HUMAN MONOCYTIC CELLS CONTAIN HIGH-LEVELS OF INTRACELLULAR FAS LIGAND- RAPID RELEASE FOLLOWING CELLULAR ACTIVATION

Human monocytes express both Fas and Fas ligand (Fast) on the cell sur face, and the interaction of these molecules induces spontaneous apopt osis, In this report we present a study of monocytic cells by FAGS and confocal microscopy using anti-Fast Abs that reveals high levels of p reformed Fast within the intracellular compartment, Further analysis b y immunoblotting of cell cytoplasmic proteins confirmed the presence o f a 37-kDa protein recognized by anti-Fast Abs, Stimulation of the mon ocytic cells with immune complexes, PHA, or superantigen gave rise to the rapid release of soluble Fast from within the cells, The presence of high levels of Fast within human monocytes suggests that, upon stim ulation, the cells can rapidly translocate intracellular Fast to the c ell surface and release it into the extracellular milieu, These findin gs indicate a novel mechanism for monocytes to respond rapidly to envi ronmental changes, resulting in the release of active, soluble FasL.