A Peptide/MHCII conformer generated in the presence of exchange peptide is substrate for HLA-DM editing

Area of Science:

• Immunology
• Molecular Biology
• Protein Chemistry

Background:

• The precise mechanism of human leukocyte antigen (HLA)-DM activity remains incompletely understood.
• Previous studies indicated HLA-DM-mediated peptide release from HLA-DR requires exchange peptides, though some release occurs without them.

Purpose of the Study:

• To elucidate the role of exchange peptides in HLA-DM activity and HLA-DR peptide release.
• To identify the specific conformer of peptide/HLA-DR complexes that serves as a substrate for HLA-DM.

Main Methods:

• Sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) was used to separate purified peptide/DR1 complexes (pDR1) into distinct conformers.
• The K(d) (dissociation constant) dependence of the conformer ratio was analyzed.
• The effect of HLA-DM and exchange peptides on peptide release from different conformers was investigated.

Main Results:

• Purified pDR1 complexes separated into two conformers via SDS-PAGE, with their ratio dependent on peptide binding affinity (K(d)).
• In the absence of exchange peptides, HLA-DM selectively released peptides from the slower migrating conformer.
• Addition of exchange peptides converted a DM-resistant conformer into the DM-labile, slower migrating conformer, which is the substrate for DM activity.

Conclusions:

• Exchange peptides are critical for generating the specific pDR1 conformer that acts as the intermediate for peptide exchange and the substrate for HLA-DM.
• This mechanism allows for the presentation of even low-affinity peptides once folded into the DM-resistant conformer, thereby broadening the repertoire of peptides presented by HLA-DR.