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 cytoto xic T lymphocytes and during immune downregulation(1). The ability of T cel ls and natural killer cells to trigger apoptosis through this mechanism is controlled by the cell surface expression of Fast (ref. 2). Because Fast ex pression is upregulated on activation(2,3), Fast was thought to be delivere d directly to the cell surface. Here we show that newly synthesized Fast is stored in specialized secretory lysosomes in both CD4(+) and CD8(+) T cell s and natural killer cells, and that polarized degranulation controls the d elivery of Fast to the cell surface. In this way, Fast-mediated apoptosis i s finely controlled by receptor-mediated target-cell recognition. The cytop lasmic tail of Fast contains signals that sort Fast to secretory lysosomes in hemopoietic cells. This pathway may provide a general mechanism for cont rolling the cell surface appearance of proteins involved in immune regulati on.