PGL-III, a Rare Intermediate of Mycobacterium leprae Phenolic Glycolipid Biosynthesis, Is a Potent Mincle Ligand

Abstract

Although leprosy (Hansen's disease) is one of the oldest known diseases, the pathogenicity of Mycobacterium leprae (M. leprae) remains enigmatic. Indeed, the cell wall components responsible for the immune response against M. leprae are as yet largely unidentified. We reveal here phenolic glycolipid-III (PGL-III) as an M. leprae-specific ligand for the immune receptor Mincle. PGL-III is a scarcely present trisaccharide intermediate in the biosynthetic pathway to PGL-I, an abundant and characteristic M. leprae glycolipid. Using activity-based purification, we identified PGL-III as a Mincle ligand that is more potent than the well-known M. tuberculosis trehalose dimycolate. The cocrystal structure of Mincle and a synthetic PGL-III analogue revealed a unique recognition mode, implying that it can engage multiple Mincle molecules. In Mincle-deficient mice infected with M. leprae, increased bacterial burden with gross pathologies were observed. These results show that PGL-III is a noncanonical ligand recognized by Mincle, triggering protective immunity.

Secondary Sites of the C‐type Lectin‐Like Fold

“…While a core monosaccharide anchors the ligand at Ca 2+ site II, proximal amino acids establish contacts with additional carbohydrate residues, as well as conjugated peptides and lipids, increasing affinity of the interaction [58] . This is particularly well illustrated by the dependency of mincle‐induced signaling on oligosaccharides di‐esterized with branched fatty acids, such as trehalose dimycolate (TDM) and phenolic glycolipid‐III (PGL‐III) [57,59,60] . Several X‐ray crystallographic and NMR studies of human and bovine mincle indicated that the interaction with these glycolipids involves Ca 2+ ‐dependent binding of the first glucose, an additional proximal binding site for the second glucose, as well as a hydrophobic grove for lipid binding [54–57,61,62] (Figure 2c).…”

“…[58] This is particularly well illustrated by the dependency of mincle-induced signaling on oligosaccharides diesterized with branched fatty acids, such as trehalose dimycolate (TDM) and phenolic glycolipid-III (PGL-III). [57,59,60] Several Xray crystallographic and NMR studies of human and bovine mincle indicated that the interaction with these glycolipids involves Ca 2 + -dependent binding of the first glucose, an additional proximal binding site for the second glucose, as well as a hydrophobic grove for lipid binding [54][55][56][57]61,62] (Figure 2c). Since these extended recognition interfaces are less conserved, they also provide selectivity, even between closely related CLRs, such as DC-SIGN and its mouse homologs SIGN-R1, 2, 3, 4, 5, 7, and 8.…”

Biophysical Interaction Landscape of Mycobacterial Mycolic Acids and Phenolic Glycolipids with Host Macrophage Membranes

“…Three major classes of MA are alpha (α-) MA, keto-MA, and methoxy (MeO-) MA (Figure A) , and play an important role both as a physical barrier and in infection biology . Another recent addition is the phenolic glycolipid (PGL), shown to boost acquired immune responses as an adjuvant in vivo . PGL is a surface lipid and glycosylated version of phthiocerol dimycocerosate (PDIM)the most extensively studied mycobacterial virulent lipid. , Its structure is composed of saccharide units (1–3 units) bound to a phenyl group, a mycocerosic acid and a phthiocerol region, Figure B.…”