C67-28 Usp4 Stabilizes the Immune Regulatory Protein Fbxo24 During Haemophilus Influenzae Type B Infection

Rational 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. Results Here, FBXO24 is protected from lysosomal degradation in THP-1 macrophage-like cells in the presence of the pathogen Haemophilus influenzae type B (HiB) by reducing its ubiquitylation levels. We identified that the ubiquitin-specific peptidase 4 (USP4) targets FBXO24 to promote its stabilization through its deubiquitylation. HiB triggered increased accumulation of FBXO24 protein levels in THP-1 macrophage-like cells mediated by USP4 in a TLR4 receptor-dependent manner. In experimental HiB pneumonia, we observed increased levels of Fbxo24 protein in lung tissue. Further, Fbxo24 heterozygous mice had increased alveolar mononuclear cells coupled with reduced bacterial loads and lung injury compared to wild-type littermates. Conclusion Collectively, these findings support 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.