CD274 and osteosarcoma: To evaluate if down-regulation of PD-L1 by K7M2 metastatic osteosarcoma cells after α-PD-L1 mAb treatment resulted in resistance to further α-PD-L1 mAb treatment, versus indirect inhibition by other tumor resident cells, metastastatic osteosarcoma cells isolated from α-PD-L1 mAb treated mice were re-implanted into naïve recipient mice that were then subsequently treated with α-PD-L1 mAb or received mock injections (Figure 2A).