Dr Feng Shao’s group identified NAIP family of NLR proteins as the immune receptors of bacterial flagellin and type III secretion apparatus to activate inflammasome-mediated anti-bacterial defenses

    April 25, 2016. Studies from Dr. Feng Shao’s laboratory at National Institute of Biological Sciences, Beijing (NIBS) show that the NAIP family of NLR proteins function as the immune receptors to recognize bacterial flagellin and type III secretion apparatus and activate inflammasome-mediated anti-bacterial defenses by using genetically knockout mice. The work entitled “Genetic functions of the NAIP family of inf­ammasome receptors for bacterial ligands in mice” is published in the Journal of Experimental Medicine.

    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 molecules, the inflammasome mediates the activation of inflammatory caspase-1, which further results in maturation of interleukin-1b (IL-1b) and interleukin-18 as well as macrophage inflammatory cell death called “pyroptosis”. Previous studies from Dr. Shao’s laboratory (Nature 2011 and PNAS 2013) report that the NAIP family of NLR proteins are inflammasome receptors for bacterial ligand molecules. While human only has one NAIP, mouse genome encodes 7 NAIPs and four of them (NAIP1, NAIP2, NAIP5 and NAIP6) are expressed in the commonly used laboratory strain. NAIP5/6 directly bind to bacterial flagellin, NAIP2 recognizes bacterial type III secretion apparatus rod proteins, and NAIP1/hNAIP are receptors for the type III secretion needle protein. NAIP engagement of these conserved bacterial molecules then recruits the downstream NLRC4 adaptor to assemble the NAIP/NLRC4 inflammasome complex and activate caspase-1-mediate inflammatory responses.

    Although above researches have demonstrated the receptor function of the NAIPs at the biochemical level, at the whole animal level it remains to be proved whether NAIP can recognize bacterial ligands and plays some important roles in inflammasome-mediated host defenses. The seven Naips in mice are located in the same genetic locus and bear more than 80% sequence homology. This poses some technical challenges in achieving individual Naip knockout in the mouse. In this study, Dr. Shao and his colleagues generated Naip5-deficient mice through conventional homologous recombination and also the Naip1 and Naip2 knockout mice using the new TALEN genome editing technology. By performing a series of functional assays in the knockout mice or macrophages derived from the knockout mice, the researchers confirmed the ligand-specific recognition property of different NAIPs. The Naip-deficient mice are specifically absent from the lethal response caused by excessive bacterial ligand-induced hyper-inflammasome activation. These experiments further reveal that the NAIPs, though highly similar in sequences, do not have functional redundancy in the mice. Infection assays performed with many different bacterial pathogens suggest that a single NAIP plays a dominant role in detecting certain bacterial pathogens while most Gram-negative pathogens are recognized by multiple NAIPs, the latter of which can help the host to cope with immune escape of the bacterial pathogen. In the mouse model of Salmonella infection, researchers from Dr. Shao’s laboratory further confirmed that the NAIPs can clear bacterial replication in mice and promote mouse survival by activating inflammasome-mediated detection of conserved bacterial products. Results reported in this study not only confirm the functional importance of the NAIP/NLRC4 inflammasome but also provide new sights into anti-bacterial therapeutics.

    PhD student Jianjin Shi from the PTN graduate program and postdoc fellow Yue Zhao from the Shao laboratory are co-first authors of this paper; PhD students Xuyan Shi and Yupeng Wang also made contributions; Dr. Fengchao Wang at NIBS transgenic facility helped to make the knockout mice. Dr. Feng Shao is the corresponding authors. The research was supported by the 973 National High-Tech. Projects, the Beijing Municipal Government, China National Science Foundation, the Strategic Priority Research Program of the Chinese Academy of Sciences and Howard Hughes Medical Institute in the States, and carried out at National Institute of Biological Sciences, Beijing.

http://jem.rupress.org/content/early/2016/04/19/jem.20160006