Peripheral CD4+ T-cell differentiation regulated by networks of cytokines and transcription factors

Abstract

CD4(+) T cells, also known as T-helper (Th) cells, play an important role in orchestrating adaptive immune responses to various infectious agents. They are also involved in the induction of autoimmune and allergic diseases. Upon T-cell receptor (TCR)-mediated cell activation, naive CD4(+) T cells can differentiate into at least four major lineages, Th1, Th2, Th17, and iTreg cells, that participate in different types of immune responses. Networks of cytokines and transcription factors are critical for determining CD4(+) T-cell fates and effector cytokine production. Here, we review collaboration and cross-regulation between various essential cytokines in the activation/induction of key transcription factors during the process of Th cell differentiation towards these distinct lineages. We also discuss the interactions of key transcription factors at both genetic and protein levels and the function of the resulting network(s) in regulating the expression of effector cytokines.

Fig. 1. The Network of transcription factors in CD4 T cells

Together with transcription factors such as NF-AT, NF-κB and AP-1 activated and/or induced by TCR-mediated signaling, the activation of STAT proteins by different cytokines plays a critical role in inducing the expression of the lineage-specific master regulators T-bet (Th1), GATA3 (Th2), RORγt (Th17) and Foxp3 (Treg). The STAT proteins also collaborate with the master regulators, the transcription factors activated/induced by TCR and some secondary transcription factors, whose expression is controlled by the master regulators, for the induction of cytokine genes. Positive or negative regulation among these transcription factors occurs at the gene expression level and/or at the protein level through protein-protein interaction, forming a sophisticated transcriptional regulatory network during Th cell differentiation. The transcription factor complexes regulate chromatin remodeling and expression of the lineage-specific cytokines; one of the lineage-specific cytokines re-enforces further differention of such cells through a positive feedback loop (note: regulation of TGFβ expression may not be at transcriptional level).

Fig. 2. Combinatorial expression of different transcription factors determines Th cell heterogeneity and plasticity

Th cells are more heterogenous and plastic than originally thought. The heterogeneity and plasticity of these Th cells may be explained by the different combinations of expression of transcription factors in a single cell. Since many Tfh and Treg cells can exhibit Th1, Th2 or Th17-like phenotype, these cells may differentiate in parallel to conventional effector T (Teff) cells. For example, Th1-like Treg cells co-express Foxp3 and T-bet although the expression level of T-bet in such cells is lower than that in Th1 Teff cells. It has been reported that Tfh cells do not express T-bet, GATA3 or RORγt; however, “do not express” may actually be “express at lower level”. Alternatively, some Tfh cells may represent a different state of another unknown Teff cells whose master regulator is yet to be defined. Similarly, Tr1 (IL-10-producing cells) and Th9 (IL-9-producing cells) may represent certain subsets of Th1, Th2, Th17 or Treg cells, in which a transcription factor that is involved in IL-10 production (i.e. c-Maf) or in IL-9 production (i.e. PU.1) is co-expressed with other lineage-specific factors. Under certain circumstances, two cytokines of different lineages may be co-expressed in the same cells; this may be explained by the combinatorial expression of two master regulators (i.e. T-bet and GATA3 expression was found in IFNγ/IL-4 co-expressors).