Stimulating the Immune system of fish by oral administration of immunomodulatory substances can prevent disease outbreaks in aquaculture. Yeast p( l ,3/1 ,6)-glucan, the active ingredient of the commercially available feed supplement MacroGard®, has been associated with production of microbicical and cytocidal oxygen radicals and the induction of apoptosis in human cancer cells. Hence it was hypothesized that the immunosuppressive effects of this substance, which were observed by some authors, could be caused by induction of apoptosis in immune cells due to oxidative stress. Utilizing molecular and immunohistochemical staining techniques it has been shown that although MacroGard® can induce apoptosis il1 vitro it is not associated with this form of cell death il1 vivo. However dietary MacroGard® influences the expression of apoptosisrelated genes in a time and organ dependent manner.
Apoptosis is also associated with disease and can be modulated by both the host as a means of controlling infection, and by pathogens in an attempt to avoid the host immune system. It was thus hypothesized that bacteria (Aeromol1as saimol1icida) and vilUses (koi herpes vilUs (KHV) and spring viremia of earp virus (SVCV)) ean modulate apoptosis in carp and that this can be affected by oral immunostimulation. In this thesis it was established that the bacterial pathogen A. saimol1icida and the SVC vilUS induce apoptosis and that this is associated with changes of apoptosis-related gene expression. KHV in contrast appeared to supress apoptosis during early stages of the infection but induced it during the later stages possibly as a means to dissen'linate the vilUs. MacroGard® enhanced gene expression in response to SVCV infection and exposure to vilUs- and bacteriaassociated molecular patterns (i.e. Poly(I:C) and LPS). In conclusion, MacroGard® can influence apoptosis-related gene expression but does not appear to induce apoptosis on its own.