Structural Evaluation of Potent NKT Cell Agonists: Implications for Design of Novel Stimulatory Ligands

Natural killer T (NKT) cells are a subset of T cells that are activated by CD1d–glycolipid complexes through a semi-invariant αβ T cell receptor (NKT TCR). Upon activation, NKT cells secrete regulatory cytokines that are implicated in T helper cell responses. α-Galactosylceramide (α-GalCer) is a potent NKT cell agonist when presented by CD1d. Phenyl ring substitutions of the α-GalCer fatty acid moiety were recently found to be superior in eliciting regulatory cytokines. Crystal structures of four new mouse CD1d–lipid complexes (five structures), a new PBS-25 complex, and CD1d with an endogenous ligand, at 1.6–1.9 Å resolution, reveal that the α-GalCer phenyl analogues impart minor structural differences to the A′-pocket, while the sphingosine and galactose moieties, important for NKT TCR recognition, remain virtually unchanged. The observed differences in cytokine-release profiles appear to be associated with increased stability of the CD1d–glycolipid complexes rather than increased affinity for the NKT TCR. Furthermore, comparison of mouse CD1d–glycolipid complexes in different crystallographic space groups reveals considerable conformational variation, particularly above the F′-pocket, the primary site of interaction with the NKT TCR. We propose that modifications of the sphingosine moiety or other substitutions that decrease α-GalCer flexibility would stabilize the F′-pocket. Such compounds might then increase CD1d affinity for the NKT TCR and further enhance the stimulatory and regulatory properties of α-GalCer derivatives.