LAUSR

Osteosarcoma is a rare malignant bone‐originating tumor that usually occurs in young people. Programmed cell death 1 ligand 1 (PD‐L1), an immune checkpoint protein, is highly expressed in osteosarcoma tissues. Several recent studies have indicated that the tumor‐related role of PD‐L1 in tumors, especially non‐plasma membrane (NPM)‐localized PD‐L1, is not limited to immune regulation in osteosarcoma. Here, mass spectrometry analysis is combined with RNA‐seq examination to identify the intracellular binding partners of PD‐L1 and elucidate the underlying mechanism of its action. It is found that the NPM‐localized PD‐L1 interacted with Insulin‐like growth factor binding protein‐3 (IGFBP3) to promote osteosarcoma tumor growth by activating mTOR signaling. This interaction is enforced after phosphoglyceratekinase1 (PGK1)‐mediated PD‐L1 phosphorylation. Based on these findings, a phosphorylation‐mimicking peptide is designed from PD‐L1 and it is encapsulated with a Cyclic RGD (cRGD)‐modified red blood cell membrane (RBCM) vesicle (Peptide@cRGD‐M). The Peptide@cRGD‐M precisely delivers the PD‐L1‐derived phosphorylation‐mimicking peptide into osteosarcoma lesions and significantly promotes its therapeutic effect on the tumor. Therefore, this investigation not only highlights the function of NPM‐localized PD‐L1, but also uses an engineering approach to synthesize a small molecular peptide capable of inhibiting osteosarcoma growth.