Steve Gerondakis

Genetic approaches in mice to understand Rel/NF-κB and IκB function : transgenics and knockouts

Rel/NF-κB transcription factors have been implicated in regulating a wide variety of genes important in cellular processes that include cell division, cell survival, differentiation and immunity. Here genetic models in which various Rel/NF-κB and IκB proteins have either been over-expressed or deleted in mice will be reviewed. Although expressed fairly ubiquitously, homozygous disruption of individual Rel/NF-κB genes generally affects the development of proper immune cell function. One exception is rela, which is essential for embryonic liver development. The disruption of genes encoding the individual subunits of the IκB kinase, namely IKKα and IKKβ, has demonstrated that IKKβ transmits the response to most common NF-κB inducing agents, whereas IKKα has an unexpected role in keratinocyte differentiation. Future studies will no doubt focus on the effect of multiple gene disruptions of members of this signaling pathway, on tissue-specific disruptions of these genes, and on the use of these mice as models for human diseases.

Regulation of an essential innate immune response by the p50 subunit of NF-kappaB.

Recognition of bacterial endotoxin (LPS) elicits multiple host responses, including activation of cells of the innate immune system. LPS exposure occurs repeatedly during septicemia, making strict regulation of gene expression necessary. Such regulation might prevent, for example, the continuous production of proinflammatory cytokines such as tumor necrosis factor (TNF), which could lead to severe vascular collapse. Tolerance to LPS is characterized by a diminished production of TNF during prolonged exposure to LPS, and is therefore likely to represent an essential control mechanism during sepsis. In the present study, which uses mice with genetic deletions of the proteins of NF-kappaB complex, we provide data demonstrating that increased expression of the p50 subunit of NF-kappaB directly results in the downregulation of LPS-induced TNF production. This contention is supported by the following observations: (1) tolerance to LPS is not induced in macrophages from p50-/- mice; (2) long-term pretreatment with LPS does not block synthesis of TNF mRNA in p50-/- macrophages (in contrast to wild-type macrophages); (3) ectopic overexpression of p50 reduces transcriptional activation of the murine TNF promoter; and (4) analysis of the four kappaB sites from the murine TNF promoter demonstrates that binding of p50 homodimers to the positively acting kappaB3 element is associated with development of the LPS-tolerant phenotype. Thus, p50 expression plays a key role in the development of LPS tolerance.

Unravelling the complexities of the NF-κB signalling pathway using mouse knockout and transgenic models

The nuclear factor-κB (NF-κB) signalling pathway serves a crucial role in regulating the transcriptional responses of physiological processes that include cell division, cell survival, differentiation, immunity and inflammation. Here we outline studies using mouse models in which the core components of the NF-κB pathway, namely the IκB kinase subunits (IKKα, IKKβ and NEMO), the IκB proteins (IκBα, IκBβ, IκBɛ and Bcl-3) and the five NF-κB transcription factors (NF-κB1, NF-κB2, c-Rel, RelA and RelB), have been genetically manipulated using transgenic and knockout technology.

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.

The regulation and roles of Rel/NF-kappa B transcription factors during lymphocyte activation.

The activation of B and T cells by a wide range of stimuli can rapidly induce specific gene expression via a mechanism that promotes the nuclear translocation of different Rel/nuclear factor-kappa B (NF-kappa B) transcription factors which are normally resident in the cytoplasm. Recent findings highlight the crucial roles of specific Rel/NF-kappa B family members in the processes of cell division, apoptosis and differentiation that accompany lymphocyte activation.

c-Rel is required for the development of thymic Foxp3+ CD4 regulatory T cells

During thymopoiesis, a unique program of gene expression promotes the development of CD4 regulatory T (T reg) cells. Although Foxp3 maintains a pattern of gene expression necessary for T reg cell function, other transcription factors are emerging as important determinants of T reg cell development. We show that the NF-κB transcription factor c-Rel is highly expressed in thymic T reg cells and that in c-rel−/− mice, thymic T reg cell numbers are markedly reduced as a result of a T cell–intrinsic defect that is manifest during thymocyte development. Although c-Rel is not essential for TGF-β conversion of peripheral CD4+CD25− T cells into CD4+Foxp3+ cells, it is required for optimal homeostatic expansion of peripheral T reg cells. Despite a lower number of peripheral T reg cells in c-rel−/− mice, the residual peripheral c-rel−/− T reg cells express normal levels of Foxp3, display a pattern of cell surface markers and gene expression similar to those of wild-type T reg cells, and effectively suppress effector T cell function in culture and in vivo. Collectively, our results indicate that c-Rel is important for both the thymic development and peripheral homeostatic proliferation of T reg cells.

Distinct roles for the NF-κB1 (p50) and c-Rel transcription factors in inflammatory arthritis

Rheumatoid arthritis (RA) is a complex disease, with contributions from systemic autoimmunity and local inflammation. Persistent synovial joint inflammation and invasive synovial pannus tissue lead to joint destruction. RA is characterized by the production of inflammatory mediators, many of which are regulated by the Rel/NF-kappaB transcription factors. Although an attractive target for therapeutic intervention in inflammatory diseases, Rel/NF-kappaB is involved in normal physiology, thus global inhibition could be harmful. An alternate approach is to identify and target the Rel/NF-kappaB subunits critical for components of disease. To assess this, mice with null mutations in c-rel or nfkb1 were used to examine directly the roles of c-Rel and p50 in models of acute and chronic inflammatory arthritis. We found c-Rel-deficient mice were resistant to collagen-induced arthritis but had a normal response in an acute, destructive arthritis model (methylated BSA/IL-1 induced arthritis) suggesting c-Rel is required for systemic but not local joint disease. In contrast, p50-deficient mice were refractory to induction of both the chronic and acute arthritis models, showing this subunit is essential for local joint inflammation and destruction. Our data suggest Rel/NF-kappaB subunits play distinct roles in the pathogenesis of inflammatory arthritis and may provide a rationale for more specific therapeutic blockade of Rel/NF-kappaB in RA.

Roles of the NF-κB Pathway in Lymphocyte Development and Function

This article focuses on the functions of NF-kappaB that vitally impact lymphocytes and thus adaptive immunity. NF-kappaB has long been known to be essential for many of the responses of mature lymphocytes to invading pathogens. In addition, NF-kappaB has important functions in shaping the immune system so it is able to generate adaptive responses to pathogens. In both contexts, NF-kappaB executes critical cell-autonomous functions within lymphocytes as well as within supportive cells, such as antigen-presenting cells or epithelial cells. It is these aspects of NF-kappaBs physiologic impact that we address in this article.

B Cell Growth Is Controlled by Phosphatidylinosotol 3-Kinase-Dependent Induction of Rel/NF-κB Regulated c-myc Transcription

Rel/NF-kappaB transcription factors regulate the division and survival of B lymphocytes. Here we show that B cells lacking NF-kappaB1 and c-Rel fail to increase in size upon mitogenic stimulation due to a reduction in induced c-myc expression. Mitogen-induced B cell growth, although not markedly impaired by FRAP/mTOR or MEK inhibitors, required phosphatidylinositol 3-kinase (PI3K) activity. Inhibition of PI3K-dependent growth coincided with a block in the nuclear import of NF-kappaB1/c-Rel dimers and a failure to upregulate c-myc. In addition, PI3K was shown to be necessary for a transcription-independent increase in c-Myc protein levels that accompanies mitogenic activation. Collectively, these findings establish a role for Rel/NF-kappaB signaling in the mitogen-induced growth of mammalian cells, which in B lymphocytes requires a PI3K/c-myc-dependent pathway.

Coordinating TLR-activated signaling pathways in cells of the immune system

Toll‐like receptor (TLR) signaling leads to the activation of mitogen‐activated protein kinase and nuclear factor‐κB signaling pathways. While the upstream signaling events initiated at the level of adaptors and the activation of the downstream signaling pathways have received a lot of attention, our understanding of how these signaling pathways are coordinated to regulate gene expression is poorly understood. This review gives a selective overview on our current understanding of signaling downstream of TLRs, with an emphasis on how the upstream kinases like the mitogen‐activated protein kinase kinase kinases (TAK1 and Tpl2) and inhibitor of κ‐B kinase (IKK) coordinate the signaling events that steer the course of an immune response.