Processing af Extracellular Antigen For Class II MHC Presentation Follows a Different Pathway
Class II MHC complexes with antigenic peptide are generated by a fundamentally different intracellular mechanism, as the APCs that interact with T‐helper cells need to sample the antigen from the extracellular compartment. In essence, a trans‐Golgi vesicle containing MHC class II has to meet up with a late endosome containing exogenous protein antigen taken into the cell by phagocytosis, macropinocytosis, or endocytosis.
Figure 5.16 Processing of exogenous antigen and presentation by class II MHC. Class II molecules with Ii are assembled (actually as a nonamer consisting of three invariant, three class II α, and three class II β chains – not shown) in the endoplasmic reticulum (ER). They are then transported through the Golgi to the trans‐Golgi reticulum The class II‐containing vacuole now fuses with a late endosome which has lysosomal characteristics and contains peptides generated by partial degradation of proteins derived from the endocytic uptake of exogenous antigen. This fusion generates a so‐called MHC class II‐enriched compartment, MIIC. Particularly implicated in the processing of exogenous antigen in the endosomes are the cysteine proteases cathepsin S and L, both of which have endopeptidase activity, as do both cathepsin D and asparagine endopeptidase (AEP) which might also partake in this process. Subsequently, the exopeptidases cathepsin B and X are thought to trim the C‐terminus, and cathepsins C and H to trim the N‐terminus of the peptides either prior to or after their binding into the MHC class II groove. Degradation of the invariant chain leaves the CLIP (class II‐associated invariant chain peptide) lying in the groove but, under the influence of the DM molecule, this is replaced by the peptides derived from exogenous antigen, and the complexes are transported to the cell surface for presentation to T‐helper cells.
Regarding the class II molecules themselves, these are assembled from α and β chains in the ER in association with the transmembrane invariant chain (Ii) (Figure 5.16), which trimerizes to recruit three MHC class II molecules into a nonameric complex. Ii has several functions. First, it acts as a dedicated chaperone to ensure correct folding of the nascent class II molecule. Second, an internal sequence of the luminal portion of Ii sits in the MHC groove to inhibit the precocious binding of peptides in the ER before the class II molecule reaches the endocytic compartment containing antigen. Additionally, combination of Ii with the αβ class II heterodimer inactivates a retention signal and allows transport to the Golgi. Finally, targeting motifs in the N-terminal cytoplasmic region of Ii ensure delivery of the class II‐containing vesicle to the endocytic pathway.
Meanwhile, exogenous protein is taken up by one of the endocytic processes referred to above. The enzyme GILT (interferon‐γ‐induced lysosomal thiol reductase) is present in the endosomes and will break any disulfide bonds that are present in the engulfed proteins. As the early endosome undergoes progressive acidification, the proteins are processed into peptides by a range of proteolytic enzymes (see Figure 5.16 legend). The early endosomes mature into late endosomes and lysosomes, both of which characteristically acquire lysosomal‐associated membrane proteins (LAMPs), including the LAMP‐2a receptor for chaperone‐mediated autophagy. These late endosomes fuse with the vacuole containing the class II–Ii complex. Under the acidic conditions within these MHC class II‐enriched compartments (MIICs), asparagine endopeptidase (AEP) and cathepsins S, L, and F degrade Ii except for the part sitting in the MHC groove that, for the time being, remains there as a peptide referred to as CLIP (class II‐associated invariant chain peptide). An MHC‐related heterodimeric molecule, DM, then catalyzes the removal of CLIP and keeps the groove open so that pep- tides generated in the endosome can be inserted (Figure 5.17). Initial peptide binding is determined by the concentration of the peptide and its on‐rate, but DM subsequently assists in the removal of lower affinity peptides to allow their replacement by high‐affinity peptides (i.e., acts as a peptide editor permitting the incorporation of peptides with the most stable binding characteristics, namely those with a slow off rate). Particularly in B‐cells and thymic epithelium, an additional MHC‐related heterodimeric molecule, DO, associates with DM bound to class II and inhibits its function. The precise role of DO remains elusive. However, in B‐cells it may transiently restrain DM in order to favor the presentation of antigens internalized via the BCR over those taken up by fluid phase endocytosis. The class II–peptide complexes are then transported to the membrane for presentation to T‐helper cells.
Figure 5.17 MHC class II transport and peptide loading illustrated by Tulp’s gently vulgar cartoon.