Visualizing the T-cell response elicited by oral administration of solubleprotein antigen

Oral administration of soluble protein antigen induces tolerance, while par ticulate antigens encountered in the intestine provoke active immunity. Alt hough the events that lead to these distinct outcomes are not yet fully cha racterized, they may reflect differences at the antigen-presenting cell (AP C) level. The role of dendritic cells (DC) in regulating responses at mucos al sites has remained largely undefined because of the low frequency of DC in mucosal-associated tissues. In this study we have used the growth factor Flt3-ligand (Flt3L) to expand DC populations in vivo, in combination with an adoptive transfer system, in order to track antigen-specific T cells dur ing oral tolerance induction. We observed rapid T-cell activation, localize d particularly in the mucosal tissues, within hours after feeding the solub le protein antigen, ovalbumin (OVA). The response was enhanced in Flt3L-tre ated mice, indicating an important role for DC during the inductive phase o f tolerance.