ChT1, an Ig superfamily molecule required for T cell differentiation

The thymus is colonized by circulating progenitor cells that differentiate into mature T cells under the influence of the thymic microenvironment. We report here the cloning and function of the avian thymocyte Ag ChT1, a member of the Ig superfamily with one V-like and one C2-like domain. ChT1-positive embryonic bone marrow cells coexpressing c-kit give rise to mature T cells upon intrathymic cell transfer. ChT1-specific Ab inhibits T cell differentiation in embryonic thymic organ cultures and in thymocyte precursor cocultures on stromal cells. Thus, we provide clear evidence that ChT1 is a novel Ag on early T cell progenitors that plays an important role in the early stages of T cell development.

Receptor-specific allelic exclusion of TCRV alpha-chains during development

Expression of a single Ag receptor on lymphocytes is maintained via allelic exclusion that generates cells with a clonal receptor repertoire. We show in normal mice and mice expressing functionally rearranged TCR alphabeta transgenes that allelic exclusion at the TCR alpha locus is not operational in immature thymocytes, whereas most mature T cells express a single TCRV alpha-chain. TCRV alpha allelic exclusion in mature thymocytes is regulated through a CD45 tyrosine phosphatase-mediated signal during positive selection. Using functional and genetic systems for selection of immature double TCRV alpha+ thymocytes, we show that peptide-specific ligand recognition provides the signal for allelic exclusion, i.e., mature T cells maintain expression of the ligand-specific TCRV alpha-chain, but lose the nonfunctional receptor. Whereas activation of TCRV beta-chains or CD3epsilon leads to receptor internalization, TCRV alpha ligation promotes retention of the TCR on the cell surface. Although both TCRV alpha- and TCRV beta-chains trigger phosphotyrosine signaling, only the TCRV beta-chain mediates membrane recruitment of the GTPase dynamin. These data indicate that TCRV alpha-directed signals for positive selection control allelic exclusion in T cells, and that developmental signals can select for single receptor usage.

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.

The interferon regulatory transcription factor IRF-1 controls positive and negative selection of CD8+ thymocytes

Little is known about the molecular mechanisms and transcriptional regulation that govern T cell selection processes and the differentiation of CD4+ and CD8+ T cells. Mice lacking the interferon regulatory transcription factor-1 (IRF-1) have reduced numbers of mature CD8+ cells within the thymus and peripheral lymphatic organs. Here we show that positive and negative T cell selection of two MHC class I-restricted TCR alphabeta transgenes, H-Y and P14, are impaired in IRF-1-/- mice. The absence of IRF-1 resulted in decreased expression of LMP2, TAP1, and MHC class I on thymic stromal cells. Despite decreased MHC class I expression on IRF-1-/- thymic stromal cells, the defect in CD8+ T cells development did not reside in the thymic environment, and IRF-1-/- stromal cells can fully support development of CD8+ thymocytes in in vivo bone marrow chimeras and in vitro reaggregation cultures. Moreover, IRF-1-/- thymocytes displayed impaired TCR-mediated signal transduction, and the induction of negative selection in TCR Tg thymocytes from IRF-1-/- mice required a 1000-fold increase in selecting peptide. We also provide evidence that IRF-1 is mainly expressed in mature, but not immature, thymocytes and that expression of IRF-1 in immature thymocytes is induced after peptide-specific TCR activation. These results indicate that IRF-1 regulates gene expression in developing thymocytes required for lineage commitment and selection of CD8+ thymocytes.