Variation in mannose-capped terminal arabinan motifs of lipoarabinomannansfrom clinical isolates of Mycobacterium tuberculosis and Mycobacterium avium complex

The unique terminal arabinan motifs of mycobacterial lipoarabinomannan (LAM ), which are mannose-capped to different extents, probably constitute the s ingle most important structural entity engaged in receptor binding and subs equent immunopathogenesis. We have developed a concerted approach of endoar abinanase digestion coupled with chromatography and mass spectrometry analy sis to rapidly identify and quantitatively map the complement of such termi nal units among the clinical isolates of different virulence and drug resis tance profiles. In comparison with LAM from laboratory strains of Mycobacte rium tuberculosis, an ethambutol (Emb) resistant clinical isolate was shown to have a significantly higher proportion of nonmannose capped arabinan te rmini, More drastically, the mannose capping was completely inhibited when an Emb-susceptible strain was grown in the presence of subminimal inhibitor y concentration of Emb. Both cases resulted in an increase of arabinose to mannose ratio in the overall glycosyl composition of LAM. Emb, therefore, n ot only could affect the complete elaboration of the arabinan as found prev iously for LAM from Mycobacterium smegmatis resistant mutant but also could inhibit the extent of mannose capping and hence its associated biological functions in M. tuberculosis. Unexpectedly, an intrinsically Emb-resistant Mycobacterium avium isolate of smooth transparent colony morphology was fou nd to have most of its arabinan termini capped with a single mannose residu e instead of the more common dimannoside as established for LAM from M, tub erculosis. This is the first report on the LAM structure from M. avium comp lex, an increasingly important opportunistic infectious agent afflicting AI DS patients.