Monoclonal antibodies to lipoarabinomannan/arabinomannan - characteristics and implications for tuberculosis research and diagnostics

Abstract

Antibodies to the mycobacterial surface lipoglycan lipoarabinomannan (LAM) and its related capsular polysaccharide arabinomannan (AM) are increasingly important for investigations focused on both understanding mechanisms of protection against Mycobacterium tuberculosis (Mtb) and developing next-generation point-of-care tuberculosis (TB) diagnostics. We provide here an overview of the growing pipeline of monoclonal antibodies (mAbs) to LAM/AM. Old and new methodologies for their generation are reviewed and we outline and discuss their glycan epitope specificity and other features with implications for the TB field.

Keywords: antigen–antibody interactions; biomarker; glycans; lipoglycans; mycobacteria; oligosaccharides; point-of-care tests.

Figure 1.. Array of synthetic oligosaccharide (OS) fragments corresponding to structural motifs in LAM/AM, organized into seven groups based on their structural/chemical similarities and location.

Numbers under each structure refer to positions on the array used for determining binding specificity [26]. LAM structure figure adapted from [27]. All glycosidic linkages are α unless otherwise indicated; all arabinose and xylose residues are in the furanose form, and all mannose residues are in the pyranose ring form. *A secondary mannan side chain of the mannan core, included in the composite LAM structure shown here, was described in 2020, but corresponding OS motifs have not yet been included in the glycan array. OSs by group and location on the glycan array (isomeric structures are differentiated by a numerical suffix (e.g., −1, −2)): Mannose-capped (Man Cap) Motifs (Group I): Man₁Ara₄ (OS#2), Man₂Ara₄ (OS#3), Man₃Ara₄ (OS#4), Man₂Ara₆-1 (OS#5), Man₄Ara₆ (OS#6), Man₂Ara₆-2 (OS#12), Man₆Ara₆ (OS#25). 5-methylthioxylofuranose (MTX)-Manp Cap Motifs (Group II): MTX₁Man₂Ara₄ (OS#7), MTX₁Man₁Ara₄ (OS#8), MTX₁Man₃Ara₄ (OS#9), MTX₁Man₁Ara₆-1 (OS#10), MTX₁Man₁Ara₆-2 (OS#11). Core Mannan Motifs (Group III): Man₄-1 (OS#17), Man₅-1 (OS#50), Ara₅Man₄ (OS#56), Ara₅Man₃ (OS#57), Man₄-2 (OS#58), Man₅-2 (OS#59). Phosphatidyl-myo-inositol mannoside (PIM; Group IV): PIM-6 (OS#23). myo-Inositol-phosphate (PI) Cap Motif (Group V): PI₁Ara₄ (OS#49). Terminal Arabinan (Ara) Motifs (Group VI): Ara₄ (OS#1), Ara₈-1 (OS#15), Ara₇ (OS#16), Ara₁₀ (OS#18), Ara₁₁ (OS#20), Ara₂₂ (OS#22), Ara₆ (OS#44), Ara₈-2 (OS#45). Other Arabinan Motifs (Group VII): Ara₁₆ (OS#19), Ara₁₈ (OS#21).

Figure 2.. Reactivities of mAbs to AM/LAM structural motifs.

Characterized anti-AM/LAM mAbs (n = 21), organized by species in which they were generated, with the structural OS motifs they recognize as reported by respective authors (see Table 1).

Figure 3.. Frequency of anti-AM/LAM mAbs reacting with individual AM/LAM structural motifs.

Total mAbs with reported reactivity to AM oligosaccharide motifs (n = 21), regardless of isotype. Motif reactivity was previously defined and reported by respective authors.

Figure I:

Four structural domains in LAM (top) and representative terminal arabinose motifs (Ara₄ and Ara₆) and capping motifs (Man₂ and MTX₁Man) shown in both line-bond structures and symbolic nomenclature (bottom). Capping sites on Ara4/Ara6 are indicated with blue shaded ovals. Sites for the attachment of succinate and other acyl groups are indicated with orange shaded ovals.