IL-1β turnover by TRIP12 and AREL1 ubiquitin ligases and UBE2L3 limits inflammation

The cytokine interleukin-1β (IL-1β) has pivotal roles in antimicrobial immunity, but also incites inflammatory pathology. Bioactive IL-1β is released following proteolytic maturation of the pro-IL-1β precursor by caspase-1 inflammasomes. UBE2L3/UBCH7, a conserved ubiquitin conjugating enzyme, promotes pro-IL-1β ubiquitylation and proteasomal disposal. However, UBE2L3 actions in vivo and ubiquitin ligases involved in this process are unknown. Here we report that deletion of Ube2l3 in mice markedly reduces pro-IL-1β turnover in macrophages, leading to excessive mature IL-1β production, neutrophilic inflammation and disease symptoms following inflammasome activation. A family-wide siRNA screen identified two ubiquitin ligases, TRIP12 and AREL1, which we show add K27-, K29- and K33- poly-ubiquitin chains on lysine residues in the ‘pro’ domain and destabilise pro-IL-1β. Mutation of ubiquitylation sites increased pro-IL-1β stability, but did not affect proteolysis by caspase-1. The extent of mature IL-1β production is therefore determined by precursor abundance, and UBE2L3, TRIP12 and AREL1 limit inflammation by shrinking the cellular pool of pro-IL-1β. Our study has uncovered fundamental processes governing IL-1β homeostasis and provided molecular insights that could be exploited to mitigate its adverse actions in disease. ### Competing Interest Statement The authors have declared no competing interest.