Molecular profile of the T cell receptors of regulatory and effector CD4+ T cells recognizing overlapping determinants on glutamic acid decarboxylase (524-543)

Glutamic acid decarboxylase (GAD65) is one of the autoantigens that initiates pathogenic T cell responses against insulin-secreting pancreatic beta cells in Type 1 diabetes (T1D). Previously it was shown that spontaneously arising pathogenic T cell responses in the NOD mouse model are confined to GAD530-543 (p530). However, regulatory T cell subpopulations, which can prevent diabetes, can also be generated, for example, by immunization with GAD524-538 (p524) or GAD524-543. Interestingly, two functionally distinct subpopulations of T cells which recognize overlapping determinants of GAD524-543, p524 and p530, utilize distinct TCR Vbeta families, Vbeta4 for pathogenic, and Vbeta12 for regulatory T cells. We characterized T cell receptors (TCRs) from each subpopulation of T cells and visualized p524-specific TCR/p524/I-A(g7) and p530-specific TCR/p530/I-A(g7) complexes via molecular modeling to help us understand, at a molecular level, the in vivo expansion of p524- or p530-specific T cells in the NOD model of T1D. The absolute restriction in Vbeta usage but not Valpha usage and conserved CDR3beta lengths for both T cell subpopulations demonstrates that the beta chains are main contributors in shaping both p524/I-A(g7) and p530/I-A(g7) restricted TCRs. However, only Vbeta4+ T cells but not Vbeta12+ T cells contain a common motif (DWG) in CDR3beta and may involve all of CDR1beta, CDR2beta, and CDR3beta in the recognition of the C-terminus of p530. These observations imply that the spontaneously arising p530-restricted TCRs may be selected under stringent structural frameworks to bind p530/I-A(g7) with high affinity. Thus, the pathogenic p530-specific T cells may arise from a small pool of autoreactive T cells upon breaking tolerance.