Macrophages suppress CTL generation in rat mixed leukocyte cultures

The generation of CTL in rat MLC was actively suppressed by a cell population present in spleen cell preparations from normal rats. These suppressor cells were characterized by using a variety of cell fractionation techniques. Suppressor cells were removed by passage of spleen cells through nylon wool columns or by treatment with carbonyl iron. Suppressive activity was present in the mononuclear cell fraction of spleen cells obtained by Ficoll-Hypaque density gradient centrifugation. After velocity sedimentation at unit gravity, enrichment of suppressive activity was demonstrated in the fractions containing large cells as compared to the fractions containing small cells. Populations rich in macrophages were shown to have similar suppressive activity upon CTL induction in MLC. These studies suggest that macrophages present in normal rat spleen cell preparations account for the difficulty in generating CTL in MLC prepared with rat cells.