Fig. 6.12 MHC class II molecules are loaded with peptide in a late endosomal compartment called the MIIC. MHC class II molecules are transported from the Golgi apparatus (labeled G in this electron micrograph of an ultrathin section of a B cell) to the cell surface via intracellular vesicles called the MHC class II compartment (MIIC). These have a complex morphology, showing internal vesicles and sheets of membrane. Antibodies labeled with different-sized gold particles identify the presence of both MHC class II molecules (visible as small dark spots) and the invariant chain (large dark spots) in the Golgi, whereas only MHC class II molecules are detectable in the MIIC. This compartment is thought to be a late endosome, an acidified compartment of the endocytic system (pH 4.5–5) in which the invariant chain is cleaved and peptide loading occurs. Photograph (×135,000) courtesy of H.J. Geuze.
This allows MHC class II molecules to more easily allow the CLIP region of Ii to pass through their binding sites. While this complex is being assembled in the endoplasmic reticulum, its component parts are associated with calnexin. Only when a nine-chain complex—three Ii chains, three α chains, and three β chains—has been assembled is the complex released from calnexin for transport out of the endoplasmic reticulum. As part of the nine-chain complex, the MHC class II molecules cannot bind peptides or unfolded proteins, so that peptides present in the endoplasmic reticulum are not usually presented by MHC class II molecules. There is evidence that in the absence of Ii many MHC class II molecules are retained in the endoplasmic reticulum as complexes with misfolded proteins.
Trafficking of membrane proteins is controlled by cytosolic sorting tags. In this regard, Ii has a second function, which is to target delivery of the MHC class II molecules to a low-pH endosomal compartment where peptide loading can occur. The complex of MHC class II α:β heterodimers with Ii trimers is retained for 2–4 hours in this compartment (see Fig. 6.11). During this time, the Ii molecule undergoes an initial cleavage by acid proteases to remove the trimerization domain, generating a truncated 22-kDa fragment of Ii called LIP22. This is further cleaved by cysteine proteases into a 10-kDa fragment called LIP10, which remains bound to the MHC class II molecule and retains it within the proteolytic compartment. A subsequent cleavage of LIP10 releases the MHC class II molecule from the membrane-associated Ii, leaving the CLIP fragment bound to the MHC molecule. This cleavage is carried out by cathepsin S in most MHC class II-positive cells but by cathepsin L in thymic epithelial cells. Being associated with CLIP, the MHC class II molecules cannot yet bind other peptides. However, since CLIP does not carry the Ii-encoded signals that retain the complex in the endocytic compartment, the MHC–CLIP complex is now free to escape to the cell surface.