Involvement of two distinct killing mechanisms in bystander target cell lysis induced by a cytotoxic T lymphocyte clone.

Cytotoxic T lymphocyte (CTL) can lyse target cells by mainly two different mechanisms including granule exocytosis- and Fas-based cytotoxicity. In addition, CTL also lyse bystander target cells. In this study, we examined whether these two different killing mechanisms also cause bystander target cell lysis. EL-4 cells, representing bystander target cells, were incubated with CD8+ CTL clone B7B7 and antigen-treated P815 cells. EGTA-MgCl2 inhibited bystander lysis and reduced the release of BLT esterase. In addition, lysis of Fas-negative bystander cells (thymocytes from C57BL/6J-lpr/lpr not expressing Fas antigen) was also detected. CTL preactivated with antigen-treated P815 cells maintained a killing activity on bystander cells in the presence of cycloheximide (CHX). Furthermore, the presence of anti-Fas mAb inhibited the lysis of EL-4 cells by CTL stimulated with anti-TCR mAb. Thus, our results suggest that granule exocytosis- and Fas-mediated signal pathways cause the lysis of bystander target cells.