Effect of different β-glucans on the respiratory burst of turbot (Psetta maxima) and gilthead seabream (Sparus aurata) phagocytes

The in vitro effect of several β-glucans on the respiratory burst of turbot and gilthead seabream phagocytes was examined. Three particulated β-glucans from yeast, Saccharomyces cerevisiae and a particulate glucan from the fungus Schizophyllum commune were used. In some experiments, cells were incubated for 1 or 2 h with a mixture of glucan (0–500 μg ml−1) and nitroblue tetrazolium (NBT). In others, cells were preincubated with glucans for 1, 3 and 6 h and then incubated for 1 h with NBT with or without PMA. Cells from gilthead seabream and turbot responded similarly to glucans, and differences in activity depended mainly on the concentration of glucans, the length of incubation period of cells and glucan, and on the glucan used. Incubation of cells with glucans for 1 h directly induced a respiratory burst which increased with the concentration of glucan. However, after 2 h incubation a decrease in NBT reduction occurred at the highest glucan concentrations. An enhancement of the respiratory burst, which increased with the concentration of glucan, was also seen when cells were preincubated with glucans and then incubated with NBT without PMA. However, when PMA was added to the NBT solution, the highest NBT reduction was found at low glucan concentrations whereas with higher concentrations of glucan the NBT reduction decreased significantly. Thus high concentrations of glucan directly induced respiratory burst and led to exhaustion. Low concentrations of glucan primed the phagocytes to be capable of enhanced production of reactive oxygen species on subsequent activation of the respiratory burst. The former may increase disease susceptibility, the latter increase resistance.