51Cr-labeled sheep erythrocytes sensitized with mouse antibody were lysed by mouse spleen cells and thymocytes. Treatment of the thymocyte donors with cortisone acetate revealed the responsible effector cells to be cortisone-resistant (medullary) thymocytes. Analysis of the thymocyte preparations for cell surface antigens revealed that fewer than 1% of the cells possessed surface Ig detectable by anti-Ig immunofluorescence and 94% of the cells were lysed by anti-thy 1 and C. Less than 0.5% of the cells in the thymocyte preparations were macrophages as determined by their ability to ingest IgG-sensitized erythrocytes. The peak of K cell activity in the thymus occurred at 48 hr after the injection of cortisone. In vitro kinetics experiments revealed that whereas splenic K cell activity reached near maximum levels in 6 to 8 hr (E:T ratio of 10:1), thymocyte-mediated ADCC continued to increase over the entire 72-hr period tested. Lower ratios of spleen cells (2:1 and 1:1) exhibited kinetics similar to thymocytes. Control thymocytes only produced low levels of cytotoxicity by 20 hr, but by 40 and 72 hr in vitro quite significant lysis was produced. Cortisone-resistant thymocytes produced higher levels of cytotoxicity than control thymocytes at all time points. By 72 hr in vitro, the cortisone-resistant thymocytes produced lysis that was equivalent to that produced by spleen cells. The kinetics observed appeared to be the true rate of lysis rather than generation of K cells from precursors in the original population; preincubation of the thymocytes in vitro for 20 hr before the addition of the target cells resulted in no greater lysis than when fresh thymocytes were used. Furthermore, no proliferative response was detectable when thymocytes were incubated with antibody-sensitized target cells.

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