Thursday, 13 December 2012

Antigen processing

Antigen processing

Antigen processing is a biological process that prepares antigens for presentation to special cells of the immune system called T lymphocytes. There are two pathways for antigen processing:
1. Endogenous pathway and
2.  Exogenous pathway.    

Endogenous pathway

Class I MHC molecules bind peptides derived from endogenous antigens that have been processed within the cytoplasm of the cell (e.g., normal cellular proteins, tumor proteins, or viral and bacterial proteins produced within infected cells).  These antigens have ubiquitin attached to them. Ubiquitin-protein conjugates can be degraded by a multifunctional protease complex called a proteasome. The proteasomes involved in antigen processing include two subunits encoded within the MHC gene cluster, LMP2 and LMP7, and a third non-MHC protein, LMP10 (also called MECL-1). All three are induced by increased levels of the T-cell cytokine IFN-γ. The peptidase activities of proteasomes containing LMP2, LMP7, and LMP10 preferentially generate peptides that bind to MHC class I molecules for expression. After the action of proteasome, the peptide is transported to the RER by TAP (transporter associated with antigen processing) as shown in Fig:1. Within the RER membrane, a newly synthesized class I α chain associates with calnexin (Fig:2a) until β2-microglobulin binds to the α chain (Fig:2b). The class I α chain/β2-microglobulin heterodimer then binds to calreticulin and the TAP-associated protein tapasin (Fig:2c-f). When a peptide delivered by TAP is bound to the class I molecule, folding of MHC class I is complete and it is released from the RER and transported through the Golgi to the surface of the cell (Fig:2i).

 Fig1: Transportation of peptide to RER.

Fig:2- Modification in MHCI during antigen processing and its     presentation to Tc cell.

 Exogenous pathway 

Class II MHC molecules bind peptides derived from exogenous antigens that are internalized by phagocytosis or endocytosis and processed within the endocytic pathway. Lysosomes are the digestive system of cells which degrades antigens into oligopeptides of about 13–18 residues,which bind to class II MHC molecules.
These vesicles are a specialized set of secretory vesicles produced by the Golgi and contain a mixture of some 40 types of digestive enzymes, including those that degrade nucleic acids, proteins and lipids. All of these enzymes have optimum activity at about pH 5. Because the hydrolytic enzymes are optimally active under acidic conditions (low pH), antigen processing can be inhibited by chemical agents that increase the pH of the compartments (e.g., chloroquine) as well as by protease inhibitors (e.g., leupeptin). This acid pH is maintained in lysosomes, as in endosomes, by proton pumps in the membrane. The membrane of the lysosome is resistant to action of its own digestive enzymes due to the extensive glycosylation of the proteins on the lumen side of the membrane (Fig:3). The Invariant Chain Guides Transport of Class II MHC Molecules to Endocytic Vesicles. The generalised mechanism for the expression of class II MHC molecules on cell surface is sown in Fig:4(left).

Fig3: Degradation of antigens by lysosomes.

Mechanism of peptide assembly on MHCII molecule by replacing clip.

Class II MHC molecules are formed in the golgi complex as class II MHC–invariant chain complexes then they are transported to the Golgi complex and trans-golgi network to the endocytic pathway from early endosomes to late endosomes, and finally to lysosomes. As the proteolytic activity increases in each successive compartment, the invariant chain is gradually degraded. However, a short fragment of the invariant chain termed CLIP (for class II–associated invariant chain peptide) remains bound to the class II molecule after the invariant chain has been cleaved within the endosomal compartment. A nonclassical class II MHC molecule called HLA-DM is required to catalyze the exchange of CLIP with antigenic peptides. This HLA-DM has quite conservative sequences. HLA-DM is not polymorphic and is not expressed at the cell membrane but is found predominantly within the endosomal compartment. The DMα and DMβ genes are located near the TAP and LMP genes in the MHC complex of humans and DM is expressed in cells that express classical class II molecules.

Other nonclassical class II MHC molecules for clip exchange:

HLA-DO is almost similar to that of HLA-DM it differs from HLA-DM in that it is expressed only by B cells and the thymus, and unlike other class II molecules, its expression is not induced by IFN-γ. An additional difference is that the genes encoding the α and the β chains of HLA-DO are not adjacent in the MHC as are all other class II α and β pairs.
An HLA-DR3 molecule associated with CLIP was isolated from a cell line that did not express HLA-DM and was therefore defective in antigen processing. Superimposing the structure of HLA-DR3–CLIP on another DR molecule bound to antigenic peptide reveals that CLIP binds to class II in the same stable manner as the antigenic peptid. The discovery of this stable complex in a cell with defective HLA-DM supports the argument that HLA-DM is required for the replacement of CLIP.

 Fig:4- Comparative study of MHCI and II molecules modification and antigen processing by them.




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