| alpha-Galactosylceramide (alpha-GalCer) is a glycolipid that stimulates natural killer T (NKT) cells to produce both T helper (Th) 1 and Th2 cytokines. This property enables alpha-GalCer to ameliorate a wide variety of infectious, neoplastic, and autoimmune diseases; however, its effectiveness against any one disease is limited by the opposing activities of the induced Th1 and Th2 cytokines. Here we report that a synthetic C-glycoside analog of alpha-GalCer, alpha-C-galactosylceramide (alpha-C-GalCer), acts as an NKT cell ligand in vivo, and stimulates an enhanced Th1-type response in mice. In two disease models requiring Th1-type responses for control, namely malaria and melanoma metastases, alpha-C-GalCer exhibited a 1000-fold more potent anti-malaria activity and a 100-fold more potent anti-metastatic activity than alpha-GalCer. Moreover, alpha-C-GalCer consistently stimulated prolonged production of the Th1 cytokines interferon (IFN)-gamma and interleukin (IL)-12, and decreased production of the Th2 cytokine IL-4 compared to alpha-GalCer.; We found that alpha-C-GalCer's enhanced therapeutic activity required the presence of IL-12, which was needed to stimulate NK cells for optimal IFN-gamma production, but did not affect IL-4. In addition, although DCs were the important APCs for NKT cells in the spleen whereas Kupffer cells were key APCs for liver NKT cells, only DCs were involved in the systemic, downstream Th1-type response to glycolipid administration. More specifically, CD8alpha+ DCs produced IL-12 in response to alpha-GalCer and alpha-C-GalCer administration, which stimulated secondary IFN-gamma production by NK cells in different organs.; Finally, we found that alpha-C-GalCer stimulated a slower DC maturation, a slower NKT cell TCR downregulation, a diminished TCR-mediated tyrosine phosphorylation, and a weaker-binding to NKT cell TCRs than alpha-GalCer. However, alpha-C-GalCer stimulated an enhanced NKT cell proliferation than did alpha-GalCer. In all, these results suggest that alpha-C-GalCer is a partial agonist for NKT cell in vivo, which paradoxically stimulates an enhanced Th1-type immune response that provides better physiological protection against disease states controlled by IFN-gamma. |
