Poxviral Targeting of Interferon Regulatory Factor Activation

Abstract

As viruses have a capacity to rapidly evolve and continually alter the coding of their protein repertoires, host cells have evolved pathways to sense viruses through the one invariable feature common to all these pathogens-their nucleic acids. These genomic and transcriptional pathogen-associated molecular patterns (PAMPs) trigger the activation of germline-encoded anti-viral pattern recognition receptors (PRRs) that can distinguish viral nucleic acids from host forms by their localization and subtle differences in their chemistry. A wide range of transmembrane and cytosolic PRRs continually probe the intracellular environment for these viral PAMPs, activating pathways leading to the activation of anti-viral gene expression. The activation of Nuclear Factor Kappa B (NFκB) and Interferon (IFN) Regulatory Factor (IRF) family transcription factors are of central importance in driving pro-inflammatory and type-I interferon (TI-IFN) gene expression required to effectively restrict spread and trigger adaptive responses leading to clearance. Poxviruses evolve complex arrays of inhibitors which target these pathways at a variety of levels. This review will focus on how poxviruses target and inhibit PRR pathways leading to the activation of IRF family transcription factors.

Keywords: immune evasion; innate immune response; interferon regulatory factor; poxvirus; virus-host interaction.

Figure 1

Detection of poxviruses by anti-viral pattern recognition receptors (PRRs) leads to the activation of IRFs. IRF3 phosphorylation by TBK1 and IKKε leads to its dimerization and nuclear translocation, where it associates with CBP/p300 to initiate transcription of anti-viral target genes, such as type I interferons (TI-IFNs). TRAF3 plays a central role in Toll-like receptor (TLR) signaling through generation of activating polyubiquitin chains. TLR3 sensing of dsRNA recruits TRIF, which activates TRAF3, in turn forming a complex with TBK1 and IKKε to promote IRF3 phosphorylation. The MyD88 adaptor protein associates with TLR7/8 and TLR9 is also able to activate TRAF3-mediated signaling. TLR4-associated TRAM can additionally activate this pathway through associations with TRIF. NFκB essential modulator (NEMO) is traditionally associated with NFκB activation to induce pro-inflammatory cytokine production but can also activate TBK1, thus inducing IRF3 phosphorylation. RIG-I-like receptors (RLRs) sense viral RNA and activate a common activator mitochondrial antiviral signaling protein (MAVS), leading to IRF activation. Cytosolic DNA sensing by cGAS, the RLR RIG-I (potentially via DNA Pol III-generated transcript from viral DNA) or Ku additionally activates this system, leading to IRF3 phosphorylation.

Figure 2

Poxviruses commonly inhibit interferon regulatory factor (IRF) signaling by targeting proximal activation complexes or IRFs directly. VACV-encoded C6 (VVC6) associates with multiple IRF3-activating adaptor proteins and kinase; TBKBP1, TANK, NAP1, TBK1 and IKKe specifically inhibit IFNβ production independent of NFκB activation. VACV-encoded N1 (VVN1) inhibits TBK1 activation, with a parallel function seen in MCV-encoded MC159 and MC160. VACV-encoded N2 (VVN2) associates with phosphorylated IRF3 in the nucleus to inhibit initiation of gene transcription. Similarly, MCV-encoded MC159 blocks nuclear IRF3 association with CBP/p300.

Figure 3

TLR adaptor-targeting by poxviruses inhibits TLR-mediated IRF activation. VACV-encoded A46 (VVA46) interacts with the Toll Il−1R (TIR) domain of MyD88, TRIF and TRAM, inhibiting IRF activation. Additionally, VACV-encoded K7 (VVK7) binds the N-terminus of DDX3, blocking TBK1 and IRF3 activation.

Figure 4

Poxviral inhibition of cytosolic nucleic acid sensing leading to IRF activation. VACV-encoded E3 (VVE3) binds dsDNA acting as a competitive inhibitor of RLR activation. Similarly, VACV-encoded C4 and C16 (VVC4/C16) inhibit DNA binding to Ku, therefore blocking DNA-PK-mediated stimulator of interferon genes (STING) activation and, hence, TBK1 activation. The VACV-encoded poxin B2 (VVB2) hydrolyses the 3′−5′ bond on cGAMP, thus inactivating this key messenger molecule in cGAS-STING activation. A further target is mTOR-dependent cGAS degradation by VACV-encoded F17 (VVF17), thus suppressing cGAS-mediated TI-IFN gene expression.