Total enantioselective synthesis and in vivo biological evaluation of a novel fluorescent BODIPY alpha-galactosylceramide.

Natural killer T (NKT) cells are a distinct subset of mature lymphocytes endowed with features of activated and regulatory T cells. alpha-Galactosylceramides (alpha-GalCers), the synthetic prototype of which is KRN7000, are the only natural reagents recognised by the T-cell receptor of NKT cells. The alpha-GalCer-activated NKT cells promptly release IFN gamma and IL-4 (IFN=interferon; IL=interleukin) and undergo apoptotic death within hours. In mice, activated NKT cells are responsible for antitumour activity and protection against autoimmune diseases. KRN7000 can thus be considered as the root of a family of novel immunoregulatory drugs. To get insights into the in vivo behaviour of alpha-galactosylceramides, an original fluorescent derivative has been prepared by following a convergent synthetic scheme. This strategy allows the introduction of different acyl chains, carbohydrate residues and various labels in the final steps of the synthesis. The fluorescent BODIPY probe derived from a versatile glycolipid precursor is as active as KRN7000 for inducing apoptosis of liver NKT cells. Fluorescence was detected in peritoneal macrophages and splenic antigen-presenting cells, in Kupffer-like cells in the liver, but not in lymphocytes.