in juvenile myelomonocytic leukemic cells

Juvenile myelomonocytic leukemia (JMML) is a severe childhood malignancy. The autocrine production of GMCSF is believed to be responsible for the spontaneous proliferation of JMML cells. A nuclear factor-kappaB (NF-kappaB)/Rel binding site within the GM-CSF gene promoter, termed the kappaB element, plays an important role in controlling transcription from the GM-CSF gene. We investigated the effect of an oligonucleotide GM3, directed to form a DNA triple helix across this kappaB element, on growth and GM-CSF production by JMML cells. Treatment of these cells, either unstimulated or induced by TNFalpha, with GM3 led to a significant and specific inhibition of both GM-CSF production and spontaneous colony formation. This constitutes the first report linking specific triplex-mediated inhibition of gene transcription with a functional outcome; i.e., cell growth. We observed the constitutive presence of NF-kappaB/Rel proteins in the nucleus of JMML cells. The constitutive and TNFalpha-induced NF-kappaB/Rel complexes were identical and were composed mainly of p50 and c-Rel proteins. Treatment of the cells with a neutralizing anti-TNFalpha monoclonal antibody completely abrogated constitutive nuclear expression of NF-kappaB/Rel proteins. These results indicate that the aberrant, constitutive GM-CSF gene activation in JMML is maintained by TNFalpha-mediated activation of NF-kappaB/Rel proteins. Our findings identify the molecular basis for the autocrine TNFalpha activation of the GM-CSF gene in JMML and suggest potential novel and specific approaches for the treatment of this aggressive childhood leukemia.

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.

Expression monitoring by hybridization to high-density oligonucleotide arrays

The human genome encodes approximately 100,000 different genes, and at least partial sequence information for nearly all will be available soon. Sequence information alone, however, is insufficient for a full understanding of gene function, expression, regulation, and splice-site variation. Because cellular processes are governed by the repertoire of expressed genes, and the levels and timing of expression, it is important to have experimental tools for the direct monitoring of large numbers of mRNAs in parallel. We have developed an approach that is based on hybridization to small, high-density arrays containing tens of thousands of synthetic oligonucleotides. The arrays are designed based on sequence information alone and are synthesized in situ using a combination of photolithography and oligonucleotide chemistry. RNAs present at a frequency of 1:300,000 are unambiguously detected, and detection is quantitative over more than three orders of magnitude. This approach provides a way to use directly the growing body of sequence information for highly parallel experimental investigations. Because of the combinatorial nature of the chemistry and the ability to synthesize small arrays containing hundreds of thousands of specifically chosen oligonucleotides, the method is readily scalable to the simultaneous monitoring of tens of thousands of genes.

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.