Abstract
In the phase IIb STEP trial an HIV-1 vaccine based on adenovirus (Ad) vectors of the human serotype 5 (AdHu5) not only failed to induce protection but also increased susceptibility to HIV-1 infection in individuals with preexisting neutralizing Abs against AdHu5. The mechanisms underlying the increased HIV-1 acquisition rates have not yet been elucidated. Furthermore, it remains unclear if the lack of the vaccine's efficacy reflects a failure of the concept of T cell-mediated protection against HIV-1 or a product failure of the vaccine. Here, we compared two vaccine regimens based on sequential use of AdHu5 vectors or two different chimpanzee-derived Ad vectors in rhesus macaques that were AdHu5 seropositive or seronegative at the onset of vaccination. Our results show that heterologous booster immunizations with the chimpanzee-derived Ad vectors induced higher T and B cell responses than did repeated immunizations with the AdHu5 vector, especially in AdHu5-preexposed macaques.