Agonist peptide modulates T cell selection thresholds through qualitative and quantitative shifts in CD8 co-receptor expression.

Engagement of the TCR is a pivotal step in thymocyte development, ultimately resulting in the survival (positive selection) or loss (negative selection) of developing T cells. The roles of peptides and stromal cell interactions necessary for these selection events, however, are still poorly understood. To investigate the effects of agonist peptide in positive selection, we used a novel cell suspension model for in vitro thymic positive selection in adults. Target thymocytes from H-2Db-restricted TCR transgenic mice, specific to the lymphocytic choriomeningitis virus (LCMV) peptide bred on a non-selecting MHC background (H-2d or TAP-1-/-), were co-cultured with freshly isolated H-2b thymic stromal cells. In the presence of selecting stroma the nominal agonist LCMV peptide induced apoptosis at high concentrations and at low concentrations enhanced the efficiency of positive selection both in numbers of cells 'rescued' and kinetics of appearance of selected single-positive cells. We further illustrate down-modulation of CD8 alpha beta or CD8 beta at high but non-deleting concentrations of agonist peptide. This highlights the ability of the T cell, within the window of positive selection, to modify surface co-receptors both qualitatively and quantitatively in response to increasing avidity TCR-peptide-MHC interactions. The direct consequence of this would be to lower the total signaling events below the threshold for apoptosis induction. Hence if self peptide were not presented in sufficient quantities in the thymus, autoreactive cells may escape deletion and may actually be positively selected.