Properties of Pathogenic Insulin Reactive T Cells That Escape Selection in Autoimmune Diabetes

How autoreactive T cells escape thymic selection and eventually become pathogenic in peripheral tissues represents a fundamental issue for understanding autoimmune diseases. It is known that APC handle proteins and peptides in different ways and through different cellular pathways. Because of these differences, the peptide epitopes generated by MHC class II formed from exogenous sources of free peptide may be distinct from those formed from the internal processing of the protein. These distinctions in peptide-MHC complexes in turn elicit unique peptide specific CD4 T cells. These peptide-specific T cells have largely been overlooked, yet their potential importance in a number of biological contexts, particularly in autoreactivity, should not be underestimated. In this thesis I examine the role of these paradoxical T cells in autoimmunity, which by their nature break the classical rules of antigen processing and presentation. We found a striking example supporting the importance of these peptide specific T cells in the autoimmune diabetes-prone NOD mouse strain. In NOD mice, we found two sets of autoreactive CD4+ T cells that recognize the dominant antigen, B:9-23 of the insulin B chain. The conventional set, called type A, recognized insulin presented by APC. These T cells were highly deleted in the thymus. The second set, called type B, did not recognize insulin protein but reacted with soluble B-chain peptide. This peptide specific set was not deleted in the thymus and moreover, APC in close contact with beta cells in the islets of Langerhans bore vesicles with the antigenic insulin peptides and robustly activated these type B insulin reactive T cells. We went on to examine the specificity of these two types of T cells. The protein reactive set (type A) recognized the stretch of residues 13-21 of the insulin B chain while the set reactive to peptide only (type B) recognized the stretch from residues 12-20. A single amino acid shift of the B chain peptide bound to I-Ag7 determined whether T cells recognized peptides generated by the processing of insulin, and consequently their escape from thymic purging. Biochemical studies indicated that the 13-21 register interacted more favorably with I-Ag7 than the 12-20 register. Thus, self reactive T cells can become pathogenic in the target organ where high concentrations of antigen and/or differences in intracellular processing present peptides in registers distinct from those found in the thymus