Mucosal and systemic antibody responses after peroral or intranasal immunization: Effects of conjugation to enterotoxin B subunits and/or of co‐administration with free toxin as adjuvant

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Abstract
The mucosa‐binding molecules cholera toxin (CT) from Vibrio cholerae and heat‐labile enterotoxin (LT) from Escherichia coli have previously been used as mucosal adjuvants and carriers for many types of antigen. However, since these molecules are toxic and cannot be used in human vaccines, it is important to study whether their non‐toxic mucosa‐binding B subunits, CTB and LTB, can be used as alternative safe mucosal adjuvants and/or carrier molecules. We have as a model protein antigen used human gammaglobulin (HGG) for admixture with or chemical conjugation to recombinantly produced CTB and LTB, respectively, and measured antigen‐specific local secretory IgA antibodies in saponin extracts from intestine and lung tissue by ELISA following intra‐nasal (i.n.) or per‐oral (p.o.) immunization. The results show that local antibody formation against HGG was increased after immunization with conjugated as compared to free HGG. However, while the conjugates alone gave rise to significant immune responses in the lung and also, to a lesser degree, in the intestine after i.n. immunization, co‐administration of a small amount of free CT/LT as adjuvant was needed to induce a significant immune response in the intestine after p.o. immunization. We also found that following i.n. immunization, the addition of CTB to HGG, without coupling, increased the mucosal immune response to some extent, indicating that CTB by itself can work as an adjuvant by the i.n. route of immunization. A striking finding was that, as a carrier, CTB was superior to LTB when the conjugates were used by the oral but not by the i.n. route of immunization. In conclusion, conjugation of an antigen to mucosa‐binding molecules such as CTB and/or LTB can dramatically increase their mucosal immunogenicity. This approach may thus be useful in the preparation of mucosal vaccines.