Abstract
Recent studies from this laboratory have demonstrated that macrophage phagocytosis of virulent strains (Erdman and H37Rv), but not the attenuated H37Ra strain of Mycobacterium tuberculosis, is mediated by phagocyte mannose receptors (MR) in addition to complement receptors (CR1 and the leukocyte integrins CR3 and CR4). Lipoarabinomannan (LAM) is a major surface lipoglycan of M. tuberculosis. LAM from the Erdman strain (Man-LAM) contains mannose oligosaccharides at the terminal portions of the molecule. This study investigated the ability of ManLAM to serve as a microbial ligand in adherence to human monocyte-derived macrophages (MDM). Polystyrene microspheres were coated with known amounts of purified ManLAM, LAM without the terminal mannosyl units from an avirulent mycobacterium (AraLAM), lipomannan (LM), or buffer and incubated with MDM monolayers in the absence of serum. The presence of LAM on microspheres was confirmed by indirect immunofluorescence studies. Microspheres coated with ManLAM demonstrated a more than threefold increase in adherence to MDM when compared with microspheres coated with AraLAM, LM, or buffer and the low levels of adherence of microspheres in the latter three groups were comparable. Compared with control monolayers, selective down-modulation of MDM MR on a mannan substrate abrogated the enhanced adherence of microspheres mediated by ManLAM. Adherence of microspheres coated with AraLAM, LM, or buffer was not influenced by MR modulation. To confirm the importance of the terminal mannosyl units of ManLAM in the enhanced adherence of ManLAM microspheres to MDM, these units were selectively removed by exomannosidase treatment. The structure of LAM products before and after enzyme treatment was confirmed by high performance anion exchange chromatography with pulsed amperometric detection. Removal of the terminal mannosyl units abolished the capacity of ManLAM to mediate enhanced adherence of microspheres to MDM. Finally, preincubation of Erdman M. tuberculosis with CS-40, a mAb directed against LAM, resulted in a consistent inhibition of adherence of the bacteria to MDM (up to 49% inhibition), confirming a role for ManLAM on intact bacteria in adherence to MDM. Thus, we provide evidence for a novel receptor-ligand pathway in phagocytosis of M. tuberculosis that consists of MR on macrophages and mannosyl units at the terminal end of ManLAM, a major microbial surface lipoglycan.