Toll-like receptor, RIG-I-like receptors and the NLRP3 inflammasome: key modulators of innate immune responses to double-stranded RNA viruses

Abstract

Double-stranded RNA (dsRNA), the genetic material for many RNA viruses, induces robust host immune responses via pattern recognition receptors, which include Toll-like receptor 3 (TLR3), retinoic acid-inducible gene-I-like receptors (RLRs) and the multi-protein NLRP3 inflammasome complex. The engagement of dsRNA receptors or inflammasome activation by viral dsRNA initiates complex intracellular signaling cascades that play essential roles in inflammation and innate immune responses, as well as the resultant development of adaptive immunity. This review focuses on signaling pathways mediated by TLR3, RLRs and the NLRP3 inflammasome, as well as the potential use of agonists and antagonists that target these pathways to treat disease.

Figure 1

Biological effects of double-stranded RNA signaling pathways.

Figure 2. Structural Characteristics of TLR3, Retinoic acid-inducible gene-I-like receptors (RLRs) and the NLRP3 inflammasome

TLR3, which is expressed both on the plasma membrane and in the intracellular endosome compartment, is comprised of a ectodomain that is capped by leucine-rich repeat (LRR) motifs that bind viral dsRNA, a transmembrane domain and an intracellular TIR (Toll/IL-1receptor) domain that binds the adaptor protein, TRIF (TIR-domain-containing adapter-inducing interferon-β) via a TIR domain interaction. Dimerization of TLR3 is required for ligand binding. The retinoic acid-inducible gene-I-like receptors (RLRs), RIG-I and MDA5, are comprised of a C-terminal RNA recognition domain (CTD) that selectively recognizes short 5′ tri-phosphates dsRNA and long dsRNA, respectively. RIG-I and MDA5 also contain a central helicase domain and two N-terminal CARDs (caspase activation and recruitment domains) that bind CARD-containing adaptors, such as MAVS (mitochondrial antiviral signaling protein). MAVS, which is localized to the outer membrane of the mitochondria via its transmembrane domain, dimerizes upon interaction of its N-terminal CARD with RIG-I or MAVS. MAVS also contains a central proline-rich region (PRR). TLR3 and RLR signaling pathways can activate the transcription factors, IRF3, IRF7 and NF-κB, with the resultant production of type I interferons (IFN-α/β) and pro-inflammatory cytokines. The NLRP3 inflammasome is comprised of three proteins, NLRP3, ASC and caspase 1. NLRP3 is comprised of C-terminal leucine-rich repeats (LRR), a central nucleotide-binding and oligomerization domain (NACHT), and a N-terminal pyrin (PYD) domain. ASC (apoptosis-associated speck-like protein containing a CARD) interacts with NLRP3 via its PYD domain and recruits caspase-1 via a CARD domain interaction, which cleaves the pro-forms of IL-1β and IL-18.

Figure 3. Double-stranded RNA-mediated signaling pathways

Both TLR and RLR signaling pathways can induce the activation of IRF3/7, which results in the production of type I interferons (IFN-α/β) and NF-κB, which induces the expression of pro-inflammatory cytokines. Upon binding of dsRNA, TLR3 dimerizes and mediates the recruitment of TRIF dimers to form a TRIF signalsome platform that leads to the activation of NF-κB and IRF3/7. In particular, the recruitment of a complex comprised of TRAF3, NAP1, TBK1 and IKKε, induces the phosphorylation and activation of IRF3/7, whereas the recruitment of RIP1, which is phosphorylated and polyubiquitinated by the E3 ubiquitin ligase, Peli1, results in the formation of a complex comprised of TRAF6, TAK1, PKR and TAB2 that activates the IKK complex and NF-κB, as well as MAP kinase signaling pathways. Recruitment of a RIP1-RIP3 complex by TRIF can also induce apoptosis via activation of FADD and caspase-8, as well as necrosis via the generation of reactive oxygen species (ROS). Similarly, upon binding of viral dsRNA, RIG-I dimerizes and interacts with MAVS, which also dimerizes and assembles a multi-protein signaling complex via TRADD (TNFR-associated death domain protein), which recruits TRAF3, TANK, FADD and RIP1. The subsequent recruitment of NEMO can induce the activation of NF-κB via IKKα and IKKβ or the activation of IRF3/7 via IKKε and TBK1. Lastly, the NLRP3 inflammasome mediates the maturation of the pro-inflammatory cytokines, IL-1β and IL-18, via the caspase-1-mediated cleavage of the cytokine pro-forms.