SUPPRESSOR CELLS IN HOMOGRAFT TOLERANT RATS

Summary If sufficient normal syngeneic lymphocytes to effect skin graft rejection are transferred to homograft tolerant rats, a prolonged period elapses before lymphoid cells from the recipient acquire normal levels of GvH responsiveness against tissues of which the donor was previously tolerant (Silvers and Billingham, 1970; Elkins, 1972; Miyamoto and McCullagh, 1974). Although the ability of lymphoid populations of such animals to mount GvH reactions can be demonstrated to reside in donor type cells during the weeks immediately after transfer, reactive cells are ultimately derived from the host itself (Elkins, 1973; Miyamoto and McCullagh, 1974). Not only are lymphoid cells from tolerant rats which have been injected recently with normal lymphocytes poorly responsive in a GvH assay, but they have been observed in some experiments to suppress the GvH activity of normal syngeneic lymphoid cells (Elkins, 1972; Atkins and Ford, 1972). It is not clear whether the cells mediating suppression of the normal lymphocytes were derived from the tolerant host itself or, alternatively, from the normal lymphocytes injected into it to terminate the tolerant state. The present experiments sought to delineate the origin of any suppressor cells within populations of lymphocytes collected from rats in which tolerance had recently been terminated. They indicate that suppression of the normal donor cells within such populations may be exerted by cells derived from the tolerant host.