Resistance to IHN virus infection in rainbow trout is increased by glucan while subsequent production of serum neutralizing activity is decreased

Protection against the pathogenic effects of viral challenge can be achieved by passive transfer of sera from individuals that have survived a viral challenge (convalescent sera). However, individuals may differ in regard to the degree of benefit each derives from components of injected sera. We tested the hypothesis that glucan-induced potentiation of the innate immune system in rainbow troutOncorhynchus mykisswould enhance the ability of an individual to benefit from passively transferred convalescent sera. Fish pre-injected with glucan, then with convalescent sera from survivors of an IHNV challenge, were fully protected when exposed to the virus. Furthermore, glucan injections resulted in higher survival even without the benefit of convalescent serum injections. These effects were both time- and concentration-dependent, evident at 15 days but peaking at 3 weeks after a dose of <80 μg glucan injected into rainbow trout weighing about 2·0 g. In fish that survived IHNV challenge, high to moderate virus neutralization titres developed in sera only if the fish had received neither glucan nor convalescent serum. We speculate that efficient inactivation and/or clearance of virus by the innate immune system can preclude orchestration of the full potential of the acquired immune system.