Helios is a marker, not a driver, of human Treg stability
Regulatory T cell (Treg) therapy holds promise as a potentially curative approach to establish immune tolerance in transplantation and autoimmune disease. An outstanding question is whether therapeutic Tregs have the potential to transdifferentiate into effector T cells and thus exacerbate rather than suppress immune responses. In mice, the transcription factor Helios is thought to promote Tregs lineage stability in a range of inflammatory contexts. In humans, the role of Helios in Tregs is less clear, in part due to the inability to enrich and study subsets of Helios-positive versus Helios-negative Tregs. Using an in vitro expansion system, we found that loss of high Helios expression and emergence of an intermediate Helios (Heliosmid)-expressing population correlated with Treg destabilization. We then used CRISPR/Cas9 to genetically ablate Helios expression in human naive or memory Tregs and found that Helios-knockout and unedited Tregs were equivalent in their suppressive function and stability in inflammation. Thus, high Helios expression is a marker, but not a driver, of human Treg stability in vitro. These data highlight the importance of monitoring Helios expression in therapeutic Treg manufacturing and provide new insight into the biological function of this transcription factor in human T cells.