Pathogen Recognition Receptors and Innate Immunity

Shizuo Akira
Laboratory of Host Defense, WPI Immunology Frontier Research Center, Osaka University

Immune response is broadly categorized into innate and adaptive immunity. The innate immune system is an evolutionally conserved host defense mechanism against pathogens. Innate immune responses are initiated by pattern recognition receptors (PRRs), which recognize specific structures of microorganisms. Among them, Toll-like receptors (TLRs) are capable of sensing organisms ranging from bacteria to fungi, protozoa and viruses, and play a major role in innate immunity. Individual TLRs recognize different microbial components, and give rise to different patterns in gene expression. Recently, cytoplasmic PRRs have been identified to detect pathogens that invade the cytosol. Two DExD/H box RNA helicases, retinoic acid inducible protein-I (RIG-I) and melanoma differentiation-associated gene 5 (mda-5) are involved in anti-viral responses by recognizing RNA in the cytoplasm. RIG-I and mda-5 recognize distinct RNA viruses. Bacterial components are also recognized by NOD-like receptors (NLRs) in the cytoplasm. Subsets of NLRs assemble and oligomerize into a common structure named inflammasome which activates caspase-1, resulting in IL-1beta production. I will talk about a recent progress in innate immunity.