Induced pluripotent stem cell-conditioned medium suppressed melanoma tumorigenicity through the enhancement of natural-killer cellular immunity

Induced pluripotent stem cells (iPSCs) can secrete cytokines that are involved in T-cell development and affect cytotoxic activity. To assess the effect of iPSC-conditioned medium on tumorigenicity, we retrieved splenocytes from B6 mice and cocultured them with or without irradiated B16 melanoma cells, mouse interleukin-2 (mIL-2), or iPSC-conditioned medium. Splenocyte cytotoxicity assays against B16 melanoma cells [as cytotoxic T lymphocyte (CTL) activity] and P815 cells [as natural killer (NK) activity] were performed. IL-10 and interferon-γ concentrations were measured. An in vivo subcutaneous B16 melanoma growth model was performed in B6 mice and treated with iPSC-conditioned medium. The lymphocyte subpopulation depletion test was performed to determine effectors against B16 melanoma cells. We found that unstimulated splenocytes had little cytotoxic activity. Without tumor cells, mIL-2 could augment iPSC-conditioned medium-treated CTL and NK activities (P < 0.01). With irradiated tumor cells, mIL-2 treatment of splenocytes could not enhance CTL or NK activity, but iPSC-conditioned medium could enhance CTL and NK activity (P < 0.001). Irradiated tumor cells induced mice splenocytes to secrete more IL-10, similar to mIL-2 treatment, but not iPSC-conditioned medium treatment. mIL-2 had better efficacy than conditioned medium in inducing splenocyte interferon-γ production. The CTL and NK cell depletion test showed that the immunostimulating effect of iPSC-conditioned medium on splenocytes was through the enhancement of NK cellular activity (P < 0.05). The subcutaneous melanoma growth model showed that B16-bearing mice treated with an iPSC-conditioned medium intraperitoneal injection had a decreased tumor growth rate (P < 0.01). Our study suggests that iPSC-conditioned medium had a protective effect against tumor-induced immunosuppression through the enhancement of host NK cellular activity.