•Binds to and activates mouse TNFR2•Stimulates proliferative expansion of Foxp3+ Tregs (in vitro assay)•Selectively activates and expands Foxp3+ Tregs in WT mice (in vivo assay)•Markedly prolongs the survival and decreases the severity of graft-versus-host disease (GVHD) (in vivo assay). This evidence concerns the gene FOXP3 and graft versus host disease.