Journal of virology 2017 09 2791(20) pii 10.1128/JVI.01184-17
Adenovirus (Ad)-based immunization is a popular approach in vaccine development, and Ad-based vectors are renowned for their potential to induce strong CD8(+) T cell responses to the encoded transgene. Surprisingly, we previously found in the mouse Friend retrovirus (FV) model that Ad-based immunization did not induce CD8(+) T cell responses to the FV Leader-Gag-derived immunodominant epitope GagL85-93 We show now that induction of GagL85-93-specific CD8(+) T cells was highly effective when leader-Gag was delivered by plasmid DNA immunization, implying a role for Ad-derived epitopes in mediating unresponsiveness. By immunizing with DNA constructs encoding strings of GagL85-93 and the two Ad-derived epitopes DNA-binding protein418-426 (DBP418-426) and hexon486-494, we confirmed that Ad epitopes prevent induction of GagL85-93-specific CD8(+) T cells. Interestingly, while DBP418-426 did not interfere with GagL85-93-specific CD8(+) T cell induction, the H-2D(d)-restricted hexon486-494 suppressed the CD8(+) T cell response to the H-2D(b)-restricted GagL85-93 strongly in H-2(b/d) mice but not in H-2(b/b) mice. This finding indicates that competition occurs at the level of responding CD8(+) T cells, and we could indeed demonstrate that coimmunization with an interleukin 2 (IL-2)-encoding plasmid restored GagL85-93-specific CD8(+) T cell responses to epitope strings in the presence of hexon486-494 IL-2 codelivery did not restore GagL85-93 responsiveness in Ad-based immunization, however, likely due to the presence of further epitopes in the Ad vector. Our findings show that seemingly immunodominant transgene epitopes can be dominated by Ad-derived epitopes. These findings underline the importance of thorough characterization of vaccine vectors, and modifications of vectors or immunogens may be required to prevent impaired transgene-specific immune responses.IMPORTANCE Ad-based vectors are widely used in experimental preclinical and clinical immunization studies against numerous infectious agents, such as human immunodeficiency virus, Ebola virus, Plasmodium falciparum, or Mycobacterium tuberculosis Preexisting immunity to Ad-based vectors is widely recognized as a hindrance to the widespread use of Ad-based vectors for immunizations in humans; however, our data show that an immune response to Ad-derived T cell epitopes can also result in loss or impairment of transgene-specific immune responses in prenaive vaccinees due to immune competition. Our results highlight that seemingly immunodominant epitopes may be affected by dominance of vector-derived epitopes, and modifications of the vector design or the immunogens employed in immunization may lead to more effective vaccines.