Understanding host-viral interaction is an essential step in developing safe and effective antimicrobials against biodefense agents and emerging pathogens. The early detection of invading viruses by the host depends on a limited number of specific receptors that detect viral patterns and activate signaling cascades, thereby triggering interferon (IFN)-mediated anti-viral defense mechanisms. Retinoic acid-inducible gene I (RIG-I) and melanoma differentiation-associated gene 5 (MDA5) emerge as key receptors for sensing RNA viruses including Paramyxoviridae, Orthomyxoviridae, Flaviviridae and Picornaviridae. In addition, members of the tripartite motif (TRIM) protein family play a major role in the inhibition of virus lifecycles.

Research in my laboratory seeks a better understanding of the molecular mechanisms underlying the IFN-mediated innate immune response against viral infection. Using molecular, biochemical and cell biological approaches, we are identifying and characterizing the regulatory mechanisms that govern the detection of RNA viruses through cellular receptors and the subsequent induction of signaling cascades leading to IFN-α/β gene expression. Another area of our research focuses on the detailed mechanisms, how viruses antagonize the host anti-viral innate immune response, and on the understanding of their roles in viral lifecycle.

Regulation of the RIG-I-mediated anti-viral innate immunity. Despite the recent rapid progress in deciphering molecular components in the RIG-I signaling pathway, the regulation of its anti-viral activity has not been well studied. Our previous work has demonstrated that the interconnection between the cytosolic viral RNA receptor RIG-I and a member of the TRIM protein family represents a novel class of regulatory pathway, which is essential for the induction of IFN-mediated host innate immunity against a wide variety of RNA viruses. Specifically, we showed that the RING-dependent ubiquitin E3 ligase TRIM25 interacts with the N-terminal Caspase recruitment domains (CARDs) of RIG-I, and this interaction effectively delivers the Lys 63-linked ubiquitin moiety to the RIG-I CARDs, resulting in a marked increase in RIG-I downstream signaling activity. Mutational analysis further demonstrated that ubiquitination of RIG-I at Lys-172 is critical for the interaction with its downstream partner MAVS/VISA/IPS-1/Cardif and for RIG-I ability to induce anti-viral signal transduction. Gene targeting also showed that TRIM25 is essential for the RIG-I anti-viral activity in response to RNA virus infection.

Viral evasion of the RIG-I-mediated innate immune response. Severity of disease and pathogenesis of influenza virus infection in humans depends on many different factors, including pre-existing immunity, strain virulence, host genetics and virus-host interactions. Among the virus-host interactions that modulate pathogenesis, the virus-mediated induction and inhibition of the type I IFN system play a critical role. Our studies revealed that the non-structural protein 1 (NS1) of human, swine and avian influenza A virus strains directly interacts with and inhibits the enzymatic activity of TRIM25, resulting in the abolished RIG-I ubiquitination and host antiviral IFN response. Importantly, a mutant NS1 recombinant influenza virus that is defective in TRIM25 binding and blocking the TRIM25/RIG-I-mediated immune response exhibited a dramatic reduction in titer in human lung epithelial cells compared to wild-type virus. Furthermore, infection of mice with this mutant NS1 recombinant virus resulted in markedly reduced pulmonary viral titers and in a complete loss of virulence, demonstrating a necessary role of NS1-mediated TRIM25 inhibition for the pathogenicity of influenza A virus infection in vivo. These findings unveil a novel immune evasion mechanism of influenza A virus and also emphasize the vital role of TRIM25 in modulating viral infections.