Decreased Vector Gene Expression from E2b Gene-Deleted Adenovirus Serotype 5 Vaccines Intensifies Proinflammatory Immune Responses

Recombinant adenovirus serotype 5 (Ad5) vectors are promising vaccine candidates due to their intrinsic immunogenicity and potent transgene expression; however, widespread preexisting Ad5 immunity has been considered a developmental impediment to the use of traditional, or conventional, E1 and E3 gene-deleted Ad5 (Ad5[E1−]) vaccines. Even in the presence of anti-Ad5 immunity, recent murine and human studies have confirmed E2b gene-deleted Ad5 (Ad5[E1−,E2b−]) vaccines to be highly efficacious inducers of transgene-specific memory responses and significantly less toxic options than Ad5[E1−] vaccines. While these findings have been substantially confirmed, the molecular mechanisms underlying the different reactions to these vaccine platforms are unknown. Using cultures of human peripheral blood mononuclear cells (hPBMCs) derived from multiple human donors, we found that Ad5[E1−,E2b−] vaccines trigger higher levels of hPBMC proinflammatory cytokine secretion than Ad5[E1−] vaccines. Interestingly, these responses were generated regardless of the donors' preexisting anti-Ad5 humoral and cell-mediated immune response status.In vitro hPBMC infection with the Ad5[E1−,E2b−] vaccine also provoked greater Th1-dominant gene responses yet smaller amounts of Ad-derived gene expression than Ad5[E1−] vaccines. These results suggest that Ad5[E1−,E2b−] vaccines, in contrast to Ad5[E1−] vaccines, do not promote activities that suppress innate immune signaling, thereby allowing for improved vaccine efficacy and a superior safety profile independently of previous Ad5 immunity.