The T-cell antigen receptor gamma gene: rearrangement and cell lineages

Resolving the mystery-How TCR transgenic mouse models shed light on the elusive case of gamma delta T cells

Cutting-edge questions in αβ T cell biology were addressed by investigating a range of different genetically modified mouse models. In comparison, the γδ T cell field lacks behind on the availability of such models. Nevertheless, transgenic mouse models proved useful for the investigation of γδ T cell biology and their stepwise development in the thymus. In general, animal models and especially mouse models give access to a wide range of opportunities of modulating γδ T cells, which is unachievable in human beings. Because of their complex biology and specific tissue tropism, it is especially challenging to investigate γδ T cells in in vitro experiments since they might not reliably reflect their behavior and phenotype under physiologic conditions. This review aims to provide a comprehensive historical overview about how different transgenic mouse models contributed in regards of the understanding of γδ T cell biology, whereby a special focus is set on studies including the elusive role of the γδTCR. Furthermore, evolutionary and translational remarks are discussed under the aspect of future implications for the field. The ultimate full understanding of γδ T cells will pave the way for their usage as a powerful new tool in immunotherapy.

Regulation of gammadelta versus alphabeta T lymphocyte differentiation by the transcription factor SOX13

alphabeta and gammadelta T cells originate from a common, multipotential precursor population in the thymus, but the molecular mechanisms regulating this lineage-fate decision are unknown. We have identified Sox13 as a gammadelta-specific gene in the immune system. Using Sox13 transgenic mice, we showed that this transcription factor promotes gammadelta T cell development while opposing alphabeta T cell differentiation. Conversely, mice deficient in Sox13 expression exhibited impaired development of gammadelta T cells but not alphabeta T cells. One mechanism of SOX13 function is the inhibition of signaling by the developmentally important Wnt/T cell factor (TCF) pathway. Our data thus reveal a dominant pathway regulating the developmental fate of these two lineages of T lymphocytes.

Emerging role of gammadelta T-cells in health and disease

The majority of T lymphocytes in the peripheral blood bear the alphabeta type of T-cell receptor (TCR) and less than 5% of circulating T lymphocytes bear the gammadelta type of TCR. The alphabeta and gammadelta T-cells contribute differently to the host immune defense. The immunologic function of gammadelta T-cell in humans is gradually unraveling. Pathologic studies have shown a relatively high proportion of gammadelta T-cells in the gastrointestinal mucosa, skin, and splenic red pulp where they appear to have an important immunologic function against pathogens attempting to enter body through the mucosal surfaces and possibly the skin. More data are emerging about the role of gammadelta T-cells in the course of infectious diseases and in the pathogenesis of autoimmunity. The possible role of these cells in immunologic surveillance against cancer has been inferred from their preferential expansion in certain malignancies. In parallel to the low frequency of these cells in normal lymphocytes, lymphomas bearing gammadelta type of T-cell receptors on cell surface are rare. These include hepatosplenic and non-hepatosplenic (including those involving the skin and those involving nasal cavity and upper aero digestive tract) and the rare non-B-cell cases of lymphomas seen in the immunosuppressed organ transplant patients. The limited available clinical data about gammadelta T-cell lymphomas suggest that these lymphomas tend to have an aggressive course particularly the hepatosplenic form with poor response to multi-agent chemotherapy and short median survival.

Molecular determinants of TCR expression and selection

The process of T cell development in the thymus is tightly regulated, being dependent on the integration of signals required for thymocyte maturation and survival. Rearrangements, expression and signaling of TCR genes play an indispensable role in this developmental program. Recent advances have provided insights into the molecular mechanisms that regulate TCR repertoire formation at the level of alphabeta versus gammadelta T cell fate and CD4(+) versus CD8(+) lineage determination.

Evidence that gammadelta versus alphabeta T cell fate determination is initiated independently of T cell receptor signaling

Two types of T cells, alphabeta and gammadelta, develop in vertebrates. How these two T cell lineages arise from a common thymic T progenitor is poorly understood. Differentiation of alphabeta lineage T cells requires the surrogate alpha chain (pTalpha), which associates with the T cell receptor (TCR) beta chain to form the pre-TCR. gammadelta lineage development does not appear to involve an obligatory surrogate chain, but instead requires productive rearrangement and expression of both TCR gamma and delta genes. It has been proposed that the quality of signals transmitted by the pre-TCR and gammadelta TCR are distinct and that these "instructive" signals determine the lineage fate of an uncommitted progenitor cell. Here we show that the thymic T progenitor cells (CD25(+)CD44(+)c-kit(+)CD3(-)CD4(-)CD8(-) thymocytes, termed pro-T cells) from young adult mice that have yet to express TCRs can be subdivided based on interleukin 7 receptor (IL-7R) expression. These subsets exhibit differential potential to develop into gammadelta versus alphabeta lineage (CD4+CD8+ cells) in the thymus. Upon intrathymic injection, IL-7R(neg-lo) pro-T cells generated a 13-fold higher ratio of alphabeta lineage to gammadelta lineage cells than did IL-7R(+) pro-T cells. Much of this difference was due to a fivefold greater potential of IL-7R(+) pro-T cells to develop into TCR-gammadelta T cells. Evidence indicates that this biased developmental potential is not a result of enhanced TCR-gamma gene rearrangement/expression in IL-7R(+) pro-T cells. These results indicate that the pro-T cells are heterogeneous in developmental potential before TCR gene rearrangement and suggest that in some precursor cells the initial lineage commitment is independent of TCR-mediated signals.

Characterization of TCR Gene Rearrangements During Adult Murine T Cell Development

Development of the αβ and γδ T cell lineages is dependent upon the rearrangement and expression of the TCRα and β or γ and δ genes, respectively. Although the timing and sequence of rearrangements of the TCRα and TCRβ loci in adult murine thymic precursors has been characterized, no similar information is available for the TCRγ and TCRδ loci. In this report, we show that approximately half of the total TCRδ alleles initiate rearrangements at the CD44highCD25+ stage, whereas the TCRβ locus is mainly in germline configuration. In the subsequent CD44lowCD25+ stage, most TCRδ alleles are fully recombined, whereas TCRβ rearrangements are only complete on 10–30% of alleles. These results indicate that rearrangement at the TCRδ locus can precede that of TCRβ locus recombination by one developmental stage. In addition, we find a bias toward productive rearrangements of both TCRδ and TCRγ genes among CD44highCD25+ thymocytes, suggesting that functional γδ TCR complexes can be formed before the rearrangement of TCRβ. These data support a model of lineage commitment in which sequential TCR gene rearrangements may influence αβ/γδ lineage decisions. Further, because TCR gene rearrangements are generally limited to T lineage cells, these analyses provide molecular evidence that irreversible commitment to the T lineage can occur as early as the CD44highCD25+ stage of development.

The role of the T-cell receptor (TCR) in alpha beta/gamma delta lineage commitment: clues from intracellular TCR staining

T cells belong to two mutually exclusive lineages expressing either alpha beta or gamma delta T-cell receptors (TCR). Although alpha beta and gamma delta cells are known to share a common precursor the role of TCR rearrangement and specificity in the lineage commitment process is controversial. Instructive lineage commitment models endow the alpha beta or gamma delta TCR with a deterministic role in lineage choice, whereas separate lineage models invoke TCR-independent lineage commitment followed by TCR-dependent selection and maturation of alpha beta and gamma delta cells. Here we review the published data pertaining to the role of the TCR in alpha beta/gamma delta lineage commitment and provide some additional information obtained from recent intracellular TCR staining studies. We conclude that a variant of the separate lineage model is best able to accommodate all of the available experimental results.

T cell receptor gamma gene regulatory sequences prevent the function of a novel TCRgamma/pTalpha pre-T cell receptor

Expression of a TCRgamma transgene in RAG-1-/- mice resulted in the development of a limited number of CD4+CD8+ (DP) thymocytes. In vivo treatments with anti-TCRgamma antibody enhanced the number of DP thymocytes, demonstrating that TCRgamma chains were expressed on the cell surface in the absence of delta, alpha, or beta chains. Mutations in pTalpha or CD3epsilon genes abolished transgene-induced DP cell development, indicating that TCRgamma can associate with pTalpha and CD3 to form a novel pre-TCR. With a transgene containing additional regulatory sequences, TCRgamma expression was down-regulated in DP cells, and little DP cell development occurred. Thus, the function of the endogenous TCRgamma/pTalpha is limited by the transcriptional down-regulation of TCRgamma genes that normally accompanies DP cell development.

The developmental fate of T cells is critically influenced by TCRgammadelta expression

Differentiation of gammadelta and alphabeta T cells from a common precursor cell depends on productive rearrangement and expression of TCRgammadelta or TCRbeta genes, but whether it is an instructive or a stochastic mechanism that is responsible for this process is unclear. We report that expression of the productively rearranged TCRgamma transgene competitively inhibits alphabeta thymocyte development under conditions where TCRbeta gene rearrangement is limiting. The status of TCRdelta gene rearrangements in the remaining alphabeta-lineage cells indicates that the effect is mediated by the intact gammadelta receptor. Paradoxically, in TCRbeta-/- mice, gammadelta receptor expression can also drive differentiation of some alphabeta-lineage cells. To resolve this paradox, we provide evidence for a minor population of gammadelta-dependent alphabeta-lineage cells in normal mice. The results indicate that the T cell lineage commitment process is either error-prone or stochastic.

The alpha beta versus gamma delta T-cell lineage choice

During thymic development, immature T cells rearrange and express the genes encoding the T-cell antigen receptor and mature as either alpha beta or gamma delta lineage T cells. In the past year, advances have been made in understanding the role of individual components of the T-cell antigen receptor complex in the development of alpha beta and gamma delta lineage T cells. In addition, the transmembrane receptor Notch has recently been implicated as a new player in alpha beta versus gamma delta lineage determination.

Function of the TCR alpha enhancer in alphabeta and gammadelta T cells

We have used gene targeted mutational approaches to assess the role of the T cell receptor alpha (TCR alpha) enhancer (E alpha) in the control of TCR alpha and TCR delta gene rearrangement and expression. We show that E alpha functions in cis to promote V alpha to J alpha rearrangement across the entire J alpha locus, a distance of greater than 70 kb. We also show that E alpha is required for normal alphabeta T cell development; in this lineage, E alpha is required for germline J alpha expression, for normal expression levels of rearranged V alpha J alpha genes, and for expression of a diverse V alpha repertoire. In gamma delta T cells, E alpha is not required for VdeltaDJdelta rearrangement, but, surprisingly, is required for normal expression levels of mature VdeltaDJdelta transcripts and for expression of germline J alpha transcripts. Our findings imply that E alpha function is not limited to the TCR alpha components of the TCRalpha/delta locus or to the alpha beta lineage; rather, E alpha function is important in both alphabeta and gammadelta lineage T cells.

Analysis of T cell receptor beta chain CDR3 size using RNA extracted from formalin fixed paraffin wax embedded tissue

AIMS

To isolate RNA and DNA simultaneously from formalin fixed paraffin wax embedded tissue to assess the clonality of enteropathy associated T cell lymphomas and to analyse it in detail by a non-radioactive method of T cell receptor complementarity determining region 3 (CDR3) spectratyping.

METHODS

DNA and RNA were isolated simultaneously from formalin fixed paraffin wax embedded tissue blocks and subjected to the polymerase chain reaction (PCR) and semi-nested reverse transcription PCR (RT-PCR), respectively. The RT-PCR T cell receptor V beta products were analysed by CDR3 spectratyping using a denaturing polyacrylamide gel and silver staining.

RESULTS

Usable DNA and RNA were isolated simultaneously from formalin fixed paraffin wax embedded tissue. The specific clonality of the tissue was successfully analysed by a non-radioactive method of T cell receptor CDR3 spectratyping of the RT-PCR products. CDR3 spectratying of the RT-PCR products demonstrated the precise clonal nature of the tumour and non-tumour tissue showing that the non-tumour tissue comprised an oligoclonal population of a number of different T cell receptor V beta families. The tumour tissue comprised two T cell subtypes of the one family, T cell receptor V beta 9.

CONCLUSIONS

RNA and DNA were isolated from formalin fixed paraffin wax embedded enteropathy associated T cell lymphoma tissue. Detailed analysis of clonality can be carried out by a non-radioactive method of CDR3 spectratyping.

Notch activity influences the alphabeta versus gammadelta T cell lineage decision

The choice between the alphabeta or gammadelta T cell fates is influenced by the production of functional, in-frame rearrangements of the TCR genes, but the mechanism that controls the lineage choice is not known. Here, we show that T cells that are heterozygous for a mutation of the Notch1 gene are more likely to develop as gammadelta T cells than as alphabeta T cells, implying that reduced Notch activity favors the gammadelta T cell fate over the alphabeta T cell fate. A constitutively activated form of Notch produces a reciprocal phenotype and induces thymocytes that have functional gammadeltaTCR gene rearrangements to adopt the alphabeta T cell fate. Our data indicate that Notch acts together with the newly formed T cell antigen receptor to direct the alphabeta versus gammadelta T cell lineage decision.

V-D-J rearrangements at the T cell receptor delta locus in mouse thymocytes of the alpha beta lineage

The T cell receptor (TCR) delta locus lies within the TCR alpha locus and is excised from the chromosome by V alpha-J alpha rearrangement. We show here that delta sequences persist in a large fraction of the DNA from mature CD4+CD8- alpha beta+ mouse thymocytes. Virtually all delta loci in these cells are rearranged and present in extrachromosomal DNA. In immature alpha beta lineage thymocytes (CD3-/loCD4+CD8+) and in CD4+CD8- alpha beta+ thymocytes expressing a transgene-encoded alpha beta receptor, rearranged delta genes are present both in chromosomal and extrachromosomal DNA. Thus, contrary to earlier proposals, commitment to the alpha beta lineage does not require recombinational silencing of the delta locus or its deletion by a site-specific mechanism prior to V alpha-J alpha rearrangement.

Evidence that productive rearrangements of TCR gamma genes influence the commitment of progenitor cells to differentiate into alpha beta or gamma delta T cells

Two models have been considered to account for the differentiation of gamma delta and alpha beta T cells from a common hematopoietic progenitor cell. In one model, progenitor cells commit to a lineage before T cell receptor (TCR) rearrangement occurs. In the other model, progenitor cells first undergo rearrangement of TCR gamma, delta, or both genes, and cells that succeed in generating a functional receptor commit to the gamma delta lineage, while those that do not proceed to attempt complete beta and subsequently alpha gene rearrangements. A prediction of the latter model is that TCR gamma rearrangements present in alpha beta T cells will be nonproductive. We tested this hypothesis by examining V gamma 2-J gamma 1C gamma 1 rearrangements, which are commonly found in alpha beta T cells. The results indicate that V gamma 2-J gamma 1C gamma 1 rearrangements in purified alpha beta T cell populations are almost all nonproductive. The low frequency of productive rearrangements of V gamma 2 in alpha beta T cells is apparently not due to a property of the rearrangement machinery, because a transgenic rearrangement substrate, in which the V gamma 2 gene harbored a frame-shift mutation that prevents expression at the protein level, was often rearranged in a productive configuration in alpha beta T cells. The results suggest that progenitor cells which undergo productive rearrangement of their endogenous V gamma 2 gene are selectively excluded from the alpha beta T cell lineage.

T-cell development. T-cell lineage commitment revisited

Years of controversy about the lineage relationship between alpha beta and gamma delta T cells may at last have been resolved: it now appears that most T cells derive from an identical T-lineage committed precursor.

In-frame TCR delta gene rearrangements play a critical role in the alpha beta/gamma delta T cell lineage decision

Using a quantitative multiprobe Southern blot analysis, we demonstrate the surprising result that a significant proportion of alpha beta T cells and thymocytes retain T cell receptor delta locus sequences. A substantial portion of the retained delta locus is in a fully V-to-D-to-J rearranged configuration and 20% of these delta rearrangements are functional, significantly less than the 33% predicted for random gene rearrangements. Our observations are in conflict with the idea that alpha beta and gamma delta T cells derive from distinct precursors and suggest that commitment of a common precursor to the gamma delta lineage depends upon expression of a gamma delta T cell receptor. We propose that the intrathymic T cell lineage decision is determined by a competition between the production of functional gamma delta and beta-pre-T cell receptor complexes.

Alpha beta and gamma delta T cells can share a late common precursor

BACKGROUND

The subdivision of T cells into alpha beta and gamma delta subtypes is conserved throughout vertebrate development. The respective alpha beta and gamma delta T-cell receptors (TCRs) are encoded by somatically rearranged genes. There has been broad speculation as to whether an individual thymocyte can become either a gamma delta T cell or an alpha beta T cell as a result of stochastic gene rearrangement processes, or whether the two types of T cell are derived from separate lineages. Many of the experimental findings are apparently conflicting, however, and the issue--a basic one in immunology and development--remains unresolved.

RESULTS

To address this issue, we have used the recently developed polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) technique, which allows us to examine quantitatively the status of TCR gamma and delta genes in postnatal alpha beta T cells and their progenitors. Interestingly, such cells are depleted of productively rearranged delta and gamma genes, which can encode delta and gamma TCR polypeptide chains. However, in mice that can rearrange TCR delta gene segments, but in which the TCR delta gene is non-functional in other respects, no such depletion of productive rearrangements is seen.

CONCLUSION

The quantitative data that we have obtained fulfill the predictions of the stochastic hypothesis: that is, a progenitor T cell first attempts to become a gamma delta T cell and, if unsuccessful, then attempts to become an alpha beta T cell. Thus, alpha beta and gamma delta T cells can derive from a common precursor thymocyte. In the simplest case, therefore, lineage-determining factors are the successful rearrangement of both gamma and delta genes before TCR alpha gene rearrangements occur, which lead to deletion of the TCR delta locus and thereby preclude further gamma delta T-cell differentiation. In contrast, successful rearrangement of the TCR beta locus remains compatible with cells becoming either gamma delta or alpha beta T cells.

Gene rearrangement patterning and DNase-I hypersensitive sites within the T-cell receptor J alpha locus

For several years, the relationship between alpha beta and gamma delta T-cell progenitors has been a topic of debate. Some argue that a subset of T-cell progenitors is "pre-committed" to the alpha beta lineage and is thus programmed to rearrange alpha, but not delta genes. It is further argued that the deletion of the delta locus by a unique rearrangement, delta rec-psi J alpha, may be the critical forerunner to V-J alpha joins in alpha beta committed cells and that a hypersensitive site (HS) termed 5'TEA might regulate such rearrangement. Here we present an alternative hypothesis. We first emphasize that directed J alpha gene rearrangements do not exclusively target the psi J alpha gene, but that clustered gene rearrangements occur throughout the J alpha locus during T-cell development. We describe the existence of not one, but at least two HS sites distributed along the J alpha locus which might serve as regulators for the gene rearrangement event. Finally, we suggest that progenitor T-cells are not committed to a particular delta or alpha gene rearrangement, but that a flexible progenitor responds to complex interactions between environmental signals and multiple regulatory elements interspersed among delta/alpha genes.

T cell receptor (beta chain) transgenic mice have selective deficits in gamma delta T cell subpopulations

TCR-beta (T cell receptor-beta chain) transgenic mice have altered lymphocyte development. TCR-beta transgenic mice are hyporesponsive to alloantigens in vivo and are deficient in gamma delta T cells. In order to begin a study of the relationship between a deficiency of alloreactive gamma delta cells and the defective function of in vivo alloantigen recognition, we analysed the gamma delta T cell development in TCR-beta mice. The presence of the TCR-V beta 8.2 chain transgene is associated with inhibition of gamma chain gene rearrangement. In order to determine how the presence of the TCR-beta transgene affects gamma delta T cell development, gamma delta T cells were studied in the skin, intestine and spleen. TCR-beta mice have dramatically reduced numbers of gamma delta T cells in the spleen and moderately reduced numbers of gamma delta T cells among intestinal intraepithelial lymphocytes. In contrast, these mice have normal numbers of gamma delta dendritic epidermal cells (DEC). These selective deficits could be due to the developmental regulation of transgene transcription during fetal life. We examined transcription of the TCR-beta transgene in the fetal thymus and found that the TCR-beta transgene is first transcribed at high levels on day 16 of fetal life, after DEC have already migrated from the thymus to the epidermis. Furthermore, mRNA from the transgene was detected in DEC, ruling out the formal possibility that DEC bear a gamma delta receptor only because they are incapable of expressing the transgene.(ABSTRACT TRUNCATED AT 250 WORDS)

Fetal thymocyte potential for T cell receptor V gamma 3-J gamma 1 junctional modification

Junctional modifications of T cell receptor (TcR) and immunoglobulin (Ig) gene joining regions provide great diversity to respective protein repertoires. The addition of non-germ-line-encoded nucleotides (N-regions) in the V-J gamma junction is one such modification which is developmentally regulated, rarely evident in the fetal animal, but common in the adult. A question has recently arisen as to whether developmentally patterned N-region additions in V-J gamma joins are a reflection of T cell progenitors which are committed to particular types of rearrangement prior to the event, or of changing environmental influences on uncommitted cell populations. To address this question with regard to the V gamma 3-J gamma 1 join, T cells were examined in the fetal thymic organ culture (FTOC), a system with which the environment of early progenitor cells could be deliberately altered. At various times following FTOC initiation, cells were isolated for examination by the polymerase chain reaction, cloning and sequencing. V gamma 3-J gamma 1 sequences within genomic DNA as well as cDNA were evaluated. Data from these studies revealed frequent N-region additions within V-J gamma joins among day 14 fetal thymocyte populations, a situation dissimilar from that in vivo. Also dissimilar from the in vivo situation was the degree of exonuclease activity evident in FTOC. The canonical V gamma 3-J gamma 1 join (a frequent junction lacking N-region addition) was recognized in all experiments, but was least common among DNA versus cDNA sequences. Results illustrate that early progenitor cell populations are not programmed to exclude junctional modifications from V gamma 3-J gamma 1 joins.

Antigen-specific, MHC-unrestricted T cells

The published record suggests that in the majority of cases the antigen is recognized by the T cell receptor (TCR) as a complex of a foreign antigen and amino acid residues contributed by the major histocompatibility complex (MHC) antigens, and the antigen-specific, MHC-restricted effector function is an unambiguous result of this process. Alternatively, the T cell receptor may recognize a particular conformational form of the antigen which is dictated by the allelic differences in the MHC, resulting also in MHC-restricted recognition. When, however, a T cell which phenotypically fulfills all the requirements necessary to perform antigen specific, MHC-restricted function, shows a lack of MHC restriction, there are two possible explanations: 1) In addition to the MHC-restricted, antigen-specific T cell receptor the cell expresses, or has newly acquired the expression of another, MHC-unrestricted (NK-like) receptor, or 2) The specific antigen recognized by the T cell receptor, is able to bind to the receptor and activate the T cell without being presented by the MHC molecule. While the first possibility has been extensively described in the literature as well as other articles in this issue, the second possibility has not been dealt with to the same extent and is the primary focus of this review.

T-cell receptor alpha/beta chain-CD3 protein complex defect in systemic lupus erythematosus: T-cell function

We describe the first case of systemic lupus erythematosus (SLE) in which peripheral blood T cells were deficient in cell surface expression of T-cell receptor alpha/beta chain (TcR alpha beta) and the CD3 protein. Because of the uncommon phenotype and because of the notion that coexpression of TcR alpha beta and CD3 is essential for antigen-specific T-cell function, in vitro functional assays were performed, showing a highly decreased proliferative response to anti-CD3 antibody and other T-cell mitogens, deficient interleukin-2 (IL-2) secretion, and impaired function to respond in autologous and allogeneic mixed lymphocyte reactions. However, the helper-inducer function of T cells was unaffected by deficient expression of the TcR alpha beta/CD3 protein complex. The relative increase of CD4+ CDw29+ helper-inducer subsets in T cells accounted for elevated secretion of two terminal B-cell stimulating factors, B-cell growth factor (BCGF) and B-cell differentiation factor (BCDF). Hence, our results suggest that the regulation of secretion of lymphokines, IL-2, and BCGF and BCDF is independently controlled in T cells, and this case illustrates the pathologic sequelae of a unique defect in T cells characteristic of SLE.

Molecular analysis of a pro-T cell clone transformed by Abelson-murine leukemia virus, displaying progressive gamma delta T cell receptor gene rearrangement and surface expression

We present a molecular analysis of T cell differentiation in a set of clones derived from in vitro Abelson murine leukemia virus (A-MuLV) infection of fetal liver cells. The parental clone had partial rearrangement of the beta and gamma loci and spontaneously displayed progressive rearrangement of V gamma genes during in vitro culture. Further differentiation of these clones leading to delta gene rearrangement and CD4 expression, then CD8, CD3 and T cell receptor gamma delta chain surface expression was obtained after intrathymic transfer followed by in vitro co-culture with thymic tissue. These A-MuLV clones, therefore, appear to represent a powerful model system for studying the early molecular events of T cell development at the clonal level.

Co-expression of T-cell receptor beta and delta mRNA detected at high frequency in hybridomas derived from adult thymus

Two different hybridoma collections from adult C3H/HeJ thymus were generated in order to analyse T-cell receptor (TcR) rearrangements, surface expression of T-cell receptors and differentiation markers as well as lymphokine production. Large, low density thymocytes were either directly fused to the thymoma BW 5147 alpha-beta- variant, or fused after stimulation with Concanavalin A in the presence of interleukin-2 for 48 h. The hybrids obtained from Concanavalin A-stimulated cells represented rather mature thymocytes, with regard to TcR rearrangements and surface T-cell receptor expression. The collection of hybrids derived from freshly isolated large thymocytes contained cells in various stages of T-cell development. An unexpectedly large number of hybrids (46 out of 84) from this group expressed full-length C beta together with full-length, or shorter, C delta mRNA. This finding suggests that a considerable proportion of alpha beta T cells proceeds through a stage in development where delta genes are being rearranged and transcribed.

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.

A transcriptional enhancer of the mouse T cell receptor delta gene locus

T cells express receptors for antigen on their surface (TcR) which consists either of alpha/beta or of gamma/delta polypeptide chains. Since the TcR delta chain gene is located within the TcR alpha chain gene locus, strict regulation of expression of the region must operate to ensure that the two loci are not concomitantly expressed in T cells bearing either alpha/beta or gamma/delta TcR. In this report it is demonstrated that elements within the mouse TcR delta gene locus, located between the J delta 2 and C delta exons, enhance transcription from a heterologous promoter five- to tenfold in a T cell hybridoma expressing a TcR gamma/delta, whereas enhancement was only twofold in an alpha/beta-bearing T cell hybridoma. No enhancement of expression was observed in a B cell hybridoma. The sequences responsible for this enhancing activity are largely confined to a 766-bp Hae III DNA restriction fragment. A region within this DNA segment shows significant homology (73% identify) to a recently identified enhancer of the human TcR delta gene locus.

T-cell receptor-gamma/delta bearing lymphocytes in normal and inflammatory human skin

Murine dendritic epidermal T cells (DETC) were recently reported to express T-cell receptor (TCR)-gamma/delta chains. In a search for the human equivalent of these cells, we tested normal and lesional skin with MoAb which react with the TCR-gamma/delta heterodimer. We performed indirect immunofluorescence (IF) on epidermal sheets, and alkaline-phosphatase-anti-alkaline-phosphatase complex (APAAP) on epidermal cell smears. Frozen skin sections from normal skin and various cutaneous lymphocyte infiltrates were also studied. A few CD3+ T lymphocytes were consistently found in normal epidermis. Most of these cells appeared to be TCR-alpha/beta +, and some CD4+ or CD8+. On epidermal sheets and cell smears, only a very small TCR-gamma/delta + cell population was visualized (less than 0.1% of the total). On normal skin sections, we observed 0 to 3 gamma/delta + cells per section. When present, they were often located in the epidermal basal layer, and were round or dendritic. Double immunolabeling revealed that gamma/delta + cells differed from CD1+ Langerhans cells, and that they had a similar phenotypic pattern as gamma/delta + peripheral lymphocytes (PBL): CD2+, CD3+, CD4-, and CD8-. Immunostaining from various inflammatory skin lesions showed that the dermal infiltrates included CD3+ T lymphocytes but virtually no gamma/delta + cells. Only a few gamma/delta + cells were found in some end-evolutive infiltrates. Taken together, these results strongly suggest that normal human epidermis occasionally harbors TCR-gamma/delta complex bearing lymphocytes, which constitute a small fraction of the CD3+ cutaneous T lymphocytes.(ABSTRACT TRUNCATED AT 250 WORDS)

T cell receptor (TcR) beta chain transgenic mice: studies on allelic exclusion and on the TcR+ gamma/delta population

To study allelic exclusion of TcR genes we analyzed two types (I and II) of TcR beta transgenic mice. T cells derived from both types of mice contained similar amounts of transgenic RNA transcripts; however, surface expression of the transgenic beta chain was drastically reduced in type II compared to type I. In type I transgenic mice, productive rearrangements and expression of endogenous TcR beta genes were suppressed whereas on T cells of type II mice, both transgenic and endogenous TcR beta chains were expressed on the surface of the same cell. These findings suggest that allelic exclusion of TcR genes in beta transgenic mice depends on amount and/or onset of transgene expression during thymic development. Furthermore, TcR gamma rearrangements and the population of TcR gamma/delta-bearing double-negative CD4-CD8- thymocytes were reduced fivefold in type I transgenic animals. However, the V gamma usage and the gamma/delta+ dendritic epidermal cell populations appeared normal. RNase protection analysis further revealed low levels of transgenic TcR beta chain transcripts in TcR+ gamma/delta CD4-CD8- thymocytes. These results suggest that the beta transgene only quantitatively influences the gamma/delta T cell compartment, and supports the independence of the gamma/delta population.

Blockage of alpha beta T-cell development by TCR gamma delta transgenes

T lymphocytes recognize antigens by means of T-cell receptors (TCR) composed of alpha beta or gamma delta heterodimers. The mechanism governing the development of alpha beta- and gamma delta-bearing T cells from a common precursor T cell is so far unknown. It has been proposed that T-cell precursors rearrange their gamma- and delta-chain genes first, and alpha beta T cells are generated only from those cells that fail to rearrange productively both gamma- and delta-chain genes. Our recent study on gamma delta-transgenic mice contradicted this hypothesis, however, and indicated that repression of gamma-chain gene expression mediated by a transcriptional silencer element has a critical role in the generation of alpha beta T cells. Here we report that the generation of alpha beta T cells is severely blocked in transgenic mice carrying gamma- and delta-chain transgenes without the associated silencer, thereby strengthening the validity of the silencer model of T-cell development.

T cell receptor gamma gene status of human alpha/beta+ and gamma/delta+ T cell clones: absence of V9JP rearrangements in alpha/beta+ clones is not a result of a lack of rearrangements involving more 5' J gamma segments

T cell receptor (TCR) gamma gene rearrangements were examined in panels of human T cell clones expressing TCR alpha/beta or gamma/delta heterodimers. Over half of the alpha/beta+ clones had both chromosomes rearranged to C gamma 2 but this was the case for only 20% of the gamma/delta+ clones. While more than half of the gamma/delta+ clones showed a V9JP rearrangement, this configuration was absent from all 49 alpha/beta+ clones analysed. However, this was not a result of all rearrangements being to the more 3' J gamma genes as 11 alpha/beta+ clones had rearrangement(s) to JP1, the most 5' J gamma gene segment. Both alpha/beta+ and gamma/delta+ clones showed a similar pattern of V gamma gene usage in rearrangements to J gamma 1 or J gamma 2 with a lower proportion of the more 3' genes being rearranged to J gamma 2 than for the more 5' genes. Several alpha/beta+ and several gamma/delta+ clones had noncoordinate patterns of rearrangement involving both C gamma 1 and C gamma 2. Eleven out of fourteen CD8+ clones tested had both chromosomes rearranged to C gamma 2 whereas all clones derived from CD4-8- cells and having unconventional phenotypes (CD4-8- or CD4+8+) had at least one C gamma 1 rearrangement. Twelve out of twenty-seven CD4+ clones also had this pattern, suggesting that CD4-8+ clones had a tendency to utilize more 3' J gamma gene segments than CD4+ clones. There was some evidence for interdonor variation in the proportions of TCR gamma rearrangements to C gamma 1 or C gamma 2 in alpha/beta+ clones as well as gamma/delta+ clones. The results illustrate the unique nature of the V9JP rearrangement in gamma/delta+ clones and the possible use of a sequential mechanism of TCR gamma gene rearrangements during T cell differentiation is discussed.

Developmentally ordered appearance of thymocytes expressing different T-cell antigen receptors

The T-cell repertoire is elaborated by a still poorly understood process during which precursor cells arising in the bone marrow seed the thymus to provide a starting point for intrathymic differentiation and selection. The products of the process are cells which express antigen receptors composed of either alpha/beta or gamma/delta heterodimers in association with CD3. The finding that the appearance of T-cell antigen receptor gamma- and delta-gene rearrangements and transcripts precedes those of full-length beta- and alpha-transcripts during ontogeny indicates that the process is ordered, a conclusion supported by the fact that the appearance of thymocytes expressing CD3-associated gamma/delta heterodimers precedes the appearance of those bearing alpha/beta heterodimers. The recent demonstrations that within the gamma- and delta-loci there is ordered and sometimes transient rearrangement and expression of specific V delta and V gamma gene segments during ontogeny raised the possibility that qualitative changes in the capacity of the differentiative process to generate components of the T-cell armamentarium might occur. We have produced a monoclonal antibody that detects an epitope of the V gamma 3 gene product, a gene segment expressed only in the early fetal thymus. In this report we demonstrate that cells expressing V gamma 3 are present transiently at the earliest stages of thymocyte development, preceding the appearance of cells bearing other gamma/delta or alpha/beta receptors. In the adult mouse, V gamma 3 expression appears to be limited to Thy-1+ cells in the epidermis. These results suggest a profound programming and staging in elaboration of the components of the T-cell system during the early stages of thymocyte development in the embryo.

In transgenic mice the introduced functional T cell receptor beta gene prevents expression of endogenous beta genes

Transgenic mice were constructed with a functional T cell receptor beta gene. Transcription of the introduced gene is largely confined to T cells, but low levels of transcripts are also seen in B cells and in other tissues. Serological analyses show that most, if not all, of the T lymphocytes express the transgenic beta chain on the cell surface and lack beta chains encoded by endogenous beta genes. Molecular genetic analyses of uncloned and cloned T lymphocytes demonstrate that rearrangement of endogenous beta genes is incomplete. Partial D beta 1-J beta 1 rearrangements are found preferentially, while complete VDJ rearrangements are not seen. These findings show that expression of the transgene regulates the rearrangement of endogenous beta genes. Although the alpha beta T cell receptors of the transgenic mice are homogeneous with respect to the beta chain, they are fully functional, at least in a variety of allogeneic responses.

The gamma T-cell antigen receptor

The gamma-TCR is encoded by genes composed of V, J, and C elements that demonstrate a limited potential for recombinational diversity. These genes are rearranged, transcribed, and translated into proteins early during thymic ontogeny. Lymphocytes express gamma-TCR proteins on the plasma membrane only in association with the CD3 complex. gamma-TCR glycoproteins usually associate with another non-gamma glycoprotein, designated delta-TCR, to form a heterodimer receptor. Both non-disulfide-bonded and disulfide-bonded gamma/delta-TCR heterodimers have been identified on the plasma membrane of human T lymphocytes. On certain gamma-TCR-bearing T cell lines, a delta-TCR protein cannot be visualized by autoradiography. It is possible that delta-TCR proteins are associated with gamma-TCR glycoproteins on these cell lines but are not efficiently radiolabeled. Alternatively, it has been suggested that homodimers of gamma-TCR proteins can assemble with CD3 and be expressed on the plasma membrane of these cells. In adult lymphoid tissues, the majority of T lymphocytes expresses a CD3, alpha/beta antigen receptor, whereas only a minor subset (less than 5% of peripheral blood lymphocytes, lymph node, spleen, and thymocytes) express a CD3, gamma/delta antigen receptor. IL-2-dependent cell lines of both murine and human CD3, gamma/delta T cells have been established. Most CD3, gamma/delta T cell lines mediate cytotoxicity against a broad spectrum of tumor-cell targets, although the functional significance of this observation remains unclear. Cytotoxicity is apparently not restricted by or directed against MHC antigens. Antibodies against CD3 or gamma-TCR can induce proliferation and IL-2 secretion and can either augment or inhibit cytotoxicity, demonstrating that the gamma/delta-TCR is a functional receptor. The ligand recognized by this receptor has not been identified. The physiological role of T lymphocytes expressing gamma/delta-TCR, the molecular and structural properties of delta-TCR, and the relationship between CD3, alpha/beta T lymphocytes and CD3, gamma/delta T lymphocytes are the major unresolved questions that will be the primary focus of further experimentation.