Lymphocyte activation and the family of NF-kappa B transcription factor complexes

Enhanced interaction between SEC2 mutant and TCR Vβ induces MHC II-independent activation of T cells via PKCθ/NF-κB and IL-2R/STAT5 signaling pathways

SEC2, a major histocompatibility complex class II (MHC II)-dependent T-cell mitogen, binds MHC II and T-cell receptor (TCR) Vβs to induce effective co-stimulating signals for clonal T-cell expansion. We previously characterized a SEC2 mutant with increased recognition of TCR Vβs, ST-4, which could intensify NF-κB signaling transduction, leading to IL-2 production and T-cell activation. In this study, we found that in contrast to SEC2, ST-4 could induce murine CD4⁺ T-cell proliferation in a Vβ8.2- and Vβ8.3-specific manner in the absence of MHC II⁺ antigen-presenting cells (APCs). Furthermore, although IL-2 secretion in response to either SEC2 or ST-4 stimulation was accompanied by up-regulation of protein kinase Cθ (PKCθ), inhibitor of κB (IκB), α and β IκB kinase (IKKα/β), IκBα, and NF-κB in mouse splenocytes, only ST-4 could activate CD4⁺ T cells in the absence of MHC II⁺ APCs through the PKCθ/NF-κB signaling pathway. The PKCθ inhibitor AEB071 significantly suppressed SEC2/ST-4-induced T-cell proliferation, CD69 and CD25 expression, and IL-2 secretion with or without MHC II⁺ APCs. Further, SEC2/ST-4-induced changes in PKCθ/NF-κB signaling were significantly relieved by AEB071 in a dose-dependent manner. Using Lck siRNA, we found that Lck controlled SEC2/ST-4-induced phosphorylation of PKCθ. We also demonstrated that the IL-2R/STAT5 pathway is essential for SEC2/ST-4-induced T-cell activation. Collectively, our data demonstrate that an enhanced ST-4-TCR interaction can compensate for lack of MHC II and stimulate MHC II-free CD4⁺ T-cell proliferation via PKCθ/NF-κB and IL-2R/STAT5 signaling pathways. Compared with SEC2, intensified PKCθ/NF-κB and IL-2R/STAT5 signals induced by ST-4 lead to enhanced T-cell activation. The results of this study will facilitate better understanding of TCR-based immunotherapies for cancer.

An analog of the natural steroidal alkaloid cortistatin A potently suppresses Tat-dependent HIV transcription

The human immunodeficiency virus type 1 (HIV) Tat protein, a potent activator of HIV gene expression, is essential for integrated viral genome expression and represents a potential antiviral target. Tat binds the 5'-terminal region of HIV mRNA's stem-bulge-loop structure, the transactivation-responsive (TAR) element, to activate transcription. We find that didehydro-Cortistatin A (dCA), an analog of a natural steroidal alkaloid from a marine sponge, inhibits Tat-mediated transactivation of the integrated provirus by binding specifically to the TAR-binding domain of Tat. Working at subnanomolar concentrations, dCA reduces Tat-mediated transcriptional initiation/elongation from the viral promoter to inhibit HIV-1 and HIV-2 replication in acutely and chronically infected cells. Importantly, dCA abrogates spontaneous viral particle release from CD4(+)T cells from virally suppressed subjects on highly active antiretroviral therapy (HAART). Thus, dCA defines a unique class of anti-HIV drugs that may inhibit viral production from stable reservoirs and reduce residual viremia during HAART.

Constitutive activity of NF-kappa B in myeloid cells drives pathogenicity of monocytes and macrophages during autoimmune neuroinflammation

The NF-κB/REL-family of transcription factors plays a central role in coordinating the expression of a wide variety of genes controlling immune responses including autoimmunity of the central nervous system (CNS). The inactive form of NF-κB consists of a heterodimer which is complexed with its inhibitor, IκB. Conditional knockout-mice for IκBα in myeloid cells (lysMCreIκBα^fl/fl) have been generated and are characterized by a constitutive activation of NF-κB proteins allowing the study of this transcription factor in myelin-oligodendrocyte-glycoprotein induced experimental autoimmune encephalomyelitis (MOG-EAE), a well established experimental model for autoimmune demyelination of the CNS.

In comparison to controls, lysMCreIκBα^fl/fl mice developed a more severe clinical course of EAE. Upon histological analysis on day 15 p.i., there was an over two fold increased infiltration of T-cells and macrophages/microglia. In addition, lysMCreIκBα^fl/fl mice displayed an increased expression of the NF-κB dependent factor inducible nitric oxide synthase in inflamed lesions. These changes in the CNS are associated with increased numbers of CD11b positive splenocytes and a higher expression of Ly6c on monocytes in the periphery. Well in accordance with these changes in the myeloid cell compartment, there was an increased production of the monocyte cytokines interleukin(IL)-12 p70, IL-6 and IL-1beta in splenocytes. In contrast, production of the T-cell associated cytokines interferon gamma (IFN-gamma) and IL-17 was not influenced.

In summary, myeloid cell derived NF-κB plays a crucial role in autoimmune inflammation of the CNS and drives a pathogenic role of monocytes and macrophages independently from T-cells.

Activation of nuclear factor kappa B (NF-kappaB) assayed by laser scanning cytometry (LSC)

Nuclear factor kappa B (NF-kappaB)/rel is the family of ubiquitous transcriptional activators involved in regulation of diverse immune and inflammatory responses. It also plays a role in control of cell growth and apoptosis. In its inactive form NF-kappaB remains in the cytoplasm sequestered through interaction with IkappaB protein. Rapid translocation of NF-kappaB from cytoplasm to nucleus that occurs in response to extracellular signals is considered to be a hallmark feature of its activation. The translocation of NF-kappaB in HL-60, U-937 and Jurkat leukemic cells as well as in human fibroblasts induced by tumor necrosis factor alpha (TNF-alpha) or phorbol myristate acetate (PMA) was presently measured by laser scanning cytometry (LSC). NF-kappaB was detected immunocytochemically with FITC-tagged antibody and its presence in the nucleus vis-a-vis cytoplasm was monitored by measuring the green fluorescence integrated over the nucleus, which was counterstained with propidium iodide (PI), and over the cytoplasm, respectively. Activation of NF-kappaB led to a rapid increase in NF-kappaB-associated fluorescence measured over the nucleus (FN) concomitant with a decrease in fluorescence over the cytoplasm (F(C)), which was reflected by an increase in F(N)/F(C) ratio. This rapid assay of NF-kappaB activation can be combined with morphological identification of the activated cells or with their immunophenotype. Bivariate analysis of NF-kappaB expression versus cellular DNA content makes it possible to correlate its activation with the cell cycle position. The described method has a potential to be used as a functional assay to monitor intracellular translocation of other transcriptional activators such as p53 tumor suppressor protein or signal transduction molecules.

Factors affecting the cytokine production of human T cells stimulated by different modes of activation

According to the widely accepted classification, human TH cell clones can be divided into two mutually exclusive subsets, TH1 and TH2, based on their profile of cytokine production. The intracellular difference between these clones is not clear. To characterize the biochemical nature of T-cell receptor (TCR)/CD3 complex-mediated signal transduction pathways, we introduced several human TH cell clones of THO- or TH1-like phenotype and analyzed the effects of various drugs and antibodies on cytokine production or proliferation of these clones. The tyrosine kinase inhibitor herbimycin inhibited the production of interferon-gamma (IFN-gamma) by THO-like clone, after stimulation with anti-CD3 monoclonal antibody alpha CD3-mAb) or with phorbol 12-myristate 13-acetate (PMA) and the calcium ionophore A23187. However, whereas herbimycin strongly inhibited the production of IL-4 and IL-5 by alpha CD3 mAb stimulated T cells, it did not affect the production of these cytokines after PMA/A23187 stimulation. Cyclosporin A inhibited the proliferation as well as the production of the cytokines, including that of IL-2, IL-4, IL-5, and IFN-gamma, irrespective of the mode of stimulation. A23187, which synergizes with PMA in the induction of IL-4 and IFN-gamma, inhibited PMA-induced IL-10 production in a dose-dependent manner. Transforming growth factor-beta and anti-IL-2 receptor monoclonal antibody partially inhibited alpha CD3 mAb-mediated T-cell proliferation, but had no effect on the proliferation induced by PMA and A23187. Cyclic adenosine monophosphate (cAMP)-elevating drugs, like prostaglandin E2 and dibutyryl cAMP, inhibited the TCR-mediated cytokine production but shifted the cytokine production profile from a TH0 to a TH2 type after stimulation with PMA and A23187. Finally, we analyzed the induction of activity of two transcription factors, nuclear factor-kappa B (NF-kappa B) and nuclear factor of activated T cells, involved in the regulation of cytokine gene expression, after a different mode of activation. The induction of NF-kappa B (p50/p65 heterodimer) by using alpha CD3-mAb stimulation but not by using PMA/A23187 stimulation was found to be inhibited by using cAMP-elevating drugs.

Coexpression of NF-kappa B/Rel and Sp1 transcription factors in human immunodeficiency virus 1-induced, dendritic cell-T-cell syncytia

Productive infection of T cells with human immunodeficiency virus 1 (HIV-1) typically requires that the T cells be stimulated with antigens or mitogens. This requirement has been attributed to the activation of the transcription factor NF-kappa B, which synergizes with the constitutive transcription factor Sp1 to drive the HIV-1 promoter. Recently, we have found that vigorous replication of HIV-1 takes place in nonactivated memory T cells after syncytium formation with dendritic cells (DCs). These syncytia lack activated cells as determined by an absence of staining for Ki-67 cell cycle antigen. The expression and activity of NF-kappa B and Sp1 were, therefore, analyzed in isolated T cells and DCs from humans and mice. We have used immunolabeling, Western blot analysis, and electrophoretic mobility shift and supershift assays. T cells lack active NF-kappa B but express Sp1 as expected. DCs express high levels of all known NF-kappa B and Rel proteins, with activity residing primarily within RelB, p50, and p65. However, DCs lack Sp1, which may explain the failure of HIV-1 to replicate in purified DCs. Coexpression of NF-kappa B and Sp1 occurs in the heterologous DC-T-cell syncytia that are induced by HIV-1. Therefore, HIV-1-induced cell fusion brings together factors that upregulate virus transcription. Since DCs and memory T cells frequently traffic together in situ, these unusual heterologous syncytia could develop in infected individuals and lead to chronic HIV-1 replication without ostensible immune stimulation.

NFATx, a novel member of the nuclear factor of activated T cells family that is expressed predominantly in the thymus

The nuclear factor of activated T cells (NFAT) regulates cytokine gene expression in T cells through cis-acting elements located in the promoters of cytokine genes. Here, we report the cDNA cloning, chromosomal localization, and initial characterization of a transcription factor related to NFATp and NFATc. The novel molecule, designated NFATx, exhibits in its middle a region very similar to the Rel homology domain in NFATc and NFATp. The amino-terminal region of NFATx also shows significant similarities to corresponding sequences in NFATc and NFATp and contains three copies of a conspicuous 17-residue motif of unknown function. We provide evidence showing that NFATx can reconstitute binding to the NFAT-binding site from the interleukin 2 promoter when combined with AP1 (c-Fos/c-Jun) polypeptides and that NFATx is capable of activating transcription of the interleukin 2 promoter in COS-7 cells when stimulated with phorbol ester and calcium ionophore. NFATx mRNA is preferentially and remarkably found in the thymus and at lower levels in peripheral blood leukocytes. The expression pattern of NFATx, together with its functional activity, strongly suggests that NFATx plays a role in the regulation of gene expression in T cells and immature thymocytes.

Kinetic analysis of human T-cell leukemia virus type I Tax-mediated activation of NF-kappa B

The human T-cell leukemia virus type I (HTLV-I) Tax protein induces the expression of cellular genes, at least in part, by activating the endogenous NF-kappa B transcription factors. Induced expression of cellular genes is thought to be important for transformation of T cells to continued growth, a prelude to the establishment of adult T-cell leukemia. However, neither underlying mechanisms nor kinetics of the Tax-mediated activation of NF-kappa B are understood. We have analyzed a permanently transfected Jurkat T-cell line in which the expression of Tax is entirely dependent on addition of heavy metals. The initial NF-kappa B binding activity seen after induction of Tax is due almost exclusively to p50/p65 heterodimers. At later times, NF-kappa B complexes containing c-Rel and/or p52 accumulate. The early activation of p50/p65 complexes is a posttranslational event, since neither mRNA nor protein levels of NF-kappa B subunits had increased at that time. We demonstrate for the first time a Tax-induced proteolytic degradation of the NF-kappa B inhibitor, I kappa B-alpha, which may trigger the initial nuclear translocation of NF-kappa B. As nuclear NF-kappa B rapidly and potently stimulates resynthesis of I kappa B-alpha, the steady-state level of I kappa B-alpha does not significantly change. Thus, the dramatic Tax-induced increase in the I kappa B-alpha turnover may continually weaken inhibition and activate NF-kappa B. Additional, distinct actions of Tax may contribute further to the high levels of NF-kappa B activity seen.

Kinetic analysis of human T-cell leukemia virus type I Tax-mediated activation of NF-kappa B

The human T-cell leukemia virus type I (HTLV-I) Tax protein induces the expression of cellular genes, at least in part, by activating the endogenous NF-kappa B transcription factors. Induced expression of cellular genes is thought to be important for transformation of T cells to continued growth, a prelude to the establishment of adult T-cell leukemia. However, neither underlying mechanisms nor kinetics of the Tax-mediated activation of NF-kappa B are understood. We have analyzed a permanently transfected Jurkat T-cell line in which the expression of Tax is entirely dependent on addition of heavy metals. The initial NF-kappa B binding activity seen after induction of Tax is due almost exclusively to p50/p65 heterodimers. At later times, NF-kappa B complexes containing c-Rel and/or p52 accumulate. The early activation of p50/p65 complexes is a posttranslational event, since neither mRNA nor protein levels of NF-kappa B subunits had increased at that time. We demonstrate for the first time a Tax-induced proteolytic degradation of the NF-kappa B inhibitor, I kappa B-alpha, which may trigger the initial nuclear translocation of NF-kappa B. As nuclear NF-kappa B rapidly and potently stimulates resynthesis of I kappa B-alpha, the steady-state level of I kappa B-alpha does not significantly change. Thus, the dramatic Tax-induced increase in the I kappa B-alpha turnover may continually weaken inhibition and activate NF-kappa B. Additional, distinct actions of Tax may contribute further to the high levels of NF-kappa B activity seen.

Bcl-3-mediated nuclear regulation of the NF-kappa B trans-activating factor

The NF-kappa B factor governs the expression of many genes encoding immunoregulatory molecules. This activity is itself controlled. In this article, novel nuclear pathways for NF-kappa B regulation are described.