The 5-Deoxy-5-methylthio-xylofuranose Residue in Mycobacterial Lipoarabinomannan. Absolute Stereochemistry, Linkage Position, Conformation, and Immunomodulatory Activity

Mycobacteria produce a cell-surface glycoconjugate, lipoarabinomannan (LAM), which has been shown to be a potent modulator of the immune response that arises from infection by these organisms. Recently, LAM from the human pathogens Mycobacterium tuberculosis and M. kansasii has been shown to contain an unusual 5-deoxy-5-methylthio-xylofuranose (MTX) residue as well as its corresponding oxidized counterpart, 5-deoxy-5-methylsulfoxy-xylofuranose (MSX). To date, the absolute configuration of these residues and their linkage position to the polysaccharide are unknown, as is their biological role. Through the combined use of chemical synthesis and NMR spectroscopy, we have established that the MTX/MSX residues in these glycoconjugates are of the d-configuration and that they are linked α-(1→4) to a mannopyranose residue in the mannan portion of the glycan. Conformational analysis of the MTX/MSX residue using NMR spectroscopy showed differences in ring conformation and as well as in the rotamer populations about the C-4−C-5 bond, as compared to the parent compound, methyl α-d-xylofuranoside. Two of the synthesized disaccharides, 3 and 34, were tested in cytokine induction assays, and neither led to the production of TNF-α or IL-12p70. In contrast, both demonstrated modest inhibitory properties when these same cytokines were induced using a preparation of Interferon-γ and Staphylococcus aureus Cowan strain (SAC/IFN-γ). These latter observations suggest that this motif may play a role in the immune response arising from mycobacterial infection.