Membrane Macrophage Colony-Stimulating Factor on MADB106 Breast Cancer Cells Does Not Activate Cytotoxic Macrophages but Immunizes Rats against Breast Cancer

Weakly immunogenic, but highly malignant, rat MADB106 breast cancer cells were retrovirally transduced with the membrane form of macrophage colony-stimulating factor (mM-CSF). The cloned mM-CSF-transfected MADB106 cells physically conjugated with macrophages, but were not killed by the macrophages in 48-h cytotoxicity assays. Macrophages killed the mM-CSF-expressing tumors in the presence of noncytotoxic doses of either taxol or taxol plus cisplatin. This indicated that macrophages bind to the mM-CSF expressed on the tumor cells, but for successful macrophage cytotoxicity to occur against mM-CSF-transduced tumor cells other factors must be present. The mM-CSF-transfected tumor cells were rejected when inoculated subcutaneously into normal rats. Cloned MADB106 tumor cells which expressed high amount of mM-CSF were rejected, while tumor cells that displayed lower levels of mM-CSF grew in 60% of the inoculated rats. The mM-CSF-transfected tumors that grew were smaller and had a greater amount of necrosis, compared to the viral vector tumors. Rats that spontaneously rejected the mM-CSF-transfected MADB106 cells showed rechallenge resistance to unmodified parental MADB106 and R3230Ac breast cancers, but not to the F98 glioma. These observations suggest that breast cancer-specific immunity was induced by the inoculation of mM-CSF-expressing MADB106 tumor cells.