Characterization of the Antigenic Heterogeneity of Lipoarabinomannan, the Major Surface Glycolipid of Mycobacterium tuberculosis, and Complexity of Antibody Specificities toward This Antigen.

Lipoarabinomannan (LAM), the major antigenic glycolipid of Mycobacterium tuberculosis, is an important immunodiagnostic target for detecting tuberculosis (TB) infection in HIV-1-coinfected patients, and is believed to mediate a number of functions that promote infection and disease development. To probe the human humoral response against LAM during TB infection, several novel LAM-specific human mAbs were molecularly cloned from memory B cells isolated from infected patients and grown in vitro. The fine epitope specificities of these Abs, along with those of a panel of previously described murine and phage-derived LAM-specific mAbs, were mapped using binding assays against LAM Ags from several mycobacterial species and a panel of synthetic glycans and glycoconjugates that represented diverse carbohydrate structures present in LAM. Multiple reactivity patterns were seen that differed in their specificity for LAM from different species, as well as in their dependence on arabinofuranoside branching and nature of capping at the nonreducing termini. Competition studies with mAbs and soluble glycans further defined these epitope specificities and guided the design of highly sensitive immunodetection assays capable of detecting LAM in urine of TB patients, even in the absence of HIV-1 coinfection. These results highlighted the complexity of the antigenic structure of LAM and the diversity of the natural Ab response against this target. The information and novel reagents described in this study will allow further optimization of diagnostic assays for LAM and may facilitate the development of potential immunotherapeutic approaches to inhibit the functional activities of specific structural motifs in LAM.

Development of an Orthogonal Protection Strategy for the Synthesis of Mycobacterial Arabinomannan Fragments.

Mycobacterium tuberculosis, the organism that causes tuberculosis (TB), has a carbohydrate-rich cell wall structure that possesses a number of immunogenic antigens. Circulating antibodies that recognize these glycans are present in patients infected by mycobacteria; detection of these antibodies could be the basis for new TB diagnostics. We describe here the synthesis of a panel of mycobacterial arabinomannan fragments for use in investigations directed at testing the feasibility of such a diagnostic method. In this study, we focused on structural motifs present in the core of the key immunogenic polysaccharide lipoarabinomannan (LAM). To access these compounds, we developed an efficient orthogonal protection strategy that allowed access to seven arabinomannan fragments of LAM (1-7). The targets included one tetrasaccharide, one pentasaccharide, three octasaccharides, and two nonasaccharides. Starting from a differentially protected trimannopyranoside derivative (8 or 9), the targets were obtained using an approach that involved selective removal of the protecting group present at the O-2 position of a single mannopyranoside residue, followed by glycosylation with a pentaarabinofuranose thioglycoside and/or a mannopyranose trichloroacetimidate.