Engagement with the TCR induces plasticity in antigenic ligands bound to MHC class I and CD1 molecules.

Complementarity-determining regions (CDRs) of αß T-cell receptors (TCRs) sense peptide-bound MHC (pMHC) complexes via chemical interactions, thereby mediating antigen specificity and MHC restriction. Flexible finger-like movement of CDR loops contributes to the establishment of optimal interactions with pMHCs. In contrast, peptide ligands captured in MHC molecules are considered more static because of the rigid hydrogen-bond network that stabilizes peptide ligands in the antigen-binding groove of MHC molecules. An array of crystal structures delineating pMHC complexes in TCR-docked and TCR-undocked forms is now available, which enables us to assess TCR engagement-induced conformational changes in peptide ligands. In this short review, we overview conformational changes in MHC class I-bound peptide ligands upon TCR docking, followed by those for CD1-bound glycolipid ligands. Finally, we analyze the co-crystal structure of the TCR:lipopeptide-bound MHC class I complex that we recently reported. We argue that TCR engagement-induced conformational changes markedly occur in lipopeptide ligands, which are essential for exposure of a primary T-cell epitope to TCRs. These conformational changes are affected by amino acid residues, such as glycine, that do not interact directly with TCRs. Thus, ligand recognition by specific TCRs involves not only T-cell epitopes but also non-epitopic amino acid residues. In light of their critical function, we propose to refer to these residues as non-epitopic residues affecting ligand plasticity and antigenicity (NR-PA).

Crystal structures of N-myristoylated lipopeptide-bound HLA class I complexes indicate reorganization of B-pocket architecture upon ligand binding.

Rhesus monkeys have evolved MHC-encoded class I allomorphs such as Mamu-B∗098 that are capable of binding N-myristoylated short lipopeptides rather than conventional long peptides; however, it remains unknown whether such antigen-binding molecules exist in other species, including humans. We herein demonstrate that human leukocyte antigen (HLA)-A∗24:02 and HLA-C∗14:02 proteins, which are known to bind conventional long peptides, also have the potential to bind N-myristoylated short lipopeptides. These HLA class I molecules shared a serine at position 9 (Ser9) with Mamu-B∗098, in contrast to most MHC class I molecules that harbor a larger amino acid residue, such as tyrosine, at this position. High resolution X-ray crystallographic analyses of lipopeptide-bound HLA-A∗24:02 and HLA-C∗14:02 complexes indicated that Ser9 was at the bottom of the B pocket with its small hydroxymethyl side chain directed away from the B-pocket cavity, thereby contributing to the formation of a deep hydrophobic cavity suitable for accommodating the long-chain fatty acid moiety of lipopeptide ligands. Upon peptide binding, however, we found the hydrogen-bond network involving Ser9 was reorganized, and the remodeled B pocket was able to capture the second amino acid residue (P2) of peptide ligands. Apart from the B pocket, virtually no marked alterations were observed for the A and F pockets upon peptide and lipopeptide binding. Thus, we concluded that the structural flexibility of the large B pocket of HLA-A∗2402 and HLA-C∗1402 primarily accounted for their previously unrecognized capacity to bind such chemically distinct ligands as conventional peptides and N-myristoylated lipopeptides.

Tocopherol suppresses 24(S)-hydroxycholesterol-induced cell death via inhibition of CaMKII phosphorylation.

Although 24(S)-hydroxycholesterol (24S-OHC) plays an important role to maintain homeostasis of cholesterol in the brain, it induces neuronal cell death at high concentrations. 24S-OHC-induced cell death was suppressed by γ-tocopherol (γ-Toc) but not by γ-tocotrienol (γ-Toc3) in a similar way to α-tocopherol (α-Toc) and α-tocotrienol (α-Toc3) in human neuroblastoma SH-SY5Y cells. Both γ-Toc and γ-Toc3 significantly inhibited cumene hydroperoxide-induced cell death, as previously shown in the case of α-Toc and α-Toc3. Lipid droplet-like structure formation induced by 24S-OHC was suppressed by neither γ-Toc nor γ-Toc3. The phosphorylation of calcium/calmodulin-dependent protein kinase II (CaMKII) was induced by 24S-OHC, which was suppressed by CaMKII phosphorylation-site inhibitor mM3 but not by calmodulin-binding-site inhibitor KN62. A calcium chelator, BAPTA-AM, inhibited calcium ionophore A23187-induced CaMKII phosphorylation but not 24S-OHC-induced CaMKII phosphorylation. Receptor-interacting protein kinase 1 (RIPK1) phosphorylation induced by 24S-OHC was not inhibited by either mM3 or KN62, suggesting that CaMKII activation does not affect RIPK1 phosphorylation. Knockdown of RIPK1 using siRNA induced not only inhibition of CaMKII phosphorylation but also reduction of total CaMKII protein levels, suggesting that RIPK1 may regulate CaMKII signalling. 24S-OHC-induced RIPK1 phosphorylation was inhibited by neither α-Toc nor α-Toc3. In contrast, CaMKII phosphorylation induced by 24S-OHC was significantly suppressed by α-Toc but not by α-Toc3. These results suggest that CaMKII activation is involved in the mechanism of 24S-OHC-induced cell death and that Toc inhibits the cell death via inhibition of CaMKII activation through a RIPK1 phosphorylation-independent pathway.