Increased infiltration of macrophages to radioresistant lung cancer cells contributes to the development of the additional resistance of tumor cells to the cytotoxic effects of NK cells.

In this study, in order to investigate the effects of increased macrophage infiltration to radioresistant lung tumors in regulating natural killer (NK) cell-mediated immunity, we examined whether the treatment of radioresistant cells with conditioned medium (CM) from phorbol myristate acetate (PMA)/interleukin (IL)-4 treated THP-1 cells (used as a tumor-associated macrophage source) leads to the development of the additional resistance of tumor cells to NK cell cytotoxicity. We found that the susceptibility of THP-1 CM-treated radioresistant cells to NK cell cytotoxicity was decreased compared to the non-treated cells. In addition, it was found that such a decreased susceptibility was associated with increased programmed death receptor ligand 1 (PD-L1) and decreased natural killer group 2D (NKG2D) ligand levels in tumor cells. We further discovered that the THP-1 cells secreted a high level of IL-6, and that blocking IL-6 action by the addition of a neutralizing antibody (Ab) for IL-6 into the THP-1 CM decreased the resistance of THP-1 CM-treated radioresistant cells to NK cell cytotoxicity. Moreover, we discovered that MEK/Erk was the most critical IL-6 downstream signaling pathway in triggering the THP-1 CM effect; thus, the addition of MEK/Erk inhibitor to THP-1 CM enhanced the susceptibility of the THP-1 CM-treated radioresistant cells to NK cell cytotoxicity. On the whole, the findings of this study suggest the existence of a malignant loop characterized by increased macrophage infiltration into radioresistant cells which, in turn, promotes the development of the additional resistance of these cells to NK cell cytotoxicity.