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Immunomodulating mechanism of oral ß-glucans in pigs could be hampered by the epithelial mucosal barrier Eric Cox1, Kim Baert1, Bruno Goddeeris1,2, Bert Devriendt1 1 UGent, 2 KULeuven, Belgium. The prophylactic use of antibiotics is prohibited in Europe since January 2006. Therefore, alternatives are searched for in order to prevent infection on problem farms. Certain ß-glucans, polymers of D-glucose, have the capacity to activate the innate immune system. In a number of experiments we aimed to determine if they could increase the resistance against infections with enterotoxigenic E. coli (ETEC), an important cause of economical losses in newly weaned piglets and we wanted to understand their mode of action. Material and methods Several experiments were performed. In a first experiments four groups of pigs, either receiving one of three different ß-glucans or no glucans were challenge infected with F4+ ETEC. In a second experiment, one ß-glucan preparation was given to 1-week-old gnotobiotic pigs during 35 days, whereafter these piglets were orally immunized with F4 fimbriae in order to see an adjuvant effect of the glucans. In a third experimental set up, the effect of seven different ß-glucans on lymphocyte proliferation, reactive oxygen species (ROS) production by neutrophils and monocytes and cytokine production was analyzed. In a fourth experiment, the role of the ß-glucan receptors dectin-1 and the complement-receptor 3 (CR3) in activation of monocytes and neutrophils by ß-glucans was examined. Results The challenge study in conventionally bred, weaned piglets showed that ßglucans can protect against an ETEC infection. Macrogard from S. cerevisiae and scleroglucan significantly reduced faecal excretion of F4+ ETEC. No effect of ß-glucans was seen on an oral F4 immunization in gnotobiotic pigs, suggesting that the glucans did not exert their protective effect by modulating the humoral mucosal immune response. Incubating different leukocytes with different ß-glucans showed that these glucans exerted different effects on the leukocytes. Macrogard induced production of reactive oxygen species (ROS) by monocytes and neutrophils, while this was not the case for scleroglucan. The latter enhanced cytokine production, similar to the induction of cytokines by macrogard. This suggest that cytokine production rather than ROS production might be important for protection. The fourth experiment demonstrated that the complement-receptor 3 (CR3) plays a cardinal role in ß-glucan receptor recognition and signaling by porcine neutrophils, while CR3 and dectin-1 were important for activation of macrophages, similar to what has been seen in man, but not in mice. Conclusion Protection of pigs against F4+ ETEC infection is ß-glucan dependent. One of the protective ß-glucans is macrogard. This ß-glucan does not exert its effect via an adjuvant effect on antibody production. It activates innate immunity via the CR3 in neutrophils and via the CR3 and dectin-1 in macrophages. Most likely crossing the intestinal mucosal barrier is crucial for ß-glucans to exert their effect on an ETEC infection