In vivo activation of invariant V alpha 14 natural killer T cells by alpha-galactosylceramide sequentially induces Fas-dependent and -independent cytotoxicity.

The present study was designed to clarify the cytotoxic capacities of invariant V alpha 14 natural killer T (iNKT) cells activated in vivo. We found that as early as 2 h after a single injection of alpha-galactosylceramide (alpha-GalCer), sorted iNKT splenocytes from treated mice kill Fas-transfected target cells. The implication of the Fas pathway in this lysis was strengthened by both the blockage of cytotoxicity in the presence of anti-Fas ligand (FasL) monoclonal antibody (mAb) and the up-regulation of FasL expression on iNKT cells. Sorted NK cells did not participate in the lytic activity at this time point. Yet, they became cytotoxic later on, 24 h post-treatment, when target cell lysis was mainly independent of the Fas pathway. This type of cell killing was predominant at this later time point, even though iNKT cells conserved a slight Fas-dependent cytotoxicity. NK cells failed to acquire the ability to kill target cells when IFN-gamma production in alpha-GalCer-injected mice was blocked by anti-IFN-gamma mAb, underscoring the major role of this cytokine. In conclusion, our findings provide the first direct evidence that iNKT cells can exert Fas-dependent cytotoxicity very shortly after in vivo alpha-GalCer activation and later, through IFN-gamma secretion, enable NK cells to kill target cells in a Fas-independent pathway.