The ligand preferences of innate immune receptors RIG-I and LGP2

The innate immunity serves as the first line against pathogen defense. Many receptors and factors play important roles in the innate immune response. A receptor family, retinoic-acid-inducible gene I (RIG-I)-like receptors (RLRs) consist of three members, RIG-I, MDA5 and LGP2. RIG-I specifically distinguishes viral RNAs in a diverse cellular RNA environment. When activated by these RNAs, RIG-I triggers downstream pathways and induces innate immune responses such as interferon production to establish an anti-viral state in host cells. Based on previous studies, the 5’ triphosphorylated (5’ppp) blunt-ended RNA bearing a double-strand (ds) panhandle structure at the 5’ end is the ligand for RIG-I. This 5’ triphosphorylation is often seen in viral RNAs. Cellular RNAs, such as messenger RNAs, are usually “capped” at the 5’ end with a 7-methyl guanidine (m7G) via a triphosphate bridge (cap-0). Since the triphosphate is “protected” by the m7G, it was believed that the structural basis of RNA discrimination by RIG-I was the presence of m7G. However, our biochemical, biophysical and cell signaling studies show that RIG-I can recognize dsRNA with cap-0 to a similar extent as 5’ppp dsRNA. Structural data of RIG-I in complex with Cap-0 RNA reveals the ability of RIG-I to accommodate the m7G moiety. In contrast, dsRNA with m7G and an additional 2’-O-methyl group on the 5’ end nucleotide ribose (cap-1) does not stimulate RIG-I and abrogates RIG-I signaling through a mechanism involving residue His830. This histidine is critical for RNA discrimination by RIG-I. Unlike RIG-I, the role of the innate immune receptor LGP2 is not well understood. Lacking domains for signaling, LGP2 is believed to be a regulator of RIG-I and MDA5. Characteristics of nucleic acid binding by LGP2 are not understood, and its relationship with RIG-I needs further studies. Our biochemical studies of LGP2 show higher ATP turnover rate in the presence of 5’OH dsRNA than viral RNA mimics, and the binding affinity of LGP2 with 5’OH dsRNA is relatively tight. 5’OH dsRNA has been shown to activate RIG-I, indicating an overlap in ligands between LGP2 and RIG-I. Competition assays show 5’OH dsRNA preferentially interacts with LGP2, suggesting a mechanism in which LGP2 sequesters non-PAMP ligand of RIG-I to diminish misactivations. Overall, the studies provide further understanding of the early steps in the anti-viral process from the activation stage, which could aid in vaccine development and the design of compound agonist to complement approaches for developing RLR based therapeutics.Ph.D.Includes bibliographical referencesby Chen Wan