Infection by Mycobacterium tuberculosis first involves its adhesion to mononuclear host phagocytes. Various macrophage opsonic and non-opsonic receptors are known to mediate this adhesion, with some specificity of mannosyl receptors for the more virulent strains. Mannosylated lipoarabinomannan, a major component of cell walls from M. tuberculosis and Mycobacterium bovis BCG, is endowed with mannooligosaccharide units that could mediate its binding to these latter receptors. To explore its interaction with murine immune cells by flow cytometry, we report a new procedure to fluorescently tag the polysaccharide molecules. We covalently labeled mannosylated lipoarabinomannan from M. bovis BCG with biotin, allowing formation of stable complexes with streptavidin coupled to a fluorochrome. In this work, we demonstrated that this major carbohydrate antigen interacts selectively with murine phagocytes, i.e. granulocytes and macrophages. This binding was affected by temperature and was serum-and divalent-cationdependent. It also appears to involve a metabolically recycling protein receptor on the phagocyte surface and mannosyl aggretopes on the mannosylated lipoarabinomannan molecule. Thus, the latter may provide a means for mycobacteria to bind to and invade their host phagocytes. This molecule could constitute one of the early factors of mycobacterial virulence. all the known ones share a basic structure composed of a mannan core linked to an arabinan chain to which are attached arabinosy1 side chains and a phosphatidylinositol anchor located at the mannan core reducing end [ 13 -151. Moreover, the majority of the terminal oligoarabinosyl motifs in lipoarabinomannan from virulent strains of M. tuberculosis and from the vaccine strain M. bovis BCG are themselves extensively capped with either mono-, di-, or trimannosyl residues [16, 171, leading to the terms mannosylated or mannose-capped lipoarabinomannan (ManLAM).
KeywordsIn an attempt to study its binding to murine immune cells by flow cytofluorimetry, the ManLAM from M. bovis BCG was covalently tagged after mild oxidation by sodium periodate. We describe the experimental procedure for the covalent coupling of biotinamidohexanoyl hydrazide to ManLAM whose functional and structural integrity was not significantly altered by this procedure. Then, the biotinylated ManLAM was recognized with high affinity by streptavidin coupled to a fluorochrome which allowed its detection at the cell surface. By this approach, we showed that ManLAM selectively interacts with granulocytes and macrophages. This interaction appears to be dependent upon the presence of serum and divalent cations, and to be sensitive to temperature. It was also found to involve a proteasesensitive membrane receptor. The binding was competitively inhibited by mannan, suggesting a mannosyl-defined interaction. Therefore, the implication of ManLAM in the first step of infection will be discussed with regard to its role in the mediation of adhesion and phagocytosis of mycobacteria.
MATERIALS AND METHODSPreparat...