CD4 and neoplasm: We next evaluated the impact of TGF‐β CAR‐T cells on the differentiation of naïve T cells into the Treg phenotype, which suppresses tumor rejection.3 TGF‐β has been shown to promote Treg differentiation,7 and this behavior was confirmed through the following co‐culture experiment: previously unactivated CD4+ T cells were primed with IL‐2, anti‐CD28, and immobilized OKT3, with or without TGF‐β.