Perturbation of azurophilic granule integrity drives NLRP3-independent IL-1β processing and release in neutrophils

IL-1β is an inflammatory cytokine secreted by myeloid cells in response to infection or sterile tissue damage. Non-canonical secretion of IL-1β from monocytes downstream of activated NLRP3 inflammasomes is the best-characterized model; this is mediated by caspase-1 cleavage of GSDMD and N-GSDMD plasma membrane (PM) pore formation to permit release of IL-1β and induction of pyroptosis. NLRP3 is a cytosolic sensor of cellular homeostasis including [K+]. In monocytes, K+ efflux mediated NLRP3 activation is triggered by agents that disrupt lysosomal integrity. Neutrophils assemble competent NLRP3 inflammasomes and release IL-1β via GSDMD-dependent mechanisms, but they resist N-GSDMD PM pore formation and pyroptosis. We tested whether lysosome-disrupting stimuli in neutrophils would phenocopy the monocyte mechanism of NLRP3-dependent IL-1β release. Surprisingly, our data indicates that lysosome disruption induces release of mature IL-1β and lytic cell death in an NLRP3-independent manner. Kinetic analysis demonstrates an alternate mechanism of release such that early phase IL-1β release is NLRP3-dependent, but prolonged stimulation leads to NLRP3-independent release. Collectively, our data supports a signaling mechanism by which azurophilic granule disruption and cytosolic accumulation of neutrophil serine proteases disrupt canonical NLRP3 inflammasome assembly and directly cleave proIL-1β as part of a novel neutrophil-specific signaling mechanism of IL-1β processing and export.