Cutting edge: Thymic positive selection and peripheral activation of islet antigen-specific T cells: Separation of two diabetogenic steps by an I- A(g7) class II MHC β-chain mutant

The diabetes-susceptible class II MHC genes (in human and mouse) share unique nonaspartic acid residues at position 57 of the class II β-chain. Transgenic expression of a mutant I-A(g7), substituting histidine and serine at position 56 and 57 of β-chain with proline and aspartic acid (I-A(g7)PD), respectively, inhibits diabetes development in the nonobese diabetic mouse model. Here, we demonstrate that immature thymocytes expressing a diabetogenic islet Ag-specific transgenic TCR are positively selected by I- A(g7)PD class II MHC to give rise to mature CD4⁺ T cells. However, splenic APCs expressing the same I-A(g7)pD fail to present pancreatic islet Ag to mature T cells bearing this diabetogenic TCR. These results indicate that nonaspartic acid residues at position 57 of class II MHC β-chain is important for diabetogenic CD4⁺ T cell activation in the periphery but is not essential for the formation of a diabetogenic T cell repertoire in the thymus.