Effects of functional polysaccharide from silkworm as an immunostimulant on transcriptional profiling and disease resistance in fish

Dietary manipulation to maintain fish health and reduce bacterial infection through the use of immunostimulants has been widely used worldwide. A broad range of bioactive substances capable of optimising animal health has been found in several insect species, including antimicrobial/antiviral peptides, polysaccharides such as chitin, lauric acid, and insect products such as honey. Recently, we identified a novel bioactive polysaccharide from Bombyx mori, termed silkrose-BM, that can activate innate immunity in mammalian RAW264.7 macrophages and provide effective protection against vibriosis in penaeid prawns. However, the efficacy of dietary silkrose-BM in teleosts remains unclear. Here, we investigated the effects of dietary inclusion of silkrose-BM in Japanese medaka (Oryzias latipes) after they were artificially challenged with Edwardsiella tarda. The survival of medaka after infection with E. tarda was significantly improved by dietary silkrose-BM at a concentration of 10, 100, and 1000 ng/g. RNA-seq analysis was performed in the intestine and liver of the medaka to identify changes in the transcriptional profiling evoked by silkrose-BM. The dietary silkrose-BM group showed 1,194 and 2,259 differentially expressed genes (DEGs) in the intestine and liver, respectively, when compared with the control group prior to E. tarda infection. Functional enrichment analysis of DEGs showed several putative genes involved in the Toll-like receptor/nuclear factor κB pathway, cytokine-cytokine receptor interactions, complement cascade, antimicrobial peptides, and junctional modification. Taken together, these results suggest that silkrose-BM used as an immunostimulant can improve the immune system and resistance to edwardsiellosis in teleosts.