Mucosal Immunology and Oral Vaccination

The mucosal surfaces of the gastrointestinal and respiratory tracts represent the principal portals of entry for most infectious agents. Hence, the development of vaccination strategies capable of inducing protective immune responses at the mucosal sites is a priority. Since the mucosal surfaces are exposed to a wide variety of antigens, the mucosal immune system has to discriminate between harmful and harmless inoffensive or beneficial antigens. For this reason, the mucosal immune surfaces are highly regulated by a complex interplay of regulatory mechanisms capable of eliciting strong immune responses against pathogens and protecting the body as well as preventing the induction of strong immune responses against dietary proteins, commensal bacteria, or environmental inoffensive antigens, which can lead to chronic diseases (Mowat 2003; Pabst and Mowat 2012). Mucosal surfaces are protected from external attacks by physicochemical defense mechanisms comprising innate and adaptive mucosal immune systems. Epithelial barriers on the mucosal surfaces at different sites in the body differ dramatically in their cellular organization, and antigen-sampling strategies at diverse mucosal sites are adapted accordingly. The intestinal mucosa is covered by only a single cell layer (type 1 epithelium), whereas multilayered squamous epithelia line the oral cavity, pharynx, esophagus, and urethra (type 2 epithelium); and the airway and vaginal linings vary from pseudo-stratified to simple epithelium (Box 2.1; Pavot et al. 2012). A major goal in vaccine design comprises the induction of protective lasting immune responses against potential pathogens on the mucosal surfaces. These responses are most effectively induced by the administration of vaccines onto mucosal surfaces through oral, nasal, rectal, or vaginal routes, when compared with those induced by parenteral routes (Neutra and Kozlowski 2006). In addition, mucosal vaccines offer