Sepsis remains a global clinical problem. By using the mouse cecal ligation and puncture model of sepsis, here we identify an important aspect of mast cell (MC)-dependent, innate immune defenses against Gram-negative bacteria by demonstrating that MC protease activity is regulated by interleukin-15 (IL-15). Mouse MCs express both constitutive and lipopolysaccharide-inducible IL-15 and store it intracellularly. Deletion of Il15 in mice markedly increases chymase activities, leading to greater MC bactericidal responses, increased processing and activation of neutrophil-recruiting chemokines, and significantly higher survival rates of mice after septic peritonitis. By showing that intracellular IL-15 acts as a specific negative transcriptional regulator of a mouse MC chymase (mast cell protease-2), we provide evidence that defined MC protease activity is transcriptionally regulated by an intracellularly retained cytokine. Our results identify an unexpected breach in MC-dependent innate immune defenses against sepsis and suggest that inhibiting intracellular IL-15 in MCs may improve survival from sepsis.

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Thus, we suggest that miR-155 plays a critical role in FcεRI-mediated MC responses by modulating components of the PI3Kγ pathway. This newly identified mechanism of miRNA-controlled MC activation may affect the initiation and maintenance of allergic disorders.

Mast cells (MCs) play a critical role in innate and adaptive immunity through the release of cytokines, chemokines, lipid mediators, biogenic amines, and proteases. We recently showed that the activities of MC proteases are transcriptionally regulated by intracellularly retained interleukin-15 (IL-15), and we provided evidence that this cytokine acts as a specific regulator of mouse mast cell protease-2 (mMCP-2). Here, we show that in wild-type bone marrow-derived mast cells (BMMCs) IL-15 inhibits mMCP-2 transcription indirectly by inducing differential expression and mMCP-2 promoter binding of the bifunctional transcription factors C/EBP␤ and YY1. In wild-type BMMCs, C/EBP␤ expression predominates over YY1 expression, and thus C/EBP␤ preferentially binds to the mMCP-2 promoter. In IL-15-deficient BMMCs, the opposite is found: YY1 expression predominates and binds to the mMCP-2 promoter at the expense of C/EBP␤. Hypertranscription of the mMCP-2 gene in IL-15-deficient BMMCs is associated with histone acetylation and, intriguingly, with methylation of non-CpG dinucleotides within the MCP-2 promoter. This suggests a novel model of cytokine-controlled protease transcription: non-CpG methylation maintains a chromosomal domain in an "open" configuration that is permissive for gene expression. Mast cells (MCs)3 are key cellular mediators of host innate defense (1). Upon activation, MC secretory granules release a number of preformed inflammatory molecules that include cytokines, histamine, serglycin proteoglycans, and several MC-specific proteases (chymases, tryptases, and carboxypeptidase A) (2, 3). Upon release into the extracellular space, these mediators regulate a broad variety of responses ranging from acute phase immediate reactions over recruitment of specific cell populations to sites of infection to long term tissue-remodeling reactions (1).In the mouse, several mast cell proteases (mMCPs) have been identified and partially characterized, including the chymases mMCP-1, -2, -4, -5, and -9 and the tryptases mMCP-6 and -7 (4). Proteases play an important role in extracellular matrix remodeling, extravascular coagulation, and fibrinolysis as well as angiogenesis. Our recent investigation of the mechanisms that regulate mMCP gene expression has shown that intracellular cytokines can regulate mMCP activities (5). Murine MCs express constitutive and lipopolysaccharide-inducible IL-15 and store IL-15 intracellularly, where it colocalizes with tumor necrosis factor-␣ and proteases in MC granula. IL-15 Ϫ/Ϫ MCs exhibit markedly elevated chymase activity, leading to increased bactericidal activity and processing of neutrophil-recruiting chemokines. In particular, intracellular IL-15 operates as a specific negative transcriptional regulator of mMCP-2 (5).In this report, we show that transcriptional repression of mMCP-2 by intracellular IL-15 is indirect. IL-15-mediated repression is effected by changing the expression levels of the transcription factors (TFs) C/EBP␤ and YY1 and thus the amount of each TF bound to the mMC...