Gene transcription in differentiating immature T cell receptor(neg) thymocytes resembles antigen-activated mature T cells

A Transgenic Dual-Luciferase Reporter Mouse for Longitudinal and Functional Monitoring of T Cells In Vivo

Adoptive T-cell therapy (ATT) efficacy is limited when targeting large solid tumors. The evaluation of ATT outcomes using accessory treatment would greatly benefit from an in vivo monitoring tool, allowing the detection of functional parameters of transferred T cells. Here, we generated transgenic bioluminescence imaging of T cells (BLITC) mice expressing an NFAT-dependent click-beetle luciferase and a constitutive Renilla luciferase, which supports concomitant in vivo analysis of migration and activation of T cells. Rapid transferability of our system to preestablished tumor models was demonstrated in the SV40-large T antigen model via both crossbreeding of BLITC mice into a T-cell receptor (TCR)-transgenic background and TCR transduction of BLITC T cells. We observed rapid tumor infiltration of BLITC CD8⁺ T cells followed by a burst-like activation that mirrored rejection kinetics. Using the BLITC reporter in the clinically relevant H-Y model, we performed female to male transfers and detected H-Y-specific alloreactivity (graft-versus-host disease) in vivo In an H-Y solid tumor model, we found migration of adoptively transferred H-Y TCR-transgenic CD4⁺ T cells into the tumor, marked by transient activation. This suggests a rapid inactivation of infiltrating T cells by the tumor microenvironment, as confirmed by their expression of inhibitory receptors. In summary, the BLITC reporter system facilitates analysis of therapeutic parameters for ATT, is rapidly transferable to models of interest not restricted to tumor research, and is suitable for rapid screening of TCR clones for tumor rejection kinetics, as well as off-target effects. Cancer Immunol Res; 6(1); 110-20. ©2018 AACR.

At the crossroads: diverse roles of early thymocyte transcriptional regulators

Transcriptional regulation of T-cell development involves successive interactions between complexes of transcriptional regulators and their binding sites within the regulatory regions of each gene. The regulatory modules that control expression of T-lineage genes frequently include binding sites for a core set of regulators that set the T-cell-specific background for signal-dependent control, including GATA-3, Notch/CSL, c-myb, TCF-1, Ikaros, HEB/E2A, Ets, and Runx factors. Additional regulators in early thymocytes include PU.1, Id-2, SCL, Spi-B, Erg, Gfi-1, and Gli. Many of these factors are involved in simultaneous regulation of non-T-lineage genes, T-lineage genes, and genes involved in cell cycle control, apoptosis, or survival. Potential and known interactions between early thymic transcription factors such as GATA-3, SCL, PU.1, Erg, and Spi-B are explored. Regulatory modules involved in the expression of several critical T-lineage genes are described, and models are presented for shifting occupancy of the DNA-binding sites in the regulatory modules of pre-Talpha, T-cell receptor beta (TCRbeta), recombinase activating genes 1 and 2 (Rag-1/2), and CD4 during T-cell development. Finally, evidence is presented that c-kit, Erg, Hes-1, and HEBAlt are expressed differently in Rag-2(-/-) thymocytes versus normal early thymocytes, which provide insight into potential regulatory interactions that occur during normal T-cell development.

T cell activation: in vivo veritas

Phenotypic changes in CD4(+) T cells undergoing antigen-dependent activation were compared in vivo and in vitro. The most obvious difference was in expression of CD25, the alpha chain of the high affinity receptor for IL-2. High level expression of CD25 in vivo is restricted to a small fraction of the cells at the leading edge of the cell division profile, whereas all activated cells express high levels of CD25 in cultures responding to antigen. Because IL-2 is known to upregulate expression of CD25 in preactivated T cells, this suggests a difference in IL-2 exposure in the two responses. A number of other markers, including CD54, show a similar difference in the pattern of expression in vivo and in vitro. Using 6-colour flow cytometry, it was demonstrated that the small percentage of cells expressing CD25 in vivo coexpresses a very high level of a number of other activation markers, including CD38, CD44 and Ly-6A/E, suggesting that these may also be upregulated by autocrine IL-2.

Regulation of thymocyte differentiation: pre-TCR signals and beta-selection

The specificity of the adaptive immune response is, in part, dependent on the clonal expression of the mature T cell receptor (TCR) on T lymphocytes. One mechanism regulating the clonality of the TCR occurs at the level of TCR-beta gene rearrangements during lymphocyte development. Expression of a nascent TCR-beta chain together with pre-Talpha (pTalpha) and CD3 molecules to form the pre-TCR complex, represents a critical checkpoint in T cell differentiation known as beta-selection. Indeed, failure to generate a functionally rearranged TCR-beta chain at this stage of development results in apoptosis. Signals derived from the pre-TCR complex trigger a maturation program within developing thymocytes that includes: rescue from apoptosis; inhibition of further DNA recombination at the TCR-beta gene locus (allowing for the clonality of antigen receptor expression; allelic exclusion); and induction of proliferation and differentiation. The signaling mechanisms that control this developmental program remain largely undefined. Here, we discuss recent evidence investigating the molecular mechanisms that regulate thymocyte differentiation downstream of pre-TCR formation.

Helix-loop-helix proteins regulate pre-TCR and TCR signaling through modulation of Rel/NF-kappaB activities

E2A and HEB are basic helix-loop-helix transcription factors essential for T cell development. Complete inhibition of their activities through transgenic overexpression of their inhibitors Id1 and Tal1 leads to a dramatic loss of thymocytes. Here, we suggest that bHLH proteins play important roles in establishing thresholds for pre-TCR and TCR signaling. Inhibition of their function allows double-negative cells to differentiate without a functional pre-TCR, while anti-CD3 stimulation downregulates bHLH activities. We also find that the transcription factor NF-kappaB becomes activated in transgenic thymocytes. Further activation of NF-kappaB exacerbates the loss of thymocytes, whereas inhibition of NF-kappaB leads to the rescue of double-positive thymocytes. Therefore, we propose that E2A and HEB negatively regulate pre-TCR and TCR signaling and their removal causes hyperactivation and apoptosis of thymocytes.

Immune-specific up-regulation of adseverin gene expression by 2,3,7,8-tetrachlorodibenzo-p-dioxin

To identify genes that are regulated by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and possibly involved in TCDD-induced immunotoxicity, we used the differential display technique to screen for differentially expressed genes in the mouse thymus. Here we show that TCDD increased the expression of adseverin, a Ca(2+)-dependent, actin-severing protein. The induction of adseverin is dose- and time-dependent in parallel with the induction of CYP1A1, which is currently the most frequently used marker for TCDD exposure. A comparison between mouse strains with different TCDD responsiveness indicated that the induction of adseverin is dependent on the aryl hydrocarbon receptor, a transcription factor known to mediate most of TCDD's biological effects. Examination of additional organs revealed that the up-regulation of the adseverin gene expression is immune-specific. Using an anti-adseverin antibody, we confirmed the induction of adseverin by TCDD at the protein level and it was confined to the thymic cortex, which harbors immature thymocytes that are known target cells of TCDD. Considering adseverin's role in actin cytoskeletal reorganization, our observations reveal new mechanistic aspects of how TCDD might exert some of its immunotoxic effects.

Allelic exclusion and differentiation by protein kinase C-mediated signals in immature thymocytes

Pre-T cell receptor (preTCR)-derived signals mediate the transition of thymocytes from the CD4(-) CD8(-) double-negative (DN) to CD4(+) CD8(+) double-positive stage of T lymphocyte development. This progression, termed beta-selection, is limited to thymocytes that have generated a functional TCR-beta chain able to associate with pTalpha to form the preTCR complex. Formation of the preTCR complex not only induces differentiation, survival, and proliferation of DN thymocytes; it also inhibits further TCR-beta gene rearrangement through an ill-defined process known as allelic exclusion. The signaling pathways controlling this critical developmental checkpoint have not been characterized. Here we demonstrate that formation of the preTCR complex leads to the activation of protein kinase C (PKC), and that activation of PKC is necessary for the differentiation and expansion of DN thymocytes. Importantly, we also show that allelic exclusion at the TCR-beta gene loci is enforced by PKC-mediated signals. These results define PKC as a central mediator of both differentiation and allelic exclusion during thymocyte development.

Allelic exclusion and differentiation by protein kinase C-mediated signals in immature thymocytes

Pre-T cell receptor (preTCR)-derived signals mediate the transition of thymocytes from the CD4(-) CD8(-) double-negative (DN) to CD4(+) CD8(+) double-positive stage of T lymphocyte development. This progression, termed beta-selection, is limited to thymocytes that have generated a functional TCR-beta chain able to associate with pTalpha to form the preTCR complex. Formation of the preTCR complex not only induces differentiation, survival, and proliferation of DN thymocytes; it also inhibits further TCR-beta gene rearrangement through an ill-defined process known as allelic exclusion. The signaling pathways controlling this critical developmental checkpoint have not been characterized. Here we demonstrate that formation of the preTCR complex leads to the activation of protein kinase C (PKC), and that activation of PKC is necessary for the differentiation and expansion of DN thymocytes. Importantly, we also show that allelic exclusion at the TCR-beta gene loci is enforced by PKC-mediated signals. These results define PKC as a central mediator of both differentiation and allelic exclusion during thymocyte development.

NF-kappa B activation by the pre-T cell receptor serves as a selective survival signal in T lymphocyte development

Activation of the transcription factor NF-kappa B and pre-T cell receptor (pre-TCR) expression is tightly correlated during thymocyte development. Inhibition of NF-kappa B in isolated thymocytes in vitro results in spontaneous apoptosis of cells expressing the pre-TCR, whereas inhibition of NF-kappa B in transgenic mice through expression of a mutated, superrepressor form of I kappa B alpha leads to a loss of beta-selected thymocytes. In contrast, the forced activation of NF-kappa B through expression of a dominant-active I kappa B kinase allows differentiation to proceed to the CD4(+)CD8(+) stage in a Rag1(-/-) mouse that cannot assemble the pre-TCR. Therefore, signals emanating from the pre-TCR are mediated at least in part by NF-kappa B, which provides a selective survival signal for developing thymocytes with productive beta chain rearrangements.

Constitutive activation of NF-kappaB and T-cell leukemia/lymphoma in Notch3 transgenic mice

The multiplicity of Notch receptors raises the question of the contribution of specific isoforms to T-cell development. Notch3 is expressed in CD4(-)8(-) thymocytes and is down-regulated across the CD4(-)8(-) to CD4(+)8(+) transition, controlled by pre-T-cell receptor signaling. To determine the effects of Notch3 on thymocyte development, transgenic mice were generated, expressing lck promoter-driven intracellular Notch3. Thymuses of young transgenics showed an increased number of thymocytes, particularly late CD4(-)8(-) cells, a failure to down-regulate CD25 in post-CD4(-)8(-) subsets and sustained activity of NF-kappaB. Subsequently, aggressive multicentric T-cell lymphomas developed with high penetrance. Tumors sustained characteristics of immature thymocytes, including expression of CD25, pTalpha and activated NF-kappaB via IKKalpha-dependent degradation of IkappaBalpha and enhancement of NF-kappaB-dependent anti-apoptotic and proliferative pathways. Together, these data identify activated Notch3 as a link between signals leading to NF-kappaB activation and T-cell tumorigenesis. The phenotypes of pre-malignant thymocytes and of lymphomas indicate a novel and particular role for Notch3 in co-ordinating growth and differentiation of thymocytes, across the pre-T/T cell transition, consistent with the normal expression pattern of Notch3.

Extracellular signal-regulated kinase (ERK) activation by the pre-T cell receptor in developing thymocytes in vivo

The first checkpoint in T cell development occurs between the CD4(-)CD8(-) and CD4(+)CD8(+) stages and is associated with formation of the pre-T cell receptor (TCR). The signaling mechanisms that drive this progression remain largely unknown. Here, we show that extracellular signal-regulated kinases (ERKs)-1/2 are activated upon engagement of the pre-TCR. Using a novel experimental system, we demonstrate that expression of the pre-TCR by developing thymocytes induces ERK-1/2 activation within the thymus. In addition, the activation of this pre-TCR signaling cascade is mediated through Lck. These findings directly link pre-TCR complex formation with specific downstream signaling components in vivo.

A dominant-negative mutant of c-Jun inhibits cell cycle progression during the transition of CD4(-)CD8(-) to CD4(+)CD8(+) thymocytes

While Jun/Fos-containing transcription factors are known to be necessary for many TCR-mediated events in mature T cells, relatively little is known about their roles in thymocyte development. We have generated transgenic mice that express a trans-dominant-negative mutant of c-Jun (TAM-67) specifically in thymocytes. Expression of TAM-67 inhibited the up-regulation of AP-1-responsive genes such as c-jun and IL-2 in stimulated thymocytes from transgenic mice. In addition, altered thymocyte development in TAM-67-expressing mice was revealed by a decrease in thymic cellularity ( approximately 50%) which could be accounted for primarily by a reduction in the number of CD4(+)CD8(+) thymocytes, a large percentage of which retained CD25. The decrease in the number of CD4(+)CD8(+) thymocytes did not appear to be due to an enhanced rate of apoptosis but rather to a decrease in the number of CD4(-)CD8(-)CD25(-) cells in the S + G(2)/M stages of the cell cycle. These results indicate that Jun/Fos-containing transcription factors promote the proliferative burst that accompanies the transition from the CD4(-)CD8(-) to the CD4(+)CD8(+) stage of thymocyte development.

Expression of heat shock protein 70 blocks thymic differentiation of T cells in transgenic mice

Heat shock protein 70 (HSP70) is involved not only in protein folding, but also in processes of differentiation and cell-cycle progression. Recently, HSP70 has been implicated in mediation of functions of some immunosuppressive agents. To study the role of HSP70 in differentiation of haematopoietic cells, we generated transgenic mice using the human inducible hsp70 gene fused to the mouse H-2K promoter. These mice develop a T-cell deficiency that is characterized by thymic hypoplasia and a significant reduction in peripheral T cells. The total number of thymocytes is about 100-fold less than that in normal mice. The majority of the thymocytes are immature T cells that express neither CD4 nor CD8 molecules, indicating that T cells are affected at an early stage of thymic differentiation. Expression of the transgenic HSP70 was detected both in bone marrow cells and in thymocytes. Furthermore, injection of normal bone marrow cells into the T-cell deficient mice led to the generation of mature T cells indicating that the T-cell deficiency was caused by the action of HSP70 in T cells. The blockage of differentiation occurred only in T cells, both alphabeta- and gammadelta-T-cell receptor (TCR)-bearing cells, but not in B cells, granulocytes, and monocytes. The observations suggest that HSP70 may inhibit a cellular process that is essential for the differentiation of early stage T cells. Further experiments using this model system will widen our understanding of HSP70 and its function on a molecular level.

Extrathymic T Cell Differentiation in the Human Intestine Early in Life

It is clear from experimental studies in mice that T cell maturation can occur outside the thymus, especially in the intestine. There is little sound evidence so far that extrathymic T cell maturation occurs to any significant extent in human gut, and, postnatally, there is abundant evidence that the gut mucosa is an immune effector organ. Here, we describe a large population of T lymphocytes in human fetal intestinal mucosa that are proliferating (Ki67+) in the absence of foreign Ag (CD3+, Ki67+ lamina propria lymphocytes (LPL) 22 ± 1.8% and CD3+, Ki67+ intraepithelial lymphocytes (IEL) 9.1 ± 1.4%), that express the T cell activation markers CD103, HLA-DR, and L-selectinlow, and that express mRNA transcripts for pre-TCR-α. There is also a substantial proportion of CD7+ LPLs that do not express CD3 (CD3−7+, 14 ± 7% of all LPLs) in the fetal gut that may be differentiating into CD3+ cells. Rearranged TCR-β transcripts of fetal LPLs, IELs, and paired blood lymphocytes were cloned and sequenced, and virtually no overlap of clonality was observed between blood and intestine, suggesting that gut T cells may not be derived from the blood. In addition, 30 days after engraftment of SCID mice with fetal intestine, CD3−7+ cells, proliferating T cells, and pre-TCR-α transcripts were abundant, and there is a threefold increase in CD3+ IELs. These data show that in the human intestine before birth a population of precursor T cells exists that may be differentiating into mature T cells in situ

Critical Involvement of Tcf-1 in Expansion of Thymocytes

T cell maturation in Tcf-1−/− mice deteriorates progressively and halts completely around 6 mo of age. During fetal development thymocyte subpopulations seem normal, although total cell numbers are lower. By 4 to 6 wk of age, obvious blockades in the differentiation of CD4−8− thymocytes are observed at two distinct stages (CD44+25+ and CD44−25−), both of which are normally characterized by extensive proliferation. This lack of thymocyte expansion and/or differentiation was also observed when Tcf-1−/− progenitor cells from the aorta-gonad-mesonephros region (embryonic day 11.5), fetal liver (embryonic day 12.5/14.5), and fetal bone marrow (embryonic day 18.5) were allowed to differentiate in normal thymic lobes (fetal thymic organ cultures) or were injected intrathymically into normal recipients. Despite these apparent defects in thymocyte differentiation and expansion, adult Tcf-1−/− mice are immunocompetent, as they generate virus neutralizing Abs at normal titers. Furthermore, their peripheral T cells have an activated phenotype (increased CD44 and decreased CD62L expression) and proliferate normally in response to Ag or mitogen, suggesting that these cells may have arisen from the early wave of development during embryogenesis and are either long lived or have subsequently been maintained by peripheral expansion. As Tcf-1 is a critical component in the Wnt/β-catenin signaling pathway, these data suggest that Wnt-like factors play a role in the expansion of double-negative thymocytes.

Lineage commitment and differentiation of T and natural killer lymphocytes in the fetal mouse

T cells and natural killer (NK) cells are presumed to share a common intrathymic precursor. The development of conventional alpha beta T lymphocytes begins within the early fetal thymus, after the colonization of multipotent CD117+ precursors. Irrevocable commitment to the T lineage is marked by thymus-induced expression of CD25. However, the contribution of the fetal thymus to NK lineage commitment and differentiation remains largely unappreciated. Recently, we demonstrated that the development of functional mouse NK cells occurs first in the fetal thymus. Moreover, the appearance of mature fetal thymic NK cells (NK1.1+/CD117-) is preceded by a thymus-induced developmental stage (NK1.1+/CD117+) that marks lineage commitment of multipotent hematopoietic precursors to the T and NK-cell fates. Commitment to the T/NK bipotent stage is induced by fetal thymic stroma, but is not thymus dependent. Recent data indicate that CD90+/CD117lo fetal blood prothymocytes exhibit NK lineage potential and are phenotypically and functionally identical to fetal thymic NK1.1+/CD117+ progenitors. This finding also indicates that full commitment of circulating precursors to the T-cell lineage occurs after thymus colonization. In this review, we discuss recent insights into the cellular and molecular events involved in fetal mouse T and NK lineage commitment and differentiation to unipotent progenitors.

Use of explant technology in the study of in vitro immune responses

The establishment of in vitro culture systems provides an accessible means to study events within the immune system. In contrast to either dispersed suspension or two-dimensional monolayer culture, the explantation of tissue fragments under organ culture conditions is, to date, the only method which allows essential three-dimensional cellular interactions to be maintained under conditions which permit controlled experimental manipulation in vitro. Recent modifications of explant technology, particularly within the area of fetal thymic organ culture, now allow the controlled reassociation of defined cellular subsets and manipulation of gene expression, under conditions where the functioning of both lymphoid and stromal cell types closely resembles that in vivo.

Dipeptidyl Peptidase I and Granzyme A Are Coordinately Expressed During CD8+ T Cell Development and Differentiation

Dipeptidyl peptidase I (DPPI) is a granule protease that plays a requisite role in processing the proenzyme form of the CTL granule serine proteases (granzymes). This study assesses DPPI mRNA and enzyme expression during T lymphocyte ontogeny and CTL differentiation. The most immature CD3−CD4−CD8− thymocytes were found to express >40-fold higher levels of DPPI mRNA, although levels of DPPI enzymatic activity in CD3−CD4−CD8− thymocytes were only modestly higher than those seen for CD4+CD8+ or CD4+CD8− thymocytes. More mature CD8+CD4− thymocytes and CD8+ splenocytes expressed significantly higher levels of DPPI mRNA and enzymatic activity than CD4+CD8+ or CD4+CD8− thymocytes. Granzyme A mRNA expression was observed in DPPI expressing CD3−CD4−CD8− and CD8+CD4− thymocytes and was also observed in CD8+CD4− splenocytes; however, expression was not observed in CD4+CD8+ or CD4+CD8− thymocytes. Both DPPI mRNA and granzyme A mRNA expression in CD8+ T cells decreased to very low or undetectable levels during the first 48 h after allostimulation in MLCs. However, peak levels of both DPPI and granzyme A expression were observed later in the course of CD8+ T cell responses to alloantigen, with DPPI mRNA expression peaking on either day 3 or day 4 and granzyme A expression peaking at the end of a 5-day MLR. These data indicate that DPPI is expressed at all stages of T cell ontogeny and differentiation in which granzyme A mRNA is detected; consequently, DPPI appears to be available for the processing and activation of granzyme A during both CD8+ T cell development and differentiation.

Identification of a novel developmental stage marking lineage commitment of progenitor thymocytes

Bipotent progenitors for T and natural killer (NK) lymphocytes are thought to exist among early precursor thymocytes. The identification and functional properties of such a progenitor population remain undefined. We report the identification of a novel developmental stage during fetal thymic ontogeny that delineates a population of T/NK-committed progenitors (NK1. 1(+)/CD117(+)/CD44(+)/CD25(-)). Thymocytes at this stage in development are phenotypically and functionally distinguishable from the pool of multipotent lymphoid-restricted (B, T, and NK) precursor thymocytes. Exposure of multipotent precursor thymocytes or fetal liver- derived hematopoietic progenitors to thymic stroma induces differentiation to the bipotent developmental stage. Continued exposure to a thymic microenvironment results in predominant commitment to the T cell lineage, whereas coculture with a bone marrow-derived stromal cell line results in the generation of mature NK cells. Thus, the restriction point to T and NK lymphocyte destinies from a multipotent progenitor stage is marked by a thymus-induced differentiation step.

Defective survival and activation of thymocytes in transgenic mice expressing a catalytically inactive form of Ca2+/calmodulin-dependent protein kinase IV

We have generated transgenic mice that express a catalytically inactive form of Ca2+/calmodulin-dependent protein kinase IV (CaMKIV) specifically in thymic T cells. The presence of this protein results in a markedly reduced thymic cellularity, although the distribution of the remaining cells is normal based on evaluation of the CD4 and CD8 cell surface antigens that are used to gauge T cell development. Isolated thymic T cells from the transgenic mice also show a dramatically decreased survival rate when evaluated in culture under conditions that do not favor activation. When challenged with an activating stimulus such as alpha-CD3 or a combination of phorbol ester plus ionophore, the cells are severely compromised in their ability to produce the cytokine interleukin-2 (IL-2). Reduction of IL-2 production is secondary to the inability to phosphorylate the cAMP response element binding protein, CREB, and induce expression of the immediate early genes such as Fos B that are required to transactivate the IL-2 promoter. Because transgene expression was regulated by the proximal promoter of the murine lck gene and this promoter is inactivated in T cells that exit the thymus, the mutant hCaMKIV is not present in peripheral T cells. Consequently, T lymphocytes present in the spleen can be activated normally in response to either stimulus mentioned above, demonstrating that the effects of the inactive CaMKIV on activation are reversible. Our results suggest that CaMKIV may represent a physiologically relevant CREB kinase in T cells and that the enzyme is also required to ensure normal expansion of T cells in the thymus. Whereas the pathway responsible for this latter role is yet to be elucidated, it is unlikely to include CREB phosphorylation.

T cell receptor (TCR)-beta gene recombination: dissociation from cell cycle regulation and developmental progression during T cell ontogeny

T cell lymphopoiesis involves extensive cell division and differentiation; these must be balanced by export and programmed cell death to maintain thymic homeostasis. Details regarding the nature of these processes, as well as their relationships to each other and to the definitive process of T cell receptor (TCR) gene recombination, are presently emerging. Two widely held concepts are that cell cycle status is inherently and inversely linked to gene recombination and that the outcomes of gene recombination regulate developmental progression. In this study, we analyze TCR-beta recombination and cell cycle status with respect to differentiation during early T cell ontogeny. We find that although differentiation, cell cycle fluctuations, and gene recombination are coincident during normal T cell development, differentiation and cell cycle status are not inherently linked to the recombination process or its products. Rather, recombination appears to occur in parallel with these events as part of a genetically patterned program of development. We propose that the outcome of gene recombination (i.e., TCR expression) may not influence developmental progression per se, but instead serves to perpetuate those developing cells that have been successful in recombination. The potential consequences of this model for the regulation of thymic lymphopoiesis and programmed cell death are discussed.

Immunosuppressive phenotype of corticotropin-releasing factor transgenic mice is reversed by adrenalectomy

Stress elicits a wide range of physiological changes involving the nervous, endocrine, and immune systems. Corticotropin-releasing factor (CRF) plays a key role in orchestrating this response, activating both the sympathetic nervous system and the hypothalamic-pituitary-adrenal axis, resulting in release of corticosteroids. The present study examines the immunological phenotype and responsiveness of CRF-transgenic (CRF-Tg) mice. The immune system of the CRF-Tg animals has profound changes compared to littermate controls, including a marked reduction in both cell number and immune responsiveness. There were also phenotypic changes in the lymphocytic composition of the various lymphoid organs, most notably in the spleen, where CRF-Tg mice had a greater percentage of T lymphocytes compared to littermate controls. Adrenalectomy of CRF-Tg reversed the immunological phenotype observed and restored immune responsiveness. These results demonstrate that CRF overexpression leads to profound impairment on lymphocyte development and function mediated via corticosteroids.

In vivo occupancy of the kappa light chain enhancers in primary pro- and pre-B cells: a model for kappa locus activation

The immunoglobulin kappa light chain locus has two enhancer elements: the intronic enhancer, which lies between the Jkappa cluster and the Ckappa exon, and the 3'kappa enhancer, which is located downstream of Ckappa. To address the contribution of these elements to the developmentally regulated activation of germline kappa locus transcription and rearrangement, we purified primary pro- and pre-B cells and determined by in vivo footprinting the sites within each enhancer that were occupied. We found that the kappa intronic enhancer NF-kappaB site is occupied in both pro- and pre-B cells, while CRE, BSAP, and PU.1/pip sites within the 3'kappa enhancer undergo changes in occupancy as cells progress from the pro-B to the pre-B cell stage. These findings suggest that regulation of the kappa locus in primary pre-B cells may be mediated by factors that bind the 3'kappa enhancer.

Delayed maturation of CD4- CD8- Fc gamma RII/III+ T and natural killer cell precursors in Fc epsilon RI gamma transgenic mice

Fc epsilon RI gamma (gamma) is a member of a group of related proteins (the zeta-family dimers) that function as signal-transducing components of both Fc receptors and the T cell antigen receptor (TCR). Analysis of gamma expression during fetal thymus ontogeny revealed that it is expressed in early thymocytes, before the initiation of clonotypic TCR-alpha and TCR-beta gene rearrangement but is down-regulated in most adult thymocytes. To explore a possible role for gamma in thymocyte development, we generated transgenic mice in which this protein was overexpressed at all stages of ontogeny. Overexpression of gamma inhibited the maturation of T cells as well as natural killer (NK) cells. The developmental effects were transgene dose related and correlated with markedly delayed maturation of fetal CD4-CD8- FcRII/III+ thymocytes, cells thought to include the progenitors of both T and NK cells. These results suggest that the zeta and gamma chains serve distinctive functions in thymocyte development and indicate that Fc receptor(s) may play an important role in regulating the differentiation of early progenitor cells within the thymus.

p53 prevents maturation to the CD4+CD8+ stage of thymocyte differentiation in the absence of T cell receptor rearrangement

Rearrangement of the immunoglobulin (Ig) and T cell receptor (TCR) gene loci allows for the generation of B and T lymphocytes with antigen-specific receptors. Complete rearrangement and expression of the TCR-beta chain enables immature thymocytes to differentiate from the CD4-CD8- to the CD4+CD8+ stage mice in which rearrangement is impaired, such as severe combined immunodeficient (SCID) mice or recombinase activating gene-deficient (RAG-/-) mice, lack mature B and T lymphocytes. Thymocytes from these mice are arrested at the CD4-CD8- stage of T cell development. We previously observed that thymocytes from RAG-2-/- mice exposed to gamma radiation differentiate from CD4-CD8- into CD4+CD8+ without TCR-beta chain rearrangement. We now report that irradiated RAG-2-/- thymocytes undergo direct somatic mutations at the p53 gene locus, and that p53 inactivation is associated with maturation of RAG2-/- thymocytes to the CD4+CD8+ stage. Generation of RAG2-/- and p53-/- double-deficient mice revealed that, in the absence of TCR-beta chain rearrangement, loss of p53 function is sufficient for CD4-CD8- thymocytes to differentiate into the CD4+CD8+ stage of T cell development. Our data provide evidence for a novel p53 mediated checkpoint in early thymocyte development that regulates the transition of CD4-CD8- into CD4+CD8+ thymocytes.

Regulation of thymocyte development from immature progenitors

T lymphocytes differentiate from hematopoietic stem cells that settle in the microenvironment of the thymus. The earliest stages of mouse alpha/beta T-cell differentiation occurring before surface expression of the TCR include three important events: proliferation, commitment to the T lineage, and rearrangement and expression of the TCR loci. Recent evidence suggests that the survival as well as differentiation of early thymocytes depends critically on molecular signals such as those generated by the recently described pre-TCR complex.

Regulation of AP-1 and NFAT transcription factors during thymic selection of T cells

The ability of thymocytes to express cytokine genes changes during the different stages of thymic development. Although CD4- CD8- thymocytes are able to produce a wide spectrum of cytokines in response to a T-cell receptor (TcR)-independent stimulus, as they approach the double-positive (DP) CD4+ CD8+ stage, they lose the ability to produce cytokine. After the DP stage, thymocytes become single-positive CD4+ or CD8+ thymocytes which reacquire the ability to secrete cytokines. In an attempt to understand the molecular basis of this specific regulatin, we use AP-1-luciferase and newly generated NFAT-luciferase transgenic mice to analyze the transcriptional and DNA-binding activities of these two transcription factors that are involved in the regulation of cytokine gene expression. Here, we show that both AP-1 and NFAT transcriptional activities are not inducible in the majority of DP cells but that during the differentiation of DP cells to the mature single-positive stage, thymocytes regain this inducibility. Subpopulation analysis demonstrates that this inducibility is reacquired at the DP stage before the down-modulation of one of the coreceptors. Indeed AP-1 inducibility, just like the ability to express the interleukin-2 gene, is reacquired during the differentiation of DP TcRlow CD69low heat-stable antigen (HSA)high thymocytes to DP TcRhigh CD69high HSAhigh cells, which is considered to be the consequence of the first signal that initiates positive selection. We therefore propose that the inability of DP thymocytes to induce AP-1 and NFAT activities is one of the causes for the lack of cytokine gene expression at this stage and that this inducibility is reacquired at the latest stage of DP differentiation as a consequence of positive selection. This could be a mechanism to prevent the activation of DP thymocytes before selection has taken place.

Insights into T cell development and signal transduction provided by TCR-zeta chain deficient mice

The T cell antigen receptor (TCR) transduces signals that mediate different responses depending on the stage of development of the T cell and the nature of the ligand it engages. The presence of multiple signal transducing subunits (CD3-gamma-delta,-epsilon and zeta chain) suggests the potential to control these responses by altering the subunit composition of the TCR. zeta chain represents an especially important signalling molecule as it contains multiple signalling motifs within its cytoplasmic tail. The generation and analysis of zeta deficient (zeta-/-) and zeta-transgenic mice has provided insight into the role of zeta as well as the CD3 subunits in TCR surface expression, T cell activation and thymocyte development. Herein, we discuss the results from such experiments which suggest distinct roles for zeta chain and the CD3 components at different stages of T cell development.

Identification of an ionomycin/cyclosporin A-responsive element within the human T cell receptor gamma enhancer

Activation through the Ca2+/calcineurin pathway is essential to the transcription of many cytokine genes. The conserved cis-acting sequence, GGAAAA, and transcription factors binding to this sequence are involved in the response to increased intracellular Ca2+ concentrations. Here we report the identification and importance of the same sequence in a non-cytokine gene, the human T cell receptor gamma (TCRG) enhancer. Results from site-directed mutations and electrophoretic mobility shift assays strongly suggest that this sequence mediates the ionomycin-induced activation of the TCRG enhancer. Our studies provide an explanation for a previous observation that TCRG mRNA levels, but not mRNA levels for T cell receptor alpha and -beta, are increased by ionomycin treatment.

Requirement for TNF-alpha and IL-1 alpha in fetal thymocyte commitment and differentiation

CD25 expression occurs early in thymocyte differentiation. The mechanism of induction of CD25 before T cell receptor rearrangement and the importance of this mechanism for T cell development are unknown. In a thymus reconstitution assay, tumor necrosis factor alpha (TNF-alpha) and interleukin-1 alpha (IL-1 alpha), two cytokines produced within the thymic microenvironment, induced CD25 expression on early immature thymocytes. Either TNF-alpha or IL-1 alpha was necessary for further thymocyte maturation and CD4+CD8+ differentiation. In irradiated mice reconstituted with CD117+CD25+ thymocytes, commitment to the T cell lineage was marked by the loss of precursor multipotency.

Dominant interfering Fas gene mutations impair apoptosis in a human autoimmune lymphoproliferative syndrome

Five unrelated children are described with a rare autoimmune lymphoproliferative syndrome (ALPS) characterized by massive nonmalignant lymphadenopathy, autoimmune phenomena, and expanded populations of TCR-CD3+CD4-CD8- lymphocytes. These findings, suggesting a genetic defect in the ability of T lymphocytes to respond to normal immunoregulatory mechanisms, prompted an evaluation of lymphocyte apoptosis. Each child had defective Fas-mediated T lymphocyte apoptosis associated with a unique, deleterious Fas gene mutation. One mutation appeared to cause a simple loss of function; however, four others had a dominant negative phenotype when coexpressed with normal Fas. Family studies demonstrated the inheritance of the mutant Fas alleles. The occurrence of Fas mutations together with abnormal T cell apoptosis in ALPS patients suggests an involvement of Fas in this recently recognized disorder of lymphocyte homeostasis and peripheral self-tolerance.

Hematopoietic lineage commitment: role of transcription factors

This review focuses on the roles of transcription factors in hematopoietic lineage commitment. A brief introduction to lineage commitment and asymmetric cell division is followed by a discussion of several methods used to identify transcription factors important in specifying hematopoietic cell types. Next is presented a discussion of the use of embryonic stem cells in the analysis of hematopoietic gene expression and the use of targeted gene disruption to analyze the role of transcription factors in hematopoiesis. Finally, the status of our current knowledge concerning the roles of transcription factors in the commitment to erythroid, myeloid and lymphoid cell types is summarized.

Transcription of granzyme A and B genes is differentially regulated during lymphoid ontogeny

During development, thymocytes express a number of genes typical for activated peripheral T lymphocytes, including granzymes. We have now analyzed by reverse transcription-polymerase chain reaction (RT-PCR), immunohistochemistry, and cytochemistry fetal liver cells and thymocytes at various developmental stages for the expression of granzyme A-G genes. At days 13-17 of gestation, only granzyme B but none of the other granzymes is expressed in fetal liver. In the most immature, Pgp-1+IL2R alpha-, thymocyte subpopulation mRNAs for granzymes A-C but not for granzymes D-G are detectable. Upon further differentiation via Pgp-1-IL-2R alpha + into more mature Pgp-1-IL-2R alpha- thymocytes the level of expression of granzymes A, B, and C gradually declines reaching its lowest level at the CD4+ 8+ double positive stage. In fetal thymic lobes depleted of lymphoid cells by treatment with deoxyguanosine, no transcripts for granzymes A, B, and C were found indicating that the PCR signals are derived exclusively from early precursor T/natural killer (NK) lineage cells rather than from residual stromal elements. In mature CD4+CD8- and CD4-CD8+ thymocytes, granzyme B mRNA is found at similar levels in both subsets whereas granzyme A mRNA is expressed selectively in the CD4-CD8+ subset. Enzymatic activity of granzyme A was only seen in a fraction of CD4-CD8+ thymocytes negative for heat stable antigen (HSA) but not in the more immature HSA+ fraction of CD4-CD8+ thymocytes. The data suggest that (a) granzyme B is a pro-thymocyte marker for all T/NK lineage cells; (b) granzyme A transcripts are associated with thymocytes with the potential to develop into the CD8+ lineage; and (c) granzyme A enzymatic activity is only expressed in the most mature CD4-CD8+ stage, suggesting that granzyme proteins are not involved in early stages of thymocyte development.

Nuclear factors that mediate intrathymic signals are developmentally regulated

Thymocytes mature through several stages of development, defined by cell surface markers such as CD3, CD4, and CD8, in response to environmental cues. Signal transduction resulting from lymphocyte-stromal cell interactions is likely to activate inducible transcription factors which in turn govern stage-specific gene expression. In this report we show that inducible transcription factors such as AP-1 and NF-AT are constitutively nuclear, in response to intrathymic signals, in freshly isolated thymocytes at all stages of maturation. In CD4+CD8+ double positive (DP), but not in the more immature CD4-CD8- double negative (DN) thymocytes, constant stimulus from the thymic environment is required to maintain nuclear AP-1. Thus, disruption of the thymus and incubation of thymocytes at 37 degrees C downregulates DNA binding by nuclear factors AP-1 and NF-AT. Similar treatment of thymocytes has previously been shown to downregulate CD3 zeta chain phosphorylation and increase T cell receptor CD3 expression on DP thymocytes, which is a feature of repertoire selection. Since mature T cells maintain inducible nuclear factors in an inactive form until an encounter with antigen, we propose that downregulation of nuclear DNA binding proteins may reflect another feature of this stage of T cell maturation.

Double-negative thymocyte subsets in CD3 zeta chain-deficient mice: absence of HSA+CD44-CD25- cells

Double-negative (DN) thymocyte subsets were examined in mice deficient in the CD3 zeta chain (zeta-/-). The HSA+CD44-CD25- subset was found to be missing, and DN thymocytes seemed to differentiate directly from HSA+CD25+CD44- cells to double-positive (DP) cells. When fetal thymic ontogeny was examined, we found a marked difference between zeta-/- embryos and heterozygous littermates from embryonic day 17.5, in terms of CD25, CD4 and CD8 expression, and thymus size. The zeta-/- thymocytes failed to down-regulate CD25 and to expand exponentially. The cell cycle status of adult thymocyte subsets indicated that although the HSA+CD25-CD44- subset was missing, the CD25+ DN population contained normal numbers of cycling cells, and the CD25+ DP cells (which were not detectable in normal mice) contained 5-10% cells in G2/M+S. Taken together these data suggest that the CD3 zeta chain might have a specific role in the control of proliferation of DN thymocytes during T cell development. Our data clearly show that one can dissociate the signal for a CD25+ DN cell to differentiate (which occurs in the absence of CD3 zeta), from a signal to proliferate and from loss of cell surface CD25.

Fetal thymic organ cultures

Thymic organ cultures are currently the only system capable of supporting a full programme of T-cell development in vitro. Unmanipulated thymus lobes are useful for studying some aspects of T-cell development but are limited for studies on interactions between thymocytes and stromal cells by their cellular heterogeneity. However, techniques have now been developed for the association of defined stromal and lymphoid populations in organ culture where optimal conditions for the interaction are maintained. This approach is now being applied to study the role of individual stromal cell types in T-cell development and selection of the T cell receptor repertoire.