Enhanced immunogenicity of HPV 16 E7 fusion proteins in DNA vaccination

DNA vaccination is a promising approach for inducing both humoral and cellular immune responses. For immunotherapy of HPV-16-associated diseases the E7 protein is considered a prime candidate, as it is expressed in all HPV-16-positive tumors. Unfortunately, the E7 protein is a very poor inducer of a cytotoxic T-cell response, when being used as antigen in DNA vaccination. Here we demonstrate that after fusion to protein export/import signals such as the herpes simplex virus ferry protein VP22, E7 can translocate in vitro from VP22-E7-expressing cells to neighboring cells that do not carry the VP22-E7 gene. In vivo, the VP22-E7 fusion shows significantly increased efficiency in inducing a cytotoxic T-cell response. Our data suggest that the export function of VP22 plays a major role in this phenomenon, since VP22 can be replaced by classical protein export signals, without impairing the induction of the E7-specific cellular immune response. However, all E7 fusion constructs showed significantly elevated protein steady-state levels, which might also account for the observed boost in immunogenicity.