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+ and Cl− efflux and led to ASC oligomerization, caspase-1 and Gasdermin-D (GSDMD) cleavage. IL-18 release was impaired by the NLRP3 inhibitor MCC950 and by the absence of NLRP3, but also by deficiency of GSDMD, suggesting that pyroptosis is the mechanism of release. This work highlights the readiness of the NLRP3 inflammasome to assemble in the absence of priming in human monocytes and hence contribute to the very early stages of the inflammatory response when IL-1β has not yet been produced. It is important to consider the unprimed setting when researching the mechanisms of NLRP3 activation, as to not overshadow the pathways that occur in the absence of priming stimuli, which might only enhance this response.
Highlights
Inflammasomes are molecular complexes formed by immune cells such as macrophages and monocytes in response to tissue injury or infection [1]
To determine if NOD-like receptor pyrin domain-containing protein 3 (NLRP3) inflammasome activation occurs in human monocytes in the absence of priming, we first compared the release of mIL-18 and mIL-1b, as pro-IL-18 gene expression is constitutive, while pro-IL-1b requires upregulation by priming [23]
Our work shows that human monocytes are able to assemble a functional NLRP3 inflammasome in response to an activating signal even in the absence of a priming step
Summary
Inflammasomes are molecular complexes formed by immune cells such as macrophages and monocytes in response to tissue injury or infection [1]. We report that primary human monocytes are equipped with a ready to assemble NLRP3 inflammasome that leads to the processing and release of constitutively expressed caspase-1 substrates such as IL-18 and GSDMD in response to just an activating signal in a sterile setting. This pathway is dependent on NLRP3, ion efflux and PTMs, which are hallmarks of the classical primed inflammasome activation. These results highlight the fact that priming might not be as necessary as initially thought for the assembly of an active inflammasome it is required to potentiate the inflammasome response and generate IL-1b dependent inflammatory pathways
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