In vitro salmonid models as tools for studying microbial-derived immunostimulants and aquaculture relevant salmonids pathogens: Current status and future perspectives

Until now, research sustainability, legal requirements, and targeted humane endpoint in animal research was a major requirement by regulatory bodies and funding agencies. However, invasive research on live animals is moving away from utilitarian justification to abolitionism altogether with the recent move by the EU parliament to phase out the use of animal experiments (2021/2784(RSP)). Thus, there is a need to find suitable alternatives to replace biological testing involving live animals. The in vitro platform is an interesting alternative for nonanimal testing of immunostimulants, which have become a preferred option for boosting fish health and to modulate host defence against pathogens in aquaculture-relevant species. This review explored the use of available in vitro salmonid models to test both microbial-derived immunostimulants (beta-glucans, LPS, poly[I:C]), and economically important salmonids pathogens. We outline key challenges with present models, future developments, and crossovers of some state-of-the-art 3D mammalian cell models, which are worth refining to suit piscine novel in vitro models. Current in vitro work are better complements to in vivo trials with animals to characterize effects of immunostimulants and pathogens. Although in vitro salmonid models cannot replace in vivo feeding trials, they can reduce and refine them, because results obtained at the cellular level when evaluating immunostimulants and pathogens are selectively comparable to those obtained from feeding trials at the transcript and protein level. Future cell models, including gene editing, ex vivo models, bioreactors, organ-onchips, and organoids are promising 3D models that are worth developing to replace live animal trials as tools to study complex processes such as digestion, host-microbiome/pathogen symbiosis, and nutrigenomics are discussed.