Dengue virus and the host innate immune response

Abstract

Dengue virus (DENV) is a mosquito-borne Flavivirus that is endemic in many tropical and sub-tropical countries where the transmission vectors Aedes spp. mosquitoes resides. There are four serotypes of the virus. Each serotype is antigenically different, meaning they elicit heterologous antibodies. Infection with one serotype will create neutralizing antibodies to the serotype. Cross-protection from other serotypes is not long term, instead heterotypic infection can cause severe disease. This review will focus on the innate immune response to DENV infection and the virus evasion of the innate immune system by escaping recognition or inhibiting the production of an antiviral state. Activated innate immune pathways includes type I interferon, complement, apoptosis, and autophagy, which the virus can evade or exploit to exacerbate disease. It is important to understand out how the immune system reacts to infection and how the virus evades immune response in order to develop effective antivirals and vaccines.

Fig. 1. The viral life cycle of dengue virus (DENV).

The virus binds to host cell receptors (exact receptors are unknown) (1) and enters the host cell (DENV permissive cells include keratinocytes, dendritic cells, endothelial cells, fibroblasts, macrophage, and mast cells), via receptor-mediated endocytosis (2). Acidification of the endosome induces conformational change of the E glycoprotein causing the virus to fuse with the endosomal membrane and release its genomic RNA material into the cytoplasm (3). DENV RNA translation and replication occur at the endoplasmic reticulum (ER) (4). The host ribosome directly translates the genomic RNA into a polyprotein, where host and viral proteases cleave the nascent protein into structural (blue) and nonstructural (red) proteins (5). RNA replication occurs in virus-induced membrane vesicles by the viral replication complex, with the transmembrane NS2a, NS2b, NS4a, and NS4b proteins acting as the scaffold (6). The viral genome is packaged into the immature virus particles during assembly (7). These particles are transported through the Golgi apparatus, where host furin-like proteases cleave the prM peptide (8), and the nascent viral particles exit the cell via exocytosis as fully mature virions (9). Some pr peptides are not cleaved resulting in immature, non-infectious virions or partially mature virions. The soluble NS1 hexamer is also secreted

Fig. 2. Innate immune response to DENV infection.

Recognition of viral genomes by cytoplasmic retinoic acid-inducible gene I (RIG-I) and melanoma differentiation-associated protein 5 (MDA5) trigger mitochondrial antiviral signaling (MAVS) activation that lead to TANK-binding kinase 1 (TBK1), IκB kinase-ε (IKKε) induction of interferon regulatory factor 3 (IRF3), and IRF7 (tan arrows). Viral genome recognition by endosomal toll-like receptor 3 (TLR3) (green arrows) and TLR7 (orange arrows) will activate TIR-domain-containing adapter-inducing IFNβ (TRIF) and myeloid differentiation primary response gene 88 (MyD88) signaling pathways, inducing IRF3/IRF7 and inhibitor of nuclear factor-κB kinase (IKK)α/IKKβ/IKKγ, and activate nuclear factor-κB (NF-κB) to produce IFNα/β and pro-inflammatory cytokines. Virus-induced mitochondria damage activates the cyclic GMP-AMP synthase (cGAS) (pink arrows) and stimulator of interferon gene (STING) pathway to induce IFNα/β production via IRF3 and IRF7. MicroRNAs (miRNAs) associate with RNA-induced silencing complex (RISC) in the cytoplasm to target viral RNA for inhibition or degradation. miRNA biogenesis starts in the nucleus as pri-miRNA and processed by Drosha into pre-miRNA. Pre-miRNAs are transported to the cytoplasm and cleaved by Dicer to produce mature miRNAs. Argonaute (Argo) and TAR RNA-binding protein (TRBP) are proteins essential to the formation of RISC (blue arrows). Double membrane vacuoles, called autophagosomes, will engulf foreign cytoplasmic material and fuse with the lysosome for degradation, inhibiting virus replication (red)

Fig. 3. Type I IFN response and complement activation from DENV infection.

a Type I IFN signaling (green): Binding of type I IFN to IFNα/β receptors (IFNARs) stimulates IFN-stimulated gene (ISG) expression that results in antiviral activity. These cytokines bind to on the surface of nearby or infected cells, activating the Janus kinase (JAK)/signal transducer and activator of transcription (STAT) pathway. JAK1 and tyrosine kinase 2 (TYK2) lead to phosphorylation and dimerization of STAT1 and STAT2, which forms a complex with interferon regulatory factor 9 (IRF9). The complex will translocate to the nucleus where they induce transcription of ISGs by the IFN-stimulated response element (ISRE). b Complement pathway (blue): Recognition of DENV by the mannose-binding lectin (MBL) complex will induce complement activation. Cleavage of C4 and C2 by MBL-associated serine protease-2 (MASP-2) make the C3 convertase and initiates the classical complement cascade, including the formation of C5 convertase and the C5b-9 membrane attack complex (MAC) to induce lysis, recruitment of phagocytes, and inflammation. c NS1 binding to TLR4 will induce vascular damage (purple)

Fig. 4. DENV evasion of innate immune response.

Nonstructural proteins block signaling pathways after virus recognition and inhibit type I IFN production. NS3 and NS4a block retinoic acid-inducible gene I (RIG-I) translocation to mitochondria (red). NS2a and NS4b from DENV1, -2, -4 (green) and NS4a from DENV1 (light green) inhibit the activation of TANK-binding kinase 1 (TBK1), blocking the RIG-I/mitochondrial antiviral signaling (MAVS) signaling pathway and IFNβ induction. NS2b targets cyclic GMP-AMP synthase (cGAS) for autophagy–lysosome-dependent degradation and prevents mitochondrial DNA sensing (purple). NS2b/3 protease inhibits IFN production by cleaving stimulator of interferon gene (STING) (pink). NS4b inhibits RNAi by binding to Dicer, preventing the biogenesis of miRNA. NS3 will also block RNAi by binding to heat-shock cognate 70, an essential protein for creating RISC, and inhibit TRBP–Argonaute interaction and RISC formation (blue). DENV will exploit the autophagy pathway and use autophagosomes for replication, assembly and maturation, and evasion of neutralizing antibodies during transmission (red)

Fig. 5. DENV inhibition of type I IFN response and complement pathways.

a Type I IFN signaling inhibition (blue): NS2a, NS4a, and NS4b complex inhibit signal transducer and activator of transcription 1 (STAT1) signaling after IFNα/β receptor activation. NS5 will cause proteasomal degradation of STAT2 by binding with the host ubiquitin protein ligase E3 component N-recognin 4 (UBR4). b Complement evasion (red): NS1 hexamer inhibits the formation of the classical pathway C3 convertase by binding to C4, C1s, and C4b. NS1 inhibits neutralization by the mannose-binding lectin (MBL) pathway by binding to MBL. NS1 blocks membrane attack complex (MAC) formation by binding to complement regulators vitronectin (VN), C9, or clusterin (Clu)