Adjuvants are included in vaccine formulations to enhance the immunogenicity and efficacy of vaccines. MF59® is an oil-in-water emulsion adjuvant and licensed for use in pandemic and seasonal influenza vaccines in many countries. MF59 is safe and well tolerated in humans. MF59-adjuvanted vaccination spares vaccine dose and enhances hemagglutination inhibiting antibodies against homologous and heterologous influenza virus strains. The mechanisms of MF59 involve rapid induction of chemokines, inflammatory cytokines, recruiting multiple immune cells, uric acid and benign apoptosis of certain innate immune cells. The adjuvant effects of MF59 on generating vaccine-specific isotype-switched IgG antibodies, effector CD8 T cells, and protective immunity were retained even in a CD4-deficient condition by inducing effective immune-competent microenvironment with various innate and antigen presenting cells in a mouse model. CD4-independent adjuvant effects of MF59 might contribute to improving the vaccine efficacy in children, the elderly, and immunocompromised patients as well as in healthy adults. Further studies will be needed to broaden the use of MF59 in various vaccine antigens and populations as well as lead to better understanding of the action mechanisms of MF59 adjuvant.
Keywords: CD4-independent; Influenza; MF59; adjuvant; immune mechanisms.