Research Note: Interactions among the MDA5, MAVS, and STING signaling pathways in chicken cells.

Innate immunity, as an organism's first line of defense, plays a crucial role in rapidly responding to and protecting the body against invading pathogens. As a cytosolic RNA sensor for viral infections, including infections caused by influenza virus, the innate immune system in chickens has 2 major pathogen-recognition receptors (PRRs): Toll-like receptor 3 (TLR3) and melanoma differentiation-associated protein 5 (MDA5). The signaling pathways activated by PRRs are complex, systemic processes that underlie the response to foreign molecules. In this study, we investigated the interactions among MDA5, mitochondrial antiviral signaling protein (MAVS), and stimulator of interferon genes (STING) signaling in chicken cells. To exclude the effects of TLR3, we transfected the clustered regularly interspaced palindromic repeats/CRISPR-associated protein 9 (CRISPR-Cas9) expression vector and TLR3-targeted gRNA plasmid into chicken DF-1 cells. We selected TLR3-knockout (KO) cell line and sequentially, we established 2 double-KO cell lines: TLR3-MAVS KO and TLR3-STING KO. After treatment with polyinosinic:polycytidylic acid (poly(I:C)), type I interferon (IFN), IFN-stimulated gene, and antiviral gene (IFN regulatory factor 7, IFNβ, Mx1, and protein kinase R1) expression was not completely activated in TLR3-MAVS KO cells, whereas it was consistently upregulated in wild-type and TLR3-STING KO DF-1 cells. These results suggest that STING is not an intermediator between MDA5 and MAVS; moreover, it does not directly interact with MDA5 during innate immune activation in chicken DF-1 cells.