Immunopathology: Processing and Presentation of Protein Antigens

Vocabulary terms and definitions covering the mechanisms of antigen processing and presentation through MHC class I and II pathways, including key molecules and specialized presentation methods.

Proteasomes

Proteolytic complexes composed of stacked rings of enzymes that degrade unfolded cytosolic and nuclear proteins into peptides.

Ubiquitin

A small polypeptide covalently linked to proteins to target them for proteasomal degradation; chains of $4$ or more are required for recognition.

TAP (Transporter associated with Antigen Processing)

A specialized transporter that actively pumps proteasome-generated peptides from the cytosolic side of the ER membrane into the interior of the ER.

Tapasin

A bridging protein that links newly synthesized class I MHC molecules to TAP molecules in the ER membrane.

MHC Class I Pathway

The pathway responsible for processing cytosolic proteins and displaying them to $CD8+$ T cells.

MHC Class II Pathway

The pathway responsible for processing extracellular proteins internalized by APCs and displaying them to $CD4+$ T cells.

Autophagy

A mechanism for degrading cellular proteins and intracellular microbes by enclosing them in vesicles and delivering them to lysosomes.

Cathepsins

Endosomal and lysosomal proteases responsible for breaking down proteins into peptides within the phagolysosomes.

Invariant chain ($Ii$)

A protein that associates with class II MHC dimers in the ER to prevent premature peptide binding and direct the molecules to late endosomes and lysosomes.

CLIP (Class II–associated $Ii$ peptide)

A $24$-amino acid remnant of the invariant chain that sits in the peptide-binding cleft of class II MHC molecules until it is displaced.

HLA-DM

A molecule that acts as a peptide exchanger, facilitating the removal of CLIP and the loading of higher affinity peptides onto class II MHC molecules.

Cross-presentation

The process where certain dendritic cells present ingested antigens on class I MHC molecules to $CD8+$ T lymphocytes.

Immunodominant epitopes

The specific peptides of a complex protein that bind most avidly to MHC molecules and elicit the strongest T cell responses.

NKT cells (Natural killer-T cells)

Cells that express markers of both NK cells and T lymphocytes and recognize lipids displayed by the nonclassical MHC molecule CD1.

\gamma\foldas T cells

A small population of T cells that recognize proteins, lipids, and small phosphorylated molecules without being restricted by MHC molecules.