Toll-like Receptor 3 is Required for Sensing Extracellular Double Stranded RNA in Salmonid Cells

The membrane-bound toll-like receptor 3 (TLR3) binds to double-stranded RNA (dsRNA) derived from viruses, as well as poly(I: C), its synthetic analogue. Upon dsRNA binding, TRIF is recruited to the TLR3 dimer and leads to the activation of the transcription factor complex NF-κB/IRF which induces the production of type I interferons. CHSE-214 is an epithelioid cell line initially isolated from Chinook salmon Oncorhynchus tshawytscha embryos and is extensively used in diagnostics of viral diseases in fish. Unlike other available salmonid fish cell lines, CHSE-214 has been found unable to respond to extra cellular poly(I: C). This deficiency also extends to the CHSE-214 -derived recombinant cell line CHSE-EC which express GFP stably. In the present study, we demonstrate that the tlr3 gene as well as 70% of the chromosome 34 on which it is located is missing from the CHSE-EC genome. Extracellular dsRNA responsiveness could be restored by stable transfection with a tlr3-expressing plasmid in a novel clonal cell line, TLR3-Pur-C4. This cell line responds to low and high molecular weight poly(I: C) in a dose-response manner. Using genome editing for loss of function, we demonstrate that the TLR3 poly(I: C) signalling is TRIF-dependent and MYD88-independent.