The effect of lysed and nonviable target cells on the experimentally determined kinetic parameters for natural and antibody-dependent cytotoxicity

In previous studies, we have demonstrated that kinetic assays of natural and antibody-dependent cellular cytotoxicity reactions can be used to obtain simultaneously estimates for both the frequency and lytic activity of the NK or K cells that lyse a given target cell. In order to generate useful information from kinetic cytotoxicity assays, it is imperative that suitable experimental conditions be used and consideration be given to factors that may alter the values of experimentally determined kinetic paramaters. In this paper we derive equations to predict the effects of unlabeled inhibitor cells on the values obtained for K_M^app and V_max. We further demonstrate that these equations can be used to interpret data obtained in the presence of a fixed concentration of added inhibitor cells and to correct for the apparent inhibition of cytotoxicity caused by nonviable cells present in ⁵¹Cr-labeled target cell preparations. We also predict that high concentrations of lysed target cells will cause product inhibition, present experimental evidence in support of this prediction, and present a method that can be used to test and correct for inhibition by lysed target cells. These results should provide for the more accurate determination of kinetic parameters for natural and antibody-dependent cytotoxicity reactions, and therby more accurate quantification of effector cell frequency and activity.