REGULATION OF MURINE MACROPHAGE EFFECTOR FUNCTIONS BY LIPOARABINOMANNAN FROM MYCOBACTERIAL STRAINS WITH DIFFERENT DEGREES OF VIRULENCE

Lipoarabinomannan (LAM) is the major arabinose- and mannose-containing phosphorylated lipopolysaccharide (LPS) in mycobacterial cell walls. LAM preparations from a virulent strain (Erdman) (LAM(Erdman)) and an attenuated strain (H37Ra) (LAM(H37Ra) of Mycobacterium tuberculosis, as well as from M. leprae (a virulent mycobacterium), were analyzed for their effects on various macrophage (Mphi) effector functions. LAM(H37Ra), like gram-negative LPS, exhibited a dose-dependent ability to induce tumor necrosis factor alpha (TNF-alpha) production in normal Mphi, and gamma interferon (IFN-gamma) priming of the Mphi greatly augmented the levels of TNF-alpha. However, the effects of LAM(H37Ra) were unaffected by polymyxin B, which totally abrogated the effects of LPS. LAM(Erdman) and LAM from M. leprae, on the other hand, induced virtually no TNF-alpha production. Analysis of Mphi mRNA by reverse transcription-polymerase chain reaction revealed that the levels of TNF-alpha mRNA induced by the various preparations correlated with the levels of TNF-alpha protein detected. Interestingly, both LAM(H37Ra) and LAM(Erdman) could block subsequent IFN-gamma- and LPS-induced Mphi activation, a previously reported measure of the potent ability of LAM to down-regulate Mphi effector functions. Two lines of evidence suggested, however, that Mphi cyclooxygenase products did not play a role in this down-regulation. LAM(H37Ra) and LPS could induce the production of NO2- in both normal and IFN-gamma-primed Mphi, whereas LAM(Erdman) could stimulate NO2- production only in primed Mphi. Both LAM(H37Ra) and LAM(Erdman) could substitute for LPS as a triggering signal for IFN-gamma-primed Mphi in a toxoplasma killing assay. The triggering ability Of LAM(Erdman), however, was abrogated by an anti-TNF-alpha antibody, suggesting that sufficient TNF-alpha production was stimulated by LAM(Erdman) to drive a Mphi function relevant in host resistance. Thus, mycobacterial LAM is a potent regulator of Mphi functions, a fact that may have important consequences in mycobacterial disease.