TCR-mediated signaling in thymocyte selection

grantor: University of TorontoTCR-mediated signals have been shown to induce both positive and negative selection of thymocytes. How the same TCR can induce both forms of selection introduces a conceptual paradox. A generally accepted model of thymocyte selection is the "strength of signal" or "affinity/avidity" hypothesis. This model predicts that the absence of any signal from the TCR results in death by neglect, a "strong" signal results in negative selection and a "weak" signal results in positive selection. My thesis examines how these TCR-peptide/MHC interactions are translated into intracellular signals that discriminate between positive and negative selection. The thesis investigates the relationship between the extent of TCR internalization in response to various peptide/MHC stimuli and thymocyte selection outcomes. Our results demonstrate a direct correlation: peptides that induce strong TCR down-regulation are most efficient at mediating negative selection, whereas ligand interactions that induce suboptimal TCR internalization are more efficient at triggering positive selection. These data suggest that T cell selection is mediated by differing intensities of the same TCR-mediated signal, rather than by distinct signals. My thesis also assesses the role of the mitogen-activated protein kinase (MAPK) signaling cascade, specifically the extracellular signal-regulated kinase (ERK) module in regulating both positive and negative selection. We find that negatively selecting conditions induce stronger ERK activation in thymocytes than do positively selecting conditions. We further demonstrate that attenuating the ERK activity by pharmacological inhibition can shift the activation threshold for negative selection to that for positive selection. Moreover, in order to study the kinetics of ERK activation upon stimulation with positive and negative selection stimuli, we devised an 'in vitro ' assay system for thymocyte selection. Data from these studies suggest that weak, sustained ERK activation is characteristic of positive selection, whereas strong but transient ERK activation leads to positive selection. Therefore, both timing and extent of ERK activation appear to be important for thymocyte selection. Since positively selecting stimuli do not induce efficient TCR internalization, the large proportion of TCR complexes that remain on the cell surface may relay continuous signals required for survival and differentiation. Together, these results add a novel viewpoint to thymocyte selection.Ph.D