AS2863619 free base enables conversion of antigen-specific effector/memory T cells into Foxp3+ regulatory T (Treg) cells for the treatment of various immunological diseases. AS2863619 free base is a potent, orally active cyclin-dependent kinase 8 (CDK8) and CDK19 inhibitor with IC50s of 0.61 nM and 4.28 nM, respectively. STAT5 activation enhanced by AS2863619 free base inhibition of CDK8/19, which consequently activates the Foxp3 gene.