Overcoming PD-1 Inhibitor Resistance with a Monoclonal Antibody to Secreted Frizzled-Related Protein 2 in Metastatic Osteosarcoma

SIMPLE SUMMARY: Osteosarcoma (OS) is the most common bone tumor in the pediatric population, and long-term survival occurs in less than a third of the population with metastatic or recurrent tumors. Secreted frizzled-related protein 2 (SFRP2) promotes metastatic OS cell migration and tumor angiogenesis. This study aimed to assess the role of antagonizing SFRP2 with a humanized monoclonal antibody to SFRP2 (hSFRP2 mAb) in OS metastases in vivo and the role of SFRP2 in T-cells. Our results demonstrate that hSFRP2 mAb treatment inhibits metastases in two metastatic models of OS and can overcome resistance to a PD-1 monoclonal antibody. hSFRP2 mAb treatment restores T-cell proliferation and, in T-cells, inhibits NFATc3, CD38 and PD-1 expression. We conclude that SFRP2-targeted immunotherapy reduces the growth of metastatic osteosarcoma, not only through a direct antitumor and antiangiogenic effect but also by impacting the immune system. ABSTRACT: Secreted frizzled-related protein 2 (SFRP2) promotes the migration/invasion of metastatic osteosarcoma (OS) cells and tube formation by endothelial cells. However, its function on T-cells is unknown. We hypothesized that blocking SFRP2 with a humanized monoclonal antibody (hSFRP2 mAb) can restore immunity by reducing CD38 and PD-1 levels, ultimately overcoming resistance to PD-1 inhibitors. Treating two metastatic murine OS cell lines in vivo, RF420 and RF577, with hSFRP2 mAb alone led to a significant reduction in the number of lung metastases, compared to IgG1 control treatment. While PD-1 mAb alone had minimal effect, hSFRP2 mAb combination with PD-1 mAb had an additive antimetastatic effect. This effect was accompanied by lower SFRP2 levels in serum, lower CD38 levels in tumor-infiltrating lymphocytes and T-cells, and lower PD-1 levels in T-cells. In vitro data confirmed that SFRP2 promotes NFATc3, CD38 and PD-1 expression in T-cells, while hSFRP2 mAb treatment counteracts these effects and increases NAD(+) levels. hSFRP2 mAb treatment further rescued the suppression of T-cell proliferation by tumor cells in a co-culture model. Finally, hSFRP2 mAb induced apoptosis in RF420 and RF577 OS cells but not in T-cells. Thus, hSFRP2 mAb therapy could potentially overcome PD-1 inhibitor resistance in metastatic osteosarcoma.