Effect of Fas+ and Fas- target cells on the ability of NK cells to repeatedly fragment DNA and trigger lysis via the Fas lytic pathway

We and others have recently shown that human NK cells express the Fas ligand (FasL) constitutively and that they can trigger the lysis of Fas positive (Fas+) target cells (TC) by apoptosis. We have also previously demonstrated that NK cells exposed to sensitive TC temporarily lose their ability to lyse sensitive TC via the granule-mediated pathway and that this loss is recovered when inactivated NK cells (NKi) are incubated in medium supplemented with IL-2, IL-12 or IL-15, In this study, we investigated the fate of the Fas-lytic pathway in NK cells exposed to either Fas+ or Fas-TC. To this end, we exposed NK cells to Jurkat (Fas-) or Jurkat (Fas+) TC for up to 6 h, separated NK cells from the TC and assessed the residual lytic activity against K562, a traditional human NK cell target, Jurkat Fas+ and Jurkat Fas- TC, Fas lytic activity was determined in calcium free medium, in the presence or absence of two distinct Fas-blocking monoclonal antibodies and a Fas.Fc fusion protein. In parallel experiments, the extent of DNA fragmentation in the three TCs was also assayed by the JAM test, Our results indicate that: (i) NK cells exposed to susceptible Fas+ TC temporarily lose most of their lytic potential due to the granule-mediated pathway, while only partially losing the Fas-lytic pathway, They also partially lose their ability to fragment DNA, (ii) NK cells exposed to Fas+ TC completely recover the Fas lytic pathway and the ability to fragment DNA via the Fas/Fas ligand when incubated in medium supplemented with IL-2 for 18 h.