Activation of Toll-like receptor 3 inhibits Marek's disease virus infection in chicken embryo fibroblast cells

Toll-like receptor 3 (TLR3) is a critical component of the innate immune system against viral infection and controls the activation of adaptive immunity. The role of TLR3 in Marek's disease virus (MDV) infection is not clear. In this study, we found that the abundance of TLR3 mRNA was significantly higher in chicken embryo fibroblast cells (CEF) infected with MDV than in a control group. Activated TLR3 signaling via TLR3 ligand stimulation inhibited replication of the RB1B strain of MDV in CEF cells. In contrast, CEF cells transfected with TLR3 siRNA promoted RB1B infection and replication. However, treatment with other TLR ligands, whether stimulatory (LPS, imiquimod and CpG) or inhibitory (TLR2/4 inhibitor and/or MyD88 inhibitor), had little effect on RB1B infection and replication. In addition, we found that the expression trend of TLR3 mRNA in RB1B-infected CEF cells was similar to that of mdv1-mir-M4-5p (a functional ortholog of oncogenic miR-155 encoded by MDV). Inconsistent with this, the TLR3 protein level was sharply reduced in RB1B-infected CEF cells at 96 hpi, while there was an at least 200-fold increase in miR-M4-5p at the same time point. Additionally, CEF cells transfected with an mdv1-mir-M4-5p mimic promoted RB1B infection and replication, while an mdv1-mir-M4-5p inhibitor inhibited RB1B infection and replication. Similar results were observed in CEF cells transfected with a gga-miR-155 mimic or inhibitor. These findings suggest that TLR3 and MDV-encoded miRNAs might be involved in MDV infection.