The major histocompatibility complex-restricted antigen receptor on T cells: distribution on thymus and peripheral T cells

Novel post-translational regulation of TCR expression in CD4+CD8+ thymocytes influenced by CD4

Expression of the multicomponent T-cell antigen receptor (TCR) complex on the surface of thymocytes is developmentally controlled. Most immature CD4-CD8- 'double negative' and CD4+CD8+ 'double positive' thymocytes express either no or few TCR on their surface, and maturation to CD4+CD8- or CD4-CD8+ 'single positive' thymocytes is accompanied by a dramatic increase in the number of surface TCR complexes. Although the initial appearance of TCR during differentiation results from rearrangement and initiation of transcription of TCR genes in the thymus, the mechanisms regulating the quantitative changes in TCR expression during intrathymic differentiation are unknown. Surface TCR levels in T-hybridoma cells can be quantitatively regulated by a series of post-translational processes, including sorting to alternative intracellular compartments and degradation, which ensure that only fully and correctly assembled receptor complexes are efficiently transported to the cell surface. Quantitative increases in TCR expression on the surface of CD4+CD8+ thymocytes occur in vivo in response to anti-CD4 antibody treatment. Here we present evidence that immature CD4+CD8+ thymocytes normally retain and degrade in the endoplasmic reticulum greater than 90% of some endogenously synthesized TCR chains, and that the increased surface TCR expression on immature CD4+CD8+ thymocytes induced by anti-CD4 is due to an increase in the escape of newly synthesized receptor chains from the endoplasmic reticulum, and is not due to increases in RNA levels, translation, or assembly. Post-translational mechanisms therefore control the levels of TCR complexes on CD4+CD8+ thymocytes, and these mechanisms can be modulated by signalling through CD4 surface molecules.

Tissue distribution and appearance in ontogeny of alpha/beta T cell receptor (TCR2) in chicken

We have performed immunoperoxidase staining on cryostat tissue sections and immunofluorescence analysis on cell suspensions to identify cells expressing the alpha/beta T cell antigen receptor during ontogeny and adult life in chickens. We used the mouse monoclonal antibody, TCR2, which was previously shown to recognize the alpha/beta TCR in chickens. TCR2+ cells were observed in thymic cortex and medulla and in T-dependent areas of spleen, intestine, and cecal tonsils of young adult chickens. Some TCR2+ cells were found in the cortex of bursal follicles and in liver. The first TCR2+ cells appear in thymus on Day 13 of the embryonic life and it is only after hatching that TCR2+ cells begin to migrate to the periphery.

Selective elimination of double-positive immature thymocytes by a thymic epithelial cell line

A cloned epithelial cell line, TEL-2, has been established from the stroma tissues of normal mouse thymus. Incubation of mouse thymocytes on TEL-2 cells resulted in the selective elimination of double-positive (CD4+CD8+) cells from the culture, whereas single-positive (CD4+CD8- or CD4-CD8+) thymocytes remaining in the culture were concentrated in non-integrated cell population. The CD3- or CD3 low-positive thymocytes were also eliminated by the TEL-2 cells from the culture, followed by the concentration of CD3 high-positive cells in the culture. Only intact viable thymocytes were integrated into TEL-2 cells. Electron microscopic examination showed that the integrated cells into TEL-2 cytoplasm were gradually degenerated. Mature single-positive T cells, mature B cells or double-negative thymocytes were not integrated into TEL-2 cells. The TEL-2 cell may provide information on the mechanism of selective disappearance of double-positive immature cells from the thymus.

Surprisingly uneven distribution of the T cell receptor V beta repertoire in wild mice

We have examined TCR V beta expression in a collection of wild mice. Many of the mice were homozygous for a large deletion at the V beta locus, and many animals also suppressed expression of several V betas using self superantigens. Expression of V beta 8.2 was unexpectedly suppressed by a self superantigen in some wild mice, which was due to the presence in these animals of a variant V beta 8.2 gene. The amino acid changes in this gene product suggest contact sites between V beta and the superantigen. Although all V betas are expressed within each wild mouse population, individual mice have a limited and variable V beta repertoire. The independent origin of multiple V beta deletions and the presence of polymorphic self superantigens suggest that this variation may be maintained by balancing selection.

Selective development of CD4+ T cells in transgenic mice expressing a class II MHC-restricted antigen receptor

T lymphocytes are predisposed to recognition of foreign protein fragments bound to cell-surface molecules encoded by the major histocompatibility complex (MHC). There is now compelling evidence that this specificity is a consequence of a selection process operating on developing T lymphocytes in the thymus. As a result of this positive selection, thymocytes that express antigen receptors with a threshold affinity for self MHC-encoded glycoproteins preferentially emigrate from the thymus and seed peripheral lymphoid organs. The specificity for both foreign antigen and MHC molecules is imparted by the alpha and beta chains of the T-cell antigen receptor (TCR). Two other T-cell surface proteins, CD4 and CD8, which bind non-polymorphic regions of class II and class I MHC molecules respectively, are also involved in these recognition events and play an integral role in thymic selection. In order to elucidate the developmental pathways of class II MHC-restricted T cells in relation to these essential accessory molecules, we have produced TCR-transgenic mice expressing a receptor specific for a fragment of pigeon cytochrome c and the Ek (class II MHC) molecule. The transgenic TCR is expressed on virtually all T cells in mice expressing Ek. The thymuses of these mice contain an abnormally high percentage of mature CD4+CD8- cells. In addition, the peripheral T-cell population is almost exclusively CD4+, demonstrating that the MHC specificity of the TCR determines the phenotype of T cells during selection in the thymus.

Positive selection of CD4+ T cells mediated by MHC class II-bearing stromal cell in the thymic cortex

T lymphocytes differentiate in the thymus, where functionally immature, CD4+CD8+ (double positive) thymocytes develop into functionally mature CD4+ helper cells and CD8+ cytotoxic (single positive) T cells. The thymus is the site where self-reactive T cells are negatively selected (clonally deleted) and where T cells with the capacity to recognize foreign antigens in association with self-proteins encoded by the major histocompatibility complex (MHC) are positively selected. The net result of these developmental pathways is a T-cell repertoire that is both self-tolerant and self-restricted. One unresolved issue is the identity of the thymic stromal cells that mediate the negative and positive selection of the T-cell repertoire. Previous work has pointed to a bone-marrow-derived macrophage or dendritic cell as the inducer of tolerance, whereas a radiation-resistant, deoxyguanosine-resistant thymic cell seems to mediate the positive selection of self-MHC restricted T cells. Thymic stromal cells in the cortex interact with the T-cell antigen receptor on thymocytes. Using several strains of transgenic mice that express the class II MHC molecule I-E in specific regions of the thymus, we show directly that the positive selection of T cells is mediated by an I-E-bearing cell in the thymic cortex.

T cell receptor expression on immature thymocytes with in vivo and in vitro precursor potential

Immature CD8-CD4- double-negative (DN) thymocytes differentiate intrathymically into CD8+CD4- and CD8-CD4+ thymocytes and migrate to the periphery. This differentiation proceeds through several intermediate phenotypic changes in the expression of CD8 and CD4. We have recently established the existence of a CD8loCD4lo cell population in murine thymus that can repopulate the irradiated thymus in vivo and differentiate rapidly in vitro to CD8+CD4+ double-positive (DP) cells. The CD8loCD4lo cells score as DN upon direct cytofluorometric analysis, yet are distinct from true DN cells by various criteria. Experimental evidence strongly suggests that they are descendants of true DN in the maturation pathway. In the experiments presented here, we further characterize this CD8loCD4lo thymocyte population. Northern blot and RNA protection analysis reveal that these cells transcribe full length mRNA for the T cell receptor (TcR)alpha chain, unlike the less mature interleukin 2 receptor-positive DN thymocytes. Surface expression of the TcR-associated CD3 molecule occurs on approximately 15% of these cells at low levels characteristic of immature cells. In the course of in vitro differentiation a vast majority (approximately 80%) of these cells convert to CD8+CD4+ and significant numbers of the brightly staining DP convertants (11%-34% on day 1 and 48%-68% on day 2) express immature levels of CD3. Our results indicate that CD8lo, CD4lo cells might be the first thymic subset to rearrange TcR alpha chain genes and express TcR alpha/beta heterodimer on the surface at levels characteristic of immature cells. Furthermore, the surface expression of TcR persists on the in vitro progeny of these thymocytes.

Intrathymic maturation of murine T lymphocytes from CD8+ precursors

The CD4-8- thymocyte subset contains immature precursors for phenotypically and functionally mature CD4+8- and CD4-8+ thymocytes and peripheral T cells, as well as nonmature CD4+8+ thymocytes, most of which die in situ. The intrathymic death of most thymocytes is probably related to selective influences that ensure that only those precursors bearing self-major histocompatibility complex (MHC)-restricted and self-tolerant T-cell antigen receptors (TCR) survive to complete the maturation process. Interactions between surface molecules on thymocytes (TCR, CD4, and CD8) and thymic stromal cells (MHC proteins) are critical to repertoire selection. To understand this process, the lineage relationships among immature, nonmature, and mature thymocytes must be defined. We have examined directly the precursor-progeny relationships among CD4+8-, CD4-8+, and CD4+8+ murine thymocyte subsets by assessing their short-term (less than 5 days) developmental potentials following intrathymic injection into Thy-1 congenic, unirradiated host mice. Our results identify TCR-/lo CD4-8+ and TCRlo CD4+8+ blast cells as sequential intermediates in the development of mature TCRhi CD4+8- and TCRhi CD4-8+ thymocytes from CD4-8- precursors, thus defining at least one intrathymic maturation pathway for T lymphocytes.

Inhibition of MHC class II-restricted T cell response by Lyt-2 alloantigen

T cell hybridomas were established by fusing a CD8+ V beta 8.1+ CTL clone and a CD4+ V beta 8.1+ helper T lymphocyte (HTL) clone to the thymoma cell line BW5147. In contrast to the HTL x BW hybridomas, which retain the same antigen specificity as the original T cell clone, the CTL x BW hybridomas lost the class I MHC-restricted antigen response but acquired a new specificity to Mlsa antigen. Mlsa reactivity of CTL x BW hybridomas was shown to be mediated by the CTL TCR as assayed by inhibition using an anticlonotypic antibody to the CTL clone. Since hybridomas established with BW5147 lose CD8 expression, we have introduced the CD8 molecule into CTL x BW5147 hybridomas by gene transfection. The CD8+ V beta 8.1+ hybridoma was no longer capable of reacting to Mlsa antigen but exhibited the same antigen specificity as the parental CTL clone. Furthermore, the presence of the transfected CD8 molecule in the HTL x BW hybridomas was found to be inhibitory to class II MHC-restricted antigen reactivity. These results demonstrate that, besides its role in increasing the overall avidity of T cell-class I MHC/antigen interaction, the CD8 molecule inhibits T cell-class II MHC gene product/antigen interaction. This negative effect of the CD8 molecule on a class II MHC-restricted response may account for the failure of CD8+ T cells using either V beta 8.1 or V beta 6, which impart reactivity to the Mlsa antigen on CD4+ T cells, to respond to the Mlsa antigen.

Positive selection determines T cell receptor V beta 14 gene usage by CD8+ T cells

We report here a mAb, 14-2, reactive with TCRs that include V beta 14. The frequency of V beta 14+ T cells varies with CD4 and CD8 subset and is controlled by the H-2 genes. Thus CD8+ T cells from H-2b mice include approximately 2.3% V beta 14+ T cells while CD8+ T cells from mice expressing K kappa include greater than 8% V beta 14+ T cells. In all strains examined, 7-8% of CD4+ T cells express V beta 14. The frequent usage of V beta 14 in CD8+ T cells of K kappa-expressing mice is a result of preferential positive selection of V beta 14+ CD8+ T cells as demonstrated by analysis of radiation chimeras. These studies demonstrate that H-2-dependent positive selection occurs in unmanipulated mice. Furthermore, the results imply that positive selection, and possibly H-2 restriction, can be strongly influenced by a V beta domain, with some independence from the beta-junctional sequence and alpha chain.

Thymocyte development: an analysis of T cell receptor gene expression in 519 newborn thymocyte hybridomas

We have examined the frequency of expression of individual alpha and beta chain V gene families in a population of immature T cells that has not been selected or tolerized. To accomplish this, we generated 519 T cell hybridomas from freshly isolated thymocytes of newborn C57BL/10 mice and subjected RNA from these hybrids to hybridization analysis with 11 V alpha, 16 V beta, C gamma and C delta probes. Comparison of the expressed repertoire of V beta gene segments in this newborn thymocyte population with similar data previously generated from adult peripheral T cells (Bill et al., Proc. Natl. Acad. Sci. USA 1988. 85: 9184.) revealed two V beta genes, V beta 12 and V beta 15, whose expression is decreased in the periphery possibly due to the effects of tolerance. An additional V beta gene segment (V beta 10) and a V beta gene family (V beta 5.1, V beta 5.2) were expressed more frequently in the mature, peripheral population than in the newborn thymus. These findings may represent two instances of positive selection of T cells. Furthermore, unlike VH gene segments, D beta-proximal V beta genes are not overrepresented on this collection of immature thymocyte hybridomas. A similar analysis of V alpha gene family expression was hampered by the fact that V alpha gene segments were expressed in only 15% of newborn thymocyte hybridomas (compared to 58% of adult hybridomas). An unexpectedly large fraction (57%) of those newborn hybrids expressing a V alpha gene segment was also found to express C delta mRNA and further examination revealed that several V alpha gene probes were actually detecting delta chain mRNA. The most notable of these was the V alpha 7 gene family, which accounted for approximately one-third of the expressed V alpha genes but was expressed exclusively as part of a delta chain mRNA. We found no examples of hybridomas co-expressing both full-length alpha and delta chain mRNA, despite significant numbers of hybridomas co-expressing full-length beta and gamma chain transcripts. This observation suggests that a large number of mature alpha/beta T cells may have proceeded through development without having gone through a stage where a delta chain message is expressed, thereby precluding their development into gamma/delta cells.

Developmental biology of T cell receptors

T cell receptors are the antigen-recognizing elements found on the effector cells of the immune system. Two isotypes have been discovered, TCR-gamma delta and TCR-alpha beta, which appear in that order during ontogeny. The maturation of prothymocytes that colonize the thymic rudiment at defined gestational stages occurs principally within the thymus, although some evidence for extrathymic maturation also exists. The maturation process includes the rearrangement and expression of the T cell receptor genes. Determination of these mechanisms, the lineages of the cells, and the subsequent thymic selection that results in self-tolerance is the central problem in developmental immunology and is important for the understanding of autoimmune diseases.

Inhibition of T-cell antigen receptor-mediated transmembrane signaling by protein kinase C activation

The murine T-lymphoma cell line LBRM-33 is known to require synergistic signals delivered through the antigen receptor (Ti-CD3) complex, together with interleukin 1 (IL-1), for activation of IL-2 gene expression and IL-2 production. Although 12-O-tetradecanoylphorbol-13-acetate (TPA) was capable of replacing IL-1 as an activating stimulus under certain conditions, biologic studies indicated that TPA failed to synergize with Ti-CD3-dependent stimuli under conditions in which IL-1 was clearly active. Acute exposure to TPA and other active phorbol esters resulted in a concentration-dependent inhibition of the increases in phosphoinositide hydrolysis and intracellular free Ca2+ concentration stimulated by phytohemagglutinin or anti-Ti antibodies. TPA treatment induced no direct alteration of phospholipase C enzymatic activities in LBRM-33 cells. In contrast, both Ti-CD3 cross-linkage and TPA rapidly stimulated the phosphorylation of identical CD3 complex polypeptides, presumably via activation of protein kinase C. Exposure of LBRM-33 cells to TPA resulted in a time-dependent, partial down-regulation of surface Ti-CD3 expression. Thus, TPA treatment inhibited the responsiveness of LBRM-33 cells to Ti-CD3-dependent stimuli by inducing an early desensitization of Ti-CD3 receptors, followed by a decrease in membrane receptor expression. These studies indicate that phorbol esters deliver bidirectional signals that both inhibit Ti-CD3-dependent phosphoinositide hydrolysis and augment IL-2 production in LBRM-33 cells.

Relative V beta transcript levels in thymus and peripheral lymphoid tissues from various mouse strains. Inverse correlation of I-E and Mls expression with relative abundance of several V beta transcripts in peripheral lymphoid tissues

We have measured the relative levels of transcripts for 15 of the 22 known V beta gene segments. The level of transcripts for the highest and lowest expressed V beta gene segment differed by greater than 20-fold in the thymus and an even larger difference was observed in the periphery. The levels of expressions were unrelated to the order of the V beta genes on the chromosome. For most of the V beta gene segments, the relative transcript levels were the same in the thymus and periphery, suggesting that thymic selection in general does not act solely upon the V beta gene segment. One V beta gene segment in the BALB and B10 mice strains was an exception to this rule. V beta 5.2 expression in the periphery of BALB and B10 mice inversely correlated with the expression of the MHC class II molecule I-E. Five V beta gene segments had reduced transcript levels in the periphery of Mls-1a mice compared with their thymic levels or to the levels found in Mls-1b mice. The peripheral level of V beta 3 transcripts vary with MHC and Mls-2 haplotypes. The observation that certain V beta transcript levels are reduced in the periphery when compared with the thymus favors the hypothesis that self tolerance at the T cell level results in the elimination of self-reactive T cells, rather than paralysis by a block at some post-transcriptional step. Finally, the wide variability of V beta gene segment expression in the thymus suggests mechanisms exist to import an early bias to the repertoire. Whether this bias results from differential V beta segment rearrangement rates, differential V beta expression rates, or events occurring after TCR-alpha/beta expression on immature/nonmature thymocyte cell surfaces is yet to be determined.

Antibodies against the T cell receptor/CD3 complex interfere with distinct intra-thymic cell-cell interactions in vivo: correlation with arrest of T cell differentiation

Postnatal treatment of mice with antibodies against the T cell receptor complex (TcR) prevents the differentiation of mature T cells in the thymic medulla without affecting the generation of most immature cortical thymocytes, thus interfering with a discrete stage of intra-thymic T cell differentiation at the cortex/medulla transition. This result has been interpreted as indicating a direct role of the TcR in the differentiation of immature to mature T cells, possibly via TcR-ligand interactions during direct cell-cell contact. Here we analyze the effect of anti-TcR (V beta 8 family) and anti-CD3 (epsilon chain) antibodies on distinct intra-thymic cell-cell interactions in vivo. We find that the maturation arrest of thymocytes correlates with a nearly complete abrogation of interactions of corresponding immature thymocyte with I-A/E+ cortical epithelial cells and I-A/E+ medullary dendritic cells, while preserving interactions with adherent I-A/E- macrophages. It is proposed that the blockade of thymocyte-epithelial cell recognition in the cortex by anti-TcR antibodies prevents the translocation of thymocytes into the medulla and their subsequent differentiation and selection, including interactions with dendritic cells. Interestingly, the anti-CD3 mAb treatment seems to spare the intra-thymic development of the CD3+, CD4-/CD8- T cell lineage.

Influence of the major histocompatibility complex on positive thymic selection of V beta 17a+ T cells

A monoclonal antibody was used to show directly positive thymic selection of the T cell repertoire in mouse strains expressing the 17a beta-chain variable domain (V beta 17a) of the T cell receptor. In the absence of the potent tolerizing class II major histocompatibility complex (MHC) molecule, I-E, peripheral expression of V beta 17a+ T cell receptors varied with the MHC haplotype of the mouse strain. In the most extreme case, H-2q mice expressed high peripheral levels of CD4+ V beta 17a+ T cells (14 to 19 percent), whereas H-2b mice expressed low levels (3 to 4 percent). Analysis of (b x q)F1 mice and chimeric mice showed that these differences were determined by positive thymic selection and implicated the thymic epithelium as the controlling cell type.

The MHC molecule I-E is necessary but not sufficient for the clonal deletion of V beta 11-bearing T cells

We have generated an mAb, RR3-15, that recognizes murine TCRs containing the V beta 11 domain. Using this antibody to stain peripheral T cells, we have demonstrated that V beta 11-bearing T cells are largely absent from strains of mice that express the class II MHC molecule, I-E. Studies with F1 mice demonstrate that this effect is dominant, consistent with tolerance. The clonal deletion of V beta 11-bearing T cells appears to occur intrathymically, as immature but not mature V beta 11+ T cells are present in the thymus of I-E-bearing mice. Examination of B6 x DBA/2 recombinant inbred strains demonstrates that the expression of I-E molecules is necessary for the clonal deletion of V beta 11-bearing T cells, but that other non-MHC genes control the clonal deletion process, as well. Paradoxically, only a small fraction of V beta 11+ T cell hybridomas are I-E reactive.

The V beta-specific superantigen staphylococcal enterotoxin B: stimulation of mature T cells and clonal deletion in neonatal mice

Staphylococcal enterotoxin B is known to be a powerful T cell stimulant in mouse and man. In this paper we show that, for mice, this is because the protein in association with major histocompatibility complex class II molecules stimulates virtually all T cells bearing V beta 3 and V beta 8.1, 8.2, and 8.3, and few others. Neonatal mice given the enterotoxin eliminate all mature, and some immature, T cells bearing these V beta s, demonstrating that tolerance to exogenously administered antigen can be caused by clonal deletion of reactive T cells. The enterotoxin shares these "superantigenic" properties with known self-antigens in mice, Mls-1a and Mls-2a, and a B cell-derived product, a shared property that is unlikely to be coincidental or inconsequential.

Parathyroid hormone and lipopolysaccharide induce murine osteoblast-like cells to secrete a cytokine indistinguishable from granulocyte-macrophage colony-stimulating factor

Osteoblasts are the cells responsible for the secretion of collagen and ultimately the formation of new bone. These cells have also been shown to regulate osteoclast activity by the secretion of cytokines, which remain to be defined. In an attempt to identify these unknown cytokines, we have induced primary murine osteoblasts with two bone active agents, parathyroid hormone (PTH) and lipopolysaccharide (LPS) and analyzed the conditioned media (CM) for the presence of specific cytokines. Analysis of the CM was accomplished by functional, biochemical, and serological techniques. The data indicate that both PTH and LPS are capable of inducing the osteoblasts to secrete a cytokine, which by all of the techniques used, is indistinguishable from granulocyte-macrophage colony-stimulating factor (GM-CSF). Secretion of GM-CSF is not constitutive and requires active induction. Production of the cytokine is dependent on the dose of PTH or LPS added. It has been demonstrated that the addition of GM-CSF to bone marrow cultures results in the formation of increased numbers of osteoclasts. Therefore, these data suggest that osteoblasts not only participate in bone remodeling by formation of new matrix but may regulate osteoclast activity indirectly by their ability to regulate hematopoiesis.

T cell receptors

Over the past few decades, a solid body of evidence has been built up linking certain autoimmune diseases to the presence of specific major histocompatibility complex haplotypes. Major histocompatibility complex products are used by the immune system to distinguish non-self from self and are important in the recognition of foreign antigen by T cells. T cells play a number of roles in the initiation and control of the immune response, leading to the suggestion that T cells may be important in the genesis of these diseases. T cell recognition is mediated by a heterodimeric cell surface receptor, the T cell antigen receptor. A better understanding of the molecular biology of this receptor may shed some light on the cellular and molecular processes involved in the initiation and progression of these diseases.

Macrophages phagocytose thymic lymphocytes with productively rearranged T cell receptor alpha and beta genes

The thymus gland is important for the formation of competent T lymphocytes. However, there is long-standing evidence that greater than 95% of newly formed thymocytes do not emigrate to peripheral lymphoid tissues but instead die locally. We have identified a rapid and selective pathway for thymocyte turnover in vitro. The mechanism entails binding, uptake, and digestion by macrophages. The susceptible cells are a subpopulation of double-positive thymocytes. These thymocytes can be enriched by virtue of their high buoyant density in Percoll and prove to have low levels of surface CD3 and little or no surface TCR. However TCR-alpha and -beta genes have undergone rearrangement, and full length alpha and beta transcripts are abundant. Therefore many double-positive cells rearrange and express TCR genes but do not have normal levels of TCR on the cell surface. We propose that thymocytes that undergo high turnover in situ are unable to form receptors that can be selected by MHC molecules in the thymus, and that these cells are recognized and cleared by the macrophage.

Involvement of the interleukin 2 pathway in the rearrangement and expression of both alpha/beta and gamma/delta T cell receptor genes in human T cell precursors

In this report, we have undertaken the phenotypic, functional and molecular characterization of a minor (less than 5%) subpopulation of adult thymocytes regarded as the earliest intrathymic T-cell precursors. Pro-T cells were immunoselected and shown to express different hematopoietic cell markers (CD45, CD38, CD7, CD5) and some activation-related molecules (4F2, Tr, HLA class II), but lack conventional T cell antigens (CD2-1-3-4-8-). TCR-gamma RNA messages are already expressed at this early ontogenic stage, while alpha and beta chain TCR genes remain in germline configuration. In vitro analyses of the growth requirements of pro-T cells demonstrated the involvement of the IL-2 pathway in promoting their proliferation and differentiation into CD3+ CD4+ or CD8+ mature thymocytes. Moreover, during the IL-2-mediated maturation process rearrangements and expression of both alpha and beta chain TCR genes occurred, and resulted in the acquisition of alpha/beta as well as gamma/delta (either disulphide-linked or non-disulphide-linked) heterodimeric TCR among the pro-T cell progeny.

Inhibition of T-cell antigen receptor-mediated transmembrane signaling by protein kinase C activation

The murine T-lymphoma cell line LBRM-33 is known to require synergistic signals delivered through the antigen receptor (Ti-CD3) complex, together with interleukin 1 (IL-1), for activation of IL-2 gene expression and IL-2 production. Although 12-O-tetradecanoylphorbol-13-acetate (TPA) was capable of replacing IL-1 as an activating stimulus under certain conditions, biologic studies indicated that TPA failed to synergize with Ti-CD3-dependent stimuli under conditions in which IL-1 was clearly active. Acute exposure to TPA and other active phorbol esters resulted in a concentration-dependent inhibition of the increases in phosphoinositide hydrolysis and intracellular free Ca2+ concentration stimulated by phytohemagglutinin or anti-Ti antibodies. TPA treatment induced no direct alteration of phospholipase C enzymatic activities in LBRM-33 cells. In contrast, both Ti-CD3 cross-linkage and TPA rapidly stimulated the phosphorylation of identical CD3 complex polypeptides, presumably via activation of protein kinase C. Exposure of LBRM-33 cells to TPA resulted in a time-dependent, partial down-regulation of surface Ti-CD3 expression. Thus, TPA treatment inhibited the responsiveness of LBRM-33 cells to Ti-CD3-dependent stimuli by inducing an early desensitization of Ti-CD3 receptors, followed by a decrease in membrane receptor expression. These studies indicate that phorbol esters deliver bidirectional signals that both inhibit Ti-CD3-dependent phosphoinositide hydrolysis and augment IL-2 production in LBRM-33 cells.

Abnormal differentiation of thymocytes in mice treated with cyclosporin A

Cyclosporin A (CsA) acts as a powerful immunosuppressive agent, and also, when given in repeated doses, can cause T-cell-dependent graft-versus-host disease and organ-specific autoimmune disease in rodents. This suggests that CsA interferes with the processes governing self-tolerance, either by nullifying the activity of T suppressor cells or by preventing the deletion of autoreactive T cells during ontogeny in the thymus. We report here that irradiated mice given repeated injections of CsA show striking dysfunction of the thymus. There are two different effects, the first of which is that CsA seems to block the differentiation of immature CD4+CD8+ thymocytes into mature CD4+CD8- and CD4-CD8+ cells expressing a high density of T-cell receptors and CD3 molecules. Second, CsA-treated mice show incomplete deletion of T cells expressing T-cell receptor molecules reactive to self H-2 I-E molecules.

Essential role of the interleukin 2-interleukin 2 receptor pathway in thymocyte maturation in vivo

The role of the IL-2-IL-2-R pathway in thymocyte differentiation in vivo is unknown. We have examined fetal thymocyte development in vivo, under conditions where all IL-2-R were saturated from day 13 of gestation with anti-IL-2-R mAbs that were previously shown to render mature T cells unable to respond to IL-2. This produced a dramatic change in the composition of developing T cells: thymocytes from day 1 neonatal mice born to anti-IL-2-R-treated mothers did not contain CD4+ or CD8+ single-positive cell populations. In addition, no generation of surface TCR beta chain-expressing T cells or antigen-reactive functional T cells occurred in treated mice. These data suggest that IL-2-IL-2-R interactions provide signals crucial to in vivo intrathymic development of mature T cells.

The T-cell repertoire is heavily influenced by tolerance to polymorphic self-antigens

T cells with V beta 3+ alpha beta receptors are deleted by self-tolerance in mice with particular major histocompatibility complex/self-antigen combinations. This also occurs for other V beta elements. Polymorphism in the major histocompatibility complex and/or the self-antigens that cause massive deletion of T cells using particular V beta elements may be maintained by the need to balance the advantage of a diverse T-cell repertoire against the potential involvement of those elements in autoimmune disease.

Effects of cyclosporine A on T cell development and clonal deletion

Cyclosporine A (CsA) is an important immunosuppressive drug that is widely used in transplantation medicine. Many of its suppressive effects on T cells appear to be related to the inhibition of T cell receptor (TCR)-mediated activation events. Paradoxically, in certain situations CsA is responsible for the induction of a T cell-mediated autoimmunity. The effects of CsA on T cell development in the thymus were investigated to elucidate the physiologic events underlying this phenomenon. Two major effects were revealed: (i) CsA inhibits the development of mature single positive (CD4+8- or CD4-8+) TCR-alpha beta+ thymocytes without discernibly affecting CD4-8- TCR-gamma delta+ thymocytes and (ii) CsA interferes with the deletion of cells bearing self-reactive TCRs in the population of single positive thymocytes that do develop. This suggests a direct mechanism for CsA-induced autoimmunity and may have implications for the relative contribution of TCR-mediated signaling events in the development of the various T cell lineages.

Interleukin-4 mediates CD8 induction on human CD4+ T-cell clones

CD4 and CD8 antigens are simultaneously expressed on most of the cortical thymocytes, that weakly express the T-cell antigen receptor(TCR)/CD3 complex. Mature peripheral T cells, however, strongly express the TCR complex and are positive for either CD4 or CD8. Nevertheless, a small percentage of peripheral CD3+ T cells express CD4 and CD8 simultaneously. These mature, double positive cells could be intermediates between CD4+CD8+ thymocytes and mature, single positive T cells, or they may originate from single positive T cells that acquire either CD4 or CD8. Here we report that activation and culturing of cloned CD4+ T cells in interleukin-4 (IL-4), results in the acquisition of CD8 due to its de novo synthesis. The IL-4-induced co-expression of CD8 on CD4+ T cells is reversible, in that CD8 disappeared from double positive T-cell clones isolated in IL-4, when they were cultured in IL-2. CD8 induced by IL-4 can be functional as a monoclonal antibody to CD8 inhibited anti-CD3-mediated cytotoxicity by a double positive T-cell clone.

In situ localization of T cell receptor beta chain in the murine thymus: changes in the intrathymic distribution of thymocytes expressing beta chain during fetal development

The expression of T cell receptor beta chain in the developing thymus was examined at the light and electron microscopic levels using the monoclonal antibody F23.1. Cells expressing cytoplasmic forms of beta chain were first observed at Day 16 of gestation, while thymocytes expressing cell surface beta chain were detected about a day later. Clustering of cortical F23.1+ cells was more pronounced in fetal thymus when compared to adult. The density of F23.1+ cells in the subcapsular areas of the thymus was initially lower than that in the rest of the cortex or the medulla. Within the subcapsular and cortical areas of the thymus there was an inverse relationship between the density of F23.1+ cells and cells labeled with the lectin from Dolichos bifloris, which binds to terminal alpha-linked N-acetylgalactosamine residues preferentially expressed by L3T4-/Lyt2- thymocytes. Although this pattern was less pronounced with increasing gestational age, it was still apparent at birth.

Predominant variable region gene usage by gamma/delta T cell receptor-bearing cells in the adult thymus

Previous studies have indicated that the diversity of gamma genes expressed by gamma/delta-bearing murine T cells is limited, but comparable information concerning the expressed diversity of delta genes is lacking. In this study, we have investigated the rearrangement and expression of delta and gamma genes in T cell hybridomas that express gamma/delta T cell receptors. Three productive delta chain cDNA clones were isolated (delta 7.3, delta 7.1, and delta 2.3) that encode new variable region sequences. Two of the delta cDNAs differ significantly from those observed in the V alpha repertoire. In addition, one cDNA expressed a new J delta region (J delta 2), which was localized between J delta 1 and C delta genes. Using these and other delta gene probes and gamma gene probes, we found that five independent hybridomas expressed four different V delta s and three different V gamma s. However, analysis of an enriched population of gamma/delta-expressing cells from the adult thymus suggests that only a few V delta genes and one V gamma gene are used by the majority of the cells. These results suggest that important components of receptor chain that contribute to specificity (i.e., the germline V gene sequences) are relatively nondiverse in the thymic gamma/delta population.

The V beta 8 gene family is preferentially used by 'naturally' activated T cells

Receptors encoded by the V beta 8 gene family detected by the monoclonal antibody F23.1 are expressed among 'naturally' activated T cells in normal spleen at frequencies significantly higher than in the total CD4+ and CD8+ cell populations. The positive selection of these clones into 'natural' T-cell activity could be the reason for the high frequencies of cells expressing V beta 8 genes. This phenotype is strain-dependent.

Evidence that multiple residues on both the alpha-helices of the class I MHC molecule are simultaneously recognized by the T cell receptor

Single amino acid substitutions at nine different positions on the H-2Kb molecules from in vitro-mutagenized, immunologically altered, somatic cell variants were correlated with their patterns of recognition by monoclonal antibodies (MAbs) and allogeneic cytotoxic T lymphocyte (CTL) clones. While MAbs were found to detect spatially discrete, domain-specific sites, CTLs interacted simultaneously with multiple residues on the alpha 1 and alpha 2 domains of the Kb molecule. The computer graphic three-dimensional Kb model structure showed that, of the seven CTL-specific residues analyzed, six residues were located on the alpha-helical regions of the two domains. Every CTL clone was found to interact with a distinct pattern of residues composed of a specific subset of the CTL-specific residues.

Preferential expression of fibronectin receptors on immature thymocytes

Fibronectin-adherent (FNR+) thymocytes are enriched for immature (CD4-8-) and large (CD4+8+) cells, and depleted of mature (CD4-8+ and CD4+8-) and nonmature small (CD4+8+) cells. Among purified CD4-8- thymocytes, cells with the surface marker J11d and the IL-2 receptor, which can give rise to all other thymocyte subsets, showed selective attachment to fibronectin. Analysis of FNR+ thymocytes showed that such cells are greatly enriched for cells in cycle. Additionally, FNR+ cells expressed low levels of T cell receptor. These results suggest a role for the fibronectin receptor during the early, proliferative phase of thymocyte differentiation. The data suggest that loss of the fibronectin receptor is a hallmark of cells that have become committed either to functional maturation or to programmed cell death.

The effect of thymus environment on T cell development and tolerance

During development in the thymus, T cells are deleted if their receptors are able to recognize self major histocompatibility complex (MHC) proteins. We show that such clonal deletion can occur because of interaction between receptors on T cells and MHC expressed on bone marrow-derived cells. In addition, development in the thymus picks out T cells to mature if their receptors will be restricted for antigen recognition in association with self MHC alleles expressed on thymus epithelial cells. This process is usually thought to involve positive selection of T cells bearing receptors with high and low affinity for MHC on thymus epithelium, and subsequent deletion of high affinity cells by interaction with bone marrow-derived cells. Our data do not fit such a model, but rather suggest that MHC molecules on thymus epithelium and bone marrow-derived cells may not be seen identically by T cell receptors.

T-cell receptor V beta use predicts reactivity and tolerance to Mlsa-encoded antigens

T lymphocytes reactive with the product of the Mlsa-allele of the minor lymphocyte stimulating (Mls) locus use a predominant T-cell receptor beta-chain variable gene segment (V beta 6). Such V beta 6-bearing T cells are selectively eliminated in the thymus of Mlsa-bearing mice, consistent with a model in which tolerance to self antigens is achieved by clonal deletion.

Self-tolerance eliminates T cells specific for Mls-modified products of the major histocompatibility complex

In mice the product of the Mlsa locus is an unusual antigen capable of interaction with certain products of the major histocompatibility locus (MHC) to form a ligand for a large portion of the T-cell alpha/beta receptor repertoire, including nearly all receptors that use V beta 8.1. The presence of Mlsa/MHC during T-cell development results in the deletion of T cells that express V beta 8.1, documenting the importance of clonal deletion in establishing tolerance to self antigens.

Analysis of CD3 and antigen receptor expression on T cell subpopulations of aged athymic mice

The expression of the T cell antigen receptor on subpopulations of extrathymically generated T cells from athymic mice was investigated and compared to antigen receptor expression in normal mice. To this end, spleen and lymph node cells from 18 individual athymic BALB/c nu/nu mice between 6 and 12 months of age and from normal controls were enriched for T cells by nylon wool filtration. Expression of the following cell surface markers was analyzed by two-color flow cytometry: Thy-1, CD4, CD8, V beta 8 and CD3. The distribution of subpopulations as defined by these markers varied much more among athymic than among normal mice. Some recurrent patterns did, however, occur that may be characteristic of the extrathymic pathway of T cell differentiation in nu/nu mice. Among Thy-1.2+ cells, CD8+ cells predominated over CD4+ cells. No CD4+8+ "double positive" cells were found, but CD4-8- "double negative" cell constituted 16% on average. All nu/nu nylon wool-nonadherent cells expressing Thy-1 at a normal level also expressed CD3, whereas Thy-1low and Thy-1- cells were CD3-. The fraction of V beta 8 T cells among the CD4+ and CD8+ subsets was near to normal in the majority of these animals. Most interestingly, the density of V beta 8 and CD3 expression was lower in athymic than in euthymic animals. This level of T cell antigen receptor expression was, however, higher than on immature CD4+8+ thymocytes. A fraction of the nude T cells presently characterized responded with proliferation to both anti-T cell receptor V beta 8 monoclonal antibody and to concanavalin A. Despite their apparently normal phenotype (with the exception of reduced T cell receptor expression), this response was, however, 4 to 10 times smaller than that of normal control cells. The presently described Thy-1+ T cell receptor intermediate cells may either be a(n) aberrant lineage(s) only generated extrathymically, or represent the accumulation of an immature intermediate stage of normal (i.e. thymic) T cell differentiation.