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

Abstract Interleukin 1-beta (IL-1β) is an inflammatory cytokine produced by myeloid cells in response to infection or sterile tissue damage. Secretion of bioactive IL-1β from macrophages (Mφ) or dendritic cells (DC) downstream of activated NLRP3/caspase-1 inflammasomes is the best characterized model; this is mediated by caspase-1 cleavage of proIL-1β and Gasdermin D. Gasdermin D pores that form in the plasma membrane mediate IL-1β release and pyroptotic cell death. NLRP3 inflammasome assembly is triggered by perturbation of ionic, metabolic or organelle homeostasis via diverse stimuli. A recent report demonstrated that NLRP3 activators in Mφ/DC include tyrosine kinase inhibitors such as imatinib mesylate used as frontline chemotherapeutics for chronic myelogenous leukemia (CML). This action of imatinib was initiated by lysosomal membrane permeabilization (LMP). As CML is characterized by high numbers of circulating immature granulocytes and neutrophils, we assessed the effects of imatinib on NLRP3 inflammasome signaling in murine and human neutrophils. We report that imatinib-treated neutrophils can process and release IL-1β independently of NLRP3 inflammasome assembly and the expression/activity of caspase-1 or Gasdermin D. Mechanistically, imatinib induces azurophilic granule permeabilization to drive robust cytosolic accumulation of granule-derived neutral serine proteases, serine protease-mediated processing of proIL-1β, and release of mature IL-1β. Together these findings elucidate a novel mechanism by which disruption of neutrophil granules can bypass the NLRP3 inflammasome pathway to drive serine protease-mediated IL-1β processing and release.