Granulocyte—macrophage colony‐stimulating factor inhibits tumour growth

The effect of granulocyte—macrophage colony‐stimulating factor (GM—CSF) on murine antitumour responses was examined. Sixty mice received Lewis lung carcinoma implants and were then randomized to receive GM—CSF 1 μg/day, GM—CSF 0·5 μg/day or saline for 10 days and studied with regard to tumour volume, carcass weight and food intake. Macrophage antitumour mechanisms including oxygen free radical production and nitric oxide release were studied in peritoneal macrophages after co‐culture with GM—CSF in vitro and in vivo. GM—CSF 1 μg/day decreased tumour growth after 5 days (mean(s.e.m.) 0·62(0·4) versus 1·24(0·19) cm³, P = 0·017). GM—CSF upregulated macrophage antitumour mechanisms by enhancing the in vivo production of superoxide radicals (mean(s.e.m.) 0·69(0·06) versus 0·45(0·10) nmol, P < 0·05) and nitric oxide (mean(s.e.m.) 48(3) versus 24(4) μmol, P < 0·01). GM—CSF functions through the enhancement of macrophage tumoricidal activity, suggesting a therapeutic potential for this cytokine in the tumour‐bearing host.