Degranulation plays an essential part in regulating cell surface expression of Fas ligand in T cells and natural killer cells.

Fas ligand (FasL) triggers apoptosis during cytotoxicity mediated by cytotoxic T lymphocytes and during immune downregulation. 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 up-regulated on activation, 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.