Lymphokine-activated human blood monocytes destroy tumor cells but not normal cells under cocultivation conditions

The possibility that tumoricidal human blood monocytes would recognize and destroy tumorigenic targets but leave bystander nontumorigenic cells unharmed was investigated. Highly purified preparations of peripheral blood monocytes isolated from normal human donors were activated in vitro by incubation with human lymphokines encapsulated in multilamellar liposomes. The cytotoxic properties of these monocytes against several tumorigenic and nontumorigenic allogeneic target cell populations were assessed by an in vitro radioisotope-release assay. Various combinations of three tumorigenic and three nontumorigenic target-cell populations, labeled with either [³H]thymidine or [¹⁴C]thymidine, were mixed and plated onto monolayers of blood monocytes. In all combinations used, activated monocytes specifically lysed only allogeneic neoplastic cells. At least in vitro, activated human blood monocytes can recognize and selectively destroy neoplastic cells.