CD8+ T cells (CTL) are essential effectors mediating killing of pancreatic beta cells. Deficiencies in central and peripheral tolerance mechanisms are proposed to contribute to the development and activation of autoreactive CTL. Peripheral mechanisms may be essential for regulating low avidity T cell receptors (TCR) that escape negative selection. While thymic insulin expression has been associated with disease risk, the role of peripheral proinsulin during tolerization of insulin-reactive CTL has not been investigated.
Utilizing NOD mice with a transgenic CD8+ TCR specific for insulin B15-23 (G9Cα-/-), we have studied the role of proinsulin expression on the development of antigen-specific CTL. G9Cα-/- transgenic mice were crossed to mice with mutations in insulin expression: G9Cα‑/‑Ins2‑/‑ or G9Cα‑/‑Ins1‑/‑Ins2‑/‑Y16Atg, an insulin transgene to ablate recognition by G9. While G9Cα-/- mice do not develop spontaneous disease, 25% of G9Cα‑/‑Ins2-/- male mice developed T1D (p=0.0046). However, the frequency of CD8+ T cells in the thymus and periphery was not altered, suggesting that negative selection mechanisms are not affected. There was no increase in CD4+CD25+FoxP3+ Tregs. Peripheral CD8+ T cells demonstrated a shift in activation marker expression as G9Cα‑/‑Ins2-/- CTL from the pancreatic lymph nodes (PLN) have decreased CD62L expression. G9 T cells produced in a system lacking native insulin expression have demonstrated increased cytotoxicity and proliferation in vitro in response to native peptide. Additionally, CTL from G9Cα-/-Ins2-/- and G9Cα-/-.Y16A exhibited increased rate of proliferation in vivo after adoptive transfer to NOD recipients compared to G9Cα‑/‑ CTL. G9Cα‑/‑.Y16A CTL have not encountered native antigen (i.e. have not been primed to cognate peptide), yet these cells exhibit the most robust response. These results suggest exposure to proinsulin during development is attenuating the effector response of insulin-reactive G9 cells. Further understanding of critical tolerogenic pathways is important for the development and application of immune-modulating therapeutics for T1D.