Suppressor activity of the human spleen and thymus.

Both the spleen and thymus of man contain a population of cells which can suppress the mixed lymphocyte reaction. Splenic cells suppress the MLC 58 +/- 4.8 percent, and the thymus is able to suppress the MLC 90 +/- 2.6 percent. However, splenic cells require stimulation by vegetable mitogens before suppressor activity can be observed, and the thymus displays spontaneous suppressor activity without prior stimulation. The suppressor effect is linearly related to the log of the cell dose (r = 0.8, p less than 0.01), and at low doses the suppressor cells stimulate rather than inhibit the MLC. The cells are exquisitely sensitive to immunosuppressive drugs, and blood levels encountered in clinical organ transplantation abrogate their suppressive activity. The suppressive activity of the cells is nonspecific, and they are able to inhibit the MLC between individuals totally unrelated to the spleen or thymus donor. Since these cells likely play a role in the development of tolerance, further characterization of their properties is required.