Viral evasion of DNA-stimulated innate immune responses

Christensen, Maria H; Paludan, Søren R

As TLR9 is expressed by only a limited number of cell types, most notably plasmacytoid dendritic cells, 5,6 this receptor is not the central sensor of viruses in the cells most often infected by viruses. RIG-I-like receptors (RLRs) recognize RNA species in the cytosol and include RIG-I, MDA5 and LGP2. Both RIG-I and MDA5 bind double-stranded RNA, but RIG-I preferentially recognizes shorter fragments while long RNA molecules activates MDA5. 7 Furthermore, to distinguish between host and pathogen RNA, RIG-I senses 5′-triphosphate containing RNA, a structure often formed in viral RNA. 8, 9 RIG-I and MDA5 both contain two caspase activation and recruitment domains, which upon ligand recognition engage in homotypic interaction with the adaptor protein mitochondrial activator of virus signaling (MAVS) leading to activation of Tank-binding kinase (TBK) 1, and IκB kinase α and β, culminating in the expression of type I IFN as well as proinflammatory proteins. [10] [11] [12] [13] Recently, attention has been drawn towards cytosolic PRRs sensing DNA. Several receptors have been proposed to be important for cytosolic DNA recognition, including absent in melanoma (AIM2), gamma-interferon-inducible protein (IFI16) and cyclic GMP-AMP synthase (cGAS) 1,2 (Table 1) . Upon DNA binding, AIM2 initiates the assembly of the inflammasome, a complex that includes the adaptor protein ASC and caspase 1. The activated inflammasome cleaves the proforms of the cytokines interleukin (IL)-1β and IL-18 to generate bioactive cytokines. 14, 15 The PYHIN protein, IFI16 and the nucleotidyltransferase, cGAS both bind DNA, mainly independent of the DNA sequence, with exceptions as described below. Signaling through cGAS or IFI16 depends on the adaptor molecule stimulator of interferon genes (STING, also known as MITA, MPYS, ERIS and TMEM173), which is localized to the endoplasmic reticulum (ER) membrane in non-activated cells. 2,16 Upon DNA binding, cGAS undergoes a conformational change, leading to activation of the enzyme and synthesis of the second messenger cyclic GMP-AMP (2′3′-cGAMP) from ATP and GTP, which is a ligand for STING. Many bacteria, including the intracellular bacteria Listeria monocytogenes, 17 also produce cyclic dinucleotides (CDN), but they contain a conventional 3′5′ linkages. These cyclic dinucleotides activate a pro-bacterial type I IFN response in mice but not humans through a STING-dependent pathway. [18] [19] [20] Importantly, the cGAS-produced 2′3′-cGAMP is a highly potent stimulator of both murine and human STING. 21, 22 cGAMP binds in a pocket at the interface of the two STING protomers, resulting in translocation of STING from the ER compartment to perinuclear autophagy-like vesicles. 22, 23 The translocation route of STING has been less defined, however recent results determine a translocation through the ER-Golgi intermediate compartments (ERGIC) and the Golgi apparatus with the activation of downstream signaling molecules taking place at or before the ERGIC. 24 TBK1 is autophosphorylated and recruited to STING, allowing the phosphorylation of STING. Interestingly, the phosphorylated residue of STING has been shown to localize in a serine cluster, which is evolutionary conserved in other adaptor molecules, including MAVS and TIR-domain-containing adapterinducing interferon-β (TRIF). The phosphorylation of STING allows the transcription factor IRF3 to dock to the phosphorylated residue, resulting in TBK1-dependent phosphorylation of IRF3 lea