Total Synthesis of Mycobacterium tuberculosis Dideoxymycobactin-838 and Stereoisomers: Diverse CD1a-Restricted T Cells Display a Common Hierarchy of Lipopeptide Recognition.

Mycobacterium tuberculosis produces dideoxymycobactin-838 (DDM-838), a lipopeptide that potently activates T cells upon binding to the MHC-like antigen-presenting molecule CD1a. M. tuberculosis produces DDM-838 in only trace amounts and a previous solid-phase synthesis provided sub-milligram quantities. We describe a high-yielding solution-phase synthesis of DDM-838 that features a Mitsunobu substitution that avoids yield-limiting epimerization at lysine during esterification, and amidation conditions that prevent double-bond isomerization of the Z-C20:1 acyl chain, and provides material with equivalent antigenicity to natural DDM-838. Isomers of DDM-838 that varied in stereochemistry at the central lysine and the C20:1 acyl chain were compared for their ability to be recognised by CD1a-restricted T cell receptors (TCRs). These TCRs, derived from unrelated human donors, exhibited a similar spectrum of reactivity towards the panel of DDM-838 isomers, highlighting the exquisite sensitivity of lipopeptide-reactive T cells for the natural DDM stereochemistry.

A divergent approach to the synthesis of iGb3 sugar and lipid analogues via a lactosyl 2-azido-sphingosine intermediate.

Isoglobotrihexosylceramide (iGb3, 1) is an immunomodulatory glycolipid that binds to CD1d and is presented to the T-cell receptor (TCR) of invariant natural killer T (iNKT) cells. To investigate how modifications to the lipid tail or terminal sugar residue of iGb3 influence iNKT cell activity, we developed an efficient and divergent synthetic route that provided access to both sugar and lipid iGb3 analogues which utilised a lactosyl 2-azido-sphingosine derivative as a common intermediate. In this way, iGb3 (1) and the unprecedented analogues 6'''-deoxy-iGb3-sphingosine 2, 6'''-deoxy-iGb3-sphinganine 3, C12 N-acyl iGb3 4 and C20:2 N-acyl iGb3 5 were prepared so that key structure-activity relationships can be explored.

Species-specific activity of glycolipid ligands for invariant NKT cells.

The immunomodulatory glycolipid α-galactosylceramide (α-GalCer) binds to CD1d and exhibits potent activity as a ligand for invariant CD1d-restricted natural killer-like T cells (iNKT cells). Structural analogues of α-GalCer have been synthesised to determine which components are required for CD1d presentation and iNKT cell activation, however, to date the importance of the phytosphingosine 4-hydroxyl for iNKT cell activation has been disputed. To clarify this, we synthesised two 4-deoxy α-GalCer analogues (sphinganine and sphingosine) and investigated their ability to activate murine and human iNKT cells. Analysis revealed that the analogues possessed comparable activity to α-GalCer in stimulating murine iNKT cells, but were severely compromised in their ability to stimulate human iNKT cells. Here we determined that species-specific glycolipid activity was due to a lack of recognition of the analogues by the T-cell receptors on human iNKT cells rather than insufficient presentation of the analogues on human CD1d molecules. From these results we suggest that glycolipids developed for potent iNKT cell activity in humans should contain a phytosphingosine base.

The NKT cell TCR repertoire can accommodate structural modifications to the lipid and orientation of the terminal carbohydrate of iGb3.

Isoglobotrihexosylceramide (iGb3) is a known NKT cell agonist, however the specific interactions required to trigger NKT cell TCR activation in response to this mammalian glycolipid are not fully understood. Here we report the synthesis of 1,3-ß-Gal-LacCer (ßG-iGb3) that displays a ß-linked terminal sugar. ßG-iGb3 activated NKT cells to a similar extent as iGb3 with a terminal α-linkage, indicating that the conformation of the terminal sugar residue of iGb3 is not essential to facilitate NKT cell TCR recognition. In addition, the immunological activity of four recently described iGb3 analogues with modifications to their terminal sugar or lipid backbone were also investigated. These iGb3 analogues all induced NKT cell proliferation, with IL-13 the predominate cytokine detected. This highlights the ability of the NKT cell TCR to accommodate variations in iGb3-based glycolipids and suggests that undiscovered NKT cell ligands may exist within the lacto-series of mammalian glycosphingolipids.

Methyl 6-azido-6-de-oxy-α-d-galactoside.

The structure of the title compound, C(7)H(13)N(3)O(5), was solved using data from a multiple fragment crystal. The galactoside ring adopts a (4)C(1) chair conformation. In the crystal, the molecules are linked by strong O-H⋯O hydrogen bonds, which build linkages around the screw axis of the cell in a similar way to the iodo analogue. These C-5 and C-6 packing motifs expand to R(2) (2)(10), C(2) (2)(7) and C(2) (2) (2)(8) motifs, as found in closely related compounds.

Methyl 6-de-oxy-6-iodo-α-d-galactoside.

In the crystal of the title compound, C(7)H(13)IO(5), the molecules are linked by O-H⋯O hydrogen bonds, which build linkages around one screw axis of the cell. These C(5) and C(6) packing motifs expand to R(2) (2)(10) and C(2) (2)(11) motifs and are similar to those found for closely related compounds. The galactoside ring has a (1)C(4) chair conformation.

An efficient synthesis of a 6″-BODIPY-α-Galactosylceramide probe for monitoring α-Galactosylceramide uptake by cells.

Herein, an efficient synthesis of BODIPY-α-Galactosylceramide 3, which can be used to study the cellular uptake of the potent immunostimulatory parent compound α-Galactosylceramide, is reported. Key in our synthetic strategy is the six-step synthesis of the core BODIPY scaffold (64% yield overall) and its quantitative conversion to an N-hydroxysuccinimidyl ester to facilitate conjugation and purification of the target glycolipid. For the preparation of the core of the glycolipid, the solubility of the lipid acceptor proved to be critical. The ability of BODIPY-αGalCer 3 to activate invariant natural killer cells was then demonstrated in vitro.

An improved synthesis of dansylated α-galactosylceramide and its use as a fluorescent probe for the monitoring of glycolipid uptake by cells.

A highly efficient synthesis of the biologically important fluorescent probe dansyl α-GalCer is presented. Key in our strategy is the incorporation of the fluorescent dansyl group at an early stage in the synthesis to facilitate in the monitoring and purification of intermediates via TLC and flash column chromatography, respectively, and the use of a high yielding α-selective glycosylation reaction between the phytosphingosine lipid and a galactosyl iodide donor. The ability of dansyl α-GalCer to activate iNKT cells and to serve as a fluorescent marker for the uptake of glycolipid by dendritic cells is also presented.