Absorption of intact proteins by the intestinal epithelium of trout,Salmo gairdneri

In the rainbow trout, a carnivorous species with a stomach, orally administered horseradish peroxidase (HRP) crossed the intestinal epithelium into the circulation. The protein first appeared in the blood 7 to 8 h after intubation, and could be assayed up to 24 h after administration. The method used, which combines ELISA (enzyme-linked immunoabsorbant assay) and chemiluminescence, enabled the transfer to be measured quantitatively. There was a direct correlation between the quantity ingested and the quantity transferred to the plasma within the experimental limits chosen. The clearance was monophasic and exponential (clearance rate: 3% per minute). Up to 6% of the ingested HRP was transferred to the blood. By cytochemistry it was possible to demonstrate that the protein crossed the intestinal cells at the level of the posterior segment, escaping the particularly intense intracellular lysosomal digestion. After entering the intercellular space, HRP was transferred to the interstitial space of the subepithelial lamina propria. During this transfer the HRP was in close contact with infiltrated macrophages and leukocytes resembling lymphoid cells. Thus, the passage of these protein particles could be the first indispensable step in the possible triggering of a local and/or a systemic immune response.