scFv6.C4 DNA vaccine with fragment C of Tetanus toxin increases protective immunity against CEA-expressing tumor.

The carcinoembryonic antigen (CEA) is the main tumor-associated antigen of colorectal cancers. Previously, we developed a DNA vaccine using scFv6.C4, a CEA surrogate, against CEA-expressing tumors; 40% of the vaccinated mice were tumor-free after tumor challenge. In order to enhance vaccine efficacy, fragment C of Tetanus Toxin (FrC) was tested as adjuvant. C57BL/6J-CEA2682 mice were electroporated intramuscularly 4 times with uP-PS/scFv6.C4-FrC or uP-PS/scFv6.C4, challenged by s.c. injection of 1 × 105 MC38-CEA cells, and tumor growth was monitored over 100 days. The humoral and cellular immune responses were assessed by ELISA, immunocytochemistry, in-vitro lymphocyte proliferation, and CTL cytotoxicity assays. Immunization with uP-PS/scFv6.C4-FrC or uP-PS/scFv6.C4 induced similar anti-CEA antibody titers. However, immunocytochemistry analysis showed stronger staining with uP-PS/scFv6.C4-FrC-immunized mice sera. When challenged with MC38-CEA cells, 63% of the FrC-vaccinated mice did not develop tumors, half of the rest had a significant tumor growth delay, and the probability of being free of tumors was on average 40% higher than that of scFv6.C4-immunized mice. Addition of the adjuvant led to higher CD4+ and CD8+ proliferative responses and strong CD8+ CTL response against MC38-CEA cells. DNA immunization with scFv6.C4 and FrC increased antitumor effect via induction of high and specific humoral and cellular immune responses to CEA.