42 Synthesis of and Immunostimulating Activity of Peptidoglycan Partial Structures

Abstract

Bacterial cell wall peptidoglycan (PGN) has been well-known as a strong immunopotentiator. We previously demonstrated that the minimum structure required for the immunostimulation is N-acetylmuramyl-L-alanyl-D-isoglutamine (muramyl dipeptide: MDP) but the activity of MDP recently proved to be not identical with that of PGN. Partial structures of PGN were hence synthesized for precise biological study. A key disaccharide glucosaminyl-β(1→4)-muramic acid was prepared by stereoselective glycosylation of an N-Troc muramic acid acceptor with N-Troc-glucosaminyl trichloroacetimidate. The disaccharide was converted to both disaccharide acceptor and donor, which were then coupled together by the same glycosylation method to give a tetrasaccharide. Octasaccharide was also obtained in a similar manner. Introduction of the dipeptide moiety to the 3-O-lactyl groups followed by deprotection afforded the peptidoglycan tetrasaccharide and octasaccahride fragments. Toll like receptor 2 (TLR2) was described as the receptor for PGN but recent studies have shown that MDP acts via a TLR2 independent pathway. The above synthetic fragments also showed TNF-α inducing activity via TLR2-independent pathway. Recently, we found that intracellular protein NOD2 is a cellular receptor for MDP. The above partial structures containing tetra- and octasaccharide also showed NOD2 dependent activity, indicating NOD2 is an intracellular receptor for PGN. We also showed that NOD1 recognized a dipeptide γ-D-glutamyl-meso-diaminopimelic acid, which is a partial structure typical of Gram-negative bacterial PGN.