It is widely accepted that novel influenza virus vaccination strategies are urgently needed both to control the spread of HPAI within fowl species and to prevent the pandemic spread of HPAI in humans should the capacity for human-to-human transmission emerge. In this study, we have tested the ability of adenovirus-based immunization to induce both broad and potent HA-specific humoral and cellular immune responses able to confer protection against lethal intranasal challenge, given its promise in other vaccine applications (31
) and recent promising results of immunization of humans with an influenza virus HA vaccine based on an adenoviral platform (37
). A broadly cross-protective vaccination could be useful for the treatment of domestic animals as well as humans, and adenoviral vectors may be a practical alternative to propagating vaccines with conventional methods in embryonated chicken eggs (2
Previous studies using inactivated whole H5N1 influenza virus vaccines in mice have indicated that strain-specific neutralizing antibodies provide long-lasting protection against homologous influenza virus challenge (33
), but protection is limited against antigenically variant strains Two recent studies have demonstrated the efficacy and immunogenicity of adenovirus-vectored influenza virus HA (H3N2) vaccines in swine and mice and have revealed that cross-protection from heterotypic challenge can occur in the absence of neutralizing humoral immunity (37
). Our study builds upon these findings, going one step further in trying to understand the role of the T-cell response to an adenovirus-based influenza virus HA vaccine. The presence of heterotypic H5N1 protection in the absence of a strong humoral neutralizing response in our initial studies, further reinforced by the ability of adenovirus-based HA2 vaccine to partially protect the immunized animal (the HA2 region is known not to induce neutralizing humoral immunity), strongly suggests a complementary role for the cellular response to its humoral counterpart. This idea is supported by recent findings showing that DNA-prime adenoviral boosting using the anti-H1N1 nucleoprotein (NP) influenza virus vaccine induced a T-cell response that was able to protect from heterosubtypic challenge (11
). Together, these data suggest that humoral immunity in the context of an influenza virus vaccine is a ready response that, when strong, fully protects an animal from homologous challenge. In contrast, a T-cell recall response is delayed in its action (7 to 8 days in mice) but has the advantage of giving broader protection against different influenza virus subtypes. The ability of DNA-based influenza virus vaccination using adenoviral delivery or other methods (1
) to induce a T-cell response able to cross-protect at least partially against heterotypic infection could represent an added value, rendering further investigation in the influenza virus recombinant vaccine development field worthwhile. Adenovirus might have some advantages over other DNA-based technologies based on its relatively safe profile, which has been proven through the vaccination of volunteers in more than 100 phase I, II, and III clinical trials. Moreover, the adenovirus production yield, particularly the ability to scale up to large quantities, make this technology one of the most promising DNA-based platforms for vaccination.
Natural vector-specific immunity of some human populations against adenovirus serotype 5 (28
) could potentially reduce vaccine efficacy in the event that global vaccination against HPAI is implemented. In a recent report, however, adenovirus serotype 5-based vaccines against influenza virus A/PR/8 (H1N1) were tested successfully in humans in a phase I clinical trial and shown to be safe (37
). Importantly, vaccination was highly effective in inducing anti-influenza virus-neutralizing antibodies despite the presence of preexisting antiadenoviral antibodies, suggesting that vector-specific immunity may be overcome (37
). Alternatively, a wide range of different human and simian adenovirus serotypes are being developed as alternative vectors, which will likely negate the issue of preexisting serotype 5-specific immunity (12
In conclusion, our findings as well as those from other adenovirus-based vaccine studies support the development of replication-defective adenovirus-based vaccines as a rapid response in the event of the pandemic spread of HPAI. Given the induction of protective immunity in chickens, widespread immunization of susceptible poultry would likely provide a significant barrier to the spread of HPAI and also be economically advantageous. Finally, in the worst-case scenario of the pandemic spread of lethal human disease, an adenovirus-based vaccine could be utilized to complement traditional inactivated influenza virus vaccine technology, which is still the primary choice but at the same time may become hampered by the limits to production capability in fertilized eggs.