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T Norton

Publications and source records attributed to T Norton.

At least 19 recordsLinked to original sources
PubMed2001

Elevated Bcl-2 is not a causal event in the positive selection of T cells.

T cell development is characterized by the induction of apoptosis in most immature thymocytes and the rescue from apoptosis of a small proportion of cells by the process of positive selection.Up-regulation of the anti-apoptotic molecule Bcl-2 is associated with thymocytes undergoing positive selection and a bcl-2 transgene promotes the generation of mature T cells. In contrast,mice transgenic for the pro-apoptotic molecule Bax show impaired T cell maturation. We have used fetal thymic organ culture to determine the action of Bcl-2 and Bax on positive selection of thymocytes. Our data show that Bcl-2 and Bax do not alter the number of thymocytes positively selected by a defined peptide ligand. This implies that Bcl-2 and Bax alter the production of mature T cells in vivo by influencing thymocyte viability rather than by direct action on positive selection. It also presents a solution to the 'chicken-and-egg' scenario relating to Bcl-2 up-regulation and positive selection. The data suggest that the up-regulation of Bcl-2 associated with T cell maturation is a consequence of positive selection rather than a cause of it.

Animals↗
PubMed2001

Watch out guinea pigs, here I come.

We live in an age of increasing emphasis of do-it-yourself, as a mere glance at the TV schedule will prove. Why not apply this same principle to your research? By becoming the guinea pig of your own experimentation you will be following a noble precedent--though maybe not a sane one!

Animals↗
PubMed2000

Early onset of CD8 transgene expression inhibits the transition from DN3 to DP thymocytes.

In this paper we show that the effects of transgenic coreceptor expression on thymocyte development depend on the onset of transgene expression. Thus, a CD8 transgene expressed on CD44+CD25+ (DN2) and CD44-CD25+ (DN3) cells causes a partial block at the stage when TCRbeta selection takes place and diminishes expansion at the subsequent developmental stages, resulting in increased DN3 and markedly reduced double-positive (DP) thymocyte numbers. This effect is evident on a polyclonal TCR repertoire as well as in TCR-transgenic mice (F5). By contrast, a CD8 transgene that leads to the same degree of overexpression on DP thymocytes, but is not expressed on double-negative subsets, has no effect on thymus size or composition. Therefore, the reduction of DP thymocyte numbers in CD8 TCRtg mice can be attributed to interferences at early developmental stages rather than to increased negative selection of DP cells.

Animals↗
PubMed2000

Activation of Cdk2 is a requirement for antigen-mediated thymic negative selection.

Apoptosis plays a critical role in T cell development and thymic selection. Thymocytes which undergo antigen-induced negative selection have been demonstrated to die by apoptosis. Despite this, relatively little is known about the specific apoptotic pathway involved in negative selection. We have examined the role of cyclin-dependent kinase 2 (Cdk2), a key regulator of thymocyte apoptosis, in this process. Stimulation of thymocytes with cognate antigen leads to a large increase in Cdk2 kinase activity. We also show that pharmacological inhibitors of Cdk2 block thymocyte apoptosis in response to antigen. Our data show that Cdk2 activity is essential for the apoptotic pathway used in negative selection.

Animals↗
PubMed2000

Inducible expression of a p56Lck transgene reveals a central role for Lck in the differentiation of CD4 SP thymocytes.

The T lymphocyte-specific protein tyrosine kinase p56lck (Lck) is an essential component of the TCR-mediated signal transduction complex. Lck knockout mice have reduced numbers of double-positive thymocytes and very few mature single-positive cells, particularly of the CD4 lineage. Here we demonstrate the ability of a tetracycline-based tissue-specific inducible Lck transgene to restore expansion of early thymocytes and maturation of single-positive cells in Lckneg mice upon induction with doxycycline. Restoration of Lck expression is particularly important for positive selection to the CD4+ lineage but has a lesser impact on selection to the CD8+ lineage, suggesting activation of Lck is an important component of the signals involved in lineage choice during thymic differentiation.

Animals↗
PubMed2000

Long term follow up of the utility of troponin T to assess cardiac risk in stable chronic hemodialysis patients.

Thirty long-term, stable hemodialysis patients were followed 24 months to identify any predictable relationship between elevated serum cTnT values and the diagnosis of coronary artery disease and/or the occurrence of a cardiac death. Patients with a baseline cTnT value of >0.1 microg/L were at high risk for life-threatening cardiac events during the 2 years follow-up. With regard to predicting a cardiac event, cTnT has a specificity of 93.75% and sensitivity of 81.8% compared to cTnI whose specificity was 87.5% but sensitivity of between 9.1 and 18.2%. CK-MB was the most specific at 100% but had a low sensitivity of 9.1%. The hemodialysis process, while causing an increase in the serum levels of all the markers studied except CK, the increase only proved significant for cTnT. The only markers whose stratification remained consistent over the 2 years where cTnT and CK-MB, for all others a gain or lose was registered. Baseline stratification using cTnT with a cut-off value of >0.1 microg/L offers opportunities to select at risk hemodialysis patients for corrective cardiovascular intervention.

Adult↗
PubMed1999

Human HMG box transcription factor HBP1: a role in hCD2 LCR function.

The locus control region (LCR) of the human CD2 gene (hCD2) confers T cell-specific, copy-dependent and position-independent gene expression in transgenic mice. This LCR consists of a strong T cell-specific enhancer and an element without enhancer activity (designated HSS3), which is required for prevention of position effect variegation (PEV) in transgenic mice. Here, we identified the HMG box containing protein-1 (HBP1) as a factor binding to HSS3 of the hCD2 LCR. Within the LCR, HBP1 binds to a novel TTCATTCATTCA sequence that is higher in affinity than other recently reported HBP1-binding sites. Mice transgenic for a hCD2 LCR construct carrying a deletion of the HBP1-binding sequences show a propensity for PEV if the transgene integrates in a heterochromatic region of the chromosome such as the centromere or telomere. We propose that HBP1 plays an important role in chromatin opening and remodelling activities by binding to and bending the DNA, thus allowing DNA-protein and/or protein-protein interactions, which increase the probability of establishing an active locus.

Amino Acid Sequence↗
PubMed1999

Th cells and Th2 responses can develop in the absence of MHC class II-CD4 interactions.

In this paper, we address the question whether CD4 and MHC class II expression are necessary for the development of the T helper lineage during thymocyte maturation and for activation-induced Th2 responses. To bypass the CD4-MHC class II interaction requirements for positive selection and activation, we used mice that are doubly transgenic for CD8 and for the MHC class I-restricted TCR F5. This transgene combination leads to MHC class I-dependent maturation of CD4 lineage cells. Upon activation, these CD4 lineage T cells secrete IL-4 and give help to B cells but show no cytotoxic activity. Remarkably, neither MHC class II nor CD4 expression are necessary for the generation and helper functions of these cells. This suggests that under normal conditions, coreceptor-MHC interactions are necessary to ensure the canonical combinations of coreceptor and function in developing thymocytes, but that they do not determine functional commitment. Our results also imply that expression of the CD4 gene does not influence, but is merely associated with the decision to establish the T helper program. In addition, we show that activation through TCR-MHC class I interactions can induce Th2 responses independently of CD4 and MHC class II expression.

Animals↗
PubMed1999

Bad can act as a key regulator of T cell apoptosis and T cell development.

Bad is a distant relative of Bcl-2 and acts to promote cell death. Here, we show that Bad expression levels are greatly increased in thymocytes during apoptosis. We generated bad transgenic mice to study the action of upregulated Bad expression on T cell apoptosis. The T cells from these mice are highly sensitive to apoptotic stimuli, including anti-CD95. The numbers of T cells are greatly depleted and the processes of T cell development and selection are perturbed. We show that the proapoptotic function of Bad in primary T cells is regulated by Akt kinase and that Bad overexpression enhances both cell cycle progression and interleukin 2 production after T cell activation. These data suggest that Bad can act as a key regulator of T cell apoptosis and that this is a consequence of its upregulation after exposure to death stimuli.

Animals↗
PubMed1999

Heterochromatin protein 1 modifies mammalian PEV in a dose- and chromosomal-context-dependent manner.

Locus control regions (LCRs) are gene regulatory elements in mammals that can overcome the highly repressive effects normally associated with heterochromatic transgene locations (for example the centromere) in mice. Deletion of essential LCR sequences renders the cognate gene susceptible to this form of repression, so a proportion of the cells from transgenic mice that would normally express the transgene are silenced-a phenomenon known as position effect variegation (PEV). We show here that PEV can also occur when the transgene is non-centromeric and that the extent of variegation can be developmentally regulated. Furthermore, by overexpressing a mammalian homologue (M31) of Drosophila melanogaster heterochromatin protein 1 (HP1; refs 7,8) in transgenic mouse lines that exhibit PEV, it is possible to modify the proportion of cells that silence the transgene in a dose-dependent manner. Thus, we show M31 overexpression to have two contrasting effects which are dependent on chromosomal context: (i) it enhanced PEV in those lines with centromeric or pericentromeric transgene locations; and (ii) it suppressed PEV when the transgene was non-centromeric. Our results indicate that components or modifiers of heterochromatin may have a chromosomal-context-dependent role in gene silencing and activation decisions in mammals.

Amino Acid Sequence↗
PubMed1999

Hypospadias in a polar bear (Ursus maritimus).

Corrective surgery on a 1.5-yr-old male polar bear (Ursus maritimus) with hypospadias included amputation of the vestigial penis, bilateral orchiectomy with scrotal ablation, and distal perineal urethrostomy. Hypospadias in other species is a congenital deformity that may be caused by extra- and intrauterine factors resulting in a disruption of the testosterone balance during urethral development, but the causative mechanism in this bear is unknown. The urethrostomy site was functional without complications 8 mo after surgery.

Animals↗
PubMed1998

The action of Bax and bcl-2 on T cell selection.

T cell development and selection in the thymus are shaped by the induction of apoptosis. However, a direct role in T cell development and selection for any of the molecules known to regulate apoptosis has remained controversial. We have studied the effect of bax and bcl-2 transgenes in recombination activation gene 1-deficient (RAG-1(-/-)) mice transgenic for the major histocompatibility complex class I-restricted F5 T cell receptor. Overexpression of a bax transgene in the thymus seriously impairs the production of mature T cells, whereas bcl-2 overexpression greatly promotes it. The effect of bax and bcl-2 overexpression on antigen-induced negative selection was studied using fetal thymic organ cultures. This analysis showed that Bcl-2 strongly inhibits negative selection, whereas Bax does not affect it. Our data directly show that Bcl-2 family members have specific roles in T cell selection and also lend support to the hypothesis that Bax and Bcl-2 can antagonize each other's action in a certain apoptosis pathway while in another they can be functionally nonreciprocal.

Animals↗
PubMed1998

Altered peptide ligands induce quantitatively but not qualitatively different intracellular signals in primary thymocytes.

Interaction of the T cell receptor (TCR) with peptide/major histocompatibility complexes (MHC) in the thymus is of critical importance for developing thymocytes. In a previous study, we described an antagonist peptide that inhibited negative selection of transgenic thymocytes induced by an agonist peptide. In this study we show that this antagonist peptide can induce positive selection of CD8(+) thymocytes more efficiently than the agonist or the weak agonist peptides, whereas the opposite is true for their ability to cause negative selection. The intracellular signals induced in thymocytes by such peptides after TCR ligation was examined in CD4(+)8(+) double-positive thymocytes from F5/beta2mo/Rag-1(o) transgenic mice. TCR ligation with either the agonist, weak agonist, or antagonist peptide variants resulted in hyperphosphorylation of CD3zeta, CD3epsilon, ZAP-70, Syk, Vav, SLP-76, and pp36-38. The extent of phosphorylation of these intracellular proteins correlated with the efficiency with which the peptide analogs induced apoptosis of immature thymocytes. Unexpectedly, there was no correlation between the upstream TCR signaling pathways analyzed and the capacity of the different peptides to induce positive selection.

Animals↗
PubMed1998

Hierarchical interactions of control elements determine CD8alpha gene expression in subsets of thymocytes and peripheral T cells.

CD4 and CD8 are crucial for the development and function of T cells. An intergenic deoxyribonuclease I hypersensitive site region (cluster CIII) directs expression in mature CD8 T cells only. Here, we show that two further independent regions from the CD8 gene locus in conjunction with cluster CIII restore transgene expression in appropriate immature thymocytes. Deletion of two of the intergenic cluster CIII DNaseI-HSS in homozygous mutant mice affects expression of CD8alphaalpha homodimers on intraepithelial T cells (IEL), particularly on the gammadeltaTCR+ subset. Surprisingly, none of the thymocyte or peripheral alphabetaTCR T cell subsets are affected by this mutation, indicating hierarchical activation of these elements within the different T cell subsets.

Alleles↗
PubMed1998

Troponin T, a predictor of death in chronic haemodialysis patients.

Of the 10 chronic haemodialysis patients whose serum TnT levels exceeded the threshold value of 0.1 microg.L(-1) at entry into the study, four were dead at 1 year and three others had a diagnosis of CAD. Of the 20 chronic haemodialysis patients with normal serum TnT levels at entry, one died and none had CAD. All five deaths were cardiac related, either arising from acute myocardial infarction or by sudden death. When serum TnT levels were compared with accepted predictors of death in chronic haemodialysis patients, such as serum creatinine, serum albumin and haematocrit, in the present study serum TnT proved to be more accurate and had excellent sensitivity and specificity. Serum TnT was also superior to serum TnI, which proved to be no more discriminating than the non-specific muscle marker, aldolase.

Adult↗
PubMed1997

A region in the CD8 gene locus that directs expression to the mature CD8 T cell subset in transgenic mice.

The coreceptors CD4 and CD8 play a crucial role during thymocyte development and T cell effector function, and their expression is developmentally regulated. To determine the underlying molecular mechanisms of CD8 gene regulation we cloned the murine CD8 gene locus from genomic libraries and analyzed this region for deoxyribonuclease (DNase I) hypersensitive sites (HSS). Here we report, using transgenic mice, deletion analysis of one of the identified clusters of DNase I hypersensitivity, consisting of three DNase I-HSS and located in the intergenic region between the CD8alpha and CD8beta genes. Our data show that at least two of the DNase I-HSS constituting this cluster are individually sufficient to direct CD8alpha or heterologous transgene expression to the mature CD8 single-positive T cell subset and that this expression coincides temporally with the appearance of positively selected T cells.

Animals↗
PubMed1997

A function for CD2 on murine B cells?

CD2 is expressed on murine B cells, probably as a result of a chromosomal translocation event during speciation. There are no activating antibodies to mouse CD2, but when activated non-specifically, only T cells up-regulate their expression of CD2. We investigated the expression and function of CD2 on B cells using mice transgenic for human CD2 under the control of a modified version of the autologous promoter/enhancer that included the immunoglobulin enhancer. This construction directs expression of human CD2 to the B cell compartment as well as to the T cell compartment. In this paper, we confirm that activating pairs of anti-CD2 monoclonal antibodies are mitogenic for mouse T cells transgenic for human CD2. In contrast, mouse B cells that express similar amounts of human CD2 are not stimulated to proliferate by equivalent doses of these antibodies. We were also unable to show any functional consequence for these B cells as a result of CD2 ligation.

Adjuvants, Immunologic↗