Through assembly with the invariant chain (Ii), MHC class II molecules bind peptides derived from proteins degraded in endocytic vesicles, but they can also acquire self antigens through autophagy. Stable peptides are bound after a process of peptide editing in the endocytic compartment involving HLA-DM and HLA-DO. CD8 T cells recognize peptide:MHC class I complexes and are activated to kill cells displaying foreign peptides derived from cytosolic pathogens, such as viruses. CD4 T cells recognize peptide:MHC class II complexes and are specialized to activate other immune effector cells, for example, B cells or macrophages, to act against the foreign antigens or pathogens that they have taken up.
For each class of MHC molecule, there are several genes arranged in clusters within a larger region known as the major histocompatibility complex (MHC). Within the MHC, the genes for the MHC molecules are closely linked to genes involved in the degradation of proteins into peptides, the formation of the complex of peptide and MHC molecule, and the transport of these complexes to the cell surface. Because the several different genes for the MHC class I and class II molecules are highly polymorphic and are expressed in a codominant fashion, each individual expresses a number of different MHC class I and class II molecules. Each different MHC molecule can bind stably to a range of different peptides, and thus the MHC repertoire of each individual can recognize and bind many different peptide antigens. Because the T-cell receptor binds a combined peptide:MHC ligand, T cells show MHC-restricted antigen recognition, such that a given T cell is specific for a particular peptide bound to a particular MHC molecule.
Unconventional T-cell subsets include iNKT cells, MAIT cells, and γ:δ T cells, which recognize nonpeptide ligands of various types. Some CD1 molecules bind self lipids and pathogen-derived lipid molecules and present them to iNKT cells. MAIT cells recognize vitamin metabolites that are specific to bacteria and yeast and that are presented by MR1. γ:δ T cells are activated by a diverse array of ligands, including MHC class Ib molecules and EPCR, that are induced by infection or cellular stress. These T-cell subsets function in the transitional area between innate and adaptive immunity, relying on a repertoire of receptors produced by somatic gene rearrangement but recognizing ligands in a manner somewhat similar to the way PAMPs are recognized by TLRs and other fully innate receptors.
Questions.
6.1 Short Answer: Dendritic cells are capable of efficiently acquiring antigens from exogenous sources and presenting these them to T cells on MHC class I molecules. How is this different from every other cell in the body and why is it important?
6.2 Matching: Match the following terms with the appropriate description:
A. Proteasome i. Displace the constitutive β subunits of the catalytic chamber as a response to interferons
B. 20S core ii. Composed of one catalytic  core and two 19S regulatory  caps
C. LMP2, LMP7, iii. Large cylindrical complex of MECL-1 28 subunits arranged in four stacked rings D. PA28 iv. Targets protein for degradation
E. Lysine 48 v.  Binds the proteasome and ubiquitin increases the rate of protein  release from the proteasome 6.3 True or False: MHC class I surface expression is not affected by the cell’s capacity to transport peptides into the endoplasmic reticulum.