Cholera toxin adjuvant promotes long-term immunological memory in the gut mucosa to unrelated immunogens after oral immunization.

This study was conducted to investigate whether cholera toxin (CT), used as a mucosal adjuvant, would promote the development in mice of immunological memory to unrelated antigens administered by the oral route. We found that oral priming immunizations with keyhole limpet haemocyanin (KLH) in combination with CT adjuvant induced long-term, at least 22 months and perhaps lifelong, immunological memory in the intestinal lamina propria (LP). In contrast, oral priming immunizations with KLH alone failed to stimulate immunological memory. Moreover, memory responses in the KLH plus CT-immunized mice were elicited by antigen alone, i.e. without CT adjuvant, suggesting that once immunological memory is established in the intestinal mucosa, e.g. by oral vaccination, elicitation of secondary-type responses does not require the presence of CT and thus could result from re-encounter with specific bacterial or viral antigens in the intestine. We also found that a single priming-dose of KLH plus CT adjuvant was sufficient to stimulate long-term, antigen-specific memory in the intestinal mucosa. Finally, the ability of CT to induce immunological memory in the gut mucosa required the whole toxin and could not be achieved by using the toxoid, the cholera toxin B subunit (CTB), which lacks the adenylate cyclase/cAMP-activating property of the whole molecule. The results support the view that mucosal adjuvants, incorporated into oral vaccines, might be an effective means to achieve long-term immunological memory and protection against pathogenic micro-organisms at mucosal surfaces.