Effectiveness of cholera toxin B subunit as an adjuvant for nasal influenza vaccination despite pre-existing immunity to CTB

In a previous paper, it was shown that cholera toxin B subunit (CTB) augments the production of protective antibodies to influenza virus when CTB is inoculated intranasally into Balb/c mice together with influenza HA vaccine¹. The present study was designed to determine whether the effectiveness of CTB as a potent adjuvant for nasal vaccination could be limited by pre-existing immunity to CTB. Mice were sensitized by intranasal inoculation of either 1 or 0.05 μg of CTB 2, 4 and/or 6 weeks before nasal vaccination. They were then vaccinated by either a single inoculation of vaccine together with 1 μg of CTB or a two-dose regimen, composed of a primary inoculation of vaccine together with 0.05 μg of CTB and a subsequent inoculation of vaccine alone¹⁻³. Levels of nasal IgA antibodies to CTB increased with the increase of the dose of CTB and the frequency of CTB-inoculation. Pre-existing immunity to CTB, however, did not significantly reduce the levels of both nasal IgA and serum haemagglutination-inhibiting (HI) antibodies to influenza virus and did not change the ability of the vaccinated mice to resist viral challenge. These results suggest that a relatively low dose of CTB could be inoculated repeatedly into animals as an adjuvant for nasal vaccination.