In vitro tumor cell destruction by syngeneic mouse macrophages: Methods for assaying cytotoxicity

Various methods for assaying in vitro cytotoxicity mediated by mouse macrophages were studied. A conventional visual cellular counting procedure was compared to two different radioactive monitoring assays where tumor target cells were prelabeled with either [³H]thymidine or [¹²⁵I] iododeoxyuridine. The radioactive monitoring methods were found to be superior to the visual assay with regard to accuracy, reproducibility and ease of operation, and labeling with [¹²⁵I]iododeoxyuridine surpassed labeling with [³h]thymidine. The results of the macrophage mediated cytotoxicity tests were similar and followed in the same direction with all methods of assay. Target cell destruction by immune syngeneic macrophages occurred at ratios exceeding 40 macrophages to one tumor cell after 5 days of incubation. In assays utilizing target cells prelabeled with radioactive compounds, cytotoxicity was monitored by either the release of radioactivity into the media or by radioactivity remaining in viable adherent target cells. Reutilization of radioactive label by macrophages was not demonstrable. It appears that exact quantitative determination of macrophage mediated cytotoxicity in vitro by radioactive monitoring offers a simplified and reproducible method of assay.