In response to cell injury, caused, for example, by trauma, several processes must be initiated simultaneously to achieve an acute inflammatory response designed to prevent sustained tissue damage and infection and to restore and maintain tissue homeostasis. Detecting cell injury is facilitated by the fact that damaged cells release intracellular molecules not normally present in the extracellular space. However, potential underlying mechanisms for the recognition of endogenous danger signals released upon cell injury have yet to be elucidated. In this study, we demonstrate that mast cells, potent promoters of acute inflammation, play a key role in responding to cell injury by recognizing IL-33 released from necrotic structural cells. In an in vitro model of cell injury, this recognition was shown to involve the T1/ST2 receptor and result in the secretion of proinflammatory leukotrienes and cytokines by mouse mast cells. Remarkably, of all of the components released upon necrosis, our results show that IL-33 alone is a key component responsible for initiating proinflammatory responses in mast cells reacting to cell injury. Our findings identify IL-33 as a key danger signal released by necrotic structural cells capable of activating mast cells, thus providing novel insights concerning the role of mast cells as sensors of cell injury.

Key Points• IL-33-induced neutrophil recruitment in vivo is mast celldependent. This is partly mediated through the mast cell release of TNF-␣.• IL-33-treated human mast cells induce neutrophil migration in vitro. IL-33 is a recently discovered cytokine involved in induction of IntroductionIL-33 is a recently described cytokine of the IL-1 family that is expressed by a variety of cell types, most notably by epithelial and endothelial cells. 1 IL-33 promotes Th2 responses 2,3 and has been suggested to function as an alarmin when released on necrosis. 4-7 IL-33 is inactivated during apoptosis, 8 and unlike IL-1␤ and IL-18, it does not require proteolytical processing for activation. 4,9 Recently, we demonstrated that of all endogenous compounds released on necrosis, IL-33 alone is a key alarmin responsible for mast-cell activation. 5 Furthermore, IL-33 has emerged as a potent regulator of mast cell activity; inducing cytokine release, adhesion, maturation, and IgE-mediated degranulation. [10][11][12][13][14][15][16][17] In addition to mast cells, IL-33 activates several other cell types by signaling through the IL-33 receptor (IL-33R), including eosinophils, 18-20 basophils, 21-24 and dendritic cells. [25][26][27] Furthermore, a new family of IL-33R-positive innate lymphoid cells, including nuocytes, has been described during the last years. 28 Moreover, IL-33 has been implicated in the pathogenesis of several diseases (reviewed by Liew et al 1 ), including asthma 29,30 and arthritis. 31,32 Importantly, mast cells might have a central role in IL-33-associated diseases, as IL-33 for instance has been shown to exacerbate collagen-induced arthritis through a mast celldependent mechanism. 32 On the other hand, IL-33 has been ascribed several beneficial functions, including a cardioprotective function, 33 a protective role in atherosclerosis, 34 as well as an important role in helminth infections. 3 Thus, IL-33, similar to mast cells, exerts beneficial or detrimental effects depending on the local environment, which makes IL-33 an interesting cytokine with therapeutic potential.Despite this accumulated knowledge, much remains to be investigated regarding IL-33 functions in vivo. It was previously reported that IL-33 participates in the recruitment of mononuclear cells, 35 and a recent study reported that mice injected intravenously with IL-33 before cecal ligation and puncture recruited more neutrophils to the peritoneum than did mice treated with PBS, 36 thus displaying a reduced sepsis mortality rate.In this study we investigated the cellular responses to intraperitoneal IL-33 injections in mice and further addressed the responsiveness of human mast cells to IL-33. Our data demonstrate that a large proportion of the IL-33R ϩ cells in the peritoneal cavity are c-Kit ϩ Fc⑀RI ϩ mast cells, and that IL-33 induces neutrophil influx in the peritoneum of mice through a mast cell-dependent mechanism partly dependent on mast cell-derived TNF. For personal use only. on May 11, 2018. by guest www.bloodjournal.org From Methods ...