Fusion of the Human Cytomegalovirus pp65 Antigen with Both Ubiquitin and Ornithine Decarboxylase Additively Enhances Antigen Presentation to CD8 ⁺ T Cells in Human Dendritic Cells

Antigens are modified for targeting to the proteasome by ubiquitin (Ub) or by a Ub-independent system such as ornithine decarboxylase (ODC) and are presented by MHC class I molecules. In this study, Park et al. compare several constructs encoding the HCMV antigen pp65 fused to Ub and/or ODC. They demonstrate that the fusion of both Ub and ODC enhances antigen presentation of pp65 by human dendritic cells to CD8+ T cells. Antigenic molecules are modified for targeting to the proteasome by ubiquitin (Ub) or by a Ub-independent system such as ornithine decarboxylase (ODC) to be presented by MHC class I molecules. In this study, we compared the immunogenicity of human cytomegalovirus pp65 antigen fused with Ub and/or ODC, using RNA electroporation of human dendritic cells. Among the C-terminal mutants of Ub (G76, A76, and V76), Ub(G) showed the best ability to enhance the degradation of a target protein and stimulate T cells. The pp65 antigens fused with either Ub(G) or ODC enhanced the stimulation to CD8+ T cells, and the effects of Ub(G) and ODC were similar. Furthermore, the fusion of both Ub and ODC additively increased immunogenicity compared with the single-fusion proteins. The fusion of Ub(G) and ODC enhanced primarily the stimulation of CD8+ rather than CD4+ T cells and more efficiently induced pp65-specific T cells in vitro. These additive effects of Ub and ODC in antigen processing may provide improved strategies to stimulate CD8+ T cells for the development of immunotherapies against the variety of viral diseases and cancers.