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To gain the capacity to activate antigen-specific T cells, MHC class II-associated CLIP must be exchanged for an antigenic peptide (Kropshofer et al. 1999). There are two CLIP variants in humans: CLIP(long) with 21-26 residues, and CLIP(short) with 14-19 residues. CLIP(long) disassociates rapidly from HLA-DR molecules at endosomal/lysosomal pH, whereas CLIP(short) displays a lower off-rate. The N-terminal 9 residues of CLIP (81-89) facilitate its rapid release (Urban et al. 1994, Kropshofer et al. 1995a, Kropshofer et al. 1995b). The non-classical MHC class II molecule HLA-DM (DM) functions as a mediator of peptide exchange by accelerating the removal of CLIP. DM mediated peptide release involves a direct interaction between DM and the class II molecule. In addition to peptide release, DM also acts as a chaperone for MHC class II molecules in endosomal/lysosomal compartments. It stabilizes the peptide-receptive empty MHC II molecules and prevents them from unfolding and also favors the generation of high-stability peptide-MHC class II complexes by promoting release of low-stability peptide ligands (Kropshofer et al. 1999, Kropshofer et al. 1997). Another non-classical MHC II molecule HLA-DO (DO), only expressed in B-cells and thymic epithelial cells, binds tightly to DM modulating DM activity both negatively and positively, depending on the amount of DO present in an APC. Heterotypic DR-DM-DO complexes are receptive for peptide loading, in these complexes DO does not appear to be inhibitory (Denzin et al. 1997, Kropshofer et al. 1998, Kropshofer et al. 1999).
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