Ex Vivo Cytokine Activation of Peripheral Blood Stem Cells: A Potential Role for Adoptive Cellular Immunotherapy

Ex vivo activation of peripheral blood stem cells (PBSC) using interleukin-2 (IL-2) results in cytotoxic effector cells that may possess beneficial in vivo effects. We proposed to evaluate ex vivo stimulation of PBSC using various cytokines alone or in combination to optimize their function. Cytokine-activated PBSC were analyzed for tumor-directed cytotoxicity and their ability to remove tumor cells from long-term clonogenic assays. Mononuclear cells were obtained from the apheresis products of normal donors and cultured with IL-2 (1000 U/ml), interferon-α (IFN-α) (1000 U/ml), or IL-12 (50 U/ml) either alone or in combinations at 37°C and 5% CO₂ for 24 h. Colony-forming unit-tumor (CFUT) assays were initiated using cytokine-activated PBSC with varying concentrations of MCF-7 or SKBR-3 human breast cancer cells. Standard 4-h ⁵¹Cr-release assays were performed with cytokineactivated PBSC using MCF-7 or SKBR-3 cells as targets. Activation of PBSC with IL-2, IFN-α , or IL-12 resulted in enhanced cytotoxicity against the two breast cancer cell lines when compared to controls. PBSC activated with IL-2 and IFN-α or IL-2 and IL-12 were more cytotoxic than PBSC activated with single cytokines (p = 0.0004 for MCF-7 cells and p < 0.001 for SKBR-3 cells). Using clonogenic assays, IL-2-activated PBSC reduced the number of CFU-T to a greater extent than did IL-12 or IFN-α-activated PBSC (p = 0.0006). However, PBSC activated with a combination of IL-2 and IFN-α or IL-2 and IL-12 demonstrated 95% and 90% reductions, respectively, compared to 79% reduction using IL-2-activated PBSC (p < 0.0001). The greatest reduction in cytotoxicity occurred in the cell populations depleted of CD56⁺ cells (p = 0.016) and CD8⁺ CD56⁺ cells (p= 0.002), suggesting that the effector cell population includes a combination of cytotoxic CD8⁺ T cells and CD56⁺ natural killer cells. These results demonstrate that the ex vivo activation of PBSC with cytokines, either alone or in combination, enhances cytotoxicity against, and removal of two human breast cancer cells. The combinations of IL-2 with IFN-α or IL-12 are most beneficial in cytotoxicity and purging assays. These results could play an important role in designing adoptive cellular immunotherapy clinical trials in the autologous hematopoietic stem cell transplant setting.