Active cancer immunotherapy relies on functional tumor-specific effector T lymphocytes for tumor elimination. Dendritic cells (DCs), as most potent antigen-presenting cells, have been popularly employed in clinical and experimental tumor treatments. We have previously demonstrated that lentiviral vector-mediated transgene delivery to DC progenitors, including bone marrow cells and hematopoietic stem cells, followed by transplantation supports systemic generation of great numbers of tumor antigenpresenting DCs. These DCs subsequently stimulate marked and systemic immune activation. Here, we examined whether this level of immune activation is sufficient to overcome tumor-induced tolerogenic environment for treating an established aggressive epithelial tumor. We showed that a combination treatment of granulocyte macrophage-colony stimulating factor and cytosinephosphate-guanine-containing oligonucleotide stimulated large numbers of tumor antigen-presenting DCs in situ from transgenemodified stem cells. Moreover, these in situ generated and activated DCs markedly stimulated activation of antigen-specific CD4 and CD8 T cells by augmenting their numbers, as well as function, even in a tumor-bearing tolerogenic environment. This leads to significant improvement in the therapeutic efficacy of established pulmonary metastases. This study suggests that lentiviral vectormodified stem cells as DC progenitors may be used as an effective therapeutic regimen for treating metastatic epithelial tumors.

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