TGFB1 and cancer: High initial Treg frequencies may be of particular concern in cancer patients, where tumor cells have the potential to induce Treg differentiation.39, 40 Our results show that while TGF‐β CAR‐Tregs can respond to TGF‐β, they do not hinder the proliferation of Teff cells and do not interfere with the stimulation of anti‐tumor T cells.