The kinetics of T cell antigen receptor expression by subgroups of CD4+8+ thymocytes: delineation of CD4+8+3(2+) thymocytes as post-selection intermediates leading to mature T cells

Negative selection of CD4+CD8+ thymocytes by T cell receptor-induced apoptosis requires a costimulatory signal that can be provided by CD28

CD4+CD8+ thymocytes expressing self-reactive T cell antigen receptors (TCR) are deleted in the thymus as a consequence of TCR/self-antigen/major histocompatibility complex interactions. However, the signals that are necessary to initiate clonal deletion have not yet been clarified. Here we demonstrate that TCR engagement does not efficiently induce apoptosis of CD4+CD8+ thymocytes, although it generates signals that increase expression of CD5, a thymocyte differentiation marker. In fact, TCR signals fail to induce thymocyte apoptosis even when augmented by simultaneous engagement with CD4 or lymphocyte function 1-associated molecules. In marked contrast, signals generated by engagement of both TCR and the costimulatory molecule CD28 potently induce apoptosis of CD4+CD8+ thymocytes. Thus, the present results define a requirement for both TCR and costimulatory signals for thymocyte apoptosis and identify CD28 as one molecule that is capable of providing the necessary costimulus. These results provide a molecular basis for differences among cell types in their ability to mediate negative selection of developing thymocytes.

Regulation of thymocyte development through CD3. II. Expression of T cell receptor beta CD3 epsilon and maturation to the CD4+8+ stage are highly correlated in individual thymocytes

Recent studies have shown that maturation of CD4-8- double negative (DN) thymocytes to the CD4+8+ double positive (DP) stage is dependent on expression of the T cell receptor (TCR)-beta polypeptide. The exact mechanism by which the TCR-beta chain regulates this maturation step remains unknown. Previous experiments had suggested that in the presence of some TCR+ thymocytes, additional DN thymocytes not expressing a TCR-beta chain may be recruited to mature to the DP stage. The recent demonstration of an immature TCR-beta-CD3 complex on early thymocytes lead to the alternative hypothesis that signal transduction through an immature TCR-CD3 complex may induce maturation to the DP stage. In the latter case, maturation to the DP stage would depend on the expression of TCR-beta-CD3 in the same cell. We examined these two hypotheses by studying the expression of the intra- and extracellular CD3 epsilon, CD3 zeta, and TCR-beta polypeptides in intrathymic subpopulations during embryogenesis. CD3 epsilon and CD3 zeta were expressed intracellularly 2 and 1 d, respectively, before intracellular expression of the TCR-beta chain, potentially allowing immediate surface expression of an immature TCR-beta-CD3 complex as soon as functional rearrangement of a TCR-beta gene locus has been accomplished. Calcium mobilization could be induced by stimulation with anti-CD3 epsilon mAb as soon as intracellular TCR-beta was detectable, suggesting that a functional TCR-beta-CD3 complex is indeed expressed on the surface of early thymocytes. From day 17 on, most cells were in the DP stage, and over 95% of the DP cells expressed on the TCR-beta chain intracellularly. At day 19 of gestation, extremely low concentrations of TCR-beta chain and CD3 epsilon were detectable on the cell surface of nearly all thymocytes previously thought to be TCR-CD3 negative. These findings strongly support the hypothesis that maturation to the DP stage depends on surface expression of and subsequent signal transduction through an immature TCR-beta-CD3 complex and suggest that maturation to the DP stage by recruitment, if it occurs at all, is of minor relevance.

Thymocyte development in major histocompatibility complex-deficient mice: evidence for stochastic commitment to the CD4 and CD8 lineages

The mechanism resulting in commitment of precursor cells in the thymus to either the CD4 or CD8 lineage remains poorly understood. In principle, this may reflect a stochastic process or may reflect instructional signals from host major histocompatibility complex (MHC) molecules. We have examined the role of MHC products in subset commitment by using mice deficient in class I or class II MHC products. Normal numbers of committed CD4 intermediates (CD4+ CD8lo) develop in the thymus in the absence of class II molecules. Similarly, CD8 transitional cells (CD4loCD8+) are present in the thymus of mice lacking class I products. These findings suggest that commitment of CD4+8+ precursor cells to either lineage is a stochastic process that does not depend on instructive signals from MHC molecules (i.e., expression of alternative differentiative options by uncommitted precursor cells is independent of this environmental signal). These studies also suggest that an interaction between the T-cell antigen receptor (TCR) and MHC molecules that is independent of CD4/CD8 coreceptor engagement enhances stochastic coreceptor downregulation substantially and leads to upregulation of TCR expression as a prelude to selective events that require joint coreceptor/TCR engagement. We suggest that this initial interaction molds the TCR repertoire of stochastically generated T-cell subsets toward recognition of self-MHC products.

Phenotype analysis of cycling and postcycling thymocytes: evaluation of detection methods for BrdUrd and surface proteins

We present a comparison of two different methods for simultaneous detection of bromodeoxyuridine and cell surface markers. Both methods use enzymatic generation of single-strand DNA with nuclease. The biological system used is the murine thymus, in which in vivo DNA synthetizing cells were labeled by injection of BrdUrd and analyzed at different time points after the nucleoside pulse. The surface proteins detected were CD4 and CD8 differentiation markers and the T-cell receptor. Extraction of DNA-associated proteins with 0.1N HCl and detergent is necessary for the action of EcoR1 and Exonuclease III, but this treatment destroys phycocyanins and induces cell aggregation, as shown using the doublet-discrimination module. For DNAse I action, cells could be treated with paraformaldehyde and a low concentration of Tween 20, and this treatment was adequate for surface staining preservation (even with phycocyanins) and BrdUrd detection. Both methods were adequate for cell cycle studies, but only 7-amino-actinomycin D could be used as total DNA dye after DNAse action, and good results needed long (48-72 h) incubation in the fixative-detergent mixture. The DNAse I method now allows three-color staining (two surface markers and Brd-Urd), analyzed in a one laser-cytometer for the study of the phenotype of cycling cells, and of their progeny, in vivo and in cell cultures. It also allows the quantitative analysis of cell surface receptor densities in conditions similar to fresh cells.

Development of mature CD8+ thymocytes: selection rather than instruction?

The role of major histocompatibility complex (MHC) molecules in T cell differentiation was investigated by comparison of thymocyte subpopulations in wild-type mice and beta 2-microglobulin (beta 2M) mutant mice deficient in MHC class I expression and mature CD8+ cells. On the basis of surface markers, glucocorticoid resistance, in vitro differentiation capacity, and absence in beta 2 M-l- mice, CD4intermediateCD8hi cells with high expression of alpha beta T cell receptor (TCR alpha beta) were identified as having been positively selected by MHC class I for development into mature CD8+ T cells. Activated CD4intCD8hi cells bearing intermediate rather than high amounts of TCR were present in both wild-type and beta 2M-l- animals. These data suggest that recognition of MHC class I molecules is required for full maturation to CD8+ T cells, but not for receptor-initiated commitment to the CD8+ lineage, consistent with a stochastic (selection) model of thymocyte development.

Multiple rearrangements in T cell receptor alpha chain genes maximize the production of useful thymocytes

Peripheral T lymphocytes each express surface T cell receptor (TCR) alpha and beta chains of a single specificity. These are produced after random somatic rearrangements in TCR alpha and beta germline genes. Published model systems using mice expressing TCR alpha and/or beta chain transgenes have shown that allelic exclusion occurs conventionally for TCR-beta. TCR alpha chain expression, however, appears to be less strictly regulated, as endogenous TCR alpha chains are often found in association with transgenic TCR beta chains in TCR alpha/beta transgenic mice. This finding, coupled with the unique structure of the TCR alpha locus, has led to the suggestion that unlike TCR beta and immunoglobulin heavy chain genes, TCR alpha genes may make multiple rearrangements on each chromosome. In the current study, we demonstrate that the majority of TCR-, noncycling thymocytes spontaneously acquire surface expression of CD3/TCR. Further, we show that cultured immature thymocytes originally expressing specific TCR alpha and beta chains may lose surface expression of the original TCR alpha, but not beta chains. These data provide evidence that not only must multiple rearrangements occur, but that TCR alpha gene rearrangement continues even after surface expression of a TCR alpha/beta heterodimer, apparently until the recombination process is halted by positive selection, or the cell dies. Sequential rearrangement of TCR alpha chain genes facilitates enhanced production of useful thymocytes, by increasing the frequency of production of both in-frame rearrangements and positively selectable TCR alpha/beta heterodimers.

Precursors of CD3+CD4+CD8+ cells in the human thymus are defined by expression of CD34. Delineation of early events in human thymic development

Studies of the most immature T cell progenitors in the human thymus have been hampered by the lack of markers and assays that define these cells. In this report we used a novel human fetal thymic organ culture system to determine the potential of T cell precursors isolated from human postnatal thymus, to differentiate into CD3+ thymocytes, and to investigate early stages of human T cell development. It was found that thymocytes that lack the markers CD3, CD4, and CD8 (triple negative [TN]) can differentiate in an allogeneic organotypic thymic culture. The capacity of TN thymocytes to differentiate was exclusively confined to the CD34+ population. CD34- TN thymocytes failed to differentiate in this system. In contrast, cloned lines of CD3- thymocytes could only be established from CD34- TN thymocytes. Five subsets of CD3- thymocytes were found with the following phenotype: CD1-TN, CD1+TN, CD1+CD4+CD8-, CD1+CD4+CD8 alpha+ beta-, and CD1+CD4+CD8 alpha beta+. These subpopulations expressed decreasing levels of CD34. The CD1-CD3- population expressed the highest levels of CD34 supporting the notion that this population is the most immature T cell precursor in the thymus, whereas the CD1+CD4+CD8 alpha+ beta+ which did not express CD34 seems to be the most mature of these CD3- populations. This notion is supported by the observations that CD34+ cells isolated from fetal liver, which differentiated into T cells in a FTOC, developed into CD3+ cells via CD1- and CD4+CD8- intermediates. Based on these data, we present a model of early stages in human intrathymic development.

Demonstration of large-scale migration of cortical thymocytes to peripheral lymphoid tissues in cyclosporin A-treated rats

Young adult Lewis rats were maintained on diets containing 0.015 or 0.027% cyclosporin A (CSA) for periods of up to 6 wk. All animals showed complete depletion of medullary thymocytes (CD4+8- and CD4-8+, T cell receptor [TCR] alpha/beta hi, Thy-1med/low, terminal deoxynucleotidyl transferase negative [TdT-]) and a 50% reduction in the number of TdT- cortical thymocytes (CD4+8+, TCR alpha/beta low, Thy-1med) within 1 wk of CSA treatment. In addition, about half of the animals displayed a 50% reduction in the number of TdT+ cortical thymocytes (CD4+8+, TCR alpha/beta low, Thy-1hi). These intrathymic changes were accompanied by a reciprocal increase in the number of double-positive (DP; CD4+8+) T cells in lymph nodes (LN) and spleens. To confirm that the latter T cells were recent thymic emigrants (RTE), CSA-treated rats were injected intrathymically with fluorescein isothiocyanate, and the phenotype of the labeled T cells appearing in LN was determined 16 h later. The results demonstrated that, in addition to those RTE exported in normal animals (> 90% medullary origin), the emigration of DP thymocytes, including large numbers of TdT+ thymocytes, was markedly increased. The presence of TdT+ cells, which normally do not leave the thymus, clearly identifies the DP RTE as originating from the thymus cortex. Intrathymic labeling studies also directly demonstrated that export of all thymocyte subsets ceases within 9 d of CSA treatment; and thymectomy experiments confirmed that the CSA-induced increase in phenotypically immature T cells resulted primarily from the disturbance of thymocyte maturation and emigration, rather than from a direct effect on preexisting T cells. These results suggest that a wave of cortical thymocytes, many of which presumably have not yet undergone negative selection, is released from the thymus during the first week of CSA treatment. The presence of these potentially unselected cells in peripheral lymphoid tissues may help to explain the increased frequency of autoreactive T cells observed in CSA-treated animals.

T cell receptor targeting to thymic cortical epithelial cells in vivo induces survival, activation and differentiation of immature thymocytes

We report that targeting of T cell receptors (TcR) to non-major histocompatibility complex (MHC) molecules on thymic cortical epithelial cells by hybrid antibodies in vivo and in fetal thymic organ cultures results in phenotypic and functional differentiation of thymocytes. A single pulse with hybrid antibodies rescues immature, CD4/8 double-positive thymocytes from their programmed death in vivo, induces expression of the early activation antigen CD69 followed by TcR up-regulation, concomitant down-regulation of CD8 or CD4 and their conversion to functional mature T cells by day 3. This temporal sequence of maturation only affects small thymocytes without co-induction of blastogenesis. TcR targeting to MHC class II-positive epithelial cells predominantly induces CD4-positive T cells. This generation of CD4 single-positive T cells occurs also in MHC class II-deficient mice and thus is independent of CD4-MHC class II interactions. Moreover, in the presence of a specific deleting antigen (Mls 1a), TcR targeting results in transient activation of immature thymocytes, however, not in subsequent TcR (V beta 6) up-regulation and development of single-positive T cells. Our findings imply that TcR cross-linking to cortical epithelial cells is sufficient to confer a differentiation signal to immature thymocytes. Furthermore, this approach distinguishes two independent TcR-mediated intrathymic events: activation and subsequent deletion of the same thymocyte subset.

Evidence for a stochastic mechanism in the differentiation of mature subsets of T lymphocytes

Thymocytes that coexpress the CD4 and CD8 glycoproteins differentiate into mature CD4+ helper or CD8+ cytotoxic cells depending on whether their antigen receptors are specific for MHC class II or class I molecules, respectively. The mechanism of this decision process was investigated in mice whose T cell development was biased toward the class II-specific lineage. We found that constitutive expression of CD4 allows a developmentally arrested population of thymocytes that have mismatched class II-specific TCRs and the CD8 coreceptor to be rescued and to acquire a cytotoxic phenotype. This result is consistent with a two-step process of thymocyte maturation, in which there is stochastic down-regulation of either CD4 or CD8 and subsequent selection based on the ability of the TCR and remaining coreceptor to engage the same MHC molecule.

Another view of the selective model of thymocyte selection

Thymocyte commitment to the CD4 helper versus CD8 cytotoxic lineage has not been satisfactorily established. Two models have been elaborated: one based on instruction, the other on selection. Most previous results support the instructive model, but our comparison of thymocyte differentiation in MHC class II-, class I- and double-deficient mice provides data challenging it. There exists a significant population of CD4 single positive cells in class II-deficient animals that is intermediate in maturity between CD4+CD8+ and end-stage CD4+CD8- thymocytes and is selected on class I molecules; an equivalent CD8+CD4- population occurs in class I-deficient animals. We propose a selective model entailing two TCR-MHC molecule engagements: the first provokes random down-modulation of either CD4 or CD8 and a degree of differentiation; the second, requiring participation of the appropriate coreceptor, permits end-stage differentiation.

Clonal deletion of thymic mature T cells induced by staphylococcal enterotoxin B in murine fetal thymus organ culture

The present study aims at investigating the intrathymic maturational stage of T cells at which clonal deletion can be induced. Staphylococcal enterotoxin B (SEB) was added to organ cultures of murine fetal thymus lobes at various time points of culture, and V beta 8-expressing cells were assayed on day 14. V beta 8 low-expressing (V beta 8lo) cells were reduced to 40-60% of the control receiving no SEB, though the reduction was ambiguous when SEB was given on day 13. In marked contrast, V beta 8 high-expressing (V beta 8hi) cells were virtually completely deleted in all groups including the group given SEB on day 13. Most of the V beta 8hi cells that were deleted by 24 h of treatment with SEB were shown to be of the CD4+8- mature phenotype, though CD4-8+V beta 8hi cells were also deleted. It was further shown that the thymic V beta 8hi CD4+8- cells recovered from organs cultured for 14 days without SEB responded to immobilized anti-V beta 8 monoclonal antibody, indicating that V beta 8hi cells, which were highly sensitive to clonal deletion, were functionally competent mature T cells. These results strongly suggest that the thymus is capable of eliminating all T cells recognizing antigen present in the thymus regardless of the maturational stage of T cells.

CD69 expression during selection and maturation of CD4+8+ thymocytes

We have analyzed the inducibility of protein kinase C (PKC)-dependent expression of CD 69 molecules in T cell receptor (TCR) transgenic thymocytes developing in the presence or absence of selecting, class I major histocompatibility complex (MHC) molecules. Small CD4+8+ thymocytes developing in the absence of selecting MHC molecules could not be induced to express CD 69 by TCR cross-linking even after spontaneous in vitro up-regulation of their TCR level which resulted in enhanced Ca++ flux. In contrast, a small proportion of CD4+8+TCRlow and most TCRhigh (CD4+8+ and CD4-8+) thymocytes developing in the presence of selecting MHC ligands could be induced to express CD 69 upon TCR cross-linking. Unlike the anti-TCR antibody, phorbol 12-myristate 13-acetate--a direct activator of PKC--induced the expression of CD 69 on all thymocytes. These results suggest that positive selection of CD4+8+ thymocytes results on coupling of TCR-mediated signals to the CD 69 expression pathway. In vitro analysis of thymocytes before and after positive selection suggests that (1) positive selection does not immediately result in resistance to deletion and (2) that sustained TCR ligation is needed to promote maturation of positively selected CD4+8+ thymocytes resulting in gradual loss of the sensitivity to deletion and acquisition of the ability to proliferate in response to TCR-mediated signals.

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.

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.

Differential effects of Bcl-2 on T and B cells in transgenic mice

We have produced bcl-2 transgenic mice by using a construct which mimics the t(14;18) translocation in human follicular lymphomas. Although lymphoid tissues from all transgenic mice contained high levels of human Bcl-2 protein, transgene expression was differentially regulated within the B- and T-cell compartments of lines derived from various founder mice. We have characterized the phenotypes of two lines of bcl-2 transgenic mice (line 2 and line 6) in which bcl-2 transgene expression was restricted primarily to the T- or B-cell lineages, respectively. Analysis of line 6 lymphocytes revealed a polyclonal expansion of B cells, and these B cells exhibited prolonged survival in vitro. In line 2 mice, numbers of T cells in the peripheral lymphoid tissues were more moderately elevated despite enhanced T-cell survival in vitro. Line 2 transgenic mice also showed significantly increased proportions of thymocytes with a mature phenotype. Taken together, these findings suggest different roles for bcl-2 in the in vivo regulation of B- and T-cell development and homeostasis.

Regulation of N-region diversity in antigen receptors through thymocyte differentiation and thymus ontogeny

The random addition of "N nucleotides" by terminal deoxynucleotidyltransferase (TdT) is an important component of the diversity of T-cell receptor genes. We have investigated the expression of TdT during thymocyte differentiation and thymus ontogeny. TdT gene transcripts are confined to immature thymocytes of the cortex, being down-regulated concomitantly with recombination-activating gene transcripts after positive selection of mature medullary T cells. According to in situ hybridization, TdT RNA is absent from the neonatal thymus, but it appears 3 to 5 days after birth, just before the appearance of significant N-region diversity in T-cell receptor junctional sequences but clearly after the thymus attains competence at clonal deletion.

CD45RA-R0+ subset is the major population of dividing thymocytes in the human

CD45, or leukocyte common antigen, is expressed in different isoforms on different subsets of thymocytes, suggesting its involvement in the process of T cell development in the thymus. We report studies on CD45 isoform expression on human thymocytes at various stages of development using three-color flow cytometric analysis and cell cycle analysis. Among CD45R0+ cells 18.4% were in S+G2/M phase and represented more than 80% of the dividing cells in the thymus. Among the CD45R0- cells 10.9% were also in cell cycle. Because the CD45R0+ population is almost exclusively CD45RA-, the CD45RA-R0+ subset constitutes the major portion of dividing cells in the thymus. Both the CD1high and the CD3- populations were actively cycling. However, the former was almost 100% and the latter only 50% CD45RA-R0+. Dividing CD45RA-R0+ cells contain, therefore, many cells that have not yet expressed the CD3/T cell receptor complex and presumably have not yet undergone selective procedures. These results are hard to reconcile with the previously presented hypothesis that CD45R0 represents a marker for cells that are destined to die in the thymus. Instead, these results suggest an alternative possibility that CD45R0+ cells may contain cells that can mature and, thus, also constitute the thymic generative lineage.

Defective maintenance of T cell tolerance to a superantigen in MRL-lpr/lpr mice

In normal mice neonatal injection of staphylococcal enterotoxin B (SEB) induces tolerance in T cells that express reactive T cell receptor (TCR) V beta regions. To determine if a T cell neonatal defect was present in MRL-lpr/lpr mice, 20 micrograms of SEB was injected intraperitoneally every other day into V beta 8.2 TCR transgenic and nontransgenic MRL(-)+/+ and MRL-lpr/lpr mice from birth to 2 wk of age. At 2 wk of age, V beta 8+ T cells were depleted, and SEB reactivity was lost, in spleen, lymph node, and thymus. These effects were equivalent in +/+ and lpr/lpr SEB-tolerized mice. However, MRL-lpr/lpr mice failed to maintain neonatal tolerance. By 4 wk of age, there was a dramatic increase in T cells expressing V beta 8.2 in the peripheral lymph nodes of MRL-lpr/lpr mice but not MRL(-)+/+ mice. In vitro stimulation with SEB or TCR crosslinking revealed a total loss of neonatal tolerance 2 wk after cessation of SEB treatment in lpr/lpr mice, but not +/+ mice. The time-course of recovery of V beta 8+ T cells and reactivity to SEB and TCR crosslinking in the thymus of MRL-lpr/lpr mice was similar to that in the lymph node. Thymectomy at 2 wk of age eliminated tolerance loss in lymph nodes of MRL-lpr/lpr mice at 4 wk of age, indicating that loss of peripheral tolerance was due to the emigration of untolerized T cells from the thymus. Challenge of neonatally tolerized MRL-lpr/lpr mice with SEB (100 micrograms, i.p.) at 8 wk of age resulted in a dramatic onset of T cell-mediated autoimmune disease characterized by 30% weight loss and 60% morality. This indicated that loss of tolerance to SEB also occurred in vivo. In contrast, neonatally tolerized MRL(-)+/+ mice remained totally unresponsive to SEB challenge and did not undergo any detectable weight loss. These results suggest that there is normal induction of neonatal tolerance to SEB in lpr/lpr mice, but that tolerance is not maintained after the tolerizing antigen is removed. This loss of neonatal tolerance can lead to severe weight loss and death on exposure to the tolerizing antigen later in life.

Phenotypic changes accompanying positive selection of CD4+CD8+ thymocytes

We know little about the way mature CD4 (helper) and CD8 (killer) T cells develop from thymic CD4+CD8+ precursors. Here we show that small but not large CD4+CD8+ cells with high levels of the alpha beta T cell receptor (TcRhigh) result from positive selection. Neither CD4+CD8+ cells with low TcR levels nor large CD4+CD8+ thymocytes with high TcR levels differentiate in vitro. However, small CD4+CD8+ cells with high TcR levels develop in vitro into mature cells by gradually decreasing the surface levels of one or the other co-receptor and acquiring the potential to respond with proliferation to ligation of the TcR. Small CD4+CD8+ cells with high levels of a major histocompatibility complex (MHC) class I-restricted transgenic TcR develop in vitro exclusively into CD4-CD8+ cells while small CD4+CD8+ TcRhigh cells with heterogeneous TcR from various mice yield both CD4 and CD8 T cells. While these experiments are consistent with an instructive model of CD4/CD8 lineage commitment they do not rule out other mechanisms which require multiple TcR-MHC ligand interactions in the generation of mature alpha beta T cells.

Inhibition of thymocyte apoptosis and negative antigenic selection in bcl-2 transgenic mice

The bcl-2 gene, which is overexpressed in human follicular B-cell lymphomas, has been found to extend cellular lifespan through inhibition of apoptosis, or programmed cell death. However, the physiological role of the Bcl-2 protein in lymphocyte development is unclear. We have established a transgenic mouse line that expresses high levels of the Bcl-2 protein in both cortical and medullary thymocytes, disrupting the normal pattern of expression of this gene. We found that in these mice, immature thymocytes became resistant to apoptosis mediated by corticosteroids and calcium ionophores. Untreated thymocytes also exhibited a survival advantage in suspension cultures compared with controls. In addition, overexpression of bcl-2 enabled a proportion of thymocytes and peripheral T cells to escape the process of clonal deletion, which normally eliminates self-reactive T cells during thymocyte maturation. These findings implicate the Bcl-2 protein in regulating the lifespan of maturing thymocytes and in the antigenic-selection process.

Differentiation of CD3-4-8- thymocytes in short-term thymic stromal cell culture

We have investigated the ability of a heterogeneous thymic stromal cell (HTSC) culture system to promote in vitro differentiation of CD3-4-8- thymocytes. Culture of purified murine CD3-4-8- thymocytes on HTSC for 1 d resulted in the appearance of CD4+8+ cells, which did not occur when the sorted cells were maintained in medium alone. It is remarkable that when the culture period was extended to 2 d, CD3-4-8- progenitors differentiated further to CD4+8- and CD4-8+ cells, which also expressed high levels of TCR-CD3. This rapid differentiation on stroma in vitro appears to outpace parallel development in vivo. The differentiation potential of a subset of CD3-4-8- thymocytes that express high levels of a marker of normal and neoplastic thymic progenitors, the 1C11 antigen, was examined next. 1C11hiCD3-4-8- cells also gave rise to CD4-8+ and CD4+8+ populations after 1 d of culture on HTSC. Extending the culture period to 2 d resulted in a significant percentage of CD3-expressing cells that were CD4+8+, CD4+8- and CD4-8+ cells. These results suggest that in the in vitro HTSC culture system, various subsets of immature thymocytes can differentiate into all the mature phenotypes of cells normally found in the adult mouse thymus. This may provide a novel and rapid assay for thymic progenitors.

Bcl-2: a repressor of lymphocyte death

The genes and mechanisms that control programmed cell death are currently the subject of intense study. The bcl-2 gene, a repressor of lymphocyte death, is perhaps the best understood of the programmed cell death associated genes. Here, Stanley Korsmeyer provides a brief overview of bcl-2, concentrating on its roles in B- and T-cell development and in oncogenesis.

CD45 isoform expression during T cell development in the thymus

Various isoforms of leukocyte common antigen, or CD45, are expressed differentially on T cells at different stages of development and activation. We report studies on CD45 isoform expression on various subsets of human T cells using two- and three-color flow cytometry and cell depletion. Bone marrow cells that were depleted of CD3+ and HLA-DR+ cells were CD45RA-RO-. The earliest CD3-CD4-CD8-CD19- thymocytes were CD45RO- with 20%-30% CD45RA+ cells. The most prominent population of CD4+CD8+ double-positive thymocytes were CD45RA-RO+. Even the CD4+CD8+ blasts were greater than 90% CD45RO+. About 80% of single-positive thymocytes (CD4+CD8- or CD4-CD8+) were also CD45RO+. Only 4.3% of CD4+ and 18% of CD8+ single-positive thymocytes were CD45RA+. In contrast, cord blood T cells which represent the stage that immediately follows single-positive thymocytes, contained 90% CD45RA+ cells. Thus, in terms of CD45 isoform expression, single-positive thymocytes are more like double-positive cells than cord blood T cells. These results suggest the following sequence of CD45 isoform switching during T cell development: CD45RA-RO- or RA+RO- (double-negative thymocytes)----RA-RO+ (double-positive and most single-positive thymocytes)----RA+RO- (cord blood T cells), the last switch from CD45RO to CD45RA occurring as a final step of maturation in the thymus.

Developmental regulation of thymocyte susceptibility to deletion by "self"-peptide

The specificity of the T cell receptor (TCR) repertoire for foreign peptide bound to self-major histocompatibility complex (MHC) molecules is determined in large part by positive and negative selection processes in the thymus, yet the mechanisms of these selection events remain unknown. Using in vitro organ culture of thymi isolated from mice transgenic for a TCR-alpha/beta specific for cytochrome c peptide bound to I-Ek, we analyzed the developmental timing of negative selection (deletion). On the basis of the experiments described below, we conclude that all CD4+8+ thymocytes, and only CD4+8+ thymocytes, are susceptible to negative selection mediated by the cytochrome c peptide antigen. First, we found that deletion of thymocytes resulting from addition of the cytochrome c peptide to the thymic organ cultures can occur at the earliest stage of TCR, CD4, and CD8 coexpression. Second, we found that CD4+8+ thymocytes isolated from positively selecting or nonselecting MHC haplotypes were equally efficiently deleted in vitro, suggesting that positive selection is not a prerequisite for deletion. Third, we examined the effects of TCR/ligand avidity on the developmental timing of deletion by varying the concentration of cytochrome c peptide added to the organ cultures. We detected deletion only at the CD4+8+ stage: intermediate concentrations of peptide that resulted in partial deletion of CD4+8+ cells did not eliminate the appearance of mature CD4+8- cells. Finally, we found that CD4+8- thymocytes were resistant to deletion as well as activation by peptide antigen added to the intact organ cultures. Nevertheless, the CD4+8- thymocytes isolated from the peptide-treated organ cultures responded vigorously to peptide presented by spleen cells in vitro. Thus, the T cells were tolerant of (but not anergized by) self-antigen encountered in thymic organ culture. Together, these results indicate that thymocytes susceptible to negative selection are not developmentally distinct from those susceptible to positive selection, and further, that the thymic microenvironment plays a role in regulating the outcome of TCR/ligand interactions.

The extent of clonal structure in different lymphoid organs

To gain insight into the clonal organization of lymphoid organs, we studied the distribution in situ of donor-derived cells in near-physiological chimeras. We introduced RT7b fetal liver cells into nonirradiated congenic RT7a neonatal rats. The chimerism 6-20 wk after injection ranged from 0.3 to 20%. The numbers of cell clones simultaneously contributing to cell generation in a particular histological feature were deduced from the variance in donor cell distribution. In bone marrow and thymus, donor-derived lymphoid cells were found scattered among host cells, indicating a high mobility of cells. In bone marrow, donor cells were evenly distributed over the entire marrow, even at low chimerism. This indicates that leukopoiesis is maintained by the proliferation of many clones. In the thymus, the various lobules showed different quantities of donor-derived lymphoid cells. Mathematical analysis of these differences indicated that 17-18 cell division cycles occur in the cortex. In spleen, the distribution of donor-derived cells over the germinal centers indicated that 5 d after antigenic stimulation, germinal centers develop oligoclonally. The main conclusions of this work are that (a) bone marrow and thymus are highly polyclonal; (b) 17-18 divisions occur between prothymocyte and mature T cell; and (c) lymphoid cells disperse rapidly while proliferating and differentiating.

Exclusion and inclusion of alpha and beta T cell receptor alleles

Exclusion and inclusion of T cell receptor (TCR) genes were analyzed in alpha beta TCR transgenic mice. Both transgenes are expressed unusually early on the surface of CD4-8-, HSA+, IL-2R- thymocytes. These progenitor cells give rise to progeny, which at the single-cell level contains endogenous alpha but not beta TCR-RNA as well as protein, in addition to products encoded by the transgenes. Thus, the surface expression of an alpha beta TCR does not prevent further alpha TCR rearrangement in immature thymocytes that still transcribe RAG-1 and RAG-2 genes. Reduced levels of RAG-1 and RAG-2 RNA are detectable only in CD4+8+ TCR high cells, which result from positive selection in the thymus. The results suggest that a developing T cell may try different alpha beta TCRs for binding to thymic MHC ligands, and that recombination at the alpha locus ceases only after positive selection.

Weak positive selection of transgenic T cell receptor-bearing thymocytes: importance of major histocompatibility complex class II, T cell receptor and CD4 surface molecule densities

We have produced alpha beta T cell receptor (TcR)-transgenic mice and studied MHC-dependent positive selection of T cells bearing this receptor. The alpha and beta transgenes were isolated from an I-Ed-restricted, CD4+ BALB/c (H-2d/d) T cell clone specific for a peptide consisting of the 91-101 residues of the lambda 2 immunoglobulin light chain of MOPC315. Mice which carry the transgenes on a BALB/c background, but with H-2d/d, H-2b/d or H-2b/b major histocompatibility complex (MHC) haplotypes, were investigated for TcR expression in thymocytes and peripheral T cells. The thymocytes expressing the transgene-encoded alpha beta receptor are weakly positively selected when compared with previous findings in other TcR-transgenic mice models. Thus, alpha beta thymocytes vary in their efficacy of being positively selected by their restriction element. Furthermore, the density of TcR and CD4 on thymocytes, as well as the density of I-Ed molecules on thymic epithelial cells, appear critical for the extent of positive selection. A possible explanation is that the transgenic TcR has a marginal affinity for self-MHC molecules on thymic epithelium, and that this may be compensated for by an increase in the number of CD4/TcR/MHC ternary complexes forming between the maturing thymocyte and the cortical epithelial cells.

Activation events during thymic selection

During their differentiation in the mouse thymus, CD4+8- cells undergo several of the sequential changes observed upon normal activation of mature, peripheral CD4+ lymphocytes. Expression of CD69, an early activation marker, is first observed on a minority of cells at the T cell receptor (TCR)lo/med double-positive stage, is maximal (50-90%) on heat-stable antigen (HSA)hi TCRhi double-positive, HSAhi TCRmed CD4+8lo, and HSAhi TCRhi CD4+8- cells, and is downmodulated at the mature HSAlo CD4+8- stage. In contrast, CD44, a late activation marker, is selectively expressed at the HSAlo stage. The set of lymphokines that CD4+8- thymocytes can produce upon stimulation also characteristically expands from mainly interleukin 2 (IL-2) at the HSAhi stage, to IL-2 and very large amounts of IL-4, IL-5, IL-10, and interferon gamma (IFN-gamma) at the HSAlo stage. 1 in 30 HSAlo CD4+8- adult thymocytes secrete IL-4 upon stimulation through their TCR. This frequency is 25% of the frequency of IL-2 producers, about 100-fold above that of peripheral (mainly resting) CD4+ T cells. With time after their generation in organ culture, CD4+8- thymocytes lose their capacity to secrete IL-4, IL-5, and IFN-gamma, but not IL-2. Similarly, the frequency of IL-4, but not of IL-2, producers progressively decreases after emigration to the periphery as judged by direct comparison between thymic and splenic CD4+ cells in newborns, or by following the fate of intrathymically labeled CD4+8- cells in adults after their migration to the spleen. This sequence suggests that thymic selection results from an activation process rather than a simple rescue from death at the double-positive stage, and shows that the functional changes induced after intrathymic activation, although transient, are still evident after export to the periphery.

Age-associated increase in number of CD4+CD8+ intestinal intraepithelial lymphocytes in rats

A significant number of CD4+CD8+ T cells were detected in intestinal intraepithelial lymphocytes (IEL) of various strains of rats including Wistar, WKA, BN, LEW and F344. The site of the CD4+CD8+ population in IEL increased with age in all strains we examined. Most IEL bearing CD8 expressed no CD5 antigen in young rats, while all CD4+CD8+ IEL and some of CD8+ IEL in aged rats were of CD5+CD45RB- phenotype. In germ-free Wistar rats, age-associated increase in the number of CD4+CD8+CD5+ IEL was not evident, indicating that stimulation by the intestinal microflora was important for expansion of the CD4+CD8+CD5+CD45RB- IEL. Aged athymic F344 nude rats contained appreciable numbers of CD4+ IEL and CD8+ IEL but few CD4+CD8+ IEL, suggesting that the CD4+CD8+ IEL may be derived from thymus-dependent populations. Unlike a majority of CD4+CD8+ thymocytes bearing a low intensity of CD3/T cell receptor (TcR) alpha/beta, the CD4+CD8+ T cells in IEL expressed a high intensity of CD3/TcR alpha/beta on their surface. The CD4+CD8+ IEL appear to contribute to the spontaneous proliferation of the IEL in aged rats as assessed by tritiated thymidine incorporation after in vitro culture with medium only. These results suggest that with aging a unique CD4+CD8+ IEL may expand at a local site of the intestine under the influence of intestinal microflora and may contribute to the first line of defense against various pathogens in the epithelium.

Different expression of the recombination activity gene RAG-1 in various populations of thymocytes, peripheral T cells and gut thymus-independent intraepithelial lymphocytes suggests two pathways of T cell receptor rearrangement

The presence of transcripts of the recombination activating gene RAG-1 was studied by in situ hybridization on selected populations of murine thymocytes, peripheral lymphocytes and gut intraepithelial lymphocytes (IEL), obtained by cell sorting. RAG-1 mRNA was found in a majority of "double-positive" (DP) thymocytes, but was absent in "single-positive" thymocytes and peripheral T lymphocytes. The only other T lineages in which about 10%-20% of the cells contained RAG-1 mRNA, and in smaller amounts, were "double-negative" (DN), T cell receptor (TcR) gamma delta- cortical thymocytes and gut CD3- IEL. These observations suggest that (a) the high expression of RAG-1 transcripts in DP thymocytes is related to the process of expansion-selection of these cells, probably accompanied by repeated TcR rearrangements, and that (b) in contrast, CD3- IEL from the gut (which are thymus independent) as well as some DN thymocytes undergo limited TcR rearrangement giving rise locally to TcR+ T cells without prior extensive process of local expansion-selection. A small percentage of peripheral B cells also contained RAG-1 mRNA, raising the possibility that this protein may also be involved in immunoglobulin class switching.

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)

Extrathymic positive selection of αβ T-cell precursors in nude mice

T lymphocytes expressing alpha beta T-cell receptors with sufficient affinity to major histocompatibility complex (MHC) molecules expressed on thymus epithelial cells are positively selected and mature to functional T cells. But several studies have demonstrated that athymic nude mice grafted with MHC-incompatible thymuses developed T cells specific for nude host rather than thymic MHC. We examined this paradox by analysing the specificity of T lymphocytes derived from nude mice. We report here that nude T lymphocyte precursors transferred to allogeneic SCID (severe combined immunodeficiency) mice with a functioning thymus (but lacking T or B cells) generated host MHC-restricted effector T cells but also contained T cells restricted to donor MHC. If nude T cells were depleted from nude lymphohaemopoietic donor cells before or after transfer, only host MHC-specific T cells matured. The results may explain the unusual MHC specificities of nude T lymphocytes described in earlier studies and demonstrate two separate differentiation steps: in nude mice, T cells may be positively selected for self-MHC restriction specificity extrathymically; then a functional thymus is required for efficient T cell maturation.

bcl-2 inhibits multiple forms of apoptosis but not negative selection in thymocytes

The vast majority of cortical thymocytes die during T cell development while those that survive this selective process accumulate in the medulla. bcl-2, an inner mitochondrial membrane protein, has been shown to inhibit apoptosis in certain cell lines. In the thymus, bcl-2 is regionally localized to the mature T cells of the medulla. To assess the role of bcl-2 in the programmed death of thymocytes, we generated transgenic mice that redirected bcl-2 expression to cortical thymocytes. bcl-2 protected immature CD4+8+ thymocytes from glucocorticoid, radiation, and anti-CD3-induced apoptosis. Moreover, bcl-2 altered T cell maturation, resulting in increased percentages of CD3hi and CD4-8+ thymocytes. Despite this, clonal deletion of T cells that recognize endogenous superantigens still occurred. This transgenic model indicates that multiple death pathways operate within the thymus that can be distinguished by their dependence on bcl-2.

CD4+CD8+CD3high thymocytes appear transiently during ontogeny: evidence from phenotypic and functional studies

During T cell development thymocyte subsets emerge in a defined order, reflective of their maturational stage. In this study we determined the timing of appearance of CD4+CD8+CD3high thymocytes during in vivo and in vitro embryonic development, and thymic reconstitution after cortisone treatment. In these models, CD4+CD8+CD3high cells followed CD4+CD8+CD3low and preceded mature CD4+CD8-CD3high/CD4-CD8+CD3high thymocytes, while cortisone resistance was first seen among CD4+CD8+CD3high cells. CD4+CD8+CD3high thymocytes were also shown to display a pattern of antigen receptor-mediated calcium influx intermediate between that induced in other CD4+CD8+ cells and mature thymocytes. These results are consistent with a precursor-progeny relationship between CD4+CD8+CD3low and CD4+CD8+CD3high thymocytes, the latter developing to mature thymocytes (Hugo, P. et al., Int. Immunol. 1991. 3: 265).

Kinetics and efficacy of positive selection in the thymus of normal and T cell receptor transgenic mice

DNA-labeling studies in alpha beta T cell receptor (TCR) transgenic mice show that the lifespan of immature CD4+8+ thymocytes is 3.5 days irrespective of whether they are selected for maturation or not. While nonselected cells die, the binding of the TCR to thymic major histocompatibility complex molecules rescues CD4+8+ cells from programmed cell death and induces first upregulation of the TCR level and then differentiation into CD4+8- or CD4-8+ cells in the absence of any cell division. When most CD4+8+ thymocytes express a selectable transgenic TCR the formation of mature cells with high TCR levels is 10-20 times as efficient as observed in normal mice, yet still only 20% of the CD4+8+ cells become mature. This is due to the limited availability of selecting 'niches': most CD4+8+ thymocytes with a selectable transgenic TCR will undergo maturation when they represent only 5% or less of all CD4+8+ cells.