Quantitation of T-cell receptor frequencies by competitive polymerase chain reaction: dynamics of T-cell clonotype frequencies in an expanding tumor-infiltrating lymphocyte culture

The use of T-cell receptor (TCR) genes as markers for antigen-reactive T cells is dependent on the ability of the TCR genes to rapidly identify antigen-reactive T-cell clonotypes in patient samples. We recently reported a competitive reverse-transcriptase polymerase chain reaction (cRT-PCR) method that can measure the frequency of individual TCRBV subfamilies and clonotypes in mixed lymphocyte populations more accurately than other semiquantitative PCR assays. However, it is impractical to measure changes in the absolute frequency of each TCRBV subfamily to identify those T cells with increasing frequency after antigen stimulation in vivo or in vitro. Therefore, we have modified our cRT-PCR method to more rapidly identify expanding T-cell populations by combining all of the TCRBV subfamily-specific competitors into a single sample to determine the relative abundance of each TCRBV subfamily. Using an expanding TIL 620 culture, we identified four TCRBV (BV2, BV12, BV17, and BV23) subfamilies that expanded over a 23-day period. These subfamilies accounted for 23% of the T cells in the day 35 culture and increased to 57%, 92%, and 80% of the days 44, 51, and 58 cultures respectively. Analysis of DNA sequences demonstrated that the observed expansion was caused primarily by a single clonotype within each subfamily. T cells expressing BV17 and BV23 recognized gp100 and MART-1 respectively. Therefore, this cRT-PCR method can detect expanding T-cell populations based solely on their TCRBV subfamily expression. Furthermore, T-cell expansion in a mixed TIL population was a good predictor of antigen reactivity.

MHC class I-restricted recognition of a melanoma antigen by a human CD4+ tumor infiltrating lymphocyte

It is generally considered that MHC class I-restricted antigens are recognized by CD8+ T cells, whereas MHC class II-restricted antigens are recognized by CD4+ T cells. In the present study, we report an MHC class I-restricted CD4+ T cell isolated from the tumor infiltrating lymphocytes (TILs) of a patient with metastatic melanoma. TIL 1383 I recognized HLA-A2+ melanoma cell lines but not autologous transformed B cells or fibroblasts. The antigen recognized by TIL 1383 I was tyrosinase, and the epitope was the 368-376 peptide. Antibody blocking assays confirmed that TIL 1383 I was MHC class I restricted, and the CD4 and CD8 coreceptors did not contribute significantly to antigen recognition. TIL 1383 I was weakly cytolytic and secreted cytokines in a pattern consistent with it being a Th1 cell. The avidity of TIL 1383 I for peptide pulsed targets is 10-100-fold lower than most melanoma-reactive CD8+ T cell clones. These CD4+ T cells may represent a relatively rare population of T cells that express a T-cell receptor capable of cross-reacting with an MHC class I/peptide complex with sufficient affinity to allow triggering in the absence of the CD4 coreceptor.

Precise developmental regulation of Ets family transcription factors during specification and commitment to the T cell lineage

Ets family transcription factors control the expression of a large number of genes in hematopoietic cells. Here we show strikingly precise differential expression of a subset of these genes marking critical, early stages of mouse lymphocyte cell-type specification. Initially, the Ets family member factor Erg was identified during an arrayed cDNA library screen for genes encoding transcription factors expressed specifically during T cell lineage commitment. Multiparameter fluorescence-activated cell sorting for over a dozen cell surface markers was used to isolate 18 distinct primary-cell populations representing discrete T cell and B cell developmental stages, pluripotent lymphoid precursors, immature NK-like cells and myeloid hematopoietic cells. These populations were monitored for mRNA expression of the Erg, Ets-1, Ets-2, Fli-1, Tel, Elf-1, GABPalpha, PU.1 and Spi-B genes. The earliest stages in T cell differentiation show particularly dynamic Ets family gene regulation, with sharp transitions in expression correlating with specification and commitment events. Ets, Spi-B and PU.1 are expressed in these stages but not by later T-lineage cells. Erg is induced during T-lineage specification and then silenced permanently, after commitment, at the beta-selection checkpoint. Spi-B is transiently upregulated during commitment and then silenced at the same stage as Erg. T-lineage commitment itself is marked by repression of PU.1, a factor that regulates B-cell and myeloid genes. These results show that the set of Ets factors mobilized during T-lineage specification and commitment is different from the set that maintains T cell gene expression during thymocyte repertoire selection and in all classes of mature T cells.

Precocious expression of T cell functional response genes in vivo in primitive thymocytes before T lineage commitment

The genes encoding effector molecules of mature T cells, IL-2, perforin and IL-4, were found to be expressed in vivo in the most primitive subsets of thymocytes of adult mice. These subsets have previously been identified by their cell surface markers and by their expression of other T lineage-associated genes. While IL-2, perforin and IL-4 are expressed in distinct patterns, all three are expressed before the induction of RAG-1 and pre-TCR alpha mRNA expression, and are confined to subsets of cells that apparently have not yet undergone commitment to the T lineage. Thus, expression of T cell response genes appears to be one of the earliest markers of lymphocyte differentiation. Activation events marked by CD69 induction occur in these early cell types, but the response gene expression by these cells is separable from CD69 expression. IL-2 and perforin are induced again much later in thymocyte development, during TCR-dependent repertoire selection. At those stages, IL-2 protein and RNA levels per cell are higher, but the fraction of cells expressing IL-2 appears to be much lower than in the most immature stages. In addition, a striking feature of the immature populations is the robust IL-2 expression by presumptive immature NK cells. These findings are discussed in terms of the developmental origins of lineage specificity in T cell response gene regulation.

Cross-lineage expression of Ig-beta (B29) in thymocytes: positive and negative gene regulation to establish T cell identity

Developmental commitment involves activation of lineage-specific genes, stabilization of a lineage-specific gene expression program, and permanent inhibition of inappropriate characteristics. To determine how these processes are coordinated in early T cell development, the expression of T and B lineage-specific genes was assessed in staged subsets of immature thymocytes. T lineage characteristics are acquired sequentially, with germ-line T cell antigen receptor-beta transcripts detected very early, followed by CD3epsilon and terminal deoxynucleotidyl transferase, then pTalpha, and finally RAG1. Only RAG1 expression coincides with commitment. Thus, much T lineage gene expression precedes commitment and does not depend on it. Early in the course of commitment to the T lineage, thymocytes lose the ability to develop into B cells. To understand how this occurs, we also examined expression of well defined B lineage-specific genes. Although lambda5 and Ig-alpha are not expressed, the mu 0 and I mu transcripts from the unrearranged IgH locus are expressed early, in distinct patterns, then repressed just before RAG1 expression. By contrast, RNA encoding the B cell receptor component Ig-beta was found to be transcribed in all immature thymocyte subpopulations and throughout most thymocyte differentiation. Ig-beta expression is down-regulated only during positive selection of CD4(+)CD8(-) cells. Thus several key participants in the B cell developmental program are expressed in non-B lineage-committed cells, and one is maintained even through commitment to an alternative lineage, and repressed only after extensive T lineage differentiation. The results show that transcriptional activation of "lymphocyte-specific" genes can occur in uncommitted precursors, and that T lineage commitment is a composite of distinct positive and negative regulatory events.

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.

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.

Developmental regulation of cAMP signaling pathways in thymocyte development

Major developmental transitions in thymocyte differentiation are accompanied by sharp alterations in cAMP metabolism. We have analyzed the cAMP accumulation responses of cell populations representing successive stages of T-cell development, namely: immature TcR- thymocytes from SCID mice, proliferating cortical blasts, small cortical thymocytes, medullary thymocytes and peripheral T cells. We find that all classes of thymocytes exhibit higher cAMP synthesis in response to forskolin than peripheral T cells. In immature TcR- thymocytes, this high capacity is buffered by efficient phosphodiesterase activity, but in CD4+CD8+TcRlow thymocytes, phosphodiesterase activity becomes much less effective. Phosphodiesterase activity then rises again after positive selection. The ability of thymocytes to respond to prostaglandin E is regulated distinctly from their ability to respond to forskolin. Unlike forskolin, PGE1 induces cAMP synthesis to similar levels in all classes of thymocytes, possibly due to partial activation of phosphodiesterase in cortical thymocytes by PGE1. Finally, we report a novel effect of Ca2+/protein kinase C signaling on cAMP accumulation, which occurs selectively in the proliferating cortical blasts.

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.

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.

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.

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.

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.

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.

Changes in inducibility of IL-2 receptor alpha-chain and T cell-receptor expression during thymocyte differentiation in the mouse

Within the thymus, developing T cells must acquire the competence to respond to appropriate signals by inducing the expression of genes required for immunologic function; one such gene encodes the 55-kDa-chain of the IL-2R (IL-2R alpha). Previously, we showed that most cortical-type thymocytes lack the competence to make this particular response, while most medullary-type cells respond like mature T lymphocytes. The noninducibility of cortical-type cells was striking, because most of their presumed precursors were inducible. To test the relationship between this apparent loss of competence and the positive and negative selection processes that may occur in the thymic cortex, we have assayed the inducibility of thymocyte populations, staged carefully with respect to their expression of TCR. Using size fractionation to enrich for dividing cells, we concentrated and thereby revealed defined developmental intermediates. We report that, although CD4+CD8- thymocytes behave as mature T cells, a significant fraction of CD4-CD8+ cells are noninducible. These noninducible thymocytes are dividing cells, which appear to be in a major developmental continuum between CD4-CD8- blasts and CD4+CD8+ blasts. Furthermore, the noninducible blasts as yet lack surface TCR expression. We also demonstrate the functional similarity of these CD4-CD8+ cells to a major subset of dividing CD4-CD8- precursor cells, which appear to have lost IL-2R alpha expression. These results suggest that precursors of cortical thymocytes lose competence to be induced to express IL-2R alpha several stages before their acquisition of cell-surface TCR complexes. The implications of this characterization are discussed in terms of the possible relationships between IL-2R alpha gene regulation and intrathymic fate determination.

Changes in inducibility of IL-2 receptor alpha-chain and T cell-receptor expression during thymocyte differentiation in the mouse

Within the thymus, developing T cells must acquire the competence to respond to appropriate signals by inducing the expression of genes required for immunologic function; one such gene encodes the 55-kDa-chain of the IL-2R (IL-2R alpha). Previously, we showed that most cortical-type thymocytes lack the competence to make this particular response, while most medullary-type cells respond like mature T lymphocytes. The noninducibility of cortical-type cells was striking, because most of their presumed precursors were inducible. To test the relationship between this apparent loss of competence and the positive and negative selection processes that may occur in the thymic cortex, we have assayed the inducibility of thymocyte populations, staged carefully with respect to their expression of TCR. Using size fractionation to enrich for dividing cells, we concentrated and thereby revealed defined developmental intermediates. We report that, although CD4+CD8- thymocytes behave as mature T cells, a significant fraction of CD4-CD8+ cells are noninducible. These noninducible thymocytes are dividing cells, which appear to be in a major developmental continuum between CD4-CD8- blasts and CD4+CD8+ blasts. Furthermore, the noninducible blasts as yet lack surface TCR expression. We also demonstrate the functional similarity of these CD4-CD8+ cells to a major subset of dividing CD4-CD8- precursor cells, which appear to have lost IL-2R alpha expression. These results suggest that precursors of cortical thymocytes lose competence to be induced to express IL-2R alpha several stages before their acquisition of cell-surface TCR complexes. The implications of this characterization are discussed in terms of the possible relationships between IL-2R alpha gene regulation and intrathymic fate determination.

Activation of T cell antigen receptor alpha- and beta-chain genes in the thymus: implications for the lineages of developing cortical thymocytes

Mammalian T lymphocytes mature in the thymus through a series of differentiation events that involve both rapid proliferation and extensive cell death. The mechanisms that govern these processes are currently unknown; however, both mitogenesis and death affect particular subpopulations of cells, suggesting the selective amplification and destruction of specific T cell clones. In mature peripheral T cells, proliferation is most commonly triggered by the recognition of antigen through the T cell antigen receptor complex. If antigen recognition also controls proliferation in the thymus, the differential expression of antigen receptor genes during maturation could play some role in determining the fate of developing T cells. In this study, we examined the expression of the alpha- and beta-chain genes of the T cell antigen receptor in different subpopulations of adult thymocytes. We compared two postmitotic populations--one that appears committed to die and one that appears mature--and several blast cell populations that are enriched for precursors of one or another presumptive lineage. We have found that Lyt-2-, L3T4- precursor thymocytes express much lower levels of both alpha- and beta-chain mRNA than the cells likely to be their immediate descendents. Furthermore, our results show that the cells of the major cortical lineage, which have at least a 95% probability of death, nevertheless express high levels of mature mRNA encoding both the alpha- and the beta-chains of the T cell antigen receptor. These results have important implications for the mechanisms involved in the overproduction and elimination of this major class of T lymphocyte.

Activation of T cell antigen receptor alpha- and beta-chain genes in the thymus: implications for the lineages of developing cortical thymocytes

Mammalian T lymphocytes mature in the thymus through a series of differentiation events that involve both rapid proliferation and extensive cell death. The mechanisms that govern these processes are currently unknown; however, both mitogenesis and death affect particular subpopulations of cells, suggesting the selective amplification and destruction of specific T cell clones. In mature peripheral T cells, proliferation is most commonly triggered by the recognition of antigen through the T cell antigen receptor complex. If antigen recognition also controls proliferation in the thymus, the differential expression of antigen receptor genes during maturation could play some role in determining the fate of developing T cells. In this study, we examined the expression of the alpha- and beta-chain genes of the T cell antigen receptor in different subpopulations of adult thymocytes. We compared two postmitotic populations--one that appears committed to die and one that appears mature--and several blast cell populations that are enriched for precursors of one or another presumptive lineage. We have found that Lyt-2-, L3T4- precursor thymocytes express much lower levels of both alpha- and beta-chain mRNA than the cells likely to be their immediate descendents. Furthermore, our results show that the cells of the major cortical lineage, which have at least a 95% probability of death, nevertheless express high levels of mature mRNA encoding both the alpha- and the beta-chains of the T cell antigen receptor. These results have important implications for the mechanisms involved in the overproduction and elimination of this major class of T lymphocyte.