Direct interaction between BM and tumor cells is required for PD-L1 expression. Cell surface PD-L1 expression was detected on B16F10 cells in monoculture or co-culture with BM cells by staining with isotype control or PD-L1 antibody and flow cytometry. Data was represented with (A) Bar Graph, (B) Histogram. 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.