Neutrophils use a novel NLRP3 inflammasome-independent mechanism to release bioactive IL-1β in response to lysosomal disruption

Abstract

Abstract IL-1β is an inflammatory cytokine mainly secreted by myeloid cells in response to infection or sterile tissue damage. Non-canonical secretion of IL-1β from macrophages downstream of activated NLRP3/caspase-1 inflammasomes is the best-characterized model; this is mediated by caspase-1 cleavage of GSDMD allowing N-GSDMD to form pores in the plasma membrane that act as conduits for IL-1β release and inducers of pyroptosis as a lytic cell death. The NLRP3 initiator acts as a sensor of perturbed cellular homeostasis including decreased cytosolic [K+]. In macrophages, this K+ efflux mediated NLRP3 activation is also triggered by agents that disrupt lysosomal integrity. While neutrophils also assemble competent NLRP3 inflammasomes and release bioactive IL-1β via GSDMD-dependent mechanisms, they resist the formation of plasma membrane N-GSDMD pores and progression to pyroptosis. We tested whether lysosome disrupting stimuli in neutrophils would phenocopy macrophage mechanisms to secrete IL-1β in an NLRP3 and GSDMD-dependent manner. Surprisingly, our data indicate that lysosome-disrupting stimuli induce neutrophils to release mature IL-1β and undergo rapid lytic cell death independently of NLRP3 inflammasome assembly and caspase-1 activity with only partial dependence on GSDMD. Ongoing studies testing effects of serine protease inhibitors support a mechanism in which cytosolic accumulation of the multiple serine proteases stored in the lysosome-like azurophilic granules coordinately disrupt canonical NLRP3 inflammasome assembly and directly cleave proIL-1β and GSDMD as part of a novel neutrophil-specific signaling mechanism for lysosomal disruption-induced processing and export of IL-1β. P01-AI141350 R01-EY014362 R21-EY032662