Direct demonstration in intestinal secretions of an IgA memory response to orally administered Shigella flexneri antigens

In the present studies our previously characterized, isolated intestinal loop model in rabbits was used to follow kinetically the local IgA immune response of the intestine and to determine whether a local (IgA) memory response could be elicited after peroral immunization with S. flexneri antigens. Nineteen rabbits received 10¹⁰ live S. flexneri strain X16 (hybrid of S. flexneri and E. coli) orally on the day after surgical creation of the isolated ileal loop. The isolated intestinal loops were never exposed directly to the shigella antigens, and shigella were not cultured from these loops. The kinetics of the local IgA response (detected by enzyme-linked immunosorbent assay) in daily secretion from these isolated loops paralleled the IgA responses we described previously when the isolated loops were directly immunized with S. flexneri strain X16. These findings are consistent with the known recirculation of sensitized gut-associated lymphoid cells to other (not directly stimulated) mucosal surfaces. A group of 10 rabbits that did not have chronically isolated ileal loops were given three weekly peroral doses of 10¹⁰ live S. flexneri strain X16 and then were allowed to rest for 60 days after the third oral immunization. At this point, chronically isolated loops were created, and a single oral challenge with 10¹⁰ live S. flexneri strain X16 was given. A striking IgA memory response was found in secretions from this group of rabbits. No such memory response was seen when killed shigella were used to prime and challenge the rabbits. These studies provide conclusive data that peroral immunization will produce a local IgA anamnestic response.