Food Additives as Novel Influenza Vaccine Adjuvants

Abstract

Influenza is a major threat to public health. Vaccination is an effective strategy to control influenza; however, the current inactivated influenza vaccine has mild immunogenicity and exhibits suboptimal efficacy in clinical use. Vaccine efficacy can be improved by the addition of adjuvants, but few adjuvants have been approved for human use. To explore novel and effective adjuvants for influenza vaccines, here we screened 145 compounds from food additives approved in Japan. Of these 145 candidates, we identified 41 compounds that enhanced the efficacy of the split influenza hemagglutinin (HA) vaccine against lethal virus challenge in a mouse model. These 41 compounds included 18 novel adjuvant candidates and 15 compounds with previously reported adjuvant effects for other antigens but not for the influenza vaccine. Our results are of value to the development of novel and effective adjuvanted influenza or other vaccines for human use.

Keywords: adjuvants; food additives; influenza; mice; vaccine.

Figure 1

Virus-specific antibody titers induced in mice by the 59 hit compounds in combination with the HA vaccine. Six-week-old BALB/c mice were immunized with the influenza HA vaccine with or without compounds twice with a two-week interval between the vaccinations. Blood samples were collected two weeks after the second immunization. Virus-specific antibodies were measured by using an ELISA with inactivated and purified CA07 virus as the coating antigen. Depicted are the antibody titers obtained from the mice immunized with the vaccine with the hit compounds. Each dot represents one mouse, and the individual antibody titers were divided by the average of the antibody titers of the mice immunized with the vaccine plus alum in the same batch. This procedure normalizes the values of the animals immunized with candidate compounds to their respective positive controls, and the log transformation aids in the representation of the values. Values above zero indicate that the antibody titers of the mice treated with the vaccine plus the hit compounds were higher than those of their positive controls. The black horizontal line represents the mean antibody titers from individual mice (n = 3, except sage oil n = 4). The dotted line represents the reference vaccine plus alum. The status of the 59 hits is indicated by the color used for the compound names: red compound names represent novel adjuvant candidates; blue compound names represent novel adjuvant candidates for the influenza vaccine; compounds with green names indicate that their adjuvant effect for the influenza vaccine has been reported previously. MW: molecular weight.

Figure 2

Body weight changes and survival rates of immunized mice after lethal challenge. Six-week-old BALB/c mice were mock-immunized with phosphate-buffered saline (PBS) or compounds only, or they were immunized with the HA vaccine alone or the compound-adjuvanted HA vaccine twice with a two-week interval. Mice were intranasally challenged with 10 MLD₅₀ of MA-CA04 virus three weeks after the second immunization. Body weight and survival were monitored for 14 days. The body weight data shown are means and standard deviations (n = 4). Green asterisks indicate a significant difference between the vaccine alone and the vaccine plus compound groups (vaccine plus compound versus vaccine alone); purple asterisks indicate a significant difference between the vaccine plus alum and the vaccine plus compound groups (vaccine plus alum versus vaccine plus compound); * p < 0.05.