Thymic negative selection is functional in NOD mice

Based on analyses of multiple TCR transgenic (tg) models, the emergence of pathogenic T cells in diabetes-prone NOD mice has been ascribed to a failure to censure autoreactive clones in the thymus. In contrast, using isolated and preselected thymocytes, we show that nonobese diabetic (NOD) genetic variation impairs neither clonal deletion nor downstream transcriptional programs. However, we find that NOD genetic variation influences alpha beta/gamma delta-lineage decisions promoted by early expression of tg alpha beta-TCRs at the double-negative (DN) stage. In B6 and other genetic backgrounds, tg alpha beta-TCRs behave like gamma delta-TCRs and commit a large fraction of DNs toward the gamma delta-lineage, thereby decreasing the size of the double-positive (DP) pool, which is efficiently positively and negatively selected. In NOD DNs, alpha beta-TCR signalosomes instead behave like preTCRs, resulting in high numbers of DPs competing for limited selection niches, and poor positive and negative selection. Once niche effects are neutralized in mixed bone marrow chimeras, positive and negative selection are equally efficient on B6 and NOD backgrounds. Biochemical analysis revealed a selective defect in the activation of Erk1/2 downstream of NOD alpha beta-TCR signalosomes. Therefore, NOD genetic variation influences alpha beta/gamma delta-lineage decisions when the alpha beta-TCR heterodimer is prematurely expressed, but not the process of negative selection.