Raffi Gugasyan

The anti‐apoptotic activities of Rel and RelA required during B‐cell maturation involve the regulation of Bcl‐2 expression

Rel and RelA, individually dispensable for lymphopoiesis, serve unique functions in activated B and T cells. Here their combined roles in lymphocyte development were examined in chimeric mice repopulated with c‐rel−/− rela−/− fetal liver hemopoietic stem cells. Mice engrafted with double‐mutant cells lacked mature IgMloIgDhi B cells, and numbers of peripheral CD4+ and CD8+ T cells were markedly reduced. The absence of mature B cells was associated with impaired survival that coincided with reduced expression of bcl‐2 and A1. bcl‐2 transgene expression not only prevented apoptosis and increased peripheral B‐cell numbers, but also induced further maturation to an IgMloIgDhi phenotype. In contrast, the survival of double‐mutant T cells was normal and the bcl‐2 transgene could not rectify the peripheral T‐cell deficit. These findings indicate that Rel and RelA serve essential, albeit redundant, functions during the later antigen‐independent stages of B‐ and T‐cell maturation, with these transcription factors promoting the survival of peripheral B cells in part by upregulating Bcl‐2.

Fatal Hepatitis Mediated by Tumor Necrosis Factor TNFα Requires Caspase-8 and Involves the BH3-Only Proteins Bid and Bim

Apoptotic death of hepatocytes, a contributor to many chronic and acute liver diseases, can be a consequence of overactivation of the immune system and is often mediated by TNFalpha. Injection with lipopolysaccharide (LPS) plus the transcriptional inhibitor D(+)-galactosamine (GalN) or mitogenic T cell activation causes fatal hepatocyte apoptosis in mice, which is mediated by TNFalpha, but the effector mechanisms remain unclear. Our analysis of gene-targeted mice showed that caspase-8 is essential for hepatocyte killing in both settings. Loss of Bid, the proapoptotic BH3-only protein activated by caspase-8 and essential for Fas ligand-induced hepatocyte killing, resulted only in a minor reduction of liver damage. However, combined loss of Bid and another BH3-only protein, Bim, activated by c-Jun N-terminal kinase (JNK), protected mice from LPS+GalN-induced hepatitis. These observations identify caspase-8 and the BH3-only proteins Bid and Bim as potential therapeutic targets for treatment of inflammatory liver diseases.

NF-κB1 and c-Rel cooperate to promote the survival of TLR4-activated B cells by neutralizing Bim via distinct mechanisms

The nuclear factor-kappaB (NF-kappaB) pathway is crucial for the survival of B cells stimulated through Toll-like receptors (TLRs). Here, we show that the heightened death of TLR4-activated nfkb1(-/-) B cells is the result of a failure of the Tpl(2)/MEK/ERK pathway to phosphorylate the proapo-ptotic BH3-only protein Bim and target it for degradation. ERK inactivation of Bim after TLR4 stimulation is accompanied by an increase in A1/Bim and Bcl-x(L)/Bim complexes that we propose represents a c-Rel-dependent mechanism for neutralizing Bim. Together these findings establish that optimal survival of TLR4-activated B cells depends on the NF-kappaB pathway neutralizing Bim through a combination of Bcl-2 prosurvival protein induction and Tpl2/ERK-dependent Bim phosphorylation and degradation.

NF-κB subunit specificity in hemopoiesis

Summary:u2002 Although the diverse functions served by the nuclear factor‐κB (NF‐κB) pathway in virtually all cell types are typically employed to deal with stress responses, NF‐κB transcription factors also play key roles in the development of hemopoietic cells. This review focuses on how NF‐κB transcription factors control various aspects of thymic T‐cell and myeloid cell differentiation that include its roles in hemopoietic precursors, conventional αβ T cells, CD4+ regulatory T cells, natural killer T cells, γδ T cells, macrophages, and dendritic cells.

Distinct roles in NKT cell maturation and function for the different transcription factors in the classical NF-κB pathway.

The nuclear factor (NF)‐κB signalling pathway is known to be critical for natural killer T (NKT) cell differentiation; however, the role of individual NF‐κB transcription factors and the precise developmental stages that they control remain unclear. We have investigated the influence of the classical NF‐κB transcription factors NF‐κB1, c‐Rel and RelA on NKT cell development and function, using gene‐deleted mice. Individually, none of these factors were essential for the requirement of NF‐κB signalling in early NKT cell development before NK1.1 expression, in contrast to earlier reports in which the classical NF‐κB pathway was globally disrupted. Instead, we found that each factor played a non‐redundant role in later stages of NKT cell maturation and function. Although NF‐κB1 deficiency resulted in a moderate reduction in mature NK1.1+ NKT cells, this was found to be more subtle than previously reported. RelA deficiency had a more profound effect on the NK1.1+ stage of NKT cell development, whereas c‐Rel‐deficient mice had normal NKT cell numbers. All three factors (NF‐κB1, RelA and c‐Rel) were necessary for normal NKT cell cytokine production. Notably, IL‐17, which is produced by a specific subset of NKT cells (NKT‐17 cells), defined as NK1.1−CD4−, was not impaired by a lack of these individual NF‐κB transcription factors, nor was this subset depleted, suggesting that NKT‐17 cells are regulated independently of the NF‐κB pathway. Thus, individual NF‐κB family members have a largely redundant role in early NKT cell development, but each of them has an important and distinct role in NKT cell maturation and/or function.

The NF-κB transcription factor RelA is required for the tolerogenic function of Foxp3+ regulatory T cells

The properties of CD4(+) regulatory T cell (Treg) subsets are dictated by distinct patterns of gene expression determined by FOXP3 and different combinations of various transcription factors. Here we show the NF-κB transcription factor RelA is constitutively active in naïve and effector Tregs. The conditional inactivation of Rela in murine FOXP3(+) cells induces a rapid onset, multi-focal autoimmune disease that depends on RelA being expressed in conventional T cells. In addition to promoting Treg lineage stability, RelA determines the size of the effector Treg population, a function influenced by the presence or absence of RelA in conventional T cells. These findings showing that RelA controls Treg stability and promotes the competitive fitness of effector Tregs highlight the importance of RelA activity in peripheral Treg induced tolerance.

The effect of 2-acetyl-4-tetrahydroxybutylimidazole on lymphocyte subsets during a contact hypersensitivity response in the NOD mouse

The compound 2-acetyl-4-tetrahydroxybutylimidazole (THI) is known to suppress a contact hypersensitivity (CH) response. The effect of THI on lymphocyte subsets during treatment and in a CH response has not been shown in mice. To further define the immunosuppressive potential of THI, a time-course study during the CH response to oxazolone (OX) was performed. While THI can prevent the induction of CH, if treatment is started before sensitization, it has a low therapeutic capability since it could not significantly inhibit the response when continuous oral treatment was commenced during the course of CH. We report that during this response continuous oral treatment of non-obese diabetic (NOD) mice with THI reduced the number of CD4+ and CD8+ T cells in the peripheral blood. In the draining lymph nodes THI treatment had no effect on lymphocyte subsets prior to contact sensitization, but subsequent sensitization and elicitation with OX could not stimulate a significant increase in the number of CD4+ T cells in the treated mice, whereas untreated control mice showed elevated numbers of these lymphocytes. These findings suggest that THI can inhibit an CH response by preventing the recruitment of CD4+ T cells in the draining lymph nodes through an unknown mechanism.

Loss of the BH3-only protein Bid does not rescue RelA-deficient embryos from TNF-R1-mediated fatal hepatocyte destruction

Loss of the BH3-only protein Bid does not rescue RelA-deficient embryos from TNF-R1-mediated fatal hepatocyte destruction

Prevention of splenic granuloma formation in adjuvant arthritis by 2-acetyl-4-tetrahydroxybutylimidazole (THI).

Adjuvant-induced arthritis (AA) in Lewis rats is a widely used model of chronic inflammatory arthritis. Non-articular features such as weight loss and necrotizing granulomas of the spleen and lymph nodes also occur in this model. The compound 2-acetyl-4-tetrahydroxybutylimidazole (THI) marginally delayed the development of AA. However, this agent had no effect on the incidence or severity of disease. In contrast, THI totally prevented granuloma formation in the spleen and associated splenomegaly. We conclude that THI may be a useful adjunctive agent for some inflammatory diseases.

Bcl-2 transgene expression fails to prevent fatal hepatocyte apoptosis induced by endogenous TNFα in mice lacking RelA

Bcl -2 transgene expression fails to prevent fatal hepatocyte apoptosis induced by endogenous TNF α in mice lacking RelA