YEAST BETA-GLUCAN STIMULATES RESPIRATORY BURST ACTIVITY OF ATLANTIC SALMON (SALMO-SALAR L) MACROPHAGES

Previous studies have shown that head kidney macrophages isolated from glucan injected rainbow trout (Oncorhychus mykiss) and Atlantic salmon (Salmo salar L.) have increased ability to kill Aeromonas salmonicida. The present work was aimed at investigating the in vitro effects of glucan on the respiratory burst and bactericidal potential of Atlantic salmon head kidney macrophages. Salmon macrophages were incubated for 1-7 days with various concentrations of yeast beta-glucan (MacroGard(R)) and tested for respiratory burst activity by the reduction of nitroblue-tetrazolium (NBT) after exposure to phorbol myristate acetate (PMA) or opsonized zymosan. The macrophages showed a marked increase in respiratory burst activity 4 to 7 days after addition of glucan. Macrophages treated with 0.1-1 mu g mL(-1) gave a maximum respiratory burst response, whereas 10 mu g mL(-1) had no effect and 50 mu g mL(-1) was inhibitory, The glucan also triggered respiratory burst activity directly, but this occurred only at relative high concentrations with a maximal effect at greater than or equal to 200 mu g mL(-1). The validity of using the NET-assay as a measure of respiratory burst activity was confirmed by using inhibitors of O-2(-) production (superoxide dismutase, trifluoperazine and diphenylene iodonium). Despite the stimulatory effect of glucan on the respiratory burst activity of salmon macrophages, these cells did not show Introduction increased bactericidal activity against the avirulent and virulent strain of A. salmonicida. Upregulation of burst activity alone is thus apparently not sufficient to enhance bactericidal activity against this pathogen by Atlantic salmon macrophages.