Fas ligand (FasL) triggers apoptosis during cytotoxicity mediated by cytotoxic T lymphocytes and during immune downregulation1. The ability of T cells and natural killer cells to trigger apoptosis through this mechanism is controlled by the cell surface expression of FasL (ref. 2). Because FasL expression is upregulated on activation2,
3, FasL was thought to be delivered directly to the cell surface. Here we show that newly synthesized FasL is stored in specialized secretory lysosomes in both CD4+ and CD8+ T cells and natural killer cells, and that polarized degranulation controls the delivery of FasL to the cell surface. In this way, FasL-mediated apoptosis is finely controlled by receptor-mediated target-cell recognition. The cytoplasmic tail of FasL contains signals that sort FasL to secretory lysosomes in hemopoietic cells. This pathway may provide a general mechanism for controlling the cell surface appearance of proteins involved in immune regulation.
