AS2863619 free base enables the conversion of antigen-specific effector memory T cells into Foxp3+ regulatory T (Treg) cells. It is a potent, orally active CDK8 and CDK19 inhibitor (IC50s: 0.61 nM and 4.28 nM). STAT5 activation enhanced by AS2863619 free base inhibition of CDK8 19, which consequently activates the Foxp3 gene.