Liposomal glycosphingolipids activate natural killer T cell-mediated immune responses through the endosomal pathway

Natural killer T (NKT) cells recognize lipid antigens, such as glycosphingolipids (GSLs), via CD1d and contribute to host defense against various pathogens. Here, we demonstrate that GSLs isolated from Sphingomonas bacteria and inserted into liposomes (GSL-liposomes) enhance the activation of NKT cells and dendritic cells (DCs). GSL-liposomes remarkably enhanced the production of IFN-γ from splenocytes in vitro and this enhancement depended on the content of the pH-sensitive lipid dioleoyl-phosphoethanolamine (DOPE) in the liposomes. GSL-liposomes containing DOPE were clearly broken in late endosomes and this may facilitate effective loading of GSLs onto CD1 molecules. Treatment with GSL-liposomes also activated NKT cells and DCs in vivo. Collectively, our results strongly suggest that GSL-liposomes can effectively induce NKT cell-mediated immune responses and may be useful as an immune adjuvant for inducing protective immunity.