Induction and assessment of immunity at enteromucosal surfaces in humans: implications for vaccine development

It is now almost axiomatic that vaccines against enteric infections must be able to stimulate the gut lymphoid tissue to be efficacious and that this goal is usually better achieved by administering immunogens orally rather than parenterally. On the basis of the notion of a common mucosal immunologic system that provides immune reactivity not only at the site of antigen deposition but also at remote mucosal sites, there is much interest in developing oral vaccines against infections in the respiratory and urogenital tracts. Recent studies indicate that oral administration of small amounts of protein antigens that are chemically or genetically linked to intestinal binding carrier molecules (such as the B subunit of cholera toxin) can evoke vigorous mucosal and extramucosal immune responses. The apparent compartmentalization of systemic and mucosal immune responses may explain not only why parenteral vaccines are just partly effective in protecting against mucosal pathogens but also why currently employed immunoanalytical methods do not provide accurate information regarding mucosal immune status. Following the developments of oral vaccines against infections due to Vibrio cholerae and enterotoxigenic Escherichia coli, two archetypes of enteropathogens, we have devoted extensive efforts to gaining insight into the humoral and cellular events involved in the development of protective immunity in the human intestinal tract. We have developed preparative and analytical methods for studying vaccine-induced intestinal and extraintestinal immune responses in humans. These include techniques for collecting intestinal fluid and lymphoid cells as well as procedures to quantitate secretory antibodies and lymphokines secreted by activated intestinal immunocytes. These developments should serve the dual purpose of facilitating analyses of human mucosal immune responses in general and assessing the immunogenicity of enteric vaccines in particular.