Macrophage activation: lipoarabinomannan from avirulent and virulent strains of Mycobacterium tuberculosis differentially induces the early genes c-fos, KC, JE, and tumor necrosis factor-alpha.
Lipoarabinomannan (LAM) is a major cell-wall associated glycolipid produced by Mycobacterium tuberculosis and Mycobacterium leprae. Previous work demonstrated that LAM from avirulent (H37Ra) and virulent (Erdman) strains of M. tuberculosis differ in structure at their non-reducing termini. In this study the effects of the H37Ra and Erdman LAM on the activation of murine bone marrow-derived macrophages has been investigated. Their abilities to elicit immediate early gene responses at mRNA (c-fos, JE, KC) and protein (TNF-alpha secretion) levels, and nitrite production, was examined. H37Ra LAM, but not Erdman LAM, elicited TNF-alpha secretion at 1000 ng/ml. Neither stimulated production of reactive nitrogen intermediates (RNI). Addition of 25 U/ml IFN-gamma enhanced TNF-alpha secretion in response to H37Ra LAM, reducing the threshold level of LAM required to 10 to 100 ng/ml. In contrast, Erdman LAM at concentrations up to 1000 ng/ml could not induce macrophage TNF-alpha secretion even in the presence of 25 U/ml IFN-gamma. H37Ra LAM also synergized with IFN-gamma to stimulate enhanced production of RNI, whereas IFN-gamma and Erdman LAM did not elicit RNI production. Examination of events before TNF-alpha and RNI production revealed that H37Ra LAM, like LPS, was able to induce increased levels of mRNA expression for c-fos, KC, and JE, with similar kinetics but reduced potency compared with LPS. Erdman LAM in concentrations up to 2500 ng/ml was unable to stimulate c-fos, KC, or JE expression. IFN-gamma at 25 U/ml was itself a potent stimulus of JE expression, and synergized with 1000 ng/ml H37Ra, and to a lesser extent, Erdman LAM for the induction of JE. In contrast, IFN-gamma inhibited H37Ra LAM stimulation of KC expression. The phenomenon of avoiding the stimulation of macrophage immediate early gene expression may be an important determinant of mycobacterial virulence.