Crystal structure of the giant panda MHC class I complex: First insights into the viral peptide presentation profile in the bear family

Crystal Structure of a Classical MHC Class I Molecule in Dogs; Comparison of DLA-88*0 and DLA-88*5 Category Molecules

DLA-88 is a classical major histocompatibility complex (MHC) class I gene in dogs, and allelic DLA-88 molecules have been divided into two categories named "DLA-88*0" and "DLA-88*5." The defining difference between the two categories concerns an LQW motif in the α2 domain helical region of the DLA-88*5 molecules that includes the insertion of an extra amino acid compared to MHC class I consensus length. We here show that this motif has been exchanged by recombination between different DLA-88 evolutionary lineages. Previously, with pDLA-88*508:01, the structure of a molecule of the DLA-88*5 category was elucidated. The present study is the first to elucidate a structure, using X-ray crystallography, of the DLA-88*0 category, namely DLA-88*001:04 complexed with β2m and a nonamer peptide derived from canine distemper virus (CDV). The LQW motif that distinguishes DLA-88*5 from DLA-88*0 causes a shallower peptide binding groove (PBG) and a leucine exposed at the top of the α2 domain helix expected to affect T cell selection. Peptide ligand amino acid substitution and pMHC-I complex formation and stability analyses revealed that P2 and P3 are the major anchor residue positions for binding to DLA-88*001:04. We speculate that the distribution pattern of the LQW motif among canine classical MHC class I alleles represents a strategy to enhance allogeneic rejection by T cells of transmissible cancers such as canine transmissible venereal tumor (CTVT).

Crystal structure of the giant panda MHC class I complex: First insights into the viral peptide presentation profile in the bear family

The viral cytotoxic T lymphocyte (CTL) epitope peptides presented by classical MHC-I molecules require the assembly of a peptide-MHC-I-β2m (pMHC-I) trimolecular complex for T cell receptor (TCR) recognition, which is the critical activation link for triggering antiviral T cell immunity. Research on T cell immunology in the Ursidae family, especially structural immunology, is still lacking. In this study, the structure of the key trimolecular complex pMHC-I, which binds a peptide from canine distemper virus, was solved for the first time using giant panda as a representative species of Ursidae. The structural characteristics of the giant panda pMHC-I complex (pAime-128), including the unique pockets in the peptide-binding groove (PBG), were analyzed in detail. Comparing the pAime-128 to others in the bear family and extending the comparison to other mammals revealed distinct features. The interaction between MHC-I and β2m, the features of pAime-128 involved in TCR docking and cluster of differentiation 8 (CD8) binding, the anchor sites in the PBG, and the CTL epitopes of potential viruses that infect pandas were clarified. Unique features of pMHC-I viral antigen presentation in the panda were revealed by solving the three-dimensional (3D) structure of pAime-128. The distinct characteristics of pAime-128 indicate an unusual event that emerged during the evolution of the MHC system in the bear family. These results provide a new platform for research on panda CTL immunity and the design of vaccines for application in the bear family.