Primary Human Macrophages Is Dependent Cutting Edge: Inflammasome Activation in

Murine NLR family, apoptosis inhibitory protein (Naip)1, Naip2, and Naip5/6 are host sensors that detect the cytosolic presence of needle and rod proteins from bacterial type III secretion systems and flagellin, respectively. Previous studies using human-derived macrophage-like cell lines indicate that human macro-phages sense the cytosolic needle protein, but not bacterial flagellin. In this study, we show that primary human macrophages readily sense cytosolic flagellin. Infection of primary human macrophages with Salmonella elicits robust cell death and IL-1b secretion that is dependent on flagellin. We show that flagellin detection requires a full-length isoform of human Naip. This full-length Naip isoform is robustly expressed in primary macro-phages from healthy human donors, but it is drastically reduced in monocytic tumor cells, THP-1, and U937, rendering them insensitive to cytosolic flagellin. However , ectopic expression of full-length Naip rescues the ability of U937 cells to sense flagellin. In conclusion, human Naip functions to activate the inflammasome in response to flagellin, similar to murine Naip5/6. W hereas certain aspects of innate immunity can be regarded as nonspecific, multicellular organisms have evolved specific recognition mechanisms as part of their defense system to respond to conserved bacterial structures and facilitate the clearance of invading pathogens (1). Flagella are surface appendages that facilitate bacterial motility and are required for the facultative intracellu-lar pathogens Salmonella enterica serovars Typhimurium (S. Typhimurium) and Typhi (S. Typhi) to target host cells in the intestinal epithelium (2). During bacterial infections, conserved structures of the flagella-forming protein flagellin are recognized by several host pattern recognition receptors (1). Extracellular flagellin is recognized by the cell-surface sensor TLR5 (3), which promotes the activation of NF-kB and subsequent secretion of IL-8 (2). Flagellin monomers are also translocated into the host cell cytosol by a mechanism that requires bacterial secretion systems. For example, during Salmonella infections, flagellin is translocated into the host cell cytosol by the Salmonella pathogenicity island 1 (SPI-1) type III secretion system (T3SS) (4). In mice, this triggers the formation of the NLR family CARD domain– containing protein (NLRC)4 inflammasome (5, 6), which promotes two major caspase-1–dependent events: release of the proinflammatory cytokines IL-1b and IL-18, and the induction of a proinflammatory form of cell death termed pyroptosis (7). The NLRC4 inflammasome can also detect the rod (PrgJ) and needle (PrgI) proteins of the SPI-1 T3SS apparatus (8, 9). The ability of mice to sense flagellin and components of the T3SS is facilitated by …