Immunological differences in intestine and rectum of Atlantic cod (Gadus morhua L.)

The defence system of the distal gut (hindgut and rectum) of Atlantic cod, (Gadus morhua L.) was studied using (immuno)histochemical, electron microscopical and real-time quantitative PCR techniques. The uptake and transport of macromolecules in the intestinal epithelium was also investigated.In this study we observed that cod has many and large goblet cells in its intestinal epithelium and that IgM+ cells are present in the lamina propria and their number is considerably higher in the rectum than in the intestine. Myeloperoxidase staining revealed low numbers of granulocytes in and under the epithelium of the distal intestine, whereas high numbers were found clustered in the submucosa of the rectum. Electron microscopy not only confirmed these observations, but also revealed the presence of lymphoid cells and macrophages within the intestinal epithelium. Acid phosphatase staining demonstrated more positive macrophage-like cells in the rectum than in the distal intestine. Antigen uptake studies showed a diffused absorption of horse radish peroxidase (HRP) and LTB-GFP, whereas ferritin uptake could not be detected.Basal gene expression of cytokines (IL-1β, IL-8 and IL-10) and immune relevant molecules (hepcidin and BPI/LPB) were compared in both the intestine and rectum and revealed approximately 2–9 times higher expression in the rectum, of which IL-1β expression showed the most prominent difference.The present results clearly indicate that intestinal immunity is very prominent in the rectum of cod.