Distinct peptide loading pathways for MHC class II molecules associated with alternative Ii chain isoforms

Area of Science:

• Immunology
• Molecular Biology
• MHC Class II Pathway

Background:

• The invariant chain (Ii) is crucial for MHC class II molecule assembly, trafficking, and antigen presentation.
• Alternative splicing of the Ii chain gene generates isoforms (p31 and p41) with potentially distinct functions.
• H2-DM is a critical regulator of peptide loading onto MHC class II molecules.

Purpose of the Study:

• To investigate the divergent roles of p31 and p41 Ii chain isoforms in MHC class II function.
• To compare class II structure and function in mice deficient for H2-DM and expressing specific Ii chain isoforms.

Main Methods:

• Generation of double mutant mouse strains lacking H2-DM and expressing either p31 or p41 Ii chain.
• Analysis of MHC class II (Aalpha(b)Abeta(b)) dimer occupancy with CLIP (class II-associated Ii chain-derived peptides).
• Functional assessment of antigen presentation and peptide-loading capabilities in the generated mouse models.

Main Results:

• DM-deficient mice expressing p31 or wild-type Ii chain showed severely impaired peptide presentation.
• Conversely, DM-deficient mice expressing the p41 Ii chain isoform exhibited significantly enhanced peptide-loading capacity.
• Class II dimer occupancy by CLIP was similar across DM-deficient Ii chain mutant strains.

Conclusions:

• Alternative splicing of the invariant chain generates functionally distinct isoforms.
• The p41 Ii chain isoform plays a critical role in enhancing peptide loading onto MHC class II molecules.
• These findings reveal distinct MHC class II presentation pathways mediated by alternatively spliced Ii chain forms.