Control of autoreactivity by a humoral factor of the thymus (THF)

Anti-self reactivity of autosensitized lymphocytes was evaluated by assay of the capacity of these cells to induce an in vitro GVH response against syngeneic tissue, and confirmed by induction of a syngeneic GVH response in vivo and by measurement of the proliferation of the sensitized cells in a syngeneic MLC. The anti-self reactivity manifested by spleen or thymus cells after sensitization on monolayers of syngeneic fibroblasts was clearly inhibited by addition of the thymic humoral factor (THF) to the lymphocytes undergoing or about to undergo sensitization. When these lymphocytes were obtained from neonatally thymectomized mice, inhibition of self reactivity by THF occurred simultaneously to THF-mediated acquisition of allogeneic reactivity in the same cell suspension. Spleen cells from neonatally thymectomized mice also exhibited directly the capacity to induce a syngeneic GVH reaction, indicating that in the absence of the thymus these cells had undergone autosensitization in vivo. This autoreactivity was prevented by incubating the in vivo-sensitized cells with THF. Also the in vivo GVH response elicited by spleen cells from neonatally thymectomized mice in syngeneic animals was inhibited by injection of THF to the donor mice. It is postulated here that thymic involvement in the processes controlling autoreactivity is at least partially mediated by THF.