Helios enhances the preferential differentiation of human fetal CD4 + naïve T cells into regulatory T cells

T cell receptor (TCR) stimulation and cytokine cues drive the differentiation of CD4 + naïve T cells into effector T cell populations with distinct proinflammatory or regulatory functions. Unlike adult naïve T cells, human fetal naïve CD4 + T cells preferentially differentiate into FOXP3 + regulatory T (T reg ) cells upon TCR activation independent of exogenous cytokine signaling. This cell-intrinsic predisposition for T reg differentiation is implicated in the generation of tolerance in utero; however, the underlying mechanisms remain largely unknown. Here, we identify epigenetic and transcriptional programs shared between fetal naïve T and committed T reg cells that are inactive in adult naïve T cells and show that fetal-derived induced T reg (iT reg ) cells retain this transcriptional program. We show that a subset of T reg -specific enhancers is accessible in fetal naïve T cells, including two active superenhancers at Helios Helios is expressed in fetal naïve T cells but not in adult naïve T cells, and fetal iT reg cells maintain Helios expression. CRISPR-Cas9 ablation of Helios in fetal naïve T cells impaired their differentiation into iT reg cells upon TCR stimulation, reduced expression of immunosuppressive genes in fetal iT reg cells such as IL10 , and increased expression of proinflammatory genes including IFNG Consequently, Helios knockout fetal iT reg cells had reduced IL-10 and increased IFN-γ cytokine production. Together, our results reveal important roles for Helios in enhancing preferential fetal T reg differentiation and fine-tuning eventual T reg function. The T reg -biased programs identified within fetal naïve T cells could potentially be used to engineer enhanced iT reg populations for adoptive cellular therapies.