SLPI suppresses ATP-mediated release of IL-1β from human monocytes – evidence for a novel anti-inflammatory mechanism

Background: IL-1β is a multifunctional pro-inflammatory cytokine associated with diverse inflammatory disorders. IL-1β release requires two consecutive danger signals. LPS is a prototypical first signal inducing pro-IL-1β synthesis, whereas extracellular ATP is a typical second signal triggering inflammasome activation, proteolytic cleavage of pro-IL-1β by activated caspase-1, and release of mature IL-1β. Secretory leukocyte protease inhibitor (SLPI), in addition to its well-known function as major anti-protease of the lung, possesses potent anti-microbial and anti-inflammatory properties. Here, we test the hypothesis that IL-1β release is inhibited by SLPI.

Methods: LPS-primed monocytic U937 cells were stimulated with BzATP or with the bacterial pore-forming toxin, nigericine, in the presence and absence of SLPI. IL-1β release was monitored by ELISA. Furthermore, pharmacological inhibitors and a siRNA approach were used to investigate the signal transduction pathway involved in the control of inflammasome activation.

Results: We demonstrated that SLPI inhibited ATP-mediated inflammasome activation in a dose-dependent manner, whereas nigericine-induced IL-1β release was unimpaired. Using a panel of receptor-specific nicotinic antagonists let us identify involvement of nicotinic acetylcholine receptors containing subunits α7, 9 and/or 10 in the SLPI-dependent inhibition of IL-1β release. These results were further corroborated by siRNA silencing of these subunits. Moreover, we showed that this mechanism is dependent on Src kinase activity. Overexpression of dominant negative Src or application of PP2, a Src inhibitor, abolished the inhibitory effect of SLPI.

Conclusions: We propose a novel anti-inflammatory mechanism, induced by SLPI, which efficiently inhibits ATP-dependent secretion of IL-1β.

*Presenting author