Biophysical parameters of self-reactive TCR engagement in T1D

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ABSTRACT Major histocompatibility loci (MHC) are the largest genetic contributors to autoimmune susceptibility, including type 1 diabetes. Our novel observations show that biophysical parameters of T cell receptor interactions with peptide-MHC are altered in the context of susceptible MHC alleles. Dissecting the interaction between T cell receptor and self-antigens requires sensitive technologies to measure the affinity and bond lifetimes. T cells apply force to the bond between TCR and pMHC antigenic complex, which is ultimately reflected by changes in how long the proteins interact. We have surveyed affinities, bond lifetimes, and force that form during T cell receptor interaction with model, infectious, and self-antigens presented on non-autoimmune I-Ab and compared these observations with I-Ag7 autoimmune MHC restricted T cells. We consistently observed 2-fold difference in force between effector and Foxp3+ regulatory T cells in non-autoimmune MHC restricted responses, with Tregs pulling higher 20pN force. However, I-Ag7 restricted effector T cells are capable of pulling similar high 20pN force, and Tregs loose the 2-fold force advantage. Moreover, human-derived beta cell antigen specific TCR restricted to autoimmune HLA-DR4 exhibited similar high 20pN force. Our overall hypothesis is that Foxp3+ Treg efficacy is dependent on peak TCR force levels that differ 2-fold from T conventional cells, this difference is absent in the context of autoimmune MHCs. How structural components of autoimmune MHC, peptide, and TCR control the stability of the TCR/pMHC interaction is not fully resolved, especially in the context of force measurements. Our recently published and unpublished observations point to CDR2 loops of the TCR as important in establishing the level of force. Moreover, it is unknown how susceptible MHC effects Treg suppressive function, and why Tregs falter during T1D. Therefore, a thorough understanding of the beta cell- specific reactivity of effector and regulatory T cells is needed to fully understand and potentially exploit their therapeutic potential for treatment of autoimmune diabetes. We have devised two aims to test this hypothesis: Aim 1. Determine TCR biophysical parameters restricted by autoimmune MHC and their impact on autoimmune and regulatory T cell function; and Aim 2. Determine structural components of TCR that specifically regulate the force and bond-lifetime, but do not influence specificity or affinity of the interaction. This project will be the first to investigate various levels of force/bond lifetimes as indicators for T cell function and loss of Treg function in autoimmunity, and connect TCR affinity vs force to the ultimate outcome in disease. Furthermore, it will provide novel insight into the mechanisms governing dysfunction of T cell tolerance during T1D.