Natural Killer T (NKT) cells have been identified as a novel lymphocyte lineage, and they recognize lipid antigens presented by CD1d protein with their T cell receptors. KRN7000 (1, Fig. 1) developed by Kirin Brewery Co. is known as one of the external stimulant to NKT cells, and has phytosphingosine (11, Fig. 3) as its sphingoid base part. In nature, as the C_<18>-sphingoid bases, sphingosine (10), sphinganine (12), and 6-hydroxysphingosine (13) exist abundantly, and 11 is a minor sphingosine base in mammals. We synthesized the analogs 18, 20, and 21 of KRN7000 (1), possessing natural sphingoid base 10, 12, and 13, respectively, and investigated their immunostimulatory activity. Among 1, 18, 20, and 21, sphingosine-type analog (18, RCAI-161) showed the most potent immunostimuratory activity, and induced ca. two times larger amount of IFN-γ production than 1 did in mice in vivo (iv). In 2011, Brenner and co-workers reported the internal natural lipid ligand of NKT cells to be β-GluCer (9, Fig. 3). According to their report, 9 was accumulated during infection, and activates NKT cells in mice and humans. We synthesized 13-glucosylceramide (26) from the properly protected glucose donor (22) and the ceramide (15) as shown in Figure 3. The chemical purity of 26 was determined as >99.9% by HPLC analysis. Synthesized 26 induced almost no IFN-γ production with very small production of IL-4. In addition, we synthesized sphingosine-type β-galactosylceramide (19), and found 19 to induce very small IFN-γ production, while a small amount of IL-4 secretion was observed.
