Molecular mechanisms of pre-T cell receptor-induced survival

A critical regulator of Bcl2 revealed by systematic transcript discovery of lncRNAs associated with T-cell differentiation

Normal T-cell differentiation requires a complex regulatory network which supports a series of maturation steps, including lineage commitment, T-cell receptor (TCR) gene rearrangement, and thymic positive and negative selection. However, the underlying molecular mechanisms are difficult to assess due to limited T-cell models. Here we explore the use of the pro-T-cell line P5424 to study early T-cell differentiation. Stimulation of P5424 cells by the calcium ionophore ionomycin together with PMA resulted in gene regulation of T-cell differentiation and activation markers, partially mimicking the CD4^-CD8^- double negative (DN) to double positive (DP) transition and some aspects of subsequent T-cell maturation and activation. Global analysis of gene expression, along with kinetic experiments, revealed a significant association between the dynamic expression of coding genes and neighbor lncRNAs including many newly-discovered transcripts, thus suggesting potential co-regulation. CRISPR/Cas9-mediated genetic deletion of Robnr, an inducible lncRNA located downstream of the anti-apoptotic gene Bcl2, demonstrated a critical role of the Robnr locus in the induction of Bcl2. Thus, the pro-T-cell line P5424 is a powerful model system to characterize regulatory networks involved in early T-cell differentiation and maturation.

Targeted deletion of p73 in mice reveals its role in T cell development and lymphomagenesis

Transcriptional silencing of the p73 gene through methylation has been demonstrated in human leukemias and lymphomas. However, the role of p73 in the malignant process remains to be explored. We show here that p73 acts as a T cell-specific tumor suppressor in a genetically defined mouse model, and that concomitant ablation of p53 and p73 predisposes mice to an increased incidence of thymic lymphomas compared to the loss of p53 alone. Our results demonstrate a causal role for loss of p73 in progression of T cell lymphomas to the stage of aggressive, disseminated disease. We provide evidence that tumorigenesis in mice lacking p53 and p73 proceeds through mechanisms involving altered patterns of gene expression, defects in early T cell development, impaired apoptosis, and the ensuing accumulation of chromosomal aberrations. Collectively, our data imply that tumor suppressive properties of p73 are highly dependent on cellular context, wherein p73 plays a major role in T cell development and neoplasia.

Regulation of lymphocyte progenitor survival by the proapoptotic activities of Bim and Bid

On their entry into the thymus, developing lymphocyte progenitors depend on signaling from the pre-T cell receptor (pre-TCR), which orchestrates differentiation, cell proliferation, and survival. The exact mechanism of pre-TCR-mediated suppression of T cell death remains unclear and controversial. Here, we identify Bim and Bid, 2 members of the BH3-only group of the BCL2 family, as important regulators of pre-T cell death. Both factors are highly expressed in proapoptotic thymocytes and their expression is suppressed on signaling through the pre-TCR. Their expression is directly regulated by the transcription factors FoxO3a and p53. Bid expression and p53 activity are related to the ongoing rearrangement of the TCR loci and induced DNA damage responses. Bim expression and FoxO3a nuclear translocation are directly controlled by the pre-TCR by means of its downstream kinase Akt/PKB. Interestingly, deletion of either gene on a pre-TCR(-/-) background rescues survival, but fails to induce further progenitor differentiation uncoupling the 2 processes.

Wip1 Phosphatase-Deficient Mice Exhibit Defective T Cell Maturation Due To Sustained p53 Activation

The PP2C phosphatase Wip1 dephosphorylates p38 and blocks UV-induced p53 activation in cultured human cells. Although the level of TCR-induced p38 MAPK activity is initially comparable between Wip1-/- and wild-type thymocytes, phosphatase-deficient cells failed to down-regulate p38 MAPK activity after 6 h. Analysis of young Wip1-deficient mice showed that they had fewer splenic T cells. Their thymi were smaller, contained significantly fewer cells, and failed to undergo age-dependent involution compared with wild-type animals. Analysis of thymocyte subset numbers by flow cytometry suggested that cell numbers starting at the double-negative (DN)4 stage are significantly reduced in Wip1-deficient mice, and p53 activity is elevated in cell-sorted DN4 and double-positive subpopulations. Although apoptosis and proliferation was normal in Wip1-/- DN4 cells, they appeared to be in cell cycle arrest. In contrast, a significantly higher percentage of apoptotic cells were found in the double-positive population, and down-regulation of thymocyte p38 MAPK activation by anti-CD3 was delayed. To examine the role of p38 MAPK in early thymic subpopulations, fetal thymic organ cultures cultured in the presence/absence of a p38 MAPK inhibitor did not correct the thymic phenotype. In contrast, the abnormal thymic phenotype of Wip1-deficient mice was reversed in the absence of p53. These data suggest that Wip1 down-regulates p53 activation in the thymus and is required for normal alphabeta T cell development.

The BCL2A1 gene as a pre-T cell receptor-induced regulator of thymocyte survival

The pre–T cell receptor (TCR) is expressed early during T cell development and imposes a tight selection for differentiating T cell progenitors. Pre-TCR–expressing cells are selected to survive and differentiate further, whereas pre-TCR⁻ cells are “negatively” selected to die. The mechanisms of pre-TCR–mediated survival are poorly understood. Here, we describe the induction of the antiapoptotic gene BCL2A1 (A1) as a potential mechanism regulating inhibition of pre–T cell death. We characterize in detail the signaling pathway involved in A1 induction and show that A1 expression can induce pre–T cell survival by inhibiting activation of caspase-3. Moreover, we show that in vitro “knockdown” of A1 expression can compromise survival even in the presence of a functional pre-TCR. Finally, we suggest that pre-TCR–induced A1 overexpression can contribute to T cell leukemia in both mice and humans.

p53 prevents maturation of T cell development to the immature CD4-CD8+ stage in Bcl11b-/- mice

Signaling pathways such as the pre-TCR and Wnt pathways regulate alpha/beta T cell differentiation in thymus. Mice lacking an essential component of the pre-TCR exhibit arrest at the (CD4(-)CD8(-)) (CD44(-)CD25(+)) stage (DN3) of thymocyte development, and introduction of p53 deficiency into those mice abrogates this arrest, resulting in transition to the (CD4(+)CD8(+)) double-positive (DP) stage. This paper examines the effect of inactivation of p53 on thymocyte development in Bcl11b(-/-) mice that exhibit arrest at the DN3 or (CD4(-)CD8(+)) immature single-positive (ISP) stage. No DP thymocytes were detected in thymocytes of adoptive transfer experiments in scid mice that were derived from p53(-/-)Bcl11b(-/-) precursors but ISP thymocytes increased in the proportion and in the cell number approximately three times higher than those from Bcl11b(-/-) precursors. Consistently, the level of apoptosis decreased to the level of wild-type precursors. These results suggest that inactivation of p53 is sufficient for DN3 thymocytes to differentiate into the ISP, but not to DP, stage of thymocyte development in Bcl11b(-/-) mice. This provides evidence for a novel p53-mediated checkpoint that regulates the transition from the DN3 to ISP stage of thymocyte development.

MAPK p38 alpha is dispensable for lymphocyte development and proliferation

Signals mediated by the p38alpha MAPK have been implicated in many processes required for the development and effector functions of innate and adaptive immune responses. As mice deficient in p38alpha exhibit embryonic lethality, most analyses of p38alpha function in lymphocytes have relied on the use of pharmacologic inhibitors and dominant-negative or constitutively active transgenes. In this study, we have generated a panel of low passage p38alpha(+/+), p38alpha(+/-), and p38alpha(-/-) embryonic stem (ES) cells through the intercrossing of p38alpha(+/-) mice. These ES cells were used to generate chimeric mice by RAG-deficient blastocyst complementation, with the lymphocytes in these mice being derived entirely from the ES cells. Surprisingly, B and T cell development were indistinguishable when comparing chimeric mice generated with p38alpha(+/+), p38alpha(+/-), and p38alpha(-/-) ES cell lines. Moreover, proliferation of p38alpha(-/-) B and T cells in response to Ag receptor and non-Ag receptor stimuli was intact. Thus, p38alpha is not an essential component of signaling pathways required for robust B and T lymphocyte developmental, nor is p38alpha essential for the proliferation of mature B and T cells.

Fas ligand is enriched in the caveolae membrane domains of thymic epithelial cells

Both Fas and Fas ligand (FasL) are expressed in the thymus. Although reports suggest that they are important throughout the thymocyte maturation process their precise role remains elusive. The present paper characterizes the expression of FasL in the thymus and in the TEA3A1 and BT1B functional thymic epithelial cell (TEC) lines. FasL expression by thymus fractions, TEA3A1, and BT1B cells was detected by Northern blot analysis. In TEA3A1 cells, we discovered that FasL protein expression was localized to caveolae membrane domains. This restricted subcellular localization of FasL, together with reports describing the localization of the major histocompatibility complex proteins, the T cell receptor and Fas to caveolae membrane domains, may provide a mechanism for the deletion of thymocytes during negative selection. Finally, using semi-quantitative RT-PCR we found that FasL expression by TECs is regulated by glucocorticoids.

NF-kappa B controls cell fate specification, survival, and molecular differentiation of immunoregulatory natural T lymphocytes

Ontogenetic, homeostatic, and functional deficiencies within immunoregulatory natural T (iNKT) lymphocytes underlie various inflammatory immune disorders including autoimmunity. Signaling events that control cell fate specification and molecular differentiation of iNKT cells are only partly understood. Here we demonstrate that these processes within iNKT cells require classical NF-kappaB signaling. Inhibition of NF-kappaB signaling blocks iNKT cell ontogeny at an immature stage and reveals an apparent, novel precursor in which negative selection occurs. Most importantly, this block occurs due to a lack of survival signals, as Bcl-x(L) overexpression rescues iNKT cell ontogeny. Maturation of immature iNKT cell precursors induces Bcl-2 expression, which is defective in the absence of NF-kappaB signaling. Bcl-x(L) overexpression also rescues this maturation-induced Bcl-2 expression. Thus, antiapoptotic signals relayed by NF-kappaB critically control cell fate specification and molecular differentiation of iNKT cells and, hence, reveal a novel role for such signals within the immune system.