Differentiation of immunologically specific cytotoxic macrophages into two types on the basis of radiosensitivity

Macrophages which are cytotoxic in vitro to tumour cells in an immunologically specific manner can be classified into (1) radiosensitive, cytotoxicity totally abolished by 500 R of X-rays; and (2) radioresistant, cytotoxicity unaffected by 500 R but partially ablated by 1,000 R. Radiosensitive macrophages are found in both syngeneic and allogeneic mice immunized with radiation-killed lymphoma cells. Radioresistant macrophages are produced by immunizing mice with allogeneic living lymphoma cells. In a syngeneic system radioresistant macrophages are produced when mice, which have first been rendered immune by immunization with radiation-killed cells, have rejected an inoculation of living cells. Immune or armed macrophages which have been rendered nonspecifically cytotoxic (i.e., "activated " by incubation with the antigen to which they are sensitized) or normal macrophages activated by treatment with endotoxin show very great resistance to irradiation and retain activity after exposure to 5,000 R in vitro.