IL-33 sensitizes mast cells to respond to a second alarmin, ATP, but not PAMPs.

Abstract

Abstract Barrier defenses must respond rapidly to pathogen infection but avoid responses to innocuous antigens or microbes. Aberrant recognition can lead to hypersensitivity and/or chronic inflammatory conditions. Barrier tissues actively release alarmins such as IL-33 in response to cell stress helping to identify pathogenic stimuli. Additional alarmin molecules such as ATP, uric acid crystals, and HMBG protein have also been identified but it remains unclear how the innate immune system integrates these multiple signals. We have used mast cells to address how IL-33 and ATP signals interact. While best known for their role in IgE-mediated allergic responses, mast cells readily produce inflammatory cytokines in response to either IL-33 or ATP. Measuring secreted IL-6, TNF alpha and IL-13 we found that bone marrow-derived and peritoneal mast cells had a 3–6-fold increased response to ATP when sensitized by pretreatment with IL-33. This synergistic effect was observed at relatively low ATP concentrations (<300μM) and required <1hr exposure to IL-33. Stimulating with PAMPs (LPS, LTA, poly I:C, ß-glucan) failed to show any enhancement with IL-33 pretreatment. Inhibitor studies indicated that the P2X3 receptor and the NFAT transcription factor mediated the ATP response. To test IL-33 and ATP synergy in vivo, we assessed eosinophil mobilization to the peritoneal cavity in mice. Consistent with the in vitro studies, co-administration of IL-33 and ATP increased eosinophil numbers compared to either alarmin alone. These results support the hypothesis that alarmins collaborate to surpass a threshold level of barrier stress necessary to initiate an inflammatory immune response.