A proposal for safety standards for human use of cholera toxin (or Escherichia coli heat-labile enterotoxin) derivatives as an adjuvant of nasal inactivated influenza vaccine

Cholera toxin (CT) and Escherichia coli heat-labile toxin (LT) are not only the causative agents of diarrhea but are also strong mucosal adjuvants which enhance immune responses to mucosally coadministered bystander antigens. One of the most promising applications of these toxins would be as mucosal adjuvant of nasal influenza vaccine. In comparison to current inactivated vaccines, the nasal vaccine provides superior cross-protection by inducing production of cross-reacting anti-viral IgA antibodies in the respiratory tract even when the vaccine strain is different from the epidemic strain. On the use of the toxins as mucosal adjuvants in humans, toxicity and allergenicity of the toxins are problems which impinge on safety. To resolve these problems, various approaches have been attempted to produce less toxic and less allergenic CT (or LT) derivatives. We now propose the following standards for human use of safer CT (or LT) derivatives as an adjuvant of a nasal influenza vaccine. Thus, CT (or LT) derivatives can be administered intranasally together with a current inactivated influenza vaccine, provided they meet the following criteria. 1) A single dose of the derivatives, administered intranasally by spraying, should be around 100 mu g/adult in a volume of less than 0.5 mi. 2) CT (or LT) derivatives should retain the properties of the native CT (or LT), i.e., the ability to augment secretory IgA and serum IgG Ab responses to viral surface glycoproteins, when administered intranasally together with an inactivated influenza vaccine. 3) CT (or LT) derivatives should not induce IgE Ab responses to the vaccine, as well as to the CT (or LT) itself. 4) The CT (or LT) should be nontoxic; the toxicity of the derivatives, as determined by the Y-1 adrenal cell assay, should not exceed 1/100 EC50 of the native CT (or 1/1000 ECi of the native CT). 5) CT (or LT) derivatives should not cause serious disease in guinea pigs when administered intranasally or intraperitoneally at the dose used in humans (around 100 mu g).