124. Optimizing Adenoviral-Based Vaccination by Combining an RGD-Modified Capsid with a Helper-Dependent Adenovirus
Authors: Airi Harui, Michael D. Roth, Saroj K. Basak
Journal: Molecular Therapy (2006)

Replication-deficient adenoviral vectors (AdV) provide a potentially important approach to gene-based vaccination. However, several limitations have been identified with AdV vectors. The first generation of AdV vectors (FG-AdV), with the entire viral genome except for deletion of the E1/E3 regions, are replication incompetent but still express residual adenoviral genes that act as targets for anti-viral immune responses, limit antigen expression, suppress MHC expression and limit antigen-presenting cells function. Finally, since dendritic cells do not express the Coxsackie-Adenoviral Receptor (CAR), they are relatively refractory to transduction by FG-AdV, thus limiting antigen delivery and vaccination efficiency. So-called gutless or helper-dependent AdV vectors (HD-AdV), devoid of all viral genome were developed to address some of these issues. We recently evaluated HD-AdV and FG-ADV for the induction of anti-viral antibody responses and transgene-specific CTL response in a mouse vaccination model. Surprisingly, injection of 108 BFU of either vector resulted in high titers of anti-viral antibodies directed against viral capsid antigens. However, significantly higher CTL responses against the transgene antigen were induced by the HD-AdV. Next, to evaluate the impact as a cancer vaccine, mice were vaccinated with 108 BFU of HD-AdV and then challenged with a tumor expressing the transgene antigen (β-gal). Complete tumor rejection was achieved with HD-AdV, but not by the same dose of FG-AdV and 60% higher CTL responses was induced with HD-AdV than mice vaccinated with FG-AdV. By comparing expression of vector DNA to expression of transgene mRNA, we determined that HD-AdV transduced DC with the same efficiency as conventional FG-AdV, but produced dramatically higher transgene expression. As such, we proposed that the potency of a HD-AdV could be further enhanced by packaging it in a RGD-modified capsid. Preliminary studies demonstrate that the interaction of RGD motifs on the viral capsid with integrin molecules on DC promote viral entry. By expressing a RGD sequence on the fiber knob, where it can optimally interact, we demonstrated that transduction of DC is enhanced 10 to 50-fold. Our current hypothesis is that the vaccine efficiency of a HD-AdV can be further enhanced by packaging it in an RGD-modified capsid, which would promote entry into DC. A HD-AdV with GFP transgene has been packaged into a RGD-modified capsid to develop RGD-HD-AdV/GFP vector. The capacity for this combined vector to transduce DC, express transgene antigen, and induce anti-tumor immune responses is currently being determined. We propose that the RGD-HD-AdV vector will prove superior when employed for vaccine purposes. This vector does not express any viral coding genes, does not suppress MHC or DC function, produces very high levels of transgene antigen expression, and with the use of RGD-sequences should demonstrate a much higher transduction efficiency for DC. As a result, very small viral inoculations should produce potent responses while limiting the induction of innate or adaptive anti-viral responses. Supported by the UCLA SPORE in Prostate Cancer, NIH/NCI (MDR) and the UCLA Human Gene Medicine Grant (SKB).