Viral infection triggers induction of type I interferons (IFNs), which are critical mediators of innate antiviral immune response. Mediator of IRF3 activation (MITA, also called STING) is an adapter essential for virus-triggered IFN induction pathways. How post-translational modifications regulate the activity of MITA is not fully elucidated. In expression screens, we identified RING finger protein 26 (RNF26), an E3 ubiquitin ligase, could mediate polyubiquitination of MITA. Interestingly, RNF26 promoted K11-linked polyubiquitination of MITA at lysine 150, a residue also targeted by RNF5 for K48-linked polyubiquitination. Further experiments indicated that RNF26 protected MITA from RNF5-mediated K48-linked polyubiquitination and degradation that was required for quick and efficient type I IFN and proinflammatory cytokine induction after viral infection. On the other hand, RNF26 was required to limit excessive type I IFN response but not proinflammatory cytokine induction by promoting autophagic degradation of IRF3. Consistently, knockdown of RNF26 inhibited the expression of IFNB1 gene in various cells at the early phase and promoted it at the late phase of viral infection, respectively. Furthermore, knockdown of RNF26 inhibited viral replication, indicating that RNF26 antagonizes cellular antiviral response. Our findings thus suggest that RNF26 temporally regulates innate antiviral response by two distinct mechanisms.

Toll-like receptor (TLR)-mediated signaling are critical for host defense against pathogen invasion. However, excessive responses would cause harmful damages to the host. Here we show that deficiency of the E3 ubiquitin ligase TRIM32 increases poly(I:C)- and LPS-induced transcription of downstream genes such as type I interferons (IFNs) and proinflammatory cytokines in both primary mouse immune cells and in mice. Trim32-/- mice produced higher levels of serum inflammatory cytokines and were more sensitive to loss of body weight and inflammatory death upon Salmonella typhimurium infection. TRIM32 interacts with and mediates the degradation of TRIF, a critical adaptor protein for TLR3/4, in an E3 activity-independent manner. TRIM32-mediated as well as poly(I:C)- and LPS-induced degradation of TRIF is inhibited by deficiency of TAX1BP1, a receptor for selective autophagy. Furthermore, TRIM32 links TRIF and TAX1BP1 through distinct domains. These findings suggest that TRIM32 negatively regulates TLR3/4-mediated immune responses by targeting TRIF to TAX1BP1-mediated selective autophagic degradation.

Graphical abstract

Interferon-induced transmembrane protein 3 (IFITM3) is a restriction factor that can be induced by viral infection and interferons (IFNs). It inhibits the entry and replication of many viruses, which are independent of receptor usage but dependent on processes that occur in endosomes. In this study, we demonstrate that IFITM3 plays important roles in regulating the RNA-virus-triggered production of IFN-β in a negative-feedback manner. Overexpression of IFITM3 inhibited Sendai virus-triggered induction of IFN-β, whereas knockdown of IFITM3 had the opposite effect. We also showed that IFITM3 was constitutively associated with IRF3 and regulated the homeostasis of IRF3 by mediating the autophagic degradation of IRF3. These findings suggest a novel inhibitory function of IFITM3 on the RNA-virus-triggered production of type I IFNs and cellular antiviral responses.