Effect of a new immunosuppressive agent, FK506, on human lymphocyte responses in vitro. I. Inhibition of expression of alloantigen-activated suppressor cells, as well as induction of alloreactivity

The effect of FK506 on in vitro human lymphocyte responses was assessed in comparison with cyclosporine. FK506 suppressed, in a dose-dependent fashion, the lymphocyte response to stimulation with PHA and with alloantigens in primary mixed lymphocyte reactions at a 70-100-fold lower concentration than CsA--namely, 50% inhibition (IC50) was obtained with 8.6 nM FK506 and with 750 nM CsA in the PHA response, and with 0.21 nM FK506 and with 20 nM CsA in MLR. Allocytolytic T lymphocyte induction was also inhibited by FK506, whereas the ability of CTL to lyse targets was not affected by the agent, indicating that FK506 did not affect the recognition and binding of alloantigen by CTL. FK506 inhibited, in a dose-dependent fashion, both IL-2 receptor and transferrin receptor expression on the alloactivated lymphocytes--whereas this agent inhibited only incompletely both expression of both receptors on lymphocytes stimulated with PHA. Lymphocytes from primary MLR cultured in the presence of FK506 were tested for suppressor cell activity on day 8 of culture. FK506 did not allow for the expression of alloantigen-activated suppressor cells when used in a dose sufficient to inhibit CTL generation.