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Cyclophilins together with FK-506-binding proteins and parvulins, belong to a group of proteins that have peptidyl-prolyl cis-trans isomerase activity. They also belong to proteins that are collectively known as immunophilins. Cyclophilins are found in all cells of all organisms studied, in both prokaryotes and eukaryotes. The first member of the cyclophilins to be identified in mammals, cyclophilin A, is the major cellular target for the immunosuppressive drug cyclosporin A. This review discusses recently available data about proteins with cyclophilin domain (CBD). Recent studies have implicated a diverse array of additional cellular functions for cyclophilins, including roles as chaperones and in cell signalling as well as in several human diseases.

Members of the FKBP family play various functions within the cell. For T cell biology essential is their involvement in the regulation of cytokine genes transcription, mainly at the level of nucleocytoplasmic transport of transcription factors. FKBP12 is the mediator of immunosuppressive action of FK506. When complexed with the drug, FKBP12 blocks nuclear import of NFAT and formation of AP-1 heterodimer, due to inhibition of calcium-dependent phosphatase calcineurin and JNK/p38 pathways. Suppression of these two, and possibly some other signaling pathways leads to prevention of IL-2 expression and T cell activation. FKBP51 and FKBP52 are natural components of glucocorticoid receptor complex and direct regulators of its activity. Upon ligand binding FKBP51, maintaining receptor in the cytoplasm, is exchanged by FKBP52, which allows translocation of the complex to the nucleus. Thereby FKBPs take a part in the regulation of immune response by glucocorticoids.