The Action of Bax and Bcl-2 on T Cell Selection

T cell development and selection in the thymus are shaped by the induction of apoptosis. However, a direct role in T cell development and selection for any of the molecules known to regulate apoptosis has remained controversial. We have studied the effect of bax and bcl-2 transgenes in recombination activation gene 1–deficient (RAG-1−/−) mice transgenic for the major histocompatibility complex class I–restricted F5 T cell receptor. Overexpression of a bax transgene in the thymus seriously impairs the production of mature T cells, whereas bcl-2 overexpression greatly promotes it. The effect of bax and bcl-2 overexpression on antigen-induced negative selection was studied using fetal thymic organ cultures. This analysis showed that Bcl-2 strongly inhibits negative selection, whereas Bax does not affect it. Our data directly show that Bcl-2 family members have specific roles in T cell selection and also lend support to the hypothesis that Bax and Bcl-2 can antagonize each other's action in a certain apoptosis pathway while in another they can be functionally nonreciprocal

Developmental and Molecular Characterization of Emerging β- and γδ-Selected Pre-T Cells in the Adult Mouse Thymus

The first checkpoint in T cell development, β selection, has remained incompletely characterized for lack of specific surface markers. We show that CD27 is upregulated in DN3 thymocytes initiating β selection, concomitant with intracellular TCR-β expression. Clonal analysis determined that CD27^(high) DN3 cells generate CD4^+CD8^+ progeny with more than 90% efficiency, faster and more efficiently than the CD27^(low) majority. CD27 upregulation also occurs in γδ-selected DN3 thymocytes in TCR-β−/− mice and in IL2-GFP transgenic reporter mice where GFP marks the earliest emerging TCR-γδ cells from DN3 thymocytes. With CD27 to distinguish pre- and postselection DN3 cells, a detailed gene expression analysis defined regulatory changes associated with checkpoint arrest, with β selection, and with γδ selection. γδ selection induces higher CD5, Egr, and Runx3 expression as compared to β selection, but it triggers less proliferation. Our results also reveal differences in Notch/Delta dependence at the earliest stages of divergence between developing αβ and γδ T-lineage cells

Glucocorticoids in T cell apoptosis and function

Glucocorticoids (GCs) are a class of steroid hormones which regulate a variety of essential biological functions. The profound anti-inflammatory and immunosuppressive activity of synthetic GCs, combined with their power to induce lymphocyte apoptosis place them among the most commonly prescribed drugs worldwide. Endogenous GCs also exert a wide range of immunomodulatory activities, including the control of T cell homeostasis. Most, if not all of these effects are mediated through the glucocorticoid receptor, a member of the nuclear receptor superfamily. However, the signaling pathways and their cell type specificity remain poorly defined. In this review, we summarize our present knowledge on GC action, the mechanisms employed to induce apoptosis and the currently discussed models of how they may participate in thymocyte development. Although our knowledge in this field has substantially increased during recent years, we are still far from a comprehensive picture of the role that GCs play in T lymphocytes