Inhibition of T-lymphocyte activation by the immunosuppressive drug FK-506

Nanamolar concentrations of the immunosuppressive drug FK-506 inhibit the induction of T-lymphocyte proliferation by the lectins concanavalin A (Con A) and phytohaemagglutinin (PHA). Activation by Con A is more sensitive to inhibition than the response to PHA. FK-506 inhibits an early Ca2+-dependent step in the activation process, and its effects are not reversible by the addition of recombinant interleukin-2 (IL-2) or lymphokine-rich culture supernatant. While the effects of suboptimal concentrations of FK-506 and cyclosporin A (CsA) are additive, FK-506 does not enhance the effects of optimal concentrations of CsA. Both drugs also have similar effects on the expression of specific mRNA in Con A-activated lymphocytes. A brief preincubation of unstimulated cells with FK-506 irreversibly inhibits their subsequent responsiveness to Con A. The mechanism of action of FK-506 thus resembles that of CsA, except that it is effective at two to three orders of magnitude lower concentrations and its effects are much less readily reversible.