Cytokinetics of macrophage-mediated cytotoxicity

A variety of methods have been described to measure cytotoxicity of host effector cells against tumor targets. While these methods have proven their value, they have certain limitations, most notably that the measured parameter cannot be related to the overall rate of tumor cell growth. Accordingly, we have developed a new method for measuring cytotoxicity based upon the rates of cell replication and target cell loss. While technically more demanding, this new method has the advantages that the data are biologically relevant to tumor cell growth and are appropriate for refined statistical analysis of measurements comparing treatment groups. This methodology was applied to macrophage-mediated inhibition of tumor cell growth. Proteose peptone-elicited macrophages decreased the rate of tumor cell loss but also tended to reduce the replicative rate of the tumor cells so that overall tumor cell growth was unaffected. In contrast, Bacillus Calmette-Guérin-activated macrophages caused an overall reduction in tumor cell numbers by increasing the rate of tumor cell loss (cytolysis) and decreasing the rate of tumor cell replication (cytostasis). Simultaneously conducted isotope release assays revealed that the percentage released increased with time but that this did not reflect a change in the rate of cell death. An equation is given relating the rate of survival of control and experimental tumor cell populations to the commonly used percent specific isotope release. This relationship explains the dependency of isotope release on time and provides an explanation why isotope release did not reliably indicate the relative efficiency of killing by B. Calmette-Guérin-activated macrophages for four different tumor targets.