Transcription of germ line V alpha segments correlates with ongoing T-cell receptor alpha-chain rearrangement

Control of V(D)J Recombination through Transcriptional Elongation and Changes in Locus Chromatin Structure and Nuclear Organization

V(D)J recombination is the assembly of gene segments at the antigen receptor loci to generate antigen receptor diversity in T and B lymphocytes. This process is regulated, according to defined developmental programs, by the action of a single specific recombinase complex formed by the recombination antigen gene (RAG-1/2) proteins that are expressed in immature lymphocytes. V(D)J recombination is strictly controlled by RAG-1/2 accessibility to specific recombination signal sequences in chromatin at several levels: cellular lineage, temporal regulation, gene segment order, and allelic exclusion. DNA cleavage by RAG-1/2 is regulated by the chromatin structure, transcriptional elongation, and three-dimensional architecture and position of the antigen receptor loci in the nucleus. Cis-elements specifically direct transcription and V(D)J recombination at these loci through interactions with transacting factors that form molecular machines that mediate a sequence of structural events. These events open chromatin to activate transcriptional elongation and to permit the access of RAG-1/2 to their recombination signal sequences to drive the juxtaposition of the V, D, and J segments and the recombination reaction itself. This chapter summarizes the advances in this area and the important role of the structure and position of antigen receptor loci within the nucleus to control this process.

Germline transcription: a key regulator of accessibility and recombination

The developmental control of V(D)J recombination is imposed at the level of chromatin accessibility of recombination signal sequences (RSSs) to the recombinase machinery. Cis-acting transcriptional regulatory elements such as promoters and enhancers play a central role in the control of accessibility in vivo. However, the molecular mechanisms by which these elements influence accessibility are still under investigation. Although accessibility for V(D)J recombination is usually accompanied by germline transcription at antigen receptor loci, the functional significance of this transcription in directing RSS accessibility has been elusive. In this chapter, we review past studies outlining the complex relationship between V(D)J recombination and transcription as well as our current understanding on how chromatin structure is regulated during gene expression. We then summarize recent work that directly addresses the functional role of transcription in V(D)J recombination.

Expression of T-cell receptor genes during early T-cell development

Lymphoid cell development is an ordered process that begins in the embryo in specific sites and progresses through multiple differentiative steps to production of T- and B-cells. Lymphoid cell production is marked by the rearrangement process, which gives rise to mature cells expressing antigen-specific T-cell receptors (TCR) and immunoglobulins (Ig). While most transcripts arising from TCR or Ig loci reflect fully rearranged genes, germline transcripts have been identified, but these have always been thought to have no specific purpose. Germline transcription from either unrearranged TCR or unrearranged Ig loci was commonly associated with an open chromatin configuration during VDJ recombination. Since only early T and B cells undergo rearrangement, the association of germline transcription with the rearrangement process has served as an appropriate explanation for expression of these transcripts in early T- and B-cell progenitors. However, germline TCR-V beta 8.2 transcripts have now been identified in cells from RAG(-/-) mice, in the absence of the VDJ rearrangement event and recombinase activity. Recent data now suggest that germline TCR-V beta transcription is a developmentally regulated lymphoid cell phenomenon. Germline transcripts could also encode a protein that plays a functional role during lymphoid cell development. In the least, germline transcripts serve as markers of early lymphoid progenitors.

Detection of spliced and unspliced forms of germline TCR-Vbeta transcripts in extrathymic lymphoid sites

Germline TCR-Vbeta transcription is commonly considered an event coupled with rearrangement of TCR genes in T cells. The extent of germline Vbeta transcription is studied here in a range of cell types and in several mouse strains. A sensitive semi-quantitative RT-PCR method was developed to specifically detect germline and not rearranged transcripts. Germline transcription of a range of different Vbeta genes was detected along with rearranged transcripts in bone marrow, thymus, mesenteric lymph node and spleen. Some transcripts were also detected in low level in non-lymphoid tissues including heart, liver and brain. Expression was also studied in the C57BL/6J-beta2microglobulin-/- (C57BL/6J-beta2M-/-) mouse model that lacks NK1.1 T cells and predominantly utilises Vbeta8.2 in the formation of a TCR. beta2M-/- mice, which lack both CD1-dependent NK1.1 T cells and CD8+ T cells, showed germline TCR-Vbeta8 transcription in most tissues indicating that germline transcription is not specifically related to CD1-dependent NK1.1 T cells. In many tissues, multiple transcripts were amplified representing both spliced and unspliced forms of germline Vbeta. For most Vbeta genes, the expression of spliced and unspliced forms was equivalent. Given an abundance of unspliced transcripts, the presence of alternative ORFs encoding a novel protein was investigated within the TCR-Vbeta genes. Sequence analysis of ORFs showed only genes with a high level of similarity to TCR-beta. All data reflect the prevalence of germline transcripts in vivo and raise questions about their functional role.

Cell surface expression of a peptide encoded by the unrearranged TCR-Vβ8.2 gene

Germline transcription of T-cell receptor (TCR) genes has been described in early lymphoid cells. The most common explanation for this phenomenon is that transcription of unrearranged Vbeta genes directs gene usage during the rearrangement event. Germline transcription of the TCR-Vbeta8.2 gene has been detected in a precursor T-cell line, C1-V13D, which shows no rearrangement at any of the TCR gene loci. This cell line also shows weak binding of specific anti-Vbeta8.2 antibody to the cell surface, consistent with expression of a truncated TCRbeta chain. RT-PCR has been used to confirm expression of spliced germline transcripts of TCR-Vbeta8.2 in C1-V13D initiated from both leader (L)5.1 and L8.2. Transcripts initiated from L8.2 were also detectable in unspliced form. In order to test expression and subcellular localisation of any encoded peptides, amplified germline transcripts in both spliced and unspliced form were cloned into the pEGFP-N1 fusion vector for stable transfection and overexpression in C1-V13D. Cell surface expression of a fusion protein between EGFP and a Vbeta peptide has been confirmed in C1-V13D but not in control COS-7 cells. Results presented here raise the possibility of a new pre-TCR structure specific to early lymphoid cells and based on TCR-Vbeta8.2 gene expression.

Mechanism and control of V(D)J recombination versus class switch recombination: similarities and differences

V(D)J recombination is the process by which the variable region exons encoding the antigen recognition sites of receptors expressed on B and T lymphocytes are generated during early development via somatic assembly of component gene segments. In response to antigen, somatic hypermutation (SHM) and class switch recombination (CSR) induce further modifications of immunoglobulin genes in B cells. CSR changes the IgH constant region for an alternate set that confers distinct antibody effector functions. SHM introduces mutations, at a high rate, into variable region exons, ultimately allowing affinity maturation. All of these genomic alteration processes require tight regulatory control mechanisms, both to ensure development of a normal immune system and to prevent potentially oncogenic processes, such as translocations, caused by errors in the recombination/mutation processes. In this regard, transcription of substrate sequences plays a significant role in target specificity, and transcription is mechanistically coupled to CSR and SHM. However, there are many mechanistic differences in these reactions. V(D)J recombination proceeds via precise DNA cleavage initiated by the RAG proteins at short conserved signal sequences, whereas CSR and SHM are initiated over large target regions via activation-induced cytidine deaminase (AID)-mediated DNA deamination of transcribed target DNA. Yet, new evidence suggests that AID cofactors may help provide an additional layer of specificity for both SHM and CSR. Whereas repair of RAG-induced double-strand breaks (DSBs) involves the general nonhomologous end-joining DNA repair pathway, and CSR also depends on at least some of these factors, CSR requires induction of certain general DSB response factors, whereas V(D)J recombination does not. In this review, we compare and contrast V(D)J recombination and CSR, with particular emphasis on the role of the initiating enzymes and DNA repair proteins in these processes.

Germline transcription of multiple TCR‐Vβ genes in cloned T‐cell lines

The functional significance of germline transcription of T cell receptor (TCR) beta chain variable (V) region genes is under investigation. The accepted model is that transcriptional activation of germline TCR genes is associated with the rearrangement process during T-cell development. By this model, germline expression of a subset of TCR-Vbeta genes might be expected in early T cells which have not yet undergone rearrangement. Germline transcription of TCR-Vbeta genes was analysed using the reverse transcriptase (RT)-PCR in a clonal T-cell precursor line C1-V13D, a clonal pre-B cell line RAW112 and a mature T helper cell line D10.G4.1. Evidence is presented for germline transcription of TCR-Vbeta8.2 and TCR-Vbeta2.1 genes in all three cell lines, although expression in RAW112 was very weak. C1-V13D cells expressed very high levels of the whole range of transcripts including Vbeta2.1, Vbeta5.1, Vbeta5.2, Vbeta6.1, Vbeta7.1, Vbeta8.1, Vbeta8.2, Vbeta8.3 and Vbeta13.1. However, D10.G4.1 cells expressed a subset of transcripts with apparently lower levels of expression, including Vbeta2.1, Vbeta5.1, Vbeta5.2, Vbeta6.1, Vbeta8.2 and Vbeta8.3. These results raise questions about the significance and possible function of germline transcripts and/or their encoded products in early lymphoid cells and in T cells at different stages of development.

Allelic exclusion of the T cell receptor alpha-chain: developmental regulation of a post-translational event

Allelic exclusion of the alpha and beta chains of the T cell receptor is maintained by different mechanisms. Exclusion of the beta-chain is primarily by allowing the successful rearrangement of only one of the two beta-chain loci. In the case of the alpha-chain, rearrangement on both chromosomes is very common, as is expression of alpha-chain mRNA and protein encoded by both loci. For the most part, however, functional alpha-chain allelic exclusion is maintained at the cell surface after positive selection in the thymus. The mechanism by which this is accomplished is not yet known, but recent evidence indicates that it is an active process coupled to signalling through the T cell receptor.

Interleukin 7 receptor control of T cell receptor gamma gene rearrangement: role of receptor-associated chains and locus accessibility

VDJ recombination of T cell receptor and immunoglobulin loci occurs in immature lymphoid cells. Although the molecular mechanisms of DNA cleavage and ligation have become more clear, it is not understood what controls which target loci undergo rearrangement. In interleukin 7 receptor (IL-7R)alpha-/- murine thymocytes, it has been shown that rearrangement of the T cell receptor (TCR)-gamma locus is virtually abrogated, whereas other rearranging loci are less severely affected. By examining different strains of mice with targeted mutations, we now observe that the signaling pathway leading from IL-7Ralpha to rearrangement of the TCR-gamma locus requires the gammac receptor chain and the gammac-associated Janus kinase Jak3. Production of sterile transcripts from the TCR-gamma locus, a process that generally precedes rearrangement of a locus, was greatly repressed in IL-7Ralpha-/- thymocytes. The repressed transcription was not due to a lack in transcription factors since the three transcription factors known to regulate this locus were readily detected in IL-7Ralpha-/- thymocytes. Instead, the TCR-gamma locus was shown to be methylated in IL-7Ralpha-/- thymocytes. Treatment of IL-7Ralpha-/- precursor T cells with the specific histone deacetylase inhibitor trichostatin A released the block of TCR-gamma gene rearrangement. This data supports the model that IL-7R promotes TCR-gamma gene rearrangement by regulating accessibility of the locus via demethylation and histone acetylation of the locus.

Developmental regulation of V(D)J recombination at the TCR alpha/delta locus

The T-cell receptor (TCR) alpha/delta locus includes a large number of V, D, J and C gene segments that are used to produce functional TCR delta and TCR alpha chains expressed by distinct subsets of T lymphocytes. V(D)J recombination events within the locus are regulated as a function of developmental stage and cell lineage during T-lymphocyte differentiation in the thymus. The process of V(D)J recombination is regulated by cis-acting elements that modulate the accessibility of chromosomal substrates to the recombinase. Here we evaluate how the assembly of transcription factor complexes onto enhancers, promoters and other regulatory elements within the TCR alpha/delta locus imparts developmental control to VDJ delta and VJ alpha rearrangement events. Furthermore, we develop the notion that within a complex locus such as the TCR alpha/delta locus, highly localized and region-specific control is likely to require an interplay between positive regulatory elements and blocking or boundary elements that restrict the influence of the positive elements to defined regions of the locus.

Characterization of human TCR Vbeta gene promoter. Role of the dodecamer motif in promoter activity

During T-lymphocyte development, the T-cell antigen receptor (TCR) gene expression is controlled by its promoter and enhancer elements and regulated in tissue- and development stage-specific manner. To uncover the promoter function and to define positive and negative regulatory elements in TCR gene promoters, the promoter activities from 13 human TCR Vbeta genes were determined by the transient transfection system and luciferase reporter assay. Although most of the TCR Vbeta gene promoters that we tested are inactive by themselves, some promoters were found to be constitutively strong. Among them, Vbeta6.7 is the strongest. 5'-Deletion and fragmentation experiments have narrowed the full promoter activity of Vbeta6.7 to a fragment of 147 base pairs immediately 5' to the transcription initiation site. A decanucleotide motif with the consensus sequence AGTGAYRTCA has been found to be conserved in most TCR Vbeta gene promoters. There are three such decamer motifs in the promoter region of Vbeta6.7, and the contribution of each such motif to the promoter activity has been examined. Further site-directed mutagenesis analyses showed that: 1) when two Ts in the decamer were mutated, the promoter activity was totally abolished; 2) when two additional nucleotides 3' to the end of decamer were mutated, the promoter activity was decreased to two-thirds of the full level; and 3) when the element with the sequence AGTGATGTCACT was inserted into other promoters, the original weak promoters become very strong. Taken together, our data suggest that the positive regulatory element in Vbeta6.7 should be considered a dodecamer rather than a decamer and that it confers strong basal transcriptional activity on TCR Vbeta genes.

Efficient Recombination of a Switch Substrate Retrovector in CD40-Activated B Lymphocytes: Implications for the Control of CH Gene Switch Recombination

Maturing B lymphocytes possess a recombination activity that switches the class of heavy chain Ig. The nature of the recombination activity, its molecular requirements and regulation remain elusive questions about B lymphocyte biology and development. Class switch recombination is controlled by cytokine response elements that are required to differentially activate CH gene transcription before their subsequent recombination. Here, we show that cultures of purified murine and human B cells, stimulated only by CD40 receptor engagement, possess a potent switch recombination activity. CD40 ligand-stimulated murine and human B lymphocytes were infected with recombinant retroviruses containing Sμ and Sγ2b sequences. Chromosomally integrated switch substrate retrovectors (SSRs), harboring constitutively transcribed S sequences, underwent extensive recombinations restricted to their S sequences with structural features akin to endogenous switching. SSR recombination commenced 4 days postinfection (5 days poststimulation) with extensive switch sequence recombination over the next 2 to 3 days. In contrast, endogenous Sγ2b and Sγ1 sequences did not undergo appreciable switch recombination upon CD40 signaling alone. As expected, IL-4 induced endogenous Sμ to Sγ1 switching, while endogenous Sμ to Sγ2b fusions remained undetectable. Surprisingly, IL-4 enhanced the onset of SSR recombination in CD40-stimulated murine B cells, with S-S products appearing only 2 days postinfection and reaching a maximum within 2 to 3 days. The efficiency of switch recombination with SSRs ressembles that seen for endogenous CH class switching.

Regulatory Elements in the Promoter of a Murine TCRD V Gene Segment

TCRD V segments rearrange in an ordered fashion during human and murine thymic development. Recombination requires the accessibility of substrate gene segments, and transcriptional enhancers and promoters have been shown to regulate the accessible chromatin configuration. We therefore investigated the regulation of TCRD V rearrangements by characterizing the promoter of the first TCRD V segment to be rearranged, DV101S1, under the influence of its own enhancer. Sequences required for full promoter activity were identified by transient transfections of normal and mutated promoters into a human γδ lymphoma, and necessary elements fall between −86 and +66 nt, relative to the major transcription start site. They include a cAMP responsive element (CRE) at −62, an Ets site at −39, a TATA box at −26, the major transcriptional start site sequence (−8 to −5 and −2 to +11), and a downstream sequence (+12 to +33). Gel shift analyses and in vitro DNase I footprinting showed that nuclear proteins bind to the functionally relevant CRE, Ets, +1 to +10 sequence, and the +17 to +21 sequence. Nuclear proteins also bind to an E box at −52, and GATA-3 binds to a GATA motif at −5, as shown by Ab ablation-supershift experiments, but mutations that abrogated protein binding to these sites failed to affect DV101S1 promoter activity. We conclude that not all protein-binding sites within the DV101S1 minimal promoter are important for enhancer driven TCRD gene transcription. Further, the possibility remains that the GATA and E box sites function in enhancer independent DV101S1 germline transcription.

Germ-line transcription and methylation status of the TCR-J alpha locus in its accessible configuration

We have generated two in vivo mouse models to study the regulation of DNA accessibility to the V(D)J recombinase machinery in the T cell receptor (TCR)-J alpha locus. In recombination activating gene (RAG)-deficient mice, both injection of a TCR-beta chain transgene (RTB mice) or anti-CD3-epsilon treatment in vivo (RT3 mice) lead to the same phenotype with homogeneous thymocyte populations blocked at the CD4+ CD8+ double positive (DP) stage. At this developmental stage, the TCR-alpha rearrangements are about to start, and the TCR-J alpha locus is frozen in an accessible but yet unrearranged configuration in these mice. We show high level of TCR-alpha germ-line transcription in thymocytes from RTB and RT3 mice. Transcripts are skewed towards the 5' end of the TCR-J alpha locus, and the T early alpha (TEA) sterile transcript is predominant and therefore provides a useful marker for the TCR-J alpha locus opening. Analysis of the DNA methylation status reveals a global surmethylation of the TCR-J alpha locus in the thymus in comparison with non-lymphoid cells in these mice. We propose that hypermethylation of the locus could precede a progressive demethylation, providing a specific protective and regulatory role in the rearrangement events.

Transcription of germline VH gene elements by normal human fetal liver

Transcription of the gene elements that form the variable region of immunoglobulin heavy chains has been proposed to represent the process that controls access for the recombination enzymes in their sequential steps of catalysis. Evidence for germline transcription of VH gene elements, as part of VH to DJH recombination, has been limited to transcripts of only a few gene elements. We have examined normal fetal liver mRNA by Northern blotting and present evidence for germline transcripts from six human VH gene families. The candidate VH4 transcripts have been confirmed as germline transcripts by hybridization with 3' flanking sequences that would have been removed by recombination from mature VHDJH genes. The candidate transcripts for VH1, VH3, VH4 and VH6 have been confirmed by polymerase chain reaction amplification with primers from the 3' flanking sequences of these gene families and determination of the sequence of these products. Determination of sequence from two clones of VH1, VH3 and VH4 indicates that more than one gene from each of these families is transcribed. PCR amplification of VH4 and VH6 with primers specific for the leader sequence (exon 1) and 3' flanking sequence indicate that these transcripts are spliced, representing RNA processing. Germline transcripts from these families are also present in normal human bone marrow. These results indicate that transcriptional activation of germline VH gene elements is a general phenomenon in tissues undergoing V to DJ recombination.

Defect in rearrangement of the most 5' TCR-J alpha following targeted deletion of T early alpha (TEA): implications for TCR alpha locus accessibility

To address the role of the TEA germline transcription, which initiates upstream of the TCR-J alpha S, in the regulation of TCR-J alpha locus accessibility, we created a mouse in which this region has been removed by homologous recombination. Normal development of T alpha beta cells and the expression of other TCR alpha germline transcripts in TEA-/- mice ruled out an exclusive role for TEA in the overall accessibility of the J alpha cluster. However, the rearrangement of the most 5' J alpha (J alpha 61 to J alpha 53) was severely impaired, indicating that TEA may control the DNA accessibility of a particular J alpha window. Moreover, the relative usage of every J alpha segment was affected. These results are consistent with TEA acting as a "rearrangement-focusing" element, targeting the primary waves of V alpha-J alpha recombination to the most 5' J alpha S in an ongoing TCR-J alpha rearrangement model.

Accessibility control of antigen-receptor variable-region gene assembly: role of cis-acting elements

Antigen receptor variable region genes are assembled from germline variable (V), diversity (D), and joining (J) gene segments. This process requires expression of V(D)J recombinase activity, and "accessibility" of variable gene segments to this recombinase. The exact mechanism by which variable gene segments become accessible during development is not known. However, several studies have shown that cis-acting elements that regulate transcription may also function to regulate accessibility. Here we review the evidence that transcriptional promoters, enhancers, and silencers are involved in regulation of accessibility. The manner in which these elements may combine to regulate accessibility is addressed. In addition, current and potential strategies for identifying and analyzing cis-acting elements that mediate locus accessibility are discussed.

Germ-line transcription of the T cell receptor delta gene in mouse hematopoietic cell lines

T cell receptor (TCR) genes are initially activated in hematopoietic stem cells that have been committed to the T cell lineage during ontogeny. We have established cell lines derived from hematopoietic organs which express truncated TCR delta mRNA (1.3 kbp). In this study, cDNA clones of this mRNA were analyzed to characterize TCR delta gene activation in early T cell development. Transcription started from an intron between D delta 2 and J delta 1, 72 bp upstream from J delta 1, and continued to J delta 1 and C delta exons, which were directly spliced to J delta 1. Thus, the truncated mRNA expressed by the cell lines was determined to be a germ-line transcript. The promoter activity of the DNA fragment between D delta 2 and J delta was assessed by its ability to drive the transcription of a reporter gene linked to it in the cell line expressing the TCR delta germ-line transcript. TCR delta germ-line transcription was found not only in these cell lines but also in fetal liver and thymus cells. These findings indicate that the TCR delta germ-line transcription is a naturally occurring event in developing T cells. The significance of germ-line transcription of TCR delta genes is unknown, but it may be an event that follows T-lineage commitment or, at least, may be closely associated with this commitment.

T cell receptor V beta 8.2 gene germ-line transcription: an early event of lymphocyte differentiation

Rearrangement of the T cell antigen receptor (TCR) beta chain genes is highly regulated in both a developmental and a tissue-specific manner. T cell precursors originate from the yolk sac or fetal liver during gestation and from the bone marrow during adulthood. They initiate the recombination of TCR genes primarily during differentiation in the thymus. It has previously been suggested that transcription of immunoglobulin genes in germ-line configuration is linked to recombination events within these loci. Here, we examine whether germ-line transcription of TCR variable genes coincides with their rearrangement or whether it marks even earlier stages of T lymphocyte development. During gestation, we found V beta 8.2 germ-line transcripts in the fetal liver and the fetal thymus, but not in the yolk sac. This transcription precedes V beta 8.2 to D beta J beta rearrangement. In adult animals, we found these transcripts in the thymus, the spleen, the liver and the bone marrow. However, in the liver, this transcription is dependent on the presence of mature lymphocytes. This transcription does not happen in non-lymphoid cells. In the B lymphocyte lineage, V beta 8.2 germ-line transcripts are detected only in the earliest stages of differentiation (pre-pro- and pro-B cells), but not in pre-B cells and mature B lymphocytes. Altogether, our results show that transcription of the unrearranged V beta 8.2 gene is an early event of lymphocyte development, taking place in lymphocyte precursors, long before V beta 8.2 rearrangement.

A locus control region in the T cell receptor alpha/delta locus

A locus control region (LCR) is a set of cis-acting elements that regulate chromatin accessibility of a gene locus. In the T cell receptor (TCR) alpha/delta locus, an LCR might regulate the differential tissue and developmental expression and the rearrangement of TCR alpha and delta genes. We have localized a region 3' of the TCR alpha/delta locus containing eight T cell-specific nuclease hypersensitive domains (HS-1 to HS-8), which fit the characteristics of an LCR. In transgenic mice, a TCR alpha gene linked to this region is expressed at a high level, independent of the site of integration and correlates with gene copy number. The transgene is expressed in the alpha beta but not the gamma delta T cell subset and is activated at the right time during development. Proper LCR function requires the region containing HS-2 to HS-6. We propose a model of LCR competition to explain the differential regulation of TCR alpha versus delta genes during development.

Functional characterization of the promoter for the human germ-line T cell receptor J alpha (TEA) transcript

The T cell receptor (TCR)-alpha and -delta loci are contained on the same chromosomal region, and yet are developmentally and genetically independent. The first element of the J alpha cluster (psi J alpha) is the site of an active rearrangement in the human thymus (delta Rec-psi J alpha rearrangement) and is localized downstream of a region expressed as a germ-line sterile transcript (TEA) in the human developing thymus. We hypothesized that the transcription of TEA could be indicative of (or responsible for) the opening of the J alpha to the V(D)J recombinase and undertook to analyze cis-acting sequences controlling the TEA transcription. The promoter of TEA was characterized. It was part of a region that is highly conserved between human and mouse and contained many sites for the putative binding of T cell-specific transcription factors. The in vitro activity of this promoter was dependent on the association with an enhancer. A strong DNase I hypersensitive site was found in the vicinity of this promoter again suggesting the possible presence of protein-DNA interactions in this region. The implications of these results in the general perspective of TCR-alpha/delta gene regulation is discussed.

Regulation of TCR alpha and beta gene allelic exclusion during T-cell development

Early in their development, most T cells become committed to the expression of one, and only one, TCR alpha beta combination. How do T cells achieve this TCR allelic exclusion? This article discusses the configuration and expression of TCR alpha and beta genes in mature T-cell lines and TCR alpha beta transgenic mice, and proposes three nonexclusive models to account for the significant occurrence of T cells with two productive alpha gene rearrangements.