The outermost capsular arabinomannans and other mannoconjugates of virulent and avirulent tubercle bacilli

Abstract

It has been shown that phagocyte mannose receptors play an important role in phagocytosis of virulent tubercle bacilli, but not of avirulent strains. Accordingly, we investigated the occurrence and structure of the outermost mannoconjugates of the capsule of five strains of the tubercle bacillus differing in their degrees of virulence. The extracellular and surface-exposed arabinomannan-containing polysaccharides were chemically characterized as being composed mainly of neutral fatty-acyl-free arabinomannans (AMs) possessing a reducing end consisting of mannose. Although no lipoarabinomannan (LAM) was detected, small amounts of acidic polysaccharides, exhibiting the same electrophoretic mobility as LAM, were identified as succinylated AMs (two to three residues per molecule) lacking the phosphatidylinositol anchor of LAM. AMs from the different strains shared the same structural features, notably the capping of a large portion of the arabinan segments with mannosyl residues. However, no correlation was observed between either the percentage of capping or the amount of AMs and the degrees of virulence of the strains. The occurrence and amounts of other mannoconjugates (phosphatidylinositol mannosides and the mannoseassociated 19 and 38 kDa lipoproteins) in the various tubercle bacilli were also examined. Although both classes of compounds were identified in all the examined strains, a correlation between the amounts of the glycoconjugates and the degrees of virulence of the strains could not be established. These data do not support the implication of these promising mannosylated molecules in the selective phagocytosis of virulent tubercle bacilli and indicate that the involvement of mannose receptors in phagocytosis of virulent M. tuberculosis needs to be re-investigated.