Analysis of immunostimulatory responses and immune tolerance to β-glucans in rainbow trout cell lines

beta-glucans are widely used in fish diets as immunomodulators to enhance the defensive capacities of farmed animals and improve their resistance to pathogens. Despite their extensive use, the precise effects that beta-glucans provoke on immune and intestinal cells in teleost fish have not yet been fully elucidated. In the current work, we have used the RTS11 monocyte-macrophage cell line and the RTgutGC intestinal epithelial cell line to further investigate the effects of beta-glucans on rainbow trout (Oncorhynchus mykiss), determining the levels of transcription of a wide range of immune genes that code for pro-inflammatory cytokines/molecules, antimicrobial peptides or adhesion molecules. Our results show that RTS11 monocytes/macrophages are more reactive to beta-glucans than intestinal epithelial cells. Despite this, beta-glucans were able to enhance some of the transcriptional effects of Aeromonas salmonicida on RTgutGC cells as they did on RTS11 cells, evidencing some immunostimulatory effects on gut epithelial cells. Finally, we have also used these two cell lines to establish whether beta-glucans induce tolerance, comparing the effects to those exerted by LPS. Pre-exposure of both cell lines to LPS strongly inhibited a secondary response to LPS that affected the transcription of many of the genes studied, confirming the potential of these cell lines to study mechanisms of immune tolerance. However, when both cell lines were stimulated with beta-glucans, only interleukin 6 (IL6) transcription was significantly lower in cells that had been previously exposed to beta-glucans in comparison to cells that had not been previously stimulated. These results provide valuable information concerning the immunostimulatory and potential tolerogenic effects exerted by beta-glucans in both macrophages and gut epithelial cells.