Distribution of antigenic glycolipids among Mycobacterium tuberculosis strains and their contribution to virulence

Abstract

Tuberculosis is a chronic disease caused by Mycobacterium tuberculosis infection. The major pathological changes are immunologically hypersensitive granuloma formation due to the local proliferation or infiltration of immune cells. However, the mechanism for the development of the disease has not yet been fully understood. The first step of infection in intracellular survival in the phagocytic cells and this process has been reported to be regulated by cell surface glycolipid virulence factors. As genetical heterogeneity of M. tuberculosis among strains has been reported recently based on DNA fragmentation pattern, I have examined the distribution of cell surface glycolipids (cord factor, sulfolipids and penta acyl trehaloses) among the virulent (M. tuberculosis H37Rv and M. tuberculosis Aoyama B) and virulent (M. tuberculosis H37Ra) strains by two dimensional thin-layer chromatography of silica gel. Seven characteristic glycolipid components of the virulent strains were detected and separated by thin-layer chromatography of silica gel. Each glycolipid was identified by fast-atom-bombardment mass-spectrometry (FAB/MS) analysis of the intact lipid and gas-chromatography mass-spectrometry (GC/MS) analysis of the fatty acid or the carbohydrate moiety. As the result, molecular weight (m/z, 1,200-3,000) of each glycolipid was determined clearly by FAB/MS analysis. The structure of fatty acids (C16-C40) or mycolic acids (C76-C88) were determined by GC/MS analysis. Cord factor (TDM, trehalose 6,6'-dimycolate) and trehalose 6-monomycolate (TMM) showed strong granuloma forming activity, but other glycolipids practically did not. On the other hand, cord factor and trehalose 6-monomycolate showed phagocytosis inhibition (but showed promotion in the presence of complement) and marked inhibition of phagosome-lysosome fusion, while sulfolipids showed strong phagocytosis promotion and marked inhibition of phagosome-lysosome fusion. Penta acyl trehaloses showed phagocytosis promotion but no effect on phagosome-lysosome fusion. Cord factor and trehalose 6-monomycolate existed ubiquitously among virulent and avirulent strains, while sulfolipids and penta acyl trehaloses were detected in only virulent strains (M. tuberculosis H37Rv and M. tuberculosis Aoyama B). These results indicate that the existence of these toxic glycolipids contributes to the virulence of M. tuberculosis, profoundly. It is suggested that these glycolipids play an important role as virulence factors in the early stage of infection and expression of pathogenicity.