Manipulation of the nuclear factor-kappaB pathway and the innate immune response by viruses

Abstract

Viral and microbial constituents contain specific motifs or pathogen-associated molecular patterns (PAMPs) that are recognized by cell surface- and endosome-associated Toll-like receptors (TLRs). In addition, intracellular viral double-stranded RNA is detected by two recently characterized DExD/H box RNA helicases, RIG-I and Mda-5. Both TLR-dependent and -independent pathways engage the IkappaB kinase (IKK) complex and related kinases TBK-1 and IKKvarepsilon. Activation of the nuclear factor kappaB (NF-kappaB) and interferon regulatory factor (IRF) transcription factor pathways are essential immediate early steps of immune activation; as a result, both pathways represent prime candidates for viral interference. Many viruses have developed strategies to manipulate NF-kappaB signaling through the use of multifunctional viral proteins that target the host innate immune response pathways. This review discusses three rapidly evolving areas of research on viral pathogenesis: the recognition and signaling in response to virus infection through TLR-dependent and -independent mechanisms, the involvement of NF-kappaB in the host innate immune response and the multitude of strategies used by different viruses to short circuit the NF-kappaB pathway.

Figure 1

Comparison of the IKK family: classical IKK kinases vs IKK-related kinases. The four IKK kinases are classified into two subgroups, based on sequence homology and substrate specificity. Numbers below the figure indicate the amino-acid residue number and important domains are indicated above (catalytic kinase domain; leucine zipper; helix–loop–helix). Classical IKK kinases – IKKα and IKKβ – share 52% overall homology to each other and the IKK-related kinases IKKɛ and TBK-1 share 61% overall homology to each other. Homology between the subgroups is limited, with 27% overall homology and approximately 33% homology within the catalytic kinase domain. The positions of critical residues involved in catalytic activity are represented. Mutation K44A (IKKα and IKKβ) or K38A (IKKɛ and TBK-1) within the ATP-binding pocket of the kinase domain generates a dominant-negative kinase; phosphomimetic mutations S176/180E (IKKα) or S177/181E (IKKβ) within the kinase activation loop generates a constitutively active kinase.

Figure 2

Summary of the signaling pathways that recognize virus infection. Virus replication results in the production of PAMPs such as single- and double-stranded RNA. Viral nucleic acids trigger multiple signaling cascades through Toll-like-receptor-dependent (TLR3, TLR7 and TLR9) and TLR-independent (RIG-I and Mda-5) pathways leading to kinase activation through TRAF family members. In pDCs, TLR7 or TLR9 engagement by ssRNA leads to direct activation of IRF-7 through MyD88/TRAF6/IRAK4/IRAK1 recruitment. TRIF and MyD88 are the adaptors linking TLRs to the TRAF proteins, whereas MAVS links RIG-1 and Mda-5 to TRAF3. TRAF-dependent induction of the kinases JNK, IKKα, IKKβ, IKKɛ, TBK-1 and IRAK-1 induce the binding of ATF2-cJun, NF-κB (p50-RelA), IRF-3 and IRF-7 to sequence-specific PRD located upstream of the IFNβ start site. Coordinated assembly of these factors forms the IFNβ enhanceosome, which is responsible for the transcriptional induction of this antiviral cytokine (modified from tenOever and Maniatis, 2006).

Figure 3

HTLV-1 Tax interactions with the canonical and non-canonical NF-κB pathways. Tax affects NF-κB ignaling in both the nucleus and the cytoplasm. In the cytoplasm, Tax dimers interact with the non-catalytic IKK subunit NEMO, and facilitate Tax recruitment to the catalytic IKK subunits (α or β), leading to subsequent phosphorylation, ubiquitination and proteasomal degradation of IκB or processing of the C-terminal inhibitory region (p100C) of p100 in the canonical and non-canonical pathways, respectively. At the transcriptional level, Tax interacts with the NF-κB subunits and recruits the transcriptional coactivators CBP/p300, leading to the transcription of NF-κB-dependent cytokines, cell cycle regulators, genes modulating apoptosis and others (modified from Sun and Yamaoka, 2005).

Figure 4

TLR3-dependent and RIG-I-dependent signaling to the innate immune response: specific cleavage of signaling adapters by HCV NS3–4A protease. Engagement of endosome-associated TLR3 by dsRNA recruits the TRIF adaptor, resulting in the activation of TBK-1 and IKKɛ kinases that phosphorylate IRF-3 and IRF-7. TRIF also signals NF-κB activation via the IKKα/β complex, which phosphorylates IκBα, resulting in the release of the NF-κB DNA-binding subunits. The RIG-1 pathway activates NF-κB and IRF-3/7, following the recognition of incoming viral ribonucleoprotein complexes. RIG-I, through C-terminal RNA helicase domain, interacts with viral dsRNA and through the CARD domains interacts with the MAVS/IPS/VISA/Cardif adaptor. MAVS contains a transmembrane domain (TM) that localizes this adaptor to the mitochondria. NS3–4A protease activity of HCV cleaves the C-terminal domain of MAVS at Cys-508, disrupts RIG-I signaling to IFN activation and establishes persistent infection. NS3–4A also targets the TRIF adaptor molecule in the TLR3-dependent pathway (modified from Hiscott et al., 2006).

Figure 5

Viruses inhibit distinct aspects of the antiviral response. Influenza virus NS1 protein, as well as Vaccinia E3L prevents viral dsRNA from being recognized. The V proteins of paramyxoviruses, including SV5, Mumps and Hendra viruses, bind to Mda-5 and block downstream signaling. Ebola virus VP35 bids to RIG-I and blocks downstream signaling. RSV and RV interfere with activation of IRF-3 in virus-infected cells. The RV P protein inhibits TBK-1-mediated phosphorylation of IRF-3. VSV M protein blocks nuclear to cytoplasmic export of IFN and cellular mRNA (modified from Conzelmann, 2005). HCV core protein and ASFV block NF-κB activation. Measles virus and RSV block IFNα and IFNβ production.