Persistence of betanodavirus in Barramundi brain (BB) cell line involves the induction of Interferon response

The BB cell line derived from the brain tissue of a barramundi (Lates calcarifer) that survived nervous necrosis virus (NNV) infection is persistently infected with NNV. To elucidate whether interferon (IFN) plays a role in the mechanism of NNV-persistent infection in BB cell line, a virus-negative control cell line was obtained by treating BB cells with NNV-specific rabbit antiserum for 5 subcultures. After the treatment, NNV titer or RNA or capsid protein was no longer detected in the cured BB (cBB) cells. Expression of Mx gene, encoding a type I IFN-inducible antiviral protein, was found in BB cells and cBB cells following NNV infection, but not in NNV-free cBB cells. Moreover, expression of Mx gene and antiviral activity against NNV were induced in cBB cells by the treatment with MAb-neutralized BB cell supernatant. Furthermore, NNV persistent infection was induced again in cBB cell culture if multiplicity of infection (MOI) was low (≦1). These experimental results indicated that IFN-like cytokines existed in the culture supernatant of BB cells, and IFN-induced response played an important role in protecting the majority of cells from virus lytic infection and regulating NNV persistence in the BB cell line.