DNA vaccination against the ovarian carcinoma-associated antigen folate receptor α (FRα) induces cytotoxic T lymphocyte and antibody responses in mice

Human folate receptor α (FRα) is a folate-binding protein that is selectively overexpressed in ovarian carcinoma and has been regarded as a suitable target antigen for immunotherapy purposes. To study the possible use of this antigen in DNA vaccination, FRα cDNA was ligated into the VR1012 (Vical) expression vector under the transcriptional control of the cytomegalovirus promoter. A total of 100 μg of purified plasmid DNA was injected intramuscularly in BALB/c mice three times at 14-day intervals. At 10 days after the second injection, the sera of the animals (100%) displayed significant antibody titers (by indirect immunofluorescence and fluorescence-activated cell sorter analysis) against syngeneic C26 cells transduced with FRα, but not against unmodified C26 cells, Immunoglobulin G2a was the predominant isotype. In addition, specific cytotoxic T lymphocyte activity against FRα-transduced C26 cells could be detected in splenocytes from all immunized animals. Coinjection of a plasmid containing interleukin-2 cDNA increased both antibody titers and cytotoxic T lymphocyte activity. Challenge by subcutaneous injection with FRα-transduced C26 cells (performed 10 days after the third injection) showed a statistically significant delay in tumor growth. Vaccination with the FRα and interleukin-2 cDNA mixture, which was performed after an intravenous injection of FRα-transduced cells, enhanced the mean survival time and reduced the number of lung metastases, thus suggesting that such vaccination is effective even against preexisting tumor cells.