CD4-CD8 lineage differentiation: Thpok-ing into the nucleus

Abstract

The mature alphabeta T cell population is divided into two main lineages that are defined by the mutually exclusive expression of CD4 and CD8 surface molecules (coreceptors) and that differ in their MHC restriction and function. CD4 T cells are typically MHC-II restricted and helper (or regulatory), whereas CD8 T cells are typically cytotoxic. Several transcription factors are known to control the emergence of CD4 and CD8 lineages, including the zinc finger proteins Thpok and Gata3, which are required for CD4 lineage differentiation, and the Runx factors Runx1 and Runx3, which contribute to CD8 lineage differentiation. This review summarizes recent advances on the function of these transcription factors in lineage differentiation. We also discuss how the "circuitry" connecting these factors could operate to match the expression of the lineage-committing factors Thpok and Runx3, and therefore lineage differentiation, to MHC specificity.

Figure 1. Thpok, Gata3 and Runx3 expression and function during lineage differentiation

T cell differentiation stages are depicted on a schematic two-parameter plot of CD4 and CD8 expression. Expression levels of Gata3, Thpok and Runx3 are summarized from negative to high. Checkpoints controled by each factor are indicated (arrows). CD4-differentiating cells are shown in purple, CD8-differentiating cells in green. There is no experimental distinction yet between specification and commitment steps during CD8 lineage differentiation. Although inactivation of both Runx1 and Runx3 is required to prevent CD8 cell development, this presumably reflects functional redundancy between these factors; thus, Runx3 is indicated as the ‘check-point keeper’.

Figure 2. Runx3 and Thpok loci

(A). Schematic representation of the Runx3 locus showing the distal and proximal promoters (arrows), and the splicing from exon 1 (distal promoter) into the coding sequence of exon 2. As a result, Runx3 proteins translated from either mRNA only differ by their amino-terminal extremity (bottom graphs, cyan and yellow coloring). Dashed lines indicate splicing. Exons are shown as boxes, with coding sequences depicted as thicker rectangles. (B). The Thpok locus is schematically depicted. Coding sequences are in exons 2 and 3 and depicted as thick orange rectangles. Exons 1a and 1b are transcribed from alternative start sites. The proximal (3′) site is used preferentially in mature CD4 T cells, whereas the distal site is preferentially active in early thymocytes (57). Binding sites for Gata3, Runx and Thpok proteins are shown as boxes underneath the sequence. Cis-regulatory elements identified by knockout or transgenic reporter analyses are indicated at the bottom of the graph and include the silencer, part of the distal regulatory element, the general T lymphoid element (GTE), active throughout T cell development (57), and the proximal enhancer (or proximal regulatory element [PRE], Refs. 19, 57). Knockout analyses have shown that the proximal enhancer is required for sustained Thpok transcription in mature CD4-lineage thymocytes and T cells (48). Deletion of the segment encompassing both Gata3 sites (and the proximal enhancer) severely reduced Thpok transcription in BAC reporter analyses (bracket) (40). Drawings are not on scale.

Figure 3. Transcriptional ‘circuitry’ controling CD4-CD8 lineage differentiation

Transcription factors and selected target genes expressed in CD4-differentiating (purple) or CD8-differentiating (green) or in a non-lineage specific manner (black) are connected in a draft transcriptional regulatory network controling lineage differentiation. Connections depict a functionally relevant gene activation (arrow-ending lines) or inhibition (bar-ending lines). Plain lines indicate direct transcription factor binding validated by ChIP assays. Connections between Thpok and Runx are shown in red, and the Gata3-Thpok link in blue. Note that the repression of Gata3 by Runx1 has so far been documented in peripheral cells only. ‘X’ refers to putative transcription factors required for Runx-mediated Cd4 silencing. The antagonism by Thpok of Runx-mediated Cd4 repression involves direct binding of Thpok to the Cd4 silencer (48), presumably protecting the silencer from Runx-mediated repression, and has also been proposed (71) to involve the Thpok-mediated repression of putative ‘X’ factors that contribute with Runx proteins to repress Cd4 and Thpok expression.