Priming Is Dispensable for NLRP3 Inflammasome Activation in Human Monocytes In Vitro

TLDR

IL‑18 and IL‑1β are pro‑inflammatory cytokines produced as inactive precursors that are activated by inflammasomes, and NLRP3 inflammasome activation is traditionally thought to require a priming step that upregulates NLRP3 and IL‑1β expression before assembly and caspase‑1–mediated cleavage. Human monocytes can form canonical NLRP3 inflammasomes and release IL‑18 in response to nigericin without prior priming, whereas monocyte‑derived macrophages cannot.

Abstract

Interleukin (IL)-18 and IL-1β are potent pro-inflammatory cytokines that contribute to inflammatory conditions such as rheumatoid arthritis and Alzheimer's disease. They are produced as inactive precursors that are activated by large macromolecular complexes called inflammasomes upon sensing damage or pathogenic signals. NLRP3 inflammasome activation is regarded to require a priming step that causes NLRP3 and IL-1β gene upregulation, and also NLRP3 post-translational licencing. A subsequent activation step leads to the assembly of the complex and the cleavage of pro-IL-18 and pro-IL-1β by caspase-1 into their mature forms, allowing their release. Here we show that human monocytes, but not monocyte derived macrophages, are able to form canonical NLRP3 inflammasomes in the absence of priming. NLRP3 activator nigericin caused the processing and release of constitutively expressed IL-18 in an unprimed setting. This was mediated by the canonical NLRP3 inflammasome that was dependent on K