LRR protein RNH1 inhibits inflammasome activation through proteasome-mediated degradation of Caspase-1 and is associated with adverse clinical outcomes in COVID-19 patients.

Inflammasomes are cytosolic innate immune sensors of pathogen infection and cellular damage that induce caspase-1 mediated inflammation upon activation. Although inflammation is protective, uncontrolled excessive inflammation can cause inflammatory diseases and can be detrimental, such as in COVID-19. However, the underlying mechanisms that control inflammasome activation are incompletely understood. Here we report that the leucine rich repeat (LRR) protein Ribonuclease inhibitor (RNH1), which shares homology with LRRs of NLRP proteins, attenuates inflammasome activation. Deletion of RNH1 in macrophages increases IL-1{beta} production and caspase-1 activation for inflammasome stimuli. Mechanistically, RNH1 decreases pro-IL-1{beta} expression and induces proteasome-mediated caspase-1 degradation. Corroborating this, mouse models of monosodium urate (MSU)-induced peritonitis and LPS-induced endotoxemia, which are dependent on caspase-1, respectively show increased neutrophil infiltration and lethality in Rnh1-/- mice compared to WT mice. Furthermore, RNH1 protein levels are negatively correlated with inflammation and disease severity in hospitalized COVID-19 patients. We propose that RNH1 is a new inflammasome regulator with relevance to COVID-19 severity.