A cell culture system was used to investigate the mechanism of action of the feedback inhibition caused by specific 7S antibody. It was found that preincubation of spleen cells with specific 7S antibody led to a marked reduction in the in vitro response of the treated spleen cells to the antigen used to prepare the antibody. The inhibition was not caused by a carry-over of free antibody nor by the release of 7S antibody from the cells. Rather, the preincubation appeared to specifically inactivate one of the cells required for initiation of an in vitro response. Since the suppression could be reversed by addition of untreated cells, it was possible to characterize the properties of the reconstituting cell. This cell is identified as the nonlymphoid accessory cell (A cell) by several criteria. (a) Suppression can be demonstrated only in assay systems requiring functional A cells. (b) The most active sources for reconstitution are also good sources for A cells. (c) The sedimentation velocity of the reconstituting cell is identical with that for A cells. (d) Like A cells, the reconstituting cell is resistant to high doses of ionizing radiation. (e) The reconstituting ability is not affected by anti-θ antibody. Of the three cells required for the initiation of an immune response, A cells, bone marrow-derived cells, and thymus-derived cells, the data are only compatible with the reconstituting cell being an A cell.
Additional experiments suggest that the Fc portion of 7S antibody binds to the surface of A cells. Thus, fluorescein isothiocyanate-labeled 7S antibody binds specifically to cells with properties similar to those described above, and F(ab')2 fragments, lacking Fc portion, are unable to cause immunosuppression when they are preincubated with spleen cells. It is possible that this binding is related to the specific suppression caused by 7S antibody molecules.