RESEARCH ARTICLE Oral Priming with Replicating Adenovirus Serotype 4 Followed by Subunit H5N1 Vaccine Boost Promotes Antibody Affinity Maturation and Expands H5N1 Cross-Clade

A Phase I trial conducted in 2009–2010 demonstrated that oral vaccination with a replica-tion competent Ad4-H5 (A/Vietnam) vector with dosages ranging from 107-1011 viral parti-cles was well tolerated. HA-specific T-cell responses were efficiently induced, but very limited hemagglutination-inhibiting (HI) humoral responses were measured. However, a sin-gle boost of Ad4-H5-Vtn vaccinated individuals with a unadjuvanted licensed H5N1 (A/Viet-nam) subunit vaccine resulted in superior HI titers compared with unprimed subjects. In the current study, the impact of Ad4-H5 priming on the quality of the polyclonal humoral immune response was evaluated using a real-time kinetics assay by surface plasmon resonance (SPR). Total binding of serum polyclonal antibodies from the Ad4-H5-Vtn primed groups against both homologous H5N1-A/Vietnam/1194/2004 (clade 1) and heterologous A/Indo-nesia-5/2005 (clade 2.1) HA1 head domain was significantly higher compared with sera from individuals that received subunit H5N1 vaccination alone. SPR measurements also demonstrated that the antigen-antibody complex dissociation rates (a surrogate for anti-body affinity) of serum antibodies against the HA1 of H5N1-A/Vietnam were significantl