Melanoma inhibits dendritic cell induction of cell-mediated immunity

Abstract

Cell-mediated immunity is essential for tumor defense. Mature dendritic cells promote cell-mediated immunity through induction of Th1 responses from naı̈ve CD4+ T cells. This requires dendritic cell production of IL-12. Th1 responses facilitate generation of cytotoxic T cells from CD8+ precursors. Melanoma inhibits peritoneal macrophage function in vivo and in vitro. Peritoneal macrophages harvested from melanoma-bearing mice are less cytotoxic to B16 melanoma cells, have less candidacidal capacity, and produce less TNF-α, nitric oxide, and superoxide. Similar impairment of macrophage function has been demonstrated in vitro using B16 melanoma conditioned media. The purpose of this study was to determine whether melanoma inhibits dendritic cell maturation, IL-12 production, and/or subsequent T cell activation. Methods: Bone marrow-derived dendritic cells (DCs) were cultured from C57BL/6 mice using media containing GM-CSF and IL-4. Melanoma conditioned media (MCM) was generated by incubating B16 melanoma cells in control media for 24 hours. DCs were exposed to control or MCM for 24 hours prior to maturation with LPS (100 ng/ml). DCs and supernatants were harvested 24 hours after stimulation. DC maturation was measured by flow cytometry using antibodies to CD40, CD54, CD80, CD86, MHC class I, and MHC class II. IL-12 levels were measured by ELISA. DCs were pulsed with antigen, fixed, and co-cultured with T cells in the presence of control or MCM. IL-2 levels (a marker of T cell activation) were measured by ELISA from 24-hour supernatants. Results: Dendritic cells exposed to MCM produce less IL-12 in response to LPS than controls (82 pg/ml vs. 110 pg/ml, p = 0.0015). LPS induced DC maturation was not affected by exposure to MCM as surface expression of CD40, CD54, CD80, CD86, MHC class I, and MHC class II was similar. T cells activated by DCs in the presence of MCM produced 45% less IL-2 than controls (29 pg/ml vs. 53 pg/ml). Conclusion: Melanoma inhibits dendritic cell induction of cell-mediated immunity by decreasing IL-12 production and inhibiting T cell activation. However, dendritic cell maturation, as assessed by surface expression of classical markers, was not affected by MCM.