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)‐1β but not IL‐18. The ingested CA‐MRSA needed to be viable, and phagocytosis alone was insufficient to stimulate IL‐1β secretion from PMN fed CA‐MRSA. In contrast to PMN response to the canonical NLRP3 inflammasome agonist nigericin, IL‐1β 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‐1β cleavage in PMN fed CA‐MRSA. Taken together, our data suggest that with respect to secretion of IL‐1β and IL‐18, PMN differ from human macrophages and exhibit agonist‐specific responses. After phagocytosis of CA‐MRSA, human PMN secreted IL‐1β through a previously unrecognized mechanism dependent on RIPK3 and serine proteases but independent of canonical NLRP3 inflammasome and caspase‐1 activation.