[Studies on the possible causes of decreasing NK cell activity in tumor-bearing mice. II. Serum and peritoneal exudate from ascitic tumor-bearing mice suppress NK cell activity].

Previous studies showed that the depressed NK cell activity in tumor-bearing mice was not due to the decrease of NK cell number. The results of this report indicate that the depressed NK cell activity in tumor-bearing mice might be due to an inhibitory factor, which probably was secreted by tumor-cells. We found that sera and peritoneal exudates from S180 or P388 tumor-bearing mice were able to inhibit the normal NK cell activity. Spleen cells incubated with the peritoneal exudate for 24 hours before assay demonstrated significantly depressed NK activity. Treatment of peritoneal exudate with a large number of spleen cells did not alter the inhibitory effect of peritoneal fluid, indicating that the inhibitory factor might not be membrane antigen shedding from the tumor cells, but may be actively secreted by them.