Post-Translational Modifications in Transcription Factors that Determine T Helper Cell Differentiation

Abstract

CD4+ T helper (Th) cells play a crucial role in the modulation of innate and adaptive immune responses through the differentiation of Th precursor cells into several subsets, including Th1, Th2, Th17, and regulatory T (Treg) cells. Effector Th and Treg cells are distinguished by the production of signature cytokines and are important for eliminating intracellular and extracellular pathogens and maintaining immune homeostasis. Stimulation of naïve Th cells by T cell receptor and specific cytokines activates master transcription factors and induces lineage specification during the differentiation of Th cells. The master transcription factors directly activate the transcription of signature cytokine genes and also undergo post-translational modifications to fine-tune cytokine production and maintain immune balance through cross-regulation with each other. This review highlights the post-translational modifications of master transcription factors that control the differentiation of effector Th and Treg cells and provides additional insights on the immune regulation mediated by protein arginine-modifying enzymes in effector Th cells.

Keywords: CD4 T cell differentiation; arginine-modifying enzyme; effector Th and Treg cell; master regulatory transcription factor; post-translational modifications.

Fig. 1. Immune regulation by Th cell lineage specification.

(A) TCR triggering with pathogenic antigen provided by antigen-presenting cell (APC) induces activation and differentiation of CD4+ Th cells, which subsequently boost the activity of cytotoxic T lymphocytes (CTL) and plasma B cells. Immune response properly coordinated by effector Th and Treg cells is important for eliminating intracellular and extracellular pathogens and maintaining immune homeostasis against disease. (B) Th cell specification is epigenetically and transcriptionally modulated in response to TCR and cytokines stimulation. Despite the importance of histone modifications, the quantitative increase and activation of master transcription factors determine Th cell fate specification.

Fig. 2. General PTMs in proteins.

Specific amino acids such as Ser (S), Thr (T), Lys (K), Arg (R), and Tyr (Y) are reversibly changed by various modifying enzymes. Ser, Thr, and Tyr residues are commonly phosphorylated by kinase and dephosphorylated by phosphatase, and OGT glycosylation of Ser and Thr is induced by OGT but is reversibly removed by O-GlcNAcase (OGA). Both Lys and Arg are methylated by transferring methyl group from S-adenosylmethionine (SAM) and selectively acetylated. While Lys is conjugated with ubiquitin (ub) and SUMO by the action of ligases, Arg is converted to citrulline by PADs. Ac, acetylation; Cit, citrullination; G, glycosylation; Me, mono- & di-methylation; SUMO, SUMOylation; P, phosphorylation; ub, ubiquitination.

Fig. 3. PTMs in master transcription factors.

(A) T-bet functions in the Th cell specification. Upon TCR and IL-12 stimulation, T-bet is subjected to PTMs, including phosphorylation at Ser, Thr, and Tyr for inducing optimal Th cell development. T-bet is also ubiquitinated at Lys and undergoes proteasomal degradation. (B) In the presence of IL-4, GATA-3 increases histone acetylation to open the chromatin structure and strongly binds to the promoter of IL-4/IL-5/IL-13 genes as a form with acetylation and SUMOylation. Methylated GATA-3 selectively promotes IL-5 and unknown modification of GATA-3 also represses IFNγ gene transcription. IFNγ repression is inhibited by multiple Ser phosphorylation, which allows partial production of IFNγ in the memory Th2 cells. The GATA-3 protein stability is also regulated by ubiquitination. (C) Stimulation with TCR and IL-4 increases c-MAF expression and enhances c-MAF-mediated IL-4 production. While ubiquitination controls the protein stability of c-MAF, Tyr phosphorylation and SUMOylation are important in context-dependent modulation of IL-21 expression. (D) Blockade of IL-4 and IFNγ signaling and stimulation with TGFβ and IL-6 induces the expression of RORγt. For Th17 cell development, RORγt enhances IL-17 transcription but inhibits IL-2 expression, whereas acetylation reverses these effects. SUMOylation and K63-linked polyubiquitination stabilize the RORγt protein to elevate IL-17 production, while K48-linked polyubiquitination induces its proteasomal degradation. (E) Strong TGFβ stimulation in the absence of IL-4 and IFNγ signaling increases various PTMs in FoxP3. Hyperacetylation, Arg methylation, O-GlcNAcylation, and Ser418 phosphorylation increase the FoxP3 stability to promote the Treg cell development. However, polyubiquitination and Ser422 phosphorylation reduce the amount and activity of FoxP3.

Fig. 4. Protein arginine-modifications in CD4+ Th cells.

Either TCR stimulation or cytokines may induce the expression and activity of protein arginine-modifying enzymes in CD4+ Th cells. PADs induce the conversion of Arg to citrulline of GATA-3 and RORγt and PRMTs integrate monomethyl and dimethyl group into Arg of GATA-3, RORγt, and FoxP3. In addition to the methylation and citrullination, GATA-3 is additionally acetylated by p300 HAT and subsequently promotes the Th2 cell development. Ac, acetylation; Cit, citrullination; G, glycosylation; Me, mono- & di-methylation.