Squalene or squalane (hydrogenated form of squalene) has been used as an alternative to mineral oil for its tolerance and investigated extensively in vaccine adjuvant applications. A number of clinical trials indicate that squalene-in-water emulsions outperform aluminum salts or W/O emulsions at increasing vaccine immunogenicity and affording cross-reactivity without causing unacceptable adverse reactions. The most successful adjuvants of influenza vaccines for human use are squalene-based oil-in-water emulsions, such as MF59 (Novartis) and ASO3 (GlaxoSmithKline). In this study, swine influenza virus was used as a model antigen to investigate the new adjuvant’s immunogenicity. Hence, developing new adjuvant formulations with improved potency and safety will be of great value and impact both in the swine industry and for global public health. In addition, post-immunization reactions associated with mineral oil-based adjuvants have been reported, such as edema, abscess/granuloma formation or necrosis at injection sites, which render pork unfit for consumption and greatly limits the wide application of these vaccines. However, previous studies demonstrate that commercial vaccines in a water-in-oil (W/O) formulation is not highly effective in preventing vaccinated pigs from wild-type virus infection. Vaccination is the primary strategy for prevention and control of swine influenza in China and other countries. Furthermore, it has been hypothesized that pigs can serve as an intermediate host for the adaptation of avian influenza viruses to humans or as mixing vessels for the generation of reassortant viruses. Although mortality is usually low, swine influenza may result in poor growth, weight loss, immunosuppression of infected pigs and economic loss in the pig industry. Swine influenza is an acute respiratory disease of pigs mainly caused by the influenza A virus. Developing new adjuvant formulations with improved potency and safety will be of great value. However, traditional oil-based emulsion adjuvants, such as Freund’s complete or incomplete adjuvant, have been reported with post-immunization reactions. Emulsions have a long history as adjuvants for both human and animal vaccines. Vaccination is the principal strategy for prevention and control of diseases, and adjuvant use is an effective strategy to enhance vaccine efficacy. The superiority of EAS to the conventional W/O formulation in adjuvant activity, safety and stability will make it a potential veterinary adjuvant. The O/W emulsion EAS developed in the present work induced potent humoral and cellular immune responses against inactivated swine influenza virus, conferred effective protection after homologous virus challenge and showed low toxicity in mice, indicating that EAS is as good as the commercial adjuvant MF59. Mice immunized with EAS-adjuvanted influenza vaccine conferred potent protection after homologous challenge. EAS-adjuvanted vaccine elicited significantly stronger IgG1 and IgG2a antibodies and higher concentrations of Th1 (IFN-γ and IL-2) cytokines compared to the W/O vaccine or antigen alone. The results demonstrated that in mice EAS-adjuvanted vaccine induced significantly higher titers of hemagglutination inhibition (HI) and IgG antibodies than water-in-oil (W/O) vaccines or antigen alone, respectively, at day 42 post vaccination (dpv) ( P < 0.05). EAS-adjuvanted H3N2 swine influenza vaccine was administrated in mice subcutaneously to assess the adjuvant potency of EAS. The data show that EAS is a homogeneous nanoemulsion with small particle size (~ 105 nm), low viscosity (2.04 ± 0.24 cP at 20 ☌), excellent stability (at least 24 months at 4 ☌) and low toxicity. In the study reported herein, a novel oil-in-water (O/W) Emulsion Adjuvant containing Squalane (termed EAS) was developed, characterized and investigated for swine influenza virus immunization. Traditional mineral oil-based adjuvants have been reported with post-immunization reactions.
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