Nora Pauwels

Galactose-modified iNKT cell agonists stabilized by an induced fit of CD1d prevent tumour metastasis

Invariant natural killer T (iNKT) cells are known to have marked immunomodulatory capacity due to their ability to produce copious amounts of effector cytokines. Here, we report the structure and function of a novel class of aromatic α‐galactosylceramide structurally related glycolipids with marked Th1 bias in both mice and men, leading to superior tumour protection in vivo. The strength of the Th1 response correlates well with enhanced lipid binding to CD1d as a result of an induced fit mechanism that binds the aromatic substitution as a third anchor, in addition to the two lipid chains. This induced fit is in contrast to another Th1‐biasing glycolipid, α‐C‐GalCer, whose CD1d binding follows a conventional key‐lock principle. These findings highlight the previously unexploited flexibility of CD1d in accommodating galactose‐modified glycolipids and broaden the range of glycolipids that can stimulate iNKT cells. We speculate that glycolipids can be designed that induce a similar fit, thereby leading to superior and more sustained iNKT cell responses in vivo.

Enhanced TCR Footprint by a Novel Glycolipid Increases NKT-Dependent Tumor Protection

NKT cells, a unique type of regulatory T cells, respond to structurally diverse glycolipids presented by CD1d. Although it was previously thought that recognition of glycolipids such as α-galactosylceramide (α-GalCer) by the NKT cell TCR (NKTCR) obeys a key–lock principle, it is now clear this interaction is much more flexible. In this article, we report the structure–function analysis of a series of novel 6′′-OH analogs of α-GalCer with more potent antitumor characteristics. Surprisingly, one of the novel carbamate analogs, α-GalCer-6′′-(pyridin-4-yl)carbamate, formed novel interactions with the NKTCR. This interaction was associated with an extremely high level of Th1 polarization and superior antitumor responses. These data highlight the in vivo relevance of adding aromatic moieties to the 6′′-OH position of the sugar and additionally show that judiciously chosen linkers are a promising strategy to generate strong Th1-polarizing glycolipids through increased binding either to CD1d or to NKTCR.

Crystal structures of bovine CD1d reveal altered αGalCer presentation and a restricted A' pocket unable to bind long-chain glycolipids.

NKT cells play important roles in immune surveillance. They rapidly respond to pathogens by detecting microbial glycolipids when presented by the non-classical MHC I homolog CD1d. Previously, ruminants were considered to lack NKT cells due to the lack of a functional CD1D gene. However, recent data suggest that cattle express CD1d with unknown function. In an attempt to characterize the function of bovine CD1d, we assessed the lipid binding properties of recombinant Bos taurus CD1d (boCD1d) in vitro. BoCD1d is able to bind glycosphingolipids (GSLs) with fatty acid chain lengths of C18, while GSLs with fatty acids of C24 do not bind. Crystal structures of boCD1d bound to a short-chain C12-di-sulfatide antigen, as well as short-chain C16-αGalCer revealed that the Á pocket of boCD1d is restricted in size compared to that of both mouse and human CD1d, explaining the inability of long chain GSL’s to bind to boCD1d. Moreover, while di-sulfatide is presented similarly compared to the presentation of sulfatide by mouse CD1d, αGalCer is presented differently at the cell surface, due to an amino acid Asp151Asn substitution that results in loss of intimate contacts between the αGalCer headgroup and CD1d. The altered αGalCer presentation by boCD1d also explains its lack of cross-activation of mouse iNKT cells and raises the interesting question of the nature and function of bovine lipid-reactive T cells.

Synthesis of C-5″ and C-6″-modified α-GalCer analogues as iNKT-cell agonists

Alpha-galactosyl ceramide (α-GalCer) is a prototypical synthetic ligand of invariant natural killer T (iNKT) cells. Upon presentation by the MHC class I-like molecule CD1d, this glycolipid stimulates iNKT cells to secrete a vast amount of both pro-inflammatory Th1 and anti-inflammatory Th2 cytokines. Recently, we discovered that selected 6″-modified α-GalCer analogues may produce markedly Th1-biased responses due to the formation of either an additional anchor with CD1d or by establishing extra interactions with the T-cell receptor of iNKT cells. Here, we report a practical synthesis towards 6″-O-carbamate and galacturonamide analogues of α-GalCer and their evaluation as iNKT cell agonists in mice.