Fish cells persistently infected with nervous necrosis virus produce a small-molecule substance for reducing cellular metabolism and suppressing viral multiplication.

Nervous necrosis virus (NNV) of the genus Betanodavirus is one of the simplest RNA viruses pathogenic to a wide range of fish species. We established the SeGF, SeGE-22 and SeGB cell lines persistently infected with NNV (PI-SeGF, PI-SeGE-22 and PI-SeGB cells) by repeatedly subculturing the cells that survived NNV infection. The PI-SeGF and PI-SeGE-22 cells continued to stably yield NNV in culture fluids at 10 to 10 median tissue culture infectious dose (TCID)/ml even after 30-50 subcultures. The PI-SeGB cells initially yielded NNV at 10 TCID/ml but stopped yielding NNV after several passages. No significant morphological differences were observed between the naïve and PI-cells in either cell line. Antiviral activity suppressing the multiplication of NNV was detected in the culture fluids of all PI-cell lines. It significantly suppressed the growth (metabolism) of each cell line but did not directly influence NNV infectivity. However, this antiviral substance was not an interferon but a heat-stable (100 ºC for 3 min), small molecule with M < 1000. When the PI-SeGB cells stopped yielding NNV after subculturing several times, the production of the antiviral substance also ceased, indicating that the production of antiviral substance is initiated by NNV infection.