OCH, a sphingosine-truncated analog of alpha-galactosylceramide (alphaGC), is a potential therapeutic reagent for a variety of Th1-mediated autoimmune diseases through its selective induction of Th2 cytokines from natural killer T (NKT) cells. We demonstrate here that the NKT cell production of IFN-gamma is more susceptible to the sphingosine length of glycolipid ligand than that of IL-4 and that the length of the sphingosine chain determines the duration of NKT cell stimulation by CD1d-associated glycolipids. Furthermore, IFN-gamma production by NKT cells requires longer T cell receptor stimulation than is required for IL-4 production by NKT cells stimulated either with immobilized mAb to CD3 or with immobilized "alphaGC-loaded" CD1d molecules. Interestingly, transcription of IFN-gamma but not that of IL-4 was sensitive to cycloheximide treatment, indicating the intrinsic involvement of de novo protein synthesis for IFN-gamma production by NKT cells. Finally, we determined c-Rel was preferentially transcribed in alphaGC-stimulated but not in OCH-stimulated NKT cells and was essential for IFN-gamma production by activated NKT cells. Given the dominant immune regulation by the remarkable cytokine production of ligand-stimulated NKT cells in vivo, in comparison with that of (antigen-specific) T cells or NK cells, the current study confirms OCH as a likely therapeutic reagent for use against Th1-mediated autoimmune diseases and provides a novel clue for the design of drugs targeting NKT cells.