Genetically modulating the RNA-binding protein Regnase-1 reveals its critical role in regulatory T cell homeostasis...

Regnase-1 is a CCCH-type RNA-binding protein that constantly degrades mRNA species via its ribonuclease domain to maintain immune quiescence. But how and by what mechanisms Regnase-1 regulates Foxp3⁺ Tregs remains poorly studied. Here, we generated a conditional knock-in strain in which a degradation-resistant Regnase-1 mutant (R111A) was selectively expressed in Foxp3⁺ Tregs as a transgene (Foxp3^YFP-CreLSL-R111A^fl/fl) to investigate the roles of Regnase-1 in Tregs. We found that all male Foxp3^YFP-CreLSL-R111A^fl/fl mice died of fatal autoimmune diseases by 4 weeks of age, which was accompanied by massive activation of T effector cells in vivo. We also found that the Foxp3⁺ Tregs in male Foxp3^YFP-CreLSL-R111A^fl/fl mice were progressively depleted in the periphery. Further analyses showed that the Foxp3⁺ Tregs expressing the R111A mutant displayed an intrinsic survival defect and that wild-type Foxp3⁺ Tregs adoptively transferred into young Foxp3^YFP-CreLSL-R111A^fl/fl mice could rescue the lethal autoimmune phenotype. Mechanistically, we demonstrated that the Regnase-1 readily degraded Bcl2l1 mRNA (encoding Bcl-xL) via binding to the Bcl2l1 mRNA stem loop structure, thus preventing Bcl-xL expression in Tregs. Thus, Tregs expressing the R111A mutant exhibited not only survival defects but also failure to respond to IL-2, resulting in their eventual demise in vivo. Together, our studies suggest that Regnase-1 can have a profound impact on Foxp3⁺ Tregs and that modulating Regnase-1 in Tregs may have important therapeutic implications.