The deubiquitinase USP4 stabilizes the SCFFBXO24 ubiquitin E3 ligase to regulate immunity in experimental pneumonia

The ubiquitin E3 ligase SCFFBXO24 targets key proteins for degradation that regulate important biological processes, but the mechanisms controlling cellular concentrations of its receptor subunit, F-box only protein 24 (FBXO24), remain unknown. Here, Haemophilus influenzae type B (HiB), an important cause of pneumonia, protects FBXO24 from lysosomal degradation in THP-1 macrophage cells by reducing its ubiquitylation levels. We identified that the ubiquitin-specific peptidase 4 (USP4) mediates FBXO24 deubiquitylation and stabilization in response to HiB infection in a TLR4 receptor-dependent manner. In experimental HiB pneumonia, either nanoparticle delivery of encapsulated USP4 siRNA or targeted genetic disruption of Fbxo24 in mice led to increased alveolar mononuclear cells coupled with reduced bacterial loads, attenuated pulmonary edema and decreased lung injury severity compared to control mice. Collectively, these findings demonstrate the ability of a bacterial pathogen to exploit a deubiquitylation mechanism to preserve cellular levels of an E3 ligase component, thereby impairing innate host defense responses. In this work, the authors identified a unique mechanism of molecular interplay between an E3 ligase subunit and a deubiquitinase that modulates innate immunity and severity of lung injury in experimental pneumonia.