The role of the T cell receptor in positive and negative selection of developing T cells

Rearrangement and junctional-site sequence analyses of T-cell receptor gamma genes in intestinal intraepithelial lymphocytes from murine athymic chimeras

The molecular organization of rearranged T-cell receptor (TCR) gamma genes intraepithelial lymphocytes (IEL) was studied in athymic radiation chimeras and was compared with the organization of gamma gene rearrangements in IEL from thymus-bearing animals by polymerase chain reaction and by sequence analyses of DNA spanning the junction of the variable (V) and joining (J) genes. In both thymus-bearing mice and athymic chimeras, IEL V-J gamma-gene rearrangements occurred for V gamma 1.2, V gamma 2, and V gamma 5 but not for V gamma 3 or V gamma 4. Sequence analyses of cloned V-J polymerase chain reaction-amplified products indicated that in both thymus-bearing mice and athymic chimeras, rearrangement of V gamma 1.2 and V gamma 5 resulted in in-frame as well as out-of-frame genes, whereas nearly all V gamma 2 rearrangements were out of frame from either type of animal. V-segment nucleotide removal occurred in most V gamma 1.2, V gamma 2, and V gamma 5 rearrangements; J-segment nucleotide removal was common in V gamma 1.2 but not in V gamma 2 or V gamma 5 rearrangements. N-segment nucleotide insertions were present in V gamma 1.2, V gamma 2, and V gamma 5 IEL rearrangements in both thymus-bearing mice and athymic chimeras, resulting in a predominant in-frame sequence for V gamma 5 and a predominant out-of-frame sequence for V gamma 2 genes. These findings demonstrate that (i) TCR gamma-gene rearrangement occurs extrathymically in IEL, (ii) rearrangements of TCR gamma genes involve the same V gene regardless of thymus influence; and (iii) the thymus does not determine the degree to which functional or nonfunctional rearrangements occur in IEL.

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.

bcl-2 transgene inhibits T cell death and perturbs thymic self-censorship

Early death is the fate of most developing T lymphocytes. Because bcl-2 can promote cell survival, we tested its impact in mice expressing an E mu-bcl-2 transgene within the T lymphoid compartment. The T cells showed remarkably sustained viability and some spontaneous differentiation in vitro. They also resisted killing by lymphotoxic agents. Although total T cell numbers and the rate of thymic involution were unaltered, the response to immunization was enhanced, consistent with reduced death of activated T cells. No T cells reactive with self-superantigens appeared in the lymph nodes, but an excess was found in the thymus. These observations, together with previous findings on B cells, suggest that modulated bcl-2 expression is a determinant of life and death in normal lymphocytes.

The fail-safe paradigm of immunological self-tolerance

Immunological self-tolerance is guaranteed by several complementary mechanisms arranged in a fail-safe hierarchy--namely, clonal deletion, inactivation, and suppression. Only failure of all these self-tolerance-preserving systems allows autoimmune disease to develop. The notion of such a fail-safe organisation of the immune system is compatible with several features of autoimmune disease: its multicausal nature, its slowly progressive and relapsing course, and the diverse range of interventions available to restore autotolerance. This hypothesis has practical implications for the understanding of both the pathogenesis of iatrogenic autoimmune disease and the design of strategies to re-establish self-tolerance.

Future approaches to factor VIII inhibitor therapy

Future progress in our ability to treat acquired factor VIII (FVIII) inhibitors must be based on advances in knowledge of both the FVIII molecule and the nature of the human immune response. New therapeutic approaches to patients with acquired FVIII inhibition likely will emphasize modifications of the immune response. This concept holds considerable promise, because studies have characterized the critical steps leading to tolerance of self-antigens. Development of FVIII inhibitors represents a loss of self-tolerance, which any successful therapy must restore. Conceivably, restoration of self-tolerance can be accomplished in many ways: prevention of antigen binding to helper T lymphocytes, deletion of self-antigen-reactive T cells, inhibition of major histocompatibility complex (MHC) recognition, or enhancement of the antigen-specific suppressor T lymphocyte population. Recent data have demonstrated that highly specific methods can suppress ongoing immune responses against defined autoantigens. Antibodies that inhibit T-cell activation, peptides that block self-antigen binding, and antibodies that inhibit MHC recognition all have been successful in modifying experimentally induced autoimmune diseases. Whether any of these immunotherapeutic approaches will be effective in the treatment of acquired FVIII inhibition remains to be determined. Until data from animal model systems establish the feasibility of immune intervention, scrutiny of other new therapeutic approaches to patients with spontaneous inhibitors will continue to be important. Administration of FVIII-bypassing procoagulant proteins shows promise, as does removal of inhibitors by affinity reagents, such as FVIII peptides containing relevant epitopes (antigenic sites). Farther on the horizon is development of recombinant FVIII molecules so modified as to remove antigenic determinants while preserving procoagulant function. Articles in this supplement summarize several avenues for treatment of patients with acquired FVIII inhibitors. Alternatives include treatment with sufficient human or porcine FVIII to offset inhibitors, use of materials that reestablish hemostasis even though FVIII levels are not increased (the so-called FVIII-bypassing agents), manipulation of immune responses through physical depletion of inhibitor by plasmapheresis or affinity chromatography, and administration of intravenous immunoglobulin or immunosuppressive cytotoxic drugs. Thus, the heterogeneous clinical presentation is paralleled by the wide range of available therapeutic approaches.

The V beta 17+ T cell repertoire: skewed J beta usage after thymic selection; dissimilar CDR3s in CD4+ versus CD8+ cells

To ascertain how the actual repertoire of T cell receptors (TCRs) deviates from the theoretical, we have generated a large number of junctional region sequences from TCRs carrying the V beta 17 variable region. The greater than 600 sequences analyzed represent transcripts from nine different cell populations, permitting several comparisons: transcripts from an expressed vs. a non-expressed V beta 17 allele, those from E+ vs. E- mice, transcripts from immature vs. mature thymocytes, those from thymic vs. peripheral T cells, and those from CD4+ vs. CD8+ cells. These comparisons have allowed us to distinguish between the influence of molecular events involved in TCR gene rearrangement and that of various selection events that shape the T cell repertoire. Our most striking findings are: (a) that J beta usage is markedly skewed, partly due to recombination mechanics and partly due to selection forces: in particular, those mediated by the class II E molecule in the thymus; and (b) that TCRs on CD4+ and CD8+ cells show intriguing dissimilarities. In addition, we present evidence that N nucleotide additions occur with clear biases, probably due to idiosyncrasies of the recombination enzymes, and provide arguments that TCR and immunoglobulin CDR3s have distinct structures.

T cell receptor V-segment frequencies in peripheral blood T cells correlate with human leukocyte antigen type

We compared T cell receptor (TCR) V-segment frequencies in human leukocyte antigen (HLA) identical siblings to sibling pairs who differ at one or both HLA haplotypes using four V beta-specific and one V alpha-specific monoclonal antibody. In every one of nine families HLA-identical sibs had the most similar patterns of V-segment frequencies in their peripheral blood, whereas totally mismatched sibs were, in general, the most dissimilar; HLA haploidentical sibs tended to be intermediate between the two groups. The degree of similarity among HLA-identical sibs was comparable to that observed among three pairs of identical twins suggesting that HLA is the major genetic component influencing TCR V-segment frequency. Consistent with this observation, it was found that the frequency of T cells expressing particular V beta segments was skewed towards either CD4+ or CD8+ cells indicating that T cells expressing some V beta genes may be positively selected primarily by class I or class II major histocompatibility complex proteins. Finally, it was observed that individuals who express the HLA class I specificity, B38, tend to express high levels of V alpha 2.3+ cells among their CD8+ T cells. These observations represent definitive proof that human V-segment frequencies are profoundly influenced by the HLA complex.

A single amino acid change in a myelin basic protein peptide confers the capacity to prevent rather than induce experimental autoimmune encephalomyelitis

Experimental autoimmune encephalomyelitis (EAE) is an experimental demyelinating disease of rodents. In (PL/J x SJL) F1 mice, it is induced by immunization with the myelin basic protein peptide Ac1-11. Ac1-11 [4A], a myelin basic protein peptide analog with a single amino acid substitution, (i) binds to class II major histocompatibility complex molecules and stimulates encephalitogenic T cells in vitro better than Ac1-11, (ii) is nonimmunogenic and nonencephalitogenic in vivo in (PL/J x SJL)F1 mice, (iii) prevents EAE when administered before or at the time of immunization with Ac1-11, and (iv) prevents EAE when administered later, near the time of disease onset. Initial studies suggest that Ac1-11 [4A] does not prevent EAE by competitive inhibition or by activation of regulatory cells. Thus, substitution of a single amino acid in a myelin basic protein peptide confers the capacity to prevent rather than induce EAE, even after peptide-specific encephalitogenic T cells have been activated.

An unusual lineage of alpha/beta T cells that contains autoreactive cells

In male mice that express a transgenic alpha/beta T cell receptor (TCR) specific for a male-specific peptide presented by class I Db major histocompatibility complex (MHC) molecules, we describe an unusual lineage of alpha/beta T cells that are thymus dependent but do not require selection by Db MHC molecules on thymic epithelium in the absence of the specific peptide (positive selection). These cells express the transgenic alpha/beta TCR and have the CD4-8- or CD4-8low phenotype. Cells with the latter phenotype are only detected when hemopoietic cells express both the male-specific peptide as well as Db MHC molecules. In fact, these cells are autoreactive, as they expand relatively slowly after transfer into male nude mice. Also in male but not female alpha/beta TCR transgenic mice, the CD8+ cells with the transgenic TCR bear the Pgp1 marker characteristic of mature T cells activated by antigen. CD4-8- as well as CD4-8low cells do not respond significantly when cultured with male stimulator cells but proliferate vigorously when stimulated by TCR antibodies. By this latter criterion, cells in the periphery of male alpha/beta TCR transgenic mice differ from mature male-specific T cells from female alpha/beta TCR transgenic, which become intrinsically anergic when transferred into male nude mice and cannot be stimulated significantly by TCR antibodies. Thus, intrathymic deletion does not eliminate all autoreactive T cells and it is possible that cells with an apparently "benign" autoreactivity may be involved in certain forms of autoimmunity.

T-cell receptor variable beta genes show differential expression in CD4 and CD8 T cells

Studies in transgenic and inbred strains of mice have shown that the critical molecular interactions controlling positive selection involve major histocompatibility complex (MHC), T-cell receptor (TCR), and CD4 or CD8 coreceptor molecules. Correlations have been established between MHC gene products and the percentage of CD4 or CD8 T cells that express specific variable (V) beta-gene products as part of the alpha beta heterodimer. These studies have important implications regarding potential mechanisms of HLA-linked autoimmune diseases in humans. If similar interactions are required for positive selection in humans, one would predict that the TCR repertoire expressed by mature, peripheral blood CD4 and CD8 T cells would vary. To test this hypothesis the expression of specific TCR V beta-region genes by CD4 and CD8 T cells from healthy individuals was compared using both triple-color flow cytometry and polymerase chain reaction based experimental approaches. The results show that the TCR repertoire does vary as a function of CD4 and CD8 T-cell subsets. Among unrelated individuals certain V beta genes were consistently overrepresented in the CD4 population (V beta-5.1, -6.7a, and -18); some were skewed to the CD8 population (V beta-14) while others showed variable patterns (V beta-12 and -17). Deletion of entire V beta gene families was not observed suggesting that this is a rare event in humans. Attempts to correlate the expressed TCR repertoire in humans with HLA alleles will require consideration of these differences in expression as a function of subset.

HLA-DR and -DQ gene polymorphism in West Africans is twice as extensive as in north European Caucasians: evolutionary implications

The HLA genes are the most polymorphic coding loci known in humans. DRB-DQA-DQB gene polymorphism was investigated by Taq I restriction fragment length polymorphism analysis in more than 700 West Africans and found to be almost twice as extensive in West Africans as in North European Caucasians. This finding indicates that Africans comprise the oldest and genetically most diverse human population and supports the hypothesis of the occurrence of a population bottleneck in the emergence of the White race. As in Caucasians, less than one-third of possible cis-encoded DQA-DQB combinations were encountered, indicating constraints on the pairing of DQ alpha and beta polypeptides. Heterozygote advantage (i.e., positive selection) was found for DRB, DQA, and DQB alleles as well as for DQA-DQB combinations. However, in West Africans as well as in North Europeans the observed frequencies of DRB-DQA-DQB homozygotes were close to neutrality expectations. Although the hypothesis that HLA polymorphism is maintained by parasite-driven overdominant selection is attractive, there is little evidence to support that view. We propose instead that one of the forces maintaining a low frequency of HLA homozygotes might be a decreased likelihood of potentially autoreactive T-cell clones escaping thymic selection in HLA heterozygotes. This would be consistent with the central role of HLA molecules as self/non-self discriminators.

The T-cell-receptor repertoire in the synovial fluid of a patient with rheumatoid arthritis is polyclonal

We have analyzed the T-cell-receptor repertoire expressed in the synovial fluid of a patient with rheumatoid arthritis by using an inverse polymerase chain reaction. Total RNA was isolated from Ficoll-purified mononuclear cells and converted into circularized double-stranded cDNA. Specific amplification of alpha- and beta-chain variable regions (V alpha and V beta) was achieved with inverted alpha- and beta-chain constant region (C alpha and C beta) primer pairs, and the amplification products were cloned into phage vectors. A total of 78 alpha and 76 beta clones were sequenced, and 67 and 72 productively rearranged alpha and beta genes were identified, respectively. Thirty-one V alpha, 33 alpha-chain joining region (J alpha), 29 V beta, and 12 beta-chain joining region (J beta) gene segments were found in the productively rearranged clones, indicating that the T-cell repertoire expressed in the synovial fluid of this RA patient is highly heterogenous and polyclonal. Comparison of peripheral blood and synovial fluid repertoires showed that the most abundant V beta sequences, V beta 2.1 and V beta 3.1, were enriched in the inflamed joint by a factor of 2 to 3. It is possible that T cells expressing these V beta gene segments, which recognize bacterial superantigens, play a role in the disease.

Surface expression of a T cell receptor beta (TCR-beta) chain in the absence of TCR-alpha, -delta, and -gamma proteins

The antigen receptor expressed by mature T cells has been described as a disulfide-linked alpha/beta or gamma/delta heterodimer noncovalently associated with CD3, a complex of transmembrane proteins that communicates signals from the T cell receptor (TCR) to the cell interior. Studies suggest that all component chains must assemble intracellularly before surface expression can be achieved. We described, however, a CD4+/CD8+ transformed murine thymocyte, KKF, that expresses surface TCR-beta chains in the absence of gamma, delta, and alpha proteins; these beta chains are only weakly associated with CD3-epsilon and CD3-zeta. Furthermore, KKF responds differently to stimulation through TCR-beta and CD3-epsilon, a functional dissociation that has been ascribed to a CD4+/CD8+ subpopulation of normal thymocytes. KKF's unique TCR structure may offer an explanation for the functional anomalies observed.

Self-nonself discrimination and repertoire selection of human T cells differentiated in an HLA-semiallogeneic environment following bone marrow transplantation for severe combined immunodeficiency

We have analyzed allorecognition, HLA restriction and T cell receptor (TcR) diversity in an HLA-heterozygous (HLA-DRw6,7) severe combined immunodeficiency (SCID) patient whose T cell system had been repopulated by HLA-homozygous (HLA-DRw6) paternal T cells following T cell-depleted bone marrow transplantation (BMT). Donor origin of T cells and host origin of antigen-presenting cells (APC) in peripheral blood and BM is shown by HLA typing of separated cell populations and two-color immunofluorescence using an anti-HLA monoclonal antibody (mAb). Peripheral blood lymphocytes (PBL) from the chimeric patient proliferate normally against PHA, anti-TcR/CD3 mAb, pooled allogeneic PBL, and also against the recall antigen (Ag) tetanus toxoid and purified protein derivative of tuberculin (PPD) following immunization, suggesting recognition by donor (DRw6) T cells of Ag presented by host (DRw6,7) APC. PPD-specific cytotoxic T lymphocytes generated in vitro from patient PBL post-BMT display specific cytotoxicity against targets expressing DRw6 and DR7, but not against DR-mismatched targets, suggesting that HLA restriction of Ag recognition may occur through determinants expressed by the host and not by the donor. Donor T cells differentiated in the HLA-semiallogeneic host show specific proliferative and cytotoxic responses against HLA-mismatched stimulators, but not against stimulators taken from the host, expressing the host-specific HLA-haplotype, or expressing the host-specific HLA-DR7 antigens. Compared to T cells directly taken from the donor, differentiation of donor T cells in the host is associated with a significant decrease of T cells expressing TcR V beta 5 and V alpha 2 determinants, while no differences in the abundance of of TcR V beta 6, V beta 8 and V beta 12 subsets were noticed. We conclude that allorecognition, major histocompatibility complex (MHC) restriction and TcR diversity generation of human T cells can be modulated through differentiation in an MHC-different environment, as had been previously shown to be the case in murine model systems.

Mechanisms of IgE tolerance: dual regulatory T cell lesions in perinatal IgE tolerance

The mechanisms of genetic control of IgE responses are exercised at different immuno-physiological levels. This study centered upon the development of IgE lineage-specific regulatory T cells. Herein, we demonstrate the following points: (a) perinatal administration of soluble self IgE molecule or self IgE complexed with foreign antigen induces IgE tolerance as manifested by antigen-specific IgE unresponsiveness and a generalized IgE immunodeficiency, and the induction of IgE tolerance does not affect antigen-specific IgG1, IgG, and IgA responses; (b) inducibility of perinatal IgE tolerance is correlated with complete absence of endogenously secreted IgE in the neonates; and the state of persistent IgE tolerance also does not correlate with the presence of high levels of circulating anti-IgE autoantibodies; (c) The lesions induced during the ontogeny of IgE antibody system do not appear to result from an imbalance of production of interleukin 4 and interferon-gamma by T helper Th2 and Th1 cells in antigen-stimulated cultures; the dual immunoregulatory lesions in T cell subsets are demonstrated: clonal anergy/deletion of CD4+ IgE Th cells and the presence of CD8+ IgE suppressor cells induced by perinatal IgE treatment. We propose that antigen/interleukin 4 activated B cells are controlled by IgE lineage-specific regulatory T cells which recognize self IgE determinant(s) on IgE committed B cells. Life-long IgE tolerance ensues as a result of a new steady state of IgE lineage-specific CD4+ and CD8+ T cell subsets.

Rat "first-wave" mature thymocytes: cycling lymphoblasts that are sensitive to activation-induced cell death but rescued by interleukin 2

The sequential appearance of thymocyte subsets in rat ontogeny was studied using the surface markers CD4, CD8, and the alpha/beta T cell receptor (here referred to as TcR). It was noted that the first TcRhigh thymocytes, appearing around birth, are not resting lymphocytes but cycling blast cells. These "first wave" TcRhigh cells are medullary in location and predominantly of the CD4/CD8 "single-positive" phenotype. Only about 5% express the light chain of the interleukin (IL)2 receptor, indicating that binding of IL 2 to high-affinity receptors is not driving proliferation of these blasts. Newborn TcR high blast cells were purified and analyzed in vitro. When cultured without further additions, they rapidly stopped cycling. Stimulation with cross-linked anti-TcR monoclonal antibody plus IL 2 resulted in vigorous and rapid proliferation that exhibited accelerated kinetics as compared to peripheral resting T cells. In contrast, TcR cross-linking without exogenous IL 2 induced cell death. This TcR-induced cell death involved fragmentation of DNA characteristic of apoptosis that was readily detectable within 18 h of culture. Addition of IL 2 to these short-term cultures prevented TcR-induced DNA fragmentation. Together, these results suggest that in newly formed TcRhigh thymocytes, TcR engagement results in clonal deletion if the IL 2 receptor remains unoccupied but allows clonal expansion if IL 2 is provided. This mechanism may be operative in the establishment of self-tolerance during T cell development.

Evidence for clonal selection of gamma/delta T cells in response to a human pathogen

T cells bearing gamma/delta antigen receptors comprise a resident population of intraepithelial lymphocytes in organs such as skin, gut, and lungs, where they are strategically located to contribute to the initial defense against infection. An important unsolved question about antigen-driven gamma/delta T cell responses regards the breadth of their T cell receptor (TCR) repertoire, since many specific epithelial compartments in mice display limited diversity. We have examined the diversity of TCR delta gene expression among human gamma/delta T cells from skin lesions induced by intradermal challenge with Mycobacterium leprae. We show that the vast majority of gamma/delta cells from M. leprae lesions use either V delta 1-J delta 1 or V delta 2-J delta 1 gene rearrangements and, within a given region of the lesion, display limited junctional diversity. This contrasts markedly with the extensive diversity of gamma/delta T cells from peripheral blood of these same individuals, as well as skin from normal donors. These results indicate that the gamma/delta response to M. leprae involves the selection of a limited number of clones from among a diverse repertoire, probably in response to specific mycobacterial and/or host antigens.

Requirement for positive selection of gamma delta receptor-bearing T cells

The alpha beta and gamma delta T cell receptors for antigen (TCR) delineate distinct T cell populations. TCR alpha beta-bearing thymocytes must be positively selected by binding of the TCR to major histocompatibility complex (MHC) molecules on thymic epithelium. To examine the requirement for positive selection of TCR gamma delta T cells, mice bearing a class I MHC-specific gamma delta transgene (Tg) were crossed to mice with disrupted beta 2 microglobulin (beta 2M) genes. The Tg+beta 2M- (class I MHC-) offspring had Tg+ thymocytes that did not proliferate to antigen or Tg-specific monoclonal antibody and few peripheral Tg+ cells. This is evidence for positive selection within the gamma delta T cell subset.

Thymocyte expression of RAG-1 and RAG-2: termination by T cell receptor cross-linking

The expression of the V(D)J [variable (diversity) joining elements] recombination activating genes, RAG-1 and RAG-2, has been examined during T cell development in the thymus. In situ hybridization to intact thymus and RNA blot analysis of isolated thymic subpopulations separated on the basis of T cell receptor (TCR) expression demonstrated that both TCR- and TCR+ cortical thymocytes express RAG-1 and RAG-2 messenger RNA's. Within the TCR+ population, RAG expression was observed in immature CD4+CD8+ (double positive) cells, but not in the more mature CD4+CD8- or CD4-CD8+ (single positive) subpopulations. Thus, although cortical thymocytes that bear TCR on their surface continue to express RAG-1 and RAG-2, it appears that the expression of both genes is normally terminated during subsequent thymic maturation. Since thymocyte maturation in vivo is thought to be regulated through the interaction of the TCR complex with self major histocompatibility complex (MHC) antigens, these data suggest that signals transduced by the TCR complex might result in the termination of RAG expression. Consistent with this hypothesis, thymocyte TCR cross-linking in vitro led to rapid termination of RAG-1 and RAG-2 expression, whereas cross-linking of other T cell surface antigens such as CD4, CD8, or HLA class I had no effect.

BCL2 protein is topographically restricted in tissues characterized by apoptotic cell death

The BCL2 protooncogene encodes an inner mitochondrial membrane protein that blocks programmed cell death. BCL2 was isolated from the chromosomal breakpoint of follicular B-cell lymphoma. Transgenic mice that overexpress BCL2 display extended survival of resting B cells. In this study we use a monospecific anti-human BCL2 antibody to define the distribution of BCL2 protein within organized tissues. BCL2 is restricted within germinal centers to the follicular mantle and to portions of the light zone implicated in the selection and maintenance of plasma cells and memory B cells. BCL2 is present in the surviving T cells in the thymic medulla. All hematopoietic lineages that derive from a renewing stem cell also display BCL2. A limited number of nonlymphoid tissues demonstrate BCL2 and can be grouped as (i) glandular epithelium in which hormones or growth factors regulate hyperplasia and involution, (ii) complex differentiating epithelium such as skin and intestine characterized by long-lived stem cells, and (iii) long-lived postmitotic cells such as neurons. Within these tissues that demonstrate apoptotic cell turnover, BCL2 is often topographically restricted to long-lived or proliferating cell zones. BCL2's function as an antidote to apoptosis may confer longevity to progenitor and effector cells in these tissues.

Tolerance to liver-specific antigens

We have described a TG model for peripheral tolerance of alloreactive CTL. Expression of Q10/L on hepatocytes renders mice functionally tolerant, although in vitro we observe that TG animals have normal numbers of CTL.Pf directed against this antigen. The basis for the tolerance presumably resides in the fact that the TG mice are lacking a subpopulation, either through deletion or anergy, that is responsible for recognition of the antigen on hepatocytes in vivo. The data are consistent with a tolerance model where cells with high affinity receptors are silenced. The presumed low affinity antigen-specific cells remaining in TG mice cannot be primed in vivo when immunized with antigen on spleen cells. Further, these CTL generate poor lytic activity in vitro. This failure to prime TG CTL in vivo could be attributed to primed cells traveling to the liver where they become tolerized when exposed to antigen on hepatocytes. However, we show that TG cells, after transfer to non-TG recipients, cannot be primed in vivo, indicating that the presumed low-affinity cells remaining in TG mice are not readily activable in this milieu. These data also indicate that this tolerance is not readily reversible during a 10- to 17-d time interval.

Abnormal thymic development, impaired immune function and gamma delta T cell lymphomas in a TL transgenic mouse strain

During derivation of transgenic mouse strains with various TL and TL/H-2 chimeric genes, one strain, Tg.Tlaa-3-1, introduced with a TL gene (Tlaa-3), was found to have an abnormal thymic T cell population and to develop a high incidence of T cell lymphomas. To investigate the etiology of the thymic abnormalities and of the lymphomas, the development of lymphoid organs in transgenic mice was studied. The thymus of these mice goes through three unusual successive events: perturbation of thymic development during embryogenesis, disappearance of thymocytes between day 14 and day 21 after birth, and subsequent proliferation of large blast-like cells. These events are associated with the abolishment of T cell receptor (TCR) alpha beta lineage of the T cell differentiation, leading to preponderance of cells belonging to the TCR gamma delta L3T4-Lyt-2- double negative (DN) lineage. Bone marrow transplantation and thymic graft experiments demonstrate that the abnormality resides in the bone marrow stem cells rather than in the thymic environment. The expression of TL antigen in the transgenic mice is greatly increased and TL is expressed in a wide range of T cells, including normally TL- DN cells and L3T4+ Lyt-2- and L3T4-Lyt-2+ single positive cells. These quantitative and qualitative abnormalities in TL expression most likely cause the abnormal T cell differentiation. The gamma delta DN cells migrate into peripheral lymphoid organs and constitute nearly 50% of peripheral T cells. Immune function of the transgenic mice is severely impaired, as T cell function is defective in antibody production to sheep red blood cells, in mixed lymphocyte culture reaction to allogenic spleen cells and also in stimulation with concanavalin A. These results indicate that the gamma delta cells are incapable of participating in these reactions. Molecular and serological analysis of T cell lymphomas reveal that they belong to the gamma delta lineage, suggesting that the gamma delta DN cells in this strain are susceptible to leukemic transformation. Based on cell surface phenotype and TCR expression of the DN thymocytes and T cell lymphomas, a map of the sequential steps involved in the differentiation of gamma delta DN cells is proposed.(ABSTRACT TRUNCATED AT 400 WORDS)

T-cell tolerance to viruses

T-cell tolerance to self antigens is maintained by events that occur within the thymus and in the periphery. Mechanisms that operate on immature T cells within the thymus are effective in induction of tolerance to viruses, but mechanisms of tolerizing mature T cells are likely to break down. This failure of peripheral mechanisms to induce T-cell tolerance to viruses has implications for autoimmunity and for treatment of chronic viral infections.

Interferon-beta specifically inhibits interferon-gamma-induced class II major histocompatibility complex gene transcription in a human astrocytoma cell line

We established cultures of human astrocytes and astrocytoma cells from surgical specimens, to study regulation of class II major histocompatibility (MHC) complex antigen expression by interferons. Using these cultures we previously showed that expression of the class II MHC determinant HLA-DR could be induced by interferon-gamma and this induction was inhibited by interferon-beta. In this report, we extend these observations by showing that the inhibitory effect of interferon-beta on interferon-gamma induction of the class II MHC gene HLA-DR alpha was exerted at the transcriptional level, as documented by nuclear run-on experiments and confirmed with blot hybridization analysis. Astrocyte expression of intercellular adhesion molecule-1 (ICAM-1) was induced efficiently by interferon-gamma, but not by interferon-beta, and induction of ICAM-1 expression by interferon-gamma could not be impaired by interferon-beta, suggesting that the suppressive effect on induction of HLA-DR was relatively gene-specific. Furthermore, interferon-beta did not antagonize interferon-gamma induction of HLA-DR expression in human monocytes, suggesting that the inhibition observed in astrocytes was relatively tissue-specific.

Altered antigen receptor signaling in anergic T cells from self-tolerant T-cell receptor beta-chain transgenic mice

T-cell tolerance to the minor lymphocyte-stimulating antigen Mls-1a in a T-cell receptor (TcR) V beta 8.1 transgenic line of mice is maintained by both clonal deletion and clonal anergy. Approximately 20-50% of peripheral CD4+ (but not CD8+) T cells isolated from these mice are anergic and fail to proliferate following TcR ligation. We have examined key events in T-cell signaling in peripheral T cells isolated from these mice. In this report, we show that the anergic CD4+ T cells did not mobilize calcium or express receptors for interleukin 2 (IL-2) following TcR ligation. However, the cells retained viability and functional potential because stimulation with phorbol 12-myristate 13-acetate and ionomycin bypassed the block in receptor-mediated signaling and induced IL-2 receptor expression and proliferation of the anergic cells.

A transgenic approach to the study of peripheral T-cell tolerance

There is now convincing evidence for the imposition of self tolerance by means of the clonal deletion of self-reactive T cells operating within the thymus. Since not all self components may be encountered there, the question must be asked whether tolerance can occur post-thymically. To test this, we and other investigators have used transgenic technology to direct expression of a known "nonself" gene to a given extrathymic tissue. No lymphocytic infiltration was ever seen in transgene-expressing tissues, even if the mice were given normal syngeneic (nontransgenic) spleen cells intravenously or were stimulated with H-2Kb spleen cells. Infiltration did, however, occur in irradiated transgenic recipients of H-2Kb immune spleen cells. In MET-Kb mice, this infiltrate diminished with time, raising the possibility that peripheral tolerance may even have been induced in immune cells. H-2Kb-bearing skin was accepted in young RIP-Kb mice but rejected in older mice, which had lost more than 75% of their beta cells as a result of the overexpression of H-2Kb. This loss of tolerance thus occurred when the concentration of the tolerogen, H-2Kb, fell below some critical threshold. Following in vitro stimulation, spleen cells from young RIP-Kb mice could not kill H-2Kb-bearing targets, but could respond to third party targets. Thymus cells, on the other hand, could be stimulated to kill both targets, clearly indicating that tolerance was not imposed intrathymically. Spleen cells from older RIP-Kb mice (those that had lost most of their beta cells) killed both targets, which is in agreement with the in vivo data. Reactivity to H-2Kb was restored to young spleen cells by providing them with IL-2. Two hypotheses were proposed to account for the above findings: tolerance results either from the deletion or functional silencing of high-affinity effector cells or of regulatory, IL-2-producing helper T cells. As it is difficult to distinguish between these, we have produced a second series of transgenic mice (F3+) with rearranged TCR genes encoding an anti-H-2Kb TCR and derived "double-transgenic" (F3+RIP+) offspring by mating these mice with RIP-Kb mice. The transgenic TCR utilized the V beta 11 segment which can be detected by a monoclonal antibody. There were in the thymus very few CD4+ and very few CD4+8+ cells in both F3+ and F3+RIP+ mice and, in the double-transgenic mice, there was no evidence of deletion of CD8+V beta 11+ cells in the periphery although they showed tolerance to H-2Kb-bearing skin.(ABSTRACT TRUNCATED AT 400 WORDS)

Developmental T cell receptor gene rearrangements: relatedness of the alpha/beta and gamma/delta T cell precursor

To examine the relationships between T cell populations at various stages of development, T cell receptor (TcR) gene rearrangements were compared between the four murine populations of (a) early thymocytes, (b) early splenocytes, (c) adult thymocytes and (d) adult splenocytes. TcR alpha gene rearrangements were shown to progress from 5' to 3' regions of the J alpha locus and from 3' to 5' regions of the V alpha locus during the development of T cells in both the thymus and spleen. Thus, the gene rearrangement potentials of proximal genes varied with age, yielding a biased repertoire in the young vs. adult animal. As evidence that gamma/delta and alpha/beta gene rearrangements appeared concomitantly in individual precursors, it was found that: (a) multiple adult thymocytes bore alpha gene rearrangements on one chromosome and delta gene rearrangements on the homologous chromosome, and (b) V gamma 3-J gamma 1 rearrangements, prominent joins in the early gamma/delta T cell population, were also prominent in the early alpha/beta T cell subset. These data illustrate the non-random nature of the developmental TcR gene rearrangement and suggest that alpha/beta and gamma/delta T cell populations derive from related, if not identical, T cell precursor populations.

Junctional sequences of fetal T cell receptor beta chains have few N regions

T cell receptors (TCRs) and immunoglobulins (Igs) derive a large fraction of their repertoire from diversity generated at the junctions of the V, D, and J coding segments. This diversity is derived both from the random deletion of nucleotides from the ends of coding regions and from the subsequent addition of nontemplated N region nucleotides. While the vast majority of TCRs and Igs from adult mice have N regions, less than 5% of both TCR-gamma/delta and Ig from fetal and neonatal mice have N regions. This study analyzed the ontogeny of junctional diversity of TCR-alpha/beta. Genomic DNA or C beta-primed cDNA was prepared from thymocytes of mice at varying stages in ontogeny, and the rearranged V beta 8 or V beta 5 sequences were amplified by polymerase chain reactions. Sequencing of the V beta-D beta-J beta junctions showed few N regions early in ontogeny, although the fraction of sequences with N regions exceeded that previously reported for Ig and for TCR-gamma/delta. N regions were found in 13% of V beta junctional sequences from day 18-19 fetal thymocytes, 33% of sequences from newborn thymocytes, 76% of sequences from day 4 postnatal thymocytes, and 88% of sequences from 5-wk-old thymocytes. In addition, nonrandom usage of the D beta and J beta segments was observed in both fetal and adult TCR sequences. While the usage of each of the six J beta 2 segments was different, the same pattern of usage was seen regardless of whether D beta 1 or D beta 2 was used, suggesting that a factor controlling the rate of usage of each J segment is intrinsic to the J gene itself. Since TCRs derive so much of their diversity from N regions, the relative paucity of N regions in fetal alpha/beta T cells would create a fetal TCR-alpha/beta repertoire that would be quite different from, and smaller than, the adult repertoire. The lack of N regions might be predicted to limit the range of affinities of TCR-MHC + peptide interactions, which may have important consequences for positive and negative selection of fetal and newborn T cells.

A novel class II MHC molecule with unusual tissue distribution

The repertoire of mature class II-restricted T cells is generated through a complex process of selection whereby early T cells confront class II molecules in the thymus, especially on epithelial cells. Expression of class II molecules on such cells is prominent both in the cortex and in the medulla. We have identified a novel class II molecule, H-20, which is expressed only in epithelial cells of the thymic medulla and in B cells. The unusual tissue distribution and the nonpolymorphic nature of H-20 suggest that its function is different from that of classical class II molecules.

Genetic control of immune response and disease susceptibility by the HLA-DQ gene

The particular alleles of the HLA-DQ locus may control the low immune response to natural antigens by a dominant genetic trait through the immune suppression mediated by CD8+ suppressor T cells. The suppressor T cells may be activated by DQ-restricted and antigen-specific CD4+ suppressor/inducer T cells, because (1) a statistically significant association and linkage between low immune responsiveness to the natural antigens and the HLA-DQ gene were observed; (2) antigen-specific CD4+ T cells restricted by the DQ molecules encoded for by the HLA-DQ allele associated with low responsiveness were evidenced in many low responders; and (3) anti-HLA-DQ mAb restored the immune response to natural antigens, in some low responders. This HLA-DQ-controlled polymorphism of immune response to the natural antigens may account for the association between HLA-DQ alleles and organ-specific autoimmune diseases.

Influence of antigen processing on thymic T-cell selection

The design of a specific blocking peptide for the immunosuppressive therapy of an autoimmune disease requires the identification of peptides of an autoantigen that are physiologically processed in vivo and bind to MHC-encoded membrane glycoproteins. However, knowledge of how an antigen is physiologically processed by antigen-presenting cells (APC) in vivo, particularly in the thymus, is lacking. It is also unknown whether the processing of an antigen by different APC in the thymus can influence thymic T-cell selection. This is an important consideration for attempts to delete or inactivate autoreactive T cells that elicit autoimmune disease. To address these issues, we investigated the processing of biosynthetically labelled recombinant human insulin (rHI), a model autoantigen, injected into mice and characterized the insulin peptides associated with MHC class II molecules on thymic epithelial cells and dendritic cells. These APC were found to differ in the way they process insulin. The detection of MHC-class-II-bound insulin peptides on the surface of the epithelial cells but not the dendritic cells correlated with their capacity to either present or not present insulin to T cells, respectively. Thus, antigen processing may control the appearance of different peptide-MHC class II complexes on thymic APC that mediate positive and negative selection, and thereby influence the development of the T-cell repertoire. Our findings could have important bearing on the future design of synthetic blocking peptides that reduce or eliminate the onset of autoimmune disease.

Interleukin 2 abrogates the nonresponsive state of T cells expressing a forbidden T cell receptor repertoire and induces autoimmune disease in neonatally thymectomized mice

Under physiological conditions, the vast majority of T cells differentiate in the thymus, an organ that provides an optimal microenvironment for T cell maturation and shapes the T cell repertoire via positive and negative selection processes. In the present report, we demonstrate that neonatal thymectomy of CBA/H mice results in a diminution of T cells in peripheral lymphoid organs (spleen, lymph nodes), but is followed by a marked transient (12 wk) increase in Thy-1+ CD3+ cells in the peritoneal cavity. These cells exhibit predominantly a double-negative (CD4-CD8-) phenotype among which products of the T cell receptor (TCR) V beta 11 gene family (i.e., an I-E-reactive TCR normally deleted in I-E-bearing CBA/H mice) are selectively overexpressed. This observation suggests that, under athymic conditions, T cell differentiation and/or accumulation may occur in the peritoneal cavity. Intraperitoneal inoculation of an interleukin 2 (IL-2) vaccinia virus construct that releases high titers of human IL-2 in vivo induces conversion of these double-negative T cells to either CD4+ CD8- or CD4- CD8+ single positives, and allows in vitro stimulation of TCR V beta 11-bearing cells with a clonotypic anti-V beta antibody. Since IL-2 induces autoimmune manifestations (DNA autoantibodies, rheumatoid factors, and interstitial nephritis) in thymectomized CBA/H mice, but not in sham-treated littermates, this lymphokine is likely to enhance the autoaggressive function of T cells that bear forbidden, potentially autoreactive TCR gene products and that are normally deleted in the thymus.

Aberrant colonic expression of MHC class II antigens in Hirschsprung's disease

The major histocompatibility complex (MHC) Class I and II cell surface antigens responsible for the recognition of self vs non-self were studied in patients with documented Hirschsprung's disease. Monoclonal antibodies reactive with monomorphic determinants of human lymphocyte antigen (HLA)-A,B,C (Class I) and HLA-DR (Class II) were used to demonstrate immunohistochemically the expression of MHC antigens in 27 biopsy specimens from a variety of colorectal disorders. The rectal specimens examined from patients with Hirschsprung's disease showed an unexpected, marked elevation of Class II antigens with abnormal localization in the mucosa and lamina propria. This ectopic expression was not seen in any portion of the small or large bowel of patients who did not have Hirschsprung's disease. Furthermore, proximal normal colon of children with Hirschsprung's disease failed to show increased expression of Class II antigen. In an attempt to better define the effector arm at a cellular level, the distribution of helper T cells (CD4+), cytotoxic/suppressor T cells (CD8+) and natural killer cells (NK; CD16+) was examined in 5 cases. In Hirschsprung's disease, rectal infiltration of CD8+ and CD16+ cells was found, but not CD4+ cells. Ectopic expression of Class II antigen with increased numbers of rectal T cells and NK cells suggested that an early immunologic event may be causal in Hirschsprung's disease.

LEF-1, a gene encoding a lymphoid-specific protein with an HMG domain, regulates T-cell receptor alpha enhancer function [corrected]

Lymphoid-specific cDNA clones were isolated that encode a nuclear protein with homology to the chromosomal nonhistone protein HMG-1 and to putative regulators of cell specialization, including the mammalian testis-determining factor SRY and fungal mating-type proteins. The gene represented by the isolated cDNA clones, termed LEF-1 (lymphoid enhancer-binding factor 1), is developmentally regulated and expressed in pre-B and T lymphocytes but not in later-stage B cells or nonlymphoid tissues. Both endogenous and recombinant LEF-1 were shown to bind to a functionally important site in the T-cell antigen receptor (TCR) alpha enhancer. Maximal TCR alpha enhancer activity was found to parallel the cell type-specific expression pattern of LEF-1. Moreover, forced expression of recombinant LEF-1 in late stage B cells increases TCR alpha enhancer function. Taken together, these data suggest that LEF-1 is a regulatory participant in lymphocyte gene expression and differentiation.

Regulation of T cell receptor signaling by a src family protein-tyrosine kinase (p59fyn)

Engagement of the clonotypic antigen receptor (TCR) on T lymphocytes provokes an activation response leading to cell proliferation and lymphokine secretion. To examine the molecular basis of T cell signaling, we generated transgenic animals in which a lymphocyte-specific nonreceptor protein-tyrosine kinase p59fyn(T) is 20-fold overexpressed in developing T lineage cells. Thymocytes from these mice, analyzed using both cellular and biochemical assays, were remarkably hyperstimulable. Moreover, the responsiveness of normal thymocytes to TCR-derived signals correlated well with the extent to which p59fyn was expressed in these cells. Overexpression of a catalytically inactive form of p59fyn substantially inhibited TCR-mediated activation in otherwise normal thymocytes. These effects are unique to p59fyn; overexpression of a closely related T cell-specific tyrosine kinase, p56lck, elicits dramatically different phenotypes. Our results suggest that p59fyn is a critically important component of the TCR signal transduction apparatus.

Characterization of a CD4-positive T-cell line derived from an athymic (nu/nu) mouse

We have isolated a Thy-1+, CD3+, CD4+ T-cell line from the spleen of a 12-week-old nu/nu (nude) BALB/c mouse. The cell line is clonal, and it expresses an alpha beta T-cell antigen receptor. Upon activation, these cells secrete IL-2 but not IL-4, putting them in the Th1 category. The cells can be triggered to proliferate and secrete lymphokines in the presence of irradiated syngeneic or allogeneic splenic feeder cells that express a variety of MHC haplotypes. This response is MHC class II-specific, because it can be blocked by either anti-Ia or anti-CD4 antibodies. From the response pattern of this T-cell line, we conclude that it recognizes a common determinant on class II MHC antigens. This nude mouse T-lymphocyte presumably has not undergone thymic selection. Therefore its unique specificity may reflect both the bias of T-cell antigen receptor genes for encoding receptors that recognize MHC molecules and the requirement for functional thymic epithelial cells for the efficient education of a self-MHC-restricted repertoire.

Cell dysfunction and depletion in AIDS: the programmed cell death hypothesis

Normal immature thymocytes respond to activation by undergoing programmed cell death (apoptosis), a physiological deletion mechanism involved in the selection of the T-cell repertoire. In this article, Jean Claude Ameisen and André Capron suggest that inappropriate induction of a form of programmed T-cell death could account for both qualitative and quantitative helper T-cell defects of HIV-infected patients. A model of AIDS pathogenesis is presented that may explain several features of HIV infection, including evolution of the disease and the development of defects in nonimmunological organs.

Genomically imposed and somatically modified human thymocyte V beta gene repertoires

The effect of thymic selection on the expressed human T-cell antigen receptor beta-chain variable region (V beta) gene repertoire was examined by using a multiprobe RNase protection assay. The relative abundance of transcripts for 22 V beta genes (encompassing 17 of the 20 human V beta gene subfamilies) within a thymus, and among 17 thymuses, was variable. On the basis of the presence of corresponding mRNAs, no genomic deletions were detected, but several coding region polymorphisms were identified. Analysis of mature T-cell subsets revealed the absence of complete "superantigen"-mediated V beta deletions, suggesting that this phenomenon, in contrast to mouse, is uncommon or absent in humans. However, several V beta genes were over- or underexpressed in one or both mature single-positive (CD4+8- or CD8+4-) thymocyte subsets compared to syngeneic total, mostly immature thymocytes. Whether these changes are induced by relatively weak superantigens or conventional antigens and whether the downshifts are caused by negative selection or lack of positive selection remains to be determined.

Antigen presentation: structural themes and functional variations

T cells recognize nonnative processed fragments of antigens presented in association with major histocompatibility complex (MHC) class I or class II molecules. Recently, an accumulating body of evidence has provided a functional linkage between antigen presentation events and the cell biology of MHC molecule assembly and transport. In this review Thomas and Vivian Braciale synthesize these developments into a cohesive model of MHC assembly and antigen presentation pathways.

Split anergy in a CD8+ T cell: receptor-dependent cytolysis in the absence of interleukin-2 production

Engagement of the antigen-specific receptor (TCR) of CD4+ T lymphocytes without a second (costimulatory) signal prevents the subsequent production of interleukin-2 (IL-2) by these cells. Because IL-2 is a key immunoregulatory lymphokine and is also produced by a subset of CD8+ T cells that are able to kill target cells, the effect of engaging the TCR of one such clone in the absence of costimulatory signals was examined. The capacity for TCR-dependent IL-2 production was lost, indicating comparable costimulator-dependent signaling requirements for IL-2 production in CD4+ and CD8+ T cells. However, TCR-mediated cytotoxicity was not impaired, implying that costimulation is required for only certain TCR-dependent effector functions.

Rescue of thymocytes and T cell hybridomas from glucocorticoid-induced apoptosis by stimulation via the T cell receptor/CD3 complex: a possible in vitro model for positive selection of the T cell repertoire

Positive and negative selection events are involved in determining useful T cell clones to mature in the thymus. Accumulating evidence suggests that immature self-reactive thymocytes undergo apoptotic death (negative selection) upon stimulation via the T cell receptors (TcR). A similar phenomenon of activation-induced death has been reported in T cell hybridomas. On the other hand, little is known about the mechanism of the positive selection. Apoptosis in rodent thymocytes or T cell hybridomas is also known to be induced by glucocorticoids in vitro at concentrations within the physiologic range. We report here that the TcR/CD3-mediated stimulation and glucocorticoids mutually inhibit the apoptosis in T cell hybridomas. The production of interleukin 2 by the rescued cells indicated that the TcR/CD3-mediated signal was transduced into the cells. Thymocytes were also rescued from glucocorticoid-induced apoptosis by the stimulation with antibodies to TcR/CD3 molecules. The rescue of thymocytes, however, was observed only at a narrow concentration range of each of the antibodies, suggesting that the proper stimulation via the TcR/CD3 is required for the rescue. If thymocytes in situ are differentially stimulated according to the affinity of the TcR towards self, only the thymocytes whose TcR have proper affinity towards self may be rescued from glucocorticoid-induced apoptosis. Therefore, we propose a hypothesis that the positive selection of the T cell repertoire is based on the inhibition of glucocorticoid-induced apoptosis in immature thymocytes bearing TcR with proper affinity for self by the TcR-mediated signals in situ. Furthermore, the selection may be influenced by the peak level of glucocorticoid concentration, since the proper concentration range of the anti-TcR/CD3 antibody for the rescue was variable depending on the glucocorticoid concentration.

T cell receptor-mediated selection of functional rat CD8 T cells from defined immature thymocyte precursors in short-term suspension culture

Recent results have indicated that positive and negative repertoire selection act on the major population of CD4,8 double-positive (DP) thymocytes that express 5-10-fold less T cell receptor (TCR) than mature T cells (i.e., they are TCRlow). Since DP cells obtained ex vivo are heterogeneous with regard to their stage within thymic selection, a homogeneous population of virgin DP cells suitable for selection studies was generated in vitro from their immediate precursors, the CD8 single-positive (SP) immature blast cells. To mimic TCR-mediated selection signals, these virgin DP cells were then cultured for another 2 d in the presence of immobilized anti-TCR monoclonal antibodies with or without interleukin 2 (IL-2). Daily monitoring of recovery and phenotype showed that without TCR stimulation, the cells remained DP and became small, TCRlow cells that were lost with a half-life of 1 d, regardless of the presence of IL-2. TCR stimulation resulted in rapid downregulation of CD4 and CD8, maintenance of a larger cell size, and induction of the CD53 antigen that marks mature and CD4,8 double-negative rat thymocytes. In the absence of IL-2, viability decreased as rapidly as without TCR stimulation. Addition of IL-2 rescued TCR-stimulated virgin DP cells and prevented CD8 downregulation, so that 50-80% of input DP cells were recovered after 2 d as CD4-8+53+ cells. After release from modulation, these in vitro generated CD8 SP cells quantitatively upregulated the TCR to the TCRhigh phenotype and were readily induced to proliferate and exhibit cytotoxic T lymphocyte (CTL) activity in a polyclonal readout. Evidence is presented implicating an IL-2 receptor (IL-2R) not containing the p55 chain (i.e., most likely the p70 intermediate affinity IL-2R) in the TCR plus IL-2-driven in vitro differentiation of virgin DP cells towards the mature CD8 SP phenotype.