ABSTRACT
The innate immune system employs a limited number of germ line-encoded pattern recognition receptors (PRR) that sense molecular patterns such as microbial molecules and unphysiological concentrations or structures of self molecules. Sensing of molecular patterns leads to cell autonomous defense mechanisms and triggers innate and adaptive immune responses. The family of RIG-I (retinoic acid-inducible gene I) like receptors (RLR) comprises RIG-I, MDA5 and Lgp2 which control the immune recognition of most pathogenic RNA viruses. These highly specialized immunosensors are expressed in the cytosol ofboth immune cells and non-immune cells and sense viral nucleic acids. As viral RNA is located in the same cellular compartment as host RNA, RLRs must be equipped to discriminate between viral and self RNA. Specifications of RLR ligands include nucleic acid structure, backbone modifications, and unusual cellular locations of nucleic acids. Here we review the tremendous progress that has been made towards defining the molecular characteristics of RLR ligands.
