Proliferating human medullary thymocytes can exhibit characteristic T helper cell type 1 cytokine responses exemplified by the immediate early expression of interleukin-2, interferon-␥, tumor necrosis factor-␣, and lymphotoxin-␤. 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-␥ 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, granulocytemacrophage colony-stimulating factor, and tumor necrosis factor-␣. 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 colonystimulating factor, and tumor necrosis factor-␣ 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.It is well established that cAMP signaling is inhibitory to T cell proliferation and effector functions. In particular, cAMP inhibits the expression of T helper-1 cytokine genes (1-3). Earlier reported studies of fibroblasts showed that elevated levels of intracellular cAMP inhibit upstream signal transduction pathways involved in cell growth and differentiation (4, 5). In contrast to fibroblasts, in which elevated levels of intracellular cAMP inhibit extracellular signal-regulated kinases 1 and 2 and c-Jun NH 2 -terminal kinases involved in the signal transduction of mitogen-activated protein kinase pathways, T cell extracellular signal-regulated kinases 1 and 2 are insensitive to elevated levels of intracellular cAMP (6). Moreover, the cAMP-mediated inhibition of c-Jun NH 2 -terminal kinase in T cells shows delayed kinetics, an observation that correlates with the induction of the cAMP-inducible early repressor ICER 1 (7). In addition, overexpression of NFAT achieved by transfection of NFAT-encoding cDNAs to lymphoma cells abrogates the sensitivity of cAMP-mediated inhibition of IL-2 gene expression (8, 9). Importantly, phosphorylation of aminoterminal serines of NFAT by protein kinase A does no...

No abstract

Inducible cAMP early repressor (ICER) is a transcriptional repressor, which, because of alternate promoter use, is generated from the 3′ region of the cAMP response modulator (Crem) gene. Its expression and nuclear occurrence are elevated by high cAMP levels in naturally occurring regulatory T cells (nTregs). Using two mouse models, we demonstrate that nTregs control the cellular localization of ICER/CREM, and thereby inhibit IL-2 synthesis in conventional CD4 + T cells. Ablation of nTregs in depletion of regulatory T-cell (DEREG) mice resulted in cytosolic localization of ICER/CREM and increased IL-2 synthesis upon stimulation. Direct contacts between nTregs and conventional CD4 + T cells led to nuclear accumulation of ICER/CREM and suppression of IL-2 synthesis on administration of CD28 superagonistic (CD28SA) Ab. In a similar way, nTregs communicated with B cells and induced the cAMPdriven nuclear localization of ICER/CREM. High levels of ICER suppressed the induction of nuclear factor of activated T cell c1 (Nfatc1) gene in T cells whose inducible Nfatc1 P1 promoter bears two highly conserved cAMP-responsive elements to which ICER/ CREM can bind. These findings suggest that nTregs suppress T-cell responses by the cAMP-dependent nuclear accumulation of ICER/ CREM and inhibition of NFATc1 and IL-2 induction.adenosin 3′,5′-cyclic monophospate | transcription | lymphocytes

Elevated levels of intracellular cyclic adenosine monophosphate (cAMP) in naturally occurring T regulatory (nTreg) cells play a key role in nTreg-cell-mediated suppression. Upon contact with nTreg cells, cAMP is transferred from nTreg cells into activated target CD4 + T cells and/or antigen-presenting cells (APCs) via gap junctions to suppress CD4+ T-cell function. cAMP facilitates the expression and nuclear function of a potent transcriptional inhibitor, inducible cAMP early repressor (ICER), resulting in ICER-mediated suppression of interleukin-2 (IL-2). Furthermore, ICER inhibits transcription of nuclearfactor of activated T cell c1/α (NFATc1/α) and forms inhibitory complexes with preexisting NFATc1/c2, thereby inhibiting NFAT-driven transcription, including that of IL-2. In addition to its suppressive effects mediated via ICER, cAMP can also modulate the levels of surface-expressed cytotoxic T lymphocyte antigen-4 (CTLA-4) and its cognate B7 ligands on conventional CD4 + T cells and/or APCs, fine-tuning suppression. These cAMP-driven nTreg-cell suppression mechanisms are the focus of this review. Keywords: CD4+ T cells r Costimulatory molecules r Gene regulation r Regulatory T (Treg) cells r Signal transduction IntroductionNaturally occurring CD4 + T regulatory (nTreg) cells are essential for maintaining peripheral tolerance; they prevent autoimmunity and limit chronic inflammatory diseases [1]. Immune responses, both protective and harmful, are principally mediated by T and B cells, which possess enormous diversity in antigen recognition, potent effector functions, and long-lasting immunologic memory. Every adaptive immune response involves the recruitment and activation of not only effector T and B cells but also nTreg cells, and the balance between effector and regulatory lymphocytes is criticalCorrespondence: Dr. Josef Bodor e-mail: bodor@uni-mainz.de for the proper control of adaptive immune responses. This balance is also critical for establishing or breaching tolerance against selfand nonself-antigens. Aside from peripheral conversion, the majority of nTreg cells are generated in the thymus with their frequency increasing from the late CD4 + CD8 + double positive to the CD4 + CD8 − single positive stage [2,3]. Remarkably, this coincides with the stage of thymic development during which human medullary thymocytes acquire cyclic AMP (cAMP) mediated expression of inducible cAMP early repressor (ICER) [4]. As ICER is a mediator of nTregcell suppression (see below and the section cAMP and Foxp3 direct ICER-mediated suppression), this suggests that competence to suppress develops in nTreg cells simultaneously with the ability of CD4 + T cells to be suppressed via upregulation of ICER.nTreg cells can have both beneficial effects, for example, preventing autoimmune diseases, and deleterious effects, for Eur. J. Immunol. 2012Immunol. . 42: 1375Immunol. -1384 example, impairing effective antitumor responses. Understanding the mechanisms of immunological self-tolerance, including those regulated by nTreg cells...

Here, we report that inducible cAMP early repressor/cAMP response element modulator (ICER/CREM) is induced early in CD25 + CD4 + regulatory T cell (T R ) assays mainly in activated Foxp3 -effector T cells and this induction correlates with sharp decrease in number of IL-2-expressing T cells. Importantly, RNAi targeting of ICER/CREM in responder CD25 -CD4 + T cells antagonizes T R -mediated suppression. Moreover, forced expression of Foxp3 in naive CD25 -T cells induces constitutive expression of ICER/ CREM in T cells with a regulatory phenotype. Foxp3 facilitates expression of ICER/ CREM both in Foxp3 transductants as well as CD25 -responder T cells suggesting that induction of T R function in suppression assays may utilize contact-dependent interaction. Indeed, CTLA-4 blockade or use of B7-deficient CD25 -responder T cells prevents ICER/CREM accumulation and leads to the rescue of IL-2 expression. Therefore, we propose that CTLA-4 binding to B7 ligands expressed on activated ligandbearing Foxp3 -effector T cells results in ICER/CREM-mediated transcriptional attenuation of IL-2. Collectively, these data suggest that Foxp3 expression in T R cells imposes suppression in contact-dependent fashion by induction of constitutive ICER/ CREM expression in activated CD25 + Foxp3 -T cell effectors thus preventing them from producing IL-2.

Allogeneic transplantation with CCR5-delta 32 (CCR5-d32) homozygous stem cells in an HIV infected individual in 2008, led to a sustained virus control and probably eradication of HIV. Since then there has been a high degree of interest to translate this approach to a wider population. There are two cellular ways to do this. The first one is to use a CCR5 negative cell source e.g., hematopoietic stem cells (HSC) to copy the initial finding. However, a recent case of a second allogeneic transplantation with CCR5-d32 homozygous stem cells suffered from viral escape of CXCR4 quasi-species. The second way is to knock down CCR5 expression by gene therapy. Currently, there are five promising techniques, three of which are presently being tested clinically. These techniques include zinc finger nucleases (ZFN), clustered regularly interspaced palindromic repeats/CRISPR-associated protein 9 nuclease (CRISPR/Cas9), transcription activator-like effectors nuclease (TALEN), short hairpin RNA (shRNA), and a ribozyme. While there are multiple gene therapy strategies being tested, in this review we reflect on our current knowledge of inhibition of CCR5 specifically and whether this approach allows for consequent viral escape.

No abstract

How regulatory T (TR) cells dampen T cell responses remains unclear. Multiple modes of action have been proposed, including cell contact-dependent and/or cytokine-dependent mechanisms. Suppression may involve direct contact between TR cells and responder T cells. Alternatively, TR cells may act on dendritic cells to reduce their ability to prime T cells by modulating costimulation, inducing the secretion of suppressive cytokines or the increase of tryptophan metabolism. Here, we review emerging, novel mechanisms involved in contact-dependent, TR-mediated suppression of IL-2 production in responder CD25- T lymphocytes and the potential involvement of inducible cAMP early repressor (ICER) in this suppression. Finally, cytokines such as TGF-beta and IL-10, produced by TR cells or other cells, may exert local suppression, which can be conveyed by basic mechanism(s) acting in a similar manner as contact-dependent, TR-mediated suppression.