Activation of CD1d-restricted invariant NKT (iNKT) cells with the glycolipid Î±-galactosylceramide (Î±-GalCer) confers protection against disease in murine models, however, clinical trials in humans have had limited impact. We synthesized a novel thioglycoside analogue of Î±-GalCer, denoted Î±-S-GalCer, and tested its efficacy for stimulating human iNKT cells in vitro. Î±-S-GalCer stimulated cytokine release by iNKT cells in a CD1d-dependent manner and primed CD1d+ target cells for lysis. Î±-S-GalCer-stimulated iNKT cells induced maturation of monocyte-derived dendritic cells into antigen-presenting cells that released IL-12 and small amounts of IL-10. The nature and potency of Î±-S-GalCer and Î±-GalCer in human iNKT cell activation were similar. However, in contrast to Î±-GalCer, Î±-S-GalCer did not activate murine iNKT cells in vivo. Because of its enhanced stability in biological systems, Î±-S-GalCer may be superior to Î±-GalCer as a parent compound for developing adjuvant therapies for humans. Â© 2011 Elsevier Inc.