Abstract
BACKGROUND: Aside from immune checkpoint inhibitors targeting programmed cell death protein 1 (PD-1) and programmed death ligand 1 (PD-L1), intervention of CD47/Sirpα mediated 'don't eat me' signal between macrophage and tumor cell is considered as a promising therapeutic approach for cancer immunotherapy. Compared with CD47, the novel immune checkpoint CD24/Siglec-10 can also deliver 'don't eat me' signal and CD24 shows much lower expression level in normal tissue which might avoid unwanted side effects. METHODS: Cell-based phage display biopanning and D-amino acid modification strategy were used to identify the CD24/Siglec-10 blocking peptide. Cell-based blocking assay and microscale thermophoresis assay were used to validate the blocking and binding activities of the peptide. Phagocytosis and co-culture assays were used to explore the in vitro function of the peptide. Flow cytometry was performed to assess the immune microenvironment after the peptide treatment in vivo. RESULTS: A CD24/Siglec-10 blocking peptide (CSBP) with hydrolysis-resistant property was identified. Surprisingly, we found that CSBP could not only block the interaction of CD24/Siglec-10 but also PD-1/PD-L1. CSBP could induce the phagocytosis of tumor cell by both the macrophages and monocytic myeloid-derived suppressor cells (M-MDSCs), which can further activate CD8(+) T cells. Besides, combination of radiotherapy and CSBP synergistically reduced tumor growth and altered the tumor microenvironment in both anti-PD-1-responsive MC38 and anti-PD-1-resistant 4T1 tumor models. CONCLUSIONS: In summary, this is the first CD24/Siglec-10 blocking peptide which blocked PD-1/PD-L1 interaction as well, functioned via enhancing the phagocytosis of tumor cells by macrophages and M-MDSCs, and elevating the activity of CD8(+) T cells for cancer immunotherapy.