A peptide binding motif for I-E-g7, the MHC class II molecule that protects E alpha-transgenic nonobese diabetic mice from autoimmune diabetes

The nonobese diabetic (NOD) mouse, a model of spontaneous insulin-dependent diabetes mellitus;(IDDM), fails to express surface MHC class II I-E-g7 mol ecules due a deletion in the E alpha gene promoter, E alpha-transgenic NOD mice express the E alpha E beta(g7) dimer and fail to develop either insuli tis or IDDM, A number of hypotheses have been proposed to explain the mecha nisms of protection, most of which require peptide binding to I-E-g7. TO de fine the requirements for peptide binding to I-E-g7, we first identified an I-E-g7-restricted T cell epitope corresponding to the sequence 4-13 of Myc obacterium tuberculosis 65-kDa heat shock protein (hsp). Single amino acid substitutions at individual positions revealed a moth for peptide binding t o I-E-g7 characterized by two primary anchors at relative position (p) 1 an d 4, and two secondary anchors at p6 and p9, This motif is present in eight of nine hsp peptides that bind to I-E-g7 with high affinity. The I-E-g7 bi nding motif displays a unique p4 anchor compared with the other known I-E m otifs, and major,differences are found between I-E-g7 and I-A(g7) binding m otifs, Analysis of peptide binding to I-E-g7 and I-A(g7) molecules as well as proliferative responses of draining lymph node cells from hsp-primed NOD and E alpha-transgenic NOD mice to overlapping hsp peptides revealed that the two MHC molecules bind different peptides, Of 80 hsp peptides tested, n one bind with high affinity to both MHC molecules, arguing against some of the mechanisms hypothesized to explain protection from IDDM in E alpha-tran sgenic NOD mice.