Abstract
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(NNV), PI-SeGE-22(NNV) and PI-SeGB(NNV) cells) by repeatedly subculturing the cells that survived NNV infection. The PI-SeGF(NNV) and PI-SeGE-22(NNV) cells continued to stably yield NNV in culture fluids at 10(6) to 10(7) median tissue culture infectious dose (TCID(50))/ml even after 30-50 subcultures. The PI-SeGB(NNV) cells initially yielded NNV at 10(3.9) TCID(50)/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(r) < 1000. When the PI-SeGB(NNV) 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.