Limited diversity of peptides related to an alloreactive T cell epitope in the HLA-B27-bound peptide repertoire results from restrictions at multiple steps along the processing-loading pathway.
The influence of various factors along the processing-loading pathway in limiting the diversity of HLA-B27-bound peptides around a core protein sequence was analyzed. The C5 proteasome subunit-derived RRFFPYYV and RRFFPYYVY peptides are natural B*2705 ligands. The octamer is an allospecific CTL epitope. Digestion of a 27-mer fragment of C5 revealed that both ligands are generated from this precursor substrate with the 20S proteasome in vitro in a ratio comparable to that in the B*2705-bound peptide pool. The C5 sequence allowed to derive a nested set of six additional peptides with 8-11 residues containing the core octamer sequence and the Arg2 motif of HLA-B27, none of which was found in the B27-bound pool. Together, low proteasomal yield, disfavored TAP-binding motifs, and low affinity for B*2705 accounted for the absence of four of the six peptides. The two remaining differed from the natural octamer or nonamer ligands only by an additional N-terminal Ser residue. Their stability in complex with B*2705 was lower than the respective natural ligands, raising the possibility that N-terminal trimming might have favored a shift toward the more stable peptides. The results suggest that the B*2705-bound peptide repertoire has a highly restricted diversity around a core alloantigenic sequence. This is not explained by a single bottleneck feature, but by multiple factors, including proteasomal generation, TAP-binding motifs, MHC-binding efficiency, and perhaps optimized stability through N-terminal trimming. Tapasin-dependent restrictions, although not excluded, were not required to explain the absence in vivo of the particular peptide set in this study.
Paradela
A
A
Centro de Biología Molecular Severo Ochoa, Universidad Autónoma de Madrid, Facultad de Ciencias, Spain.
Alvarez
I
I
García-Peydró
M
M
Sesma
L
L
Ramos
M
M
Vázquez
J
J
López De Castro
J A
JA
D000595:Amino Acid Sequence; D017951:Antigen Presentation / Q000276:immunology*; D002460:Cell Line; D002999:Clone Cells; D003546:Cysteine Endopeptidases / Q000378:metabolism / Q000502:physiology; D018984:Epitopes, T-Lymphocyte / Q000276:immunology / Q000378:metabolism*; D015796:HLA-B27 Antigen / Q000235:genetics / Q000276:immunology / Q000378:metabolism*; D006801:Humans; D007519:Isoantigens / Q000276:immunology*; D008024:Ligands; D046911:Macromolecular Substances; D008969:Molecular Sequence Data; D009097:Multienzyme Complexes / Q000378:metabolism / Q000502:physiology; D009842:Oligopeptides / Q000276:immunology* / Q000378:metabolism*; D046988:Proteasome Endopeptidase Complex; D011485:Protein Binding / Q000276:immunology; D013602:T-Lymphocytes, Cytotoxic / Q000201:enzymology / Q000276:immunology / Q000378:metabolism; D014162:Transfection
D018984:Epitopes, T-Lymphocyte; D015796:HLA-B27 Antigen; D007519:Isoantigens; D008024:Ligands; D046911:Macromolecular Substances; D009097:Multienzyme Complexes; D009842:Oligopeptides; D003546:Cysteine Endopeptidases; D046988:Proteasome Endopeptidase Complex