Human melanoma-reactive CD4+ and CD8+ CTL clones resist Fas ligand-induced apoptosis and use Fas/Fas ligand-independent mechanisms for tumor killing

Tumor cells have been shown recently to escape immune recognition by developing resistance to Fas-mediated apoptosis and acquiring expression of Fas ligand (FasL) molecule that they may use for eliminating activated Fas+ lymphocytes. In this study, we report that tumor-specific T lymphocytes isolated from tumor lesions by repeated in vitro TCR stimulation with relevant Ags (mostly represented by normal self proteins, such as MART-1/Melan A and gp100) can develop strategies for overcoming these escape mechanisms. Melanoma cells (and normal melanocytes) express heterogeneous levels of Fas molecule, but they result homogeneously resistant to Fas-induced apoptosis. However, CD4+ and CD8+ CTL clones kill melanoma cells through Fas/FasL-independent, granule-dependent lytic pathway. In these lymphocytes, Ag/MHC complex interaction with TCR does not lead to functional involvement of FasL, triggered, on the contrary, by T cell activation with nonspecific stimuli such as PMA/ionomycin. Additionally, melanoma cells express significant levels of FasL (detectable on the cell surface only after treatment with metalloprotease inhibitors), although to a lesser extent than professional immune cells such as Thl clones. Nevertheless, antimelanoma CTL clones resist apoptosis mediated by FasL either in soluble form or expressed by Thl lymphocytes or FasL+ melanoma cells. These results demonstrate that CD4+ and CD8+ antimelanoma T cell clones can be protected against Fas-dependent apoptosis, and thus be useful reagents of immunotherapeutic strategies aimed to potentiate tumor-specific T cell responses.     

Immature thymocyte antigen-1: a novel thymocyte marker discriminating pre- and post-selected thymocytes

Previously, we described a mAb (1-23) reacting with a novel cell surface antigen expressed on thymocytes at late CD4-CD8- [(double negative (DN)] to early CD4+CD8+ [(double positive (DP)] differentiation stage. Since the expression of this molecule was restricted to immature thymocytes, we designated it as immature thymocyte antigen-1 (IMT-1). In this study, we have investigated the relevance of IMT-1 expression to thymocyte selection using TCR transgenic mice, scid mice or RAG-2-/- mice. The IMT-1+ population in DP thymocytes was decreased in the thymuses of MHC class I-restricted or class II-restricted TCR transgenic mice with a positively selecting MHC background when compared with that of the mice with a non-selecting MHC background. IMT-1+ DP thymocytes were also decreased in TCR transgenic mice in which negative selection occurs. When DP thymocytes in H-Y TCR transgenic mice were stimulated with CD3epsilon mAb in vitro as well as in vivo, the expression of IMT-1 on DP thymocytes was decreased. Furthermore, the expression of IMT-1 on DN thymocytes from RAG-2-/- mice was drastically reduced when CD3epsilon mAb was challenged in vivo. These results suggest that the expression of IMT-1 on DP or DN thymocytes is down-regulated by stimulation through TCR as well as pre-TCR. Taken together, these results show that IMT-1 is a unique surface marker which exquisitely separates pre-selected thymocytes from post-selected thymocytes.     

Fas expression and apoptosis in human B cells

Mechanisms of B cell apoptosis are critical in reducing aberrant B cell proliferations such as those that arise in autoimmune disease and in B cell malignancies. The physiologic interaction of CD4+ helper T cells and B lymphocytes has been extensively studied over the past two decades. Although CD4+ T cells are considered primarily to offer positive costimulatory signals for B cell differentiation into active immunoglobulin-secreting cells, recent studies have shown that CD4+ T cells are crucial in downregulating the humoral immune response. In the course of cognate interaction between CD40 ligand (CD40L)-bearing CD4+ T cells and CD40-expressing germinal center B cells, CD40 ligation results in augmented Fas expression at the B cell surface. Like CD40L, Fas ligand is expressed on activated CD4+ Th1 cells and when bound to Fas receptor on the B cell surface, initiates an apoptotic signal in that cell. Thus, CD4+ T cells limit the growth of autologous germinal center B cells by first inducing Fas expression and then instigating a death signal via Fas ligand. In this work, we will consider these observations about CD4+ T-cell-induced, Fas-mediated B cell death in the context of other factors that affect apoptosis in B cells, normal and malignant.     

Galectin-1 specifically modulates TCR signals to enhance TCR apoptosis but inhibit IL-2 production and proliferation

Galectin-1 is an endogenous lectin expressed by thymic and lymph node stromal cells at sites of Ag presentation and T cell death during normal development. It is known to have immunomodulatory activity in vivo and can induce apoptosis in thymocytes and activated T cells (1-3). Here we demonstrate that galectin-1 stimulation cooperates with TCR engagement to induce apoptosis, but antagonizes TCR-induced IL-2 production and proliferation in a murine T cell hybridoma and freshly isolated mouse thymocytes, respectively. Although CD4+ CD8+ double positive cells are the primary thymic subpopulation susceptible to galectin-1 treatment alone, concomitant CD3 engagement and galectin-1 stimulation broaden susceptible thymocyte subpopulations to include a subset of each CD4- CD8-, CD4+ CD8+, CD4- CD8+, and CD4+ CD8- subpopulations. Furthermore, CD3 engagement cooperates with suboptimal galectin-1 stimulation to enhance cell death in the CD4+ CD8+ subpopulation. Galectin-1 stimulation is shown to synergize with TCR engagement to dramatically and specifically enhance extracellular signal-regulated kinase-2 (ERK-2) activation, though it does not uniformly enhance TCR-induced tyrosine phosphorylation. Unlike TCR-induced IL-2 production, TCR/galectin-1-induced apoptosis is not modulated by the expression of kinase inactive or constitutively activated Lck. These data support a role for galectin-1 as a potent modulator of TCR signals and functions and indicate that individual TCR-induced signals can be independently modulated to specifically affect distinct TCR functions.     

Separately expressed T cell receptor alpha and beta chain transgenes exert opposite effects on T cell differentiation and neoplastic transformation

Two aspects of T cell differentiation in T cell receptor (TCR)-transgenic mice, the generation of an unusual population of CD4-CD8-TCR+ thymocytes and the absence of gamma delta cells, have been the focus of extensive investigation. To examine the basis for these phenomena, we investigated the effects of separate expression of a transgenic TCR alpha chain and a transgenic TCR beta chain on thymocyte differentiation. Our data indicate that expression of a transgenic TCR alpha chain causes thymocytes to differentiate into a CD4-CD8-TCR+ lineage at an early developmental stage, depleting the number of thymocytes that differentiate into the alpha beta lineage. Surprisingly, expression of the TCR alpha chain transgene is also associated with the development of T cell lymphosarcoma. In contrast, expression of the transgenic TCR beta chain causes immature T cells to accelerate differentiation into the alpha beta lineage and thus inhibits the generation of gamma delta cells. Our observations provide a model for understanding T cell differentiation in TCR-transgenic mice.     

Ecologic studies of rodent reservoirs: their relevance for human health

To gain insight into the immunomodulatory effects of vitamin E (VE), immune cell population analyses were conducted using thymus and spleen from male broilers fed diets with various levels of VE supplementation (0, 17, 46, and 87 mg dl-alpha-tocopherol acetate/kg of feed). At 2 and 7 wk of age, the percentages of B cells, macrophages, and T cell subsets, delineated by the expression of CD4, CD8, and T cell receptor (TCR) isotype, in thymus and spleen were determined by flow cytometry. The percentages of thymic and splenic B cells and macrophages from 2- and 7-wk-old chickens, as well as the percentage of thymic T cells in 2-wk-old chickens, were unaffected by VE treatment. However, 7-wk-old broilers maintained on 87 mg VE/kg feed had a higher percentage of CD4+CD8- thymocytes, a higher CD4+CD8- to CD4-CD8+ thymocyte ratio, and a lower percentage of CD4+CD8+ thymocytes than chickens receiving no dietary VE supplementation. The VE-induced increase in the percentage of CD4+CD8- thymocytes was due to an increase in the TCR2+CD4+CD8- thymocyte subset, whereas the decrease in the percentage of CD4+CD8+ thymocytes involved all TCR defined T cell subsets. In the spleen, the percentage of CD4+CD8- T cells was lower in 2-wk-old chickens and higher in 7-wk-old chickens maintained on 87 mg/kg feed than in chickens receiving no dietary VE supplementation. The decrease in CD4+CD8- splenocytes at 2 wk of age was due to a decline in the percentage of TCR2+CD4+CD8- splenocytes, whereas the increase in CD4+CD8- splenocytes in 7-wk-old chicks was due to an increase in the percentages of all TCR defined CD4+CD8- T cell subsets. These data support an immunomodulatory effect of VE on CD4+CD8- T cells.     

Requirement for p56lck tyrosine kinase activation in T cell receptor-mediated thymic selection

The nonreceptor protein tyrosine kinase p56lck (Lck) serves as a fundamental regulator of thymocyte development by delivering signals from the pre-T cell receptor (pre-TCR) that permit subsequent maturation. However, considerable evidence supports the view that Lck also participates in signal transduction from the mature TCR. We have tested this conjecture by expressing a dominant-negative form of Lck under the control of a promoter element (the distal lck promoter) that directs high expression in CD4+CD8+ thymocytes, mature thymocytes, and peripheral T cells, thereby avoiding, complications that result from the well-documented ability of dominant-negative Lck to block very early events in thymocyte maturation. Here we report that expression of the catalytically inactive Lck protein at twice normal concentrations inhibits thymocyte positive selection by as much as 80%, while leaving other aspects of cell maturation intact. This effect was studied in more detail in mice simultaneously bearing the male-specific H-Y alpha/beta TCR transgene and ovalbumin-specific DO10 alpha/beta TCR transgene, where even equimolar expression of the dominant-negative Lck protein substantially vitiated the positive selection process. Although deletion of H-Y alpha/beta thymocytes proceeded normally in male mice despite the presence of catalytically inactive Lck, modest inhibition of superantigen-mediated deletion was in some cases observed. These data further implicate Lck in the propagation of all TCR-derived signals, and indicate that even very modest deficiencies in the representation of functional Lck molecules could in humans, profoundly alter the character of the peripheral TCR repertoire.     

A role for Fas in negative selection of thymocytes in vivo

To seek information on the role of Fas in negative selection, we examined subsets of thymocytes from normal neonatal mice versus Fas-deficient lpr/lpr mice injected with graded doses of antigen. In normal mice, injection of 1-100 microg of staphylococcal enterotoxin B (SEB) induced clonal elimination of SEB-reactive Vbeta8+ cells at the level of the semi-mature population of HSAhi CD4+ 8- cells found in the thymic medulla; deletion of CD4+ 8+ cells was minimal. SEB injection also caused marked elimination of Vbeta8+ HSAhi CD4+ 8- thymocytes in lpr/lpr mice. Paradoxically, however, elimination of these cells in lpr/lpr mice was induced by low-to-moderate doses of SEB (相似文献   

Protein kinase C regulates Fas (CD95/APO-1) expression

Fas (CD95/APO-1) is a transmembrane protein of the TNF/neuron growth factor receptor family. Ligation of Fas by specific Abs or Fas ligand (FasL/CD95 ligand) induces rapid apoptotic cell death in a variety of cell types. Despite progress in understanding the death signals transduced from Fas, very little is known with regard to the mechanisms by which Fas expression is regulated. Using our previously established murine T cell hybridoma model A1.1, we show that specific protein kinase C (PKC) inhibitors could block activation-induced Fas expression and apoptosis. The activation of PKC with PMA or 1-oleoyl-2-acetyl-sn-glycerol could mimic the TCR signal by inducing the expression of Fas but not FasL. PKC-dependent Fas expression was also observed in several murine and human tumor cell lines. Since the inhibition of Ca2+ redistribution by an inhibitor of intracellular Ca2+ mobilization, 8-(diethylamino)-octyl-3,4,5-trimethoxybenzoate hydrochloride, inhibited TCR-induced FasL but not Fas, the expression of Fas appears to be independent of Ca2+ mobilization. Significantly, expression of the newly identified Fas-regulatory gene, TDAG51, was found to be dependent upon the activity of PKC. PKC activation only induced Fas expression in cells expressing wild-type TDAG51. Thus, Fas expression is likely mediated by PKC through TDAG51.     

Preferential requirement of CD3 zeta-mediated signals for development of immature rather than mature thymocytes

Antigen recognition signals by the TCR are transduced through activation motifs present in the cytoplasmic region of CD3 chains. In vitro analysis has suggested that the CD3zeta chain mediates different signals from other CD3 chains. To analyze the in vivo function of CD3zeta-mediated signals for T cell development, mice expressing a mutant CD3zeta chain lacking all the activation motifs were generated by introducing the transgene into zeta-knockout mice. Mature CD4(+) single-positive (SP) thymocytes in these mice were greater in number than in zeta-deficient mice, and the promoted differentiation was indicated by the changes of CD69 and HSA phenotypes. We found that even in the absence of activation motifs in CD3zeta, these mature cells became functional, being able to induce Ca2+ mobilization and proliferation upon stimulation. On the other hand, CD4(-)CD8(-) double-negative (DN) thymocytes, most of which were arrested at the CD44(-)CD25(+) stage similarly to those in zeta-deficient mice, could not be promoted for differentiation into CD4(+)CD8(+) double-positive thymocytes in these mice in spite of the fact that the expression of the transgene in DN thymocytes was higher than that of zeta in wild-type mice. These results demonstrate the preferential dependence of the promotion of development and/or expansion of DN thymocytes rather than mature thymocytes upon the activation signals through the zeta chain and suggest differential requirements of TCR signaling for mature SP and immature DN thymocyte developments in vivo.     

Fas (CD95)-transduced signal preferentially stimulates lupus peripheral T lymphocytes

Fas (CD95) is a cell surface receptor whose biological function in circulating peripheral T cells is not well understood. To address the question of abnormal T cell sensitivity to Fas stimulation in systemic lupus erythematosus (SLE), we studied Fas-transduced stimulation and apoptosis in peripheral blood T cells from patients with SLE and normal control. Immobilized anti-Fas monoclonal antibodies (mAb) (imCH-11; IgM type) significantly stimulated SLE T cell proliferation compared to T cells from normal donors and patients with rheumatoid arthritis (p < 0.003 and p < 0.005, respectively). The soluble form of CH-11 and other immobilized anti-Fas mAb (UB-2, ZB-4; IgG type) failed to stimulate lupus T cells while immobilized human Fas ligand did. Furthermore, imCH-11 induced IL-2 and IL-6 mRNA expression. However, imCH-11 activation failed to induce expression of the T cell activation surface molecules CD25 and CD69. Addition of exogenous ceramide, a second messenger for Fas-mediated apoptosis signaling, also induced T cell proliferation in SLE and normal controls. Moreover, fumonisin B1, a specific ceramide synthase inhibitor, and caspase inhibitors markedly suppressed imCH-11 induced T cell proliferation, suggesting that the ceramide pathway may be involved in Fas-transduced stimulation signals in SLE T cells. These results show that SLE T cells have an alteration in the Fas signal transduction pathway leading to cell proliferation. This defect may be important in Fas-mediated peripheral immune homeostasis.     

CD8+ T cells clear influenza virus by perforin or Fas-dependent processes

Influenza virus infection is controlled in CD4-depleted mice that are also defective for the expression of either Fas (Fas-/-) or perforin (P-/-). Virus-immune P+/+ and P-/- CD8+ T cells can thus function in, respectively, a Fas-/- or Fas+/+ lung environment. The obvious question is whether the P-/- CD8+ set is effective in Fas-/- mice, a conclusion that would tend to favor cytokine secretion as the mode of virus clearance. Short term chimeras were made with P-/- bone marrow, P+/+ or P-/- T cells, and Fas+/+ or Fas-/- irradiated recipients. While the P+/+ CD8+ population cleared the virus from Fas+/+ and Fas-/- respiratory epithelium, the P-/- effectors were operational only if there was the potential for Fas to be expressed on radiation-resistant lung cells. Target cell destruction mediated via the Fas or perforin pathways is clearly the primary mechanism used by CD8+ T cells to terminate this viral pneumonia.     

Critical role for the common cytokine receptor gamma chain in intrathymic and peripheral T cell selection

The common cytokine receptor gamma chain (gammac), which is a functional subunit of the receptors for interleukins (IL)-2, -4, -7, -9, and -15, plays an important role in lymphoid development. Inactivation of this molecule in mice leads to abnormal T cell lymphopoiesis characterized by thymic hypoplasia and reduced numbers of peripheral T cells. To determine whether T cell development in the absence of gammac is associated with alterations of intrathymic and peripheral T cell selection, we have analyzed gammac-deficient mice made transgenic for the male-specific T cell receptor (TCR) HY (HY/gammac- mice). In HY/gammac- male mice, negative selection of autoreactive thymocytes was not diminished; however, peripheral T cells expressing transgenic TCR-alpha and -beta chains (TCR-alphaT/betaT) were absent, and extrathymic T cell development was completely abrogated. In HY/gammac- female mice, the expression of the transgenic TCR partially reversed the profound thymic hypoplasia observed in nontransgenic gammac- mice, generating increased numbers of thymocytes in all subsets, particularly the TCR-alphaT/betaT CD8+ single-positive thymocytes. Despite efficient positive selection, however, naive CD8+ TCR-alphaT/betaT T cells were severely reduced in the peripheral lymphoid organs of HY/gammac- female mice. These results not only underscore the indispensible role of gammac in thymocyte development, but also demonstrate the critical role of gammac in the maintenance and/or expansion of peripheral T cell pools.     

Linomide enhances apoptosis in CD4+CD8+ thymocytes

Linomide, a quinoline-3-carboxamide, has a pleiotropic immune modulating capacity and inhibits development as well as progression of disease in animal models of autoimmunity. Linomide treatment of mice resulted in a dramatic, dose-dependent decrease of the thymic cell number shortly after the start of administration. Flow cytometric analysis revealed that the major thymocyte subset, the early immature type CD4+CD8+ thymocytes, were reduced in number by 75%, mature CD4+CD8- or CD4-CD8+ thymocytes were less sensitive to treatment. The polyclonal T cell activator Con A (Concanavalin A) was used together with IL-2 to evaluate the potential proliferative responsiveness of ex vivo thymocytes. Thymocytes from mice treated with Linomide exhibited a more vigorous proliferation than control cultures. An effect shown to not only be due to the enrichment of mature thymocytes in the cultures from Linomide treated animals, but also when purified, mature thymocytes (CD4+CD8- and CD4-CD8+) were cultured with Con A and IL-2, these cells responded with a significantly enhanced proliferation. In vivo Linomide treatment did not result in increased plasma concentrations of corticosterone and treatment of adrenalectomized mice resulted in a reduction of thymocytes which was comparable to the effect in intact mice, indicating that glucocorticoids (GC) are not major mediators of Linomide-induced thymocyte deletion. In addition to this, and supporting a glucocorticoid independent mode of action, Linomide treatment of thymocytes in vitro resulted in a significant increase in the number of apoptotic cells, specifically in the CD4+CD8+ subset, implicating apopotosis as one component in the course of thymocyte reduction. In addition to this, in vivo treatment with Linomide resulted in an identical pattern to that seen in vitro in that there was significantly increased apoptosis only in the CD4+CD8+. These data indicate that Linomide modifies thymocyte development using a glucocorticoid independent pathway and results in the increased apoptosis of the CD4+CD8+ subset.     

The Fas counterattack in vivo: apoptotic depletion of tumor-infiltrating lymphocytes associated with Fas ligand expression by human esophageal carcinoma

Various cancer cell lines express Fas ligand (FasL) and can kill lymphoid cells by Fas-mediated apoptosis in vitro. FasL expression has been demonstrated in several human malignancies in vivo. We sought to determine whether human esophageal carcinomas express FasL, and whether FasL expression is associated with increased apoptosis of tumor-infiltrating lymphocytes (TIL) in vivo, thereby contributing to the immune privilege of the tumor. Using in situ hybridization and immunohistochemistry, respectively, FasL mRNA and protein were colocalized to neoplastic esophageal epithelial cells in all esophageal carcinomas (squamous, n = 6; adenocarcinoma, n = 2). The Extent of FasL expression was variable, with both FasL-positive and FasL-negative neoplastic regions occurring within tumors. TIL were detected by immunohistochemical staining for the leukocyte common Ag, CD45. FasL expression was associated with a mean fourfold depletion of TIL when compared with FasL-negative areas within the same tumors (range 1.6- to 12-fold, n = 6,p < 0.05). Cell death of TIL was detected by dual staining of CD45 (immunohistochemistry) and DNA strand breaks (TUNEL, terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling). There was a mean twofold increase in detectable cell death among TIL in FasL-positive areas compared with FasL-negative areas (range 1.6- to 2.4-fold, n = 6, p < 0.05). In conclusion, we demonstrate a statistically significant, quantitative reduction of TIL concomitant with significantly increased TIL apoptosis within FasL-expressing areas of esophageal tumors. Our findings suggest Fas-mediated apoptotic depletion of TIL in response to FasL expression by esophageal cancers, and provide the first direct, quantitative evidence to support the Fas counterattack as a mechanism of immune privilege in vivo in human cancer.     

A NF-kappa B/c-myc-dependent survival pathway is targeted by corticosteroids in immature thymocytes

Glucocorticoid hormones modulate T cell maturation in vivo. While low levels of hormones are required for appropriate T cell development, high levels of glucocorticoid hormones target immature developing thymocytes for cell death during systemic stress. In this report, we propose a molecular mechanism for the induction of apoptosis in CD4+CD8+ double-positive thymocytes by dexamethasone in vivo. Dexamethasone injection induced the expression of IkappaBalpha and IkappaBbeta in thymocytes and down-regulated NF-kappaB DNA binding activated by intrathymic signals. Down-regulation of NF-kappaB DNA binding preceded cell death, suggesting that NF-kappaB may be important for the survival of immature thymocytes. In addition, ex vivo treatment of thymocyte single-cell suspension with dexamethasone accelerated p65/RelA down-regulation and cell death. Conversely, NF-kappaB induction diminished dexamethasone-induced death. Expression of the c-myc proto-oncogene, a NF-kappaB target, was also reduced in thymocytes of dexamethasone-treated animals, and ectopic transgenic expression of c-myc in mice provided partial rescue of double-positive thymocytes from dexamethasone mediated cell death. These observations suggest that viability of CD4+CD8+ thymocytes may be maintained by an NF-kappaB/c-myc-dependent pathway in vivo.     

Retroviral-mediated expression of an MHC class I-restricted T cell receptor in the CD8 T cell compartment of bone marrow-reconstituted mice

The introduction of cloned T cell receptor (TCR) genes into bone marrow cells could provide a way to increase the frequency of tumor- or pathogen-specific cytotoxic T lymphocyte (CTL) precursors. We demonstrate here the ability of a retroviral vector to direct expression of a Valpha15/Vbeta13 MHC class I-restricted TCR in lethally irradiated mice reconstituted with transduced bone marrow cells. We have detected retroviral-mediated TCR expression by flow cytometry 6-19 weeks after transplantation in C57L (Vbeta13(-/-)) and Rag1(-/-) bone marrow-reconstituted mice, and in C57BL/6 hosts reconstituted with transduced C57BL/6-Rag1(-/-) bone marrow. Southern analysis confirmed the presence of integrated provirus and revealed that the frequency of transduction is greater than the frequency of cell surface TCR expression. Although TCR expression on Vbeta13+ transduced cells is lower than endogenous TCR levels, it is largely confined to CD4+CD8+ (thymus) and CD8+ (thymus and spleen) T cells. In Rag1(-/-) mice, which display a developmental arrest of thymocytes at the immature CD4-CD8- stage, retrovirus-mediated TCR expression selectively rescues CD4+CD8+ and CD8+ populations. These results indicate that the ectopically expressed TCR is functional during T cell development. Furthermore, we have observed Vbeta13+ TCR expression by up to 13% of peripheral CD8+ T cells in C57L and C57BL/6 hosts. This represents a substantial increase relative to total Vbeta13 frequency in normal C57BL/6 mice (3-5%), and an even greater increase over the estimated frequency of CTL precursors of a defined specificity (10(-5)-10(-4)). Our findings indicate that TCR gene transfer can be used to develop new approaches to immunotherapy, and provide the basis for further studies examining the contribution of retrovirus-mediated TCR expression to an antigen-specific CTL response.     

CD5 expression is developmentally regulated by T cell receptor (TCR) signals and TCR avidity

Recent data indicate that the cell surface glycoprotein CD5 functions as a negative regulator of T cell receptor (TCR)-mediated signaling. In this study, we examined the regulation of CD5 surface expression during normal thymocyte ontogeny and in mice with developmental and/or signal transduction defects. The results demonstrate that low level expression of CD5 on CD4(-)CD8(-) (double negative, DN) thymocytes is independent of TCR gene rearrangement; however, induction of CD5 surface expression on DN thymocytes requires engagement of the pre-TCR and is dependent upon the activity of p56(lck). At the CD4(+)CD8(+) (double positive, DP) stage, intermediate CD5 levels are maintained by low affinity TCR-major histocompatibility complex (MHC) interactions, and CD5 surface expression is proportional to both the surface level and signaling capacity of the TCR. High-level expression of CD5 on DP and CD4(+) or CD8(+) (single positive, SP) thymocytes is induced by engagement of the alpha/beta-TCR by (positively or negatively) selecting ligands. Significantly, CD5 surface expression on mature SP thymocytes and T cells was found to directly parallel the avidity or signaling intensity of the positively selecting TCR-MHC-ligand interaction. Taken together, these observations suggest that the developmental regulation of CD5 in response to TCR signaling and TCR avidity represents a mechanism for fine tuning of the TCR signaling response.