The poultry sector provides a major contribution to the world food supplies and its meat production is expected to increase sharply in the next ten years, leading to poultry becoming the most widely consumed meat. Hence, the health of broiler chickens is of great importance, however, the current ban on preventive treatment of antimicrobials have led to an increase in intestinal health problems in poultry. This urges for alternative strategies to improve health of broiler chickens. Young chickens are more susceptibility to infectious diseases than adult ones since the adaptive immune system is not fully developed yet. Therefore, strategies should aim to increase the resistance to pathogens early in the life of chickens. At that stage, the immune responsiveness of chickens to invading pathogens relies on the innate immune system and natural killer (NK) cells, abundantly present within the intestinal intraepithelial lymphocyte (IEL) population, are among the first to respond. The aim of this thesis was to investigate stimulation of immune responsiveness, modulation of intestinal microbiota and the interaction between these by nutritional supplementation in order to increase the resistance and consequently improve the health of young broiler chickens.
For this purpose, number and functions of natural killer (NK) and T cell subsets in the intestine and other tissues of broiler chickens were assessed and used for subsequent studies pertaining modulation of these cells. Furthermore, the interaction between the immune system and intestinal microbiota in young broiler chickens was investigated and early exposure to adult-derived microbiota was shown to increase NK cell responsiveness. Then, the potential of nutritional strategies to modulate immune responsiveness and microbiota composition was identified. A significant role for NK cells in the immune response to Salmonella enterica serotype Enteritidis (SE), an important intestinal pathogen in chickens, was recognized. Finally, nutritional supplementation resulted in increased resistance to SE infection in young broiler chickens by stimulation of NK cell responsiveness, modulation of intestinal microbiota as well as the interaction between these.