Gemcitabine promotes tumor cell-derived inflammatory responses leading to immunosuppression

Abstract Some chemotherapeutic agents can selectively deplete MDSC in vivo. However, recent studies suggest that Gemcitabine (GEM) and 5-Fluorouracil (5-FU) treatment decrease the anticancer efficacy by activating the inflammasome pathway in MDSC. In addition, chemotherapy can modify the tumor microenvironment by induction of chronic inflammation. Therefore, regulations of MDSC by chemotherapy still remain elusive. Here, we explored the mechanisms by which multi-dose clinical regimen of GEM treatment induce an ongoing inflammation leading to the enhanced immunosuppressive activity of monocytic MDSC (M-MDSC). We found that repeated GEM treatment promoted the expansion and differentiation of immunosuppressive M-MDSC although tumor development was delayed in E0771 tumor-bearing mice. Accordingly, IFN-γ-producing CD4 and CD8 T cells were significantly decreased in the TME of GEM-treated mice. The conditioned medium of GEM-treated tumor cells (GEM-CM) promoted the differentiation of immunosuppressive mouse BM Ly6Chigh cells. Tumor-derived soluble factors, such as GM-CSF and soluble ICAM-1(sICAM-1), were up-regulated upon chemo-drug treatment. Blocking of GM-CSF or ICAM-1 with neutralizing Abs could partially reduce immunosuppression of Ly6Chigh cells induced by GEM-CM. Enhanced production of reactive oxygen species (ROS) and activation of NF-κB were observed in GEM-treated tumor cells. Treatment with the inhibitor of NF-κB signaling could abrogate GEM-induced hyperexpression of GM-CSF in E0771 cells. Together, our results define an undesired effect of GEM-induced ROS which leads to NF-κB activation, up-regulation of GM-CSF, and subsequent differentiation of immunosuppressive myeloid cells.