Structural Characterization of Mycobacterial Phosphatidylinositol Mannoside Binding to Mouse CD1d

The mechanisms permitting nonpolymorphic CD1 molecules to present lipid antigens that differ considerably in polar head and aliphatic tails remain elusive. It is also unclear why hydrophobic motifs in the aliphatic tails of some antigens, which presumably embed inside CD1 pockets, contribute to determinants for T-cell recognition. The 1.9-Å crystal structure of an active complex of CD1b and a mycobacterial diacylsulfoglycolipid presented here provides some clues. Upon antigen binding, endogenous spacers of CD1b, which consist of a mixture of diradylglycerols, moved considerably within the lipid-binding groove. Spacer displacement was accompanied by F' pocket closure and an extensive rearrangement of residues exposed to T-cell receptors. Such structural reorganization resulted in reduction of the A' pocket capacity and led to incomplete embedding of the methyl-ramified portion of the phthioceranoyl chain of the antigen, explaining why such hydrophobic motifs are critical for T-cell receptor recognition. Mutagenesis experiments supported the functional importance of the observed structural alterations for T-cell stimulation. Overall, our data delineate a complex molecular mechanism combining spacer repositioning and ligandinduced conformational changes that, together with pocket intricacy, endows CD1b with the required molecular plasticity to present a broad range of structurally diverse antigens.three-dimensional structure | groove shrinking | diacylglycerol endogenous ligand | T lymphocyte activation | CD1b mutant transfectant T lymphocytes have developed the capacity to recognize as antigens a large variety of molecules including peptides, (glyco)lipids, and phosphorylated metabolites (1). Specific recognition of peptides or lipids by T-cell receptors (TCR) occurs when these molecules form antigenic complexes with dedicated antigen-presenting molecules belonging to MHC or CD1 families, respectively. Diversity has forced the immune system to develop appropriate strategies to present antigens in immunogenic form. Polymorphic MHC molecules cope with the peptide repertoire by constraining the ligand conformational space (2). Less clear is how the immune system adapts to the large glycolipid antigenic range and forms antigenic complexes using the functionally nonpolymorphic CD1 molecules.Human antigen-presenting cells (APC) display the CD1a, CD1b, CD1c, and CD1d proteins on their plasma membranes (1, 3). CD1 ectodomains consist of a heavy chain, which folds into three extracellular domains (α1-α3) noncovalently associated with β2-microglobulin (4). Antigen-binding grooves nestle between the α1 and α2 domains and are mostly lined by hydrophobic residues. This allows the antigenic lipids to be anchored via their hydrophobic chains, so that polar motifs protrude toward the aqueous milieu. Consequently, polar heads but not hydrophobic tails are assumed to establish stimulatory contacts with TCRs. Nevertheless, modifications in the lipid chains may also indirectly impact on TCR recognition (5).The number, shape, and conn...

Section: Resultsmentioning

confidence: 89%

Structural reorganization of the antigen-binding groove of human CD1b for presentation of mycobacterial sulfoglycolipids

Garcia-Alles

Collmann

Versluis

et al. 2011

“…We estimate that no molecule larger than a single-chain C 18 lipid molecule could fit within this pocket and still coordinate a charged head group with the conserved Arg-82 residue near the opening of the pocket. Larger head groups that project into the solvent above the pocket opening are possible, as observed for the binding of mycobacterial phosphatidylinositol mannoside with CD1d (30). However, the volume constraints of chCD1-2 protein would limit the length of any bound hydrocarbon chain.…”

Section: Discussionmentioning

confidence: 99%

“…The crystal structure of chCD1-2 with an endogenously bound ligand, presumably palmitic acid, was determined to a resolution of 2.0 Å [supporting information (SI) Table S1]. The overall structure of chCD1-2 resembles that of mammalian CD1 molecules (8,17,18,21,26,(30)(31)(32)(33). Briefly, 2 ␣ helices (␣1 and ␣2) sit atop a central 6-stranded, anti-parallel, ␤-sheet platform, thus forming the ␣1-␣2 superdomain.…”

Section: Resultsmentioning

confidence: 99%

The crystal structure of avian CD1 reveals a smaller, more primordial antigen-binding pocket compared to mammalian CD1

Zajonc

Striegl

Dascher

et al. 2008

Self Cite

The molecular details of glycolipid presentation by CD1 antigenpresenting molecules are well studied in mammalian systems. However, little is known about how these non-classical MHC class I (MHCI) molecules diverged from the MHC locus to create a more complex, hydrophobic binding groove that binds lipids rather than peptides. To address this fundamental question, we have determined the crystal structure of an avian CD1 (chCD1-2) that shares common ancestry with mammalian CD1 from Ϸ310 million years ago. The chCD1-2 antigen-binding site consists of a compact, narrow, central hydrophobic groove or pore rather than the more open, 2-pocket architecture observed in mammalian CD1s. Potential antigens then would be restricted in size to single-chain lipids or glycolipids. An endogenous ligand, possibly palmitic acid, serves to illuminate the mode and mechanism of ligand interaction with chCD1-2. The palmitate alkyl chain is inserted into the relatively shallow hydrophobic pore; its carboxyl group emerges at the receptor surface and is stabilized by electrostatic and hydrogen bond interactions with an arginine residue that is conserved in all known CD1 proteins. In addition, other novel features, such as an A loop that interrupts and segments the normally long, continuous ␣1 helix, are unique to chCD1-2 and contribute to the unusually narrow binding groove, thereby limiting its size. Because birds and mammals share a common ancestor, but the rate of evolution is slower in birds than in mammals, the chCD1-2-binding groove probably represents a more primordial CD1-binding groove.evolution ͉ glycolipid

“…Recent crystal structure studies of glycolipid-CD1d complexes have revealed that the lipid portion of glycolipids fits into the CD1d binding groove with some diversity (28)(29)(30)(31). Biological studies on α-GalCer analogues have shown that the modification of the lipid chain determines Th1/Th2 balance (32,33).…”

Section: Discussionmentioning

confidence: 99%

Design of a potent CD1d-binding NKT cell ligand as a vaccine adjuvant

Fujio

Imamura

et al. 2010

161

201

The glycolipid α-galactosylceramide (α-GalCer) has been shown to bind CD1d molecules to activate invariant natural killer T (iNKT) cells, and subsequently induce activation of various immune-competent cells, including dendritic cells, thereby providing a significant adjuvant effect for various vaccines. However, in phase I clinical trials, α-GalCer was shown to display only marginal biological activity. In our search for a glycolipid that can exert more potent stimulatory activity against iNKT cells and dendritic cells and produce an adjuvant effect superior to α-GalCer, we performed step-wise screening assays on a focused library of 25 α-GalCer analogues. Assays included quantification of the magnitude of stimulatory activity against human iNKT cells in vitro, binding affinity to human and murine CD1d molecules, and binding affinity to the invariant t cell receptor of human iNKT cells. Through this rigorous and iterative screening process, we have identified a lead candidate glycolipid, 7DW8-5, that exhibits a superior adjuvant effect than α-GalCer on HIV and malaria vaccines in mice.