BackgroundIn the Friend retrovirus mouse model we developed potent adenovirus-based vaccines that were designed to induce either strong Friend virus GagL85–93-specific CD8+ T cell or antibody responses, respectively. To optimize the immunization outcome we evaluated vaccination strategies using combinations of these vaccines.ResultsWhile the vaccines on their own confer strong protection from a subsequent Friend virus challenge, the simple combination of the vaccines for the establishment of an optimized immunization protocol did not result in a further improvement of vaccine effectivity. We demonstrate that the co-immunization with GagL85–93/leader-gag encoding vectors together with envelope-encoding vectors abrogates the induction of GagL85–93-specific CD8+ T cells, and in successive immunization protocols the immunization with the GagL85–93/leader-gag encoding vector had to precede the immunization with an envelope encoding vector for the efficient induction of GagL85–93-specific CD8+ T cells. Importantly, the antibody response to envelope was in fact enhanced when the mice were adenovirus-experienced from a prior immunization, highlighting the expedience of this approach.ConclusionsTo circumvent the immunosuppressive effect of envelope on immune responses to simultaneously or subsequently administered immunogens, we developed a two immunizations-based vaccination protocol that induces strong immune responses and confers robust protection of highly Friend virus-susceptible mice from a lethal Friend virus challenge.Electronic supplementary materialThe online version of this article (doi:10.1186/s12977-017-0336-7) contains supplementary material, which is available to authorized users.

Human immunodeficiency virus is notorious for its ability to avoid clearance by therapeutic interventions, which is partly attributed to the establishment of reservoirs in latently infected cells and cells that reside in immunologically privileged B cell follicles. In the work presented here, we show that cells of the B cell follicle are equally infected by a simple mouse gammaretrovirus. Using fluorescently labeled Friend retrovirus, we found that B cells and T cells in the B cell follicle, while not carrying the bulk of the virus load, were indeed infected by Friend virus in the early acute phase of the infection and persisted in the chronic infection. Our results suggest that infection of follicular cells may be a shared property of lymphotropic viruses and propose the FV infection of mice as a useful model to study strategies for follicular reservoir elimination.

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 GagL We show now that induction of GagL-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 GagL and the two Ad-derived epitopes DNA-binding protein (DBP) and hexon, we confirmed that Ad epitopes prevent induction of GagL-specific CD8 T cells. Interestingly, while DBP did not interfere with GagL-specific CD8 T cell induction, the H-2D-restricted hexon suppressed the CD8 T cell response to the H-2D-restricted GagL strongly in H-2 mice but not in H-2 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 GagL-specific CD8 T cell responses to epitope strings in the presence of hexon IL-2 codelivery did not restore GagL 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. Ad-based vectors are widely used in experimental preclinical and clinical immunization studies against numerous infectious agents, such as human immunodeficiency virus, Ebola virus, , or 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.

Adenovirus (AdV)-based vectors are popular experimental vaccine vectors, but despite their ability to induce strong immune responses, their application is impeded by widespread preexisting immunity against many AdV types that can impair or even abrogate the induction of transgene-specific immune responses. Therefore, the development of vectors based on AdV types with a low seroprevalence is important for effective AdV-based immunization in humans. We investigated the immunization efficacy of vectors based on AdV type 48 (Ad48) and Ad50 in the ovalbumin (ova) model as well as the Friend retrovirus (FV) model, which allows testing of the protective effect of vaccine-induced immunity. Using ova-encoding vectors, we found a significantly lower induction of ova-specific CD8+ T cells and antibody responses by Ad48- and Ad50-based vectors than by Ad5-based vectors. Similarly, we found a reduced induction of FV-specific CD8+ T cell responses in Ad48- and Ad50.Leader-Gag-immunized mice compared with that in Ad5-immunized mice; however, some of those mice were able to control the FV infection, and protection correlated with the level of neutralizing antibodies 10 days after FV challenge. Analyses of the AdV-specific antibodies and CD8+ T cells induced by the individual AdV types revealed a high level of cross-reactivity, and the efficacy of Ad48-based immunization was impaired in Ad5-preimmune mice. Our results show that the immunity induced by Ad48- and Ad50-based vectors is reduced compared to that induced by Ad5 and is sufficient to control FV infection in only some of the immunized mice. A high level of cross-reactivity suggests that AdV preimmunity must be considered even when applying rare AdV-based vectors. IMPORTANCE AdV-based vectors are important tools for the development of vaccines against a wide range of pathogens. While AdV vectors are generally considered safe and highly effective, their application can be severely impaired by preexisting immunity due to the widespread seroprevalence of some AdV types. The characterization of different AdV types with regard to immunogenicity and efficacy in challenge models is of great importance for the development of improved AdV-based vectors that allow for efficient immunization despite anti-AdV immunity. We show that the immunity induced by an Ad48-based vector is inferior to that induced by an Ad5-based vector but can still mediate the control of an FV infection in highly FV-susceptible mice. However, the efficacy of Ad48-based immunization was impaired in Ad5-preimmune mice. Importantly, we found cross-reactivity of both the humoral and cellular immune responses raised by the individual AdV types, suggesting that switching to a different AdV type may not be sufficient to circumvent preexisting anti-AdV immunity.

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