The Generation of the Cellular Response to Virus Infection
The efficient and effective clearance of an invading viral pathogen involves a multifaceted system of defenses for a directed response at the primary site of infection resulting in the activation of innate immune defenses and the eventual education and priming of the adaptive immune system. This cellular response to virus infection begins with identification of the invading virus through recognition of pathogen associated molecular patterns (PAMP) by cytosolic and membrane bound sentinel proteins collectively known as pattern recognition receptors (PRRs). Upon recognition, the PRRs initiate signal transduction cascades to activate antiviral transcription factors that result in large scale changes in gene ex
The PRRs consist of multiple classes of proteins including Toll-like receptors (TLRs), Rig-I like receptors (RLRs), and Nod-like receptors (NLRs).
Figure 1. The Cellular Antiviral Response.
The cellular antiviral response begins with recognition of foreign nucleic acid by Rig-I like receptors, Rig-I and MDA5, and toll-like receptors (TLR), TLR3 and TLR7. Upon binding to the foreign nucleic acid, these molecules initiate signal transduction cascades to activate transcription factors IRF3 and NF-κB in order to drive transcription of the antiviral cytokine IFNβ. Rig-I and MDA5 signal activate IRF3 through TRAF3 and NF-κB through TRAF6, through a mitochondrial adaptor protein, MAVS. TLR3 signals through TRIF and TLR7 signals through MyD88 in order to activate NF-κB and IRF transcription factors. Both pathways converge to cause transcription of the antiviral cytokine IFNβ.
TLRs are responsible for recognition of a variety of bacterial and viral PAMPs including lipid, protein, and nucleic acid motifs present in extracellular space or endosomal compartments. The RLRs function to recognize foreign nucleic acid motifs in the cytoplasm. The NLRs function to recognize bacterial peptidoglycans in the cytoplasm. Together, TLRs and RLRs detect invading viral pathogens and initiate a signal transduction cascade to activate an antiviral gene ex
In addition to the membrane bound TLRs, the Rig-I like receptor (RLR) RNA helicases serve as PRRs for identifying foreign nucleic acid motifs in the cytosol. The RLR family consists of three intracellular helicases: retinoic acid inducible gene I (RIG-I), melanoma differentiation associated gene 5 (MDA5), and laboratory of genetics and physiology 2 (LGP2). These proteins have all been demonstrated to bind double stranded RNA and have been implicated in sensing RNA virus infection despite initially being characterized in other cellular processes. The RLRs share highly conserved amino acid sequences as well as structural similarities. All members share a central conserved DExD/H RNA helicase domain and a C-terminal regulatory domain. RIG-I and MDA5 contain two caspase activation and recruitment domains (CARDs) at their N-terminal, which is lacking in LGP2. The CARDs are responsible for generating a signal transduction cascade by interacting with the mitochondrial antiviral signaling protein (MAVS) for the activation of NF-κB and IRFs. Rig-I has been shown to have a preference for RNA ligands with 5’-triphosphorylated (5’PPP) RNA with short stretches of dsRNA and the poly-uridine motif located in the 3’region of hepatitis C virus genome. MDA5 has been identified to bind to long dsRNA with secondary structure. Knockout mouse studies have provided insights into the endogenous ligands recognized by these PRRs, and revealed largely non-redundant functions. Rig-I was found to recognize infections by Rhabdoviridae, Orthomyxoviridae, Paramyxoviridae, Flaviviridae, and Reoviridae. MDA5 was found to detect infections by Picornaviridae and Calciviridae. The role of LGP2 is more complicated as it has been determined to bind dsRNA and function in both positive and negative aspects of RLR signaling, which is likely dependent upon its concentration.
Influenza evasion of immune system
The PRRs Rig-I and TLR7 are responsible for recognition of influenza virus genomic material during infection. Together, these two PRRs signal for the production of IFNβ and other antiviral genes. In order to combat the effectiveness of the host IFN system, influenza virus encodes several mechanisms to evade host detection and to inhibit the cellular response to virus infection. The viral polymerase complex helps to shut down the host response by limiting host gene ex
The NS1 protein is multifunctional protein that contains a RNA binding domain and an effector domain. A major responsibility of NS1 during infection is to evade and disable the host IFN response. Evidence suggests that NS1 is responsible for inhibiting interferon production because recombinant viruses that lack NS1 produce a stronger IFN response in both tissue culture and mouse models. NS1 has evolved many mechanisms to prevent the production of IFN and to inhibit its function. The first mechanism is its ability to prevent the activation of antiviral transcription factors IRF3/IRF7 and NF-κB by inhibiting Rig-I and its ubiquitin modifying protein TRIM25 from signaling through MAVS. In addition, NS1 also functions to inhibit 2’5’OAS/RNaseL by competing for binding of dsRNA, and inhibiting PKR activation. NS1 has also been identified to directly interact with PKR and prevent it from inhibiting the host translational machinery. Additional NS1 mechanisms for inhibiting the host response include preventing 3’ end processing of pre-mRNAs through interaction with CPSF30, preventing nuclear export of polyadenylated mRNA, and interacting with EIF4GI and poly-A binding protein to specifically enhance viral mRNA translation. Through these many interactions, NS1 is able to prevent IFN production and inhibit the cellular antiviral response in order to enhance viral replication.