PDCD1LG2 and osteosarcoma: Apatinib reduces the doxorubicin resistance of osteosarcoma via the STAT3-mediated SRY-box transcription factor 2 (Sox2) pathway and represses the programmed cell death 1 ligand 2 (PD-L2) mediated immune escape in osteosarcoma by mediating the VEGFR2 and STAT3/ras homolog family member A (RhoA)/rho-associated coiled-coil containing protein kinase 1 (ROCK1)/LIM domain kinase 2 (LIMK2) pathways [15, 16].