Characterization of protective immune responses induced by nasal influenza vaccine containing mutant cholera toxin as a safe adjuvant (CT112K).

Immune responses induced by a nasal influenza vaccine with a mutant cholera toxin (CT112K), known to be a safe adjuvant, were characterized in BALB/c mice to confirm the most suitable regimen of this vaccine for humans. Mice received a primary intranasal administration of the adjuvant (0.1μg)-combined PR8 vaccine (0.1μg) and a secondary administration of the PR8 vaccine alone (0.1μg) 4 weeks later. Two weeks after the secondary immunization, the mice were infected with a nonlethal or a lethal dose of PR8 viruses. Nasal and lung wash virus titers 1 or 3 days after infection indicated that complete protection could be provided by secondary immune responses, which had an immediate effect of preventing infection 2 weeks after the secondary immunization. In this two-dose regimen, high levels of secondary IgA, IgG and IgM antibody-forming cell (AFC) responses were induced in the nasal-associated lymphoid tissue and the spleen. In parallel with the AFC responses, high levels of nasal wash anti-PR8 HA IgA, and lung and serum IgG antibody (Ab) responses were induced 2 weeks after the secondary immunization. The two-dose regimen also induced accelerated delayed-type hypersensitivity responses, which exhibited almost the same peak height as that in the case of the primary response. In addition, the two-dose regimen induced a low memory cell activity of cytotoxic T lymphocytes, detected by in vitro culture of spleen cells. Thus, the immediate effect of preventing infection was mainly provided by the secondary Ab responses. Moreover, the levels of nasal wash IgA Abs correlated well with cross-protection against infection with variant viruses in the upper respiratory tract (RT). These results suggest that the major protective factors among Ab and T cell-mediated immune responses, which are induced by the two-dose regimen using CT112K-combined vaccines, are the cross-reactive IgA Abs in the upper RT and the less cross-reactive IgG Abs in the lower RT, and that the two-dose regimen is a suitable vaccination condition for humans.