Highly efficient induction of protective immunity by a vaccinia virus vector defective in late gene expression

Vaccinia viruses defective in the essential gene coding for the enzyme urac il DNA glycosylase (UDG) do not undergo DNA replication and do not express late genes in wild-type cells. A UDG-deficient vaccinia virus vector carryi ng the tick-borne encephalitis (TBE) virus prM/E gene, termed vD4-prME, was constructed, and its potential as a vaccine vector was evaluated. High-lev el expression of the prM/E antigens could be demonstrated in infected compl ementing cells, and moderate levels were found under noncomplementing condi tions. The vD4-prME vector was used to vaccinate mice; animals receiving si ngle vaccination doses as low as 10(4) PFU were fully protected against cha llenge with high doses of virulent TEE virus. Single vaccination doses of 1 0(3) PFU were sufficient to induce significant neutralizing antibody titers , With the corresponding replicating virus, doses at least 10-fold higher w ere needed to achieve protection. The data indicate that late gene expressi on of the vaccine vector is not required for successful vaccination; early vaccinia virus gene expression induces a potent protective immune response. The new vaccinia virus-based defective vectors are therefore promising liv e vaccines for prophylaxis and cancer immunotherapy.