Enhanced immunogenicity of B cell lymphoma genetically engineered to express both B7-1 and interleukin-12

The A20 murine B cell lymphoma was transfected with B7-1 and subsequently these variants and vector control variants were retrovirally infected to express murine interleukin-12 (mIL-12). In vitro data showed that the B7-1 variants enhanced secretion of IL-2 and IL-4 by allogeneic T cells in mixed lymphocyte tumor cultures. While IL-12 variants stimulated IFN-gamma, variants expressing both B7-1 and IL-12 stimulated IFN-gamma, IL-2, and IL-4 secretion. Tumorigenicity experiments showed that whereas B7-1 delayed tumor onset, only the mIL-12 variants with or without B7-1 were completely rejected in syngeneic hosts. In addition, tumor-free mice were protected against subsequent challenge with the parental unmodified cells and had enhanced cytotoxic T lymphocyte (CTL) lysis activity. Results from minimal disease mixing experiments demonstrated that only the A20/B7-1/mIL-12 variant was able to reject A20 unmodified cells inoculated at the same site, whereas prolonged survival was observed when the A20 parental cells were inoculated at different sites. Depletion studies and injections into nu-/nu- mice demonstrated that both CD4+ and CD8+ T cells may mediate immunity. These data suggest that vaccinations with tumor cells genetically modified to express both B7-1 and IL-12 may alter cytokine profiles and generate CTL activity and, thus, the mechanisms of enhanced antitumor immunity may be multifactorial.