PDCD1 and osteosarcoma: In order to determine if T cell responses to osteosarcoma metastases in α-PD-L1 mAb treated mice may have become tolerized through engagement of other inhibitory ligands, we evaluated expression of PD-1, CTLA-4, and LAG3 on tumor reactive T cells, as these inhibitory proteins have been shown to be critically important in other tumor tolerance settings [14-18].