Uptake and transport of soluble (ferritin) and particulate (Vibrio anguillarum) antigen from intestinal lumen to mucosal macrophages was studied using immunocytochemical and electronmicroscopical techniques. Both antigens were taken up by epithelial cells of the second gut segment, reached the supranuclear vacuoles and were finally transported to large intraepithelial macrophages. In contrast with particulate antigen, antigenic determinants of ferritin were demonstrated in the basal part of the epithelium and appeared to be released at the mucosal site.After anal intubation many small macrophages penetrated the intestinal epithelium, took up antigen and after 24 h disappeared again from the intestinal mucosa. This feature was antigenindependent and also occurred after anal PBS-intubation. The larger, less mobile macrophages stayed in the intestinal epithelium and finally exposed antigenic determinants (of both antigens) at their outer surface, suggesting an antigen-presenting function. Whether these large macrophages can induce a local or mucosal immune response and whether the smaller mobile macrophages are involved in a systemic response is discussed.

Mucosal and systemic (serum) immune responses were studied after oral, anal or intramuscular ( i m ) immunization with particulate (Vibrio anguillarum) or soluble (ferritin) antigen. Antigenspecific antibodies were found by ELISA in skin mucus after repeated oral or anal administration of bacteria, but not after immunization with ferritin. Daily feeding with bacteria did not give detectable antibodies in serum, while regular oral administration of ferritin resulted in an increase of specific antibodies during the first 3 weeks. From that time immunosuppression was observed, as the antibody titre decreased despite the continued ferritin feeding. Immunosuppression was also found after a second anal intubation or i.m. injection with ferritin, independent of the route of priming(i.m. or anal). On thecontrary, asecondanalintubationofbacteriaresultedinasecondary serum response. Theseresultscombined withthosereportedin PartsIandIIofthestudyindicatean important immunological role for the second gut segment. Because mucosal as well as serum responses can be obtained by anal immunization with bacteria, the significance for oral vaccination is discussed.