Dr. Feng Shao's group identified NLR family members human NAIP and mouse NAIP1 and their role in inflammasome-mediated innate immunity.

    On Aug. 12, 2013, Studies from Dr. Feng Shao's laboratory at National Institute of Biological Sciences, Beijing (NIBS) identify human NAIP and mouse NAIP1, members of NOD-like receptor (NLR) family, that specifically recognize bacterial type III secretion needle proteins and activate inflammasome-mediated innate immunity. The work entitled "Human NAIP and mouse NAIP1 recognize bacterial type III secretion needle protein for inflammasome activation" is published in the journal of Proc Natl Acad Sci USA.

    The inflammasome pathway is an emerging innate immune system as important as the well-characterized Toll-like receptor (TLR) pathway. Upon sensing of pathogen-derived or endogenous danger molecules, the inflammasome mediates the activation of inflammatory caspase-1, which further results in maturation of interleukin-1β(IL-1β) and interleukin-18 as well as macrophage inflammatory death called "pyroptosis". The inflammasome plays a critical role in immune defense against various microbial infections. Assembly of the inflammasome complex is believed to be mediated by cytosolic NOD-like receptor (NLR) family that contains 23 members in humans and 34 members in mice. Many of NLR family are functionally unknown.

    A previous study from Dr. Shao's laboratory (Nature 2011) for the first time reports that the NAIP family of NLR proteins are inflammasome receptors that directly recognize bacterial molecules. Mouse genome encodes 7 NAIPs and four of them (NAIP1, NAIP2, NAIP5 and NAIP6) are known to be expressed in the commonly used laboratory strain. NAIP5 and NAIP6 directly bind to bacterial flagellin and NAIP2 recognizes bacterial type III secretion rod proteins, which mediates caspase-1 activation through direct interaction with another NLR protein NLRC4. The function of NAIP1 and sole NAIP family member in human (hNAIP) was unknown before this study.

    In this study, Dr. Shao and his colleagues discover that bacterial type III secretion needle proteins from various bacterial pathogens all can induce inflammasome activation in both human and mouse macrophage cells. Through a series of genetic and biochemical experiments, they demonstrate that hNAIP and mouse NAIP1 function as the receptors that directly recognize cytosolic bacterial type III secretion needle proteins, thereby stimulating inflammasome assembly, caspase-1 activation and IL-1β production. They further show that NAIP-mediated sensing of type III secretion needle proteins, upon infection by Gram-negative bacterial pathogens such as Shigella flexneri and Salmonella typhimurium, is a general innate immune response in both human monocyte-derived and mouse bone marrow macrophages.

    Dr. Shao's team also discovers that different NAIPs have differential expression level in different types of mouse immune cells. For example, NAIP1 is expressed higher in dendritic cells than in macrophages, while the situation with NAIP5 is reverse. Consistently, inflammasome activation by the needle protein in dendritic cells is more robust than in macrophages. They further notice that the inflammasome-stimulation activity of a certain NAIP ligand is also highly dependent on the bacterium that bears it and bacterial infection-induced NLRC4 inflammasome activation is often a mixture contribution from multiple NAIP receptors. These not only lead to demonstration that NAIP1 sensing of the needle protein dominates S. flexneri-induced inflammasome activation but also establish a new framework for better understating bacterial infection-induced inflammation.

    This study for the first time identifies the immunological function of hNAIP and mouse NAIP1 and further confirms that NAIP-family NLR proteins are cytosolic innate immune receptors for bacteria-derived molecules. The study also uncovers a general immunogenic activity of bacterial type III secretion needle protein in both human and mouse cells, providing guidance in vaccine and adjuvant development.

    PhD student Jieling Yang and postdoc fellow Yue Zhao are co-first authors of this paper; PhD student Jianjin Shi also made important contributions. Dr. Feng Shao is the corresponding author. The study was supported by the 973 National High-Tech. Projects as well as by funding from the Beijing Municipal Government, and carried out at National Institute of Biological Sciences, Beijing.