Structural elements of a protein antigen determine immunogenicity of the embedded MHC class I-restricted T cell epitope

Substantial effort has been invested into optimization of vector structure, DNA formulation, or delivery methods to increase the effectiveness of DNA vaccines. In contrast, it has been only insufficiently explored how the higher order structure of an antigenic protein influences immunogenicity of embedded epitopes in vivo. Potent CD8(+) T cell responses specific for a single immunogenic epitope are induced upon electrovaccination with plasmid DNA encoding the full-length heavy chain of the human HLA-Cw3 molecule. Contrary to expectations, a minimal construct, which provoked a substantial release of IFN-gamma from specific CTLs in vitro, did not induce a significant response in vivo. Systematically altered variants of the Cw3 molecule were thus tested both in vivo and in vitro to determine which structural parts are responsible for this discrepancy. In complementation experiments the participation of trans-acting helper epitopes was ruled out. Successive C-terminal truncations, human/mouse domain swap variants, and subdomain modifications defined the alpha3 region of the HLA heavy chain and membrane anchoring as critical elements. Based on these data, refined minimal constructs were engineered that triggered very high in vivo responses. The most advanced variant consisted only of an adenoviral leader, antigenic epitope, alpha3 domain, and 16 aa of the transmembrane domain. When a tumor Ag epitope was incorporated into one of these high performer minimal constructs, protection against melanoma metastases was attained upon vaccination. Thus, structural elements of the Ag can dominantly influence immunogenicity in vivo. These elements can also markedly improve the immunogenicity of unrelated Ags and may form the basis of a new generation of DNA vaccines.