Glucocorticoid in T cell differentiation

• Thymocytes differentiate into mature T cells with an immunological restriction for self major histocompatibility complex (MHC) antigens. Cells that do not recognize self MHC expressed on thymic epithelial cells (TEC) die by a rapid induction of apoptosis (default selection). Auto-reactive cells that recognize MHC presented endogenous peptides on bone-marrow derived antigen presenting cells also die by apoptosis (negative selection). Default selection accounts for the major loss of developing thymocytes out of which only a few percent develop into mature peripheral T cells. The present thesis deals with some aspects of thymic selection mechanisms with particular focus on the potential role of glucocorticoids (GC) in this process. GCs have earlier been shown to potently induce apoptosis in immature CD4+/CD8+ thymocytes. We have found that the GC receptor (GR) antagonist RU486 have an inhibitory effect in two model systems for thymic negative selection. In DO11.10 transgenic mice, in which the TCR transgene recognize an OVA derived peptide presented by the I-Ad MHC class II molecule, RU486 completely inhibited thymic apoptosis induced by peptide treatment (Paper I). In normal mice treated with the anti-CD3 mAb 2C 11, RU486 was earlier found to partially inhibit thymic apoptosis and this was now found to be dependent on the Fc domain of the mAb (Paper II). In untreated DO11.10 mice the TCR transgene was found to mediate a positive selection effect, correlating with a low expression of rnRNA for the nuclear receptor Nur77 (Paper IV). On further analysis of thymocytes from DO11.10 mice, these cells were found to be comparatively resistant to corticosterone (CS) in vitro and to contain reduced levels of GR as well as an increased expression of CD28 and increased DNA binding activity of transcription factors AP-1 and NFkB (Paper V). Based on our results, and some earlier publications, we postulated that TEC might secrete GC. This was tested by a reporter cell system in which COS cells, containing GR and GRE upstream of a reporter gene, were co-cultivated with explanted TEC in vitro (Paper III). TEC were found to generate a clear, specific signal in this system. I discuss the potential role of GC for default, and for different types of negative selection in thymus in relation to our own findings.

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