Cutting Edge: Autoantigen Ro52 Is an Interferon Inducible E3 Ligase That Ubiquitinates IRF-8 and Enhances Cytokine Expression in Macrophages

IFN regulatory factor (IRF)-8 is a transcription factor important for the development and function of macrophages. It plays a critical role in the induction of cytokine genes, including IL-12p40. Immunopurification and mass spectrometry analysis found that IRF-8 interacted with Ro52 in murine macrophages upon IFN-gamma and TLR stimulation. Ro52 is an IFN-inducible protein of the tripartite motif (TRIM) family and is an autoantigen present in patients with Sjögren's syndrome and systemic lupus erythematosus. Ro52 has a RING motif and is capable of ubiquitinating itself. We show that IRF-8 is ubiquitinated by Ro52 both in vivo and in vitro. Ectopic expression of Ro52 enhanced IL-12p40 expression in IFN-gamma/TLR-stimulated macrophages in an IRF-8-dependent manner. Together, Ro52 is an E3 ligase for IRF-8 that acts in a non-degradation pathway of ubiquitination, and contributes to the elicitation of innate immunity in macrophages.