The in vitro influence of thymus cells on natural killer cell activity of spleen cells against prelabeled target cells (YAC-I and RL♂I) has been studied in syngeneic as well as in allogeneic murine models. In mixing experiments to demonstrate suppression, total thymocytes have been found to have no effect on NK activity of syngeneic or allogeneic spleen cells. Among several thymocyte fractions separated by velocity sedimentation, a relatively faster sedimenting fraction showed remarkable suppression of NK activity by spleen cells against two target cells. The suppressive effect of this particular fraction on NK activity was demonstrated to be proportional to the cell dose. The suppressive function was resistant to irradiation at 1000 or 2000 rad administered in vitro and was not restricted by the major histocompatibility complex. Moreover, the thymocyte fraction which induced suppression was not sensitive to NK-mediated cytolysiś by syngeneic spleen cells. The suppression of NK cytolysis in vitro by certain subpopulations of thymocytes as observed in the present studies may be consistent with a role for the thymus in regulating NK activity in vivo.
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