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CLIP or Class II-associated invariant chain peptide is the part of the invariant chain (Ii) that binds MHC class II groove and remains there until the MHC receptor is fully assembled. The purpose of CLIP is to prevent the binding of self-peptide fragments prior to MHC II localization within the endo/lysosome.
During MHC II assembly, invariant chain complexes with the MHC II complex. In a special endosome called the MHC II compartment, cathepsin S cleaves the invariant chain, leaving CLIP bound to the MHC II complex. In presence of antigen peptide fragments, HLA-DM then binds to the MHC II molecule, releasing CLIP and allowing peptides to bind. MHC II with bound antigen is then transported to the cell membrane for presentation. HLA-DO can inhibit the reaction catalysed by HLA-DM.
As a possible reference could be cited: Abbas K, Lichtman S, Pillai: Cellular and Molecular Immunology. Seventh edition, Elsevier/Saunders 2012 page 133.