The role of interleukin-2 in regulating the sensitivity of natural killer cells for Fas-mediated apoptosis

The Fas/Fas-ligand (FasL) system seems to play a key role in regulating imm unoresponses. Highly purified CD56(+)CD3(-) natural killer (NK) cells were found to be resistant to the apoptosis-inducing Fas mAb CH11 in the absence or in the presence of interleukin-2 (IL-2) for up to 3 days. However, NK c ells activated with IL-2 for 3 days became apoptotic following combined tre atment with CH11 and actinomycin D, suggesting the presence of an intact ap optotic machinery. In contrast, NK cells cultivated in IL-2 for 6 days beca me sensitive to CH11-induced apoptosis without addition of actinomycin D. A t this time, a pronounced up-regulation of the Fas protein on the NK cell m embrane was detected. By using reverse transcription/polymerase chain react ion it was found that the anti-apoptotic gene FLIP was strongly expressed i n NK cells for up to 6 days of IL-2 stimulation. After day 6, a time-depend ent decrease in the expression of FLIP was observed concomitantly with incr eased sensitivity for Fas-mediated apoptosis. The amount of apoptotic and n ecrotic NK cells in the presence of IL-2 increased in a time-dependent mann er, reaching 40% at day 6 of culture. The amount of apoptotic and necrotic NK cells was reduced in the presence of Fas-Fc protein. In addition, IL-2 s timulated the NK cells to release soluble Fast in a time-dependent manner, whereas membrane Fast did not seem to increase in a similar manner. These r esults indicate that Fas/FasL interactions are involved in the down-regulat ion of IL-2-activated human NK cells.