Inhibition of cytokine production and cytokine-stimulated T-cell activation by FK506 (Tacrolimus)1

Insofar as it exerted its immuno suppressive effect by inhibiting cytokine expression, we assessed the effect of FK506 (Tacrolimus) on cytokine-stimul ated T-cell activation. Human T cells, treated with FK506, or controls were stimulated with the mitogens PHA + PMA, Con A, and the "CD3-bypass" stimul ation regimen, PMA + ionomycin. T-cell proliferation was quantitated by mea suring the uptake of tritiated thymidine, and mRNA expression was assessed by RT-PCR. FK506, in a concentration-dependent fashion, inhibited T-cell pr oliferation and steady-state mRNA expression of IL-2 and IL-7; half-maximal suppression was obtained at 10(-7) to 5 x 10(-8) M. We tested whether FK50 6 antiproliferative effect could be overcome with exogenously reconstituted rIL-2 and/or rIL-7. Neither rIL-2 nor rIL-7, individually in conjunction w ith suboptimal concentrations of PHA or Con A, or in combination without an y costimulus, was capable of abrogating FK506 antiproliferative effect, ind icating that FK506 also acted by inhibiting cytokine-stimulated T-cell acti vation. To confirm this, T cells were treated with FK506 and stimulated by rIL-2 and rIL-7, individually in conjunction with suboptimal concentration of PHA and Con A. In addition, T cells were stimulated with rIL-2 and rIL-7 without any costimuli. FK506 inhibited T-cell activation stimulated by rIL -2 and by rIL-7, individually and in combination. This confirms that, in ex erting its antiproliferative effect, FK506 acts at two levels, by inhibitin g cytokine availability and by suppressing cytokine effect on target cells, and explains the beneficial effect of FK506 in attenuating ongoing immune responses.