Mouse Myeloma Cell Line Secreting Bifunctional Fusion Protein RM4/IFN-τ Elicits Antitumor CD8 MHC Class I-Restricted T Cells That Are Cytolytic In Vitro and Tumoricidal In Vivo

Mouse myeloma cell line V_kC_k/RM4-IFN-τ secreting the bifunctional fusion protein RM4/IFN-τ was used to study the relationship between IFN-τ secretion of tumor cells and its tumorigenicity and to study the potential mechanism responsible for the immune response. IFN-τ secretion of V_kC_k/RM4-IFN-τ tumor cells was estimated at 90 U/ml using an antiviral assay. To evaluate tumorigenicity, 5×10⁵ viable IFN-τ-secreting V_kC_k/RM4-IFN-τ and non-IFN-τ-secreting V_KC_K tumor cells were injected s.c. into syngeneic BALB/c mice and V_KC_K/RM4-IFN-τ-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-τ secretion by V_kC_k/RM4-IFN-τ tumor cells curtailed its tumorigenicity in BALB/c mice and induced a persistant protective immune response against a subsequent graft of parental V_kC_k 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-τ in tumors to induce potent antitumor immunity and further suggests that the bifunctional fusion protein RM4/IFN-τ may be useful in cancer immunotherapy because of its capacity of targeting IFN-τ to human tumors expressing the human tumor-associated TAG72 antigen.