Lineage-restricted function of nuclear factor kappaB-inducing kinase (NIK) in transducing signals via CD40

The NFκB signaling system in the generation of B-cell subsets: from germinal center B cells to memory B cells and plasma cells

Memory B cells and antibody-secreting cells are the two prime effector B cell populations that drive infection- and vaccine-induced long-term antibody-mediated immunity. The antibody-mediated immunity mostly relies on the formation of specialized structures within secondary lymphoid organs, called germinal centers (GCs), that facilitate the interactions between B cells, T cells, and antigen-presenting cells. Antigen-activated B cells may proliferate and differentiate into GC-independent plasmablasts and memory B cells or differentiate into GC B cells. The GC B cells undergo proliferation coupled to somatic hypermutation of their immunoglobulin genes for antibody affinity maturation. Subsequently, affinity mature GC B cells differentiate into GC-dependent plasma cells and memory B cells. Here, we review how the NFκB signaling system controls B cell proliferation and the generation of GC B cells, plasmablasts/plasma cells, and memory B cells. We also identify and discuss some important unanswered questions in this connection.

The canonical and non-canonical NF-κB pathways and their crosstalk: A comparative study based on Petri nets

NF-κB is a protein complex that occurs in almost all animal cell types. It regulates the cellular immune responses to stimuli in the nucleus. Dysregulation of NF-κB can cause severe diseases like chronic inflammation, autoimmune diseases or cancer. We modeled the two major pathways leading from the external cellular stimulation of the CD40 receptor to the nuclear translocation of NF-κB dimers, the canonical and non-canonical pathway. Based on literature data, we developed two Petri net models describing these pathways. In a third Petri net, we combined the two models, introducing crosstalk specific in CD40L-stimulated B cells. In terms of structural properties, we checked the Petri nets for their consistency and correctness. To explore differences and similarities, we compared structural properties and the simulation behavior of the models. The non-canonical NF-κB pathway exhibited a more diverse regulation than the canonical pathway. Applying in silico knockout analyses, we were able to quantify the relevance of individual biochemical processes. We predicted interrelationships, e.g., between the synthesis of the protein NF-κB-inducing kinase and the processing of the precursor protein p100. The activation of the transcription factors, p50-RelA and p52-RelB, was affected by most of the knockouts. The results of the in silico knockout were in accordance with experimental studies. The Petri net models provide a basis for further analyses and could be extended to include gene expression, additional pathways, molecular processes, and kinetic data.

Map3k14 as a Regulator of Innate and Adaptive Immune Response during Acute Viral Infection

The replication of virus in secondary lymphoid organs is crucial for the activation of antigen-presenting cells. Balanced viral replication ensures the sufficient availability of antigens and production of cytokines, and both of which are needed for virus-specific immune activation and viral elimination. Host factors that regulate coordinated viral replication are not fully understood. In the study reported here, we identified Map3k14 as an important regulator of enforced viral replication in the spleen while performing genome-wide association studies of various inbred mouse lines in a model of lymphocytic choriomeningitis virus (LCMV) infection. When alymphoplasia mice (aly/aly, Map3k14^aly/aly, or Nik^aly/aly), which carry a mutation in Map3k14, were infected with LCMV or vesicular stomatitis virus (VSV), they display early reductions in early viral replication in the spleen, reduced innate and adaptive immune activation, and lack of viral control. Histologically, scant B cells and the lack of CD169⁺ macrophages correlated with reduced immune activation in Map3k14^aly/aly mice. The transfer of wildtype B cells into Map3k14^aly/aly mice repopulated CD169⁺ macrophages, restored enforced viral replication, and resulted in enhanced immune activation and faster viral control.

NF-κB inducing kinase is a therapeutic target for systemic lupus erythematosus

NF-κB-inducing kinase (NIK) mediates non-canonical NF-κB signaling downstream of multiple TNF family members, including BAFF, TWEAK, CD40, and OX40, which are implicated in the pathogenesis of systemic lupus erythematosus (SLE). Here, we show that experimental lupus in NZB/W F1 mice can be treated with a highly selective and potent NIK small molecule inhibitor. Both in vitro as well as in vivo, NIK inhibition recapitulates the pharmacological effects of BAFF blockade, which is clinically efficacious in SLE. Furthermore, NIK inhibition also affects T cell parameters in the spleen and proinflammatory gene expression in the kidney, which may be attributable to inhibition of OX40 and TWEAK signaling, respectively. As a consequence, NIK inhibition results in improved survival, reduced renal pathology, and lower proteinuria scores. Collectively, our data suggest that NIK inhibition is a potential therapeutic approach for SLE.

Dendritic cells require NIK for CD40-dependent cross-priming of CD8+ T cells

Dendritic cells (DCs) link innate and adaptive immunity and use a host of innate immune and inflammatory receptors to respond to pathogens and inflammatory stimuli. Although DC maturation via canonical NF-κB signaling is critical for many of these functions, the role of noncanonical NF-κB signaling via the serine/threonine kinase NIK (NF-κB-inducing kinase) remains unclear. Because NIK-deficient mice lack secondary lymphoid organs, we generated transgenic mice with targeted NIK deletion in CD11c(+) cells. Although these mice exhibited normal lymphoid organs, they were defective in cross-priming naive CD8(+) T cells following vaccination, even in the presence of anti-CD40 or polyinosinic:polycytidylic acid to induce DC maturation. This impairment reflected two intrinsic defects observed in splenic CD8(+) DCs in vitro, namely antigen cross-presentation to CD8(+) T cells and secretion of IL-12p40, a cytokine known to promote cross-priming in vivo. In contrast, antigen presentation to CD4(+) T cells was not affected. These findings reveal that NIK, and thus probably the noncanonical NF-κB pathway, is critical to allow DCs to acquire the capacity to cross-present antigen and prime CD8 T cells after exposure to licensing stimuli, such as an agonistic anti-CD40 antibody or Toll-like receptor 3 ligand.

Conditional Deletion of NF-κB-Inducing Kinase (NIK) in Adult Mice Disrupts Mature B Cell Survival and Activation

NF-κB-inducing kinase (NIK) is a primary regulator of the noncanonical NF-κB signaling pathway, which plays a vital role downstream of BAFF, CD40L, lymphotoxin, and other inflammatory mediators. Germline deletion or inactivation of NIK in mice results in the defective development of B cells and secondary lymphoid organs, but the role of NIK in adult animals has not been studied. To address this, we generated mice containing a conditional allele of NIK. Deletion of NIK in adult mice results in decreases in B cell populations in lymph nodes and spleen, similar to what is observed upon blockade of BAFF. Consistent with this, B cells from mice in which NIK is acutely deleted fail to respond to BAFF stimulation in vitro and in vivo. In addition, mice with induced NIK deletion exhibit a significant decrease in germinal center B cells and serum IgA, which is indicative of roles for NIK in additional pathways beyond BAFF signaling. Our conditional NIK-knockout mice may be broadly useful for assessing the postdevelopmental and cell-specific roles of NIK and the noncanonical NF-κB pathway in mice.

Structure of the nuclear factor κB-inducing kinase (NIK) kinase domain reveals a constitutively active conformation

NF-κB-inducing kinase (NIK) is a central component in the non-canonical NF-κB signaling pathway. Excessive NIK activity is implicated in various disorders, such as autoimmune conditions and cancers. Here, we report the first crystal structure of truncated human NIK in complex with adenosine 5'-O-(thiotriphosphate) at a resolution of 2.5 Å. This truncated protein is a catalytically active construct, including an N-terminal extension of 60 residues prior to the kinase domain, the kinase domain, and 20 residues afterward. The structure reveals that the NIK kinase domain assumes an active conformation in the absence of any phosphorylation. Analysis of the structure uncovers a unique role for the N-terminal extension sequence, which stabilizes helix αC in the active orientation and keeps the kinase domain in the catalytically competent conformation. Our findings shed light on the long-standing debate over whether NIK is a constitutively active kinase. They also provide a molecular basis for the recent observation of gain-of-function activity for an N-terminal deletion mutant (ΔN324) of NIK, leading to constitutive non-canonical NF-κB signaling with enhanced B-cell adhesion and apoptosis resistance.

Splenic extramedullary hemopoiesis caused by a dysfunctional mutation in the NF-κB-inducing kinase gene

NF-κB-inducing kinase (NIK) plays critical roles in the development of lymph nodes and Peyer's patches, and microarchitecture of the thymus and spleen via NF-κB activation. Alymphoplasia (aly/aly) mice have a point mutation in the NIK gene that causes a defect in the activation of an NF-κB member RelB. Here, we developed a novel method to determine the aly mutation by genetic typing using PCR. This method facilitated the easy establishment of a congeneic aly/aly mouse line. Indeed, we generated a mouse line with aly mutation on a BALB/cA background (BALB/cA-aly/aly). BALB/cA-aly/aly mice showed significant splenomegaly with extramedullary hemopoiesis, which was not significant in aly/aly mice on a C57BL/6 background. Interestingly, the splenomegaly and extramedullary hemopoiesis caused by the aly mutation was gender-dependent. These data together with previous reports on extramedullary hemopoiesis in RelB-deficient mice suggest that NIK-RelB signaling may be involved in the suppression of extramedullary hemopoiesis in adult mice.

Non-canonical NF-κB signaling activation and regulation: principles and perspectives

Nuclear-factor κB (NF-κB) transcription factors are activated by a wide variety of stimuli in diverse cell types and control key aspects of immune function and development. Receptor-mediated activation of NF-κB appears to occur through two distinct signaling pathways termed as the canonical and non-canonical NF-κB pathways. Although much work has demonstrated the physiological importance of non-canonical NF-κB signaling to immunity and its involvement in diverse pathologies, such as cancers and autoimmune disease, the architecture and regulation of the pathway is only beginning to be understood. The non-canonical pathway appears to be activated by a select set of receptors within the tumor necrosis factor superfamily, and we discuss the molecular mechanisms that connect ligation of these receptors to pathway activation. It has become increasingly clear that the key regulatory step of the pathway involves modulation of the post-translational degradation of NF-κB-inducing kinase (NIK), the central activating kinase of non-canonical NF-κB signaling. How NIK post-translational stability is controlled before and after receptor ligation is an important aspect of understanding non-canonical NF-κB signaling. Furthermore, how release of NF-κB dimers downstream of the pathway's activation is actually connected to its identified physiological and pathological roles is a key remaining question in the field.

Protein kinase R is a novel mediator of CD40 signaling and plays a critical role in modulating immunoglobulin expression during respiratory syncytial virus infection

Effective immunoglobulin responses play a vital role in protection against most pathogens. However, the molecular mediators and mechanisms responsible for signaling and selective expression of immunoglobulin types remain to be elucidated. Previous studies in our laboratory have demonstrated that protein kinase R (PKR) plays a crucial role in IgE responses to double-stranded RNA (dsRNA) in vitro. In this study, we show that PKR plays a critical role in IgG expression both in vivo and in vitro. PKR(-/-) mice show significantly altered serum IgG levels during respiratory syncytial virus (RSV) infection. IgG2a expression is particularly sensitive to a lack of PKR and is below the detection level in mock- or RSV-infected PKR(-/-) mice. Interestingly, we show that upon activation by anti-CD40 and gamma interferon (IFN-γ), B cells from PKR(-/-) mice show diminished major histocompatibility complex class II (MHC II), CD80, and CD86 levels on the cell surface compared to wild-type (WT) mice. Our data also show that PKR is necessary for optimal expression of adhesion molecules, such as CD11a and ICAM-1, that are necessary for homotypic aggregation of B cells. Furthermore, in this report we demonstrate for the first time that upon CD40 ligation, PKR is rapidly phosphorylated and activated, indicating that PKR is an early and novel downstream mediator of CD40 signaling pathways.

NIK signaling in dendritic cells but not in T cells is required for the development of effector T cells and cell-mediated immune responses

The canonical NF-κB pathway is a driving force for virtually all aspects of inflammation. Conversely, the role of the noncanonical NF-κB pathway and its central mediator NF-κB-inducing kinase (NIK) remains poorly defined. NIK has been proposed to be involved in the formation of T(H)17 cells, and its absence in T(H) cells renders them incapable of inducing autoimmune responses, suggesting a T cell-intrinsic role for NIK. Upon systematic analysis of NIK function in cell-mediated immunity, we found that NIK signaling is dispensable within CD4(+) T cells but played a pivotal role in dendritic cells (DCs). We discovered that NIK signaling is required in DCs to deliver co-stimulatory signals to CD4(+) T cells and that DC-restricted expression of NIK is sufficient to restore T(H)1 and T(H)17 responses as well as cell-mediated immunity in NIK(-/-) mice. When CD4(+) T cells developed in the absence of NIK-sufficient DCs, they were rendered anergic. Reintroduction of NIK into DCs allowed developing NIK(-/-) CD4(+) T cells to become functional effector populations and restored the development of autoimmune disease. Therefore, our data suggest that a population of thymic DCs requires NIK to shape the formation of most αβ CD4(+) T effector lineages during early development.

The kinase NIK as a therapeutic target in multiple myeloma

INTRODUCTION

Multiple myeloma (MM) is a neoplasm derived from B lymphocytes and often results in uncontrolled clonal expansion of antibody-secreting cells. While current treatments are able to prolong survival, MM remains incurable. Excessive NF-κB activity in MM contributes to tumor progression and survival.

AREAS COVERED

The contribution of NF-κB-inducing kinase (NIK) to alternative NF-κB signaling, where it is the key kinase, and classical NF-κB signaling. Modulation of NIK by natural and chemical factors and current and potential therapies for MM that target NIK.

EXPERT OPINION

Mutations affecting the activation of NIK have been identified in MM samples and cell lines, suggesting that NIK may be an important target for therapy of MM. NIK contributes to activation of both NF-κB pathways in MM, giving us the opportunity to limit two pathways contributing to oncogenic survival with a single therapeutic. Many of the mutations identified in MM cells result in the same outcome, hyperactive NIK, thus a single therapeutic may be effective in many patients even though they carry differing mutations. As NIK appears only to activate classical NF-κB when overexpressed, and in normal cells NIK levels are usually low, it is possible that therapeutics designed to limit the amount of NIK may not produce serious side effects in healthy cells.

Differential B-lymphocyte regulation by CD40 and its viral mimic, latent membrane protein 1

CD40 plays a vital role in humoral immunity, via its potent and multifaceted function as an activating receptor of various immune cells, most notably B lymphocytes. The Epstein-Barr virus-encoded transforming protein latent membrane protein 1 (LMP1) serves as a functional mimic of CD40 signals to B cells but lacks key regulatory controls that restrain CD40 signaling. This allows LMP1 to activate B cells in an abnormal manner that can contribute to the pathogenesis of human B-cell lymphoma and autoimmune disease. This review focuses upon a comparative analysis of CD40 versus LMP1 functions and mechanisms of action in B lymphocytes, discussing how this comparison can provide valuable information on both how CD40 signaling is normally regulated and how LMP1 disrupts the normal CD40 pathways, which can provide information of value to therapeutic design.

NF-κB inducing kinase: a key regulator in the immune system and in cancer

NF-κB inducing kinase (NIK) is a kinase that activates the canonical and non-canonical NF-κB pathways to control transcriptional expression of certain proteins such as cytokines, chemokines and NF-κB signaling molecules. Many advances have been made in understanding the molecular mechanisms by which the stability of NIK is regulated to affect downstream signaling. Genetic mouse models suggest that NIK plays an essential role in the regulation of the immune system as well as in the bone microenvironment. Increasing evidence links NIK to the tumorigenesis of hematological cancers, such as multiple myeloma, and solid tumors, such as pancreatic carcinoma and melanoma. Understanding the mechanism by which NIK is de-regulated will potentially provide therapeutic options for certain diseases such as autoimmunity and cancer.

B-cells need a proper house, whereas T-cells are happy in a cave: the dependence of lymphocytes on secondary lymphoid tissues during evolution

A fundamental tenet of immunology is that adaptive immune responses are initiated in secondary lymphoid tissues. This dogma has been challenged by several recent reports. We discuss how successful T cell-mediated immunity can be initiated outside of such dedicated structures, whereas they are required for adaptive humoral immunity. This resembles an ancient immune pathway in the oldest cold-blooded vertebrates, which lack lymph nodes and sophisticated B-cell responses including optimal affinity maturation. The T-cell, however, has retained the capacity to recognize antigen in a lymph node-free environment. Besides bone marrow and lung, the liver is one organ that can potentially serve as a surrogate lymphoid organ and could represent a remnant from the time before lymph nodes developed.

Neo-lymphoid aggregates in the adult liver can initiate potent cell-mediated immunity

Are lymph nodes really essential for successful immunizations? We found that the liver can compensate for missing lymphoid structures in initiating cellular, but not antibody-mediated, immunity.

Subcutaneous immunization delivers antigen (Ag) to local Ag-presenting cells that subsequently migrate into draining lymph nodes (LNs). There, they initiate the activation and expansion of lymphocytes specific for their cognate Ag. In mammals, the structural environment of secondary lymphoid tissues (SLTs) is considered essential for the initiation of adaptive immunity. Nevertheless, cold-blooded vertebrates can initiate potent systemic immune responses even though they lack conventional SLTs. The emergence of lymph nodes provided mammals with drastically improved affinity maturation of B cells. Here, we combine the use of different strains of alymphoplastic mice and T cell migration mutants with an experimental paradigm in which the site of Ag delivery is distant from the site of priming and inflammation. We demonstrate that in mammals, SLTs serve primarily B cell priming and affinity maturation, whereas the induction of T cell-driven immune responses can occur outside of SLTs. We found that mice lacking conventional SLTs generate productive systemic CD4- as well as CD8-mediated responses, even under conditions in which draining LNs are considered compulsory for the initiation of adaptive immunity. We describe an alternative pathway for the induction of cell-mediated immunity (CMI), in which Ag-presenting cells sample Ag and migrate into the liver where they induce neo-lymphoid aggregates. These structures are insufficient to support antibody affinity maturation and class switching, but provide a novel surrogate environment for the initiation of CMI.

Lymph nodes (LNs) are believed to be the most important tissues initiating immune responses by facilitating the activation of T and B lymphocytes. Mice lacking such LNs (called alymphoplastic) are severely immune compromised and resistant to immunizations. We discovered that the immune-deficiency of such alymphoplastic mice is actually not caused by the loss of LNs, but rather by the underlying genetic lesion. Surprisingly, mice lacking all lymph nodes can still mount potent T cell-mediated immune responses. We also discovered that T and B cells have completely different structural requirements for their activation/maturation. Whereas B cells rely on LNs to become efficient antibody-producing cells, T cells can be activated successfully outside of such dedicated tissues. So—in the absence of LNs—antigens delivered by immunization are actively transported into the liver where cellular immunity is initiated. The mammalian fetal liver is responsible for the early formation of blood and immune cells, and we propose that the adult liver can still provide a niche for T cell–antigen encounters. During evolution, T and B cells emerged simultaneously, allowing cold-blooded vertebrates (which lack LNs) to launch adaptive immune responses. The development of LNs in mammals coincided with a drastic improvement in antibody affinity maturation, whereas T cells remain LN-independent to this day.

Dendritic cells require the NF-kappaB2 pathway for cross-presentation of soluble antigens

NF-kappaB-inducing kinase (NIK) is responsible for activation of the non-canonical p100 processing pathway of NF-kappaB activation. This kinase has been shown to be critical for activation of this pathway after signaling through several TNF family members including CD40. The functional importance of this pathway in CD40 and TLR-induced dendritic cell (DC) differentiation was studied in vivo in the alymphoplasia (Aly) mouse. The Aly mouse expresses a mutant NIK molecule that prohibits the induction of the non-canonical pathway. We show that while MHC class II presentation and in vivo migration of Aly DCs is intact, these cells are unable to cross-prime CD8+ T cells to exogenous Ag. Gene expression array analysis of DCs matured in vivo indicates multiple defects in Ag processing pathways after maturation and provide a global view of the genes that are regulated by the NF-kappaB2 pathway in DCs. These experiments indicate a possible role for NIK in mediating cross-priming of soluble Ag. In addition, our findings explain the profound immune unresponsiveness of the Aly mouse.

Regulation of late B cell differentiation by intrinsic IKKalpha-dependent signals

NF-kappaB-inducing kinase (NIK)-mediated IKKalpha phosphorylation activates the alternative NF-kappaB pathway, which is characterized by nuclear translocation of p52:RelB heterodimers. This alternative pathway is initiated by a select few receptors, including LT-betaR, BAFF-R, and CD40. Although NIK, IKKalpha, and p52 are all critical regulators of LT-betaR signaling in stromal cells during humoral immune responses, lymphocytes require NIK, but not p52, for optimal Ig production. This disparity suggests that NIK possesses critical cell-type-specific functions that do not depend on NF-kappaB. Here we use mice bearing targeted mutations of the IKKalpha activation loop Ser(176/180) (IKKalpha(AA)) to address the B cell-intrinsic functions of NIK-IKKalpha signaling in vivo. We find that IKKalpha(AA) B cells mount normal primary antibody responses but do not enter germinal centers. This defect likely derives from ineffective early T-B cell collaboration and leads to impaired generation of humoral memory and relatively short-lived, low-affinity antibody production. Our findings contrast with those obtained by using p52(-/-) B cells, which mount normal Ig responses, and alymphoplasia (NIK mutant) B cells, which produce very little primary Ig. Thus, the NIK-IKKalpha-p52 axis is not as linear and exclusive as previous studies suggest, and IKKalpha possesses critical NF-kappaB-independent functions in B cells.

Distinct regulation of CD40-mediated interleukin-6 and interleukin-12 productions via mitogen-activated protein kinase and nuclear factor kappaB-inducing kinase in mature dendritic cells

The role of mitogen-activated protein kinase (MAPK) and nuclear factor kappaB (NF-kappaB) pathways, especially NF-kappaB-inducing kinase (NIK)-mediated alternative pathway, in CD40-mediated interleukin (IL)-6 and IL-12 productions by immature or mature dendritic cells (DCs) was investigated. Murine myeloid DCs were matured by treatment with lipopolysaccharide. CD40 ligation induced modest or vigorous cytokine productions in immature or mature DCs, respectively. After CD40 ligation, p38 MAPK was significantly activated in either immature or mature DCs. SB203580, a p38 MAPK inhibitor, markedly decreased CD40-mediated IL-6 and IL-12 productions in immature DCs. In mature DCs, SB203580 significantly decreased CD40-mediated IL-6 but not IL-12 production. On the other hand, CD40 ligation induced vigorous activation of the NF-kappaB alternative pathway including p100 phosphorylation and subsequent nuclear translocations of p52, a processed form of p100, and RelB in mature but not immature DCs. The CD40-mediated phosphorylation of p100 was completely abolished in NIK-mutated mature DCs. The NIK mutation markedly reduced CD40-mediated IL-12 but not IL-6 production by mature DCs. Taken together, we concluded that IL-6 and IL-12 productions in response to CD40 ligation were controlled by p38 MAPK and NIK mediated alternative pathway, respectively, in mature DCs.

New lessons from old pathogens: what parasitic infections have taught us about the role of nuclear factor-kappaB in the regulation of immunity

The nuclear factor-kappaB (NF-kappaB) family of transcription factors is activated by many infectious and inflammatory stimuli. This family regulates the expression of multiple genes, whose products include cytokines, chemokines, adhesion molecules, and antiapoptotic factors that are important components of the innate and adaptive immune response. A prominent role of NF-kappaB transcription factors in resistance to a variety of infectious diseases was revealed by studies with mice that lack individual family members. However, little is known about the basis for these effects or about the role of individual family members during a coordinated immune response to infection. Diverse parasites such as Toxoplasma gondii, Leishmania major, and Trichuris muris provide a unique opportunity to understand the role of the NF-kappaB system in the development of innate and adaptive immunity to these infections. The basis for resistance and susceptibility to these parasites is well understood, and studies using these experimental systems have provided unique insights into the role of NF-kappaB in the regulation of T-helper 1 cell (Th1) and Th2 type responses. It has become clear that NF-kappaB family members have cell lineage-specific functions and that their relative importance varies with type of infection as well as route of pathogen entry. Thus, studies with models of parasitic infection have revealed that individual NF-kappaB family members perform distinct, nonoverlapping, and biologically significant functions in the regulation of immunity and inflammation.

Act1, a negative regulator in CD40- and BAFF-mediated B cell survival

TNF receptor (TNFR) superfamily members, CD40, and BAFFR play critical roles in B cell survival and differentiation. Genetic deficiency in a novel adaptor molecule, Act1, for CD40 and BAFF results in a dramatic increase in peripheral B cells, which culminates in lymphadenopathy and splenomegaly, hypergammaglobulinemia, and autoantibodies. While the B cell-specific Act1 knockout mice displayed a similar phenotype with less severity, the pathology of the Act1-deficient mice was mostly blocked in CD40-Act1 and BAFF-Act1 double knockout mice. CD40- and BAFF-mediated survival is significantly increased in Act1-deficent B cells, with stronger IkappaB phosphorylation, processing of NF-kappaB2 (p100/p52), and activation of JNK, ERK, and p38 pathways, indicating that Act1 negatively regulates CD40- and BAFF-mediated signaling events. These findings demonstrate that Act1 plays an important role in the homeostasis of B cells by attenuating CD40 and BAFFR signaling.

The multifaceted roles of TRAFs in the regulation of B-cell function

Tumour-necrosis factor receptor (TNFR)-associated factors (TRAFs) are cytoplasmic adaptor proteins that are important in lymphocyte activation and apoptosis. Many studies of TRAFs have used models of exogenous overexpression by non-lymphoid cells. However, the actions of TRAFs present at normal levels in lymphoid cells often differ considerably from those that have been established in non-lymphocyte overexpression models. As I discuss here, information obtained from studying these molecules in physiological settings in B cells reveals that they have several roles, which are both unique and overlapping. These include activation of kinases and transcription factors, and interactions with other signalling proteins, culminating in the induction or inhibition of biological functions.

Receptor-specific signaling for both the alternative and the canonical NF-kappaB activation pathways by NF-kappaB-inducing kinase

The NF-kappaB-inducing kinase (NIK) induces proteolytic processing of NF-kappaB2/p100 and, hence, the generation of NF-kappaB dimers such as p52:RelB but was suggested not to signal for the processing of IkappaB. Here, we show that although the induction of IkappaB degradation in lymphocytes by TNF is independent of NIK, its induction by CD70, CD40 ligand, and BLyS/BAFF, which all also induce NF-kappaB2/p100 processing, does depend on NIK function. Both CD70 and TNF induce recruitment of the IKK kinase complex to their receptors. In the case of CD70, but not TNF, this process is associated with NIK recruitment and is followed by prolonged receptor association of just IKK1 and NIK. Recruitment of the IKK complex to CD27, but not that of NIK, depends on NIK kinase function. Our findings indicate that NIK participates in a unique set of proximal signaling events initiated by specific inducers, which activate both canonical and noncanonical NF-kappaB dimers.

NF-kappaB-inducing kinase restores defective IkappaB kinase activity and NF-kappaB signaling in intestinal epithelial cells

Cytokine-stimulated IkappaBalpha degradation is impaired in HT-29 and primary intestinal epithelial cells. To gain more insight into the mechanism of this defect, we dissected cytokine-induced NF-kappaB signaling pathway in HT-29 cells. IL-1beta and TNF, alone or in combination with IFNgamma, failed to induce IkappaBalpha or IkappaBbeta degradation in HT-29 cells. Despite similar 125I-IL-1beta binding, HT-29 cells displayed no IRAK degradation, a 75% reduction of IKK activity, and decreased IkappaBalpha phosphorylation, NF-kappaB DNA binding activity and IL-8 mRNA accumulation in response to IL-1beta compared to Caco-2 cells. Selective activation of NF-kappaB pathway by adenoviral delivery of NF-kappaB-inducing kinase (Ad5NIK) or IKKbeta (Ad5IKKbeta) strongly activated IKK activity (>20 fold) in HT-29 cells with concomitant endogenous IkappaBalpha serine 32 phosphorylation and total IkappaBalpha degradation. In addition, NF-kappaB DNA binding activity and IL-8 secretion is higher in Ad5NIK-infected than in IL-1beta-stimulated HT-29 cells. These data show that altered NF-kappaB signaling is associated with impaired stimulation of an upstream IKK activator.

The Role of the p38 Mitogen-Activated Protein Kinase, Extracellular Signal-Regulated Kinase, and Phosphoinositide-3-OH Kinase Signal Transduction Pathways in CD40 Ligand-Induced Dendritic Cell Activation and Expansion of Virus-Specific CD8+T Cell Memory Responses

Mature dendritic cells (DCs) are central to the development of optimal T cell immune responses. CD40 ligand (CD40L, CD154) is one of the most potent maturation stimuli for immature DCs. We studied the role of three signaling pathways, p38 mitogen-activated protein kinase (MAPK), extracellular signal-regulated kinase (ERK), and phosphoinositide-3-OH kinase (PI3K), in CD40L-induced monocyte-derived DC activation, survival, and expansion of virus-specific CD8(+) T cell responses. p38 MAPK pathway was critical for CD40L-mediated up-regulation of CD83, a marker of DC maturation. CD40L-induced monocyte-derived DC IL-12 production was mediated by both the p38 MAPK and PI3K pathways. CD40L-mediated DC survival was mostly mediated by the PI3K pathway, with smaller contributions by p38 MAPK and ERK pathways. Finally, the p38 MAPK pathway was most important in mediating CD40L-stimulated DCs to induce strong allogeneic responses as well as expanding virus-specific memory CD8(+) T cell responses. Thus, although the p38 MAPK, PI3K, and ERK pathways independently affect various parameters of DC maturation induced by CD40L, the p38 MAPK pathway within CD40L-conditioned DCs is the most important pathway to maximally elicit T cell immune responses. This pathway should be exploited in vivo to either completely suppress or enhance CD8(+) T cell immune responses.

Epstein-Barr virus-encoded latent infection membrane protein 1 regulates the processing of p100 NF-kappaB2 to p52 via an IKKgamma/NEMO-independent signalling pathway

The oncogenic Epstein-Barr virus (EBV)-encoded latent infection membrane protein 1 (LMP1) constitutively activates the 'canonical' NF-kappaB pathway that involves the phosphorylation and degradation of IkappaBalpha downstream of the IkappaB kinases (IKKs). In this study, we show that LMP1 also promotes the proteasome-mediated proteolysis of p100 NF-kappaB2 resulting in the generation of active p52, which translocates to the nucleus in complex with the p65 and RelB NF-kappaB subunits. LMP1-induced NF-kappaB transactivation is reduced in nf-kb2(-/-) mouse embryo fibroblasts, suggesting that p100 processing contributes to LMP1-mediated NF-kappaB transcriptional effects. This pathway is likely to operate in vivo, as the expression of LMP1 in primary EBV-positive Hodgkin's lymphoma and nasopharyngeal carcinoma biopsies correlates with the nuclear accumulation of p52. Interestingly, while the ability of LMP1 to activate the canonical NF-kappaB pathway is impaired in cells lacking IKKgamma/NEMO, the regulatory subunit of the IKK complex, p100 processing remains unaffected. As a result, nuclear translocation of p52, but not p65, occurs in the absence of IKKgamma. These data point to the existence of a novel signalling pathway that regulates NF-kappaB in LMP1-expressing cells, and may thereby play a role in both oncogenic transformation and the establishment of persistent EBV infection.

The IkappaB function of NF-kappaB2 p100 controls stimulated osteoclastogenesis

The prototranscription factor p100 represents an intersection of the NF-kappaB and IkappaB families, potentially serving as both the precursor for the active NF-kappaB subunit p52 and as an IkappaB capable of retaining NF-kappaB in the cytoplasm. NF-kappaB-inducing kinase (NIK) controls processing of p100 to generate p52, and thus NIK-deficient mice can be used to examine the biological effects of a failure in such processing. We demonstrate that treatment of wild-type osteoclast precursors with the osteoclastogenic cytokine receptor activator of NF-kappaB ligand (RANKL) increases both expression of p100 and its conversion to p52, resulting in unchanged net levels of p100. In the absence of NIK, p100 expression is increased by RANKL, but its conversion to p52 is blocked, leading to cytosolic accumulation of p100, which, acting as an IkappaB protein, binds NF-kappaB complexes and prevents their nuclear translocation. High levels of unprocessed p100 in osteoclast precursors from NIK-/- mice or a nonprocessable form of the protein in wild-type cells impair RANKL-mediated osteoclastogenesis. Conversely, p100-deficient osteoclast precursors show enhanced sensitivity to RANKL. These data demonstrate a novel, biologically relevant means of regulating NF-kappaB signaling, with upstream control and kinetics distinct from the classical IkappaBalpha pathway.

Recent advances on the role of CD40 and dendritic cells in immunity and tolerance

CD40 is a key signaling pathway for the function of B cells, monocytes, and dendritic cells in the immune system, and plays an important role in inflammatory pathways of nonhemopoietic cells. The NFkappaB family of transcription factors is a critical mediator in inflammation. NFkappaB is involved both in the regulation of CD40 expression and in cell signaling after CD40 ligation. This positive feedback loop linking NFkappaB and CD40 plays an important role in the control of the adaptive immune response, with fundamental implications for immunity and tolerance in vivo.

The CD40-CD154 interaction in B cell-T cell liaisons

The CD40 receptor is expressed constitutively on B lymphocytes, for which it provides important signals regulating clonal expansion, antibody production and isotype switching, as well as the development of humoral memory. The major source of CD154, the ligand for CD40, is activated T lymphocytes. Interactions between CD40 and CD154 provide a number of signals that play important roles in regulating the complex and multifactorial interactions between these two major cell types of the adaptive immune response. Understanding both the biological effects of this receptor-ligand interaction, as well as how CD40 signaling pathways are controlled, adds to our detailed picture of the complex interplay between B and T cells.

The NF-kappa B activation in lymphotoxin beta receptor signaling depends on the phosphorylation of p65 at serine 536

NF-kappaB-inducing kinase (NIK) has been shown to play an essential role in the NF-kappaB activation cascade elicited by lymphotoxin beta receptor (LTbetaR) signaling. However, the molecular mechanism of this pathway remains unclear. In this report we demonstrate that both NIK and IkappaB kinase alpha (IKKalpha) are involved in LTbetaR signaling and that the phosphorylation of the p65 subunit at serine 536 in its transactivation domain 1 (TA1) plays an essential role. We also found that NF-kappaB could be activated in the LTbetaR pathway without altering the level of the phosphorylation of IkappaB and nuclear localization of p65. By using a heterologous transactivation system in which Gal4-dependent reporter gene is activated by the Gal4 DNA-binding domain in fusion with various portions of p65, we found that TA1 serves as a direct target in the NIK-IKKalpha pathway. In addition, mutation studies have revealed the essential role of Ser-536 within TA1 of p65 in transcriptional control mediated by NIK-IKKalpha. Furthermore, we found that Ser-536 was phosphorylated following the stimulation of LTbetaR, and this phosphorylation was inhibited by the kinase-dead dominant-negative mutant of either NIK or IKKalpha. These observations provide evidence for a crucial role of the NIK-IKKalpha cascade for NF-kappaB activation in LTbetaR signaling.

The Notch ligand Jagged-1 is able to induce maturation of monocyte-derived human dendritic cells

Notch receptors play a key role in several cellular processes including differentiation, proliferation, and apoptosis. This study investigated whether the activation of Notch signaling would affect the maturation of dendritic cells (DCs). Direct stimulation of Notch signaling in DCs with a peptide ligand induced DC maturation, similar to LPS: DCs up-regulated maturation markers, produced IL-12, lost endocytosis capacity, and became able to activate allogeneic T cells. Furthermore, coculture of DCs with cells expressing Notch ligand Jagged-1 induced up-regulation of maturation markers, IL-12 production, T cell proliferative responses, and IFN-gamma production. Our data suggest that activation of Notch by Jagged-1 plays an important role in maturation of human DCs. Additionally, they reveal a novel role for Notch signaling in cell maturation events distal to the cell fate decision fork. These data may have important medical implications, since they provide new reagents to induce DC activity, which may be beneficial as adjuvants in situations where an immune response needs to be elicited, such as tumor immunotherapy.

CD40 regulates the processing of NF-kappaB2 p100 to p52

The nf-kb2 gene encodes the cytoplasmic NF-kappaB inhibitory protein p100 from which the active p52 NF-kappaB subunit is derived by proteasome-mediated proteolysis. Ligands which stimulate p100 processing to p52 have not been defined. Here, ligation of CD40 on transfected 293 cells is shown to trigger p52 production by stimulating p100 ubiquitylation and subsequent proteasome-mediated proteolysis. CD40-mediated p52 accumulation is dependent on de novo protein synthesis and triggers p52 translocation into the nucleus to generate active NF-kappaB dimers. Endogenous CD40 ligation on primary murine splenic B cells also stimulates p100 processing, which results in the delayed nuclear translocation of p52-RelB dimers. In both 293 cells and primary splenic B cells, the ability of CD40 to trigger p100 processing requires functional NF-kappaB-inducing kinase (NIK). In contrast, NIK activity is not required for CD40 to stimulate the degradation of IkappaBalpha in either cell type. The regulation of p100 processing by CD40 is likely to be important for the transcriptional regulation of CD40 target genes in adaptive immune responses.

Role of NF kappa B activator Act1 in CD40-mediated signaling in epithelial cells

CD40, a cell surface receptor in the tumor necrosis factor receptor family, first identified and functionally characterized on B lymphocytes, is also expressed on epithelial and other cells and is now thought to play a more general role in immune regulation. Overexpression of the NF kappa B activator 1 (Act1) leads to the activation of both NF kappa B and Jun kinase in epithelial cell lines. Endogenous Act1 is recruited to the CD40 receptor in human intestinal (HT29) and cervical (HeLa) epithelial cells upon stimulation with CD40 ligand, indicating that Act1 is involved in this signaling pathway. Act1 also interacts with tumor necrosis factor receptor-associated factor 3, a component involved in CD40-activated pathway. Furthermore, transfection of Act1 into C33A cervical epithelial cells, which do not express it, renders these cells sensitive to CD40 ligand-induced NF kappa B activation and protects them from CD40 ligand-induced apoptosis. We conclude that Act1 plays an important role in CD40-mediated signaling in epithelial cells.

Differential requirement for NF-kappaB-inducing kinase in the induction of NF-kappaB by IL-1beta, TNF-alpha, and Fas

In this study, we examined the role of the nuclear factor-kappaB (NF-kappaB)-inducing kinase (NIK) in distinct signaling pathways leading to NF-kappaB activation. We show that a dominant-negative form of NIK (dnNIK) delivered by adenoviral (Ad5dnNIK) vector inhibits Fas-induced IkappaBalpha phosphorylation and NF-kappaB-dependent gene expression in HT-29 and HeLa cells. Interleukin (IL)-1beta- and tumor necrosis factor-alpha (TNF-alpha)-induced NF-kappaB activation and kappaB-dependent gene expression are inhibited in HeLa cells but not in Ad5dnNIK-infected HT-29 cells. Moreover, Ad5dnNIK failed to sensitize HT-29 cells to TNF-alpha-induced apoptosis at an early time point. However, cytokine- and Fas-induced signals to NF-kappaB are finally integrated by the IkappaB kinase (IKK) complex, since IkappaBalpha phosphorylation, NF-kappaB DNA binding activity, and IL-8 gene expression were strongly inhibited in HT-29 and HeLa cells overexpressing dominant-negative IKKbeta (Ad5dnIKKbeta). Our findings support the concept that cytokine signaling to NF-kappaB is redundant at the level of NIK. In addition, this study demonstrates for the first time the critical role of NIK and IKKbeta in Fas-induced NF-kappaB signaling cascade.

Protein kinase C beta controls nuclear factor kappaB activation in B cells through selective regulation of the IkappaB kinase alpha

Activation of the nuclear factor (NF)-kappaB transcription complex by signals derived from the surface expressed B cell antigen receptor controls B cell development, survival, and antigenic responses. Activation of NF-kappaB is critically dependent on serine phosphorylation of the IkappaB protein by the multi-component IkappaB kinase (IKK) containing two catalytic subunits (IKKalpha and IKKbeta) and one regulatory subunit (IKKgamma). Using mice deficient for protein kinase C beta (PKCbeta) we show an essential role of PKCbeta in the phosphorylation of IKKalpha and the subsequent activation of NF-kappaB in B cells. Defective IKKalpha phosphorylation correlates with impaired B cell antigen receptor-mediated induction of the pro-survival protein Bcl-xL. Lack of IKKalpha phosphorylation and defective NF-kappaB induction in the absence of PKCbeta explains the similarity in immunodeficiencies caused by PKCbeta or IKKalpha ablation in B cells. Furthermore, the well established functional cooperation between the protein tyrosine kinase Bruton's tyrosine kinase (Btk), which regulates the activity of NF-kappaB and PKCbeta, suggests PKCbeta as a likely serine/threonine kinase component of the Btk-dependent NF-kappaB activating signal transduction chain downstream of the BCR.

CD40 ligation conditions dendritic cell antigen-presenting function through sustained activation of NF-kappaB

An understanding of the biochemical control of dendritic cell (DC) differentiation/activation is essential for improving T cell immunity by various immunotherapeutic approaches, including DC immunization. Ligation of CD40 enhances DC function, including conditioning for CTL priming. NF-kappaB, and particularly RelB, is an essential control pathway for myeloid DC differentiation. Furthermore, RelB regulates B cell Ag-presenting function. We hypothesized that CD40 ligand (CD40L) and TNF-alpha, which differ in their capacity to condition DC, would also differ in their capacity to activate NF-kappaB. DC differentiated for 2 days from monocytes in the presence of GM-CSF and IL-4 were used as a model, as NF-kappaB activity was constitutively low. The capacity of DC to activate T cells following CD40L treatment was enhanced compared with TNF-alpha treatment, and this was NF-kappaB dependent. Whereas RelB/p50 translocation induced by TNF-alpha was attenuated after 6 h, RelB/p50 nuclear translocation induced by CD40L was sustained for at least 24 h. The mechanism of this difference related to enhanced degradation of IkappaBalpha following CD40L stimulation. However, NF-kappaB activation induced by TNF-alpha could be sustained by blocking autocrine IL-10. These data indicate that NF-kappaB activation is essential for T cell activation by DC, and that this function is enhanced if DC NF-kappaB activation is prolonged. Because IL-10 moderates DC NF-kappaB activation by TNF-alpha, sustained NF-kappaB activation can be achieved by blocking IL-10 in the presence of stimuli that induce TNF-alpha.

The oncogenic protein kinase Tpl-2/Cot contributes to Epstein-Barr virus-encoded latent infection membrane protein 1-induced NF-kappaB signaling downstream of TRAF2

The Epstein-Barr virus-encoded latent infection membrane protein 1 (LMP1) is a pleiotropic protein, the activities of which include effects on cell transformation and phenotype, growth, and survival. The ability of LMP1 to mediate at least some of these phenomena could be attributed to the activation of the transcription factor NF-kappaB. LMP1 promotes NF-kappaB activation through the recruitment of the adapter protein TRAF2 and the formation of a dynamic multiprotein complex that includes the NF-kappaB kinase, the IkappaB kinases, and their downstream targets, IkappaBs and p105. In this study, we have identified the oncogenic kinase Tpl-2/Cot as a novel component of LMP1-induced NF-kappaB signaling. We show that Tpl-2 is expressed in primary biopsies from patients with nasopharyngeal carcinoma and Hodgkin's disease, where LMP1 is also found. Inducible expression of LMP1 promotes the activation of Tpl-2, and a catalytically inactive Tpl-2 mutant suppresses LMP1-induced NF-kappaB signaling. In colocalization and coimmunoprecipitation experiments, Tpl-2 and TRAF2 were found to interact with Tpl-2 functioning downstream of TRAF2. Consistent with this observation, catalytically inactive Tpl-2 also blocked CD40-mediated NF-kappaB activation, which largely depends on TRAF2. The ability of Tpl-2 to influence LMP1-induced NF-kappaB occurs through modulation of both IkappaBalpha and p105 functions. Furthermore, Tpl-2 was found to influence the expression of angiogenic mediators, such as COX-2 in LMP1-transfected cells. These data identify Tpl-2 as a component of LMP1 signaling downstream of TRAF2 and as a modulator of LMP1-mediated effects.

NF-kappa B2 is required for optimal CD40-induced IL-12 production but dispensable for Th1 cell Differentiation

NF-kappa B is a ubiquitously expressed transcription factor involved in the regulation of innate and adaptive immunity. As part of studies to define the role of various NF-kappa B family members in Th cell development and maintenance, we infected NF-kappa B2(-/-) and control mice with Leishmania major and followed disease progression. NF-kappa B2(-/-) mice on a normally resistant background develop chronic nonhealing lesions associated with uncontrolled parasite replication and a failure to develop an IFN-gamma response. We show that there are no intrinsic defects in Th cell differentiation in the absence of NF-kappa B2. Indeed, NF-kappa B2(-/-) T cells are able to develop a Th1 phenotype and protect recombination-activating gene(-/-) mice from progressive cutaneous leishmaniasis. We demonstrate instead that the susceptibility of NF-kappa B2(-/-) mice to L. major is the result of an IL-12 deficiency, and we provide evidence for a specific impairment in CD40-induced IL-12 production by macrophages lacking this transcription factor.

The cross-priming APC requires a Rel-dependent signal to induce CTL

Induction of OVA-specific CTL by cross-priming requires help from CD4 T cells, which use CD154 to signal CD40 on the APC. To further dissect the molecular pathways involved in cross-priming, we examined the role of Rel, an NF-kappaB family member. c-rel(-/-) mice failed to generate OVA-specific CTL by cross-priming, but could induce CTL to HSV-1. Using chimeric mice, Rel expression was shown to be required by the APC, but not by the T cells. Notably, the deficiency in Rel could be overcome by triggering CD40, implying that the APC required Rel before receipt of the CD40 signal. These data suggest that the cross-priming APC must receive two signals before it can stimulate CTL. The first signal is Rel dependent and is required before activation of CD4 helper T cells, which then deliver the second signal using CD154 to trigger CD40.

Expression of CD40 identifies a unique pathogenic T cell population in type 1 diabetes

Juvenile diabetes (type 1) is an autoimmune disease in which CD4(+) T cells play a major role in pathogenesis characterized by insulitis and beta cell destruction leading to clinical hyperglycemia. To date, no marker for autoimmune T cells has been described, although it was previously demonstrated that autoimmune mice have a large population of CD4(+) cells that express CD40. We show here that established, diabetogenic T cell clones of either the Th1 or Th2 phenotype are CD40-positive, whereas nondiabetogenic clones are CD40-negative. CD40 functionally signals T cell clones, inducing rapid activation of the transcription factor NFkappaB. We show that autoimmune diabetes-prone nonobese diabetic mice have high levels of CD40(+)CD4(+) T cells in the thymus, spleen, and importantly, in the pancreas. Finally, as demonstrated by adoptive transfers, CD4(+)CD40(+) cells infiltrate the pancreatic islets causing beta-cell degranulation and ultimately diabetes.

Formation of Peyer's patches

Formation of Peyer's patches requires complex interactions between the gut epithelium, the mesenchyme, and bone-marrow-derived hematopoietic progenitors. The first Peyer's patches anlage appear around embryonic day 15.5, when the endoderm has undergone transition to a simple epithelium, the lymphatic vessels have reached the intestinal mucosa, and mesenchymal cells have started to form clusters. Recent data using knockout mice provide insight into the molecular nature of the signals that mediate Peyer's patch ontogeny. These include members of the tumor-necrosis factor family and homeostatic chemokines.

CD40 activates NF-kappa B and c-Jun N-terminal kinase and enhances chemokine secretion on activated human hepatic stellate cells

Activated hepatic stellate cells (HSCs) are the main producers of extracellular matrix in the fibrotic liver and contribute to hepatic inflammation through the secretion of chemokines and the recruitment of leukocytes. This study assesses the function of CD40 on human HSCS: Activated human HSCs express CD40 in culture and in fibrotic liver, as determined by flow cytometry, RT-PCR, and immunohistochemistry. CD40 expression is strongly enhanced by IFN-gamma. Stimulation of CD40 with CD40 ligand (CD40L)-transfected baby hamster kidney cells induces NF-kappaB, as demonstrated by the activation of I-kappaB kinase (IKK), increased NF-kappaB DNA binding, and p65 nuclear translocation. CD40-activated IKK also phosphorylates a GST-p65 substrate at serine 536 in the transactivation domain 1. Concomitant with the activation of IKK, CD40L-transfected baby hamster kidney cell treatment strongly activates c-Jun N-terminal kinase. CD40 activation increases the secretion of IL-8 and monocyte chemoattractant protein-1 by HSCs 10- and 2-fold, respectively. Adenovirally delivered dominant negative (dn) IKK2 and TNFR-associated factor 2dn inhibit IKK-mediated GST-I-kappaB and GST-p65 phosphorylation, NF-kappaB binding, and IL-8 secretion, whereas IKK1dn and NF-kappaB-inducing kinase dominant negative do not have inhibitory effects. We conclude that the CD40-CD40L receptor-ligand pair is involved in a cross-talk between HSCs and immune effector cells that contributes to the perpetuation of HSC activation in liver fibrosis through TNFR-associated factor 2- and IKK2-dependent pathways.

Dendritic cells from nonobese diabetic mice exhibit a defect in NF-kappa B regulation due to a hyperactive I kappa B kinase

Insulin-dependent diabetes mellitus (IDDM) is characterized by the T cell-mediated destruction of insulin-producing beta cells. Accordingly, APCs, such as macrophage, have also been shown to be important in the disease process. However, the role(s) of dendritic cells (DCs) that exhibit potent APC function remains undefined in IDDM. Here we demonstrate that DCs derived from nonobese diabetic (NOD) mice, a model for IDDM, are more sensitive to various forms of stimulation compared with those from C57BL/6 and BALB/c mice, resulting in increased IL-12 secretion. This property is a consequence of hyperactivation of NF-kappaB, a transcription factor known to regulate IL-12 gene expression. Specifically, NOD DCs exhibit persistent hyperactivation of both IkappaB kinase and NF-kappaB in response to stimuli, in addition to selective degradation of IkappaBepsilon. Transfection of NOD DCs with a modified form of IkappaBalpha significantly reduced IL-12 secretion, suggesting that hyperactivation of NF-kappaB was in part responsible for increased IL-12 production. An enhanced capacity of NOD DCs to secrete IL-12 would be expected to contribute to the development of pathogenic Th1 (Tc1) cells during the diabetogenic response.

B lymphocyte activation by contact-mediated interactions with T lymphocytes

T cell dependent B lymphocyte activation requires interactions between numerous receptor-ligand pairs on the two cell types. Recently, advances have been made both in understanding how these various signals regulate B cell effector functions and in identifying many new receptor-ligand pairs that contribute to the regulation of B cell function by T lymphocytes.

Effects of the NIK aly mutation on NF-kappaB activation by the Epstein-Barr virus latent infection membrane protein, lymphotoxin beta receptor, and CD40

Homozygosity for the aly point mutation in NF-kappaB-inducing kinase (NIK) results in alymphoplasia in mice, a phenotype similar to that of homozygosity for deletion of the lymphotoxin beta receptor (LTbetaR). We now find that NF-kappaB activation by Epstein-Barr virus latent membrane protein 1 (LMP1) or by an LMP1 transmembrane domain chimera with the LTbetaR signaling domain in human embryonic kidney 293 cells is selectively inhibited by a wild type dominant negative NIK comprised of amino acids 624-947 (DN-NIK) and not by aly DN-NIK. In contrast, LMP1/CD40 is inhibited by both wild type (wt) and aly DN-NIK. LMP1, an LMP1 transmembrane domain chimera with the LTbetaR signaling domain, and LMP1/CD40 activate NF-kappaB in wt or aly murine embryo fibroblasts. Although wt and aly NIK do not differ in their in vitro binding to tumor necrosis factor receptor-associated factor 1, 2, 3, or 6 or in their in vivo association with tumor necrosis factor receptor-associated factor 2 and differ marginally in their very poor binding to IkappaB kinase beta (IKKbeta), only wt NIK is able to bind to IKKalpha. These data are compatible with a model in which activation of NF-kappaB by LMP1 and LTbetaR is mediated by an interaction of NIK or a NIK-like kinase with IKKalpha that is abrogated by the aly mutation. On the other hand, CD40 mediates NF-kappaB activation through a kinase that interacts with a different component of the IKK complex.

Defective lymphotoxin-beta receptor-induced NF-kappaB transcriptional activity in NIK-deficient mice

The role of NF-kappaB-inducing kinase (NIK) in cytokine signaling remains controversial. To identify the physiologic functions of NIK, we disrupted the NIK locus by gene targeting. Although NIK-/- mice displayed abnormalities in both lymphoid tissue development and antibody responses, NIK-/- cells manifested normal NF-kappaB DNA binding activity when treated with a variety of cytokines, including tumor necrosis factor (TNF), interleukin-1 (IL-1), and lymphotoxin-beta (LTbeta). However, NIK was selectively required for gene transcription induced through ligation of LTbeta receptor but not TNF receptors. These results reveal that NIK regulates the transcriptional activity of NF-kappaB in a receptor-restricted manner.

Dendritic cell longevity and T cell persistence is controlled by CD154-CD40 interactions

Inflammatory mediators facilitate the maturation of dendritic cells (DC), enabling them to induce the activation, proliferation and differentiation of cognate T cells. The role of CD40 on DC and CD154 on T cells has been studied by the co-adoptive transfer of antigen-pulsed DC and TCR-transgenic (Tg) T cells in vivo. It is shown that in the absence of CD40-CD154 interactions, initial Tg T cell expansion occurs in vivo, but over time, T cell expansion cannot be sustained. The basis for the demise of the T cell population is likely due to the disappearance of the antigen-pulsed DC in the draining lymph nodes when CD154-CD40 interactions are interrupted. These findings show that both T cell and DC persistence in vivo is dependent on CD40-CD154 interactions. In addition to the physical persistence of the DC, CD40 triggering of DC also greatly increases the period for which they can productively present antigen to Tg T cells. Hence DC persistence and antigen-presenting cell capacity are both dependent on CD40 signaling. While TNF-alpha can mature DC as measured by a variety of criteria, the unique capacity of CD40 signaling to sustain T cell responses and induce DC maturation is underscored by the inability of TNF-alpha to rescue the immune deficiency of CD40(-/-) DC. Hence, the profound impact of CD154 deficiency on cell-mediated immunity may be due to its ability to limit the duration of antigen presentation in vivo and cause the premature demise of antigen-specific T cells.

p21-activated kinase 1 activates the nuclear factor kappa B (NF-kappa B)-inducing kinase-Ikappa B kinases NF-kappa B pathway and proinflammatory cytokines in Helicobacter pylori infection

Helicobacter pylori, the causative agent of several human gastric diseases, induces activation of the immediate early response transcription factor nuclear factor kappaB (NF-kappaB), which subsequently triggers release of proinflammatory cytokines in colonized epithelial cells. Here we report that in H. pylori infection p21-activated kinase 1 (PAK1) activates NF-kappaB. Activated PAK1 associates with NF-kappaB-inducing kinase, which upon activation directs the activity of IkappaB kinases to IkappaBalpha. Our results indicate that in epithelial cells PAK1 participates in a unique pathway that links H. pylori-dependent effector molecules to the activation of NF-kappaB and the induction of the innate immune response.