The non-canonical NF-κB pathway in immunity and inflammation

Abstract

The nuclear factor-κB (NF-κB) family of transcription factors is activated by canonical and non-canonical signalling pathways, which differ in both signalling components and biological functions. Recent studies have revealed important roles for the non-canonical NF-κB pathway in regulating different aspects of immune functions. Defects in non-canonical NF-κB signalling are associated with severe immune deficiencies, whereas dysregulated activation of this pathway contributes to the pathogenesis of various autoimmune and inflammatory diseases. Here we review the signalling mechanisms and the biological function of the non-canonical NF-κB pathway. We also discuss recent progress in elucidating the molecular mechanisms regulating non-canonical NF-κB pathway activation, which may provide new opportunities for therapeutic strategies.

Figure 1. Canonical and non-canonical NF-κB pathways

The canonical nuclear factor-κB (NF-κB) pathway is triggered by signals from a large variety of immune receptors, which activate the kinase TGFβ -activated kinase 1 (TAK1). TAK1 then activates a trimeric IκB kinase (IKK) complex, composed of catalytic (IKKα and IKKβ) and regulatory (IKKγ) subunits, via phosphorylation of IKKβ. Upon stimulation, the IKK complex, largely through IKKβ, phosphorylates members of the inhibitor of κB (IκB) family, such as the prototypical IκB member IκBα and the I κB-like molecule p105, which sequester NF-κB members in the cytoplasm. IκBα associates with dimers of p50 and members of the REL family (RELA or c-REL), whereas p105 associates with p50 or REL (RELA or c-REL). Upon phosphorylation by IKK, IκBα and p105 are targeted for ubiquitin (Ub)-dependent degradation in the proteasome, resulting in the nuclear translocation of canonical NF-κB family members, which bind to specific DNA elements, termed κB enhancers of target genes, in the form of various dimeric complexes, including RELA–p50, c-REL–p50, and p50–p50 (REF. 1). By contrast, non-canonical NF- κB signalling is based on the processing of p100, an IκB-like molecule that predominantly, although not exclusively, regulates RELB. The non-canonical NF- κB pathway selectively responds to a subset of tumour necrosis factor receptor (TNFR) superfamily members that target the activation of the kinase NFκB-inducing kinase (NIK). NIK phosphorylates and activates IKKα, which in turn phosphorylates carboxy-terminal serine residues of p100, triggering selective degradation of the C-terminal IκB-like structure of p100 and leading to the generation of p52 and the nuclear translocation of p52 and RELB. PRRs, pattern recognition receptors.

Figure 2. Regulation and activation of the non-canonical NF-κB pathway

a | Under unstimulated conditions, newly synthesized NF-κB-inducing kinase (NIK) is rapidly bound by TNFR associated factor 3 (TRAF3) and targeted for ubiquitylation (Ub) by the cellular inhibitor of apoptosis (cIAP)–TRAF2–TRAF3 E3 ubiquitin ligase complex. The continuous degradation of NIK in the proteasome prevents NIK accumulation and non-canonical nuclear factor-κB (NF-κB) pathway activation. b | Ligation of specific TNFR superfamily members by their ligands (TNF family members) stimulates the recruitment of TRAF2, TRAF3, cIAP1 and cIAP2 (shown as cIAP in the figure) to the receptor complex, in which activated cIAP mediates K48 ubiquitylation and proteasomal degradation of TRAF3, resulting in stabilization and accumulation of NIK and the induction of p100 processing, followed by the translocation of NF-κB p52–RELB heterodimers into the nucleus. Receptor-mediated non-canonical NF-κB activation is negatively regulated via TRAF3 deubiquitylation by OTUD7B and phosphorylation-dependent degradation of NIK is mediated by IκB kinase-α (IKKα) and TANK-binding kinase 1 (TBK1)^–. c | Complement membrane activation complex (MAC) activates the non-canonical NF-κB pathway via a TRAF-independent mechanism that involves the formation of an endosome-based signalling complex containing MAC, the kinase AKT and NIK, in which activated AKT mediates NIK stabilization. NLRP12, NACHT, LRR and PYD domain-containing protein 12.

Figure 3. The non-canonical NF-κB pathway regulates GC reactions in multiple steps

Immune responses to protein antigens involve the formation of germinal centres (GCs) within B cell follicles, in which activated B cells undergo clonal expansion and a series of differentiation events, which leads to the generation of antibody-secreting plasma cells or memory B cells. The initial step of GC formation involves interactions between antigen-primed T cells and B cells on the border of the T cell zone and the B cell follicle, which are required for differentiation of the CD4⁺ T cells into follicular helper T (T_FH) cells. The T cell–B cell interaction is initiated through TCR interaction with the MHC–antigen complex and also requires the ICOS–ICOSL interaction. Ligation of CD40 and BAFFR by their ligands, CD40L and BAFF, induces non-canonical NF-κB signalling in B cells, which is important for maintaining high levels of ICOSL expression and, thus, T_FH cell differentiation. T_FH cells, in turn, promote the clonal expansion and differentiation of B cells in the subsequent steps of GC reactions. Non-canonical nuclear factor-κB (NF-κB) signalling regulates GC reactions through promoting the survival of naive B cells, the differentiation of T_FH cells by inducing the expression of inducible co-stimulator ligand (ICOSL) in B cells, GC cell expansion, GC B cell survival, immunoglobulin (Ig) class switching, somatic hypermutation (SHM) and plasma cell survival^,,,–,. In addition, non-canonical NF-κB signalling also functions in stromal cells to mediate the induction of chemokines required for the formation of B cell follicles and the follicular dendritic cell network, which in turn are crucial for GC formation (not shown in Figure). BAFF, B cell activating factor; BAFFR, BAFF receptor; BCR, B cell receptor; CXCR5, CXC-chemokine ligand 5; DC, dendritic cell; FDC, follicular dendritic cell; TCR, T cell receptor.

Figure 4. Regulation of T cell responses by canonical and non-canonical NF-κB pathways

Naive T cell activation via the T cell receptor (TCR) and signalling through the co-receptor CD28 leads to the rapid activation of the canonical nuclear factor-κB (NF-κB) pathway, which contributes to the initial activation and clonal expansion of T cells. T cell activation is associated with TCR stimulation and the inducible expression of several TNF receptor (TNFR) superfamily members such as CD27, CD30, OX40 and 41BB, which mediate the activation of non-canonical NF-κB signalling upon ligation by their ligands on antigen-presenting cells and activated T cells. Non-canonical NF-κB signalling is crucial for the generation and maintenance of effector and memory T cells and also has a role in mediating the induction of effector cytokines^,,. Since TNFRs also activate canonical NF-κB signalling, it is likely that the two pathways function cooperatively during the effector and memory phases of T cell responses.

Figure 5. The non-canonical NF-κB pathway regulates inflammation in different cell types

Under physiological conditions, non-canonical nuclear factor-κB (NF-κB) signalling mediates the survival and homeostasis of B cells, the generation and effector function of T helper 17 (T_H17) cells, the differentiation of osteoclasts from monocytes, chemokine production in endothelial cells, and glucagon responses in hepatocytes^,,,,,,,. These cellular events become pathogenic in situations in which the non-canonical NF-κB pathway is aberrantly activated owing to the uncontrolled production of its inducing agents or genetic deficiencies in its negative regulators. Aberrant B cell survival renders self-reactive B cells resistant to negative selection, contributing to the accumulation of autoantibodies associated with inflammatory diseases. Autoimmune T_H17 cells mediate inflammation in the central nervous system (CNS) and other tissues. Excessive and chronic production of chemokines by endothelial cells promotes the recruitment of inflammatory cells and the formation of tertiary lymphoid structures in tissue-specific inflammation^,. Aberrant osteoclast generation is a pathological mechanism of inflammatory bone loss, and aberrant glucagon responses are associated with metabolic diseases^,. BAFF, B cell activating factor; CD40L, CD40 ligand; CREB1, cAMP-responsive element-binding protein; GM-CSF, granulocyte–macrophage colony-stimulating factor; Ig, immunoglobulin; LTα 1β2, lymphotoxin α 1 β2; RANKL, RANK, receptor activator for NF- κB ligand; TNF, tumour necrosis factor.