Abstract
The overall survival rate of patients with osteosarcoma has remained stagnant at 15-30% for several decades. Although immunotherapy has revolutionized the oncology field, largely attributed to the success of immune-checkpoint blockade, the durability and efficacy of anti-PD1 (programmed cell death protein 1) mAb vary across different malignancies. Among the major reasons for tumor resistance to this immune checkpoint therapy is the absence of tumor-infiltrating cytotoxic T lymphocytes. However, the presence of intratumor exhausted PD1hi T cells also contributes to insensitivity to anti-PD1 treatment. In this study, we established the osteosarcoma mouse tumor model resistant to anti-PD1 mAb that harbored PD1hi T cells. Furthermore, flow cytometry analysis of tumor infiltrating leukocytes after treatment was used as a screening platform to identify agents that could re-sensitize T cells to anti-PD1 mAb. Results showed that anti-CD40 mAb treatment converted PD1hi T cells to PD1lo T cells, reversing phenotypic T cell exhaustion and sensitizing anti-PD1 refractory tumors to respond to anti-PD1 mAb. Results also showed that intratumor Treg presented with a less activated and attenuated suppressive phenotype after anti-CD40 mAb treatment. Our study provides proof of concept to systematically identify immune conditioning agents, which are able to convert PD1hi T cells to PD1lo T cells, with clinical implications in the treatment against refractory osteosarcoma to anti-PD1 mAb.