The Different Tactics of Foot-and-Mouth Disease Virus to Evade Innate Immunity

Abstract

Like all pathogens, foot-and-mouth disease virus (FMDV) is recognized by the immune system inducing a heightened immune response mainly mediated by type I and type III IFNs. To overcome the strong antiviral response induced by these cytokines, FMDV has evolved many strategies exploiting each region of its small RNA genome. These include: (a) inhibition of IFN induction at the transcriptional and translational level, (b) inhibition of protein trafficking; (c) blockage of specific post-translational modifications in proteins that regulate innate immune signaling; (d) modulation of autophagy; (e) inhibition of stress granule formation; and (f) in vivo modulation of immune cell function. Here, we summarize and discuss FMDV virulence factors and the host immune footprint that characterize infection in cell culture and in the natural hosts.

Keywords: FMDV; IFN; apoptosis; autophagy; cattle/swine; evasion innate immunity; ubiquitin/ISG; virulence factors.

FIGURE 1

Genome organization of FMDV. A schematic representation of linear (+) stranded FMDV RNA is depicted as thin solid lines and dotted lines represent ORF. Viral genomic RNA contains elements shown at the 5′ and 3′ UTR and are represented as thin lines. The 5′UTR consists of S fragment, poly (C) tract, pseudoknots (PKs), cis-acting replicative element (cre) and internal ribosome entry site (IRES). The single ORF encodes a polyprotein and is represented in outlined-open boxes. Filled triangles, squares and diamonds indicate processing sites for 3C^pro, 2A, and L^pro, respectively. Post-translational proteolytic cleavages are shown as partial products. Asterisks describe the two AUG initiation codons. The 3′UTR consist of a short stretch of RNA and a poly (A) sequence. The P1 region encodes the structural polypeptides. The P2 and P3 regions encode the non-structural proteins associated with replication. 3B (VPg) protein is shown as covalently linked to the 5′end of the genomic RNA.

FIGURE 2

Innate antiviral immune responses during viral infection. Upon viral infection, viral RNA is detected by cytosolic RLRs (RIG-I, MDA-5 or LGP2) and/or membrane bound TLRs, triggering a signal transduction cascade that culminates in the transcription of innate molecules including IFN, pro-inflammatory cytokines and ISGs. Cytosolic RNA is detected by RIG-I, MDA5 and presumably LGP2. RIG-I recognizes RNAs containing 5′ppp and panhandle-like secondary structures. MDA5 recognizes dsRNA. LGP2 might regulate the functions of RIG-I and MDA5. Interaction of RIG-I and MDA5 RNA exposes their CARD domains. RIG-I is subsequently ubiquitinated with unanchored K63-linked Ub chains. MDA5 stacks along dsRNA and form helical filaments. Discrete CARD oligomers (with or without Ub) align on CARD domains of mitochondrial MAVS inducing its polymerization. Endosomal RNAs are detected by TLR3 or TLR7/8, which interact with adaptor proteins TRIF and MyD88, respectively. MyD88 uses other adaptors, IRAK1/4 to allow for interaction with TRAF proteins. Polymerization of MAVS or conformational changes on TRIF and MyD88/IRAK1/4 recruit E3 ligases, mainly TRAF3 and TRAF6. These E3 ligases synthesize poly-Ub that are sensed by NEMO to recruit TBK1 or IKK. NEMO/TBK1 complexes lead mainly to phosphorylation of IRF3/7 (although IRF1 or IRF5 might also be phosphorylated). Phosphorylated IRFs dimerize and translocate to the nucleus. The IKK complex phosphorylates IκB causing its degradation and detachment of NF-κB which then translocates to the nucleus. The same IKK complex can phosphorylate IRF7. Nuclear homo and heterodimers of IRF3 and IRF7 bind to the IFNβ promoter to stimulate its transcription. Nuclear NF-κB binds the IFNβ promoter but also promoters of many proinflammatory cytokines such as TNFα, IL-1, etc. Binding of dimers of phosphorylated IRF7 is essential for the expression of IFNα. In parallel, E3 ligases can activate MAPK3 and subsequently other kinases including ERK1/2 and JNK which phosphorylate the components of the AP1 heterodimer followed by translocation to the nucleus to cooperate with the induction of IFNβ transcription. Secreted IFN can act in an autocrine and paracrine manner by binding to its receptor (IFNR). Upon binding JAKs (JAK1 and TYK2) are activated to phosphorylate the IFNAR receptor, also allowing for JAK dependent phosphorylation of STAT1 and STAT2 (although other STATs may also be affected). Heterodimers of STAT1/2 interact with IRF9 to form the ISGF3G complex which translocate to the nucleus and binds DNA to drive the expression of over 300 ISGs, many of which display antiviral activity. Homodimers of STAT1 translocate to the nucleus and mainly enhance transcription of proinflammatory cytokines. Many proteins of the pathway (RIG-I, MDA5, MAVS, TRAF3/6, MyD88, and IRF3/7/1/5) are targets for deUbiquitination and/or deISGylation and serve as negative regulators of the IFN signaling pathway. For instance, USP18 remove ISG15 from ISG15-conjugated substrates by cleaving the isopeptide bond between ISG15 and its targeted substrate. Other negative regulators of IFN signaling include SOCS and PIAS. Formation of SGs and activation of autophagy can be detected during innate immunity signaling responses.

FIGURE 3

Foot-and-mouth disease virus modulates early stages of immune response in vivo. Upon FMDV infection and first round of replication in epithelial cells-the primary infection site-, FMDV gets in contact with cells of the innate immune response inducing functional consequences that affect the host response. FMDV interacts with different players of the immune response either as a result of lytic infection of epithelial cells and subsequent phagocytosis, and/or lytic action on damaged infected tissue or by direct infection of immune cells through a direct or an antibody-dependent internalization process in Mφ, DCs, or NK or γδ T cells. After this stage, DCs, pDCs and Mφ produce IFN and other cytokines that modulate the immune response. Possible interaction of DC, NK and γδ T cells in the innate immune response is depicted, although clear interaction is thus far not completely understood. Ultimately, after FMDV infection and replication, systemic IL-10 produced by cDCs is detected, thus directing the adaptive immune response toward a stronger humoral stimulation rather than a T-cell mediated response.