Comparison of the reactivities and immunogenicities of procholeragenoid and the B subunit of cholera toxin in Thai volunteers

Procholeragenoid, a stable high-molecular-weight aggregate of cholera toxin derived by heat treatment, was evaluated for reactivity and immunogenicity in adult Thai volunteers. Since procholeragenoid is known to retain some residual activity of cholera toxin, increasing amounts were ingested until diarrhea occurred; 250 micrograms induced diarrhea, but 100 micrograms did not. Procholeragenoid and cholera toxin B subunit, both in 100-micrograms amounts, were then compared for systemic and intestinal antitoxin responses. When three peroral doses were given, these immunogens gave comparable responses. The secretory immunoglobulin A antitoxin responses to three doses of 100 micrograms of B subunit did not differ significantly from responses found in previous studies of Thai adults given 1 or 5 mg of B subunit, but serum antitoxin responses were less after 1 or 2 doses of 100 micrograms than after doses of 1 or 5 mg. Serum antitoxin levels were similar after 3 doses of B subunit. Procholeragenoid in the maximum safe dose of 100 micrograms does not offer any immunologic advantage over B subunit, although it may be less expensive and easier to produce. However, these studies suggest that higher amounts of B subunit are more immunogenic and may be preferable, if found to be sufficiently cost effective, when added to oral killed whole Vibrio cholerae vaccines.