Vaccination by mouth is highly preferable because of its ease of application but also because this way the most natural and most adequate immune responses against pathogens from respiratory and gastro-intestinal tract will be induced. Oral vaccines will also solve some of the problems related to the demands for more vaccinations. A problem with oral vaccines is the natural tolerance of the immune system along our mucosae. We have found that natural protease inhibitors in the mucosa are involved in the downregulation of antigen presentation during an immune response, thereby leading to tolerance. In this project we want to further explore this important finding to develop better oral vaccines by interference with these inhibitors during the induction of an mucosal immune response. This way the mucosal immune response will be enhanced, leading to an improvement of the efficiency of mucosal vaccination.