Development of diabetogenic T cells in NOD mice

Abstract

Insulin dependent diabetes mellitus (IDDM), type 1 diabetes, results from the destruction of pancreatic p cells by p cell-specific autoreactive T cells. Defects in negative selection in the thymus are thought to result in the failure to delete potential p cell-reactive T cells, contributing to the development of autoimmune diabetes. This possibility was investigated by comparing the deletion profile of CD4+CD8+ double positive (DP) thymocytes in NOD mice, a diabetes-prone animal model of IDDM, with that of diabetes-resistant strains of mice after anti-CD3 antibody treatment to trigger T cell activation. These investigations showed that immature NOD DP thymocytes had a comparatively lower activation threshold, resulting in efficient phosphorylation of ERK and JNK and efficient differentiation into CD69+CD3high DP thymocytes, and this property depended on the particular MHC class II haplotype of NOD mice (H-2g7). In NOD fetal thymic organ cultures (FTOC), single positive (SP) thymocytes can be efficiently generated from DP thymocytes, and these cells could induce diabetes when adoptively transferred into T cell deficient NOD.scid mice. These studies suggest that the low activation threshold of DP thymocytes in NOD mice results in inordinate positive selection, which ultimately contributes to the development of autoreactive diabetogenic T cells. Cytotoxic T lymphocyte antigen-4 (CTLA-4) is known as a negative regulator of T cell activation in the periphery, but its role in the development of thymocytes is poorly understood. Antibody-mediated blockade of CTLA-4 in NOD FTOC prevented the efficient deletion of DP thymocytes. In a second series of investigations using NOD FTOC, it was shown that blockade of CTLA-4 prevented the efficient deletion of DP thymocytes. Expression of CTLA-4 in NOD DP thymocytes was found to increase after TCR engagement, and blockade of CTLA-4 suppressed the activation of DP thymocytes, resulting in the inhibition of: 1) CD69 expression, 2) activation-induced CTLA-4 expression, 3) phosphorylation of ERK, and 4) deletion of DP thymocytes after TCR engagement. The number of DP thymocytes in CTLA-4 deficient C57BL/6 FTOC was significantly higher than that of littermate controls. These results suggest that CTLA-4- mediated positive signaling acts in concert with strong TCR stimulation to cause deletion of DP thymocytes in NOD and other strains of mice. This mechanism may be involved in the complete deletion of DP thymocytes bearing very high affinity/avidity TCR for self­peptide/MHC complex in T cell development, resulting in the peripheral tolerance.