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

Gene expression profiling in conjunction with physiological rescues of IKKalpha-null cells with wild type or mutant IKKalpha reveals distinct classes of IKKalpha/NF-kappaB-dependent genes

Cellular responses to stress-like stimuli require the IkappaB kinase (IKK) signalsome (IKKalpha, IKKbeta, and NEMO/IKKgamma) to activate NF-kappaB-dependent genes. IKKbeta and NEMO/IKKgamma are required to release NF-kappaB p65/p50 heterodimers from IkappaBalpha, resulting in their nuclear migration and sequence-specific DNA binding; but IKKalpha was found to be dispensable for this initial phase of canonical NF-kappaB activation. Nevertheless, IKKalpha-/- mouse embryonic fibroblasts (MEFs) fail to express NF-kappaB targets in response to proinflammatory stimuli, uncovering a nuclear role for IKKalpha in NF-kappaB activation. However, it remains unknown whether the global defect in NF-kappaB-dependent gene expression of IKKalpha-/- cells is caused by the absence of IKKalpha kinase activity. We show by gene expression profiling that rescue of near physiological levels of wild type IKKalpha in IKKalpha-/- MEFs globally restores expression of their canonical NF-kappaB target genes. To prove that the kinase activity of IKKalpha was required on a genomic scale, the same physiological rescue was performed with a kinase-dead, ATP binding domain IKKalpha mutant (IKKalpha(K44M)). Remarkably, the IKKalpha(K44M) protein rescued approximately 28% of these genes, albeit in a largely stimulus-independent manner with the notable exception of several genes that also acquired tumor necrosis factor-alpha responsiveness. Thus the IKKalpha-containing signalsome unexpectedly functions in the presence and absence of extracellular signals in both kinase-dependent and -independent modes to differentially modulate the expression of five distinct classes of IKKalpha/NF-kappaB-dependent genes.

NF-kappaB: a multifaceted transcription factor regulated at several levels

NF-kappaB is a generic name for an evolutionarily conserved transcription-factor system that contributes to the mounting of an effective immune response but is also involved in the regulation of cell proliferation, development, and apoptosis. The implication of NF-kappaB in central biological processes and its extraordinary connectivity to other signaling pathways raise a need for highly controlled regulation of NF-kappaB activity at several levels. While all NF-kappaB activation pathways share a central and critical proteasome-mediated step that leads to the degradation of inhibitory proteins and the release of DNA-binding subunits, there is evidence for a downstream level of NF-kappaB regulation that employs several mechanisms. These include promoter-specific exchange of dimers and modification of the transactivating p65 subunit by phosphorylation, acetylation, ubiquitination, or prolyl isomerization. The signaling pathways and enzymes controlling this second level of regulation and their potential use as therapeutic targets for the treatment of NF-kappaB-associated pathologies are discussed here.

Cellular Interactions in Lymph Node Development

The organized accumulation of lymphocytes is a biological phenomenon used to optimize both homeostatic immune surveillance, as well as chronic responses to pathogenic stimuli. During embryonic development, circulating hemopoietic cells gather at predestined sites throughout the body, where they are subsequently arranged in T and B cell-specific areas characteristic of secondary lymphoid organs. In contrast, the body seems to harbor a limited second set of selected sites that support formation of organized lymphoid aggregates. However, these are only revealed at times of local, chronic inflammation, when so-called tertiary lymphoid structures appear. Once thought of as two distinct phenomena, recent insights suggest that highly similar networks of paracrine interactions regulate the formation of both secondary and tertiary lymphoid structures. This review will focus on these cellular interactions between organizing and inducing cell populations leading to the formation of lymph nodes or organized inflammatory infiltrates.

I kappa B kinase complex alpha kinase activity controls chemokine and high endothelial venule gene expression in lymph nodes and nasal-associated lymphoid tissue

The lymphotoxin (LT) beta receptor plays a critical role in secondary lymphoid organogenesis and the classical and alternative NF-kappaB pathways have been implicated in this process. IKKalpha is a key molecule for the activation of the alternative NF-kappaB pathway. However, its precise role and target genes in secondary lymphoid organogenesis remain unknown, particularly with regard to high endothelial venules (HEV). In this study, we show that IKKalpha(AA) mutant mice, who lack inducible kinase activity, have hypocellular lymph nodes (LN) and nasal-associated lymphoid (NALT) tissue characterized by marked defects in microarchitecture and HEV. In addition, IKKalpha(AA) LNs showed reduced lymphoid chemokine CCL19, CCL21, and CXCL13 expression. IKKalpha(AA) LN- and NALT-HEV were abnormal in appearance with reduced expression of peripheral node addressin (PNAd) explained by a severe reduction in the HEV-associated proteins, glycosylation-dependent cell adhesion molecule 1 (GlyCAM-1), and high endothelial cell sulfotransferase, a PNAd-generating enzyme that is a target of LTalphabeta. In this study, analysis of LTbeta(-/-) mice identifies GlyCAM-1 as another LTbeta-dependent gene. In contrast, TNFRI(-/-) mice, which lose classical NF-kappaB pathway activity but retain alternative NF-kappaB pathway activity, showed relatively normal GlyCAM-1 and HEC-6ST expression in LN-HEV. In addition, in this communication, it is demonstrated that LTbetaR is prominently expressed on LN- and NALT-HEV. Thus, these data reveal a critical role for IKKalpha in LN and NALT development, identify GlyCAM-1 and high endothelial cell sulfotransferase as new IKKalpha-dependent target genes, and suggest that LTbetaR signaling on HEV can regulate HEV-specific gene expression.

AIRE and immunological tolerance: insights from the study of autoimmune polyendocrinopathy candidiasis and ectodermal dystrophy

PURPOSE OF REVIEW

To review the clinical and molecular features of autoimmune polyendocrinopathy candidiasis and ectodermal dystrophy and discuss recent advances in the function of the AIRE protein. We will summarize how AIRE contributes to immunological tolerance, and thus to the prevention of autoimmunity.

RECENT FINDINGS

The organization of a well-structured thymic microenvironment and the interaction between nascent thymocytes and thymic epithelial cells have been shown to be essential for AIRE expression. AIRE is involved in the expression of ectopic proteins by medullary thymic epithelial cells. This allows the establishment of central tolerance and contributes to the prevention of organ-specific autoimmunity, as shown by findings in patients with autoimmune polyendocrinopathy candidiasis and ectodermal dystrophy (a disease caused by AIRE gene mutations) and in aire (-/-) mice.

SUMMARY

Autoimmune polyendocrinopathy candidiasis and ectodermal dystrophy represents a unique model to investigate the cellular and molecular mechanisms that govern central tolerance and help prevent autoimmunity. Recent findings indicate that the compartmentalization of AIRE and interaction with other proteins are involved in this mechanism. The disturbance of AIRE expression may also be responsible for autoimmune manifestations in disorders with disrupted thymic structure other than autoimmune polyendocrinopathy candidiasis and ectodermal dystrophy alone.

Lymph node fibroblastic reticular cells construct the stromal reticulum via contact with lymphocytes

The sophisticated microarchitecture of the lymph node, which is largely supported by a reticular network of fibroblastic reticular cells (FRCs) and extracellular matrix, is essential for immune function. How FRCs form the elaborate network and remodel it in response to lymphocyte activation is not understood. In this work, we established ERTR7⁺gp38⁺VCAM-1⁺ FRC lines and examined the production of the ER-TR7 antigen. Multiple chemokines produced by FRCs induced T cell and dendritic cell chemotaxis and adhesion to the FRC surface. FRCs can secrete the ER-TR7 antigen as an extracellular matrix component to make a reticular meshwork in response to contact with lymphocytes. The formation of the meshwork is induced by stimulation with tumor necrosis factor-α or lymphotoxin-α in combination with agonistic antibody to lymphotoxin-β receptor in a nuclear factor-κB (RelA)–dependent manner. These findings suggest that signals from lymphocytes induce FRCs to form the network that supports the movement and interactions of immune effectors within the lymph node.

Localization of Marginal Zone Macrophages Is Regulated by C-C Chemokine Ligands 21/19

The marginal zone (MZ) of the spleen is an important site for the capture of blood-borne pathogens and a gateway for lymphocytes entering the white pulp. We have recently reported that Leishmania donovani infection results in a remarkably selective loss of MZ macrophages (MZM) from the MZ. To understand the basis of this observation, we have investigated how MZM maintain their anatomical distribution in the steady state in uninfected mice. We now report that plt/plt mice, which lack functional CCL19 and CCL21, have significantly reduced numbers of MZM compared with normal C57BL/6 (B6) mice. Similarly, in B6.CD45.1-->plt/plt chimeras, donor-derived MZM were rare compared with the number observed in reciprocal plt/plt-->B6.CD45.1 chimeras. Moreover, we show that administration of pertussis toxin, an inhibitor of chemokine receptor signaling, to B6 mice results in exit of MZM from the MZ, that MZM can migrate in response to CCL19 and CCL21 in vitro, and that MZM colocalize with CD31+CCL21+ endothelial cells. Collectively, these data indicate that CCL21 and, to a lesser extent, CCL19 play significant roles in the distinctive localization of MZM within the splenic MZ. Deficiency of CCL19 and CCL21, as also previously observed in mice infected with L. donovani, may thus account for the selective loss of MZM seen during this infection.

Functions of NF-kappaB1 and NF-kappaB2 in immune cell biology

Two members of the NF-kappaB (nuclear factor kappaB)/Rel transcription factor family, NF-kappaB1 and NF-kappaB2, are produced as precursor proteins, NF-kappaB1 p105 and NF-kappaB2 p100 respectively. These are proteolytically processed by the proteasome to produce the mature transcription factors NF-kappaB1 p50 and NF-kappaB2 p52. p105 and p100 are known to function additionally as IkappaBs (inhibitors of NF-kappaB), which retain associated NF-kappaB subunits in the cytoplasm of unstimulated cells. The present review focuses on the latest advances in research on the function of NF-kappaB1 and NF-kappaB2 in immune cells. NF-kappaB2 p100 processing has recently been shown to be stimulated by a subset of NF-kappaB inducers, including lymphotoxin-beta, B-cell activating factor and CD40 ligand, via a novel signalling pathway. This promotes the nuclear translocation of p52-containing NF-kappaB dimers, which regulate peripheral lymphoid organogenesis and B-lymphocyte differentiation. Increased p100 processing also contributes to the malignant phenotype of certain T- and B-cell lymphomas. NF-kappaB1 has a distinct function from NF-kappaB2, and is important in controlling lymphocyte and macrophage function in immune and inflammatory responses. In contrast with p100, p105 is constitutively processed to p50. However, after stimulation with agonists, such as tumour necrosis factor-alpha and lipopolysaccharide, p105 is completely degraded by the proteasome. This releases associated p50, which translocates into the nucleus to modulate target gene expression. p105 degradation also liberates the p105-associated MAP kinase (mitogen-activated protein kinase) kinase kinase TPL-2 (tumour progression locus-2), which can then activate the ERK (extracellular-signal-regulated kinase)/MAP kinase cascade. Thus, in addition to its role in NF-kappaB activation, p105 functions as a regulator of MAP kinase signalling.

A stroma-derived defect in NF-kappaB2-/- mice causes impaired lymph node development and lymphocyte recruitment

The NF-kappaB family of transcription factors is vital to all aspects of immune function and regulation in both the hemopoietic and stromal compartments of immune environments. Recent studies of mouse models deficient for specific members of the NF-kappaB family have revealed critical roles for these proteins in the process of secondary lymphoid tissue organogenesis. In this study, we investigate the role of NF-kappaB family member NF-kappaB2 in lymph node development and lymphocyte recruitment. Inguinal lymph nodes in nfkappab2(-/-) mice are reduced in size and cellularity, most notably in the B cell compartment. Using in vitro and in vivo lymph node grafting assays, we show that the defect resides in the stromal compartment. Further examination of the nfkappab2(-/-) inguinal lymph nodes revealed that expression of peripheral node addressin components CD34 and glycosylation-dependent cell adhesion molecule-1 along with the high endothelial venule-restricted sulfotransferase HEC-GlcNAc6ST was markedly reduced. Furthermore, expression of the lymphocyte homing chemokines CCL19, CCL21, and CXCL13 was down-regulated. These data highlight the role of NF-kappaB2 in inguinal lymph node organogenesis and recruitment of lymphocytes to these organs due to its role in up-regulation of essential cell adhesion molecules and chemokines, while suggesting a potential role for NF-kappaB2 in organization of lymph node endothelium.

Inhibition of p38 MAP kinase- and RICK/NF-kappaB-signaling suppresses inflammatory bowel disease

Ulcerative colitis and Crohn's disease are the two entities of chronic inflammatory bowel diseases (IBD). One of the main pathogenic mechanisms is probably a dysregulated immune response triggered by products of the enteric bacterial flora. The goal of this study was to evaluate the effects of the p38 mitogen-activated protein kinase (MAPK) inhibitor SB203580 on inflammatory responses using the DSS-induced experimental colitis model in mice reflecting human IBD. We found that SB203580 improved the clinical score, ameliorates the histological alterations, and reduces the mRNA levels of proinflammatory cytokines. In addition to p38 kinase activity, the "classical" and the "alternative" NF-kappaB pathways were also strongly activated during colitis induction. All three pathways were drastically down-regulated by SB203580 treatment. An analysis of the molecular basis of NF-kappaB activation revealed that Rip-like interacting caspase-like apoptosis-regulatory protein kinase (RICK), a key component of a pathway leading to NF-kappaB induction, is also strongly inhibited by SB203580. Since RICK is an effector kinase of NOD2, an intracellular receptor of bacterial peptidoglycan, these results support the notion that NOD signaling could play a pivotal role in the IBD pathogenesis. Thus, RICK could represent a novel target for future therapies in human IBD.

Transient and selective NF-kappa B p65 serine 536 phosphorylation induced by T cell costimulation is mediated by I kappa B kinase beta and controls the kinetics of p65 nuclear import

Full transcriptional activity of the nuclear, DNA-bound form of NF-kappaB requires additional posttranslational modifications. In this study, we systematically mapped the T cell costimulation-induced phosphorylation sites within the C-terminal half of the strongly trans-activating NF-kappaB p65 subunit and identified serine 536 as the main phosphorylation site. The transient kinetics of serine 536 phosphorylation paralleled the kinetics of IkappaBalpha and IkappaB kinase (IKK) phosphorylation and also mirrored the principle of T cell costimulation. The TCR-induced pathway leading to serine 536 phosphorylation is regulated by the kinases Cot (Tpl2), receptor interacting protein, protein kinase Ctheta, and NF-kappaB-inducing kinase, but is independent from the phosphatidylinositol 3-kinase/Akt signaling pathway. Loss-of-function and gain-of-function experiments showed phosphorylation of p65 serine 536 by IKKbeta, but not by IKKalpha. Phosphorylation occurs within the cytoplasmic and intact NF-kappaB/IkappaBalpha complex and requires prior phosphorylation of IkappaBalpha at serines 32 and 36. Reconstitution of p65(-/-) cells either with wild-type p65 or a p65 mutant containing a serine to alanine mutation revealed the importance of this phosphorylation site for cytosolic IkappaBalpha localization and the kinetics of p65 nuclear import.