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 that, upon activation, induce caspase-1 mediated inflammation. Although inflammation is protective, uncontrolled excessive inflammation can cause inflammatory diseases and is also detrimental in COVID-19 infection. 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 NOD-like receptor family pyrin domain (PYD)-containing (NLRP) proteins, attenuates inflammasome activation. Mechanistically, RNH1 decreased pro-IL1{beta} expression and induced 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 showed increased neutrophil infiltration and lethality in Rnh1-/- mice compared to WT mice. Further, RNH1 protein levels were 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.