Oral tolerance in birds and mammals: Digestive tract development determines the strategy

Development of oral (or airway) vaccines to be used for the vaccination of poultry is clearly a necessity because the process of individual bird vaccination is time-consuming and costly. One of the key issues to resolve in this context is whether the vaccine is protected from oral tolerance. Oral tolerance is a form of immune tolerance that is induced following the feeding of protein antigens dissolved in water or aqueous buffers. The issue is particularly relevant in cases in which nonpathogenic subunit protein vaccines are considered as vaccines. These short peptides/proteins contain the immunogenic moiety of the parent pathogen, but they lack the capacity to induce inflammation, and an antigen in the absence of inflammation might induce tolerance. The following review summarizes our main observations on mechanisms of oral tolerance in the chicken, compares these findings to what is known in the mammal, and provides some insight for developing strategies for oral vaccination of poultry.