Regulatory T (Treg) cells play an important role in the establishment and maintenance of immune tolerance. Most Treg Cells express CD25 molecule, which has a high affinity with IL-2. After activation of CD25, it signals by inducing the activation of transcription factor STAT5. IL-2 plays a central role in many aspects of Treg cell biology.
In addition to TCR signaling, IL-2 – STAT5 is also required to induce Foxp3 expression in Treg cells. Foxp3 is a transcription factor marker of Treg. Its continuous expression in differentiated Treg cells is necessary for the stability and function of Treg cell lineage. In addition, IL-2R signaling supports the persistence and immunosuppressive ability of mature Treg cells.
Previous studies have shown that the differentiation of Treg cells needs a “two-step” model, in which the relatively strong TCR stimulation induces the expression of CD25 in the precursor cells, so as to make them accept the subsequent IL-2 stimulation, and finally induce the expression of Foxp3. Consistent with this model, IL-2 produced by self-reactive CD4SP thymocytes can limit the differentiation of Treg cells in the thymus.
The fact that IL-2 – STAT5 signal induces and maintains Foxp3 expression indicates that this pathway directly acts on the FOXP3 gene. However, in view of the pleiotropic transcription of the IL-2 signal on hundreds of targets, it is still unclear to what extent the IL-2 – STAT5 pathway directly affects the FOXP3 gene, thereby affecting Treg expression. Previous studies have revealed that the intron enhancer element in CNS2 (conserved non-coding sequence 2) of FOXP3 gene cluster can bind to STAT5, which is a necessary condition for maintaining Foxp3 expression in mature Treg cells, but its induction is not necessary. Therefore, IL-2 – STAT5 signaling must induce Foxp3 expression in Treg precursor cells before DNA demethylation activates CNS2.
In a recent study, the Yongqing Feng team from St. Jude Children’s hospital identified STAT5 binding enhancers and explored their biological functions. The results were published in Immunology.
In fact, this conserved regulatory element basically overlaps with the previously identified SATB1 (CNS0) and MLL4 binding sites (– 8.5kb MLL4). Although the role of SATB1 in the differentiation of thymic Treg cells has been demonstrated, the importance of SATB1 binding to this region has not been verified. Given the genome-wide binding of SATB1, it is not clear which Foxp3 enhancer explains the function of SATB1. Similarly, the binding of MLL4 to this region has been shown to promote the polarization of Treg cells in vitro, but its role in vivo is still unclear.
Using the mouse genetic model, we observed that the deletion of CNS0 resulted in the selective damage of thymic Treg cells and the decrease of peripheral Treg cells, which was most obvious in the perinatal period. With the increase of age, this defect can be alleviated, which may be due to the expansion of peripheral Treg cells. Although the deletion of this enhancer will lead to a sustained slight increase in immunity without significant autoimmunity, it will significantly enhance autoimmunity when it is co-existing with AIRE deficiency. The results show that this IL-2 – STAT5 response element can ensure the strong differentiation of Treg cells in the early postnatal period, and can achieve the maximum remission of autoimmune diseases by cooperating with other tolerance mechanisms.
Stanislav Dikiy, Jun Li, Lu Bai. et al., A distal Foxp3 enhancer enables interleukin-2 dependent thymic Treg cell lineage commitment for robust immune tolerance. Immunity (2021). DOI:https://doi.org/10.1016/j.immuni.2021.03.020