Conditioned media from the renal cell carcinoma cell line 786.O drives human blood monocytes to a monocytic myeloid-derived suppressor cell phenotype

Myeloid-derived suppressor cells (MDSCs) are immature myeloid cells critical in mediating immune suppression in cancer patients. To develop an in vitro assay system that functionally mimics the tumor microenvironment, we cultured human monocytes with conditioned media from several cancer cell lines. Conditioned media from five tumor cell lines induced survival and differentiation of monocytes into cells characteristically similar to macrophages and MDSCs. Notably, media from the 786.O renal cell carcinoma line induced monocytes to acquire a monocytic MDSC phenotype characterized by decreased HLA-DR expression, increased nitric oxide production, enhanced proliferation, and ability to suppress autologous CD3(+) T cell proliferation. We further demonstrated that these in vitro MDSCs are phenotypically and functionally similar to patient-derived MDSCs. Inhibitors of STAT3, CK2, and GM-CSF resulted in partial reversal of the MDSC phenotype. MDSCs generated in vitro from 786.O tumor conditioned media represent a platform to identify potential therapeutics that inhibit MDSC activities.