Progress towards a molecular understanding of cyclosporin A-mediated immunosuppression.

Immunosuppressive drugs like cyclosporin A (CsA), FK506 and rapamycin exert their immunosuppressive potential primarily by interfering with the activation and proliferation of T cells. These substances bind to intracellular receptor proteins, called immunophilins. Immunophilins are ubiquitous and abundant proteins, found in all prokaryotic and eukaryotic cells investigated, as well as in many subcellular compartments. Immunophilins display an inherent enzymatic activity, the prolyl-peptidyl cis-trans isomerase (PPIase). The eukaryotic PPIases are inhibited upon the binding of immunosuppressants. In addition, the complex of immunophilins and CsA or FK506 acquires a new activity, namely the binding and inhibition of the cytoplasmic Ca(2+)-binding phosphatase calcineurin. This inhibition is postulated to prevent the proper assembly and nuclear positioning of the transcription factor NF-AT (nuclear factor of activated T cells). The correct DNA binding of NF-AT to regulatory elements of the interleukin 2 (IL-2) promoter normally contributes to the transcriptional activation of this gene. Thus, immunosuppressive drugs prevent T-cell activation by interfering with Ca(2+)-dependent signal transduction pathways which regulate gene activities.