Rolf Engstad

Yeast glucan induces increase in lysozyme and complement-mediated haemolytic activity in Atlantic salmon blood.

The activities of lysozyme, alkaline and acid phosphatase, N-acetyl-glucosaminidase and complement (as spontaneous haemolytic activity) were measured in plasma of Atlantic salmon, Salmo salar L., after intraperitoneal injection of a β -1,3 and β -1,6 linked glucan from Saccharomyces cerevisiae . The glucan has previously been shown to induce resistance of Atlantic salmon to several bacterial pathogens. The glucan treatment resulted in increased activities of lysozyme 1–3 weeks and complement 2–4 weeks after injection. The activities of alkaline phosphatase and N-acetyl-glucosaminidase showed a decrease 1 week after injection and then slowly restored to normal levels, whereas acid phosphatase was only slightly altered during the experimental period. These results indicate that the glucan induces a selective increase in lysozyme rather than a general release of lysosomal enzymes. The time intervals of enhanced activities of lysozyme and complement partially overlapped with the time intervals of expected enhanced protection against pathogens. Thus, induction of increased activities of lysozyme and complement may, at least in part, explain the mechanisms by which glucan induces protection against bacterial pathogens in salmon.

Recognition of yeast cell wall glucan by Atlantic salmon (Salmo salar L. ) macrophages.

Phagocytosis of yeast (Saccharomyces cerevisiae) glucan particles by Atlantic salmon (Salmo salar L.) pronephric macrophages was studied. The particles contained > 95% glucose linked through beta-1,3- and beta-1,6-glycosidic linkages. The macrophages rapidly phagocytized both native and opsonized glucan particles although the latter were taken up at a higher rate. Within 30 min, 40-60% of the macrophages had taken up > 1 native glucan particle. The uptake of native glucan particles could be inhibited by preincubating the macrophages with laminarin, a soluble beta-1,3-linked glucan, and a soluble yeast glucan made by partial formolysis of glucan particles. Soluble yeast glucan, on the other hand, did not inhibit uptake of serum opsonized glucan particles or sheep red blood cells, which showed that it did not interfere with phagocytosis in general or inhibit phagocytosis through complement receptors. Polyglucoses with glycosidic linkages other than beta-1,3, like dextran, glycogen, and pustulan or the polymannose mannan, showed little or no inhibition of phagocytosis of native glucan particles. Altogether these observations indicate that Atlantic salmon macrophages may have a specific receptor for yeast glucan. Studies with chelator- and heat-treated salmon serum showed that glucan particles were opsonized primarily by activation of the alternative complement pathway. However, the data indicate that serum components other than complement may also be involved in the opsonization of glucan particles.

Specificity of a β-glucan receptor on macrophages from Atlantic salmon (Salmo salar L.)

Abstract This study was undertaken to study the specificity of a β-glucan receptor on Atlantic salmon macrophages. Previous in vitro studies have shown that Atlantic salmon macrophages express a receptor that rapidly recognizes and mediates uptake of nonopsonized β-glucan particles. The ingestion of particles was shown to be inhibited by preincubating the macrophages with glucans containing β-1,3-linkages, but not by glucans containing other linkages. In the present study we have shown that small oligomers from formolyzed β-glucan particles, and linear β-1,3-linked oligomers with a degree of polymerization (DP) ⩾ 3, were efficient inhibitors of uptake of glucan particles. Oligomers from β-1,6-linked pustulan, or small size oligomers with linkages other than β-1,3, were not able to inhibit uptake of glucan particles. The inhibitory effect of laminarin and laminariheptaose was abolished by degrading the nonreducing terminal ends by sodium periodate treatment. The inhibitory effect of laminarin was regained by a complete Smith degradation; that is, periodate oxidation followed by reduction and hydrolysis. Modification of the reducing end of laminariheptaose had no effect on its ability to inhibit uptake. Furthermore, it was shown that periodate-oxidized glucan particles were not taken up by salmon macrophages, and that the uptake was regained when the particles were hydrolyzed to recover the nonreducing terminal end. Lastly, it was shown that endo-β-1,6-glucanase treatment of the yeast glucan particles did not reduce uptake, confirming that β-1,6-linkages are not involved in the recognition. These results suggest that Atlantic salmon macrophages possess a receptor that may recognize even very short β-1,3-linked glucosyl chains extending from yeast cell walls.