Two approaches have been used to assess the hypothesis that granzymes secreted by cytotoxic lymphocytes act within the target cells to trigger an internal disintegration pathway leading to target cell lysis. The lytic properties of several clones of rat basophilic leukemia cells transfected with either cytolysin (perforin) alone (RBL-cy) or a combination of cytolysin and granzyme A (RBL-cy-gza) were compared with cloned CTL. Analysis of the kinetics of target cell 125I-DNA vs 51Cr release with

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... hree tumor targets showed negligible DNA release with RBL-cy, less extensive 125I-DNA release relative to 51Cr release with RBL-cy-gza effector cells, whereas CTL caused greater or equal DNA release than 51Cr release at all time points. Using three different tumor target cells, comparison of RBL-cy-gza and RBL-cy clones in multiple experiments shows that RBL-cy-gza are on average more than threefold more lytic than RBL-cy. This distinction was not seen with red cell targets, in which an internal disintegration pathway does not operate. A second approach to this issue consisted of cytoplasmic loading of tumor target cells with aprotinin, a macromolecular protease inhibitor known to inhibit granzyme A and probably other granzymes. Although the control BSA-loaded target cells were essentially identical to nonloaded targets in all cases, aprotinin-loaded targets showed substantially lower release of both 51Cr and 125I-DNA with both CTL and RBL-cy-gza effector cells. In contrast, aprotinin-loaded targets were lysed with the same efficiency as control targets by RBL-cy effector cells. We conclude that secreted granzymes contribute to target lysis by triggering a target cell internal disintegration pathway that leads to both lysis and DNA breakdown.