REVIEWS Modulation of NF‑κB signalling by microbial pathogens Masmudur M. Rahman and Grant McFadden Abstract | The nuclear factor-κB (NF‑κB) family of transcription factors plays a central part in the host response to infection by microbial pathogens, by orchestrating the innate and acquired host immune responses. The NF‑κB proteins are activated by diverse signalling pathways that originate from many different cellular receptors and sensors. Many successful pathogens have acquired sophisticated mechanisms to regulate the NF‑κB signalling pathways by deploying subversive proteins or hijacking the host signalling molecules. Here, we describe the mechanisms by which viruses and bacteria micromanage the host NF‑κB signalling circuitry to favour the continued survival of the pathogen. The nuclear factor-κB (NF-κB) family of transcription Signalling targets upstream of NF‑κB factors regulates the expression of hundreds of genes that NF-κB proteins are tightly regulated in both the cyto- are associated with diverse cellular processes, such as pro- plasm and the nucleus6. Under normal physiological liferation, differentiation and death, as well as innate and conditions, NF‑κB complexes remain inactive in the adaptive immune responses. The mammalian NF‑κB cytoplasm through a direct interaction with proteins proteins are members of the Rel domain-containing pro- of the inhibitor of NF-κB (IκB) family, including IκBα, tein family: RELA (also known as p65), RELB, c‑REL, IκBβ and IκBε (also known as NF-κBIα, NF-κBIβ and the NF-κB p105 subunit (also known as NF‑κB1; which NF-κBIε, respectively); IκB proteins mask the nuclear is cleaved into the p50 subunit) and the NF-κB p100 localization domains in the NF‑κB complex, thus subunit (also known as NF‑κB2; which is cleaved into retaining the transcription complex in the cytoplasm. the p52 subunit); these proteins can homodimerize or In response to diverse stimuli, various cellular immune heterodimerize through their conserved Rel homology receptors (such as Toll-like receptors (TLRs)) and domain to mediate gene transcription1,2. NF‑κB pro- cytokine receptors (such as the interleukin‑1 receptors teins are activated by a variety of diverse extracellular (IL‑1Rs), TNF receptors (TNFRs) and other TNFR-like or intracellular stimuli, including microbial pathogens receptors) can rapidly activate the NF‑κB complex fol- and pathogen-associated molecular patterns (PAMPs)3. lowing the appropriate pro-inflammatory stimulation. The NF‑κB signalling pathway is an attractive target for This activation is mediated by a signalling cascade that exploitation by microbial pathogens in order to mod- uses multiple adaptors (including TNFR-associated fac- ulate host cell events, as activation of NF‑κB is such a tors (TRAFs), myeloid differentiation primary response rapid response. Cytoplasmic NF‑κB complexes are trans- protein 88 (MYD88) and TIR domain-containing adap- ferred to the nucleus within minutes after exposure to tor protein (TIRAP)), as well as intermediate transduc- a pathogen or PAMPs, even in the absence of de novo ing molecules and kinases (including IL‑1R‑associated protein synthesis, and induce the expression of a broad kinases (IRAKs), receptor-interacting proteins (RIPs; spectrum of antimicrobial pro-inflammatory cellular also known as RIPKs) and NF-κB-inducing kinase (NIK; Department of Molecular response genes4,5. The central role of these transcription also known as MAP3K14)), to eventually lead to degra- Genetics and Microbiology, College of Medicine, factors in pathogen defence is highlighted by the fact dation of the inhibitory protein IκBα, thus liberating the University of Florida, that the NF‑κB signalling cascade is probably the most NF‑κB complexes for transport to the nucleus, where 1600 SW Archer Road, frequently targeted intracellular pathway for subver- they undergo further layers of regulation7 (FIG. 1). PO Box 100266, sion by anti-immune modulators that are encoded by a The receptor-mediated signalling events converge Gainesville, Florida, USA. wide spectrum of microbial pathogens4. In this Review, on the same core components of the NF‑κB activation Correspondence to G.M. e‑mail: [email protected] we describe some of the recent advances in our under- apparatus: the IκB kinase (IKK) complex, which is com- doi:10.1038/nrmicro2539 standing of the various mechanisms used by pathogens posed of two catalytic subunits, IKKα and IKKβ, and a Published online 8 March 2011 to modulate NF‑κB signalling. regulatory subunit, NEMO (NF-κB essential modulator; NATURE REVIEWS | MICROBIOLOGY VOLUME 9 | APRIL 2011 | 291 © 2011 Macmillan Publishers Limited. All rights reserved REVIEWS Classical pathway Alternative pathway kinases — including IKKi (inducible IKK; also known Diverse as IKKε) and TBK1 (TANK-binding kinase 1) — can TNFRs Antigen microbial TLRs Immune cytokine receptors and IL-1Rs receptors activate the NF‑κB pathway in response to microbial inducers infection11. TBK1 interacts with TANK, a TRAF-binding protein that activates NF‑κB by modulating the func- tion of TRAF2 and that also interacts with IKKi. TBK1 Adaptors enhances the enzymatic activity of IKKβ by direct phos- NIK phorylation, and thus contributes to NF‑κB activation12. Activation of the Activation Because of the essential role of IKKs and IκBα in the IKK complex of IKKα activation and regulation of NF‑κB signalling, these pro- NEMO IKKα IKKα teins (or their post-translational modifications) are often P P targeted directly by microbial pathogens to control the IKKα IKKβ IκBα Phosphorylation host immune responses. p105 RELA Phosphorylation of of p100 The NF‑κB pathway is classified as either classi- the IκBα and p105 cal (canonical) or alternative (non-canonical) on the P P IκBα p100 p100 basis of the IKK subunits that get activated by upstream P kinases13,14 (FIG. 1). In the classical pathway (for exam- p105 RELA ple, triggered by TNFR1 signalling), IKKβ and NEMO RELB Degradation of IκBα Processing of p100 become activated by adaptors (such as TRAFs) and then Processing of p105 phosphorylate p105 and IκBα to release the prototypi- cal heterodimer p50–RELA. In the alternative pathway p50 RELA p52 RELB (for example, triggered by lymphotoxin‑β receptor), Nuclear translocation Nuclear translocation IKKα is activated by NIK and then phosphorylates p100, which is subsequently cleaved to form p52. p52 then forms a heterodimer with RELB and translocates to the nucleus. This alternative pathway is triggered by a Nucleus p50 RELA p52 RELB subset of tumour necrosis factor (TNF) family members, including CD40, lymphotoxin‑β, B cell-activating factor (BAFF), receptor activator of NF‑κB ligand (RANKL) Activation of NF-κB-responsive genes and TNF-related weak inducer of apoptosis (TWEAK). By contrast, inflammatory cytokines, genotoxic stress, • Inflammation antigens and TLR stimulation tend to activate the clas- • Cell survival • Cytokines • Cell proliferation sical pathway. Both the classical and alternative pathways • Chemokines • Cell differentiation are modulated by microbial pathogens, as the two path- • Enzymes • Cell cycle arrest ways induce coordinated immune responses following • Adhesion molecules • Innate immune responses diverse infections. • Adaptive immune responses Outcome of NF‑κB activation Activation of NF‑κB is considered to be the central Figure 1 | The classical and alternative NF‑ B signalling pathways use a wide κ initiating cellular event of host responses to invasion variety of signals to control a diverse set of cellular responses.Nature Re viewsProtein | Micr levelsobiology and activity of signalling molecules can be regulated through post-translational by microbial pathogens. The presence of a functional modifications such as phosphorylation, ubiquitylation and acetylation. The activation of NF‑κB signalling cascade in the horseshoe crab, a species nuclear factor-κB (NF-κB) ultimately results in the transcription of genes that encode that is known as a ‘living fossil’, suggests that the pro- pro-inflammatory factors and factors that influence cell proliferation. IκBα, NF-κB teins involved are the evolutionarily conserved immune inhibitor-α (also known as NF-κBIα); IKK, IκB kinase; IL‑1R, interleukin‑1 receptor; NEMO, defence molecules15, and highlights the central role of NF-κB essential modulator (also known as IKKγ); NIK, NF-κB-inducing kinase (also known NF‑κB in upregulating the expression of genes encod- as MAP3K14); TLR, Toll-like receptor; TNFR, TNF receptor. ing chemokines, cytokines, adhesion molecules (such as intercellular adhesion molecule 1 (ICAM1)), enzymes that produce secondary inflammatory mediators, and inhibi- also known as IKKγ)8,9. Activation of the IKK complex tors of apoptosis. These molecules are key components induces phosphorylation of IκBα, followed by ubiquityl­ of the innate immune response to invading microorgan- ation by a specific cullin-RING ubiquitin ligase (CRL) isms and are required for the migration of inflammatory family SCFβTRCP complex, which contains SKP1 (S phase and phagocytic cells to the site of infection, where NF‑κB kinase-associated protein 1), cullin 1 and the F-box has been activated. The activated phagocytic cells kill, protein βTRCP as substrate adaptor; this SCFβTRCP ingest and degrade microbial pathogens and eventually complex specifically recognizes and degrades IκBα present the antigens to T cells after they re-migrate to that is phosphorylated on Ser32 and Ser36. As well as secondary lymphoid organs. The secreted cytokines, processing IκBα, the ubiquitylation–proteasome