Invariant-cognate peptide exchange restores class II dimer stability in HLA-DM mutants

Area of Science:

• Immunology
• Molecular Biology
• Cell Biology

Background:

• Class II presentation mutants, with defects in HLA-DMA or HLA-B genes, exhibit impaired antigen presentation restricted by HLA-DR, -DQ, and -DP.
• These functional defects correlate with altered cell surface class II molecule conformation and unstable extracted class II dimers.

Purpose of the Study:

• To investigate the mechanism by which acidic pH and cognate peptide treatment correct class II dimer instability in DMB mutants.
• To assess the extent to which this treatment rectifies the class II conformational defect in DMB mutants.

Main Methods:

• Incubation of mutant cells in acidic pH with cognate peptide.
• Analysis of class II molecule conformation and dimer stability using SDS-PAGE.
• Epitope mapping using monoclonal antibodies (mAbs) 7.3.19.1 and 16.23.

Main Results:

• Acidic pH elutes invariant chain (li)-derived peptides, creating binding sites for cognate peptides in DMB mutants.
• Cognate peptide binding stabilizes class II dimers in a pH-independent manner.
• Acid/peptide treatment restored the 7.3.19.1 epitope but not the 16.23 epitope; low pH degraded the 16.23 epitope in nonmutant cells.
• A mutant (10.24.6) with a DRA mutation also showed unstable DR dimers corrected by acid/peptide treatment.

Conclusions:

• The primary defect in DMB and 10.24.6 mutants is an excess of li peptides and a deficiency of bound cognate peptides on class II molecules.
• Acidic pH and cognate peptide treatment represent a potential strategy to correct specific defects in antigen presentation pathways.