Induction of innate immunity by nasal influenza vaccine administered in combination with an adjuvant (cholera toxin).

Inactivated influenza vaccine was administered intranasally to BALB/c mice together with an adjuvant (cholera toxin B subunit [CTB] supplemented with a trace amount of the whole toxin, CTB*) and its ability to induce innate immunity and confer protection against influenza was examined. Nasal wash virus titres 3 days after inoculation of homologous viruses were measured as an index of the ability of the vaccine to confer protection in mice immunized with either CTB*-combined vaccine or CTB* alone 1-21 days previously. The results were as follows. (1) Partial but significant reduction of the nasal-wash virus titres (prevention) was detected beginning 3 days after the vaccination, that is, 2 days earlier than the appearance of both virus-specific antibody-forming cells (AFCs) in the nasal-associated lymphoid tissue (NALT) and virus-specific IgA antibody responses in the nasal washes of mice immunized with the CTB*-combined vaccine. (2) The protection, detected on day 3 and peaking on day 5 but lost by day 21, was also conferred in mice immunized with CTB* alone. (3) The non-specific prevention was detected at doses of more than 0.3 microg of CTB*/mouse. (4) The nonspecific protection beginning 3 days after the immunization involved the enhanced expression of cytokine mRNAs (IL-15 and IL-18), considered responsible for natural killer (NK) cell activation, by the non-T cell populations in the NALT. (5) Normal NALT cells, when cultured in vitro with CTB*, secreted IL-1beta within a few hours in culture. These results demonstrate that the CTB*-combined vaccine, when given intranasally into mice, can confer nonspecific protection against influenza beginning 3 days after the vaccination and that CTB* also possessed this ability to confer protection non-specifically and temporarily by inducing the secretion of IL-1beta, one of the most important cytokines that initiates both innate and adaptive immunity, and also NK cell activity.