Impaired induction of type II interferon in tumor-bearing mice

The response of mice to type II interferon inducers such as purified protein derivative of tuberculin or OK-432 was significantly impaired after either the implantation of tumor cells or the injection of cell-free tumor ascitic fluids. Complete inhibition of type II interferon induction occurred in 3 to 5 days. This inhibitory effect persisted until the death of mice implanted with tumors or for 7 days after i.p. injection of 0.2 ml of tumor ascitic fluid. The inhibitory activity was seen even with 0.2 ml of the 10,000-fold dilution of the fluid and with its filtrate passed through a Diaflo UM-10 membrane (greater than 10,000-dalton exclusion), However, such tumor ascitic fluids did not affect the induction of type I interferon in mice by polyriboinosinic-polyribocytidylic acid or tilorone-HCl. Moreover, adjuvant-induced nontumorous ascitic fluid had no inhibitory activity. These results indicate that the in vivo induction of type II interferon is specifically impaired by tumor-mediated factor(s) with molecular weights of less than 10,000.