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MHC Class II Antigen Presentation — MFDS Part 1 MCQ

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HardPhysiologyMHC Class II Antigen PresentationMFDS Part 1

A dendritic cell endocytoses an extracellular streptococcal protein. In an acidified late endosome, invariant chain is degraded to CLIP, and HLA-DM facilitates exchange of CLIP for a bacterial peptide. Which dendritic-cell surface structure then provides the antigen-specific ligand recognised by a conventional CD4+ T-helper cell?

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Correct answer: A — A peptide-loaded MHC class II molecule

The correct answer is A. The extracellular protein enters the endosomal–lysosomal pathway, where HLA-DM promotes replacement of CLIP by an antigenic peptide on MHC class II. The resulting peptide–MHC class II complex is transported to the dendritic-cell surface and recognised by the antigen-specific T-cell receptor with the CD4 co-receptor. MHC class I principally presents peptides to CD8+ T cells, although dendritic cells can also cross-present some extracellular antigens through that pathway. HLA-DM acts mainly within peptide-loading compartments rather than serving as the surface antigenic ligand. CLIP is a temporary placeholder that is normally displaced before presentation of the bacterial peptide. The alpha-beta T-cell receptor–CD3 complex is expressed by the responding T cell, not by the dendritic cell.

Reference: Pishesha N, Harmand TJ, Ploegh HL. A guide to antigen processing and presentation. Nature Reviews Immunology. 2022;22:751–764. https://pubmed.ncbi.nlm.nih.gov/35418563/