RUNX proteins in transcription factor networks that regulate T-cell lineage choice

Abstract

Recent research has uncovered complex transcription factor networks that control the processes of T-cell development and differentiation. RUNX (runt-related transcription factor) proteins are among the many factors that have crucial roles in these networks. In this Review, we examine the mechanisms by which RUNX complexes act together with other transcription factors, such as Th-POK (T-helper-inducing POZ/Kruppel-like factor) and GATA-binding protein 3 (GATA3) in determining the CD4/CD8 lineage choice of developing thymocytes. In addition, we discuss evidence indicating that RUNX complexes are also involved in the differentiation of effector T-cell subsets and that the molecular mechanisms by which RUNX proteins regulate T-cell fate decisions are conserved between the thymus and periphery.

Figure 1. The structure of Cd4 and Cd8 loci, illustrating important cis-acting elements

a | The expression of CD4 is driven by the Cd4 proximal enhancer in all T-cell populations and is restricted to the double-positive and CD4 single-positive thymocyte populations owing to the activity of the Cd4 silencer, which is found in intron 1. The binding of RUNX (runt-related transcription factor) proteins to the Cd4 silencer is necessary for silencer activity. b | The expression of Cd8ab is driven by a series of stage-specific enhancers (known as E8_I – E8_IV), which have overlapping activity.

Figure 2. The structure of the Zbtb7b locus, illustrating important cis-acting elements

The Zbtb7b locus encodes Th-POK (T-helper-inducing POZ/Kruppel-like factor). The Zbtb7b silencer (designated the distal response element; DRE or RUNX (runt-related transcription factor)-binding site 1 (RBS-1)) restricts the expression of Th-POK to MHC class II-specific thymocytes. The binding of RUNX to this region is necessary but not sufficient for silencer activity. The Zbtb7b silencer region also contains enhancer activity which, together with the general T-lymphoid element (GTE) may be involved in initiating the expression of Th-POK. The proximal enhancer (PE; also known as the proximal regulatory element (PRE) and/or RUNX- binding site 2 (RBS-2)^,) is not required to initiate the expression of Th-POK following positive selection, but is required to maintain Th-POK expression in MHC class II-selected thymocytes that are destined to become CD4⁺ T cells. Loss of the PE in the Zbtb7b locus leads to gradual loss of Th-POK expression and thereby causes partial redirection of MHC class II-selected thymocytes to the CD8 lineage. GATA3, GATA-binding protein 3.

Figure 3. The transcription factor network that underlies T-cell lineage choice

Th-POK (T-helper-inducing POZ/Kruppel-like factor) ‘seals’ CD4⁺ T-cell fate, which is specified by GATA-binding protein 3 (GATA3) and thymocyte selection-associated high-mobility group box (TOX), and represses CD8⁺ T-cell fate in part by repressing the expression of RUNX3 (runt-related transcription factor 3). In CD8⁺ T cells, RUNX3 maintains silencing of Th-POK expression, thereby repressing the CD4⁺ T-cell fate and irreversibly committing cells to the CD8 lineage. MYB is involved in promoting CD4 lineage choice, possibly by inducing the expression of GATA3 following T-cell-receptor activation.

Figure 4. Transcription factor networks that underlie the differentiation of peripheral naive CD4⁺ T cells into T-helper- and regulatory T-cell subsets

RUNX3 (runt-related transcription factor 3) represses the production of interleukin-4 (IL-4) by binding to the Il4 silencer in the T helper 2 (T_H2) cytokine locus while cooperating with T-bet to promote the production of interferon-γ (IFNγ) in T_H1 cells. GATA-binding protein 3 (GATA3), which is required for the expression of T_H2-type cytokines (encoded by the T_H2-cytokine locus), represses the expression of the T_H1-cell-associated gene Il12rb2 in T_H2 cells. In T_H17 cells, RUNX1 cooperates with the transcription factor retinoic-acid-receptor-related orphan receptor-γt (RORγt) to induce the production of IL-17. In regulatory T (T_Reg) cells, forkhead box P3 (FOXP3) represses the RUNX1-mediated production of IL-2.