Frontline Science: Staphylococcus aureus promotes receptor-interacting protein kinase 3-and protease-dependent production of IL-1β in human neutrophils

Microbial infection elicits robust immune responses that initially depend on polymorphonuclear neutrophils (PMN), which ingest and kill invading bacteria. However, community-associated methicillin-resistant Staphylococcus aureus (CA-MRSA) remain viable within PMN and prompt their lysis with concomitant release of damage-associated molecular patterns and proinflammatory cytokines that promote additional inflammation. Here, we show that ultrapure human PMN (>99.8% pure) that have ingested CA-MRSA released interleukin (IL)-1 beta but not IL-18. The ingested CA-MRSA needed to be viable, and phagocytosis alone was insufficient to stimulate IL-1 beta secretion from PMN fed CA-MRSA. In contrast to PMN response to the canonical NLRP3 inflammasome agonist nigericin, IL-1 beta secretion by PMN fed CA-MRSA occurred independently of NLRP3 inflammasome or caspase-1 activation and required instead active receptor-interacting protein kinase 3 (RIPK3) but not RIPK1. Furthermore, inhibition of neutrophil serine proteases blocked pro-IL-1 beta cleavage in PMN fed CA-MRSA. Taken together, our data suggest that with respect to secretion of IL-1 beta and IL-18, PMN differ from human macrophages and exhibit agonist-specific responses. After phagocytosis of CA-MRSA, human PMN secreted IL-1 beta through a previously unrecognized mechanism dependent on RIPK3 and serine proteases but independent of canonical NLRP3 inflammasome and caspase-1 activation.