Novel immunosuppressive agent, FK506. In vitro effects on the cloned T cell activation

The present study shows the in vitro effects of a novel immunosuppressive agent, FK506, in comparison with cyclosporin A (CsA). FK506 inhibited concanavalin A response and allo-mixed lymphocyte reaction of murine splenic lymphocytes in a dose-dependent manner, and at 40- to 200-fold lower concentrations than CsA. Allo-cytolytic T lymphocyte induction from murine thymocytes was also inhibited by FK506, whereas the ability of cytolytic T lymphocyte to lyse targets was not affected by the agent. Immunosuppressive effects of FK506 were further characterized by using antigen specific-proliferative T lymphocyte clones, BC.21 and KO.6. FK506 inhibited the proliferation of T cell clones stimulated with specific antigens in a dose-dependent manner, and at about 100-fold lower concentrations than CsA. However, cloned T cells, once activated, were scarcely affected by the agent; interleukin-2 (IL-2) driven proliferation of cloned T cells was not inhibited. On the other hand, it was found that FK506 inhibited both IL-2 secretion and IL-2 receptor expression of BC.21 after stimulation with the specific antigen. FK506 also inhibited the proliferation of BC.21 stimulated with phorbol 12-myristate 13-acetate plus calcium ionophore, indicating that it directly affected the signaling pathway downward from the perturbation of the Ti/T3 complex. Finally, it was suggested that FK506 and CsA synergistically inhibited the antigen-driven proliferation of cloned T cells. These results indicate that the novel immunosuppressive agent, FK506, affects T cell activation with mechanisms similar to those of CsA but at considerably lower concentrations.