The antigen-inexperienced thymic suppressor cells: a class of lymphocytes in the young chicken thymus that inhibits antibody production and cell-mediated immune responses.

Transfer of thymus cells from young chickens in combination with a light whole body irradiation (360 R) was found to suppress the rejection of skin grafts across strong histocompatibility (B) differences. On the average, the suppressed animals also showed decreased serum hemagglutinin titers against erythrocytes of the skin donor strain and a decreased graft-versus-host (GvH) reactivity against embryos of this strain. The thymic suppressor cells can be obtained from animals that have not experienced the antigen under test. However, after transfer and contact to the antigen (skin graft) they can lead to the formation of specific ("activated") suppressor cells and can mediate in the long run a specific inhibition of the response to this antigen. The suppressive activity is associated with a bursa-dependent cellular subpopulation in the thymus that is different from B lymphocytes, B precursor cells or GvH-reactive T cells. The bursa dependency of the thymic suppressor cell suggests that functionally different lineages of thymic and thymus-derived lymphocytes are derived from different sources of prethymic stem cells. The suppressor cells are predominantly found in the young chicken thymus and already detectable in the 16-day-old embryo, while poor suppressive activity is found in the adult thymus. The suppressive effect can be obtained with thymus cells from either syngeneic or allogeneic donors. Embryonic allogeneic donors provide suppressive cell preparations free of GvH reactivity. The possibility that the thymus suppressor cells mediate self tolerance and "neonatal tolerance" is discussed.