Role of dietary Spirulina platensis in improving mucosal immune responses and disease resistance of rainbow trout (Oncorhynchus mykiss)

Abstract

In the present study, the effects of dietary Spirulina platensis (SP) on immune responses, histological structure of the mucosal surfaces and disease resistance of rainbow trout (Oncorhynchus mykiss) was investigated. Three practical diets containing three levels of SP (0%, 2.5% and 5%) were formulated and each diet was randomly assigned to triplicate groups of 45 fish. At the end of seven-week feeding trial, the expression of immune-related genes [lysozyme, component 3 (C3), immunoglobulin M (IgM), omDB-3 and tumor necrosis factor-α (TNF-α)] and histological characteristics of the skin, gill and posterior intestine of fish (18 fish form each diet group) were studied. The remaining fish were challenged intraperitoneally with Yersinia ruckeri. The results showed that SP supplemented diets did not have side effects on the normal structure of mucosal surfaces in intestine, skin and gill. The percentage of acidic mucin producing goblet cells of intestine was higher in both treatment groups whereas the percentage of neutral mucin goblet cells decreased in these groups compared to the control group. In the intestine of fish fed 5% SP higher goblet cell density, villus height, absorption surface area and intraepithelial lymphocytes than the control were observed. The expression of immune-relevant genes showed an up-regulation of lysozyme gene in skin and intestine of fish fed SP supplemented diets. The expression of genes encoding C3, omDB-3 and TNF-α genes were up-regulated in skin and gill tissues of fish fed 2.5% SP supplemented diet, while in fish received 5% SP supplemented diet, up-regulation of these genes was noted in skin tissue. IgM gene expression was also up-regulated in all the examined mucosal tissues in fish fed with 2.5% SP whereas diet with 5%SP caused an up-regulation of this gene in gill and skin tissues. The challenge experiment showed that the cumulative mortality was significantly lower in fish fed SP supplemented diets than control group after infection with Y. ruckeri. These results showed that the dietary supplementation of fish diet especially with 2.5% SP had the potential to improve mucosal immune responses and disease resistance of rainbow trout.