Biomedical subjects

E V Rothenberg

Publications and source records attributed to E V Rothenberg.

At least 37 records · Page 2Linked to original sources

PubMed1997

Different developmental arrest points in RAG-2 -/- and SCID thymocytes on two genetic backgrounds: developmental choices and cell death mechanisms before TCR gene rearrangement.

To analyze the early development of T cell precursors in the absence of TCR gene rearrangement, recombinase-activating gene-deficient (RAG-2 -/-) thymocytes were compared with thymocytes from SCID mice on the C.B-17 (BALB) and B6 genetic backgrounds. RAG-2 -/- thymocytes accumulate as quiescent cells with a heat-stable Ag (HSA)-positive CD25+ CD44- c-kit(low) phenotype, resembling normal cells just before selection for functional TCR beta-chain expression. CD44 and c-kit progressively down-regulate in the HSA+ subset, providing a background-independent and TCR-independent developmental clock. On this basis, compared with RAG-2 -/- thymocytes, SCID thymocytes 1) arrest at more heterogeneous, and generally earlier, stages; 2) accumulate to lower overall cell numbers; and 3) maintain higher populations of cycling and activated G1 cells, showing both increased responsiveness and increased cell death. B6-SCID thymocytes appear to die particularly early. Low levels of Fas were observed on "advanced" HSA+ SCID thymocytes but not on any RAG-2 -/- thymocytes, suggesting a potential difference in activation state or mechanism of death. In both RAG-2 -/- and SCID thymocytes, there are also two discrete subsets of HSA(low) CD25- CD44+ c-kit+ cells: a Sca-1+ CD44++ CD122- NK1.1- putative progenitor subset and an NK-like Sca-1- CD44+(+) CD122+ NK1.1+ subset. The absolute cell numbers in these HSA(low) subsets and the extent of NK cell differentiation, measured by perforin expression, are nearly constant in all the mutant strains analyzed, in contrast to the HSA+ CD25+ population, which was expanded in the RAG-2 -/-. Thus, the SCID thymocytes appear to undergo a normal generation but a premature death as compared with the RAG-2 -/- thymocytes.

Animals↗

PubMed1996

A dynamic assembly of diverse transcription factors integrates activation and cell-type information for interleukin 2 gene regulation.

The interleukin 2 (IL-2) gene is subject to two types of regulation: its expression is T-lymphocyte-specific and it is acutely dependent on specific activation signals. The IL-2 transcriptional apparatus integrates multiple types of biochemical information in determining whether or not the gene will be expressed, using multiple diverse transcription factors that are each optimally activated or inhibited by different signaling pathways. When activation of one or two of these factors is blocked IL-2 expression is completely inhibited. The inability of the other, unaffected factors to work is explained by the striking finding that none of the factors interacts stably with its target site in the IL-2 enhancer unless all the factors are present. Coordinate occupancy of all the sites in the minimal enhancer is apparently maintained by continuous assembly and disassembly cycles that respond to the instantaneous levels of each factor in the nuclear compartment. In addition, the minimal enhancer undergoes specific increases in DNase I accessibility, consistent with dramatic changes in chromatin structure upon activation. Still to be resolved is what interaction(s) conveys T-lineage specificity. In the absence of activating signals, the minimal IL-2 enhancer region in mature T cells is apparently unoccupied, exactly as in non-T lineage cells. However, in a conserved but poorly studied upstream region, we have now mapped several novel sites of DNase I hypersensitivity in vivo that constitutively distinguish IL-2 producer type T cells from cell types that cannot express IL-2. Thus a distinct domain of the IL-2 regulatory sequence may contain sites for competence- or lineage-marking protein contacts.

Animals↗

PubMed1996

How T cells count.

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Antigen-Presenting Cells↗

PubMed1996

Regulated expression and function of CD122 (interleukin-2/interleukin-15R-beta) during lymphoid development.

To determine whether signaling via CD122 (interleukin-2 [IL-2]/IL-15 receptor beta-chain) plays a role in regulating the expansion and differentiation of lymphocyte precursors, we have characterized its expression and evaluated its ability to influence the activity of developing lymphoid cells. A significant fraction of Sca1+Lin- hematopoietic stem cells in day 12 fetal liver were found to be CD122+. CD122-mRNA+ and IL-2-mRNA+ cells were also localized in embryo sections within pharyngeal blood vessels adjacent to and surrounding the thymic analgen. This distribution is consistent with the migration of CD122+ progenitor cells from the liver to the developing thymus where a majority of Sca1+ intrathymic T-cell progenitors were CD122+. Analysis of CD122 expression in the day 12 fetal liver revealed that the majority of B220+ cells were CD122+. Furthermore, CD122 expression was restricted to the earliest B220+ cells (CD43+CD24-; prepro B cells; fraction A) that proliferate vigorously to IL-2 in the absence of any stromal cells, but not to IL-15. Consistent with a role for the IL-2/IL-2R pathway in lymphocyte development is the progressive loss of B cells seen in IL-2-deficient mice. Together, these observations suggest that CD122 plays a role in regulating normal lymphocyte development in vivo.

Animals↗

PubMed1994

Interleukin 2 transcription factors as molecular targets of cAMP inhibition: delayed inhibition kinetics and combinatorial transcription roles.

Elevation of cAMP can cause gene-specific inhibition of interleukin 2 (IL-2) expression. To investigate the mechanism of this effect, we have combined electrophoretic mobility shift assays and in vivo genomic footprinting to assess both the availability of putative IL-2 transcription factors in forskolin-treated cells and the functional capacity of these factors to engage their sites in vivo. All observed effects of forskolin depended upon protein kinase A, for they were blocked by introduction of a dominant negative mutant subunit of protein kinase A. In the EL4.E1 cell line, we report specific inhibitory effects of cAMP elevation both on NF-kappa B/Rel family factors binding at -200 bp, and on a novel, biochemically distinct "TGGGC" factor binding at -225 bp with respect to the IL-2 transcriptional start site. Neither NF-AT nor AP-1 binding activities are detectably inhibited in gel mobility shift assays. Elevation of cAMP inhibits NF-kappa B activity with delayed kinetics in association with a delayed inhibition of IL-2 RNA accumulation. Activation of cells in the presence of forskolin prevents the maintenance of stable protein-DNA interactions in vivo, not only at the NF-kappa B and TGGGC sites of the IL-2 enhancer, but also at the NF-AT, AP-1, and other sites. This result, and similar results in cyclosporin A-treated cells, imply that individual IL-2 transcription factors cannot stably bind their target sequences in vivo without coengagement of all other distinct factors at neighboring sites. It is proposed that nonhierarchical, cooperative enhancement of binding is a structural basis of combinatorial transcription factor action at the IL-2 locus.

Base Sequence↗

PubMed1994

Costimulation by interleukin-1 of multiple activation responses in a developmentally restricted subset of immature thymocytes.

An intriguing feature of thymocyte differentiation is that the competence to express both interleukin-(IL)2 and CD25 is acquired even prior to T cell receptor (TcR) expression. When T cell receptor-independent stimuli are used, immature cells can express IL-2 at levels comparable to mature cells, but unlike the mature cells, immature cells require IL-1 as a costimulus. Here we present evidence that IL-1 affects a variety of responses by members of the CD25+ subset of immature thymocytes. Cells in this population are IL-1 dependent not only for induction of IL-2 expression, but also for high-level maintenance of CD25 expression. CD25 expression is amplified by IL-1 through a mechanism highly sensitive to changes in Ca2+ ionophore concentration. The effects of IL-1 on CD25 maintenance are not mediated by IL-2, because of the divergent effects of cAMP on IL-2 and CD25 expression. IL-1 costimulation also increases RNA accumulation in the cell cycle, and this effect too seems to be separable from the effects on IL-2 and CD25 expression. All these effects of IL-1 are developmentally stage-specific, manifest in the CD25+ subset of immature thymocytes but not in later-stage thymocytes or splenic T cells. Multiparameter cell sorting experiments that dissect the transitional stages between immature and TcR+ thymocytes imply that all immature cells pass through an IL-1 responsive state. Responsiveness to IL-1 costimulation is then lost by these cells, apparently irreversibly, at a stage just prior to detectable cell-surface TcR expression. These results indicate that IL-1 responsiveness is a defining characteristic of the activation physiology of cells in a particularly important developmental stage.

Animals↗

PubMed1994

Signaling mechanisms in thymocyte selection.

The processes known as positive and negative selection that determine the fate of T and B cells depend on finely tuned interactions between the T-cell receptor complex, CD4 or CD8 co-receptors, and a peptide-MHC complex. New work indicates that the avidity of this interaction is critical in the determination of its outcome. The effects of these interactions on developing thymocytes are also a function of the unique activation properties with which thymocytes are programmed just before they undergo selection.

Animals↗

PubMed1994

Interleukin-2 transcription is regulated in vivo at the level of coordinated binding of both constitutive and regulated factors.

Interleukin-2 (IL-2) transcription is developmentally restricted to T cells and physiologically dependent on specific stimuli such as antigen recognition. Prior studies have shown that this stringent two-tiered regulation is mediated through a transcriptional promoter/enhancer DNA segment which is composed of diverse recognition elements. Factors binding to some of these elements are present constitutively in many cell types, while others are signal dependent, T cell specific, or both. This raises several questions about the molecular mechanism by which IL-2 expression is regulated. Is the developmental commitment of T cells reflected molecularly by stable interaction between available factors and the IL-2 enhancer prior to signal-dependent induction? At which level, factor binding to DNA or factor activity once bound, are individual regulatory elements within the native enhancer regulated? By what mechanism is developmental and physiological specificity enforced, given the participation of many relatively nonspecific elements? To answer these questions, we have used in vivo footprinting to determine and compare patterns of protein-DNA interactions at the native IL-2 locus in cell environments, including EL4 T-lymphoma cells and 32D clone 5 premast cells, which express differing subsets of IL-2 DNA-binding factors. We also used the immunosuppressant cyclosporin A as a pharmacological agent to further dissect the roles played by cyclosporin A-sensitive factors in the assembly and maintenance of protein-DNA complexes. Occupancy of all site types was observed exclusively in T cells and then only upon excitation of signal transduction pathways. This was true even though partially overlapping subsets of IL-2-binding activities were shown to be present in 32D clone 5 premast cells. This observation was especially striking in 32D cells because, upon signal stimulation, they mobilized a substantial set of IL-2 DNA-binding activities, as measured by in vitro assays using nuclear extracts. We conclude that binding activities of all classes fail to stably occupy their cognate sites in IL-2, except following activation of T cells, and that specificity of IL-2 transcription is enforced at the level of chromosomal occupancy, which appears to be an all-or-nothing phenomenon.

Animals↗

PubMed1994

Programming for recognition and programming for response. Separate developmental subroutines in the murine thymus.

Pre-T cells become programmed with the capacity to make functional responses to activating stimuli in a process that occurs prior to, and independently of, T-cell receptor gene rearrangement and T-cell receptor-dependent positive selection. In spite of this early programming, as differentiation proceeds further the cells enter a stage in which they appear to be unable to make any functional responses. This 'eclipse' phase begins when the cells undergo successful T-cell receptor beta-chain rearrangement and ends, with the return of their functional competence, only when they successfully traverse positive selection. These results suggest that pre-T cells are subject to two distinct subroutines of differentiation, which cannot operate at the same time: one which confers function and one which confers and selects recognition specificity. To provide a possible molecular basis for the relationship between these two processes, we consider specific alterations in response-associated transcription factors that may cause the changes in responsiveness observed during programming for recognition. The interplay of the two differentiation subroutines is proposed to be a consequence of the use of common transcription factors in different combinatorial contexts for functional responses, assembly of T-cell receptor complexes, and selection.

Aging↗

PubMed1993

Functional and phenotypic analysis of thymocytes in SCID mice. Evidence for functional response transitions before and after the SCID arrest point.

Thymocytes from severe combined immune deficient (SCID) mice undergo developmental arrest at an early stage, before most TCR gene rearrangement. They therefore represent a natural test case to assess those aspects of T cell development that are TCR independent. Multiparameter flow cytometry was used to analyze the array of immature phenotypes present in the SCID thymus at steady state, as defined by the markers CD4, CD5, Sca-1, NK1.1, CD44, heat-stable antigen (HSA), and IL-2R alpha. The results suggest a simple developmental block in SCID mice rather than a program of aberrant differentiation. SCID thymocytes displayed efficient, developmentally regulated functional responses. Approximately 20-25% of the cells, mostly within the IL-2R alpha +HSA+CD44low fraction, could be induced to express IL-2. This IL-2 inducibility was highly dependent on IL-1 costimulation, in agreement with the behavior of normal immature thymocytes. These results formally demonstrate that competence to express IL-2 is developed independently of TCR expression or gene rearrangement. Comparison of the response properties of various SCID thymocyte subsets indicated that IL-2 inducibility is first likely to be acquired at an early (Sca-1++CD44++HSAlow) stage. A later functional transition was revealed by comparing patterns of IL-2R alpha regulation in normal and SCID IL-2R alpha +HSA+CD44low thymocytes. The SCID thymocytes uniformly maintained IL-2R alpha expression on in vitro stimulation, whereas only a minority of the normal cells in the corresponding subset could do so unless IL-1 was also added. The SCID arrest point thus appears to separate the IL-2R alpha +HSA+CD44low stage into distinct early (TCR independent) and late phases. Normal cells that progress beyond the SCID arrest point appear to lose, rather than gain, competence to make various responses, even before they leave the IL-2R alpha +HSA+CD44low stage. A model is proposed in which discrete changes in functional competence define novel transitions in early thymocyte development, at least some of which may be linked to TCR-beta gene rearrangement before positive or negative selection.

Animals↗

PubMed1993

Molecular basis for developmental changes in interleukin-2 gene inducibility.

At least three stages in the intrathymic development of pre-T cells are demarcated by differences in the competence to express the interleukin-2 (IL-2) gene as an acute response to stimulation. IL-2 inducibility appears to be acquired relatively early, prior to T-cell receptor (TcR) gene rearrangement. It is then abrogated during the stage when cells are subject to positive and negative selection, i.e., the fate determination processes that select cells for maturation or death. IL-2 inducibility finally reappears in mature classes of thymocytes that have undergone positive selection. To provide a basis for a molecular explanation of these developmental transitions, we have examined the representation in different thymocyte subsets of a set of DNA-binding proteins implicated in IL-2 gene regulation. As the DNA-binding activities of many factors are elicited only by inductive stimuli, the cells were cultured in the presence or absence of the calcium ionophore A23187 and phorbol ester. Our results separate these factors into four regulatory classes: (i) constitutive factors, such as Oct-1 and probably Sp1, that are expressed in thymocytes at all stages; (ii) inducible factors, such as NF-kappa B and complexes binding to the region of a CD28 response element, that can be activated in all thymocytes, including those cells (CD4+ CD8+ TcRlow) that can undergo selection; (iii) inducible factors, such as NF-AT and AP-1, that can be activated in mature (CD4+ CD8- TcRhigh) and immature (CD4- CD8- TcR-) thymocytes alike but not in the transitional stages when the cells (CD4+ CD8+ TcRlow) are subject to selection; and (iv) a factor containing CREB, which can be activated in thymocytes of all developmental stages by culture but does not require specific induction. These results verify that inducible transcription factors are targets of intrathymic developmental change. They also identify NF-AT and AP-1 as factors that are particularly sensitive to the mechanism altering thymocyte responses during the stages when thymocytes may undergo positive and negative selection.

Animals↗

PubMed1993

Developmental and anatomical patterns of IL-2 gene expression in vivo in the murine thymus.

Interleukin-2 (IL-2) is a potent growth factor that mature T lymphocytes synthesize and use as a proliferation signal. Much controversy has arisen concerning whether it is used to drive the extensive proliferation of immature pre-T cells in the thymus. Immature thymocytes acquire the competence to express IL-2 at an early stage, but it has remained uncertain whether they are activated to exercise this competence in vivo. Therefore, we have used in situ hybridization and immunohistochemistry on serial sections obtained from fetal and adult thymuses of normal C57BL/6 mice and of mice bearing the scid defect to determine where, when, and whether IL-2 is expressed in vivo. Our results show a striking spatial and temporal pattern of IL-2 expression in the normal fetal thymus. We detected a burst of IL-2 mRNA accumulation at day 14.5 of gestation, which rapidly decreased by day 15. At day 15, we observed maximal IL-2 protein production that subsequently decreased by day 16 of gestation. Both in situ hybridization and immunohistochemical staining revealed an unexpectedly strict localization of IL-2 expressing cells to patches around the periphery of the fetal thymus, creating a previously unrecognized compartment of high IL-2 protein content. IL-2 production in the day-15 fetal thymus appeared to be unaffected by the scid mutation, indicating that this response is likely to be T-cell receptor (TcR)-independent. Several features distinguish the IL-2 induction pattern in the adult thymus from that in the fetal thymus. In the normal adult thymus, IL-2-expressing cells are extremely rare (found at a frequency of 10(-7)), but they are reproducibly detectable as isolated cells in the outer cortex and subcapsular region of the thymus. Unlike the fetal thymic IL-2 producers, the IL-2 producers in the adult thymus are completely eliminated in mice homozygous for the scid mutation. This suggests that the IL-2-expressing cells in the normal adult thymus are of a more mature phenotype than the immature, TcR-negative cells that accumulate in the scid adult thymus. Thus, our work demonstrates that two developmentally distinct types of cell interactions induce IL-2 expression in vivo: one, a broadly localized interaction in day 14-15 fetal thymus that is unaffected by the scid mutation; the other, a rare event that occurs asynchronously from late fetal through adult life, but which is completely eliminated by the scid defect.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗

PubMed1992

The development of functionally responsive T cells.

The work reviewed in this article separates T cell development into four phases. First is an expansion phase prior to TCR rearrangement, which appears to be correlated with programming of at least some response genes for inducibility. This phase can occur to some extent outside of the thymus. However, the profound T cell deficit of nude mice indicates that the thymus is by far the most potent site for inducing the expansion per se, even if other sites can induce some response acquisition. Second is a controlled phase of TCR gene rearrangement. The details of the regulatory mechanism that selects particular loci for rearrangement are still not known. It seems that the rearrangement of the TCR gamma loci in the gamma delta lineage may not always take place at a developmental stage strictly equivalent to the rearrangement of TCR beta in the alpha beta lineage, and it is not clear just how early the two lineages diverge. In the TCR alpha beta lineage, however, the final gene rearrangement events are accompanied by rapid proliferation and an interruption in cellular response gene inducibility. The loss of conventional responsiveness is probably caused by alterations at the level of signaling, and may be a manifestation of the physiological state that is a precondition for selection. Third is the complex process of selection. Whereas peripheral T cells can undergo forms of positive selection (by antigen-driven clonal expansion) and negative selection (by abortive stimulation leading to anergy or death), neither is exactly the same phenomenon that occurs in the thymic cortex. Negative selection in the cortex appears to be a suicidal inversion of antigen responsiveness: instead of turning on IL-2 expression, the activated cell destroys its own chromatin. The genes that need to be induced for this response are not yet identified, but it is unquestionably a form of activation. It is interesting that in humans and rats, cortical thymocytes undergoing negative selection can still induce IL-2R alpha expression and even be rescued in vitro, if exogenous IL-2 is provided. Perhaps murine thymocytes are denied this form of rescue because they shut off IL-2R beta chain expression at an earlier stage or because they may be uncommonly Bcl-2 deficient (cf. Sentman et al., 1991; Strasser et al., 1991). Even so, medullary thymocytes remain at least partially susceptible to negative selection even as they continue to mature.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗

PubMed1992

In vitro transfection of fresh thymocytes and T cells shows subset-specific expression of viral promoters.

We describe conditions under which exogenous DNA templates can be introduced for transient expression into primary murine T lymphocytes. T cells at various stages of development, including concanavalin A-activated splenic T cells, immature pre-T cells, and even small cortical thymocytes, could be successfully transfected. A variety of model DNA constructs were compared in which different viral promoter regions were used to drive expression of the chloramphenicol acetyltransferase (CAT) reporter gene. All showed enhanced expression in cells that had been acutely stimulated with the Ca2+ ionophore A23187 and phorbol ester as chemical proxies for T-cell receptor-mediated signals. In addition, splenocytes but not thymocytes required prior treatment with a mitogen and interleukin-2 in order to express these constructs, implying that even postmitotic thymocytes may be held in a quasiactivated state. A most striking result was the finding that the viral regulatory sequences in the Rous sarcoma virus long terminal repeat and the simian virus 40 early region were subject to sharply differential regulation, with a rank order that changed depending on the developmental stage of the T cells. The most immature thymic blasts and several lymphoma cell lines expressed the pRSV-Cat and pSV2-Cat constructs similarly, but cortical thymocytes exhibited a strong preference for pSV2-Cat. Splenic concanavalin A-stimulated blasts, on the other hand, slightly preferred pRSV-Cat, a tendency which became exaggerated in factor-dependent T-cell lines. The ratio of pRSV-Cat to pSV2-Cat expression varied according to cell type by as much as 500-fold. These results argue against a trivial linkage of promoter preference to cell cycle status but instead provide evidence that activation of T cells at distinct stages of differentiation results in the expression of different ensembles of nuclear regulatory proteins. In contrast to the simian virus 40 and Rous sarcoma virus promoter regions, the long terminal repeats of the retroviruses mink cell focus-forming virus and Akv were expressed well in all primary T-lineage cells. Thus, they represent excellent model promoters for engineering developmental stage-independent expression of exogenous genes in murine T cells.

Animals↗

PubMed1991

Acquisition of mature functional responsiveness in T cells: programming for function via signaling.

The results discussed here provide strong evidence that different T-cell effector gene programs are activated by different signals, and that in several cases their responses to the same exogenous stimuli shift during the development and antigen responses of the cells. T-cell responses are thus conditional and plastic at the individual cell level. In the formalism of the introductory section, the results support elements of Models 2 and 3, and suggest a fusion between them as differentiation is explained in terms of alteration in the relative strengths of different intracellular signaling pathways. Returning to an initial question, how are different functional capabilities assigned nonrandomly to cells with different antigen recognition specificities? This question has not been answered, but it can be reformulated. If all virgin T cells can transiently make IL-2, then we must ask what features of cell biology explain the preferential preservation of IL-2 inducibility in CD4+ cells as opposed to CD8+ cells. If the capacity to induce IL-4 expression is not acquired in the thymus, then we may ask whether the initial opening of this locus depends on a CD4-transmitted signal. Similarly, the CD8 molecule itself might participate in inducing the initial differentiation events that render CTL-p inducible for granzyme C and perforin. This would be in accord with a large literature showing that CD8 engagement is much more important in the initial induction of CTL activity than in the exercise of function by pre-primed CTL effectors. The subtext of each of these "questions", however, is that intrathymic events may not directly affect the genes used by terminal effectors for function at all. They may instead bias a cell's complement of triggering receptors, thus rendering it differentially sensitive to particular signals generated during antigen reception. This view is extreme, and will probably turn out to be an overstatement. But it does inspire a unique set of investigations into the basis of T-cell function. It lends urgency to the question of whether CD4+ and CD8+ cells differ in their G proteins, kinases, or inducible proto-oncogenes. If they do, we can then ask whether such differences themselves arise in the periphery, or whether they can be traced back to thymocytes fresh from positive selection--or before.

Animals↗

PubMed1990

Regulatory anatomy of the murine interleukin-2 gene.

We have cloned the mouse IL2 gene and sequenced 2800 bp of 5' flanking DNA. Comparison to the previously reported human sequence revealed extensive identity (approximately 86%) between the two genes from +1 to -580 with additional small islands of homology further upstream. Proximal sites which have been shown to be important in regulation of the human IL2 gene are well conserved in sequence and location. Transfection experiments using hybrid gene constructs containing varying lengths of the mouse 5' flanking DNA linked to a CAT reporter gene have demonstrated the presence of several novel positive and negative regulatory elements. One negative regulatory region lying between -750 and -1000 consists primarily of alternating purines and pyrimidines and is absent from the human gene. The conserved region from -321 and -578, an upstream segment from -1219 to -1332, and another region of approximately 450 bp from -1449 to -1890, which contained a well-conserved sequence of 60 bp, were each associated with enhanced levels of expression. We found no evidence for intragenic or downstream enhancer elements in this gene. All the elements identified affect only the magnitude of the inducible response, for no region when deleted had the effect of altering either the need for induction, the kinetics of stimulation, or the cell-type specificity of expression. Deletion studies suggest a strong requirement for NFAT binding even in the presence of extensive 5' flanking sequence. Therefore we conclude that IL2 gene expression is controlled primarily through a central TH1-specific signaling pathway, which acts through proximal elements, while distal cis-elements exert a secondary modulating effect.

Animals↗

PubMed1990

IL-2 gene inducibility in T cells before T cell receptor expression. Changes in signaling pathways and gene expression requirements during intrathymic maturation.

The ability to express the growth hormone IL-2 upon stimulation gives T lymphocytes one of their major effector functions in the immune system. IL-2 is apparently synthesized only by T cells, and only by a subset of T cells which constitutes a "helper" class. It remains unknown how and when the IL-2-producing lineage becomes distinct from other functional effector lineages. We have therefore examined immature T cell precursors to determine when IL-2 inducibility is acquired in relation to other maturation events, such as expression of an Ag-binding TCR, which is suspected to play an influential role in the determination of subclass commitment. In mature T cells, IL-2 is inducible via agonists of the phosphoinositide pathway, a network of signaling mediators shared by a wide variety of metazoan cell types. The universality of this activation pathway makes it seem less likely, a priori, to be a target of developmental change than the intrinsic susceptibility to induction of the IL-2 locus. However, our results presented here refute this expectation. In this report, we show that both TCR+ cells and pre-T cells too immature to express TCR can be induced to express IL-2 at high levels. The induction requirements for IL-2 expression, however, are different in TCR- and TCR+ cells. Even by using Ca2+ ionophore and phorbol ester to bypass the requirement for the TCR in cell activation, the TCR- cells also require the presence of the polypeptide hormone IL-1. By contrast, TCR+ mature cells not only can express IL-2 without IL-1, but also show no response to IL-1 when Ca2+ ionophore and phorbol ester are present. IL-1-dependent IL-2 producers appear in the thymus of repopulating radiation chimeras before "mature" (TCR+) T cells, whereas IL-1-independent IL-2 production is found only afterward. Thus, IL-2 inducibility per se apparently precedes TCR expression and all TCR-associated fate determination events. However, developmental alteration of signal transduction pathways may play a vital regulatory role in the later allocation of particular functional responses to appropriate lineages of T cells.

Animals↗

PubMed1990

Death and transfiguration of cortical thymocytes: a reconsideration.

The fate of most cells in the mammalian thymus, namely CD4+CD8+ (double positive) cortical cells, is unknown. A fraction of the cells in this population serve as developmental intermediates, but the vast majority will die. In this article Ellen Rothenberg discusses how and when the cells are condemned or promoted to maturity, in the light of intriguing new data that suggests that they may be able to be rescued for longer than was previously thought.

Animals↗