Overexpression of Cellular Prion Protein Induces an Antioxidant Environment Altering T Cell Development in the Thymus

Cellular prion protein (PrP(C)) is an ubiquitously expressed glycoprotein whose roles are still widely discussed, particularly in the field of immunology. Using TgA20- and Tg33-transgenic mice overexpressing PrP(C), we investigated the consequences of this overexpression on T cell development. In both models, overexpression of PrP(C) induces strong alterations at different steps of T cell maturation. On TgA20 mice, we observed that these alterations are cell autonomous and lead to a decrease of alphabeta T cells and a concomitant increase of gammadelta T cell numbers. PrP(C) has been shown to bind and chelate copper and, interestingly, under a copper supplementation diet, TgA20 mice presented a partial restoration of the alphabeta T cell development, suggesting that PrP(C) overexpression, by chelating copper, generates an antioxidant context differentially impacting on alphabeta and gammadelta T cell lineage.

Role of the T cell receptor alpha chain in the development and phenotype of naturally arising CD4+CD25+ T cells

The T cell receptor alpha chain repertoire and the possible influence of the alpha chain on the development and phenotype of naturally arising mouse CD4+CD25+ T cells have not been extensively analysed. We used all available Valpha-specific monoclonal antibodies and a sensitive multiplex genomic DNA PCR assay to study the Valpha repertoire of CD4+CD25+ T cells in normal mice. To address whether CD4+CD25+ T cells express two TCR alpha chains, we have carried out four-colour flow cytometry using combinations of the available anti-Valpha reagents in mice where one allele of the TCRA locus had been inactivated. Results indicate that the Valpha repertoire of CD4+CD25+ T cells is as diverse as their CD25- partners. In addition, CD4+CD25+ T cells develop normally in Tcralpha+/- mice and we show for the first time that despite expressing only one TCRalpha chain, they retain their characteristic CD4(low), CD3(low), TCRbeta(low), CD5(high), CD45RB(low) and cytoplasmic CD152(high) phenotype.

Correlation between disease severity and in vitro cytokine production mediated by MSRV (Multiple Sclerosis associated RetroViral element) envelope protein in patients with multiple sclerosis

MSRV is a retroviral element previously isolated in cell cultures from patients with multiple sclerosis. It is part of a new multi-copy endogenous retrovirus family named HERV-W and displays pro-inflammatory properties both in vitro in human PBMC cultures and in vivo in a humanized SCID mice model. In the present study, we have evaluated potential links between the pro-inflammatory properties of MSRV envelope protein and MS disease. Thus, cytokine productions mediated by the surface unit of MSRV envelope protein were evaluated in PBMC of MS patients and compared with healthy controls. Divergent reactivity to ENV-SU between MS and control PBMC was observed and was reflected by a significant increase of IFN-gamma, IL-6 and IL-12p40 production by the tested MS population. Interestingly, the overproduction of IL-6 and IL-12p40 was found to correlate with disease severity (EDSS) in most patients. Altogether our data suggest that MSRV envelope protein may induce an abnormal cytokine secretion, thus contributing to the inflammatory process in MS.

Standard Nomenclature for the Superantigens Expressed byStaphylococcus

The International Nomenclature Committee for Staphylococcal Superantigens proposes an international procedure for the designation of newly described superantigens and putative superantigens, a procedure that will be compatible with the new age of genomics.

p53-dependent and p53-independent pathways for radiation-induced immature thymocyte differentiation

The pre-T-cell receptor (TCR) delivers essential survival/differentiation signals to the developing thymocytes. Severe combined immunodeficient (SCID) and recombination-activating gene (RAG)-deficient mice are unable to assemble antigen receptor genes, and therefore cannot express a pre-TCR. Consequently, T lymphocyte differentiation is arrested at an early stage in the thymus of these animals, and immature thymocytes are eliminated through apoptotic processes. This maturation arrest can be relieved and thymocyte differentiation rescued after the exposure of these mice to whole-body gamma-irradiation. Whereas the promotion of immature thymocyte survival/differentiation was shown to require p53 activity in irradiated SCID mice, it was suggested, on the other hand, that p53 activation prevents immature thymocytes survival/differentiation in irradiated RAG-deficient mice. However, SCID mice have impaired responses to ionizing radiation. In this paper, we analysed p53 requirement in radiation-induced thymocyte differentiation in CD3epsilon(Delta5/Delta5) mice, where pre-TCR deficiency also results in an early block of lymphocyte development. Our results show at the cellular and molecular levels that, in this DNA repair-proficient model, irradiation-induced thymocyte differentiation proceeds either by a p53-dependent or by a p53-independent pathway, which differ in their sensitivity to the radiation dose delivered.

IMGT/GeneInfo: enhancing V(D)J recombination database accessibility

IMGT/GeneInfo is a user-friendly online information system that provides information on data resulting from the complex mechanisms of immunoglobulin (IG) and T cell receptor (TR) V(D)J recombinations. For the first time, it is possible to visualize all the rearrangement parameters on a single page. IMGT/GeneInfo is part of the international ImMunoGeneTics information system (IMGT), a high-quality integrated knowledge resource specializing in IG, TR, major histocompatibility complex (MHC), and related proteins of the immune system of human and other vertebrate species. The IMGT/GeneInfo system was developed by the TIMC and ICH laboratories (with the collaboration of LIGM), and is the first example of an external system being incorporated into IMGT. In this paper, we report the first part of this work. IMGT/GeneInfo_TR deals with the human and mouse TRA/TRD and TRB loci of the TR. Data handling and visualization are complementary to the current data and tools in IMGT, and will subsequently allow the modelling of V(D)J gene use, and thus, to predict non-standard recombination profiles which may eventually be found in conditions such as leukaemias or lymphomas. Access to IMGT/GeneInfo is free and can be found at http://imgt.cines.fr/GeneInfo.

LAT regulates gammadelta T cell homeostasis and differentiation

LAT (linker for activation of T cells) is essential for T cell receptor signaling. Mice homozygous for a mutation of the three C-terminal LAT tyrosine residues showed a block in alphabeta T cell development and a partially impaired gammadelta T cell development. Without intentional immunization, they accumulated gammadelta T cells in the spleen and lymph nodes that chronically produced T helper type 2 cytokines in large amounts, and caused the maturation of plasma cells secreting immunoglobulin E (IgE) and IgG1. These effects are very similar to that triggered in the alphabeta lineage by a mutation involving a distinct LAT tyrosine. Thus, LAT is an essential regulator of T cell homeostasis and terminal differentiation.

Phenotypic and functional characterization of intrahepatic T lymphocytes during chronic hepatitis C

The pathogenesis of liver cell injury during chronic hepatitis C (CHC) is poorly understood. The cellular immune response is thought to play a key role in both inhibition of viral replication and liver pathology. However, little is currently known about which lymphocyte populations and which immune effectors contribute to or control liver damage. We investigated a panel of 15 phenotypic and functional markers of intrahepatic T-lymphocyte subsets irrespective of their antigen specificity in 48 hepatitis C virus (HCV)-infected patients and 8 healthy control subjects. Lymphocyte characteristics were evaluated from liver biopsy specimens both at gene expression level by real-time quantitative reverse-transcription polymerase chain reaction (RT-PCR) and by immunochemistry, in relation with the degree of liver injury and with intrahepatic HCV-RNA levels. As compared with controls, we found major changes in T-lymphocyte subsets in HCV-infected patients, with a significant decrease of T-cell antigen receptor (TCR) delta and CD56 gene expression, associated with a concomitant increase of TCRalpha and CD8beta that were correlated with cytotoxic factors, proinflammatory chemokines, and chemokine receptors including peforin, interferon gamma (IFN-gamma), tumor necrosis factor alpha (TNF-alpha), RANTES, and CXCR3. The gene expression of CD8beta, a specific marker for conventional TCRalpha+CD8+ lymphocytes, was correlated by multivariate analysis with both alanine aminotransferase (ALT) serum levels and histologic activity index. Furthermore, CD8 staining was observed by immunochemistry in the areas of lobular and piecemeal necrosis. In contrast, no lymphocyte marker was correlated with viral load, measured both in serum and in liver. In conclusion, these results strongly suggest key roles for CD8+ T cells as effectors of liver damage during chronic HCV infection and for their inability to control viral replication.

T lymphocytes infiltrating the liver during chronic hepatitis C infection express a broad range of T-cell receptor beta chain diversity

BACKGROUND/AIMS

During viral chronic hepatitis C (CHC), the intra-hepatic lymphocyte infiltrate is mainly composed of T lymphocytes expressing alphabeta T-cell receptors (TCR). Since little is known about the TCRalphabeta diversity of intra-hepatic T lymphocytes (IHL), we evaluated the IHL repertoire from CHC patients (n=8) as compared to healthy subjects (n=4), total peripheral blood mononuclear cells, and purified peripheral and intra-hepatic CD8(+) cells (n=2).

METHODS

The diversity of TCRalphabeta receptors was evaluated by determining the size and the sequence of the TCRbeta chain complementarity determining region 3 (CDR3). The number of total T lymphocytes in liver was estimated by real-time quantitative reverse transcription-polymerase chain reaction of TCRalpha and CD3 epsilon transcripts.

RESULTS

Our results show that transcripts encoding all TCR V beta (BV) families and all TCR J beta (BJ) segments were present in healthy and CHC livers. No biased TCR repertoire, in terms of preferential BV or BJ gene use or restricted CDR3 sequence, was observed in infected livers. When corrected for equivalent numbers of T lymphocytes, BJ segments utilization and CDR3 length diversity were similar in IHL and PBMC, indicating that the TCRbeta chain diversity is comparable in both cases. In addition, TCR diversity was similar in both peripheral and intra-hepatic CD8+ T cells.

CONCLUSIONS

This study shows limited expansions of intra-hepatic T lymphocytes in CHC patients. The increase of T lymphocytes in infected livers correlates with diversification of TCR, arguing for the establishment of a multi-specific immune response.

Multiple sclerosis-associated retrovirus particles cause T lymphocyte-dependent death with brain hemorrhage in humanized SCID mice model

A retroviral element (multiple sclerosis-associated retrovirus, MSRV) defining a family of genetically inherited endogenous retroviruses (human endogenous retrovirus type W, HERV-W) has been characterized in cell cultures from patients with multiple sclerosis. Recently, MSRV retroviral particles or the envelope recombinant protein were shown to display superantigen activity in vitro, but no animal model has yet been set up for studying the pathogenicity of this retrovirus. In the present study, the pathogenicity of different sources of MSRV retroviral particles has been evaluated in a hybrid animal model: severe combined immunodeficiency (SCID) mice grafted with human lymphocytes and injected intraperitoneally with MSRV virion or mock controls. MSRV-injected mice presented with acute neurological symptoms and died within 5 to 10 days post injection. Necropsy revealed disseminated and major brain hemorrhages, whereas control animals did not show abnormalities (P <.001). In ill animals, reverse transcriptase-polymerase chain reaction (RT-PCR) analyses showed circulating MSRV RNA in serum, whereas overexpression of proinflammatory cytokines such as tumor necrosis factor (TNF)-alpha and interferon (IFN)-gamma was evidenced in spleen RNA. Neuropathological examination confirmed that hemorrhages occurred prior to death in multifocal areas of brain parenchyma and meninges. Further series addressed the question of immune-mediated pathogenicity, by inoculating virion to SCID mice grafted with total and T lymphocyte-depleted cells in parallel: dramatic and statistically significant reduction in the number of affected mice was observed in T-depleted series (P <.001). This in vivo study suggests that MSRV retroviral particles from MS cultures have potent immunopathogenic properties mediated by T cells compatible with the previously reported superantigen activity in vitro, which appear to be mediated by an overexpression of proinflammatory cytokines.

Simultaneous evaluation of lymphocyte subpopulations in the liver and in peripheral blood mononuclear cells of HCV-infected patients: relationship with histological lesions

ABBREVIATIONS

Intrahepatic lymphocytes are believed to be involved in the immunopathogenesis of hepatitis C virus (HCV) infection and the evolution of HCV-induced hepatitis. In the present study, we examined the three main intrahepatic lymphocyte subsets, namely CD3+CD56- conventional T lymphocytes, CD3+CD56+ natural T (NT) lymphocytes and CD3-CD56+ natural killer (NK) lymphocytes in HCV-infected patients. The proportion of each lymphocyte subset was evaluated both in liver biopsies and in samples of peripheral blood lymphocytes (PBL) by flow cytometry in 21 patients with histologically proven chronic hepatitis C. Simultaneously, alanine aminotransferase (ALT) levels, viral load and histological lesions were assessed. Neither NT nor NK populations correlated with any biochemical, viral or histological parameters. Furthermore, Valpha24+ NT lymphocytes showed no preferential enrichment in the liver of HCV-infected patients. Regarding conventional T lymphocytes, a highly significant linear correlation was found between intrahepatic CD3+CD56- T lymphocytes and the Knodell score, a numerical score for assessing histological activity and fibrosis (r = 0.715, P < 0.0001) and more specifically with the periportal necrosis parameter, which is the main lesion of chronic hepatitis C. In addition, analysis of the peripheral compartment revealed a high correlation between values of CD3+CD56- lymphocytes and both Knodell score (r = 0.624, P = 0.003) and serum ALT levels and again with periportal necrosis. The strong correlation between the proportion of peripheral CD3+CD56- conventional T lymphocytes and the severity of hepatic lesions leads us to propose that evaluation of this accessible peripheral population could be used as an indicator test for the severity of histological lesions in chronic hepatitis C.

Quantitative and qualitative changes in V-J alpha rearrangements during mouse thymocytes differentiation: implication for a limited T cell receptor alpha chain repertoire

Knowledge of the complete nucleotide sequence of the mouse TCRAD locus allows an accurate determination V-J rearrangement status. Using multiplex genomic PCR assays and real time PCR analysis, we report a comprehensive and systematic analysis of the V-J recombination of TCR alpha chain in normal mouse thymocytes during development. These respective qualitative and quantitative approaches give rise to four major points describing the control of gene rearrangements. (a) The V-J recombination pattern is not random during ontogeny and generates a limited TCR alpha repertoire; (b) V-J rearrangement control is intrinsic to the thymus; (c) each V gene rearranges to a set of contiguous J segments with a gaussian-like frequency; (d) there are more rearrangements involving V genes at the 3' side than 5' end of V region. Taken together, this reflects a preferential association of V and J gene segments according to their respective positions in the locus, indicating that accessibility of both V and J regions is coordinately regulated, but in different ways. These results provide a new insight into TCR alpha repertoire size and suggest a scenario for V usage during differentiation.

Monocyte chemotactic protein-1 secretion and expression after Toxoplasma gondii infection in vitro depend on the stage of the parasite

Infection of human fibroblasts with tachyzoites of RH and Prugniaud strains, two different strains of Toxoplasma gondii, significantly increased monocyte chemotactic protein (MCP)-1 secretion contrary to what happened with bradyzoites of the cystogenetic strain. Quantification of MCP-1 mRNA by RT-PCR showed that this phenomenon is regulated at the transcriptional level. Thus, the stage of parasite can be deciding in MCP-1 induction since only tachyzoites induced MCP-1 expression and secretion. MCP-1 induced by tachyzoites could be involved in cell recruitment, as shown by the quantification of MCP1 ARNm by real-time PCR (LightCycler, Roche Diagnostics), in the pathogenesis of T. gondii infection.

Immunotherapy by non-myeloablative stem cell transplantation: study of the immune reconstitution. Arguments for distinct cell subsets in skin and blood

INTRODUCTION

Non-myeloablative peripheral stem cell transplantation has been shown to induce tumour rejection in patients with acute leukaemia. However, the immunological mechanisms involved and the immune reconstitution achieved have not been investigated.

MATERIALS AND METHODS

We describe the cases of two patients for whom we have studied the lymphocyte reconstitution achieved, using both phenotypic and genetic analyses of the T-cell repertoire, after peripheral stem cell transplantation.

RESULTS

: In both cases we observed immune reconstitution with T-cell repertoire evolution and presence of activated CD8(+) T cells. In one of the patients an activated clone expressing Vbeta8 represents 46% of the CD8(+) cells. Expansion of this clone occurred in the absence of graft vs host disease symptoms. In the second case a skin lesion typical of graft vs host disease appeared after complete remission had been achieved. The T-cell repertoire in a biopsy of the lesion was distinct from that observed in the blood.

CONCLUSION

Our study indicates that peripheral donor cells can effectively reconstitute a grafted patient while inducing an immune response against antigens expressed by the leukaemic/myeloma cells. Our data provide arguments for different populations of T cells associated with graft vs leukaemia/lymphoma and GVH effects.

Toxoplasma gondii infection can regulate the expression of tumour necrosis factor-alpha receptors on human cells in vitro

The in vitro regulation of tumour necrosis factor (TNF)-alpha receptors during Toxoplasma gondii infection of human MRC5 fibroblasts and human myelomonocytic THP-1 cells was investigated. Cells were infected with the virulent RH of T. gondii. TNFR membrane receptors were analysed by flow cytometry with biotinylated TNF-alpha. Shedding of the soluble form of TNFR1 and TNFR2 in cell culture supernatants was measured by enzyme-linked immunosorbent assay, and expression of mRNA production of TNFR1 and TNFR2 was analysed by quantitative real-time polymerase chain reaction, 1 h after infection. In the MRC5 cell line, T. gondii infection did not induce any up- or down-regulation of membrane TNFRs, soluble TNFRs or mRNA of TNFRs. However, THP-1 cell infection with living parasites induced a significant soluble TNFR1 release by THP-1 cells after 1 h. We detected an approximately 50% up-regulation (P < 0.01) of soluble TNFR1 in infected THP-1 cells compared to controls. No change in soluble TNFR2 levels was observed in the same conditions. Moreover, infection decreased the level of TNF membrane receptors, but had no effect on TNFR1 and TNFR2 mRNA levels. TNFR modulation by T. gondii infection, in vitro, depends on the cell type. Furthermore, our data suggest that living parasites control the shedding of the soluble form of TNFR1. This mechanism may influence the role of TNF-alpha in toxoplasmosis.

TCRA gene rearrangement in immature thymocytes in absence of CD3, pre-TCR, and TCR signaling

During thymocyte differentiation, TCRA genes are massively rearranged only after productively rearranged TCRB genes are expressed in association with pTalpha and CD3 complex molecules within a pre-TCR. Signaling from the pre-TCR via the CD3 complex is thought to be required to promote TCRA gene accessibility and recombination. However, alphabeta(+) thymocytes do develop in pTalpha-deficient mice, showing that TCRalpha-chain genes are rearranged, either in CD4(-)CD8(-) or CD4(+)CD8(+) thymocytes, in the absence of pre-TCR expression. In this study, we analyzed the TCRA gene recombination status of early immature thymocytes in mutant mice with arrested thymocyte development, deficient for either CD3 or pTalpha and gammac expression. ADV genes belonging to different families were found rearranged to multiple AJ segments in both cases. Thus, TCRA gene rearrangement is independent of CD3 and gammac signaling. However, CD3 expression was found to play a role in transcription of rearranged TCRalpha-chain genes in CD4(-)CD8(-) thymocytes. Taken together, these results provide new insights into the molecular control of early T cell differentiation.

Multiple sclerosis retrovirus particles and recombinant envelope trigger an abnormal immune response in vitro, by inducing polyclonal Vbeta16 T-lymphocyte activation

A retroviral element (MSRV) defining a family of genetically inherited endogenous retroviruses (HERV-W) has recently been characterized in cell cultures from patients with multiple sclerosis (MS). To address the possible relationship with MS, direct detection of circulating virion RNA was proposed but revealed technically difficult to perform in standardized conditions, in the face of multiple endogenous HERV-W copies. A parallel approach has evaluated MSRV potential pathogenicity in relation to characteristic features of multiple sclerosis, in particular, T-lymphocyte-mediated immunopathology. We report here that MSRV particles induce T-lymphocyte response with a bias in the Vbeta16 chain usage in surface receptor, whatever the HLA DR of the donor. A recombinant MSRV envelope-but not core-protein reproduced similar nonconventional activation. Molecular analysis of Vbeta CDR3 showed that Vbeta16 expansions are polyclonal. Our results thus provide evidence that MSRV envelope protein can trigger an abnormal immune response with similar characteristics to that of superantigens.

Both TCR alpha and TCR delta chain diversity are regulated during thymic ontogeny

TCRalpha and TCRdelta chains are coded by a common genetic locus using a single set of V gene segments (ADV segments). This article addresses the question of regulation of the use of the ADV segments by the TCRalpha and TCRdelta chains. Using both qualitative and quantitative analyses we have studied the use of 23 ADV gene families as part of TCRalpha and TCRdelta transcripts. A number of previously undetected rearrangement and transcription events are described, indicating that the intrathymic TCRdelta repertoire is much more diverse than previously supposed. Repertoire analysis at several developmental time points allowed the description of regulated waves of ADV gene use, not only for TCRdelta chains, but also for TCRalpha chains, during thymic ontogeny. Control of these waves appears to be linked directly to the ADV segments and their local chromatin environment, which may change over the course of T cell differentiation.

Preferential ADV-AJ association during recombination in the mouse T-cell receptor alpha/delta locus

The gene coding for a T-cell receptor (TCR) alpha chain is assembled from variable (ADV) and joining (AJ) genes located on Chromosome 14. Each of the 90 ADV genes can rearrange with any one of the 61 AJ genes. We have previously demonstrated that ADV and AJ gene segment use evolves with time, with a progressive opening of ADV and AJ regions of the locus. To define the rules governing the use of AJ genes by ADV genes belonging to one family, we carried out a detailed analysis of 268 combinations of ADV2 BALB/c transcripts. We found that the different ADV2 members use different sets of AJ genes depending on their location within the ADV locus: ADV2S7 (the most AJ proximal ADV2 member) rearranges mainly with the AJ genes located close to the TEA element, whereas 50% of the sequences for ADV2S8, which is distal to the AJ locus, use the most distal AJ genes. ADV2S5, an ADV2 member located in the middle of the ADV locus, is associated with a wider set of AJ genes, located in the center of the AJ locus. Taken together, our results indicate that, in addition to the progressive opening of the ADV and AJ loci, the chromosomal location of ADV and AJ genes is a factor affecting AJ use in BALB/c mice.

Pairing of Vbeta6 with certain Valpha2 family members prevents T cell deletion by Mtv-7 superantigen

Superantigens (SAg) are proteins of bacterial or viral origin able to activate T cells by forming a trimolecular complex with both MHC class II molecules and the T cell receptor (TCR), leading to clonal deletion of reactive T cells in the thymus. SAg interact with the TCR through the beta chain variable region (Vbeta), but the TCR alpha chain has been shown to have an influence on the T cell reactivity. We have investigated here the role of the TCR alpha chain in the modulation of T cell reactivity to Mtv-7 SAg by comparing the peripheral usage of Valpha2 in Vbeta6(+) (SAg-reactive) and Vbeta8.2(+) (SAg non-reactive) T cells, in either BALB/D2 (Mtv-7(+)) or BALB/c (Mtv-7(-)) mice. The results show, first, that pairing of Vbeta6 with certain Valpha2 family members prevents T cell deletion by Mtv-7 SAg. Second, there is a strikingly different distribution of the Valpha2 family members in CD4 and CD8 populations of Vbeta6 but not of Vbeta8.2 T cells, irrespective of the presence of Mtv-7 SAg. Third, the alpha chain may play a role in the overall stability of the TCR/SAg/MHC complex. Taken together, these results suggest that the Valpha domain contributes to the selective process by its role in the TCR reactivity to SAg/MHC class II complexes, most likely by influencing the orientation of the Vbeta domain in the TCR alphabeta heterodimer.

TCR alpha-chain repertoire in pTalpha-deficient mice is diverse and developmentally regulated: implications for pre-TCR functions and TCRA gene rearrangement

Pre-TCR expression on developing thymocytes allows cells with productive TCRB gene rearrangements to further differentiate. In wild-type mice, most TCRA gene rearrangements are initiated after pre-TCR expression. However, in pTalpha-deficient mice, a substantial number of alphabeta+ thymocytes are still produced, in part because early TCR alpha-chain expression can rescue immature thymocytes from cell death. In this study, the nature of these TCR alpha-chains, produced and expressed in the absence of pre-TCR expression, have been analyzed. We show, by FACS analysis and sequencing of rearranged transcripts, that the TCRA repertoire is diverse in pTalpha-/- mice and that the developmental regulation of AJ segment use is maintained, yet slightly delayed around birth when compared with wild-type mice. We also found that T cell differentiation is more affected by pTalpha inactivation during late gestation than later in life. These data suggest that the pre-TCR is not functionally required for the initiation and regulation of TCRA gene rearrangement and that fetal thymocytes are more dependent than adult cells on pTalpha-derived signals for their differentiation.

TCR α-Chain Repertoire in pTα-Deficient Mice Is Diverse and Developmentally Regulated: Implications for Pre-TCR Functions and TCRA Gene Rearrangement

Pre-TCR expression on developing thymocytes allows cells with productive TCRB gene rearrangements to further differentiate. In wild-type mice, most TCRA gene rearrangements are initiated after pre-TCR expression. However, in pTα-deficient mice, a substantial number of αβ+ thymocytes are still produced, in part because early TCR α-chain expression can rescue immature thymocytes from cell death. In this study, the nature of these TCR α-chains, produced and expressed in the absence of pre-TCR expression, have been analyzed. We show, by FACS analysis and sequencing of rearranged transcripts, that the TCRA repertoire is diverse in pTα−/− mice and that the developmental regulation of AJ segment use is maintained, yet slightly delayed around birth when compared with wild-type mice. We also found that T cell differentiation is more affected by pTα inactivation during late gestation than later in life. These data suggest that the pre-TCR is not functionally required for the initiation and regulation of TCRA gene rearrangement and that fetal thymocytes are more dependent than adult cells on pTα-derived signals for their differentiation.

Use of TCR ADV gene segments by the delta chain is independent of their position and of CD3 expression

The CD3 signaling complex is required for cell surface expression and selection of both alphabeta and gammadelta TCR. In this study we analyzed TCRD transcripts in both wild-type and CD3-epsilon-deficient mice. We show that the repertoire of ADV segments used by the delta chain is unchanged in the latter. Not all ADV genes participate in making up the TCRD repertoire. However, their use does not depend on their distance from the other TCRD-forming segments. For example ADV12, situated at more than 870 kb from the DD region, is expressed as part of TCRD transcripts, whereas ADV8, members of which are proximal to the DD region, is not. These data suggest that the accessibility of ADV8 gene segments is differentially regulated during T cell development in the thymus. Taken together, our results suggest that TCRA and TCRD rearrangements are independently controlled, and that the absence of TCRA expression in CD3-epsilon-deficient mice is not due to a lack of accessibility of the ADV gene segments but rather to inaccessibility of the AJ gene region.

Differential chronology of TCRADV2 gene use by alpha and delta chains of the mouse TCR

The genes coding for TCR alpha and delta chains share the same genetic locus (TCRA/D). The rules governing the utilization of a V gene with the alpha and delta chains have not been established. More specifically, it is not known whether the position of a gene within the locus influences its utilization in alpha and delta TCR. To elucidate these points, we mapped ADV2 genes in the TCRA/D locus of BALB/c mice and analyzed their utilization in TCR alpha and delta transcripts from thymi isolated from mice of different ages. Our results show that all ADV2 genes can be used by the two chains, but with strikingly different patterns. Moreover, ADV2 utilization by the alpha chain proceeds in successive concentric waves during development, suggesting a progressive regulation of gene accessibility and utilization. These results support independent control of TCRA and TCRD gene assembly.