Generation of tumor-specific cytotoxic T lymphocyte and prolongation of the survival of tumor-bearing mice using interleukin-18-secreting fibroblasts loaded with an epitope peptide

There is currently much interest in generating cytotoxic T lymphocyte (CTL) responses against tumor antigens as a therapy for cancer. In this study mouse fibroblasts (H-2^b) were genetically modified to express a costimulatory B7.1 and a mature interleukin (IL)-18, and then loaded with an ovalbumin (OVA) epitope (SIINFEKL, H-2K^b restricted) as a model antigen, and tested for the induction of OVA-specific CTLs in C57BL/6 mice (H-2^b). The genetically modified fibroblasts lacking either IL-18 or B7.1 were also constructed. Immunization with the IL-18/B7.1-transfected fibroblasts induced strong cytotoxic activities against OVA-expressing EL4 (EG7) tumor cells, but not against other H-2^b tumor cells such as EL4, C1498, and B16F1 cells. The magnitude of the cytotoxic response in mice with the IL-18/B7.1-transfected fibroblasts was significantly higher than the response in mice immunized with any other cell constructs. CD8⁺ T cells with OVA-specific cytotoxic activities were predominant in mice immunized with the IL-18/B7.1-transfected fibroblasts. Furthermore, treatment with the IL-18/B7.1-transfected fibroblasts significantly prolonged the survival period of EG7 tumor-bearing mice. Anti-tumor CTL immunity by the IL-18/B7.1-transfected fibroblasts could be induced without the help of host antigen-presenting cells (APCs) and NK1.1⁺ cells, whereas partially decreased by the depletion of CD4⁺ T cells at the inductive stage. These results support the ability of IL-18/B7.1 gene transfer to enhance the antigen-presenting capacity of fibroblasts for inducing antigen-specific CTL response.