This molecule is a heterodimer of the HLA-DOα chain and the HLA-DOβ chain. HLA-DO is not present at the cell surface, being found only in intracellular vesicles, and it does not seem to bind peptides. HLA-DO acts as a negative regulator of HLA-DM. HLA-DO binds to HLA-DM in the same manner as MHC class II molecules (see Fig. 6.14), and it must be bound to HLA-DM in order to leave the endoplasmic reticulum. When the DM–DO dimer reaches an acidified endocytic compartment, HLA-DO appears to dissociate slowly from HLA-DM, which is then free to catalyze peptide editing for MHC class II molecules. Moreover, IFN-γ increases the expression of HLA-DM, but not of the HLA-DOβ chain. Thus, during inflammatory responses, IFN-γ produced by T cells and NK cells can increase the expression of HLA-DM, and so overcome the inhibitory effects of HLA-DO. Why HLA-DO is expressed in this way remains obscure. The loss of HLA-DO in mice does not dramatically alter adaptive immunity, but does cause a spontaneous production of autoantibodies with age. As thymic epithelial cells function in the selection of developing CD4 T cells, perhaps HLA-DO influences the repertoire of self peptides that these T cells encounter at different stages, as discussed further in Chapter 8.
The role of HLA-DM in peptide editing for MHC class II molecules parallels the role of tapasin in facilitating peptide binding to MHC class I molecules. HLA-DM carries out this function by mediating peptide exchange and driving the association of high-affinity peptides. Thus, it seems likely that specialized mechanisms of delivering peptides have coevolved with the MHC molecules themselves. It is also likely that pathogens have evolved strategies to inhibit the loading of peptides onto MHC class II molecules, much as viruses have found ways of subverting antigen processing and presentation through the MHC class I molecules. We will return to these topics in Chapter 13 when we discuss pathogen immunoevasion mechanisms.
The peptide editing conferred by DM and removing unstable MHC molecules provide important safeguards. To reveal the presence of an intracellular pathogen, the peptide:MHC complex must be stable at the cell surface. If peptides were to dissociate too readily, an infected cell could escape detection, and if peptides could too easily be acquired from other cells, then healthy cells might be mistakenly targeted for destruction. Tight binding of peptides to MHC molecules reduces the chance of these unwanted outcomes. MHC class I molecules display peptides derived largely from cytosolic proteins, so it is important that dissociation of a peptide from a cell-surface MHC molecule does not allow extracellular peptides to bind in the empty peptide-binding site. Fortunately, when an MHC class I molecule at the surface of a living cell loses its peptide, its conformation changes, the β2-microglobulin dissociates, and the α chain is internalized and rapidly degraded. Thus, most empty MHC class I molecules are quickly lost from the cell surface, largely preventing them from acquiring peptides directly from the surrounding extracellular fluid. This helps ensure that primed T cells target infected cells while sparing surrounding healthy cells.
Empty MHC class II molecules are also removed from the cell surface. Although at neutral pH, empty MHC class II molecules are more stable than empty MHC class I molecules, they aggregate readily, and internalization of such aggregates may account for their removal. Moreover, peptide loss from MHC class II molecules is most likely when the molecules are transiting through acidified endosomes as part of the normal process of cell-membrane recycling. At acidic pH, MHC class II molecules are able to bind peptides that are present in the vesicles, but those that fail to do so are rapidly degraded.