Susceptibility to HLA-DM protein is determined by a dynamic conformation of major histocompatibility complex class II...

Area of Science:

• Immunology
• Molecular Biology
• Structural Biology

Background:

• Human Leukocyte Antigen - Antigen Presentation System (HLA-DM) plays a critical role in adaptive immunity by mediating peptide exchange on Major Histocompatibility Complex class II (MHCII) molecules.
• The precise mechanism by which HLA-DM identifies and facilitates the exchange of peptides loaded onto MHCII remains incompletely understood and debated.
• Non-optimal peptide binding to MHCII can lead to unstable complexes, impacting antigen presentation efficiency.

Purpose of the Study:

• To elucidate the sequence and structural determinants governing the interaction between HLA-DM and MHCII-peptide complexes.
• To investigate the role of peptide binding pockets, particularly P1 and P9, in MHCII susceptibility to HLA-DM-mediated peptide exchange.
• To understand the conformational dynamics of MHCII molecules that influence their interaction with HLA-DM.

Main Methods:

• Investigated the impact of non-optimal peptide binding on MHCII affinity and stability.
• Utilized a suite of biophysical techniques including surface plasmon resonance (SPR), SDS resistance assays, antibody binding assays, gel filtration, dynamic light scattering (DLS), small-angle X-ray scattering (SAXS), and Nuclear Magnetic Resonance (NMR) spectroscopy.
• Examined the effects of specific MHCII mutations, including those in the P1 and P9 pockets and outside the HLA-DM interaction site, on HLA-DM susceptibility.

Main Results:

• Non-optimally bound peptides in the P1 pocket resulted in low MHCII binding affinity, kinetic instability, and increased susceptibility to HLA-DM-mediated exchange.
• These changes were associated with detectable conformational alterations in MHCII molecules.
• Crucially, these conformational changes and susceptibility to exchange were reversible by altering the P9 pocket anchor residue, and mutations outside the P1 pocket also increased susceptibility, highlighting a dynamic conformational determinant.

Conclusions:

• Susceptibility to HLA-DM-mediated peptide exchange is primarily determined by dynamic MHCII conformational states rather than solely by P1 pocket occupancy.
• HLA-DM efficiently targets and edits unstable MHCII-peptide complexes, replacing them with more stable peptide ligands.
• This study provides a molecular mechanism for HLA-DM's function in optimizing antigen presentation through conformational surveillance of MHCII molecules.