An investigation of antigen processing in the rainbow trout gastrointestine was carried out to provide a rational basis for the design of oral delivery systems for protein antigens. Using in vitro systems involving isolated lumenal enzymes and gut cell suspensions the degradation of human gamma globulin (HGG) and bovine serum albumin (BSA) was analysed by Western blotting and laser densitometry. Proteolysis by lumenal enzymes was dependant on pH and temperature and serine proteases were found to be partly responsible for antigen degradation in the intestine. The extent of intracellular proteolysis depended on the antigen used and on the gut region from which the cells were isolated. To test the predictive value of results obtained from the in vitro studies, the processing of HGG in the digestive tract after oral administration was investigated. The findings indicated that different regions of the gut perform distinct bur complementary roles in proteolysis. Measurement of the uptake of HGG into the bloodstream of these fish by enzyme linked immunosorbent assay (ELISA) and Western blotting indicated that the nature of proteins. absorbed from the gut could be influenced by altering the conditions in the gastrointestine. After parenteral and oral immunisation of HGG the antibody response was investigated in plasma and in mucosal and biliary secretions and found that a fragment of HGG produced by partial digestion with intestinal enzymes was highly antigenic in trout. The methods developed to study antigen processing in the gut were applied to assess the potential value of modern enteric delivery systems in teleosts. Encapsulation of HGG in poly lactide-co-glycolide (PLG) microparticles partially protected HGG from degradation in the gut and increased its absorption into the bloodstream. A live attenuated strain of Aeromonas salmonicida was shown to adhere to and invade isolated trout enterocytes and Atlantic Salmon tissue culture cells using a range of light - and electron microscopical techniques. These results indicate that an investigation into antigen processing by the gut is a valuable preliminary step in the formulation of oral delivery systems for teleosts.

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