Tumor cell threshold required for suppression of macrophage inflammation.

In DBA/2 mice bearing transplanted, syngeneic P815 mastocytoma, macrophage accumulation was impaired in the tumor when the cancer grew intraperitoneally. By varying the number of transplanted mastocytoma cells and quantitating the macrophage response to a standardized stimulus of proteose peptone, we determined that 4-16x10(6) mastocytoma cells were required to inhibit monocyte inflammation. That interference with monocyte inflammation required a threshold number of tumor cells was consistent with an increase in tumor-associated macrophages proportional to tumor growth during early cancer. It was also consonant with a greater number of macrophages in tumors initiated with small rather than large tumor inocula. Impairment of monocyte inflammation could be passively transferred with ascitic fluid.