Potential involvement of persistently infected cells in protective immunity of grouper fish convalesced from nervous necrosis virus infection.

Nervous necrosis virus (NNV) is a simple RNA virus with high pathogenicity across over 170 fish species. Two experimental models have been developed to efficiently induce convalescent fish using live viruses: (1) pre-administering polyinosinic:polycytidylic acid (poly(I:C)) to induce a non-specific antiviral state, and (2) maintaining fish at suboptimal temperatures for disease onset. Notably, these convalescent fish exhibit robust long-term protective immunity despite having undetectable levels of neutralizing antibodies. This study synthesizes established in vivo and in vitro data on the kinetics of NNV multiplication in grouper fish thereby elucidates the mechanisms by which the host survives and acquires robust immunity against NNV infection, particularly focusing on persistently infected (PI) cells. In vitro analyses demonstrate that the NNV multiplication rate is governed more by cellular metabolism than by temperature alone. PI cells continuously produce a small-molecule antiviral substance that suppresses viral multiplication by down-regulating host cellular metabolism. Therefore, NNV kinetics in PI cells is governed by a dynamic equilibrium between the production of this antiviral substance and metabolic activity. The striking similarity between in vitro NNV kinetics and those observed in convalescent fish suggests that persistent infection frequently occurs in vivo. The continuous presence of the virus during persistent infection likely provides a sustained stimulation for cellular immunity. We propose that persistent infection plays a pivotal role in limiting viral multiplication and in establishing long-term protection in the absence of neutralizing antibodies. These findings provide a mechanistic framework for understanding how convalescent fish survive NNV infection and maintain enduring immunity.