Two distinct action mechanisms of immunophilin-ligand complexes for the blockade of T-cell activation.

The immunosuppressive effects of cyclosporin A (CsA) and FK506 are mediated through binding to immunophilins. Here we show that FK506-FKBP complex suppresses the activation of JNK and p38 pathways at a level upstream of mitogen-activated protein kinase (MAPK) kinase kinase (MAPKK-K) besides the calcineurin-NFAT pathway. A238L, a viral gene product that binds to immunophilin, also blocks activation of both pathways. In contrast, direct inhibitors of calcineurin, Cabin 1 and FR901725, suppress the activation of NFAT but not the JNK or p38 pathway. We further demonstrate that co-expression of a constitutively active NFAT and a constitutively active MEKK1 renders the interleukin-2 promoter in Jurkat T lymphocytes resistant to CsA and FK506, whereas Jurkat cells expressing a constitutively active NFAT alone are still sensitive to CsA or FK506. Therefore, CsA and FK506 exert their immunosuppressive effects through targeting both the calcineurin-dependent NFAT pathway and calcineurin-independent activation pathway for JNK and p38.