Antigen uptake and immune responses after oral vaccination

Oral delivery of vaccine, with antigen simply mixed with feed, is the easiest method of mass vaccination of fish of all sizes. It is time and labour-saving, and avoids any manipulation stress. However, large quantities of antigen are necessary and it is not possible to know the exact dose absorbed by each fish. Moreover the resulting protection is generally weak and of short duration. To improve the effectiveness of oral vaccination a considerable amount of work has been done in the last 15 years. It is now well established that the second segment of the hindgut of fish has the ability to absorb soluble or particulate antigens. These antigens reach the intraepithelial macrophages which show antigenic determinants on their cell membrane, suggesting an antigen-presenting function. Moreover, a gut-associated lymphoid tissue composed of several lymphoid cell types, macrophages, granulocytes and plasma cells has been described in this region of the gut and recent studies indicate that it is involved in an intestinal immune response. Both mucosal and systemic immune responses seem to develop as indicated by the presence of antibodies in gut mucus, bile and serum. However, these responses are higher following anal delivery of antigen than oral delivery. It is generally considered that during oral delivery antigens are digested in the foregut and/or in the stomach. Thus studies are being carried out now on ways to protect antigens, for instance through the encapsulation of antigen, the neutralisation of gastric secretions and the use of oral adjuvants.