The CLIP region

Evidence for class II association with the so-called CLIP region originally came from peptide elution studies. Transfection assays similarly demonstrate this portion of Ii chain is essential for class II chap-erone activity. Recent X-ray crystal studies have given us a clear picture of CLIP peptide bound to the human DR3 class II molecule. Interestingly, the overall structure of this ternary complex shows a striking similarity to the class II groove occupied by a conventional antigenic peptide. Thus, CLIP peptide is bound in an extended conformation and has extensive atomic contacts with class II residues contributed by both a and P chains. The CLIP region present on intact Ii chain is part of an unfolded flexible domain and is thus accessible for binding to the MHC class II peptide groove.

CLIP peptide release is mediated by DM, the non-conventional class II product originally described as a facilitator of antigen presentation in mutant human cell lines. CLIP/class II complexes have also been found to accumulate on the surface of mutant mouse spleen cells lacking DM expression. Interestingly, CLIP/class II complexes in these DM mutant mice are not immunogenic and fail to stimulate CD4+ T cells. This demonstration of immunological tolerance provides strong evidence that under normal physiological conditions, class II maturation including CLIP removal, is an extremely efficient process.

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