Stabilized beta-catenin potentiates Fas-mediated T cell apoptosis

Stabilized β-Catenin Ameliorates ALPS-Like Symptoms of B6/lpr Mice

Autoimmune lymphoproliferative syndrome (ALPS) is an incurable disease mainly caused by the defect of Fas-mediated apoptosis and characterized by nonmalignant autoimmune lymphoproliferation. Stabilized β-catenin could not only potentiate Fas-mediated T cell apoptosis via upregulating the expression of Fas on activated T cells, but also potentiate T cell apoptosis via intrinsic apoptotic pathway. In the present study, we introduced β-cat^Tg into lpr/lpr mice and aimed to explore the potential role of stabilized β-catenin (β-cat^Tg) in the development of ALPS-like phenotypes of lpr/lpr mice. We found that the total splenocyte cells and some compositions were slightly downregulated in β-cat^Tglpr/lpr mice, especially the CD4 and CD8 T_EM cells were significantly reduced. Meanwhile, stabilized β-catenin obviously decreased the numbers of spleen TCRβ⁺CD4⁻CD8⁻ T (DNT) cells, and the levels of some serum proinflammatory factors also were lowered in β-cat^Tglpr/lpr mice. Beyond that, stabilized β-catenin slightly lowered the levels of the serum autoantibodies and the scores of kidney histopathology of β-cat^Tglpr/lpr mice compared with lpr/lpr mice. Our study suggested that stabilized β-catenin ameliorated some ALPS-like symptoms of lpr/lpr mice by potentiating Fas-independent signal-mediated T cell apoptosis, which might uncover a potential novel therapeutic direction for ALPS.

Leukocyte Beta-Catenin Expression Is Disturbed in Systemic Lupus Erythematosus

Wnt/β-catenin signaling is relatively understudied in immunity and autoimmunity. β-catenin blocks inflammatory mediators and favors tolerogenic dendritic cell (DC) phenotypes. We show here that leukocytes from lupus-prone mice and SLE patients express diminished β-catenin transcriptional activity, particularly in myeloid cells, although other leukocytes revealed similar trends. Serum levels of DKK-1, an inhibitor under transcriptional control of Wnt/β-catenin, were also decreased in lupus-prone mice. Surprisingly, however, preemptive deletion of β-catenin from macrophages appears to have no effect on lupus development, even in mice with varying genetic loads for lupus. Although myeloid-specific loss of β-catenin does not seem to be important for lupus development, the potential role of this transcription factor in other leukocytes and renal cells remain to be elucidated.

Identification of Tetraazacyclic Compounds as Novel Potent Inhibitors Antagonizing RORγt Activity and Suppressing Th17 Cell Differentiation

CD4⁺ T-helper cells that produce interleukin-17 (Th17 cells) are characterized as pathological T-helper cells in autoimmune diseases. Differentiation of human and mouse Th17 cells requires a key transcription regulator, retinoic acid receptor-related orphan receptor γt (RORγt), which is a potential therapeutic target for autoimmune diseases. To develop a therapeutic agent for Th17-mediated autoimmune diseases, we have established a high-throughput screening (HTS) assay for candidate screening, in which the luciferase activity in RORγt-LBD positive and negative Jurkat cells were analyzed to evaluate induction of RORγt activity by compounds. This technique was applied to screen a commercially-available drug-like chemical compound library (Enamine) which contains 20155 compounds. The screening identified 17 compounds that can inhibit RORγt function in the HTS screen system. Of these, three tetraazacyclic compounds can potently inhibit RORγt activity, and suppress Th17 differentiation and IL-17 production. These three candidate compounds could significantly attenuate the expression of the Il17a by 65%- 90%, and inhibit IL-17A secretion by 47%, 63%, and 74%, respectively. These compounds also exhibited a potent anti-RORγt activity, with EC₅₀ values of 0.25 μM, 0.67 μM and 2.6 μM, respectively. Our data demonstrated the feasibility of targeting the RORγt to inhibit Th17 cell differentiation and function with these tetraazacyclic compounds, and the potential to improve the structure of these compounds for autoimmune diseases therapeutics.

Inhibition of RORγt activity and Th17 differentiation by a set of novel compounds

BACKGROUND

Retinoic acid receptor-related orphan receptor gamma t (RORγt) is the master regulator of Th17 cell differentiation, which plays a critical role in the pathology of several autoimmune diseases. By directing Th17 cells function, RORγt could be a potential target for drug development for Th17 related autoimmune disease.

METHODS

A Jurkat cell-based reporter assay system was used for screening RORγt inhibitors from a drug-like chemical library, following with mouse Th17 cells differentiation study to identify the effect of targeted compounds in primary T cells. 293T cell-based reporter assay was conducted to determine the cell specificity, and MTT assay was performed to determine the cell toxicity of those compounds.

RESULTS

In this study, we identified four lead compounds that suppressed RORγt activity, Th17 differentiation and IL-17A secretion. These candidates displayed inhibition ability on RORγt activity in T cell derived Jurkat cell, but not in 293 T cell, which indicated the restricted effects of these compounds to other cells or tissues. Futhermore, our results demonstrated that these candidates exhibited more robust inhibitory on IL-17 F transcription expression than IL-17A, which is different from one reported compound, SR1001, that mainly suppressed IL-17A, rather than IL-17 F production.

CONCLUSIONS

Our study discovered four novel compounds that inhibited RORγt activity and Th17 function, which indicates their potential in therapeutic application of Th17 related autoimmune disorders.

Microgravity inhibits resting T cell immunity in an exposure time-dependent manner

BACKGROUND

Decline immune function is well documented after spaceflights. Microgravity is one of the key factors directly suppressing the function of immune system. Though T cell immune response was inhibited by microgravity, it is not clearly whether activation would be inhibited after a pre-exposure of microgravity on T lymphocytes at the resting state.

METHODS

We herein investigated the response ability of resting CD4⁺ and CD8⁺ T cells experiencing pre-exposure of modeled microgravity (MMg) for 0, 8, 16 and 24 hrs to concanavalin A (ConA) stimulation. The phenotypes and subsets of immune cells were determined by flow cytometry.

RESULTS

Both CD4⁺ and CD8⁺ T cells with an MMg pre-exposure exhibited decreased expressions of activation-markers including CD25, CD69 and CD71, inflammatory cytokine secretion and cell proliferation in response to ConA compared with T cells with 1g controls in an MMg exposure time- dependent manner. Moreover, short term MMg treatment caused more severe decreased proliferation in CD4⁺ T cells than in CD8⁺ T cells.

CONCLUSIONS

MMg can directly impact on resting T cell subsets. CD4⁺ T cells were more sensitive to the microgravity inhibition than CD8⁺ T cells in respect of cell proliferation. These results offered new insights for the MMg-caused T cell functional defects.

CEACAM1 regulates Fas-mediated apoptosis in Jurkat T-cells via its interaction with β-catenin

CEACAM1 (Carcinoembryonic Antigen Cell Adhesion molecule 1), an activation induced cell surface marker of T-cells, modulates the T-cell immune response by inhibition of the T-cell and IL-2 receptors. Since T-cells undergo activation induced cell death via Fas activation, it was of interest to determine if this pathway was also affected by CEACAM1. Previously, we identified a novel biochemical interaction between CEACAM1 and the armadillo repeats of β-catenin in Jurkat cells, in which two critical residues, H469 and K470 of the cytoplasmic domain of CEACAM1-4L played an essential role; while in other studies, β-catenin was shown to regulate Fas-mediated apoptosis in Jurkat cells. CEACAM1 expression in Jurkat cells leads to the re-distribution of β-catenin to the actin cytoskeleton as well as inhibition of β-catenin tyrosine phosphorylation and its degradation after Fas stimulation. As a result, Fas-mediated apoptosis in these cells was inhibited. The K470A mutation of CEACAM1 partially rescued the inhibitory effect, in agreement with the prediction that a CEACAM1-β-catenin interaction pathway is involved. Although CEACAM1 has two ITIMs, they were not tyrosine-phosphorylated upon Fas ligation, indicating an ITIM independent mechanism; however, mutation of the critical residue S508, located between the ITIMs, to aspartic acid and a prerequisite for ITIM activation, abrogates the inhibitory activity of CEACAM1 to Fas-mediated apoptosis. Since Fas-mediated apoptosis is a major form of activation-induced cell death, our finding supports the idea that CEACAM1 is a general inhibitory molecule for T-cell activation utilizing a variety of pathways.

β-catenin/TCF-1 pathway in T cell development and differentiation

T cells must undergo two critical differentiation processes before they become competent effectors that can mediate actual immune responses. Progenitor T cells undergo defined stages of differentiation in the thymus, which include positive and negative selection, to generate a repertoire of T cells that will respond to foreign but not self antigens. When these immunocompetent T cells first migrate out of thymus into peripheral lymphoid tissues, they are naïve and are unable to mediate immune responses. However, upon antigen encounter, peripheral CD4+ naïve T cells undergo another differentiation process to become armed effector T cells including Th1, Th2, Th17 or regulatory T cells, all of which are capable of regulating immune responses. A canonical Wnt/β-catenin/T cell factor (TCF) pathway has been shown to regulate T cell differentiation in both the thymus and in peripheral lymphoid tissues. Dysfunction of this pathway at any stage of T cell differentiation could lead to severe autoimmunity including experimental autoimmune encephalomyelitis or immune deficiency. Understanding the role played by β-catenin/TCF-1 in T cell differentiation will facilitate our understanding of the mechanisms that regulate T cell function and assist in identifying novel therapy targets for treating both autoimmune and immune diseases. Therefore, in this review, we will focus on the function of β-catenin/TCF-1 pathway in the regulation of thymic and peripheral T cell differentiation processes.

Transcription factor network regulating CD(+)CD8(+) thymocyte survival

More than 80% of thymocytes are CD4(+)CD8(+) double positive (DP) cells subject to positive/ negative selection. The lifespan of DP thymocytes is critical in shaping the peripheral T-cell repertoire essential for mounting immune responses against foreign, but not self, antigens. During T-cell maturation, if the first round of T-cell receptor (TCR) α chain rearrangement fails to generate a productive T-cell receptor, DP cells start another round of α chain rearrangement until positive selection or cell death intervenes. Thus, the lifespan of DP cells determines how many rounds of α chain rearrangement can be carried out, and influences the likelihood of completing positive selection. The antiapoptotic protein Bcl-x(L) is the ultimate effector regulating DP cell survival, and several transcription factors critical for T-cell development, such as TCF-1, E proteins, c-Myb, and RORγt, regulate DP survival via a Bcl-x(L)-dependent pathway. However, the relationship between these transcription factors in this process is largely unclear. Recent results are revealing an interactive network among these critical factors during regulation of DP thymocyte survival. This review will discuss how these transcription factors potentially work together to control DP thymocyte survival that is critical for successful completion of T-cell development.

T cell factor 1 regulates thymocyte survival via a RORγt-dependent pathway

Survival of CD4(+)CD8(+) double-positive (DP) thymocytes plays a critical role in shaping the peripheral T cell repertoire. However, the mechanisms responsible for the regulation of DP thymocyte lifespan remain poorly understood. In this work, we demonstrate that T cell factor (TCF)-1 regulates DP thymocyte survival by upregulating RORγt. Microarray analysis revealed that RORγt was significantly downregulated in TCF-1(-/-) thymocytes that underwent accelerated apoptosis, whereas RORγt was greatly upregulated in thymocytes that had enhanced survival due to transgenic expression of a stabilized β-catenin (β-cat(Tg)), a TCF-1 activator. Both TCF-1(-/-) and RORγt(-/-) DP thymocytes underwent similar accelerated apoptosis. Forced expression of RORγt successfully rescued TCF-1(-/-) DP thymocytes from apoptosis, whereas ectopically expressed TCF-1 was not able to rescue the defective T cell development because of the lack of RORγt-supported survival. Furthermore, activation of TCF-1 by stabilized β-catenin was able to enhance DP thymocyte survival only in the presence of RORγt, indicating that RORγt acts downstream of TCF-1 in the regulation of DP thymocyte survival. Moreover, β-catenin/TCF-1 directly interacted with the RORγt promoter region and stimulated its activity. Therefore, our data demonstrated that TCF-1 enhances DP thymocyte survival through transcriptional upregulation of RORγt, which we previously showed is an essential prosurvival molecule for DP thymocytes.

Critical role of TCF-1 in repression of the IL-17 gene

Overwhelming activation of IL-17, a gene involved in inflammation, leads to exaggerated Th17 responses associated with numerous autoimmune conditions, such as experimental autoimmune encephalomyelitis (EAE). Here we show that TCF-1 is a critical factor to repress IL-17 gene locus by chromatin modifications during T cell development. Deletion of TCF-1 resulted in increased IL-17 gene expression both in thymus and peripheral T cells, which led to enhanced Th17 differentiation. As a result, TCF-1^-/- mice were susceptible to Th17-dependent EAE induction. Rag1^-/- mice reconstituted with TCF-1^-/- T cells were also susceptible to EAE, indicating TCF-1 is intrinsically required to repress IL-17. However, expression of wild-type TCF-1 or dominant negative TCF-1 did not interfere with Th17 differentiation in mature T cells. Furthermore, expression of TCF-1 in TCF-1^-/- T cells could not restore Th17 differentiation to wild-type levels, indicating that TCF-1 cannot affect IL-17 production at the mature T cell stage. This is also supported by the normal up-regulation or activation in mature TCF-1^-/- T cells of factors known to regulate Th17 differentiation, including RORγt and Stat3. We observed hyperacetylation together with trimethylation of Lys-4 at the IL-17 locus in TCF-1^-/- thymocytes, two epigenetic modifications indicating an open active state of the gene. Such epigenetic modifications were preserved even when TCF-1^-/- T cells migrated out of thymus. Therefore, TCF-1 mediates an active process to repress IL-17 gene expression via epigenetic modifications during T cell development. This TCF-1-mediated repression of IL-17 is critical for peripheral T cells to generate balanced immune responses.

Transient activation of Wnt/{beta}-catenin signaling induces abnormal growth plate closure and articular cartilage thickening in postnatal mice

Wnt/beta-catenin signaling is required for skeletal development and organization and for function of the growth plate and articular cartilage. To further clarify these roles and their possible pathophysiological importance, we created a new transgenic mouse model in which Wnt/beta-catenin signaling can be activated in cartilage for specific periods of time. These transgenic mice expressed a constitutive active form of beta-catenin fused to a modified estrogen receptor ligand-binding domain under the control of cartilage-specific collagen 11alpha2 promoter/enhancer. Transient Wnt/beta-catenin signaling activation in young adult mice by tamoxifen injections induced growth retardation and severe deformities in knee joints. Tibial and femoral growth plates displayed an excessive number of apoptotic cells and eventually underwent abnormal regression. Articular cartilage exhibited an initial acute loss of proteoglycan matrix that was followed by increases in thickness, cell density, and cell proliferation. In reciprocal studies, we found that conditional ablation of beta-catenin in postnatal mice using a Col2-CreER strategy led to hypocellularity in articular cartilage, growth plate disorganization, and a severe reduction in bone volume. Together, these data provide evidence that Wnt/beta-catenin signaling has important and distinct roles in growth plate and articular cartilage and that postnatal dysregulation of this signaling pathway causes diverse structural and functional changes in the two cartilaginous structures.