Mutually antagonistic signals regulate selection of the T cell repertoire

Prenatal Administration of Betamethasone Causes Changes in the T Cell Receptor Repertoire Influencing Development of Autoimmunity

Prenatal glucocorticoids are routinely administered to pregnant women at risk of preterm delivery in order to improve survival of the newborn. However, in half of the cases, birth occurs outside the beneficial period for lung development. Glucocorticoids are potent immune modulators and cause apoptotic death of immature T cells, and we have previously shown that prenatal betamethasone treatment at doses eliciting lung maturation induce profound thymocyte apoptosis in the offspring. Here, we asked if there are long-term consequences on the offspring’s immunity after this treatment. In the non-obese diabetic mouse model, prenatal betamethasone clearly decreased the frequency of pathogenic T cells and the incidence of type 1 diabetes (T1D). In contrast, in the lupus-prone MRL/lpr strain, prenatal glucocorticoids induced changes in the T cell repertoire that resulted in more autoreactive cells. Even though glucocorticoids transiently enhanced regulatory T cell (Treg) development, these cells did not have a protective effect in a model for multiple sclerosis which relies on a limited repertoire of pathogenic T cells for disease induction that were not affected by prenatal betamethasone. We conclude that prenatal steroid treatment, by inducing changes in the T cell receptor repertoire, has unforeseeable consequences on development of autoimmune disease. Our data should encourage further research to fully understand the consequences of this widely used treatment.

Thymocyte responsiveness to endogenous glucocorticoids is required for immunological fitness

Generation of a self-tolerant but antigen-responsive T cell repertoire occurs in the thymus. Although glucocorticoids are usually considered immunosuppressive, there is also evidence that they play a positive role in thymocyte selection. To address the question of how endogenous glucocorticoids might influence the adaptive immune response, we generated GRlck-Cre mice, in which the glucocorticoid receptor gene (GR) is deleted in thymocytes prior to selection. These mice were immunocompromised, with reduced polyclonal T cell proliferative responses to alloantigen, defined peptide antigens, and viral infection. This was not due to an intrinsic proliferation defect, because GR-deficient T cells responded normally when the TCR was cross-linked with antibodies or when the T cell repertoire was "fixed" with αβ TCR transgenes. Varying the affinity of self ligands in αβ TCR transgenic mice showed that affinities that would normally lead to thymocyte-positive selection caused negative selection, and alterations in the TCR repertoire of polyclonal T cells were confirmed by analysis of TCR Vβ CDR3 regions. Thus, endogenous glucocorticoids are required for a robust adaptive immune response because of their promotion of the selection of T cells that have sufficient affinity for self, and the absence of thymocyte glucocorticoid signaling results in an immunocompromised state.

The role of the glucocorticoid receptor in inflammation and immunity

Glucocorticoids are potent immunosuppressive agents with complex actions on immune cells evoking the following effects: inducing apoptosis, changing differentiation fate, inhibition of cytokine release, inhibition of migration and other features. Distinct molecular mechanisms of the glucocorticoid receptor (GR) contribute to different anti-inflammatory effects. Recently inflammatory models have been investigated using conditional knockout and function selective mice shedding light on critical cell types and molecular mechanisms of endogenous and therapeutic GC actions. Here we review the multiple effects of GCs on major immune cells, dendritic cells, myeloid cells and B- and T-lymphocytes and give a summary of studies using conditional GR knockout mice.

Developmental shift in TcR-mediated rescue of thymocytes from glucocorticoid-induced apoptosis

Glucocorticoid hormone (GC) production by thymic epithelial cells influences TcR signalling in DP thymocytes and modifies their survival. In the present work, we focused on exploring details of GC effects on DP thymocyte apoptosis with or without parallel TcR activation in AND transgenic mice, carrying TcR specific for pigeon cytochrome C, in vivo. Here we show that the glucocorticoid receptor (GR) protein level was the lowest in DP thymocytes, and it was slightly down-regulated by GC analogue, anti-CD3, PCC and combined treatments as well. Exogenous GC analogue treatment or TcR stimulation alone lead to marked DP cell depletion, coupled with a significant increase of early apoptotic cell ratio (AnnexinV staining), marked abrogation of the mitochondrial function in DP cells (CMXRos staining), and significant decrease in the Bcl-2(high) DP thymocyte numbers, respectively. On the other hand, the simultaneous exposure to these two proapototic signals effectively reversed all the above-described changes. The parallel analysis of CD4 SP cell numbers, AnnexinV, CMXRos, Bcl-2 and GR stainings revealed, that the GR and TcR signals were not antagonistic on the mature thymocytes. These data provide experimental evidence in TcR transgenic mice, in vivo, that when TcR activation and GR signals are present simultaneously, they rescue double positive thymocytes from programmed cell death. The two separate signalling pathways merge in DP thymocytes at such important apoptosis regulating points as the Bcl-2 and GR, showing that their balanced interplay is essential in DP cell survival.

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.

T Cell Receptor Signaling Inhibits Glucocorticoid-induced Apoptosis by Repressing the SRG3 Expression via Ras Activation

Activation of T cell antigen receptor (TCR) signaling inhibits glucocorticoid (GC)-induced apoptosis of T cells. However, the detailed mechanism regarding how activated T cells are protected from GC-induced apoptosis is unclear. Previously, we have shown that the expression level of SRG3, a murine homolog of BAF155 in humans, correlated well with the GC sensitivity of T cells either in vitro or in vivo. Intriguingly, the expression of SRG3 decreased upon positive selection in the thymus. Here we have shown that TCR signaling inhibits the SRG3 expression via Ras activation and thereby renders primary thymocytes and some thymoma cells resistant to GC-mediated apoptosis. By using pharmacological inhibitors, we have shown that Ras-mediated down-regulation of the SRG3 gene expression is mediated by MEK/ERK and phosphatidylinositol 3-kinase pathways. Moreover, TCR signals repressed the SRG3 transcription through the putative binding sites for E proteins and Ets family transcription factors in the proximal region of the SRG3 promoter. Introduction of mutations in these elements rendered the SRG3 promoter immune to the Ras or TCR signals. Taken together, these observations suggest that TCR signals result in GC desensitization in immature T cells by repressing SRG3 gene expression via Ras activation.

Antigen and glucocorticoid hormone (GC) induce positive selection of DP thymocytes in a TcR transgenic mouse model

Thymocyte maturation in the thymus is controlled by stromal and humoral components. Among the humoral regulators locally produced glucocorticoids (GCs) seem to have a key role in the positive selection of thymocytes. Our previous studies have shown that the administration of GCs or the stimulation through the CD3 complex can induce apoptosis of double positive (DP) cells, but the combined presence of these stimuli induces positive selection. In this work our aim was to investigate the effects of antigen exposure and synthetic GC hormone (dexamethasone, DX) administration on the selection processes of DP cells in TcR transgenic mice. In our model, AND-pigeon cytochrome c (PCC)-specific I-E(k) (MHC-II) restricted Vbeta3, Valpha11 TcR expressing transgenic mice were treated with PCC, with high or low dose DX, or with PCC and DX together, followed by the analysis of total thymocyte numbers, thymocyte composition, with regard to their CD69, Vbeta3 and Annexin V expression. The administration of PCC and/or DX for 2 days resulted in a decreased DP cell number and a significantly increased CD4 SP cell ratio. However, in both cases the total thymocyte numbers decreased. CD69 expression increased on both DP and CD4 SP cells after PCC and/or DX treatments. We found that after DX or combined treatment, the percentage of Annexin V positive cells increased. The ratio of Vbeta3 TcR bearing DP thymocytes showed no change after DX or PCC administrations alone, but it decreased significantly after combined treatment. MHC-II bound PCC peptides in the presence of GCs enhanced the maturation of Vbeta3+ DP cells into CD4 SP stage, therefore, the Vbeta3- cells remained mostly in the DP immature stage. These data indicate that both antigen and low dose GC alone are capable of inducing positive selection of DP cells, but together they gave a stronger effect in promoting positive selection. From these we conclude that GCs influence the maturation and selection processes of thymocytes.

Overexpression of murine TSLP impairs lymphopoiesis and myelopoiesis

The role of thymic stromal cell-derived lymphopoietin (TSLP) in regulating hematopoiesis is poorly characterized, so we investigated its regulatory effects in vivo using TSLP transgenic mice. Overexpression of TSLP disrupted hematopoietic homeostasis by causing imbalances in lymphopoiesis and myelopoiesis. Mice harboring a TSLP transgene had 5- to 700-fold fewer B and T precursors and no detectable pre-B lymphocyte colonyforming activity in the marrow or spleen. Conversely, TSLP transgenic mice possessed 15 to 20 times more splenic myeloid precursors than their littermates, and progenitor activity of the granulocyteerythrocyte-macrophage-megakaryocyte colony-forming units was significantly elevated. The arrest in lymphopoiesis and the expansion of myeloid progenitor cells in TSLP transgenic mice suggest that TSLP has negative and positive regulatory effects on lymphoid and myeloid development, respectively.