Nonobese diabetic mice display elevated levels of class II-associated invariant chain peptide associated with I-A(g7) on the cell surface

Peptide presentation by MHC class II molecules plays a pivotal role in dete rmining the peripheral T cell repertoire as a result of both positive and n egative selection in the thymus. Homozygous I-A(g7) expression imparts susc eptibility to autoimmune diabetes in the nonobese diabetic mouse, and recen tly, it has been proposed that this arises from ineffectual peptide binding . Following biosynthesis, class II molecules are complexed with class II-as sociated invariant chain peptides (CLIP), which remain associated until dis placed by Ag-derived peptides. If I-A(a7) is a poor peptide binder, then th is may result in continued occupation by CLIP to the point of translocation to the cell surface. To test this hypothesis we generated affinity-purifie d polyclonal antisera that recognized murine CLIP bound to class II molecul es in an allele-independent fashion. We have found abnormally high natural levels of cell surface class II occupancy by CLIP on nonobese diabetic sple nic B cells. Experiments using I-A-transfected M12.C3 cells showed that I-A (g7) alone was associated with elevated levels of CLIP, suggesting that thi s was determined solely by the amino acid sequence of the class II molecule . These results indicated that an intrinsic property of I-A(g7) would affec t both the quantity and the repertoire of self-peptides presented during th ymic selection.