Tumor growth is associated with a profound alteration in myelopoiesis, leading to recruitment of immunosuppressive cells known as myeloid-derived suppressor cells (MDSCs). We showed that among factors produced by various experimental tumors, the cytokines GM-CSF, G-CSF, and IL-6 allowed a rapid generation of MDSCs from precursors present in mouse and human bone marrow (BM). BM-MDSCs induced by GM-CSF+IL-6 possessed the highest tolerogenic activity, as revealed by the ability to impair the priming of CD8+ T cells and allow long term acceptance of pancreatic islet allografts. Cytokines inducing MDSCs acted on a common molecular pathway and the immunoregulatory activity of both tumor-induced and BM-derived MDSCs was entirely dependent on the C/EBPβ transcription factor. Adoptive transfer of tumor antigen-specific CD8+ T lymphocytes resulted in therapy of established tumors only in mice lacking C/EBPβ in the myeloid compartment, suggesting that C/EBPβ is a critical regulator of the immunosuppressive environment created by growing cancers.
► Tumor environment releases factors activating C/EBPbeta that controls MDSC activity ► GM-CSF, G-CSF, and IL-6 are sufficient to generate MDSCs from mouse and human BM ► Adoptive transfer of BM-derived MDSCs induce tolerance and allograft acceptance ► C/EBPbeta deficiency creates a favorable milieu for CTL-dependent cancer rejection
