Clinical model: interferons activate human monocytes to an eradicative tumor cell level in vitro

Eradicative levels of antitumor activity by cytokines and leukocytes have not yet been reached experimentally and are needed clinically. Only a limited number of human cancers respond to therapy with interferon (IFN), other cytokines, or mononuclear leukocytes despite significant antitumor activity in vitro. We studied the IFN and monocytic cell conditions that would lead to an eradicative effect using human cells in vitro. Targets of the IFN-activated monocytic cells were either four human tumor cell lines (human osteosarcoma [HOS], LOX melanoma, A549 lung tumor, and SNB-19 glioblastoma) or two diploid cell lines (WI38 and MRC5). An average of 30-90 colony-forming tumor target cells were cultured overnight in 96-well tissue culture plates prior to treatment with serially diluted IFN with or without activated elutriation-purified monocytes or lymphocytes. The target cell colonies were treated for 3 days. The colonies were then stained with crystal violet to determine the levels of antitumor activity. IFN-activated human monocytes reached an eradicative level (95%-100%) against three of four tumor cell lines. The eradicative level (1) was induced best in human monocytes activated by combined type I and II IFNs, (2) was effective against tumor cells that were growing for 24 h, (3) was specific for human tumors, as diploid human cells were not inhibited, and (4) required contact between the macrophage and the tumor cells. Also, for the first time, the minimal effective concentration (MEC) of IFNs to activate monocytes can approach those needed for antiviral activity. To our knowledge, this is the first report of near total eradication of many tumor cells, but not diploid cells, by IFN-activated monocytes. Because of its potency and specificity, the IFN-activated monocyte arm of the innate immune system may be a candidate for therapy of established tumors.