HOIL‐1‐catalysed, ester‐linked ubiquitylation restricts IL‐18 signaling in cytotoxic T cells but promotes TLR signalling in macrophages.

The atypical E3 ligase HOIL‐1 forms ester bonds between ubiquitin and serine/threonine residues in proteins, but the physiological roles of this unusual modification are unknown. We now report that IL‐18 signalling leading to the production of interferon γ (IFNγ) and granulocyte–macrophage colony‐stimulating factor (GM‐CSF) is enhanced in cytotoxic T cells from knock‐in mice expressing the E3 ligase‐inactive HOIL‐1[C458S] mutant, demonstrating that the formation of HOIL‐1‐catalysed ester‐linked ubiquitin bonds restricts the activation of this pathway. We show that the interaction of IRAK2 with TRAF6 is required for IL‐18‐stimulated IFN‐γ and GM‐CSF production, and that the increased production of these cytokines in cytotoxic T cells from HOIL‐1[C458S] mice correlates with an increase in both the number and size of the Lys63/Met1‐linked hybrid ubiquitin chains attached to IRAK2 in these cells. In contrast, the secretion of IL‐12 and IL‐6 and the formation of il‐12 and il‐6 mRNA induced in bone marrow‐derived macrophages (BMDMs) by prolonged stimulation with TLR‐activating ligands that signal via myddosomes, which also requires the interaction of IRAK2 with TRAF6, were not increased but modestly reduced in HOIL‐1[C458S] BMDM. The decreased production of these cytokines correlated with reduced ubiquitylation of IRAK2. Our results establish that changes in HOIL‐1‐catalysed ester‐linked ubiquitylation can promote or reduce cytokine production depending on the ligand, receptor and immune cell and may be explained by differences in the ubiquitylation of IRAK2. [ABSTRACT FROM AUTHOR]