Vaccines with MF59 Adjuvant Expand the Antibody Repertoire to Target Protective Sites of Pandemic Avian H5N1 Influenza Virus

An oil-based adjuvant improves the efficacy of an H5N1 vaccine by inducing antibodies against additional sites on influenza surface proteins. Revving Up a Flu Vaccine By preventing incalculable illnesses, vaccines are one of medicine’s great triumphs. Part of the credit goes to the adjuvants, substances included in vaccine preparations that boost the immune response but have no effect on their own. These agents—as various as aluminum salts and lipids—are thought to activate the innate immune system, the generalized protective response most animals show to pathogens. But the precise mechanisms by which these agents augment the immune response continue to elude scientists. Khurana and colleagues have now closely compared the immune response to an avian influenza vaccine without adjuvant to that induced with an oil-in-water adjuvant. Mixing the adjuvant with the vaccine induced antibodies that recognized a wider variety of flu antigens, including some known to inactivate the virus. The adjuvant examined by these authors was MF59—an emulsion of squalene, a natural hydrocarbon, and several fatty acids—which is used to boost influenza virus vaccines marketed in Europe. Flu vaccines that include MF59 engender more neutralizing antibodies than those that do not. To better understand the characteristics of these additional antibodies to flu proteins, the authors analyzed sera from subjects in two clinical trials of vaccines against the avian flu H5N1, with and without MF59. As expected, MF59 increased the amount of antibody against H5N1 flu, but the diversity of the new antibodies was also amplified. The authors looked specifically for antibodies directed at regions of the hemagglutinin protein and at neuraminidase, through which the virus binds to and infects its host’s cells. [The H5N1 name of the influenza strain refers to the exact type of hemagglutinin (H) and neuraminidase (N) proteins carried by the virus.] MF59 caused production of more antibodies directed against the hemagglutinin region that binds host cells, rather than the region that anchors the protein in the membrane, and many more regions within these areas were targeted. Only vaccines with MF59 generated antibodies against long sequences and against the native conformation of the host cell binding region. These particular antibodies are important because they are the ones that successfully block infection—the ultimate goal of any vaccine. They were also able to bind to and neutralize hemagglutinin from other strains of H5N1 flu—from Indonesia and China—much more effectively than were antibodies induced by vaccines without adjuvant; this finding showed that MF59-containing vaccines also have the desirable feature of extending protection beyond the particular flu strain used for vaccination. This study does not reveal exactly how the wider immune repertoire induced by MF59 is generated. But it does begin to define precisely the nature of the adjuvant-induced antibodies to influenza and to explain why they are more effective than those made in response to vaccines without adjuvant. MF59 is the adjuvant used in some vaccines against the current pandemic H1N1 flu, and the conclusions from this study are likely to apply to those vaccines as well. Vaccines against influenza viruses with pandemic potential, including H5N1, are under development. Because of a lack of preexisting immunity to these viruses, adjuvants (immune potentiators or enhancers) are needed to improve immune responses, to conserve scarce vaccine, and for cross-protection against strains that have drifted evolutionarily from the original. Aluminum-based adjuvants do not improve vaccine immunogenicity for influenza subunit vaccines, whereas oil-in-water adjuvants are effective, especially with H5N1-inactivated vaccines. We used whole-genome-fragment phage display libraries followed by surface plasmon resonance (SPR) technologies to elucidate the effect of different adjuvants on the antibody repertoire against H5N1 vaccine in humans. The oil-in-water adjuvant MF59 induced epitope spreading from HA2 to HA1 in hemagglutinin (HA) and neuraminidase relative to unadjuvanted or aluminum-adjuvanted vaccines. Moreover, we observed an increase by a factor of 20 in the frequency of HA1-to-HA2–specific phage clones in sera after MF59-adjuvanted vaccine administration and a factor of 2 to 3 increase in the avidity of antibodies binding to properly folded HA1(28–319), as measured by SPR. The adjuvant-dependent increase in binding to conformational HA1 epitopes correlated with broadening of cross-clade neutralization and predicted improved in vivo protection. Thus, MF59 adjuvant improves the immune response to a H5N1 vaccine by inducing qualitative and quantitative expansion of the antibody repertoires with protective potential.