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Structure and antigenicity of lipoarabinomannan from Mycobacterium bovis BCG
S Prinzis1, D Chatterjee, P J Brennan
1Department of Microbiology, Colorado State University, Fort Collins 80523.
Abstract:
Lipoarabinomannan (LAM), a major lipoglycan of the mycobacterial cell envelope, was previously recognized as existing in two major forms: LAM with arabinofuranosyl (Araf)-containing termini (AraLAM) and a mannose-capped version (ManLAM) in which the majority of these termini are modified by additional mannose residues. Since ManLAM was first recognized in the virulent (Erdman) strain of Mycobacterium tuberculosis and the noncapped version in a rapidly growing, attenuated, H37Ra strain, it was thought that mannose capping may be a key factor in virulence. In the present study, LAM from M. bovis BCG was isolated and the non-reducing termini sequenced through differential O-alkylation, partial depolymerization and gas chromatography-mass spectrometric analyses of fragments. LAM from M. bovis BCG contains a short mannan backbone, highly branched arabinofuranosyl-containing side chains and several mannosyl residues capping the non-reducing termini of these side chains. Thus, LAM from M. bovis BCG is of the ManLAM type, showing no major structural differences at the non-reducing ends from the M. tuberculosis Erdman product. This observation led us to examine the earlier strain and to conclude that it showed little resemblance to conventional strains of M. tuberculosis. Thus, the absence of mannose caps may be more a feature of rapid growth than of avirulence. These results demonstrate that the relationship between mannose capping and disease induction is not a simple one. However, use of a panel of LAM-specific monoclonal antibodies showed antigenic differences between the BCG and the Erdman products, suggesting the presence of features specific to the different strains and pointing to LAM as a molecule within which further species and strain variations reside.
Insights
Lipoarabinomannan (LAM) structure varies between Mycobacterium strains. Mannose capping on LAM may relate to rapid growth rather than virulence, with antigenic differences found between strains.
Area of Science:
- Mycobacterial cell envelope structure
- Lipoglycan analysis
- Immunochemistry
Background:
- Lipoarabinomannan (LAM) is a key lipoglycan in mycobacteria, previously classified into arabinofuranosyl-capped (AraLAM) and mannose-capped (ManLAM) forms.
- Mannose capping was initially linked to virulence, observed in virulent Mycobacterium tuberculosis (Erdman strain) and absent in attenuated H37Ra.
- Previous understanding suggested mannose capping as a critical factor in mycobacterial virulence.
Purpose of the Study:
- To investigate the structural characteristics of LAM from Mycobacterium bovis BCG, specifically its non-reducing termini.
- To re-evaluate the relationship between mannose capping of LAM and mycobacterial virulence.
- To explore antigenic variations within LAM across different mycobacterial strains.
Main Methods:
- Isolation and structural sequencing of LAM from M. bovis BCG.
- Utilized differential O-alkylation, partial depolymerization, and gas chromatography-mass spectrometry (GC-MS) for terminal structure analysis.
- Employed a panel of LAM-specific monoclonal antibodies to assess antigenic differences.
Main Results:
- LAM from M. bovis BCG exhibits ManLAM characteristics, with mannosyl residues capping the termini of branched arabinofuranosyl-containing side chains.
- The structure of BCG LAM shows no significant difference at the non-reducing ends compared to M. tuberculosis Erdman strain.
- Antigenic differences were detected between BCG and Erdman LAM using monoclonal antibodies, indicating strain-specific features.
Conclusions:
- The absence of mannose capping may be associated with rapid growth rather than avirulence.
- The relationship between mannose capping and disease induction is complex and not a simple direct correlation.
- LAM possesses inherent species and strain variations, as evidenced by antigenic differences between M. bovis BCG and M. tuberculosis Erdman.