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 that can be found in high numbers in the tumor microenvironment and are critical in mediating immune suppression in cancer patients. To develop an in vitro assay system that functionally mimics the tumor microenvironment, we cultured human blood monocytes with conditioned media from several cancer cell lines. Conditioned media from 4 out of 8 tumor cell lines tested induced survival and differentiation of monocytes into cells with characteristics similar to macrophages and MDSCs. Media from one cell line in particular, 786.O (a renal cell carcinoma line), induced the monocytes to acquire a monocytic MDSC phenotype characterized by a decrease in cell surface expression of HLA-DR, an increase in nitric oxide (NO) production, and changes in morphology similar to an immature myeloid cell. Additionally, these in vitro generated MDSCs suppressed autologous CD3+ T cell proliferation. The tumor conditioned MDSCs were compared, functionally and phenotypically, to MDSCs derived using GM-CSF and IL-6 and to M1 and M2 differentiated and polarized macrophages. We further demonstrated that the in vitro MDSCs are phenotypically and functionally similar to patient-derived MDSCs. MDSCs differentiated in vitro from 786.O tumor conditioned media represent a platform to identify potential therapeutics that inhibit MDSC activities.