Gely unresponsive to the toxin (270, 271). A similar study proposed that HlgCB interacts with Toll-likereceptor 4 (TLR4) (a SCR7 molecular weight pattern recognition receptor known for recognizing lipopolysaccharide [LPS] of Gram-negative bacteria to induce inflammation) to induce the production of IL-12-p40 and tumor necrosis factor alpha (TNF- ) from murine bone marrow-derived dendritic cells (Fig. 6) (272, 273). Thus, it appears from these studies that the leucocidins are likely to engage classical pattern recognition receptors, in nonclassical ways, to induce inflammatory responses. More in-depth investigation will serve to validate these findings and determine how such altered proinflammatory signaling through TLR2 and TLR4 Pedalitin permethyl ether web impacts global immune responses to S. aureus.Inflammasome ActivationIn addition to its described proinflammatory priming of PMNs, PVL is also known to bind to both monocytes and macrophages (likely due to its recognition of C5aR on the cell surface) and elicitmmbr.asm.orgMicrobiology and Molecular Biology ReviewsS. aureus Leucocidinscellular responses (174, 253). When PVL binds to the surface of monocytes and macrophages, significant increases in IL-1 release are observed (Table 1). IL-1 is a major proinflammatory cytokine that is produced during a cellular process known as inflammasome activation (Fig. 6) (253). This cytokine can activate neutrophils and induce the expression of additional proinflammatory cytokines such as TNF and IL-6 (for detailed information on the inflammasome, the production of IL-1 , and its influence on the host response to infection, see references 274?78). One major inflammasome complex, known to respond to pore-forming toxins to induce the release of IL-1 , is the NLRP3 inflammasome (278?80). PVL-dependent induction of IL-1 release from monocytes and macrophages appears to be directly dependent on NLRP3 inflammasome activation, similar to what has been observed for alpha-hemolysin (Fig. 6) (253, 281). In support of this study, others have determined that PVL is the primary leucocidin responsible for the release of IL-1 by primary human macrophages, although it was found that HlgCB also induces IL-1 release albeit to a lesser extent (266). Leucocidin synergism with other toxic molecules produced by S. aureus was also found to effectively enhance the release of IL-1 induced by PVL, highlighting the complex nature of the inflammatory response that likely occurs during S. aureus infection (266). The increased IL-1 release by macrophages in response to a sublytic administration of PVL was shown to stimulate cocultured cells (in this case, alveolar epithelial cells) to release the proinflammatory cytokines IL-8 and macrophage chemotactic protein 1 (MCP-1) via cytokine-dependent activation of the IL-1 receptor (266). This study is particularly informative, as it directly links the functional consequences of IL-1 release by immune cells in response to PVL to the induction of proinflammatory signaling by epithelial cells, which ultimately leads to increased immune cell recruitment during infection. An important difference between the above-described two studies is their use of moderately lytic concentrations (253) versus sublytic concentrations (266) of PVL to induce IL-1 production. The fact that sublytic toxin concentrations are capable of inducing IL-1 release supports the hypothesis that toxin-mediated signaling events occur through direct cellular recognition strategies and are not simply an effect of overt to.Gely unresponsive to the toxin (270, 271). A similar study proposed that HlgCB interacts with Toll-likereceptor 4 (TLR4) (a pattern recognition receptor known for recognizing lipopolysaccharide [LPS] of Gram-negative bacteria to induce inflammation) to induce the production of IL-12-p40 and tumor necrosis factor alpha (TNF- ) from murine bone marrow-derived dendritic cells (Fig. 6) (272, 273). Thus, it appears from these studies that the leucocidins are likely to engage classical pattern recognition receptors, in nonclassical ways, to induce inflammatory responses. More in-depth investigation will serve to validate these findings and determine how such altered proinflammatory signaling through TLR2 and TLR4 impacts global immune responses to S. aureus.Inflammasome ActivationIn addition to its described proinflammatory priming of PMNs, PVL is also known to bind to both monocytes and macrophages (likely due to its recognition of C5aR on the cell surface) and elicitmmbr.asm.orgMicrobiology and Molecular Biology ReviewsS. aureus Leucocidinscellular responses (174, 253). When PVL binds to the surface of monocytes and macrophages, significant increases in IL-1 release are observed (Table 1). IL-1 is a major proinflammatory cytokine that is produced during a cellular process known as inflammasome activation (Fig. 6) (253). This cytokine can activate neutrophils and induce the expression of additional proinflammatory cytokines such as TNF and IL-6 (for detailed information on the inflammasome, the production of IL-1 , and its influence on the host response to infection, see references 274?78). One major inflammasome complex, known to respond to pore-forming toxins to induce the release of IL-1 , is the NLRP3 inflammasome (278?80). PVL-dependent induction of IL-1 release from monocytes and macrophages appears to be directly dependent on NLRP3 inflammasome activation, similar to what has been observed for alpha-hemolysin (Fig. 6) (253, 281). In support of this study, others have determined that PVL is the primary leucocidin responsible for the release of IL-1 by primary human macrophages, although it was found that HlgCB also induces IL-1 release albeit to a lesser extent (266). Leucocidin synergism with other toxic molecules produced by S. aureus was also found to effectively enhance the release of IL-1 induced by PVL, highlighting the complex nature of the inflammatory response that likely occurs during S. aureus infection (266). The increased IL-1 release by macrophages in response to a sublytic administration of PVL was shown to stimulate cocultured cells (in this case, alveolar epithelial cells) to release the proinflammatory cytokines IL-8 and macrophage chemotactic protein 1 (MCP-1) via cytokine-dependent activation of the IL-1 receptor (266). This study is particularly informative, as it directly links the functional consequences of IL-1 release by immune cells in response to PVL to the induction of proinflammatory signaling by epithelial cells, which ultimately leads to increased immune cell recruitment during infection. An important difference between the above-described two studies is their use of moderately lytic concentrations (253) versus sublytic concentrations (266) of PVL to induce IL-1 production. The fact that sublytic toxin concentrations are capable of inducing IL-1 release supports the hypothesis that toxin-mediated signaling events occur through direct cellular recognition strategies and are not simply an effect of overt to.
