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Mouse myeloma cell line secreting bifunctional fusion protein RM4/IFN-gamma [corrected] elicits antitumor CD8 MHC class I-restricted T cells that are cytolytic in vitro and tumoricidal in vivo.

Mouse myeloma cell line VKCK/RM4-IFN-gamma secreting the bifunctional fusion protein RM4/IFN-gamma was used to study the relationship between IFN-gamma secretion of tumor cells and its tumorigenicity and to study the potential mechanism responsible for the immune response. IFN-gamma secretion of VKCK/RM4-IFN-gamma tumor cells was estimated at 90 U/ml using an antiviral assay. To evaluate tumorigenicity, 5 x 10(5) viable IFN-gamma-secreting VKCK/RM4-IFN-gamma and non-IFN-gamma-secreting VKCK tumor cells were injected s.c. into syngeneic BALB/c mice and VKCK/RM4-IFN-gamma-immunized or T cell subset-depleted BALB/c mice, respectively. Tumor progression or regression was evaluated 2 weeks after tumor inoculation. Our animal studies showed that RM4/IFN-gamma secretion by VKCK/RM4-IFN-gamma tumor cells curtailed its tumorigenicity in BALB/c mice and induced a persistant protective immune response against a subsequent graft of parental VKCK tumor. This protective immunity is long term and tumor specific as measured in a 51Cr-release assay. In addition, our animal studies in T cell subset-depleted BALB/c mice showed that CD8 CTL play a major role in the reduction of tumorigenicity. This study thus highlights the potential advantages of localized IFN-gamma in tumors to induce potent antitumor immunity and further suggests that the bifunctional fusion protein RM4/IFN-gamma may be useful in cancer immunotherapy because of its capacity of targeting IFN-gamma to human tumors expressing the human tumor-associated TAG72 antigen [corrected].