Ikaros Zinc Finger Transcription Factors: Regulators of Cytokine Signaling Pathways and CD4+ T Helper Cell Differentiation

Abstract

CD4⁺ T helper cells are capable of differentiating into a number of effector subsets that perform diverse functions during adaptive immune responses. The differentiation of each of these subsets is governed, in large part, by environmental cytokine signals and the subsequent activation of downstream, cell-intrinsic transcription factor networks. Ikaros zinc finger (IkZF) transcription factors are known regulators of immune cell development, including that of CD4⁺ T cell subsets. Over the past decade, members of the IkZF family have also been implicated in the differentiation and function of individual T helper cell subsets, including T helper 1 (T_H1), T_H2, T_H17, T follicular (T_FH), and T regulatory (T_REG) cells. Now, an increasing body of literature suggests that the distinct cell-specific cytokine environments responsible for the development of each subset result in differential expression of IkZF factors across T helper populations. Intriguingly, recent studies suggest that IkZF members influence T helper subset differentiation in a feed-forward fashion through the regulation of these same cytokine-signaling pathways. Here, we review the increasingly prominent role for IkZF transcription factors in the differentiation of effector CD4⁺ T helper cell subsets.

Keywords: CD4+ T helper cells; Ikaros zinc finger (IkZF) transcription factors; cytokines; differentiation; gene regulation.

Figure 1

Structure of Ikaros Zinc Finger transcription factor family members. Members of the Ikaros Zinc Finger (IkZF) family of transcription factors are known regulators of hematopoietic cell development, including that of CD4⁺ T cells. The IkZF family consists of five members: Ikaros (encoded by the gene Ikzf1), Helios (Ikzf2), Aiolos (Ikzf3), Eos (Ikzf4), and Pegasus (Ikzf5). These factors contain N-terminal zinc finger (ZF) domains, which are responsible for mediating direct interactions with DNA, and C-terminal ZFs, which facilitate homo- and heterodimerization between IkZF family members. Of the N-terminal zinc fingers, ZF2 and ZF3 mediate direct interaction with the core IkZF DNA binding motif (GGGAA), while ZF1 and ZF4 regulate factor binding site/target gene specificity.

Figure 2

Regulation of CD4⁺ T cell differentiation programs by IkZF family members. Tabulated summary of the known effects of IkZF family members on expression of the T_H1, T_H2, T_H17, T_FH, and T_REG gene programs, including associated cytokines and transcription factors. The broad impact of each IkZF factor on the differentiation program of individual T helper cell subsets is also provided, if known.

Figure 3

Transcriptional regulation of the interleukin-2 locus by IkZF transcription factors. Signals from the pro-inflammatory cytokine interleukin-2 (IL-2) differentially regulate the expression of T helper cell programs. IL-2 signaling supports the differentiation of T_H1, T_H2, and T_REG cell subsets, but represses the differentiation of T_H17 and T_FH populations. The Ikaros zinc finger family members Ikaros, Helios, Aiolos, and Eos have all been implicated in regulating IL-2 expression. (A) In anergic CD4⁺ and T_H17 cells, respectively, Ikaros and Aiolos have been shown to directly associate with the Il2 promoter to repress IL-2 expression. For Ikaros, this association correlates with reduced H3 and H4 acetylation. Aiolos association has been linked to a decrease in both acetylation and the positive histone mark H3K4^me3 at the Il2 promoter and a concurrent increase in H3K9^me3, which is indicative of a transcriptionally inactive locus. In regulatory T (T_REG) cells, Eos and Helios have both been implicated in IL-2 repression. Mechanistically, Eos forms a protein complex with the T_REG lineage-defining transcription factor FOXP3 and C-terminal binding protein (CtBP) to repress Il2. As with Aiolos, this repression is associated with reduced H3 and H4 acetylation, reduced H3K4^me3, and increased H3K9^me3 enrichment at the Il2 promoter. Similarly, Helios and FOXP3 are co-enriched at the Il2 locus in T_REG cells and correlate with reduced H3 acetylation, consistent with gene repression. (B) In conventional T helper cell populations, in the absence of FOXP3, Eos has been positively linked to the expression of IL-2. However, the mechanism by which Eos influences IL-2 expression is currently unclear.

Figure 4

STAT and IkZF regulatory networks underlying the expression of T helper cell gene programs. This schematic depicts representative cytokines implicated in the development of murine T helper cell populations, their downstream STAT signaling pathways, and the Ikaros zinc finger transcription factors recently associated with the differentiation and functions of each subset. A recent study demonstrated that the IkZF factor Aiolos cooperates with STAT3 to induce the expression of Bcl6, the lineage-defining transcription factor for the T_FH cell type. The differential expression and/or activity of IkZF and STAT factors across T helper cell subsets is suggestive of the intriguing possibility that additional, cell type-specific STAT/IkZF regulatory modules may regulate T helper cell differentiation decisions across subsets.