Bcl-3 and NFκB p50-p50 Homodimers Act as Transcriptional Repressors in Tolerant CD4+ T Cells

Abstract

The transcriptional events that control T cell tolerance are still poorly understood. To investigate why tolerant T cells fail to produce interleukin (IL)-2, we analyzed the regulation of NF B-mediated transcription in CD4 ؉ T cells after tolerance induction in vivo. We demonstrate that a predominance of p50-p50 homodimers binding to the IL-2 promoter B site in tolerant T cells correlated with repression of NF B-driven transcription. Impaired translocation of the p65 subunit in tolerant T cells was a result from reduced activation of I B kinase and poor phosphorylation and degradation of cytosolic I Bs. Moreover, tolerant T cells expressed high amounts of the p50 protein. However, the increased expression of p50 could not be explained by activation-induced de novo synthesis of the precursor p105, which was constitutively expressed in tolerant T cells. We also demonstrate the exclusive induction of the I B protein B cell lymphoma 3 (Bcl-3) in tolerant T cells as well as its specific binding to the NF B site. These results suggest that the cellular ratio of NF B dimers, and thus the repression of NF B activity and IL-2 production, are regulated at several levels in tolerant CD4 ؉ T cells in vivo.Induction of T cell tolerance may result in either deletion or anergy, the latter being characterized by a block in proliferation and diminished expression of Th1 cytokines, particularly interleukin-2 (IL-2) 1 (1). Although T cell anergy has been described both in vitro and in vivo (2-5), recent studies have shown that rather than being functionally inert, the anergic T cells may adopt the role of regulatory T cells with a Tr1-like phenotype (6, 7). Importantly, anergic T cells may be functionally regulated at several different levels, e.g. in vivo tolerized T cells may neither produce IL-2 nor respond to signaling through the IL-2 receptor (8).Because the failure of tolerant T cells to produce IL-2 is a hallmark of anergy, it is particularly important to delineate the molecular explanation for this defect. Downstream of the T cell receptor (TCR), several intracellular signaling pathways converge at the level of transcriptional regulation of the IL-2 gene. AP-1, NFAT, and NF B families of transcription factors are essential in T cell activation as coordinators of IL-2 transcription (9) and tolerant CD4 ϩ T cell clones as well as in vivo tolerized T cells possess reduced AP-1 binding to the IL-2 promoter (10 -12). In addition, we have previously shown that CD4 ϩ T cells from mice tolerized in vivo express qualitative alterations in the composition of the NF B complex (13). Defective NF B expression is also a feature of tolerant B cells (14,15).The NF B family consists of five members, p65, c-rel, RelB, p105/p50, and p100/p52, and the NF B complex is formed by hetero-or homodimerization of these different proteins (16). p65, c-rel, and RelB are synthesized as mature proteins and contain transactivation domains, whereas p105/p50 and p100/ p52 are produced as inactive precursor proteins and lack transactivation domains. Activa...

NF-κB in immunobiology

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Tumor Necrosis Factor α Induces Spermidine/Spermine N1-Acetyltransferase through Nuclear Factor κBin Non-small Cell Lung Cancer Cells

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