Both in vivo and in vitro, the efficiency of osteosarcoma-reactive CTLs has been reported to be improved significantly through blocking the interaction of PD-1/PD-L1, which results in reduced burden of the tumor as well as enhanced rate of survival in the osteosarcoma metastasis models (Lussier et al., 2015[37]). Here, PDCD1 is linked to osteosarcoma.