Abstract
Phagocytosis of the virulent Erdman and H37Rv strains of Mycobacterium tuberculosis, but not that of the attenuated H37Ra strain, by human macrophages is mediated by the mannose receptor (MR) in addition to complement receptors. We have recently determined that a major capsular lipoglycan, lipoarabinomannan (LAM), from the Erdman strain serves as a ligand for the MR during phagocytosis of bacteria. In this study we directly compare uptake of Erdman, H37Rv, and H37Ra LAM by human macrophages and assess the relative contribution of the MR in this process. Microspheres coated with LAM served as model phagocytic particles for studies of LAM as a capsular ligand. Uptake (37 degrees C) of LAM microspheres by monocyte-derived macrophages was greatest for Erdman LAM and intermediate for H37Rv and H37Ra LAM compared with that of buffer microspheres or microspheres coated with LAM from a nontuberculosis strain of mycobacterium (AraLAM). Inhibition of microsphere uptake in the presence of mannan or mannose-BSA was highest for Erdman LAM (75 +/- 8 and 50 +/- 7%, respectively) and H37Rv LAM (57 +/- 13 and 21 +/- 5%, respectively) relative to H37Ra LAM (36 +/- 16 and 22 +/- 11 %, respectively). Inhibition of microsphere uptake in the presence of anti-MR Ab followed a similar pattern: Erdman LAM (80 +/- 9%) > H37Rv LAM (53 +/- 1%) > H37Ra LAM (26 +/- 12%). Attachment (4 degrees C) of microspheres coated with Erdman LAM, H37Rv LAM, and H37Ra LAM was enhanced 12-, 5-, and 4-fold, respectively, compared with that of microspheres coated with AraLAM, and mannose-BSA inhibited attachment of these microspheres by 82 +/- 7, 69 +/- 8, and 12 +/- 17%. Galactose-BSA did not inhibit attachment of any LAM microsphere groups. Chromatographic analyses of mild acid hydrolysates of LAM from Erdman, H37Rv, and H37Ra all revealed the major terminal dimannosyl units. These studies demonstrate differences in the ability of LAM from different M. tuberculosis strains to mediate adherence to macrophages and to serve as ligands for the macrophage MR despite the presence of terminal dimannosyl units. Thus, these studies point toward other subtle structural alterations in LAM among strains that influence initial interactions with human phagocytes.