Ectopic Expression of Self-Antigen Drives Regulatory T Cell Development and Not Deletion of Autoimmune T Cells

Abstract

Type 1 Diabetes (T1D) is a T cell-mediated autoimmune disease that is characterized by antigen specific targeting and destruction of insulin producing β cells. While multiple studies have characterized the pathogenic potential of β-cell specific T cells, we have limited mechanistic insight into self-reactive autoimmune T cell development and their escape from negative selection in the thymus. Here we demonstrate that ectopic expression of insulin B:9-23 (InsB9-23) by thymic antigen presenting cells is insufficient to induce deletion of high or low affinity InsB9-23 reactive CD4+ T cells; however, we observe an increase in the proportion and number of thymic and peripheral Foxp3+ regulatory cells T cells. In contrast, the MHC stable insulin mimetope (InsB9-23 R22E) efficiently deletes insulin specific T cells and prevents escape of high affinity thymocytes. Collectively, these results suggest that antigen dose and peptide:MHC complex stability can lead to multiple fates of insulin reactive CD4+ T cell development and autoimmune disease outcome.

Type 1 diabetes pathogenesis and the role of inhibitory receptors in islet tolerance

“…The remaining thymocytes have TCRs with medium affinity, and these T cells proceed through development and differentiate into mature CD4 + or CD8 + T cells (positive selection). CD4 + single positive thymocytes with moderate‐to‐high affinity for self‐antigen can also undergo agonist‐driven differentiation into regulatory T cells …”

“…Thymus‐specific deletion of insulin promotes T1D, while transgenic expression of proinsulin under the MHCII promoter protects NOD mice from disease development . By extension, promoting low expression of insulin‐derived peptides in the thymi of insulin‐specific CD4 + TCR retrogenic mice allows the escape of insulin‐specific CD4 + T cells, while high expression of insulin‐derived peptides promotes the deletion of cognate T cells . Using transgenic mice that express enhanced green fluorescent protein (eGFP) under the control of different promoters, and tetramers to track GFP‐specific CD4 + T cells, Malhotra and colleagues linked the level of antigen expression in the thymus to a specific mode of tolerance induction; ubiquitin‐ or beta actin–driven GFP expression promoted efficient deletion of GFP‐specific CD4 + T cells (tolerance cluster 3).…”

Understanding Autoimmune Diabetes through the Prism of the Tri-Molecular Complex

“…Surprisingly, we did not observe any increase in thymic deletion of the relatively high (4-8) or low (12-4.1) TCRs, although the increase in insulin expression did protect mice from developing autoimmune diabetes. Protection from disease appeared to be due to an increase in Treg development with a significant increase in thymic, splenic, and pancreas-residing insulin-specific Tregs ( 72 ). These findings pose an intriguing possibility that the amount or stability of self-peptide:MHC complexes during thymic selection is more important for Treg development rather than deletion of self-reactive T cells (Figure 2 A).…”

“…It is currently unknown whether tissue-specific PTM antigens are transported and expressed in the thymus. In order to model how the presence of post-translationally modified peptides in the thymus could alter the selection of insulin-reactive TCRs (4-8 and 12-4.1) that normally escape negative selection, we ectopically expressed the R22E insulin mimetope in bone marrow-derived APCs ( 72 ). In the presence of R22E, the high-affinity 4-8 TCR bearing thymocytes were efficiently deleted, while the low affinity 12-4.1 population was affected to a lesser degree, albeit still showing an increase in negative selection based on Annexin V staining.…”

Generation of T Cell Receptor Retrogenic Mice

“…Importantly, retroviral gene delivery has made investigation of single TCRs in vivo more accessible to a higher number of laboratories and is compatible with an unlimited number of genetic backgrounds. Our laboratory and others have successfully applied the TCR retrogenic approach to study T cell responses to infectious, autoimmune, and model antigens (Abraham, Flickinger, Waldman, & Snook, 2019;Lee, Sprouse, Banerjee, Bettini, & Bettini, 2017;Sprouse et al, 2018;Yang et al, 2018). Over the years, the efficiency of generating TCR retrogenic mice has improved.…”