Figure 1. Schematic representation of the major histocompatibility complex (MHC) class I and class II pathways for antigen processing and presentation. (a) Peptides generated in the cytosol by the proteasome (1) are translocated into the ER by the peptide transporter associated with antigen processing (TAP) (2) and loaded on to MHC class I molecules. Subsequently, the class I complexes are transported through the Golgi apparatus (3) to the cell surface. (b) MHC class II molecules are associated with the invariant chain (Ii) in the ER (1). The Ii-associated complex is targeted to the endosomal compartment (2), where peptides generated from endocytosed proteins are loaded into MHC class II molecules (3) and transferred to the cell surface (4).

Figure 2. Endosomal processing of antigens and MHC class II molecules for presentation of endocytosed antigens. (a) The invariant chain (Ii) targets a MHC class II dimer to the MHC class II endocytic compartment. (b) Proteolytic processing of antigens and of Ii to generate MHC class II with bound CLIP. (c) Human leucocyte antigen (HLA)-DM catalysed exchange of CLIP for antigenic peptide. (d) Proteolytic trimming of the peptide termini and transport of the complex to the cell surface.

Figure 3. The proteasome, a cytosolic protease involved in production of peptide ligands for MHC class I molecules. (a) The eukaryotic 20S proteasome consists of four stacked rings of and subunits (7, 7, 7, 7). (b) Schematic representation of the 20S and 26S proteasomes. The 26S proteasome is formed by the addition of two 19S complexes to the 20S proteasome.