Regulation of the early stages of T-cell development

High throughput technologies for the functional discovery of mammalian enhancers: new approaches for understanding transcriptional regulatory network dynamics

Completion of the human and mouse genomes has inspired new initiatives to obtain a global understanding of the functional regulatory networks governing gene expression. Enhancers are primary regulatory DNA elements determining precise spatio- and temporal gene expression patterns, but the observation that they can function at any distance from the gene(s) they regulate has made their genome-wide characterization challenging. Since traditional, single reporter approaches would be unable to accomplish this enormous task, high throughput technologies for mapping chromatin features associated with enhancers have emerged as an effective surrogate for enhancer discovery. However, the last few years have witnessed the development of several new innovative approaches that can effectively screen for and discover enhancers based on their functional activation of transcription using massively parallel reporter systems. In addition to their application for genome annotation, these new high throughput functional approaches open new and exciting avenues for modeling gene regulatory networks.

T cell receptor delta gene rearrangement and T early alpha (TEA) expression in immature alpha beta lineage thymocytes: implications for alpha beta/gamma delta lineage commitment

Mature T cells comprise two mutually exclusive lineages expressing heterodimeric alpha beta or gamma delta antigen receptors. During development, beta, gamma, and delta genes rearrange before alpha, and mature gamma delta cells arise in the thymus prior to alpha beta cells. The mechanism underlying commitment of immature T cells to the alpha beta or gamma delta lineage is controversial. Since the delta locus is located within the alpha locus, rearrangement of alpha genes leads to deletion of delta. We have examined the rearrangement status of the delta locus immediately prior to alpha rearrangement. We find that many thymic precursors of alpha beta cells undergo VDJ delta rearrangements. Furthermore, the same cells frequently coexpress sterile T early alpha (TEA) transcripts originating 3' of C delta and 5' of the most upstream J alpha, thus implying that individual alpha beta lineage cells undergo sequential VDJ delta and VJ alpha rearrangements. Finally, VDJ delta rearrangements in immature alpha beta cells appear to be random, supporting models in which alpha beta lineage commitment is determined independently of the rearrangement status at the TCR delta locus.

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.

Development of gamma delta T cells in the adult murine thymus

The development of T cells belonging to the gamma delta lineage is not well understood. We have analyzed the cells in the adult murine thymus which express the gamma delta TcR on the surface in order to learn more about this process. Our data demonstrate a number of clear subpopulations of gamma delta expressing cells in the thymus based on the expression of Thy-1 and HSA (heat-stable antigen). Only one of these subpopulations, the one expressing both Thy-1 and HSA, contains dividing cells or has a significant rate of turnover. Together with the fact that emigrant gamma delta cells are HSA+Thy-1+, this suggests that this thymic subpopulation is the sole, or major, source of exported cells. However, the turnover of cells from this population is 5 x 10(4) - 10 x 10(4) cells per day, while previous estimates of the rate of export of gamma delta cells are in the order of 10(4) cells per day. Furthermore the V gamma profile of recent gamma delta+ emigrants differs from that of the thymic HSA+Thy-1+ cells. This raises the possibility that only a selected subpopulation of the thymic gamma delta+HSA+Thy-1+ population is exported, and that some gamma delta cells may die in situ in the thymus. The function of the other gamma delta thymic subpopulations, which are turning over very slowly or not at all, (i.e. the HSA-Thy-1- and HSA-Thy-1+ subpopulations) remains unclear.

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.