The superfamily of tripartite motif-containing (TRIM) proteins is conserved throughout the metazoan kingdom and has expanded rapidly during vertebrate evolution; there are now more than 60 TRIM proteins known in humans and mice. Many TRIM proteins are induced by type I and type II interferons, which are crucial for many aspects of resistance to pathogens, and several are known to be required for the restriction of infection by lentiviruses. In this Review, we describe recent data that reveal broader antiviral and antimicrobial activities of TRIM proteins and discuss their involvement in the regulation of pathogen-recognition and transcriptional pathways in host defence.

Dengue is one of the most important arboviral diseases caused by infection of four serotypes of dengue virus (DEN). We found that activation of interferon regulatory factor 3 (IRF3) triggered by viral infection and by foreign DNA and RNA stimulation was blocked by DEN-encoded NS2B3 through a protease-dependent mechanism. The key adaptor protein in type I interferon pathway, human mediator of IRF3 activation (MITA) but not the murine homologue MPYS, was cleaved in cells infected with DEN-1 or DEN-2 and with expression of the enzymatically active protease NS2B3. The cleavage site of MITA was mapped to LRR↓ 96 G and the function of MITA was suppressed by dengue protease. DEN replication was reduced with overexpression of MPYS but not with MITA, while DEN replication was enhanced by MPYS knockdown, indicating an antiviral role of MITA/MPYS against DEN infection. The involvement of MITA in DEN-triggered innate immune response was evidenced by reduction of IRF3 activation and IFN induction in cells with MITA knockdown upon DEN-2 infection. NS2B3 physically interacted with MITA, and the interaction and cleavage of MITA could be further enhanced by poly(dA:dT) stimulation. Thus, we identified MITA as a novel host target of DEN protease and provide the molecular mechanism of how DEN subverts the host innate immunity.

The tripartite motif (TRIM) family member, TRIM21, is an E3 ubiquitin ligase for IFN regulatory factor (IRF)3 and IRF8 that functions in both innate and acquired immunity. It is also an autoantigen known as Ro52/SS-A. The function of TRIM21 in vivo, however, has remained elusive. We generated Trim21−/− mice with the Trim21 gene replaced by an enhanced GFP (EGFP) reporter. EGFP expression analyses showed that Trim21 was widely expressed in many tissues, with the highest levels in immune cells. Studies of Trim21−/− embryonic fibroblasts demonstrated that TLR-mediated induction of proinflammatory cytokines, including IL-1β, IL-6, TNF-α, and CXCL10, was consistently up-regulated relative to wild-type cells. Reporter analyses demonstrated that TLR-mediated NF-κB activation was higher in Trim21−/− cells than in wild-type cells, most likely accounting for their enhanced cytokine expression. In contrast, functional analyses of immune cells from Trim21−/− mice revealed no abnormalities in their composition or function, even though ubiquitylation of IRF3 and IRF8 was impaired. These results suggested possible redundancies in activities mediated by TRIM21. In keeping with this concept, we found that a number of TRIM family members were up-regulated in Trim21−/− cells. Taken together, these findings demonstrate that TRIM21 plays a previously unrecognized role in the negative regulation of NF-κB-dependent proinflammatory cytokine responses, and suggest that multiple TRIM proteins contribute to the maintenance of functional equilibrium in inflammatory responses, in part through functional redundancy.

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Interferon regulatory factor 7 (IRF7) is a potent transcription factor of type I interferons (IFNs) and IFN stimulated genes (ISGs) and is known as the master regulator of type I IFN-dependent immune responses. Because excessive responses could harm the host, IRF7 itself is delicately regulated at the transcriptional, translational, and posttranslational levels. Modification of IRF7 by small ubiquitin-related modifiers (SUMOs) has been shown to regulate IFN expression and antiviral responses negatively, but the specific E3 ligase needed for IRF7 SUMOylation has remained unknown. As reported here, we have identified the tripartite motif–containing (TRIM) protein 28 (TRIM28) as a binding partner of IRF7. We have demonstrated that TRIM28 also interacts with the SUMO E2 enzyme and increases SUMOylation of IRF7 both in vivo and in vitro, suggesting it acts as a SUMO E3 ligase of IRF7. Unlike the common SUMO E3 ligase, protein inhibitor of activated STAT 1(PIAS1), the E3 activity of TRIM28 is specific to IRF7, because it has little effect on IRF7’s close relative IRF3. TRIM28 is therefore, so far as we know, the first IRF7-specific SUMO E3 reported. TRIM28-mediated SUMOylation of IRF7 is increased during viral infection, and SUMOylation of transcription factors usually results in transcriptional repression. Overexpression of TRIM28 therefore inhibits IRF7 transactivation activity, whereas knockdown of TRIM28 has the opposite effect and potentiates IFN production and antiviral responses. Collectively, our results suggest that TRIM28 is a specific SUMO E3 ligase and negative regulator of IRF7.

Viral infection activates

Viperin is identified as an antiviral protein induced by interferon (IFN), viral infections, and pathogenassociated molecules. In this study, we found that viperin is highly induced at the RNA level by Japanese encephalitis virus (JEV) and Sindbis virus (SIN) and that viperin protein is degraded in JEV-infected cells through a proteasome-dependent mechanism. Promoter analysis revealed that SIN induces viperin expression in an IFN-dependent manner but that JEV by itself activates the viperin promoter through IFN regulatory factor-3 and AP-1. The overexpression of viperin significantly decreased the production of SIN, but not of JEV, whereas the proteasome inhibitor MG132 sustained the protein level and antiviral effect of viperin in JEVinfected cells. Knockdown of viperin expression by RNA interference also enhanced the replication of SIN, but not that of JEV. Our results suggest that even though viperin gene expression is highly induced by JEV, it is negatively regulated at the protein level to counteract its antiviral effect. In contrast, SIN induces viperin through the action of IFN, and viperin exhibits potent antiviral activity against SIN.Viperin, an interferon (IFN)-induced cellular antiviral protein, was identified originally as cig5 in human cytomegalovirus (HCMV)-infected human fibroblasts (53). A similar gene called vig-1 was identified subsequently in viral hemorrhagic septicemia virus-infected rainbow trout leukocytes (3) and in mouse dendritic cells infected with vesicular stomatitis virus (VSV) and pseudorabies virus (4). Viperin was then found to be induced by both type I and type II IFN and to exhibit antiviral activity against HCMV (9). Furthermore, the results of several gene-profiling microarray studies showed that the viperin gene is one of those that is highly induced by a range of different viruses (20,25,38,40,42,46) and microbial products, such as lipopolysaccharide (35,42), the double-stranded RNA analog poly(I-C) (39), and double-stranded B-form DNA (22), in various cell types.The human viperin gene encodes a protein of 361 amino acids with a predicted molecular mass of 42.2 kDa. Protein sequence analysis reveals a CX 3 CX 2 C motif, which is found in the superfamily of S-adenosyl methionine (SAM)-dependent radical enzymes (44) in residues 83 to 90 of human viperin. Thus, viperin is also called radical SAM domain-containing 2 (RSAD2). The radical SAM superfamily comprises more than 600 members (44, 47), and evidence suggests that the three conserved cysteine residues are part of an unusual iron-sulfur cluster that uses SAM as a cofactor to form a radical that is involved in catalysis (19,27). Although the precise function of viperin remains to be elucidated, recent data have proven its cellular antiviral effects. The overexpression of viperin reduces productive HCMV infection in human fibroblasts by downregulating several structural proteins that are critical for viral assembly and maturation (9), inhibits influenza virus release by perturbing lipid rafts (48), decreases human hepatitis...

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