Identification and characterization of novel inflammasome molecules and pathways
Final Report Abstract
Rapid detection of intracellular pathogens is crucial for the innate immune system to establish a robust response towards potentially harmful bacteria. Murine Naip (NLR family, apoptosis inhibitory protein) 1, Naip2 and Naip5/6 are host sensors that detect the cytosolic presence of bacterial type III secretion system (T3SS) needle, T3SS rod protein and flagellin, respectively. In mice, this triggers formation of the NLRC4 (NLR family CARD domain-containing protein) inflammasome. NLRC4 acts as a platform for activation of Caspase-1, which promotes two major events: release of the proinflammatory cytokines IL-1and IL-18, and the induction of a pro-inflammatory form of cell death termed pyroptosis. Previous studies using human-derived macrophage-like cell lines indicate that human macrophages sense the cytosolic needle protein, but not bacterial flagellin. In this DFG project report we show that primary human macrophages readily sense cytosolic flagellin. Infection of primary human macrophages with Salmonella elicits robust cell death and IL-1secretion that is dependent on flagellin. We provide evidence that flagellin-detection requires a full-length isoform of the human Naip sensor. This full-length Naip isoform is robustly expressed in primary macrophages from healthy human donors, but drastically (>30 fold) reduced in monocytic tumor cells, U937 cells, rendering them insensitive to cytosolic flagellin. However, ectopic expression of the Naip isoform rescues the ability of U937 cells to sense flagellin. Strikingly, the depletion of the human Naip isoform dramatically reduced inflammasome activation in response to intracellular flagellin delivered via the anthrax Protective Antigen pore or by Salmonella infection. In conclusion, human Naip functions to activate the inflammasome in response to flagellin, similar to murine Naip5/6. These results provide compelling evidence that primary human macrophages are equipped with enhanced defense mechanisms. The identification of different human inflammasome receptor isoforms originating from a single gene could be a major step in understanding the diverse innate immune pathways. These findings could lead to new treatments for infectious diseases and to a better understanding of the pathogenesis of severe autoinflammatory diseases.
Publications
- (2013) Policing the cytosol—bacterial-sensing inflammasome receptors and pathways. Curr Opin Immunol, 25(1), 34-39
Ng TM, Kortmann J, Monack DM
