IL‐6 and IL‐17A degradation by mast cells is mediated by a serglycin:serine protease axis

Mast cells contain large amounts of fully active proteases that are stored in complex with serglycin proteoglycan in their secretory granules. Upon degranulation, such serglycin:protease complexes are released to the extracellular space and can potentially have an impact on the local inflammatory reaction, either through direct effects of serglycin proteoglycan or through effects mediated by its bound proteases. The objective of this study was to address this scenario by investigating the possibility that serglycin‐associated proteases can regulate levels of pro‐inflammatory cytokines. Indeed, we show here that activated cultured peritoneal mast cells from wild type mice efficiently reduced the levels of exogenously administered IL‐6 and IL‐17A, whereas serglycin‐deficient mast cells lacked this ability. Furthermore, our data suggest that the reduction of IL‐6 and IL‐17A concentrations is due to proteolytic degradation mediated by serglycin‐dependent serine proteases. Moreover, we show that activated mast cells have the capacity to release IL‐6 and that the levels of this cytokine in supernatants were markedly higher in cultures of serglycin‐deficient versus serglycin‐sufficient mast cells, suggesting that serglycin‐dependent serine proteases also participate in the regulation of endogenously produced IL‐6. In summary, although the general consensus is that mast cells have a pathogenic impact on inflammatory settings, this study identifies a role for a mast cell‐derived serglycin:serine protease axis in down‐regulating levels of major inflammatory cytokines. These findings support the notion that mast cells could have a dual role in inflammatory settings, by both being able to secrete pathogenic compounds and being able to regulate their levels after release. In this study we report that, upon activation and degranulation, mast cells efficiently and specifically reduce extracellular concentrations of the inflammatory cytokines IL‐6 and IL‐17A, whereas a number of other cytokines were unaffected. Moreover, we demonstrate that the ability of mast cells to regulate the levels of IL‐6 and IL‐17A was dependent on serglycin proteoglycan and could be blocked by a serine protease inhibitor. Thus, our data strongly suggest that the mechanism behind mast cell‐mediated reduction of IL‐6 and IL‐17A involves proteolytic degradation of IL‐6 and IL‐17A mediated by the serglycin:serine protease axis.