Mechanism of enhancement of the immune responses to influenza vaccine with cholera toxin B subunit and a trace amount of holotoxin.

Cholera toxin B subunit (CTB) (1 microgram) and a trace amount of cholera toxin (CT) (0.1-10 ng), when inoculated intranasally into Balb/c mice together with influenza vaccine, induced synergistically a greater delayed-type hypersensitivity (DTH) response to the vaccine than did a trace amount of CT alone. In parallel with the in vivo response, normal peritoneal macrophages that were incubated in vitro with the vaccine and the CT-containing CTB, induced a higher adenylate cyclase activity and a greater ability to transfer DTH response into naive recipient mice than did the macrophages incubated with the vaccine and CT. The treatment of macrophages with the vaccine and CTB failed to induce either adenylate cyclase or DTH response. From these results, the mechanism by which CTB and a trace amount of CT enhance immune responses synergistically could be explained by the enhancement of the CT action on macrophages or by the efficient binding of a trace amount of CT to antigen-presenting cells in the presence of a relatively large amount of CTB, resulting in enhanced cyclic AMP formation followed by enhanced antigen presentation.