The effect of cyclophosphamide and irradiation on cells which suppress contact sensitivity in the mouse.

Contact sensitivity was produced in mice by painting the skin with picryl chloride and was assessed by the increase in ear thickness following local challenge. Contact sensitivity was passively transferred by immune lymph node and spleen cells taken at 4 days. The mice were then challenged immediately and the reactions read at 24 and 48 hr. Immune lymph node and spleen cells taken at day 8 virtually fail to transfer. Experiment showed that they contain cells which suppress passive transfer. These are demonstrated by mixing approximately equal numbers of 4-day cells, which transfer contact sensitivity, and cells taken at later times and injecting them intravenously into recipients. These 'suppressor cells' can be demonstrated by day 6 and are still present at day 11 after immunization. The precursors of the suppressor cells are sensitive to cyclophosphamide. Irradiation of immune mice 2 days before taking cells also selectively inactivates the suppressor cells. When mice are pretreated with cyclophosphamide before immunization or irradiated 2 days before transfer, the lymph node and spleen cells taken on day 9 after immunization transfer contact sensitivity. In contrast the same number of cells from untreated mice were inactive. This suggests that the cells which mediate passive transfer or their precursors may occur in an inhibited form in lymph nodes and spleen at later times after immunization. These suppressor cells in immune mice differ from the T suppressor cells produced by the injection of picryl sulphonic acid--an agent which causes unresponsiveness: (1) the precursors of the T suppressor cells resist cyclophosphamide; (2) the T suppressor cells are found in mice treated so as to produce unresponsiveness while the other type of suppressor cells occurs in mice immunized for contact sensitivity. However, both types of suppressor cells are selectively inactivated by irradiation as compared with the cells which mediate contact sensitivity and both are able to act on the effector stage of contact sensitivity.