Figure 6 |. Direct recognition of CD1a molecules carrying permissive ligands.

a | The T cell receptor (TCR) BK6 forms a left-sided footprint (lower panel, red) that contacts CD1a itself rather than the lipid ligand lysophosphatidylcholine (LPC)⁵⁸. b | Permissive lipid ligands act through absence of interference using two proposed mechanisms. Some endogenous lipids have small head groups that are predicted not to protrude substantially from the groove, whereas LPC does protrude from the groove, but takes a rightward position on the surface so that it does not contact a left-binding TCR. c | Based on binary TCR structures^{60, 62}, non-permissive ligands, such as sphingomyelin, sulfatide and the mycobactin-like lipopeptide, bind in the groove and are thought to have an anti-antigenic function by blocking TCR docking to CD1a. Other non-permissive ligands disrupt a triad of amino acids (R76, E154 and R73) within the A’ roof⁵⁸.