CD11b-positive BM cells are critical for PD-L1 expression. B16F10 tumor cell surface PD-L1 expression was determined after co-culture with BM cells isolated from knockout mice, including (A) B−/−, (B) CD11b−/−, (C) CD28−/−, (D) Rag2−/−, and (E) perforin−/− mice. (F) B16F10 cells were pre-incubated with anti-rat-IgG or CD11b-neutralizing antibody and co-cultured with BM cells. PD-L1 expression was determined by staining with isotype control or PE/Cy7-PD-L1 antibody and using flow cytometry. (G) CD11b-positive BM cells were separated from the mixed BM cells using PE-CD11b-coupled nanoparticles. B16F10 cells were co-cultured with CD11b-positive BM cells for 48 hours and then stained with isotype control or PE/Cy7-PD-L1 antibody. PD-L1 expression level was determined by flow cytometry. Data are presented as mean ± standard error (n = 3), *P <0.05 versus B16F10 alone, student t test. MFI = Median Fluorescence Intensity, BM = Bone Marrow.