Direct regulation of TCR rearrangement and expression by E proteins during early T cell development

Abstract

γδ T cells are widely distributed throughout mucosal and epithelial cell-rich tissues and are an important early source of IL-17 in response to several pathogens. Like αβ T cells, γδ T cells undergo a stepwise process of development in the thymus that requires recombination of genome-encoded segments to assemble mature T cell receptor (TCR) genes. This process is tightly controlled on multiple levels to enable TCR segment assembly while preventing the genomic instability inherent in the double-stranded DNA breaks that occur during this process. Each TCR locus has unique aspects in its structure and requirements, with different types of regulation before and after the αβ/γδ T cell fate choice. It has been known that Runx and Myb are critical transcriptional regulators of TCRγ and TCRδ expression, but the roles of E proteins in TCRγ and TCRδ regulation have been less well explored. Multiple lines of evidence show that E proteins are involved in TCR expression at many different levels, including the regulation of Rag recombinase gene expression and protein stability, induction of germline V segment expression, chromatin remodeling, and restriction of the fetal and adult γδTCR repertoires. Importantly, E proteins interact directly with the cis-regulatory elements of the TCRγ and TCRδ loci, controlling the predisposition of a cell to become an αβ T cell or a γδ T cell, even before the lineage-dictating TCR signaling events. This article is categorized under: Immune System Diseases > Stem Cells and Development Immune System Diseases > Genetics/Genomics/Epigenetics.

Keywords: E2A; T cell receptor; gamma delta T cell; gene rearrangement; thymus.

Figure 1.

Stochastic patterns of TCR rearrangement can lead to γδ T cells with TCRβ locus rearrangements and αβ T cells with rearrangements in the TCRγ locus. The TCRγ (pink) and the TCRδ (orange) loci usually rearrange at the DN2 stage, whereas the TCRβ locus (blue) undergoes rearrangement primarily in DN3 cells. A. Selection of γδ T cells prior to TCRβ locus rearrangement. B. Selection of γδ T cells after TCRβ locus rearrangement. C. Selection of αβ T cells after non-productive TCRγ and/or TCRδ locus rearrangement. D. Selection of αβ T cells before TCRγ or TCRδ locus rearrangement. E. Restricted timing of TCRβ, TCRδ, TCRγ, and TCRα rearrangement. Colored dashed lines indicate genomic rearrangement, colored circles indicate cell surface expression. Lightning signs indicate TCR signaling, green for γδTCR signaling, yellow for pre-TCR signaling, and gray for αβTCR signaling.

Figure 2.

E protein transcription factors E2A and/or HEB form dimers that bind the “E-box” sequence CANNTG and activate transcription. A. Binding of E protein dimers drives expression of E protein target genes, which include many T-lineage genes. B. Id proteins disrupt E protein dimers to form inactive dimers that cannot bind DNA, causing the loss of E protein target gene expression. Green oval=E protein, red oval = Id protein.

Figure 3.

TCR signaling causes downregulation of E protein target genes by upregulating Id3 in response to the RAS-ERK-EGR signaling pathway.

Figure 4.

Germline configuration of the mouse TCRβ locus. Vertical lines indicate the positions of elements or groups of elements. V elements=Trbv, J elements=Trbj, C exon=Trbc. Yellow oval = TCRβ enhancer (E_β).

Figure 5.

Germline configuration of the mouse TCRγ locus. Vertical lines indicate the placement of elements or exons. V elements=Trgv, J elements=Trgj, C exons=Trgc. Yellow ovals = enhancers. Pink oval = promoter.

Figure 6.

Germline configuration of the mouse TCRα/δ locus. Vertical lines on the top indicate V elements that are specific to TCRδ (Trdv) rearrangement (purple or green text) and those that can be used during either TCRα or TCRδ rearrangement (black text). The blue bars on bottom indicate positional groupings of the Trav elements. Purple=Trdv elements expressed in adult thymus. Green=Trdv elements expressed in fetal thymus. Brown=Trdv elements expressed in fetal and adult thymus. Pink oval=promoter, yellow ovals=enhancers. Note that this structure represents the TCRδ locus in the 129 mouse strain. While there are some differences, the general structure and regulatory elements of the TCRδ locus are conserved in the C57Bl/6 mouse strain.

Figure 7.

Differential control of TCRγ and TCRδ loci is regulated by Runx factors, Myb, and E2A, in combination with signaling through Notch and IL-7 receptors, respectively. Core regions of TCRγ Cluster 1 (A) and the TCRδ locus (B). Vertical arrow downward = positive regulation, flat horizontal line = negative regulation. Key for symbols is shown to the right.