Delineating JunB's Crucial Function in Mature Th17 Cells through Inducible Targeted Protein Degradation

The AP-1 transcription factor JunB is essential for the differentiation of pathogenic T helper 17 (Th17) cells, which are key mediators of autoimmune diseases such as multiple sclerosis and colitis. While the importance of JunB during Th17 polarization is known, its role in mature Th17 cells--critical therapeutic targets in these diseases--remains unclear. In this study, we employed the dTAG system, a targeted protein degradation approach, to deplete JunB in Th17 cells generated both in vitro and in vivo. During pathogenic Th17 cell differentiation, JunB degradation replicated known effects of JunB deficiency, including reduced expression of interleukin (IL)-17A and the genes encoding ROR{gamma}t (Rorc) and the IL-23 receptor (Il23r). In contrast, in mature pathogenic Th17 cells, JunB degradation downregulated Il23r without affecting IL-17A or Rorc expression. Furthermore, JunB degradation compromised the viability of mature pathogenic Th17 cells. Transcriptomic analyses revealed that JunB regulates distinct gene sets during Th17 polarization compared to mature Th17 cells. The gene Inhba, which encodes activin A, was identified as a JunB target in both stages. Supplementation with activin A restored IL-17A and Rorc expression during pathogenic Th17 cell differentiation. These findings demonstrate that JunB maintains mature pathogenic Th17 cell phenotypes, including IL-23 receptor expression, and supports pathogenic Th17 cell survival. As IL-23 signaling is crucial for sustaining pathogenic Th17 cells, targeting JunB may offer a therapeutic strategy to limit Th17-driven autoimmune inflammation.