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Department of NBE Discovery, Boehringer Ingelheim Austria GmbH, Vienna, Austria
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-
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
3 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,
3 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.
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