Regulation of secondary lymphoid organ development by the nuclear factor-kappaB signal transduction pathway

RelB, together with RelA, sustains cell survival and confers proteasome inhibitor sensitivity of chronic lymphocytic leukemia cells from bone marrow

Although the biological factors that contribute to the pathogenesis of chronic lymphocytic leukemia (CLL) remain widely unresolved, it has been suggested that dysregulated cell survival and proliferation are fundamental to this process. Constitutive classical nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB) activation protects CLL B-cells from cell death and plays a critical role in the acquisition of chemoresistance. RelB, representing the alternative NF-κB activity, functions specifically in lymphoid organogenesis and B-cell maturation. RelB indeed plays a tumor-supportive role and confers radiation resistance in tumors. However, the involvement of RelB in CLL has not been addressed. Here, we analyzed the NF-κB activation in 67 of CLL bone marrow (BM). Both the RelA and RelB activity were detected in CLL B-cells from BM, in spite of inevitable variability. Low RelB activity was linked to a favorable prognosis of CLL. The migration and adhesion abilities of CLL B-cells were not affected by the RelB activity. High RelB activity, together with the RelA activity, maintained basal survival of cells. The induction of RelA and RelB expression in the nucleus was responsible for better survival of CLL B-cells supported by bone marrow stromal cells. In addition, the presence of high RelB activity in CLL B-cells was correlated with sensitivity to proteasome inhibitor but not fludarabine. Taken together, we provided evidences that not only RelA but also RelB, subunits of NF-κB family, played an important role in the cellular behaviors of CLL cells from BM. The strength of RelB activity influenced the prognosis of CLL patients.

KEY MESSAGE

RelB, with RelA activity, maintained the basal survival of CLL cells from BM. RelB, with RelA, conferred the proteasome inhibitor sensitivity of CLL cells. Induction of RelA and RelB was responsible for the better survival of CLL B-cells. The strength of RelB activity influenced the prognosis of CLL patients.

Muramyl dipeptide responsive pathways in Crohn's disease: from NOD2 and beyond

Crohn's disease (CD) is one of main disease entities under the umbrella term chronic inflammatory bowel disease. The etiology of CD involves alterations in genetic, microbiological, and immunological factors. This review is devoted to the role of the bacterial wall compound muramyl dipeptide (MDP) for the activation of inflammatory pathways involved in the pathogenesis of CD. The importance of this molecule is underscored by the fact that (1) MDP, which is found in most Gram-negative and -positive bacteria, is able to trigger several immunological responses in the intestinal system, and (2) that alterations in several mediators of the MDP response including-but not restricted to-nucleotide oligomerization domain 2 (NOD2) are associated with CD. The normalization of MDP signaling is one of several important factors that influence the intestinal inflammatory response, a fact which emphasizes the pathogenic importance of MDP signaling for the pathogenesis of CD. The important aspects of NOD2 and non-NOD2 mediated effects of MDP for the development of CD are highlighted, as well as how alterations in these pathways might translate into the development of new therapeutic strategies.

Preventive Effects of a Natural Anti-Inflammatory Agent, Astragaloside IV, on Ischemic Acute Kidney Injury in Rats

This study investigated the anti-inflammatory effects of astragaloside IV(AS-IV) on ischemia/reperfusion (IR) induced acute kidney injury (AKI) in rats. Experimental model of ischemic AKI was induced in rats by bilateral renal artery clamp for 45 min followed by reperfusion of 12 h and 24 h, respectively. AS-IV was orally administered once a day to rats at 10 and 20 mg·kg⁻¹·d⁻¹ for 7 days prior to ischemia. AS-IV pretreatment significantly decreased serum urea, creatinine, and cystatin C levels at 12 h and 24 h of reperfusion in AKI rats. AS-IV pretreatment also ameliorated tubular damage and suppressed the phosphorylation of p65 subunit of NF-κB in AKI rats. Moreover, NF-κB and MPO activity as well as serum and tissue levels of TNF-α, MCP-1, and ICAM-1 were elevated in AKI rats. All of these abnormalities were prevented by AS-IV. Furthermore, AS-IV downregulated the mRNA expression of NF-κB, TNF-α, MCP-1, and ICAM-1 in AKI rats. These results suggest that AS-IV might be developed as a novel therapeutic approach to prevent ischemic AKI through inhibition of NF-κB mediated inflammatory genes expression.

SHARPIN is a key regulator of immune and inflammatory responses

Mice with spontaneous mutations in the Sharpin gene develop chronic proliferative dermatitis that is characterized by eosinophilic inflammation of the skin and other organs with increased expression of type 2 cytokines and dysregulated development of lymphoid tissues. The mutant mice share phenotypic features with human hypereosinophilic syndromes. The biological function of SHARPIN and how its absence leads to such a complex inflammatory phenotype in mice are poorly understood. However, recent studies identified SHARPIN as a novel modulator of immune and inflammatory responses. The emerging mechanistic model suggests that SHARPIN functions as an important adaptor component of the linear ubiquitin chain assembly complex that modulates activation of NF-κB signalling pathway, thereby regulating cell survival and apoptosis, cytokine production and development of lymphoid tissues. In this review, we will summarize the current understanding of the ubiquitin-dependent regulatory mechanisms involved in NF-κB signalling, and incorporate the recently obtained molecular insights of SHARPIN into this pathway. Recent studies identified SHARPIN as an inhibitor of β1-integrin activation and signalling, and this may be another mechanism by which SHARPIN regulates inflammation. Furthermore, the disrupted lymphoid organogenesis in SHARPIN-deficient mice suggests that SHARPIN-mediated NF-κB regulation is important for de novo development of lymphoid tissues.

Mechanisms of thymus medulla development and function

The development of CD4(+) helper and CD8(+) cytotoxic T-cells expressing the αβ form of the T-cell receptor (αβTCR) takes place in the thymus, a primary lymphoid organ containing distinct cortical and medullary microenvironments. While the cortex represents a site of early T-cell precursor development, and the positive selection of CD4(+)8(+) thymocytes, the thymic medulla plays a key role in tolerance induction, ensuring that thymic emigrants are purged of autoreactive αβTCR specificities. In recent years, advances have been made in understanding the development and function of thymic medullary epithelial cells, most notably the subset defined by expression of the Autoimmune Regulator (Aire) gene. Here, we summarize current knowledge of the developmental mechanisms regulating thymus medulla development, and examine the role of the thymus medulla in recessive (negative selection) and dominant (T-regulatory cell) tolerance.

Mesenchymal cell differentiation during lymph node organogenesis

Secondary lymphoid tissues such as lymph nodes are essential for the interactions between antigen presenting cells and lymphocytes that result in adaptive immune responses that protect the host against invading pathogens. The specialized architecture of these organs facilitates the cognate interactions between antigen-loaded dendritic cells and lymphocytes expressing their specific receptor as well as B-T cell interactions that are at the core of long lasting adaptive immune responses. Lymph nodes develop during embryogenesis as a result of a series of cross-talk interactions between a hematopoietically derived cell lineage called lymphoid tissue inducer cells and stromal cells of mesenchymal origin to form the anlagen of these organs. This review will present an overview of the different signaling pathways and maturation steps that mesenchymal cells undergo during the process of lymph node formation such as cell specification, priming, and maturation to become lymphoid tissue stromal organizer cells.

SCF-mediated degradation of p100 (NF-κB2): mechanisms and relevance in multiple myeloma

On the basis of differential analysis of affinity purifications by mass spectrometry, we identified the nuclear factor κB (NF-κB) protein p100 (NF-κB2) as an interactor of the F-box protein FBXW7α. The NF-κB pathway is important for cell growth, differentiation, and survival. p100, which shuttles between the cytoplasm and nucleus, functions as the primary inhibitor of the noncanonical NF-κB pathway by sequestering NF-κB heterodimers in the cytoplasm. In the absence of NF-κB stimulation, the nuclear pool of p100 is constitutively targeted for degradation by FBXW7α, which recognizes a conserved motif that is phosphorylated by glycogen synthase kinase 3 (GSK3). Efficient activation of noncanonical NF-κB signaling depends on the clearance of nuclear p100, either through FBXW7α-mediated degradation or nuclear export mediated by a signal in the C terminus of p100. Upon prolonged stimulation of the NF-κB pathway, p100 is stabilized and retained in the nucleus, contributing to the cessation of noncanonical NF-κB signaling. The molecular mechanism of p100 degradation has implications in multiple myeloma, a disease with constitutive activation of the noncanonical NF-κB pathway. Accordingly, expression of a stable p100 mutant, FBXW7α depletion, or chemical inhibition of GSK3 in multiple myeloma cells results in cell death in vitro and in a xenotransplant model. Thus, the FBXW7α-dependent degradation of p100 functions as a prosurvival mechanism through control of NF-κB activity.

Biphasic activation of nuclear factor-kappa B in experimental models of subarachnoid hemorrhage in vivo and in vitro

It has been proven that nuclear factor-kappa B (NF-κB) is activated as a well-known transcription factor after subarachnoid hemorrhage (SAH). However, the panoramic view of NF-κB activity after SAH remained obscure. Cultured neurons were signed into control group and six hemoglobin- (Hb-) incubated groups. One-hemorrhage rabbit SAH model was produced, and the rabbits were divided randomly into one control group and five SAH groups. NF-κB activity was detected by electrophoretic mobility shift assay (EMSA) and immunohistochemistry. Real-time polymerase chain reaction (PCR) was performed to assess the downstream genes of NF-κB. NeuN immunofluorescence and lactate dehydrogenase (LDH) quantification were used to estimate the neuron injury. Double drastically elevated NF-κB activity peaks were detected in rabbit brains and cultured neurons. The downstream gene expressions showed an accordant phase peaks. NeuN-positive cells decreased significantly in day 3 and day 10 groups. LDH leakage exhibited a significant increase in Hb-incubated groups, but no significant difference was found between the Hb incubated groups. These results suggested that biphasic increasing of NF-κB activity was induced after SAH, and the early NF-κB activity peak indicated the injury role on neurons; however, the late peak might not be involved in the deteriorated effect on neurons.

p100 Deficiency is insufficient for full activation of the alternative NF-κB pathway: TNF cooperates with p52-RelB in target gene transcription

Background

Constitutive activation of the alternative NF-κB pathway leads to marginal zone B cell expansion and disorganized spleen microarchitecture. Furthermore, uncontrolled alternative NF-κB signaling may result in the development and progression of cancer. Here, we focused on the question how does the constitutive alternative NF-κB signaling exert its effects in these malignant processes.

Methodology/Principal Findings

To explore the consequences of unrestricted alternative NF-κB activation on genome-wide transcription, we compared gene expression profiles of wild-type and NF-κB2/p100-deficient (p100^−/−) primary mouse embryonic fibroblasts (MEFs) and spleens. Microarray experiments revealed only 73 differentially regulated genes in p100^−/− vs. wild-type MEFs. Chromatin immunoprecipitation (ChIP) assays showed in p100^−/− MEFs direct binding of p52 and RelB to the promoter of the Enpp2 gene encoding ENPP2/Autotaxin, a protein with an important role in lymphocyte homing and cell migration. Gene ontology analysis revealed upregulation of genes with anti-apoptotic/proliferative activity (Enpp2/Atx, Serpina3g, Traf1, Rrad), chemotactic/locomotory activity (Enpp2/Atx, Ccl8), and lymphocyte homing activity (Enpp2/Atx, Cd34). Most importantly, biochemical and gene expression analyses of MEFs and spleen, respectively, indicated a marked crosstalk between classical and alternative NF-κB pathways.

Conclusions/Significance

Our results show that p100 deficiency alone was insufficient for full induction of genes regulated by the alternative NF-κB pathway. Moreover, alternative NF-κB signaling strongly synergized both in vitro and in vivo with classical NF-κB activation, thereby extending the number of genes under the control of the p100 inhibitor of the alternative NF-κB signaling pathway.

Control of dichotomic innate and adaptive immune responses by artery tertiary lymphoid organs in atherosclerosis

Tertiary lymphoid organs (TLOs) emerge in tissues in response to non-resolving inflammation such as chronic infection, graft rejection, and autoimmune disease. We identified artery TLOs (ATLOs) in the adventitia adjacent to atherosclerotic plaques of aged hyperlipidemic ApoE^−/− mice. ATLOs are structured into T cell areas harboring conventional dendritic cells and monocyte-derived DCs; B cell follicles containing follicular dendritic cells within activated germinal centers; and peripheral niches of plasma cells. ATLOs also show extensive neoangiogenesis, aberrant lymphangiogenesis, and high endothelial venule (HEV) neogenesis. Newly formed conduit networks connect the external lamina of the artery with HEVs in T cell areas. ATLOs recruit and generate lymphocyte subsets with opposing activities including activated CD4⁺ and CD8⁺ effector T cells, natural and induced CD4⁺ T regulatory (nTregs; iTregs) cells as well as B-1 and B-2 cells at different stages of differentiation. These data indicate that ATLOs organize dichotomic innate and adaptive immune responses in atherosclerosis. In this review we discuss the novel concept that dichotomic immune responses toward atherosclerosis-specific antigens are carried out by ATLOs in the adventitia of the arterial wall and that malfunction of the tolerogenic arm of ATLO immunity triggers transition from silent autoimmune reactivity to clinically overt disease.

IKKα-mediated signaling circuitry regulates early B lymphopoiesis during hematopoiesis

Multiple transcription factors regulate B-cell commitment, which is coordinated with myeloid-erythroid lineage differentiation. NF-κB has long been speculated to regulate early B-cell development; however, this issue remains controversial. IκB kinase-α (IKKα) is required for splenic B-cell maturation but not for BM B-cell development. In the present study, we unexpectedly found defective BM B-cell development and increased myeloid-erythroid lineages in kinase-dead IKKα (KA/KA) knock-in mice. Markedly increased cytosolic p100, an NF-κB2-inhibitory form, and reduced nuclear NF-κB p65, RelB, p50, and p52, and IKKα were observed in KA/KA splenic and BM B cells. Several B- and myeloid-erythroid-cell regulators, including Pax5, were deregulated in KA/KA BM B cells. Using fetal liver and BM congenic transplantations and deleting IKKα from early hematopoietic cells in mice, this defect was identified as being B cell-intrinsic and an early event during hematopoiesis. Reintroducing IKKα, Pax5, or combined NF-κB molecules promoted B-cell development but repressed myeloid-erythroid cell differentiation in KA/KA BM B cells. The results of the present study demonstrate that IKKα regulates B-lineage commitment via combined canonical and noncanonical NF-κB transcriptional activities to target Pax5 expression during hematopoiesis.

The mucosal immune system of the respiratory tract

Most viruses use host mucosal surfaces as their initial portals of infection. The respiratory tract has the body's second-largest mucosal surface area after the digestive tract. An understanding of the unique nature of the mucosal immune system of respiratory organs is therefore extremely important for the development of new-generation vaccines and novel methods of preventing and treating respiratory infectious diseases, including viral infections.

Fbxw7α- and GSK3-mediated degradation of p100 is a pro-survival mechanism in multiple myeloma

Fbxw7α is a member of the F-box family of proteins, which function as the substrate-targeting subunits of SCF (Skp1/Cul1/F-box protein) ubiquitin ligase complexes. Using differential purifications and mass spectrometry, we identified p100, an inhibitor of NF-κB signalling, as an interactor of Fbxw7α. p100 is constitutively targeted in the nucleus for proteasomal degradation by Fbxw7α, which recognizes a conserved motif phosphorylated by GSK3. Efficient activation of non-canonical NF-κB signalling is dependent on the elimination of nuclear p100 through either degradation by Fbxw7α or exclusion by a newly identified nuclear export signal in the carboxy terminus of p100. Expression of a stable p100 mutant, expression of a constitutively nuclear p100 mutant, Fbxw7α silencing or inhibition of GSK3 in multiple myeloma cells with constitutive non-canonical NF-κB activity results in apoptosis both in cell systems and xenotransplant models. Thus, in multiple myeloma, Fbxw7α and GSK3 function as pro-survival factors through the control of p100 degradation.

The noncanonical NF-κB pathway

The noncanonical nuclear factor-κB (NF-κB) signaling pathway mediates activation of the p52/RelB NF-κB complex and, thereby, regulates specific immunological processes. This NF-κB pathway relies on the inducible processing of NF-κB2 precursor protein, p100, as opposed to the degradation of IκBα in the canonical NF-κB pathway. A central signaling component of the noncanonical NF-κB pathway is NF-κB-inducing kinase (NIK), which functions together with a downstream kinase, IKKα (inhibitor of NF-κB kinase α), to induce phosphorylation-dependent ubiquitination and processing of p100. Under normal conditions, NIK is targeted for continuous degradation by a tumor necrosis factor (TNF) receptor-associated factor-3 (TRAF3)-dependent E3 ubiquitin ligase. In response to signals mediated by a subset of TNF receptor superfamily members, NIK becomes stabilized as a result of TRAF3 degradation, leading to the activation of noncanonical NF-κB. This review discusses both the historical perspectives and the recent progress in the regulation and biological function of the noncanonical NF-κB pathway.

Transcriptional control of HIV replication by multiple modulators and their implication for a novel antiviral therapy

Transcriptional regulation is critical for the human immunodeficiency virus 1 (HIV-1) life cycle and is the only step at which the virus amplifies the content of its genetic information. Numerous known and still unknown transcriptional factors, both host and viral, regulate HIV-1 gene expression and latency. This article is a comprehensive review of transcription factors involved in HIV-1 gene expression and presents the significant implications of nuclear factor kappa B (NF-κB) and the HIV-1 transactivator of transcription (Tat) protein. We include recent findings on chromatin remodeling toward HIV transcription and its therapeutic implication is also discussed. The current status of small-molecular-weight compounds that affect HIV transcription is also described.

Induction of the alternative NF-κB pathway by lymphotoxin αβ (LTαβ) relies on internalization of LTβ receptor

Several tumor necrosis factor receptor (TNFR) family members activate both the classical and the alternative NF-κB pathways. However, how a single receptor engages these two distinct pathways is still poorly understood. Using lymphotoxin β receptor (LTβR) as a prototype, we showed that activation of the alternative, but not the classical, NF-κB pathway relied on internalization of the receptor. Further molecular analyses revealed a specific cytosolic region of LTβR essential for its internalization, TRAF3 recruitment, and p100 processing. Interestingly, we found that dynamin-dependent, but clathrin-independent, internalization of LTβR appeared to be required for the activation of the alternative, but not the classical, NF-κB pathway. In vivo, ligand-induced internalization of LTβR in mesenteric lymph node stromal cells correlated with induction of alternative NF-κB target genes. Thus, our data shed light on LTβR cellular trafficking as a process required for specific biological functions of NF-κB.

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.

Stromal cell-immune cell interactions

Interaction between different types of hematopoietic cells is essential for proper functioning of the immune system. For instance, the cytokines produced by antigen-presenting dendritic cells will determine the type of T cell response that is induced. However, hematopoietic cells are also strongly influenced by the surrounding nonhematopoietic cells. The cells that form these microenvironments are collectively called stromal cells. Here, we focus on the stromal cells present within secondary lymphoid organs and discuss their importance for various aspects of the immune system.

Transcription factor Nkx2-3 controls the vascular identity and lymphocyte homing in the spleen

The vasculature in the spleen and peripheral lymph nodes (pLNs) is considerably different, which affects both homing of lymphocytes and antigenic access to these peripheral lymphoid organs. In this paper, we demonstrate that in mice lacking the homeodomain transcription factor Nkx2-3, the spleen develops a pLN-like mRNA expression signature, coupled with the appearance of high endothelial venules (HEVs) that mediate L-selectin-dependent homing of lymphocytes into the mutant spleen. These ectopic HEV-like vessels undergo postnatal maturation and progressively replace MAdCAM-1 by pLN addressin together with the display of CCL21 arrest chemokine in a process that is reminiscent of HEV formation in pLNs. Similarly to pLNs, development of HEV-like vessels in the Nkx2-3-deficient spleen depends on lymphotoxin-β receptor-mediated signaling. The replacement of splenic vessels with a pLN-patterned vasculature impairs the recirculation of adoptively transferred lymphocytes and reduces the uptake of blood-borne pathogens. The Nkx2-3 mutation in BALB/c background causes a particularly disturbed splenic architecture, characterized by the near complete lack of the red pulp, without affecting lymph nodes. Thus, our observations reveal that the organ-specific patterning of splenic vasculature is critically regulated by Nkx2-3, thereby profoundly affecting the lymphocyte homing mechanism and blood filtering capacity of the spleen in a tissue-specific manner.

Steady state migratory RelB+ langerin+ dermal dendritic cells mediate peripheral induction of antigen-specific CD4+ CD25+ Foxp3+ regulatory T cells

Tolerance to self-antigens expressed in peripheral organs is maintained by CD4(+) CD25(+) Foxp3(+) Treg cells, which are generated as a result of thymic selection or peripheral induction. Here, we demonstrate that steady-state migratory DCs from the skin mediated Treg conversion in draining lymph nodes of mice. These DCs displayed a partially mature MHC II(int) CD86(int) CD40(hi) CCR7(+) phenotype, used endogenous TGF-β for conversion and showed nuclear RelB translocation. Deficiency of the alternative NF-κB signaling pathway (RelB/p52) reduced steady-state migration of DCs. These DCs transported and directly presented soluble OVA provided by s.c. implanted osmotic minipumps, as well as cell-associated epidermal OVA in transgenic K5-mOVA mice to CD4(+) OVA-specific TCR-transgenic OT-II T cells. The langerin(+) dermal DC subset, but not epidermal Langerhans cells, mediated conversion of naive OT-II×RAG-1(-/-) T cells into proliferating CD4(+) CD25(+) Foxp3(+) Tregs. Thus, our data suggest that steady-state migratory RelB(+) TGF-β(+) langerin(+) dermal DCs mediate peripheral Treg conversion in response to epidermal antigen in skin-draining lymph nodes.

Nuclear export of the NF-κB inhibitor IκBα is required for proper B cell and secondary lymphoid tissue formation

The N-terminal nuclear export sequence (NES) of inhibitor of nuclear factor kappa B (NF-κB) alpha (IκBα) promotes NF-κB export from the cell nucleus to the cytoplasm, but the physiological role of this export regulation remains unknown. Here we report the derivation and analysis of genetically targeted mice harboring a germline mutation in IκBα NES. Mature B cells in the mutant mice displayed nuclear accumulation of inactive IκBα complexes containing a NF-κB family member, cRel, causing their spatial separation from the cytoplasmic IκB kinase. This resulted in severe reductions in constitutive and canonical NF-κB activities, synthesis of p100 and RelB NF-κB members, noncanonical NF-κB activity, NF-κB target gene induction, and proliferation and survival responses in B cells. Consequently, mice displayed defective B cell maturation, antibody production, and formation of secondary lymphoid organs and tissues. Thus, IκBα nuclear export is essential to maintain constitutive, canonical, and noncanonical NF-κB activation potentials in mature B cells in vivo.

Small interfering RNA targeting IKKβ prevents renal ischemia-reperfusion injury in rats

The transcription factor NF-κB has been found critical to the pathogenesis of renal ischemia-reperfusion injury, which is a major cause of acute kidney injury (AKI). Activation of NF-κB is dependent upon the activation of the specific inhibitory κB kinase (IKK) subunit IKKβ. Here, we investigate whether small interfering RNA (siRNA) targeting IKKβ protects rats from renal ischemia- reperfusion injury in vivo. Renal ischemia-reperfusion injury was induced by clamping the renal artery for 45 min. Rats were treated before ischemia with IKKβ siRNA or scrambled siRNA, administered by renal artery injection. Treated animals were evaluated for renal IKKβ protein and mRNA expression, blood biochemistry, tissue histopathology, NF-κB/DNA binding activity, and expression of two downstream inflammatory cytokines, neutrophil gelatinase-associated lipocalin (NGAL) and IL-18. A local injection of IKKβ siRNA resulted in inhibition of renal IKKβ gene expression, NF-κB/DNA binding activity, and expression of NGAL and IL-18. Rats pretreated with IKKβ siRNA had significantly less blood urea nitrogen and serum creatinine levels and less renal tubular damage scores. Consequently, our data confirm that targeted silencing of IKKβ using siRNA substantially diminishes kidney injury and inflammation following ischemia-reperfusion.

Nuclear initiated NF-κB signaling: NEMO and ATM take center stage

A large body of literature describes elaborate NF-κB signaling networks induced by inflammatory and immune signals. Decades of research has revealed that transcriptionally functional NF-κB dimers are activated by two major pathways, canonical and non-canonical. Both pathways involve the release of NF-κB dimers from inactive cytoplasmic complexes to cause their nuclear translocation to modulate gene expression programs and biological responses. NF-κB is also responsive to genotoxic agents; however, signal communication networks that are initiated in the nucleus following DNA damage induction are less defined. Evidence in the literature supports the presence of such signaling pathways induced by multiple distinct genotoxic agents, resulting in the activation of cytoplasmic IKK complex. An example is a pathway that involves the DNA damage-responsive kinase ataxia telangiectasia mutated (ATM) and a series of post-translational modifications of NF-κB essential modulator (NEMO) in the nucleus of a genotoxin-exposed cell. Recent evidence also suggests that this nuclear-initiated NF-κB signaling pathway plays significant physiological and pathological roles, particularly in lymphocyte development and human cancer progression. This review will summarize these new developments, while identifying significant unanswered questions and providing new hypotheses that may be addressed in future studies.

A single NFκB system for both canonical and non-canonical signaling

Two distinct nuclear factor κB (NFκB) signaling pathways have been described; the canonical pathway that mediates inflammatory responses, and the non-canonical pathway that is involved in immune cell differentiation and maturation and secondary lymphoid organogenesis. The former is dependent on the IκB kinase adaptor molecule NEMO, the latter is independent of it. Here, we review the molecular mechanisms of regulation in each signaling axis and attempt to relate the apparent regulatory logic to the physiological function. Further, we review the recent evidence for extensive cross-regulation between these two signaling axes and summarize them in a wiring diagram. These observations suggest that NEMO-dependent and -independent signaling should be viewed within the context of a single NFκB signaling system, which mediates signaling from both inflammatory and organogenic stimuli in an integrated manner. As in other regulatory biological systems, a systems approach including mathematical models that include quantitative and kinetic information will be necessary to characterize the network properties that mediate physiological function, and that may break down to cause or contribute to pathology.

Different expression patterns of pAkt, NF-κB and cyclin D1 proteins during the invasion process of head and neck squamous cell carcinoma: an in vitro approach

BACKGROUND

Several signaling pathways are involved in the progression of squamous cell carcinoma. Among them, activated PI3K/Akt may result in NF-κB nuclear translocation, thus leading to the transcription of genes enrolled in cellular invasion and proliferation, such as cyclin D1. This study sought to evaluate the expression of pAkt, NF-κB and cyclin D1 proteins in head and neck squamous cell carcinoma cell lines and their respective in vitro-obtained invasive clones.

METHODS

Squamous cell carcinoma cell lines originating from the tongue, pharynx and the metastatic lymph node were submitted to an in vitro invasion assay to select invasive clones. All experimental groups were submitted to immunofluorescence and Western blot assays. Statistical analysis was performed through a Student's t-test with a significance level of 5%.

RESULTS

The pAkt and NF-κB expression differed from cytoplasm and nucleus depending on the studied cell line. The invasive clone from the tongue presented a network-like structure of pAkt's cytoplasmic expression. This lineage as well as the invasive clone from pharynx also showed pAkt and NF-κB nuclear transportation. Significant pAkt and NF-κB increases were observed in the tongue and pharynx invasive clones. Cyclin D1 was detected in the nucleus of all studied cells and was significantly enhanced in the invasive clones from tongue and pharynx.

CONCLUSION

This study suggests the participation of pAkt, NF-κB and cyclin D1 in the invasion process of head and neck squamous cell carcinoma. Moreover, cytoplasmic pAkt network-like structure was probably related to cytoskeleton changes presented during invasion.

Direct and indirect roles of the LTbetaR pathway in central tolerance induction

Medullary thymic epithelial cells (mTECs) play a critical role in thymic negative selection of autoreactive thymocytes, especially for thymocytes specific for peripheral tissue-restricted self-antigens (TRA). Deficiency in lymphotoxin b receptor (LTbetaR) is associated with peripheral tissue inflammation, but whether this is caused by defective negative selection has been unclear; the significance of the LTbetaR pathway for negative selection is evident in some models but not others. Here, we revisit the data and clarify the role of LTbetaR in mTEC development and function and thymic TRA expression. These processes are discussed as potential mechanisms for LTbetaR-mediated control of negative selection.

Cellular source and molecular form of TNF specify its distinct functions in organization of secondary lymphoid organs

Secondary lymphoid organs provide a unique microenvironment for generation of immune responses. Using a cell type-specific conditional knockout approach, we have dissected contributions of tumor necrosis factor (TNF) produced by B cells (B-TNF) or T cells (T-TNF) to the genesis and homeostatic organization of secondary lymphoid organs. In spleen, lymph nodes and Peyer patches, the cellular source of TNF, and its molecular form (soluble versus membrane-bound) appeared distinct. In spleen, in addition to major B-TNF signal, a complementary T-TNF signal contributed to the microstructure. In contrast, B-TNF predominantly controlled the development of follicular dendritic cells and B-cell follicles in Peyer patches. In lymph nodes, cooperation between TNF expressed by B and T cells was necessary for the maintenance of microarchitecture and for generation of an efficient humoral immune response. Unexpectedly, soluble but not membrane TNF expressed by B cells was essential for the organization of the secondary lymphoid organs. Thus, the maintenance of each type of secondary lymphoid organ is orchestrated by distinct contributions of membrane-bound and soluble TNF produced by B and T lymphocytes.

The unexpected role of lymphotoxin beta receptor signaling in carcinogenesis: from lymphoid tissue formation to liver and prostate cancer development

The cytokines lymphotoxin (LT) alpha, beta and their receptor (LTbetaR) belong to the tumor necrosis factor (TNF) superfamily, whose founder-TNFalpha-was initially discovered due to its tumor necrotizing activity. LTbetaR signaling serves pleiotropic functions including the control of lymphoid organ development, support of efficient immune responses against pathogens due to maintenance of intact lymphoid structures, induction of tertiary lymphoid organs, liver regeneration or control of lipid homeostasis. Signaling through LTbetaR comprises the noncanonical/canonical nuclear factor-kappaB (NF-kappaB) pathways thus inducing chemokine, cytokine or adhesion molecule expression, cell proliferation and cell survival. Blocking LTbetaR signaling or Fcgamma-receptor mediated immunoablation of LT-expressing cells was demonstrated to be beneficial in various infectious or noninfectious inflammatory or autoimmune disorders. Only recently, LTbetaR signaling was shown to initiate inflammation-induced carcinogenesis, to influence primary tumorigenesis and to control reemergence of carcinoma in various cancer models through distinct mechanisms. Indeed, LTbetaR signaling inhibition has already been used as efficient anti-inflammatory, anti-cancer therapy in some experimental models. Here, we review the pleiotropic functions attributed to LT, the effects of its deregulation and extensively discuss the recent literature on LT's link to carcinogenesis.

Sitagliptin (MK0431) inhibition of dipeptidyl peptidase IV decreases nonobese diabetic mouse CD4+ T-cell migration through incretin-dependent and -independent pathways

OBJECTIVE

Treatment of NOD mice with the dipeptidyl peptidase-IV (DPP-IV) inhibitor sitagliptin preserved islet transplants through a pathway involving modulation of splenic CD4(+) T-cell migration. In the current study, effects of sitagliptin on migration of additional subsets of CD4(+) T-cells were examined and underlying molecular mechanisms were further defined.

RESEARCH DESIGN AND METHODS

Effects of sitagliptin on migration of NOD mouse splenic, thymic, and lymph node CD4(+) T-cells were determined. Signaling modules involved in DPP-IV-, Sitagliptin- and incretin-mediated modulation of CD4(+) T-cell migration were studied using Western blot and Rac1 and nuclear factor-kappaB (NF-kappaB) activity assays.

RESULTS

Migration of splenic and lymph node CD4(+) T-cells of diabetic NOD mice was reduced by sitagliptin treatment. In vitro treatment of splenic, but not thymic or lymph node CD4(+) T-cells, from nondiabetic NOD mice with soluble (s) DPP-IV increased migration. Sitagliptin abolished sDPP-IV effects on splenic CD4(+) T-cell migration, whereas incretins decreased migration of lymph node, but not splenic, CD4(+) T-cells. Splenic CD4(+) T-cells demonstrating increased in vitro migration in response to sDPP-IV and lymph node CD4(+) T-cells that were nonresponsive to incretins selectively infiltrated islets of NOD mice, after injection. Sitagliptin decreases migration of splenic CD4(+) T-cells through a pathway involving Rac1/vasodilator-stimulated phosphoprotein, whereas its inhibitory effects on the migration of lymph node CD4(+) T-cells involve incretin-activation of the NF-kappaB pathway.

CONCLUSIONS

Benefits of sitagliptin treatment in diabetic NOD mice may be mediated through selective effects on subpopulations of T-cells that are related to autoimmunity.

Loss of negative feedback control of nuclear factor-kappaB2 activity in lymphocytes leads to fatal lung inflammation

Proteolytic processing of the nuclear factor (NF)-kappaB2 precursor protein p100 generates the active NF-kappaB2 subunit p52, which in turn transcriptionally up-regulates p100 expression. p100 also functions as an IkappaB molecule capable of repressing p52 activity. The biological significance of this negative feedback control loop has yet to be demonstrated in vivo. Here we show that mice deficient in p100 but with constitutive expression of p52 in lymphocytes developed fatal lung inflammation characterized by diffuse alveolar damage with marked peribronchial fibrosis. In contrast, their littermates with only p100 deficiency or constitutive expression of p52 in lymphocytes developed mild lung inflammation with perivascular lymphocyte infiltration and had a normal life span. The fatal lung inflammation is associated with high-level induction of interferon-gamma and its inducible inflammatory chemokines, suggesting the involvement of a T-helper-1 immune response. These findings demonstrate the physiological relevance of the NF-kappaB2 p100 precursor protein in limiting the potentially detrimental effects of constitutive NF-kappaB2 signaling in lymphocytes.

Ontogeny of stromal organizer cells during lymph node development

The development of secondary lymphoid organs, such as lymph nodes (LNs), in the embryo results from the reciprocal action between lymphoid tissue inducer (LTi) cells and stromal cells. However, the initial events inducing LN anlagen formation before the LTi stromal cells cross-talk interactions take place are not fully elucidated. In this study, we show that the inguinal LN anlagen in mouse embryos developed from mesenchymal cells surrounding the lymph sacs, spherical structures of endothelial cells that bud from veins. Using inguinal and mesenteric LNs (mLNs), we provide evidence supporting a two-step maturation model for stromal cells: first, ICAM-1(-)VCAM-1(-) mesenchymal precursor cells become ICAM-1(int)VCAM-1(int) cells, in a process independent of LTi cells and lymphotoxin beta receptor (LTbetaR) signaling. The second step involves the maturation of ICAM-1(int)VCAM-1(int) cells to ICAM-1(high)VCAM-1(high) mucosal addressin cell adhesion molecule-1(+) organizer cells and depends on both LTi cells and LTbetaR. Addition of alphaLTbetaR agonist to LN organ cultures was sufficient to induce ICAM-1(int)VCAM-1(int) cells to mature. In LtbetaR(-/-) embryos, both inguinal and mLN stromal cells showed a block at the ICAM-1(int)VCAM-1(int) stage, and, contrary to inguinal LNs, mLNs persist longer and contained LTi cells, which correlated with the sustained gene expression of Il-7, Cxcl13, and, to a lesser degree, Ccl21. Taken together, these results highlight the importance of the signals and cellular interactions that induce the maturation of stromal cells and ultimately lead to the formation of lymphoid tissues.

The complicated role of NF-kappaB in T-cell selection

The nuclear factor (NF)-kappaB transcription factor family plays important roles in the immune system. Aberrant NF-kappaB signaling is frequently associated with inflammation and autoimmune diseases but the underlying mechanisms are not fully understood. Recent studies show that NF-kappaB plays a critical role in T-cell central tolerance. Two NF-kappaB signaling pathways have been identified: the canonical pathway and the alternative pathway. In the establishment of T-cell central tolerance, the alternative pathway appears to be the key signaling component in thymic stromal cells for their development and function, while the canonical pathway exerts its function more in autonomous T-cell selection. This review intends to summarize the current understanding of the role of NF-kappaB in establishing T-cell central tolerance and highlight unsolved intriguing questions for future work.

Mouse aorta smooth muscle cells differentiate into lymphoid tissue organizer-like cells on combined tumor necrosis factor receptor-1/lymphotoxin beta-receptor NF-kappaB signaling

OBJECTIVE

Mouse aorta smooth muscle cells (SMC) express tumor necrosis factor receptor superfamily member 1A (TNFR-1) and lymphotoxin beta-receptor (LTbetaR). Circumstantial evidence has linked the SMC LTbetaR to tertiary lymphoid organogenesis in hyperlipidemic mice. Here, we explored TNFR-1 and LTbetaR signaling in cultured SMC.

METHODS AND RESULTS

TNFR-1 signaling activated the classical RelA NF-kappaB pathway, whereas LTbetaR signaling activated the classical RelA and alternative RelB NF-kappaB pathways, and both signaling pathways synergized to enhance p100 inhibitor processing to the p52 subunit of NF-kappaB. Microarrays showed that simultaneous TNFR-1/LTbetaR activation resulted in elevated mRNA encoding leukocyte homeostatic chemokines CCL2, CCL5, CXCL1, and CX3CL1. Importantly, SMC acquired features of lymphoid tissue organizers, which control tertiary lymphoid organogenesis in autoimmune diseases through hyperinduction of CCL7, CCL9, CXCL13, CCL19, CXCL16, vascular cell adhesion molecule-1, and intercellular adhesion molecule-1. TNFR-1/LTbetaR cross-talk resulted in augmented secretion of lymphorganogenic chemokine proteins. Supernatants of TNFR-1/LTbetaR-activated SMC markedly supported migration of splenic T cells, B cells, and macrophages/dendritic cells. Experiments with ltbr(-/-) SMC indicated that LTbetaR-RelB activation was obligatory to generate the lymphoid tissue organizer phenotype.

CONCLUSIONS

SMC may participate in the formation of tertiary lymphoid tissue in atherosclerosis by upregulation of lymphorganogenic chemokines involved in T-lymphocyte, B-lymphocyte, and macrophage/dendritic cell attraction.

Roles of the NF-kappaB 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-kappaB's physiologic impact that we address in this article.

Her2 activates NF-kappaB and induces invasion through the canonical pathway involving IKKalpha

The membrane bound receptor tyrosine kinase Her2 is overexpressed in approximately 30% of human breast cancers, which correlates with poor prognosis. Her2-induced signaling pathways include MAPK and PI3K/Akt, of which the latter has been shown to be critical for Her2(+) breast cancer cell growth and survival. In addition, the NF-kappaB pathway has been shown to be activated downstream of Her2 overexpression; however, the mechanisms leading to this activation are not currently clear. Using Her2(+)/ER(-) breast cancer cells, we show that Her2 activates NF-kappaB through the canonical pathway which, surprisingly, involves IKKalpha. Knockdown of IKKalpha led to a significant decrease in transcription levels of multiple NF-kappaB-regulated cytokine and chemokine genes. siRNA-mediated knockdown of IKKalpha resulted in a decrease in cancer cell invasion, but had no effect on cell proliferation. Inhibition of the PI3K/Akt pathway had no effect on NF-kappaB activation, but significantly inhibited cell proliferation. Our study suggests different roles for the NF-kappaB and PI3K pathways downstream of Her2, leading to changes in invasion and proliferation of breast cancer cells. In addition this work indicates the importance of IKKalpha as a mediator of Her2-induced tumor progression.

Secondary lymphoid organs: responding to genetic and environmental cues in ontogeny and the immune response

Secondary lymphoid organs (SLOs) include lymph nodes, spleen, Peyer's patches, and mucosal tissues such as the nasal-associated lymphoid tissue, adenoids, and tonsils. Less discretely anatomically defined cellular accumulations include the bronchus-associated lymphoid tissue, cryptopatches, and isolated lymphoid follicles. All SLOs serve to generate immune responses and tolerance. SLO development depends on the precisely regulated expression of cooperating lymphoid chemokines and cytokines such as LTalpha, LTbeta, RANKL, TNF, IL-7, and perhaps IL-17. The relative importance of these factors varies between the individual lymphoid organs. Participating in the process are lymphoid tissue initiator, lymphoid tissue inducer, and lymphoid tissue organizer cells. These cells and others that produce crucial cytokines maintain SLOs in the adult. Similar signals regulate the transition from inflammation to ectopic or tertiary lymphoid tissues.

Siva-1 promotes K-48 polyubiquitination of TRAF2 and inhibits TCR-mediated activation of NF-kappaB

The proapoptotic protein Siva-1 plays an important role in some of the extrinsic and intrinsic apoptosis signaling pathways in cancer cells. Previously, we showed that Siva-1 inhibited the activity of the prosurvival transcription factor NF-kappaB. In the present study, upon TCR cross-linking of Jurkat T leukemia cells, we demonstrated that the inhibitory target of Siva-1 is upstream of the IKK complex in the NF-kappaB signaling pathway. Additionally, Siva-1 also suppressed the activity of another crucial transcription factor AP-1, and a common mediator of both these pathways is the adaptor protein TRAF2. Further, we observed that Siva-1 indeed interacted with TRAF2 and negatively regulated its activity by promoting K48-hnked polyubiquitination. Siva-1 specifically interacted with the ring finger domain of TRAF2, which is essential for its E3 hgase activity and its ability to subsequently activate NF-kappaB. TCR cross-linking of Jurkat T cells that lacked Siva-1 revealed significantly lowered K48- but elevated K63-ubiquitinated TRAF2 levels upon TCR cross-linking, suggesting that the differential pattern of ubiquitination in these cells essentially contributed to a robust and sustained activation of NF-kappaB. The above results demonstrated an important role for endogenous Siva-1 in negatively regulating NF-kappaB activation by targeting TRAF2.

TRAF1 is involved in the classical NF-kappaB activation and CD30-induced alternative activity in Hodgkin's lymphoma cells

TNFR-associated factors (TRAFs) participate in diverse biological processes, such as adaptive and innate immunity, stress response, and bone metabolism. We report that all TRAFs except TRAF3 are expressed at mRNA and protein levels in B cell-derived Hodgkin's lymphoma cell lines (L428 and KM-H2). Both the classical (p50-RelA) and the alternative NF-kappaB activity (p52-RelB) are sustained in L428 and KM-H2 cells. A successful depletion of TRAF1 protein expression by means of RNA interference abrogates the anti-apoptosis activity in L428 cells. The TRAF1-deficiency reduces the classical NF-kappaB activity but not the alternative activity. The expression of the NF-kappaB targeting genes, such as ICAM-1, c-Flip, and Cyclin D1, is suppressed in the TRAF1-depleted cells. On the other hand, CD30 signaling upregulates the TRAF1 expression while reducing the expression of TRAF2 and TRAF5. Importantly, the CD30-induced alternative NF-kappaB activation is inhibited by the depletion of the TRAF1 expression. We also demonstrate that the phosphorylation of the extracellular signal-regulated kinase (ERK) upon CD30 stimulation in Hodgkin's lymphoma cells is independent of TRAF1 expression. Our data shed new light on the function of TRAF1 in B cell-derived lymphoma cells. We conclude that TRAF1 is an important molecule mediating both the CD30 signaling-dependent and independent NF-kappaB activation, which prevents the lymphoma cells from spontaneous and induced apoptosis.

Induction of intestinal lymphoid tissue formation by intrinsic and extrinsic signals

Since the discovery of inducer cells as a separate lineage for organogenesis of Peyer's patches in the small intestine of fetal mice, a lot of progress has been made in understanding the molecular pathways involved in the generation of lymphoid tissue and the maintenance of the lymphoid architecture. The findings that inducer cells also exist in adult mice and in humans, have a lineage relationship to natural killer cells, and can be stimulated during infections highlight their possible role in establishing innate and adaptive immune responses. Novel concepts in the development of intestinal lymphoid tissues have been made in the past few years suggesting that lymphoid organs are more plastic as previously thought and depend on antigenic stimulation. In addition, the generation of novel lymphoid organs in the gut under inflammatory conditions indicates a function in chronic diseases. The present review summarizes current knowledge on the basic framework of signals required for developing lymphoid tissue under normal and inflammatory conditions.

Regulation and function of NF-kappaB transcription factors in the immune system

The mammalian Rel/NF-kappaB family of transcription factors, including RelA, c-Rel, RelB, NF-kappaB1 (p50 and its precursor p105), and NF-kappaB2 (p52 and its precursor p100), plays a central role in the immune system by regulating several processes ranging from the development and survival of lymphocytes and lymphoid organs to the control of immune responses and malignant transformation. The five members of the NF-kappaB family are normally kept inactive in the cytoplasm by interaction with inhibitors called IkappaBs or the unprocessed forms of NF-kappaB1 and NF-kappaB2. A wide variety of signals emanating from antigen receptors, pattern-recognition receptors, receptors for the members of TNF and IL-1 cytokine families, and others induce differential activation of NF-kappaB heterodimers. Although work over the past two decades has shed significant light on the regulation of NF-kappaB transcription factors and their functions, much progress has been made in the past two years revealing new insights into the regulation and functions of NF-kappaB. This recent progress is covered in this review.

NF-kappaB pathways in the immune system: control of the germinal center reaction

The NF-kappaB signaling pathway plays a critical role in regulating innate and adaptive immunity. This is clearly evident as mouse models deficient for numerous NF-kappaB subunits and upstream activators exhibit defects in the immune system ranging from impaired development of lymphocytes to defective adaptive immune responses. In this review, we focus on the role that NF-kappaB plays in the germinal center (GC) reaction. Specifically, we discuss the major NF-kappaB subunits and the IkappaB homolog, Bcl-3. Recent findings reveal that Bcl-6, an unrelated transcriptional repressor, is functionally similar to Bcl-3 as both factors may suppress p53 activity to allow for efficient GC formation to occur. We discuss potential mechanisms of action for Bcl-3 and Bcl-6 in this highly complex, but important process of B-cell affinity maturation.

Targeting NF-kappaB: a promising molecular therapy in inflammatory arthritis

The nuclear factor-kappa B family of transcription factors is intimately involved in the regulation of the inflammatory responses that play a fundamental role in the damage of articular tissues. Thus, many studies have examined the important contributions of components of the NF-kappaB signaling pathways to the pathogenesis of various rheumatic diseases and their pharmacologic modulation. Currently available therapeutic agents including nonsteroidal anti-inflammatory drugs, corticosteroids, nutraceuticals, and disease-modifying antirheumatic drugs, as well as novel specific small-molecule inhibitors have been employed. In addition, promising nucleic acid-based strategies have shown encouraging results. However, further research will be needed before NF-kappaB-aimed strategies become an effective therapy for inflammatory arthritis.

Lymphotoxin beta receptor signaling promotes tertiary lymphoid organogenesis in the aorta adventitia of aged ApoE-/- mice

Atherosclerosis involves a macrophage-rich inflammation in the aortic intima. It is increasingly recognized that this intimal inflammation is paralleled over time by a distinct inflammatory reaction in adjacent adventitia. Though cross talk between the coordinated inflammatory foci in the intima and the adventitia seems implicit, the mechanism(s) underlying their communication is unclear. Here, using detailed imaging analysis, microarray analyses, laser-capture microdissection, adoptive lymphocyte transfers, and functional blocking studies, we undertook to identify this mechanism. We show that in aged apoE^−/− mice, medial smooth muscle cells (SMCs) beneath intimal plaques in abdominal aortae become activated through lymphotoxin β receptor (LTβR) to express the lymphorganogenic chemokines CXCL13 and CCL21. These signals in turn trigger the development of elaborate bona fide adventitial aortic tertiary lymphoid organs (ATLOs) containing functional conduit meshworks, germinal centers within B cell follicles, clusters of plasma cells, high endothelial venules (HEVs) in T cell areas, and a high proportion of T regulatory cells. Treatment of apoE^−/− mice with LTβR-Ig to interrupt LTβR signaling in SMCs strongly reduced HEV abundance, CXCL13, and CCL21 expression, and disrupted the structure and maintenance of ATLOs. Thus, the LTβR pathway has a major role in shaping the immunological characteristics and overall integrity of the arterial wall.

Differential RelA- and RelB-dependent gene transcription in LTbetaR-stimulated mouse embryonic fibroblasts

BACKGROUND

Lymphotoxin signaling via the lymphotoxin-beta receptor (LTbetaR) has been implicated in biological processes ranging from development of secondary lymphoid organs, maintenance of spleen architecture, host defense against pathogens, autoimmunity, and lipid homeostasis. The major transcription factor that is activated by LTbetaR crosslinking is NF-kappaB. Two signaling pathways have been described, the classical inhibitor of NF-kappaB alpha (IkappaBalpha)-regulated and the alternative p100-regulated pathway that result in the activation of p50-RelA and p52-RelB NF-kappaB heterodimers, respectively.

RESULTS

Using microarray analysis, we investigated the transcriptional response downstream of the LTbetaR in mouse embryonic fibroblasts (MEFs) and its regulation by the RelA and RelB subunits of NF-kappaB. We describe novel LTbetaR-responsive genes that were regulated by RelA and/or RelB. The majority of LTbetaR-regulated genes required the presence of both RelA and RelB, revealing significant crosstalk between the two NF-kappaB activation pathways. Gene Ontology (GO) analysis confirmed that LTbetaR-NF-kappaB target genes are predominantly involved in the regulation of immune responses. However, other biological processes, such as apoptosis/cell death, cell cycle, angiogenesis, and taxis were also regulated by LTbetaR signaling. Moreover, LTbetaR activation inhibited expression of a key adipogenic transcription factor, peroxisome proliferator activated receptor-gamma (pparg), suggesting that LTbetaR signaling may interfere with adipogenic differentiation.

CONCLUSION

Microarray analysis of LTbetaR-stimulated fibroblasts provided comprehensive insight into the transcriptional response of LTbetaR signaling and its regulation by the NF-kappaB family members RelA and RelB.