Immunomodulating marine glycosphingolipids:combining natural product isolation and chemical synthesis in search of chemical diversity

It is now well established that sponges of the genus Agelas and Axinella produce α-galactoglycosphingolipids (α-Gal-GSLs, e.g. 1–3), unique glycosphingolipids which are different from common β-glucosyl- or β-galactosyl ceramides from higher animals and plants in that they show an α-galactose as the first sugar of the carbohydrate chain. Natural α-Gal-GSLs possess interesting immunomodulating activities. They are potent ligands of the CD1d antigen presenting protein, and are capable to specifically activate natural killer T cells (N KT cells) in vivo. KRN7000 (4), a synthetic analogue the simplest α-Gal GSLs agelasphin (1) is under clinical trial as a novel anticancer agent, acting through stimulation of the immune system. As a part of our ongoing search of glycolipids from sponges, we discovered a number of more complex alpha-Gal-GSLs, which were tested on murine T-cell populations. The obtained data suggested that the immunostimulating activity is greatly reduced by glycosylation of the galactose 2-OH group, although some subsequent results obtained by other researchers seemed to be in contrast with these findings. This inconsistency was solved by a recent study, demonstrating that alpha-Gal-GSLs glycosylated at 2' or 3' positions are not active themselves, but are processed by the antigen presenting cells (APCs) with removal of the sugars linked to the galactose. In this scenario, we felt that the availability of analogs of alpha-Gal-GSLs, modified at the key position 2' in a way that cannot be affected by APC processing (e.g. 5), would be important to obtain clear-cut results. Therefore, we carried out the stereoselective total synthesis of the alpha-Gal-GSL analogs 5, and evaluated the influence of the structural modification on their immunostimulating activity. The synthetic procedures, the results of the assays, and their implications on the structural requirements for the immunostimulating activity will be discussed.Meanwhile, the analysis of natural alpha-Gal-GSLs from Agelas and Axinella sponges is far from being concluded, and these species are continuing to provide new glycosphingolipids with unprecedented glycosylation patterns. The latest results of this studies, which also contributed to clarify the structure-activity relationship of alpha-Gal-GSLs will be discussed.