Naturally Processed Non-Canonical HLA-A*02:01 Presented Peptides [Protein Structure and Folding]

December 12th, 2014 by Hassan, C., Chabrol, E., Jahn, L., Kester, M. G. D., de Ru, A. H., Drijfhout, J. W., Rossjohn, J., Falkenburg, J. H. F., Heemskerk, M. H. M., Gras, S., van Veelen, P. A.

Human Leukocyte Antigen (HLA) class I molecules generally present peptides (p) of 8 to 11 amino acids (aa) in length. Although an increasing number of examples with lengthy (>11 aa) peptides, presented mostly by HLA-B alleles, have been reported. Here we characterise HLA-A*02:01 restricted, in addition to the HLA-B*0702 and HLA-B*4402 restricted, lengthy peptides (>11 aa) arising from the B-cell ligandome. We analysed a number of 15-mer peptides presented by HLA-A*02:01, and confirmed pHLA-I-formation by HLA-folding and thermal stability assays. Surprisingly the binding affinity and stability of the 15-mer epitopes in complex with HLA-A*02:01 were comparable to the values observed for canonical length (8 to 11 aa) HLA-A*02:01-restricted peptides. We solved the structures of two 15-mer epitopes in complex with HLA-A*02:01, within which the peptides adopted distinct super-bulged conformations. Moreover, we demonstrate that T-cells can recognize the 15-mer peptides in the context of HLA-A*02:01, indicating that these 15-mer peptides represent immunogenic ligands. Collectively, our data expand our understanding of longer epitopes in the context of HLA-I, highlighting that they are not limited to HLA-B family, but can bind the ubiquitous HLA-A*02:01 molecule, and play an important role in T-cell immunity.