PDCD1 and neoplasm: We found that the level of PD‐L1 expression in B16F10 cells in bone metastases was higher than that in B16F10 cells cultured in vitro (Figure S7A, Supporting Information), which may be due to the microenvironment within bone metastases promoting tumor cells to overexpress PD‐L1.[27] To test the binding ability of HSCs‐PD‐1 to PD‐L1 in tumors, we used lentiviral transfection to overexpress PD‐L1 in B16F10 cells in vitro (Figure S7B,C, Supporting Information) and then co‐cultured HSCs‐PD‐1 or HSCs with adherent B16F10‐PD‐L1 cells for 3 h.