TRIM56 Promotes Antiviral Responses Downstream of TLR4

The ubiquitin ligase protein tripartite-motif containing 56 (TRIM56) positively regulates Toll-like receptor-3 (TLR3) signaling by forming a complex with Toll-Interleukin-1 receptor domain-containing adapter protein inducing interferon (IFN)-beta (TRIF), independent of its E3 ligase activity. Whether TRIM56 modulates other TLR pathways in innate antiviral immunity, however, is unclear. Herein, we show ectopic expression of TRIM56 augments activation of IFN regulatory factor-3 (IRF3)-dependent promoters following stimulation by lipopolysaccharide (LPS) in HEK293-TLR4-MD2-CD14 cells while leaving activation of NF-κB-dependent promoter unaffected, suggesting TRIM56 specifically promotes immune signaling through the TLR4-TRIF axis but not the MYD88 arm downstream of this TLR. Confirming its impact on endogenous antiviral responses in immune sentinel cells naturally harboring the TLR4 pathway, we demonstrated enforced expression of TRIM56 enhanced LPS-induced expression of IFN-beta and IFN-stimulated genes (ISGs) and establishment of an antiviral state in bone marrow-derived macrophages. Importantly, depletion of endogenous TRIM56 impaired LPS-induced antiviral gene expression and cellular antiviral defense. Altogether, these data add to understanding of the role of TRIM56 in TLR-mediated innate immune responses. Given that TRIM56 is an ISG and that many immune adjuvants and some viral proteins activate TLR4, the findings of this study could have implications for designing immunotherapies, especially those against viral infections.