The study investigates the recognition of lysosomal glycosphingolipids by natural killer T (NKT) cells. NKT cells are a unique subset of T cells that recognize lipid antigens presented by CD1d, a molecule similar to the major histocompatibility complex class I. The authors found that isoglobotrihexosylceramide (iGb3), a lysosomal glycosphingolipid, is recognized by both mouse and human NKT cells. In mice lacking the enzyme β-hexosaminidase b, which degrades iGb3, there is a severe reduction in NKT cell numbers, suggesting that iGb3 is essential for NKT cell development. The study also shows that iGb3 can stimulate NKT cells in vitro and that its presentation requires lysosomal trafficking and the function of saposins, which are lipid transfer proteins. The findings suggest that iGb3 may play a crucial role in controlling NKT cell responses to infections, malignancies, and autoimmune diseases. The study provides insights into the mechanisms underlying NKT cell activation and highlights the potential of iGb3 as a therapeutic target in various diseases.The study investigates the recognition of lysosomal glycosphingolipids by natural killer T (NKT) cells. NKT cells are a unique subset of T cells that recognize lipid antigens presented by CD1d, a molecule similar to the major histocompatibility complex class I. The authors found that isoglobotrihexosylceramide (iGb3), a lysosomal glycosphingolipid, is recognized by both mouse and human NKT cells. In mice lacking the enzyme β-hexosaminidase b, which degrades iGb3, there is a severe reduction in NKT cell numbers, suggesting that iGb3 is essential for NKT cell development. The study also shows that iGb3 can stimulate NKT cells in vitro and that its presentation requires lysosomal trafficking and the function of saposins, which are lipid transfer proteins. The findings suggest that iGb3 may play a crucial role in controlling NKT cell responses to infections, malignancies, and autoimmune diseases. The study provides insights into the mechanisms underlying NKT cell activation and highlights the potential of iGb3 as a therapeutic target in various diseases.