The Molecular Basis for Recognition of CD1d/α-Galactosylceramide by a Human Non-Vα24 T Cell Receptor

CD1d-mediated presentation of glycolipid antigens to T cells is capable of initiating powerful immune responses that can have a beneficial impact on many diseases. Molecular analyses have recently detailed the lipid antigen recognition strategies utilized by the invariant V alpha 24-J alpha 18 TCR rearrangements of iNKT cells, which comprise a subset of the human CD1d-restricted T cell population. In contrast, little is known about how lipid antigens are recognized by functionally distinct CD1d-restricted T cells bearing different TCR alpha chain rearrangements. Here we present crystallographic and biophysical analyses of alpha-galactosylceramide (alpha-GalCer) recognition by a human CD1d-restricted TCR that utilizes a V alpha 3.1-J alpha 18 rearrangement and displays a more restricted specificity for alpha-linked glycolipids than that of iNKT TCRs. Despite having sequence divergence in the CDR1 alpha and CDR2 alpha loops, this TCR employs a convergent recognition strategy to engage CD1d/alpha GalCer, with a binding affinity (similar to 2 mM) almost identical to that of an iNKT TCR used in this study. The CDR3 alpha loop, similar in sequence to iNKT-TCRs, engages CD1d/alpha GalCer in a similar position as that seen with iNKT-TCRs, however fewer actual contacts are made. Instead, the CDR1 alpha loop contributes important contacts to CD1d/alpha GalCer, with an emphasis on the 4'OH of the galactose headgroup. This is consistent with the inability of V alpha 24- T cells to respond to a-glucosylceramide, which differs from alpha GalCer in the position of the 4'OH. These data illustrate how fine specificity for a lipid containing alpha-linked galactose is achieved by a TCR structurally distinct from that of iNKT cells.