INHIBITORY EFFECT ON TUMOUR COLONY FORMATION OF MOUSE SERUM ASSOCIATED WITH TUMOUR RESISTANCE IN VIVO IN SEMI SYNGENEIC MICE

Differences in tumour susceptibility between strains of mice (C57B1/6 and C57B1/6 × DBA/2 = B6D2F1) could be demonstrated for several tumours of C57B1 origin, both solid tumours (B16 melanoma and Lewis lung carcinoma) and lymphomas (RBL-5, 136-3 and ALC). Serum from mice with high tumour resistance in vivo (B6D2F1) showed an inhibitory effect on tumour colony formation in a soft agar colony assay. Serum from mice with lower tumour resistance (C57B1/6) had no effect. When other F1 hybrids of C57B1/6 parental origin were tested, the same correlation between in vitro inhibition of tumour colony formation and in vivo susceptibility was found. The serum factor was species non-specific, since the activity was expressed against in vitro grown cell lines of human origin. The tumour colony inhibitory activity was heat sensitive (56°C for 30 minutes), precipitable by (NH4)2SO4, and not removed by adsorption on tumour cells. These results demonstrate the existance of a naturally-ocurring humoral tumerostatic factort(s) which correlates to in vivo susceptibility to tumour cells. Its relationship to NK cell activity is discussed.