Abstract
Background Since ionizing radiation has showed the dramatic effect to kill the cancer cells by direct DNA damage as well as triggering anti-cancer immune responses through release of various tumor antigens and induction of NK activating molecules, it has been used for long time to treat many cancer patients including patients with melanoma. However, it has been known that radiotherapy might promote the remnant cancer cells to escape immune system. One of the suggested ways is induction of a ligand for programmed death-1 (PD-L1) after radiotherapy in head and neck cancer, bladder cancer and lung cancer cells which engages the receptor, programmed death-1 (PD-1) in immune cells. PD-1/PD-L1 axis transduces the inhibitory signal and suppresses the adaptive immunity in T cells. However, their role in innate immunity remains poorly understood. Therefore, we investigated whether ionizing radiation could change the expression of PD-L1/2 in malignant melanoma cells and the receptor, PD-1, in NK-92 cells. Results Surface PD-L1/2 levels on melanoma cells were increased by ionizing radiation in a dose-dependent manner but the level of PD-L1 was not changed significantly in NK-92 cells. Radiation-induced PD-L1/2 suppressed the activity of the NK-92 cells against melanoma cells despite of upregulation of NKG2D ligands. Furthermore, activated NK cells had high level of PD-1 and could not kill PD-L1/2+ melanoma cells effectively. When we used PD-L1 inhibitor or silenced PD-L1 gene to inhibit PD-1/PD-L1 axis, they reversed the activity of the suppressed NK cells. Conclusions Through these results, we supposed that PD-1/PD-L1 blockade could enhance the immune responses of NK cells against melanoma cells after radiotherapy and might overcome the PD-L1 mediated radioresistance of cancer cells.
Blocking of programmed death ligand 1 enhances natural killer cell-mediated immunity against malignant melanoma cells