Differentiation of effector CD4 T cell populations (*)

Abstract

CD4 T cells play critical roles in mediating adaptive immunity to a variety of pathogens. They are also involved in autoimmunity, asthma, and allergic responses as well as in tumor immunity. During TCR activation in a particular cytokine milieu, naive CD4 T cells may differentiate into one of several lineages of T helper (Th) cells, including Th1, Th2, Th17, and iTreg, as defined by their pattern of cytokine production and function. In this review, we summarize the discovery, functions, and relationships among Th cells; the cytokine and signaling requirements for their development; the networks of transcription factors involved in their differentiation; the epigenetic regulation of their key cytokines and transcription factors; and human diseases involving defective CD4 T cell differentiation.

Figure 1

Cytokines play critical roles in differentiation and effector functions of Th1, Th2, and Th17 cells. Upon TCR activation triggered by antigen-presenting cells, naive CD4 T cells differentiate into distinct Th lineages in the context of combinations of cytokines. The differentiation processes involve upregulation of master transcriptional regulators and activation of STAT proteins (185). Each lineage expresses unique cytokine receptors, which can respond to cytokines produced by accessory cells. At later stages of Th cell differentiation, different Th cells preferentially express an IL-1 family receptor. Together with a STAT activator, an IL-1 family cytokine is capable of inducing effector cytokine production from Th cells in a TCR-independent manner (–49).

Figure 2

Networks of transcription factors for Th cell differentiation. Critical transcription factors involved in Th1, Th2, and Th17 differentiation and the relationships among these factors are shown. The cell fate of each Th lineage is determined by many transcription factors, including master regulators and STAT family proteins. There are collaborations and positive regulations among the transcription factors during Th cell differentiation and lineage-specific cytokine production. Some factors negatively regulate expression or function of transcription factors of other Th lineages.

Figure 3

Positive regulatory circuits for Th cell differentiation. Both TCR- and cytokine-mediated signaling, through activation of NFAT/NFκB/AP-1 and STAT proteins, respectively, are critical for early cytokine production and upregulation of a master transcription factor. The master transcription factor induces secondary transcription factors, which collaborate with the master transcription factor to enhance the expression of cytokine and cytokine receptors. In some cases, the master transcription factor also promotes its own expression. Elevated cytokine production and cytokine receptor expression provide powerful positive feedback loops for promoting Th cell fate determination as well as for selective expansion of committed Th cells.

Figure 4

Important cis-regulatory elements at the Ifng, Il4/Il13, and Il17a/Il17f loci and binding of transcription factors to these sites. (a) Within ~140 kb flanking the Ifng gene, many conserved noncoding sequences and their epigenetic modifications have been studied (259). CNS–22 has been shown to be critical for IFN-γ production (261). Many transcription factors, including T-bet, STAT4, Runx3, STAT5, and CTCF, bind to different regions of the Ifng gene (65, 131, 144, 181, 243, 261, 262). Our unpublished data suggest that Runx3 also binds to other regulatory elements in addition to the Ifng promoter. (b) Within ~70 kb flanking the Il4/Il13 genes, several important regulatory elements, including the locus control region (LCR), CNS1, Il4 HSII, Il4 HSIV, and Il4 HSV_A/V, have been identified (, , –267). Many transcription factors, including GATA3, c-Maf, STAT5, STAT6, Runx3, NFAT, and Notch/CSL, directly bind to different regions of the Il4/Il13 locus (61, 62, 122, 137, 144, 165, 233). Our unpublished data obtained from anti-GATA3 ChIPseq showed six GATA3-binding sites across this ~70-kb region in Th2 cells, with three located in LCR at RHS4, 5, and 6 and the other three at ~1.6 kb upstream of Il13, Il4 HSII, and Il4 HSV_A. (c) Epigenetic modifications of the CNSs across ~160 kb of Il17a/Il17f have been reported (268). Some transcription factors critical for regulation of IL-17 expression, including RORγt, STAT3, BATF, Runx1, NFAT, and Gfi-1/LSD1, directly bind to the CNS or promoter regions of Il17a/Il17f (90, 110, 152, 160, 177, 205).

Figure 5

Possible relationships among classic Th2, IL-4-producing Tfh, and Th9 cells. Depending on the cytokine milieu, naive CD4 T cells may differentiate directly into Th2, Tfh, and Th9 cells once they are activated (33, 121, 280, 281). Classic Th2 cells may become IL-4-producing Tfh cells upon receiving IL-6/IL-21 and ICOSL stimulation or IL-9-producing cells upon receiving TGF-β signaling. Whether Th9 cells or IL-4-producing Tfh cells can become classic Th2 cells is not known. The transcription factor directly responsible for IL-9 production has not been identified.

Figure 6

Complexity of Tregs. Foxp3 is the master regulator for Tregs. Recent reports and our unpublished data show that the transcription factors for Th1, Th2, and Th17 cells, T-bet, GATA3, and RORγt, respectively, can also be coexpressed in some Tregs (36, 113, 182). A relatively high expression ratio of Foxp3 over another master transcription factor prevents the induction of effector cytokines. Different combinations of transcription factors subdivide the Tregs with possible distinct regulatory functions. In some cases, the level of Foxp3 expression may fall, leading to expression of effector cytokines controlled by the other master regulator expressed by the cell. Effector cells derived from Tregs, because of their unique antigen specificity, may participate in normal immune responses to infections or contribute to autoimmune diseases (287).