CD4 and neoplasm: Our results showed that the frequency of Tregs in the group of IL-21high/PD-L1high tumor explants + CD4+ T cells was higher than that of the group of IL-21low/PD-L1low tumor explants + CD4+ T cells (38.8 ± 8.5% vs. 25.8 ± 2.4%, respectively, P < 0.05) (Figures 5A, B).