Role of the thymus in suppression of immune responses in newborn rats.

The role of thymus-derived cells in controlling the process of immunological maturation in the neonatal rat has been examined. Thymectomy performed on the day of birth resembled irradiation given at this time in that it permitted adoptive immune responses to sheep erythrocytes to be initiated in the neonatal host. Whereas the consequences of irradiation could be demonstrated if antigenic challenge was given of the following day, adoptive responses were not sustained unless an interval in excess of 24 h had elapsed after thymectomy, suggesting that extra-thymic suppressor cells persist for at least this period. Suppression of adoptive responses was achieved in irradiated hosts with thymus cells obtained from rats on their day of birth but not with cells from older donors. Using the transfer of allogeneic lymphocytes as a stimulus, antibody-forming cell precursors were shown to be plentiful in the newborn rat and it was accordingly suggested that thymic suppressor cells are reponsible for the poor responsiveness of newborn rats to antigenic challenge.