Mechanism of cell-mediated cytotoxicity at the single cell level. VIII. Kinetics of lysis of target cells bound by more than one cytotoxic T lymphocyte.

We measured the effects of having multiple cytotoxic T lymphocytes (CTL) bound to one target cell by using the single-cell cytotoxicity in agarose assay. We found that even though there is variability in the time at which individual target cells are lysed, we can identify a general trend: the mean rate of lysis increases with the number of CTL bound per target cell, reaching a maximum when the CTL-target cell ratio is three. Combining a quantitative model for the rate of lethal hitting in multicellular conjugates with a multi-event model for the rate of target cell disintegration, we developed a new multistage kinetic model for predicting the rate of target cell lysis in multiple lymphocyte-target cell conjugates. The variability in the time at which target cells are hit and the variability in the time until they disintegrate are incorporated into the model. By analyzing our measured data in the context of the multistage kinetic model, we were able to estimate via nonlinear least squares regression the target cell disintegration rate, but not the lethal hitting rate. Lethal hitting appeared to be too fast, when compared with disintegration, to significantly affect the time of target cell lysis. By using previously determined values of the lethal hitting rate for single lymphocyte-target cell conjugates and by postulating that lymphocytes act independently of each other in delivering lethal hits, we were able to estimate the rate at which target cells are hit in multiple-lymphocyte single target cell conjugates. By using this estimate of the lethal hitting rate and the regression estimate of the disintegration rate, the multistage kinetic model gave a quantitative fit to our data. From this analysis, we found that the rate at which a target cell disintegrates after being lethally hit increases with the number of CTL per conjugate. This result is quite surprising, because once the first hit has been received, a target cell can disintegrate in a killer cell-independent manner. Under the conditions of our experiment, it appears as if target cell disintegration is not killer cell-independent. Furthermore, our analysis of the time course of target cell disintegration suggests that the process is not governed by simple first order kinetics, but rather by a more complex multistep mechanism.