A long-term memory obtained by genetic immunization results in full protection from a mammary adenocarcinoma expressing an EBV gene

We have tested the capability of a plasmid DNA (pDNA) expressing the EBV nu clear Ag-4 (EBNA-4) to evoke a T cell response-associated protective immune response against a tumor expressing this gene. We have found that ACA mice immunized with EBNA-4-expressing plasmid were partially protected against syngeneic mammary carcinoma line (S6C) expressing EBNA-4 (S6C-E4), This pro tection was enhanced by coimmunizing mice with EBNA-4- and GM-CSF-expressin g plasmids, and a full protection was achieved by coimmunizing mice with EB NA-4- and IFN-gamma-expressing plasmids, Furthermore, mice that have reject ed the EBNA-4-positive tumor were also resistant against a subsequent chall enge with the original nontransfected tumor line. We then checked for the a bility of pDNA immunization to provide a protective long-term memory respon se. We indeed found that even after 3 mo from the last immunization, full p rotection was obtained by this method, as compared with full tumor outgrowt h in the control-immunized group. These findings support the concept that a nonviral, pDNA-based vaccination strategy is useful to fully protect from the outgrowth of tumors expressing this EBV gene.