MUSICA CLASICO-ROMANTICA:

MHC class II molecule α and β chains (see Chapter 5) are found in the ER complexed to a polypeptide called the invariant chain (Ii). This protein is encoded outside the MHC. The αβ–Ii complex is transported through the Golgi complex to an acidic endosomal or lysosomal compartment called MIIC. These MIIC vesicles appear to be specialized for the transport and loading of MHC class II molecules. They have characteristics of both endosomes and lysosomes and have an onion-skin appearance under the electron microscope, comprising multiple membrane structures. The αβ complex spends 1–3 hours in this compartment before reaching the cell surface. The Ii chain is cleaved to small fragments, one of which, termed CLIP (class II-associated invariant peptide), is located in the groove of the class II molecule until replaced by peptides destined for presentation (Figs 7.11 and 7.12).

How do antigenic peptides derived from exogenous proteins meet MHC molecules in the appropriate compartment?

The answer to this question lies in the intracellular traffic routes of MHC molecules. After synthesis in the ER both types of MHC molecule are transported through the Golgi compartment, class I in association with antigenic peptide and class II bound to invariant chain Ii. Class II molecules segregate from class I molecules in the trans- Golgi network. They then join the endosomal/lysosomal MIIC compartment en route to the plasma membrane.

Exogenous antigen can also enter APCs via an endocytic route (either receptor mediated or fluid phase, see Fig. 7.11) where in some cells, such as DCs, it can load onto:

• MHC class II molecules in MIIC vesicles; and

In rats, TAP genes are polymorphic and different alleles are linked in cis to the appropriate class I allele

Fig. 7.10 Different MHC class I molecules in rats can accommodate peptides (blue bars) with either a positive charge at the C terminus (+) or a neutral amino acid (o). Similarly, TAP molecules (orange) come in two forms, which differ in the types of peptide they preferentially transport into the ER. Most rat strains have the appropriate TAP allele on the same haplotype as the class I gene that it serves best.

peptide charged at C terminus Ia Tapa peptide neutral at C terminus Ib Tapb

• MHC class I molecules in the ER, in a TAP-dependent manner.

The exchange of CLIP for other peptides is orchestrated by a class II-related molecule called HLA-DM (Fig. 7.13). This glycoprotein consists of an α chain and a β chain, both of which are encoded in the class II region of the MHC (see Fig. 7.8).

HLA-DM acts by stabilizing empty MHC class II molecules so that when CLIP is released other peptides get a chance to associate. The DM molecule itself has a closed groove and it is not capable of binding peptide.

In cell lines lacking DMA and DMB genes, class II molecules are unstable and the cells no longer process and present proteins. Their class II molecules end up at the cell surface occupied by CLIP fragments of the invariant chain (Fig. 7.14).

MHC CLASS II MOLECULES ARE LOADED WITH EXOGENOUS PEPTIDES

Proposed routes of intracellular trafficking of MHC molecules involved in antigen presentation

Fig. 7.11 Newly synthesized MHC class I molecules are loaded with peptide (1). MHC class II molecules associate with Ii in the ER (2). Ii prevents loading with peptide and contains sequences that enable the MHC class II molecule to exit from the rough endoplasmic reticulum (RER). MHC class I and class II molecules segregate after transit through the Golgi (3). Class I molecules go directly to the cell surface (4). Class II

molecules enter an acidic compartment called MIIC, where they are loaded with peptide derived from exogenous antigen, and the CLIP peptide that occupies the binding groove dissociates (5). cell surface surface expression transport vesicle antigen processing antigen displaces Ii from MHC molecule class II storage vesicle trans-Golgi network trans medial cis Golgi apparatus cytoplasmic antigen ribosomes ER antigen 3 2 1 4 5 nucleus class I class II DM late endosome/ liposome MIIC vesicle early endosome endocytosis

MHC class II molecule processing compartment

Fig. 7.12 Electron micrograph of ultrathin cryosections from B cells showing a multilaminar MIIC vesicle. The bar represents 100 nm. MHC class II molecules are revealed by antibodies coupled to 10 nm gold particles and HLA-DM by large gold particles (15 nm). (Courtesy of Dr Monique Kleijmeer)

N

HLA-DM acts like a catalyst to influence binding of peptides in exchange for CLIP

Fig. 7.13 An MHC class II molecule is shown loaded with the Ii chain. The Ii chain is cleaved to the CLIP fragment. Association of the complex with HLA-DM then allows CLIP to be exchanged for other peptides derived from endocytosed proteins present in MIIC vesicles. class II Ii class II DM DM CLIP class II class II recycled peptide peptide CLIP

A further MHC-encoded molecule, HLA-DO (see Fig. 7.8), which associates with DM, is also involved in peptide loading. Like conventional MHC class II molecules, HLA-DO is a heterodimer (see Chapter 5), consisting of the DOA (formerly DNA) and DOB chains. Similar genes and molecules to DM and DO are found in other species, for example in mouse they are encoded by the Ma, Mb, and Oa, Ob genes, respectively.

CROSS-PRESENTATION ALLOWS APCs TO ACQUIRE ANTIGENS FROM INFECTED CELLS

Cross-presentation occurs when exogenous peptides are presented by MHC class I molecules. The principle that endogenously synthesized proteins are the source of peptides for MHC class I molecules appears to be violated by the phenomenon of cross-presentation. There is a specialized pathway in which T cells recognize MHC class I molecule–peptide complexes formed from exogenous protein antigens endocytically acquired by DCs.

Q. What function could cross-presentation fulfil in defense against viral infections?

A. Presumably such a mechanism is required so that, even if a virus fails to infect an APC directly, a successful cytotoxic CD8 T cell-mediated immune response can be generated.

In this process of cross-presentation, APCs acquire remnants of virus-infected cells by phagocytosis and present them on MHC class I molecules. Internalized proteins appear to be transferred from the endocytic

pathway into the cytosol where they are then subject to processing and transport by proteasomes and TAP, respectively, as for other peptides (Fig. 7.15).

CO-STIMULATORY MOLECULES ARE