Figure 1.

HLA class II antigen-processing and presentation pathway. In APCs, newly synthesized HLA class II molecules are assembled in the ER and bind the Ii. The Ii direct the transport of HLA class II molecules directly or indirectly into the MIIC, where the li is degraded by different proteases leaving the peptide fragment CLIP still embedded in the HLA class II binding groove. An HLA class II-like molecule, HLA-DM, facilitates the release of CLIP and assists in the exchange of CLIP with relevant exogeneous antigenic fragments. Then the HLA class II peptide complex is transported to cell surface for presentation to CD4⁺ T cells.