CD8 is required during positive selection of CD4-/CD8+ T cells

Potential targeting of B7-H4 for the treatment of cancer

Observations noting the presence of white blood cell infiltrates within tumors date back more than a century, however the cellular and molecular mechanisms regulating tumor immunity continue to be elucidated. The recent successful use of monoclonal antibodies to block immune regulatory pathways to enhance tumor-specific immune responses for the treatment of cancer has encouraged the identification of additional immune regulatory receptor/ligand pathways. Over the past several years, a growing body of data has identified B7-H4 (VTCN1/B7x/B7S1) as a potential therapeutic target for the treatment of cancer. The potential clinical significance of B7-H4 is supported by the high levels of B7-H4 expression found in numerous tumor tissues and correlation of the level of expression on tumor cells with adverse clinical and pathologic features, including tumor aggressiveness. The biological activity of B7-H4 has been associated with decreased inflammatory CD4⁺ T-cell responses and a correlation between B7-H4-expressing tumor-associated macrophages and FoxP3⁺ regulatory T cells (Tregs) within the tumor microenvironment. Since B7-H4 is expressed on tumor cells and tumor-associated macrophages in various cancer types, therapeutic blockade of B7-H4 could favorably alter the tumor microenvironment allowing for antigen-specific clearance tumor cells. The present review highlights the therapeutic potential of targeting B7-H4.

The multiple roles of the CD8 coreceptor in T cell biology: opportunities for the selective modulation of self-reactive cytotoxic T cells

Short peptide fragments generated by intracellular protein cleavage are presented on the surface of most nucleated cells bound to highly polymorphic MHCI molecules. These pMHCI complexes constitute an interface that allows the immune system to identify and eradicate anomalous cells, such as those that harbor infectious agents, through the activation of CTLs. Molecular recognition of pMHCI complexes is mediated primarily by clonally distributed TCRs expressed on the surface of CTLs. The coreceptor CD8 contributes to this antigen-recognition process by binding to a largely invariant region of the MHCI molecule and by promoting intracellular signaling, the effects of which serve to enhance TCR stimuli triggered by cognate ligands. Recent investigations have shed light on the role of CD8 in the activation of MHCI-restricted, antigen-experienced T cells and in the processes of T cell selection and lineage commitment in the thymus. Here, we review these data and discuss their implications for the development of potential therapeutic strategies that selectively target pathogenic CTL responses erroneously directed against self-derived antigens.

The role of CD4 in regulating homeostasis of T helper cells

Intrathymic T cell selection and peripheral activation of mature T cells are crucial for self-recognition and the general immune response to viral, bacterial, and tumor antigens. The T cell coreceptors, CD4 and CD8, contribute to the regulation of these processes. The importance of interactions between CD4 and molecules encoded by the class II major histocompatibility complex (MHC) for thymic T cell selection has been clearly established, however, the role of CD4-MHC class II interactions in T helper (TH) cell differentiation, in the maintenance of homeostasis in the peripheral immune system, and in the generation of memory TH cells is largely unclear. Here, we present evidence for a role of CD4 in controlling homeostasis in the peripheral immune system. We also demonstrate the importance of CD4-MHC class II interactions in inducing these previously not recognized functions of CD4.

MPK38 expression is upregulated in immature T cells activated by concanavalin A

We have previously identified a cDNA clone from a murine teratocarcinoma PCC4 cDNA library, which we have termed MPK38 for murine protein kinase 38. MPK38 is a new member of the SNF1 serine/threonine kinase family. To understand the role of the MPK38 in thymocytes, we have investigated the level of MPK38 expression by Northern blot analysis. Interestingly, incubation of thymocytes at 37 degrees C resulted in the loss of the MPK38 transcript, however the transcript could be reinduced by treatment with Con A or PHA, but not with PMA and growth factors such as IL-2, IL-7, TGF-beta, TNF-alpha, and EGF. In addition, stimulation of mature T cells from the spleen with Con A failed to reinduce the MPK38 transcript, indicating a developmental regulation of MPK38 expression, whereas non-T cell populations significantly reinduced the transcript. These results suggest that MPK38 may play a functional role as one of the signal regulators in early T cell activation and, potentially, certain lineages of hematopoietic cell activation induced by Con A.

Distinct Stage-Specific cis-Active Transcriptional Mechanisms Control Expression of T Cell Coreceptor CD8α at Double- and Single-Positive Stages of Thymic Development

Developing thymocytes that give rise to CD8+ (cytotoxic) and CD4+ (helper) αβ-TCR T lymphocytes go through progressive stages of expression of coreceptors CD8 and CD4 from being negative for both (the double-negative stage), to coexpressing both (the double-positive (DP) stage), to a mutually exclusive sublineage-specific expression of one or the other (the single-positive (SP) stage). To delineate the mechanisms underlying regulation of CD8 during these developmental transitions, we have examined expression of a series of mouse CD8α gene constructs in developing T cells of conventional and CD8α “knock-out” transgenic mice. Our results indicate that cis-active transcriptional control sequences essential for stage- and sublineage-specific expression lie within a 5′ 40-kb segment of the CD8 locus, ∼12 kb upstream of the CD8α gene. Studies to characterize and sublocalize these cis sequences showed that a 17-kb 5′ subfragment is able to direct expression of the CD8α gene up to the CD3intermediate DP stage but not in more mature DP or SP cells. These results indicate that stage-specific expression of CD8α in developing T cells is mediated by the differential activity of multiple functionally distinct cis-active transcriptional control mechanisms. It will be important to determine the relationship of “switching” between these cis mechanisms and selection.

Unexpectedly complex regulation of CD4/CD8 coreceptor expression supports a revised model for CD4+CD8+ thymocyte differentiation

CD4+ CD8+ TCRlo thymocytes are the precursors of CD4+ and CD8+ mature T cells, whose receptors show specific recognition of peptide-MHC class II and MHC class I complexes, respectively. How T cells emerge from the intrathymic differentiation process with selective expression of either CD8 molecule or CD4 molecule coordinated with the MHC class specificity of the TCR has been the subject of intense examination. Many previous studies of this question have been based on the assumption that extinction of CD4 or CD8 expression by the precursor thymocytes was a steady, uninterrupted process. Here we show that this is an incorrect assumption, with CD4 and CD8 expression undergoing an unexpectedly complex series of expression changes involving down-modulation, kinetically asymmetric up-regulation, and then selective loss. Based on these data, we propose a model for the differentiation pathway of alphabeta TCR thymocytes that explains previous, apparently contradictory findings and establishes useful parameters for future studies at the cellular and gene level.

Enhancement of CD3-mediated thymocyte apoptosis by the cross-linkage of heat-stable antigen

Heat-stable antigen (HSA) is a murine differentiating antigen that is expressed on both CD4-CD8- double-negative and CD4+CD8+ double-positive thymocytes but not CD4+ or CD8+ single-positive thymocytes. Effects of anti-HSA monoclonal antibody, R13, on thymocyte apoptosis induced by various stimulations were investigated by a single-cell suspension culture system. Immobilized R13 enhanced the CD3-mediated DNA fragmentation and killing of thymocytes but not the dexamethasone-induced or phorbol myristate acetate-induced killing of thymocytes. Immobilized R13 by itself could not induce thymocyte apoptosis. Soluble R13 enhanced CD3-mediated apoptosis when HSA and T-cell receptor (TCR)/CD3 were co-cross-linked by a cross-reactive secondary antibody. Even without the cross-reactive secondary antibody, soluble R13 enhanced CD3-mediated apoptosis, although a greater than 100-fold increase in the amount of R13 was needed to give a similar enhancement compared with immobilized R13. Neither R13 by itself nor R13 plus secondary antibody induced cytosolic calcium influx, whereas R13 enhanced CD3-mediated cytosolic calcium increase. These results suggest a functional role of HSA in promoting the activation-induced apoptosis of thymocytes and the involvement of HSA in negative selection.

Characterization of the mouse CD8 beta chain-encoding gene promoter region

We identified a regulatory region of the mouse CD8 beta chain-encoding gene (CD8b) promoter. The CD8b 5' upstream sequence could not drive the expression of the bacterial chloramphenicol acetyltransferase (CAT) gene without T-cell receptor or SV40 enhancer elements. The results of transient transfection assays indicated that the dominant transcription-activating element within the CD8b-promoter is located at -45 to -40 base pairs (CCGCCC) from the transcriptional initiation site. Elimination of this element, by deletion or specific point mutation, significantly reduced transcriptional activity from this promoter. The sequence of this core region corresponds to a GC box motif known to act as a binding site for a ubiquitously expressed transcriptional activator, Sp1. However, the promoter activity appeared to be T-cell-specific, and the gel retardation assay using the core sequence as a probe revealed formation of complexes with multiple nuclear factors, one of them being specific to T lineage cells. These data suggest that the CD8b promoter requires a cis-acting element as well as several nuclear factors for full-range, tissue-specific transcription.

The regulation and function of the CD4 coreceptor during T lymphocyte development

The data reviewed in this chapter suggest that the primary developmental function of the CD4 and CD8 coreceptors is to improve the efficacy by which a thymocyte recognizes peptide/MHC. During positive selection, DP thymocytes down-regulate expression of either CD4 or CD8 in response to signals that originate from the TCR/coreceptor complex. Experiments with transgenic and MHC-null mice have shown that coreceptor down-regulation and lineage commitment can occur stochastically in a manner that is independent of TCR specificity for MHC. Nevertheless, the positive selection of a given thymocyte is contingent on sustained expression of the coreceptor that is appropriate for the MHC specificity of its TCR. In most cases, loss of the required coreceptor blocks developmental progression and results in thymocyte apoptosis. CD4 expression is controlled by both positive and negative regulatory sequences embedded in the CD4 gene and it is likely that similar sequences regulate the CD8 gene. The down-regulation of coreceptor expression is coupled to a functional commitment which ensures that mature CD4+ T cells have a helper phenotype and CD8+ T cells have a cytotoxic phenotype. The molecular basis for this coupling and the identity of the switching mechanism which governs coreceptor regulation remain to be determined.

Human CD8 transgene regulation of HLA recognition by murine T cells

A series of human CD8 transgenic (hCD8 Tg) mice with differential expression in the thymus and periphery were produced to investigate CD8 coreceptor regulation of repertoire selection and T cell responses. Expression of hCD8 markedly enhanced responses to both HLA class I molecules and hybrid A2/Kb molecules providing functional evidence for a second interaction site, outside of the alpha 3 domain, which is essential for optimal coreceptor function. Peripheral T cell expression of hCD8 was sufficient to augment responsiveness to HLA class I, as hCD8 Tg mice which lacked thymic expression responded as well as mice expressing hCD8 in the thymus and periphery. Both murine CD8+ and CD4+ T cells expressing hCD8 transgenes exhibited markedly enhanced responses to foreign HLA class I, revealing the ability of T cell receptor repertoires selected on either murine class I or class II to recognize human class I major histocompatibility complex (MHC). In contrast to recognition of foreign class I, thymic expression of hCD8 transgenes was absolutely required to enhance recognition of antigenic peptide restricted by self-HLA class I. Thus, our studies revealed disparate requirements for CD8 coreceptor expression in the thymus for selection of a T cell repertoire responsive to foreign MHC and to antigenic peptides bound to self-MHC, providing a novel demonstration of positive selection that is dependent on human CD8.

The function of the CD4 coreceptor in the development of T cells

T cells with helper activity can be found in mice that lack expression of the CD4 glycoprotein. The CD4 promoter is active in these cells; they respond to antigens presented by MHC class II molecules; they do not express CD8 and they do not depend on MHC class I for their development. By such criteria, these CD8- T cells resemble normal CD4+ helper T cells. The development of the helper lineage in CD4-null mice can be potentiated by expression of transgenes that encode either wild type CD4, or a deletion mutant of CD4 that lacks the cytoplasmic tail and therefore cannot interact with the tyrosine kinase p56lck. These observations suggest that CD4 is not absolutely required for the specification of the helper cell lineage. The role of the CD4 molecule in the development of T cells and possible mechanisms by which it achieves its functions are discussed.

Reduced thymic maturation but normal effector function of CD8+ T cells in CD8 beta gene-targeted mice

CD8 is a cell surface glycoprotein on major histocompatibility complex class I-restricted T cells. Thymocytes and most peripheral T cells express CD8 as heterodimers of CD8 alpha and CD8 beta. The intestinal intraepithelial lymphocytes (IEL), which have been suggested to be generated extrathymically, express CD8 predominantly as homodimers of CD8 alpha. We have generated CD8 beta gene-targeted mice. CD8 alpha+ T cell population in the thymus and in most peripheral lymphoid organs was reduced to 20-30% of that in wild-type littermates. CD8 alpha expression on thymocytes and peripheral T cells also decreased to 44 and 53% of the normal levels, respectively. In contrast, neither the population size nor the CD8 alpha expression level of CD8 alpha+ IEL was reduced. This finding indicates that CD8 beta is important only for thymic-derived CD8+ T cells. The lack of CD8 beta reduces but does not completely abolish thymic maturation of CD8+ T cells. Our result also reveals the role of CD8 beta in regulating CD8 alpha expression on thymic derived T cells. Peripheral T cells in these mice were efficient in cytotoxic activity against lymphocytic choriomeningitis virus and vesicular stomatitis virus, suggesting that CD8 beta is not essential for the effector function of CD8+ T cells.

Human CD4 restores normal T cell development and function in mice deficient in murine CD4

The ability of a human coreceptor to function in mice was investigated by generating human CD4 (hCD4)-expressing transgenic mice on a mouse CD4-deficient (mCD4-/-) background. From developing thymocyte to matured T lymphocyte functions, hCD4 was shown to be physiologically active. By examining the expansion and deletion of specific V beta T cell families in mutated mice with and without hCD4, it was found that hCD4 can participate in positive and negative selection. Mature hCD4 single positive cells also were found in the periphery and they were shown to restore MHC class II-restricted alloreactive and antigen-specific T cell responses that were deficient in the mCD4 (-/-) mice. In addition, these hCD4 reconstituted mice can generate a secondary immunoglobulin G humoral response matching that of mCD4 wild-type mice. The fact that human CD4 is functional in mice and can be studied in the absence of murine CD4 should facilitate studies of human CD4 activity in general and human immunodeficiency virus 1 gp120-mediated pathogenesis in acquired immune deficiency syndrome specifically.

The role of charge and multiple faces of the CD8 alpha/alpha homodimer in binding to major histocompatibility complex class I molecules: support for a bivalent model

The CD8 dimer interacts with the alpha 3 domain of major histocompatibility complex class I molecules through two immunoglobulin variable-like domains. In this study a crystal structure-informed mutational analysis has been performed to identify amino acids in the CD8 alpha/alpha homodimer that are likely to be involved in binding to class I. Several key residues are situated on the top face of the dimer within loops analogous to the complementarity-determining regions (CDRs) of immunoglobulin. In addition, other important amino acids are located in the A and B beta-strands on the sides of the dimer. The potential involvement of amino acids on both the top and the side faces of the molecule is consistent with a bivalent model for the interaction between a single CD8 alpha/alpha homodimer and two class I molecules and may have important implications for signal transduction in class I-expressing cells. This study also demonstrates a role for the positive surface potential of CD8 in class I binding and complements previous work demonstrating the importance of a negatively charged loop on the alpha 3 domain of class I for CD8 alpha/alpha-class I interaction. We propose a model whereby residues located on the CDR-like loops of the CD8 homodimer interact with the alpha 3 domain of MHC class I while amino acids on the side of the molecule containing the A and B beta-strands contact the alpha 2 domain of class I.

Separation of thymic education from antigen presenting functions of major histocompatibility complex class I molecules

Participation of transmembrane (TM) and glycosyl-phosphatidylinositol (GPI) anchored H-2Db molecules in antigen presentation and thymic selection events was investigated using transgenic mice. Both GPI-Db and TM-Db can efficiently present H-Y antigen, influenza and lymphocytic choriomeningitis virus (LCMV) peptides to primed cytotoxic, H-2Db-restricted T cells. Transgenic mice expressing GPI-Db, although unable to reject TM-Db skin grafts, nevertheless generate secondary CTL responses which can lyse TM-Db-bearing targets, indicating that GPI-Db mice fail to delete all TM-Db-reactive T cells. Furthermore, double-transgenic mice bearing GPI-Db and a T-cell receptor (TcR) for H-2Db+LCMV do not positively select receptor positive, CD8+CD4- T cells. This paradoxical behaviour of GPI-Db molecules suggests that the structural requirements for antigen presentation and thymic selection by class I molecules are different and may explain why GPI-linked class I molecules, such as Qa-2, do not appear to function as restriction elements in vivo.

Identification and characterization of an Alu-containing, T-cell-specific enhancer located in the last intron of the human CD8 alpha gene

Expression of the human CD8 alpha gene is restricted to cells of the lymphoid lineage and developmentally regulated during thymopoiesis. As an initial step towards understanding the molecular basis for tissue-specific expression of this gene, we surveyed the surrounding chromatin structure for potential cis-acting regulatory regions by DNase I hypersensitivity mapping and found four hypersensitive sites, three of which were T cell restricted. By using a reporter-based expression approach, a T-cell-specific enhancer was identified by its close association with a prominent T-cell-restricted hypersensitive sites in the last intron of the CD8 alpha gene. Deletion studies demonstrated that the minimal enhancer is adjacent to a negative regulatory element. DNA sequence analysis of the minimal enhancer revealed a striking cluster of consensus binding sites for Ets-1, TCF-1, CRE, GATA-3, LyF-1, and bHLH proteins which were verified by electrophoretic mobility shift assays. In addition, the 5' end of the enhancer was composed of an Alu repeat which contained the GATA-3, bHLH, and LyF-1 binding sites. Site-directed mutation of the Ets-1 and GATA-3 sites dramatically reduced enhancer activity. The functional importance of the other binding sites only became apparent when combinations of mutations were analyzed. Taken together, these results suggest that the human CD8 alpha gene is regulated by the interaction of multiple T-cell nuclear proteins with a transcriptional enhancer located in the last intron of the gene. Comparison of the CD8 alpha enhancer with other recently identified T-cell-specific regulatory elements suggests that a common set of transcription factors regulates several T-cell genes.

Identification and characterization of an Alu-containing, T-cell-specific enhancer located in the last intron of the human CD8 alpha gene

Expression of the human CD8 alpha gene is restricted to cells of the lymphoid lineage and developmentally regulated during thymopoiesis. As an initial step towards understanding the molecular basis for tissue-specific expression of this gene, we surveyed the surrounding chromatin structure for potential cis-acting regulatory regions by DNase I hypersensitivity mapping and found four hypersensitive sites, three of which were T cell restricted. By using a reporter-based expression approach, a T-cell-specific enhancer was identified by its close association with a prominent T-cell-restricted hypersensitive sites in the last intron of the CD8 alpha gene. Deletion studies demonstrated that the minimal enhancer is adjacent to a negative regulatory element. DNA sequence analysis of the minimal enhancer revealed a striking cluster of consensus binding sites for Ets-1, TCF-1, CRE, GATA-3, LyF-1, and bHLH proteins which were verified by electrophoretic mobility shift assays. In addition, the 5' end of the enhancer was composed of an Alu repeat which contained the GATA-3, bHLH, and LyF-1 binding sites. Site-directed mutation of the Ets-1 and GATA-3 sites dramatically reduced enhancer activity. The functional importance of the other binding sites only became apparent when combinations of mutations were analyzed. Taken together, these results suggest that the human CD8 alpha gene is regulated by the interaction of multiple T-cell nuclear proteins with a transcriptional enhancer located in the last intron of the gene. Comparison of the CD8 alpha enhancer with other recently identified T-cell-specific regulatory elements suggests that a common set of transcription factors regulates several T-cell genes.

The lack of CD8 alpha cytoplasmic domain resulted in a dramatic decrease in efficiency in thymic maturation but only a moderate reduction in cytotoxic function of CD8+ T lymphocytes

The glycoprotein CD8 is believed to play an important role in the maturation and function of MHC class I-restricted T lymphocytes. CD8 has been proposed to function as a co-receptor of the TcR to participate in signal transduction, possibly through its cytoplasmic domain that binds to protein tyrosine kinase p56lck. A T cell-specific transgene encoding CD8 alpha truncated at the cytoplasmic domain ("tailless CD8 alpha"), was introduced into CD8 alpha-deficient mice. This animal model was used to study the role of the CD8 cytoplasmic domain in T cell ontogeny and function. "Tailless CD8 alpha" was expressed on the cell surface of thymocytes and peripheral T cells. A small population of peripheral CD4- T cells (6% of T lymphocytes) was found to have cell surface expression of "tailless CD8 alpha" and endogenous CD8 beta, indicating that these cells may belong to the CD8+ T cell lineage. A consistent result was obtained from CD8 alpha-deficient mice bearing the "tailless CD8 alpha" and the MHC class I-restricted 2C TcR transgenes. A small population of CD4- T cells expressing CD8 beta, the "tailless CD8 alpha" and the 2C TcR transgenes was present in the periphery of these mice in a selecting background, but was absent in a deleting background. When "tailless CD8 alpha" mice were infected with lymphocytic choriomeningitis virus (LCMV), the peripheral CD8+ CD4- T cell subset expanded dramatically and a significant LCMV-specific cytolytic activity was detected. The results suggest that the cytoplasmic portion of CD8 alpha is not absolutely required but dramatically enhances the efficiency of thymic maturation of CD8+ T cells. The lack of CD8 alpha cytoplasmic domain in peripheral CD8+ T cells does not abolish the generation of cytotoxicity in response to an in vivo LCMV infection, although the cytolytic activity is slightly reduced compared to that in control mice.

Functional GATA-3 binding sites within murine CD8 alpha upstream regulatory sequences

Genes encoding the accessory molecules CD8 and CD4 are activated early in thymocyte development, generating CD4+8+ double positive intermediates, which give rise to two functionally distinct mature T cell subsets that express either CD4 or CD8. The mechanisms that govern the activation or suppression of the CD8 gene are likely to be central to the T cell development program. To identify the key regulatory factors, we have initiated an analysis of the transcriptional regulation of the murine CD8 alpha gene. We have identified three CD8+ cell-specific DNAase I hypersensitive sites (HSS) located upstream of the murine CD8 alpha gene. In vitro mobility shift analysis of the -4.0-kb HSS region has revealed multiple binding sites for the T cell-restricted transcription factor GATA-3. In vitro translated murine GATA-3 binds specifically to both CD8 GATA sites, and coexpression of this factor in transient transfection assays transactivates a reporter construct containing these sequences. These results provide the first evidence for the role of a T cell-restricted factor in the regulation of either CD8 or CD4 genes.

Thymic selection and thymic major histocompatibility complex class II expression are abnormal in mice undergoing graft-versus-host reactions

The graft-vs.-host reaction (GVHR) results in damage to the epithelial and lymphoid compartments of the thymus and thus in abnormal maturation and function of thymocytes in mice undergoing GVHR. In this report, the effects of GVHR on thymic T cell receptor (TCR) expression and usage have been investigated. GVHR was induced in unirradiated F1 hybrid mice by the intravenous transfer of parental lymphoid cells. Expression of the CD3/TCR complex on thymocyte subsets defined by CD4 and CD8 was studied by three-color flow cytometry. The level of CD3/TCR was decreased on CD4+CD8-, but not CD4-CD8+, mature thymocytes. The lack of upregulation of CD3/TCR on CD4 single-positive thymocytes, but not on their CD8+ counterparts, suggested an abnormality of class II major histocompatibility complex (MHC) expression in the thymuses of mice undergoing GVHR. Immunofluorescence staining of thymic frozen sections revealed that MHC class II expression was dramatically decreased in GVH-reactive mice. GVHR-induced changes in positive and negative selection were evaluated by determining the incidence of specific V beta TCR segment usage in the thymus. In normal mice, thymocyte usage of any given V beta segment was highly consistent between individuals of the same strain and age; however, a marked divergence in the incidence of TCR V beta 6hi and V beta 8hi cells between GVH-reactive littermate mice was observed, suggesting that thymic positive selection had become disregulated in these animals. Furthermore, negative selection was defective; the incidence of phenotypically self-reactive V beta 6hi T cells was significantly greater in the thymuses of GVH-reactive mice bearing the endogenous superantigen Mls-1a than in untreated controls. Thus, mice undergoing GVHR showed defective TCR upregulation on CD4+CD8- thymocytes and changes in TCR usage reflecting aberrant thymic selection, in conjunction with decreased expression of MHC class II. Most abnormalities of TCR expression and usage on CD4+ thymocytes observed in GVH-reactive mice were analogous to those of class II knockout mice.

In vivo depletion of chicken T-cell subsets

In the chicken three types of T-cell receptors can be defined by monoclonal antibodies TCR1, TCR2 and TCR3, which recognize gamma delta T cells, and V beta 1- and V beta 2-expressing alpha beta T cells, respectively. In the present report we have analysed means of selectively depleting the gamma delta T cells and the V beta 1+ alpha beta T cells. gamma delta T cells, which represent up to 66% of all T cells in blood of a 6-month-old chicken, can be effectively depleted by neonatal thymectomy (Tx) to levels as low as 1%. Immunohistology demonstrates a similar depletion in lymphoid organs while intestinal epithelium-associated gamma delta T cells are affected by Tx to a lesser extent. V beta 1-bearing alpha beta T cells, which comprise about 80% of the alpha beta T cells, were depleted by embryonic and neonatal injection of the TCR2 antibody. In the thymus such treatment depleted only the V beta 1+ alpha beta T cells with high density expression of T-cell receptor. Therefore, we thymectomized TCR2-treated animals in order to prevent development of mature V beta 1+ alpha beta T cells from the low density immature thymocytes. Treatment of chickens with a total of 22 mg of TCR2 antibody plus Tx reduced V beta 1+ alpha beta T cells from an average of 65% to 10% of all T cells. In these TCR2 antibody-treated animals the V beta 2-expressing alpha beta T cells become the predominant type of T cell (average 85%).

Alloreactive cytotoxic T cells can develop and function in mice lacking both CD4 and CD8

Using the technique of homologous recombination in embryonic stem cells, a mouse strain without functional CD4 and CD8 genes has been generated. Surprisingly, these mice contain significant numbers of alpha beta T cells. Although mice deficient for CD8 only do not show any cytotoxic response when their T cells are stimulated with either alloantigen or viral antigen, the CD4-8- mice do generate alloreactive cytotoxic T cells. These cytotoxic T cells bear the alpha beta T cell receptor and recognize major histocompatibility complex class I antigens. In addition fully allogeneic skin transplants were rejected but skin transplants expressing only minor transplantation antigens were not. Virus-specific cytotoxic T lymphocytes were also not detected. It seems that alloreactive cytotoxic T cells can be induced and exert their effector function in vitro and in vivo in the absence of CD8, and that they can develop and mature in vivo without the CD8 molecule or the signals it might provide.

Reconstitution of the subclass-specific expression of CD4 in thymocytes and peripheral T cells of transgenic mice: identification of a human CD4 enhancer

During thymic maturation, CD4-CD8-TCR- immature thymocytes differentiate through a CD4+CD8+TCRlo intermediate into two functionally distinct mature T cell subsets: helper T cells expressing CD4 and a major histocompatibility complex (MHC) class II-restricted T cell receptor (TCR), and cytotoxic T cells expressing CD8 and and MHC class I-restricted TCR. The mutually exclusive expression of CD4 and CD8 is maintained in the periphery during expansion of these mature T cell subsets. To elucidate the mechanisms controlling CD4 and CD8 expression on differentiating thymocytes and mature peripheral T cells, we have examined the expression of human CD4 gene constructs in the lymphoid tissues of transgenic mice. Our analyses demonstrate that sequences contained within or closely linked to the human CD4 gene are sufficient to reconstitute the appropriate regulation of human CD4 expression on all thymocyte and mature peripheral T cell subsets. Specifically, appropriate developmental regulation was dependent on two sets of sequences, one contained within a 1.3-kb restriction fragment located 6.5 kb upstream of the human CD4 gene, and the other present within or immediately flanking the gene. Nucleotide sequence analysis identified the 1.3-kb restriction fragment as the likely human homologue of an enhancer found 13 kb upstream of the mouse CD4 transcription initiation site. The human CD4 transgenic mice provide a useful system for the identification and characterization of additional sequence elements that participate in human CD4 gene regulation and for the elucidation of regulatory mechanisms governing the developmental program mediating the maturation of the CD4+ and CD8+ peripheral T cell subsets.

Phenotypic and functional analysis of positive selection in the gamma/delta T cell lineage

Recent evidence suggests that T cells expressing gamma/delta antigen receptors (T cell receptor [TCR]) are subject to positive selection during development. We have shown that T cells expressing a class I major histocompatibility complex (MHC)-specific gamma/delta TCR transgene (tg) are not positively selected in class I MHC-deficient, beta 2-microglobulin (beta 2m) gene knockout mice (tg+ beta 2m-). In this report, we examine phenotypic and functional parameters of gamma/delta positive selection in this transgenic model system. TCR-gamma/delta tg+ thymocytes of mature surface phenotype (heat stable antigen-, CD5hi) were found in beta 2m+ but not in beta 2m- mice. Moreover, subsets of tg+ thymocytes with the phenotype of activated T cells (interleukin [IL]2R+, CD44hi, or Mel-14lo) were also present only in the beta 2m+ mice. Cyclosporine A, which blocks positive selection of TCR-alpha/beta T cells, also inhibited gamma/delta tg+ T cell development. These results support the idea that positive selection of TCR-gamma/delta requires active TCR-mediated signal transduction. Whereas tg+ beta 2m+ thymocytes produced IL-2 and proliferated when stimulated by alloantigen, TCR engagement of tg+ beta 2m- thymocytes by antigen induced IL-2R expression but was uncoupled from the signal transduction pathway leading to IL-2 production and autocrine proliferation. Overall, these results demonstrate significant parallels between gamma/delta and alpha/beta lineage development, and suggest a general role for TCR signaling in thymic maturation.

CD4, CD8 and tyrosine kinases in thymic selection

Analysis of T-cell development in transgenic and gene-deficient mice suggests that the co-receptor function of CD8 is essential for positive selection. Recent data also demonstrate that the requirement for CD4 and CD8 in negative selection of T cells is not absolute and may be regulated by T-cell receptor affinity for the deleting ligand, an interpretation consistent with the affinity model of thymic selection. In addition to its association with CD4 and CD8, it appears that p56lck is also important during the early stages of thymic development.

CD8 is needed for positive selection but differentially required for negative selection of T cells during thymic ontogeny

During thymic development, immature thymocytes expressing major histocompatibility complex (MHC) class I-restricted T cell receptors (TcR) differentiate into CD8+ T cells with cytolytic functions. To evaluate the role of CD8 in positive and negative selection during thymic ontogeny, mice rendered CD8-null by gene targeting were bred with three lines of transgenic mice expressing unique MHC class I-restricted TcR. In all three instances CD8 was required for positive selection of MHC class I-restricted transgenic T cells. The efficiency of positive selection decreased in accordance with a reduced level of CD8 expression on thymocytes. Surprisingly, there was a differential requirement for CD8 expression in negative selection of MHC class I-restricted thymocytes, depending on the antigen specificity of TcR. These observations show that CD8 is essential for positive selection but is differentially required for negative selection of MHC class I-restricted T cells. Thus thymic selection, at least for negative selection, can occur in the absence of the CD8 accessory molecule.

Requirement for CD8-major histocompatibility complex class I interaction in positive and negative selection of developing T cells

The interaction of the T cell surface glycoprotein CD8 with major histocompatibility complex (MHC) class I molecules on target cells is required for effective T cell activation. Mutations in the alpha 3 domain of the MHC class I molecule can disrupt binding to CD8, yet leave antigen presentation unaffected. Here we show that such a mutation can interfere with positive and negative selection of T cells bearing T cell receptors (TCRs) that interact specifically with the mutant class I molecule. Autoreactive T cells in male mice expressing a transgenic TCR specific for the male antigen H-Y and H-2Db were not deleted in the context of a transgenic Db molecule bearing a mutation at residue 227. Similarly, CD8+ cells were not positively selected in female mice expressing both the TCR and mutant class I transgenes. Endogenous MHC class I molecules were competent to bind CD8, but were unable to rescue the defect, indicating a requirement for coordinate recognition of antigen/MHC by a complex of the TCR and CD8 coreceptor for both positive and negative selection of thymocytes.

The level of CD8 expression can determine the outcome of thymic selection

During thymic development, thymocytes that can recognize major histocompatability complex (MHC) molecules on thymic epithelial cells are selected to survive and mature (positive selection), whereas thymocytes that recognize MHC on hematopoietic cells are destroyed (negative selection). It is not known how MHC recognition can mediate both death and survival. One model to explain this paradox proposes that thymocytes whose T cell antigen receptors (TCRs) recognize MHC with high affinity are eliminated by negative selection, whereas low affinity TCR-MHC interactions are sufficient to mediate positive selection. Here we report that, while the expression of a 2C TCR transgene leads to positive selection of thymocytes in H-2b mice, expression of both a CD8 transgene and a 2C TCR transgene causes negative selection. This observation indicates that quantitative differences in TCR-MHC recognition are a critical determinant of T cell fate, a finding predicted by the affinity model for thymic selection.

Crystal structure of a soluble form of the human T cell coreceptor CD8 at 2.6 A resolution

A secreted fragment of the extracellular portion of human CD8 alpha has been expressed in CHO cells, and a deglycosylated and proteolyzed form of this fragment has been crystallized. We report here the crystal structure of this fragment as refined at 2.6 A resolution. The structure was solved by molecular replacement using a superposition of ten variable domains from immunoglobulin light chains as the search model. Only the N-terminal 114 amino acids of CD8 alpha are visible in the electron density maps. The domain formed by these residues possesses a fold typical of immunoglobulin variable domains and associates to form Fv-like homodimers.

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

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

Disruption of CD8-dependent negative and positive selection of thymocytes is correlated with a decreased association between CD8 and the protein tyrosine kinase, p56lck

The CD4 and CD8 coreceptor molecules on immature thymocytes participate in T cell repertoire selection. To examine more definitively the role of CD4 and CD8 in the negative and positive selection of immature thymocytes, we generated transgenic mice with elevated surface CD4 expression and mated them with mice expressing a transgenic T cell receptor. Augmented CD4 expression was found to markedly alter CD8-dependent negative and positive selection of T cells specific for the male (H-Y) antigen presented by H-2Db major histocompatibility complex class I molecules. Moreover, the cytoplasmic tail of CD4 was essential for effecting these alterations, since the overexpression of tailless CD4 molecules failed to influence the outcome of CD8-dependent selection. The inhibition of positive and negative selection in double-transgenic mice expressing the full-length CD4 molecule was associated with a decreased interaction between the protein tyrosine kinase p56lck and CD8. These results strongly implicate p56lck in T cell repertoire selection.

The development of functionally responsive T cells

The work reviewed in this article separates T cell development into four phases. First is an expansion phase prior to TCR rearrangement, which appears to be correlated with programming of at least some response genes for inducibility. This phase can occur to some extent outside of the thymus. However, the profound T cell deficit of nude mice indicates that the thymus is by far the most potent site for inducing the expansion per se, even if other sites can induce some response acquisition. Second is a controlled phase of TCR gene rearrangement. The details of the regulatory mechanism that selects particular loci for rearrangement are still not known. It seems that the rearrangement of the TCR gamma loci in the gamma delta lineage may not always take place at a developmental stage strictly equivalent to the rearrangement of TCR beta in the alpha beta lineage, and it is not clear just how early the two lineages diverge. In the TCR alpha beta lineage, however, the final gene rearrangement events are accompanied by rapid proliferation and an interruption in cellular response gene inducibility. The loss of conventional responsiveness is probably caused by alterations at the level of signaling, and may be a manifestation of the physiological state that is a precondition for selection. Third is the complex process of selection. Whereas peripheral T cells can undergo forms of positive selection (by antigen-driven clonal expansion) and negative selection (by abortive stimulation leading to anergy or death), neither is exactly the same phenomenon that occurs in the thymic cortex. Negative selection in the cortex appears to be a suicidal inversion of antigen responsiveness: instead of turning on IL-2 expression, the activated cell destroys its own chromatin. The genes that need to be induced for this response are not yet identified, but it is unquestionably a form of activation. It is interesting that in humans and rats, cortical thymocytes undergoing negative selection can still induce IL-2R alpha expression and even be rescued in vitro, if exogenous IL-2 is provided. Perhaps murine thymocytes are denied this form of rescue because they shut off IL-2R beta chain expression at an earlier stage or because they may be uncommonly Bcl-2 deficient (cf. Sentman et al., 1991; Strasser et al., 1991). Even so, medullary thymocytes remain at least partially susceptible to negative selection even as they continue to mature.(ABSTRACT TRUNCATED AT 400 WORDS)

Ontogeny of T cell maturation in LEC mutant rats which bear a congenital arrest of maturation from CD4+CD8+ to CD4+CD8- thymocytes

LEC rats bear a congenital deficiency in CD4+CD8- thymocytes and peripheral CD4+ T cells, and consequently a deficiency in Th cell functions. Ontogeny of T cell maturation in normal and LEC mutant rats was, therefore, investigated. Prenatal development of thymocytes in normal rat strains, with respect to the expression of CD4/CD8 and TcR antigens, was similar to that of mice except that its kinetics was delayed by approximately 24 h. The kinetics of T cell maturation in LEC rats was comparable with that of normal rats up to day 19 of gestation, at which stage double-negative thymocytes (CD4-CD8-) developed into double positives (CD4+CD8+) through immature CD4-CD8+ subset. At day 19 of gestation in LEC as well as normal rats, double positives occupied approximately 80% of the total thymocytes, half of which were TcR-dull positive, indicating that TcR was normally rearranged and then expressed in LEC rat thymocytes. These data indicate that double negatives normally mature into at least double positives in LEC rats. Both single positives appeared after day 19 of gestation in normal rats, while in LEC rats CD4+CD8- cells did not appear, suggesting that the deficiency in CD4+CD8- cells is due to a congenital arrest of maturation from CD4+CD8+ to CD4+CD8- cells, but not due to a postnatal deletion.

Bone marrow-derived progenitor T cells convey the origin of maturational arrest from CD4+CD8+ to CD4-CD8+ thymocytes in LEC mutant rats

A mutant strain of rats, LEC, shows a novel arrest of T cell maturation from CD4+CD8+ to CD4+CD8- but not to CD4-CD8+ cells in the thymus. Transplantation of LEC rat fetal thymuses into the subcapsule of the kidney of athymic nude rats resulted in a normal maturation of thymocytes in the thymus graft. Furthermore, both single-positive thymocytes and peripheral lymph node T cells expressed T cell receptor alpha/beta antigen, and lymph node T cells acquired the ability to produce interleukin 2 upon mitogen stimulation. Transplantation of fetal thymuses from LEA rats, which express the same major histocompatibility complex haplotype as LEC rats, into LEC rat kidney subcapsule resulted in the maturational arrest from CD4+CD8+ to CD4+CD8- cells in the thymus graft. These data strongly suggest that bone marrow-derived progenitor T cells carry the cause of maturational arrest and that the thymic stroma of LEC rats has a normal potential to nurse thymocytes.

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.

Negative and positive selection of antigen-specific cytotoxic T lymphocytes affected by the alpha 3 domain of MHC I molecules

The alpha 1 and alpha 2 domains of major histocompatibility complex (MHC) class I molecules function in the binding and presentation of foreign peptides to the T-cell antigen receptor and control both negative and positive selection of the T-cell repertoire. Although the alpha 3 domain of class I is not involved in peptide binding, it does interact with the T-cell accessory molecule, CD8. CD8 is important in the selection of T cells as anti-CD8 antibody injected into perinatal mice interferes with this process. We previously used a hybrid class I molecule with the alpha 1/alpha 2 domains from Ld and the alpha 3 domain from Q7b and showed that this molecule binds an Ld-restricted peptide but does not interact with CD8-dependent cytotoxic T lymphocytes. Expression of this molecule in transgenic mice fails to negatively select a subpopulation of anti-Ld cytotoxic T lymphocytes. In addition, positive selection of virus-specific Ld-restricted cytotoxic T lymphocytes does not occur. We conclude that besides the alpha 1/alpha 2 domains of class I, the alpha 3 domain plays an important part in both positive and negative selection of antigen-specific cells.

Mutations in CD8 that affect interactions with HLA class I and monoclonal anti-CD8 antibodies

The T cell co-receptor, CD8, binds to the alpha 3 domain of HLA class I (Salter, R.D., R.J. Benjamin, P.K. Wesley, S.E. Buxton, T.P.J. Garrett, C. Clayberger, A.M. Krensky, A.M. Norman, D.R. Littman, and P. Parham. 1990. Nature [Lond.]. 345:41; Connolly, J.M., T.A. Potter, E.M. Wormstall, and T.H. Hansen. 1988. J. Exp. Med. 168:325; and Potter, T.A., T.V. Rajan, R.F. Dick II, and J.A. Bluestone. 1989. Nature [Lond.]. 337:73). To identify regions of CD8 that are important for binding to HLA class I, we performed a mutational analysis of the CD8 molecule in the immunoglobulin (Ig)-like variable domain. Our mutational analysis was based on our finding that using a cell-cell adhesion assay murine CD8 (Lyt-2) did not bind to human class I. Since the interaction of human CD8 with HLA class I is species specific, we substituted nonconservative amino acids from mouse CD8 and analyzed the ability of the mutated CD8 molecules expressed in COS 7 cells to bind HLA class I-bearing B lymphoblastoid cells, UC. Mutants with the greatest effect on binding were located in a portion of the molecule homologous to the first and second hypervariable regions of an antibody combining site. In addition, a panel of 12 anti-CD8 monoclonal antibodies were used to stain the 10 CD8 mutants, and amino acids that affected antibody binding were localized on the crystal structure of the Bence-Jones homodimer, REI. Support for an Ig-like structure of CD8 can be found in the pattern of substitutions affecting antibody binding. This work supports the similar tertiary structure of the CD8 alpha-terminal domain and an Ig variable domain.

CD8 is needed for development of cytotoxic T cells but not helper T cells

A mutant mouse strain without CD8 (Lyt-2 and Lyt-3) expression on the cell surface has been generated by disrupting the Lyt-2 gene using embryonic stem cell technology. In these mice, CD8+ T lymphocytes are not present in peripheral lymphoid organs, but the CD4+ T lymphocyte population seems to be unaltered. Cytotoxic response of T lymphocytes from these mice against alloantigens and viral antigens is dramatically decreased. Proliferative response against alloantigens and in vivo help to B lymphocytes, however, are not affected. These data suggest that CD8 is necessary for the maturation and positive selection of class I MHC restricted cytotoxic T lymphocytes but is not required on any of the intermediate thymocyte populations (CD8+CD4-TcR- or CD4+CD8+TcRlow) during the development of functional class II MHC restricted helper T cells.

T-cell subset relationships in thymocyte development

During the past couple of years there has been significant progress in our understanding of the development of different lineages of T cells within the thymus. Pathways, subpopulations and cellular dynamics are all becoming clearer. Signal transduction through primary and accessory receptors is also beginning to be understood. However, the exact nature of the events that lead uncommitted cells to choose a particular lineage (either alpha beta/gamma delta or CD4+/CD8+) has still not been determined.

Positive and negative selection of the alpha beta T-cell repertoire in vivo

The selection of developing and mature alpha beta T cells, by intrathymic and extra-thymic ligands expressed on cells of hemopoietic and other origins, has been studied in a variety of systems. These experiments have increased our knowledge of the selection of alpha beta T cells in the thymus, in the presence or absence of specific peptides bound to MHC, and also made clear that additional selective mechanisms exist outside the thymus that can be exploited to silence mature T cells.

Biased expression of individual T cell receptor V gene segments in CD4+ and CD8+ human peripheral blood T lymphocytes

The expression of seven different alpha and beta gene segments of the T cell receptor on normal human CD4+ and CD8+ T lymphocytes, respectively, was examined using V gene-specific monoclonal antibodies. We found a statistically significant (p less than 0.001) bias of the expression of four V gene products towards the CD4+ subpopulation. In every individual analyzed, the V beta 5.1 gene segment was expressed to a higher degree among CD4+ compared to CD8+ cells, with a median value of 4.8% among CD4+ cells and 1.5% among CD8+ cells. There was also a statistically significant skewness in the usage of the V beta 6.7, V beta 8 and V beta 12 gene segments towards the CD4+ T cell population, but not as dominating as for the V beta 5.1. Lymphocytes from umbilical cord blood showed similar skewed reactivities for the V beta 5.1, V beta 6.7 and V beta 12-specific monoclonal antibodies. Aspects of positive and negative selection, as possible explanations for these findings, are discussed.

Requirement for association of p56lck with CD4 in antigen-specific signal transduction in T cells

The T cell-specific transmembrane glycoprotein CD4 interacts with class II MHC molecules via its external domain and is associated with tyrosine kinase p56lck via a cysteine motif in its cytoplasmic domain. We have assessed the ability of CD4 to synergize with the antigen-specific T cell receptor (TCR) for induction of transmembrane signals that result in lymphokine production. Mutant CD4 molecules were introduced into T cells that lacked endogenous CD4 but expressed TCRs specific for lysozyme peptides or the superantigen SEA bound to Ab or Abm12 class II MHC molecules. With either ligand, T cell activation occurred only when CD4 was associated with p56lck. These results demonstrate that residues within the cytoplasmic domain of CD4 are required for its coreceptor function in TCR-mediated signal transduction and strongly support the notion that the association of CD4 with p56lck is critical in this process.

Unresponsiveness to Mlsa induced in newborn Mlsb mice by maternal preimmunization

Female BALB/c (H-2d, Mlsb) mice alloimmunized prior to and during syngeneic pregnancy with DBA/2 (H-2d, Mlsa) splenocytes gave rise to offspring with severely reduced responsiveness in adult life to DBA/2 stimulation in vitro mixed lymphocyte cultures. The offspring of the hyperimmunized mothers were also tolerant to neonatal challenge with large numbers of DBA/2 splenocytes, which resulted in runting disease of control neonatal BALB/c mice. Both challenged and unchallenged offspring of the immunized BALB/c mothers were hyporesponsive to DBA/2 but both stimulated and responded to normal BALB/c lymphocytes, indicating alteration in their T-cell repertoire. There was no reduction in the V beta 6-positive thymocyte subpopulation in the challenged or unchallenged offspring of the alloimmunized BALB/c mothers compared to normal controls, suggesting that hyporesponsiveness to DBA/2 is not due to thymic deletion of Mlsa-responsive clones. Examination of the T-cell subset composition of the hydrocortisone-resistant thymocytes and peripheral lymphocytes of the challenged and unchallenged groups of experimental mice revealed large increases in the percentage of Lyt-2+ T cells, sometimes accompanied by a decrease in the L3T4+ T-cell subset compared to age-matched control BALB/c. Lymphocytes from the hyporesponsive mice specifically suppressed the proliferative responses of control BALB/c to DBA/2 but not to AKR. The data indicate that maternal hyperimmunization can induce tolerance by a mechanism involving intrathymic selection of suppressor cells which can be combined with a negative selection of helper cells.

Towards an integrated view of thymopoiesis

One prediction from the complex series of steps in intrathymic T-cell differentiation is that to regulate it the stroma controlling the process must be equally complex: the attraction of precursors, commitment to the T-cell lineage, induction of T-cell receptor (TCR) gene rearrangement, accessory molecule expression, repertoire expansion, major histocompatibility complex (MHC) molecule-based selection (positive and negative), acquisition of functional maturity and migratory capacity must all be controlled. In this review, Richard Boyd and Patrice Hugo combine knowledge of T-cell differentiation with thymic stromal cell heterogeneity to offer an integrated view of thymopoiesis within the thymic microenvironment.

Participation of CD4 coreceptor molecules in T-cell repertoire selection

During thymocyte development, progenitor cells bearing both CD4 and CD8 coreceptor molecules mature into functional T lymphocytes that express these proteins in a mutually exclusive way. Although T-cell specificity is determined primarily by the structure of the T-cell antigen receptor (TCR) heterodimer, a developmentally regulated process acts to ensure that cells bearing class II-restricted TCRs are CD4+ and those bearing class I-restricted TCRs express only CD8. To investigate this maturation process, we have engineered transgenic mice in which CD4 is expressed in all thymocyte subsets and in all peripheral T cells. Peripheral CD4+8+ T lymphocytes from these mice react with both class I and class II alloantigens. Moreover, expression of the CD4 transgene disrupts the positive selection of doubly transgenic thymocytes bearing a class I-restricted TCR specific for the male (H-Y) antigen. Hence the CD4 coreceptor participates directly in T-cell repertoire selection.

Thymic selection in CD8 transgenic mice supports an instructive model for commitment to a CD4 or CD8 lineage

Immature thymocytes, which coexpress CD4 and CD8, give rise to mature CD4+CD8- and CD4-CD8+ T cells. Only those T cells that recognize self-MHC are selected to mature, a process known as positive selection. The specificity of the T cell antigen receptor (TCR) for class I or class II MHC influences the commitment to a CD4 or CD8 lineage. This may occur by a directed mechanism or by stochastic commitment followed by a selection step that allows only CD8+, class I-specific and CD4+, class II-specific cells to survive. We have generated a mouse line expressing a CD8 transgene under the control of the T cell-specific CD2 regulatory sequences. Although constitutive CD8 expression does not affect thymic selection of CD4+ cells, selection of a class I-specific TCR in the CD8 subset is substantially improved. This outcome is consistent with a model for positive selection in which selection occurs at a developmental stage in which both CD4 and CD8 are expressed, and positive selection by class I MHC generates an instructive signal that directs differentiation to a CD8 lineage.

Maturational arrest from CD4+8+ to CD4+8- thymocytes in a mutant strain (LEC) of rat

A mutant strain (LEC) of rats was found to have a novel defect in T cell maturation, that is, arrest of differentiation from CD4+8+ to CD4+8- but not to CD4-8+ thymocytes. FACS analyses demonstrated a deficiency in the CD4+8- T cell subset in the thymus and a marked decrease in CD4+ T cells in peripheral lymphoid organs. Expression of the T cell receptor (TCR)/CD3 complex in CD4+8+ and CD4-8+ thymocytes of LEC rats was normal. Expression of class II major histocompatibility complex (MHC) in the thymus of LEC rats was also the same as that of normal rats. These results indicate that maturational arrest occurs only in the transition pathway from CD4+8+ to CD4+8- thymocytes, and that this mutation can not be attributed to the default of expression of either TCR/CD3, CD4, or class II MHC antigen. Consequently, dysfunction of helper T cells was observed in LEC rats, while killer T cells and B cells functioned normally. Although the complete identification of the origin of this mutation requires further studies, it is hoped that such investigations will throw light on the mechanism of positive selection.