How are T(H)1 and T(H)2 effector cells made?

Abstract

Differentiation of T(H)1 and T(H)2 effector cells proceeds through several phases: First, naïve CD4(+) precursor cells are instructed to differentiate as appropriate to optimally fight the infectious threat encountered. This process is governed by the IL12 and IL4 cytokines, as well as by signaling through the Notch receptor. In response to these signals, transcription is initiated of lineage specific cytokine genes including the Ifngamma and Il4 genes as well as of genes encoding transcriptional regulators, such as T-bet and Gata3. The respective differentiation programs are reinforced by both positive and negative feedback mechanisms. Furthermore, epigenetic modifications of the lineage specific genes result in the emergence of regulatory elements, which control high level lineage restricted expression by both intrachromosomal and interchromosomal associations. Together, these mechanisms ensure stable inheritance of the differentiated fate in the numerous progeny of the original naïve CD4(+) T cells.

Figure 1. IL12 and IL4 driven T helper differentiation

Th1 induction by IL12: Initial TCR activation induces low grade expression of the Ifng and the Tbx21 genes (1). Signaling through the IL12 receptor results in STAT4 mediated promotion of IFNγ expression (2). Binding of the IFNγ receptor by low initial auto/paracrine produced IFNγ activates STAT1 (3), which strongly promotes expression of the Tbx21 gene (4). T-bet then enhances the transcriptional competence of the Ifng gene (5) leading to increased production of this cytokine (6). In addition, T-bet prevents Th2 differentiation by inhibiting Gata3 (7). Finally, T-bet promotes expression of the IL12 receptor β2 chain (8), resulting in greater IL12 responsiveness (9) and yet further elevated production of IFNγ (10). Th2 induction by IL4: Initial TCR signaling induces low level expression of the Il4 and Gata3 genes (11). IL4 receptor signaling strongly promotes expression of these two genes (12). Gata3 reorganizes chromatin structure in the Th2 locus, encompassing the Il4, Il5 and Il13 genes, enhancing their transcription competence (13). Increased IL4 production further enhances TH2-cell differentiation in a fed forward loop (14). Finally, Gata3 prevents the Th1 differentiation program by inhibiting expression of the IL12 receptor β2 chain (14) and of the Stat4 gene (not depicted). Primary events are indicated with black arrows, secondary events with red arrows and tertiary events with blue arrows.

Figure 2. Notch mediated T_H-cell differentiation

Notch signaling: Two families of Notch ligands exist, called DLL and Jagged. Notch is present as a heterodimer on the surface of the T cells. Binding of ligand to the extracellular domain of Notch results in γ-secretase mediated cleavage of the transmembrane portion of Notch, allowing the intracellular domain (NICD) to translocate to the nucleus, where it forms a trimolecular complex with the DNA binding protein RBP-J and the transcriptional coactivator Mastermind-like (MAM). T_H1 (left): Members of the DLL family of Notch ligands are expressed on APC in response to microbial stimuli that promote T_H1-cell induction by APC. Notch may promote T_H1-cell differentiation by direct transactivation of the Tbx21 gene (1), leading to expression of T-bet, transactivation of the Ifnγ gene via binding to NFkB family members like p50 (2), and indirectly though inhibition of IL4 receptor signaling (3). T_H2: Expression of Jagged family members is induced on APC by T_H2 promoting microbial and pro-inflammatory stimuli. Notch directly promotes activity of the upstream Gata3 promoter resulting in transcription of the Gata3 gene (4). Furthermore, Notch regulates transcription of the Il4 gene by binding to the HS5 enhancer (5). Please note that critical events, such as TCR signaling have been omitted for clarity.

Figure 3. Genomic organization of the T_H2 cytokine locus on chromosome 11 and the ifnγ locus on chromosome 10

A. Blue bold arrows indicate constitutive (present in naïve CD4⁺, T_H1 and T_H2 cells) DNAse I hypersensitive sites and black arrows indicate DNAse I hypersensitive sites specific in T_H2 cells (for the T_H2 locus) or T_H1 cells (for the Ifnγ locus). Vertical lines and boxes on the loci indicate gene exons. B. Evolutionary Conserved regions in the IFNγ locus. Cross species conservation analysis of at least 100bp and 70% DNA homology between mouse and frog, fugu, chicken, opossum, dog, Rhesus macaque, human species for the genomic regions displaying the IFNγ locus on mouse chromosome 10 using the ECR browser (http://ecrbrowser.dcode.org). (Colors depicting the conserved regions: blue-coding regions, yellow-untranslated regions, red-intergenic region, green-transposons and simple repeats, pink-intronic regions). Arrows on top depict DNAse I hypersensitive sites in naïve CD4⁺ cells (black), T_H1 cells (blue) and T_H2 cells (red).

Figure 4. Dynamic interplay of chromosomal interactions in the nucleus of T cells

A. Arrows on top of the T_H2 cytokine locus represent the intrachromosomal interactions between the RHS7 DNAse I hypersensitive site of the T_H2 LCR and several other genomic regions of the T_H2 locus in T_H2 cells. Arrows on top of the Ifnγ locus depict the intrachromosomal as well as the interchromosomal interactions captured for the Ifnγ – T_H2 cytokine locus. The arrow with the discontinuous line represents the T_H1 specific intrachromosomal interaction between two regions of the Ifnγ locus. B. Schematic hypothetical representation of the territories for mouse chromosome 10 and 11. The T_H2 cytokine locus is located on mouse chromosome 11 and the Ifnγ locus is located on mouse chromosome 10. We hypothesize that the interchromosomal interactions between the T_H2 cytokine locus and the Ifnγ locus are taking place in the interchromosome domain compartment in naïve CD4⁺ cells potentially keeping the two loci poised and at the same time transcriptionally silent for rapid expression upon TCR stimulation. During differentiation of naïve CD4⁺ T cells to T_H1 cells the Ifnγ gene is mainly regulated through intachromosomal interactions with regulatory elements in the locus and in TH2 cells intrachromosomal interactions between the T_H2 LCR and the cytokine gene promoters upon the action of T_H2 cell lineage specific factors regulate transcription.