Cytotoxic activity of normal mouse serum on mouse tumor cells in vitro

Cytotoxic effects of normal mouse serum on mouse tumor cells were investigated in vitro. When FE melanoma cells of C57BL/6 mouse origin, were cultured in medium containing 1% fetal calf serum (FCS) and 10-30% C57BL/6 mouse serum, number of viable FE cells markedly decreased after a little increase in their number, indicating cell death of FE cells in culture with mouse serum. Phase-contrast microscopic examination showed appearance of fatty degeneration in FE cells after 24 h, and an increase in cell death after 48 h. Electron microscopic examination, and agarose gel electrophoresis of DNA at 72 h of culture showed that their cell death occurred as necrosis. This cytotoxic effect of mouse serum was also found in culture of combinations of C57BL/6 mouse serum and C57BL/6 mouse melanoma cells (G6 cells), and BALB/c mouse serum and various BALB/c mouse tumor cells (G-5 and G-1 liver tumor cells, and Colon 26 cells). Furthermore, sera of BALB/c and B10D2 mice also showed the cytotoxic effect on FE cells. The cytotoxic effect of mouse serum was not ascribed to complement activity because all mouse sera were treated at 56 degrees C for 30 min before use, and this heat treatment completely abolished complement activity, and because serum of C5-deficient mice also showed the cytotoxic effect. This cytotoxic activity was stable at heat treatment at 100 degrees C for 10 min, and was in a serum fraction of molecular weights more than 30,000 dalton. The present results show that normal mouse serum has a factor(s) inducing fatty degeneration and necrosis of mouse tumor cells.