Revisiting the Structure of the Anti-neoplastic Glucans of Mycobacterium bovis Bacille Calmette-Guérin

Abstract

The attenuated strain of Mycobacterium bovis Bacille Calmette-Gué rin (BCG), used worldwide to prevent tuberculosis and leprosy, is also clinically used as an immunotherapeutic agent against superficial bladder cancer. An anti-tumor polysaccharide has been isolated from the boiling water extract of the Tice substrain of BCG and tentatively characterized as consisting primarily of repeating units of 6-linked-glucosyl residues. Mycobacterium tuberculosis and other mycobacterial species produce a glycogen-like ␣-glucan composed of repeating units of 4-linked glucosyl residues substituted at some 6 positions by short oligoglucosyl units that also exhibits an anti-tumor activity. Therefore, the impression prevails that mycobacteria synthesize different types of anti-neoplastic glucans or, alternatively, the BCG substrains are singular in producing a unique type of glucan that may confer to them their immunotherapeutic property. The present study addresses this question through the comparative analysis of ␣-glucans purified from the extracellular materials and boiling water extracts of three vaccine substrains. The polysaccharides were purified, and their structural features were established by mono-and two-dimensional NMR spectroscopy and matrix-assisted laser desorption/ionization time-of-flight mass spectrometry of the enzymatic and chemical degradation products of the purified compounds. The glucans isolated by the two methods from the three substrains of BCG were shown to exhibit identical structural features shared with the glycogen-like ␣-glucan of M. tuberculosis and other mycobacteria. Incidentally, we observed an occasional release of dextrans from Sephadex columns that may explain the reported occurrence of 6-substituted ␣-glucans in mycobacteria.

The Pattern Recognition Receptor Dectin-1: From Fungi to Mycobacteria

“…In regard to the Dectin-1 ligand, there are no published reports of mycobacteria expressing β-1,3-glucans, although mycobacteria have been shown to express α-glucans [46,47]. However, biochemical studies do not support a role for α-glucans as Dectin-1 ligands [48].…”

Structure of Streptomyces Maltosyltransferase GlgE, a Homologue of a Genetically Validated Anti-tuberculosis Target

“…A ligand-bound structure was solved to 2.1 Å resolution, which will be referred to as the mal-GlgE structure. The maltose was situated at the C-terminal ends of the ␤-strands making up the center of the (␤/␣) 8 barrel of domain A, typical of active sites within the ␣-amylase family (36). The maltose was bound in a pocket (Fig.…”

“…6), four of which have been observed before in members of the GH13 ␣-amylase family of enzymes in the GH-H clan (36). Domain A is a (␤/␣) 8 barrel, typical of the catalytic domain of this family of enzymes, that forms part of the dimer interface. Domain B corresponds to an insertion after the third ␤-strand of domain A (␤12 in supplemental Fig.…”

Genetic Basis for the Biosynthesis of Methylglucose Lipopolysaccharides in Mycobacterium tuberculosis

“…The extracellular ␣-D-glucan content was determined by gas chromatography of sugar derivatives resulting from the acid hydrolysis of the lyophilized macromolecules. Further purification of ␣-D-glucan was carried out by anion-exchange chromatography on a DEAE-trisacryl gel, and the neutral fractions were rechromatographed on a Bio-Gel P-60 column, as previously described (30).…”