Induction of the Inflammasome by the SARS-CoV-2 Accessory Protein ORF9b, Abrogated by Small-Molecule ORF9b Homodimerization Inhibitors

Viral accessory proteins play critical roles in viral escape from host innate immune responses and in viral inflammatory pathogenesis. Here we show that the SARS-CoV-2 accessory protein, ORF9b, but not other SARS-CoV-2 accessory proteins (ORF3a, ORF3b, ORF6, ORF7, ORF8, ORF9c, and ORF10), strongly activates inflammasome-dependent caspase-1 in A549 lung carcinoma cells and THP-1 monocyte-macrophage cells. Exposure to lipopolysaccharide (LPS) and ATP additively enhanced the activation of caspase-1 by ORF9b, suggesting that ORF9b and LPS follow parallel pathways in the activation of the inflammasome and caspase-1. Following rational in silico approaches, we have designed small molecules capable of inhibiting the homodimerization of ORF9b, which experimentally inhibited ORF9b-ORF9b homotypic interactions, caused mitochondrial eviction of ORF9b, inhibited ORF9b-induced activation of caspase-1 in A549 and THP-1 cells, cytokine release in THP-1 cells, and restored type I interferon (IFN-I) signaling suppressed by ORF9b in both cell models. These small molecules are first-in-class compounds targeting a viral accessory protein critical for viral-induced exacerbated inflammation and escape from innate immune responses, with the potential of mitigating the severe immunopathogenic damage induced by highly pathogenic coronaviruses and restoring antiviral innate immune responses curtailed by viral infection.