IL-10 and M-CSF up-regulate Gas6 expression. (A) Cocultures of CT26 tumor cells with tumor-infiltrating macrophages isolated from CT26 tumors grown in WT and Gas6^−/− hosts reveal that absence of Gas6 in macrophages significantly reduces their ability to stimulate proliferation of tumor cells in vitro (n = 3, P < .05). (B) ELISA revealing that IL-10 and M-CSF, but not IL-4, up-regulate Gas6 levels in the medium of cultured peritoneal macrophages after 24 hours (n = 3; P < .005). (C-D) ELISA measurement of CT26 tumor lysates showing similar IL-10 and M-CSF protein levels in WT and Gas6^−/− mice (n = 8-9; P = NS).