Fas Involvement in Cytotoxicity Mediated by Human NK Cells

Lysis of target cells (TC) by cytolytic lymphocytes involves the secretion of cytoplasmic granules containing perforin and serine esterases by the effector cell (EC). Recently, a granule-independent cytolytic mechanism involving the interaction of the apoptosis-triggering Fas antigen (CD95) with Fas ligand (FasL) has been revealed in T cells. However, whether the Fas lytic pathway also functions in NK cells has not been established. We purified human peripheral NK cells (>98% CD56+) and found that PMA and ionomycin treatment upregulated FasL message and stimulated the NK cells to lyse a Fas+TC. This lysis was partially inhibited by the anti-Fas-blocking antibody M3 or by Fas·Fc fusion protein. We also found that FasL is constitutively expressed on the human NK-like leukemia cell line YT-INDY and that YT-INDY utilizes a Ca2+-independent Fas lytic pathway, as well as the granule pathway. We have previously shown that CD28/B7 interactions are involved in TC recognition by YT-INDY. K562 cotransfected with Fas and B7-1 (K562/Fas/B7) was lysed by YT-INDY at a higher level than a vector-transfected K562 line, whereas K562 transfected with Fas alone was not. Lysis of K562/Fas/B7 cotransfectants was partially Fas-mediated, as indicated by the presence of Ca2+-independent, M3-inhibitable lysis. Ca2+-independent, Fas-mediated lysis of several TC by YT-INDY was inhibited by anti-CD28 antibody. Anti-LFA-1 also inhibited Fas-mediated cytotoxicity in YT-INDY. Thus, fresh human NK cells and the human NK-like cell line YT-INDY are capable of using the Fas lytic pathway. In YT-INDY, CD28/B7 and LFA-1/ICAM interactions appear to influence the Fas lytic pathway.