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Target Discovery and Validation Reviews and Protocols

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Overview of Immune System 295<br />

Fig. 9. Schematic of class II pathway of antigen processing. The invariant chain<br />

transports the major histocompatability complex (MHC) class II molecules from the<br />

endoplasmic reticulum to a specialized endosomal compartment where the invariant<br />

chain is cleaved leaving a small peptide CLIP. Exogenous antigens that are taken up<br />

by endocytosis are cleaved into small peptide fragments within the endosomes.<br />

Within the endosomes, the CLIP fragment is eventually replaced by the cleaved<br />

peptides <strong>and</strong> then complexes are delivered to the cell surface. CPL, compartment for<br />

peptide loading.<br />

peptides are generated by the proteasomes, multicatalytic protease assemblies<br />

that are responsible for the major pathway of protein turnover in the cytoplasm<br />

(38). The peptides generated by the proteasome are subsequently translocated<br />

into the lumen of the endoplasmic reticulum (ER) by the transporter associated<br />

with antigen presentation (TAP) heterodimer, which may display a certain<br />

degree of specificity by selecting peptides with length <strong>and</strong> termini optimal for<br />

the class I binding (39). The binding between the peptide <strong>and</strong> class I molecule<br />

takes place inside the ER. Tumor cells therefore present peptides from tumor<br />

antigens in the context of class I MHC to CD8+ T-cells.<br />

6.2. Class II Pathway of Antigen Processing<br />

Class II molecules are present only on the surface of specialized APCs,<br />

including DCs, B-cells, <strong>and</strong> macrophages. These are predominantly loaded with<br />

peptides derived from exogenous proteins that enter the endocytic pathway of

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