Investigating the effects of synthetic peptides on aquatic pathogens and rainbow trout immunity

The demand for fish and seafood has increased exponentially over the past decade and continues to increase. Feeding fish has also become problematic as currently, the industry is dependent on fish meal which is not sustainable. Therefore, alternative protein sources are being investigated. However, some alternative protein sources such as plant-based protein has been shown to cause gut inflammation in salmonids. Aquaculture operations are also in a constant battle with keeping fish free of disease and pathogens have developed resistance to current drugs. Novel drugs are desperately needed to combat aquatic pathogens. The current work explores fish immunology by using novel peptide sequences derived from aquatic organisms and has identified one sequence, rtVWF, to possess antibacterial activity against an important fish pathogen, S. iniae. rtVWF was also found to be non-hemolytic which is a good indicator for future in vivo use. The monocyte/macrophage-like cell line, RTS11, upregulates tumor necrosis factor alpha in response to S. ininae. Besides immune cells, the gut, liver and gill epithelium of fish also play an important role in immune defense. In the current work, the immune response of these cell types: RTgutGC, RTL-W1 and RT-gillW1, respectively, was explored. First, in the presence of another aquatic pathogen, F. psychrophilum and subsequently, in the presence of alternative feedstuffs such as soybean protein and mealworm protein. In the presence of F. psychrophilum, RTgutGC cells significantly upregulate the proinflammatory cytokine interleukin 1 beta. When the same cells were co-cultured with RTS11, the RTS11 cells significantly upregulated tumor necrosis factor alpha. This suggests the gut epithelium of fish may depend on immune cells to activate tumor necrosis factor alpha signalling during F. psychrophilum infection. Exposing RTgutGC cells and RTL-W1 cells to soybean and mealworm protein did not have a significant effect on cell viability, cell adhesion, immune gene expression or wound healing but nevertheless this model provides a new model to study nutrient-liver interactions. Collectively, the results of this study expand our current understanding of host-pathogen interactions in fish immunology but also provide new avenues for investigation in fish physiology