Cyclophilin A (leading to calcineurin inhibition)

Class

Small molecule calcineurin inhibitor (cyclosporine-class immunosuppressant, oral)

Pathway

Calcineurin-NFAT signaling; blocks IL-2 gene transcription in T cells

Cyclophilin A is an intracellular protein belonging to the immunophilin family, present in T lymphocytes and many other cell types. On its own, cyclophilin A has enzymatic activity (peptidyl-prolyl isomerase) involved in protein folding, but it also serves as the binding partner for cyclosporine-class drugs. When a drug such as voclosporin binds cyclophilin A, the resulting complex inhibits calcineurin, a calcium-dependent phosphatase. Calcineurin normally removes phosphate groups from a transcription factor called NFAT (nuclear factor of activated T cells), allowing NFAT to move into the nucleus and switch on genes, most notably interleukin-2, that drive T cell activation and proliferation. By blocking calcineurin, cyclophilin A-binding drugs prevent this step, dampening the immune response at its source. This mechanism is central to preventing organ transplant rejection, where excessive T cell activation against foreign tissue must be controlled, and it is also exploited in autoimmune and inflammatory diseases where T cell-driven immune activity contributes to tissue damage, such as certain kidney disorders. Because calcineurin inhibition acts broadly on T cell signaling rather than on a single antigen, this class of immunosuppressant provides potent, relatively rapid control of immune activation across many T cell-mediated conditions.