In vivo reactivity of mouse natural killer (NK) cells against normal bone marrow cells

An in vivo role for mouse natural killer (NK) cells in the rapid rejection of transplantable tumors has been previously demonstrated, using an assay of elimination of [¹²⁵I]iododeoxyuridine-labeled tumor cells from the lungs and other organs. We have now used the same technique to examine the role of NK cells in in vivo clearance of syngeneic or allogeneic bone marrow cells from normal mice. The degree of clearance from the lungs or liver, at 4 hr after intravenous inoculation of radiolabeled bone marrow cells, correlated with the levels of NK activity in the recipients. Young CBA mice, with high NK activity, showed substantially more clearance of bone marrow cells than SJL mice, with low NK activity. Within the same strain, mice at 7 weeks of age had higher in vivo as well as in vitro reactivity than did 30-week-old mice. These differences were seen with syngeneic bone marrow cells, but there was stronger in vivo reactivity against parental cells by F₁ hybrid recipients. Depression of NK activity by pretreatment of mice with cyclophosphamide, silica, or carrageenan also caused decreased clearance of syngeneic or parental bone marrow cells. Conversely, augmentation of NK activity by pretreatment of the mice with polyinosine:polycytidine resulted in increased in vivo clearance. These results indicate that NK cells may be involved in the in vivo control of growth of some normal cells as well as of some tumor cells.