CD4 and cancer: Although tumoral exosome‐PD‐L1 has been reported to be deleterious for cancer patients (Chen et al., 2018; Poggio et al., 2019; Ricklefs et al., 2018), our findings indicate that WJMSC sEV‐PD‐L1 might indeed be beneficial for treating aGvHD patients through targeting CD4+ T cells.